Taniguchi N, Kawahara K, Yone K, et al

.
The dramatic role of tumor necrosis factor (TNF) inhibition in treatment of rheumatoid arthritis (RA) has led to a more thorough evaluation of the "innate" immune system. Although the innate system was first recognized for its role in septic shock, it is clear that it plays a role in autoimmune disorders. The innate immune system is an ancient system for early response to certain repetitive sequences found on infectious agents through a family of Toll receptors.[1]

Now, Taniguchi and colleagues report on another cytokine, termed the "high mobility group box chromosomal protein 1" (HMGB-1), which was initially identified as part of the septic shock response and was recently rediscovered as a new proinflammatory cytokine. They demonstrated increased HMGB-1 concentrations in synovial fluid of RA patients compared with those of osteoarthritis patients. It was difficult to measure this molecule in whole serum, but fractionated plasma has elevated levels in RA patients.[2] Using immunohistologic methods, the current study found HMGB-1 in CD68-positive cells (macrophage-like cells) of RA synovium. Using in vitro assays with synovial macrophages, they found that TNF-alpha stimulation translocated HMGB-1 from the nucleus to the cytosol, which changes its biologic properties to proinflammatory. Blockade of the receptor for HMGB-1 inhibited the release of both HMGB-1 and TNF-alpha from synovial macrophages.

This article extends to human RA a previous study on the role of HMGB-1 in the rat model of arthritis.[3] These studies are important for rheumatologists, because their results suggest that HMGB-1 is central to the pathogenesis of RA and may provide a novel target for therapy of RA and other autoimmune disorders. Also, a great deal of emphasis has been placed on the role of apoptotic products and immune stimulation. HMGB-1 appears stimulated by necrotic rather than apoptotic cells and may help provide another link between cellular damage and autoimmunity.
HMGB-1, historically known as an abundant, nonhistone architectural chromosomal protein, is extremely conserved across species.[4] As a nuclear protein, HMGB-1 stabilizes nucleosomes and allows bending of DNA that facilitates gene transcription.[5] Chromosomal proteins HMG-14 and HMG-17 enhance the transcriptional potential of chromatin when incorporated into nucleosomes during, but not after, chromatin assembly on replicating DNA. Two molecules of either HMG-14 or HMG-17 can bind to nucleosome cores, independently of the underlying DNA sequence, in a cooperative manner to limit nucleosome mobility and stabilize the structure of the nucleosome core without stabilizing the higher-order chromatin structure.[6] By modifying the structure of nucleosomes, the proteins affect the local structure of the chromatin fiber leading to an increase in the rate of transcriptional elongation but not initiation.

Unexpectedly, HMG-1 also stimulates DNA binding by p53Delta30, a carboxy-terminal-deleted form of the protein that is considered to be constitutively active, suggesting that HMG-1 stimulates p53 by a mechanism that is distinct from other known activators of p53. Finally, using transient transfection assays, it has been shown that HMG-1 can increase p53- and p53Delta30-mediated transactivation in vivo. HMG-1 promotes the assembly of higher-order p53 nucleoprotein structures, and these data, along with the finding that HMG-1 is capable of bending DNA, suggest that HMG-1 may activate p53 DNA binding by a novel mechanism involving a structural change in the target DNA.
Also unexpectedly, recent studies identified extracellular HMGB-1 as a potent macrophage-activating factor.[7,8] It is released as a late mediator during inflammation and participates in the pathogenesis of systemic inflammation after the early mediator response has resolved. Thus HMGB-1 occupies a critical role as a proinflammatory mediator passively released by necrotic but not apoptotic cells.[6] Stimulated macrophages actively secrete HMGB-1 to promote inflammation and in turn, stimulate production of multiple, proinflammatory cytokines.[2] Also, the binding of p53 protein to DNA is stimulated by its interaction with HMG-1.[9]

