Induction of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity in human fibroblasts incubated with compactin (ML-236B), a competitive inhibitor of the reductase.

The two compounds compactin and ML-236B are identical fungal metabolites isolated from strains of Penicilliunz brevicompactum and Penicillium citrinum, respectively. ML-236B has been shown to be a potent competitive inhibitor of rat liver microsomal 3-hydroxy-3-methylgluiaryl coenzyme A reductase (HMG-CoA reductase), the rate-controlling enzyme in cholesterol biosynthesis (Endo, A., Kuroda, M., and Tanzawa, K. (1976) FEBS Lett. 72, 323-326). In the current studies we demonstrate that compactin is a potent competitive inhibitor of HMG-CoA reductase in extracts of human fibroblasts W, = 1.1 nu). Incubation of intact fibroblasts with 2.6 PM compactin produced a complete but readily reversible inhibition of [Wacetate incorporation into [Wlcholesterol without affecting the incorporation of [Wlmevalonate into [lClcholesterol. The compactin-mediated inhibition of HMG-CoA reductase activity in intact fibroblasts led to the production of large amounts of enzyme that was not active in the cell because it was inhibited by compactin. The presence of this enzyme could be demonstrated, however, by assays of HMG-CoA reductase activity in cell-free extracts under conditions in which the inhibitory effect of compactin was overcome by dilution. The appearance of this latent HMG-CoA reductase activity was prevented by cycloheximide. The latent HMG-CoA reductase that appeared in the presence of compactin could not be suppressed fully by the addition of low density lipoprotein to the culture medium. However, the enzyme could be completely suppressed by the addition of small amounts of mevalonate together with low density lipoprotein. From these data we suggest that incubation of intact fibroblasts with compactin may unmask a new regulatory system for HMG-CoA reductase that does not depend on exogenous cholesterol derived from low density lipoprotein but may depend on an endogenous compound generated from the metabolism of mevalonate.