Truncated Estrogen Receptor α 46-kDa Isoform in Human Endothelial Cells

Background— Estrogen acutely activates endothelial nitric oxide synthase (eNOS). However, the identity of the receptors involved in this rapid response remains unclear. Methods and Results— We detected an estrogen receptor α (ERα) transcript in human endothelial cells that encodes a truncated 46-kDa ERα (Δ1a-hERα-46). A corresponding 46-kDa ERα protein was identified in endothelial cell lysates. Transfection of cDNAs encoding the full-length ERα (ERα-66) and Δ1a-hERα-46 resulted in appropriately sized recombinant proteins identified by anti-ERα antibodies. Confocal microscopy revealed that a proportion of both ERα-66 and hERα-46 was localized outside the nucleus and mediated specific cell-surface binding of estrogen as assessed by FITC-conjugated, BSA-estrogen binding studies. Both ERα isoforms colocalized with eNOS and mediated acute activation of eNOS in response to estrogen stimulation. However, estrogen-stimulated transcriptional activation mediated by Δ1a-hERα-46 was much less than with ERα-66. Furthermore, Δ1a-hERα-46 inhibited classical hERα-66–mediated transcriptional activation in a dominant-negative fashion. Conclusions— These findings suggest that expression of an alternatively spliced, truncated ERα isoform in human endothelial cells confers a unique ability to mediate acute but not transcriptional responses to estrogen.