PU.1 Activity Determines Fate of Myeloid Progenitor Cells during Lineage Commitment.

Myeloid cell commitment is regulated by factors interacting with chromatin in a progenitor cell entering differentiation. PU.1 is an ETS family transcription factor that has been well characterized in inducing myelopoiesis and blocking erythroid differentiation. Conditionally activated PU.1-Estrogen Receptor transgene in mouse PU.1 knockout-derived hematopoietic progenitors is known to induce macrophage differentiation. We observed that manipulation of PU.1 activity by using different levels of PU.1-ER activator, tamoxifen, was capable of producing major populations of myeloid progeny including macrophages, granulocytes, lymphocytes and mastocytes. Gradual activation of PU.1 produce distinct committed populations that require presence of PU.1 activator. Withdrawal of tamoxifen from macrophages restored cycling and pluripotent potential resembling parental cell line. Furthermore, very low PU.1 activity induced frequent chromosomal aberations such as polyploidy. To determine that PU.1 directly caused these phenotypic changes we determined its occupancy at genes representing PU.1 targets by quantitative chromatin immunoprecipitation (ChIP). We demonstrate that PU.1 is enriched at regulatory regions of lineage restricted cell populations including promoters of CD11b, CD14, IL7R, PAX5, RAG1, Gelatinase, Lysozyme, and MCP. Different PU.1 levels promote distinct cell populations that have different localizations of PU.1 in chromatin. We also tested mRNA levels of PU.1 target genes in progenitor cell line induced by PU.1. Macrophage-specific target genes such as CD11b and CD14 are induced at high PU.1 levels and granulocytic PU.1 targets such as Gelatinase and Lysozyme are induced at intermediate and high PU.1 levels. B-cell specific mRNAs such as IL7-R, PAX5, and RAG1 were induced at low and high PU.1 levels. Our data suggest that PU.1 is recruited to and transactivates its lineage-restricted target genes differently at distinct PU.1 levels and such mechanism may be involved in myeloid cell commitment and leukemogenesis.