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

In the corpus luteum, prostaglandin F-2 alpha (PGF(2 alpha)) appears to 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 mitochondrial cytochrome P450 side-chain cleavage enzyme (P450scc). However, the 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 progesterone 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) (single or multiple injections; 100-250 mu g each) or left untreated. Ovarian SCP2, P450scc, and 3 beta HSD protein and mRNA levels were examined 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-base pair SCP2-mRNA specific sequence. The results indicate that serum progesterone was significantly reduced 4 and 8 h after PGF(2 alpha) injections (P < 0.001; n = 6/time point). The decline in progesterone paralleled 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 expression was also reduced at 4 h (44-54%), but by 8 h, both protein and mRNA levels had increased above the normal control levels (P < 0.02). In contrast; the 0.8-kilobase SCP2-specific mRNA transcript was reduced to 50% and 80% of the pre-PGF(2 alpha) treatment level at 4 and 8 h, respectively (P < 0.01). SCP2 ribonuclease protection assay analysis also 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 expression, Western blot analysis indicated that a 15-kilodalton SCP2-immunoreactive protein (presumably the pro-SCP2 form) was significantly reduced 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 confirms that PGF(2 alpha) alters ovarian cholesterol transport capacity as part of its antisteroidogenic action.