NIK Is Involved In the Activation of the Classical and Alternative NF-κB Pathways In Diffuse Large B Cell Lymphoma

Abstract 3099

Deregulated NF-κB activity plays a role in the lymphoma pathogenesis, and has been proposed to constitute a cardinal feature of some subtypes of diffuse large B cell lymphoma (DLBCL). The NF-κB-Inducing Kinase (NIK) is essential for the activation of the alternative NF-κB pathway by inducing the phosphorylation of the NF-κB member p100, which leads to its processing to p52 and its subsequent nuclear translocation. A role for NIK in the classical NF-κB pathway as well has been shown, suggesting NIK as an attractive therapeutic target in lymphomas. Here, we study the frequency and extent of alternative and classical NF-κB activation in diffuse large B cell lymphoma, and the implication of NIK in both pathways. The activation of the classical and alternative NF-κB pathways was present in 28 and 34% of DLBCL cases respectively, as assessed by nuclear expression of p50 (classical pathway) and p52 (alternative pathway) by immunohistochemistry in a series of 301 samples. Activation of both NF-κB pathways was observed in germinal centre B-cell like (GC) and activated B-cell like (ABC) subtypes, with a slight predominance, although not significant, in ABC subtype. In contrast, the levels of p52 and p50 were significantly higher in ABC-DLBCL cell lines than those of GC subtype. The activation of both pathways was mostly overlapped and there was a strong positive correlation between nuclear p52 and p50 (p<0.001). Eighteen % of the cases expressed both p50 and p52 while only 8 and 16% expressed exclusively p50 or p52, respectively. Activation of the alternative NF-κB pathway was strongly associated with Epstein-Barr virus (EBV), since 93% of EBV+ cases expressed nuclear p52 (p<0.001). In our study, no TRAF3 deletions were detected in a panel of 25 DLBCL samples, although absence of TRAF3 was observed in one DLBCL cell line. Since NIK acts as a bottleneck in the activation of the alternative pathway but has also been described to play a role in the classical pathway, we wanted to analyze the effect of the knockdown of NIK on both pathways. Using small interference RNA in two lymphoma cell lines, we observed that the silencing of NIK had an effect on both pathways, decreasing the processing of p100 as well as p105. Taken together, our results show that the activation of NF-κB distinguishes a subset of DLBCL cases, comprising both ABC and GC subtypes, suggest a frequent overlap between the classical and alternative NF-κB pathway in DLBCL, and identify a possible role for NIK in the activation of both pathways.