Modulation of the mevalonate pathway and cell growth by pravastatin and d-limonene in a human hepatoma cell line (Hep G2).

Modulation of cell growth by a combination of pravastatin [a 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitor] and d-limonene (an inhibitor of protein isoprenylation) was studied using Hep G2, a human hepatoma-derived cell line. Pravastatin, at 0.1 mM, produced 85% inhibition of cholesterol biosynthesis in Hep G2 cells. The combination of 0.1 mM pravastatin and 1.0 mM d-limonene had no further effect on the reduction seen with pravastatin alone. Addition of 0.1 mM pravastatin or 1.0 mM d-limonene did not significantly suppress DNA synthesis by the cells, whereas the combination suppressed it to 50% of the control level. Production of m-p21ras was markedly decreased to 35% of the control level by the combination of these two inhibitors. Both the reduction by pravastatin of farnesylpyrophosphate as substrate for protein:farnesyl transferase and inhibition of protein farnesylation by d-limonene seem to be responsible for the profound suppression of m-p21ras formation in the cells. However, dolichol synthesis was not suppressed by the combination of these inhibitors. In human fibroblasts, the combination suppressed m-p21ras production but not DNA synthesis. These findings suggest that the combination of pravastatin and d-limonene acts on cancer cell growth through inhibition of the post-translational processing of cellular proteins including p21ras, rather than through the suppression of cholesterol and dolichol biosynthesis. Thus, the combination of an HMG-CoA reductase inhibitor and an inhibitor of protein isoprenylation offers potential as a new approach for cancer therapy.