Much More Than Just Diabetes: The Pivotal Role of Dipeptidyl-Peptidase 4 in Skin Fibrosis

View Large Image Figure ViewerDownload Hi-res image Download (PPT)Clinical Implications•Epidermolysis bullosa (EB)–derived skin fibroblasts show elevated dipeptidyl-peptidase 4 activity.•Sitagliptin restores the levels of fibronectin and fibril deposition in EB-derived fibroblasts.•Sitagliptin presents an interesting treatment strategy for skin fibrosis and scarring. •Epidermolysis bullosa (EB)–derived skin fibroblasts show elevated dipeptidyl-peptidase 4 activity.•Sitagliptin restores the levels of fibronectin and fibril deposition in EB-derived fibroblasts.•Sitagliptin presents an interesting treatment strategy for skin fibrosis and scarring. Researchers have long recognized dipeptidyl-peptidase 4 (DPP4) as a therapeutic target in diabetes mellitus attributable to its role in glucose metabolism. However, recent research has unveiled its multifaceted involvement in various physiological processes beyond glycemic control, particularly in fibrotic disorders (Figure 1). In their recent study, Zeyer et al., 2024Zeyer K.A. Bornert O. Nelea V. Bao X. Leytens A. Sharoyan S. et al.Dipeptidyl peptidase-4-mediated fibronectin processing evokes a profibrotic extracellular matrix [e-pub ahead of print].J Invest Dermatol. 2024; (accessed 27 May 2024)https://doi.org/10.1016/j.jid.2024.03.020Google Scholar shed light on the significance of DPP4 in skin fibrosis, particularly in the context of epidermolysis bullosa (EB), and the potential therapeutic implications of targeting DPP4 in mitigating fibrosis and scarring. This commentary discusses the broader implications of DPP4 inhibition in fibrosis and highlights future research and clinical application directions. DPP4, an S9 proline oligopeptidase family member, is a widespread serine protease cleaving N-terminal dipeptides adjacent to proline or alanine residues. When matrix metalloproteinases cleave DPP4 from the cell surfaces, it becomes soluble (soluble DPP4 [sDPP4]), retaining its catalytic and enzymatic capabilities. sDPP4 accounts for 95% of dipeptidyl peptidase activity in human serum. DPP4 is found in various organs and cell types, influencing T-cell functions such as proliferation, activation, and inflammation while also affecting wound healing by modulating macrophage recruitment and fibrosis. By cleaving N-terminal dipeptides, DPP4 deactivates active peptides and processes substrates, including chemokines and vasoactive peptides such as glucagon-like peptide 1, glucose-dependent insulinotropic polypeptide, and substance P. In addition, DPP4 binds to adenosine deaminase and extracellular matrix (ECM) components such as fibronectin (Ohm et al., 2023Ohm B. Moneke I. Jungraithmayr W. Targeting cluster of differentiation 26 / dipeptidyl peptidase 4 (CD26/DPP4) in organ fibrosis.Br J Pharmacol. 2023; 180: 2846-2861Google Scholar). The expression and function of DPP4 in the skin have been controversial. During embryonic and early postnatal development, researchers detected DPP4 in the papillary dermis of the mouse skin; however, in adult mice, DPP4 was predominantly expressed in the reticular dermis. In contrast to studies in mice, there are limited and conflicting data on the expression and function of DPP4 in human skin. Rinkevich et al., 2015Rinkevich Y. Walmsley G.G. Hu M.S. Maan Z.N. Newman A.M. Drukker M. et al.Skin fibrosis. Identification and isolation of a dermal lineage with intrinsic fibrogenic potential.Science. 2015; 348aaa2151Google Scholar were the first to describe a distinct fibroblast (FB) cell population with intrinsic fibrotic potential characterized by the expression of DPP4 (Rinkevich et al., 2015Rinkevich Y. Walmsley G.G. Hu M.S. Maan Z.N. Newman A.M. Drukker M. et al.Skin fibrosis. Identification and isolation of a dermal lineage with intrinsic fibrogenic potential.Science. 