Carolyn L. Geczy

Functions of S100 Proteins

The S100 protein family consists of 24 members functionally distributed into three main subgroups: those that only exert intracellular regulatory effects, those with intracellular and extracellular functions and those which mainly exert extracellular regulatory effects. S100 proteins are only expressed in vertebrates and show cell-specific expression patterns. In some instances, a particular S100 protein can be induced in pathological circumstances in a cell type that does not express it in normal physiological conditions. Within cells, S100 proteins are involved in aspects of regulation of proliferation, differentiation, apoptosis, Ca2+ homeostasis, energy metabolism, inflammation and migration/invasion through interactions with a variety of target proteins including enzymes, cytoskeletal subunits, receptors, transcription factors and nucleic acids. Some S100 proteins are secreted or released and regulate cell functions in an autocrine and paracrine manner via activation of surface receptors (e.g. the receptor for advanced glycation end-products and toll-like receptor 4), G-protein-coupled receptors, scavenger receptors, or heparan sulfate proteoglycans and N-glycans. Extracellular S100A4 and S100B also interact with epidermal growth factor and basic fibroblast growth factor, respectively, thereby enhancing the activity of the corresponding receptors. Thus, extracellular S100 proteins exert regulatory activities on monocytes/macrophages/microglia, neutrophils, lymphocytes, mast cells, articular chondrocytes, endothelial and vascular smooth muscle cells, neurons, astrocytes, Schwann cells, epithelial cells, myoblasts and cardiomyocytes, thereby participating in innate and adaptive immune responses, cell migration and chemotaxis, tissue development and repair, and leukocyte and tumor cell invasion.

Proinflammatory properties of the human S100 protein S100A12

S100 proteins represent a new class of chemoattractants. Here we extend earlier evidence for the proinflammatory properties of human S100A12. A12 induced migration of monocytoid cells, with optimal activity at 10−10 M and potency of >10−9 M C5a. Neutrophils were poorly responsive, and lymphocyte migration was not affected. Actin polymerization in monocytoid cells was accompanied by a sustained [Ca2+]i flux of a magnitude comparable with C5a. A12 elicited a transient infiltration of neutrophils (4–8 h) and more delayed recruitment of monocytes (8–24 h) in vivo. A12 (∼70 nM) was present in synovial fluid (SF) from rheumatoid arthritis patients, and synovium contained A12‐positive neutrophils in the sublining and interstitial region, often surrounding the perivasculature but rarely in the synovial lining layer, although some macrophages were positive. The A12 gene was transiently up‐regulated in monocytes by tumor necrosis factor α (6 h); induction by lipopolysaccharide (LPS) was sustained (12–48 h). A12 may contribute to leukocyte migration in chronic inflammatory responses.

Inflammation-associated S100 proteins: new mechanisms that regulate function

This review focuses on new aspects of extracellular roles of the calgranulins. S100A8, S100A9 and S100A12 are constitutively expressed in neutrophils and induced in several cell types. The S100A8 and S100A9 genes are regulated by pro- and anti-inflammatory mediators and their functions may depend on cell type, mediators within a particular inflammatory milieu, receptors involved in their recognition and their post-translational modification. The S100A8 gene induction in macrophages is dependent on IL-10 and potentiated by immunosuppressive agents. S100A8 and S100A9 are oxidized by peroxide, hypochlorite and nitric oxide (NO). HOCl generates intra-chain sulfinamide bonds; stronger oxidation promotes cross-linked forms that are seen in human atheroma. S100A8 is >200-fold more sensitive to oxidative cross-linking than low-density lipoprotein and may reduce oxidative damage. S100A8 and S100A9 can be S-nitrosylated. S100A8–SNO suppresses mast cell activation and inflammation in the microcirculation and may act as an NO transporter to regulate vessel tone in inflammatory lesions. S100A12 activates mast cells and is a monocyte and mast cell chemoattractant; a G-protein-coupled mechanism may be involved. Structure–function studies are discussed in relation to conservation and divergence of functions in S100A8. S100A12 induces cytokines in mast cells, but not monocytes/macrophages. It forms complexes with Zn2+ and, by chelating Zn2+, S100A12 significantly inhibits MMPs. Zn2+ in S100A12 complexes co-localize with MMP-9 in foam cells in atheroma. In summary, S100A12 has pro-inflammatory properties that are likely to be stable in an oxidative environment, because it lacks Cys and Met residues. Conversely, S100A8 and S100A9 oxidation and S-nitrosylation may have important protective mechanisms in inflammation.

