Identification of a novel isoform of estrogen receptor, a potential inhibitor of estrogen action, in vascular smooth muscle cells.

Clinical and experimental studies showed that estrogen has antiatherogenic effects. We previously demonstrated that the estrogen receptor (ER) mRNA and protein are expressed in vascular smooth muscle cells (VSMC) derived from rat aorta. Here, the expression of isoforms of the ER was examined in VSMC. Reverse transcriptase-polymerase chain reaction using specific primers for rat ER cDNA was performed from RNA of rat VSMC. This revealed the existence of ER cDNA that is shorter than the wild-type ER cDNA. Sequencing of the amplified products identified three isoforms of the ER and the wild-type ER. These ER mRNA isoforms lacked the region corresponding to exon 4, exon 4 and 5, and exon 3 and 4. Therefore, they were designated as ERdelta4 isoform, ERdelta4/5 isoform and ERdelta3/4 isoform, respectively. Chloramphenicol acetyltransferase assay was performed with these ER isoforms constructed into the expression vector and the reporter plasmid containing the estrogen responsive element. The assay showed that these ER isoforms lost estrogen-dependent transactivation activities and that ERdelta4/5 isoform has a inhibitory effect on normal estrogen action when it was cotransfected with the wild-type ER. These ER isoforms might be involved in the regulation of VSMC by estrogen.