Hormonal regulation of steroidogenic acute regulatory (StAR) protein messenger ribonucleic acid expression in the rat ovary

Steroidogenic acute regulatory (StAR) protein is thought to mediate the rapid increase in steroid hormone biosynthesis in response to tropic hormones by facilitating cholesterol transport to the inner mitochondrial membrane where the P450 side-chain cleavage enzyme (P450scc) is located. Since cholesterol delivery is the regulated step in steroidogenesis and is dependent on de novo protein synthesis, StAR mRNA levels were examined in response to the tropic hormones, pregnant mare's serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG). The results of this investigation revealed that major StAR mRNA transcripts of 3.4 and 1.6 kb and a less abundant transcript of 1.2 kb were detected in the adrenal, ovary, and testis. Within the ovary, StAB mRNA levels were regulated by PMSG and hCG. The two major transcripts were increased in the immature rat ovary following PMSC administration and in the ovary, 8 d after ovulation, in response to stimulation by hCG. Serum progesterone levels were increased following hCG treatment in parallel with the enhanced expression of StAR. Following PMSG treatment, ovarian StAR transcripts at 3.4 and 1.6 kb were each increased twofold. In the ovary, 8 d following ovulation, basal ovarian StAR mRNA levels were elevated up to sixfold relative to the preovulatory StAR mRNA levels. Even with the enhanced basal level of StAR mRNA within the ovary 8 d postovulation, hCG administration still resulted in a 2.5- and 7-fold increase in the 3.4 and 1.6 kb (p < 0.025) transcripts, respectively, and a 58% increase in serum progesterone. In contrast to the dramatic alterations in StAR mRNA expression following hormonal stimulation, P450scc mRNA levels remained unchanged in response to hCG stimulation. The levels of serum progesterone paralleled the change in ovarian StAR mRNA in all experiments. This study provides the first evidence that StAR mRNA expression in the rat ovary is mediated by gonadotropins, further supporting its important role in the regulation of steroid hormone biosynthesis.