Mitogenic signaling by lysophosphatidic acid (LPA) involves Gα12

Lysophosphatidic acid (LPA), a major G protein coupled receptor(GPCR)-activating ligand present in serum, elicits growth factor like responses by stimulating specific GPCRs coupled to heterotrimeric G proteins such as G(i), G(q), and G(12/13). Previous studies have shown that the overexpression of wild-type G alpha(12) (G alpha 12WT) results in the oncogenic transformation of NIH3T3 cells (G alpha 12WT-NIH3T3) in a serum-dependent manner. Based on the potent growth-stimulating activity of LPA and the presence of LPA and LPA-like molecules in the serum, we hypothesized that the serum-dependent neoplastic transformation of G alpha 12WT-NIH3T3 cells was mediated by the stimulation of LPA-receptors (LPARs) by LPA in the serum. In the present study, using guanine nucleotide exchange assay and GST-TPR binding assay, we show that the treatment of G alpha 12WT-NIH3T3 with 2 mu M LPA leads to the activation of G alpha(12). Stimulation of these cells with LPA promotes JNK-activation, a critical component of G alpha(12)-response and cell proliferation. We also show that LPA can substitute for serum in stimulating JNK-activity, DNA synthesis, and proliferation of G alpha 12WT-NIH3T3 cells. LPA-mediated proliferative response in NIH3T3 cells involves G alpha(12), but not the closely related G alpha(13). Pretreatment of G alpha 12WT-NIH3T3 cells with suramin (100 mu M), a receptor-uncoupling agent, inhibited LPA-stimulated proliferation of these cells by 55% demonstrating the signal coupling between cell surface LPAR and Ga-12 in the neoplastic proliferation of NIH3T3 cells. As LPA and LPAR mediated mitogenic pathways have been shown to play a major role in tumor genesis and progression, a mechanistic understanding of the signal coupling between LPAR, Ga-12, and the downstream effectors is likely to unravel additional targets for novel cancer chemotherapies.