BAK and NOXA Are Critical Determinants of Mitochondrial Apoptosis Induced by Bortezomib in Mesothelioma

Sara Busacca,Giovanni Gaudino,Vignesh K Gunasekharan,Astero Klabatsa,Julia J Gorski,Michael Sheaff,Kenneth Arthur,Dean A Fennell,V Courtney Broaddus,Alex D Chacko,Mohamed El-Tanani

doi:10.1371/journal.pone.0065489

Abstract

Based on promising preclinical efficacy associated with the 20S proteasome inhibitor bortezomib in malignant pleural mesothelioma (MPM), two phase II clinical trials have been initiated (EORTC 08052 and ICORG 05–10). However, the potential mechanisms underlying resistance to this targeted drug in MPM are still unknown. Functional genetic analyses were conducted to determine the key mitochondrial apoptotic regulators required for bortezomib sensitivity and to establish how their dysregulation may confer resistance. The multidomain proapoptotic protein BAK, but not its orthologue BAX, was found to be essential for bortezomib-induced apoptosis in MPM cell lines. Immunohistochemistry was performed on tissues from the ICORG-05 phase II trial and a TMA of archived mesotheliomas. Loss of BAK was found in 39% of specimens and loss of both BAX/BAK in 37% of samples. However, MPM tissues from patients who failed to respond to bortezomib and MPM cell lines selected for resistance to bortezomib conserved BAK expression. In contrast, c-Myc dependent transactivation of NOXA was abrogated in the resistant cell lines. In summary, the block of mitochondrial apoptosis is a limiting factor for achieving efficacy of bortezomib in MPM, and the observed loss of BAK expression or NOXA transactivation may be relevant mechanisms of resistance in the clinic.

Highlights

Malignant pleural mesothelioma (MPM) is an aggressive cancer caused by exposure to asbestos
BAK is an Essential Regulator of Bortezomib-induced Apoptosis in MPM Cells
Bortezomib induced apoptosis in wild type mouse embryonic fibroblasts (WT MEF), but this effect was dramatically reduced in cells with homozygous deletion of BAX and BAK (BAX/BAK double knockout (DKO) MEF), as evidenced by PARP cleavage (Figure 1A) and caspase 3 activation (4.13 and 1.13 fold increase in Wild type (WT) and DKO respectively) (Figure 1B)

Summary

Introduction

Malignant pleural mesothelioma (MPM) is an aggressive cancer caused by exposure to asbestos. It is increasing in incidence worldwide there is a paucity of effective therapy [1]. Pemetrexed or raltitrexed when combined with cisplatin have been shown to lead to modest improvements in overall survival [2,3]. Patients universally relapse; following which, there is no agreed standard of care. MPM is a highly drug resistant cancer, and this is correlated with apoptosis resistance [4]. There is a pressing need for new, more effective therapies, where there is an unmet clinical need after first-line chemotherapy

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Results

Conclusion