Interferon-γ and Lipopolysaccharide Potentiate Monocyte Tissue Factor Induction by C-Reactive Protein Relationship With Age, Sex, and Hormone Replacement Treatment

BACKGROUND Elevated plasma levels of C-reactive protein (CRP) in population studies and in patients with unstable coronary syndromes are predictive of future adverse events, including cardiac death and myocardial infarction, implicating inflammation in pathogenesis. Although CRP is considered a marker of inflammation, it induces monocyte tissue factor (TF) and may play a prothrombotic role in atherosclerosis and its complications. METHODS AND RESULTS Peripheral blood mononuclear cells (PBMCs) from 79 healthy men and women aged 26 to 83 years and 21 healthy postmenopausal women taking hormone replacement therapy (HRT) were stimulated with CRP, lipopolysaccharide (LPS), interferon-gamma (IFN), or their combination. Levels of CRP in the normal range (1 to 5 microg/mL) increased basal monocyte TF 4- to 6-fold and 40-fold at higher concentrations (25 microg/mL). Coincubation of LPS with CRP produced a greater-than-additive response. IFN did not induce TF but synergized with CRP to approximately double activity. There was a striking positive correlation between age and monocyte TF induction, with a dramatic rise on monocytes from postmenopausal women that was not apparent on cells from women taking HRT. CONCLUSIONS Synergy between CRP and inflammatory mediators may play a direct prothrombotic role in the pathogenesis of coronary atherosclerosis and its acute complications by increasing monocyte/macrophage TF. This may contribute to age and sex differences in coronary events and to the protective effects of HRT.

Inflammation and hyperalgesia induced by nerve injury in the rat: a key role of mast cells

&NA; Inflammatory cells and their mediators are known to contribute to neuropathic pain following nerve injury. Mast cells play a key role in non‐neural models of inflammation and we propose that mast cells and their mediators (in particular histamine) are important in the development of neuropathic pain. In rats, where the sciatic nerve was partially ligated, we showed that stabilisation of mast cells with sodium cromoglycate reduced the recruitment of neutrophils and monocytes to the injured nerve and suppressed the development of hyperalgesia. Treatment with histamine receptor antagonists suppressed the development of hyperalgesia following nerve injury and alleviated hyperalgesia once it was established. These results suggest that mast cell mediators such as histamine released within hours of nerve injury contribute to the recruitment of leukocytes and the development of hyperalgesia.

Serum and mucosal S100 proteins, calprotectin (S100A8/S100A9) and S100A12, are elevated at diagnosis in children with inflammatory bowel disease.

Objective. Various markers characterize the complex inflammatory processes seen in chronic inflammatory bowel disease (IBD) including calprotectin, a complex of two S100 proteins, which has been evaluated and validated as a faecal marker of inflammation. However, the systemic and mucosal expression patterns of calprotectin and related S100 proteins are not well characterized in this disease. The objective of this study was to assess serum and mucosal levels of calprotectin, S100A12 and soluble receptor for advanced glycation end products (sRAGE), a putative S100 ligand, in a paediatric population with IBD. Material and methods. Children were enrolled at diagnosis of IBD, along with groups of children without IBD. Standard inflammatory markers and disease activity scores were collated. Calprotectin, S100A12 and sRAGE levels in serum and biopsy culture supernatants were measured by ELISA and tissue distribution of S100 proteins was investigated by immunohistochemistry. Results. Serum and mucosal calprotectin and S100A12 levels were increased in children with IBD as compared with non-IBD controls. Serum calprotectin levels correlated with S100A12 levels and with disease activity scores in children with IBD. sRAGE levels were not increased in IBD. S100A8, S100A9 and S100A12 were abundantly expressed throughout the lamina propria and epithelium in inflamed mucosa. In contrast, these proteins were present in the lamina propria, but not the epithelium, in non-inflamed mucosa. Conclusions. Serum calprotectin and S100A12 are increased in children with IBD and indicate disease activity. Elevated levels of these proteins are present in the colonic mucosa and may contribute to the pathogenesis of IBD. Furthermore, an imbalance between sRAGE and S100A12 may contribute to inflammatory changes present in IBD.

S100A8 and S100A9 in Human Arterial Wall IMPLICATIONS FOR ATHEROGENESIS

Atherogenesis is a complex process involving inflammation. S100A8 and S100A9, the Ca2+-binding neutrophil cytosolic proteins, are associated with innate immunity and regulate processes leading to leukocyte adhesion and transmigration. In neutrophils and monocytes the S100A8-S100A9 complex regulates phosphorylation, NADPH-oxidase activity, and fatty acid transport. The proteins have anti-microbial properties, and S100A8 may play a role in oxidant defense in inflammation. Murine S100A8 is regulated by inflammatory mediators and recruits macrophages with a proatherogenic phenotype. S100A9 but not S100A8 was found in macrophages in ApoE-/- murine atherosclerotic lesions, whereas both proteins are expressed in human giant cell arteritis. Here we demonstrate S100A8 and S100A9 protein and mRNA in macrophages, foam cells, and neovessels in human atheroma. Monomeric and complexed forms were detected in plaque extracts. S100A9 was strongly expressed in calcifying areas and the surrounding extracellular matrix. Vascular matrix vesicles contain high levels of Ca2+-binding proteins and phospholipids that regulate calcification. Matrix vesicles characterized by electron microscopy, x-ray microanalysis, nucleoside triphosphate pyrophosphohydrolase assay and cholesterol/phospholipid analysis contained predominantly S100A9. We propose that S100A9 associated with lipid structures in matrix vesicles may influence phospholipid-Ca2+ binding properties to promote dystrophic calcification. S100A8 and S100A9 were more sensitive to hypochlorite oxidation than albumin or low density lipoprotein and immunoaffinity confirmed S100A8-S100A9 complexes; some were resistant to reduction, suggesting that hypochlorite may contribute to protein cross-linking. S100A8 and S100A9 in atherosclerotic plaque and calcifying matrix vesicles may significantly influence redox- and Ca2+-dependent processes during atherogenesis and its chronic complications, particularly dystrophic calcification.

