TIF1β functions as a coactivator for C/EBPβ and is required for induced differentiation in the myelomonocytic cell line U937

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Abstract
Representational difference analysis (RDA) cloning has identified transcriptional intermediary factor 1 beta (TIF1β) as a gene inducibly expressed early during myeloid differentiation of the promyelocytic cell lines HL-60 and U937. To assess the role of TIF1β, U937 cell lines were made that expressed antisense-hammerhead ribozymes targeted specifically against TIF1β mRNA. These cells failed to differentiate into macrophages, as determined by several criteria: a nonadherent morphology, a failure to arrest cell cycle, lowered levels of macrophage-specific cell surface markers, resistance toLegionella pneumophilainfection, a loss of the ability to phagocytose and chemotax, and decreased expression of chemokine mRNAs. One way TIF1β acts in macrophage differentiation is to augment C/EBPβ transcriptional activity. Furthermore, we show by EMSA supershifts and coimmunoprecipitation that C/EBPβ and TIF1β physically interact. Although TIF1β is necessary for macrophage differentiation of U937 cells, it is not sufficient, based on the inability of ectopically expressed TIF1β to induce or augment phorbol ester-induced macrophage differentiation. We conclude that TIF1β plays an important role in the terminal differentiation program of macrophages, which involves the coactivation of C/EBPβ and induction of C/EBPβ-responsive myeloid genes.