Role of FoxO1 in FFA-induced oxidative stress in adipocytes

Uncoupling and reactive oxygen species (ROS)--a double-edged sword for β-cell function? "Moderation in all things"

The ability of the mitochondrion to (a) manage fuel import to oxidize for adenosine tri-phosphate (ATP) generation while (b) protecting itself and the cellular environment from electron leak, which can generate highly reactive oxygen species (ROS) is a delicate balancing act. ATP is the currency of the cell and as such serves a signaling function as a substrate partner to many kinases and ion channels. While various ROS species have been viewed as a dangerous and toxic group of molecules, it also has a role as a signal derived from mitochondria, as well as other enzymatic sources: a double-edged sword. Current efforts to understand the biochemical mechanisms affected by ROS as a signal--usually noted to be hydrogen peroxide (H(2)O(2))--are exciting, but this duality of ROS effects also pose challenges in managing its levels to protect cells. The mitochondrial uncoupling protein-2 (UCP2), UCP3, and the permeability transition pore have been integral to efforts to try to understand what role mitochondrial-derived ROS have in cells. In this piece we reflect on mitochondrial ROS and uncoupling proteins as signaling regulators. It seems that when it comes to ROS and uncoupling the proverb "Moderation in all things" is apt.

Genistein and daidzein, typical soy isoflavones, inhibit TNF-α-mediated downregulation of adiponectin expression via different mechanisms in 3T3-L1 adipocytes

SCOPE

Previous reports suggest that soy isoflavones have multiple biological functions and may help to restore adiponectin expression and insulin sensitivity. However, little is known about whether soy isoflavones can inhibit the downregulation of adiponectin and their molecular mechanisms. In the present study, we demonstrate that genistein (Gen) or daidzein (Dai) can significantly inhibit the downregulation of adiponectin expression via unique and different molecular mechanisms.

METHODS AND RESULTS

Pretreatment with Gen or Dai significantly inhibited the tumor necrosis factor-alpha (TNF-α)-mediated downregulation of adiponectin expression in 3T3-L1 adipocytes. Gen inhibited the TNF-α-induced c-Jun-NH(2) -terminal kinase (JNK) signaling that is involved in adiponectin expression. Molecular docking studies based on JNK1 with Gen or Dai clearly supported our experimental results. However, Dai did not significantly inhibit JNK signaling. Dai did, however, inhibit the TNF-α-induced downregulation of forkhead box-containg protein O1, which is also involved in adiponectin expression.

CONCLUSION

These data demonstrate that: (i) both Gen and Dai significantly inhibit the TNF-α-mediated downregulation of adiponectin in adipocytes; (ii) Gen is an effective inhibitor of JNK activation, thus inhibiting the TNF-α-mediated downregulation of adiponectin; and (iii) Dai can inhibit the downregulation of adiponectin by restoring the TNF-α-mediated reduction of forkhead box-containg protein O1 protein expression.

Mechanisms linking obesity, inflammation and altered metabolism to colon carcinogenesis

Due to its prevalence, obesity is now considered a global epidemic. It is linked to increased risk of colorectal cancer, the third most common cancer and the second leading cause of death among adults in Western countries. Obese adipose tissue differs from lean adipose tissue in its immunogenic profile, body fat distribution and metabolic profile. Obese adipose tissue releases free fatty acids, adipokines and many pro-inflammatory chemokines. These factors are known to play a key role in regulating malignant transformation and cancer progression. Obese adipose tissue is infiltrated by macrophages that participate in inflammatory pathways activated within the tissue. Adipose tissue macrophages consist of two different phenotypes. M1 macrophages reside in obese adipose tissue and produce pro-inflammatory cytokines, and M2 macrophages reside in lean adipose tissue and produce anti-inflammatory cytokines, such as interleukin-10 (IL-10). The metabolic networks that confer tumour cells with their oncogenic properties, such as increased proliferation and the ability to avoid apoptosis are still not well understood. We review the interactions between adipocytes and immune cells that may alter the metabolism towards promotion of colorectal cancer.

