Induction of the mitochondrial permeability transition by the DNA alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine. Sorting cause and consequence of mitochondrial dysfunction.

The alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) alters DNA and stimulates the activity of poly(ADP-ribose) polymerase-1 (PARP-1), a nuclear enzyme involved in DNA repair. The consumption of cellular NAD(+) by PARP-1 is accompanied by ATP depletion, mitochondrial depolarization and release of proapoptotic proteins, but whether a causal relationship exists among these events remains an open question. Most of cellular NAD(+) is stored in the mitochondrial matrix and becomes available for cytosolic and nuclear processes only after its release through the permeability transition pore (PTP), a voltage-gated inner membrane channel. Here we have explored whether MNNG affects mitochondrial function upstream of PARP-1 activation. We show that MNNG has a dual effect on isolated mitochondria. At relatively low concentrations (up to 0.1 mM), it selectively sensitizes the PTP to opening, while at higher concentrations (above 0.5 mM) it inhibits carbonyl cyanide 4-(trifluoromethoxy)phenylhydrazone (FCCP)-stimulated respiration. MNNG caused PTP opening and activation of the mitochondrial proapoptotic pathway in intact HeLa cells, which resulted in cell death that could be prevented by the PTP inhibitor CsA. We conclude that a key event in MNNG-dependent cell death is induction of PTP opening that occurs independently of PARP-1 activation.