Cardiolipin synthesis is required to support human cholesterol biosynthesis from palmitate upon serum removal in Hela cells

We examined whether cardiolipin (CL) synthesis was required to support cholesterol (CH) production from palmitate in Hela cells. Knockdown of human cardiolipin synthase-1 (hCLS1) in Hela cells has been shown to reduce CL synthesis. Therefore Hela cells stably expressing shRNA for hCLS1 and mock control cells were incubated for 16 h with [¹⁴C(U)]palmitate bound to albumin (1:1 molar ratio) in the absence or presence of serum. Knockdown of hCLS1 in Hela cells resulted in a reduction in [¹⁴C(U)]palmitate incorporation into CL and CH. This reduction in [¹⁴C(U)]palmitate incorporation into CH was most pronounced during incubation under serum-free conditions. The reduction in [¹⁴C(U)]palmitate incorporation into CH was not due to alterations in total uptake of [¹⁴C(U)]palmitate into cells or altered palmitate metabolism, since [¹⁴C(U)]palmitate incorporation into phosphatidylcholine, the major [¹⁴C(U)]palmitate- containing lipid, and its immediate precursor, 1,2-diacyl-sn-glycerol, were unaffected by hCLS1 knockdown. In addition, knockdown of hCLS1 did not affect CH pool size, indicating that CH catabolism was unaltered. Hydroxymethylglutaryl coenzyme A reductase enzyme activity and its mRNA expression were reduced by knockdown of hCLS1 and this was most pronounced in Hela cells cultured under serum-free conditions. These data indicate that CL synthesis is required to support human de novo CH biosynthesis under conditions of increased demand for CH.