Thiazolidinediones (TZDs) are effective therapies for type 2 diabetes, which has reached epidemic proportions in industrialized societies. TZD treatment reduces circulating free fatty acids (FFAs), which oppose insulin actions in skeletal muscle and other insulin target tissues. Here we report that TZDs, acting as ligands for the nuclear receptor peroxisome proliferator-activated receptor (PPAR)-gamma, markedly induce adipocyte glycerol kinase (GyK) gene expression. This is surprising, as standard textbooks indicate that adipocytes lack GyK and thereby avoid futile cycles of triglyceride breakdown and resynthesis from glycerol and FFAs. By inducing GyK, TZDs markedly stimulate glycerol incorporation into triglyceride and reduce FFA secretion from adipocytes. The 'futile' fuel cycle resulting from expression of GyK in adipocytes is thus a novel mechanism contributing to reduced FFA levels and perhaps insulin sensitization by antidiabetic therapies.

5'-Adenosine monophosphate-activated protein kinase (AMPK) is a master regulator of energy homeostasis in eukaryotes. Despite three decades of investigation, the biological roles of AMPK and its potential as a drug target remain incompletely understood, largely because of a lack of optimized pharmacological tools. We developed MK-8722, a potent, direct, allosteric activator of all 12 mammalian AMPK complexes. In rodents and rhesus monkeys, MK-8722-mediated AMPK activation in skeletal muscle induced robust, durable, insulin-independent glucose uptake and glycogen synthesis, with resultant improvements in glycemia and no evidence of hypoglycemia. These effects translated across species, including diabetic rhesus monkeys, but manifested with concomitant cardiac hypertrophy and increased cardiac glycogen without apparent functional sequelae.

Peroxisome proliferator-activated receptor ␥ (PPAR␥) is the master regulator of adipogenesis as well as the target of thiazolidinedione (TZD) antidiabetic drugs. Many PPAR␥ target genes are induced during adipogenesis, but others, such as glycerol kinase (GyK), are expressed at low levels in adipocytes and dramatically up-regulated by TZDs. Here, we have explored the mechanism whereby an exogenous PPAR␥ ligand is selectively required for adipocyte gene expression. The GyK gene contains a functional PPAR␥-response element to which endogenous PPAR␥ is recruited in adipocytes. However, unlike the classic PPAR␥-target gene aP2, which is constitutively associated with coactivators, the GyK gene is targeted by nuclear receptor corepressors in adipocytes. TZDs trigger the dismissal of corepressor histone deacetylase (HDAC) complexes and the recruitment of coactivators to the GyK gene. TZDs also induce PPAR␥-Coactivator 1␣ (PGC-1␣), whose recruitment to the GyK gene is sufficient to release the corepressors. Thus, selective modulation of adipocyte PPAR␥ target genes by TZDs involves the dissociation of corepressors by direct and indirect mechanisms.

activated receptor-␥ (PPAR␥) regulates adipocyte genes involved in adipogenesis and lipid metabolism and is the molecular target for thiazolidinedione (TZD) antidiabetic agents. Adipose triglyceride lipase (ATGL) is a recently described triglyceride-specific lipase that is induced during adipogenesis and remains highly expressed in mature adipocytes. This study evaluates the ability of PPAR␥ to directly regulate ATGL expression in adipocytes in vitro and in vivo. In fully differentiated 3T3-L1 adipocytes, ATGL mRNA and protein are increased by TZD and non-TZD PPAR␥ agonists in a dose-and time-dependent manner. Rosiglitazone-mediated induction of ATGL mRNA is rapid and is not inhibited by the protein synthesis inhibitor cycloheximide, indicating that intervening protein synthesis is not required for this effect. Rosiglitazone-mediated induction of ATGL mRNA and protein is inhibited by the PPAR␥-specific antagonist GW-9662 and is also significantly reduced following siRNA-mediated knockdown of PPAR␥, supporting the direct transcriptional regulation of ATGL by PPAR␥. In vivo, ATGL mRNA and protein are increased by rosiglitazone treatment in white and brown adipose tissue of mice with and without obesity due to high-fat diet or leptin deficiency. Thus, PPAR␥ positively regulates ATGL mRNA and protein expression in mature adipocytes in vitro and in adipose tissue in vivo, suggesting a role for ATGL in mediating PPAR␥'s effects on lipid metabolism. peroxisome proliferator-activated receptor-␥; thiazolidinediones; desnutrin; patatin-like phopholipase domain-containing protein A2; calcium-independent phospholipase A2-ADIPOSE TISSUE PLAYS A CRITICAL ROLE in energy homeostasis in higher organisms. Not only does it serve as the main site for energy storage in the form of triglycerides, but it also contributes to systemic glucose and lipid metabolism via its function as an endocrine organ (24). In the normal physiological state, excess fuel substrate is partitioned to adipose tissue, where it is stored as triglycerides until its subsequent release as nonesterified fatty acids (FAs) in the setting of increased metabolic fuel requirement. Pathophysiological disorders of adipose tissue such as obesity and lipodystrophy are associated with dysregulation of this process. As a result, excess FAs are released into the circulation and accumulate in extra-adipose tissue depots such as muscle and liver, ultimately contributing to dyslipidemia, insulin resistance, and overt diabetes (3).Peroxisome proliferator-activated receptor-␥ (PPAR␥) is a member of the steroid/thyroid/retinoid receptor superfamily of ligand-activated nuclear transcription factors and is enriched in adipose tissue, where it serves as an essential regulator of adipocyte differentiation and maintenance of the mature adipocyte phenotype (39,45). PPAR␥ is the molecular target for thiazolidinedione (TZD) antidiabetic agents that improve insulin sensitivity, glucose tolerance, and lipid homeostasis in vivo (31, 52). A potential mechanism for these beneficial metabolic e...

