Interaction of S100A8/S100A9-arachidonic acid complexes with the scavenger receptor CD36 may facilitate fatty acid uptake by endothelial cells

S100 Proteins in the Epidermis

The S100 proteins comprise a family of 21 low molecular weight (9-13 kDa) proteins that are characterized by the presence of two calcium-binding EF-hand motifs. Fourteen S100 protein genes are located within the epidermal differentiation complex on human chromosome 1q21 and 13 S100 proteins (S100A2, S100A3, S100A4, S100A6, S100A7, S100A8, S100A9, S100A10, S100A11, S100A12, S100A15, S100B, and S100P) are expressed in normal and/or diseased epidermis. S100 proteins exist in cells as anti-parallel hetero- and homodimers and upon calcium binding interact with target proteins to regulate cell function. S100 proteins are of interest as mediators of calcium-associated signal transduction and undergo changes in subcellular distribution in response to extracellular stimuli. They also function as chemotactic agents and may play a role in the pathogenesis of epidermal disease, as selected S100 proteins are markedly overexpressed in psoriasis, wound healing, skin cancer, inflammation, cellular stress, and other epidermal states.

Mechanism of apoptosis induced by S100A8/A9 in colon cancer cell lines: the role of ROS and the effect of metal ions

The protein complex S100A8/A9, abundant in the cytosol of neutrophils, is secreted from the cells upon cellular activation and induces apoptosis in tumor cell lines and normal fibroblasts in a zinc-reversible manner. In the present study, we present evidence that the S100A8/A9 also exerts its apoptotic effect by a zinc-independent mechanism. Treatment of the colon carcinoma cells with different concentrations of human S100A8/A9 or the metal ion chelator diethylenetriaminepentacetic acid (DTPA) resulted in a significant increase of cell death. Annexin V/phosphatidylinositol and Hoechst 33258 staining revealed that cell death was mainly of the apoptotic type. A significant increase in the activity of caspase-3 and -9 was observed in both cell lines after treatment. Caspase-8 activation was negligible in both cell lines. The cytotoxicity/apoptotic effect of human S100A8/A9 and DTPA was inhibited significantly (P<0.05) by Zn(+2) and Cu(+2), more effectively than by Ca(2+) and Mg(2+). The antioxidant N-acetyl-L-cysteine inhibited the cytotoxicity/apoptotic effect of S100A8/A9 and DTPA. However, as a result of the different time-courses of both agents and that the S100A8/A9-induced apoptosis was not completely reversed, we conclude that S100A8/A9 exerts its apoptotic effect on two colon carcinoma cell lines through a dual mechanism: one via zinc exclusion from the target cells and the other through a yet-undefined mechanism, probably relaying on the cell-surface receptor(s).

The pivotal role of scavenger receptor CD36 and phagocyte-derived oxidants in oxidized low density lipoprotein-induced adhesion to endothelial cells

Adhesion of phagocytes to endothelial cells constitutes a crucial step in atherogenesis. Oxidized low density lipoproteins (LDL) are supposed to facilitate the adhesion process. We investigated the molecular mechanisms by which mildly and extensively hypochlorite-oxidized LDL force adhesion of murine macrophages and human neutrophils to human umbilical venous endothelial cells. After 1h of co-incubation of macrophages, endothelial cells, and lipoproteins adhesion significantly increased to 160+/-13% (S.E.M., n=5) in the presence of mildly oxidized lipoprotein, and 210+/-11% (S.E.M., n=5) in the presence of extensively oxidized lipoprotein. Similar results were obtained with neutrophils. CD36 antibody (FA6-152) significantly reduced adhesion to 102+/-7% (S.E.M., n=5) using mildly oxidized low density lipoprotein and to 179+/-16% (S.E.M., n=5) using extensively oxidized low density lipoprotein. Native high density lipoprotein and to a lesser extent methionine-oxidized high density lipoprotein significantly counteracted the effects of low density lipoprotein. Prior incubation of endothelial cells with modified lipoproteins followed by their removal and subsequent incubation with macrophages or neutrophils resulted in only minor changes of adhesion. This suggests that the direct contact of low density lipoprotein with phagocytes followed by activation of a respiratory burst with release of reactive oxygen species facilitates the adhesion process. Accordingly, the addition of antioxidants (superoxide dismutase and catalase) to the co-incubation medium was followed by a significant decrease in phagocyte adhesion. It is concluded that oxidized low density lipoprotein-induced respiratory burst activation of phagocytes with subsequent release of oxidants constitutes a crucial step in promoting the adhesion of phagocytes to endothelial cells.

