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ARTICLE INFO Keywords: Saponin Trillium tschonoskii Hepatocellular carcinoma Chemotherapy Multidrug resistance ABSTRACT Combating with multidrug resistance (MDR) is a major part of hepatocellular carcinoma (HCC) chemotherapy. Steroidal saponin from Trillium [...]

The effects of expression of mutant (Q227L)activated G[[Alpha].sub.s] and elevation 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 epidermal 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 expression of mutant (Q227L)activated G[[Alpha].sub.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 culture medium also blocked estrogen stimulation of MCF-7 cell proliferation in vitro. MCF-7 cells expressing Q227L-G[[Alpha].sub.s] grew very slowly in vitro, and when these cells were injected s.c. into athymic mice implanted with estrogen pellets, the frequency of tumor formation was reduced 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 levels of cAMP in transformed mammary epithelial cells can be a crucial factor in determining the expression of the transformed phenotype. Interactions between the [G.sub.s]/adenylyl cyclase and MAPK-1,2 signaling pathways could be one mechanism by which expression of the transformed phenotype in mammary epithelial cells are regulated.

Conversion of external signals into proliferative responses may be mediated by interactions between signaling pathways that control cell proliferation. Interactions between Gα.sub.a], th α subunit of the heterotrimeric guanine nucleotide binding protein that stimulates adenylyl cyclase, and Ras, an important element in growth factor signaling, were studied. Expression of activated G[α.sub.s], in NIH 3T3 cells increased intracellular concentrations of adenosine 3',5'-monophosphate (cAMP) and inhibited H-Ras-stimulated DNA synthesis and mitogen-activated protein kinase activity. Activated Gas and 8-Br-cAMP suppressed H-Ras-induced transformation of NIH 3T3 cells. Apparently, G[α.sub.s] inhibits proliferative signals from Ras by stimulating cAMP production and activating protein kinase A., Many heterotrimeric guanine nucleotide binding proteins (G proteins) participate in mitogenic signaling (1). Of these, the [G.sub.s] and its signaling pathway are probably the most enigmatic. In a few systems, [...]