- Email: [email protected]
Epidermal desquamation
Journal of Dermatological Science (2004) 36, 131—140
REVIEW ARTICLE
Epidermal desquamation Leonard M. Milstone Department of Dermatology, Yale University School of Medicine, P.O. Box 208059, New Haven, CT 06520-8059, USA Received 5 April 2004; received in revised form 23 April 2004; accepted 11 May 2004
Summary Epidermal desquamation, a continuous but insensible bodily activity, is largely ignored unless the rate or amount of scale production becomes abnormal. It is the last topic to be considered in any serious discussion of epidermal growth and differentiation, but is becoming an increasingly fertile ground for investigation. This review summarizes: (a) methods for measuring desquamation; (b) variables that affect normal desquamation; (c) mechanisms of desquamation; (d) the role of desquamation in nutritional homeostasis; and (e) the role of desquamation as a first line of defense. Consideration is given to whether desquamation might be harnessed to eliminate or remediate toxins that have accumulated in the body. ß 2004 Japanese Society for Investigative Dermatology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Epidermal desquamation is a fact of life for all mammals. This continuous but insensible bodily activity is largely ignored unless the rate or amount of scale production becomes abnormal. Little consideration has been given to basic questions such as: Why do mammals continuously shed scale? What function does desquamation serve? How does desquamation contribute to overall energy and nutritional balance? Surely, we do not make scale just to assist dermatologists in diagnosing disease or to provide a niche for house dust mites or to assist bloodhounds in their search for fugitives. In this review, I will summarize what we do know with regard to: (a) methods for measuring desquamation; (b) variables that affect normal desquamation; (c) mechanisms of desquamation; (d) the role of desquamation in nutritional homeostasis; and (e) the role of desquamation as a first line of defense. With this information as background, rather than taking desquamation for granted, we can begin to E-mail address: [email protected] (L.M. Milstone).
contemplate whether it offers unique therapeutic options. For example, might desquamation be harnessed to eliminate or remediate toxins that have accumulated in the body?
2. Measuring desquamation In many early studies, scale was collected from pajamas and bed sheets [1,2] or from specially fabricated garments [3]. The methods chosen for those studies had the potential to provide total number of shed cells and were particularly appropriate for metabolic studies. Unfortunately, those methods of collection and analysis of shed scale were cumbersome and were prone to sampling and interpretive error. Therefore, investigators sought methods that would be quicker, more easily controlled and less inconvenient for the investigator and the subject. Collection chambers were devised to collect scale reproducibly from small areas of skin, and a variety of methods confirmed their reliability for quantifying scale (reviewed in [4]). The relevance of those methods was validated by
0923–1811/$30.00 ß 2004 Japanese Society for Investigative Dermatology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.jdermsci.2004.05.004
132
L.M. Milstone
the observation that the number of squames removed by a short, controlled rinse of the epidermal surface with mild detergent correlated closely with the number of squames shed spontaneously over a longer period of time [5]. Extrapolation of data from such sampling methods is the origin of estimates that humans shed between 2 108 and 10 108 cells per day [5]. Much of the interesting information that we have about epidermal scale production comes from measurements that are not direct measures of desquamation. The continuous epidermal renewal that generates shed squames is a steady-state process– —that which is produced must be eliminated. Therefore, in the absence of significant apoptosis, scale production will be directly related to parameters that can be measured on epidermis in situ, such as labeling or mitotic indices, transit times and stratum corneum thickness. Stratum corneum cohesiveness and corneocyte size are additional measurements that provide unique insights into the process of desquamation and are sometimes used to make inferences about desquamation. Each of these parameters is related to the others, but each also can vary independently. As a result, there is no fixed relationship between these parameters, and none should serve as a true surrogate for counting exfoliated cells.
3. Effect of common variables on desquamation It is surprisingly difficult to assemble an authoritative list of how common variables affect desquamation. Many factors contribute to this unsatisfactory state: the number of studies that actually measure shed cells is small and confirmatory data are often completely lacking; few studies of the same variable use the same measures of desquamation; most studies focus on one or two variables and do not control for other variables which may be important. Finally, the standard deviation for shed cell counts Table 1
Age Sex Race Season Location
between individuals in most studies is often very high, suggesting that some critical variable is being overlooked. For example, many investigators accept the observation that the diameter of squames shed from normal epidermis is inversely proportional to the rate of desquamation, as measured by stratum corneum turnover [6]. Women are said to shed larger squames than men [7], suggesting slower desquamation in women. Yet rates of desquamation in men and women are the same [8]. Stratum corneum cohesiveness must be an important determinant of desquamation, and stratum cohesiveness is reportedly greater in women than men [9]. Yet, available data show no difference between men and women in the cell layers of the stratum corneum [10]. The accuracy of those reports has never been challenged; the apparent inconsistencies remain unexplained. Measurements relevant to epidermal desquamation are summarized in Table 1. Most investigations concur that anatomic location has a measurable effect on the size [7,11] and number [8] of desquamated cells, although there is disagreement about the precise rank order of desquamation at different sites. Inexplicably, African Americans desquamate at a greater rate than Caucasians or Orientals [12,13]. It is reported that desquamation is greater in summer than winter [8], though there is great individual variability in those data. Rates of desquamation were faster in younger than the older individuals in one study [14] but equal in another [5]. Limited data comparing desquamation from men and women revealed no difference, but were not the focus of the paper in which it was reported [8].
4. Mechanism of epidermal desquamation A growing body of literature indicates that desquamation is an active, not a passive process. In one of the earliest experiments to address this question,
Effect of common variables on desquamation Layers
Turnover time
Squame size
Desquamation
No difference [63—65] or " with age (males only) [10] No difference [10] — —
Slower (longer) in elderly [5,63] — — —
" with age [7] Female > male [7] — Summer < winter [8]
Limb > trunk > face > genital [10,64]
Forehead < arm < trunk [5,66]
Thigh ¼ abdomen > arm > forehead [7]
Young > old [14] or no difference [5] No difference [8] AA > Cauc ¼ Or [12,13] Summer > winter (elderly, only) [8] Forearm > back > abdomen [5]
AA, African American; Cauc, Caucasian; Or, Oriental.
Epidermal desquamation
the epidermal surface was covered and carefully protected from external influences. Recovery of readily detached, fully mature corneocytes led to the conclusion that neither friction nor grooming was required for shedding [15]. Likewise, spontaneous shedding of differentiated cells from stratified cultures of human keratinocytes emphasized that desquamation was the final, active step in the keratinocytes’ program of differentiation and required neither friction nor desiccation (Fig. 1) [16,17]. Ultrastructural examination of the stratum corneum provided the initial clues into the mechanism of spontaneous desquamation. One of the critical observations was that desmosomes decrease in size and distribution as the corneocytes approach the outer surface in this otherwise inert-appearing structure [18]. Since desmosomes have a central role in keratinocyte cell—cell adhesion, active
133
destruction or dissolution of desmosomes has been the root hypothesis of most mechanistic studies. The biochemistry and cell biology underlying desquamation are likely to be quite complex, but outlines of the program are beginning to emerge. Beyond the morphologic dissolution of the proteinaceous desmosomes [19], a number of observations point to the importance of more than one hydrolytic enzyme in desquamation. In experiments using pieces of stratum corneum and an ultrastructural endpoint, proteases alone were ineffective in digesting or altering desmosomes, but glycosidase treatment followed by protease treatment succeeded in significantly reducing the size of those desmosomes [20]. Analysis of proteins in the stratum corneum has been used to demonstrate the progressive degradation of the desmosomal proteins desmoglein I [19,21,22], desmocollin [23],
Fig. 1 Epidermal desquamation is the final event in a program of differentiation lasting approximately 4 weeks. Nearly a billion cells are lost each day from the surface of adult skin. Critical events in the active process that lead to normal desquamation are the rearrangement of desmosomes to the lateral edges of squames in the stratum corneum and their ultimate enzymatic digestion. Under appropriate conditions, stacked and shed squames have the shape of a tetrakaidecahedron [105].
134
desmoplakin (Bernard, unpublished, cited in [24]) and corneodesmosin [25]. Proteases have been isolated from stratum corneum and several are expressed or activated in epidermis as late products of terminal differentiation: stratum corneum chymotryptic enzyme (SCCE) [26]; stratum corneum tryptic enzyme (SCTE) [27,28]; cathepsin D [29]; and stratum corneum thiol protease (SCTP/cathepsinL2) [24,30]. Topical application of peptide protease inhibitors, chymostatin and leupeptin, increases stratum corneum thickness in mice [31]. Secreted lamellar bodies spill proteinases into the intercellular space between keratinocytes in the lower stratum corneum [32]. Isolated lamellar bodies contain proteases and glycosidases, leading to the postulate that this organelle might play a critical role in desquamation [33]. Whether the lamellar bodies contain the full complement of hydrolytic enzymes needed to degrade desmosomes remains an open question. Desmosomes may not be the only target substrate for proteases important in desquamation. In a murine model, absence of matripase/MT-SP1 is associated with loss of proteolytically processed filaggrin and a very cohesive stratum corneum [34]. Genetic deficiency of a cathepsin has been associated with thickened stratum corneum [35,36]. Conversely, genetic deficiency of a protease inhibitor causes a defective stratum corneum with a tendency to exfoliate prematurely [37]. In the latter two genetic ‘‘models’’, specific epidermal substrates or enzyme targets have not been identified. Future investigations into the subcellular distribution, activation and inhibition of these proteases promise to yield interesting cell biology. Lipids clearly play important roles in regulating desquamation and may do so by several distinct mechanisms. The contribution of ceramides to corneocyte adhesion is still not established, though they are not likely to be the central adhesive molecules in stratum corneum as once proposed [38]. It is now well established that intrinsic (inherited) or extrinsic (acquired) defects in the intercellular lipid barrier to transepidermal water loss leads to epidermal hyperplasia and abnormal desquamation and scale. Other inherited defects in lipid metabolism are not associated with epidermal hyperplasia and, therefore, must alter desquamation through mechanism(s) other than hyperproliferation. Steroid sulfatase deficiency, the cause of recessive Xlinked ichthyosis (RXLI), is associated with abnormal accumulation of cholesterol sulfate in the stratum corneum and the persistence of intact desmosomes high into the stratum corneum [39]. Topical application of cholesterol sulfate to mouse skin mimics those changes and causes scaliness [40], and cho-
L.M. Milstone
lesterol sulfate inhibits both trypsin and chymotrypsin in the test tube [31]. Several mechanisms have been proposed to explain how intrinsic lipid abnormalities might lead to abnormal intercellular lipid organization that, in turn, would cause abnormal scale (reviewed in [32]). Those hypotheses, which largely lack experimental verification, include (a) abnormal hydrogen bonding causing phase separations, (b) abnormal liquid—crystalline phase transitions, and (c) restricted access of proteases to their substrates.
