Ikaros zinc finger transcription factors as novel regulators of STAT factor activity

Abstract

Abstract Cytokine signaling is important for the differentiation and function of a number of cell types, including lymphocyte populations. One mechanism by which cytokine signals are propagated is via phosphorylation-mediated activation of Signal Transducer and Activator of Transcription (STAT) factors, which directly regulate cell type-specific gene programs. We recently identified novel regulatory modules composed of STAT and Ikaros Zinc Finger (IkZF) transcription factors in CD4+ T cell populations. Specifically, we found that IkZF/STAT modules composed of Aiolos/STAT3 and Eos/STAT5 directly activate genes associated with T follicular helper (TFH) and T helper 1 (TH1) gene programs, respectively. Here, we extend these findings and show that IkZF factor expression correlates with STAT activation, suggesting that IkZF proteins may regulate STAT factor activity in CD4+ T cells. We find that overexpression of Aiolos, but not the IkZF factors Ikaros and Eos, results in increased STAT3 activation. Conversely, overexpression of Eos, but not Ikaros or Aiolos, results in increased activation of STAT5. Importantly, when we performed these studies using IkZF protein mutants incapable of interacting with STAT factors, the observed increase in STAT factor phosphorylation was lost. We also find that in CD4+ T cells, Eos deficiency correlates with reduced STAT5 activation, while Aiolos-deficient cells display reduced STAT3 activation. Finally, we observe differential IkZF factor expression across CD4+ T cell subsets. When coupled with known differences in STAT factor activation, these findings suggest that IkZF/STAT regulatory modules may broadly regulate T helper cell differentiation.