CONTEXT:In animal models, estrogen inhibits atherogenesis by inhibiting many of the early steps of atherosclerotic plaque formation. However, the lack of cardioprotective effect by postmenopausal hormone replacement therapy and possible increase in cardiovascular events observed during the first year after the initiation of hormone replacement therapy may suggest that once the plaque is formed, estrogen may have additional effects that may counteract its beneficial outcomes. Indeed, the effect of estrogen on plaque stability has not been identified. OBJECTIVE:We hypothesized that 17beta-estradiol (E2) may cause increased apoptosis in human coronary artery endothelial cells (HCAECs). This effect would explain an adverse effect on plaque stability in vivo. INTERVENTION(S) AND MAIN OUTCOME MEASURE(S):The effect of E2 on apoptosis, cell proliferation, and expression of proapoptotic molecules Fas and Fas ligand (FasL) in cultured HCAECs was evaluated. RESULTS:HCAECs in culture treated with E2 showed an increase in DNA strand breaks and nuclear fragmentation indicative of apoptosis. E2 treatment also induced a significant concentration-dependent increase in Fas mRNA and protein expressions in HCAECs. Moreover, the expression of FasL mRNA and secretion of FasL protein by HCAECs were enhanced in response to E2 treatments. CONCLUSIONS:E2 increases the apoptosis in cultured HCAECs. Enhanced Fas and FasL expressions in response to E2 suggest that activation of the Fas/FasL pathway may be a mediator of the proapoptotic effects of E2 in these cells.