The endothelial cell binding site for advanced glycation end products consists of a complex: an integral membrane protein and a lactoferrin-like polypeptide

Advanced glycation end products (AGEs), formed as the result of the extended interaction of proteins with ketoses, modulate central properties of endothelial cells and mononuclear phagocytes by interacting with a cell surface binding site comprised of a novel integral membrane protein (receptor for AGE = RAGE) and a lactoferrin-like polypeptide (LF-L), the latter having sequence identity to lactoferrin (LF). To further understand this cellular binding site, the interaction of RAGE with LF-L and LF was characterized. By ligand blotting and a solid state competitive binding assay, 125I-LF-L and 125I-LF bound to RAGE immobilized on nitrocellulose membranes or polypropylene tubes in a time-dependent and reversible manner, demonstrating a high affinity component with Kd approximately 100 pM. The interaction of 125I-LF-L and 125I-LF with RAGE was independent of iron in LF and was competed by addition of an excess of unlabeled carboxyl-terminal portion of LF. Cross-linking studies with purified 125I-LF-L and RAGE, in the presence of disuccinimidyl suberate, showed a new, slowly migrating band, corresponding to a complex of RAGE and LF-L, and cross-linking on mouse aortic endothelial cells showed two new slowly migrating bands on immunoblotting visualized with both anti-RAGE IgG and anti-LF-L IgG. These data lead us to propose that the endothelial cell surface binding site for AGEs consists of LF-L bound noncovalently to RAGE anchored in the cell membrane.

Survey of the distribution of a newly characterized receptor for advanced glycation end products in tissues

Advanced glycation end products (AGEs), the final products of nonenzymatic glycation and oxidation of proteins, are found in the plasma and accumulate in the tissues during aging and at an accelerated rate in diabetes. A novel integral membrane protein, termed receptor for AGE (RAGE), forms a central part of the cell surface binding site for AGEs. Using monospecific, polyclonal antibody raised to human recombinant and bovine RAGE, immunostaining of bovine tissues showed RAGE in the vasculature, endothelium, and smooth muscle cells and in mononuclear cells in the tissues. Consistent with these data, RAGE antigen and mRNA were identified in cultured bovine endothelium, vascular smooth muscle, and monocyte-derived macrophages. RAGE antigen was also visualized in bovine cardiac myocytes as well as in cultures of neonatal rat cardiac myocytes and in neural tissue where motor neurons, peripheral nerves, and a population of cortical neurons were positive. In situ hybridization confirmed the presence of RAGE mRNA in the tissues, and studies with rat PC12 pheochromocytes indicated that they provide a neuronal-related cell culture model for examining RAGE expression. Pathological studies of human atherosclerotic plaques showed infiltration of RAGE-expressing cells in the expanded intima. These results indicate that RAGE is present in multiple tissues and suggest the potential relevance of AGE-RAGE interactions for modulating properties of the vasculature as well as neural and cardiac function, prominent areas of involvement in diabetes and in the normal aging process.

Regulation of human mononuclear phagocyte migration by cell surface-binding proteins for advanced glycation end products

Nonenzymatic glycation of proteins occurs at an accelerated rate in diabetes and can lead to the formation of advanced glycation end products of proteins (AGEs), which bind to mononuclear phagocytes (MPs) and induce chemotaxis. We have isolated two cell surface-associated binding proteins that mediate the interaction of AGEs with bovine endothelial cells. One of these proteins is a new member of the immunoglobulin superfamily of receptors (termed receptor for AGEs or RAGE); and the second is a lactoferrin-like polypeptide (LF-L). Using monospecific antibodies to these two AGE-binding proteins, we detected immunoreactive material on Western blots of detergent extracts from human MPs. Radioligand-binding studies demonstrated that antibody to the binding proteins blocked 125I-AGE-albumin binding and endocytosis by MPs. Chemotaxis of human MPs induced by soluble AGE-albumin was prevented in a dose-dependent manner by intact antibodies raised to the AGE-binding proteins, F(ab')2 fragments of these antibodies and by soluble RAGE. When MP migration in response to N-formyl-Met-Leu-Phe was studied in a chemotaxis chamber with AGE-albumin adsorbed to the upper surface of the chamber membrane, movement of MPs to the lower compartment was decreased because of interaction of the glycated proteins with RAGE and LF-L on the cell surface. The capacity of AGEs to attract and retain MPs was shown by implanting polytetrafluoroethylene (PTFE) mesh impregnated with AGE-albumin into rats: within 4 d a florid mononuclear cell infiltrate was evident in contrast to the lack of a significant cellular response to PTFE with adsorbed native albumin. These data indicate that RAGE and LF-L have a central role in the interaction of AGEs with human mononuclear cells and that AGEs can serve as a nidus to attract MPs in vivo.

Cloning and expression of a cell surface receptor for advanced glycosylation end products of proteins

Advanced glycosylation end products of proteins (AGEs) are nonenzymatically glycosylated proteins which accumulate in vascular tissue in aging and at an accelerated rate in diabetes. A approximately 35-kDa polypeptide with a unique NH2-terminal sequence has been isolated from bovine lung and found to be present on the surface of endothelial cells where it mediates the binding of AGEs (receptor for advanced glycosylation end product or RAGE). Using an oligonucleotide probe based on the amino-terminal sequence of RAGE, an apparently full-length cDNA of 1.5 kilobases was isolated from a bovine lung cDNA library. This cDNA encoded a 394 amino acid mature protein comprised of the following putative domains: an extracellular domain of 332 amino acids, a single hydrophobic membrane spanning domain of 19 amino acids, and a carboxyl-terminal domain of 43 amino acids. A partial clone encoding the human counterpart of RAGE, isolated from a human lung library, was found to be approximately 90% homologous to the bovine molecule. Based on computer analysis of the amino acid sequence of RAGE and comparison with databases, RAGE is a new member of the immunoglobulin superfamily of cell surface molecules and shares significant homology with MUC 18, NCAM, and the cytoplasmic domain of CD20. Expression of the RAGE cDNA in 293 cells allowed them to bind 125I-AGE-albumin in a saturable and dose-dependent manner (Kd approximately 100 nM), blocked by antibody to RAGE. Western blots of 293 cells transfected with RAGE cDNA probed with anti-RAGE IgG demonstrated expression of immunoreactive protein compared to its absence in mock-transfected cells. These results suggest that RAGE functions as a cell surface receptor for AGEs, which could potentially mediate cellular effects of this class of glycosylated proteins.

