The aim of this study was to examine whether the expression levels of mRNA of the three estrogen receptor (ER) subtypes, ERalpha, ERbeta, and truncated ER product-1 (TERP-1) found in the rat pituitary gland were modified during gestation, lactation, and postlactation periods. By using relative quantitative RT-PCR, we found that ERalpha mRNA significantly peaked in midpregnancy. However, the ERalpha protein level remained constant. ERbeta gene expression did not change throughout pregnancy, suggesting that it was not related to estradiol levels during this reproductive period. In contrast, both TERP-1 mRNA and protein levels dramatically increased throughout the second half of gestation, being faintly detectable in early pregnancy. TERP-1 expression was rapidly reversed by lactation, whereas neither pituitary ERalpha nor ERbeta relative levels were significantly altered. In addition, pup removal for 24-96 h on d 9 postpartum significantly reduced the expression of both ERalpha and ERbeta mRNA compared with that in lactating animals, but the expression of TERP-1 mRNA was no longer detected. Collectively, our data indicate that 1) TERP-1, ERalpha, and ERbeta expression levels are differentially regulated in the pituitary; 2) TERP-1 is variably expressed depending on the hormonal environment related to the estrous cycle, pregnancy, and lactation; and 3) TERP-1/ERalpha ratios dramatically change depending on reproductive periods, suggesting a critical role for TERP-1 in reproductive events.

We have characterized the intronic promoter of the rat estrogen receptor (ER) alpha gene, responsible for the lactotrope-specific truncated ER product (TERP)-1 isoform expression. Transcriptional regulation was investigated by transient transfections using 5'-deletion constructs. TERP promoter constructs were highly active in MMQ cells, a pure lactotrope cell line, whereas a low basal activity was detected in alphaT3-1 gonadotrope cells or in COS-7 monkey kidney cells. Serial deletion analysis revealed that 1) a minimal -693-bp region encompassing the TATA box is sufficient to allow lactotrope-specific expression; 2) the promoter contains strong positive cis-acting elements both in the distal and proximal regions, and 3) the region spanning the -1698/-1194 region includes repressor elements. Transient transfection studies, EMSAs, and gel shifts demonstrated that estrogen activates the TERP promoter via an estrogen-responsive element (ERE1) located within the proximal region. Mutation of ERE1 site completely abolishes the estradiol-dependent transcription, indicating that ERE1 site is sufficient to confer estrogen responsiveness to TERP promoter. In addition, ERalpha action was synergized by transfection of the pituitary-specific factor Pit-1. EMSAs showed that a single Pit-1 DNA binding element in the vicinity of the TATA box is sufficient to confer response by the TERP promoter. In conclusion, we demonstrated, for the first time, that TERP promoter regulation involves ERE and Pit-1 cis-elements and corresponding trans-acting factors, which could play a role in the physiological changes that occur in TERP-1 transcription in lactotrope cells.

To analyze the molecular origin of an ER variant, the truncated ER product-1, transiently expressed at the proestrus in lactotrope cells, we generated a 2.5-kb sequence of a genomic region upstream and downstream the specific sequence truncated ER product-1. Genomic Southern blot analysis showed that truncated ER product-1 is spliced from a noncoding leader exon localized within the intron 4 of the ER alpha gene. Analysis of the promoter sequence revealed the presence of a major transcriptional start site, a canonical TATA box and putative cis regulatory elements for pituitary specific expression as well as an E-responsive element. In transient transfection, the truncated ER product-1 promoter was transcriptionally the most active in the lactotrope cell lines (MMQ). Analysis of truncated ER product-1 functionality showed that: 1) the protein inhibited ER alpha binding to the E-responsive element in electromobility shift assays, 2) inhibited the E2 binding to ER alpha in binding assays, 3) the truncated ER product-1/ER alpha complex antagonized the transcriptional activity elicited by E2, 4) nuclear localization of green fluorescent protein-ER alpha was altered in Chinese hamster ovary cell lines stably expressing truncated ER product-1. Collectively, these data demonstrated that the protein exerts full dominant negative activity against ER alpha. Moreover, truncated ER product-1/ER alpha complex also repressed the activity of all promoters tested to date, suggesting a general inhibitory effect toward transcription. In conclusion, the data suggest that truncated ER product-1 could regulate estrogen signaling via a specific promoter in lactotrope cells.