SEX-SPECIFIC AND CELL-SPECIFIC EXPRESSION OF AN ESTROGEN-RECEPTOR ISOFORM IN THE PITUITARY-GLAND

The presence of multiple monomeric forms has been described for the es trogen receptor (ER) in the pituitary gland. We analyzed ER mRNA forms in male and female rat pituitary. A single 6.2-kb ER mRNA species was detected in the male rat pituitary, whereas the female rat pituitary exhibited two ER mRNA. forms of 6.2 and 5.5 kb, respectively. The 6.2- kb mRNA was present throughout the different stages of the estrous cyc le, while the 5.5-kb mRNA appeared to be restricted to proestrus, sugg esting an acute regulation of ER transcription at this stage. The 5.5- kb ER mRNA could be rapidly induced either by 17 beta-estradiol replac ement in ovariectomized adult female rats or by priming immature rats with pregnant-mare serum gonadotropin. Using enriched cell populations , an inverse and strong correlation was established between the presen ce of the 5.5-kb ER mRNA form and the number of gonadotropes. Converse ly, the localization of the 5.5-kb mRNA form was demonstrated in lacto trope populations. In order to elucidate the structural modifications in the transiently expressed ER mRNA, a series of reverse-transcriptas e polymerase chain reaction amplifications was carried out using sever al pairs of primers corresponding to the entire ER-coding region. The data showed that no alternative splicing was occurring in the PR-codin g region involving a potential role of either 3'- or 5'-untranslated r egions. Thus, ER presents a 17 beta-estradiol-dependent transcriptiona l mechanism triggered on proestrous day and specific to the female lac totropes.