D Stern

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.

Tumor necrosis factor/cachectin (TNF) is a mediator of the septic state, which involves diffuse abnormalities of coagulation throughout the vasculature. Since previous studies have shown that endothelial cells can play an active role in coagulation, we wished to determine whether TNF could modulate endothelial cell hemostatic properties. Incubation of purified recombinant TNF with cultured endothelial cells resulted in a time- and dose-dependent acquisition of tissue factor procoagulant activity. Concomitant with enhanced procoagulant activity, TNF also suppressed endothelial cell cofactor activity for the anticoagulant protein C pathway; both thrombin-mediated protein C activation and formation of functional activated protein C-protein S complex on the cell surface were considerably attenuated. Comparable concentrations of TNF (half-maximal affect at approximately 50 pM) and incubation times (half-maximal affect by 4 h after addition to cultures) were required for each of these changes in endothelial cell coagulant properties. This unidirectional shift in cell surface hemostatic properties favoring promotion of clot formation indicates that, in addition to leukocyte procoagulants, endothelium can potentially be instrumental in the pathogenesis of the thrombotic state associated with inflammatory and malignant disorders.

Attack by reactive oxygen intermediates, common to many kinds of cell/tissue injury, has been implicated in the development of diabetic and other vascular diseases. Such oxygen-free radicals can be generated by advanced glycation end products (AGEs), which are nonenzymatically glycated and oxidized proteins. Since cellular interactions of AGEs are mediated by specific cellular binding proteins, receptor for AGE (RAGE) and the lactoferrin-like polypeptide (LF-L), we tested the hypothesis that AGE ligands tethered to the complex of RAGE and LF-L could induce oxidant stress. AGE albumin or AGEs immunoisolated from diabetic plasma resulted in induction of endothelial cell (EC) oxidant stress, including the generation of thiobarbituric acid reactive substances (TBARS) and resulted in the activation of NF-kappa B, each of which was blocked by antibodies to AGE receptor polypeptides and by antioxidants. Infusion of AGE albumin into normal animals led to the appearance of malondialdehyde determinants in the vessel wall and increased TBARS in the tissues, activation of NF-kappa B, and induction of heme oxygenase mRNA. AGE-induced oxidant stress was inhibited by pretreatment of animals with either antibodies to the AGE receptor/binding proteins or antioxidants. These data indicate that interaction of AGEs with cellular targets, such as ECs, leads to oxidant stress resulting in changes in gene expression and other cellular properties, potentially contributing to the development of vascular lesions. Further studies will be required to dissect whether oxidant stress occurs on the cell surface or at an intracellular locus.

Vascular cell adhesion molecule-1 (VCAM-1), an inducible cell-cell recognition protein on the endothelial cell surface (EC), has been associated with early stages of atherosclerosis. In view of the accelerated vascular disease observed in patients with diabetes, and the enhanced expression of VCAM-1 in diabetic rabbits, we examined whether irreversible advanced glycation endproducts (AGEs), could mediate VCAM-1 expression by interacting with their endothelial cell receptor (receptor for AGE, RAGE). Exposure of cultured human ECs to AGEs induced expression of VCAM-1, increased adhesivity of the monolayer for Molt-4 cells, and was associated with increased levels of VCAM-1 transcripts. The inhibitory effect of anti-RAGE IgG, a truncated form of the receptor (soluble RAGE) or N-acetylcysteine on VCAM-1 expression indicated that AGE-RAGE-induced oxidant stress was central to VCAM-1 induction. Electrophoretic mobility shift assays on nuclear extracts from AGE-treated ECs showed induction of specific DNA binding activity for NF-kB in the VCAM-1 promoter, which was blocked by anti-RAGE IgG or N-acetylcysteine. Soluble VCAM-1 antigen was elevated in human diabetic plasma. These data are consistent with the hypothesis that AGE-RAGE interaction induces expression of VCAM-1 which can prime diabetic vasculature for enhanced interaction with circulating monocytes.

Systemic infusion of low concentrations of tumor necrosis factor/cachectin (TNF) into mice that bear TNF-sensitive tumors leads to activation of coagulation, fibrin formation, and occlusive thrombosis exclusively within the tumor vascular bed. To identify mechanisms underlying the localization of this vascular procoagulant response, a tumor-derived polypeptide has been purified to homogeneity from supernatants of murine methylcholanthrene A-induced fibrosarcomas that induces endothelial tissue factor synthesis and expression (half-maximal response at approximately 300 pM), and augments the procoagulant response to TNF in a synergistic fashion. This tumor-derived polypeptide was identified as the murine homologue of vascular permeability factor (VPF) based on similar mobility on SDS-PAGE, an homologous NH2-terminal amino acid sequence, and recognition by a monospecific antibody to guinea pig VPF. In addition, VPF was shown to induce monocyte activation, as evidenced by expression of tissue factor. Finally, VPF was shown to induce monocyte chemotaxis across collagen membranes and endothelial cell monolayers. Taken together, these results indicate that VPF can modulate the coagulant properties of endothelium and monocytes, and can promote monocyte migration into the tumor bed. This suggests one mechanism through which tumor-derived mediators can alter properties of the vessel wall.