| Estrogen has been shown to be neuroprotective in various studies. Estrogen replacement therapy has provided promising beneficial effects in neurodegenerative diseases, such as Alzheimer's disease. The current studies investigate the neuroprotective actions of estrogen in several models of neurotoxicity induced by different insults with the human neuroblastoma cell line, SK-N-SH cells. The insults include beta-amyloid protein 25--35 (Abeta), MPTP, serum deprivation, and high density culture model. Our studies demonstrate that physiologically relevant concentrations of 17beta-estradiol (E2) are neuroprotective in all cytotoxic models established. MPTP, serum deprivation, and the high density culture, but not AP, induce apoptosis as detected with ELISA quantification of oligonucleosomes, and DNA laddering methods. The protective effects of E2 are abolished by the addition of ICI 182,780 in the MPTP treated cells, but not in the other models, suggesting that the effect of E2 in the MPTP model is probably associated with activation of estrogen receptors.; The L-type calcium channel blocker nifedipine nearly competely blocks Abeta-induced cell death, but not MPTP. Decreased extracellular Ca ++ concentration also restores Abeta induced cell viability loss. Western blotting analysis with anti L-type Ca++ channel alpha 1-subunit antibodies demonstrated that Abeta increases the expression of the alpha1-dihydropyridine binding complex, and neuronal alpha 1C and alpha1D subunits of L-type channels. Both E2 and nifedipine inhibit the increase in channel protein expression. MPTP induces an overexpression in alpha1C and alpha1D but the increases were not modified by E2 or nifedipine. Collectively, the observed phenomena help to illustrate the mechanisms of the neuroprotective effects of E2 in both apoptotic and necrotic cell death related to neurodegenerative changes and ischemic- or growth factor withdrawal-induced brain injury. It is proposed that E2 protection against Abeta induced cytotoxicity may occur at least partially through the regulation of expression of L-type Ca ++ channels; the protective effects of E2 in the MPTP model is through ER-mediated anti-apoptotic pathways. Moreover, the current studies reveal that nifedipine, the acute Ca++ channel blocker protects neurons from Abeta toxicity through restoration of the overexpression of the channel protein. A new fundamental role of dihydropyridines will be considered in the regulation of calcium homeostasis. |
