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Hormonal control of thermogenesis and energy dissipation
Storm T, Thamsborg G, Steiniche T, Genant HK, Sorensen OH: 1990. Effect of intermittent cyclical etidronate therapy on bone mass and fracture rate in women with postmenopausal osteoporosis. N Engl J Med 322:1265-1271.
from skeletal metastases in breast cancer patients during long-term bisphosphonates (APD) treatment. Lancet 2:983-985. Van Holten-Verzantvoort AT, Zwinderman AH, Aaronson NK, et al.: 1991. The effect of supportive pamidronate treatment on aspects of quality of life of patients with advanced breast cancer. Eur J Cancer 27:544549.
Urwin GH, Yates AJP, Gray RES, et al.: 1987. Treatment of hypercalcemia of malignancy with intravenous clodronate. Bone b(Supp1 l):S43-51.
Watts NB, Harris ST, Genant HK, et al.: 1990. Intermittent cyclical etidronate treatment of postmenopausal osteoporosis. N Engl J Med 323~73-79.
Valkema R, Vismans FJFE, Papapoulos SE, Pauwels EKJ, Bijvoet OLM: 1989. Maintained improvement in calcium balance and bone mineral content in patients with osteoporosis treated with the bisphosphonate APD. Bone Miner 5:183-192.
Yates AJP, Percival RC, Gray RES, et al.: 1985. Intravenous clodronate in the treatment and retreatment of Paget’s disease of bone. Lancet 1: 1474-l 477. TEM
Van Holten-Verzantvoort AT, Bijvoet OLM, Cleton FJ, et al.: 1987. Reduced morbidity
Hormonal Control of Thermogenesis and Energy Dissipation (adaptive)
sympathetic
nervous
hormones
adipose
important
system
is primarily
(SNS).
in adaptive thermogenesis
most, permissive. brown
thermogenesis
The participation
has been considered
The finding of type II-thyroxine tissue
controlled
(BAT) has opened
role for thyroid hormone
of
by the thyroid
minor or, at
(T,) S-deiodinase
a way to uncover
in
a more
in adaptive thermogenesis.
This
enzyme is activated by the SNS and insulin. When activated, it generates high BAT concentrations
of triiodothyronine
(T,) from plasma T4. T,,
intrinsically 10 times more active than T4 has been shown essential for the expression
of the key protein
protein (UCP). The multihovmonal
in BAT thermogenesis,
uncoupling
control of BAT type-II S-deiodinase
and the marked influence of T3 on UCP and BAT thermogenesis that the local control of T3 generation
may be an important
variability in the potential of mammals
Some Basic Concepts of Thermogenesis
In transformations form to another,
source of
temperature
and
(Trends Endocrinol Metab 1993;4:25-32)
dissipate energy.
??
to maintain
suggest
of energy from one a substantial
fraction of
J. Enrique Silva is Director of the Endocrine Division, Department of Medicine, Sir Mortimer B. Davis Jewish General Hospital; and Professor of Medicine at McGill University, Montreal, Quebec H3T lE2, Canada.
TEM Vol. 4, No. 1, 1993
the energy involved is lost as heat. Living organisms are not exempted from this basic thermodynamic law, and in this case the energy dissipated has appropriately been termed obligatory thermogenesis, which, in the resting state, is largely accounted for by the basal metabolic rate (BMR). Obligatory thermogenesis is sufficient to maintain body temperature without the participation of other regulatory mechanisms at a certain ambient
called thermoneutrality tem-
the same species as well. To maintain body temperature in colder environments, a number of mechanisms are activated, one of which, the most important for cold acclimation, is nonshivering facultative thermogenesis. In addition, facultative thermogenesis is activated by overfeeding, serving the purpose of dissipating the excess of ingested calories as heat. When stimulated by food, facultative thermogenesis is more descriptively called diet-induced thermogenesis. Not surprisingly, facultative thermogenesis is turned down and eventually off by high environmental temperatures and by fasting (Landsberg and Young 1983) (Figure 1).
??
J. Enrique Silva Facultative
temperature
perature, which varies depending upon size and thermal insulation among species and probably among individuals of
Brown Adipose Tissue, a Thermogenic Organ
The smaller the animal, the greater is the surface area to volume ratio and the need for facultative thermogenesis to maintain body temperature in ambient temperatures colder than thermoneutrality. In small mammals, including the newborn human, the main site of facultative thermogenesis is the brown adipose tissue (BAT) [for reviews, see HimmsHagen (1985 and 1989), Nicholls and Locke (1984), and Cannon and NederFigure 1. Schematic representation of obligatory and facultative thermogenesis as the functionofambient temperature. Thermoneutrality temperature, indicated by the arrow pointing to the abscissa, is defined as that ambient temperature at which obligatory thermogenesis alone is able to maintain body temperature. Below and above this temperature, adaptive mechanisms are activated. For simplicity, mechanisms to save heat in cold temperatures, such as vasoconstriction and hair ruffling, have been omitted. See the text for details. The effect of thyrotoxicosis is discussed at the end of the article.
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I
Amblent Temperature
25
gaard (1985)]. BAT cells owe their brown color to the cytochrome from their abundant and large mitochondria (Nechad 1986). Over 60% of the excess heat produced in response to cold in rats is generated in BAT (Foster and Frydman 1977). Upon stimulation, BAT can substantially increase substrate oxidation with a lower thermodynamic efficiency, that is, with greater heat production per molecule of substrate. This is made possible by the presence of thermogenin or uncoupling protein (UCP), a protein uniquely expressed in BAT mitochondria and located in the inner membrane of this organelle. Acting as an ionic transporter, UCP dissipates the proton gradient created by mitochondrial respiration, bypassing ATP synthetase. The dissipation of the proton gradient causes a major increase in the respiratory rate uncoupled from ATP synthesis, as ATP synthetase is bypassed (Nicholls et al. 1986). When BAT is fully stimulated, ATP generation accounts for far less than 10% of BAT oxygen consumption (Nicholls and Locke 1984, Nicholls et al. 1986). In addition to being activated by cold, BAT thermogenesis is stimulated by overfeeding, seemingly by the same sympathetic distal pathways and biochemical mechanisms (Himms-Hagen 1985 and 1989, Nicholls and Locke 1984, Rothwell and Stock 1979 and 1980). Information regarding body temperature and nutritional status is integrated in the hypothalamus, from where impulses are fired into BAT via sympathetic nerves (Nicholls and Locke 1984). The release of norepinephrine (NE) from the sympathetic nerve endings activates the tissue via fl- and a-adrenergic receptors (Cannon and Nedergaard 1985). The role of BAT in cold- or diet-induced facultative thermogenesis is dramatically illustrated by the animal models of genetic obesity, the ob/ob mouse and fdfa rat. In these animals, there is a central defect (Young and Landsberg 1983, Martin et al. 1986) in the stimulation of BAT, as a consequence of which they develop obesity and cold intolerance. Although the activity of UCP is modulated by an interplay between fatty acids and nucleotides, UCP concentration determines the thermogenic potential of BAT (Cunningham et al. 1985, Nicholls and Locke 1984, Nicholls et al. 1986). Accordingly, UCP synthesis is an integral
26
part of BAT responses, being evident within 15 min of cold exposure in rats (Ricquier et al. 1986, Bianco et al. 1988, Rehnmark et al. 1992). The importance of BAT in cold adaptation in small animals and human newborns has been well established, but in adult humans the site of facultative thermogenesis is not as clear, and it is a matter of debate how relevant the physiologic role of BAT is. Nonetheless, BAT is present in adult human beings (Ito et al. 1991, Lean et al. 1986, Huttunen et al. 1981), and it is activated by the same stimuli as in rodents (Garruti and Ricquier 1992, Ricquier et al. 1982, Huttunen et al. 1981). Moreover, in cadavers of adults dying in situations representing thermal stress, UCP concentration has been found in the same range as in human neonates and coldstimulated rodents (Lean et al. 1986 and
THE SMALLER
THE ANIMAL,
THE GREATER IS THE SURFACE AREA TO VOLUME RATIO AND THE NEED FOR FACULTATIVE THERMOGENESIS TEMPERATURE.
TO MAINTAIN BODY IN SMALL MAMMALS,
INCLUDING THE NEWBORN HUMAN, THE MAIN SITE OF FACULTATIVE THERMOGENESIS
IS BROWN ADIPOSE
TISSUE.
1987). Recent studies combining immunodetection of UCP and specific analysis of UCP mRNA show that both are readily detectable in perirenal fat of >50% of adult humans, at levels compa-
rable to those of rodents
at room tem-
perature (Garruti and Ricquier 1992). Probably because of our relatively large size, and our ability to control ambient temperature and shelter ourselves from cold, BAT in most of us adult humans is in a less active state, reminiscent of that of warm-adapted rats (Foster and Frydman 1979, Peachey et al. 1988), but it is evident that the level of activation varies widely (Garruti and Ricquier 1992, Lean et al. 1986) and conceivably could account for a good part of the variability of diet-induced thermogenesis and the ability of humans to withstand cold (Jequier 1987, Ravussin et al. 1988, Katzeff and Danforth 1989).
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??
