Heart estrogen receptor alpha: Distinct membrane and nuclear distribution patterns and regulation by estrogen

Estrogen receptor alpha (ER alpha) is present in the heart consistent with estrogen-induced modulation of cardiac function by genomic and nongenomic mechanisms, and with estrogen-mediated cardioprotective effects. We show that, in heart from adult male rats, ER alpha is detected mainly as two distinct isoforms: (i) a similar to 66 kDa isoform with the expected mass of the classical full-length ER alpha and (ii) an additional isoform of similar to 45 kDa. Differential centrifugation separated the 66 kDa isoform into the cytosolic fraction; while the 45 kDa isoform was enriched in the membrane fraction. High-resolution confocal studies show that ERa is distributed in the nucleus, cytosol, and various membranes including the plasmalemma. Notoriously, ER alpha labeling was very prominent in T-tubular membranes defined by alpha-actinin staining and the intercalated disks. In the T-tubules, ERa degree of association to alpha-actinin depends on the distribution pattern of the receptor along the T-tubules; association is high when ER alpha pattern is continuous, while it is low when the receptor has a discontinuous granular distribution. Nuclear ERa has a distinct trabecular distribution and it is excluded from the heterochromatin, consistent with an active transcription factor. Treatment with estrogen (similar to 4 h) produced an overall decrease in both nuclear and non-nuclear ER alpha levels and made more evident discrete ER alpha nuclear puncta uncovering cellular mechanism(s) of short term action of estrogen in the heart. The results indicate that the levels of the cardiac ER alpha isoforms are downregulated by estrogen and are differentially distributed: the full-length ER alpha is mainly compartmentalized in the cytosol and nucleus, while the 45 kDa isoform is mainly present in membrane structures. The membrane localization of ER alpha may support the rapid effects of estrogens on heart function. (c) 2006 Elsevier Inc. All rights reserved.