In addition to its role in energy storage, adipose tissue also accumulates cholesterol. Concentrations of cholesterol and triglycerides are strongly correlated in the adipocyte, but little is known about mechanisms regulating cholesterol metabolism in fat cells. Here we report that antidiabetic thiazolidinediones (TZDs) and other ligands for the nuclear receptor PPARγ dramatically upregulate oxidized LDL receptor 1 (OLR1) in adipocytes by facilitating the exchange of coactivators for corepressors on the OLR1 gene in cultured mouse adipocytes. TZDs markedly stimulate the uptake of oxidized LDL (oxLDL) into adipocytes, and this requires OLR1. Increased OLR1 expression, resulting either from TZD treatment or adenoviral gene delivery, significantly augments adipocyte cholesterol content and enhances fatty acid uptake. OLR1 expression in white adipose tissue is increased in obesity and is further induced by PPARγ ligand treatment in vivo. Serum oxLDL levels are decreased in both lean and obese diabetic animals treated with TZDs. These data identify OLR1 as a novel PPARγ target gene in adipocytes. While the physiological role of adipose tissue in cholesterol and oxLDL metabolism remains to be established, the induction of OLR1 is a potential means by which PPARγ ligands regulate lipid metabolism and insulin sensitivity in adipocytes. IntroductionThe adipocyte is the major site of fatty acid storage in the body and plays a critical role in maintaining normal glucose and lipid homeostasis. In a healthy person, excess fat is stored as triglycerides in the adipose tissue, and fatty acids are released into the bloodstream only in response to an increased energy requirement, for example, during fasting. If the capacity of the adipocyte to store lipids is exceeded, it can no longer regulate the release of FFAs into the circulation, which ultimately leads to the abnormal accumulation of lipid in nonadipose depots. A buildup of triglycerides in the liver, pancreatic islets, and the muscle is thought to lead to metabolic dysregulation of these tissues (1); in particular, increased plasma FFA levels and elevated intramyocellular lipids are highly correlated with insulin resistance (2, 3).Obesity can be viewed as a state of long-term lipid disequilibrium that is marked by massive adipocyte hypertrophy and is a major risk factor for developing insulin resistance and type 2 diabetes. When compared with small fat cells from lean controls, enlarged adipocytes isolated from obese animals or humans demonstrate a decreased ability to store triglycerides (4), increased insulin resistance (5), and increased secretion of leptin and TNF-α (6). Interestingly, adipose tissue from ob/ob mice also exhibits an increase in cholesterol biosynthesis (7), and hypertrophied adipocytes from 2 obese rodent models showed elevated mRNA levels of SREBP-2, 3-hydroxy-3-methylglutaryl-CoA (HMG CoA) reductase, and the LDL receptor (8, 9), which suggests that these cells are relatively cholesterol deficient. Adipocytes normally contain a significant amount of ...

