There is a lowered risk for a variety of cancers found among populations consuming large amounts of plant foods. Our overall objective is to test the hypothesis that suppression of tumor associated 3-hydroxy-3- methylglutaryl coenzyme A (HMG CoA) reductase by mevalonate-derived end products of plant metabolism (collectively called isoprenoids) is a major determinant for this reduced risk of developing cancer. HMG CoA reductase, the major rate-limiting component of mevalonate pathway activities, is essential for the growth of all cells and is subject to multivalent feedback regulation. Tumor cell growth is associated with elevated reductase activity due to an attenuation of sterol-feedback regulation. However, tumor associated reductase activity is uniquely sensitive to isoprenoid-mediated suppression at the posttranscriptional level. It is proposed in this study that decreased levels of a mevalonate-derived isoprenoid, farnesyl diphosphate, induces cell cycle arrest at the G2/M interface. This cell cycle arrest initiates apoptotic cell death in tumors.
Aim I will determine if there is a tumor-specific aberration at the transcriptional or post transcriptional level of regulation which permits increased reductase expression in B16 melanomas and in human CEM leukemic and Hep G2 cells. The molecular basis of tumor reductase regulation at the transcriptional level will be examined in two ways: first, by comparing methylation states of reductase genes in tumor and normal cells using methylation-sensitive restriction endonucleases, and secondly by in vivo protein/promoter region sequence interactions in both tumor and normal cells utilizing ligation-mediated PCR analysis. Posttranscriptional regulation will be investigated by comparing heterogeneity in reductase 5'-untranslated regions in tumor versus normal cells and the distribution of heterogeneous reductase 5'-untranslated region in the polysome fractions in tumor and normal cells.
Aim 2 explores the biochemical consequences of isoprenoid-mediated suppression of mevalonate synthesis (HMG CoA reductase activity) in tumor cells. Mevalonate-derived farnesyl diphosphate is essential for the maturation of nuclear lamins. Impairment of posttranslational processing (farnesylation) interferes with cell cycle progression and diverts cells towards apoptosis. Recognizing that it is unlikely that common dietary practices permit the consumption of individual isoprenoids in tumor- suppressive quantity, these studies explore the possibility that diverse isoprenoids acting synergistically through a common action have tumor- suppressive activity at dietary relevant levels of intake. These analyses offer a novel rationale for understanding the unique sensitivity of tumor cells to isoprenoid constituents of the diet. Such information will add support to dietary guidelines that stress the role of plant foods in reducing cancer risk.
