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Does lysozyme play a role in the pathogenesis of COPD?
Medical Hypotheses 84 (2015) 551–554
Contents lists available at ScienceDirect
Medical Hypotheses journal homepage: www.elsevier.com/locate/mehy
Does lysozyme play a role in the pathogenesis of COPD? Jerome Cantor ⇑, Bronislava Shteyngart St John’s University College of Pharmacy and Health Sciences, Queens, NY 11439, United States
a r t i c l e
i n f o
Article history: Received 14 December 2014 Accepted 25 February 2015
a b s t r a c t Elastic fiber injury is an important process in the pathogenesis of chronic obstructive pulmonary disease (COPD), particularly with regard to the development of pulmonary emphysema. Damage to these fibers results in uneven distribution of mechanical forces in the lung, leading to dilatation and rupture of alveolar walls. While the role of various enzymes and oxidants in this process has been well-documented, we propose that a previously unsuspected agent, lysozyme, may contribute significantly to the changes in elastic fibers observed in this disease. Studies from our laboratory have previously shown that lysozyme preferentially binds to elastic fibers in human emphysematous lungs. On the basis of this finding, it is hypothesized that the attachment of lysozyme to these fibers enhances their susceptibility to injury, and further impairs the transfer of mechanical forces in the lung, leading to increased alveolar wall damage and enhanced progression of COPD. The hypothesized effects of lysozyme are predicated on its interaction with hyaluronan (HA), a long-chain polysaccharide that is found in close proximity to elastic fibers. By preventing the binding of HA to elastic fibers in COPD, lysozyme may interfere with the protective effect of this polysaccharide against enzymes and oxidants that degrade these fibers. Furthermore, the loss of the hydrating effect of HA on these fibers may impair their elastic properties, greatly increasing the probability of their fragmentation in response to mechanical forces. The proposed hypothesis may explain why the content of HA is significantly lower in the lungs of COPD patients. It may also contribute to the design of clinical trials involving the use of exogenously administered HA as a potential treatment for COPD. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction The pathogenesis of chronic obstructive pulmonary disease (COPD) is complex, involving multiple types of injurious agents acting on a number of tissue constituents. Nevertheless, numerous studies suggest that a primary feature of the disease is damage to the elastic fiber network of the lung as a result of chronic inflammation [1–3]. Both neutrophils and macrophages recruited to the lung by tobacco smoke and other toxic substances release a variety of enzymes and oxidants that degrade elastic fibers, thereby disrupting the mechanical forces that control the expansion and contraction of alveoli. Over a period of many years, the continual degradation of these fibers leads to the dilatation and rupture of alveolar walls, which reduces lung surface area and impairs ventilation and perfusion in the lung. In the current paper, we add a previously unsuspected protein to the list of agents that may have adverse effects on pulmonary elastic fibers by proposing a role for lysozyme in this process. It
⇑ Corresponding author at: Department of Pharmacy and Health Sciences, St John’s University, 8000 Utopia Pkwy, Queens, NY 11439, United States. E-mail address: [email protected] (J. Cantor). http://dx.doi.org/10.1016/j.mehy.2015.02.015 0306-9877/Ó 2015 Elsevier Ltd. All rights reserved.
is hypothesized that lysozyme released by inflammatory cells in the lung binds to elastic fibers, makes them more susceptible to injury, and impairs their ability to regulate the transfer of mechanical forces in the lung, leading to increased alveolar wall damage and enhanced progression of COPD. Support for this concept is based on a number of findings, some of which are derived from experiments in our laboratory over the past 20 years. In particular, we will propose that the relationship between lysozyme and the polysaccharide, hyaluronan (HA), is critical to the changes in elastic fibers that are involved in the pathogenesis of COPD.
