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The Skinny on Fats in Wound Healing
COMMENTARY focus of an article in this issue by Hellmann et al. (2018).
See related article on page 2051
Role of lipid mediators in normal and wounded skin
The Skinny on Fats in Wound Healing Praveen R. Arany1 The sequential wound healing cascade promotes optimal repair. However, prolonged or overly vigorous individual phases impede healing. Although most wound therapies focus on initiating the healing process by supplementing growth factors or matrices, the study by Hellmann et al. highlights the role of lipid molecules termed resolvins, specifically RvD1 and RvD2, as promoters of epithelial wound closure. Journal of Investigative Dermatology (2018) 138, 1909e1910. doi:10.1016/j.jid.2018.04.012
Wound healing is essential for survival. Routine breaches in external skin and internal mucosa need to be rapidly and effectively repaired to maintain physiological functions. These routine healing responses involve both cellular and noncellular components that mediate an overlapping cascade of events that is essential for optimal healing (Eming et al., 2014). Exogenous assaults by physical, chemical, or microbiological agents, as well as deficiencies in local or systemic host factors, often result in suboptimal wound healing. This can lead to cosmetic disfigurements such as scarring and keloids or major morbidities, including losses of limbs or deficiencies in organ function. Hence, effective promotion of tissue healing is at the core of optimal patient care. To promote healing, efforts have been directed at supplementing or substituting various factors. Among the noncellular mediators, the four major biological macromolecules (namely proteins, nucleic acids, carbohydrates, and lipids) have all been shown to play roles in wound healing. Proteins function as an integral constituents of both soluble (growth factors, peptides) and insoluble (cytoskeleton, extracellular matrix) components of tissue repair (Barrientos et al., 2008). These are among the best understood macromolecules, and are widely exploited in
Essentially, this approach appears to lend itself to putative combinations that promote epithelial migration and maturation along with other wound phase-targeted therapies such as those improving local circulation or induction of connective tissue formation. clinical approaches that range from local or systemic application of growth factors to sophisticated extracellular scaffold systems, to wound dressings. The roles of specific nucleic acids as genetic elements controlling several aspects of tissue healing have also been explored. Recently, regulatory nucleic acids (e.g., microRNAs) have been examined, and are now being used to promote healing (Luan et al., 2018). The roles of the other two groups of macromolecules, namely carbohydrates and lipids, are not as well characterized. The involvement of selected lipids in the skin healing response is the
1
School of Dental Medicine, Oral Biology and Biomedical Engineering, University at Buffalo, Buffalo, New York, USA Correspondence: Praveen R. Arany, 3435 Main Street, B36A Foster Hall, Buffalo, NY 14214. E-mail: [email protected] ª 2018 The Authors. Published by Elsevier, Inc. on behalf of the Society for Investigative Dermatology.
The investigators outlined key roles of three major lipid metabolomes (arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid) in normal and wounded skin in mice, pigs, and humans. Characterization of these molecules highlighted critical involvement of D-series resolvins, specifically RvD1, RvD2, RvD3, and RvD4. The investigators had previously documented a key role for RvD3 in lung epithelial wound healing. Hence, this work focuses primarily on the roles of RvD1 and RvD2 as mediators of skin wound healing. Hellman et al. (2018) first observe that skin epithelial cells synthesize these resolvins, suggesting that they may influence normal epithelial functions. They also note that these molecules are present in both sexes and all three species (humans, mice, and pigs) they examined. They go on to investigate the presence of the specific cognate receptors and observe that ALX/FPR2 (that binds RvD1) and DRV2/GPR18 (that binds RvD2) are present in normal skin. Next, they assess the function of these molecules in epidermis using a threedimensional air-water interface epithelial stratification model. Expression of resolvin receptors appears to increase as epithelial cells differentiate, suggesting correlation with terminal keratinocyte maturation and formation of a functional skin barrier. Therapeutic applications to promote wound healing
Having established presumptive roles for these molecules in normal skin, Hellman et al. (2018) treat wounded mouse skin with exogenous RvD1, RvD2, and RvD4. They observe a significant improvement in wound closure with rapid clearance of these lipids from the surface of these wounds. To further explore the mechanisms of the healing-promoting responses, the investigators examined wound cytokine, chemokine, and growth factor profiles. Levels of various wound growth factors, such as CTGF, FGF2 and 7, TGF-b1, and cyclooxygenase-2 were not altered by resolvin treatment. However, significantly reduced IL-1b expression was noted in treated wounds. Both the increased expression of resolvins in www.jidonline.org 1909
COMMENTARY late-phase healing responses involved in reorganization and remodeling. Clinical impact and translational path forward
Figure 1. Multifaceted role for resolvins in wound healing. The presence of inflammation in wounds has multiple roles, including removal of necrosis or infections and creating a cytokine-rich milieu to promote wound closure. The secretion of resolvins in microparticles from these cells are shown to promote epithelial migration. This work suggests that exogenously applied resolvins may selectively improve epithelial migration without invoking the other wound-inhibiting inflammatory responses.
