Accumulating evidence highlights intriguing interplays between circadian and metabolic pathways. We show that PER2 directly and specifically represses PPARγ, a nuclear receptor critical in adipogenesis, insulin sensitivity and inflammatory response. PER2-deficient mice display altered lipid metabolism, with drastic reduction of total triacylglycerol and non-esterified fatty acids. PER2 exerts its inhibitory function by blocking PPARγ recruitment to target promoters and thereby transcriptional activation. Whole-genome microarray profiling demonstrates that PER2 dictates the specificity of PPARγ transcriptional activity. Indeed, lack of PER2 results in enhanced adipocyte differentiation of cultured fibroblasts. PER2 targets S112 in PPARγ, a residue whose mutation has been associated to altered lipid metabolism. Lipidomic profiling demonstrates that PER2 is necessary for normal lipid metabolism in white adipocyte tissue. Our findings support a scenario in which PER2 controls the pro-adipogenic activity of PPARγ by operating as its natural modulator, thereby revealing potential avenues of pharmacological and therapeutic intervention.

Overexpression of plasma apolipoprotein CIII (apo CIII) causes hypertriglyceridemia in transgenic mice. A genetically variant form of the human apo CIII promoter, containing five single base pair changes, has been shown to be associated with severe hypertriglyceridemia in a patient population. In animals and in cultured cells the apo CIII gene is transcriptionally downregulated by insulin. In this study we demonstrate that, unlike the wild-type promoter, the variant promoter was defective in its response to insulin treatment, remaining constitutively active at all concentrations of insulin. The loss of insulin regulation was mapped to polymorphic sites at -482 and -455, which fall within a previously identified insulin response element. Loss of insulin regulation could result in overexpression of the apo CIII gene and contribute to the development of hypertriglyceridemia. The variant apo Cm promoter is common in the human population and may represent a major contributing factor to the development of hypertriglyceridemia. (J. Clin. Invest. 1995. 96:2601-2605

Background: Adrenergic activation of brown adipocytes mobilizes fatty acids for oxidation and promotes transcription of oxidative genes. Results: Activation of adipocyte lipases generates agonists of PPAR␣ and PPAR␦ that promote transcription of oxidative genes. Conclusion: Lipolytic products signal via PPAR␣ and PPAR␦. Significance: Lipolytic activation of PPAR␣ and PPAR␦ provides a mechanism for matching oxidative capacity to substrate supply.

Autosomal dominant familial partial lipodystrophy (FPLD) due to mutant LMNA encoding nuclear lamin A/C is characterized by adipose tissue repartitioning together with multiple metabolic disturbances, including insulin resistance and dyslipidemia. There is emerging evidence that some rare mutations in peroxisome proliferator-activated receptor-␥ (PPAR-␥), encoded by PPARG, might be associated with human lipodystrophy. We report a three-generation Canadian kindred ascertained based upon partial lipodystrophy, with a normal LMNA gene sequence. Candidate gene sequencing showed that all four affected subjects were heterozygous for a novel T3 A mutation at PPARG nucleotide 1164 in exon 5 that predicted substitution of phenylalanine at codon 388 by leucine (F388L). The mutation was absent from normal family members and normal unrelated subjects, and altered a highly conserved residue within helix 8 of the predicted ligand-binding pocket of PPAR-␥. The mutant receptor had significantly decreased basal transcriptional activity and impaired stimulation by a synthetic ligand. The germline transmission of a transactivation-deficient mutation in PPARG suggests that autosomal dominant partial lipodystrophy is genetically heterogeneous. Our findings are consistent with the idea that mutant PPARG can underlie the partial lipodystrophy phenotype. Diabetes 51: 3586 -3590, 2002

