Thrombin interactions with cells of the megakaryocytic lineage

Thrombin is a potent physiological platelet agonist, critical for haemostasis and thrombosis. Several studies have recently demonstrated that thrombin also displays functional effects on bone marrow platelet progenitors, i.e. the megakaryocytes, and possibly on megakaryocyte precursors. Following a brief description of the different cell systems used for these investigations, this paper focuses on data related to thrombin receptors, their coupling mechanisms, and biological responses due to their activation on cells of the megakaryocytic lineage. In spite of some differences, whose significance remains to be established, most data reported give evidence of considerable homologies in thrombin receptors and signal transduction mechanisms between platelets and cells belonging to the megakaryocytic lineage. In addition, a potential functional role for thrombin in the regulation of megakaryocytopoiesis is emerging. The physiological relevance of these observations are discussed.