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

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