17α-Estradiol down-regulates glutathione synthesis in serum deprived PC-12 cells

During the last decades it has been shown that estrogen may have neuroprotective functions in the CNS. However, we have previously reported that pretreatment with estradiol abolishes its protection of cultured cerebellar granule neurons from glutamate-induced cell death due to down-regulation of endogenous glutathione. 17α-Estradiol is considered a hormonally inactive isomer of 17β-estradiol still containing its antioxidant potential. Here, we demonstrate that 17α-estradiol enhanced serum deprivation-induced cell death in the rat pheochromocytoma cell line PC-12, while antioxidants vitamins C and E in combination (vitamins C/E) tended to protect. We further examined mechanisms behind the glutathione lowering effect of 17α-estradiol in serum deprived PC-12 cells. Endogenous glutathione levels were reduced in the serum deprived cells. Serum deprivation-induced cell death seemed to depend partly on this reduction as supplemented N-acetylcysteine, a cysteine precursor with potential to restore glutathione levels, reduced cell death. 17α-Estradiol down-regulated glutathione, promoter activity of the rate-limiting enzyme in glutathione production, glutamate cysteine ligase (GCL), as well as c-Fos protein levels in serum deprived cells. The c-Fos transcription factor normally binds to the AP-1 response element in the GCL promoter resulting in increased production of glutathione as a stress response. Over-expression of AP-1 proteins partly restored the GCL promoter activity in serum deprived cells treated with 17α-estradiol. Nrf2, a transcription factor binding another response element in the GCL promoter was unaffected by 17α-estradiol. Conclusively, 17α-estradiol may have a long-term negative effect on the endogenous glutathione level through its ability to down-regulate the glutathione synthesis during serum deprivation.