Abstract 549: Estrogen Deficiency Down Regulates Cardiac Estrogen Receptor β in Specific Subcellular Compartments
Numerous animal and human observational studies have reported that estrogen is cardioprotective. Clinical trials, however, failed to show a cardiovascular benefit of hormone therapy. The reason for this discontinuity is unclear, but timing of hormone therapy initiation appears to be critical for cardioprotection. Since estrogen receptors (ER) may be involved, the hypothesis tested in this study is that estrogen deficiency reduces cardiac ER levels in specific compartments, causing insensitivity to estrogen cardioprotection. Adult female open-chest rats were given vehicle (n=3) or the ER antagonist ICI (1 mg/kg, iv, n=2) prior to 25 minute ischemia and 2 hour reperfusion. Adult male rats were administered vehicle (n=4), β-estradiol (E2, 10 μg/kg, iv, n=3), ERβ agonist DPN (5 μg/kg, iv, n=3), or ERα agonist PPT (5 mg/kg, iv, n=3) prior to in vivo ischemia/reperfusion. Female rats had much lower infarct size (22.6 ± 2.5%) than male rats (47.6 ± 5.2%), and the ER antagonist ICI blocked this protection. E2 and ERβ agonist DPN significantly reduced infarct size to 28.9 ± 2.9% and 25.5 ± 2.4%, respectively. There was no effect of the ERα agonist PPT. After estrogen deficiency was confirmed by serum E2 levels, ovariectomized (OVX) rats were given vehicle (n=3), E2 (n=3), or DPN (n=3). Infarct size in OVX rats (52.5 ± 4.7%) was similar to males, but E2 or DPN had no effect. Cardiac ERβ levels were next measured in cytosolic (cyto), mitochondrial (mito), membrane (memb), and nuclear/myofilament (nuc) fractions from female (n=4) and OVX (n=2) hearts by Western blot analysis. Cardiac ERβ was present in all the female fractions. ERβ levels in OVX hearts were significantly reduced by 69%, 59%, and 71% in cyto, mito, and memb fractions, respectively. OVX rats were then treated with a long-acting E2, estradiol benzoate (8 μg, sc), for 24 or 72 hours. Memb ERβ levels were restored by 72 hour E2 treatment, while cyto and mito levels were still lower than sham female levels. Thus, these results show that estrogen deficiency in female rats causes down-regulation of ERβ levels in specific compartments which is partially restored by E2 therapy. The results also suggest that reduced cardiac ERβ levels may cause insensitivity to estrogen cardioprotection.