Contrary Functions of NF-κB Signaling in Anti-Cancer Treatment Outcome

Abstract 1652

Nuclear factor-κB (NF-κB) transcription factors are involved in cancer-relevant processes such as suppression of apoptosis, growth promotion, enhanced migration and invasiveness, although their actual role as oncogenic or tumor-suppressive activities remains controversial. Moreover, NF-κB-mediated suppression of apoptosis has been linked to chemoresistance. Interestingly, cellular senescence, a terminal cell-cycle arrest initiated via DNA-damaging chemotherapy as well, and known to improve long-term outcome, is associated with the massive induction of secretable NF-κB target genes, which, in turn, potentially reinforce the senescence phenotype.

In this study, primary Eμ-myc transgenic mouse lymphomas as a well established model for human aggressive B-NHL, and information from human diffuse large B-cell lymphomas (DLBCL) were used in a cross-species approach to identify oncogenic networks in which chemotherapy-activated NF-κB signaling no longer mediates resistance but promotes therapy-induced senescence (TIS) and contributes to the outcome of anti-cancer treatment.

Primary Myc-driven lymphoma cells, stably bcl2-transduced to block apoptosis, were exposed to the DNA-damaging chemotherapeutic agent adriamycin (ADR) to induce TIS. Gene set enrichment analysis of microarray-based gene expression profiles from drug-senescent vs. untreated cells found NF-κB target genes strongly skewed towards the TIS group, and multiplex ELISA-based analysis detected significantly higher DNA binding activities for the NF-κB family subunits p50, p52, p65 (RelA) and RelB in senescent cells.

Inactivation of NF-κB by stable expression of the NF-κB super-repressor IκBα-δN (SR) lowered expression levels of NF-κB target genes in ADR-treated lymphomas. Matched pairs of individual primary lymphomas differing only in their SR status displayed compromised senescence induction in vivo when expressing the SR, indicating that TIS depends on intact NF-κB function. To assess the contribution of endogenous NF-κB signaling to long-term outcome, we grouped primary Myc-lymphomas by their NF-κB activity levels as “NF-κB low” (NL) or “NF-κB high” (NH). ADR-treated NH, but not NL lymphomas presented selective vulnerability to the SR moiety. Recapitulating the clinical outcome of patients suffering from DLBCL, around 60% of the mice harboring Eμ-myc lymphomas achieved long-term remissions, while the remaining 40% encountered a relapse after chemotherapy. Relapsing Myc-lymphomas exhibited substantially higher expression of the NF-κB targets IκBα and bcl2, reminiscent of activated B-cell-like (ABC) DLBCL, the clinically inferior subtype characterized by constitutively active NF-κB signaling. In contrast, germinal center B-cell-like (GCB) DLBCL rarely possess activating NF-κB mutations, but frequently develop in the context of a t(14;18) translocation that drives Bcl2 overexpression independent of NF-κB. Induced NF-κB target gene expression and increased TIS induction after overexpression of an NF-κB-activating CARD11 mutant suggested that higher NF-κB activity may contribute to treatment outcome via TIS promotion. Indeed, stratifying a large dataset of untreated GEP and corresponding clinical data after immunochemotherapy from 233 DLBCL patients by a 63-gene NF-κB expression signature (Shaffer-AL et al, Immunol Rev, 2006; Lenz-G et al, N Eng J Med, 2008), confirmed for the subset of GCB patients with above-median Bcl2 expression – the group whose genetic features we modeled before in the mouse – a significantly superior progression-free survival.

In essence, our “cross-species” investigations demonstrate opposing roles of NF-κB in treatment outcome and have important ramifications for clinical trials that aim at inhibiting NF-κB signaling in DLBCL.