Formulating and evaluating the hypothesis The relationship between lysozyme and elastic fibers Previous studies from this laboratory have shown that lysozyme preferentially binds to elastic fibers in lungs with pulmonary emphysema (Fig. 1) [4]. However, the significance of this finding is complicated by the fact that this protein has no recognized physiological function beyond its role in bacteriolysis. The increased levels of lysozyme that accompany inflammation may only reflect nonspecific release from lysosomes. Furthermore, the
552
J. Cantor, B. Shteyngart / Medical Hypotheses 84 (2015) 551–554
Fig. 1. (A) Photomicrograph of human emphysematous lung showing intense immunostaining following treatment with anti-lysozyme antibodies. (B) Photomicrograph of normal lung showing absence of immunostaining with anti-lysozyme antibodies. Only focal intracellular staining of leukocytes is present.
association of lysozyme with components of inflamed tissues may simply be due to its strongly cationic nature, resulting in electrostatic binding to various anionic molecules rather than substrate-specific interactions. With regard to elastic fibers, lysozyme may form electrostatic or hydrogen bonds with carbohydrate residues in elastic fibers. Alternatively, it could specifically bind to N-acetyl-D-glucosamine, a component of bacterial cells susceptible to degradation by lysozyme, which has also been found in glycoproteins associated with these fibers. The low-grade inflammatory process that occurs in pulmonary emphysema, involving release of elastases and other injurious agents, might increase the access of lysozyme to these carbohydrate components, thereby facilitating the binding of this protein. This concept has some experimental support in studies showing that lysozyme only adheres to skin elastic fibers that have been previously damaged by either UV radiation or advanced glycation end-products [5].
Interaction of HA and lysozyme in elastic fiber injury This laboratory has also shown that hyaluronidase treatment of human lung tissues in vitro increases lysozyme binding to elastic fibers, presumably by degrading HA and exposing specific attachment sites on the elastic fiber [4]. In the case of pulmonary emphysema, the presence of lysozyme on elastic fibers might disrupt the normal interactions between these fibers and HA, thereby increasing elastolysis and enhancing the progression of the disease. This hypothesis was tested by our laboratory, using the elastase model of pulmonary emphysema. Animals exposed to aerosolized lysozyme prior to elastase administration showed significantly increased airspace enlargement [11]. The ability of lysozyme to enhance elastolysis was further examined in vitro, using an extracellular matrix preparation, mainly composed of elastic fibers. Treatment of the matrix with lysozyme resulted in attachment of the protein to elastic fibers, which impaired the ability of HA to protect them [11].
The role of HA in protecting elastic fibers How lysozyme may disrupt hydration of elastic fibers The hypothesized effects of lysozyme are predicated on its interaction with HA, a long-chain polysaccharide that is found in close proximity to elastic fibers. HA is best known for its capacity to retain large amounts of water, but it has a number of other important functions as well. As a constituent of the extracellular matrix, HA acts to stabilize proteoglycans and contributes to tissue growth and repair [6,7]. The ability of HA to expand its domain in a fluid environment may have several consequences for inflammatory reactions involving elastic fibers. By creating a more viscous milieu, HA may impede the movement of cells and molecules through the extracellular matrix, thereby reducing the activity of leukocytes and their proinflammatory products. The consequences of this ‘‘physical barrier” effect of HA have been explored in our laboratory, where aerosolized HA was found to significantly reduce airspace enlargement in experimental models of pulmonary emphysema induced by either intratracheal instillation of elastase or long-term exposure to cigarette smoke [8–10]. Studies using fluorescent-labeled HA indicate that the polysaccharide coats elastic fibers, and may replace endogenous HA that is removed from these fibers during the inflammatory process [8].
The ability of lysozyme to interfere with the binding of HA to elastic fibers may also adversely affect their mechanical properties, due to a loss of hydration in the absence of this polysaccharide. The absorption of water onto nonpolar hydrophobic groups during the extension of elastic fibers contributes to the storage of elastic energy [12,13]. A decrease in the availability of water can compromise this process, reducing elastic fiber recoil. Furthermore, water facilitates the swelling of the core elastin protein, thereby increasing its random energy state and enhancing recoil as a result of the increased loss of entropy during distention [14]. Resistance to stretching of the elastic fibers may also be dependent on the intramolecular forces of attraction among the elastin peptide chains themselves. Removing water molecules decreases the distance between the elastin peptide chains, enhancing their cohesion and reducing the distensibility of the fibers [15]. At the extreme, the complete absence of water yields an elastic modulus approaching that of glass [16]. The binding of lysozyme to elastic fibers may therefore cause a reduction in their recoil capacity by either blocking the attachment of HA to the fibers or forming complexes with this polysaccharide
J. Cantor, B. Shteyngart / Medical Hypotheses 84 (2015) 551–554
that limit its retention of water [17]. The continued accumulation of lysozyme around elastic fibers could eventually reduce hydration to the point where the fibers become brittle, making them vulnerable to rupture under strain (Fig. 2). The resulting breakage of the fibers could mimic the effects of enzymatic degradation, with resultant alveolar distention and rupture. This mechanism could provide an additional reason why pulmonary emphysema continues to progress even when the causative agent is removed [18].