Hormones
and Thermogenesis
Obligatory thermogencsis, iable among individuals,
although vat’is rclativcly
constant in a given person. Given the low thermodynamic efficiency of animals, 30% at best, synthetic processes (for example, growth or repair), turnover 01 substrates (for instance, lipogenesis and lipolysis), or other energy-requiring processes, such as the maintenance of ion gradients through membranes, will necessarily result in heat production. By stimulating some or all of these processes, most hormones enhance obligatory thermogenesis. In this regard, probably because of its widespread effects, thyroid hormone seems to have the most pronounced effect on BMR and obligatory thermogenesis. Although attempts have been made to relate the thermogenic effect of thyroid hormone to some specific cellular effects, none by itself seems to account for all its thermogenic potential [see Sestoft (1980), for review]. Thus, its thermogenic effect has been related to the stimulation of Na/K ATPase (IsmailBeigi et al. 1989), but this enzyme does not seem to account for but a small fraction of the increase in oxygen consumption in thyrotoxicosis (Valdemarsson et al. 1992, Clark et al. 1982). Other biochemical processes considered candidates to explain the thermogenic effect of thyroid hormone have been the NADH shuttle through the cytosolic and mitochondrial glycerophosphate dehydrogenases (Lee and Lardy 1965, Sestoft 1980); the increase in protein synthesis, amino acid turnover, and gluconeogenesis (Sestoft 1980); and the stimulation of both lipolysis and lipogenesis. In an elegant recent study, the energetic cost of lipogenesis has been estimated as 3%4% of the calorigenesis induced by thyroid hormone (Oppenheimer et al. 1991). On the other hand, the increase in oxygen consumption observed in hyperthyroid states is highly coupled to ATP production, refuting the old concept that the calorigenic effect of thyroid hormone was due to uncoupling of phosphorylation (Sestoft 1980, Schwartz and Oppenheimer 1978). It would appear that the pronounced effect of thyroid hormone on obligatory thermogenesis can be best explained by the stimulation by this hormone of innumerable pathways in intermediary metabolism and, to a sig-
TEM Vol. 4, No. I. 1993
nificant extent, by the increased myocardial work necessary to deliver more oxygen to the tissues (Sestoft 1980). It is likely that the effect of other hormones on BMR and obligatory thermogenesis is similarly explained: they stimulate various metabolic pathways, directly or indirectly, and this is accompanied by a proportional increase in heat production, with no perceptible variation of the thermodynamic efficiency. Physiologic fluctuations in the plasma concentrations of thyroid hormone, gonadal steroids, glucocorticoids, and GH, all of which can have long-term effects on BMR, do not seem sufficient to cause day-to-day variations in energy expenditure and thermogenesis; that is, these hormones, via affecting BMR, may play a role in long-term energy balance, but do not seem appropriate for adjustments in heat production for temperature regulation or energy dissipation in overfeeding. In contrast, insulin, glucagon, and epinephrine may cause rapid changes in metabolic rate and thermogenesis, but in this case, rather than adaptive changes in thermogenesis to adjust body temperature or balance the energy economy, they merely reflect the mobilization and transformation of substrates. Glucagon and epinephrine also contribute to overall thermogenesis by increasing cardiac work. Thus, in general, hormones directly affect obligatory thermogenesis and play an important albeit permissive role in facultative, adaptive (to cold or diet) thermogenesis by contributing to the availability of substrates. In contrast, the sympathetic nervous system (SNS), via the neurotransmitter NE, directly controls heat production in homeostasis, that is, the SNS maintains body temperature in response to cold, and balances energy intake with thermogenesis. Over the last several years, it has become evident that certain hormones may also play a more direct role on facultative thermogenesis. In the subsequent paragraphs, I discuss relatively recent developments in our laboratory that show previously unsuspected relations between the SNS and thyroid hormone. These studies have disclosed synergistic interactions between thyroid hormone and the SNS that are essential for energy balance and temperature regulation, and demonstrate a critical role for thyroid hormone in facultative thermogenesis. Along with this, I mention
TEM Vol. 4, No. 1, 1993
other studies that show how other hormones such as glucocorticoids and insulin can modulate the sympathetic stimulation of BAT
??
The Activation of Thyroid Hormone Is a Critical Step Modulating the Effects of This Hormone in Selected Tissues
Although the major secretory product of the thyroid gland is T,, T, accounts for most of the thyromimetic potency of the thyroidal secretion. T, is intrinsically 10 times more active than T,, as a result of a proportionally higher affinity for the nuclear thyroid hormone receptors (Samuels et al. 1974, Oppenheimer et al. 1976). Normally, free T, concentration in the cell does not appear sufficiently high for T, to occupy more than a relatively minor proportion of receptors (Surks and Oppenheimer 1977, Silva and Larsen 1977).
WITHIN 4 HOURS OF COLD EXPOSURE, RECEPTORS
THE NUCLEAR T3 OF BROWN ADIPOSE
TISSUE (BAT) AT ARE NEARLY SATURATED WITH T, PRODUCED BY BAT 5 ‘D-II.
A large proportion of the T, produced is the result of extrathyroidal S-deiodination of T4, a reaction catalyzed by two types of S-deiodinase activities (Silva et al. 1982) called type-1 and -11 5’deiodinases (Leonard and Visser 1986). The type-1 5’-deiodinase (S’D-I) has been recently cloned and found to be a selenocysteine-containing protein (Berry et al. 1991), and type-II 5’-deiodinase (SD-II)
is now known to be a different
protein. While 5’D-I generates T, largely for the plasma pool, S/D-II’s main physiologic function seems to be to provide T, for the tissue where it is present, in a regulated fashion, the most dramatic evidence for which is in brain (Silva and Matthews 1984, Calvo et al. 1990) and BAT (Silva and Larsen 1985 and 1986a and b). T, can be found in the brain of fetuses before it is detectable in plasma (Calvo et al. 1990). In BAT, S’D-II is under complex hormonal regulation (Silva and Larsen 1986a and b). In addition to the SNS, hypothyroxinemia and insulin are potent stimulators of its
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activity. The stimulation by insulin is pronounced, is direct, requires ongoing protein synthesis (Silva and Larsen 1986b, Mills et al. 1987), and may play a role in diet-induced thermogenesis, as discussed below. In the realm of thermogenesis, it was most exciting to find that BAT contained SD-II (Leonard et al. 1983) and that this was activated by the SNS (Silva and Larsen 1983). The sympathetic stimulation of BAT induced either by cold exposure or the injection of NE results in severalfold increases in BAT T, (Silva and Larsen 1985). Within 4 h of cold exposure, the nuclear T, receptors (NTRs) of BAT are nearly saturated with T, produced by BAT SD-II; if anything, in this condition the contribution of plasma T, to BAT T, is less than in the nonstimulated condition (Bianco and Silva 1988). This immediately suggested a synergism between the SNS and thyroid hormone in BAT, whereby the adrenergic stimulation of BAT increased the concentration of the more potent T, in the tissue, which in turn could enhance the thermogenic response. As mentioned, the thermogenic potential of BAT appears to be determined by UCP concentration. To demonstrate the importance of UCP for cold adaptation, and the importance of S’D-II and its activation for this response, we took advantage of our observation that the cold-induced BAT stimulation caused a severalfold increase in BAT T, (Silva and Larsen 1985). [See Figure 2 and Bianco and Silva (1987a and 1987b) for details.] The top panel of Figure 2 shows the core temperature of euthyroid or hypothyroid rats maintained at either 4°C or 23°C. The middle panel shows the liver mitochondrial a-glycerophosphate dehydrogenase (a-GPD) as evidence of thyroid action in peripheral tissues. a-GPD is very sensitive to the thyroid status and is specifically stimulated by thyroid hormone (Lee and Lardy 1965). The bottom panel shows the mitochondrial concentration of UCP in BAT measured by a very sensitive and specific assay (Bianco and Silva 1987a). Treatments are indicated at the top. At room temperature (23”C), there is no difference in core temperature between euthyroid and hypothyroid rats, yet there is marked difference in both a-GPD activity and UCP concentration. This suggests that the mechanisms to
27
Temp
4-c +
33-C
T,
+
T4
+
+
IOP CORE TEMPERATURE
BAT
UCP
A P .w
IILL-
s’.
"10 0
Euthyrad
??Thyrotdectomlzed
Figure 2. Effects of ambient temperature and
replacement with T, or T4 on core body temperature, liver mitochondrial a-glycerophosphate dehydmgenase (a-GPD), and brown adipose tissue (BAT) uncoupling protein (UCP) in normal and thyroidectomized rats. Rats were in the cold for 48 h. T,, 0.15 @lo0 g, was given twice daily, by the S.C. route, for 5 days. T,, 0.4 pg/lOOg, was given twice daily, by the S.C. route, for the 2 days that rats remained in the cold. Iopanoic acid (IOP), an inhibitor of SD-II, was given intraperitoneally, 5 mg/l 00 g twice daily. For discussion, see the text and for experimental details (Bianco and Silva 1987a and b).
reduce heat dissipation are probably sufficient to maintain body temperature in this environment in spite reduced obligatory thermogenesis.
of the When
ambient temperature was reduced to 4°C for 48 h, however, euthyroid rats maintained their core temperature, whereas hypothyroid rats developed severe hypothermia. Cold exposure did not affect a-GPD, but had different effects on UCP in eu- and hypothyroid rats: whereas in the former UCP increased -3-fold, it remained virtually unchanged in the
28
latter. That the lack of UCP response to cold is caused by a failure of BAT to respond rather than to insufficient adrenergic stimulation is evidenced by the observations that in hypothyroid rats NE turnover is increased (Young et al. 1984) and UCP does not respond to exogenous NE (Silva 1988). Replacement of hypothyroid rats with T, for 5 days before the experiment caused no significant increase in core temperature, which was still significantly lower that in the intact rats. The persistent hypothermia contrasted with normalization of serum T, concentration [OS2 + 0.03 (SEM) vs 0.57 f 0.03 ng/mL in intact rats] and that of hepatic a-GPD activity. Thus, the correction of systemic hypothyroidism did not suffice to bring about a normal response to the cold stress. A different situation was observed when T, was replaced just for the 2 days of cold exposure: core temperature was maintained normal and a-GPD activity was not restored, but UCP concentration was normalized. This short treatment with T, was insufficient to normalize either serum T, or T, concentrations, which were -30% and -50% of that in intact rats. Thus, with T, we observed prevention of hypothermia in the face of persistent systemic hypothyroidism, but paralleling a normalization of the UCP response to cold. Iopanoic acid (IOP), an inhibitor of SD-II, abolished T,-induced restoration of both UCP response and normothermia. Appropriate controls (Bianco and Silva 1987a and 1987b) demonstrated that, at the dosage used, IOP effectively blocked SD-II and that this substance did not prevent the entry of T, or T, into BAT. Furthermore, the addition of T, to the T, regimen normalized serum T, and liver a-GPD, but did not prevent the hypothermia and, when given along with IOP and T,, T, did not prevent IOP from blocking the effect of T,. In other experiments (Car&ho et al. 1991), the specific obliteration of the adrenergic activation of SD-II with prazosin (Silva and Larsen 1983) prevented the increase in oxygen consumption induced by cold in T,-replaced thyroidectomized rats, adding support to the physiologic relevance of S’D-II activation to the UCP response, and of this to the thermogenic response. A T, dose-UCP response analysis in thyroidectomized rats showed that UCP
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response to cold bears a peculiar relation with BAT T,. UCP response is not linear with the NTR occupancy, and for a full response, as seen in euthyroid rats or in hypothyroid rats treated with T,, full nuclear receptor occupancy is necessary, as illustrated in Figure 3. Here, the UCP responses to cold and liver a-GPD activities (for comparison) have been plotted against the integrated occupancy of the receptors by different doses of T, given for 48 h to thyroidectomized rats maintained previously on T, replacement (Bianco and Silva 1987a and 1987b). For a full UCP response to cold, near-saturation of NTR is necessary, and it is not until occupancy exceeds 60%70% that the UCP response increases significantly. From the response in intact rats and thyroidectomized T,-replaced rats, it was inferred that in these two situations the adrenergic activation of SD-II by cold results in near-saturation of the receptors, which was later confirmed in kinetic studies in euthyroid rats exposed to cold, where we found that NTR occupancy is maximal within 4 h of initiating cold exposure (Bianco and Silva 1988). Thus, T, appears a much more efficient source of T, for BAT than plasma T, when SD-II is activated. Most importantly, these studies have shown that the concentration of T, is locally regulated in BAT. Rapid changes in BAT T, may occur without changes in plasma T,. This local regulation may increase BAT NTR occupancy to levels seen in severe thyrotoxicosis, yet without it being present. Thus, to reach 98% NTR occupancy, plasma T, must be elevated to 50 ng/mL, a level 100 times higher than the euthyroid plasma T, (Bianco and Silva 1987b). The stimulation of 5’D-II makes this level of receptor occupancy possible without systemic thyrotoxicosis. Note in Figure 3 that euthyroid levels of a-GPD activity are achieved with 50% saturation, which occurs with a serum T, of 0.6 ng/mL, obtained with the lowest dose of T,. With higher plasma T, levels, the liver is made hyperthyroid. In recent years, our laboratory has been engaged in unraveling the mechanisms whereby T, and NE regulate UCP levels. We have found that T, amplifies the stimulation of NE on UCP gene transcription rather than being merely a permissive factor for the ex-
TEM Vol. 4,No.1,1993
.
