Lovastatin and simvastatin are modulators of the proteasome

Lovastatin and simvastatin are HMG-CoA reductase inhibitors widely used as antihyperlipidemic drugs, which also display antiproliferative properties. In the present paper, we provide evidence that both lovastatin and simvastatin are modulators of the purified bovine pituitary 20 S proteasome, since they mildly stimulate the chymotrypsin-like activity and inhibit the peptidylglutamylpeptide hydrolyzing activity without interfering with the trypsin-like activity. However, those effects are only observed when the closed ring forms of the drugs are used, while the opened ring form of lovastatin acts as a mild inhibitor of the chymotrypsin like activity. The closed ring form of lovastatin is much more potent as a cytotoxic agent on the Colon-26 (C-26) colon carcinoma cell line than the opened ring form, which is only mildly cytostatic. Moreover, neither the cytotoxic effects nor the effects on 20 S proteasome activities are prevented by mevalonate, which by itself inhibits the trypsin-like activity of the proteasome. Neither the opened ring nor the closed ring form of lovastatin induces an accumulation of ubiquitin-protein conjugates, which is observed after treatment with lactacystin, a selective proteasome inhibitor. In contrast with the opened ring form of lovastatin, the closed ring form induces the disappearance of detectable p27(kip1) from C-26 cells. Altogether, our results indicate that the closed ring form of lovastatin induces cytotoxic effects independent of its HMG-CoA inhibiting activity, however, those effects are mediated by a complex modulation of proteasome activity rather than by inhibition of the 20 S proteasome.