differentiated epithelial cells and the correlation with reduced inflammation suggest they are involved in later wound healing stages. Resolvins are known to actively promote resolution of inflammation that eventually promotes reepithelization and wound closure (Figure 1) (Dalli and Serhan, 2012). To substantiate the effects of resolvin treatments in wound healing, Hellman et al. (2018) examined wounds in transgenic mice lacking ALK/FPR2 and DRV2/ GPR18 and noted abrogation of therapeutic responses. This directly implicates resolvin receptor complexes in mediating the healing process. Additionally, Hellman et al. attempted to characterize the precise molecular pathways mediating the wound-promoting responses of RvD2 treatments using primary human keratinocytes. In these studies, RvD2 promoted epithelial cell migration via the PI3K-Akt-mTor pathway without significantly affecting cell proliferation. This is an interesting subversion of this signaling pathway that is classically associated with proliferative-survival responses. The implication is that resolvins participate in
Most current wound treatments are focused on the initiation phases of healing to either induce tissue formation or modulate inflammation that would allow healing to proceed. In contrast, the study by Hellmann et al. (2018) promotes the use of resolvins to actively promote the later differentiation and maturation healing phase (Widgerow, 2012). This approach may lend itself to putative combinations in which resolvins promote epithelial migration and maturation and agents that target other wound phases, such as those that improve local circulation or induce connective tissue formation, are combined. Carefully chosen combinations could maximally affect specific clinical scenarios such as chronic wounds with excessive inflammation that delay tissue healing (Figure 1). The ability to use resolvins topically may promote healing without invoking systemic inflammatory responses or reactions (Serhan, 2014). The critical role of lipids in cell membranes suggests that additional investigations into their permissive roles in cell motility for both vertical stratification (during differentiation) and horizontal matrix migration (on connective tissue) may be indicated. Given the translational potential of this work, there are several possible future directions for development of resolvins as novel wound treatments. The detection of plasma levels of topically applied resolvins raises some concerns regarding potential systemic toxicity and potential effects on tumors. The lack of an induced epithelial proliferative response suggests that tumor initiation may not be a concern. However, the pro-migratory effects of resolvins raise questions regarding potential effects on preexisting local or metastatic cancer cells. With regard to the wound healing responses themselves, utilization of resolvins as a
1910 Journal of Investigative Dermatology (2018), Volume 138
monotherapy or as a combination during the specific inflammatory resolving phase would be optimal, as observed in this study. Future studies could examine effects of resolvins in inflamed or infected wounds such as diabetic and burn wounds that are particularly attractive clinical applications (Bohr et al., 2013; Mustafa et al., 2013; Tang et al., 2013). The work in this issue is a concrete step forward toward a more detailed understanding of the roles and rationale for using resolvins as components of wound therapy. ORCID Praveen Arany: http://orcid.org/0000-0002-61162340
CONFLICT OF INTEREST The author states no conflict of interest.
