Thrombin-stimulated calcium mobilization is inhibited by thrombospondin via CD36

Activation of the G-protein-linked thrombin receptor in endothelial cells normally leads to an increase in free intracellular calcium, [Ca2+]i, which is the proximate stimulus for many important cell functions. We present evidence showing that signals from CD36, the thrombospondin (TSP) receptor, can inhibit this thrombin-mediated calcium response. Human endothelial cells preloaded with Indo-1 exhibited rapid calcium mobilization in response to thrombin. The presence of TSP inhibited the thrombin-stimulated calcium response in CD36-positive microvascular endothelial cells but not in CD36-negative umbilical vein endothelial cells. This TSP effect was mimicked by anti-CD36 antibodies and a TSP peptide (CSVTCG), but not by an alternative CD36 ligand (collagen IV) or an antibody to an alternative TSP receptor (alphavbeta3). TSP also inhibited the calcium response to the thrombin receptor-tethered ligand peptide, SFLLRN. In addition, TSP and anti-CD36 antibodies inhibited the calcium response of a closely related receptor, the trypsin/SLIGKVD-activated receptor PAR-2. TSP did not indiscriminately inhibit all calcium release pathways, since histamine- or VEGF-stimulated calcium responses were not inhibited by TSP. We conclude that cross-talk from the CD36 receptor influences the responsive state of the endothelial thrombin receptor family and/or its signaling pathway.