-)
or 0
Euthymd
UCP
response
+ 95%
CL
60 80 20 40 Nuclear T, receptor occupancy (X)
Figure 3. Uncoupling protein (UCP, 0) response to cold and liver mitochondrial aglycerophosphate dehydrogenase (a-GPD, 0) as a function of nuclear T, receptor occupancy in BAT and liver, respectively, in thyroidectomized rats treated with different doses of T,. Bars indicate the mean f SEM of 3-4 rats. All rats were previously maintained on T, replacement (0.15 gg/lOOg, s.c., twice daily) for 5 days and then given different doses of T, (0, 0.3, 0.5, 0.8, 1.3,2.1, and 4.7 pg/lOOg day, divided into two S.C.doses, from .?eji to right) during the 48 h of cold exposure. The boxes represent the mean UCP response to cold or a-CPD activity in euthyroid rats equally exposed to cold f95% confidence limits (CL). Nuclear receptor occupancy has been integrated for the 48-h cold-exposure period and expressed as a fraction of the maximal binding capacity. For experimental details, see Bianco and Silva (1987a and b).
pression of UCP (Bianco et al. 1988), as
has been postulated in the past (Triandafillou et al. 1982, Mory et al. 1981). Furthermore, T, enhances the effect of NE not only at a transcriptional level, but at a posttranscriptional level as well (Rehnmark et al. 1992). Studies in progress in our laboratory show that UCP gene has functional thyroid hormoneand CAMP-responsive elements, corroborating the hypothesis previously advanced by us (Bianco et al. 1988) that thyroid hormone, via its receptor, and the second messenger of NE, CAMP, probably via a specific phosphorylated protein, interact synergistically at the level of the UCP gene to stimulate its expression. What is not clear at present is whether thyroid hormone affects the strength of the NE signal. Seydoux et al. (1982) reported a reduction in BAT f%adrenergic receptors in hypothyroid rats, but concluded that this defect could not explain but a minor fraction of the defective BAT response to sympathetic stimulation. Sundin et al. (1984) later reported that CAMP accumulation 15 min after isoproterenol exposure in isolated brown adipocytes from hypothyroid rats was not less than in cells obtained from normal rats, but when a similar experiment was later performed
TEM Vol. 4, No. 1, 1993
by Raasmaja and Larsen (1989), 2 h after stimulation with isoproterenol or NE, CAMP was significantly reduced. Although it remains open whether reduced CAMP generation in hypothyroid BAT can account for part of its insufficient response to SNS stimulation, the reduced number of receptors does not appear to be relevant, since both the respiratory and UCP responses to forskolin, acting directly on adenylate cyclase, are as blunted as those to adrenergic receptor agonists. Furthermore, we have recently found (unpublished observation) that the short regimens of T, or T, that restore BAT UCP and thermogenic responses to cold or NE in thyroidectomized rats fail to increase the number of fl-adrenergic receptors. Only after prolonged treatment with T, do we observe an increase in the number of these receptors. The complex interactions among NE, thyroid hormone, insulin, BAT S’D-II and BAT thermogenesis are schematically summarized in Figure 4.
??
Sympathetic-Hormonal Interactions in the Central Nervous System
While there is a synergism among SNS, thyroid hormone, and insulin in BAT,
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central stimulation of BAT seems to be subjected to different interactions. It was already mentioned that in hypothyroidism adrenergic input to BAT is increased, as reflected by NE turnover (Young et al. 1984), and, indeed, hypothyroid BAT shows signs of enhanced adrenergic stimulation (Mary et al 1981), yet with blunted end responses, as summarized in the foregoing paragraphs. In hyperthyroidism, in contrast, SNS stimulation is reduced and, in spite of the higher levels of plasma T,, there is a reduction of GDP binding by BAT mitochondria, an expression of functional UCP (Triandafillou et al. 1982), and reduced responses to cold exposure (Sundin 1981). It would appear that sympathetic BAT stimulation is inversely related to the magnitude of obligatory thermogenesis: the higher the latter, the lower is the need for facultative thermogenesis and, hence, of BAT activity, to maintain body temperature. These relations are schematically represented in Figure 1. In addition to obligatory thermogenesis and ambient temperature, other hormones seem to affect BAT stimulation and responses at a central level. CRH may have a stimulator-y effect opposed by glucocorticoids on BAT sympathetic stimulation (Vander Tuig et al. 1984, Walker and Romsos 1992). It is not unlikely that the inhibitory effects of glucocorticoids and the beneficial effects of adrenalectomy on BAT thermogenesis in obese mice and rats are mediated through CRH.
??
Concluding Remarks
It is evident that the role of thyroid hormone on BAT function and on facultative thermogenesis would have remained inapparent without the knowledge of S’D-II. It also follows that the response of S’D-II is an important source of variation in the thermogenic response of BAT. The steepness of the UCP response with nuclear T, receptor occupancy (Figure 3) suggests that BAT T, content plays a critical role in the responses of BAT The concept that low energy expenditure is a major, genetically determined risk factor for obesity (Jequier 1987, Katzeff and Danforth 1989, Ravussin et al. 1988, Bogardus et al. 1986, Bouchard et al. 1990, Stunkard et al. 1990) is receiving increasing support. Food ingestion stimulates BAT in a 29
1990. The response to long-tern1 overfeeding in identical twins. N Engl J Med 322:1477-1482.
Plasma membrane
Calvo R, Obregon MJ, Ruiz de 0, Escobar de1 Rey F, Morreale de Escobar G: 1990. Congenital hypothyroidism, as studied in rats: crucial role of maternal thyroxine but not of 3,5,3’-triiodothyronine in the protection of the fetal brain. J Clin Invest 86:889899.
+ Glucose uptake Insulin
w/
+
Lipogenesis
Cannon B, Nedergaard J: 1985. The biochemistry of an inefficient tissue: brown adipose tissue. Essays Biochem 2: 110-164.
Figure 4 Schematic representation of the effects of sympathetic nerve stimulation (via norepinephrine, NE), T4, T,, and insulin on brown adipose tissue (BAT) thermogenesis. See the text for discussion. The a and g indicate the adrenergic receptors involved in mediating NE effects. CAMPis the second messenger carrying the NE signal to the uncoupling protein (UCP) gene. CAMPand T, are essential for the full expression of the UCP gene and hence to determine BAT thermogenic potential. The type-II T, S-deiodinase (5’D-II) catalyzing T4 to T, conversion, the main source of T, for the brown adipocyte, is activated by both NE and insulin. Both insulin and CAMP also contribute to the therrnogenic response by providing substrate, glucose, and fatty acids. T, is also important for the expression of lipogenic enzymes and a-glycerophosphate dehydrogenase (a-GPD) in BAT (not shown here), which are also important for the overall thermogenic response (Bianco and Silva 1987b).
manner similar to cold. Cafeteria diet, a form of inducing overfeeding in animals, stimulates BAT in a sustained fashion (Rothwell and Stock 1980); as little as one meal perceptibly stimulates S’D-II, GDP binding, and NE turnover in BAT (Glick 1987); and S’D-II response is attenuated in the genetically obese fa/f rat (Wu et al. 1987). Moreover, BAT S’D-II is under complex hormonal regulation (Silva and Larsen 1986a and b), with insulin being a very potent stimulator (Silva and Larsen 1986b, Mills et al. 1987) (Figure 4), and both diabetes and insulin resistance have been associated with reduced BAT responses to adrenergic stimulation (Howland and Bond 1987, Geloen and Trayhurn 1990, Walker and Romsos 1992, Marie et al. 1992). Thus, the studies summarized here offer powerful insights into previously unsuspected sources of variability in energy expenditure.
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Acknowledgments
Initial studies described here were performed while J.E.S. was an Associate Investigator in the Howard Hughes Medical Institute. Subsequent work has been supported by PHS grant DK-42431 and
30
MRC (Canada) grant MT 11550 (to J.E.S.) and FAPESP, of Brazil (to Dr. A.C. Bianco).
