IntroductionMantle cell lymphoma (MCL) represents ϳ 5%-10% of all nonHodgkin lymphomas. It is characterized by the expansion of mature B-clonal lymphocytes harboring the t(11;14)(q13;q32) translocation, which induces overexpression of cyclin D1, and consequent cell cycle deregulation. In addition, MCL tumor cells carry a high number of secondary chromosomal and molecular alterations affecting proteins involved in cell cycle progression, senescence, and cellular response to DNA damage. 1 The prognosis for this disease is in most cases extremely poor, with a median survival of 5-7 years. 2 Conventional chemotherapeutic regimens have been the standard treatment of MCL. However, all these strategies are rapidly confronted with the onset of resistance, and allogenic bone marrow transplantation represents the only potential curative approach in young patients. 3 Recently, new therapeutic approaches have been entered into the clinic, and the proteasome inhibitor bortezomib was approved for the treatment of relapsed/refractory MCL. 4 Bortezomib induces a p53-independent cytotoxicity, mediated by the proapoptotic BH3-only protein NOXA, and the generation of reactive oxygen species in MCL cells. 5 Further studies have shown a synergism between bortezomib and the BH3-mimetics, obatoclax and ABT-737, highlighting the relevance of NOXA induction to counteract the protective effect of MCL-1 accumulation in proteasomecompromised cells. 6,7 Hypotheses to explain how MCL cells can acquire resistance to bortezomib include the overexpression and/or accumulation of antiapoptotic BCL-2 proteins, the rise of a bortezomib-resistant nuclear factor-B activity, and an increased proteasomal activity. [8][9][10] Despite these studies, the precise mechanism by which bortezomib-resistant MCL cases could lose the capacity to undergo NOXA-mediated mitochondrial apoptosis remains unclear.A key defect might reside within the endoplasmic reticulum (ER) stress pathway, because its activation in MCL cells exposed to bortezomib has been recently shown to elicit NOXA transcription. 11 ER homeostasis is controlled by the immunoglobulin heavy chain binding protein (BiP), also referred as 78-kDa glucose-regulated protein (Grp78). BiP/Grp78 forms a large multiprotein complex with a set of other ER molecular chaperones, including the heat shock protein of 90 kDa (Hsp90) ER homolog, Grp94, protein disulfide isomerase, calcium binding protein, and cyclophilin B. 12 Under nonstressed conditions, BiP/Grp78 binds to and maintains in an inactive monomeric state the ER transmembrane PKR-like ER kinase, inositol requiring protein 1 (IRE1), and the bZIP activating transcription factor 6. 13,14 After proteasome inhibition, the accumulation of polyubiquitinated and misfolded proteins within the ER lumen leads to BiP/Grp78 dissociation from the luminal domains of these sensor proteins, leading to the initiation of the unfolded protein response (UPR): the cytosolic domain of activating transcription factor 6 is cleaved, enabling the protein to translocate and to activ...

Bortezomib therapy has shown promising clinical activity in mantle cell lymphoma (MCL), but the development of resistance to proteasome inhibition may limit its efficacy. To unravel the factors involved in the acquisition of bortezomib resistance in vivo, immunodeficient mice were engrafted with a set of MCL cell lines with different levels of sensitivity to the drug, followed by gene expression profiling of the tumors and functional validation of the identified gene signatures. We observed an increased tumorigenicity of bortezomib-resistant MCL cells in vivo, which was associated with plasmacytic differentiation features, like interferon regulatory factor 4 (IRF4) and Blimp-1 upregulation. Lenalidomide was particularly active in this subgroup of tumors, targeting IRF4 expression and plasmacytic differentiation program, thus overcoming bortezomib resistance. Moreover, repression of the IRF4 target gene MYC in bortezomib-resistant cells by gene knockdown or treatment with CPI203, a BET (bromodomain and extra terminal) bromodomain inhibitor, synergistically induced cell death when combined with lenalidomide. In mice, addition of CPI203 to lenalidomide therapy further decreased tumor burden, involving simultaneous MYC and IRF4 downregulation and apoptosis induction. Together, these results suggest that exacerbated IRF4/MYC signaling is associated to bortezomib resistance in MCL in vivo and warrant clinical evaluation of lenalidomide plus BET inhibitor combination in MCL cases refractory to proteasome inhibition.

Purpose: Mantle cell lymphoma (MCL) is an aggressive B-cell neoplasm with generally poor prognosis, for which current therapies have shown limited efficacy. Vorinostat is a histone deacetylase inhibitor (HDACi) that has been approved for the treatment of cutaneous T-cell lymphoma. Our purpose was to describe the molecular mechanism whereby vorinostat induces apoptosis in MCL with particular emphasis on the role of proapoptotic BH3-only proteins.Experimental Design: The sensitivity to vorinostat was analyzed in eight MCL cell lines and primary cells from 10 MCL patients. Determination of vorinostat mechanism of action was done by flow cytometry, immunoblotting, HDAC activity assay kit, quantitative reverse transcription PCR, chromatin immunoprecipitation, and siRNA-mediated transfection.Results: Vorinostat inhibited total histone deacetylase activity leading to selective toxicity toward tumor cells. Vorinostat-mediated cell death implied the activation of mitochondrial apoptosis, as attested by BAX and BAK conformational changes, mitochondrial depolarization, reactive oxygen species generation, and subsequent caspase-dependent cell death. This phenomenon was linked to H4 hyperacetylation on promoter regions and consequent transcriptional activation of the proapoptotic BH3-only genes BIM, BMF, and NOXA. Selective knockdown of the three corresponding proteins rescued cells from vorinostatinduced apoptosis. Moreover, vorinostat enhanced the activity of the BH3-mimetic ABT-263 in MCL cells, leading to synergistic apoptosis induction.Conclusion: These results indicated that transcriptional upregulation of BH3-only proteins plays an important role in the antitumoral activity of vorinostat in MCL, and that HDACi alone or in combination with BH3-mimetizing agents may represent a promising therapeutic approach for MCL patients.