REFERENCES Barrientos S, Stojadinovic O, Golinko MS, Brem H, Tomic-Canic M. Growth factors and cytokines in wound healing. Wound Repair Regen 2008;16:585e601. Bohr S, Patel SJ, Sarin D, Irimia D, Yarmush ML, Berthiaume F. Resolvin D2 prevents secondary thrombosis and necrosis in a mouse burn wound model. Wound Repair Regen 2013;21:35e43. Dalli J, Serhan CN. Specific lipid mediator signatures of human phagocytes: microparticles stimulate macrophage efferocytosis and proresolving mediators. Blood 2012;120:e60e72. Eming SA, Martin P, Tomic-Canic M. Wound repair and regeneration: mechanisms, signaling, and translation. Sci Transl Med 2014;6:265sr6. Hellmann J, Sansbury BE, Wong B, Li X, Singh M, Nuutila K, et al. Biosynthesis of D-series resolvins in skin provides insights into their roles in tissue repair. J Invest Dermatol 2018;138:2051e60. Luan A, Hu MS, Leavitt T, Brett EA, Wang KC, Longaker MT, et al. Noncoding RNAs in wound healing: a new and vast frontier. Adv Wound Care (New Rochelle) 2018;7:19e27. Mustafa M, Zarrough A, Bolstad AI, Lygre H, Mustafa K, Hasturk H, et al. Resolvin D1 protects periodontal ligament. Am J Physiol Cell Physiol 2013;305:C673e9. Serhan CN. Pro-resolving lipid mediators are leads for resolution physiology. Nature 2014;510:92e101. Tang Y, Zhang MJ, Hellmann J, Kosuri M, Bhatnagar A, Spite M. Proresolution therapy for the treatment of delayed healing of diabetic wounds. Diabetes 2013;62:618e27. Widgerow AD. Cellular resolution of inflammation—catabasis. Wound Repair Regen 2012;20: 2e7.
See related article on page 2051
Role of lipid mediators in normal and wounded skin
The Skinny on Fats in Wound Healing Praveen R. Arany1 The sequential wound healing cascade promotes optimal repair. However, prolonged or overly vigorous individual phases impede healing. Although most wound therapies focus on initiating the healing process by supplementing growth factors or matrices, the study by Hellmann et al. highlights the role of lipid molecules termed resolvins, specifically RvD1 and RvD2, as promoters of epithelial wound closure. Journal of Investigative Dermatology (2018) 138, 1909e1910. doi:10.1016/j.jid.2018.04.012
Wound healing is essential for survival. Routine breaches in external skin and internal mucosa need to be rapidly and effectively repaired to maintain physiological functions. These routine healing responses involve both cellular and noncellular components that mediate an overlapping cascade of events that is essential for optimal healing (Eming et al., 2014). Exogenous assaults by physical, chemical, or microbiological agents, as well as deficiencies in local or systemic host factors, often result in suboptimal wound healing. This can lead to cosmetic disfigurements such as scarring and keloids or major morbidities, including losses of limbs or deficiencies in organ function. Hence, effective promotion of tissue healing is at the core of optimal patient care. To promote healing, efforts have been directed at supplementing or substituting various factors. Among the noncellular mediators, the four major biological macromolecules (namely proteins, nucleic acids, carbohydrates, and lipids) have all been shown to play roles in wound healing. Proteins function as an integral constituents of both soluble (growth factors, peptides) and insoluble (cytoskeleton, extracellular matrix) components of tissue repair (Barrientos et al., 2008). These are among the best understood macromolecules, and are widely exploited in
Essentially, this approach appears to lend itself to putative combinations that promote epithelial migration and maturation along with other wound phase-targeted therapies such as those improving local circulation or induction of connective tissue formation. clinical approaches that range from local or systemic application of growth factors to sophisticated extracellular scaffold systems, to wound dressings. The roles of specific nucleic acids as genetic elements controlling several aspects of tissue healing have also been explored. Recently, regulatory nucleic acids (e.g., microRNAs) have been examined, and are now being used to promote healing (Luan et al., 2018). The roles of the other two groups of macromolecules, namely carbohydrates and lipids, are not as well characterized. The involvement of selected lipids in the skin healing response is the
1
School of Dental Medicine, Oral Biology and Biomedical Engineering, University at Buffalo, Buffalo, New York, USA Correspondence: Praveen R. Arany, 3435 Main Street, B36A Foster Hall, Buffalo, NY 14214. E-mail: [email protected] ª 2018 The Authors. Published by Elsevier, Inc. on behalf of the Society for Investigative Dermatology.
