A putative mitochondrial mechanism for antioxidative cytoprotection by 17beta-estradiol

It has been demonstrated that estrogens are potent antioxidants and protect against H₂O₂-mediated depletion of intracellular ATP in human lens epithelial cells (HLE-B3) [Invest. Ophthalmol. Vis. Sci. 44 (2003) 2067]. To investigate the mechanism by which 17β-estradiol (17β-E₂) protects against oxidative stress, HLE-B3 cells were exposed to insult with H₂O₂ at physiological (50μM) and moderately supra- physiological (100μM) levels over a time course of several hours, with and without pretreatment with 17β-E₂. The ability of 17β-E₂ to prevent H₂O₂-induced injury to several oxidant susceptible components of the cellular ATP generating machinery, including abundances of mitochondrial gene transcripts encoding respiratory chain subunits and cytochrome c, the glycolytic pathway enzyme, glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the energy-shuttling creatine kinase (CK) system, and mitochondrial membrane potential (ΔΨ_m), a measure of mitochondrial membrane integrity, were determined 3hr after oxidative insult. Northern blot analysis revealed H ₂O₂-induced reductions in mitochondrial transcripts for nicotinamide adenine dinucleotide dehydrogenase (NADH) subunits 4 and 5 and cytochrome c. H₂O₂ also inactivated GAPDH but did not alter CK activity. Pretreatment and simultaneous addition of 17β-E ₂ with H₂O₂ did not prevent the reductions in mitochondrial transcript levels and GAPDH activity. 17β-Estradiol did moderate the collapse of mitochondrial membrane potential (ΔΨ _m) in response to H₂O₂ as demonstrated by JC-1 staining and fluorescence microscopy. Although the precise mode of action responsible for protection by estradiols against oxidative stress remains to be determined, these results indicate that the hormone stabilizes the mitochondrial membrane, thereby preserving the driving force for oxidative ATP synthesis.