2015; 348aaa2151Google Scholar). Traditionally, DPP4 has been considered a marker of reticular FBs. However, recent single-cell sequencing studies revealed DPP4 expression in FBs of both dermal layers in human skin (Tabib et al., 2018Tabib T. Morse C. Wang T. Chen W. Lafyatis R. SFRP2/DPP4 and FMO1/LSP1 define major fibroblast populations in human skin.J Invest Dermatol. 2018; 138: 802-810Google Scholar; Vorstandlechner et al., 2020Vorstandlechner V. Laggner M. Kalinina P. Haslik W. Radtke C. Shaw L. et al.Deciphering the functional heterogeneity of skin fibroblasts using single-cell RNA sequencing.FASEB J. 2020; 34: 3677-3692Google Scholar). Tabib et al., 2018Tabib T. Morse C. Wang T. Chen W. Lafyatis R. SFRP2/DPP4 and FMO1/LSP1 define major fibroblast populations in human skin.J Invest Dermatol. 2018; 138: 802-810Google Scholar suggested a new major FB population characterized by DPP4 and SFRP2 expressions, whereas Korosec et al., 2019Korosec A. Frech S. Gesslbauer B. Vierhapper M. Radtke C. Petzelbauer P. et al.Lineage identity and location within the dermis determine the function of papillary and reticular fibroblasts in human skin.J Invest Dermatol. 2019; 139: 342-351Google Scholar found a broader distribution of DPP4+ FBs mainly in the reticular dermis but also in the papillary dermis. In 1995, researchers proposed DPP4 inhibition as a therapeutic approach for treating type 2 diabetes (Ohm et al., 2023Ohm B. Moneke I. Jungraithmayr W. Targeting cluster of differentiation 26 / dipeptidyl peptidase 4 (CD26/DPP4) in organ fibrosis.Br J Pharmacol. 2023; 180: 2846-2861Google Scholar). In the early 2000s, studies introduced gliptins as small molecules targeting DPP4, and the Food and Drugs Administration approved sitagliptin in 2006. Gliptins are highly specific pharmacological inhibitors of DPP4 activity that effectively control blood glucose levels by inhibiting the cleavage of glucagon-like peptide 1 and glucose-dependent insulinotropic polypeptide. As a result, they are now widely used as an oral medication for treating diabetes mellitus and have a favorable profile of good tolerability and low side effects (Pathak and Bridgeman, 2010Pathak R. Bridgeman M.B. Dipeptidyl peptidase-4 (DPP-4) inhibitors in the management of diabetes.P T. 2010; 35: 509-513Google Scholar). In addition, they do not cause hypoglycemic episodes and have very few drug interactions (Pathak and Bridgeman, 2010Pathak R. Bridgeman M.B. Dipeptidyl peptidase-4 (DPP-4) inhibitors in the management of diabetes.P T. 2010; 35: 509-513Google Scholar). Moreover, gliptins have shown antihypertensive, anti-inflammatory, antiapoptotic, and immunomodulatory effects independent of the incretin pathway (Pathak and Bridgeman, 2010Pathak R. Bridgeman M.B. Dipeptidyl peptidase-4 (DPP-4) inhibitors in the management of diabetes.P T. 2010; 35: 509-513Google Scholar). However, physicians increasingly use gliptins for their benefit in diabetes and various fibrotic diseases, including the kidney, heart, lung, and liver diseases (Ohm et al., 2023Ohm B. Moneke I. Jungraithmayr W. Targeting cluster of differentiation 26 / dipeptidyl peptidase 4 (CD26/DPP4) in organ fibrosis.Br J Pharmacol. 2023; 180: 2846-2861Google Scholar). Initially, anecdotal evidence showed that patients treated with gliptins experienced improved wound healing. Interestingly, scar formation was also significantly improved in these patients. Subsequently, retrospective studies showed a significant reduction in the risk of hypertrophic scars and keloid development in gliptin-treated patients undergoing sternotomy (Suwanai et al., 2020Suwanai H. Watanabe R. Sato M. Odawara M. Matsumura H. Dipeptidyl peptidase-4 inhibitor reduces the risk of developing hypertrophic scars and keloids following median sternotomy in diabetic patients: a nationwide retrospective cohort study using the National Database of Health Insurance