Presence of functionally active tissue plasminogen activator in human CFU-M derived megakaryocytes in vitro

Plasminogen activator inhibitor-1 mRNA is expressed in platelets and megakaryocytes and the megakaryoblastic cell line CHRF-288

Plasminogen activator inhibitor-1 (PAI-1) is present in the platelet alpha-granule and is released on platelet activation. Platelet PAI-1 could either be synthesized by the megakaryocyte or taken up from the plasma. In this report we confirm the presence of PAI-1 protein in human megakaryocytes by Western blot analysis and show its synthesis in guinea pig megakaryocytes by metabolic labeling. We document the presence of PAI-1 mRNA in human platelets and show a 3-kb mRNA species on Northern blot analysis of guinea pig megakaryocytes. Neither untreated CHRF-288 cells, a megakaryoblastic cell line, nor human erythroleukemia (HEL) cells expressed PAI-1 mRNA. Phorbol ester (phorbol 12-myristate 13-acetate, 160 nM) treatment of CHRF-288 and HEL cells for 4 days induced PAI-1 mRNA expression in CHRF-288 cells but not in HEL cells. These studies show that PAI-1 is synthesized by megakaryocytes. Megakaryocytes most likely determine the PAI-1 content of platelets and thereby establish the antifibrinolytic potential of the platelet.

Tissue-plasminogen activator RNA detected in megakaryocytes by in situ hybridization and biotinylated probe

Tissue-plasminogen activator (t-PA) has been identified in human megakaryocytes isolated from human rib fragments or from culture cells. The presence of t-PA antigen in the endoplasmic reticulum of mature cells was demonstrated by ultrastructural immunochemistry. These results suggest that t-PA can be synthesized by megakaryocytes as well as by vascular endothelial cells. In order to confirm this possibility, in situ hybridization using antisense RNA biotinylated probe was used in megakaryocyte culture at different steps of maturation. The presence of t-PA-RNA was clearly demonstrated at an early step of differentiation. Biotin staining was enhanced in mature cells and this was correlated with the detection of t-PA antigen by immunocytochemistry.