Life after death: targeting high mobility group box 1 in emergent cancer therapies

Abstract

High mobility group box 1 (HMGB1), an evolutionarily highly conserved and abundant nuclear protein also has roles within the cytoplasm and as an extracellular damage-associated molecular pattern (DAMP) molecule. Extracellular HMGB1 is the prototypic endogenous 'danger signal' that triggers inflammation and immunity. Recent findings suggest that posttranslational modifications dictate the cellular localization and secretion of HMGB1. HMGB1 is actively secreted from immune cells and stressed cancer cells, or passively released from necrotic cells. During cancer development or administration of therapeutic agents including chemotherapy, radiation, epigenetic drugs, oncolytic viruses, or immunotherapy, the released HMGB1 may either promote or limit cancer growth, depending on the state of progression and vascularization of the tumor. Extracellular HMGB1 enhances autophagy and promotes persistence of surviving cancer cells following initial activation. When oxidized, it chronically suppresses the immune system to promote cancer growth and progression, thereby enhancing resistance to cancer therapeutics. In its reduced form, it can facilitate and elicit innate and adaptive anti-tumor immunity, recruiting and activating immune cells, in conjunction with cytotoxic agents, particularly in early transplantable tumor models. We hypothesize that HMGB1 also functions as an epigenetic modifier, mainly through regulation of NF-kB-dependent signaling pathways, to modulate the behavior of surviving cancer cells as well as the immune cells found within the tumor microenvironment. This has significant implications for developing novel cancer therapeutics.

Keywords: CD8+ T cells; Cancer; HMGB1; NF-kB signaling; activation; dendritic cells; epigenetic pathways; innate immunity.

Figure 1

A schematic representation of gene structure, protein domains and their associated functions of human HMGB1. Upper Panel: The 5 exons of the gene are indicated by boxes (open for translated regions and solid for untranslated regions). Middle Panel: The human HMGB1 has 215 amino acid residues and can be divided into three domains. Box A domain at the N-terminal is required for DNA binding, Box B is also required for DNA binding, and a fragment peptide within (#80 to 123) is required for cytokine activity. Peptide #89-108 is required for TRL4 binding and peptide #150-183 for RAGE-binding. Peptide Hp91 (#91 to 108) is an immune adjuvant peptide [113]. The C tail is acidic and codes for regulatory functions. There are two nuclear localization signals (NLS1, #27-43, and NLS2, #178-184) [6]. Three cysteine residues, C23, C45 and C106 are labeled. C23 and C45 can form an intramolecular disulfite bridge, while C106 is essential for binding to TLR4. Lower Panel: Listed are main functions of four different localized forms of HMGB1. The interplay between mitochondria and two forms of HMGB1 are based on two recent studies [41,42].

Figure 2

Cancer promoting properties of HMGB1. In cancer, HMGB1 dysfunction is associated with each of the six hallmarks of cancer the latter of which were summarized by Hanahan and Weinberg [88]. We have presented an earlier version of this association in our previous review [89].

Figure 3

HMGB1 in the tumor microenvironment and hypothesis of HMGB1 functioning via NF-kB and epigenetic pathways. A. The release of HMGB1 in the tumor microenvironment. HMGB1 has yin-yang effects on cancer. Acute release of HMGB1 after cancer therapies promotes maturation of DCs through interaction with TLR4 and clonal expansion of tumor antigen-specific T cells and thus elicits antitumor responses. In contrast, persistent hypoxia in growing tumors leads to necrosis, causing chronic release of HMGB1, which promotes angiogenesis and tumor growth through the recruitment of macrophages (TAM) and endothelial precursor cells (EPC) and activation of local endothelial cells through RAGE signaling. The acutely and chronically released HMGB1 molecules are represented in red and black, indicating that they may be differently modified by redox or other mechanisms. This figure is modified from a figure by Srikrishna and Freeze [58]. B. The hypothesis of HMGB1 acting via NF-kB and epigenetic pathways to exert its long lasting effects on surviving tumor cells, immune cells and stromal cells in the tumor microenvironment. The NF-kB pathway is regulated partly via epigenetic pathways (EP) both upstream and downstream. STAT3 and NF-kB, both inflammation-regulated genes, regulate genes to promote cancer cell proliferation, survival, caner angiogenesis and metastasis. As an example to illustrate the point of role of epigenetic mechanisms, we showed that IL-6, produced and released from macrophage stimulated by HMGB1 and modulated via epigenetic mechanisms, regulates transcription of DNMT1, one of the key enzymes for DNA methylation in cancer cells. The TNF-stimulated and phosphorylated p65 binds and recruits DNMT1 to the promoter of BRMS1 gene, thus inhibits the production of BRMS1, an inhibitor of tumor metastasis.