DOES THE RAT CORPUS-LUTEUM EXPRESS THE PROGESTERONE-RECEPTOR GENE

To determine whether the progesterone receptor (PR) gene is expressed in rat corpora lutea (CL), PR messenger RNA (mRNA) and protein levels were examined in CL of both cycling and pregnant rats using in situ hy bridization, reverse transcription-polymerase chain reaction, and West ern blotting analyses. During the estrous cycle, levels of luteal PR m RNA were below the sensitivity of in situ hybridization. Although no s ignal could be detected in luteal cells, granulosa cells of preovulato ry follicles of the same ovary expressed PR mRNA at high levels during the evening of proestrus. Likewise, CL of pregnant rats expressed und etectable levels of PR mRNA, which remained unchanged throughout pregn ancy, as determined by in situ hybridization and reverse transcription -polymerase chain reaction. In addition, no PR protein could be detect ed in rat CL by Western analysis, indicating that luteal expression of the PR gene, if any, is negligible. To determine whether the lack of PR mRNA in the rat CL is due to progesterone-induced down-regulation o f PR mRNA or to low levels of estrogen, aminoglutethimide was used to block the synthesis of progesterone in the presence and absence of exo genous estrogen, and PR mRNA levels were examined in the CL as well as the placenta. Inhibition of progesterone synthesis did increase PR mR NA levels in the placenta, and additional estrogen treatment further i ncreased PR mRNA levels in this tissue. In contrast, neither aminoglut ethimide alone nor aminoglutethimide plus estrogen induced PR mRNA exp ression in CL of the same rats. The temperature-sensitive luteal cell line derived from the rat CL, which produces very low levels of proges terone, also did not express PR mRNA. These results indicate that the rat corpus luteum expresses undetectable levels of PR mRNA and protein , which is not attributable to progesterone-induced down-regulation of PR mRNA or to a lack of estrogen-induced up-regulation of PR mRNA in this tissue.