Neuroprotective effects of estrogen and tamoxifen in vitro: A facilitative role for glia

Selective estrogen receptor modulators (SERMs) are steroidal or nonsteroidal compounds that can exhibit either estrogen-like agonistic effects or estrogen-antagonistic effects depending on the target tissue. While SERM actions in the breast, bone, and uterus have been well characterized, their effects in the brain are considerably less well understood. Previous work by our laboratory has demonstrated a beneficial effect of tamoxifen in the reduction of ischemic stroke damage in ovariectomized female rats. The present study utilized neuronal cell culture models to attempt to understand the mechanisms of tamoxifen-mediated neuroprotection. Neither physiologic doses of 17β-E₂ nor clinically therapeutic doses of tamoxifen directly protected GT1-7 neurons or purified cultures of rat cerebrocortical neurons from several forms of cell death. Reverse transcriptase polymerase chain reaction and Western blot analysis revealed that GT1-7 neurons possessed both estrogen receptor-or (ERα) and ERβ mRNA and protein, whereas purified embryonic rat cortical neurons only expressed appreciable levels of ERα transcript and protein, with little to no expression of ERβ. In contrast to the lack of protection in the purified neuronal cultures, both 17β-E₂ and tamoxifen significantly protected mixed glial/neuronal cortical cultures from cell death, suggesting that glia may facilitate 17βE₂- and tamoxifen-mediated neuroprotection. Furthermore, astrocyte-conditioned media and exogenous transforming growth factor-β1, a documented astrocyte-derived cytokine, were shown to rescue purified cortical neurons from cell death. Together, these findings support a role for astrocytes in neuroprotection and raise the intriguing possibility that astrocytes may help mediate the neuroprotective effect of 17β-E₂ and tamoxifen.