Phosphorylation-deficient Stat1 inhibits retinoic acid–induced differentiation and cell cycle arrest in U-937 monoblasts

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Abstract
All-trans retinoic acid (ATRA) is a potent inducer of terminal differentiation of immature leukemic cell lines in vitro and of acute promyelocytic leukemia (APL) cells in vivo. Recent reports have shown that ATRA induces the expression of several interferon-regulated genes, including signal transducer and activator of transcription (Stat)1. To investigate the role of Stat1 activation in ATRA signaling, sublines were established for the human monoblastic cell line U-937 constitutively expressing wild-type or phosphorylation-defective Stat1, mutated in the conserved tyrosine 701 required for dimerization and nuclear translocation. Results showed that ATRA induction leads to activation of Stat1 by the phosphorylation of tyrosine 701 and subsequent nuclear translocation. Consistent with a functional importance of this activation, ectopic expression of Stat1Y701F suppressed ATRA-induced morphologic differentiation and expression of the monocytic surface markers CD11c and the granulocyte colony-stimulating factor receptor. Moreover, ATRA-induced growth arrest in the G0/G1phase of the cell cycle was inhibited by phosphorylation-deficient Stat1. Taken together, these results indicate that Stat1 is a key mediator of ATRA-induced cell cycle arrest and differentiation of U-937 cells.