Alterations in lipid signaling underlie lipodystrophy secondary to AGPAT2 mutations

Congenital generalized lipodystrophy (CGL), secondary to AGPAT2 mutation is characterized by the absence of adipocytes and development of severe insulin resistance. In the current study, we investigated the adipogenic defect associated with AGPAT2 mutations. Adipogenesis was studied in muscle-derived multipotent cells (MDMCs) isolated from vastus lateralis biopsies obtained from controls and subjects harboring AGPAT2 mutations and in 3T3-L1 preadipocytes after knockdown or overexpression of AGPAT2. We demonstrate an adipogenic defect using MDMCs from control and CGL human subjects with mutated AGPAT2. This defect was rescued in CGL MDMCs with a retrovirus expressing AGPAT2. Both CGL-derived MDMCs and 3T3-L1 cells with knockdown of AGPAT2 demonstrated an increase in cell death after induction of adipogenesis. Lack of AGPAT2 activity reduces Akt activation, and overexpression of constitutively active Akt can partially restore lipogenesis. AGPAT2 modulated the levels of phosphatidic acid, lysophosphatidic acid, phosphatidylinositol species, as well as the peroxisome proliferator-activated receptor γ (PPARγ) inhibitor cyclic phosphatidic acid. The PPARγ agonist pioglitazone partially rescued the adipogenic defect in CGL cells. We conclude that AGPAT2 regulates adipogenesis through the modulation of the lipome, altering normal activation of phosphatidylinositol 3-kinase (PI3K)/Akt and PPARγ pathways in the early stages of adipogenesis.

Sirtuin 1 and sirtuin 3: physiological modulators of metabolism

The sirtuins are a family of highly conserved NAD(+)-dependent deacetylases that act as cellular sensors to detect energy availability and modulate metabolic processes. Two sirtuins that are central to the control of metabolic processes are mammalian sirtuin 1 (SIRT1) and sirtuin 3 (SIRT3), which are localized to the nucleus and mitochondria, respectively. Both are activated by high NAD(+) levels, a condition caused by low cellular energy status. By deacetylating a variety of proteins that induce catabolic processes while inhibiting anabolic processes, SIRT1 and SIRT3 coordinately increase cellular energy stores and ultimately maintain cellular energy homeostasis. Defects in the pathways controlled by SIRT1 and SIRT3 are known to result in various metabolic disorders. Consequently, activation of sirtuins by genetic or pharmacological means can elicit multiple metabolic benefits that protect mice from diet-induced obesity, type 2 diabetes, and nonalcoholic fatty liver disease.

Phytochemicals and their impact on adipose tissue inflammation and diabetes

Type 2 diabetes mellitus is an inflammatory disease and the mechanisms that underlie this disease, although still incompletely understood, take place in the adipose tissue of obese subjects. Concurrently, the prevalence of obesity caused by Western diet's excessive energy intake and the lack of exercise escalates, and is believed to be causative for the chronic inflammatory state in adipose tissue. Overnutrition itself as an overload of energy may induce the adipocytes to secrete chemokines activating and attracting immune cells to adipose tissue. But also inflammation-mediating food ingredients like saturated fatty acids are believed to directly initiate the inflammatory cascade. In addition, hypoxia in adipose tissue as a direct consequence of obesity, and its effect on gene expression in adipocytes and surrounding cells in fat tissue of obese subjects appears to play a central role in this inflammatory response too. In contrast, revisiting diet all over the world, there are also some natural food products and beverages which are associated with curative effects on human health. Several natural compounds known as spices such as curcumin, capsaicin, and gingerol, or secondary plant metabolites catechin, resveratrol, genistein, and quercetin have been reported to provide an improved health status to their consumers, especially with regard to diabetes, and therefore have been investigated for their anti-inflammatory effect. In this review, we will give an overview about these phytochemicals and their role to interfere with inflammatory cascades in adipose tissue and their potential for fighting against inflammatory diseases like diabetes as investigated in vivo.

