Sustained activation of the extracellular signal-regulated kinase mitogen-activated protein kinase pathway is required for megakaryocytic differentiation of K562 cells

The extracellular signal-regulated kinase (ERK), originally identified as a participant in mitogenic signaling, has recently been implicated in the signaling of cellular differentiation. To examine the role of the ERK/MAP kinase pathway in megakaryocytic differentiation of K562 cells, the effects of 12-O-tetradecanoylphorbol-13-acetate (TPA) and bryostatin on ERK activation were determined. Both TPA and bryostatin are known to activate PBC but paradoxically have opposing effects on megakaryocytic differentiation, TPA, a differentiation inducer, caused sustained activation of ERK (>24 h), whereas bryostatin, a differentiation blocker, only transiently activated ERK (similar to 6 h) and attenuated the activation of ERK by TPA, To confirm a requirement for sustained ERK activation for megakaryocytic differentiation, PD098059, a synthetic inhibitor of the MAP kinase kinase 1 (MEK1) was employed. Introduction of PD098059 at any time during the first 18 h of TPA treatment completely abrogated megakaryocytic differentiation of K562 cells, After 24 h of TPA treatment, introduction of PD098059 failed to block differentiation. Differentiation blockade by PD098059 occurred via inhibition of MEK because transfection of a constitutively active mutant of MEK2 could override the PD098059 blockade. Experiments with conditioned media suggested that sustained activation of the ERK/MAP kinase pathway promoted the autocrine secretion of megakaryocytic lineage determination factors.