Mutant alpha subunit of Gz transforms Swiss 3T3 cells

Many hormones and neurotransmitters induce cell proliferation by regulating signaling pathways controlled by heterotrimeric G proteins. Mutations that activate the alpha subunits of Gs and Gi2 produce the gsp and gip2 oncogenes that are found in certain human endocrine tumors. Similar mutations have conferred on other G alpha subunits the ability to promote neoplastic transformation in cultured mammalian cells. Gz, a G protein whose normal signaling function is poorly understood, shares with Gi2 the ability to inhibit adenylyl cyclase. We asked whether mutationally activated alpha z can stimulate cell proliferation in a cell line in which stimulation adenylyl cyclase is mitogenic, Swiss 3T3 cells. Stable expression of alpha z-Q205L in Swiss 3T3 cells induced focus formation, a faster growth rate with a higher saturation density, anchorage-independent growth in soft agar, and increased [3H]thymidine incorporation in the absence of growth factors. alpha z-Q205L produced a similar but less extensively transformed phenotype in NIH3T3 cells--increased saturation density in culture, a smaller number of foci and few colonies in soft agar. Stimulation of thymidine incorporation by alpha z-Q205L in Swiss 3T3 cells was increased by co-treatment with cholera toxin, a stimulator of adenylyl cyclase. Taken together, our results indicate that alpha z stimulates one or more mitogenic pathways in Swiss 3T3 cells, and that effectiveness of these mitogenic pathways does not require reducing the concentration of cellular cAMP.