Expression and hormonal regulation of the high-density lipoprotein (HDL) receptor scavenger receptor class B type I messenger ribonucleic acid in the rat ovary

Since cholesterol delivery to the ovary is an essential regulated step in steroidogenesis, mRNA levels for the Scavenger Receptor Class B Type I (SR-BI), a putative high-density lipoprotein receptor (HDL-R), were examined in response to tropic hormones and the luteolytic agent prostaglandin F2alpha (PGF2alpha). For this, the rat SR-BI cDNA was isolated and cloned. The results of this investigation revealed that a single SR-BI mRNA transcript of 2.4 kb was highly expressed in the rat adrenal, ovary, and testis. The SR-BI transcript was increased (twofold) in the immature rat ovary following pregnant mare's serum gonadotropin (PMSG) administration and in the ovary, 8 d after ovulation, in response to stimulation by human chorionic gonadotropin (hCG). In the ovary 8 d following ovulation, basal ovarian SR-BI mRNA levels were elevated up to sixfold relative to the preovulatory SR-BI mRNA levels. Even with the enhanced basal level of SR-BI mRNA within the ovary, hCG administration still resulted in a 2.5- (p < 0.025) and sevenfold (p < 0.01) increase in the 2.4-kb transcript, 3 and 6 h postinjection, respectively. This increase corresponded to a 58% increase in serum progesterone. In contrast, when PGF2alpha was administered, SR-BI mRNA levels were significantly reduced (3.5-fold; p < 0.01) in concert with a fourfold reduction (p < 0.001) in serum progesterone secretion. Furthermore, PGF2alpha blocked the hCG-induced increase in SR-BI mRNA levels when administered 30 min prior to hCG injection. The results of this study demonstrate that SR-BI mRNA levels are dramatically increased following exposure to gonadotropins in the ovary, whereas PGF2alpha exposure significantly reduced SR-BI mRNA levels.