Regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase: search for the enzyme’s repressor derived from mevalonate

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Abstract
Three inhibitors of squalene 2,3-oxide--lanosterol cyclase (AMO 1618, 4,4, 10$\beta $-trimethyl-trans-decal-3$\beta $-ol (TMD) and 2,3-iminosqualene (ISq)) were used to study effects on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, and on sterol and polyprenyl synthesis from [$^{14}$C]acetate and [$^{14}$C]mevalonate in cultured rat hepatoma (H4) cells. After a 4 h exposure of cultures to AMO 1618 or TMD, followed by removal of the inhibitors, the utilization of [$^{14}$C]acetate for synthesis of digitonin-precipitable sterols increased about twofold, an increase parallelled by the rise in HMG-CoA reductase. Mevalonate at 2.3 mM counteracted the effects of these inhibitors on the reductase. When (R)-[2-$^{14}$C] mevalonate at 2.3 mM was included with the two inhibitors in the culture media, the cells were still able to synthesize cholesterol although in lesser amounts than the controls. In the presence of TMD the H4 cells also accumulated [$^{14}$C]squalene 2,3-oxide and [$^{14}$C]squalene 2,3-22,23-dioxide. ISq added to cells kept in full-growth medium (10 $\mu $g ml $^{-1}$) caused an almost complete and irreversible inactivation of the squalene oxide-lanosterol cyclase but did not inhibit polyprenyl synthesis, as the amount of [$^{14}$C]mevalonate converted into squalene, squalene 2,3-oxide, squalene 2,3-22,23-dioxide plus a little cholesterol was equal to the amount converted by control cells into cholesterol plus squalene. After a 24 h exposure of cells kept in full-growth medium to ISq (10 $\mu $g ml$^{-1}$),, the levels of HMG-CoA reductase rose about twofold. ISq completely abolished the suppressive effect of 2.3 mM (R)-mevalonate on the reductase. Chromatin isolated from cell nuclei contains cholesterol, which is renewed biosynthetically. It is argued that the suppressor of HMG-CoA reductase, derived from mevalonate, is a sterol and not a non-steroidal product of mevalonate metabolism.