Identification of a common regulatory pathway that determines cell survival and cell cycle progression in cardiac fibroblasts

Relative resistance to apoptosis and the ability to proliferate and produce a collagen-rich scar determine the critical role of cardiac fibroblasts in wound healing and tissue remodeling following myocardial injury. Identification of cardiac fibroblast-specific factors and mechanisms underlying these aspects of cardiac fibroblast function is therefore of considerable scientific and clinical interest. In the present study, gene knockdown and over-expression approaches, and promoter binding assays, showed that DDR2, a mesenchymal cell-specific collagen receptor tyrosine kinase localized predominantly in fibroblasts in the heart, acts via ERK1/2 MAPK-activated SRF transcription factor to enhance the expression of anti-apoptotic cIAP2 in cardiac fibroblasts, conferring resistance against oxidative injury. Further, DDR2 was found to act via ERK1/2 MAPK-activated SRF to transcriptionally up regulate Skp2 that in turn facilitated post-translational degradation of p27, the cyclin dependent kinase inhibitor that causes cell cycle arrest, to promote G1-S transition, as evidenced by Rb phosphorylation, increased PCNA levels and flow cytometry. DDR2-dependent ERK1/2 MAPK activation also suppressed FoxO3a-mediated transcriptional induction of p27. Inhibition of the binding of collagen type I to DDR2 using WRG-28 indicated the obligate role of collagen type I in the activation of DDR2 and its regulatory role in cell survival and cell cycle protein expression. Notably, DDR2 levels positively correlated with SRF, cIAP2 and PCNA levels in cardiac fibroblasts from Spontaneously Hypertensive Rats. To conclude, DDR2-mediated ERK1/2MAPK activation facilitates coordinated regulation of cell survival and cell cycle progression in cardiac fibroblasts via SRF. New & NoteworthyRelative resistance to apoptosis and the ability to proliferate and produce a collagen-rich scar enable cardiac fibroblasts to play a central role in myocardial response to injury. This study reports novel findings that mitogen-stimulated cardiac fibroblasts exploit a common regulatory mechanism involving collagen receptor (DDR2)-dependent activation of ERK1/2 MAPK and SRF to achieve coordinated regulation of apoptosis resistance and cell cycle progression, which could facilitate their survival and function in the injured myocardium.