Proteasome Inhibition with Bortezomib Enhances Activity of Topoisomerase I-targeting Drugs by NF-κB-independent Mechanisms

Abstract

Purpose: The potentiation of topoisomerase (topo)-I-induced apoptosis by proteasome inhibitors is dependent on the treatment sequence, but not on NF-κB. In this study, alternate mechanisms modulating apoptosis induced with the topo I-targeting drug, SN-38, when followed by the proteasome inhibitor bortezomib (PS-341) were investigated. Materials and Methods: Human non-small cell lung carcinoma (NSCLC-3) cells transfected with a control vector (NSCLC-3/neo) or a vector containing dominant negative IκBα (NSCLC-3/mIκBα) were treated with SN-38 for 1 h followed by PS-341 for 4 h (SN-38→PS-341), or with either drug alone. The functional role of the anti-apoptotic protein survivin was tested using NSCLC-3 transfected with myc-tagged wild-type (NSCLC-3/myc-survivin), or dominant negative mutant T34A survivin (NSCLC-3/myc-T34A). Results: In NSCLC-3/neo or NSCLC-3/mIκBα cells, treatment with SN-38→PS-341 led to down-regulation of the survivin transcript and protein, enhanced apoptosis and reduced (>3-fold) survival compared to SN-38 or PS-341 alone. In contrast to the cells transfected with wild-type survivin, or the control NSCLC-3/neo, those cells transfected with mutant survivin and treated with SN-38→PS-341 exhibited enhanced caspase 9 activity (>2-fold), caspase 3 (>2- to 3-fold) activity and cytotoxicity compared to the NSCLC-3/neo cells. Conclusion: In contrast to inhibition of NF-κB activity, down-regulation of the anti-apoptotic survivin was correlated with modulation of the sequence-dependent synergistic effects of PS-341 in SN-38-induced apoptosis.