553
would result in the progressive loss of HA from the lung as injury to elastic fibers continues and more lysozyme becomes bound to them. While there is no direct evidence to support this concept, a preliminary study from this laboratory demonstrated a correlation between the amount of HA in human lung tissue and diffusing capacity of the lung for carbon monoxide (DLCO), which reflects the extent of emphysema (unpublished observations). The implications of the hypothesis regarding aerosolized HA as a therapy for COPD
Consequences of the hypothesis Using the hypothesis to explain why elastolysis is decreased in advanced COPD One of the poorly understood features of COPD concerns the observed reduction in elastolysis as the disease becomes more severe [19]. While it has been proposed that this finding may be due to the eventual exhaustion of the elastolytic process, several studies suggest that human emphysematous lungs still have large amounts of elastic fibers [20–23]. Consequently, there should not be a significant reduction in elastolysis. An alternative explanation for this phenomenon could involve the binding of lysozyme to elastic fibers. A number of studies have shown that elastic fibers from UV-damaged skin, which contain large amounts of lysozyme, are more resistant to elastolysis [24]. While similar studies have not been performed in patients with COPD, it is quite possible that the continued accumulation of lysozyme around damaged pulmonary elastic fibers could make them more resistant to enzymatic breakdown, despite the loss of HA. This process would provide a more reasonable explanation for the marked reduction in elastolysis in advanced cases of this disease. Using the hypothesis to explain why lung levels of HA correlate with severity of COPD Several studies have shown that HA is decreased in human lungs with pulmonary emphysema [25,26]. Furthermore, there is relatively less HA in comparison to other lung glycosaminoglycans, which is consistent with the possibility that deposition of HA in the extracellular matrix is blocked by lysozyme [26]. This process
Our laboratory is exploring the use of aerosolized HA as a treatment for COPD, and clinical trials are currently underway to test the safety and efficacy of this agent. In designing these studies, it is important to determine which patients should be included in the trial. While our findings of markedly decreased HA in patients with advanced COPD suggest that this group might benefit most from this type of therapy, the potential blockage of HA binding to elastic fibers by lysozyme would contradict this conclusion. In fact, our hypothesis argues in favor of designing the clinical trial around a population of patients with mild to moderate COPD, who retain a large proportion of elastic fibers that have not been subjected to injury and subsequent deposition of lysozyme. In these patients, treatment with HA might be significantly more effective with regard to preventing loss of lung function. Conclusions The presence of lysozyme on elastic fibers is a well-documented phenomenon. However, the role of this phenomenon in the pathogenesis of diseases involving injury and repair of these fibers remains poorly understood. Our hypothesis provides a framework for testing the role of lysozyme in COPD, and determining how this protein could affect the progression and treatment of this disorder. The data from our laboratory demonstrate a connection between lysozyme and HA, and provide an explanation for the seemingly contradictory findings of decreased elastolysis and loss of HA in advanced COPD. The proposed hypothesis also explains the continued progression of the disease despite this reduction in elastolysis. Finally, this hypothesis provides a rationale for selecting a suitable patient population for testing the efficacy of aerosolized HA as a potential treatment for COPD.
Conflict of interest The authors declare that there is no conflict of interest regarding the material presented in this manuscript. References
Fig. 2. Photomicrograph of human emphysematous lung showing immunostaining of fragmented elastic fibers with anti-lysozyme antibodies (arrowheads). Deposition of lysozyme on elastic fibers in COPD may reduce their hydration and increase their susceptibility to mechanical failure.
