Hormonal regulation of messenger ribonucleic acid encoding steroidogenic acute regulatory protein in ovine corpora lutea

Steroidogenic acute regulatory protein (STAR), proposed to be involved in the transport of cholesterol to the inner mitochondrial membrane, has recently been cloned from MA-10 cells. Using reverse transcription-polymerase chain reaction, we generated a complementary DNA encoding 404 base pairs of StAR from ovine luteal tissue to perform studies regarding regulation of the messenger RNA (mRNA) encoding this protein. In Exp 1, ewes were hypophysectomized (HPX) on day 5 of the estrous cycle and administered saline or physiological regimens of LH and/or GH until collection of luteal tissue on day 12 of the estrous cycle (n = 4/group). Luteal concentrations [mean ± SEM; femtomoles per μg poly(A)⁺ RNA] of mRNA encoding StAR were lower (P < 0.05) in the HPX plus saline-treated ewes (26.4 ± 7.3) than in day 12 pituitary-intact ewes (n = 4; 77.7 ± 9.3). Replacement of LH (59.1 ± 13.1), GH (59.1 ± 12.8), or LH and GH (69.9 ± 4.5) in HPX ewes increased (P < 0.05) concentrations of mRNA encoding StAR to values not different from those in day 12 controls. In Exp 2, ewes on day 11 or 12 of the estrous cycle were injected with prostaglandin F(2α) (PGF(2α)) to induce luteal regression. Corpora lutea were collected 4, 12, or 24 h after injection (n = 4-5/time point) and from untreated control ewes (n = 4) or 24 h after injection of saline (n = 4). Treatment with PGF(2α) decreased (P < 0.05) concentrations of progesterone in serum 4, 12, and 24 h after injection. Concentrations of StAR mRNA were decreased (P < 0.01) to 47%, 19%, and 8% of control values 4, 12, and 24 h after PGF(2α) injection, respectively. In Exp 3, ewes received ovarian arterial infusions of saline, PGF(2α), or phorbol 12-myristate 13- acetate (PMA), and luteal tissue was collected 0 (no infusion), 4, 12, or 24 h later (n = 3-4/group). Treatment with PGF(2α) or PMA decreased (P < 0.05) concentrations of progesterone in serum 4, 12, and 24 h postinjection. Steady state concentrations of mRNA encoding StAR (P < 0.05) were 36% and 25% of the control value 12 and 24 h after PGF(2α),, injection. Injection of PMA decreased (P < 0.05) concentrations of StAR mRNA to 75% and 50% of control values at 4 and 12 h, but concentrations of mRNA encoding StAR were not different from control values at 24 h. In Exp 4, steady state concentrations of mRNA encoding StAR were not different in luteal tissue collected on days 4 (70.5 ± 8.9), 10 (61.3 ± 6.3), or 15 (51.0 ± 13.3; n = 5/day) of the estrous cycle. Concentrations of mRNA encoding StAR on day 15 of pregnancy (84.8 ± 6.4; n = 7) were similar to values obtained on day 4, tended to be higher (P = 0.07) than those observed on day 10, and were higher (P < 0.05) than those on day 15 of the estrous cycle. Concentrations of progesterone in sera and luteal tissues were also higher (P < 0.01) in ewes on day 15 of pregnancy than in ewes on any day of the estrous cycle examined. In summary, mRNA encoding StAR was present in high concentrations in ovine luteal tissue. Removal of the pituitary gland decreased luteal concentrations of StAR mRNA, which were restored by replacement of LH and/or GH. Administration of PGF(2α) decreased concentrations of StAR mRNA, apparently by activating protein kinase C. Increased secretion of progesterone between days 4 and 10 of the estrous cycle was not associated with increased steady state concentrations of StAR mRNA in corpora lutea; however, increased concentrations of progesterone in serum on day 15 of pregnancy were associated with increased luteal concentrations of mRNA encoding STAR.