REGULATION OF THE MYELOPEROXIDASE ENHANCER-BINDING PROTEINS PU1, C-EBP-ALPHA, C-EBP-BETA, AND C-EBP-DELTA DURING GRANULOCYTE-LINEAGE SPECIFICATION

We have compared the molecular architecture and function of the myelop eroxidase upstream enhancer in multipotential versus granulocyte-commi tted hematopoietic progenitor cells, We show that the enhancer is acce ssible in multipotential cell chromatin but functionally incompetent b efore granulocyte commitment. Multipotential cells contain both Pu1 an d C-EBP alpha as enhancer-binding activities. Pu1 is unphosphorylated in both multipotential and granulocyte-committed cells but is phosphor ylated in B lymphocytes, raising the possibility that differential pho sphorylation may play a role in specifying its lymphoid versus myeloid functions. C-EBP alpha exists as multiple phosphorylated forms in the nucleus of both multipotential and granulocyte-committed cells. C-EBP beta is unphosphorylated and cytoplasmically localized in multipotent ial cells but exists as a phosphorylated nuclear enhancer-binding acti vity in granulocyte-committed cells. Granulocyte colony-stimulating fa ctor-induced granulocytic differentiation of multipotential progenitor cells results in activation of C-EBP delta expression and functional recruitment of C-EBP delta and C-EBP beta to the nucleus, Our results implicate Pu1 and the C-EBP family as critical regulators of myelopero xidase gene expression and are consistent with a model in which a temp oral exchange of C-EBP isoforms at the myeloperoxidase enhancer mediat es the transition from a primed state in multipotential cells to a tra nscriptionally active configuration in promyelocytes.