Proteasome inhibitors induce apoptosis in human lung cancer cells through a positive feedback mechanism and the subsequent Mcl-1 protein cleavage

Proteasome inhibitors (PIs) are promising new therapeutic agents for treating non-small cell lung carcinoma (NSCLC). To investigate the mechanisms of action of PIs, we analyzed the proapoptotic activities of PIs (MG132 or Bortezomib) in NSCLC cells. We found that both MG132 (>1 mu M) and Bortezomib (>0.025 mu M) induced a significant apoptosis in NCI-H1703, a PI-sensitive NSCLC cell line, through initially activating the intrinsic apoptosis pathway, leading to the activation of a positive feedback mechanism (PFM), which then conveyed apoptosis signaling from the intrinsic pathway to the extrinsic pathway with formation of a signaling loop for maximal caspase activation. Mcl-1 and Noxa were identified to be the major anti-apoptotic and proapoptotic proteins, respectively, in PI-induced apoptosis and mutually exclusive in protein stability. Although the Mcl-1 protein was upregulated by proteasome inhibition, it was also subjected to caspase 3-dependent cleavage governed by the PFM. Moreover, it was revealed that Mcl-1 protein cleavage contributed to PFM-governed apoptosis in following inter-related ways: reducing the anti-apoptotic Mcl-1; generating the truncated proapoptotic Mcl-1(S); and inducing a shift of balance between Mcl-1 and Noxa. It was further manifested that tumor necrosis factor-related apoptosis-inducing ligand boosted MG132's proapoptotic activity through strengthening the PFM in both NCI-H1703 and NCI-H358, a PI-resistant NSCLC cell line. Therefore, this study provides a basis for enhancing the efficacy of PIs in treating NSCLC. Oncogene (2009) 28, 3775-3786; doi:10.1038/onc.2009.240; published online 17 August 2009