Activation of Classical and Alternative Nuclear Factor-kappaB (NF-kB) Pathways in Diffuse Large B-Cell Lymphomas.

Activation of the NF-kB pathway is involved in many human neoplasms. In this study we examined the status of the NF-kB canonical (IkB, p50/p65) and non-canonical (p52, RelB) pathways in diffuse large B-cell lymphomas (DLBCL), which are a common and heterogeneous group of lymphoid malignancies. DLBCL have been divided into activated B-cell (ABC) like, and germinal center B-cell (GCB) like subgroups, which have been reported to have high and low NF-kB activity, respectively. However, the nature of the NF-kB complexes in this lymphoma entity has not been previously evaluated. Therefore we performed Western blotting, electrophoretic mobility shift assays (EMSA) and real time quantitative RT-PCRs on nuclear and cytoplasmic protein and total RNA extracts in 20 primary tumor samples and 9 different DLBCL cell lines. In the cell lines, presence of NF-kB proteins in nuclear extracts correlated with expression of NF-kB target genes (CCR7, IkBa, CCND2, BCL-2, IRF4) as determined by RT-PCR. Therefore, these could be assigned into GCB and ABC-like categories. In primary DLBCLs, EMSA showed NF-kB binding in all but one case. The same cases (19/20) had high p52 in the nucleus indicating activation of the alternative pathway. TNF family ligand BAFF was also found to be expressed in all primary samples and most cell lines. The classical pathway, as determined by nuclear p50, was also present in these cases. Levels of p65 and RelB expression were variable, but did not correlate with the mRNA expression of NF-kB target genes. In conclusion, while DLBCL cell lines may be divided into two distinct categories, the primary samples represented a spectrum of NF-kB target gene activity, while levels of NF-kB expression were high. BAFF expression and activation of the alternative pathway may be important in the pathogenesis of DLBCL.