Novel intronic promoter in the rat ER alpha gene responsible for the transient transcription of a variant receptor

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 tha t 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 T ATA box and putative cis regulatory elements for pituitary specific express ion as well as an E-responsive element. In transient transfection, the trun cated ER product-1 promoter was transcriptionally the most active in the la ctotrope 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 an tagonized the transcriptional activity elicited by E2, 4) nuclear localizat ion of green fluorescent protein-ER alpha was altered in Chinese hamster ov ary cell lines stably expressing truncated ER product-1. Collectively, thes e data demonstrated that the protein exerts full dominant negative activity against ER alpha. Moreover, truncated ER product-1/ER alpha complex also r epressed the activity of all promoters tested to date, suggesting a general inhibitory effect toward transcription. In conclusion, the data suggest th at truncated ER product-1 could regulate estrogen signaling via a specific promoter in lactotrope cells.