Geranylgeraniol restores cell growth and Ras processing in mevalonate-depleted fibroblast cells

Eukaryotic cells are dependent upon the availability of mevalonate for normal growth. This requirement supplies mevalonate-derived isoprenoids that are necessary for the posttranslational modification of a large number of growth-regulating proteins, including those of the Ras family. Recently, metabolic labeling experiments have suggested that some mammalian cells can utilize geranylgeraniol (GGOH) and farnesol (FOH) for protein isoprenylation although the metabolic pathway underlying this phenomenon is not described. To further characterize isoprenoid metabolism we incubated wild-type and H-ras transfected (EJ-6-2-Bam-6a) NIH 3T3 cells with lovastatin, an inhibitor of hydroxymethylglutaryl coenzyme A reductase that depletes cells of mevalonate, for 24 hours. In both cell types proliferation, assessed as radiolabeled thymidine incorporation into DNA, was impaired at lovastatin levels greater than 1 μM in a concentration-dependent fashion; 5 mM mevalonate restored cell proliferation. The growth inhibition induced by 100 μM lovastatin was reversed by 5 μM GGOH, but not by FOH. The impairment of Ras isoprenylation, determined by Western blot analysis of fractionated cell lysate using a pan-anti-Ras antibody linked to a horseradish peroxidase detection system, by 100 μM lovastatin was partially reversed by 50 μM GGOH, but not by FOH, in NIH 3T3 cells only. These results are consistent with the existence of a metabolic pathway for deriving isoprenoid intermediates from GGOH but suggest that differences in this pathway exist between wild-type and H-ras transformed NIH 3T3 cells.