Cytochrome P450 2E1-dependent oxidant stress and upregulation of anti-oxidant defense in liver cells.

Induction of cytochrome P450 2E1 (CYP2E1) is a central pathway by which ethanol generates oxidative stress. Cytochrome P450 2E1 metabolizes many other toxicologic compounds. Toxicity of these agents is enhanced by ethanol, due to induction of CYP2E1. Cytochrome P450 2E1 is induced under a variety of physiological and pathophysiological conditions. The laboratory at Mount Sinai School of Medicine established HepG2 cell lines that constitutively express human CYP2E1. Ethanol, polyunsaturated fatty acids and iron were toxic to the HepG2 cells that express CYP2E1 (E47 cells) but not control HepG2 cells. The E47 cells had higher glutathione levels than control HepG2 cells due to activation of the genes encoding the heavy and light subunits of gamma glutamyl cysteine synthetase (GCLC and GCLM). There was also a twofold increase in catalase, cytosolic and microsomal glutathione transferase, and heme oxygenase-1 (HO-1) in the E47 cells due to activation of their respective genes. These activations were prevented by anti-oxidants, suggesting that the upregulation of these anti-oxidant genes may reflect an adaptive mechanism to remove CYP2E1-derived oxidants. Increases in nuclear factor erythroid 2-related factor 2 (Nrf2) protein and mRNA were observed in livers of hepatocytes of chronic alcohol-fed and of pyrazole-treated rats or mice, conditions known to elevate CYP2E1. E47 cells showed increased Nrf2 mRNA and protein expression. Upregulation of GCLC and HO-1 in E47 cells is dependent on Nrf2. These results suggest that Nrf2 is activated and its levels are increased when CYP2E1 is elevated. It is suggested that Nrf2 plays a key role in the adaptive response against increased oxidative stress caused by CYP2E1.