S100A8: emerging functions and regulation

The functional importance of members of the S100 Ca2+‐binding protein family is becoming apparent. Murine (m)S100A8 (initially named CP‐10) is a potent chemoattractant (10−13 to 10−11 M) for myeloid cells and the chemotactic activity of other S100s has since been reported, suggesting a new class of chemoattractants. Murine S100A8 has been associated with a number of acute and chronic inflammatory conditions including bacterial infection, atherogenesis, and cystic fibrosis. It is expressed constitutively with S100A9 in neutrophils and is regulated by inflammatory stimulants in macrophages and microvascular endothelial cells. The lack of co‐expression of S100A9 with S100A8 in activated macrophages suggests distinct functions for the proteins expressed by different cell types. Glucocorticoids up‐regulate induction of mS100A8 by inflammatory mediators, and its exquisite sensitivity to oxidation suggests that it may protect against oxidative tissue damage. Inactivation of the mS100A8 gene is embryonic lethal, providing the first evidence for non‐redundant function of a member of the S100 gene family. S100A8 may have an immunoregulatory role by contributing to the regulation of fetal‐maternal interactions. It may play a protective role and its absence may allow infiltration by maternal cells, a process eventually manifesting as resorption. This review focuses on the variety of emerging functions attributed to murine S100A8, a protein implicated in embryogenesis, growth, differentiation, and immune and inflammatory processes. J. Leukoc. Biol. 66: 549–556; 1999.

Oxidation regulates the inflammatory properties of the murine S100 protein S100A8.

The myeloid cell-derived calcium-binding murine protein, S100A8, is secreted to act as a chemotactic factor at picomolar concentrations, stimulating recruitment of myeloid cells to inflammatory sites. S100A8 may be exposed to oxygen metabolites, particularly hypochlorite, the major oxidant generated by activated neutrophils at inflammatory sites. Here we show that hypochlorite oxidizes the single Cys residue (Cys41) of S100A8. Electrospray mass spectrometry and SDS-polyacrylamide gel electrophoresis analysis indicated that low concentrations of hypochlorite (40 μm) converted 70–80% of S100A8 to the disulfide-linked homodimer. The mass was 20,707 Da, 92 Da more than expected, indicating additional oxidation of susceptible amino acids (possibly methionine). Phorbol 12-myristate 13-acetate activation of differentiated HL-60 granulocytic cells generated an oxidative burst that was sufficient to efficiently oxidize exogenous S100A8 within 10 min, and results implicate involvement of the myeloperoxidase system. Moreover, disulfide-linked dimer was identified in lung lavage fluid of mice with endotoxin-induced pulmonary injury. S100A8 dimer was inactive in chemotaxis and failed to recruit leukocytes in vivo. Positive chemotactic activity of recombinant Ala41S100A8 indicated that Cys41 was not essential for function and suggested that covalent dimerization may structurally modify accessibility of the chemotactic hinge domain. Disulfide-dependent dimerization may be a physiologically significant regulatory mechanism controlling S100A8-provoked leukocyte recruitment.

Oxidative modifications of S100 proteins: functional regulation by redox

Several S100 Ca2+–binding proteins undergo various post–translational modifications that may alter their intracellular and extracellular functions. S100A8 and S100A9, two members of this family, are particularly susceptible to oxidative modification. These proteins, abundantly expressed in neutrophils and activated macrophages, are associated with acute and chronic inflammatory conditions, including microbial infections, cystic fibrosis, rheumatoid arthritis, and atherosclerosis. They have diverse intracellular roles including NADPH oxidase activation and arachidonic acid transport and can be secreted via a Golgi–independent pathway to exert extracellular functions. Many pro–inflammatory functions have been described for S100A8 and S100A9, but they are also implicated in anti–inflammatory roles in wound–healing and protection against excessive oxidative tissue damage, the latter as a result of their exquisite capacity to scavenge oxidants. Similarly, their genes are induced by proinflammatory (LPS and TNF–α) stimuli, but induction is IL–10–dependent, and anti–inflammatory glucocorticoids induce or amplify expression. S100A8 and S100A9 were described recently as damage–associated molecular pattern molecules, which provide a novel, conceptual framework for understanding their functions. However, because of this designation, recent reviews focus solely on their pro–inflammatory functions. Here, we summarize the mounting evidence from functional and gene regulation studies that these proteins may also play protective roles. This review offers an explanation for the disparate, functional roles of S100A8 and S100A9 based on emerging data that post–translational, oxidative modifications may act as a regulatory switch.