5. Role of desquamation in nutritional homeostasis The contribution made by epidermal turnover and desquamation to the body’s caloric and nutritional balance has been largely ignored or generally dismissed as minor. Throughout the first half of the last century, there was an interest in nutritional imbalance as a cause of skin disease. Far less attention was given to the role in overall homeostasis of normal or diseased skin. Direct measurements relevant to this subject are really quite limited in scope and number. Studies published in the past 50 years generally have focused on protein and iron homeostasis and have addressed one of two questions: (1) Can exfoliative dermatitis or psoriasis result in negative nitrogen balance or iron deficiency anemia? (2) What role does loss through the skin have in normal iron homeostasis?. There is no doubt that patients with severe scaling dermatoses lose increased amounts of protein in epidermal scale compared to those with less severe disease or to normal individuals. At the turn of the last century, Schamberg et al. studied protein metabolism over many weeks in psoriatic patients on different protein diets [2]. They came to the remarkable conclusions that their rather debilitated patients with psoriasis were in positive nitrogen balance and that low protein diets had a beneficial effect on psoriasis. Their method for collecting epidermal scale was admittedly rudimentary and incomplete, and their focus was on demonstrating that nitrogen loss through scale could not account for the overall observed positive nitrogen balance. However, they clearly did show that more than 10 times as much nitrogen was lost in scales during periods of active compared to periods of quiescent disease. This subject was revisited 50 years later and when investigators found no evidence for positive nitrogen balance in erythrodermic patients, they considered the opposite hypothesis: Could increased epidermal turnover and desquamation in erythroderma cause negative
Epidermal desquamation
nitrogen balance? For those studies, subjects were either confined to bed so that scale could be periodically collected [1] or were placed in ‘‘spacesuits’’ to collect desquamated scale [3]. An erythrodermic patient lost significantly more protein in scale compared to either a psoriatic with limited guttate lesions or a control subject [1]; likewise, individual patients lost considerably more scale when they had active exfoliative erythroderma than when they went into a spontaneous or drug-induced clinical remission [3]. The increased protein loss through desquamation (5—10-fold more than normal) was commensurate with the increased turnover in psoriasis. Nonetheless, this amounted to no more than 20 g of scale per day, and the authors concluded that, relative to daily protein intake, this increased amount of lost scale would not represent sufficient protein loss to cause negative nitrogen balance. The role of epidermal desquamation in iron balance was recognized early from studies using parenterally administered radioactive iron to understand iron turnover in both normal people and in patients with iron overload. Normal individuals lose a total of 1—1.5 mg of iron each day [41], and cutaneous losses account for 20—25% of that total [42—44]. Iron lost through the skin is found principally in shed cells, not in sweat [42,45,46]. Iron in cell-free sweat normally accounts for no more than 10% of cutaneous iron loss [47], although sweat losses do vary with environmental conditions [45]. In situations of iron overload, daily iron loss through skin is greater than in normal individuals [42]. Unfortunately, increased iron loss through skin in hemochromatosis or other iron overload conditions is not sufficient to prevent iron accumulation in internal organs. If epidermal desquamation accounts for a significant proportion of daily iron loss, is it possible that exfoliative dermatitis or psoriasis might cause iron deficiency anemia? Substantial amounts of a parenterally administered radioactive iron tracer were eliminated through epidermal desquamation in patients with exfoliative dermatitis [44]. Several groups have reported increased amount of iron in epidermis of psoriatic plaques compared to uninvolved epidermis or normal epidermis [48,49]. In contrast, another group found much less iron in psoriatic plaques than in normal skin [50]. Any conclusions drawn from those studies regarding total-body iron homeostasis must be tempered by the fact that neither were there measurements of iron stores in other tissues (particularly bone marrow) nor was there consideration of the possibility that increased losses through desquamation might be compensated by increased absorption. Many
135
years ago, Marks and Shuster determined that patients with extensive skin disease lost more iron through their skin than normal individuals and that some of those patients were anemic [51]. They concluded–—after careful evaluation of the anemia, the iron stores and the iron turnover in those patients–—that increased iron loss from desquamation was rarely, if ever, the cause of iron deficiency anemia in patients with extensive skin disease (reviewed in [52]). Many of the studies of iron loss through skin, cited above [42,46,48—50], were based on measurements of full-thickness epidermal biopsies rather than on desquamated cells. Such methodology requires the authors to assume that the keratinocyte is parsimonious but that the epidermis as a whole is not. That assumption is reasonable, since there are no conflicting data. Thus, once a nutrient makes its way into a keratinocyte, it is neither returned to the circulation nor locally reutilized, but rather is held tightly by the (parsimonious) keratinocyte and gets swept away in the relentless tide of differentiation that terminates in (wasteful) desquamation.
6. Role of desquamation as a first line of defense We return now to the question of just why is continuous and active desquamation a property of normal epidermis? Is desquamation, like molting, just another evolutionary solution to the problem of how to produce a fresh, intact barrier in the face of accumulated minor damage and/or changing body mass? Or does it reflect the need for great flexibility in continuously modifying the composition of a barrier that will be exposed to a wide variety of environmental niches and challenges? There is no answer to these questions, but a few lines of investigation suggest that continuous desquamation might be of great advantage as a first line of defense against a myriad of known as well as unanticipated or novel physical, chemical or toxic assailants. The first clue comes from the observation that a very large proportion of intrinsic (genetic) defects in the epidermis cause abnormalities in desquamation. Table 2 demonstrates that abnormal scale production is not the result of defects in one class of epidermal protein but rather in a very wide array of proteins involved in structure, signaling and metabolism. Together, these defined defects suggest that excess scale or desquamation is the default pathway for trouble in the epidermis. Likewise, clinical experience reveals that abnormal desquamation is one common response to a variety
136
Table 2
L.M. Milstone
Gene mutations responsible for scaly skin
Disease
Inheritance
Gene/protein
References
AD AD AD AD AD AD AD HI AR AD
KRT5,14/keratins 5,14 KRT1,10/keratins 1,10 KRT2e/keratin 2e KRT9/keratin 9 KRT6A,6B,16,17/keratins 6A,6B,16,17 KRT1, KRT16/keratins 1,16 KRT4,13/keratins 4,13 DSP/desmoplakin DSP/desmoplakin DSG1/desmoglein 1
[67] [68—70] [71,72] [73] [74,75] [76,77] [78] [79] [80] [81]
Cornified envelope Lamellar ichthyosis Keratoderma (Camissa)
AR AD
TGM1/transglutaminase 1 LOR/loricrin
[82] [83]
Membrane transporters Darier disease Acrokeratosis verruciformis Hailey—Hailey disease Lamellar ichthyosis
AD AD AD AR
ATP2A2/SERcalciumATPase ATP2A2/SERcalciumATPase ATP2C1/??Pmr1 ABCA12
[84] [85] [86] [87]
Cell junction proteins PPK with deafness (Vohlwinkle) Erythrokeratoderma variabilis Keratitis ichthyosis deafness (KID)
AD AD AD
GJB2/connexin 26 GJB3/connexin 31 GBJ2/connexin 26
[88] [89] [88,90]
Metabolic enzymes X-linked ichthyosis Tyrosinemia II (Richner—Hanhart) Sjogren—Larsson Refsum Conradi—Hunermann CHILD
X-LR AR AR AR X-LD X-LD
STS/steroid sulfatase TAT/tyrosine animotransferase FALDH/fatty aldehyde dehydrogenase PAXH/phytanoyl CoA Hydroxylase EBP/3b-hydroxysterol d7-isomerase EBP/3b-hydroxysterol d7-isomerase NASDHL/3b-hydroxysterol dehydrogenase CGI-58/?? ALOXE3/lipoxegenase 3 ALOX12B/12 (R)-lipoxygenase
[91] [92] [93] [94] [95,96] [97] [98]
Organelle=function Cytoskeleton Epidermolysis bullosa simplex Epidermolytic hyperkeratosis Ichthyosis bullosa of Siemans Epidermolytic PPK Pachyonychia congenita Non-epidermolytic PPK White sponge nevus Non-epidermolytic PPK (striate)
Chanarin—Dorfman (neutral lipid storage) Congenital ichthyosiform erythroderma
AR AR
Protease/protease inhibitors Papillion—Lefevre keratoderma Ichthyosis linearis circumflexa (Netherton)
AR AR
CTSC/cathepsin C SPINK5/serine protease inhibitor 5
[35,36] [101]
Secreted proteins Mal de Meleda keratoderma
AR
SLURP1/??
[102]
DNA repair/transcription Trichothiodystrophy
AR
ERCC2/XPD ERCC3/XPB
[103] [104]
of extrinsic assaults or challenges, such as friction, abrasion or chemical disruption of the lipid barrier. A second clue comes from toxicology. After years of studying the mutagenic potential of synthetic chemicals, Bruce Ames et al. turned their attention to natural products. To everyone’s surprise, they found that many chemicals isolated from dietary or garden plants had significant mutagenic potential
[99] [100] [100]
[53]. In an attempt to reconcile man’s ability to live in such an apparently toxic environment, they proposed a list of mechanisms for defending against chemical assault. At the top of their list was one that had, to the best of my knowledge, previously been given little attention. ‘‘Defenses that animals have evolved are mostly of a general type, since the number of natural
Epidermal desquamation
chemicals that might have toxic effects is so large. General defenses offer protection not only against natural but also against synthetic chemicals. These defenses include the following: (a) the continuous shedding of cells exposed to toxins; (b) the induction of a wide variety of general detoxifying mechanisms; (c) the active excretion of planar hydrophobic molecules out of liver and intestinal cells; (d) DNA repair; (e) animals’ olfactory and gustatory perception of bitter, acrid, astringent, and pungent chemicals. . ... That defenses are usually general, rather than specific for each chemical, makes good evolutionary sense.’’ [54]. A similar broad view of desquamation has been proposed as a generic way to resist assault by microbes. Increased desquamation, whether part of an innate or acquired response, is considered to be an important factor in locally limiting and ultimately clearing human dermatophyte infections [55,56]. To explain the inherently high rates of epidermal turnover and desquamation in the bottlenose dolphin, it has been suggested that rapid sloughing might limit colonization by surface microorganisms [57].
7. Proposal to harness desquamation to remediate internal toxins Given the preceding perspective on the functions and consequences of epidermal desquamation, let us consider whether there might be circumstances in which desquamation might be extended beyond its usual protective function and be harnessed to eliminate internal toxins. The concept is that if the epidermis could be turned into a sponge or sink for an internal toxin, then the renewal process that culminates in desquamation could eliminate that toxin. Iron can be used as a model toxin to illustrate these requirements, since iron overload is common, iron is quite toxic and a great deal is known about the physiology of iron uptake and elimination. For such a scheme to be feasible, several minimum requirements must be met. The first requirement is that there must be natural or pharmacological mechanisms for transporting the toxin into keratinocytes. Iron is an essential nutrient for all proliferating cells and is normally transported into basal keratinocytes bound to its transport protein, transferrin, via receptormediated endocytosis. There is considerable information about the expression and regulation of the transferrin receptor, making manipulation of receptor function one potential pathway to increase iron transport into keratinocytes. Pharmacologically, there are several chemical relatives of nifedipine
137
[58,59] that act as iron ionophores and increase iron transport into keratinocytes [60]. The second requirement is that the toxin must be retained in the keratinocyte. Either the transport mechanism must be unidirectional or there must be an intracellular mechanism for sequestering the toxin. Iron is sequestered and retained in cells by the inducible storage protein, ferritin. The increased iron that is transported into cultured keratinocytes by the nitroso-derivative of nifedipine is retained in those cells, resulting in a substantial accumulation of iron over many days [60]. In addition, it must be assumed that epidermal differentiation does not result in cellular release of the toxin and reentry into the general body pool. As indicated above, there has never been a demonstration (which is not to say that it does not or cannot happen) that terminal differentiation of keratinocytes results in salvage of nutrients or other chemicals from the epidermis into the general circulation. The third requirement is that quantitative considerations should make this route of elimination feasible. Only a small fraction of dietary iron is normally absorbed, and absorption is generally limited by feedback mechanisms that sense body stores of iron [41]. Patients with hereditary hemochromatosis generally absorb four to five times as much iron as normal individuals, but their ability to eliminate that additional iron is quite limited. One quarter of absorbed dietary iron is eliminated through the skin in normal individuals (see above); more iron is eliminated through the skin in hemochromatosis [42], but not enough to prevent net iron accumulation. Periodic iron depletion through menstruation or phlebotomy reduces tissue accumulation of iron and delays onset of organ damage. Since one unit of blood contains 240 mg iron, phlebotomy every other month eliminates about 4 mg of iron per day and maintains iron homeostasis in hereditary hemochromatosis. Thus, increasing iron elimination through epidermis by 16-fold (0.25—4.0 mg per day) should be sufficient to prevent iron accumulation. The fourth and final requirement is that increased levels of the systemic toxin should have relatively little toxic effect on normal epidermis. Iron overload is a fairly common problem; yet, skin disease caused by iron overload rarely provokes visits to the dermatologist. A single report indicates an association between iron overload and thin, dry epidermis [61]. Another group has shown that iron overload in mice increases susceptibility to chemical carcinogenesis [18,62]. Neither of those studies addressed the issue of whether the effect of iron overload acted directly on the epidermis or indirectly through a systemic effect. Epidermis normally stores 1/10 the amount of iron stored in liver, and
138
liver can increase its store of iron 10-fold before toxic levels are reached. Thus, unless keratinocytes are much more susceptible to toxic effects of iron than hepatocytes, there should be ample room to increase iron levels in keratinocytes without incurring toxic effects. It would seem that harnessing desquamation to remediate iron overload is theoretically feasible, if not yet proven practical.
