Expression and hormonal regulation of the high-density lipoprotein (HDL) receptor Scavenger Receptor Class B Type I messenger ribonucleic acid in therat ovary

Since cholesterol delivery to the ovary is an essential regulated step in s teroidogenesis, mRNA levels for the Scavenger Receptor Class B Type I (SR-B I), a putative high-density lipoprotein receptor (HDL-R), were examined in response to tropic hormones and the luteolytic agent prostaglandin F2 alpha (PGF2 alpha), For this, the rat SR-BI cDNA was isolated and cloned. The re sults 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 p regnant mare's serum gonadotropin (PMSG) administration and in the ovary, 8 d after ovulation, in response to stimulation by human chorionic gonadotro pin (hCG). In the ovary 8 d following ovulation, basal ovarian SR-BI mRNA l evels were elevated up to sixfold relative to the preovulatory SR-BI mRNA l evels. Even with the enhanced basal level of SR-BI mRNA within the ovary, h CG 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, respective ly. This increase corresponded to a 58% increase in serum progesterone, In contrast, when PCF2 alpha was administered, SR-BI mRNA levels were signific antly reduced (3.5-fold; p < 0.01) in concert with a fourfold reduction (p <0.001) in serum progesterone secretion. Furthermore, PCF2a blocked the hCG -induced increase in SR-BI mRNA levels when administered 30 min prior to hC G injection. The results of this study demonstrate that SR-BI mRNA levels a re dramatically increased following exposure to gonadotropins in the ovary, whereas PGF2a exposure significantly reduced SR-BI mRNA levels.