EXPRESSION OF Q227L-G-ALPHA(S) IN MCF-7 HUMAN BREAST-CANCER CELLS INHIBITS TUMORIGENESIS

The effects of expression of mutant (Q227L)-activated G alpha(s) and e levation of cAMP on mitogen-activating protein kinase (MAPK) activity and the transformed phenotype were studied in the MCF-7 human mammary epithelial cell line, Elevation of cAMP partially inhibited the epider mal growth factor-stimulated DNA synthesis and the intrinsic MAPK (ERK -1 and ERK-2) of serum-starved MCF-7 cells, Addition of 8Br-cAMP or ex pression of mutant (Q227L)-activated G alpha(s) in MCF-7 cells blocked the ability of these cells to grow in an anchorage-independent manner , as assessed by colony formation in soft agar, 8Br-cAMP in the cultur e medium also blocked estrogen stimulation of MCF-7 cell proliferation in vitro, MCF-7 cells expressing Q227L-G alpha(s) grew very slowly in vitro, and when these cells were injected s.c. into athymic mice impl anted with estrogen pellets, the frequency of tumor formation was redu ced greatly and the sizes of the tumors formed were much smaller than those in mice injected with MCF-7 cells that had been transfected with the empty vector, These results indicate that the intracellular level s of cAMP in transformed mammary epithelial cells can be a crucial fac tor in determining the expression of the transformed phenotype, Intera ctions between the G(s)/adenylyl cyclase and MAPK-1,2 signaling pathwa ys could be one mechanism by which expression of the transformed pheno type in mammary epithelial cells are regulated.