Mantle cell lymphoma (MCL) is a lymphoproliferative disorder derived from a subset of naive pregerminal center cells with a mature B-cell phenotype, characterized by the t(11;14)(q13;q32) translocation which results in cyclin D1 overexpression and cell cycle deregulation. MCL is an aggressive disease and current therapies do not bring long survival benefit to patients. In spite of the promising introduction of the proteasome inhibitor bortezomib (bz) in the clinical practice, not all the patients respond and resistance often appears after initial treatment. In order to improve MCL response to bz, we have generated two bz-resistant MCL cell lines, JBR and ZBR, characterized by a 10 and 4 fold increase of the LD50 for bz, when compared to their respective parental cell lines Jeko-1 and Z-138. Following treatment with bz, JBR and ZBR cells did not exhibit apparent defects in proteasome activity or in the cell death machinery, but presented increased levels of the prosurvival factor BiP/Grp78, an endoplasmic reticulum (ER) chaperone involved in the activation of the unfolded protein response (UPR) pathway and in tumor protection from stress and drug-induced cell death. Analysis of BiP induction in a set of 12 MCL cell lines and primary cultures further confirmed a tight correlation between an increase in BiP protein expression and the loss of sensitivity to bz. Unexpectedly, immunoprecipitation assays showed an increased chaperoning activity of the molecular chaperone Hsp90 toward BiP in bz-resistant cells. We assessed the activity of two Hsp90 inhibitors in the ansamycin class, 17-AAG and IPI-504 (Infinity Pharmaceuticals) on a panel of 20 MCL samples, with distinct cytogenetic abnormalities, growth characteristics and sensitivity to bz. IPI-504 and 17-AAG were found to be cytostatic with IC50 ranging from 0.06 to 15.4 µM at 72h, irrespective of p53 status and expression of the Hsp90 client proteins cyclin D1, Bcl-2, Akt and IKK\#946;. When combined with bz, IPI-504 allowed JBR and ZBR cells to overcome resistance to proteasome inhibition and led to a synergistic induction of apoptotic cell death, with increased loss of mitochondrial membrane potential, activation of caspases and phosphatidylserine exposure, reaching combination indexes (CIs) between 0.03 and 1.094. At the molecular level, addition of IPI-504 to bz treatment led to complete dissociation of Hsp90/BiP complexes followed by BiP depletion and consequent inhibition of hallmarks of UPR activation (eIF2\#945; phosphorylation, XBP-1 splicing and CHOP upregulation), and synergistic upregulation of the proapoptotic BH3-only protein Noxa. In summary, our results suggest that targeting UPR activation via the inhibition of Hsp90 may represent an attractive model for the design of a new and rational bortezomib-based combination therapy for MCL.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 5650.

100th AACR Annual Meeting-- Apr 18-22, 2009; Denver, CO

American Association for Cancer Research
2009