Mechanistic study of the anti-cancer effect of Gynostemma pentaphyllum saponins in Rat 6 fibroblast cell system.

摘要

The medicinal herb, Gynostemma pentaphyllum (Gp) has been shown to have anti-cancer effects. However, the precise mechanism is not known. The key active components of Gp are a group of saponins, structurally related to ginsenosides. In the present study, the anti-cancer effect of Gp saponins was assessed by testing their abilities to prevent the formation of foci induced by an activated c-Ha-ras oncogene in normal Rat 6 fibroblasts. The results indicated that the Gp saponins inhibited ras-induced foci in dosage and time dependent manners. To facilitate the investigation of the mode of inhibition of Gp in living cells, a green fluorescent protein-ras fusion construct (pGFP-ras ) was used to substitute ras in the focus formation assay. Gp saponins posted a strong inhibition against the growth of the ras-transformed cells that were co-cultivated with normal R6 cells. The data strongly suggest that Gp might stimulate R6 cells to secret factor(s) that presumably has growth inhibitory effect against the transformed cells. Indeed, an active fraction against the transformed cells has been identified from the conditioned medium collected from Gp-treated R6 cultures. The bioactive component(s) seems to be heat-labile, with molecular weight larger than 14,000. The level of Raf-1 protein was sharply down-regulated within 2 days of Gp treatment. Further investigation indicated that Gp treatment induced instability, instead of transcriptional inactivation of the Raf-1 expression. Besides Raf-1, the immediate downstream gene, Erk seemed to be dephosphorylated upon Gp treatment, while the total protein remained unchanged. Four genes: beta2-microglobulin, GST7-7, gelatinase A and cathepsin L were up-regulated, while three genes: Erk-1, gammaIGFBP-6, and 14-3-3 zeta were down-regulated upon treatment with Gp saponins. The finding that an anti-cancer effect of a non-toxic drug may be mediated through the surrounding normal cells is conceptually novel and should have a broad implication in the future development of drugs or dietary supplements with cancer prevention function. (Abstract shortened by UMI.)