The DNA-Binding Protein High Mobility Group Box 1 (HMGB1) Interacts with RelB to Maintain Non-Canonical NF-κb Signaling, Cell Survival and Immune Evasion in Infant Acute Lymphoblastic Leukemia

• Published in: Blood 2016-12, Vol.128 (22), p.3913-3913
• Main Authors: Toia, Liana M, Magno, Jinno Antonio, Shand, Jessica C
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood.V128.22.3913.3913

Disordered NF-κB signaling is a hallmark of many hematologic malignancies but has not been well-studied in MLL-rearranged infant lymphoblastic leukemia (iMLL-ALL), a subtype of pediatric ALL with an extremely poor prognosis in need of new therapy. Given the stable genome and short latency of iMLL-ALL, targeting of oncoproteins that play a coordinated role in regulation of the leukemic transcriptome represent ideal targets. Here, we present one such potential target: a complex formed between the nucleosome-stabilizing protein high mobility group box 1 (HMGB1) and RelB, a transcriptionally active subunit of the noncanonical NF-κB pathway. Disruptions in the non-canonical NF-κB pathway, which normally functions to support lymphogenesis, are known to drive lymphoid malignancies. Specifically, changes in proteasomal processing of the p100 subunit modulate nuclear p52/RelB regulation of the NF-κB2 gene. In stable short-term culture, we observed that primary infant MLL-ALL cells (derived from the diagnostic pheresis specimen of a 9 month old with t(4;11)) express an endogenous NF-κB subunit profile consistent with constitutive non-canonical activation, including significantly increased levels of IKKa, RelB, and NF-κB2 by Western Blot and qPCR compared to healthy human B cells and cytogenetically normal ALL. First, we asked whether exogenous "drivers" of the canonical (p65/RelA) pathway could alter this expression profile by treating iMLL-ALL with TNFalpha and bacterial LPS. Exogenous cytokine treatment did not alter expression levels of IKKa, RelB or NF-κB2, nor did it increase p65RelA, likely due to the lack of pattern recognition receptors on precursor lymphoblastic cells. We have previously demonstrated that infant MLL-ALL express high intracellular levels of the nucleosome-stabilizing protein high mobility group box-1 (HMGB1). During infection, HMGB1 forms a transcription regulatory complex with RelB that, by epigenetic means, silences inflammatory cytokine promoters as a negative feedback. In this study, we hypothesized that HMGB1-RelB interactions stabilize and maintain a pro-leukemic NF-κB program. Because the function of HMGB1 is determined by post-translational modifications, we first verified the DNA-binding disulfide form of the protein in the nuclear fraction of iMLL-ALL cells by immunoprecipitation. Next, we proved a physiologically relevant interaction between HMGB1 and RelB by co-immunoprecipitation; both HMGB1 and RelB could be identified by immun