(114c) Nicotinamide (Vitamin B3) Increases the Ploidy and Proplatelet Production of Human Megakaryocytes

Megakaryocytic cells (Mks) are derived from hematopoietic stem cells (HSCs). As the immediate precursors to platelets, Mks are central to blood coagulation and hemostasis. The sequence of Mk differentiation includes progression through several rounds of endomitosis to form polyploid cells and the production of cytoplasmic proplatelet extensions from which platelets are released. Thrombopoietin (Tpo) is the major cytokine controlling Mk commitment, maturation, and platelet production. Culture of stem and progenitor (CD34+) cells with Tpo alone yields a high purity of Mks. However, the size and ploidy of human Mks produced in culture are much smaller than those observed in vivo. This is problematic because there is a direct correlation between Mk DNA content and platelet production. We have discovered that nicotinamide (NIC), one of the two principle forms of niacin (vitamin B3), greatly increases Mk ploidy and proplatelet production in Tpo-stimulated cultures of human CD34+ cells. By day 13, the percentage of high-ploidy (8N or greater) Mks was twice as great with NIC than for Tpo alone. Furthermore, cells treated with NIC reached ploidy levels of 64N and 32N for 6.25 and 3 mM NIC, respectively, compared to 16N without NIC. NIC-treated cells also exhibited dramatic morphologic evidence of greater Mk maturation, as characterized by larger cells, more highly lobated nuclei, and a greater frequency of proplatelet-forming cells. Remarkably, while Mk polyploidization and apoptosis are typically correlated events, the increase in DNA content observed for NIC-treated cells was not accompanied by any changes in the kinetics of apoptosis. This indicates that the rate of apoptosis can be at least partially decoupled from the rate of Mk maturation. I will also discuss the mechanisms that underlie the effects of nicotinamide on Mk maturation. Elucidating these mechanisms could provide the basis for clinical advances for the treatment of Mk and platelet disorders. Furthermore, ex vivo culture of HSCs under conditions that promote Mk commitment, expansion, and differentiation would enable the production of immature and mature Mks for transplantation therapies and, in the longer term, may allow for the production of large numbers of platelets for use in platelet transfusions.