Peroxisome proliferator-activated receptor γ-dependent repression of the inducible nitric oxide synthase gene

The peroxisome proliferator-activated receptor gamma (PPAR gamma) is a member of the nuclear receptor superfamily that activates target gene transcription in a ligand-dependent manner. In addition, liganded PPAR gamma can inhibit transcription of genes induced by gamma interferon (IFN-gamma) and/or lipopolysaccharides (LPSs), including the inducible nitric oxide synthase (iNOS) gene. Inhibition of the iNOS promoter is achieved partially through antagonizing the activities of NF-kappa B, AP-1, and STAT1, which are known to mediate effects of LPS and IFN-gamma. Previous studies have suggested that transrepression of these factors by nuclear receptors involves competition for limiting amounts of the general coactivators CREB-binding protein (CBP) and p300. CBP and p300 are thought to be recruited to nuclear receptors through bridging factors that include SRC-1, although CBP also interacts directly with PPAR gamma through its amino terminus. These observations have raised questions concerning the involvement of SRC-1-like factors in CBP recruitment and transrepression. We here provide evidence that PPAR gamma's ability to repress iNOS transcription requires the ligand-dependent charge clamp that mediates interactions with CBP and SRC-1. Single amino acid mutations in PPAR gamma that abolished ligand-dependent interactions with SRC-1 and CBP not only resulted in complete loss of transactivation activity but also abolished transrepression. Conversely, a CBP deletion mutant containing the SRC-1 interaction domain but lacking the N-terminal PPAR gamma interaction domain was inactive as a PPAR gamma coactivator and failed to rescue transrepression. Together, these findings are consistent with a model in which transrepression by PPAR gamma is achieved by targeting CBP through direct interaction with its N-terminal domain and via SRC-1-like bridge factors.