The investigators outlined key roles of three major lipid metabolomes (arachidonic acid, eicosapentaenoic acid, and docosahexaenoic acid) in normal and wounded skin in mice, pigs, and humans. Characterization of these molecules highlighted critical involvement of D-series resolvins, specifically RvD1, RvD2, RvD3, and RvD4. The investigators had previously documented a key role for RvD3 in lung epithelial wound healing. Hence, this work focuses primarily on the roles of RvD1 and RvD2 as mediators of skin wound healing. Hellman et al. (2018) first observe that skin epithelial cells synthesize these resolvins, suggesting that they may influence normal epithelial functions. They also note that these molecules are present in both sexes and all three species (humans, mice, and pigs) they examined. They go on to investigate the presence of the specific cognate receptors and observe that ALX/FPR2 (that binds RvD1) and DRV2/GPR18 (that binds RvD2) are present in normal skin. Next, they assess the function of these molecules in epidermis using a threedimensional air-water interface epithelial stratification model. Expression of resolvin receptors appears to increase as epithelial cells differentiate, suggesting correlation with terminal keratinocyte maturation and formation of a functional skin barrier. Therapeutic applications to promote wound healing
Having established presumptive roles for these molecules in normal skin, Hellman et al. (2018) treat wounded mouse skin with exogenous RvD1, RvD2, and RvD4. They observe a significant improvement in wound closure with rapid clearance of these lipids from the surface of these wounds. To further explore the mechanisms of the healing-promoting responses, the investigators examined wound cytokine, chemokine, and growth factor profiles. Levels of various wound growth factors, such as CTGF, FGF2 and 7, TGF-b1, and cyclooxygenase-2 were not altered by resolvin treatment. However, significantly reduced IL-1b expression was noted in treated wounds. Both the increased expression of resolvins in www.jidonline.org 1909
COMMENTARY late-phase healing responses involved in reorganization and remodeling. Clinical impact and translational path forward
Figure 1. Multifaceted role for resolvins in wound healing. The presence of inflammation in wounds has multiple roles, including removal of necrosis or infections and creating a cytokine-rich milieu to promote wound closure. The secretion of resolvins in microparticles from these cells are shown to promote epithelial migration. This work suggests that exogenously applied resolvins may selectively improve epithelial migration without invoking the other wound-inhibiting inflammatory responses.
