CCN1 expression by fibroblasts is required for bleomycin induced skin fibrosis

Abstract The microenvironment contributes to the excessive connective tissue deposition characterizing fibrosis. The CCN family of matricellular proteins are secreted by fibroblasts into the fibrotic microenvironment; however, the role of endogenous CCN1 in skin fibrosis is unknown. Mice harboring a fibroblast-specific deletion for CCN1 were used to assess if CCN1 contributes to dermal homeostasis, the dermal punch model of wound healing, and bleomycin-induced skin fibrosis. Mice with a fibroblast-specific CCN1 deletion showed progressive skin thinning and reduced accumulation of type I collagen; however, the overall mechanical property of skin (Young's modulus) was not significantly reduced. Real time-polymerase chain reaction analysis revealed that CCN1-deficient skin displayed reduced expression of mRNAs encoding enzymes that promote collagen stability (including prolyl-4-hydroxylase and PLOD2), although COL1A1 mRNA expression was unaltered. CCN1-deficent skin showed reduced hydroxyproline levels. Electron microscopy revealed that collagen fibers were disorganized in CCN1-deficient skin. CCN1-deficient mice were resistant to bleomycin-induced skin fibrosis, as visualized by reduced collagen accumulation and skin thickness suggesting that deposition/accumulation of collagen in impaired in the absence of CCN1. Conversely, CCN1-deficient mice showed unaltered wound closure kinetics, suggesting de novo collagen production in response to injury was not impaired in the absence of CCN1. In response to either wounding or bleomycin, induction of α-smooth muscle actin-positive myofibroblasts was unaffected by loss of CCN1. CCN1 protein is overexpressed by dermal fibroblasts isolated from lesional (i.e., fibrotic) areas of patients with early onset scleroderma. Thus, CCN1 expression by fibroblasts, being essential for skin fibrosis, is a viable anti-fibrotic target.