Introduction: NF-kB family of transcription factors are constitutively active in mantle cell lymphoma (MCL), being the NF-kB pathway implicated in survival and apoptosis inhibition. Activation of this pathway is regulated by IKKa and IKKb, that phosphorylates the NF-kB inhibitor IkBa, leading to its degradation and activation of NF-kB transcription factors. We analyzed the effect of two specific NF-kB inhibitors, the selective inhibitor of IKKs BMS-345514 (IKKa IC50=3.5uM; IKKb IC50=1uM), and the specific inhibitor of the IKKb subunit ML120B (IC50=3.3uM), on cell proliferation and apoptosis in MCL. Results: Whereas treatment of MCL primary cells and MCL cell lines (Jeko and Z138) with BMS-345541 (4-6 uM) were cytotoxic, the IKKb inhibitor ML120B (10-20-40 uM) showed no effect. BMS-345541 (2-4-6 uM) and ML120B (5-10 uM) decreased cell proliferation as well induced apoptosis in the DLBCL cell line OCI-Ly3, being highly sensitive to IKKb inhibition, in concordance with the constitutive expression of IKKb of DLBCL cells. IKKb has a higher activity for IkBa than IKKa, being also more critical during NF-kB-mediated cell proliferation induced by TNF-a. Both inhibitors, ML120B and BMS-345541 lead to a complete inhibition of cell proliferation upon TNF-a stimulation in all the cell lines analyzed (Jeko, Z-138, Granta-519, JVM-2 and OCI-Ly3). The NF-kB transcription factors p65 and p50 are retained in the cytosol by its binding with the IkBa protein. Both IKK inhibitors decreased the basal levels of phosphorylated IkBa, decrease that was higher after treatment with ML120B. IKKa and IKKb are able to phosphorylate p65, leading to activation of p65/p50. ML120B and BMS-345541 decreased nuclear levels of p50, p65 and phospho-p65 (Ser536) in the DLBCL cell line, whereas in MCL cells only BMS-345541 was able to affect the levels of p50 and phospho-p65, despite a complete decrease on phospho-IkBa exerted by ML120B. Total, nuclear and DNA-bound basal levels of IkBa, IKKb, p65 and p50 showed a different pattern between MCL and DLBCL. Despite MCL cells express higher levels of IKKb, all the IkBa protein is phosphorylated in the DLBCL cell line that also showed higher levels of phospho-p65, p65 and p50. These differences could explain the cytotoxic effect of ML120B and the crucial contribution of IKK b/NF-kB pathway on cell proliferation of DLBCL. MCL cells overexpresses cyclin D1, cyclin implicated in cell proliferation. It has been described that IKKa positively regulates the levels of b-catenin, which controls cyclin D1 expression and cell proliferation in several tumors. Treatment of MCL cells with BMS-345541 (2-4-6 uM) decreased the levels of b-catenin and cyclin D1, whereas ML120B had no effect. Conclusions: Our results show differences between the contribution of IKKa and IKK b on NF-kB pathway in MCL and DLBCL, suggesting an important role of IKKa in MCL.

Citation Information: In: Proc Am Assoc Cancer Res; 2009 Apr 18-22; Denver, CO. Philadelphia (PA): AACR; 2009. Abstract nr 1790.

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