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 [14C]
acetate and [14C]
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 [14C]
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)-[2-14C]
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 [14C]
squalene 2,3-
oxide and [14C]
squalene 2,3-22,23-dioxide. ISq added to cells kept in full-growth medium (10 micrograms 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 [14C]
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 micrograms 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.