Expression and activation of protein kinase C isoforms in a human megakaryocytic cell line

Megakaryocytes undergo a unique differentiation program, becoming polyploid through repeated cycles of DNA synthesis without concomitant cell division. We have shown previously that phorbol 12-myristate 13-acetate (PMA) induces the Dami human megakaryocytic cell line to become polyploid and to express platelet-specific proteins, including von Willebrand factor (vWF) and glycoprotein Ib (GpIb). Phorbol esters are thought to regulate gene expression principally through the activation of protein kinase C (PKC), a family of structurally related kinases with potentially unique activation requirements and substrate specificities. A survey of PKC isoforms in Dami cells revealed that, by both Western and Northern analyses, PKC isoforms alpha, beta, delta, epsilon, eta, theta, and zeta were reproducibly detected. PKC-gamma was not detected. In order to define the role of individual PKC isoforms in megakaryocytic maturation, PMA and 2-deoxyphorbol 13-phenylacetate 20-acetate (dPPA), a putative selective activator of the PKC-beta 1 isotype, were compared for their effects on Dami cell maturation. Treatment with either dPPA or PMA caused Dami cells to cease proliferating, to become polyploid, and to express vWF. We also examined dPPA and PMA for their ability to activate and to downregulate expression of different PKC isoforms. Fifteen-minute treatment with PMA resulted in the translocation of PKC isoforms alpha, epsilon, and theta from the cytosolic to the membrane fraction; twenty-four hour treatment resulted in the downregulation of these isoforms. In contrast, dPPA was found to be a potent activator of PKC-epsilon alone and exhibited weaker effects on alpha and theta. These data suggest that PKC isoforms beta, delta, eta, and zeta, which appear not to be activated by either phorbol ester, are unlikely to be primarily involved in megakaryocytic maturation in response to these agents. The isoforms that are translocated by both phorbol esters-PKC isoforms alpha and theta, and particularly epsilon-are more likely to transduce the signals that stimulate Dami cell differentiation.