Gα13-mediated transformation and apoptosis are permissively dependent on basal ERK activity

We previously reported that the alpha-subunit of heterotrimeric G(13) protein induces either mitogenesis and neoplastic transformation or apoptosis in a cell-dependent manner. Here, we analyzed which signaling pathways are required for Galpha(13)-induced mitogenesis or apoptosis using a novel mutant of Galpha(13). We have identified that in human cell line LoVo, the mutation encoding substitution of Arg260 to stop codon in mRNA of Galpha(13) subunit produced a mutant protein (Galpha(13)-T) that lacks a COOH terminus and is endogenously expressed in LoVo cells as a polypeptide of 30 kDa. We found that Galpha(13)-T lost its ability to promote proliferation and transformation but retained its ability to induce apoptosis. We found that full-length Galpha(13) could stimulate Elk1 transcription factor, whereas truncated Galpha(13) lost this ability. Galpha(13)-dependent stimulation of Elk1 was inhibited by dominant-negative extracellular signal-regulated kinase (MEK) but not by dominant-negative MEKK1. Similarly, MEK inhibitor PD-98059 blocked Galpha(13)-induced Elk1 stimulation, whereas JNK inhibitor SB-203580 was ineffective. In Rat-1 fibroblasts, Galpha(13)-induced cell proliferation and foci formation were also inhibited by dominant-negative MEK and PD-98059 but not by dominant-negative MEKK1 and SB-203580. Whereas Galpha(13)-T alone did not induce transformation, coexpression with constitutively active MEK partially restored its ability to transform Rat-1 cells. Importantly, full-length but not Galpha(13)-T could stimulate Src kinase activity. Moreover, Galpha(13)-dependent stimulation of Elk1, cell proliferation, and foci formation were inhibited by tyrosine kinase inhibitor, genistein, or by dominant-negative Src kinase, suggesting the involvement of a Src-dependent pathway in the Galpha(13)-mediated cell proliferation and transformation. Importantly, truncated Galpha(13) retained its ability to stimulate apoptosis signal-regulated kinase ASK1 and c-Jun terminal kinase, JNK. Interestingly, the apoptosis induced by Galpha(13)-T was inhibited by dominant-negative ASK1 or by SB-203580.