Peroxisome proliferator-activated receptor-gamma (PPAR-gamma) has essential roles in adipogenesis and glucose homeostasis, and is a molecular target of insulin-sensitizing drugs. Although the ability of PPAR-gamma agonists to antagonize inflammatory responses by transrepression of nuclear factor kappa B (NF-kappaB) target genes is linked to antidiabetic and antiatherogenic actions, the mechanisms remain poorly understood. Here we report the identification of a molecular pathway by which PPAR-gamma represses the transcriptional activation of inflammatory response genes in mouse macrophages. The initial step of this pathway involves ligand-dependent SUMOylation of the PPAR-gamma ligand-binding domain, which targets PPAR-gamma to nuclear receptor corepressor (NCoR)-histone deacetylase-3 (HDAC3) complexes on inflammatory gene promoters. This in turn prevents recruitment of the ubiquitylation/19S proteosome machinery that normally mediates the signal-dependent removal of corepressor complexes required for gene activation. As a result, NCoR complexes are not cleared from the promoter and target genes are maintained in a repressed state. This mechanism provides an explanation for how an agonist-bound nuclear receptor can be converted from an activator of transcription to a promoter-specific repressor of NF-kappaB target genes that regulate immunity and homeostasis.

Multiple enzymatic activities are required for transcriptional initiation. The enzyme DNA topoisomerase II associates with gene promoter regions and can generate breaks in double-stranded DNA (dsDNA). Therefore, it is of interest to know whether this enzyme is critical for regulated gene activation. We report that the signal-dependent activation of gene transcription by nuclear receptors and other classes of DNA binding transcription factors, including activating protein 1, requires DNA topoisomerase IIbeta-dependent, transient, site-specific dsDNA break formation. Subsequent to the break, poly(adenosine diphosphate-ribose) polymerase-1 enzymatic activity is induced, which is required for a nucleosome-specific histone H1-high-mobility group B exchange event and for local changes of chromatin architecture. Our data mechanistically link DNA topoisomerase IIbeta-dependent dsDNA breaks and the components of the DNA damage and repair machinery in regulated gene transcription.

The mechanisms that control the precisely regulated switch from gene repression to gene activation represent a central question in mammalian development. Here, we report that transcriptional activation mediated by liganded nuclear receptors unexpectedly requires the actions of two highly related F box/WD-40-containing factors, TBL1 and TBLR1, initially identified as components of an N-CoR corepressor complex. TBL1/TBLR1 serve as specific adaptors for the recruitment of the ubiquitin conjugating/19S proteasome complex, with TBLR1 selectively serving to mediate a required exchange of the nuclear receptor corepressors, N-CoR and SMRT, for coactivators upon ligand binding. Tbl1 gene deletion in embryonic stem cells severely impairs PPARgamma-induced adipogenic differentiation, indicating that TBL1 function is also biologically indispensable for specific nuclear receptor-mediated gene activation events. The role of TBLR1 and TBL1 in cofactor exchange appears to also operate for c-Jun and NFkappaB and is therefore likely to be prototypic of similar mechanisms for other signal-dependent transcription factors.