Failed Degradation of JunB Contributes to Overproduction of Type I Collagen and Development of Dermal Fibrosis in Patients With Systemic Sclerosis

Systemic sclerosis (SSc; scleroderma) is a complex autoimmune disease with unknown etiology. Pathogenic processes include vascular damage, autoimmunity, and widespread fibrosis of the skin and internal organs. Transforming growth factor β (TGFβ) has been implicated in the development of many fibrotic diseases, including SSc. TGFβ is a pleiotropic mediator with a critical role in wound healing and tissue remodeling. Consequently, it is of major importance in pathologic conditions that are characterized by tissue remodeling, scarring, and fibrosis. TGFβ regulates the expression of genes that are part of, or regulate the formation of, the extracellular matrix (ECM). Type I collagen is an integral structural component of the ECM, with a major role in wound healing and connective tissue remodeling. Dysregulated or excessive production and deposition of type I collagen lead to ECM accumulation and, eventually, tissue fibrosis. TGFβ is a potent inducer of the human procollagen type I α2 chain gene (COL1A2). We have recently described in detail the in vivo mechanism underlying the transcriptional control of COL1A2, through a complex interaction between its distal enhancer and proximal promoter, in response to TGFβ. This far-upstream enhancer (FUE) region is highly homologous to the mouse Col1a2 enhancer region (1–4), which is activated in adult mice during wound healing and fibrosis (5). Studies exploring the enhancer function have shown that TGFβ can also activate COL1A2 via a noncanonical (Smad-independent) signaling pathway requiring enhancer/promoter cooperation. This interaction appears to involve activator protein 1 (AP-1) family members and, in particular, an exchange of c-Jun for JunB in the critical AP-1 site of the enhancer, resulting in enhancer/promoter coalescence and transactivation of the transcriptional machinery bound in the promoter, by the factors bound to the enhancer. Moreover, using transgenesis, we have shown that interfering with this mechanism results in the abolition of COL1A2 expression by fibroblasts in vivo (6). The AP-1 family of transcription factors (c-Jun, JunB, JunD, cFos, FosB, Fra-1, and Fra-2) form homodimers and heterodimers as part of a complex mode of transcriptional regulation and are induced by a large variety of cellular signals (7–12). AP-1 transcriptional regulation is known to be involved in many normal and pathogenic cellular processes (13–16). Recently, the important roles of the AP-1 family members c-Jun, c-Fos, and JunD in dermal fibrosis have been described in patients with scleroderma (17–20). In this study, we observed that the novel TGFβ response element (TβRE) located far upstream in the enhancer of human COL1A2 was active in fibroblasts from the fibrotic lesions of patients with SSc. We also found that JunB was activated by TGFβ and detected constitutive expression of JunB in SSc dermal fibroblasts. Furthermore, inhibition of JunB resulted in the down-regulation of type I collagen expression by SSc dermal fibroblasts and reduced the ability of dermal fibroblasts to migrate into an in vitro wound. We also delineated a mechanism of JunB overexpression in SSc dermal fibroblasts, involving mammalian target of rapamycin (mTOR), Akt, and glycogen synthase kinase 3β (GSK-3β) signaling.