Adipocyte iron regulates adiponectin and insulin sensitivity

Iron overload is associated with increased diabetes risk. We therefore investigated the effect of iron on adiponectin, an insulin-sensitizing adipokine that is decreased in diabetic patients. In humans, normal-range serum ferritin levels were inversely associated with adiponectin, independent of inflammation. Ferritin was increased and adiponectin was decreased in type 2 diabetic and in obese diabetic subjects compared with those in equally obese individuals without metabolic syndrome. Mice fed a high-iron diet and cultured adipocytes treated with iron exhibited decreased adiponectin mRNA and protein. We found that iron negatively regulated adiponectin transcription via FOXO1-mediated repression. Further, loss of the adipocyte iron export channel, ferroportin, in mice resulted in adipocyte iron loading, decreased adiponectin, and insulin resistance. Conversely, organismal iron overload and increased adipocyte ferroportin expression because of hemochromatosis are associated with decreased adipocyte iron, increased adiponectin, improved glucose tolerance, and increased insulin sensitivity. Phlebotomy of humans with impaired glucose tolerance and ferritin values in the highest quartile of normal increased adiponectin and improved glucose tolerance. These findings demonstrate a causal role for iron as a risk factor for metabolic syndrome and a role for adipocytes in modulating metabolism through adiponectin in response to iron stores.

Body iron stores as predictors of insulin resistance in apparently healthy urban Colombian men

The aim of this study was to evaluate body iron stores as predictors of insulin resistance. We developed a cross-sectional study among 123 men, 25-64 years of age and determined fasting plasma glucose, insulin, serum ferritin, and C-reactive protein levels. A survey was performed to record personal antecedents and family history of non-transmissible chronic diseases. Log-transformed ferritin levels was an independent predictor for log-transformed insulin resistance index assessed by homeostatic model assessment when body mass index or waist circumference were not included in multiple linear regression models. Sedentarism, heart attack family history, and log-C reactive protein levels were also significant predictors for insulin resistance. In conclusion, documented anthropometric predictors affect the significance of ferritin as a potential prediction variable for insulin resistance. Mechanisms of how body fat could influence ferritin levels should be evaluated. To our knowledge, this is the first evaluation of the relationship between body iron stores and insulin resistance in a Latin American population.

The effects of obesity on venous thromboembolism: A review

Obesity has emerged as a global health issue that is associated with wide spectrum of disorders, including coronary artery disease, diabetes mellitus, hypertension, stroke, and venous thromboembolism (VTE). VTE is one of the most common vascular disorders in the United States and Europe and is associated with significant mortality. Although the association between obesity and VTE appears to be moderate, obesity can interact with other environmental or genetic factors and pose a significantly greater risk of VTE among individuals who are obese and who are exposed simultaneously to several other risk factors for VTE. Therefore, identification of potential interactions between obesity and certain VTE risk factors might offer some critical points for VTE interventions and thus minimize VTE morbidity and mortality among patients who are obese. However, current obesity measurements have limitations and can introduce contradictory results in the outcome of obesity. To overcome these limitations, this review proposes several future directions and suggests some avenues for prevention of VTE associated with obesity as well.

Mitochondrial dysregulation in the pathogenesis of diabetes: potential for mitochondrial biogenesis-mediated interventions

Muscle mitochondrial metabolism is a tightly controlled process that involves the coordination of signaling pathways and factors from both the nuclear and mitochondrial genomes. Perhaps the most important pathway regulating metabolism in muscle is mitochondrial biogenesis. In response to physiological stimuli such as exercise, retrograde signaling pathways are activated that allow crosstalk between the nucleus and mitochondria, upregulating hundreds of genes and leading to higher mitochondrial content and increased oxidation of substrates. With type 2 diabetes, these processes can become dysregulated and the ability of the cell to respond to nutrient and energy fluctuations is diminished. This, coupled with reduced mitochondrial content and altered mitochondrial morphology, has been directly linked to the pathogenesis of this disease. In this paper, we will discuss our current understanding of mitochondrial dysregulation in skeletal muscle as it relates to type 2 diabetes, placing particular emphasis on the pathways of mitochondrial biogenesis and mitochondrial dynamics, and the therapeutic value of exercise and other interventions.

