There is substantial evidence which associates murine and human skin carcinogenesis with point mutations in ras oncogene that result in its activation. In recent years, therefore, there has been considerable interest in elucidating the functioning of ras in the induction of cell transformation with an approach that this might be useful in the identification of novel anticancer agents. Ras oncogenes encode a 21 kDa binding protein termed p21 which transforms mammalian cells only after subcellular localization at the inner side of the plasma membrane. This process requires at least three post-transnational modifications which mainly occur at the carboxyl terminus of p21 which has a consensus motif Cys-Aaa-Aaa-Xaa and is known as the CAAX box. The major post- transnational modification of p21 is the transfer of a farnesyl group to the cysteine residue of the CAAX box by a specific cytosolic enzyme farnesyltransferase (FTase). Identification and characterization of FTase, therefore, provides a useful starting point for the development of potential chemical inhibitors of carcinogenesis. These inhibitors could interfere with p21 processing and thereby could selectively inhibit its function without completely inhibiting overall cellular activities. In preliminary studies we observed that 1) all trans retinoic acid (tRA) protects against malignant conversion of papillomas to carcinomas in murine skin, and 2) FTase is involved in the conversion of papillomas to malignant carcinomas during skin carcinogenesis. Based on these studies, we hypothesize that chemopreventive effects of tRA against cutaneous malignancy may involve the inhibition of p21 processing by 1) inhibiting the active site of the enzyme, and/or 2) functioning as a pseudo substrate with farnesyl pyrophosphate and/or p21. The objective of this proposal is to study the kinetic of inhibition of FTase activity by structurally-related retinoids employing purified FTase from chemically- induced murine skin tumors, and to assess the role and effectiveness of retinoids in inhibiting the FTase activity and the processing of p21 in Ha-ras transformed cell line. Studies in the present Pilot and Feasibility Application are designed as a starting point to test our hypothesis, and its verification could aid in the development of new strategies for the intervention of cutaneous malignancy.
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