PROSTAGLANDIN-F2-ALPHA MEDIATES OVARIAN STEROL CARRIER PROTEIN-2 EXPRESSION DURING LUTEOLYSIS

In the corpus luteum, prostaglandin F-2 alpha (PGF(2 alpha)) appears t o be a physiological agent with both antisteroidogenic and luteolytic actions. It is hypothesized that the antisteroidogenic action of PGF(2 alpha) acts through altered transport of cholesterol to the mitochond rial cytochrome P450 side-chain cleavage enzyme (P450scc). However, th e effect of PGF(2 alpha) on the expression of the putative cholesterol transport protein, sterol carrier protein-2 (SCP2; 13.2 kilodaltons), has not been examined. In this study, the decline in serum progestero ne after PGF(2 alpha) injection was examined in parallel with altered ovarian SCP2, P450scc, and 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) protein and messenger RNA (mRNA) levels. Rats (28 days old) were treated with 8 IU PMSG to induce follicular development and ovulation . Ten days after ovulation, animals were treated with PGF(2 alpha) (si ngle or multiple injections; 100-250 mu g each) or left untreated. Ova rian SCP2, P450scc, and 3 beta HSD protein and mRNA levels were examin ed 0 (time zero), 4, and 8 h post-PGF(2 alpha) treatment using Western and Northern blot analysis. SCP2 mRNA levels were also examined using a highly sensitive ribonuclease protection assay that detects a 429-b ase pair SCP2-mRNA specific sequence. The results indicate that serum progesterone was significantly reduced 4 and 8 h after PGF(2 alpha) in jections (P < 0.001; n = 6/time point). The decline in progesterone pa ralleled a 50-60% reduction in 3 beta HSD protein and mRNA levels by 4 h post-PGF(2 alpha). Protein and mRNA levels for 3 beta HSD returned to control values; by 8 h post-PGF(2 alpha) treatment. P450scc express ion was also reduced at 4 h (44-54%), but by 8 h, both protein and mRN A levels had increased above the normal control levels (P < 0.02). In contrast; the 0.8-kilobase SCP2-specific mRNA transcript was reduced t o 50% and 80% of the pre-PGF(2 alpha) treatment level at 4 and 8 h, re spectively (P < 0.01). SCP2 ribonuclease protection assay analysis als o indicated that SCP2 mRNA levels were reduced 65% (P < 0.03) and 85% (P < 0.01) by 4 and 8 h post-PGF(2 alpha) treatment compared to those in time zero ovarian tissue. Consistent with the loss of SCP2 mRNA exp ression, Western blot analysis indicated that a 15-kilodalton SCP2-imm unoreactive protein (presumably the pro-SCP2 form) was significantly r educed or absent in the PGF(2 alpha)-treated animals (P < 0.04). These results are the first to demonstrate that ovarian SCP2 expression is significantly altered after PGF(2 alpha) treatment, and this study con firms that PGF(2 alpha) alters ovarian cholesterol transport capacity as part of its antisteroidogenic action.