Mechanistic studies on the effects of nicotinamide on megakaryocytic polyploidization and the roles of NAD⁺ levels and SIRT inhibition

Objective: Megakaryocytic cells (Mks) undergo endomitosis and become polyploid. Mk ploidy correlates with platelet production. We previously showed that nicotinamide (NIC) greatly increases Mk ploidy in cultures of human mobilized peripheral blood CD34⁺ cells. This study aims to examine the generality of NIC effects, NIC's impact on Mk ultrastructure, and potential mechanisms for the increased ploidy. Materials and Methods: We used electron microscopy to examine Mk ultrastructure and flow cytometry to evaluate NIC effects on Mk differentiation and ploidy in mobilized peripheral blood CD34⁺ cell cultures under diverse megakaryopoietic conditions. Mk ploidy and NAD(H) content were evaluated for NIC and other NAD⁺ precursors. We tested additional inhibitors of the sirtuin (or SIRT) 1 and SIRT2 histone/protein deacetylases and, after treatment with NIC, evaluated changes in the acetylation of SIRT1/2 targets. Results: NIC increased ploidy under diverse culture conditions and did not alter Mk ultrastructure; 6.25 mM NIC increased NAD⁺ levels fivefold. Quinolinic acid increased NAD⁺ similar to that for 1 mM NIC, but yielded a much smaller ploidy increase. Similar increases in Mk ploidy were obtained using NIC or the SIRT1/2 inhibitor cambinol, while the SIRT2 inhibitor AGK2 moderately increased ploidy. SIRT1/2 inhibition in cells treated with NIC was evidenced by increased acetylation of nucleosomes and p53. Greater p53 acetylation with NIC was associated with increased binding of p53 to its consensus DNA binding sequence. Conclusion: NIC greatly increases Mk ploidy under a wide range of conditions without altering Mk morphology. Inhibition of SIRT1 and/or SIRT2 is primarily responsible for NIC effects on Mk maturation.