EVIDENCE FOR A SELECTIVE INHIBITORY EFFECT OF THROMBIN ON MEGAKARYOCYTE PROGENITOR GROWTH MEDIATED BY THE THROMBIN RECEPTOR

An efficient method for the culture of human megakaryocyte precursors in serum-free medium has been developed facilitating study of the effe ct of regulators of megakaryocyte growth and maturation without interf erence by serum-derived factors. We have investigated how megakaryocyt es and their precursors respond to the procoagulant enzyme, thrombin. In addition to its already documented agonist effect on mature megakar yocytes, thrombin was found to have a marked inhibitory effect on the growth of megakaryocyte colonies from CD34(+) bone marrow cells stimul ated by IL3. This inhibitory effect, not previously reported, was sele ctive for megakaryocytic cells. The growth of granulomonocytic and ery throid colonies was not affected. A monoclonal antibody which neutrali zed the effect of exogenous transforming growth factor beta (TGF beta) was unable to fully neutralize the inhibitory effect of thrombin. Wit h the use of a synthetic peptide, corresponding to the tethered thromb in receptor ligand, and of a recombinant inactive form of thrombin, we provide direct evidence that both the inhibitory effect of thrombin o n megakaryocyte proliferation and its agonist effect on mature magakar yocytes are mediated by a receptor analogous to the recently cloned pl atelet thrombin receptor.