Thus HMGB-1 has become a major therapeutic target due to its key role in endotoxin-mediated lethality through acute lung injury, activation of macrophages, smooth muscle cell, and epithelial cell barrier dysfunction. Now a role in pathogenesis of RA seems possible.[2] Furthermore, different tissues may express different isoforms of HMGB-1.[10]
HMGB-1 is structurally composed of 3 different domains: 2 homologous DNA-binding sequences entitled box A and box B and a highly negatively charged C terminus.[4] The B box domain contains the proinflammatory cytokine functionality of the molecule, whereas the A box region has an antagonistic, anti-inflammatory effect with therapeutic potential. Administration of highly purified, recombinant DNA neutralizing antibodies against HMGB-1-rescued mice from lethal sepsis, even when initial treatment was delayed for 24 hours after the onset of infection, establishing a clinically relevant therapeutic window that is significantly wider than for other known cytokines. In addition, the finding of a natural inhibitor (the A box) should provide a novel therapeutic approach for treatment options. As an additional therapeutic approach for other cytokines, inhibitors of the receptor may prove useful. The action of HMGB-1 appears to require RAGE receptor.[4] This receptor activates an intracellular signal transduction pathway within minutes after ligand exposure and can promote up-regulation of expression of TNF mRNA several hours later.
The findings about HMGB-1 raise important questions about the role of this molecule in arthritis. There is difficulty in measuring the serum levels, perhaps due to factors that may interfere with detection.[2] Perhaps the circulating form of HMGB-1 that is pathogenetic for inflammation is different from the form produced by normal cells, because the molecule may be modified during the death of the cell. Alternatively, the molecule may be bound to specific molecules in the circulation that may modulate its activity as well as make its detection difficult. For example, the measurement of free tumor necrosis factor is complicated by its binding to macroglobulins as well as to specific binding proteins and naturally occurring neutralizing inhibitors.[7]

In summary, the relationship between proinflammatory cytokines HMGB-1 and TNF-alpha is an important new area for understanding pathogenesis and developing new therapy. The finding of cytokine activity in the "historical" molecule suggests that we may uncover additional targets for autoimmune disease. After so many years of few new therapeutic targets, potential sites are blooming.

--------------
References
1. Medzhitov R, Janeway C Jr. Innate immune recognition: mechanisms and pathways. Immunol Rev. 2000;173:89-97.
2. Ulloa L, Batliwalla FM, Andersson U, Gregersen PK, Tracey KJ. High mobility group box chromosomal protein 1 as a nuclear protein, cytokine, and potential therapeutic target in arthritis. Arthritis Rheum. 2003;48:876-881.
3. Kokkola R, Sundberg E, Ulfgren AK, et al. High mobility group box chromosomal protein 1: a novel proinflammatory mediator in synovitis. Arthritis Rheum. 2002;46:2598-2603.
4. Andersson U, Erlandsson-Harris H, Yang H, Tracey KJ. HMGB1 as a DNA-binding cytokine. J Leukoc Biol. 2002;72:1084-1091.
5. Bustin M. Revised nomenclature for high mobility group (HMG) chromosomal proteins. Trends Biochem Sci. 2001;26:152-153.
6. Bustin M, Trieschmann L, Postnikov YV. The HMG-14/-17 chromosomal protein family: architectural elements that enhance transcription from chromatin templates. Semin Cell Biol. 1995;6:247-255.
7. Agnello D, Wang H, Yang H, Tracey KJ, Ghezzi P. HMGB-1, a DNA-binding protein with cytokine activity, induces brain TNF and IL-6 production, and mediates anorexia and taste aversion. Cytokine. 2002;18:231-236.
8. Veilleux S, Boissonneault G. Dynamics of reporter gene stimulation by HMG box proteins. DNA Cell Biol. 2002;21:199-212.
9. Jayaraman L, Moorthy NC, Murthy KG, Manley JL, Bustin M, Prives C. High mobility group protein-1 (HMG-1) is a unique activator of p53. Genes Dev. 1998;12:462-472.
10. West KL, Ito Y, Birger Y, Postnikov Y, Shirakawa H, Bustin M. HMGN3a and HMGN3b, two protein isoforms with a tissue-specific expression pattern, expand the cellular repertoire of nucleosome-binding proteins. J Biol Chem. 2001;276:25959-25969.

---------

Arthritis & Rheumatism. 2003;48(4):971-981