Caspase-3-mediated secretion of connective tissue growth factor by apoptotic endothelial cells promotes fibrosis.

Apoptosis of endothelial cells (ECs) is an early pathogenic event in various fibrotic diseases. In this study, we evaluated whether paracrine mediators produced by apoptotic ECs play direct roles in fibrogenesis. C3H mice injected subcutaneously with serum-free medium conditioned by apoptotic ECs (SSC) showed increased skin thickness and heightened protein levels of alpha-smooth-muscle actin (alphaSMA), vimentin and collagen I as compared with mice injected with medium conditioned by non-apoptotic ECs. Fibroblasts exposed to SSC in vitro showed cardinal features of myofibroblast differentiation with increased stress fiber formation and expression of alphaSMA. Caspase-3 silencing in ECs prevented the release of mediators favoring myofibroblast differentiation. To identify the fibrogenic factor(s) released by ECs, the protein contents of media conditioned by either apoptotic or non-apoptotic ECs were compared using SDS-PAGE-liquid chromatography (LC)-tandem mass spectrometry (MS/MS) and two-dimensional LC-MS/MS. Connective tissue growth factor (CTGF) was the only fibrogenic protein found increased in SSC. Pan-caspase inhibition with ZVAD-FMK or caspase-3 silencing in ECs confirmed that CTGF was released downstream of caspase-3 activation. The fibrogenic signaling signatures of SSC and CTGF on fibroblasts in vitro were similarly Pyk2-, Src-family kinases- and PI3K dependent, but TGF-beta-independent. CTGF-immunodepleted SSC failed to induce myofibroblast differentiation in vitro and skin fibrosis in vivo. These results identify caspase-3 activation in ECs as a novel inducer of CTGF release and fibrogenesis.