Obesity is an alarming primary health problem and is an independent risk factor for type II diabetes, cardiovascular diseases, and hypertension. Although the pathologic mechanisms linking obesity with these co-morbidities are most likely multifactorial, increasing evidence indicates that altered secretion of adipose-derived signaling molecules (adipokines; e.g. adiponectin, leptin, and tumor necrosis factor ␣) and local inflammatory responses are contributing factors. Chemerin (RARRES2 or TIG2) is a recently discovered chemoattractant protein that serves as a ligand for the G protein-coupled receptor CMKLR1 (ChemR23 or DEZ) and has a role in adaptive and innate immunity. Here we show an unexpected, high level expression of chemerin and its cognate receptor CMKLR1 in mouse and human adipocytes. Cultured 3T3-L1 adipocytes secrete chemerin protein, which triggers CMKLR1 signaling in adipocytes and other cell types and stimulates chemotaxis of CMKLR1-expressing cells. Adenoviral small hairpin RNA targeted knockdown of chemerin or CMKLR1 expression impairs differentiation of 3T3-L1 cells into adipocytes, reduces the expression of adipocyte genes involved in glucose and lipid homeostasis, and alters metabolic functions in mature adipocytes. We conclude that chemerin is a novel adipose-derived signaling molecule that regulates adipogenesis and adipocyte metabolism.

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Intracellular heme levels must be tightly regulated to maintain proper mitochondrial respiration while minimizing toxicity, but the homeostatic mechanisms are not well understood. Here we report a novel negative feedback mechanism whereby the nuclear heme receptor Rev-erba tightly controls the level of its own ligand. Heme binding to Rev-erba recruits the NCoR/histone deacetylase 3 (HDAC3) corepressor complex to repress the transcription of the coactivator PGC-1a, a potent inducer of heme synthesis. Depletion of Rev-erba derepresses PGC-1a, resulting in increased heme levels. Conversely, increased Rev-erba reduces intracellular heme, and impairs mitochondrial respiration in a heme-dependent manner. Consistent with this bioenergetic impairment, overexpression of Rev-erba dramatically inhibits cell growth due to a cell cycle arrest. Thus, Rev-erba modulates the synthesis of its own ligand in a negative feedback pathway that maintains heme levels and regulates cellular energy metabolism.[Keywords: Rev-erba; PGC-1a; heme homeostasis; mitochondria; energy] Supplemental material is available at http://www.genesdev.org.

Nuclear peroxisome proliferator-activated receptor-␥ (PPAR␥) is required for adipocyte differentiation, but its role in mature adipocytes is less clear. Here, we report that knockdown of PPAR␥ expression in 3T3-L1 adipocytes returned the expression of most adipocyte genes to preadipocyte levels. Consistently, down-regulated but not up-regulated genes showed strong enrichment of PPAR␥ binding. Surprisingly, not all adipocyte genes were reversed, and the adipocyte morphology was maintained for an extended period after PPAR␥ depletion. To explain this, we focused on transcriptional regulators whose adipogenic regulation was not reversed upon PPAR␥ depletion. We identified GATA2, a transcription factor whose down-regulation early in adipogenesis is required for preadipocyte differentiation and whose levels remain low after PPAR␥ knockdown. Forced expression of GATA2 in mature adipocytes complemented PPAR␥ depletion and impaired adipocyte functionality with a more preadipocyte-like gene expression profile. Ectopic expression of GATA2 in adipose tissue in vivo had a similar effect on adipogenic gene expression. These results suggest that PPAR␥-independent down-regulation of GATA2 prevents reversion of mature adipocytes after PPAR␥ depletion.Peroxisome proliferator-activated receptors (PPARs) 2 are ligand-activated transcription factors that bind to PPAR response elements as heterodimers with the retinoid X receptor ␣ (1). Among all PPARs, the expression of PPAR␥ exhibits the greatest specificity for adipose tissue (2, 3), and the antidiabetic thiazolidinedione drugs are high affinity PPAR␥ ligands that promote adipogenesis (4). One of the most common models used to study adipocyte differentiation is the mouse 3T3-L1 cell line. After hormonal stimulation of growth-arrested preadipocytes, cells undergo a clonal expansion phase before they permanently exit the cell cycle for the final adipocyte commitment (5). During this process, PPAR␥ is induced and stimulates the expression of many adipocyte-specific genes. PPAR␥ is both necessary (6) and sufficient (7) for the differentiation of murine fibroblasts into adipocytes, and no factor is known to stimulate adipocyte differentiation in the absence of PPAR␥.PPAR␥ itself is activated mainly by hormonally induced changes in the expression of transcriptional activators and repressors. Upstream of the induction of PPAR␥ are, for instance, EGR2 (early growth response 2) and members of the Krüppel-like factors and C/EBP (CCAAT/enhancer binding protein) families. C/EBP␤ has been shown to directly or indirectly induce PPAR␥ expression (5). On the other hand, several inhibiting factors upstream of PPAR␥ are regulators of alternative cell fates, including some members of the WNT family (8) as well as factors highly expressed in preadipocytes and downregulated during differentiation, including PREF-1 and GATA2/3 (9 -11).In contrast to the thoroughly studied role of PPAR␥ during adipocyte differentiation, our knowledge about the role of PPAR␥ in maintaining the adipocyte phenotype in mat...

Amiodarone, an efficacious and widely used antiarrhythmic agent, has been reported to cause hepatotoxicity in some patients. To gain insight into the mechanism of this unwanted effect, mice were administered various doses of amiodarone and examined for changes in hepatic histology and gene regulation. Amiodarone induced hepatomegaly, hepatocyte microvesicular lipid accumulation, and a significant decrease in serum triglycerides and glucose. Northern blot analysis of hepatic RNA revealed a dose-dependent increase in the expression of a number of genes critical for fatty acid oxidation, lipoprotein assembly, and lipid transport. Many of these genes are regulated by the peroxisome proliferator-activated receptor-␣ (PPAR␣), a ligand-activated nuclear hormone receptor transcription factor. The absence of induction of these genes as well as hepatomegaly in PPAR␣ knockout [PPAR␣(Ϫ/Ϫ)] mice indicated that the effects of amiodarone were dependent upon the presence of a functional PPAR␣ gene. Compared to wildtype mice, treatment of PPAR␣(Ϫ/Ϫ) mice with amiodarone resulted in an increased rate and extent of total body weight loss. The inability of amiodarone to directly activate either human or mouse PPAR␣ transiently expressed in human HepG2 hepatoma cells indicates that the effects of amiodarone on the function of this receptor were indirect. Based upon these results, we conclude that amiodarone disrupts hepatic lipid homeostasis and that the increased expression of PPAR␣ target genes is secondary to this toxic effect. These results provide important new mechanistic information regarding the hepatotoxic effects of amiodarone and indicate that PPAR␣ protects against amiodarone-induced hepatotoxicity.