B cell antigen receptor signaling enhances IFN-gamma-induced Stat1 target gene expression through calcium mobilization and activation of multiple serine kinase pathways.

The signal transducers and activators of transcription 1 (Stat1) are essential for the majority of interferon-gamma (IFN-gamma)-regulated gene expression. Phosphorylation of serine 727 in the transcription activation domain of Stat1 is induced in response to IFN-gamma for maximal transcription activity. In this report, we show that crosslinking of B cell antigen receptor (BCR) or T cell antigen receptor (TCR) can enhance S727 phosphorylation in Stat1 and result in increased expression of Stat1 target genes. We further demonstrate that this enhancement by BCR cross-linking involves the widely used secondary messenger Ca2+ and simultaneous activation of multiple serine kinase pathways. When cells are exposed to both IFN-gamma and a Ca2+ fluxing reagent, the level of S727 phosphorylation is enhanced, resulting in increased transcription activation of Stat1 target genes. We directly demonstrate that the biochemical function of phospho-Ser-727 is to enhance the recruitment of transcription coactivator CBP/p300 to the promoters of Stat1 target genes. Furthermore, we show that both the p38 mitogen-activated protein kinase (MAPK) and the Ca(2+)/calmodulin-dependent kinase (CaMKII) are activated in response to BCR signaling to converge on Stat1 S727 for maximal gene expression. These studies demonstrate that a wide variety of noncytokine signaling pathways can modulate cytokine signaling through modulation of Stat1 serine phosphorylation.