Mechanism of interaction of thrombospondin with human endothelium and inhibition of sickle erythrocyte adhesion to human endothelial cells by heparin

Thrombospondin (TSP) mediates sickle erythrocyte adhesion to endothelium, b ut the mechanism remains unknown. Since TSP is comprised of heterogeneously distinct domains, this adhesion may depend on the interaction of specific regions of TSP with different cell surface receptors. To examine the mechan isms of interaction of TSP with human umbilical vein endothelial cells (HUV EC), we performed binding studies using soluble [I-125]TSP. Our data showed that (i) monoclonal antibodies (MoAbs) against cell surface heparan sulfat e (HS) or the heparin-binding domain of TSP, or cleavage of HS on HUVEC by heparitinase reduced TSP binding by 28-40%, (ii) the RGD peptide or MoAbs a gainst integrin alpha(v)beta(3) or the calcium binding region of TSP inhibi ted binding by 18-28%, and (iii) a MoAb against the cell-binding domain of TSP inhibited binding by 36%. Unmodified heparin inhibited the binding of T SP to endothelial cells by 70% and did so far more effectively than selecti vely desulfated heparins, HS or chondroitin sulfate. Heparin inhibited TSP binding to HUVEC at much lower concentrations than were required to inhibit TSP binding to sickle erythrocytes. Unmodified heparin effectively inhibit ed the TSP-mediated adhesion of sickle erythrocytes to HUVEC. These data im ply that cell surface MS-mediated mechanisms play a key role in TSP-mediate d sickle erythrocyte adhesion to endothelium, and heparin may be of use for inhibition of this adhesion. (C) 1999 Elsevier Science B.V. All rights res erved.