Although gender disparities in cardiac disease are recognized, the mechanisms through which pre-menopausal females are protected have not been fully elucidated. Cardiac disease incidence in females increases post-menopause, suggesting a role for estrogen in pre-menopausal cardioprotection. However, clinical trials found no beneficial cardiovascular outcomes from hormone replacement therapy, indicating a better mechanistic understanding is needed. Thus the goal of this study is to understand the mechanism responsible for the male-female differences in ischemia-reperfusion injury, cardioprotection and hypertrophy. Heart failure preceded by hypertrophy is a leading cause of death, and sex differences in hypertrophy are well known, although the basis for these sex differences is poorly understood. We used a systems biology approach to investigate mechanisms underlying sex differences in cardiac hypertrophy. Male and female mice were treated for 2 and 3 weeks with angiotensin II to induce hypertrophy. Sex differences in cardiac hypertrophy were apparent after 3 weeks of treatment. RNA sequencing was performed on hearts, and sex differences in mRNA expression at baseline and following hypertrophy were observed, as well as within-sex differences between baseline and hypertrophy. Sex differences in mRNA were substantial at baseline and reduced somewhat with hypertrophy, as the mRNA differences induced by hypertrophy tended to overwhelm the sex differences. We performed an integrative analysis to identify mRNA networks that were differentially regulated in the 2 sexes by hypertrophy and obtained a network centered on PPARa (peroxisome proliferator-activated receptor a). Mouse experiments further showed that acute inhibition of PPARa blocked sex differences in the development of hypertrophy. The data in this study suggest that PPARa is involved in the sex-dimorphic regulation of cardiac hypertrophy. We are currently studying cardiac specific PPARa null mice to examine if PPARa plays an important role in sex differences. Steroid hormone receptors, ER-alpha and ER-beta, classically function as transcription factors regulating gene expression. Recent data indicate that estrogen can also elicit effects by binding to estrogen receptors (ER-alpha, ER-beta; and GPR30) at the plasma membrane and initiate kinase signaling. We investigate the hypothesis that that non-nuclear ER activation reduces cardiac I-R injury in mice. We used an estrogen-dendrimer conjugate (EDC), which has been demonstrated in mice to be a non-nuclear selective ER modulator. We treated ovariectomized wild type mice with EDC, estradiol or dendrimer control for two weeks using Alzet osmotic minipumps. Isolated hearts were perfused in the Langendorff model and subjected to 30 minutes ischemia and 90 minutes reperfusion. Two weeks of treatment with estradiol significantly decreased infarct size and improved post-ischemic contractile dysfunction (40.42.5% vs. 62.95.8% for infarct and 44.74.0% vs. 27.02.7% for post-ischemic functional recovery). Similarly, EDC treatment significantly decreased infarct size (40.93.6% for EDC vs 63.84.7% vehicle) and increased post-ischemic functional recovery (48.83.0% EDC vs. 28.62.5% vehicle) compared to vehicle-treated hearts. Uterine weight was significantly increased by estrogen treatment but not by EDC. To distinguish whether protection was mediated by GPER or classic ER we used ICI 182,780, which is an antagonist of ER-alpha and ER-beta, but an agonist of GPER. ICI 182,780 significantly blocked the EDC-mediated cardioprotection. To better understand which ER was involved in cardioprotection, we generated cardiac-specific ER-alpha; knockout mice. In these mice, EDC treatment significantly decreased infarct size (20.11.9% vs. 51.27.8% dendrimer) and improved functional recovery (65.84.2% vs. 36.85.2% dendrimer) compared to the vehicle-treated ER-alpha; knockout mice. These results indicate that EDC is effective in providing cardioprotection during ischemia-reperfusion injury in mice, indicating that non-nuclear ER actions play a major role in this protection. Moreover, non-nuclear ERs; and/or GPR30 (or ERs; in other cell types) are likely candidates to mediate cardioprotection during ischemia-reperfusion injury. Thus, EDC could be utilized clinically to provide cardiovascular benefit without the classical steroid hormone side effect, such as uterine and breast cancer.
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