Induction of platelet formation from megakaryocytoid cells by nitric oxide

Although the growth factors that regulate megakaryocytopoiesis are well kno wn, the molecular determinants of platelet formation from mature megakaryoc ytes remain poorly understood. Morphological changes in megakaryocytes asso ciated with platelet formation and removal of senescent megakaryocytes are suggestive of an apoptotic process. Previously, we have established that ni tric oxide (NO) can induce apoptosis in megakaryocytoid cell lines. To dete rmine whether there is an association between NO-induced apoptosis and plat elet production, we exposed Meg-01 cells to S-nitrosoglutathione (GSNO) wit h or without thrombopoeitin (TPO) pretreatment and used flow cytometry and electron microscopy to assess platelet-sized particle formation. Meg-01 cel ls treated with TPO alone produced few platelet-sized particles (<3% of tot al counts), whereas treatment with GSNO alone produced a significant percen tage of platelet-sized particles (22 +/- 4% of total counts); when combined with TPO pretreatment, however, GSNO led to a marked increase in platelet- sized particle production (48 +/- 3% of total counts). Electron microscopy confirmed that Meg-01 cells treated with TPO and GSNO yielded platelet-size d particles with morphological features specific for platelet forms. The pl atelet-sized particle population appears to be functional, because addition of calcium, fibrinogen, and thrombin receptor-activating peptide led to ag gregation. These results demonstrate that NO facilitates platelet productio n, thereby establishing the essential role of NO in megakaryocyte developme nt and thrombopoiesis.