CYP2E1 overexpression inhibits microsomal Ca²⁺-ATPase activity in HepG2 cells

Cytochrome P450 2E1 (CYP2E1) is a microsomal enzyme that generates reactive oxygen species during its catalytic cycle. We previously found an important role for calcium in CYP2E1-potentiated injury in HepG2 cells. The possibility that CYP2E1 may oxidatively damage and inactivate the microsomal Ca²⁺-ATPase in intact liver cells was evaluated, in order to explain why calcium is elevated during CYP2E1 toxicity. Microsomes were isolated by differential centrifugation from two liver cell line: E47 cells (HepG2 cells transfected with the pCI neo expression vector containing the human CYP2E1 cDNA, which overexpress active microsomal CYP2E1), and control C34 cells (HepG2 cells transfected with the pCI neo expression vector alone, which do not express significantly any cytochrome P450). The Ca²⁺-dependent ATPase activity was determined by measuring the accumulation of inorganic phosphate from ATP hydrolysis. CYP2E1 overexpression produced a 45% decrease in Ca²⁺-dependent ATPase activity (8.6 nmol Pi/min/mg protein in C34 microsomes versus 4.7 nmol Pi/min/mg protein in microsomes). Saturation curves with Ca²⁺ or ATP showed that CYP2E1 overexpression produced a decrease in V_max but did not affect the K_m for either Ca²⁺ or ATP. The decrease in activity was not associated with a decrease in SERCA protein levels. The ATP-dependent microsomal calcium uptake was evaluated by fluorimetry using fluo-3 as the fluorogenic probe. Calcium uptake rate in E47 microsomes was 28% lower than in C34 microsomes. Treatment of E47 cells with 2 mM N-acetylcysteine prevented the decrease in microsomal Ca²⁺-ATPase found in E47 cells. These results suggest that CYP2E1 overexpression produces a decrease in microsomal Ca²⁺-ATPase activity in HepG2 cells mediated by reactive oxygen species. This may contribute to elevated cytosolic calcium and to CYP2E1-potentiated injury.