NF-κB inhibition enhances caspase-3 degradation of Akt1 and apoptosis in response to camptothecin

DNA damaging agents, such as camptothecin, and ionizing radiation (IR), can induce both NF-κB activation and apoptosis, however, the mechanism of their inter-regulation is not yet clear. In the present study, we discovered that Akt1 is degraded when cells deficient in Ataxia Telangiectasia mutated (ATM) were treated to CPT for apoptosis induction. While CPT-induced NF-κB activation could not be detected in ATM-deficient AT5BIVA cells, caspase-3 activation occurred and was even further enhanced by pretreatment with proteasome inhibitor-1 (Pro1), a NF-κB inhibitor. In contrast, activation of NF-κB but not of caspase-3 by CPT could be found in normal MRC5CV1 cells. NF-κB inhibition by Pro1, dominant negative mutant IκBα (S32/36) or p65 (N250), however, induced the caspase-3 activation in the normal cells, indicating the role of ATM-mediated NF-κB activation against cell apoptosis. On the other hand, interestingly, CPT significantly reduced the level of Akt1, this effect further enhanced by Pro1 pretreatment in AT5BIVA cells. In MRC5CV1 cells, however, Akt1 level could be reduced only when CPT and NF-κB inhibitors were co-treated to the cells, and this reversed by DEVD-cho treatment, demonstrating the caspase-3-mediated Akt1 degradation. Moreover, although MRC5CV1 cells were much more resistant to CPT compared with AT5BIVA, wortmannin and LY294002 significantly increased the chemosensitivity of MRC5CV1 cells to CPT. Given the accumulating evidences demonstrating Akt as a promising anticancer therapeutic target, all these results suggest that DNA damage induced apoptosis could be regulated by ATM-mediated NF-κB activation, and that Akt1 degradation be necessarily required for this apoptotic process.