differentiated epithelial cells and the correlation with reduced inflammation suggest they are involved in later wound healing stages. Resolvins are known to actively promote resolution of inflammation that eventually promotes reepithelization and wound closure (Figure 1) (Dalli and Serhan, 2012). To substantiate the effects of resolvin treatments in wound healing, Hellman et al. (2018) examined wounds in transgenic mice lacking ALK/FPR2 and DRV2/ GPR18 and noted abrogation of therapeutic responses. This directly implicates resolvin receptor complexes in mediating the healing process. Additionally, Hellman et al. attempted to characterize the precise molecular pathways mediating the wound-promoting responses of RvD2 treatments using primary human keratinocytes. In these studies, RvD2 promoted epithelial cell migration via the PI3K-Akt-mTor pathway without significantly affecting cell proliferation. This is an interesting subversion of this signaling pathway that is classically associated with proliferative-survival responses. The implication is that resolvins participate in
Most current wound treatments are focused on the initiation phases of healing to either induce tissue formation or modulate inflammation that would allow healing to proceed. In contrast, the study by Hellmann et al. (2018) promotes the use of resolvins to actively promote the later differentiation and maturation healing phase (Widgerow, 2012). This approach may lend itself to putative combinations in which resolvins promote epithelial migration and maturation and agents that target other wound phases, such as those that improve local circulation or induce connective tissue formation, are combined. Carefully chosen combinations could maximally affect specific clinical scenarios such as chronic wounds with excessive inflammation that delay tissue healing (Figure 1). The ability to use resolvins topically may promote healing without invoking systemic inflammatory responses or reactions (Serhan, 2014). The critical role of lipids in cell membranes suggests that additional investigations into their permissive roles in cell motility for both vertical stratification (during differentiation) and horizontal matrix migration (on connective tissue) may be indicated. Given the translational potential of this work, there are several possible future directions for development of resolvins as novel wound treatments. The detection of plasma levels of topically applied resolvins raises some concerns regarding potential systemic toxicity and potential effects on tumors. The lack of an induced epithelial proliferative response suggests that tumor initiation may not be a concern. However, the pro-migratory effects of resolvins raise questions regarding potential effects on preexisting local or metastatic cancer cells. With regard to the wound healing responses themselves, utilization of resolvins as a
1910 Journal of Investigative Dermatology (2018), Volume 138
monotherapy or as a combination during the specific inflammatory resolving phase would be optimal, as observed in this study. Future studies could examine effects of resolvins in inflamed or infected wounds such as diabetic and burn wounds that are particularly attractive clinical applications (Bohr et al., 2013; Mustafa et al., 2013; Tang et al., 2013). The work in this issue is a concrete step forward toward a more detailed understanding of the roles and rationale for using resolvins as components of wound therapy. ORCID Praveen Arany: http://orcid.org/0000-0002-61162340
CONFLICT OF INTEREST The author states no conflict of interest.
REFERENCES Barrientos S, Stojadinovic O, Golinko MS, Brem H, Tomic-Canic M. Growth factors and cytokines in wound healing. Wound Repair Regen 2008;16:585e601. Bohr S, Patel SJ, Sarin D, Irimia D, Yarmush ML, Berthiaume F. Resolvin D2 prevents secondary thrombosis and necrosis in a mouse burn wound model. Wound Repair Regen 2013;21:35e43. Dalli J, Serhan CN. Specific lipid mediator signatures of human phagocytes: microparticles stimulate macrophage efferocytosis and proresolving mediators. Blood 2012;120:e60e72. Eming SA, Martin P, Tomic-Canic M. Wound repair and regeneration: mechanisms, signaling, and translation. Sci Transl Med 2014;6:265sr6. Hellmann J, Sansbury BE, Wong B, Li X, Singh M, Nuutila K, et al. Biosynthesis of D-series resolvins in skin provides insights into their roles in tissue repair. J Invest Dermatol 2018;138:2051e60. Luan A, Hu MS, Leavitt T, Brett EA, Wang KC, Longaker MT, et al. Noncoding RNAs in wound healing: a new and vast frontier. Adv Wound Care (New Rochelle) 2018;7:19e27. Mustafa M, Zarrough A, Bolstad AI, Lygre H, Mustafa K, Hasturk H, et al. Resolvin D1 protects periodontal ligament. Am J Physiol Cell Physiol 2013;305:C673e9. Serhan CN. Pro-resolving lipid mediators are leads for resolution physiology. Nature 2014;510:92e101. Tang Y, Zhang MJ, Hellmann J, Kosuri M, Bhatnagar A, Spite M. Proresolution therapy for the treatment of delayed healing of diabetic wounds. Diabetes 2013;62:618e27. Widgerow AD. Cellular resolution of inflammation—catabasis. Wound Repair Regen 2012;20: 2e7.