REGULATION OF THROMBIN RECEPTORS ON HUMAN UMBILICAL VEIN ENDOTHELIAL-CELLS

Activated thrombin receptors on human umbilical vein endothelial cells rapidly undergo homologous desensitization, leaving the cells unable to respond to thrombin. The present studies examine the fate of activa ted thrombin receptors on endothelial cells and the mechanisms that re store intact receptors to the cell surface. The results show that: 1) at biologically relevant concentrations, thrombin rapidly cleaves all of its receptors on the cell surface, 2) The cleaved receptors are cle ared from the cell surface in a two-phase process, with 60% being inte rnalized within 10 min, the remainder requiring several hours, 3) The restoration of intact, thrombin-responsive receptors on the cell surfa ce initially occurs from an intracellular pool of receptors in a proce ss that is independent of protein synthesis, 4) Recycling of cleaved r eceptors either does not occur on endothelial cells or is masked by re ceptor clearance, 5) Subconfluent endothelial cells re-express intact receptors on the cell surface at a slower rate than confluent cells, 6 ) The agonist peptide, SFLLRN, also causes receptor internalization, a lthough at concentrations greater than those required for receptor act ivation and desensitization. These results are distinctly different fr om those observed with megakaryoblastic cell lines, where >90% of the activated thrombin receptors are internalized rapidly, up to 40% of th e cleaved receptors are recycled, and no intracellular pool of intact receptors has been detected, Since the primary structure of the thromb in receptor is the same in all the cell types studied, these results d emonstrate that there can be substantial differences between cell type s in the mechanisms involved in the clearance of activated receptors a nd the re-expression on the cell surface of intact receptors capable o f responding to thrombin.