L.M. Milstone
[11]
[12]
[13]
[14]
8. Conclusion Epidermal desquamation, the last topic to be considered in any serious discussion of epidermal growth and differentiation, is becoming an increasingly fertile ground for investigation. The catalogue of environmental and endogenous factors that alter desquamation is large and growing rapidly. There is lively interest in the mechanisms by which desquamation occurs and there is renewed appreciation that it is an active, not a passive, process. Desquamation is a factor in normal homeostasis of certain essential nutrients. There are hints that desquamation acts as a first-line, innate mechanism for resisting external assault. Ongoing research in desquamation promises to yield more effective and creative control over this always essential, and sometimes pathological, process.
References [1] Block WD, Lea WA, Curtis AC, Cannon EF. Nitrogen balance and sulfur excretion studies in psoriasis. J Invest Dermatol 1958;30:287—93. [2] Schamberg JF, Kolmer JA, Ringer AI, Raiziss GW. Research studies in psoriasis. J Cutan Dis 1913;31:803—912. [3] Freedberg IM, Baden HP. The metabolic response to exfoliation. J Invest Dermatol 1962;38:277—84. [4] Serup J, Jemec G. Handbook of non-invasive methods and the skin. Boca Raton: CRC Press; 1995. p. 143—58. [5] Roberts D, Marks R. The determination of regional and age variations in the rate of desquamation: a comparison of four techniques. J Invest Dermatol 1980;74:13—6. [6] Grove GL, Kligman AM. Corneocyte size as an indirect measure of epidermal proliferative activity. In: Marks R, Plewig G, editors. Stratum Corneum. Berlin (Heidelberg, New York): Springer—Verlag; 1983. p. 191—5. [7] Plewig G. Regional differences of cell sizes in the human stratum corneum. II. Effects of sex and age. J Invest Dermatol 1970;54:19—23. [8] Herrmann S, Scheuber E, Plewig G. Exfoliative cytology: effects of the seasons. In: Marks R, Plewig G, editors. Stratum Corneum. Berlin (Heidelberg, New York): Springer—Verlag; 1983. p. 181—5. [9] Nicholls S, Marks R. Novel techniques for the estimation of intracorneal cohesion in vivo. Br J Dermatol 1977;96:595— 602. [10] Ya-Xian Z, Suetake T, Tagami H. Number of cell layers of the stratum corneum in normal skin–—relationship to the
[15] [16]
[17]
[18] [19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
anatomical location on the body age sex and physical parameters. Arch Dermatol Res 1999;291:555—9. Plewig G, Marples RR. Regional differences of cell sizes in the human stratum corneum. I. J Invest Dermatol 1970;54:13—8. Weigand DA, Haygood C, Gaylor JR. Cell layers and density of Negro and Caucasian stratum corneum. J Invest Dermatol 1974;62:563—8. Corcuff P, Lotte C, Rougier A, Maibach HI. Racial differences in corneocytes. A comparison between black. Acta Derm Venereol 1991;71:146—8. Leyden JJ, McGinley KJ, Grove GL, Kligman AM. Age-related differences in the rate of desquamation of skin surface cells. Adv Exp Med Biol 1978;97:297—8 [proceedings]. Goldschmidt H, Kligman AM. Desquamation of the human horny layer. Arch Dermatol 1967;95:583—6. Sun TT, Green H. Differentiation of the epidermal keratinocyte in cell culture: formation of the cornified envelope. Cell 1976;9:511—21. Milstone LM, McGuire J, LaVigne JF. Retinoic acid causes premature desquamation of cells from confluent cultures of stratified squamous epithelia. J Invest Dermatol 1982;79:253—60. Allen TD, Potten CS. Desmosomal form, fate, and function in mammalian epidermis. J Ultrastruct Res 1975;51:94—105. Skerrow CJ, Clelland DG, Skerrow D. Changes to desmosomal antigens and lectin-binding sites during differentiation in normal human epidermis: a quantitative ultrastructural study. J Cell Sci 1989;92:667—77. Walsh A, Chapman SJ. Sugars protect desmosome and corneosome glycoproteins from proteolysis. Arch Dermatol Res 1991;283:174—9. Suzuki Y, Koyama J, Moro O, et al. The role of two endogenous proteases of the stratum corneum in degradation of desmoglein-1 and their reduced activity in the skin of ichthyotic patients. Br J Dermatol 1996;134:460—4. Lundstrom A, Egelrud T. Evidence that cell shedding from plantar stratum corneum in vitro involves endogenous proteolysis of the desmosomal protein desmoglein I. J Invest Dermatol 1990;94:216—20. King IA, Tabiowo A, Purkis P, et al. Expression of distinct desmocollin isoforms in human epidermis. J Invest Dermatol 1993;100:373—9. Bernard D, Mehul B, Thomas-Collignon A, et al. Analysis of proteins with caseinolytic activity in a human stratum corneum extract revealed a yet unidentified cysteine protease and identified the so-called ‘‘stratum corneum thiol protease’’ as cathepsin l2. J Invest Dermatol 2003;120:592—600. Simon M, Jonca N, Guerrin M, et al. Refined characterization of corneodesmosin proteolysis during terminal differentiation of human epidermis and its relationship to desquamation. J Biol Chem 2001;276:20292—9 [erratum appears in J Biol Chem December 2001;14276(50):47742—3]. Egelrud T, Lundstrom A. A chymotrypsin-like proteinase that may be involved in desquamation in plantar stratum corneum. Arch Dermatol Res 1991;283:108—12. Suzuki Y, Nomura J, Hori J, et al. Detection and characterization of endogenous protease associated with desquamation of stratum corneum. Arch Dermatol Res 1993;285:372—7. Brattsand M, Egelrud T. Purification molecular cloning, and expression of a human stratum corneum trypsin-like serine protease with possible function in desquamation. J Biol Chem 1999;274:30033—40. Horikoshi T, Igarashi S, Uchiwa H, et al. Role of endogenous cathepsin D-like and chymotrypsin-like pro-
Epidermal desquamation
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42] [43]
[44]
[45]
[46]
[47] [48] [49]
[50]
teolysis in human epidermal desquamation. Br J Dermatol 1999;141:453—9. Watkinson A. Stratum corneum thiol protease (SCTP): a novel cysteine protease of late epidermal differentiation. Arch Dermatol Res 1999;291:260—8. Sato J, Denda M, Nakanishi J, et al. Cholesterol sulfate inhibits proteases that are involved in desquamation of stratum corneum. J Invest Dermatol 1998;111:189—93. Menon GK, Ghadially R, Williams ML, Elias PM. Lamellar bodies as delivery systems of hydrolytic enzymes: implications for normal and abnormal desquamation. Br J Dermatol 1992;126:337—45. Freinkel RK, Traczyk TN. Lipid composition and acid hydrolase content of lamellar granules of fetal rat epidermis. J Invest Dermatol 1985;85:295—8. List K, Szabo R, Wertz PW, et al. Loss of proteolytically processed filaggrin caused by epidermal deletion of Matriptase/MT-SP1. J Cell Biol 2003;163:901—10. Toomes C, James J, Wood AJ, et al. Loss-of-function mutations in the cathepsin C gene result in periodontal disease and palmoplantar keratosis. Nat Genet 1999;23:421—4 [see comment]. Hart TC, Hart PS, Bowden DW, et al. Mutations of the cathepsin C gene are responsible for Papillon—Lefevre syndrome. J Med Genet 1999;36:881—7. Komatsu N, Takata M, Otsuki N, et al. Elevated stratum corneum hydrolytic activity in Netherton syndrome suggests an inhibitory regulation of desquamation by SPINK5derived peptides. J Invest Dermatol 2002;118:436—43. Wertz PW, Swartzendruber DC, Kitko DJ, et al. The role of the corneocyte lipid envelopes in cohesion of the stratum corneum. J Invest Dermatol 1989;93:169—72. Anton-Lamprecht I, Schnyder UW. Ultrastructure of inborn errors of keratinization. VI. Inherited ichthyoses–—a model system for heterogeneities in keratinization disturbances. Archiv fur Dermatologische Forschung 1974;250:207—27. Maloney ME, Williams ML, Epstein Jr EH, et al. Lipids in the pathogenesis of ichthyosis: topical cholesterol sulfateinduced scaling in hairless mice. J Invest Dermatol 1984;83:252—6. Andrews NC. Disorders of iron metabolism. N Engl J Med 1999;341:1986—95 [published erratum appears in N Engl J Med February 2000;3342(5):364][see comments]. Green R, Charlton R, Seftel H, et al. Body iron excretion in man: a collaborative study. Am J Med 1968;45:336—53. Jacob RA, Sandstead HH, Munoz JM, et al. Whole body surface loss of trace metals in normal males. Am J Clin Nutr 1981;34:1379—83. Reizenstein P, Skog E, Stigell P. Radio-iron content of epithelium and cell turnover in psoriasis. Iron excretion in man, II. Acta Derm Venereol 1968;48:70—4. Apte SV, Venkatachalam PS. Factors influencing dermal loss of iron in human volunteers. Indian J Med Res 1962;50:817—22. Weintraub LR, Demis J, Conrad ME, Crosby WH. Iron excretion by the skin. Selective localization of iron 59 in epithelial cells. Am J Pathol 1965;46:121—7. Brune M, Magnusson B, Persson H, Hallberg L. Iron losses in sweat. Am J Clin Nutr 1986;43:438—43. Molin L, Wester PO. Iron content in normal and psoriatic epidermis. Acta Derm Venereol 1973;53:473—6. Forslind B, Lindberg M, Roomans GM, et al. Aspects on the physiology of human skin: studies using particle probe analysis. Microsc Res Tech 1997;38:373—86. Kurz K, Steigleder GK, Bischof W, Gonsior B. PIXE analysis in different stages of psoriatic skin. J Invest Dermatol 1987;88:223—6.