[1] Hogg JC, Timens W. The pathology of chronic obstructive pulmonary disease. Annu Rev Pathol 2009;4:435–59. [2] Tuder RM, Petrache I. Pathogenesis of chronic obstructive pulmonary disease. J Clin Invest 2012;122:2749–55. [3] Abboud RT, Vimalanathan S. Pathogenesis of COPD. Part I. The role of proteaseantiprotease imbalance in emphysema. Int J Tuberc Lung Dis 2008;12:361–7. [4] Shteyngart B, Chaiwiriyakul S, Wong J, Cantor JO. Preferential binding of lysozyme to elastic fibers in pulmonary emphysema. Thorax 1998;53:193–6. [5] Yoshinaga E, Kawada A, Ono K, et al. NƐ-(carboxymethyl)lysine modification of elastin alters its biological properties: implications for the accumulation of abnormal elastic fibers in actinic elastosis. J Invest Dermatol 2012;132:315–23. [6] Dicker KT, Gurski LA, Pradhan-Bhatt S, Witt RL, Farach-Carson MC, Jia X. Hyaluronan: a simple polysaccharide with diverse biological functions. Acta Biomater 2014;10:1558–70. [7] Jiang D, Liang J, Noble PW. Regulation of non-infectious lung injury, inflammation, and repair by the extracellular matrix glycosaminoglycan hyaluronan. Anat Rec (Hoboken) 2010;293:982–5.
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J. Cantor, B. Shteyngart / Medical Hypotheses 84 (2015) 551–554
[8] Cantor JO, Cerreta JM, Armand G, Turino GM. Aerosolized hyaluronic acid decreases alveolar injury induced by human neutrophil elastase. Proc Soc Exp Biol Med 1998;217:471–5. [9] Cantor JO, Cerreta JM, Keller S, Turino GM. Modulation of airspace enlargement in elastase-induced emphysema by intratracheal instillment of hyaluronidase and hyaluronic acid. Exp Lung Res 1995;21:423–36. [10] Cantor JO, Cerreta JM, Ochoa M, et al. Aerosolized hyaluronan limits airspace enlargement in a mouse model of cigarette smoke-induced pulmonary emphysema. Exp Lung Res 2005;31:417–30. [11] Cantor JO, Shteyngart B, Cerreta JM, Turino GM. The effect of lysozyme on elastase-mediated injury. Exp Biol Med 2002;227:108–13. [12] Li B, Alonso DO, Bennion BJ, Daggett V. Hydrophobic hydration is an important source of elasticity in elastin-based biopolymers. J Am Chem Soc 2001;123:11991–8. [13] Lillie MA, Chalmers GW, Gosline JM. Elastin dehydration through the liquid and the vapor phase: a comparison of osmotic stress models. Biopolymers 1996;39:627–39. [14] Gosline JM. The role of hydrophobic interactions in the swelling of elastin. In: Sandberg LB, Gray WR, Franzblau C, editors. Advances in experimental medicine and biology, vol. 79. New York: Plenum; 1977. p. 621–31. [15] Kendall K. Molecular adhesion and its applications. New York: Plenum; 2001. p. 103–31. [16] Hoeve CAJ. The elasticity in the presence of diluents. In: Sandberg LB, Gray WR, Franzblau C, editors. Advances in experimental medicine and biology, vol. 79. New York: Plenum; 1977. p. 607–19. [17] Water JJ, Schack MM, Velazquez-Campoy A, Maltesen MJ, van de Weert M, Jorgensen L, et al. Complex coacervates of hyaluronic acid and lysozyme: effect
[18]
[19]
[20] [21] [22]
[23] [24]
[25] [26]
on protein structure and physical stability. Eur J Pharm Biopharm 2014;88:325–31. Miller M, Cho JY, Pham A, Friedman PJ, Ramsdell J, Broide DH. Persistent airway inflammation and emphysema progression on CT scan in ex-smokers observed for 4 years. Chest 2011;139:1380–7. Cocci F, Miniati M, Monti S, et al. Urinary desmosine excretion is inversely correlated with the extent of emphysema in patients with chronic obstructive pulmonary disease. Int J Biochem Cell Biol 2002;34:594–604. Pierce JA, Hocon JB, Ebert RV. The collagen and elastin content of the lung in emphysema. Ann Intern Med 1961;55:210–22. Johnson JR, Andrews FA. Lung scleroproteins in age and emphysema. Chest 1970;57:239–44. Fukuda Y, Masuda Y, Ishizaki M, Masugi Y, Ferrans VJ. Morphogenesis of abnormal elastic fibers in lungs of patients with panacinar and centriacinar emphysema. Hum Pathol 1989;20:652–9. Deslee G, Woods JC, Moore CM, Liu L, Conradi SH, Milne M, et al. Elastin expression in very severe human COPD. Eur Respir J 2009;34:324–31. Park PW, Biedermann K, Mecham L, Bissett DL, Mecham RP. Lysozyme binds to elastin and protects elastin from elastase-mediated degradation. J Invest Dermatol 1996;106:1075–80. Masakichi M, Hideo A, Atsunobu Y, et al. A study on glycosaminoglycans in a case of pulmonary emphysema. Z Erkr Atmungsorgane 1975;143:270–6. Konno K, Arai H, Motomiya M, et al. A biochemical study on glycosaminoglycans (mucopolysaccharides) in emphysematous and in aged lungs. Am Rev Respir Dis 1982;126:797–801.
