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

The peroxisome proliferator-activated receptor gamma (PPAR gamma) is a memb er of the nuclear receptor superfamily that activates target gene transcrip tion in a ligand-dependent manner. In addition, liganded PPAR gamma can inh ibit 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 a ntagonizing 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 th at transrepression of these factors by nuclear receptors involves competiti on for limiting amounts of the general coactivators CREB-binding protein (C BP) and p300. CBP and p300 are thought to be recruited to nuclear receptors through bridging factors that include SRC-1, although CBP also interacts d irectly with PPAR gamma through its amino terminus. These observations have raised questions concerning the involvement of SRC-1-like factors in CBP r ecruitment 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 muta tions 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 containi ng the SRC-1 interaction domain but lacking the N-terminal PPAR gamma inter action domain was inactive as a PPAR gamma coactivator and failed to rescue transrepression. Together, these findings are consistent with a model in w hich transrepression by PPAR gamma is achieved by targeting CBP through dir ect interaction with its N-terminal domain and via SRC-1-like bridge factor s.