139
[51] Marks J, Shuster S. Iron metabolism in skin disease. Arch Dermatol 1968;98:469—75. [52] Marks J. Iron metabolism in skin disease. J R Coll Physicians London 1967;1:367—72. [53] Ames BN, Profet M, Gold LS. Dietary pesticides (99.99% all natural). Proc Natl Acad Sci USA 1990;87:7777—81. [54] Ames BN, Profet M, Gold LS. Nature’s chemicals and synthetic chemicals: comparative toxicology. Proc Natl Acad Sci USA 1990;87:7782—6. [55] Jones HE, Reinhardt JH, Rinaldi MG. Acquired immunity to dermatophytes. Arch Dermatol 1974;109:840—8. [56] Berk SH, Penneys NS, Weinstein GD. Epidermal activity in annular dermatophytosis. Arch Dermatol 1976;112:485—8. [57] Hicks BD, St Aubin DJ, Geraci JR, Brown WR. Epidermal growth in the bottlenose dolphin, Tursiops truncatus. J Invest Dermatol 1985;85:60—3. [58] Savigni DL, Morgan EH. Mediation of iron uptake and release in erythroid cells by photodegradation products of nifedipine. Biochem Pharmacol 1996;51:1701—9. [59] Savigni DL, Wege D, Cliff GS, et al. Iron and transition metal transport into erythrocytes mediated by nifedipine degradation products and related compounds. Biochem Pharmacol 2003;65:1215—26. [60] Gruen AB, Zhou J, Morton KA, Milstone LM. Photodegraded nifedipine stimulates uptake and retention of iron in human epidermal keratinocytes. J Investigat Dermatol 2001;116:774—7. [61] Chevrant-Breton J, Simon M, Bourel M, Ferrand B. Cutaneous manifestations of idiopathic hermochromatosis. Study of 100 cases. Arch Dermatol 1977;113:161—5. [62] Rezazadeh H, Nayebi AR, Athar M. Role of iron-dextran tumorigenesis in normal and pregnant mice on 7,12dimethylbenz(a) anthracene-initiated and croton oil-promoted cutaneous. Hum Exp Toxicol 2001;20:471—6. [63] Grove GL, Kligman AM. Age-associated changes in human epidermal cell renewal. J Gerontol 1983;38:137—42. [64] Holbrook KA, Odland GF. Regional differences in the thickness (cell layers) of the human stratum corneum: an ultrastructural analysis. J Invest Dermatol 1974;62:415— 22. [65] Lavker RM. Structural alterations in exposed and unexposed aged skin. J Invest Dermatol 1979;73:59—66. [66] Baker H, Kligman AM. Technique for estimating turnover time of human stratum corneum. Arch Dermatol 1967;95:408—11. [67] Bonifas JM, Rothman AL, Epstein Jr EH. Epidermolysis bullosa simplex: evidence in two families for keratin gene abnormalities. Science 1991;254:1202—5 [see comment]. [68] Chipev CC, Korge BP, Markova N, et al. A leucine—proline mutation in the H1 subdomain of keratin 1 causes epidermolytic hyperkeratosis. Cell 1992;70:821—8. [69] Cheng J, Syder AJ, Yu QC, et al. The genetic basis of epidermolytic hyperkeratosis: a disorder of differentiationspecific epidermal keratin genes. Cell 1992;70:811—9. [70] Rothnagel JA, Dominey AM, Dempsey LD, et al. Mutations in the rod domains of keratins 1 and 10 in epidermolytic hyperkeratosis. Science 1992;257:1128—30. [71] Steijlen PM, Kremer H, Vakilzadeh F, et al. Genetic linkage of the keratin type II gene cluster with ichthyosis bullosa of Siemens and with autosomal dominant ichthyosis exfoliativa. J Invest Dermatol 1994;103:282—5. [72] Kremer H, Zeeuwen P, McLean WH, et al. Ichthyosis bullosa of Siemens is caused by mutations in the keratin 2e gene. J Invest Dermatol 1994;103:286—9. [73] Reis A, Hennies HC, Langbein L, et al. Keratin 9 gene mutations in epidermolytic palmoplantar keratoderma (EPPK). Nat Genet 1994;6:174—9.
140
[74] McLean WH, Rugg EL, Lunny DP, et al. Keratin 16 and keratin 17 mutations cause pachyonychia congenita. Nat Genet 1995;9:273—8. [75] Bowden PE, Haley JL, Kansky A, et al. Mutation of a type II keratin gene (K6a) in pachyonychia congenita. Nat Genet 1995;10:363—5. [76] Kimonis V, DiGiovanna JJ, Yang JM, et al. A mutation in the V1 end domain of keratin 1 in non-epidermolytic palmar-plantar keratoderma. J Invest Dermatol 1994;103:764—9. [77] Shamsher MK, Navsaria HA, Stevens HP, et al. Novel mutations in keratin 16 gene underly focal non-epidermolytic palmoplantar keratoderma (NEPPK) in two families. Hum Mol Genet 1995;4:1875—81. [78] Rugg EL, McLean WH, Allison WE, et al. A mutation in the mucosal keratin K4 is associated with oral white sponge nevus. Nat Genet 1995;11:450—2. [79] Armstrong DK, McKenna KE, Purkis PE. Haploinsufficiency of desmoplakin causes a striate subtype of palmoplantar keratoderma. Hum Mol Genet 1999;8:143—8 [erratum appears in Hum Mol Genet May 1999;8(5):943]. [80] Norgett EE, Hatsell SJ, Carvajal-Huerta L, et al. Recessive mutation in desmoplakin disrupts desmoplakin-intermediate filament interactions and causes dilated cardiomyopathy. Hum Mol Genet 2000;9:2761—6. [81] Rickman L, Simrak D, Stevens HP, et al. N-terminal deletion in a desmosomal cadherin causes the autosomal dominant skin disease striate palmoplantar keratoderma. Hum Mol Genet 1999;8:971—6. [82] Huber M, Rettler I, Bernasconi K, et al. Mutations of keratinocyte transglutaminase in lamellar ichthyosis. Science 1995;267:525—8 [see comment]. [83] Korge BP, Ishida-Yamamoto A, Punter C, et al. Loricrin mutation in Vohwinkel’s keratoderma is unique to the variant with ichthyosis. J Invest Dermatol 1997;109:604— 10. [84] Sakuntabhai A, Ruiz-Perez V, Carter S, et al. Mutations in ATP2A2, encoding a Ca2þ pump, cause Darier disease. Nat Genet 1999;21:271—7 [see comment]. [85] Dhitavat J, Macfarlane S, Dode L, et al. Acrokeratosis verruciformis of Hopf is caused by mutation in ATP2A2: evidence that it is allelic to Darier’s disease. J Invest Dermatol 2003;120:229—32 [see comment]. [86] Hu Z, Bonifas JM, Beech J, et al. Mutations in ATP2C1, encoding a calcium pump, cause Hailey—Hailey disease. Nat Genet 2000;24:61—5. [87] Lefevre C, Audebert S, Jobard F, et al. Mutations in the transporter ABCA12 are associated with lamellar ichthyosis type 2. Hum Mol Genet 2003;12:2369—78. [88] Richard G, White TW, Smith LE, et al. Functional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma. Hum Genet 1998;103:393—9. [89] Richard G, Smith LE, Bailey RA, et al. Mutations in the human connexin gene GJB3 cause erythrokeratodermia variabilis. Nat Genet 1998;20:366—9 [see comment]. [90] van Steensel MA, van Geel M, Nahuys M, et al. A novel connexin 26 mutation in a patient diagnosed with keratitis-ichthyosis-deafness syndrome. J Invest Dermatol 2002;118:724—7. [91] Webster D, France JT, Shapiro LJ, Weiss R. X-linked ichthyosis due to steroid-sulphatase deficiency. Lancet 1978;1:70—2. [92] Natt E, Kida K, Odievre M, et al. Point mutations in the tyrosine aminotransferase gene in tyrosinemia type II. Proc Natl Acad Sci USA 1992;89:9297—301.
L.M. Milstone
[93] De Laurenzi V, Rogers GR, Hamrock DJ, et al. SjogrenLarsson syndrome is caused by mutations in the fatty aldehyde dehydrogenase gene. Nat Genet 1996;12:852—7. [94] Jansen GA, Ofman R, Ferdinandusse S, et al. Refsum disease is caused by mutations in the phytanoyl-CoA hydroxylase gene. Nat Genet 1997;17:190—3. [95] Derry JM, Gormally E, Means GD, et al. Mutations in a delta 8—delta 7 sterol isomerase in the tattered mouse and X-linked dominant chondrodysplasia punctata. [email protected]. Nat Genet 1999;22:286—90. [96] Braverman N, Lin P, Moebius FF, et al. Mutations in the gene encoding 3 beta-hydroxysteroid-delta 8, delta 7isomerase cause X-linked dominant Conradi—Hunermann syndrome. Nat Genet 1999;22:291—4. [97] Grange DK, Kratz LE, Braverman NE, Kelley RI. CHILD syndrome caused by deficiency of 3beta-hydroxysteroiddelta8, delta7-isomerase. Am J Med Genet 2000;90:328— 35 [see comment]. [98] Konig A, Happle R, Bornholdt D, et al. Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome. Am J Med Genet 2000;90:339—46. [99] Lefevre C, Jobard F, Caux F, et al. Mutations in CGI-58, the gene encoding a new protein of the esterase/lipase/ thioesterase subfamily, in Chanarin-Dorfman syndrome. Am J Hum Genet 2001;69:1002—12. [100] Jobard F, Lefevre C, Karaduman A, et al. Lipoxygenase-3 (ALOXE3) and 12(R)-lipoxygenase (ALOX12B) are mutated in non-bullous congenital ichthyosiform erythroderma (NCIE) linked to chromosome 17p13.1. Hum Mol Genet 2002;11:107—13. [101] Chavanas S, Bodemer C, Rochat A, et al. Mutations in SPINK5, encoding a serine protease inhibitor, cause Netherton syndrome. Nat Genet 2000;25:141—2. [102] Fischer J, Bouadjar B, Heilig R, et al. Mutations in the gene encoding SLURP-1 in Mal de Meleda. Hum Mol Genet 2001;10:875—80. [103] Broughton BC, Steingrimsdottir H, Weber CA, Lehmann AR. Mutations in the xeroderma pigmentosum group D DNA repair/transcription gene in patients with trichothiodystrophy. Nat Genet 1994;7:189—94. [104] Weeda G, Eveno E, Donker I, et al. A mutation in the XPB/ ERCC3 DNA repair transcription gene, associated with trichothiodystrophy. Am J Hum Genet 1997;60:320—9. [105] Menton DN. A minimum-surface mechanism to account for the organization of cells into columns in the mammalian epidermis. Am J Anat 1976;145:1—22. Dr. Leonard M. Milstone received a BS degree in chemistry in 1966 and an MD in 1970 from Yale University. He did post-doctoral research training in molecular genetics at the NIH and postdoctoral clinical training in dermatology at the University of Oregon and Yale University. He has been on the faculty of dermatology at Yale since 1977, where he is Professor of Dermatology. Dr. Milstone has long-standing interests in epidermal homeostasis and ichthyosis. He chairs the medical advisory board for the Foundation for Ichthyosis and Related Skin Types (F.I.R.S.T.), and is on the editorial boards of the Journal of Dermatologic Science and the Journal of Investigative Dermatology. Current research is focused on iron in the epidermis and sequence-specific gene targeting in epidermis.
