Prevalence of bortezomib-resistant constitutive NF-kappaB activity in mantle cell lymphoma

Background: The proteasome inhibitor bortezomib can inhibit activation of the transcription factor NF-kappa B, a mechanism implicated in its anti-neoplastic effects observed in mantle cell lymphoma (MCL). However, NF-kappa B can be activated through many distinct mechanisms, including proteasome independent pathways. While MCL cells have been shown to harbor constitutive NF-kappa B activity, what fraction of this activity in primary MCL samples is sensitive or resistant to inhibition by bortezomib remains unclear. Results: Proteasome activity in the EBV-negative MCL cell lines Jeko-I and Rec-I is inhibited by greater than 80% after exposure to 20 nM bortezomib for 4 hours. This treatment decreased NF-kappa B activity in Jeko-I cells, but failed to do so in Rec-I cells when assessed by electrophoretic mobility shift assay (EMSA). Concurrently, Rec-I cells were more resistant to the cytotoxic effects of bortezomib than Jeko-I cells. Consistent with a proteasome inhibitor resistant pathway of activation described in mouse B-lymphoma cells (WEHI231) and a breast carcinoma cell line (MDA-MB-468), the bortezomib-resistant NF-kappa B activity in Rec-I cells is inhibited by calcium chelators, calmodulin inhibitors, and perillyl alcohol, a monoterpene capable of blocking L-type calcium channels. Importantly, the combination of perillyl alcohol and bortezomib is synergistic in eliciting Rec-I cell cytotoxicity. The relevance of these results is illuminated by the additional finding that a considerable fraction of primary MCL samples (8 out of 10) displayed bortezomib-resistant constitutive NF-kappa B activity. Conclusion: Our findings show that bortezomib-resistant NF-kappa B activity is frequently observed in MCL samples and suggest that this activity may be relevant to MCL biology as well as serve as a potential therapeutic target.