Rituximab-Mediated Inhibition of the p38 MAPK and NF-κB Survival Pathways In Vitro and Validation in Nude Mice Bearing the Raji B-NHL Tumor.

Treatment of B-NHL cell lines with rituximab inhibited the p38 MAPK and NF-κB pathways, resulting in chemosensitization to drug-induced apoptosis (Vega et al., 2004 Oncogene, 23:3530–40; Jazirehi et al., 2005 Cancer Res., 65:264–76). Chemosensitization was the direct result of inhibition by these pathways of the anti-apoptotic gene products Bcl-2/Bcl-_xL. Cell signaling by rituximab in B-NHL patients has not been investigated and thus, we have examined an in vivo model bearing a tumor xenograft for validation. Balb/c nude mice were transplanted s.c. with the B-NHL cell line Raji; a group of mice was left untreated and another group was treated with rituximab (1000 μg) at days 5 and 10 following implantation of 8x10⁶ tumor cells. The animals were monitored for tumor cell growth and sacrificed at day 30 and tumors were obtained for further analysis. Treatment with rituximab resulted in significant inhibition of tumor cell growth compared to control. Tumor tissues were examined by immunohistochemistry for phospho and non-phospho p38 MAPK and NF-κB (p50), Bcl-2 and Bcl-_xL expression. Analysis of tumor-derived tissues from control mice revealed overexpression of phospho-p38 MAPK and NF-κB (p50) and strong nuclear localization of phospho-p50. In addition, there was overexpression of Bcl-2 and Bcl-_xL. In contrast, tumor tissues derived from rituximab-treated mice demonstrated significant inhibition of phospho-p38 MAPK, phospho-p50-NF-κB and reduced nuclear localization of phospho-p50. In addition, Bcl-2 and Bcl-_xL expression was also significantly reduced. These findings established for the first time, in a pre-clinical model, rituximab-mediated inhibition of the cell survival pathways mediated by p38 MAPK and NF-κB. In addition, the study also corroborates the role of Bcl-2 and Bcl-_xL expression in resistance and their inhibition by rituximab. In a separate abstract, tumor tissues derived from many untreated B-NHL patients were analyzed by immunohistochemistry for the activation of both phospho-p38 MAPK and NF-κB and compared to biopsies derived from control individuals. Overall, this study suggests that the p38 MAPK and NF-κB survival pathways are targets for rituximab-mediated effects and also suggests that such targets can be used for intervention in cases of rituximab resistance.