REVIEW ARTICLE
Epidermal desquamation Leonard M. Milstone Department of Dermatology, Yale University School of Medicine, P.O. Box 208059, New Haven, CT 06520-8059, USA Received 5 April 2004; received in revised form 23 April 2004; accepted 11 May 2004
Summary Epidermal desquamation, a continuous but insensible bodily activity, is largely ignored unless the rate or amount of scale production becomes abnormal. It is the last topic to be considered in any serious discussion of epidermal growth and differentiation, but is becoming an increasingly fertile ground for investigation. This review summarizes: (a) methods for measuring desquamation; (b) variables that affect normal desquamation; (c) mechanisms of desquamation; (d) the role of desquamation in nutritional homeostasis; and (e) the role of desquamation as a first line of defense. Consideration is given to whether desquamation might be harnessed to eliminate or remediate toxins that have accumulated in the body. ß 2004 Japanese Society for Investigative Dermatology. Published by Elsevier Ireland Ltd. All rights reserved.
1. Introduction Epidermal desquamation is a fact of life for all mammals. This continuous but insensible bodily activity is largely ignored unless the rate or amount of scale production becomes abnormal. Little consideration has been given to basic questions such as: Why do mammals continuously shed scale? What function does desquamation serve? How does desquamation contribute to overall energy and nutritional balance? Surely, we do not make scale just to assist dermatologists in diagnosing disease or to provide a niche for house dust mites or to assist bloodhounds in their search for fugitives. In this review, I will summarize what we do know with regard to: (a) methods for measuring desquamation; (b) variables that affect normal desquamation; (c) mechanisms of desquamation; (d) the role of desquamation in nutritional homeostasis; and (e) the role of desquamation as a first line of defense. With this information as background, rather than taking desquamation for granted, we can begin to E-mail address: [email protected] (L.M. Milstone).
contemplate whether it offers unique therapeutic options. For example, might desquamation be harnessed to eliminate or remediate toxins that have accumulated in the body?
2. Measuring desquamation In many early studies, scale was collected from pajamas and bed sheets [1,2] or from specially fabricated garments [3]. The methods chosen for those studies had the potential to provide total number of shed cells and were particularly appropriate for metabolic studies. Unfortunately, those methods of collection and analysis of shed scale were cumbersome and were prone to sampling and interpretive error. Therefore, investigators sought methods that would be quicker, more easily controlled and less inconvenient for the investigator and the subject. Collection chambers were devised to collect scale reproducibly from small areas of skin, and a variety of methods confirmed their reliability for quantifying scale (reviewed in [4]). The relevance of those methods was validated by
0923–1811/$30.00 ß 2004 Japanese Society for Investigative Dermatology. Published by Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.jdermsci.2004.05.004
132
L.M. Milstone
the observation that the number of squames removed by a short, controlled rinse of the epidermal surface with mild detergent correlated closely with the number of squames shed spontaneously over a longer period of time [5]. Extrapolation of data from such sampling methods is the origin of estimates that humans shed between 2 108 and 10 108 cells per day [5]. Much of the interesting information that we have about epidermal scale production comes from measurements that are not direct measures of desquamation. The continuous epidermal renewal that generates shed squames is a steady-state process– —that which is produced must be eliminated. Therefore, in the absence of significant apoptosis, scale production will be directly related to parameters that can be measured on epidermis in situ, such as labeling or mitotic indices, transit times and stratum corneum thickness. Stratum corneum cohesiveness and corneocyte size are additional measurements that provide unique insights into the process of desquamation and are sometimes used to make inferences about desquamation. Each of these parameters is related to the others, but each also can vary independently. As a result, there is no fixed relationship between these parameters, and none should serve as a true surrogate for counting exfoliated cells.
3. Effect of common variables on desquamation It is surprisingly difficult to assemble an authoritative list of how common variables affect desquamation. Many factors contribute to this unsatisfactory state: the number of studies that actually measure shed cells is small and confirmatory data are often completely lacking; few studies of the same variable use the same measures of desquamation; most studies focus on one or two variables and do not control for other variables which may be important. Finally, the standard deviation for shed cell counts Table 1
Age Sex Race Season Location
between individuals in most studies is often very high, suggesting that some critical variable is being overlooked. For example, many investigators accept the observation that the diameter of squames shed from normal epidermis is inversely proportional to the rate of desquamation, as measured by stratum corneum turnover [6]. Women are said to shed larger squames than men [7], suggesting slower desquamation in women. Yet rates of desquamation in men and women are the same [8]. Stratum corneum cohesiveness must be an important determinant of desquamation, and stratum cohesiveness is reportedly greater in women than men [9]. Yet, available data show no difference between men and women in the cell layers of the stratum corneum [10]. The accuracy of those reports has never been challenged; the apparent inconsistencies remain unexplained. Measurements relevant to epidermal desquamation are summarized in Table 1. Most investigations concur that anatomic location has a measurable effect on the size [7,11] and number [8] of desquamated cells, although there is disagreement about the precise rank order of desquamation at different sites. Inexplicably, African Americans desquamate at a greater rate than Caucasians or Orientals [12,13]. It is reported that desquamation is greater in summer than winter [8], though there is great individual variability in those data. Rates of desquamation were faster in younger than the older individuals in one study [14] but equal in another [5]. Limited data comparing desquamation from men and women revealed no difference, but were not the focus of the paper in which it was reported [8].
4. Mechanism of epidermal desquamation A growing body of literature indicates that desquamation is an active, not a passive process. In one of the earliest experiments to address this question,
Effect of common variables on desquamation Layers
Turnover time
Squame size
Desquamation
No difference [63—65] or " with age (males only) [10] No difference [10] — —
Slower (longer) in elderly [5,63] — — —
" with age [7] Female > male [7] — Summer < winter [8]
Limb > trunk > face > genital [10,64]
Forehead < arm < trunk [5,66]
Thigh ¼ abdomen > arm > forehead [7]
Young > old [14] or no difference [5] No difference [8] AA > Cauc ¼ Or [12,13] Summer > winter (elderly, only) [8] Forearm > back > abdomen [5]
AA, African American; Cauc, Caucasian; Or, Oriental.
Epidermal desquamation
the epidermal surface was covered and carefully protected from external influences. Recovery of readily detached, fully mature corneocytes led to the conclusion that neither friction nor grooming was required for shedding [15]. Likewise, spontaneous shedding of differentiated cells from stratified cultures of human keratinocytes emphasized that desquamation was the final, active step in the keratinocytes’ program of differentiation and required neither friction nor desiccation (Fig. 1) [16,17]. Ultrastructural examination of the stratum corneum provided the initial clues into the mechanism of spontaneous desquamation. One of the critical observations was that desmosomes decrease in size and distribution as the corneocytes approach the outer surface in this otherwise inert-appearing structure [18]. Since desmosomes have a central role in keratinocyte cell—cell adhesion, active
133
destruction or dissolution of desmosomes has been the root hypothesis of most mechanistic studies. The biochemistry and cell biology underlying desquamation are likely to be quite complex, but outlines of the program are beginning to emerge. Beyond the morphologic dissolution of the proteinaceous desmosomes [19], a number of observations point to the importance of more than one hydrolytic enzyme in desquamation. In experiments using pieces of stratum corneum and an ultrastructural endpoint, proteases alone were ineffective in digesting or altering desmosomes, but glycosidase treatment followed by protease treatment succeeded in significantly reducing the size of those desmosomes [20]. Analysis of proteins in the stratum corneum has been used to demonstrate the progressive degradation of the desmosomal proteins desmoglein I [19,21,22], desmocollin [23],
Fig. 1 Epidermal desquamation is the final event in a program of differentiation lasting approximately 4 weeks. Nearly a billion cells are lost each day from the surface of adult skin. Critical events in the active process that lead to normal desquamation are the rearrangement of desmosomes to the lateral edges of squames in the stratum corneum and their ultimate enzymatic digestion. Under appropriate conditions, stacked and shed squames have the shape of a tetrakaidecahedron [105].
134
desmoplakin (Bernard, unpublished, cited in [24]) and corneodesmosin [25]. Proteases have been isolated from stratum corneum and several are expressed or activated in epidermis as late products of terminal differentiation: stratum corneum chymotryptic enzyme (SCCE) [26]; stratum corneum tryptic enzyme (SCTE) [27,28]; cathepsin D [29]; and stratum corneum thiol protease (SCTP/cathepsinL2) [24,30]. Topical application of peptide protease inhibitors, chymostatin and leupeptin, increases stratum corneum thickness in mice [31]. Secreted lamellar bodies spill proteinases into the intercellular space between keratinocytes in the lower stratum corneum [32]. Isolated lamellar bodies contain proteases and glycosidases, leading to the postulate that this organelle might play a critical role in desquamation [33]. Whether the lamellar bodies contain the full complement of hydrolytic enzymes needed to degrade desmosomes remains an open question. Desmosomes may not be the only target substrate for proteases important in desquamation. In a murine model, absence of matripase/MT-SP1 is associated with loss of proteolytically processed filaggrin and a very cohesive stratum corneum [34]. Genetic deficiency of a cathepsin has been associated with thickened stratum corneum [35,36]. Conversely, genetic deficiency of a protease inhibitor causes a defective stratum corneum with a tendency to exfoliate prematurely [37]. In the latter two genetic ‘‘models’’, specific epidermal substrates or enzyme targets have not been identified. Future investigations into the subcellular distribution, activation and inhibition of these proteases promise to yield interesting cell biology. Lipids clearly play important roles in regulating desquamation and may do so by several distinct mechanisms. The contribution of ceramides to corneocyte adhesion is still not established, though they are not likely to be the central adhesive molecules in stratum corneum as once proposed [38]. It is now well established that intrinsic (inherited) or extrinsic (acquired) defects in the intercellular lipid barrier to transepidermal water loss leads to epidermal hyperplasia and abnormal desquamation and scale. Other inherited defects in lipid metabolism are not associated with epidermal hyperplasia and, therefore, must alter desquamation through mechanism(s) other than hyperproliferation. Steroid sulfatase deficiency, the cause of recessive Xlinked ichthyosis (RXLI), is associated with abnormal accumulation of cholesterol sulfate in the stratum corneum and the persistence of intact desmosomes high into the stratum corneum [39]. Topical application of cholesterol sulfate to mouse skin mimics those changes and causes scaliness [40], and cho-
L.M. Milstone
lesterol sulfate inhibits both trypsin and chymotrypsin in the test tube [31]. Several mechanisms have been proposed to explain how intrinsic lipid abnormalities might lead to abnormal intercellular lipid organization that, in turn, would cause abnormal scale (reviewed in [32]). Those hypotheses, which largely lack experimental verification, include (a) abnormal hydrogen bonding causing phase separations, (b) abnormal liquid—crystalline phase transitions, and (c) restricted access of proteases to their substrates.
