Regulation of proliferation and Ras localization in transformed cells by products of mevalonate metabolism

Lovastatin, an inhibitor of 3-hydroxy-3-methylglutaryl (HMG) CoA reductase, and 6-fluoromevalonate (Fmev), an inhibitor of diphosphomevalonate decarboxylase, blocked the synthesis of downstream mevalonate products, including prenyl-derived lipids, and prevented membrane localization of Ras in the myeloid cell line U-937. In contrast to lovastatin, which induced cytosol localization of Ras in U-937 cells, Fmev failed to increase cytosolic Ras and also completely prevented the proliferation of U-937 cells. Growth of U-937 cells was restored by the addition of lovastatin to Fmev-blocked cells. These results implied that a product of mevalonate metabolism proximal to isopentenyl diphosphate was responsible for the suppression of proliferation. To delineate the action of this endogenous inhibitor of cell proliferation and determine the relationship between its impact on Ras localization and cell proliferation, the effect of Fmev on a variety of leukemia, and lymphoma-derived cells was examined. Whereas Fmev blocked the growth of these cell lines, there were more than 50-fold differences in the concentrations required to inhibit the growth of individual cell lines by 90%. Regardless of its effect on cell proliferation, the biochemical effect of Fmev was similar. Thus, Fmev uniformly prevented the conversion of radiolabeled mevalonate to isopentenyl diphosphate and other downstream products, including synthesis of sterol and nonsterol lipids and prenylation of proteins. A correlation was noted between higher intrinsic rates of mevalonate synthesis by a cell and susceptibility to inhibition by Fmev. Thus, sensitivity of a cell line to inhibition by Fmev was associated with markedly increased rates of HMG CoA reductase activity that were further increased by incubation with Fmev. Whereas Fmev depleted cellular levels of the prenylated protein Ras in the sensitive cell line U-937, there was no depletion of cellular Ras levels in the resistant cell line EL-4, but rather, there was a shift of Ras from membrane to cytosol, as expected for inhibition of prenylation. These results suggest that leukemic cells with increased HMG CoA reductase activity produce increased levels of an endogenous mevalonate-derived inhibitor that leads to Ras depletion and suppression of cell growth. As a result, inhibition of the growth of these transformed cells might be specifically accomplished by Fmev.