Contents lists available at ScienceDirect
Medical Hypotheses journal homepage: www.elsevier.com/locate/mehy
Does lysozyme play a role in the pathogenesis of COPD? Jerome Cantor ⇑, Bronislava Shteyngart St John’s University College of Pharmacy and Health Sciences, Queens, NY 11439, United States
a r t i c l e
i n f o
Article history: Received 14 December 2014 Accepted 25 February 2015
a b s t r a c t Elastic fiber injury is an important process in the pathogenesis of chronic obstructive pulmonary disease (COPD), particularly with regard to the development of pulmonary emphysema. Damage to these fibers results in uneven distribution of mechanical forces in the lung, leading to dilatation and rupture of alveolar walls. While the role of various enzymes and oxidants in this process has been well-documented, we propose that a previously unsuspected agent, lysozyme, may contribute significantly to the changes in elastic fibers observed in this disease. Studies from our laboratory have previously shown that lysozyme preferentially binds to elastic fibers in human emphysematous lungs. On the basis of this finding, it is hypothesized that the attachment of lysozyme to these fibers enhances their susceptibility to injury, and further impairs the transfer of mechanical forces in the lung, leading to increased alveolar wall damage and enhanced progression of COPD. The hypothesized effects of lysozyme are predicated on its interaction with hyaluronan (HA), a long-chain polysaccharide that is found in close proximity to elastic fibers. By preventing the binding of HA to elastic fibers in COPD, lysozyme may interfere with the protective effect of this polysaccharide against enzymes and oxidants that degrade these fibers. Furthermore, the loss of the hydrating effect of HA on these fibers may impair their elastic properties, greatly increasing the probability of their fragmentation in response to mechanical forces. The proposed hypothesis may explain why the content of HA is significantly lower in the lungs of COPD patients. It may also contribute to the design of clinical trials involving the use of exogenously administered HA as a potential treatment for COPD. Ó 2015 Elsevier Ltd. All rights reserved.
Introduction The pathogenesis of chronic obstructive pulmonary disease (COPD) is complex, involving multiple types of injurious agents acting on a number of tissue constituents. Nevertheless, numerous studies suggest that a primary feature of the disease is damage to the elastic fiber network of the lung as a result of chronic inflammation [1–3]. Both neutrophils and macrophages recruited to the lung by tobacco smoke and other toxic substances release a variety of enzymes and oxidants that degrade elastic fibers, thereby disrupting the mechanical forces that control the expansion and contraction of alveoli. Over a period of many years, the continual degradation of these fibers leads to the dilatation and rupture of alveolar walls, which reduces lung surface area and impairs ventilation and perfusion in the lung. In the current paper, we add a previously unsuspected protein to the list of agents that may have adverse effects on pulmonary elastic fibers by proposing a role for lysozyme in this process. It
⇑ Corresponding author at: Department of Pharmacy and Health Sciences, St John’s University, 8000 Utopia Pkwy, Queens, NY 11439, United States. E-mail address: [email protected] (J. Cantor). http://dx.doi.org/10.1016/j.mehy.2015.02.015 0306-9877/Ó 2015 Elsevier Ltd. All rights reserved.
is hypothesized that lysozyme released by inflammatory cells in the lung binds to elastic fibers, makes them more susceptible to injury, and impairs their ability to regulate the transfer of mechanical forces in the lung, leading to increased alveolar wall damage and enhanced progression of COPD. Support for this concept is based on a number of findings, some of which are derived from experiments in our laboratory over the past 20 years. In particular, we will propose that the relationship between lysozyme and the polysaccharide, hyaluronan (HA), is critical to the changes in elastic fibers that are involved in the pathogenesis of COPD.
