Role of PARP, NF-kB and telomerase in necrosis induced by selective methylation of N3-adenine.

Selective methylation of N3-adenine represents a novel pharmacological strategy for the treatment of mismatch repair-deficient tumors, which are tolerant to O6-methylguanine, the main cytotoxic lesion induced by methylating agents of clinical interest such as temozolomide. However, the biochemical pathways involved in cell death induced by N3-methyladenine have not been clarified, yet. In the present study we show that MeOSO2(CH2)2-lexitropsin (Me-Lex), a compound generating almost exclusively N3-methyladenine, provoked poly(ADP-ribosylation), loss of mitochondrial membrane potential, and increase of ROS production in leukemia cell lines with intact or defective mismatch repair system. These events were followed by a marked reduction of nuclear poly(ADP-ribose) polymerase-1 (PARP-1) expression and Nuclear Factor-kB (NF-kB) activity. Moreover, treatment with Me-Lex induced a profound decrease of telomerase in the cytosol that was accompanied by a transient up-regulation of activity in the nucleus. PARP-1 inhibition blocked ADP-ribose polymer formation, preserved mitochondrial membrane integrity and counteracted the reduction of NF-kB activity thus preventing the appearance of necrosis. On the other hand, the combination of Me-Lex + PARP-1 inhibitor triggered apoptosis due to disruption of base excision repair process. In conclusion, the results underline the central and paradoxical role of PARP-1 in cell death induced by N3-methyladenine: effector of necrosis and co-ordinator of methylpurine repair. Supported by: FIRB 2001, PRIN 2003 and NIH grant RO1 CA29088 projects to GG, LT and BG, respectively.