No abstract

The accumulation of triglycerides (TG) in the liver, designated hepatic steatosis, is characteristically associated with obesity and insulin resistance, but it can also develop after fasting. Here, we show that fasting-induced hepatic steatosis is under genetic control in inbred mice. After a 24-h fast, C57BL/6J mice and SJL/J mice both lost more than 20% of body weight and ϳ60% of total body TG. In C57BL/6J mice, TG accumulated in liver, producing frank steatosis. In striking contrast, SJL/J mice failed to accumulate any hepatic TG even though they lost nearly as much adipose tissue mass as the C57BL/6J mice. Mice from five other inbred strains developed fastinginduced steatosis like the C57BL/6J mice. Measurements of the uptake of free fatty acids (FA) in vivo and in vitro demonstrated that SJL/J mice were protected from steatosis because their heart and skeletal muscle took up and oxidized twice as much FA as compared with C57BL/6J mice. As a result of this muscle diversion, serum-free FA and ketone bodies rose much less after fasting in SJL/J mice as compared with C57BL/6J mice. When livers of SJL/J and C57BL/6J mice were perfused with similar concentrations of FA, the livers took up and esterified similar amounts. We conclude that SJL/J mice express one or more variant genes that lead to enhanced FA uptake and oxidation in muscle, thereby sparing the liver from FA overload in the fasting state.Liver and adipose tissue coordinate metabolic responses to oscillations in nutrient availability (1, 2). In the postprandial state, the liver secretes triglycerides (TG) 4 into the blood in very low-density lipoproteins (VLDL). In adipose tissue, lipoprotein lipase hydrolyzes the TG, producing fatty acids (FA) and monoglycerides that enter fat cells for reesterification and storage as TG (1). The activity of adipose tissue lipoprotein lipase is enhanced by the postprandial rise in insulin. At the same time, insulin inhibits lipolysis of stored TG in fat cells, assuring that the TG will be retained in the cells (3).Under fasting conditions, insulin falls and the inhibitory effect of insulin on adipose tissue lipolysis is diminished. The released FA enters the blood and is used as an energy source in liver, heart, and skeletal muscle. In the liver, excess FA are either re-esterified into TG for intracellular storage or oxidized and secreted as ketone bodies, which become the main energy source for the brain. In skeletal muscle during fasting, FA are oxidized to CO 2 (1, 2).We (4 -6) and others (7) previously reported that livers of mice accumulate large amounts of TG after fasting for 6 -24 h. In the current study, we screened 7 strains of inbred mice to study the genetic control of fasting-induced hepatic TG accumulation. Mice from 6 of 7 strains exhibited fasting-induced fatty liver. In the unique mouse strain (SJL/J), hepatic TG failed to accumulate after a 24-h fast even though the SJL/J mice lost amounts of body weight and adipose tissue that were similar to those of the other 6 strains. To trace the mechanism fo...

Background and Aims The mechanisms by which the I148M mutant variant of the patatin‐like phospholipase domain‐containing 3 (PNPLA3I148M) drives development of nonalcoholic steatohepatitis (NASH) are not known. The aim of this study was to obtain insights on mechanisms underlying PNPLA3I148M‐induced acceleration of NASH. Approach and Results Hepatocyte‐specific overexpression of empty vector (luciferase), human wild‐type PNPLA3, or PNPLA3I148M was achieved using adeno‐associated virus 8 in a diet‐induced mouse model of nonalcoholic fatty liver disease followed by chow diet or high‐fat Western diet with ad libitum administration of sugar in drinking water (WDSW) for 8 weeks. Under WDSW, PNPLA3I148M overexpression accelerated steatohepatitis with increased steatosis, inflammation ballooning, and fibrosis (P < 0.001 versus other groups for all). Silencing PNPLA3I148M after its initial overexpression abrogated these findings. PNPLA3I148M caused 22:6n3 docosahexanoic acid depletion and increased ceramides under WDSW in addition to increasing triglycerides and diglycerides, especially enriched with unsaturated fatty acids. It also increased oxidative stress and endoplasmic reticulum stress. Increased total ceramides was associated with signature of transducer and activator of transcription 3 (STAT3) activation with downstream activation of multiple immune‐inflammatory pathways at a transcriptomic level by network analyses. Silencing PNPLA3I148M reversed STAT3 activation. Conditioned media from HepG2 cells overexpressing PNPLA3I148M increased procollagen mRNA expression in LX2 cells; this was abrogated by hepatocyte STAT3 inhibition. Conclusions Under WDSW, PNPLA3I148M overexpression promotes steatosis and NASH by metabolic reprogramming characterized by increased triglycerides and diglycerides, n3 polyunsaturated fatty acid depletion, and increased ceramides with resultant STAT3 phosphorylation and downstream inflammatory pathway activation driving increased stellate cell fibrogenic activity.