Protein kinase C signaling as a survival pathway against CYP2E1-derived oxidative stress and toxicity in HepG2 cells

Hepatic induction of CYP2E1 is a major pathway involved in oxidative stress and damage caused by chronic ethanol consumption; CYP2E1 also promotes the activation of a variety of hepatotoxins to reactive intermediates. Phorbol esters activate protein kinase C (PKC), thereby blocking cell differentiation and promoting tumor growth. In this study, we examined the possible role of PKC signaling as a survival pathway against CYP2E1-mediated toxicity using transfected HepG2 hepatoma cells stably overexpressing CYP2E1 (E47 cells). Cells were exposed to arachidonic acid (AA) plus Fe, which has been previously reported to cause a synergistic toxicity in E47 cells by a mechanism dependent on CYP2E1 activity and involving oxidative stress and lipid peroxidation. Phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA), but not the inactive analog 4-alpha-TPA, prevented lipid peroxidation, glutathione depletion, and loss of viability produced by AA + Fe in E47 cells. TPA also protected against the toxicity caused by AA alone, or by iron alone, in the E47 cells. TPA did not lower but instead induced catalytically active CYP2E1 in these cells. The protective effect of TPA on CYP2E1-dependent AA + Fe toxicity seemed to involve a PKC-related survival mechanism, since PKC inhibitors such as Ro 31-8425 (bisindolylmaleimide X hydrochloride) or staurosporine abolished that protection, and activation of PKC by TPA was an early event that occurs prior to the developing toxicity. In conclusion, PKC activation by TPA prevents CYP2E1-derived acute oxidative stress and toxicity in HepG2 cells, and this appears to involve maintenance of the intracellular redox homeostasis via PKC signal transduction.