Differential effects of monoterpenes and lovastatin on RAS processing

Limonene and related monoterpenes have been shown to impair the incorporation of mevalonic acid-derived isoprene compounds, that is farnesyl pyrophosphate, into RAS and RAS-related proteins. As farnesylation is critical for RAS's membrane localization and function, the isoprenylation pathways have received attention as potential targets of anti-RAS pharmacologic maneuvers. We have expanded on these prior studies and demonstrate that one of limonene's metabolic derivatives, perillyl alcohol, decreases the levels of antigenic RAS in the human-derived myeloid THP-1 and lymphoid RPMI-8402 cell lines. Both limonene and perillyl alcohol decrease levels of [35S]methionine-labeled RAS proteins in cells that have been pulsed with radiolabeled methionine for 4 h. In contrast, lovastatin, which inhibits hydroxymethylglutaryl coenzyme A reductase and thus depletes cells of farnesyl pyrophosphate, does not diminish levels of total antigenic RAS but rather results in a shift in the RAS protein; levels of farnesylated RAS decrease whereas levels of unmodified/unfarnesylated RAS increase. As limonene and perillyl alcohol do not induce such a shift, we conclude that these monoterpenes decrease farnesylated RAS protein levels by a mechanism that is clearly distinct from that of either depleting cells of farnesyl pyrophosphate or inhibiting the enzyme farnesyl protein transferase that catalyzes the post-translational farnesylation of RAS. Perillyl alcohol decreases antigenic RAS levels but does not decrease levels of another membrane-tethered protein, the alpha subunit of the heterotrimeric G protein. Furthermore, perillyl alcohol decreases the levels of radiolabeled methionine incorporated into immunoprecipitable RAS to a greater extent than it decreases radiolabeled methionine incorporated into total cellular protein. Thus there is some degree of specificity for the activity of perillyl alcohol to depress RAS levels.