5. Role of desquamation in nutritional homeostasis The contribution made by epidermal turnover and desquamation to the body’s caloric and nutritional balance has been largely ignored or generally dismissed as minor. Throughout the first half of the last century, there was an interest in nutritional imbalance as a cause of skin disease. Far less attention was given to the role in overall homeostasis of normal or diseased skin. Direct measurements relevant to this subject are really quite limited in scope and number. Studies published in the past 50 years generally have focused on protein and iron homeostasis and have addressed one of two questions: (1) Can exfoliative dermatitis or psoriasis result in negative nitrogen balance or iron deficiency anemia? (2) What role does loss through the skin have in normal iron homeostasis?. There is no doubt that patients with severe scaling dermatoses lose increased amounts of protein in epidermal scale compared to those with less severe disease or to normal individuals. At the turn of the last century, Schamberg et al. studied protein metabolism over many weeks in psoriatic patients on different protein diets [2]. They came to the remarkable conclusions that their rather debilitated patients with psoriasis were in positive nitrogen balance and that low protein diets had a beneficial effect on psoriasis. Their method for collecting epidermal scale was admittedly rudimentary and incomplete, and their focus was on demonstrating that nitrogen loss through scale could not account for the overall observed positive nitrogen balance. However, they clearly did show that more than 10 times as much nitrogen was lost in scales during periods of active compared to periods of quiescent disease. This subject was revisited 50 years later and when investigators found no evidence for positive nitrogen balance in erythrodermic patients, they considered the opposite hypothesis: Could increased epidermal turnover and desquamation in erythroderma cause negative
Epidermal desquamation
nitrogen balance? For those studies, subjects were either confined to bed so that scale could be periodically collected [1] or were placed in ‘‘spacesuits’’ to collect desquamated scale [3]. An erythrodermic patient lost significantly more protein in scale compared to either a psoriatic with limited guttate lesions or a control subject [1]; likewise, individual patients lost considerably more scale when they had active exfoliative erythroderma than when they went into a spontaneous or drug-induced clinical remission [3]. The increased protein loss through desquamation (5—10-fold more than normal) was commensurate with the increased turnover in psoriasis. Nonetheless, this amounted to no more than 20 g of scale per day, and the authors concluded that, relative to daily protein intake, this increased amount of lost scale would not represent sufficient protein loss to cause negative nitrogen balance. The role of epidermal desquamation in iron balance was recognized early from studies using parenterally administered radioactive iron to understand iron turnover in both normal people and in patients with iron overload. Normal individuals lose a total of 1—1.5 mg of iron each day [41], and cutaneous losses account for 20—25% of that total [42—44]. Iron lost through the skin is found principally in shed cells, not in sweat [42,45,46]. Iron in cell-free sweat normally accounts for no more than 10% of cutaneous iron loss [47], although sweat losses do vary with environmental conditions [45]. In situations of iron overload, daily iron loss through skin is greater than in normal individuals [42]. Unfortunately, increased iron loss through skin in hemochromatosis or other iron overload conditions is not sufficient to prevent iron accumulation in internal organs. If epidermal desquamation accounts for a significant proportion of daily iron loss, is it possible that exfoliative dermatitis or psoriasis might cause iron deficiency anemia? Substantial amounts of a parenterally administered radioactive iron tracer were eliminated through epidermal desquamation in patients with exfoliative dermatitis [44]. Several groups have reported increased amount of iron in epidermis of psoriatic plaques compared to uninvolved epidermis or normal epidermis [48,49]. In contrast, another group found much less iron in psoriatic plaques than in normal skin [50]. Any conclusions drawn from those studies regarding total-body iron homeostasis must be tempered by the fact that neither were there measurements of iron stores in other tissues (particularly bone marrow) nor was there consideration of the possibility that increased losses through desquamation might be compensated by increased absorption. Many
135
years ago, Marks and Shuster determined that patients with extensive skin disease lost more iron through their skin than normal individuals and that some of those patients were anemic [51]. They concluded–—after careful evaluation of the anemia, the iron stores and the iron turnover in those patients–—that increased iron loss from desquamation was rarely, if ever, the cause of iron deficiency anemia in patients with extensive skin disease (reviewed in [52]). Many of the studies of iron loss through skin, cited above [42,46,48—50], were based on measurements of full-thickness epidermal biopsies rather than on desquamated cells. Such methodology requires the authors to assume that the keratinocyte is parsimonious but that the epidermis as a whole is not. That assumption is reasonable, since there are no conflicting data. Thus, once a nutrient makes its way into a keratinocyte, it is neither returned to the circulation nor locally reutilized, but rather is held tightly by the (parsimonious) keratinocyte and gets swept away in the relentless tide of differentiation that terminates in (wasteful) desquamation.
6. Role of desquamation as a first line of defense We return now to the question of just why is continuous and active desquamation a property of normal epidermis? Is desquamation, like molting, just another evolutionary solution to the problem of how to produce a fresh, intact barrier in the face of accumulated minor damage and/or changing body mass? Or does it reflect the need for great flexibility in continuously modifying the composition of a barrier that will be exposed to a wide variety of environmental niches and challenges? There is no answer to these questions, but a few lines of investigation suggest that continuous desquamation might be of great advantage as a first line of defense against a myriad of known as well as unanticipated or novel physical, chemical or toxic assailants. The first clue comes from the observation that a very large proportion of intrinsic (genetic) defects in the epidermis cause abnormalities in desquamation. Table 2 demonstrates that abnormal scale production is not the result of defects in one class of epidermal protein but rather in a very wide array of proteins involved in structure, signaling and metabolism. Together, these defined defects suggest that excess scale or desquamation is the default pathway for trouble in the epidermis. Likewise, clinical experience reveals that abnormal desquamation is one common response to a variety
136
Table 2
L.M. Milstone
Gene mutations responsible for scaly skin
Disease
Inheritance
Gene/protein
References
AD AD AD AD AD AD AD HI AR AD
KRT5,14/keratins 5,14 KRT1,10/keratins 1,10 KRT2e/keratin 2e KRT9/keratin 9 KRT6A,6B,16,17/keratins 6A,6B,16,17 KRT1, KRT16/keratins 1,16 KRT4,13/keratins 4,13 DSP/desmoplakin DSP/desmoplakin DSG1/desmoglein 1
[67] [68—70] [71,72] [73] [74,75] [76,77] [78] [79] [80] [81]
Cornified envelope Lamellar ichthyosis Keratoderma (Camissa)
AR AD
TGM1/transglutaminase 1 LOR/loricrin
[82] [83]
Membrane transporters Darier disease Acrokeratosis verruciformis Hailey—Hailey disease Lamellar ichthyosis
AD AD AD AR
ATP2A2/SERcalciumATPase ATP2A2/SERcalciumATPase ATP2C1/??Pmr1 ABCA12
[84] [85] [86] [87]
Cell junction proteins PPK with deafness (Vohlwinkle) Erythrokeratoderma variabilis Keratitis ichthyosis deafness (KID)
AD AD AD
GJB2/connexin 26 GJB3/connexin 31 GBJ2/connexin 26
[88] [89] [88,90]
Metabolic enzymes X-linked ichthyosis Tyrosinemia II (Richner—Hanhart) Sjogren—Larsson Refsum Conradi—Hunermann CHILD
X-LR AR AR AR X-LD X-LD
STS/steroid sulfatase TAT/tyrosine animotransferase FALDH/fatty aldehyde dehydrogenase PAXH/phytanoyl CoA Hydroxylase EBP/3b-hydroxysterol d7-isomerase EBP/3b-hydroxysterol d7-isomerase NASDHL/3b-hydroxysterol dehydrogenase CGI-58/?? ALOXE3/lipoxegenase 3 ALOX12B/12 (R)-lipoxygenase
[91] [92] [93] [94] [95,96] [97] [98]
Organelle=function Cytoskeleton Epidermolysis bullosa simplex Epidermolytic hyperkeratosis Ichthyosis bullosa of Siemans Epidermolytic PPK Pachyonychia congenita Non-epidermolytic PPK White sponge nevus Non-epidermolytic PPK (striate)
Chanarin—Dorfman (neutral lipid storage) Congenital ichthyosiform erythroderma
AR AR
Protease/protease inhibitors Papillion—Lefevre keratoderma Ichthyosis linearis circumflexa (Netherton)
AR AR
CTSC/cathepsin C SPINK5/serine protease inhibitor 5
[35,36] [101]
Secreted proteins Mal de Meleda keratoderma
AR
SLURP1/??
[102]
DNA repair/transcription Trichothiodystrophy
AR
ERCC2/XPD ERCC3/XPB
[103] [104]
of extrinsic assaults or challenges, such as friction, abrasion or chemical disruption of the lipid barrier. A second clue comes from toxicology. After years of studying the mutagenic potential of synthetic chemicals, Bruce Ames et al. turned their attention to natural products. To everyone’s surprise, they found that many chemicals isolated from dietary or garden plants had significant mutagenic potential
[99] [100] [100]
[53]. In an attempt to reconcile man’s ability to live in such an apparently toxic environment, they proposed a list of mechanisms for defending against chemical assault. At the top of their list was one that had, to the best of my knowledge, previously been given little attention. ‘‘Defenses that animals have evolved are mostly of a general type, since the number of natural
Epidermal desquamation
chemicals that might have toxic effects is so large. General defenses offer protection not only against natural but also against synthetic chemicals. These defenses include the following: (a) the continuous shedding of cells exposed to toxins; (b) the induction of a wide variety of general detoxifying mechanisms; (c) the active excretion of planar hydrophobic molecules out of liver and intestinal cells; (d) DNA repair; (e) animals’ olfactory and gustatory perception of bitter, acrid, astringent, and pungent chemicals. . ... That defenses are usually general, rather than specific for each chemical, makes good evolutionary sense.’’ [54]. A similar broad view of desquamation has been proposed as a generic way to resist assault by microbes. Increased desquamation, whether part of an innate or acquired response, is considered to be an important factor in locally limiting and ultimately clearing human dermatophyte infections [55,56]. To explain the inherently high rates of epidermal turnover and desquamation in the bottlenose dolphin, it has been suggested that rapid sloughing might limit colonization by surface microorganisms [57].
7. Proposal to harness desquamation to remediate internal toxins Given the preceding perspective on the functions and consequences of epidermal desquamation, let us consider whether there might be circumstances in which desquamation might be extended beyond its usual protective function and be harnessed to eliminate internal toxins. The concept is that if the epidermis could be turned into a sponge or sink for an internal toxin, then the renewal process that culminates in desquamation could eliminate that toxin. Iron can be used as a model toxin to illustrate these requirements, since iron overload is common, iron is quite toxic and a great deal is known about the physiology of iron uptake and elimination. For such a scheme to be feasible, several minimum requirements must be met. The first requirement is that there must be natural or pharmacological mechanisms for transporting the toxin into keratinocytes. Iron is an essential nutrient for all proliferating cells and is normally transported into basal keratinocytes bound to its transport protein, transferrin, via receptormediated endocytosis. There is considerable information about the expression and regulation of the transferrin receptor, making manipulation of receptor function one potential pathway to increase iron transport into keratinocytes. Pharmacologically, there are several chemical relatives of nifedipine
137
[58,59] that act as iron ionophores and increase iron transport into keratinocytes [60]. The second requirement is that the toxin must be retained in the keratinocyte. Either the transport mechanism must be unidirectional or there must be an intracellular mechanism for sequestering the toxin. Iron is sequestered and retained in cells by the inducible storage protein, ferritin. The increased iron that is transported into cultured keratinocytes by the nitroso-derivative of nifedipine is retained in those cells, resulting in a substantial accumulation of iron over many days [60]. In addition, it must be assumed that epidermal differentiation does not result in cellular release of the toxin and reentry into the general body pool. As indicated above, there has never been a demonstration (which is not to say that it does not or cannot happen) that terminal differentiation of keratinocytes results in salvage of nutrients or other chemicals from the epidermis into the general circulation. The third requirement is that quantitative considerations should make this route of elimination feasible. Only a small fraction of dietary iron is normally absorbed, and absorption is generally limited by feedback mechanisms that sense body stores of iron [41]. Patients with hereditary hemochromatosis generally absorb four to five times as much iron as normal individuals, but their ability to eliminate that additional iron is quite limited. One quarter of absorbed dietary iron is eliminated through the skin in normal individuals (see above); more iron is eliminated through the skin in hemochromatosis [42], but not enough to prevent net iron accumulation. Periodic iron depletion through menstruation or phlebotomy reduces tissue accumulation of iron and delays onset of organ damage. Since one unit of blood contains 240 mg iron, phlebotomy every other month eliminates about 4 mg of iron per day and maintains iron homeostasis in hereditary hemochromatosis. Thus, increasing iron elimination through epidermis by 16-fold (0.25—4.0 mg per day) should be sufficient to prevent iron accumulation. The fourth and final requirement is that increased levels of the systemic toxin should have relatively little toxic effect on normal epidermis. Iron overload is a fairly common problem; yet, skin disease caused by iron overload rarely provokes visits to the dermatologist. A single report indicates an association between iron overload and thin, dry epidermis [61]. Another group has shown that iron overload in mice increases susceptibility to chemical carcinogenesis [18,62]. Neither of those studies addressed the issue of whether the effect of iron overload acted directly on the epidermis or indirectly through a systemic effect. Epidermis normally stores 1/10 the amount of iron stored in liver, and
138
liver can increase its store of iron 10-fold before toxic levels are reached. Thus, unless keratinocytes are much more susceptible to toxic effects of iron than hepatocytes, there should be ample room to increase iron levels in keratinocytes without incurring toxic effects. It would seem that harnessing desquamation to remediate iron overload is theoretically feasible, if not yet proven practical.
