Protein content and number of nucleolar organizer regions are enhanced during phorbol ester-induced differentiation of cultured human megakaryocytic cells

In order to investigate the protein synthesis in megakaryocyte polyploidization, phorbol myristate acetate (PMA, 5 x 10^-9 M), a differentiation marker known to induce megakaryocyte polyploidization, was added to human megakaryocytic cell lines (DAMI, HEL and K562) and the expression of platelet/megakaryocytic integrins, the numbers of nucleolar organizer regions (AgNORs) and the total protein content were estimated. Following exposure of PMA, the expression of the platelet membrane glycoprotein GPIIIa and thrombospondin and transferrin receptors was augmented in the three cell lines. The number of AgNORs shifted from 16.4 ± 4.3, 24.4 ± 2.5 and 13.6 ± 3.1 for unstimulated cells to 20.0 ± 5.3, 38.7 ± 7.9 and 16.8 ± 2.3 for PMA-treated DAMI, HEL and K562 cells, respectively. Furthermore, after treatment with PMA, the numbers of AgNORs clusters or nucleoles increased significantly to 179%, 238% and 154% of controls in DAMI, HEL and K562 cell lines, respectively. Finally, addition of PMA culture for four days, significantly increased the protein contents to 153%, 171% and 254% of controls for DAMI, HEL and K562 cell lines, respectively (p < 0.05 by t-test). In conclusion, the increase in the total protein content and in the number of AgNORs by PMA, suggests that PMA-induced-megakaryocyte polyploidization occurs by enhanced protein production.