HMGB1 Functional Overview
High mobility group box 1 (HMGB1), a member of the high-mobility group (HMG) family, was first identified in the thymus in 1973 and is classified as a non-histone DNA-binding protein (Goodwin et al., 1973). HMGB1, a 215 amino-acid protein, is structurally composed of two HMG-box domains, which are DNA-binding, and C-terminal tail which facilitates protein-protein interactions (Park et al., 2004; Stros et al., 2010). Nuclear HMGB1 acts as a DNA chaperone and contributes to several regulatory processes in the nucleus such as transcriptional stability, nucleosome assembly and DNA replication (Agresti et al., 2003; Reeves et al., 2010). Although nuclear HMGB1 has a high affinity for DNA, structural modification of HMGB1 through hyperacetylation results in the translocation of HMGB1 from the nucleus to the cytosol, regulated by the JAK/STAT1 pathway, where its function is altered (Lu et al., 2003). The specific cellular localization of HMGB1 is reflective of its activation state, with the cytosolic translocation of HMGB1 from the nucleus occurring in response to multiple stimuli including cell death by necrosis, or following inflammatory induction. In this way HMGB1 acts as a sophisticated danger signal, and can be further secreted extracellularly in a lysosomal-dependent manner either by passive release following cell death or active release upon immune response (Yang et al., 2010).
In the cytoplasm HMGB1 can regulate autophagy upon interaction with Beclin-1 (Kang et al., 2010). Extracellular HMGB1 acts as a DAMP (danger-associated molecular pattern), triggers the inflammatory response by binding to membrane bound immune receptors such as toll-like receptor 4 (TLR4) and receptor for advanced glycation endproducts (RAGE) (Hori et al., 1995; Park et al., 2004).
Due to the pleiotropic functions of HMGB1, which are determined by post-translational modifications and dependent on its cellular localization, HMGB1 has a diverse functional role in regulating processes in inflammation and cancer.
HMGB1 and the Inflammatory Response
Accumulating evidence links HMGB1 with inflammatory pathogeneses, initially identified to have a specific role as a mediator of sepsis (Wang et al., 1999). Secreted HMGB1 is present in several forms, including a thiol or disulfide acetylated forms, or non-acetylated forms (Venerau et al., 2012), highlighting the functional importance of the specific post-translational modification associated with necrotic or inflammatory release. Importantly, HMGB1 release from apoptotic cells has been identified, however it is secreted in an oxidized from which induces cellular tolerance rather than a pro-inflammatory response (Bell et al., 2006).
HMGB1 mediated activation of TLR2 and TLR4 leads to MYD88 dependent downstream signaling to the IKK complex, subsequently inducing NF-Kappa Beta activation eliciting pro-inflammatory cytokine induction, and also promotes non-inflammatory pathways via STAT3 and Smad3 signalling (Conti et al, 2013). Cytosolic activation of TLR9 by HMGB1 also culminates in downstream NF-Kappa Beta activation in a MyD88 dependent manner, whereas HMGB1-RAGE signaling activates NF-Kappa Beta via p38 and ERKMAP kinase pathways (Bianchi et al., 2007; Dai et al., 2010).While HMGB1 alone can activate several cell surface receptors, HMGB1 can associate with molecules extracellularly with receptor specificity depending on this extracellular interaction (Müller et al., 2001). For example, HMGB1 binds to DNA initiating activation of RAGE and TLR9 (Tian et al., 2007), whereas HMGB1-nucleasome interactions lead to TLR2 activation ( Urbonaviciute et al., 2008). Additionally, HMGB1 has been found to synergize with LPS and enhance the sensitivity of TLR4, thereby increasing overall cytokine production (Youn et al., 2008).
HMGB1 and Autophagy
In addition to its DNA binding function and role as a DAMP, cytosolic HMGB1 can also regulate autophagy. Direct interaction with Beclin 1 induces downstream autophagy (Tang et al, 2010), thus highlighting a critical role for cytosolic HMGB1 in cell survival in response to stress. Interestingly, and adding to the complexity of HMGB1, cytosolic translocation induced by reactive oxygen species (ROS) in response to cellular stress initiates autophagy by HMGB1-induced disruption of the Beclin 1-Bcl2 complex (Tang et al, 2010).
HMGB1 in Disease
The role of HMGB1 in cancer is complex, with both intracellular and extracellular forms associated with tumour formation (Sparvero et al, 2009). Interestingly, increased levels of HMGB1 have been identified in both tumour tissue and the serum of cancer patients which is in contrast to other DAMPs suggesting discriminatory upregulation (Kang et al, 2013; Volp et al, 2006). Of note, HMGB1 is released from tumour cells which die from both radiation and chemotherapy cancer treatment, which has specifically shown to induce the dendritic cell mediated TLR4 response (Apetoh et al., 2007). Additionally, HMGB1 has been implicated to play an important role not only in cancer but also in autoimmune disease, with elevated levels identified in rheumatoid arthritis and systemic lupus erythematosus pathogenesis (Popovic et al, 2005, Tanaguchi et al, 2003).
HMGB1 as a Biomarker and a Therapeutic Target
HMGB1 has been the focus of drug targets in several studies to date, primarily adopting inhibition of RAGE signaling using soluble RAGE, and a HMGB1 neutralizing antibody which was shown to inhibit extracellular stimulation with HMGB1 (Kang et al., 2013; Liu et al., 2011). Several studies have shown that inhibition of HMGB1-RAGE signaling can attenuate tumour growth (Ellerman et al, 2007). However, it is important to note that selective inhibition of HMGB1 is essential to prevent loss of the protective inflammatory response to infection and damage. Regulation of the DNA-binding properties of HMGB1 can be achieved by employed an A Box domain alone to compete with the A box domain on HMGB1, which has been demonstrated to have anti-tumour properties in mice (Cottone et al., 2015; Li et al., 2003). Finally, focus on the specific post-translational protein structure of HMGB1 offers an attractive biomarker for several HMGB1-related diseases.
Figure 1: The differential roles of HMGB1. Nuclear HMGB1 interacts with and binds to DNA influencing processes such as transcriptional stability and DNA replication. Extracellular HMGB1 binds to cell surface receptors TLR2, TLR4, RAGE or endosomal receptor TLR9 to initiate downstream NF Kappa Beta activation and subsequent pro-inflammatory response. Cytoplasmic HMGB1 can be secreted either actively or passively, depending on the cell stimulation, via lysosomes, and in this way can activate cell surface receptors on surrounding cells. Cytoplasmic HMGB1 can also compete with Bcl-2 to associate with Beclin-1 and induce autophagosome formation which precedes autophagy.
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