Claims of Japan.Plast Reconstr Surg. 2020; 146: 83-89Google Scholar). In recent years, extensive research has focused on DPP4 and its interaction with TGFβ in fibrosis and scar formation. Upon TGFβ stimulation, FBs differentiate into myofibroblasts, characterized by alpha-smooth muscle actin expression. Myofibroblasts can also originate from various progenitor cells, including tissue-resident mesenchymal cells and inflammatory monocytes. TGFβ signaling and FB activation into myofibroblasts are crucial in all fibrotic conditions. However, systemic targeting of TGFβ has faced challenges attributable to its diverse effects, including autoimmune reactions and increased cancer risk (Ohm et al., 2023Ohm B. Moneke I. Jungraithmayr W. Targeting cluster of differentiation 26 / dipeptidyl peptidase 4 (CD26/DPP4) in organ fibrosis.Br J Pharmacol. 2023; 180: 2846-2861Google Scholar). Although TGFβ itself is not a direct DPP4 substrate, gliptins exhibit antifibrotic effects, possibly by modulating downstream interactions of DPP4 with TGFβ (Thielitz et al., 2008Thielitz A. Vetter R.W. Schultze B. Wrenger S. Simeoni L. Ansorge S. et al.Inhibitors of dipeptidyl peptidase IV-like activity mediate antifibrotic effects in normal and keloid-derived skin fibroblasts.J Invest Dermatol. 2008; 128: 855-866Google Scholar). Studies have shown that DPP4 inhibitors interfere with TGFβ signaling pathways and reduce TGFβ receptor expression. In addition, sDPP4 activates the NF-κB pathway (Ohm et al., 2023Ohm B. Moneke I. Jungraithmayr W. Targeting cluster of differentiation 26 / dipeptidyl peptidase 4 (CD26/DPP4) in organ fibrosis.Br J Pharmacol. 2023; 180: 2846-2861Google Scholar). Recent research has highlighted the role of DPP4 in hypertrophic scar formation in both humans and mice (Vorstandlechner et al., 2021Vorstandlechner V. Laggner M. Copic D. Klas K. Direder M. Chen Y. et al.The serine proteases dipeptidyl-peptidase 4 and urokinase are key molecules in human and mouse scar formation.Nat Commun. 2021; 12: 6242Google Scholar). In their recent study, Zeyer et al., 2024Zeyer K.A. Bornert O. Nelea V. Bao X. Leytens A. Sharoyan S. et al.Dipeptidyl peptidase-4-mediated fibronectin processing evokes a profibrotic extracellular matrix [e-pub ahead of print].J Invest Dermatol. 2024; (accessed 27 May 2024)https://doi.org/10.1016/j.jid.2024.03.020Google Scholar shed new light on the role of DPP4 in interacting with the ECM, with a particular focus on fibronectin, fibrillin, and microfibrils. They used a model of EB, a chronic inflammatory condition that leads to fibrosis, rather than traditional skin fibrotic entities such as skin scarring or scleroderma. Increased TGFβ activity and FB activation were found in EB-derived fibrosis, supporting its suitability for studying fibrotic mechanisms alongside chronic inflammation. Notably, DPP4 expression and activity were significantly increased in EB-derived FBs, further emphasizing its relevance in all fibrotic conditions. Interestingly, the researchers showed that DPP4 modulates ECM deposition through proteolysis of the fibronectin N-terminus and that treatment with sitagliptin led to the normalization of ECM deposition. The study by Zeyer et al., 2024Zeyer K.A. Bornert O. Nelea V. Bao X. Leytens A. Sharoyan S. et al.Dipeptidyl peptidase-4-mediated fibronectin processing evokes a profibrotic extracellular matrix [e-pub ahead of print].J Invest Dermatol. 2024; (accessed 27 May 2024)https://doi.org/10.1016/j.jid.2024.03.020Google Scholar certainly provides a basis for further interesting investigations. The researchers showed that inhibition of DPP4 attenuates the altered fibril deposition of EB-derived FBs and shotgun inhibition of proteases using aprotinin. Investigating different (serine) proteases would be worthwhile, particularly dipeptidyl-peptidase 7 and dipeptidyl-peptidase 8, which show a synergistic interaction with DPP4. In our work on serine proteases in hypertrophic scars, we have identified several other overexpressed serine proteases in human and mouse scars, including urokinase, serine protease 23 (PRSS23), HTRA1, and AEBP1 (Vorstandlechner et al., 2021Vorstandlechner V. Laggner M. Copic D. Klas K. Direder M. Chen Y. et al.The serine proteases dipeptidyl-peptidase 4 and urokinase are key molecules in human and mouse scar formation.Nat Commun. 