Transactivation of platelet-derived growth factor receptor at by the GTPase-deficient activated mutant of Gα12

The GTPase-deficient, activated mutant of G alpha(12) (G alpha(12)Q229L, or G alpha(12)QL) induces neoplastic growth and oncogenic transformation of NIH 3T3 cells. Using microarray analysis, we have previously identified a role for platelet-derived growth factor receptor alpha (PDGFR alpha) in G alpha(12)-mediated cell growth (R. N. Kumar et al., Cell Biochem. Biophys. 41:63-73, 2004). In the present study, we report that G alpha(12)QL stimulates the functional expression of PDGFR alpha and demonstrate that the expression of PDGFR alpha by G alpha(12)AL is dependent on the small GTPase Rho. Our results indicate that it is cell type independent as the transient expression of G alpha(12)QL or the activation of G alpha(12)-coupled receptors stimulates the expression of PDGFR alpha in NIH 3T3 as well as in human astrocytoma 1321N1 cells. Furthermore, we demonstrate the presence of an autocrine loop involving PDGF-A and PDGFR alpha in G alpha(12)QL-transformed cells. Analysis of the functional consequences of the G alpha(12)-PDGFR alpha signaling axis indicates that Got,2 stimulates the phosphatidylinositol 3-kinase (PI3K)-AKT signaling pathway through PDGFR. In addition, we show that G alpha(12)QL stimulates the phosphorylation of forkhead transcription factor FKHRL1 via AKT in a PDGFR alpha- and PI3K-dependent manner. Since AKT promotes cell growth by blocking the transcription of antiproliferative genes through the inhibitory phosphorylation of forkhead transcription factors, our results describe for the first time a PDGFR alpha-dependent signaling pathway involving PI3K-AKT-FKHRL1, regulated by G alpha(12)QL in promoting cell growth. Consistent with this view, we demonstrate that the expression of a dominant negative mutant of PDGFR alpha attenuated G alpha(12)-mediated neoplastic transformation of NIH 3T3 cells.