Down-regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA levels and synthesis in syrian hamster C100 cells by the oxidosqualene cyclase inhibitor [4'-(6-allyl-ethyl-amino-hexyloxy)-2'-fluoro-phenyl]-(4-bromophenyl)-me thanone (Ro 48-8071): comparison to simvastatin.

In vivo inhibition of 2,3-oxidosqualene:lanosterol cyclase (OSC, E.C. 5.4.99.7)--the enzyme which catalyzes the cyclization of monooxidosqualene to lanosterol--does not result in elevated 3-hydroxy-3-methylglutaryl CoA reductase (HMGR) activity. This trait is attributed to increased levels of oxysterols, produced upon partial inhibition of OSC, that suppress HMGR and other sterol-responsive genes. The OSC inhibitor [4'-(6-allyl-ethyl-amino-hexyloxy)-2'-fluoro-phenyl]-(4-bromopheny l)-methanone (Ro 48-8071) was shown earlier to lower low-density lipoprotein (LDL) cholesterol in hamsters with no increase in hepatic HMGR, in contrast to simvastatin. To delineate the regulatory mechanism(s) by which Ro 48-8071 reduces cholesterol synthesis without raising HMGR levels, Syrian hamster C100 cells were incubated with either Ro 48-8071 or simvastatin, and their effects on cholesterol synthesis and LDL uptake, as well as on HMGR mRNA levels and rates of synthesis, were determined. Using RNase protection and radioimmunoprecipitation assays, we found that, in the absence of LDL in the culture medium, both HMGR mRNA levels and synthesis were reduced with concentrations of Ro 48-8071 inhibiting cholesterol synthesis by 50-75%, whereas LDL uptake was either reduced or unchanged. In contrast, simvastatin, at concentrations inhibiting cholesterol synthesis by the same 50-75%, increased both HMGR mRNA levels and synthesis, as well as LDL uptake. In the presence of LDL, HMGR mRNA levels and synthesis along with LDL uptake were little affected after incubation with Ro 48-8071. Still, simvastatin markedly increased both HMGR mRNA levels and synthesis in cells incubated in the presence of LDL, leaving LDL uptake unaffected. These data suggest that inhibition of OSC by Ro 48-8071 results in an indirect down-regulation of HMGR mRNA levels and synthesis.