ESTROGEN-MEDIATED MITOCHONDRIAL CHOLESTEROL TRANSPORT AND METABOLISM TO PREGNENOLONE IN THE RABBIT LUTEINIZED OVARY

We investigated the mechanisms of luteotropic actions of estradiol on steroidogenesis. To this end, we examined, in vitro, the metabolism of cholesterol from endogenous or exogenous sources for pregnenolone pro duction in rabbit luteinized ovarian cell mitochondria isolated from p seudopregnant animals in various states of stimulation by estradiol. W e found that estradiol-mediated regulation of mitochondrial cholestero l metabolism for pregnenolone production differs from the mechanisms o f regulation reported for steroidogenic protein/polypeptide hormones i n the following respects: 1) in the estradiol-sensitive, luteinized-ov ary, rabbit model, temporary blockage of cytochrome P-450 cholesterol side-chain cleavage enzyme by aminoglutethimide treatment in vivo has no effect on mitochondrial pregnenolone production in vitro after the aminoglutethimide is removed, indicating no additional capacity for up stream cholesterol storage; 2) preincubating mitochondria at 37 degree s C fails to increase subsequent pregnenolone synthesis in response to the addition of isocitrate; and 3) exogenously added cholesterol does not readily enter the steroidogenic pool of cholesterol unless the en dogenous cholesterol pool is first depleted. These new observations in dicate that estradiol increases the usable steroidogenic cholesterol p ool in rabbit ovarian mitochondria. Also, 1) they are consistent with a putative requirement for the participation of one or more estrogen-s ensitive protein factors to enhance cholesterol trafficking to the inn er mitochondrial membrane, and 2) they complement the observation of e strogen-dependent expression of steroidogenic acute regulatory protein in rabbit luteal tissue.