Concentration dependent effects of estrogen and progestins on cell death in cortical cultures

Abstract

Estrogen has been shown to be neuroprotective both in vitro and in vivo. However, the concentration of estrogen used has often been non-physiological. Therefore, we characterized the effects of a wide range of estrogen (17beta-estradiol) concentrations (from 1 nM to 10 uM) on neuronal death in cortical cultures. The effects of estrogen were tested on apoptotic death induced by exposure to serum deprivation or nifedipine, and on necrotic death induced by exposure to iron, buthionine sulfoximine (BSO), NMDA, or kainate. Estrogen was protective at uM concentrations against iron, BSO, and kainate toxicity. At these concentrations estrogen acted as a free radical scavenger as measured by inhibition of BSO induced oxidative stress assayed by dichlorofluorescein (DCF) fluorescence. The only effect observed at a physiological concentration of estrogen (10 nM) was potentiation of NMDA toxicity. This effect of estrogen was associated with increased NMDA stimulated calcium influx. The potentiation of cell death and increased calcium influx were both blocked by the estrogen receptor antagonist, ICI 182,780. The protective effects of high concentration estrogen were no t blocked by ICI 182,780, in fact, ICI 182,780 enhanced the protection, suggesting that a damaging effect of estrogen receptor activation was masked by a protective non-receptor effect. These results indicate a lack of protective effects by physiological concentrations of estrogen, and raise the concern that physiological concentrations of estrogen may potentiate neuronal injury. In contrast to the effects of estrogen, progesterone was protective against NMDA toxicity at a physiological concentration (10 nM). However, the synthetic progestin, medroxyprogesterone, commonly used for hormone replacement therapy, was only protective at a high concentration (10 uM). Since hormone replacement therapy typically involves the use of both an estrogen and a progestin further study of various estrogens and progestins, both alone, and in combination, are necessary.