References Berty MJ, Banu L, Larsen PR: 1991. Type I iodothyronine deiodinase is a selenocysteinecontaining enzyme. Nature 349:438-440. Bianco AC, Silva JE: 1987a. Intracellular conversion of thyroxine to triiodothyronine is required for the optimal thermogenic function of brown adipose tissue. J Clin Invest 79:295-300. Bianco AC, Silva JE: 1987b. Optimal response of key enzymes and uncoupling protein to cold in brown adipose tissue depends on local T, generation. Am J Physiol253:E255E263. Bianco AC, Silva JE: 1988. Cold exposure rapidly produces virtual saturation of brown adipose tissue nuclear T, receptors. Am J Physiol255:E496-E508. Bianco AC, Sheng X, Silva JE: 1988. Triiodothyronine amplifies norepinephrine stimulation of uncoupling protein gene transcription by a mechanism not requiring protein synthesis. J Biol Chem 263:18,16818,175. Bogardus C, Lillioja S, Ravussin E, et al.: 1986. Familial dependence of the resting metabolic rate. N Engl J Med 315:96-100. B ouchard C, Tremblay A, Despres JP, et al.:
Carvalho SD, Kimura ET, Bianco AC, Silva JE: 1991. Central role of brown adipose tissue thyroxine 5’deiodinase on thyroid hormone-dependent thermogenic response to cold. Endocrinology 128:2149-2 159. Clark DG, Brinkman M, Filsell OH, Lewis SJ, Berry MN: 1982. No major thermogenic role for (Na+ + K+)-dependent adenosine triphosphatase apparent in hepatocytes from hyperthyroid rats. Biochem J 202:66 l665. Cunningham SA, Leslie P, Hopwood D, et al.: 1985. The characterization and energetic potential of brown adipose tissue in man. Clin Sci 69:343-348. Foster DO, Frydman ML: 1977. Non-shivering thermogenesis in the rat. II. Measurements of blood flow with microspheres point to brown adipose tissue of the dominant site of the calorigenesis induced by noradrenaline. Can J Physiol Pharmacol56:110-112. Foster DO, Frydman ML: 1979. Tissue distribution of cold-induced thermogenesis in conscious warm-or-cold acclimated rats reevaluated from changes in tissue blood flows: the dominant role of brown adipose tissue in the replacement of shivering by nonshivering thermogenesis. Can J Physiol Pharmacol57:257-270. Garruti G, Ricquier D: 1992. Analysis of uncoupling protein and its mRNA in adipose tissue deposits of adult humans. Int J Obes 16:383-390. Geloen A, Trayhum P: 1990. Regulation of the level of uncoupling protein in brown adipose tissue by insulin. Am J Physiol 258:R418-R424. Glick Z: 1987. The thermic effect of a meal. J Obes Weight Regul 6: 170-l 78. Himms-Hagen J: 1985. Brown adipose tissue metabolism and thermogenesis. Annu Rev Nutr 5:69-94. Himms-Hagen J: 1989. Brown adipose tissue thermogenesis role in thermoregulation, energy regulation and obesity. Prog Lipid Res 28:67-l 15. Howland RJ, Bond KD: 1987. Cold-acclimation increases insulin sensitivity of brown adipocytes. Hot-m Metab Res 19:503-504. Huttunen P, Hirvonen J, Kirmula V: 198 1. The
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TEA4 Vol.4,No. 1,1993
occurrence of brown adipose tissue in outdoorworkers. Eur J Appl Physiol46:339345. Ismail-Beigi F, Gick GG, Edelman IS: 1989. Thyroid hormone regulation of Na,KATPase. In Lardy H, Stratman F, eds. Hormones, Thermogenesis and Obesity. New York, Elsevier, pp 269-278. Ito T, Tanuma Y, Yamada M, Yamamoto M: 199 1. Morphological studies on brown adipose tissue in the bat and in humans of various ages. Arch Histol Cytol54:1-39. Jequier E: 1987. Energy utilization in human obesity. Ann NY Acad Sci 499~73-83. Katzeff HL, Danforth E: 1989. Decreased thermic effect of a mixed meal during overnutrition in human obesity. Am J Clin Nutr 50:915-921. Landsberg L, Young JB: 1983. Autonomic regulation of thermogenesis. In Girardier L, Stock MJ, eds. Mammalian Thermogenesis. London Chapman and Hall, pp 99-140. Lean MEJ, James WPT, Jennings G, Trayhum P: 1986. Brown adipose tissue uncoupling protein content in infants, children and adult humans. Clin Sci 71:291297. Lean MEJ, Trayhum P, Murgatroyd PR, Dixon AK. 1987. The case for brown adipose tissue function in humans: biochemistry, physiology and computed tomography. In Berry EM, Blondheim SH, Eliahou HE, et al., eds. Recent Advances in Obesity Research. London, Libbey, pp 109-l 16. Lee YP, Lardy HA: 1965. Influence of thyroid hormones on I-cr-glycerophosphate dehydrogenases and other dehydrogenases in various organs of the rat. J Biol Chem 240:1427-1436. Leonard JL, Visser TJ. 1986. Biochemistry of deiodination. In Hennemann G, ed. Thyroid Hormone Metabolism. New York, Marcel Dekker, pp 189-229. Leonard JL, Mellen SA, Larsen PR: 1983. Thyroxine 5’-deiodinase activity in brown adipose tissue. Endocrinology 112: 11531155. Marie V, Dupuy F, Bazin R: 1992. Decreased T,-to-T, conversion in brown adipose tissue of Zucker fa/f pups before the onset of obesity. Am J Physiol 263:E115E120. Martin RJ, Harris RBS, Jones DD: 1986. Evidence of a central mechanism of obesity in the Zucker fatty rat (fa/fa). Proc Sot Exp Biol Med 183:1-10. Mills I, Barge RM, Silva JE, Larsen PR: 1987. Insulin stimulation of iodothyronine 5’-deiodinase in rat brown adipocytes. Biothem Biophys Res Commun 143:81-86. Mot-y G, Ricquier D, Pesquies P, Hemon P: 1981. Effects of hypothyroidism on the brown adipose tissue of adult rats: compari-
TEM Vol. 4, No. 1, 1993
son with the effects of adaptation to the cold. J Endocrinol 91:s 15-524.
stration of putative receptors in isolated nuclei and soluble nuclear extracts. Science 184:1188-1191.
Nechad M. 1986. Structure and development of brown adipose tissue. In Trayhum P, Nicholls DG, eds. Brown Adipose Tissue. London, Edward Arnold, pp l-30.
Schwartz HL, Oppenheimer JH: 1978. Physiologic and biochemical actions of thyroid hormone. Pharmacol Ther 3:349-376.
Nicholls DG, Locke RM: 1984. Thermogenic mechanisms in brown fat. Physiol Rev 64: l-64.
Sestoft L: 1980. Metabolic aspects of the calorigenic effect of thyroid hormone in mammals. Clin Endocrinol 13:489-506.
Nicholls DG, Cunningham SA, Rial E: 1986. The bioenergetic mechanisms of brown adipose tissue thermogenesis. In Trayhum P, Nicholls DG, eds. Brown Adipose Tissue. London, Edward Arnold, pp 52-85.
Seydoux J, Giacobino J-P, Girardier L: 1982. Impaired metabolic response to nerve stimulation in brown adipose tissue of hypothyroid rats. Mol Cell Endocrinol25:2 13-226.
Oppenheimer JH, Schwartz HL, Surks MI, Koemer D, Dillmann WH: 1976. Nuclear receptors and the initiation of thyroid hormone action. Ret Prog Horm Res 32:529565. Oppenheimer JH, Schwartz HL, Lane JT, Thompson MP: 199 1. Functional relationship of thyroid hormone-induced lipogenesis, lipolysis and thermogenesis in the rat. J Clin Invest 87:125-132. Peachey T, French RR, York DA: 1988. Regulation of GDP binding and uncoupling protein concentration in brown adipose tissue mitochondria: the effects of coldacclimation, warm-reacclimation and noradrenaline. Biochemistry 249:451457. Raasmaja A, Larsen PR: 1989. a,- and gAdrenergic agents cause synergistic stimulation of the iodothyronine deiodinase in rat brown adipocytes. Endocrinology 125:2502-2509. Ravussin E, Lillioja S, Knowler WC, et al.: 1988. Reduced rate of energy expenditure as a risk factor for body-weight gain. N Engl J Med 318~467-472. Rehnmark S, Bianco AC, Kieffer JD, Silva JE: 1992. Transcriptional and post-transcriptional mechanisms in the uncoupling protein mRNA response to cold. Am J Physiol 262:E58-E67.
Silva JE: 1988. Full expression of uncoupling protein gene requires the concurrence of norepinephrine and triiodothyronine. Mol Endocrinol2:706-713. Silva JE, Larsen PR: 1977. Pituitaty nuclear 3,5,3’-triiodothyronine and thyrotropin secretion: an explanation for the effect of thyroxine. Science 198:617-6 19. Silva JE, Larsen PR: 1983. Adrenergic activation of triiodothymnine production in brown adipose tissue. Nature 305:7 12-7 13. Silva JE, Larsen PR: 1985. Potential of brown adipose tissue type II thyroxine 5’-deiodinase as a local and systemic source of triiodothyronine in tats. J Clin Invest 76:22962305. Silva JE, Larsen PR: 1986a. Interrelationships among thyroxine, growth hormone, and the sympathetic nervous system in the regulation of 5’-iodothyronine deiodinase in rat brown adipose tissue. J Clin Invest 77: 12 141223. Silva JE, Larsen PR: 1986b. Hormonal regulation of iodothyronine 5’-deiodinase in rat brown adipose tissue. Am J Physio125 l:E639E643.
Ricquier D, Nechad M, Moty G: 1982. Ultrastructural and biochemical characterization of human brown adipose tissue in pheochromocytoma. J Clin Endocrinol Metab 54:803-807. Ricquier D, Bouillaud F, Toumelin P, et al.: 1986. Expression of uncoupling protein mRNA in thermogenic or weakly thermogenie brown adipose tissue. J Biol Chem 261:13,905-13,910. Rothwell NJ, Stock MJ: 1979. A role for brown adipose tissue in diet-induced thermogenesis. Nature 281:31-35. Rothwell NJ, Stock MJ: 1980. Similarities between cold- and diet-induced thermogenesis in the rat. Can J Physiol Pharmacol 58~842-848. Samuels HH, Tsai JS, Casanova J: 1974. Thyroid hormone action: in vitro demon-
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Silva JE, Matthews PS: 1984. Production rates and turnover of triiodothyronine in rat-developing-cerebral cortex and cerebellum: responses to hypothyroidism. J Clin Invest 74:1035-1049. Silva JE, Leonard JL, Crantz FR, Larsen PR: 1982. Evidence for two tissue-specific pathways for in vivo thyroxine 5’deiodination in the rat. J Clin Invest 69: 1176-l 184. Stunkard AJ, Harris JR, Pedersen NL, McCleam GE: 1990. The body-mass index of twins who have been reared apart. N Engl J Med 322:1483-1487. Sundin U: 198 1. GDP binding to rat brown fat mitochondria: effects of thyroxine at different ambient temperature. Am J Physiol 241:C134-C139. Sundin U, Mills I, Fain JN: 1984. Thyroidcatecholamine interactions in isolated brown adipocytes. Metabolism 33:1028-1033. Surks MI, Oppenheimer JH: 1977. Concentration of L-thyroxine and r-triiodothyronine specifically bound to nuclear recep-
31
tars in rat liver and kidney. Quantitative evidence favoring a major role of T, in thyroid hormone action. J Clin Invest 60:555-562. Triandafillou J, Gwilliam C, Himms-Hagen J: 1982. Role of thyroid hormone in coldinduced changes in rat brown adipose tissue mitochondria. Can J Biochem 60:530537. Valdemarsson S, Ikomi-Kumm J, Monti M: 1992. Increased lymphocyte therrnogenesis in hyperthyroid patients: role of Na/K pump function-evaluation of aerobic/anaerobic
EU
metabolism. 126:291-295.
