La fibrose en deux parties : de la paillasse à la souris
Apoptosis of endothelial cells (EC) is an early event in various fibrotic diseases including chronic allograft vasculopathy and systemic sclerosis.  We showed previously that mediators released by apoptotic EC activate myofibroblast differentiation and resistance to apoptosis, two mechanisms pivotal to fibrogenesis.  PI3K (phospatidylinositol-3 kinase) activation was found to be central to these two mechanisms.  A C-terminal fragment of perlecan (LG3) produced by apoptotic EC was found to inhibit apoptosis of fibroblasts.  The aims of the present project were : 1. to define the receptors and pathways implicated in this anti-apoptotic response and 2. to characterize the fibrogenic mediators implicated in myofibroblast differentiation.    Concerning the anti-apoptotic response, the inhibition of 21 integrin activity in fibroblasts exposed to either medium conditioned by apoptotic EC (SSC) or LG3 prevented resistance to apoptosis and was associated with decreased levels of Akt phosphorylation.  Neutralizing Src family kinases (SFK) activity in fibroblasts produced the same effects.  These results suggest that LG3 produced by apoptotic EC initiate a state of resistance to apoptosis in fibroblasts via an α2β1integrin/SFK/PI3K dependent pathway.  LG3 did not induce myofibroblast differentiation. We went on to identify which mediators present in SSC are implicated in myofibroblast differentiation.   Media conditioned by apoptotic and non-apoptotic EC (respectively SSC and SSC-ZVAD) were analyzed comparatively by 2-dimension liquid chromatography, western blotting and mass spectrometry.  Connective tissue growth factor (CTGF) was the only known fibrogenic factor increased in SSC.  Caspase-3 silencing of EC demonstrated that CTGF is released by apoptotic EC downstream of caspase-3 activation. In fibroblasts, blocking the activation of SFK or silencing the proline-rich tyrosine kinase 2 (Pyk2) blocked myofibroblast differentiation triggered by either SSC or recombinant CTGF in vitro.  Exposure to a pan-transforming growth factor (TGF-β) neutralizing antibody failed to attenuate myofibroblast differentiation in fibroblasts exposed to either SSC or CTGF.  Subcutaneous injection of mouse SSC to C3H mice daily for three weeks led to increased skin thickness, increased protein levels of αSMA, vimentin and collagen I. This fibrogenic response was blunted in mice injected with either SSC-ZVAD or SSC immunodepleted of CTGF.    These results bring new mechanistic insights into the fibrogenic pathways activated by EC death.  Caspase activation in apoptotic EC triggers the production of LG3 and CTGF which in turn activate SFK/PI3K dependant pathways in fibroblasts thus activating a TGF-β-independent fibrogenic response.
Apoptosis Differentiation Endothelial cell Fibroblast Myofibroblast CTGF CTGF Perlecan Pyk2 Pyk2 Src Src PI3K PI3K