Obesity induced in mice by high-fat feeding activates the protein kinase cdk5 in adipose tissues. This results in phosphorylation of the nuclear receptor PPARγ, a dominant regulator of adipogenesis and fat cell gene expression, at serine 273. This modification of PPARγ does not alter its adipogenic capacity, but leads to dysregulation of a large number of genes whose expression is altered in obesity, including a reduction in the expression of the insulin-sensitizing adipokine, adiponectin. The phosphorylation of PPARγ by cdk5 is blocked by anti-diabetic PPARγ ligands, such as rosiglitazone and MRL24. This inhibition works both in vivo and in vitro, and surprisingly, is completely independent of classical receptor transcriptional agonism. Similarly, inhibition of PPARγ phosphorylation in obese patients by rosiglitazone is very tightly associated with the anti-diabetic effects of this drug. These data strongly suggest that cdk5-mediated phosphorylation of PPARγ may be involved in the pathogenesis of insulin-resistance, and present an opportunity for development of an improved generation of anti-diabetic drugs through PPARγ.

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PPARγ is the functioning receptor for the thiazolidinedione (TZD) class of anti-diabetes drugs including rosiglitazone and pioglitazone1. These drugs are full classical agonists for this nuclear receptor, but recent data has shown that many PPARγ-based drugs have a separate biochemical activity, blocking the obesity-linked phosphorylation of PPARγ by Cdk52. Here we describe novel synthetic compounds that have a unique mode of binding to PPARγ, completely lack classical transcriptional agonism and block the Cdk5-mediated phosphorylation in cultured adipocytes and in insulin-resistant mice. Moreover, one such compound, SR1664, has potent anti-diabetic activity while not causing the fluid retention and weight gain that are serious side effects of many of the PPARγ drugs. Unlike TZDs, SR1664 also does not interfere with bone formation in culture. These data illustrate that new classes of anti-diabetes drugs can be developed by specifically targeting the Cdk5-mediated phosphorylation of PPARγ.

Summary PGC1α is a key transcriptional coregulator of oxidative metabolism and thermogenesis. Through a high throughput chemical screen, we found that molecules antagonizing the TRPVs (Transient Receptor Potential Vanilloid), a family of ion channels, induced PGC1α expression in adipocytes. In particular, TRPV4 negatively regulated the expression of PGC1α, UCP1 and cellular respiration. Additionally, it potently controlled the expression of multiple proinflammatory genes involved in the development of insulin resistance. Mice with a null mutation for TRPV4 or wild-type mice treated with a TRPV4 antagonist showed elevated thermogenesis in adipose tissues and were protected from diet-induced obesity, adipose inflammation and insulin resistance. This role of TRPV4 as a cell-autonomous mediator for both the thermogenic and proinflammatory programs in adipocytes could offer a new target for treating obesity and related metabolic diseases.

Reduced peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α) expression and mitochondrial dysfunction in adipose tissue have been associated with obesity and insulin resistance. Whether this association is causally involved in the development of insulin resistance or is only a consequence of this condition has not been clearly determined. Here we studied the effects of adipose-specific deficiency of PGC-1α on systemic glucose homeostasis. Loss of PGC-1α in white fat resulted in reduced expression of the thermogenic and mitochondrial genes in mice housed at ambient temperature, whereas gene expression patterns in brown fat were not altered. When challenged with a high-fat diet, insulin resistance was observed in the mutant mice, characterized by reduced suppression of hepatic glucose output. Resistance to insulin was also associated with an increase in circulating lipids, along with a decrease in the expression of genes regulating lipid metabolism and fatty acid uptake in adipose tissues. Taken together, these data demonstrate a critical role for adipose PGC-1α in the regulation of glucose homeostasis and a potentially causal involvement in the development of insulin resistance.glucose metabolism | mitochondrial gene expression | thermogenesis | cold exposure | type 2 diabetes

OBJECTIVEThe peroxisome proliferator–activated receptor-γ coactivator (PGC)-1 family of transcriptional coactivators controls hepatic function by modulating the expression of key metabolic enzymes. Hepatic gain of function and complete genetic ablation of PGC-1α show that this coactivator is important for activating the programs of gluconeogenesis, fatty acid oxidation, oxidative phosphorylation, and lipid secretion during times of nutrient deprivation. However, how moderate changes in PGC-1α activity affect metabolism and energy homeostasis has yet to be determined.RESEARCH DESIGN AND METHODSTo identify key metabolic pathways that may be physiologically relevant in the context of reduced hepatic PGC-1α levels, we used the Cre/Lox system to create mice heterozygous for PGC-1α specifically within the liver (LH mice).RESULTSThese mice showed fasting hepatic steatosis and diminished ketogenesis associated with decreased expression of genes involved in mitochondrial β-oxidation. LH mice also exhibited high circulating levels of triglyceride that correlated with increased expression of genes involved in triglyceride-rich lipoprotein assembly. Concomitant with defects in lipid metabolism, hepatic insulin resistance was observed both in LH mice fed a high-fat diet as well as in primary hepatocytes.CONCLUSIONSThese data highlight both the dose-dependent and long-term effects of reducing hepatic PGC-1α levels, underlining the importance of tightly regulated PGC-1α expression in the maintenance of lipid homeostasis and glucose metabolism.