Acta
Endocrinol
ine 5’-monodeiodinase is attenuated in Zucker obese rat. Am J Physiol 252:E63E67.
(Copenh)
Vander Tuig JG, Oshima K, Yoshida T, Romsos DR, Bray GA: 1984. Adrenalectomy increases norepinephrine turnover in brown adipose tissue of obese (ob/ob) mice. Life Sci 34:1423-1432. Walker HC, Romsos DR: 1992. Glucocorticoids in the CNS regulate BAT metabolism and plasma insulin in ob/ob mice. Am J Physiol262:EllO-E117. Wu SY, Stern JS, Fisher DA, Glick Z: 1987. Cold-induced increase in brown fat thyrox-
Young JB, Landsberg L: 1983. Diminished sympathetic nervous system activity in genetically obese (ob/ob) mouse. Am J Physiol 245:E148-E154. Young JB, Kaplan MM, Landsberg L: 1984. Parallel changes in sympathetic nervous system (SNS) and iodothyronine 5’-deiodinase (5’D) activity in hypothyroid rats [abst]. Clin Res 32:377A. TEM
TH 21 st Annual
ng in
4th-9th July 1993 Topics will include all aspects of clinical and basic research oriented thyroidology. Cardiff is the capital city of Wales and one of Britain’s major administrative, commercial business, and educational centers. Easy access to the beautiful surrounding countryside and coastline. For further details please contact: Universal Conference Organizers PO Box 1380, Long Ashton Bristol BS18 9BF, England, UK
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TEM Vol.4,No. I,1993
from skeletal metastases in breast cancer patients during long-term bisphosphonates (APD) treatment. Lancet 2:983-985. Van Holten-Verzantvoort AT, Zwinderman AH, Aaronson NK, et al.: 1991. The effect of supportive pamidronate treatment on aspects of quality of life of patients with advanced breast cancer. Eur J Cancer 27:544549.
Urwin GH, Yates AJP, Gray RES, et al.: 1987. Treatment of hypercalcemia of malignancy with intravenous clodronate. Bone b(Supp1 l):S43-51.
Watts NB, Harris ST, Genant HK, et al.: 1990. Intermittent cyclical etidronate treatment of postmenopausal osteoporosis. N Engl J Med 323~73-79.
Valkema R, Vismans FJFE, Papapoulos SE, Pauwels EKJ, Bijvoet OLM: 1989. Maintained improvement in calcium balance and bone mineral content in patients with osteoporosis treated with the bisphosphonate APD. Bone Miner 5:183-192.
Yates AJP, Percival RC, Gray RES, et al.: 1985. Intravenous clodronate in the treatment and retreatment of Paget’s disease of bone. Lancet 1: 1474-l 477. TEM
Van Holten-Verzantvoort AT, Bijvoet OLM, Cleton FJ, et al.: 1987. Reduced morbidity
Hormonal Control of Thermogenesis and Energy Dissipation (adaptive)
sympathetic
nervous
hormones
adipose
important
system
is primarily
(SNS).
in adaptive thermogenesis
most, permissive. brown
thermogenesis
The participation
has been considered
The finding of type II-thyroxine tissue
controlled
(BAT) has opened
role for thyroid hormone
of
by the thyroid
minor or, at
(T,) S-deiodinase
a way to uncover
in
a more
in adaptive thermogenesis.
This
enzyme is activated by the SNS and insulin. When activated, it generates high BAT concentrations
of triiodothyronine
(T,) from plasma T4. T,,
intrinsically 10 times more active than T4 has been shown essential for the expression
of the key protein
protein (UCP). The multihovmonal
in BAT thermogenesis,
uncoupling
control of BAT type-II S-deiodinase
and the marked influence of T3 on UCP and BAT thermogenesis that the local control of T3 generation
may be an important
variability in the potential of mammals
Some Basic Concepts of Thermogenesis
In transformations form to another,
source of
temperature
and
(Trends Endocrinol Metab 1993;4:25-32)
dissipate energy.
??
to maintain
suggest
of energy from one a substantial
fraction of
J. Enrique Silva is Director of the Endocrine Division, Department of Medicine, Sir Mortimer B. Davis Jewish General Hospital; and Professor of Medicine at McGill University, Montreal, Quebec H3T lE2, Canada.
TEM Vol. 4, No. 1, 1993
the energy involved is lost as heat. Living organisms are not exempted from this basic thermodynamic law, and in this case the energy dissipated has appropriately been termed obligatory thermogenesis, which, in the resting state, is largely accounted for by the basal metabolic rate (BMR). Obligatory thermogenesis is sufficient to maintain body temperature without the participation of other regulatory mechanisms at a certain ambient
called thermoneutrality tem-
the same species as well. To maintain body temperature in colder environments, a number of mechanisms are activated, one of which, the most important for cold acclimation, is nonshivering facultative thermogenesis. In addition, facultative thermogenesis is activated by overfeeding, serving the purpose of dissipating the excess of ingested calories as heat. When stimulated by food, facultative thermogenesis is more descriptively called diet-induced thermogenesis. Not surprisingly, facultative thermogenesis is turned down and eventually off by high environmental temperatures and by fasting (Landsberg and Young 1983) (Figure 1).
??
J. Enrique Silva Facultative
temperature
perature, which varies depending upon size and thermal insulation among species and probably among individuals of
Brown Adipose Tissue, a Thermogenic Organ
The smaller the animal, the greater is the surface area to volume ratio and the need for facultative thermogenesis to maintain body temperature in ambient temperatures colder than thermoneutrality. In small mammals, including the newborn human, the main site of facultative thermogenesis is the brown adipose tissue (BAT) [for reviews, see HimmsHagen (1985 and 1989), Nicholls and Locke (1984), and Cannon and NederFigure 1. Schematic representation of obligatory and facultative thermogenesis as the functionofambient temperature. Thermoneutrality temperature, indicated by the arrow pointing to the abscissa, is defined as that ambient temperature at which obligatory thermogenesis alone is able to maintain body temperature. Below and above this temperature, adaptive mechanisms are activated. For simplicity, mechanisms to save heat in cold temperatures, such as vasoconstriction and hair ruffling, have been omitted. See the text for details. The effect of thyrotoxicosis is discussed at the end of the article.
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I
Amblent Temperature
25
gaard (1985)]. BAT cells owe their brown color to the cytochrome from their abundant and large mitochondria (Nechad 1986). Over 60% of the excess heat produced in response to cold in rats is generated in BAT (Foster and Frydman 1977). Upon stimulation, BAT can substantially increase substrate oxidation with a lower thermodynamic efficiency, that is, with greater heat production per molecule of substrate. This is made possible by the presence of thermogenin or uncoupling protein (UCP), a protein uniquely expressed in BAT mitochondria and located in the inner membrane of this organelle. Acting as an ionic transporter, UCP dissipates the proton gradient created by mitochondrial respiration, bypassing ATP synthetase. The dissipation of the proton gradient causes a major increase in the respiratory rate uncoupled from ATP synthesis, as ATP synthetase is bypassed (Nicholls et al. 1986). When BAT is fully stimulated, ATP generation accounts for far less than 10% of BAT oxygen consumption (Nicholls and Locke 1984, Nicholls et al. 1986). In addition to being activated by cold, BAT thermogenesis is stimulated by overfeeding, seemingly by the same sympathetic distal pathways and biochemical mechanisms (Himms-Hagen 1985 and 1989, Nicholls and Locke 1984, Rothwell and Stock 1979 and 1980). Information regarding body temperature and nutritional status is integrated in the hypothalamus, from where impulses are fired into BAT via sympathetic nerves (Nicholls and Locke 1984). The release of norepinephrine (NE) from the sympathetic nerve endings activates the tissue via fl- and a-adrenergic receptors (Cannon and Nedergaard 1985). The role of BAT in cold- or diet-induced facultative thermogenesis is dramatically illustrated by the animal models of genetic obesity, the ob/ob mouse and fdfa rat. In these animals, there is a central defect (Young and Landsberg 1983, Martin et al. 1986) in the stimulation of BAT, as a consequence of which they develop obesity and cold intolerance. Although the activity of UCP is modulated by an interplay between fatty acids and nucleotides, UCP concentration determines the thermogenic potential of BAT (Cunningham et al. 1985, Nicholls and Locke 1984, Nicholls et al. 1986). Accordingly, UCP synthesis is an integral
26
part of BAT responses, being evident within 15 min of cold exposure in rats (Ricquier et al. 1986, Bianco et al. 1988, Rehnmark et al. 1992). The importance of BAT in cold adaptation in small animals and human newborns has been well established, but in adult humans the site of facultative thermogenesis is not as clear, and it is a matter of debate how relevant the physiologic role of BAT is. Nonetheless, BAT is present in adult human beings (Ito et al. 1991, Lean et al. 1986, Huttunen et al. 1981), and it is activated by the same stimuli as in rodents (Garruti and Ricquier 1992, Ricquier et al. 1982, Huttunen et al. 1981). Moreover, in cadavers of adults dying in situations representing thermal stress, UCP concentration has been found in the same range as in human neonates and coldstimulated rodents (Lean et al. 1986 and
THE SMALLER
THE ANIMAL,
THE GREATER IS THE SURFACE AREA TO VOLUME RATIO AND THE NEED FOR FACULTATIVE THERMOGENESIS TEMPERATURE.