Formulating and evaluating the hypothesis The relationship between lysozyme and elastic fibers Previous studies from this laboratory have shown that lysozyme preferentially binds to elastic fibers in lungs with pulmonary emphysema (Fig. 1) [4]. However, the significance of this finding is complicated by the fact that this protein has no recognized physiological function beyond its role in bacteriolysis. The increased levels of lysozyme that accompany inflammation may only reflect nonspecific release from lysosomes. Furthermore, the
552
J. Cantor, B. Shteyngart / Medical Hypotheses 84 (2015) 551–554
Fig. 1. (A) Photomicrograph of human emphysematous lung showing intense immunostaining following treatment with anti-lysozyme antibodies. (B) Photomicrograph of normal lung showing absence of immunostaining with anti-lysozyme antibodies. Only focal intracellular staining of leukocytes is present.
association of lysozyme with components of inflamed tissues may simply be due to its strongly cationic nature, resulting in electrostatic binding to various anionic molecules rather than substrate-specific interactions. With regard to elastic fibers, lysozyme may form electrostatic or hydrogen bonds with carbohydrate residues in elastic fibers. Alternatively, it could specifically bind to N-acetyl-D-glucosamine, a component of bacterial cells susceptible to degradation by lysozyme, which has also been found in glycoproteins associated with these fibers. The low-grade inflammatory process that occurs in pulmonary emphysema, involving release of elastases and other injurious agents, might increase the access of lysozyme to these carbohydrate components, thereby facilitating the binding of this protein. This concept has some experimental support in studies showing that lysozyme only adheres to skin elastic fibers that have been previously damaged by either UV radiation or advanced glycation end-products [5].
Interaction of HA and lysozyme in elastic fiber injury This laboratory has also shown that hyaluronidase treatment of human lung tissues in vitro increases lysozyme binding to elastic fibers, presumably by degrading HA and exposing specific attachment sites on the elastic fiber [4]. In the case of pulmonary emphysema, the presence of lysozyme on elastic fibers might disrupt the normal interactions between these fibers and HA, thereby increasing elastolysis and enhancing the progression of the disease. This hypothesis was tested by our laboratory, using the elastase model of pulmonary emphysema. Animals exposed to aerosolized lysozyme prior to elastase administration showed significantly increased airspace enlargement [11]. The ability of lysozyme to enhance elastolysis was further examined in vitro, using an extracellular matrix preparation, mainly composed of elastic fibers. Treatment of the matrix with lysozyme resulted in attachment of the protein to elastic fibers, which impaired the ability of HA to protect them [11].
The role of HA in protecting elastic fibers How lysozyme may disrupt hydration of elastic fibers The hypothesized effects of lysozyme are predicated on its interaction with HA, a long-chain polysaccharide that is found in close proximity to elastic fibers. HA is best known for its capacity to retain large amounts of water, but it has a number of other important functions as well. As a constituent of the extracellular matrix, HA acts to stabilize proteoglycans and contributes to tissue growth and repair [6,7]. The ability of HA to expand its domain in a fluid environment may have several consequences for inflammatory reactions involving elastic fibers. By creating a more viscous milieu, HA may impede the movement of cells and molecules through the extracellular matrix, thereby reducing the activity of leukocytes and their proinflammatory products. The consequences of this ‘‘physical barrier” effect of HA have been explored in our laboratory, where aerosolized HA was found to significantly reduce airspace enlargement in experimental models of pulmonary emphysema induced by either intratracheal instillation of elastase or long-term exposure to cigarette smoke [8–10]. Studies using fluorescent-labeled HA indicate that the polysaccharide coats elastic fibers, and may replace endogenous HA that is removed from these fibers during the inflammatory process [8].
