Estrogen prevents cardiomyocyte apoptosis through inhibition of reactive oxygen species and differential regulation of p38 kinase isoforms

From human and animal studies, estrogen is known to protect the myocardium from an ischemic insult. However, there is limited knowledge regarding mechanisms by which estrogen directly protects cardiomyocytes. In this report, we employed an in vitro model, in which cultured rat cardiomyocytes underwent prolonged hypoxia followed by reoxygenation (H/R), to study the cardioprotective mechanism of estrogen. 17-beta-estradiol (E2) acting via estrogen receptors inhibited H/R-induced apoptosis of cardiomyocytes. Mitochondrial reactive oxygen species (ROS) generated from H/R activated p38 alpha MAPK, and inhibition of p38 alpha with SB203580 significantly prevented H/R-induced cell death. E2 suppressed ROS formation and p38 alpha activation by H/R and concomitantly augmented the activity of p38 beta. Unlike p38 alpha, p38 beta was little affected by H/R. Dominant negative p38 beta protein expression decreased E2-mediated cardiomyocyte survival and ROS suppression during H/R stress. The prosurvival signaling molecule, phosphoinositol-3 kinase (PI3K), has previously been linked to cell survival following ischemia-reperfusion injury. Here, E2-activated PI3K was found to inhibit ROS generated from H/R injury, leading to inhibition of downstream p38 alpha. We further linked these signaling pathways in that p38 beta was activated by E2 stimulation of PI3K. Thus, E2 differentially modulated two major isoforms of p38, leading to cardiomyocyte survival. This was achieved by signaling through PI3K, integrating cell survival mediators.