TO MAINTAIN BODY IN SMALL MAMMALS,
INCLUDING THE NEWBORN HUMAN, THE MAIN SITE OF FACULTATIVE THERMOGENESIS
IS BROWN ADIPOSE
TISSUE.
1987). Recent studies combining immunodetection of UCP and specific analysis of UCP mRNA show that both are readily detectable in perirenal fat of >50% of adult humans, at levels compa-
rable to those of rodents
at room tem-
perature (Garruti and Ricquier 1992). Probably because of our relatively large size, and our ability to control ambient temperature and shelter ourselves from cold, BAT in most of us adult humans is in a less active state, reminiscent of that of warm-adapted rats (Foster and Frydman 1979, Peachey et al. 1988), but it is evident that the level of activation varies widely (Garruti and Ricquier 1992, Lean et al. 1986) and conceivably could account for a good part of the variability of diet-induced thermogenesis and the ability of humans to withstand cold (Jequier 1987, Ravussin et al. 1988, Katzeff and Danforth 1989).
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??
Hormones
and Thermogenesis
Obligatory thermogencsis, iable among individuals,
although vat’is rclativcly
constant in a given person. Given the low thermodynamic efficiency of animals, 30% at best, synthetic processes (for example, growth or repair), turnover 01 substrates (for instance, lipogenesis and lipolysis), or other energy-requiring processes, such as the maintenance of ion gradients through membranes, will necessarily result in heat production. By stimulating some or all of these processes, most hormones enhance obligatory thermogenesis. In this regard, probably because of its widespread effects, thyroid hormone seems to have the most pronounced effect on BMR and obligatory thermogenesis. Although attempts have been made to relate the thermogenic effect of thyroid hormone to some specific cellular effects, none by itself seems to account for all its thermogenic potential [see Sestoft (1980), for review]. Thus, its thermogenic effect has been related to the stimulation of Na/K ATPase (IsmailBeigi et al. 1989), but this enzyme does not seem to account for but a small fraction of the increase in oxygen consumption in thyrotoxicosis (Valdemarsson et al. 1992, Clark et al. 1982). Other biochemical processes considered candidates to explain the thermogenic effect of thyroid hormone have been the NADH shuttle through the cytosolic and mitochondrial glycerophosphate dehydrogenases (Lee and Lardy 1965, Sestoft 1980); the increase in protein synthesis, amino acid turnover, and gluconeogenesis (Sestoft 1980); and the stimulation of both lipolysis and lipogenesis. In an elegant recent study, the energetic cost of lipogenesis has been estimated as 3%4% of the calorigenesis induced by thyroid hormone (Oppenheimer et al. 1991). On the other hand, the increase in oxygen consumption observed in hyperthyroid states is highly coupled to ATP production, refuting the old concept that the calorigenic effect of thyroid hormone was due to uncoupling of phosphorylation (Sestoft 1980, Schwartz and Oppenheimer 1978). It would appear that the pronounced effect of thyroid hormone on obligatory thermogenesis can be best explained by the stimulation by this hormone of innumerable pathways in intermediary metabolism and, to a sig-
TEM Vol. 4, No. I. 1993
nificant extent, by the increased myocardial work necessary to deliver more oxygen to the tissues (Sestoft 1980). It is likely that the effect of other hormones on BMR and obligatory thermogenesis is similarly explained: they stimulate various metabolic pathways, directly or indirectly, and this is accompanied by a proportional increase in heat production, with no perceptible variation of the thermodynamic efficiency. Physiologic fluctuations in the plasma concentrations of thyroid hormone, gonadal steroids, glucocorticoids, and GH, all of which can have long-term effects on BMR, do not seem sufficient to cause day-to-day variations in energy expenditure and thermogenesis; that is, these hormones, via affecting BMR, may play a role in long-term energy balance, but do not seem appropriate for adjustments in heat production for temperature regulation or energy dissipation in overfeeding. In contrast, insulin, glucagon, and epinephrine may cause rapid changes in metabolic rate and thermogenesis, but in this case, rather than adaptive changes in thermogenesis to adjust body temperature or balance the energy economy, they merely reflect the mobilization and transformation of substrates. Glucagon and epinephrine also contribute to overall thermogenesis by increasing cardiac work. Thus, in general, hormones directly affect obligatory thermogenesis and play an important albeit permissive role in facultative, adaptive (to cold or diet) thermogenesis by contributing to the availability of substrates. In contrast, the sympathetic nervous system (SNS), via the neurotransmitter NE, directly controls heat production in homeostasis, that is, the SNS maintains body temperature in response to cold, and balances energy intake with thermogenesis. Over the last several years, it has become evident that certain hormones may also play a more direct role on facultative thermogenesis. In the subsequent paragraphs, I discuss relatively recent developments in our laboratory that show previously unsuspected relations between the SNS and thyroid hormone. These studies have disclosed synergistic interactions between thyroid hormone and the SNS that are essential for energy balance and temperature regulation, and demonstrate a critical role for thyroid hormone in facultative thermogenesis. Along with this, I mention
TEM Vol. 4, No. 1, 1993
other studies that show how other hormones such as glucocorticoids and insulin can modulate the sympathetic stimulation of BAT
??
The Activation of Thyroid Hormone Is a Critical Step Modulating the Effects of This Hormone in Selected Tissues
Although the major secretory product of the thyroid gland is T,, T, accounts for most of the thyromimetic potency of the thyroidal secretion. T, is intrinsically 10 times more active than T,, as a result of a proportionally higher affinity for the nuclear thyroid hormone receptors (Samuels et al. 1974, Oppenheimer et al. 1976). Normally, free T, concentration in the cell does not appear sufficiently high for T, to occupy more than a relatively minor proportion of receptors (Surks and Oppenheimer 1977, Silva and Larsen 1977).
WITHIN 4 HOURS OF COLD EXPOSURE, RECEPTORS
THE NUCLEAR T3 OF BROWN ADIPOSE
TISSUE (BAT) AT ARE NEARLY SATURATED WITH T, PRODUCED BY BAT 5 ‘D-II.
A large proportion of the T, produced is the result of extrathyroidal S-deiodination of T4, a reaction catalyzed by two types of S-deiodinase activities (Silva et al. 1982) called type-1 and -11 5’deiodinases (Leonard and Visser 1986). The type-1 5’-deiodinase (S’D-I) has been recently cloned and found to be a selenocysteine-containing protein (Berry et al. 1991), and type-II 5’-deiodinase (SD-II)
is now known to be a different
protein. While 5’D-I generates T, largely for the plasma pool, S/D-II’s main physiologic function seems to be to provide T, for the tissue where it is present, in a regulated fashion, the most dramatic evidence for which is in brain (Silva and Matthews 1984, Calvo et al. 1990) and BAT (Silva and Larsen 1985 and 1986a and b). T, can be found in the brain of fetuses before it is detectable in plasma (Calvo et al. 1990). In BAT, S’D-II is under complex hormonal regulation (Silva and Larsen 1986a and b). In addition to the SNS, hypothyroxinemia and insulin are potent stimulators of its
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activity. The stimulation by insulin is pronounced, is direct, requires ongoing protein synthesis (Silva and Larsen 1986b, Mills et al. 1987), and may play a role in diet-induced thermogenesis, as discussed below. In the realm of thermogenesis, it was most exciting to find that BAT contained SD-II (Leonard et al. 1983) and that this was activated by the SNS (Silva and Larsen 1983). The sympathetic stimulation of BAT induced either by cold exposure or the injection of NE results in severalfold increases in BAT T, (Silva and Larsen 1985). Within 4 h of cold exposure, the nuclear T, receptors (NTRs) of BAT are nearly saturated with T, produced by BAT SD-II; if anything, in this condition the contribution of plasma T, to BAT T, is less than in the nonstimulated condition (Bianco and Silva 1988). This immediately suggested a synergism between the SNS and thyroid hormone in BAT, whereby the adrenergic stimulation of BAT increased the concentration of the more potent T, in the tissue, which in turn could enhance the thermogenic response. As mentioned, the thermogenic potential of BAT appears to be determined by UCP concentration. To demonstrate the importance of UCP for cold adaptation, and the importance of S’D-II and its activation for this response, we took advantage of our observation that the cold-induced BAT stimulation caused a severalfold increase in BAT T, (Silva and Larsen 1985). [See Figure 2 and Bianco and Silva (1987a and 1987b) for details.] The top panel of Figure 2 shows the core temperature of euthyroid or hypothyroid rats maintained at either 4°C or 23°C. The middle panel shows the liver mitochondrial a-glycerophosphate dehydrogenase (a-GPD) as evidence of thyroid action in peripheral tissues. a-GPD is very sensitive to the thyroid status and is specifically stimulated by thyroid hormone (Lee and Lardy 1965). The bottom panel shows the mitochondrial concentration of UCP in BAT measured by a very sensitive and specific assay (Bianco and Silva 1987a). Treatments are indicated at the top. At room temperature (23”C), there is no difference in core temperature between euthyroid and hypothyroid rats, yet there is marked difference in both a-GPD activity and UCP concentration. This suggests that the mechanisms to
27
Temp
4-c +
33-C
T,
+
T4
+
+
IOP CORE TEMPERATURE
BAT
UCP
A P .w
IILL-
s’.
