Elevated levels of endogenous pregnenolone fatty acid esters in follicular fluid high density lipoproteins support progesterone synthesis in porcine granulosa cells.

Abstract

Fatty acid esters of pregnenolone (PREG-FA) are present at high levels within ovarian follicular fluid (FF), although no physiological function has yet been attributed to these pregnenolone metabolites. We demonstrate in the following study that PREG-FA are predominantly localized in association with high density lipoproteins (HDL) within FF at levels that surpass those found in plasma HDL by approximately 200-fold. Extensive treatment with dextran-coated charcoal revealed that although nonconjugated steroids could be removed very efficiently from whole fluid and HDL, little effect could be demonstrated on the PREG-FA levels present within the fluid or its HDL component. This observed resistance to charcoal stripping may reflect a steroid-protein interaction that is altogether different from the non-conjugated steroid-protein association. Both plasma and FF-derived HDL were equally effective in supporting PROG biosynthesis in porcine granulosa cell cultures under normal culture conditions, but when PROG synthesis was dramatically blocked by inhibiting the P450 side-chain cleavage enzyme complex with the P450 enzyme inhibitor ketoconazole, only HDL of FF origin was capable of restoring PROG synthesis. Similar cultures that received HDL of plasma origin, containing similar levels of total cholesterol but far less PREG-FA, showed no significant difference in PROG accumulation over control cultures under these conditions. Several reports have described the presence of an entity within FF that was shown to resist charcoal stripping and enhanced PROG synthesis in granulosa cell cultures. Our data suggest that this entity within FF may indeed be the high levels of endogenous PREG-FA. We, therefore, propose a role for PREG-FA as a potential substrate in the biosynthesis of PROG in porcine granulosa cells.