Selective Inhibition of I kappaB Kinase Sensitizes Mantle Cell Lymphoma B Cells to TRAIL by Decreasing c-FLIP Level.

Mantle cell lymphoma (MCL) is an aggressive B lymphoid neoplasm with a mature B-cell phenotype and genetically characterized by the t(11;14)(q13;q32) leading to cyclin D1 overexpression with the consequent deregulation of cell cycle at the G₁-S checkpoint. MCL cells also present a constitutive activation of the NF-kappaB pathway which leads to the overexpression of several anti-apoptotic regulators. We have analyzed sensitivity to the extrinsic apoptotic signal triggered by tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) on six human MCL cell lines and primary cells from 10 MCL patients, which differ in their p53-dependent pathway status, growth characteristics and sensitivity to cytotoxic drugs. TRAIL has been shown to exert in vivo a selective anti-tumor activity with minimal toxicity on normal cells. We observed that TRAIL was able to trigger apoptosis in a majority of MCL cell lines and primary MCL tumor cells, while sparing normal peripheral B cells. TRAIL-induced cell death was characterized by a time- and dose-dependent loss of membrane potential, Bax and Bak activation, caspase activation and phophatidylserine exposure. MCL sensitivity to TRAIL was irrespective of TRAIL-R1 and TRAIL-R2 receptor levels, Bcl-2 family members or caspase regulators expression, but was closely linked to the activity of the NF-kappaB p50 factor and to the expression of c-FLIP, a NF-kappaB-regulated factor. C-FLIP accumulated into the TRAIL-dependent complex in resistant cells and its transient knockdown overcame MCL resistance to TRAIL. In parallel, NF-kappaB inhibitors differentially modulated TRAIL cytotoxicity. Indeed, sub-toxic doses of bortezomib increased TRAIL cytotoxic effects by up-regulating TRAIL-R2 receptor expression, but also led to the intracellular accumulation of c-FLIP, impeding full synergistic interaction in cells with highest c-FLIP basal level. In contrast, the IkappaB kinase (IKK) inhibitor BMS-354451 allowed to consistent reduction of NF-kappaB activity, decreased total and DISC-associated c-FLIP levels, and sensitized all MCL cells to TRAIL cytotoxic effects. These results indicate that pharmacological enhancement of MCL cells sensitivity to TRAIL does not depend on TRAIL receptors level but is rather regulated by NF-kappaB-regulated c-FLIP expression. Considering that both TRAIL and BMS-345541 have already demonstrated selective cytotoxicity against malignant cells, combining TRAIL, with pharmacological inhibitors of IkappaB kinase signaling may represent an attractive model for the design of a new and rational combination therapy for MCL.