Roles of NF-kappa B and 26 S proteasome in apoptotic cell death induced bytopoisomerase I and II poisons in human nonsmall cell lung carcinoma

Activation of signaling pathways after DNA damage induced by topoisomerase (topo) poisons can lead to cell death by apoptosis, Treatment of human nons mall cell lung carcinoma (NSCLC-3 or NSCLC-5) cells with the topo I poison SN-38 or the topo II poison etoposide (VP-16) leads to activation of NF-kap paB before induction of apoptosis, Inhibiting the degradation of I kappaB a lpha by pretreatment with the proteasome inhibitor MG-132 significantly inh ibited NF-kappaB activation and apoptosis but not DNA damage induced by SN- 38 or VP-16. Transfection of NSCLC-5 or NSCLC-5 cells with dominant negativ e mutant I kappaB alpha (mI kappaB alpha) inhibited SN-38 or VP-16 induced transcription and DNA binding activity of NF-kappaB without altering drug-i nduced apoptosis. Regulation of apoptosis by mitochondrial release of cytoc hrome c and activation of pro-caspase 9 followed by cleavage of poly(ADP-ri bose) polymerase by effector caspases 3 and 7 was similar in neo and mI kap paB alpha cells treated with SN-38 or VP-16. In contrast to pretreatment wi th MG-132, exposure to MG-132 after SN-38 or VP-16 treatment of neo or mI k appaB alpha cells decreased cell cycle arrest in the S/G(2) + M fraction an d enhanced apoptosis compared with drug alone. In summary, apoptosis induce d by topoisomerase poisons in NSCLC cells is not mediated by NF-kappaB but can be manipulated by proteasome inhibitors.