Objective: Basic fibroblast growth factor (bFGF)-stimulated proliferation of coronary smooth muscle cells (cSMC) contributes to the pathogenesis of arteriosclerosis and restenosis. However, the molecular mechanisms involved are not fully understood. We have shown previously that protein kinase C (PKC) and mitogen-activated protein kinase (MAPK) are required for the bFGF-stimulated mitogenic process in bovine cSMC. In this study, we determined the PKC isoform(s) involved and investigated their functional role in the bFGF-stimulated signaling and cell cycle progression in human and bovine cSMC.
Methods and results: Downregulation of PKC by phorbol 12-myristate 13-acetate (PMA) inhibited bFGF-induced DNA synthesis, the activation of MAPK, and the expression of c-myc, demonstrating the involvement of PMA-sensitive PKC isoforms in growth factor-induced proliferation and the MAPK pathway. The PMA-sensitive classical PKC isoforms alpha, beta, gamma and novel PKC isoforms delta and epsilon were found in human cSMC. Whereas blocking of the classical PKC isoforms had no influence, the suppression of PKC delta by genetic and pharmacological approaches inhibited the bFGF-stimulated c-Raf1-MEK-MAPK-c-myc signaling and DNA synthesis in cSMC. In contrast to PKC epsilon, our results showed that bFGF activated PKC delta by phosphorylation in a time-dependent manner. In addition, inhibition of PKC delta induced a hypophosphorylation of the retinoblastoma protein and suppression of the cyclins D1 and A, demonstrating the importance of PKC delta for bFGF-induced cell cycle progression through the G1 phase in cSMC.
Conclusions: Our results show that PKC delta is required for the bFGF-stimulated c-Raf1-MEK-MAPK-c-myc signaling pathway involved in the proliferation of cSMC. Therefore, it may be an interesting therapeutic target for preventing proliferative vascular disorders.