In recent years, considerable emphasis is placed on identifying new cancer chemopreventive agents which could be useful for human population. In this regard, the identification of better anti-tumor promoting agents is highly desired since these agents appear to have greater relevance in preventing against the development of neoplasms. Silymarin, an antioxidant flavonoid isolated from artichoke, has been shown to possess significant protective effects against hepatotoxicity and other disorders. Studies employing cell or organ culture systems have suggested that silymarin may possess anti- tumor promoting effects. In a recent study, we observed that silymarin significantly inhibits 12-O-tetradecanoylphorbol-13-acetate (TPA)- and other skin tumor promoter-induced epidermal ornithine decarboxylase (ODC) activity, and TPA-induced epidermal ODC mRNA expression. Primary studies also showed that topical application of silymarin prior to that of TPA affords significant protection against tumor promotion in terms of both tumor incidence and tumor multiplicity in SENCAR mouse skin. Together, these studies strengthen the possibility that silymarin could be a significantly useful anti-tumor promoting agent. Our preliminary data also show that in cell culture system silymarin inhibits farnesyltransferase (FTase) activity concomitant with the decrease in Ha-ras p21 membrane localization, and was not inhibitory to 3-hydroxy-3-methylglutaryl coenzyme A reductase. Silymarin also showed selective inhibition of the growth of vHa-ras oncogene transformed NIH3T3 cells compared to normal NIH3T3 cells. Collectively, cell culture studies suggested that silymarin selectively inhibits FTase and thereby ras p21 farnesylation leading to a significant decrease in the growth of activated ras oncogene carrying cells. In this proposal, we will extend these studies to evaluate in detail the anti-tumor promoting effects of silymarin at cellular, biochemical, and molecular levels. Studies will be performed to assess: i) the dose-dependent protective effect of topical application of silymarin against TPA- and okadaic acid-caused skin tumor promotion in SENCAR mice; ii) the inhibitory effect of silymarin against the cellular, biochemical and molecular changes associated with tumor promotion; iii) whether silymarin is a stage specific inhibitor of tumor promotion; and iv) the inhibitory effect of silymarin on FTase mRNA expression, FTase activity and Ha-ras p21 membrane localization during murine skin tumorigenesis. Studies will also be performed to assess the selectivity of silymarin in inhibiting FTase mRNA expression, FTase activity, Ha-ras p21 membrane localization and cell growth employing SP-1 (carry activated Ha-ras oncogene) and PA (not carry activated Ha-ras oncogene) cell lines derived from DMBA- TPA induced papillomas in SENCAR mouse skin. The results of the proposed studies will identify a new agent with significant anti-tumor promoting effect, and will define in particular the Ha-ras mediated mechanism of such protective effect.
