Systematic evaluation of anti-apoptotic growth factor signaling in vascular smooth muscle cells. Only phosphatidylinositol 3'-kinase is important.

ABSTRACT

Peptide growth factors contribute to the pathogenesis of cardiovascular diseases by inducing a variety of cellular responses including anti-apoptotic effects. Several of the signaling molecules that are activated by growth factor receptors such as Src family kinases (Src), phosphatidylinositol 3'-kinase (PI3K), phospholipase Cgamma (PLCgamma), Ras, and SHP-2 were shown to mediate survival signals. We systematically investigated the relative contribution of each signaling molecule for growth factor-dependent cell survival in vascular smooth muscle cells (VSMC). Our approach was the use of mutated plateletderived growth factor (PDGF) beta-receptors (betaPDGFR) in which the tyrosine residues required for binding of each signaling molecule were individually mutated to phenylalanine. To bypass endogenous PDGFR in VSMC we used chimeric receptors (ChiRs), containing the extracellular domain of the macrophage colony-stimulating factor (M-CSF) receptor and the cytoplasmic domain of the wild type (WT) or mutated betaPDGFR. Selective activation of the ChiR-WT with M-CSF significantly reduced apoptosis to the same extent as PDGF-BB in non-transfected cells. Deletion of the binding site for PI3K, but not for Src, RasGAP, SHP-2, or PLCgamma, completely abolished the anti-apoptotic effect. Consistently, a ChiR mutant that only binds PI3K was fully able to mediate cell survival as efficiently as the ChiR-WT. Furthermore, the PDGF-dependent anti-apoptotic effect in non-transfected cells was completely abolished by the PI3K inhibitor wortmannin, whereas inhibitors of Src, PLCgamma, ERK, or p38 MAP kinase had no effect. The exploration of downstream signaling events revealed that PDGF-BB activates the anti-apoptotic Akt signaling pathway in a PI3K-dependent manner. Moreover, Akt phosphorylates and thus inactivates the pro-apoptotic proteins BAD and Forkhead transcription factors (FKHR, FKHRL1). We conclude that growth factor-dependent cell survival in VSMC is mediated only by activation of the PI3K/Akt pathway, whereas all other receptor-associated signaling molecules do not play a significant role.