From the Cover: PPARγ and PPARδ negatively regulate specific subsets of lipopolysaccharide and IFN-γ target genes in macrophages

PPARγ is a member of the nuclear receptor superfamily of ligand-dependent transcription factors that regulates adipocyte differentiation and glucose homeostasis (1–3). Although the endogenous ligands that regulate peroxisome proliferator-activated receptor (PPAR) γ activity in vivo remain poorly characterized, several naturally occurring polyunsaturated fatty acids and their metabolites have been identified that activate PPARγ, including products that are generated through the actions of specific lipoxygenases (e.g., 13-hydroxyoctadecadienoic acid and 15-hydroxyeicosatetraenoic acid) and prostaglandin synthases (e.g., 15 deoxy-Δ12,14 prostaglandin J2) (4–7). In addition, numerous synthetic PPARγ agonists have been identified, including the thiazolidinedione class of drugs used clinically in the treatment of type 2 diabetes mellitus (1, 3). Natural and synthetic PPARγ ligands have been also been shown to exert antiinflammatory effects in models of athero-sclerosis (8–10), inflammatory bowel disease (11, 12), and allergic encephalomyelitis (13–15). The investigation of potential antiinflammatory effects of PPARγ agonists in these settings was initially based on studies demonstrating that they could inhibit transcriptional activation of inflammatory response genes by activators such as lipopolysaccharide (LPS), IL-1β, and IFN-γ in macrophages and other cell types (16–18). PPARγ expression is dramatically up-regulated in macrophages and T cells during inflammatory responses, and can be induced by IL-4 and other immunoregulatory molecules (19–21). Overexpression of PPARγ potentiates the ability of diverse PPARγ agonists to inhibit the expression of inflammatory response genes, consistent with it mediating antiinflammatory effects (16, 22). However, 15-deoxyΔ12,14 prostaglandin J2 was found to inhibit NF-κB-dependent transcription by PPARγ-independent mechanisms (23, 24), and doses of thiazolidinediones that exert maximal inhibitory effects on LPS-inducible genes are significantly higher than would be expected based on their binding affinity for PPARγ in vitro (16). Furthermore, treatment of macrophages derived from PPARγ-null embryonic stem cells with high concentrations of synthetic PPARγ ligands was recently reported to inhibit the induction of the inducible nitric-oxide synthase (iNOS) and cyclooxygenase 2 (COX2) genes by IFN-γ to approximately the same extent as in macrophages derived from wild-type embryonic stem cells (25). These observations raise a number of questions regarding the roles of PPARγ in regulating macrophage gene expression during inflammatory responses. To address these questions, we used a combination of microarray analysis and Cre-mediated disruption of the PPARγ gene in primary mouse macrophages to characterize PPARγ-dependent and -independent transcriptional programs regulated by PPARγ agonists in these cells. Surprisingly, relatively few genes were positively regulated by synthetic PPARγ ligands in thioglycollate-elicited peritoneal macrophages. Most of the genes in this group have roles in lipid metabolism and were also activated by a specific PPARδ agonist. The major action of synthetic PPARγ ligands was to inhibit induction of a subset of LPS and IFN-γ-dependent genes. This effect was largely PPARγ dependent at low concentrations of rosiglitazone (Ro), but became increasingly PPARγ independent at high concentrations. PPARγ-independent effects could be at least partially explained by activation of PPARδ, which can also potently inhibit LPS induction of COX2 and iNOS transcription. These observations support a primary role of PPARγ in mediating antiinflammatory effects of thiazolidinediones and reveal potential roles of PPARδ in contributing to this function.