"10 0
Euthyrad
??Thyrotdectomlzed
Figure 2. Effects of ambient temperature and
replacement with T, or T4 on core body temperature, liver mitochondrial a-glycerophosphate dehydmgenase (a-GPD), and brown adipose tissue (BAT) uncoupling protein (UCP) in normal and thyroidectomized rats. Rats were in the cold for 48 h. T,, 0.15 @lo0 g, was given twice daily, by the S.C. route, for 5 days. T,, 0.4 pg/lOOg, was given twice daily, by the S.C. route, for the 2 days that rats remained in the cold. Iopanoic acid (IOP), an inhibitor of SD-II, was given intraperitoneally, 5 mg/l 00 g twice daily. For discussion, see the text and for experimental details (Bianco and Silva 1987a and b).
reduce heat dissipation are probably sufficient to maintain body temperature in this environment in spite reduced obligatory thermogenesis.
of the When
ambient temperature was reduced to 4°C for 48 h, however, euthyroid rats maintained their core temperature, whereas hypothyroid rats developed severe hypothermia. Cold exposure did not affect a-GPD, but had different effects on UCP in eu- and hypothyroid rats: whereas in the former UCP increased -3-fold, it remained virtually unchanged in the
28
latter. That the lack of UCP response to cold is caused by a failure of BAT to respond rather than to insufficient adrenergic stimulation is evidenced by the observations that in hypothyroid rats NE turnover is increased (Young et al. 1984) and UCP does not respond to exogenous NE (Silva 1988). Replacement of hypothyroid rats with T, for 5 days before the experiment caused no significant increase in core temperature, which was still significantly lower that in the intact rats. The persistent hypothermia contrasted with normalization of serum T, concentration [OS2 + 0.03 (SEM) vs 0.57 f 0.03 ng/mL in intact rats] and that of hepatic a-GPD activity. Thus, the correction of systemic hypothyroidism did not suffice to bring about a normal response to the cold stress. A different situation was observed when T, was replaced just for the 2 days of cold exposure: core temperature was maintained normal and a-GPD activity was not restored, but UCP concentration was normalized. This short treatment with T, was insufficient to normalize either serum T, or T, concentrations, which were -30% and -50% of that in intact rats. Thus, with T, we observed prevention of hypothermia in the face of persistent systemic hypothyroidism, but paralleling a normalization of the UCP response to cold. Iopanoic acid (IOP), an inhibitor of SD-II, abolished T,-induced restoration of both UCP response and normothermia. Appropriate controls (Bianco and Silva 1987a and 1987b) demonstrated that, at the dosage used, IOP effectively blocked SD-II and that this substance did not prevent the entry of T, or T, into BAT. Furthermore, the addition of T, to the T, regimen normalized serum T, and liver a-GPD, but did not prevent the hypothermia and, when given along with IOP and T,, T, did not prevent IOP from blocking the effect of T,. In other experiments (Car&ho et al. 1991), the specific obliteration of the adrenergic activation of SD-II with prazosin (Silva and Larsen 1983) prevented the increase in oxygen consumption induced by cold in T,-replaced thyroidectomized rats, adding support to the physiologic relevance of S’D-II activation to the UCP response, and of this to the thermogenic response. A T, dose-UCP response analysis in thyroidectomized rats showed that UCP
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response to cold bears a peculiar relation with BAT T,. UCP response is not linear with the NTR occupancy, and for a full response, as seen in euthyroid rats or in hypothyroid rats treated with T,, full nuclear receptor occupancy is necessary, as illustrated in Figure 3. Here, the UCP responses to cold and liver a-GPD activities (for comparison) have been plotted against the integrated occupancy of the receptors by different doses of T, given for 48 h to thyroidectomized rats maintained previously on T, replacement (Bianco and Silva 1987a and 1987b). For a full UCP response to cold, near-saturation of NTR is necessary, and it is not until occupancy exceeds 60%70% that the UCP response increases significantly. From the response in intact rats and thyroidectomized T,-replaced rats, it was inferred that in these two situations the adrenergic activation of SD-II by cold results in near-saturation of the receptors, which was later confirmed in kinetic studies in euthyroid rats exposed to cold, where we found that NTR occupancy is maximal within 4 h of initiating cold exposure (Bianco and Silva 1988). Thus, T, appears a much more efficient source of T, for BAT than plasma T, when SD-II is activated. Most importantly, these studies have shown that the concentration of T, is locally regulated in BAT. Rapid changes in BAT T, may occur without changes in plasma T,. This local regulation may increase BAT NTR occupancy to levels seen in severe thyrotoxicosis, yet without it being present. Thus, to reach 98% NTR occupancy, plasma T, must be elevated to 50 ng/mL, a level 100 times higher than the euthyroid plasma T, (Bianco and Silva 1987b). The stimulation of 5’D-II makes this level of receptor occupancy possible without systemic thyrotoxicosis. Note in Figure 3 that euthyroid levels of a-GPD activity are achieved with 50% saturation, which occurs with a serum T, of 0.6 ng/mL, obtained with the lowest dose of T,. With higher plasma T, levels, the liver is made hyperthyroid. In recent years, our laboratory has been engaged in unraveling the mechanisms whereby T, and NE regulate UCP levels. We have found that T, amplifies the stimulation of NE on UCP gene transcription rather than being merely a permissive factor for the ex-
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.
-)
or 0
Euthymd
UCP
response
+ 95%
CL
60 80 20 40 Nuclear T, receptor occupancy (X)
Figure 3. Uncoupling protein (UCP, 0) response to cold and liver mitochondrial aglycerophosphate dehydrogenase (a-GPD, 0) as a function of nuclear T, receptor occupancy in BAT and liver, respectively, in thyroidectomized rats treated with different doses of T,. Bars indicate the mean f SEM of 3-4 rats. All rats were previously maintained on T, replacement (0.15 gg/lOOg, s.c., twice daily) for 5 days and then given different doses of T, (0, 0.3, 0.5, 0.8, 1.3,2.1, and 4.7 pg/lOOg day, divided into two S.C.doses, from .?eji to right) during the 48 h of cold exposure. The boxes represent the mean UCP response to cold or a-CPD activity in euthyroid rats equally exposed to cold f95% confidence limits (CL). Nuclear receptor occupancy has been integrated for the 48-h cold-exposure period and expressed as a fraction of the maximal binding capacity. For experimental details, see Bianco and Silva (1987a and b).
pression of UCP (Bianco et al. 1988), as
has been postulated in the past (Triandafillou et al. 1982, Mory et al. 1981). Furthermore, T, enhances the effect of NE not only at a transcriptional level, but at a posttranscriptional level as well (Rehnmark et al. 1992). Studies in progress in our laboratory show that UCP gene has functional thyroid hormoneand CAMP-responsive elements, corroborating the hypothesis previously advanced by us (Bianco et al. 1988) that thyroid hormone, via its receptor, and the second messenger of NE, CAMP, probably via a specific phosphorylated protein, interact synergistically at the level of the UCP gene to stimulate its expression. What is not clear at present is whether thyroid hormone affects the strength of the NE signal. Seydoux et al. (1982) reported a reduction in BAT f%adrenergic receptors in hypothyroid rats, but concluded that this defect could not explain but a minor fraction of the defective BAT response to sympathetic stimulation. Sundin et al. (1984) later reported that CAMP accumulation 15 min after isoproterenol exposure in isolated brown adipocytes from hypothyroid rats was not less than in cells obtained from normal rats, but when a similar experiment was later performed
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by Raasmaja and Larsen (1989), 2 h after stimulation with isoproterenol or NE, CAMP was significantly reduced. Although it remains open whether reduced CAMP generation in hypothyroid BAT can account for part of its insufficient response to SNS stimulation, the reduced number of receptors does not appear to be relevant, since both the respiratory and UCP responses to forskolin, acting directly on adenylate cyclase, are as blunted as those to adrenergic receptor agonists. Furthermore, we have recently found (unpublished observation) that the short regimens of T, or T, that restore BAT UCP and thermogenic responses to cold or NE in thyroidectomized rats fail to increase the number of fl-adrenergic receptors. Only after prolonged treatment with T, do we observe an increase in the number of these receptors. The complex interactions among NE, thyroid hormone, insulin, BAT S’D-II and BAT thermogenesis are schematically summarized in Figure 4.
??
Sympathetic-Hormonal Interactions in the Central Nervous System
While there is a synergism among SNS, thyroid hormone, and insulin in BAT,
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central stimulation of BAT seems to be subjected to different interactions. It was already mentioned that in hypothyroidism adrenergic input to BAT is increased, as reflected by NE turnover (Young et al. 1984), and, indeed, hypothyroid BAT shows signs of enhanced adrenergic stimulation (Mary et al 1981), yet with blunted end responses, as summarized in the foregoing paragraphs. In hyperthyroidism, in contrast, SNS stimulation is reduced and, in spite of the higher levels of plasma T,, there is a reduction of GDP binding by BAT mitochondria, an expression of functional UCP (Triandafillou et al. 1982), and reduced responses to cold exposure (Sundin 1981). It would appear that sympathetic BAT stimulation is inversely related to the magnitude of obligatory thermogenesis: the higher the latter, the lower is the need for facultative thermogenesis and, hence, of BAT activity, to maintain body temperature. These relations are schematically represented in Figure 1. In addition to obligatory thermogenesis and ambient temperature, other hormones seem to affect BAT stimulation and responses at a central level. CRH may have a stimulator-y effect opposed by glucocorticoids on BAT sympathetic stimulation (Vander Tuig et al. 1984, Walker and Romsos 1992). It is not unlikely that the inhibitory effects of glucocorticoids and the beneficial effects of adrenalectomy on BAT thermogenesis in obese mice and rats are mediated through CRH.
??
Concluding Remarks
It is evident that the role of thyroid hormone on BAT function and on facultative thermogenesis would have remained inapparent without the knowledge of S’D-II. It also follows that the response of S’D-II is an important source of variation in the thermogenic response of BAT. The steepness of the UCP response with nuclear T, receptor occupancy (Figure 3) suggests that BAT T, content plays a critical role in the responses of BAT The concept that low energy expenditure is a major, genetically determined risk factor for obesity (Jequier 1987, Katzeff and Danforth 1989, Ravussin et al. 1988, Bogardus et al. 1986, Bouchard et al. 1990, Stunkard et al. 1990) is receiving increasing support. Food ingestion stimulates BAT in a 29
1990. The response to long-tern1 overfeeding in identical twins. N Engl J Med 322:1477-1482.
Plasma membrane
Calvo R, Obregon MJ, Ruiz de 0, Escobar de1 Rey F, Morreale de Escobar G: 1990. Congenital hypothyroidism, as studied in rats: crucial role of maternal thyroxine but not of 3,5,3’-triiodothyronine in the protection of the fetal brain. J Clin Invest 86:889899.
+ Glucose uptake Insulin
w/
+
Lipogenesis
Cannon B, Nedergaard J: 1985. The biochemistry of an inefficient tissue: brown adipose tissue. Essays Biochem 2: 110-164.