Expression of calcium-binding proteins MRP8 and MRP14 in inflammatory muscle diseases

The pathophysiological role of infiltrating macrophages and their subtypes in idiopathic inflammatory myopathies such as dermatomyositis, polymyositis, and inclusion body myositis is not fully clear. Monocytes exhibit various phenotypes with different functional properties such as release of pro- or anti-inflammatory mediators. Expression of myeloid-related proteins MRP8 and MRP14, two calcium-binding S100-proteins, characterizes a proinflammatory subtype of macrophages. We immunohistochemically investigated expression of MRP8 and MRP14 in muscle biopsies of 33 patients with dermatomyositis, polymyositis, and inclusion body myositis. We found a clear association of expression of MRP8 and MRP14 by infiltrating macrophages with degeneration of myofibers. Because MRP8 and MRP14 are secreted by activated macrophages we investigated if these proteins would have direct extracellular effects on myocytes. We found that the purified MRP8/MRP14 complex inhibited proliferation and differentiation of C2C12 myoblasts and that it induced apoptosis via activation of caspase-3 in a time- and dose-dependent manner. These results indicate that in the course of inflammatory myopathies, activated macrophages can promote destruction and impair regeneration of myocytes via secretion of MRP8/MRP14.

S100A9/S100A8: Myeloid representatives of the S100 protein family as prominent players in innate immunity

Neutrophils are rapidly recruited to sites of inflammation and are thereby at the forefront of the organism's defense against numerous attacks. As unspecific phagocytes, they belong to the so-called innate immunity. Two S100 proteins, namely S100A9 (MRP14) and S100A8 (MRP8), constitute roughly 40% of the cytosolic protein in these cells, implying by their pure abundance an important role in the effector functions of neutrophils. However, despite intense research in the past 15 years, the puzzle that may embed both molecules into the neutrophil/monocyte physiology is still incomplete. One reason might be the conformational variability the S100A9 and S100A8 molecules can adopt. They readily form hetero- and homodimeric, trimeric as well as tetrameric complexes, but they evidently do also exert specific functions as monomers. An ever-increasing body of information suggests that S100A9 plays a prominent role in leukocyte trafficking and arachidonic acid metabolism. In addition, elevated levels of S100A9 and S100A8 in body fluids of inflamed tissues strengthen the view that these molecules are important players in fighting inflammation. The aim of this review is to give an update on the current developments concerning the S100A9/S100A8 molecule in biology and medicine.

Intracellular and extracellular roles of S100 proteins

S100, a multigenic family of non-ubiquitous Ca(2+)-modulated proteins of the EF-hand type expressed in vertebrates exclusively, has been implicated in intracellular and extracellular regulatory activities. Members of this protein family have been shown to interact with several effector proteins within cells thereby regulating enzyme activities, the dynamics of cytoskeleton constituents, cell growth and differentiation, and Ca(2+) homeostasis. Structural information indicates that most of S100 proteins exist in the form of antiparallelly packed homodimers (in some cases heterodimers), capable of functionally crossbridging two homologous or heterologous target proteins in a Ca(2+)-dependent (and, in some instances, Ca(2+)-independent) manner. In addition, extracellular roles have been described for several S100 members, although secretion (via an unknown mechanism) has been documented for a few of them. Extracellular S100 proteins have been shown to exert regulatory effects on inflammatory cells, neurons, astrocytes, microglia, and endothelial and epithelial cells, and a cell surface receptor, RAGE, has been identified as a potential S100A12 and S100B receptor transducing the effects of these two proteins on inflammatory cells and neurons. Other cell surface molecules with ability to interact with S100 members have been identified, suggesting that RAGE might not be a universal S100 protein receptor and/or that a single S100 protein might interact with more than one receptor. Collectively, these data indicate that members of the S100 protein family are multifunctional proteins implicated in the regulation of a variety of cellular activities.

