Isoprenylation of RhoB is necessary for its degradation -: A novel determinant in the complex regulation of RhoB expression by the mevalonate pathway

Statins improve vascular functions by mechanisms independent from their cholesterol-lowering effect. Rho GTPases are emerging as key targets for the vascular effects of statins. RhoB is a short-lived, early-response inducible protein involved in receptor endocytosis, apoptosis, and gene expression. Here we show that statins regulate RhoB expression by acting at multiple levels. Simvastatin increased RhoB protein levels by 8- to 10-fold. This effect was related to a depletion of isoprenoid intermediates, as deduced from the observation that several metabolites of the cholesterol biosynthetic pathway, namely, mevalonate and geranylgeranyl-pyrophosphate, attenuated simvastatin-induced RhoB up-regulation. Moreover, prenyltransferase inhibitors mimicked simvastatin effect. Cholesterol supplementation did not prevent simvastatin-elicited up-regulation but increased RhoB levels per se. Simvastatin moderately augmented RhoB transcript levels, but markedly impaired the degradation of RhoB protein, which accumulated in the cytosol in its non-isoprenylated form. Inhibition of RhoB isoprenylation was apparently required for simvastatin-induced up-regulation, because levels of an isoprenylation-deficient RhoB mutant were not affected by simvastatin. Moreover, this mutant was found to be markedly more stable than the wild-type protein. These results show that RhoB isoprenylation is necessary for rapid turnover of this protein and identify a novel link between the cholesterol biosynthetic pathway and the regulation of G-protein expression.