SELECTIVE-INHIBITION OF CHOLESTEROL-BIOSYNTHESIS IN BRAIN-CELLS BY SQUALESTATIN-1

The effect of squalestatin 1 (SQ) on squalene synthase and other enzym es 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 biosynthesi s in brain cells. Direct in vitro enzyme studies with membrane fractio ns from primary cultures of embryonic rat brain (IC50 = 37 nM), pig br ain (IC50 = 21 nM), and C6 glioma cells (IC50 = 35 nM) demonstrated th at SQ potently inhibited squalene synthase activity but had no effect on the long-chain cis-isoprenyltransferase catalyzing the conversion o f F-P-P to polyprenyl pyrophosphate (Poly-P-P), the precursor of dolic hyl phosphate (Dol-P). SQ also had no effect on F-P-P synthase; the co nversion of [H-3]F-P-P to geranylgeranyl pyrophosphate (GG-P-P) cataly zed by partially purified GG-P-P synthase from bovine brain; the enzym atic farnesylation of recombinant H-p21(ras) by rat brain farnesyltran sferase; or the enzymatic geranylgeranylation of recombinant Rab1A, ca talyzed by rat brain geranylgeranyltransferase. Consistent with SQ sel ectively blocking the synthesis of squalene, when C6 glial cells were metabolically labeled with [H-3] mevalonolactone, the drug inhibited t he incorporation of the labeled precursor into squalene and cholestero l (IC50 = 3-5 mu M) but either had no effect or slightly stimulated th e labeling of Dol-P, ubiquinone (CoQ), and isoprenylated proteins. The se results indicate that SQ blocks cholesterol biosynthesis in brain c ells by selectively inhibiting squalene synthase. Thus, SQ provides a useful tool for evaluating the obligatory requirement for de novo chol esterol biosynthesis in neurobiological processes without interfering with other critical reactions involving F-P-P.