Interleukin-4/STAT6 represses STAT1 and NF-κB-dependent transcription through distinct mechanisms

STAT6 mediates interleukin-4 (IL-4)-dependent positive and negative regulation of inflammatory gene expression. In the present report we examined the molecular mechanisms involved in IL-4-induced repression of reporter gene transcription driven by STAT1 and/or NF-kappaB. Transient expression of STAT6 in a STAT6-deficient cell line (HEK 293) conferred sensitivity to IL-4 for STAT6-dependent transcription and for repression of interferon-gamma (IFN gamma)/STAT1- and/or tumor necrosis factor-alpha (TNF alpha)/NF-kappaB driven reporter gene expression. In cells transfected with a deletion mutant of STAT6 lacking its transactivating domain, IL-4 could not mediate either positive or negative control of reporter gene expression. Overexpression of CREB-binding protein dramatically enhanced IL-4/STAT6-stimulated transcription and overcame IL-4-mediated repression of TNF alpha /NF-kappaB-dependent but not IFN gamma /STAT1-dependent transcription. A single amino acid change in the DNA-binding domain of STAT6 (H415A) selectively reduced the affinity of STAT6 for IL-4-responsive STAT sequence motifs (N4) without affecting the affinity for IFN gamma -responsive (GAS) sequences (N3) and, accordingly, eliminated transcription from an IL-4-responsive promoter. Interestingly, this mutation eliminated IL-4-mediated suppression of reporter gene transcription stimulated by TNF alpha /NF-kappaB but retained nearly full capacity to suppress IFN gamma /STAT1-stimulated transcription. Taken together these results demonstrate that STAT6 mediates suppression of STAT1 and NF-kappaB-dependent transcription by distinct mechanisms. Both processes are dependent upon the STAT6 transactivation domain and may involve sequestration of necessary but different transcriptional coactivator proteins. These two suppressive mechanisms are controlled differentially by the nature of the STAT6 DNA-binding site (i.e. N3 versus N4).