Phagocyte-specific S100 proteins: a novel group of proinflammatory molecules

Three members of the S100 family of calcium-binding proteins comprise a new group of proinflammatory molecules released by phagocytes. A novel inflammatory syndrome defined by extraordinarily high expression of S100A8 and S100A9 confirmed recent observations in vitro demonstrating a role of these proteins during recruitment of leukocytes. S100A12 directly activates endothelial cells, mononuclear phagocytes and lymphocytes through interaction with the receptor for advanced glycation end products. Thus, these S100-proteins are attractive targets to modulate inflammation.

Myeloid cell function in MRP-14 (S100A9) null mice

Myeloid-related protein 14 (MRP-14) and its heterodimeric partner, MRP-8, are cytosolic calcium-binding proteins, highly expressed in neutrophils and monocytes. To understand the function of MRP-14, we performed targeted disruption of the MRP-14 gene in mice. MRP-14(-/-) mice showed no obvious phenotype and were fertile. MRP-8 mRNA but not protein is present in the myeloid cells of these mice, suggesting that the stability of MRP-8 protein is dependent on MRP-14 expression. A compensatory increase in other proteins was not detected in cells lacking MRP-8 and MRP-14. Although the morphology of MRP-14(-/-) myeloid cells was not altered, they were significantly less dense. When Ca(2+) responses were investigated, there was no change in the maximal response to the chemokine MIP-2. At lower concentrations, however, there was reduced responsiveness in MRP-14(-/-) compared with MRP-14(+/+) neutrophils. This alteration in the ability to flux Ca(2+) did not impair the ability of the MRP-14(-/-) neutrophils to respond chemotactically to MIP-2. In addition, the myeloid cell functions of phagocytosis, superoxide burst, and apoptosis were unaffected in MRP-14(-/-) cells. In an in vivo model of peritonitis, MRP-14(-/-) mice showed no difference from wild-type mice in induced inflammatory response. The data indicate that MRP-14 and MRP-8 are dispensable for many myeloid cell functions.

Calcium-binding proteins MRP 8 and 14 in a Staphylococcus aureus infection model: role of therapy, inflammation, and infection persistence

Concentrations of the calcium-binding proteins of the S100 family, myeloid-related proteins 8 and MRP 14 (MRP8/14), are elevated in chronic infections, yet the role of these proteins is not clearly defined. Using commercial and developed enzyme immunoassays, we assayed for MRP8/14 in sterile-filtered abscess fluid from tissue-cage-implanted rats and rabbits. Staphylococcus aureus abscesses were created 6 weeks after the intraperitoneal implantation of tissue cages. Leukocytes, bacteria, and non-protein-bound calcium and zinc were measured in the infection exudate at day 3 or 5 of infection and after 8 days of treatment with antimicrobials beta-lactams (18 rabbits, 35 rats) and fluoroquinolone-rifampin (6 rabbits). Half of the infected rats were depleted of neutrophils; these rats exhibited significantly lower MRP 8/14 concentrations on all days sampled, regardless of the level of infection. The level of abscess MRP 8/14 is high early in the course of infection but decreases with effective antimicrobial treatment by as much as 100-fold. Thirty-day-old abscesses with log 6 bacterial counts and low neutrophil counts showed low concentrations of MRP 8/14 in these models. In abscess fluid, interleukin-6, as a representative marker of inflammation, correlated with MRP8/14, whereas ionized calcium and zinc did not. Our data suggest that infection and inflammation are not equal stimuli for MRP 8/14. The neutrophil appears to be the main source of MRP8/14 in this model.

Evidence for the involvement of the unique C-tail of S100A9 in the binding of arachidonic acid to the heterocomplex S100A8/A9

Protein complexes formed by S100A8 and S100A9 represent the only AA-binding capacity in the human neutrophilic cytosol and are involved in the intracellular arachidonic acid metabolism. The formation of S100A8/A9 protein complexes and the binding of calcium to the complexes are prerequisites for the specific binding of polyunsaturated fatty acids. The present study was undertaken to characterize the fatty acid binding site within the protein complex. Deletions at both termini and point mutations of different basic amino acids especially within the extended C-terminal tail of human S100A9 were introduced. The S100A9 mutant proteins were then analyzed with respect to protein-protein interaction (GST pull down-assay and yeast two-hybrid system) and functional properties (arachidonic acid and calcium binding). The data give strong evidence that the unique C-tail of S100A9 containing the three consecutive histidine residues (His103-His105) represents the region to which the fatty acid carboxy-group is bound to the protein complex. The localization of the AA-binding site within the unique C-tail of S100A9 correlates with the fact that fatty acid binding has not yet been reported for other S100 proteins.