L.M. Milstone
[11]
[12]
[13]
[14]
8. Conclusion Epidermal desquamation, the last topic to be considered in any serious discussion of epidermal growth and differentiation, is becoming an increasingly fertile ground for investigation. The catalogue of environmental and endogenous factors that alter desquamation is large and growing rapidly. There is lively interest in the mechanisms by which desquamation occurs and there is renewed appreciation that it is an active, not a passive, process. Desquamation is a factor in normal homeostasis of certain essential nutrients. There are hints that desquamation acts as a first-line, innate mechanism for resisting external assault. Ongoing research in desquamation promises to yield more effective and creative control over this always essential, and sometimes pathological, process.
References [1] Block WD, Lea WA, Curtis AC, Cannon EF. Nitrogen balance and sulfur excretion studies in psoriasis. J Invest Dermatol 1958;30:287—93. [2] Schamberg JF, Kolmer JA, Ringer AI, Raiziss GW. Research studies in psoriasis. J Cutan Dis 1913;31:803—912. [3] Freedberg IM, Baden HP. The metabolic response to exfoliation. J Invest Dermatol 1962;38:277—84. [4] Serup J, Jemec G. Handbook of non-invasive methods and the skin. Boca Raton: CRC Press; 1995. p. 143—58. [5] Roberts D, Marks R. The determination of regional and age variations in the rate of desquamation: a comparison of four techniques. J Invest Dermatol 1980;74:13—6. [6] Grove GL, Kligman AM. Corneocyte size as an indirect measure of epidermal proliferative activity. In: Marks R, Plewig G, editors. Stratum Corneum. Berlin (Heidelberg, New York): Springer—Verlag; 1983. p. 191—5. [7] Plewig G. Regional differences of cell sizes in the human stratum corneum. II. Effects of sex and age. J Invest Dermatol 1970;54:19—23. [8] Herrmann S, Scheuber E, Plewig G. Exfoliative cytology: effects of the seasons. In: Marks R, Plewig G, editors. Stratum Corneum. Berlin (Heidelberg, New York): Springer—Verlag; 1983. p. 181—5. [9] Nicholls S, Marks R. Novel techniques for the estimation of intracorneal cohesion in vivo. Br J Dermatol 1977;96:595— 602. [10] Ya-Xian Z, Suetake T, Tagami H. Number of cell layers of the stratum corneum in normal skin–—relationship to the
[15] [16]
[17]
[18] [19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
anatomical location on the body age sex and physical parameters. Arch Dermatol Res 1999;291:555—9. Plewig G, Marples RR. Regional differences of cell sizes in the human stratum corneum. I. J Invest Dermatol 1970;54:13—8. Weigand DA, Haygood C, Gaylor JR. Cell layers and density of Negro and Caucasian stratum corneum. J Invest Dermatol 1974;62:563—8. Corcuff P, Lotte C, Rougier A, Maibach HI. Racial differences in corneocytes. A comparison between black. Acta Derm Venereol 1991;71:146—8. Leyden JJ, McGinley KJ, Grove GL, Kligman AM. Age-related differences in the rate of desquamation of skin surface cells. Adv Exp Med Biol 1978;97:297—8 [proceedings]. Goldschmidt H, Kligman AM. Desquamation of the human horny layer. Arch Dermatol 1967;95:583—6. Sun TT, Green H. Differentiation of the epidermal keratinocyte in cell culture: formation of the cornified envelope. Cell 1976;9:511—21. Milstone LM, McGuire J, LaVigne JF. Retinoic acid causes premature desquamation of cells from confluent cultures of stratified squamous epithelia. J Invest Dermatol 1982;79:253—60. Allen TD, Potten CS. Desmosomal form, fate, and function in mammalian epidermis. J Ultrastruct Res 1975;51:94—105. Skerrow CJ, Clelland DG, Skerrow D. Changes to desmosomal antigens and lectin-binding sites during differentiation in normal human epidermis: a quantitative ultrastructural study. J Cell Sci 1989;92:667—77. Walsh A, Chapman SJ. Sugars protect desmosome and corneosome glycoproteins from proteolysis. Arch Dermatol Res 1991;283:174—9. Suzuki Y, Koyama J, Moro O, et al. The role of two endogenous proteases of the stratum corneum in degradation of desmoglein-1 and their reduced activity in the skin of ichthyotic patients. Br J Dermatol 1996;134:460—4. Lundstrom A, Egelrud T. Evidence that cell shedding from plantar stratum corneum in vitro involves endogenous proteolysis of the desmosomal protein desmoglein I. J Invest Dermatol 1990;94:216—20. King IA, Tabiowo A, Purkis P, et al. Expression of distinct desmocollin isoforms in human epidermis. J Invest Dermatol 1993;100:373—9. Bernard D, Mehul B, Thomas-Collignon A, et al. Analysis of proteins with caseinolytic activity in a human stratum corneum extract revealed a yet unidentified cysteine protease and identified the so-called ‘‘stratum corneum thiol protease’’ as cathepsin l2. J Invest Dermatol 2003;120:592—600. Simon M, Jonca N, Guerrin M, et al. Refined characterization of corneodesmosin proteolysis during terminal differentiation of human epidermis and its relationship to desquamation. J Biol Chem 2001;276:20292—9 [erratum appears in J Biol Chem December 2001;14276(50):47742—3]. Egelrud T, Lundstrom A. A chymotrypsin-like proteinase that may be involved in desquamation in plantar stratum corneum. Arch Dermatol Res 1991;283:108—12. Suzuki Y, Nomura J, Hori J, et al. Detection and characterization of endogenous protease associated with desquamation of stratum corneum. Arch Dermatol Res 1993;285:372—7. Brattsand M, Egelrud T. Purification molecular cloning, and expression of a human stratum corneum trypsin-like serine protease with possible function in desquamation. J Biol Chem 1999;274:30033—40. Horikoshi T, Igarashi S, Uchiwa H, et al. Role of endogenous cathepsin D-like and chymotrypsin-like pro-
Epidermal desquamation
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42] [43]
[44]
[45]
[46]
[47] [48] [49]
[50]
teolysis in human epidermal desquamation. Br J Dermatol 1999;141:453—9. Watkinson A. Stratum corneum thiol protease (SCTP): a novel cysteine protease of late epidermal differentiation. Arch Dermatol Res 1999;291:260—8. Sato J, Denda M, Nakanishi J, et al. Cholesterol sulfate inhibits proteases that are involved in desquamation of stratum corneum. J Invest Dermatol 1998;111:189—93. Menon GK, Ghadially R, Williams ML, Elias PM. Lamellar bodies as delivery systems of hydrolytic enzymes: implications for normal and abnormal desquamation. Br J Dermatol 1992;126:337—45. Freinkel RK, Traczyk TN. Lipid composition and acid hydrolase content of lamellar granules of fetal rat epidermis. J Invest Dermatol 1985;85:295—8. List K, Szabo R, Wertz PW, et al. Loss of proteolytically processed filaggrin caused by epidermal deletion of Matriptase/MT-SP1. J Cell Biol 2003;163:901—10. Toomes C, James J, Wood AJ, et al. Loss-of-function mutations in the cathepsin C gene result in periodontal disease and palmoplantar keratosis. Nat Genet 1999;23:421—4 [see comment]. Hart TC, Hart PS, Bowden DW, et al. Mutations of the cathepsin C gene are responsible for Papillon—Lefevre syndrome. J Med Genet 1999;36:881—7. Komatsu N, Takata M, Otsuki N, et al. Elevated stratum corneum hydrolytic activity in Netherton syndrome suggests an inhibitory regulation of desquamation by SPINK5derived peptides. J Invest Dermatol 2002;118:436—43. Wertz PW, Swartzendruber DC, Kitko DJ, et al. The role of the corneocyte lipid envelopes in cohesion of the stratum corneum. J Invest Dermatol 1989;93:169—72. Anton-Lamprecht I, Schnyder UW. Ultrastructure of inborn errors of keratinization. VI. Inherited ichthyoses–—a model system for heterogeneities in keratinization disturbances. Archiv fur Dermatologische Forschung 1974;250:207—27. Maloney ME, Williams ML, Epstein Jr EH, et al. Lipids in the pathogenesis of ichthyosis: topical cholesterol sulfateinduced scaling in hairless mice. J Invest Dermatol 1984;83:252—6. Andrews NC. Disorders of iron metabolism. N Engl J Med 1999;341:1986—95 [published erratum appears in N Engl J Med February 2000;3342(5):364][see comments]. Green R, Charlton R, Seftel H, et al. Body iron excretion in man: a collaborative study. Am J Med 1968;45:336—53. Jacob RA, Sandstead HH, Munoz JM, et al. Whole body surface loss of trace metals in normal males. Am J Clin Nutr 1981;34:1379—83. Reizenstein P, Skog E, Stigell P. Radio-iron content of epithelium and cell turnover in psoriasis. Iron excretion in man, II. Acta Derm Venereol 1968;48:70—4. Apte SV, Venkatachalam PS. Factors influencing dermal loss of iron in human volunteers. Indian J Med Res 1962;50:817—22. Weintraub LR, Demis J, Conrad ME, Crosby WH. Iron excretion by the skin. Selective localization of iron 59 in epithelial cells. Am J Pathol 1965;46:121—7. Brune M, Magnusson B, Persson H, Hallberg L. Iron losses in sweat. Am J Clin Nutr 1986;43:438—43. Molin L, Wester PO. Iron content in normal and psoriatic epidermis. Acta Derm Venereol 1973;53:473—6. Forslind B, Lindberg M, Roomans GM, et al. Aspects on the physiology of human skin: studies using particle probe analysis. Microsc Res Tech 1997;38:373—86. Kurz K, Steigleder GK, Bischof W, Gonsior B. PIXE analysis in different stages of psoriatic skin. J Invest Dermatol 1987;88:223—6.