Figure 4 Schematic representation of the effects of sympathetic nerve stimulation (via norepinephrine, NE), T4, T,, and insulin on brown adipose tissue (BAT) thermogenesis. See the text for discussion. The a and g indicate the adrenergic receptors involved in mediating NE effects. CAMPis the second messenger carrying the NE signal to the uncoupling protein (UCP) gene. CAMPand T, are essential for the full expression of the UCP gene and hence to determine BAT thermogenic potential. The type-II T, S-deiodinase (5’D-II) catalyzing T4 to T, conversion, the main source of T, for the brown adipocyte, is activated by both NE and insulin. Both insulin and CAMP also contribute to the therrnogenic response by providing substrate, glucose, and fatty acids. T, is also important for the expression of lipogenic enzymes and a-glycerophosphate dehydrogenase (a-GPD) in BAT (not shown here), which are also important for the overall thermogenic response (Bianco and Silva 1987b).
manner similar to cold. Cafeteria diet, a form of inducing overfeeding in animals, stimulates BAT in a sustained fashion (Rothwell and Stock 1980); as little as one meal perceptibly stimulates S’D-II, GDP binding, and NE turnover in BAT (Glick 1987); and S’D-II response is attenuated in the genetically obese fa/f rat (Wu et al. 1987). Moreover, BAT S’D-II is under complex hormonal regulation (Silva and Larsen 1986a and b), with insulin being a very potent stimulator (Silva and Larsen 1986b, Mills et al. 1987) (Figure 4), and both diabetes and insulin resistance have been associated with reduced BAT responses to adrenergic stimulation (Howland and Bond 1987, Geloen and Trayhurn 1990, Walker and Romsos 1992, Marie et al. 1992). Thus, the studies summarized here offer powerful insights into previously unsuspected sources of variability in energy expenditure.
??
Acknowledgments
Initial studies described here were performed while J.E.S. was an Associate Investigator in the Howard Hughes Medical Institute. Subsequent work has been supported by PHS grant DK-42431 and
30
MRC (Canada) grant MT 11550 (to J.E.S.) and FAPESP, of Brazil (to Dr. A.C. Bianco).
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Carvalho SD, Kimura ET, Bianco AC, Silva JE: 1991. Central role of brown adipose tissue thyroxine 5’deiodinase on thyroid hormone-dependent thermogenic response to cold. Endocrinology 128:2149-2 159. Clark DG, Brinkman M, Filsell OH, Lewis SJ, Berry MN: 1982. No major thermogenic role for (Na+ + K+)-dependent adenosine triphosphatase apparent in hepatocytes from hyperthyroid rats. Biochem J 202:66 l665. Cunningham SA, Leslie P, Hopwood D, et al.: 1985. The characterization and energetic potential of brown adipose tissue in man. Clin Sci 69:343-348. Foster DO, Frydman ML: 1977. Non-shivering thermogenesis in the rat. II. Measurements of blood flow with microspheres point to brown adipose tissue of the dominant site of the calorigenesis induced by noradrenaline. Can J Physiol Pharmacol56:110-112. Foster DO, Frydman ML: 1979. Tissue distribution of cold-induced thermogenesis in conscious warm-or-cold acclimated rats reevaluated from changes in tissue blood flows: the dominant role of brown adipose tissue in the replacement of shivering by nonshivering thermogenesis. Can J Physiol Pharmacol57:257-270. Garruti G, Ricquier D: 1992. Analysis of uncoupling protein and its mRNA in adipose tissue deposits of adult humans. Int J Obes 16:383-390. Geloen A, Trayhum P: 1990. Regulation of the level of uncoupling protein in brown adipose tissue by insulin. Am J Physiol 258:R418-R424. Glick Z: 1987. The thermic effect of a meal. J Obes Weight Regul 6: 170-l 78. Himms-Hagen J: 1985. Brown adipose tissue metabolism and thermogenesis. Annu Rev Nutr 5:69-94. Himms-Hagen J: 1989. Brown adipose tissue thermogenesis role in thermoregulation, energy regulation and obesity. Prog Lipid Res 28:67-l 15. Howland RJ, Bond KD: 1987. Cold-acclimation increases insulin sensitivity of brown adipocytes. Hot-m Metab Res 19:503-504. Huttunen P, Hirvonen J, Kirmula V: 198 1. The
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Nicholls DG, Cunningham SA, Rial E: 1986. The bioenergetic mechanisms of brown adipose tissue thermogenesis. In Trayhum P, Nicholls DG, eds. Brown Adipose Tissue. London, Edward Arnold, pp 52-85.
Seydoux J, Giacobino J-P, Girardier L: 1982. Impaired metabolic response to nerve stimulation in brown adipose tissue of hypothyroid rats. Mol Cell Endocrinol25:2 13-226.
Oppenheimer JH, Schwartz HL, Surks MI, Koemer D, Dillmann WH: 1976. Nuclear receptors and the initiation of thyroid hormone action. Ret Prog Horm Res 32:529565. Oppenheimer JH, Schwartz HL, Lane JT, Thompson MP: 199 1. Functional relationship of thyroid hormone-induced lipogenesis, lipolysis and thermogenesis in the rat. J Clin Invest 87:125-132. Peachey T, French RR, York DA: 1988. Regulation of GDP binding and uncoupling protein concentration in brown adipose tissue mitochondria: the effects of coldacclimation, warm-reacclimation and noradrenaline. Biochemistry 249:451457. Raasmaja A, Larsen PR: 1989. a,- and gAdrenergic agents cause synergistic stimulation of the iodothyronine deiodinase in rat brown adipocytes. Endocrinology 125:2502-2509. Ravussin E, Lillioja S, Knowler WC, et al.: 1988. Reduced rate of energy expenditure as a risk factor for body-weight gain. N Engl J Med 318~467-472. Rehnmark S, Bianco AC, Kieffer JD, Silva JE: 1992. Transcriptional and post-transcriptional mechanisms in the uncoupling protein mRNA response to cold. Am J Physiol 262:E58-E67.
Silva JE: 1988. Full expression of uncoupling protein gene requires the concurrence of norepinephrine and triiodothyronine. Mol Endocrinol2:706-713. Silva JE, Larsen PR: 1977. Pituitaty nuclear 3,5,3’-triiodothyronine and thyrotropin secretion: an explanation for the effect of thyroxine. Science 198:617-6 19. Silva JE, Larsen PR: 1983. Adrenergic activation of triiodothymnine production in brown adipose tissue. Nature 305:7 12-7 13. Silva JE, Larsen PR: 1985. Potential of brown adipose tissue type II thyroxine 5’-deiodinase as a local and systemic source of triiodothyronine in tats. J Clin Invest 76:22962305. Silva JE, Larsen PR: 1986a. Interrelationships among thyroxine, growth hormone, and the sympathetic nervous system in the regulation of 5’-iodothyronine deiodinase in rat brown adipose tissue. J Clin Invest 77: 12 141223. Silva JE, Larsen PR: 1986b. Hormonal regulation of iodothyronine 5’-deiodinase in rat brown adipose tissue. Am J Physio125 l:E639E643.
Ricquier D, Nechad M, Moty G: 1982. Ultrastructural and biochemical characterization of human brown adipose tissue in pheochromocytoma. J Clin Endocrinol Metab 54:803-807. Ricquier D, Bouillaud F, Toumelin P, et al.: 1986. Expression of uncoupling protein mRNA in thermogenic or weakly thermogenie brown adipose tissue. J Biol Chem 261:13,905-13,910. Rothwell NJ, Stock MJ: 1979. A role for brown adipose tissue in diet-induced thermogenesis. Nature 281:31-35. Rothwell NJ, Stock MJ: 1980. Similarities between cold- and diet-induced thermogenesis in the rat. Can J Physiol Pharmacol 58~842-848. Samuels HH, Tsai JS, Casanova J: 1974. Thyroid hormone action: in vitro demon-
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Silva JE, Matthews PS: 1984. Production rates and turnover of triiodothyronine in rat-developing-cerebral cortex and cerebellum: responses to hypothyroidism. J Clin Invest 74:1035-1049. Silva JE, Leonard JL, Crantz FR, Larsen PR: 1982. Evidence for two tissue-specific pathways for in vivo thyroxine 5’deiodination in the rat. J Clin Invest 69: 1176-l 184. Stunkard AJ, Harris JR, Pedersen NL, McCleam GE: 1990. The body-mass index of twins who have been reared apart. N Engl J Med 322:1483-1487. Sundin U: 198 1. GDP binding to rat brown fat mitochondria: effects of thyroxine at different ambient temperature. Am J Physiol 241:C134-C139. Sundin U, Mills I, Fain JN: 1984. Thyroidcatecholamine interactions in isolated brown adipocytes. Metabolism 33:1028-1033. Surks MI, Oppenheimer JH: 1977. Concentration of L-thyroxine and r-triiodothyronine specifically bound to nuclear recep-
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tars in rat liver and kidney. Quantitative evidence favoring a major role of T, in thyroid hormone action. J Clin Invest 60:555-562. Triandafillou J, Gwilliam C, Himms-Hagen J: 1982. Role of thyroid hormone in coldinduced changes in rat brown adipose tissue mitochondria. Can J Biochem 60:530537. Valdemarsson S, Ikomi-Kumm J, Monti M: 1992. Increased lymphocyte therrnogenesis in hyperthyroid patients: role of Na/K pump function-evaluation of aerobic/anaerobic
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Vander Tuig JG, Oshima K, Yoshida T, Romsos DR, Bray GA: 1984. Adrenalectomy increases norepinephrine turnover in brown adipose tissue of obese (ob/ob) mice. Life Sci 34:1423-1432. Walker HC, Romsos DR: 1992. Glucocorticoids in the CNS regulate BAT metabolism and plasma insulin in ob/ob mice. Am J Physiol262:EllO-E117. Wu SY, Stern JS, Fisher DA, Glick Z: 1987. Cold-induced increase in brown fat thyrox-
Young JB, Landsberg L: 1983. Diminished sympathetic nervous system activity in genetically obese (ob/ob) mouse. Am J Physiol 245:E148-E154. Young JB, Kaplan MM, Landsberg L: 1984. Parallel changes in sympathetic nervous system (SNS) and iodothyronine 5’-deiodinase (5’D) activity in hypothyroid rats [abst]. Clin Res 32:377A. TEM
TH 21 st Annual
ng in
4th-9th July 1993 Topics will include all aspects of clinical and basic research oriented thyroidology. Cardiff is the capital city of Wales and one of Britain’s major administrative, commercial business, and educational centers. Easy access to the beautiful surrounding countryside and coastline. For further details please contact: Universal Conference Organizers PO Box 1380, Long Ashton Bristol BS18 9BF, England, UK
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