NF-κB–independent down-regulation of XIAP by bortezomib sensitizes HL B cells against cytotoxic drugs

The proteasome inhibitor bortezomib has been shown to possess promising antitumor activity and significant efficacy against a variety of malignancies. Different studies demonstrated that bortezomib breaks the chemoresistance in different tumor cells basically by altering nuclear factor-kappaB (NF-kappaB) activity. NF-kappaB has been shown to be constitutively active in most primary Hodgkin-Reed-Sternberg (H-RS) cells in lymph node sections and in Hodgkin lymphoma (HL) cell lines and was suggested to be a central molecular switch in apoptosis resistance in HL. Here we report a bimodal effect of bortezomib in HL cells. Whereas high-dose bortezomib induced direct cytotoxicity that correlated with decreased NF-kappaB activity, low-dose bortezomib sensitized HL cells against a variety of cytotoxic drugs without altering NF-kappaB action. Strikingly, bortezomib induced marked XIAP down-regulation at the posttranslational level that was independent of the NF-kappaB status. Similarly, RNA interference (RNAi)-mediated XIAP down-regulation generated susceptibility to cytostatic agents. The results identify XIAP as an NF-kappaB-independent target of bortezomib action that controls the chemoresistant phenotype of HL cells.