Benzo[a]pyrene-induced necrosis in the HepG₂ cells via PARP-1 activation and NAD⁺ depletion

Benzo[a]pyrene (BaP), a member of polycyclic aromatic hydrocarbons (PAH), has been reported to induce cell death in various cell types. However, the underlying mechanisms are controversial. In the present study, we report that BaP induces necrotic cell death in human hepatoma (HepG₂) cells. The process is dependent on the activation of poly(ADP-ribose)polymerase-1 (PARP-1), a nuclear enzyme responsible for repairing DNA damage. Once activated, PARP-1 catalyzes the formation of ADP-ribose polymers on acceptor proteins at the expense of NAD⁺. Incubation of cells with high extracellular concentration of NAD⁺ (5 mM) after BaP treatment caused an elevation in intracellular NAD⁺ level and blocked cell death. Inhibitor of PARP-1 suppressed both overactivation of PARP-1 activity and NAD⁺ depletion. Moreover, addition of pyruvate (5 mM), but not glutamate (5 mM) or glutamine (5 mM), could restore ATP production and prevent cell death. These results elucidated a sequence of events linking cellular metabolism to the progression of cell death induced by this organic toxicant.