| CD40, a tumor necrosis factor receptor superfamily member, is expressed on B-lymphocytes. Interaction between CD40 and its ligand CD40L, expressed on activated T-lymphocytes, is critical for normal B cell development and malignant B cell survival. In this study, we demonstrate that CD40 signals B cell survival, in part, via transcriptional activation of the NF-kappaB RelB subunit. B cell receptor engagement on the murine WEHI 231 B lymphoma cell, a model system often used to study clonal deletion of self-reactive B cells, leads to a decrease in NF-kappaB p50/c-Rel binding and induction of apoptosis, which can be rescued by CD40L treatment. Activation of p52/RelB NF-kappaB complexes, via the alternative NF-kappaB pathway, has been implicated in rescue by CD40L. Unexpectedly, we observed that CD40L treatment of WEHI 231 cells caused a greater increase in ReIB than p52. This led us to hypothesize that CD40 signals B cell survival via the de novo RelB synthesis pathway: the synergistic action of p50/p65 NF-kappaB and c-Jun/Fra-2 AP-1 complexes which potently activated the RelB promoter in breast cancer cells and NIH 3T3 cells. CD40L treatment of WEHI 231 cells led to induction of RelB mRNA and AP-1 binding, and to activation of c-Jun, JunD, JunB and Fra-2 AP-1 members. Co-transfection of these AP-1 subunits with p50/c-Rel led to synergistic activation of the RelB promoter, demonstrating their ability to function in de novo RelB synthesis. Ectopic expression of RelB protected WEHI 231 cells from anti-IgM-induced apoptosis, while knockdown of RelB through siRNA led to increased apoptosis following anti-IgM treatment. RelB expression affected the levels of the anti-apoptotic factors Survivin and MnSOD. Similarly, CD40L treatment of B-cell chronic lymphocytic leukemia (B-CLL) cells, isolated from different patients, led to an increase in RELB mRNA and protein levels and to increased mRNA levels of MNSOD and SURVIVIN, pro-survival RelB targets identified in WEHI 231 cells. Furthermore, CD40L stimulation of B-CLL cells led to induction of various NF-kappaB and AP-1 factors, consistent with de novo RelB synthesis. Thus, induction of de novo synthesis of RelB upon CD40 engagement plays an essential role in the survival of transformed B cells. |
