Differential expression and regulation of progesterone receptor isoforms in rat and mouse pituitary cells and LβT2 gonadotropes

Manipulation of endogenous progesterone receptor (PR) does not produce equivalent physiological effects in mouse and rat pituitary cells. To test whether this may be due in part to difference in PR isoform expression, we examined hormonally regulated pituitary PR-A and PR-B mRNA levels using quantitative real-time PCR. The LβT2 mouse gonadotrope line or pituitary cells from adult, ovariectomized rats or mice were cultured with or without 0.2 nM 17β-estradiol (E₂) for 3 days. PR-A was the predominant form expressed for all groups. For mouse cells, E₂ led to an increase in both isoforms without a change in the A:B ratio; for rat cells, the PR-B response to E₂ was more robust resulting in a decrease in the A:B ratio. Exposure of E₂-treated pituitary cells to 200 nM progesterone for 6 h decreased both PR-A and PR-B levels in rat cells, but had no effiect on PR isoform expression in mouse cells even when exposure was extended to 12 h. The low level of PR expression found in LβT2 gonadotropes was unaffected by E₂, alone or with progesterone. The weak PR expression and lack of responsiveness of LβT2 cells cannot be explained by a male phenotype as was shown by the more than tenfold higher PR mRNA level in primary cultures of male mouse pituitary cells, which responded to E₂ stimulation with a proportional increase in PR isoforms similar to female cells. Functionally, E₂-stimulated changes in PR mRNA isoform ratios in rat, mouse or LβT2 cells correlated with the degree of progesterone augmentation of GnRH-stimulated LH secretion in these models. These results are consistent with the hypothesis that robust GnRH priming and progesterone augmentation of LH secretion in the rat compared to these events in the mouse are a consequence, in part, of differences in the E₂-modulated ratio of PR isoforms.