139
[51] Marks J, Shuster S. Iron metabolism in skin disease. Arch Dermatol 1968;98:469—75. [52] Marks J. Iron metabolism in skin disease. J R Coll Physicians London 1967;1:367—72. [53] Ames BN, Profet M, Gold LS. Dietary pesticides (99.99% all natural). Proc Natl Acad Sci USA 1990;87:7777—81. [54] Ames BN, Profet M, Gold LS. Nature’s chemicals and synthetic chemicals: comparative toxicology. Proc Natl Acad Sci USA 1990;87:7782—6. [55] Jones HE, Reinhardt JH, Rinaldi MG. Acquired immunity to dermatophytes. Arch Dermatol 1974;109:840—8. [56] Berk SH, Penneys NS, Weinstein GD. Epidermal activity in annular dermatophytosis. Arch Dermatol 1976;112:485—8. [57] Hicks BD, St Aubin DJ, Geraci JR, Brown WR. Epidermal growth in the bottlenose dolphin, Tursiops truncatus. J Invest Dermatol 1985;85:60—3. [58] Savigni DL, Morgan EH. Mediation of iron uptake and release in erythroid cells by photodegradation products of nifedipine. Biochem Pharmacol 1996;51:1701—9. [59] Savigni DL, Wege D, Cliff GS, et al. Iron and transition metal transport into erythrocytes mediated by nifedipine degradation products and related compounds. Biochem Pharmacol 2003;65:1215—26. [60] Gruen AB, Zhou J, Morton KA, Milstone LM. Photodegraded nifedipine stimulates uptake and retention of iron in human epidermal keratinocytes. J Investigat Dermatol 2001;116:774—7. [61] Chevrant-Breton J, Simon M, Bourel M, Ferrand B. Cutaneous manifestations of idiopathic hermochromatosis. Study of 100 cases. Arch Dermatol 1977;113:161—5. [62] Rezazadeh H, Nayebi AR, Athar M. Role of iron-dextran tumorigenesis in normal and pregnant mice on 7,12dimethylbenz(a) anthracene-initiated and croton oil-promoted cutaneous. Hum Exp Toxicol 2001;20:471—6. [63] Grove GL, Kligman AM. Age-associated changes in human epidermal cell renewal. J Gerontol 1983;38:137—42. [64] Holbrook KA, Odland GF. Regional differences in the thickness (cell layers) of the human stratum corneum: an ultrastructural analysis. J Invest Dermatol 1974;62:415— 22. [65] Lavker RM. Structural alterations in exposed and unexposed aged skin. J Invest Dermatol 1979;73:59—66. [66] Baker H, Kligman AM. Technique for estimating turnover time of human stratum corneum. Arch Dermatol 1967;95:408—11. [67] Bonifas JM, Rothman AL, Epstein Jr EH. Epidermolysis bullosa simplex: evidence in two families for keratin gene abnormalities. Science 1991;254:1202—5 [see comment]. [68] Chipev CC, Korge BP, Markova N, et al. A leucine—proline mutation in the H1 subdomain of keratin 1 causes epidermolytic hyperkeratosis. Cell 1992;70:821—8. [69] Cheng J, Syder AJ, Yu QC, et al. The genetic basis of epidermolytic hyperkeratosis: a disorder of differentiationspecific epidermal keratin genes. Cell 1992;70:811—9. [70] Rothnagel JA, Dominey AM, Dempsey LD, et al. Mutations in the rod domains of keratins 1 and 10 in epidermolytic hyperkeratosis. Science 1992;257:1128—30. [71] Steijlen PM, Kremer H, Vakilzadeh F, et al. Genetic linkage of the keratin type II gene cluster with ichthyosis bullosa of Siemens and with autosomal dominant ichthyosis exfoliativa. J Invest Dermatol 1994;103:282—5. [72] Kremer H, Zeeuwen P, McLean WH, et al. Ichthyosis bullosa of Siemens is caused by mutations in the keratin 2e gene. J Invest Dermatol 1994;103:286—9. [73] Reis A, Hennies HC, Langbein L, et al. Keratin 9 gene mutations in epidermolytic palmoplantar keratoderma (EPPK). Nat Genet 1994;6:174—9.
140
[74] McLean WH, Rugg EL, Lunny DP, et al. Keratin 16 and keratin 17 mutations cause pachyonychia congenita. Nat Genet 1995;9:273—8. [75] Bowden PE, Haley JL, Kansky A, et al. Mutation of a type II keratin gene (K6a) in pachyonychia congenita. Nat Genet 1995;10:363—5. [76] Kimonis V, DiGiovanna JJ, Yang JM, et al. A mutation in the V1 end domain of keratin 1 in non-epidermolytic palmar-plantar keratoderma. J Invest Dermatol 1994;103:764—9. [77] Shamsher MK, Navsaria HA, Stevens HP, et al. Novel mutations in keratin 16 gene underly focal non-epidermolytic palmoplantar keratoderma (NEPPK) in two families. Hum Mol Genet 1995;4:1875—81. [78] Rugg EL, McLean WH, Allison WE, et al. A mutation in the mucosal keratin K4 is associated with oral white sponge nevus. Nat Genet 1995;11:450—2. [79] Armstrong DK, McKenna KE, Purkis PE. Haploinsufficiency of desmoplakin causes a striate subtype of palmoplantar keratoderma. Hum Mol Genet 1999;8:143—8 [erratum appears in Hum Mol Genet May 1999;8(5):943]. [80] Norgett EE, Hatsell SJ, Carvajal-Huerta L, et al. Recessive mutation in desmoplakin disrupts desmoplakin-intermediate filament interactions and causes dilated cardiomyopathy. Hum Mol Genet 2000;9:2761—6. [81] Rickman L, Simrak D, Stevens HP, et al. N-terminal deletion in a desmosomal cadherin causes the autosomal dominant skin disease striate palmoplantar keratoderma. Hum Mol Genet 1999;8:971—6. [82] Huber M, Rettler I, Bernasconi K, et al. Mutations of keratinocyte transglutaminase in lamellar ichthyosis. Science 1995;267:525—8 [see comment]. [83] Korge BP, Ishida-Yamamoto A, Punter C, et al. Loricrin mutation in Vohwinkel’s keratoderma is unique to the variant with ichthyosis. J Invest Dermatol 1997;109:604— 10. [84] Sakuntabhai A, Ruiz-Perez V, Carter S, et al. Mutations in ATP2A2, encoding a Ca2þ pump, cause Darier disease. Nat Genet 1999;21:271—7 [see comment]. [85] Dhitavat J, Macfarlane S, Dode L, et al. Acrokeratosis verruciformis of Hopf is caused by mutation in ATP2A2: evidence that it is allelic to Darier’s disease. J Invest Dermatol 2003;120:229—32 [see comment]. [86] Hu Z, Bonifas JM, Beech J, et al. Mutations in ATP2C1, encoding a calcium pump, cause Hailey—Hailey disease. Nat Genet 2000;24:61—5. [87] Lefevre C, Audebert S, Jobard F, et al. Mutations in the transporter ABCA12 are associated with lamellar ichthyosis type 2. Hum Mol Genet 2003;12:2369—78. [88] Richard G, White TW, Smith LE, et al. Functional defects of Cx26 resulting from a heterozygous missense mutation in a family with dominant deaf-mutism and palmoplantar keratoderma. Hum Genet 1998;103:393—9. [89] Richard G, Smith LE, Bailey RA, et al. Mutations in the human connexin gene GJB3 cause erythrokeratodermia variabilis. Nat Genet 1998;20:366—9 [see comment]. [90] van Steensel MA, van Geel M, Nahuys M, et al. A novel connexin 26 mutation in a patient diagnosed with keratitis-ichthyosis-deafness syndrome. J Invest Dermatol 2002;118:724—7. [91] Webster D, France JT, Shapiro LJ, Weiss R. X-linked ichthyosis due to steroid-sulphatase deficiency. Lancet 1978;1:70—2. [92] Natt E, Kida K, Odievre M, et al. Point mutations in the tyrosine aminotransferase gene in tyrosinemia type II. Proc Natl Acad Sci USA 1992;89:9297—301.
L.M. Milstone
[93] De Laurenzi V, Rogers GR, Hamrock DJ, et al. SjogrenLarsson syndrome is caused by mutations in the fatty aldehyde dehydrogenase gene. Nat Genet 1996;12:852—7. [94] Jansen GA, Ofman R, Ferdinandusse S, et al. Refsum disease is caused by mutations in the phytanoyl-CoA hydroxylase gene. Nat Genet 1997;17:190—3. [95] Derry JM, Gormally E, Means GD, et al. Mutations in a delta 8—delta 7 sterol isomerase in the tattered mouse and X-linked dominant chondrodysplasia punctata. [email protected]. Nat Genet 1999;22:286—90. [96] Braverman N, Lin P, Moebius FF, et al. Mutations in the gene encoding 3 beta-hydroxysteroid-delta 8, delta 7isomerase cause X-linked dominant Conradi—Hunermann syndrome. Nat Genet 1999;22:291—4. [97] Grange DK, Kratz LE, Braverman NE, Kelley RI. CHILD syndrome caused by deficiency of 3beta-hydroxysteroiddelta8, delta7-isomerase. Am J Med Genet 2000;90:328— 35 [see comment]. [98] Konig A, Happle R, Bornholdt D, et al. Mutations in the NSDHL gene, encoding a 3beta-hydroxysteroid dehydrogenase, cause CHILD syndrome. Am J Med Genet 2000;90:339—46. [99] Lefevre C, Jobard F, Caux F, et al. Mutations in CGI-58, the gene encoding a new protein of the esterase/lipase/ thioesterase subfamily, in Chanarin-Dorfman syndrome. Am J Hum Genet 2001;69:1002—12. [100] Jobard F, Lefevre C, Karaduman A, et al. Lipoxygenase-3 (ALOXE3) and 12(R)-lipoxygenase (ALOX12B) are mutated in non-bullous congenital ichthyosiform erythroderma (NCIE) linked to chromosome 17p13.1. Hum Mol Genet 2002;11:107—13. [101] Chavanas S, Bodemer C, Rochat A, et al. Mutations in SPINK5, encoding a serine protease inhibitor, cause Netherton syndrome. Nat Genet 2000;25:141—2. [102] Fischer J, Bouadjar B, Heilig R, et al. Mutations in the gene encoding SLURP-1 in Mal de Meleda. Hum Mol Genet 2001;10:875—80. [103] Broughton BC, Steingrimsdottir H, Weber CA, Lehmann AR. Mutations in the xeroderma pigmentosum group D DNA repair/transcription gene in patients with trichothiodystrophy. Nat Genet 1994;7:189—94. [104] Weeda G, Eveno E, Donker I, et al. A mutation in the XPB/ ERCC3 DNA repair transcription gene, associated with trichothiodystrophy. Am J Hum Genet 1997;60:320—9. [105] Menton DN. A minimum-surface mechanism to account for the organization of cells into columns in the mammalian epidermis. Am J Anat 1976;145:1—22. Dr. Leonard M. Milstone received a BS degree in chemistry in 1966 and an MD in 1970 from Yale University. He did post-doctoral research training in molecular genetics at the NIH and postdoctoral clinical training in dermatology at the University of Oregon and Yale University. He has been on the faculty of dermatology at Yale since 1977, where he is Professor of Dermatology. Dr. Milstone has long-standing interests in epidermal homeostasis and ichthyosis. He chairs the medical advisory board for the Foundation for Ichthyosis and Related Skin Types (F.I.R.S.T.), and is on the editorial boards of the Journal of Dermatologic Science and the Journal of Investigative Dermatology. Current research is focused on iron in the epidermis and sequence-specific gene targeting in epidermis.