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GROWTH, DIFFERENTIATION, AND APOPTOSIS

G₁₃-mediated transformation and apoptosis are permissively dependent on basal ERK activity

Departments of Pharmacology and Anesthesiology, University of Illinois, Chicago, Illinois 60612

Submitted 25 March 2003 ; accepted in final form 1 May 2003

We previously reported that the -subunit of heterotrimeric G₁₃ protein induces either mitogenesis and neoplastic transformation or apoptosis in a cell-dependent manner. Here, we analyzed which signaling pathways are required for G₁₃-induced mitogenesis or apoptosis using a novel mutant of G₁₃. We have identified that in human cell line LoVo, the mutation encoding substitution of Arg260 to stop codon in mRNA of G₁₃ subunit produced a mutant protein (G₁₃-T) that lacks a COOH terminus and is endogenously expressed in LoVo cells as a polypeptide of 30 kDa. We found that G₁₃-T lost its ability to promote proliferation and transformation but retained its ability to induce apoptosis. We found that full-length G₁₃ could stimulate Elk1 transcription factor, whereas truncated G₁₃ lost this ability. G₁₃-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 G₁₃-induced Elk1 stimulation, whereas JNK inhibitor SB-203580 was ineffective. In Rat-1 fibroblasts, G₁₃-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 G₁₃-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 G₁₃-T could stimulate Src kinase activity. Moreover, G₁₃-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 G₁₃-mediated cell proliferation and transformation. Importantly, truncated G₁₃ retained its ability to stimulate apoptosis signal-regulated kinase ASK1 and c-Jun terminal kinase, JNK. Interestingly, the apoptosis induced by G₁₃-T was inhibited by dominant-negative ASK1 or by SB-203580.

mitogen-activated kinase; effector mutant; Src kinase

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