Fatty acid transport across membranes: relevance to nutrition and metabolic pathology

Long-chain fatty acids are an important constituent of the diet and they contribute to a multitude of cellular pathways and functions. Uptake of long-chain fatty acids across plasma membranes is the first step in fatty acid utilization, and recent evidence supports an important regulatory role for this process. Although uptake of fatty acids involves two components, passive diffusion through the lipid bilayer and protein-facilitated transfer, the latter component appears to play the major role in mediating uptake by key tissues. Identification of several proteins as fatty acid transporters, and emerging evidence from genetically altered animal models for some of these proteins, has contributed significant insight towards understanding the limiting role of transport in the regulation of fatty acid utilization. We are also beginning to better appreciate how disturbances in fatty acid utilization influence general metabolism and contribute to metabolic pathology.

Binding of two nuclear complexes to a novel regulatory element within the human S100A9 promoter drives the S100A9 gene expression

S100A9, also referred to as MRP14, is a calcium-binding protein whose expression is tightly regulated during differentiation of myeloid cells. The present study was performed to study the cell type- and differentiation-specific transcriptional regulation of the S100A9 gene. Analysis of the S100A9 promoter in MonoMac-6 cells revealed evidence for a novel regulatory region from position -400 to -374 bp, termed myeloid-related protein regulatory element (MRE). MRE deletion resulted in a 5.2-fold reduction of promoter activity. By electrophoretic mobility shift analysis two nuclear complexes binding to this region were identified and referred to as MRE-binding complex A (MbcA) and MRE-binding complex B (MbcB). By mutagenesis the MRE-binding motif could be narrowed to a 12-bp region. The relevance of MRE is deduced from the observations that the formation of either MRE-binding complex A or MRE-binding complex B strongly correlated with S100A9 gene expression in a cell type-specific, activation- and differentiation-dependent manner. Moreover, DNA affinity chromatography and Western blot studies indicate that a Kruppel-related zinc finger protein and the transcriptional intermediary factor 1beta (TIF1beta) are involved in an MRE-binding complex, thereby regulating the S100A9 gene expression.

Peroxisome-proliferator-activated-receptor gamma (PPARgamma) independent induction of CD36 in THP-1 monocytes by retinoic acid

Retinoic acid (RA) has been shown to regulate cellular growth and differentiation of a variety of cell types, including cells of the myelomonocytic lineage. We used the monocytic leukaemia cell line THP-1, which differentiates to macrophages in response to phorbol 12-myristate 13-acetate (PMA), to investigate the regulation by RA of genes in the scavenger receptor type B family (CD36) in human monocyte/macrophages. Reverse transcription-polymerase chain reaction and flow cytometry demonstrated that, like PMA and the natural peroxisome-proliferator-activated receptor-gamma (PPARgamma) ligand 15d-PGJ2, RA induced CD36 gene expression in these cells. Moreover, RA plus 15d-PGJ2 further enhanced CD36 protein and mRNA levels over that seen with the RA or PPARgamma compounds alone. The PPARgamma antagonist GW9662 was shown to block completely PPARgamma-ligand induction of CD36 gene expression, but had little effect on the action of RA. Our data indicated that RXR- and RAR-specific ligands (LG153 and TTNPB, respectively) were each alone able to increase CD36 mRNA and surface protein levels. By using calphostin C, a specific protein kinase C (PKC) inhibitor, we demonstrated that induction of CD36 by PMA, as well as by PPARgamma and RXR ligands were dependent upon PKC activation. In contrast, activation of CD36 through the RAR pathway was not affected by inhibition of PKC activity. Taken together, these data demonstrate that RA can up-regulate CD36 expression in human monocytes/macrophages. This regulation appears to be predominantly mediated through the RAR/RXR pathway of action and, unlike previously described methods of CD36 modulation, is independent of PPARgamma and PKC signalling. This study suggests a possible role for RA in physiological processes involving the scavenger receptor function in cells of the monocyte/macrophage lineage.

