Transactivation of Platelet-Derived Growth Factor Receptor α by the GTPase-Deficient Activated Mutant of Gα₁₂

Abstract

Heterotrimeric G proteins regulate diverse cellular responses by coupling heptahelical receptors to intracellular effectors (11,25,35). Of the different ␣-subunits that have been analyzed thus far, the ␣-subunit of the heterotrimeric G protein G12 (G␣ 12 ) shows the most potent mitogenic and oncogenic activities (5, 35). The initial identification of G␣ 12 as the transforming oncogene in Ewing's sarcoma cell lines indicated the critical role of G␣ 12 in oncogenic signaling (5). Consistent with these observations, serum stimulation or mutational activation of G␣ 12 has been shown to stimulate mitogenic pathways in different cell types in addition to inducing neoplastic transformation of 35). NIH 3T3 cells transformed by the GTPase-deficient activated mutant of G␣ 12 (G␣ 12 Q229L, or G␣ 12 QL) show the characteristic oncogenic phenotype defined by the increased proliferation, anchorage-independent growth, reduced growth factor dependency, attenuation of apoptotic signals, and neoplastic cytoskeletal changes (35). Previously, we have shown that the neoplastic growth of NIH 3T3 cells mediated by the activated mutant G␣ 12 involves the expression several unique genes, including platelet-derived growth factor receptor ␣ (PDGFR␣) (25). This finding is of critical interest since the signaling pathways involving PDGFR have been strongly correlated with cell proliferation and neoplastic transformation (18), thus pointing to a possible role of PDGFR␣ in G␣ 12 QL-mediated oncogenic signaling pathways.The receptor kinases PDGFR␣ and PDGFR␤ are activated by dimers of PDGF isoforms, PDGF-A, PDGF-B, PDGF-C, and PDGF-D (18). Distinct homo-or heterodimers of PDGFs activate specific homo-or heterodimers of PDGFR␣ and PDGFR␤ in a tissue-specific and context-specific manner (18). The activation of PDGFRs, leading to their dimerization and transphosphorylation at specific tyrosine residues, provides docking sites for different effector molecules such as Shp, phospholipase C-gamma, and phosphatidylinositol 3-kinase (PI3K) (18). Overexpression of PDGFR␣ or PDGFR␤ and the associated autocrine signaling pathways appear to play an etiological role in tumorigenesis and tumorangiogenesis of different neoplasms including basal cell carcinoma (45), gastrointestinal stromal tumors (17), and ovarian cancers (19,30). It has also been observed that PDGFR, specifically PDGFR␤, is transactivated by G protein-coupled receptors (GPCRs) stimulated by lysophosphatidic acid (LPA) (20), sphingosine-1 phosphate (2), serotonin (33), and angiotensin II (39). However, the underlying signaling mechanisms and the role of the specific G proteins in mediating such transactivation are not fully understood.In the present study, we demonstrate that G␣ 12 QL stimulates the expression and transactivation of PDGFR␣ in NIH 3T3 cells. The ability of transiently expressed G␣ 12 QL to stimulate the expression of PDGFR␣ in 1321N1 astrocytoma cells indicates that such a nexus between G␣ 12 and PDGFR␣ is not restricted to one specific cell type. Our results indicate that...

G protein-coupled receptor 56 regulates mechanical overload-induced muscle hypertrophy

“…Downstream signaling through Gα13/Rho has been associated with muscle hypertrophy in pressure-overloaded cardiac muscle (10), supporting the concept that Gα12/13 is a mechanosensitive anabolic pathway. Moreover, Gα12 signaling has been shown to activate the PI3K/Akt pathway in 3T3 cells (22), supporting our findings and confirming the Gα12/13 signaling as a regulatory pathway for mTOR. The loss of GPR56 could limit activation of Gα12/13 signaling during overload and prevent full activation of mTOR, similar to our observations in overloaded Gpr56-KO muscle (see model in Fig.…”

Human Bone Marrow Activates the Akt Pathway in Metastatic Prostate Cells through Transactivation of the α-Platelet–Derived Growth Factor Receptor

“…This would corroborate the idea that the PDGF-AA and PDGF-BB we detected in the bone marrow aspirates provide a negligible contribution to the activation of a-PDGFR in bone-metastatic prostate cells, and therefore confirm the concept of a-PDGFR transactivation. The neutralization of a-PDGFR in the absence of internalization would also exclude that additional isoforms of PDGF (i.e., PDGF-AB and PDGF-CC) could be present in the bone marrow aspirates and activate a-PDGFR in PC3-ML cells as well as rule out the possibility of an autocrine stimulation loop, which has previously been shown as a mechanism of a-PDGFR transactivation (19).…”

Lysophosphatidic Acid Stimulates the Proliferation of Ovarian Cancer Cells via the gep Proto-Oncogene G 12

“…Our results indicate that LPA specifically stimulates the proliferation of ovarian cancer cell lines but not the nonmalignant IOSE or HOSE cell lines. Such LPA-mediated proliferation of ovarian cancer cells is inhibited by the expression of a competitively inhibitory minigene of Gα 12 that is known to disrupt receptor-Gα gep proto-oncogene family, 7 in transmitting LPA-mediated proliferation signals to ovarian cancer cells. Thus, our studies establish for the first time an unequivocal role for LPA and Gα 12 signaling axis in ovarian cancer cell proliferation.…”