Positive and negative transcriptional regulation of the Foxp3 gene is mediated by access and binding of the Smad3 protein to enhancer I.

The molecular mechanisms underlying retinoic acid (RA) augmentation of T cell receptor (TCR) and transforming growth factor-β (TGF-β)-induced Foxp3 transcription and inhibition of the latter by cytokines such as IL-27 were here shown to be related processes involving modifications of baseline (TGF-β-induced) phosphorylated Smad3 (pSmad3) binding to a conserved enhancer region (enhancer I). RA augmentation involved the binding of retinoic acid receptor (RAR) and retinoid X receptor (RXR) to a dominant site in enhancer I and a subordinate site in the promoter. This led to increased histone acetylation in the region of the Smad3 binding site and increased binding of pSmad3. Cytokine (IL-27) inhibition involved binding of pStat3 to a gene silencer in a second conserved enhancer region (enhancer II) downstream from enhancer I; this led to loss of pSmad3 binding to enhancer I. Thus, control of accessibility and binding of pSmad3 provides a common framework for positive and negative regulation of TGF-β-induced Foxp3 transcription.