The ability of lysozyme to interfere with the binding of HA to elastic fibers may also adversely affect their mechanical properties, due to a loss of hydration in the absence of this polysaccharide. The absorption of water onto nonpolar hydrophobic groups during the extension of elastic fibers contributes to the storage of elastic energy [12,13]. A decrease in the availability of water can compromise this process, reducing elastic fiber recoil. Furthermore, water facilitates the swelling of the core elastin protein, thereby increasing its random energy state and enhancing recoil as a result of the increased loss of entropy during distention [14]. Resistance to stretching of the elastic fibers may also be dependent on the intramolecular forces of attraction among the elastin peptide chains themselves. Removing water molecules decreases the distance between the elastin peptide chains, enhancing their cohesion and reducing the distensibility of the fibers [15]. At the extreme, the complete absence of water yields an elastic modulus approaching that of glass [16]. The binding of lysozyme to elastic fibers may therefore cause a reduction in their recoil capacity by either blocking the attachment of HA to the fibers or forming complexes with this polysaccharide
J. Cantor, B. Shteyngart / Medical Hypotheses 84 (2015) 551–554
that limit its retention of water [17]. The continued accumulation of lysozyme around elastic fibers could eventually reduce hydration to the point where the fibers become brittle, making them vulnerable to rupture under strain (Fig. 2). The resulting breakage of the fibers could mimic the effects of enzymatic degradation, with resultant alveolar distention and rupture. This mechanism could provide an additional reason why pulmonary emphysema continues to progress even when the causative agent is removed [18].
553
would result in the progressive loss of HA from the lung as injury to elastic fibers continues and more lysozyme becomes bound to them. While there is no direct evidence to support this concept, a preliminary study from this laboratory demonstrated a correlation between the amount of HA in human lung tissue and diffusing capacity of the lung for carbon monoxide (DLCO), which reflects the extent of emphysema (unpublished observations). The implications of the hypothesis regarding aerosolized HA as a therapy for COPD
Consequences of the hypothesis Using the hypothesis to explain why elastolysis is decreased in advanced COPD One of the poorly understood features of COPD concerns the observed reduction in elastolysis as the disease becomes more severe [19]. While it has been proposed that this finding may be due to the eventual exhaustion of the elastolytic process, several studies suggest that human emphysematous lungs still have large amounts of elastic fibers [20–23]. Consequently, there should not be a significant reduction in elastolysis. An alternative explanation for this phenomenon could involve the binding of lysozyme to elastic fibers. A number of studies have shown that elastic fibers from UV-damaged skin, which contain large amounts of lysozyme, are more resistant to elastolysis [24]. While similar studies have not been performed in patients with COPD, it is quite possible that the continued accumulation of lysozyme around damaged pulmonary elastic fibers could make them more resistant to enzymatic breakdown, despite the loss of HA. This process would provide a more reasonable explanation for the marked reduction in elastolysis in advanced cases of this disease. Using the hypothesis to explain why lung levels of HA correlate with severity of COPD Several studies have shown that HA is decreased in human lungs with pulmonary emphysema [25,26]. Furthermore, there is relatively less HA in comparison to other lung glycosaminoglycans, which is consistent with the possibility that deposition of HA in the extracellular matrix is blocked by lysozyme [26]. This process
Our laboratory is exploring the use of aerosolized HA as a treatment for COPD, and clinical trials are currently underway to test the safety and efficacy of this agent. In designing these studies, it is important to determine which patients should be included in the trial. While our findings of markedly decreased HA in patients with advanced COPD suggest that this group might benefit most from this type of therapy, the potential blockage of HA binding to elastic fibers by lysozyme would contradict this conclusion. In fact, our hypothesis argues in favor of designing the clinical trial around a population of patients with mild to moderate COPD, who retain a large proportion of elastic fibers that have not been subjected to injury and subsequent deposition of lysozyme. In these patients, treatment with HA might be significantly more effective with regard to preventing loss of lung function. Conclusions The presence of lysozyme on elastic fibers is a well-documented phenomenon. However, the role of this phenomenon in the pathogenesis of diseases involving injury and repair of these fibers remains poorly understood. Our hypothesis provides a framework for testing the role of lysozyme in COPD, and determining how this protein could affect the progression and treatment of this disorder. The data from our laboratory demonstrate a connection between lysozyme and HA, and provide an explanation for the seemingly contradictory findings of decreased elastolysis and loss of HA in advanced COPD. The proposed hypothesis also explains the continued progression of the disease despite this reduction in elastolysis. Finally, this hypothesis provides a rationale for selecting a suitable patient population for testing the efficacy of aerosolized HA as a potential treatment for COPD.
