TIF1 beta functions as a coactivator for C/EBP beta and is required for induced differentiation in the myelomonocytic cell line U937

Representational difference analysis (RDA) cloning has identified transcrip tional intermediary factor 1 beta (TIF1 beta) as a gene inducibly expressed early during myeloid differentiation of the promyelocytic cell lines HL-60 and U937. To assess the role of TIF1 beta, U937 cell lines were made that expressed antisense-hammerhead ribozymes targeted specifically against TIF1 beta mRNA. These cells failed to differentiate into macrophages, as determ ined by several criteria: a nonadherent morphology, a failure to arrest cel l cycle, lowered levels of macrophage-specific cell surface markers, resist ance to Legionella pneumophila infection, a loss of the ability to phagocyt ose and chemotax, and decreased expression of chemokine mRNAs. One way TIF1 beta acts in macrophage differentiation is to augment C/EBP beta transcrip tional activity. Furthermore, we show by EMSA supershifts and coimmunopreci pitation that C/EBP beta and TIF1 beta physically interact. Although TIF1 b eta is necessary for macrophage differentiation of U937 cells, it is not su fficient, based on the inability of ectopically expressed TIF1 beta to indu ce or augment phorbol ester-induced macrophage differentiation. We conclude that TIF1 beta plays an important role in the terminal differentiation pro gram of macrophages, which involves the coactivation of C/EBP beta and indu ction of C/EBP beta -responsive myeloid genes.