Role of CD36 in membrane transport of long-chain fatty acids

CD36 is a multispecific membrane glycoprotein that has been postulated to have a variety of functions. Evidence generated in isolated cells and in mice and rat models of altered CD36 expression has indicated an important role for CD36 in membrane transport of long-chain fatty acids. The cumulative data indicate that CD36 facilitates a major fraction of fatty acid uptake by muscle and fat, and that CD36 deficiency is associated with a large (60-80%) defect in fatty acid uptake by those tissues. In humans, polymorphisms in the CD36 gene may underlie defective fatty acid metabolism and some forms of heart disease. Herein we review our current understanding of the transport function and regulation of CD36. The realization that the transport step rate limits cellular fatty acid utilization suggests that abnormalities in CD36 expression or function may impact on susceptibility to certain metabolic diseases such as obesity and insulin resistance.

The crystal structure of human MRP14 (S100A9), a Ca(2+)-dependent regulator protein in inflammatory process

Human MRP14 (hMRP14) is a Ca(2+)-binding protein from the S100 family of proteins. This protein is co-expressed with human MRP8 (hMRP8), a homologue protein in myeloid cells, and plays an indispensable role in Ca(2+)-dependent functions during inflammation. This role includes the activation of Mac-1, the beta(2) integrin which is involved in neutrophil adhesion to endothelial cells. The crystal structure of the holo form of hMRP14 was analyzed at 2.1 A resolution. hMRP14 is distinguished from other S100 member proteins by its long C-terminal region, and its structure shows that the region is extensively flexible. In this crystal structure of hMRP14, Chaps molecules bind to the hinge region that connects two EF-hand motifs, which suggests that this region is a target-binding site of this protein. Based on a structural comparison of hMRP14 with hMRP8 and human S100A12 (hS100A12) that is another homologue protein, the character of MRP8/14 hetero-complex and the functional significance of the flexibility of the C-terminal region of hMRP14 are discussed.

The S100 family heterodimer, MRP-8/14, binds with high affinity to heparin and heparan sulfate glycosaminoglycans on endothelial cells

The S100 family proteins MRP-8 (S100A8) and MRP-14 (S100A9) form a heterodimer that is abundantly expressed in neutrophils, monocytes, and some secretory epithelia. In inflamed tissues, the MRP-8/14 complex is deposited onto the endothelium of venules associated with extravasating leukocytes. To explore the receptor interactions of MRP-8/14, we use a model system in which the purified MRP-8/14 complex binds to the cell surface of an endothelial cell line, HMEC-1. This interaction is mediated by the MRP-14 subunit and is mirrored by recombinant MRP-14 alone. The cell surface binding of MRP-14 was blocked by heparin, heparan sulfate, and chondroitin sulfate B, and the binding sites were sensitive to heparinase I and trypsin treatment but not to chondroitinase ABC. Furthermore MRP-8/14 and MRP-14 did not bind to a glycosaminoglycan-minus cell line. MRP-14 has a high affinity for heparin (K(d) = 6.1 +/- 3.4 nm), and this interaction mimicked that with the endothelial cells. We therefore conclude that the MRP-8/14 complex binds to endothelial cells via the MRP-14 subunit interacting chiefly with heparan sulfate proteoglycans. CD36 and RAGE, two other putative receptors for MRP-8/14, were not expressed by HMEC-1 cells. This binding activity may explain the immobilization of the MRP-8/14 complex on endothelium that is observed in vivo.

Unravelling the significance of cellular fatty acid-binding proteins

Cellular long-chain fatty acid (FA) transport and metabolism are believed to be regulated by membrane-associated and soluble proteins that bind and transport FAs. Several different classes of membrane proteins have been proposed as FA acceptors or transmembrane FA transporters. New evidence from in-vitro and whole-animal studies supports the existence of protein-mediated transmembrane transport of FAs, which is likely to coexist with passive diffusional uptake. The trafficking of FAs by intracellular fatty acid-binding proteins may involve their interaction with specific membrane or protein targets. Evidence is also emerging for concerted actions between the membrane and cytoplasmic fatty acid-binding proteins that allow for efficient regulation of FA transport and metabolism.