Overexpression of catalase in the mitochondrial or cytosolic compartment increases sensitivity of HepG2 cells to tumor necrosis factor-α-induced apoptosis

The sensitivity of HepG2 cells overexpressing catalase in either the cytosolic or mitochondrial compartment to tumor necrosis factor-alpha (TNP-alpha) and cycloheximide was studied. Cells overexpressing catalase in the cytosol (C33 cells) and especially in mitochondria (mC5 cells) were more sensitive to TNF-alpha-induced apoptosis than were control cells (Hp cells). The activities of caspase-3 and -8 were increased by TNF-alpha, with the highest activities found in mC5 cells. Sodium azide, an inhibitor of catalase, reduced the increased sensitivity of mC5 and C33 cells to TNF-alpha to the level of toxicity found with control Hp cells. Azide also decreased the elevated caspase-3 activity of mC5 cells. A pan-caspase inhibitor prevented the TNF-alpha-induced apoptosis and toxicity produced by catalase overexpression, Addition of H2O2 prevented TNF-alpha-induced apoptosis and caspase activation, an effect prevented by simultaneous addition of catalase, TNF-alpha plus cycloheximide increased ATP levels, with higher levels in C33 and mC5 cells compared with Hp cells. TNF-alpha did not produce apoptosis in mC5 cells maintained in a low energy state. TNF-alpha signaling was not altered by the overexpression of catalase, as activation of nuclear factor kappa B and AP-1 by TNF-alpha was similar in the three cell lines. These results suggest that catalase, overexpressed in the cytosolic or especially the mitochondrial compartment, potentiates TNF-alpha induced apoptosis and activation of caspases by removal of H2O2.