Lymphotoxin in human middle ear cholesteatoma

The progressive growth of epidermis appears to be a crucial factor in the pathogenesis of cholesteatoma. How this growth is encouraged is still not well understood. In the present study, lymphotoxin was found in human middle ear cholesteatoma tissues by the immunoperoxidase method using rabbit anti-human lymphotoxin immunoglobulin G (IgG). Lymphotoxin was localized in the epithelium and connective tissues of the cholesteatomas studied. The epithelium of normal external ear canal skin was lightly stained, but the stain was much weaker than that of the cholesteatomas. In vitro, recombinant human lymphotoxin stimulated proliferation, protein synthesis, and terminal differentiation of basal keratinocytes. These findings suggest that lymphotoxin may be one of the mediators involved in the development of cholesteatoma.

The role of tumor necrosis factor-alpha in bone resorption of cholesteatoma

Recent investigations have indicated that cytokines such as tumor necrosis factor-alpha (TNF-alpha) play a potential role in the bone resorption associated with inflammatory diseases. In this immunoperoxidase study, TNF-alpha was localized in mononuclear cells, macrophages, fibroblasts, osteoblasts, and osteoclasts adjacent to bone resorption areas in both human and experimental middle ear cholesteatomas. In vitro, TNF-alpha stimulated monocytes to form multinucleated cells that demonstrate tartrate-resistant acid phosphatase activity, a marker enzyme for osteoclasts. These multinucleated osteoclast-like cells induce resorption of devitalized bone. The extent of bone resorption was increased by the co-cultures of osteoblasts and osteoclasts in the presence of TNF-alpha, suggesting that cell to cell interaction plays a significant role in bone resorption. Moreover, TNF-alpha was capable of stimulating macrophages to produce acid phosphatase and collagenase, and osteoblasts to produce prostaglandin E2 and collagenase. These chemical mediators have been known to lead to bone resorption. Our findings suggest that TNF-alpha may play an important clinical role in the destructive process of cholesteatoma.

Tumor necrosis factor alpha in middle ear cholesteatoma and its effect on keratinocytes in vitro

Tumor necrosis factor alpha (TNF-alpha) was found in human middle ear cholesteatoma tissues by the immunoperoxidase method using rabbit anti human TNF-alpha immunoglobulin G. It appeared to be localized in the epithelium and connective tissue of cholesteatomas studied, particularly on basal and spinous cells, and macrophages. The epithelium of normal external ear canal skin was lightly stained, but the staining was much weaker than that of the cholesteatomas observed. In vitro, recombinant mouse TNF-alpha stimulated proliferation, protein synthesis, and terminal differentiation of basal keratinocytes. These stimulatory effects were blocked by the anti-TNF-alpha immunoglobulin G, which by itself had no effect on basal keratinocytes. These findings suggest that TNF-alpha plays an important role in the development of cholesteatomas seen clinically.

Effects of indomethacin and calcitonin on bone absorption in type II collagen-induced otosclerosis-like lesions in rats

In this study, we determined the effects of indomethacin and calcitonin on bone resorption in otosclerosis-like lesions in rats. Morphometric analysis showed that both indomethacin and calcitonin inhibited active otosclerosis-like lesions (bone resorption) and rats immunologically induced with type II collagen, and indomethacin had a much higher inhibitory effect than calcitonin. In in vitro studies we found that conditioned medium from splenic lymphocytes of rats immunized with type II collagen stimulated collagenase production by macrophages and fibroblasts. Collagenase is the major enzyme for degradation of the organic components of bone. Treatment of the immunized rats with indomethacin and calcitonin significantly reduced the stimulatory effect of the lymphocyte-conditioned medium on collagenase production. Indomethacin caused a greater reduction of the stimulatory effect of the lymphocytes on collagenase production than calcitonin. These findings are in agreement with results of the morphometric study. Results of the present study also suggest that cell-to-cell interaction plays an important role in collagenase production for degradation of organic components of bone resorption in otosclerotic lesions.

Colony-stimulating factor in middle ear cholesteatoma

Granulocyte-macrophage colony stimulating factor (GM-CSF) was found in human middle ear cholesteatoma tissues by immunohistochemical technique using mouse monoclonal anti-(human) GM-CSF IgG. Immunofluorescent staining showed the presence of GM-CSF in the peribasal area, in some suprabasal cells of the epithelium, and in the inflammatory connective tissue, especially in the monocytes and fibroblasts. These findings were confirmed by the immunoperoxidase method. Staining of the external ear canal epithelium, however, was significantly weaker than that of the cholesteatoma epithelium. The presence of GM-CSF in cholesteatoma appears to be a response to inflammation occurring in the middle ear cavity. Moreover, our in vitro study showed that GM-CSF induced the proliferation and protein synthesis of basal keratinocytes. This study suggests that GM-CSF is involved in the development and destructive effects of middle ear cholesteatoma.