Previous studies have documented that, in malignant B cells, rituximab elicits a complex and not yet totally understood signaling network contributing to its antitumor effect. In this context, we investigated the role of protein kinase C (PKC), an atypical PKC isoform, in the cellular response to rituximab. We found that follicular lymphoma cells displayed an increase in PKC expression and activity levels, compared with nonmalignant B cells, and that this enzyme was a critical regulator of the classical MAPK module by stimulating Raf-1 kinase activity. PKC appeared to be a significant contributor of abnormal mTOR regulation in follicular lymphoma cells through a MAPK-dependent mechanism. Rituximab was found to inhibit the PKC/MAPK/mTOR module in these cells but not in other B-cell lymphomas. Importantly, the expression of a constitutively active form of PKC resulted in an efficient protection of these cells toward rituximab. Altogether, our study describes a new regulatory component of mTOR pathway in follicular cell lymphoma and demonstrates that PKC is a target for rituximab. Therefore, PKC could represent an important parameter for rituximab efficacy and a promising target for future targeted therapy in follicular lymphoma.
IntroductionRituximab (RTX) is an anti-CD20 mouse/human chimeric IgG1-antibody that targets the CD20 antigen found on the surface of malignant and normal B cells. RTX used as a single agent has demonstrated efficacy in patients with various lymphoid malignancies, including indolent and aggressive forms of non-Hodgkin lymphoma (NHL), as well as in chronic lymphocytic leukemia (CLL). Moreover, it has some therapeutic activity in antibodybased autoimmune diseases. 1 However, the most significant contribution of this new agent is that it greatly improves the efficacy of chemotherapy in the treatment of various forms of lymphoid neoplasias, including follicular lymphoma (FL), mantle cell lymphoma (MCL), or diffuse large B-cell lymphoma (DLBCL). [2][3][4] The mechanism by which RTX facilitates tumor eradication by chemotherapy remains unknown. Several hypotheses have been raised, including a simple additive effect or a true synergistic mechanism mediated by RTX-induced activation of proapoptotic signals. Indeed, as documented by Bonavida and coworkers, RTX-mediated CD20 engagement resulted in reduced expression of Bcl-2 or Bcl-xL, enhanced expression of Fas death receptor, as well as in the negative regulation of NF-B or MAPK signaling pathways, depending on the cellular models (Jazirehi et al 5 and Vega et al 6 ). In parallel, our group has described that RTX can also activate the sphingomyelin cycle, a well-defined metabolic process resulting in the accumulation of ceramide, a lipid second messenger that primes cell to inhibition of proliferation and/or apoptosis. 7 Based on these findings, we raised the possibility that, in the context of RTX-treated FL cells, ceramide targets a common regulator of one or several of these prosurvival signaling pathways. This question presents an evident prac...
