Selective inhibition of cholesterol biosynthesis in brain cells by squalestatin 1

The effect of squalestatin 1 (SQ) on squalene synthase and other enzymes utilizing farnesyl pyrophosphate (F-P-P) as substrate was evaluated by in vitro enzymological and in vivo metabolic labeling experiments to determine if the drug selectively inhibited cholesterol biosynthesis in brain cells. Direct in vitro enzyme studies with membrane fractions from primary cultures of embryonic rat brain (IC₅₀ = 37 nM), pig brain (IC₅₀ = 21 nM), and C6 glioma cells (IC₅₀ = 35 nM) demonstrated that SQ potently inhibited squalene synthase activity but had no effect on the long-chain cis- isoprenyltransferase catalyzing the conversion of F-P-P to polyprenyl pyrophosphate (Poly-P-P), the precursor of dolichyl phosphate (Dol-P). SQ also had no effect on F-P-P synthase; the conversion of [³H]F-P-P to geranylgeranyl pyrephosphate (GG-P-P) catalyzed by partially purified GG-P-P synthase from bovine brain; the enzymatic farnesylation of recombinant H- p21(ras) by rat brain farnesyltransferase; or the enzymatic geranylgeranylation of recombinant Rab1A, catalyzed by rat brain geranylgeranyltransferase. Consistent with SQ selectively blocking the synthesis of squalene, when C6 glial cells were metabolically labeled with [³H]mevalonolactone, the drug inhibited the incorporation of the labeled precursor into squalene and cholesterol (IC₅₀ = 3-5 μM) but either had no effect or slightly stimulated the labeling of Dol-P, ubiquinone (COQ), and isoprenylated proteins. These results indicate that SQ blocks cholesterol biosynthesis in brain cells by selectively inhibiting squalene synthase. Thus, SQ provides a useful tool for evaluating the obligatory requirement for de hove cholesterol biosynthesis in neurobiological processes without interfering with other critical reactions involving F-P-P.