ATRA treatment induces an early transient phosphorylation of Stat1 Ser727 in U-937 cells. (A-C) U-937 cells were treated with ATRA for 0 to 96 hours (A), 0 to 24 hours (B), or 0 to 6 hours (C). Lysates were prepared and analyzed by Western blot using antibodies directed against Ser727-phosphorylated Stat1. The blots were stripped and reprobed with antibodies targeted against total Stat1. A lysate of U-937 cells induced by 100 U IFN-_γ for 30 minutes was included as a positive control for Stat1 Ser727 phosphorylation (C).