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Fibroblasts are polymorphic cells with pleiotropic roles in organ morphogenesis, tissue homeostasis and immune responses. In fibrotic diseases, fibroblasts synthesize abundant amounts of extracellular matrix which lead to scaring and organ failure. In contrast, the hallmark feature of fibroblasts in arthritis is matrix degradation by the release of metalloproteinases and degrading enzymes, and subsequent tissue destruction. The mechanisms driving these functionally opposing pro-fibrotic and pro-inflammatory phenotypes of fibroblasts are enigmatic. We identified the transcription factor PU.1 as an essential orchestrator of the pro-fibrotic gene expression program. The interplay between transcriptional and post-transcriptional mechanisms which normally control PU.1 expression is perturbed in various fibrotic diseases, resulting in upregulation of PU.1, induction of fibrosis-associated gene sets, and a phenotypic switch in matrix-producing pro-fibrotic fibroblasts. In contrast, pharmacological and genetic inactivation of PU.1 disrupts the fibrotic network and enables re-programming of fibrotic fibroblasts into resting fibroblasts with regression of fibrosis in different organs.

Objective Expression of dipeptidylpeptidase 4 (DPP‐4) identifies a dermal fibroblast lineage involved in scarring during wound healing. The role of DDP‐4 in tissue fibrosis is, however, unknown. The aim of the present study was to evaluate DPP‐4 as a potential target for the treatment of fibrosis in patients with systemic sclerosis (SSc). Methods Expression of DPP‐4 in skin biopsy samples and dermal fibroblasts was analyzed by real‐time polymerase chain reaction, immunofluorescence, and Western blot analyses. The activity of DPP‐4 was modulated by overexpression, knockdown, and pharmacologic inhibition of DPP4 using sitagliptin and vildagliptin. The effects of DPP4 inhibition were analyzed in human dermal fibroblasts and in different mouse models of SSc (each n = 6). Results The expression of DPP‐4 and the number of DPP‐4–positive fibroblasts were increased in the fibrotic skin of SSc patients, in a transforming growth factor β (TGFβ)–dependent manner. DPP‐4–positive fibroblasts expressed higher levels of myofibroblast markers and collagen (each P < 0.001 versus healthy controls). Overexpression of DPP4 promoted fibroblast activation, whereas pharmacologic inhibition or genetic inactivation of DPP4 reduced the proliferation, migration, and expression of contractile proteins and release of collagen (each P < 0.001 versus control mice) by interfering with TGFβ‐induced ERK signaling. DPP4‐knockout mice were less sensitive to bleomycin‐induced dermal and pulmonary fibrosis (P < 0.0001 versus wild‐type controls). Treatment with DPP4 inhibitors promoted regression of fibrosis in mice that had received bleomycin challenge and mice with chronic graft‐versus‐host disease, and ameliorated fibrosis in TSK1 mice (each P < 0.001 versus untreated controls). These antifibrotic effects were associated with a reduction in inflammation. Conclusion DPP‐4 characterizes a population of activated fibroblasts and shows that DPP‐4 regulates TGFβ‐induced fibroblast activation in the fibrotic skin of SSc patients. Inhibition of DPP4 exerts potent antifibrotic effects when administered in well‐tolerated doses. As DPP4 inhibitors are already in clinical use for diabetes, these results may have direct translational implications for the treatment of fibrosis in patients with SSc.

Uncontrolled activation of TGFβ signaling is a common denominator of fibrotic tissue remodeling. Here we characterize the tyrosine phosphatase SHP2 as a molecular checkpoint for TGFβ-induced JAK2/STAT3 signaling and as a potential target for the treatment of fibrosis. TGFβ stimulates the phosphatase activity of SHP2, although this effect is in part counterbalanced by inhibitory effects on SHP2 expression. Stimulation with TGFβ promotes recruitment of SHP2 to JAK2 in fibroblasts with subsequent dephosphorylation of JAK2 at Y570 and activation of STAT3. The effects of SHP2 on STAT3 activation translate into major regulatory effects of SHP2 on fibroblast activation and tissue fibrosis. Genetic or pharmacologic inactivation of SHP2 promotes accumulation of JAK2 phosphorylated at Y570, reduces JAK2/STAT3 signaling, inhibits TGFβ-induced fibroblast activation and ameliorates dermal and pulmonary fibrosis. Given the availability of potent SHP2 inhibitors, SHP2 might thus be a potential target for the treatment of fibrosis.