Estrogen neuroprotection: the involvement of the Bcl-2 binding protein BNIP2

To identify genes selectively induced by estrogens in cells of neural origin we have treated with a low concentration of 17 beta-estradiol (E2) the estrogen-receptor positive SK-ER3 neuroblastoma cells and we have isolated messages modulated by the hormonal treatment at short (1 h) and longer (17 h) times. By using the ddPCR approach we identified numerous messages which content was significantly and reproducibly altered by the hormonal treatment. Among these messages we focused our attention on bnip2, which expression was inhibited by estradiol. bnip2 was found to be a member of the BNIP family of genes of unknown physiological activity at the time. Investigations carried out in our laboratory proved a strong correlation between the increased expression of bnip2 gene and cell death induced by toxic stimuli. Furthermore, we showed that transfection of the bnip2 cDNA results in massive cell death and Bcl-2 overexpression counteracts the toxic effect of bnip2. These findings suggest that the proteins encoded by these two genes either interact or act in an opposite manner on the same mechanisms triggering the apoptotic cascade of events. Time-course experiments carried out in different cell systems and with a variety of neurotoxic agents proved a strong correlation between estrogen-induced decrease in bnip2 expression and the time required for estrogen to exert its protective effect. These observations led us to hypothesize an involvement of bnip2 in estrogen effects on cell survival. The finding that bnip2 is developmentally regulated may suggest a role of this gene in those brain areas where the differentiation is orchestrated by estradiol. Investigations in non-neural cells show that bnip2 is the mediator of the anti-apoptotic activity of estrogens in a variety of cells and thus might represent an important target for the evaluation of the activity of novel synthetic ligands for the estrogen receptor.