PRESENCE OF THE 7 TRANSMEMBRANE THROMBIN RECEPTOR ON HUMAN TUMOR-CELLS - EFFECT OF ACTIVATION ON TUMOR ADHESION TO PLATELETS AND TUMOR TYROSINE PHOSPHORYLATION

Thrombin-treated tumour cells enhance their adhesion to platelets, fib ronectin and von Willebrand factor in vitro, and enhance their pulmona ry metastasis in mice in vivo. A unique seven transmembrane spanning t hrombin receptor has recently been cloned which is activated following thrombin proteolysis of the N-terminal end of the receptor with expos ure of a tethered ligand. An N-terminal 14-mer (SFLLRNPNDKYEPF) or 6-m er (SFLLRN) of the tethered ligand can serve as a thrombin receptor ac tivation peptide (TRAP) by mimicking the action of thrombin on platele ts, endothelial cells and smooth muscle cells. We have examined six hu man tumour cell lines for their response to TRAP, for the presence of this thrombin receptor mRNA by RT-PCR, protein by immunoblot and for t heir in vitro and in vivo response to TRAP. All six cell lines contain the receptor mRNA, and when treated with 100 mu M 6-mer TRAP or 1 u/m l thrombin increase their adhesion to platelets 2-3-fold. Four of the six cell lines undergo tyrosine phosphorylation within 30s to 1 min af ter exposure to 6-mer TRAP or thrombin. Thus tumour cells respond to t hrombin via activation of their seven transmembrane spanning thrombin receptor.