We have reported previously that
NB-598 competitively inhibits human
squalene epoxidase and strongly inhibits
cholesterol synthesis from [14C]
acetate in cultured cells. Furthermore, multiple
oral administration of
NB-598 decreased serum
cholesterol levels in dogs (Horie, M., Tsuchiya, Y., Hayashi, M., Iida, Y., Iwasawa, Y., Nagata, Y., Sawasaki, Y., Fukuzumi, H., Kitani, K., and Kamei, T. (1990) J. Biol. Chem. 265, 18075-18078). In the present study, the effects of
NB-598 on 3-hydroxy-3-methylglutaryl
coenzyme A (
HMG-CoA) reductase and
low-density-lipoprotein (
LDL) receptor were examined using a human
hepatoma cell line Hep G2. Incubation of Hep G2 cells with
NB-598 for 18 h increased
HMG-CoA reductase activity in a dose-dependent manner. However, the increase in activity induced by
NB-598 was lower than that induced by L-654,969 (a potent
HMG-CoA reductase inhibitor), although
NB-598 inhibited
cholesterol synthesis more potently than L-654,969. On the other hand,
HMG-CoA reductase mRNA was increased to the same extent by both inhibitors. These results demonstrate that
NB-598 does not inhibit the synthesis of non-
sterol derivative(s) of
mevalonate, which regulate
HMG-CoA reductase activity at the post-transcriptional level.
NB-598 increased the binding of 125I-LDL to Hep G2 cells.
LDL receptor mRNA was also induced by
NB-598. In the presence of
LDL or
cycloheximide,
NB-598 did not increase
LDL receptor activity. These results demonstrate that the induction of
LDL receptor activity by
NB-598 is due to increases in
mRNA and
protein through the inhibition of
cholesterol synthesis at the
squalene epoxidase step. From these observations,
squalene epoxidase inhibitor is expected to be highly effective in the treatment of
hypercholesterolemia and also is very useful as a research tool for studying the regulation of
cholesterol metabolism.