NAD+ depletion and cytotoxicity in isolated hepatocytes

Activation of poly(ADP-ribose)polymerase by DNA damaging agents causes a depletion of intracellular NAD+ and subsequent lowering of ATP pools, which if extensive may lead to cell death. We have studied the cytotoxicity to isolated hepatocytes of dimethyl sulphate, a direct-acting carcinogen and mutagen, hydrogen peroxide, generated by glucose/glucose oxidase, and menadione (2-methyl-1,4-naphthoquinone) in relation to their effects on intracellular NAD+ and ATP levels. Both dimethyl sulphate and glucose/glucose oxidase caused a depletion of NAD+, which was apparently due to an activation of poly(ADP-ribose)polymerase as it was prevented by inhibitors of the polymerase, i.e. 3-aminobenzamide and nicotinamide. This protection of intracellular NAD+ was accompanied by a prevention of the cytotoxicity of both dimethyl sulphate and glucose/glucose oxidase, while it did not alter the decrease in intracellular ATP they induced. This apparent dissociation of effects on ATP from NAD+ does not support the suggestion that activation of poly(ADP-ribose)polymerase leads to a decrease in cellular ATP as a consequence of NAD+ depletion. Menadione also caused a depletion of NAD+ which preceded cytotoxicity, but in contrast to dimethyl sulphate and H2O2 this depletion did not involve poly(ADP-ribose)polymerase as it was not prevented by inhibitors of the enzyme. Our results also indicate that the cytotoxicity of menadione is not mediated by H2O2 alone. Marked depletion of intracellular NAD+ prior to toxicity and a protection against toxicity associated with maintenance of NAD+ suggest a possible role for the maintenance of intracellular NAD+ in cellular integrity.