Inhibition of Interleukin-4 Production in CD4⁺ T Cells by Peroxisome Proliferator-Activated Receptor-γ (PPAR-γ) Ligands: Involvement of Physical Association between PPAR-γ and the Nuclear Factor of Activated T Cells Transcription Factor

Peroxisome proliferator-activated receptor-γ (PPAR-γ) has been implicated in the regulation of multiple inflammatory processes. However, little is known of PPAR-γ in the regulation of interleukin (IL)-4 expression in T cells. In this study, the effects of PPAR-γ ligands on production of IL-4, a pro-inflammatory cytokine associated with the pathophysiology of allergic diseases, were investigated. 15-Deoxy-Δ ^12,14 prostaglandin J₂ (15d-PGJ₂) and ciglitazone, two representative PPAR-γ ligands, significantly inhibited IL-4 production in both antigen-stimulated primary CD4⁺ T cells and the phorbol 12-myristate 13-acetate (PMA)/ionomycin-activated EL-4 T cell line. 15d-PGJ₂ and ciglitazone inhibited the activation of IL-4 gene promoter in EL-4 T cells transiently transfected with IL-4 promoter/reporter constructs, and the repressive effect mapped to a region in the IL-4 promoter containing binding sites for nuclear factor of activated T cells (NF-AT). The activation of T cells by PMA/ionomycin resulted in a marked enhancement of the binding activities to the NF-AT site that was significantly inhibited by the addition of PPAR-γ ligands. In cotransfected EL-4 T cells, PPAR-γ also inhibited the activation of the IL-4 promoter at multiple NF-AT sites in a ligand-dependent manner. NF-ATc1 bound PPAR-γ both in vivo and in vitro, and the interaction interfaces involved the Rel similarity domain of NF-ATc1. In cotransfections of HeLa cells, PPAR-γ inhibited the NF-ATc1 transactivation in a ligand-dependent manner. Coexpression of p300 or AP-1 relieved the PPAR-γ ligand-mediated inhibition of the NF-AT transactivation. From these results, we propose that PPAR-γ ligand-mediated suppression of IL-4 production in CD4⁺ T cells may involve both inhibition of the NFAT-DNA interactions and competitive recruitment of transcription integrators between NF-AT and PPAR-γ.