Neuroprotective effects in hippocampal neurons vary with estrogen concentrations

Neuroprotective effects of estrogens on cell survival and regeneration have frequently been shown. Here, we have studied the effects of low, gonad-derived serum estrogen concentrations, of intermediate concentrations, which are provided by hippocampal cells, and of high doses of estrogen on proliferation, apoptosis, and axon outgrowth of rat hippocampal neurons. No effects of low concentrations were found, neither in vitro nor after ovariectomy in vivo. To study the effects of hippocampus-derived estradiol we inhibited estrogen synthesis by treatment of hippocampal cell cultures with letrozole, an aromatase inhibitor. Alternatively, we used siRNA against Steroidogenic acute regulatory protein (StAR). Axon outgrowth as shown by quantitative immunohistochemistry of growth associated protein 43 and by measurements of axon length was significantly downregulated in response to letrozole and in siRNA-StAR transfected cells. Similarly, counting of Ki67-positive cells, a parameter of proliferation revealed a decrease in the number of proliferative cells. The number of TUNEL and of caspase-3 positive cells, parameters of apoptosis, increased in response to inhibition of estrogen synthesis. Application of high, supraphysiological doses of estradiol promoted axon outgrowth but did not affect proliferation and apoptosis. The estrogen receptor antagonist ICI 182,780 antagonized estrogen-induced axon outgrowth but had no effect on proliferation, suggesting different pathways of estrogen signaling in neuroprotection. Our findings demonstrate that axon outgrowth appears to be promoted by estradiol in a wide range of estrogen concentrations. In contrast, proliferation and apoptosis in the hippocampus are balanced in a restricted range of concentration as provided by hippocampus-derived estradiol. Support contributed by the DFG.