Abstract
Hep G2 cells were incubated under conditions known to influence the HMG-CoA (3-hydroxy-3-methylglutaryl-CoA) reductase activity, e.g. in the presence of compactin (a competitive inhibitor of HMG-CoA reductase itself) and U18666A (a squalene-2,3-epoxide cyclase inhibitor). We studied the effects of these conditions both on the HMG-CoA reductase activity and on the reductase mRNA content. In the presence of compactin the mRNA content increased, but less than the enzyme activity, as determined after removal of the inhibitor. The increase in mRNA could be prevented by addition of mevalonate or by a combination of low-density lipoprotein (LDL) plus a low concentration of mevalonate. LDL alone prevented the compactin-induced increases in mRNA and activity only partially. The effect of U18666A on reductase mRNA content and activity was biphasic, i.e. a slight decrease at low (0.3-0.5 microM) concentrations, with a concomitant formation of polar sterols [Boogaard, Griffioen & Cohen (1987) Biochem. J. 241, 345-351], and an increase at high (20-30 microM) concentrations, with complete blockage of sterol formation. At these high concentrations of U18666A, additional compactin (2 microM) increased the reductase activity, but not the mRNA content. We conclude that non-sterol metabolites of mevalonate regulate exclusively at the enzyme level, whereas sterol metabolites regulate at the reductase mRNA level. In the latter group of regulators we distinguish mevalonate metabolites which can, and metabolites which cannot, be replaced by exogenous LDL.