Constitutive and inducible nitric oxide synthases in human megakaryoblastic cells.

Human megakaryoblastic cells (Meg-01) were found to possess constitutive and express inducible nitric oxide (NO) synthase activities. The constitutive NO synthase was Ca+(+)- and NADPH-dependent, as is the NO synthase found previously in human platelets. Stimulation of Meg-01 cells by the cytokines interleukin-1 beta (0.15-32.5 ng/ml) and tumor necrosis factor-alpha (0.15-10 ng/ml) resulted in expression of the inducible, Ca+(+)-independent, NO synthase. This activity was increased by the addition of NADPH, tetrahydrobiopterin and sepiapterin as cofactors. Induction of this enzyme was accompanied by a decrease in the constitutive NO synthase activity, a phenomenon which was prevented or reversed by dexamethasone (1 microM). Thus, human early differentiated megakaryocytic cells can synthesize NO from L-arginine by both the constitutive and the inducible NO synthases. These findings indicate that these enzymes may play an important biological role in megakaryocyte and platelet functions.