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 antiatherog enic effects. We previously demonstrated that the estrogen receptor (E R) mRNA and protein are expressed in vascular smooth muscle cells (VSM C) 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 V SMC. This revealed the existence of ER cDNA that is shorter than the w ild-type ER cDNA. Sequencing of the amplified products identified thre e isoforms of the ER and the wild-type ER. These ER mRNA isoforms lack ed the region corresponding to exon 4, exon 4 and 5, and exon 3 and 4. Therefore, they were designated as ER Delta 4 isoform, ER Delta 4/5 i soform and ER Delta 3/4 isoform, respectively. Chloramphenicol acetylt ransferase assay was performed with these ER isoforms constructed into the expression vector and the reporter plasmid containing the estroge n responsive element. The assay showed that these ER isoforms lost est rogen-dependent transactivation activities and that ER Delta 4/5 isofo rm has a inhibitory effect on normal estrogen action when it was cotra nsfected with the wild-type ER. These ER isoforms might be involved in the regulation of VSMC by estrogen. (C) 1996 Academic Press, Inc.