Polyamine release and vesicular polyamine transporter expression in megakaryoblastic cells and platelets

Polyamines not only play essential roles in cell growth and function of living organisms but are also released into the extracellular space and function as regulators of chemical transduction, although the cells from which they are released and their mode of release are not well understood. The vesicular polyamine transporter (VPAT), encoded by the SLC18B1 is responsible for the vesicular storage of spermine and spermidine, followed by their vesicular release from secretory cells. Focusing on VPAT will help identify polyamine-secreting cells and new polyamine functions. In this study, we investigated the possible involvement of VPAT in vesicular release of polyamines in MEG-01 clonal megakaryoblastic cells and platelets. RT-PCR, western blotting, and immunohistochemistry revealed VPAT expression in MEG-01 cells. MEG-01 cells secreted polyamines upon A23187 stimulation in the presence of Ca²⁺, which is temperature-dependent and sensitive to bafilomycin A₁. A23187-induced polyamine secretion from MEG-01 cells was reduced by treatment with reserpine, VPAT inhibitors, or VPAT RNA interference. Platelets also expressed VPAT, displaying a punctate distribution, and released spermidine upon A23187 and thrombin stimulation. These findings have demonstrated VPAT-mediated vesicular polyamine release from MEG-01 cells, suggesting the presence of similar vesicular polyamine release mechanisms in platelets.