2021; 12: 6242Google Scholar). It would be interesting to see whether these also interact with fibronectin or integrins to influence TGFβ activity. However, apart from urokinase, no commercially available pharmacological inhibitors for these serine proteases exist, which makes further investigation difficult. In addition, we wonder whether DPP4 binds to or cleaves other ECM components, as the researchers have shown with fibronectin. Indeed, DPP4 has been shown to form complexes with seprase (FB activation protein-α), thereby allowing the degradation of ECM proteins, further facilitating cell invasion to the site of injury (Ohm et al., 2023Ohm B. Moneke I. Jungraithmayr W. Targeting cluster of differentiation 26 / dipeptidyl peptidase 4 (CD26/DPP4) in organ fibrosis.Br J Pharmacol. 2023; 180: 2846-2861Google Scholar). We believe that separately testing the binding dynamics of DPP4 to, for example, collagens I and III, elastin, but also glycoproteins or matricellular components would provide valuable insights to better understand the potential cleavage products resulting from DPP4 interaction with ECM. Furthermore, it would be interesting to see whether the observed changes in fibronectin/fibril deposition and the corresponding rescue by DPP4 inhibitors would also hold for FBs derived from other fibrotic pathologies, such as scleroderma and renal, cardiac, and hepatic sclerosis. If this is not the scenario, it would suggest a distinct mechanism in EB, possibly involving a specific interplay between the chronic inflammatory environment and the absence of collagen VII. Finally, in every context of DPP4 and fibrosis, we have asked ourselves whether there are physiological regulators of DPP4 activity, such as endogenous DPP4 inhibitors. Future studies could evaluate whether some of the ECM/fibronectin or other DPP4 substrate cleavage products or plasma/cellular molecules exist that could inhibit DPP4 activity, thus allowing a negative feedback loop to control DPP4 activity. Despite the growing evidence and understanding of DPP4 in fibrosis, all current gliptin studies listed on ClinicalTrials.gov are in diabetes or fibrotic complications of diabetes, such as cystic fibrosis–related diabetes. We believe that the time is ripe to bring DPP4 into clinical use in nondiabetic patients with fibrosis, particularly skin fibrosis and scarring. In the next few years, we hope that a phase I topical safety and tolerability study of gliptins will be initiated, followed soon by phase II efficacy studies in skin fibrosis. Vera Vorstandlechner: http://orcid.org/0000-0002-5938-1552 Michael Mildner: http://orcid.org/0000-0002-6892-925X The authors state no conflict of interest. Dipeptidyl Peptidase-4–Mediated Fibronectin Processing Evokes a Profibrotic Extracellular MatrixJournal of Investigative DermatologyPreviewFibronectin serves as a platform to guide and facilitate deposition of collagen and fibrillin microfibrils. During development of fibrotic diseases, altered fibronectin deposition in the extracellular matrix (ECM) is generally an early event. After this, dysregulated organization of fibrillins and fibrillar collagens occurs. Because fibronectin is an essential orchestrator of healthy ECM, perturbation of its ECM-organizational capacity may be involved in development of fibrosis. To investigate this, we employed recessive dystrophic epidermolysis bullosa as a disease model with progressive, severe dermal fibrosis.