AMP-activated protein kinase (AMPK) is the central component of a cellular signaling system that regulates multiple metabolic enzymes and pathways in response to reduced intracellular energy levels. The transcription factor hepatic nuclear factor 4␣ (HNF4␣) is an orphan nuclear receptor that regulates the expression of genes involved in energy metabolism in the liver, intestine, and endocrine pancreas. Inheritance of a single null allele of HNF4␣ causes diabetes in humans. Here we demonstrate that AMPK directly phosphorylates HNF4␣ and represses its transcriptional activity. AMPK-mediated phosphorylation of HNF4␣ on serine 304 had a 2-fold effect, reducing the ability of the transcription factor to form homodimers and bind DNA and increasing its degradation rate in vivo. These results demonstrate that HNF4␣ is a downstream target of AMPK and raise the possibility that one of the effects of AMPK activation is reduced expression of HNF4␣ target genes.The transcription factor HNF4␣ 1 (NR2A1), a member of the nuclear receptor superfamily, plays a key role in regulating the expression of metabolic genes in multiple tissues including the liver, intestine, kidney, and endocrine pancreas (for review see Ref. 1). In humans, a nonsense mutation in a single allele of the HNF4␣ gene causes an inherited form of diabetes known as maturity onset diabetes of the young (MODY) (for a review see Ref. 2). This syndrome is characterized by insufficient insulin secretion (3, 4), indicating a significant defect in pancreatic function. Surprisingly, there is not a major deficit in liver function, despite the important role of HNF4␣ in hepatic gene expression (5). These observations suggest that HNF4␣ regulates pancreatic genes involved in glucose sensing and/or insulin production. This hypothesis is supported by experiments demonstrating that HNF4␣ regulates expression of genes encoding enzymes of glucose metabolism and insulin secretion (6 -8). Because MODY1 patients have a single null allele that does not produce a dominant-negative acting protein (9), the physiological abnormalities in these patients must be caused by a relatively mild (ϳ50%) decrease in the amount of HNF4␣ protein. Taken together, these observations raise the possibility that any modification of HNF4␣ that causes a reduction in its transcriptional activity could have significant effects on pancreatic function.HNF4␣ has also been shown recently to play an important role in the regulation of hepatic glucose output, a key component of the maintenance of plasma glucose levels. Yoon et al. (10) recently demonstrated that the transcriptional regulation of gene for the key gluconeogenic enzyme phosphoenolpyruvate carboxykinase by cAMP was mediated by the transcriptional co-activator PGC-1 acting through HNF4␣. These findings suggest that HNF4␣ may play a role in the transcriptional response of the liver to metabolic hormones and that factors regulating the activity of HNF4␣ could have an effect on hepatic glucose output.AMPK is the mammalian homolog of the yeast SNF1 pr...

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The antidiabetic thiazolidinediones, which include troglitazone and rosiglitazone, are ligands for the nuclear receptor peroxisome proliferator-a c t i v a t e d receptor (PPA R )-and exert their antihyperglycemic e ffects by regulation of PPA R--responsive genes. We report here that PPA R-activation by troglitazone depends on the experimental setting. Troglitazone acts as a partial agonist for PPA R-in transfected muscle (C2C12) and kidney (HEK 293T) cells, producing a submaximal transcriptional response (1.8-to 2.5-fold activation) compared with rosiglitazone (7.4-to 13-fold activation). Additionally, troglitazone antagonizes rosiglitazone-stimulated PPA R-transcriptional activity. Limited protease digestion of PPA R-suggests conformational differences in the receptor bound to troglitazone versus rosiglitazone. Consistent with this fin d i n g , an in vitro coactivator association assay demonstrated that troglitazone-bound PPA R-recruited the transcriptional coactivators p300 and steroid receptor coactivator 1 less efficiently than rosiglitazone-bound receptor. In contrast to these observations, troglitazone behaves as a full agonist of PPA R-in 3T3L1 adipocytes. Tw odimensional protein gel electrophoresis demonstrated that troglitazone and rosiglitazone regulated distinct but overlapping sets of genes in several cell types. Thus, troglitazone may behave as a partial agonist under certain physiological circumstances and as a full agonist in others. These differences could be caused by variations in the amount of specific cofactors, differences in PPA R response elements, or the presence of different isoforms of PPA R-. D i a b e t e s 4 9 :5 3 9-547, 2000 T ype 2 diabetes is characterized by decreased insulin sensitivity of peripheral tissues. Glucose homeostasis is maintained under these circumstances by increased insulin secretion from pancreatic -cells. In some cases, the -cell is unable to maintain increased output. The antidiabetic thiazolidinediones (TZDs), such as troglitazone, improve peripheral insulin sensitivity, leading to reduced blood glucose and insulin levels and the preservation of pancreatic function (1-4). Improvement of insulin sensitivity by TZDs is most likely due to the activation of the peroxisome proliferator-activated receptor (PPA R )-(5). The TZDs are high-affinity ligands for PPA R-in vitro, and the rank order of receptor affinity correlates with their in vivo hypoglycemic activity (6), with one reported exception (7). Although many of the molecular details are not clearly understood, a model has emerged in which activated PPA R-m o dulates the transcriptional activity of a set of genes encoding proteins that are important in glucose and lipid metabolism. H o w e v e r, the identity of these genes and the precise pathways leading to the normalization of insulin sensitivity remain largely unknown.R e c e n t l y, the X-ray crystal structure of the ligand-binding domain of PPA R-has been elucidated (3,8), revealing that ligand binding causes a conformational change within P PA R-such...

One of the primary functions of AMP-activated protein kinase (AMPK) is to regulate the metabolic pathways in response to reduced cellular energy charge. Most of the known targets of the kinase are cytoplasmic enzymes involved in both catabolic and anabolic metabolism. In addition, activation of AMPK in many cells results in changes in the pattern of gene expression. Although some of these effects are undoubtedly secondary responses to modified cellular metabolism, it is possible that in addition to its well-characterized function in the cytoplasm, AMPK also directly phosphorylates and regulates proteins involved in gene transcription. There are now several examples of transcription factors, cofactors and components of the transcriptional core machinery that are directly phosphorylated and regulated by AMPK. Here I review these examples and discuss the significance of AMPK activity in the nucleus.