FGF21 is a hormone produced in liver and fat that dramatically improves peripheral insulin sensitivity and lipid metabolism. We show here that obese mice with genetically reduced levels of a key hepatic transcriptional coactivator, PGC-1␣, have improved wholebody insulin sensitivity with increased levels of hepatic and circulating FGF21. Gain-and loss-of-function studies in primary mouse hepatocytes show that hepatic FGF21 levels are regulated by the expression of PGC-1␣. Importantly, PGC-1␣-mediated reduction of FGF21 expression is dependent on Rev-Erb␣ and the expression of ALAS-1. ALAS-1 is a PGC-1␣ target gene and the rate-limiting enzyme in the synthesis of heme, a ligand for Rev-Erb␣. Modulation of intracellular heme levels mimics the effect of PGC-1␣ on FGF21 expression, and inhibition of heme biosynthesis completely abrogates the down-regulation of FGF21 in response to PGC-1␣. Thus, PGC-1␣ can impact hepatic and systemic metabolism by regulating the levels of a nuclear receptor ligand.aintenance of hepatic energy homeostasis is complex and under precise molecular control. Adaptive changes in enzyme expression levels are often controlled at the level of transcription by nuclear hormone receptors and other transcription factors (1). Members of the peroxisome proliferator activated receptor (PPAR)-gamma coactivator-1 (PGC-1s) family play an important role in the tight regulation of enzyme expression within the liver. Fasting induces PGC-1␣ expression, allowing this protein to coactivate several transcription factors including FOXO1, glucocorticoid receptor, nuclear respiratory factor-1 (NRF-1), hepatocyte nuclear factor-4␣, retinoid-related orphan receptors (RORs), and PPAR␣. This leads to increased expression of key enzymes involved in gluconeogenesis, fatty acid oxidation, heme biosynthesis, and the circadian clock (2-4).The importance of the PGC-1 coactivators in the maintenance of liver metabolism is illustrated in several mouse models. Mice with a tissue-specific loss of one allele of hepatic PGC-1␣ expression exhibit fasting-induced steatosis and develop hepatic insulin resistance on a high-fat diet (5). Hepatic PGC-1␣ levels are increased in mouse models of diabetes and obesity (6-9), and are inversely correlated with insulin resistance in humans (10). Although it is clear that the PGC-1s play a key role in regulating the hepatic response to nutritional cues, the molecular pathways are complex.So far, PGC-1s have been shown only to act as potent positive regulators of transcription, because they promote local chromatinremodeling events and formation of the preinitiation complex (11). PGC-1s recruit histone acetyltransferases (HAT)-containing protein complexes (through interactions with CBP/p300), and the TRAP/Mediator complex (by interacting with TRAP220/Med1) (12) in response to hormonal or physiological cues (reviewed in refs. 3 and 13). PGC-1␣ can associate with proteins that negatively affect its coactivator function (e.g., p160/Mib) (14, 15), but there is no evidence that PGC-1s can directly med...

PGC-1a is a transcriptional coactivator that powerfully regulates many pathways linked to energy homeostasis. Specifically, PGC-1a controls mitochondrial biogenesis in most tissues but also initiates important tissue-specific functions, including fiber type switching in skeletal muscle and gluconeogenesis and fatty acid oxidation in the liver. We show here that S6 kinase, activated in the liver upon feeding, can phosphorylate PGC-1a directly on two sites within its arginine/serine-rich (RS) domain. This phosphorylation significantly attenuates the ability of PGC1a to turn on genes of gluconeogenesis in cultured hepatocytes and in vivo, while leaving the functions of PGC-1a as an activator of mitochondrial and fatty acid oxidation genes completely intact. These phosphorylations interfere with the ability of PGC-1a to bind to HNF4a, a transcription factor required for gluconeogenesis, while leaving undisturbed the interactions of PGC-1a with ERRa and PPARa, factors important for mitochondrial biogenesis and fatty acid oxidation. These data illustrate that S6 kinase can modify PGC-1a and thus allow molecular dissection of its functions, providing metabolic flexibility needed for dietary adaptation.