Discrepant NOXA (PMAIP1) transcript and NOXA protein levels: a potential Achilles’ heel in mantle cell lymphoma

A Weilbacher,A M Staiger,M Gutekunst,H Van Der Kuip,W E Aulitzky,G Ott,M C Vöhringer,M A Dengler,H Horn

doi:10.1038/cddis.2013.552

Abstract

Mantle cell lymphoma (MCL) is an aggressive lymphoid neoplasm with transient response to conventional chemotherapy. We here investigated the role of the Bcl-2 homology domain 3-only protein NOXA for life–death decision in MCL. Surprisingly, NOXA (PMAIP1) mRNA and NOXA protein levels were extremely discrepant in MCL cells: NOXA mRNA was found to be highly expressed whereas NOXA protein levels were low. Chronic active B-cell receptor signaling and to a minor degree cyclin D1 overexpression contributed to high NOXA mRNA expression levels in MCL cells. The phoshatidyl-inositol-3 kinase/AKT/mammalian target of rapamycin pathway was identified as the major downstream signaling pathway involved in the maintenance of NOXA gene expression. Interestingly, MCL cells adapt to this constitutive pro-apoptotic signal by extensive ubiquitination and rapid proteasomal degradation of NOXA protein (T½∼15–30 min). In addition to the proteasome inhibitor Bortezomib, we identified the neddylation inhibitor MLN4924 and the fatty acid synthase inhibitor Orlistat as potent inducers of NOXA protein expression leading to apoptosis in MCL. All inhibitors targeted NOXA protein turnover. In contrast to Bortezomib, MLN4924 and Orlistat interfered with the ubiquitination process of NOXA protein thereby offering new strategies to kill Bortezomib-resistant MCL cells. Our data, therefore, highlight a critical role of NOXA in the balance between life and death in MCL. The discrepancy between NOXA transcript and protein levels is essential for sensitivity of MCL to ubiquitin-proteasome system inhibitors and could therefore provide a druggable Achilles' heel of MCL cells.

Highlights

We found a stunning discrepancy between constitutive NOXA mRNA and NOXA protein expression in Mantle cell lymphoma (MCL) cell lines and primary cells
MCL cells constitutively express high levels of NOXA transcript mediated by an active phoshatidyl-inositol-3 kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) signaling pathway
NOXA protein expression was found to be low because of rapid ubiquitin-proteasome system (UPS)-mediated degradation. Targeting this high NOXA protein turnover with different inhibitors of the UPS led to accumulation of NOXA protein and induction of NOXA-dependent apoptosis

Summary

Introduction

This alteration causes cell cycle deregulation and is considered the primary oncogenic event in MCL pathogenesis.[1,2] MCL cells carry a high number of secondary genetic alterations further disturbing cell cycle regulation and/or DNA damage response.[3,4] In addition, deregulation of different oncogenic signaling pathways involved in proliferation and/or survival,[2,3] such as constitutive activation of B-cell receptor (BCR),[5] phoshatidyl-inositol-3 kinase (PI3K)/ AKT/mammalian target of rapamycin (mTOR)[6] and NF-kB signaling pathways,[7] and defects in apoptosis regulation[8,9] have been described in MCL. We recently demonstrated that high constitutive NOXA levels are central for hypersensitivity of embryonal carcinoma cells to cisplatin.[19]

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Results

Conclusion