| The low incidence of cardiovascular diseases in women before menopause or during hormone replacement therapy suggests a protective effect of estrogen. The protective mechanisms provided by estrogen are still not clear but may involve its effects on improving lipid profile, and vascular activities. Many studies using supraphysiological concentrations (pM) of estrogen suggested a direct antioxidant activity of estrogen. In this study, we investigate the protective effects of estrogen on endothelial cells under physiological concentrations (nM). Both male and female bovine aortic endothelial cells (BAEC) were incubated with 17β-estradiol (E2) from 1 nM to 10 μM for 24 hours. We found that E2 significantly decreased cell death caused by paraquat treatment in female BAEC, but not in the male BAEC. Interestingly, the optimal protection was observed at low E2 concentration (1–10 nM). The protective effect gradually decreased upon increasing E2 concentration. In addition, incubation with E2 (1 nM–10 μM) significantly increased the activities of antioxidant enzymes such as superoxide dismutase (SOD); catalase (CAT); glutathione peroxidase (GPX); and glutathione reductase (GR). Compared with control, the activities of SOD, CAT, GPX, and GR were increased by 4x, 2x, 2x, and 3x with 1 nM E2 treatment, respectively. Again, low levels of E2 (1–10 nM) produced a greater induction of antioxidant enzymes (AOE) than higher levels of E 2 (1–10 μM). Furthermore, 1–10 nM E2 preincubation of female BAEC provided a protection against thermokilling. This E2-mediated thermal and/or oxidative protection corresponded with the induction of the inducible form of heat shock protein 70 (HSP70i) and heme oxygenase-I (HO-1, or HSP32), but not the constitutive form of HSP70 (HSC70), HSP27, or HSP90. Interestingly, the E2-mediated cytoprotection could be blocked by 4-OH-tamoxifen, but not by raloxifene. More significantly, 4-OH-tamoxifen, but not raloxifene, selectively blocked the induction of HSP70i, endothelial nitric oxide synthase (eNOS), and HO-1 activities by E2. These results suggest that in physiological situations, E2 provides cardioprotection through the induction of multiple cellular defense systems, including AOEs, HSPs, eNOS, and HO-1. Our finding that the E2-mediated cytoprotection could be mimicked by phytoestrogens, such as resveratrol, and genistein, may provide a future direction for the development of estrogenic compounds in the battle against cardiovascular diseases. |
