The peroxisome proliferator-activated receptors (PPARs) are nuclear hormone receptors that regulate glucose and lipid homeostasis. The PPAR␥ subtype plays a central role in the regulation of adipogenesis and is the molecular target for the 2,4-thiazolidinedione class of antidiabetic drugs. Structural studies have revealed that agonist ligands activate the PPARs through direct interactions with the C-terminal region of the ligand-binding domain, which includes the activation function 2 helix. GW0072 was identified as a high-affinity PPAR␥ ligand that was a weak partial agonist of PPAR␥ transactivation. X-ray crystallography revealed that GW0072 occupied the ligand-binding pocket by using different epitopes than the known PPAR agonists and did not interact with the activation function 2 helix. In cell culture, GW0072 was a potent antagonist of adipocyte differentiation. These results establish an approach to the design of PPAR ligands with modified biological activities.The nuclear hormone receptors are ligand-activated transcription factors that regulate target genes essential for mammalian physiology and development (1). The peroxisome proliferatoractivated receptors (PPARs) are nuclear receptors activated by fatty acids and their eicosanoids metabolites, which regulate genes involved in the biosynthesis, storage, and metabolism of these ligands (2). The pharmacology of synthetic PPAR ligands demonstrated the role of these receptors in regulating glucose and lipid homeostasis and established their utility as molecular targets for the development of drugs for the treatment of diabetes and cardiovascular disease (3).Biochemical and structural studies with several nuclear receptors revealed that hormone binding induces allosteric changes in the conformation of the ligand-binding domain, which promote recruitment of transcriptional coactivator proteins such as steroid receptor coactivator 1 (SRC1) (4) and CREB binding protein (CBP) (5). We recently reported x-ray crystallographic analysis of the ternary complex of PPAR␥ with the 2,4-thiazolidinedione (TZD) rosiglitazone (Fig. 1A) and the coactivator SRC1 (6), as well as the complexes of PPAR␦ with either the fibrate GW2433 or the essential fatty acid eicosapentaenoic acid (7). Despite differences in their gross chemical structure, all of these small molecule PPAR agonists share a common binding mode, in which the acidic head groups form a network of hydrogen bonds with Y473, H449, and H323 within the ligand-binding pocket. These interactions stabilize a charge clamp (6) between the Cterminal activation function 2 (AF-2) helix and a conserved lysine residue on the surface of the receptor, through which coactivator proteins are recruited to the receptor.

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The transcriptional activity of the human androgen receptor (hAR), like other nuclear receptors, is dependent on accessory factors. One such factor is c-Jun, which has been shown to have a selective function of mediating androgen receptor-dependent transactivation. This c-Jun activity is inhibited by c-Fos, another protooncoprotein that can dimerize with c-Jun to form the transcription factor AP-1. Here we show that c-jun mediates hAR-induced transactivation from the promoter of the androgen-regulated gene, human kallikrein-2 (hKLK2), and c-Fos blocks this activity. Using c-Fos truncation mutants and measuring hKLK2-dependent transcription, we have determined that the bZIP region of c-Fos is required and sufficient for inhibiting c-Jun enhancement of hAR transactivation. Further truncation analysis of the bZIP shows that the c-Fos dimerization function, mediated through the leucine zipper, is essential for the negative activity, whereas DNA binding, mediated through the basic region, is dispensable. These results suggest that heterodimerization by c-Fos with c-Jun blocks c-Jun's ability to enhance hAR-induced transactivation.