Expression of two progesterone receptor isoforms in cumulus cells and their roles during meiotic resumption of porcine oocytes.

The present study aimed to investigate progesterone receptor (PR) gene expression in cumulus cells and their roles during meiotic resumption of porcine oocytes. The amount of PR-A or PR-B mRNA was analyzed by RT-PCR using primer sets for the PR-B region or the PR-A/B common region. The level of PR-B mRNA in cumulus cells was up-regulated by FSH and LH during the first 8 h of cultivation but the level significantly decreased at 12 h. However, a high level of total PR mRNA was maintained up to a cultivation period of 20 h. The level of PR-B protein in cumulus cells reached its maximum at 4 to 12 h, whereas PR-A predominated in cumulus cells of cumulus-oocyte complexes (COCs) at 20 h. Accompanying the shift in expression of PR isoforms, progesterone production in cumulus cells was significantly increased, and both the proliferative activity of cumulus cells during a 10- to 20-h cultivation period and the level of connexin-43, a major component of the gap junction, in cumulus cells significantly decreased. When COCs were cultured with FSH and LH for 10 h and then further cultured with additional RU486, there was a significant suppression in the shift in PR isoforms and in progesterone production, a loss of proliferative activity, and a decrease in connexin-43 mRNA in cumulus cells. Moreover, treatment with RU486 after 10-h cultivation of COCs inhibited the meiotic resumption of oocytes and cumulus cell expansion. These results suggest that the induction of PR isoforms in cumulus cells and their binding to progesterone appear to impact on proliferation and differentiation in a time-dependent manner, and the shift from PR-B to PR-A may help mediate certain events.