Carl Barsigian

Endothelial Cell VE-cadherin Functions as a Receptor for the β15–42 Sequence of Fibrin

The contact of fibrin with the apical surface of human umbilical vein endothelial cells (HUVEC) can induce capillary tube formation via the interaction of fibrin β15–42 with a putative cell receptor (Chalupowicz, D. G., Chowdhury, Z. A., Bach, T. L., Barsigian, C., and Martinez, J. (1995) J. Cell Biol. 130, 207–215). To characterize this interaction, we studied the binding of the thrombin-cleaved N-terminal disulfide knot of fibrin (NDSK II), a dimeric fragment with exposed β15–42, to HUVEC in three separate assay systems. Time-course binding of125I-NDSK II to HUVEC monolayers or suspensions revealed that binding was specific at 50–60%, as determined by the addition of unlabeled NDSK II. Specific binding of 125I-NDSK II to HUVEC was 70% reversible by dilution or by competition, and was found to be divalent cation-independent. Binding plateaued after 10 min at a saturation of 15–20 nm. Scatchard analysis using the LIGAND computer program defined a single population of receptors with aK D of 7.7 ± 1.6 nm and approximately 21,000 ± 7000 binding sites/cell. N-terminal disulfide knot derivatives in which β15–42 was absent (NDSK 325) or unexposed (NDSK, NDSK I) did not show specific binding. Specific binding of 125I-NDSK II could not be inhibited by RGDS or by antibodies to the αvβ3 or β1 integrins, PECAM-1, ICAM-1, or N-cadherin. In contrast, a synthetic β15–42/ovalbumin conjugate inhibited total125I-NDSK II binding by 47 ± 19% (corresponding to 95% of specific 125I-NDSK II bound) and a monoclonal antibody to vascular endothelial cadherin (VE-cadherin) inhibited binding by 35 ± 8% (corresponding to 70% of specific125I-NDSK II bound). Another assay was based on the capture of cadherins from HUVEC lysates by a polyclonal pan-cadherin antibody immobilized on plastic dishes. Binding of NDSK II to the captured cadherins was 89 ± 5% specific, while specific binding of NDSK 325 and NDSK was negligible. An immortalized line of human adipose-derived microvascular endothelial cells, which express N-cadherin but not VE-cadherin, demonstrated no specific binding of NDSK II by the capture assay. These data define a novel interaction of fibrin with VE-cadherin, which is mediated by the fibrin N-terminal β15–42 sequence, and may contribute to the mechanism through which fibrin induces angiogenesis.

Catabolic properties of aglycofibrinogen synthesized by tunicamycin-treated human hepatoma (HepG2) cells and rabbit hepatocytes

Human hepatoma cell (HepG2) or rabbit hepatocyte monolayers were incubated with [35S]methionine in presence or absence of tunicamycin, a potent inhibitor of asparagine-linked glycosylation. The 35S-labeled nonglycosylated and control fibrinogens purified from the media were used to evaluate the influence of the oligosaccharide on the catabolic properties of this glycoprotein. Plasmin, pronase, cathepsin D or cathepsin B each degraded the nonglycosylated and control fibrinogens similarly, as evidenced by the release of trichloroacetic acid-soluble radioactivity and by SDS-polyacrylamide gel electrophoresis and autoradiography of plasmic digests. Nonglycosylated and control fibrin clots also showed no differences in susceptibility to plasmic digestion. The two forms of fibrinogen demonstrated the same plasma half-life in rabbits. These data indicate that the oligosaccharide does not influence the proteolytic stability or the in vivo plasma survival of fibrinogen, and suggest that other biochemical determinants may influence the catabolic properties of this molecule.