Effect of Cholesterol Transport Inhibitors on Steroidogenesis and Plasma Membrane Cholesterol Transport in Cultured MA-10 Ley dig Tumor Cells*

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Abstract
These studies were directed toward understanding the cellular actions of inhibitor drugs that affect steroidogenesis and cholesterol transport. We investigated the microfilament inhibitor cytochalasin-D, the microtubule inhibitor colchicine, the calmodulin antagonist trifluoperazine, and the inhibitor of acidic vesicle function nigericin. We found that all of these compounds caused dose-dependent inhibition of progesterone synthesis in the MA-10 cells. Each compound also inhibited (Bu)2cAMP-stimulated pregnenolone synthesis, indicating that each inhibited a fundamental process required for steroidogenesis. Each compound was next evaluated for inhibitory actions on cholesterol transport to and from the plasma membrane. On the basis of inhibitor sensitivity, two different categories of cholesterol transport were defined. Transport of newly synthesized or low density lipoprotein-derived cholesterol from the interior to the plasma membrane was inhibitor insensitive. Plasma membrane cholesterol internalization, however, was sensitive to all of the inhibitors and did not result because of any drug effect on the acyl-coenzyme-A-cholesterol acyl transferase. Cycling of cholesteryl ester-derived cholesterol through the plasma membrane appeared to occur before its use for steroidogenesis. Thus, inhibition of plasma membrane internalization would prevent utilization of both plasma, membrane cholesterol and cholesteryl ester-derived cholesterol, the two major substrate sources for steroid hormone synthesis. Consistent with this interpretation was the finding that inhibition of plasma membrane cholesterol internalization by each inhibitor paralleled the inhibitor''s effect on steroidogenesis.