MULTIPLE STAT COMPLEXES INTERACT AT THE INTERFERON REGULATORY FACTOR-I INTERFERON-GAMMA ACTIVATION SEQUENCE IN PROLACTIN-STIMULATED NB2 T-CELLS

Interferon regulatory factor-1 (IRF-1) is a major immediate early gene induced by prolactin (PRL) in a biphasic, cell cycle-dependent manner in Nb2 T cells. This biphasic expression (30 min and 10 h) is mediate d in part by an interferon-gamma activation sequence (GAS) in the IRF- 1 promoter which binds factors belonging to the Signal Transducers and Activators of Transcription (Stat) family. By electrophoretic mobilit y shift assays (EMSA), Stat1 alpha was found to be the major and Stat5 a a minor component of the 30 min complex. At 10 h, Stat-like factors were again found at the IRF-1 GAS. Western blot analyses show that Sta t5a was rapidly induced by PRL to enter the nucleus, but unexpectedly, Stat1 alpha and the alternatively-spliced Stat1 beta were already pre sent in the uninduced nucleus. Further, Stat1 alpha but not Stat1 beta is preferentially tyrosine phosphorylated in response to PRL stimulat ion. Our studies suggest that multiple Stat complexes may contribute t o the biphasic transcription of the IRF-1 gene in PRL-stimulated T cel ls.