Conflict of interest The authors declare that there is no conflict of interest regarding the material presented in this manuscript. References
Fig. 2. Photomicrograph of human emphysematous lung showing immunostaining of fragmented elastic fibers with anti-lysozyme antibodies (arrowheads). Deposition of lysozyme on elastic fibers in COPD may reduce their hydration and increase their susceptibility to mechanical failure.
[1] Hogg JC, Timens W. The pathology of chronic obstructive pulmonary disease. Annu Rev Pathol 2009;4:435–59. [2] Tuder RM, Petrache I. Pathogenesis of chronic obstructive pulmonary disease. J Clin Invest 2012;122:2749–55. [3] Abboud RT, Vimalanathan S. Pathogenesis of COPD. Part I. The role of proteaseantiprotease imbalance in emphysema. Int J Tuberc Lung Dis 2008;12:361–7. [4] Shteyngart B, Chaiwiriyakul S, Wong J, Cantor JO. Preferential binding of lysozyme to elastic fibers in pulmonary emphysema. Thorax 1998;53:193–6. [5] Yoshinaga E, Kawada A, Ono K, et al. NƐ-(carboxymethyl)lysine modification of elastin alters its biological properties: implications for the accumulation of abnormal elastic fibers in actinic elastosis. J Invest Dermatol 2012;132:315–23. [6] Dicker KT, Gurski LA, Pradhan-Bhatt S, Witt RL, Farach-Carson MC, Jia X. Hyaluronan: a simple polysaccharide with diverse biological functions. Acta Biomater 2014;10:1558–70. [7] Jiang D, Liang J, Noble PW. Regulation of non-infectious lung injury, inflammation, and repair by the extracellular matrix glycosaminoglycan hyaluronan. Anat Rec (Hoboken) 2010;293:982–5.
554
J. Cantor, B. Shteyngart / Medical Hypotheses 84 (2015) 551–554
[8] Cantor JO, Cerreta JM, Armand G, Turino GM. Aerosolized hyaluronic acid decreases alveolar injury induced by human neutrophil elastase. Proc Soc Exp Biol Med 1998;217:471–5. [9] Cantor JO, Cerreta JM, Keller S, Turino GM. Modulation of airspace enlargement in elastase-induced emphysema by intratracheal instillment of hyaluronidase and hyaluronic acid. Exp Lung Res 1995;21:423–36. [10] Cantor JO, Cerreta JM, Ochoa M, et al. Aerosolized hyaluronan limits airspace enlargement in a mouse model of cigarette smoke-induced pulmonary emphysema. Exp Lung Res 2005;31:417–30. [11] Cantor JO, Shteyngart B, Cerreta JM, Turino GM. The effect of lysozyme on elastase-mediated injury. Exp Biol Med 2002;227:108–13. [12] Li B, Alonso DO, Bennion BJ, Daggett V. Hydrophobic hydration is an important source of elasticity in elastin-based biopolymers. J Am Chem Soc 2001;123:11991–8. [13] Lillie MA, Chalmers GW, Gosline JM. Elastin dehydration through the liquid and the vapor phase: a comparison of osmotic stress models. Biopolymers 1996;39:627–39. [14] Gosline JM. The role of hydrophobic interactions in the swelling of elastin. In: Sandberg LB, Gray WR, Franzblau C, editors. Advances in experimental medicine and biology, vol. 79. New York: Plenum; 1977. p. 621–31. [15] Kendall K. Molecular adhesion and its applications. New York: Plenum; 2001. p. 103–31. [16] Hoeve CAJ. The elasticity in the presence of diluents. In: Sandberg LB, Gray WR, Franzblau C, editors. Advances in experimental medicine and biology, vol. 79. New York: Plenum; 1977. p. 607–19. [17] Water JJ, Schack MM, Velazquez-Campoy A, Maltesen MJ, van de Weert M, Jorgensen L, et al. Complex coacervates of hyaluronic acid and lysozyme: effect
[18]
[19]
[20] [21] [22]
[23] [24]
[25] [26]
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