Thrombin stimulates melanoma tumor‐cell binding to endothelial cells and subendothelial matrix

Thrombin has been shown to activate tumor‐cell adhesion to platelets, fibronectin and von Willebrand factor 2‐ to 3‐fold in vitro, and enhance metastasis 10‐ to 156‐fold in vivo. We therefore elected to determine whether thrombin binds to tumor cells and whether thrombin‐treated tumor cells enhance their adhesion to endothelial cells, the first barrier to tumor invasion and metastasis. Thrombin‐treated human and hamster melanoma cells (SK‐Mel‐28 and HM‐29) enhanced their adhesion to bovine aortic endothelial cells 2.1‐ to 2.3‐fold, respectively. Similar results were obtained with bovine capillary endothelial cells. Thrombin activation of tumor cells was rapid, reaching its peak 15 min after thrombin activation; and transient, declining to baseline levels by 60 min. ¹²⁵I‐thrombin bound to both SK‐Mel‐28 and HM‐29 cells in a saturation‐dependent manner, was inhibitable by unlabelled thrombin, and could be 90% washed away with buffer following 30 min of incubation. Electron microscopy of tumor cells bound to fibronectin‐coated millipore filters revealed adhesion of naive as well as thrombin‐treated tumor cells to endothelial cells and subendothelial matrix between endothelial cells. Neither mode of adhesion was preferentially enhanced by thrombin‐treated tumor cells. Both naive and thrombin‐treated SK‐Mel‐28 cells had the adhesive ligand integrin receptors: α3β1 (fibronectin, laminin, collagen); α5β1 (fibronectin); α,β_x (vitronectin). Receptors for the β2 integrin family (LFA‐1 and Mac‐1) were not found, nor were receptors of the β3 integrin family, GPIIIa. The receptor ligands fibronectin and vitronectin were present. None of the above receptors or ligands increased their density or appeared de novo after thrombin stimulation. Thus, 2 melanoma cell lines have thrombin receptors which, when occupied, lead to enhanced adhesion of tumor cells to endothelium and subendothelial matrix.