Prostaglandin F_2α, mediates ovarian sterol carrier protein-2 expression during luteolysis

In the corpus luteum, prostaglandin F_2α (PGF_2α) appears to be a physiological agent with both antisteroidogenic and luteolytic actions. It is hypothesized that the antisteroidogenic action of PGF_2α acts through altered transport of cholesterol to the mitochondrial cytochrome P450 side-chain cleavage enzyme (P450scc). However, the effect of PGF_2α, 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_2α injection was examined in parallel with altered ovarian SCP2, P450scc, and 3β-hydroxysteroid dehydrogenase (3βHSD) 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_2α (single or multiple injections; 100-250 μg each) or left untreated. Ovarian SCP2, P450scc, and 3βHSD protein and mRNA levels were examined 0 (time zero), 4, and 8 h post-PGF_2α 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_2α injections (P < 0.001; n = 6/time point). The decline in progesterone paralleled a 50-60% reduction in 3βHSD protein and mRNA levels by 4 h post-PGF_2α. Protein and mRNA levels for 3βHSD returned to control values by 8 h post-PGF_2α 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_2α 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_2α 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_2α-treated animals (P < 0.04). These results are the first to demonstrate that ovarian SCP2 expression is significantly altered after PGF_2α treatment, and this study confirms that PGF_2α alters ovarian cholesterol transport capacity as part of its antisteroidogenic action.