Activity, pharmacological inhibition and biological regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase in Trypanosoma brucei

Activity of hydroxymethylglutaryl-coenzyme A (HMG-CoA) reductase, the key enzyme in the biosynthesis of steroids and polyisoprenoids in mammalian cells, has been detected in both the bloodstream form and the culture-adapted procyclic form of Trypanosoma brucei (3.7 ± 0.6 and 12.7 ± 1.8 pmol mevalonate produced min^-1 (mg cell protein)^-1, respectively). The enzyme activity is enriched 6-fold in microsomal fractions. Several competitive inhibitors of mammalian HMG-CoA reductase, including synvinolin (simvastatin), inhibit the multiplication of both forms of trypanosome in vitro (IC₅₀, approx. 25-50 μM after 2-3 days). This growth inhibition is potentiated by agents interfering with the exogenous supply of cholesterol, such as antibodies blocking the low-density lipoprotein (LDL) receptor, or 5 μM chloroquine. Conversely, growth inhibition by synvinolin can be largely reverted either by 300 nM LDL or by products of the mevalonate pathway, such as 20 mM mevalonate and in procyclics by 100 μM squalene or cholesterol. In procyclics, low concentrations of synvinolin selectively inhibit the incorporation of [¹⁴C]acetate into sterols, but not into fatty acids. These results argue for a critical role in trypanosomes of a mevalonate pathway, that is involved in the biosynthesis of sterol and probably of other metabolites. The HMG-CoA reductase activity is decreased 2-fold in procyclics incubated with 4 mM mevalonate and increased 2-fold in the presence of 2.5 μM synvinolin. Synvinolin also upregulates LDL binding up to 4-fold. These data suggest that HMG-CoA reductase and LDL receptor expression are regulated in T. brucei as in mammalian cells, to ensure sterol homeostasis.