Regulation of LYRM1 gene expression by free fatty acids, adipokines, and rosiglitazone in 3T3-L1 adipocytes

LYR motif containing 1 (LYRM1) is a novel gene that is abundantly expressed in the adipose tissue of obese subjects and is involved in insulin resistance. In this study, free fatty acids (FFAs) and tumor necrosis factor-α (TNF-α) are shown to upregulate LYRM1 mRNA expression in 3T3-L1 adipocytes. Conversely, resistin and rosiglitazone exert an inhibitory effect on LYRM1 mRNA expression. These results suggest that the expression of LYRM1 mRNA is affected by a variety of factors that are related to insulin sensitivity. LYRM1 may be an important mediator in the development of obesity-related insulin resistance.

Companion Animals Symposium: Obesity in dogs and cats: What is wrong with being fat?

Few diseases in modern pets are diet induced. One possible exception to this is obesity, which is ultimately caused by consuming more calories than needed by the dog or cat. Although fat is the most concentrated and efficiently stored source of calories, and protein least so, an excess of calories from any source will contribute to adiposity. Obesity is an excess of body fat sufficient to result in impairment of health or body function. In people, this is generally recognized as 20 to 25% above ideal BW. This degree of excess is important in dogs as well. A lifelong study in dogs showed that even moderately overweight dogs were at greater risk for earlier morbidity; these dogs required medication for chronic health problems sooner than their lean-fed siblings. The average difference in BW between groups was approximately 25%. Obese cats also face increased health risks, including an increased risk of arthritis, diabetes mellitus, hepatic lipidosis, and early mortality. The risk for development of diabetes increases about 2-fold in overweight cats and about 4-fold [corrected] in obese cats. Altered adipokine secretion appears to be an important mechanism for the link between excess BW and many diseases. Once considered to be physiologically inert, adipose tissue is an active producer of hormones, such as leptin and resistin, and cytokines, including many inflammatory cytokines such as tumor necrosis factor-α, IL-1β and IL-6, and C-reactive protein. The persistent, low-grade inflammation secondary to obesity is thought to play a causal role in chronic diseases such as osteoarthritis, cardiovascular disease, diabetes mellitus, and others. For example, tumor necrosis factor-α alters insulin sensitivity by blocking activation of insulin receptors. In addition, obesity is associated with increased oxidative stress, which also may contribute to obesity-related diseases. Management of obesity involves nutritional modification as well as behavioral modification. Increased protein intake combined with reduced calorie intake facilitates loss of body fat while minimizing loss of lean body mass. Limiting treats to 10% of calorie intake and increasing exercise both aid in successful BW management.

Pathogenic obesity and nutraceuticals

Over a decade of intense research in the field of obesity has led to the knowledge that chronic, excessive adipose tissue expansion leads to an increase in the risk for CVD, type 2 diabetes mellitus and cancer. This is primarily thought to stem from the low-grade, systemic inflammatory response syndrome that characterises adipose tissue in obesity, and this itself is thought to arise from the complex interplay of factors including metabolic endotoxaemia, increased plasma NEFA, hypertrophic adipocytes and localised hypoxia. Plasma concentrations of vitamins and antioxidants are lower in obese individuals than in the non-obese, which is hypothesised to negatively affect the development of inflammation and disease in obesity. This paper provides a review of the current literature investigating the potential of nutraceuticals to ameliorate the development of oxidative stress and inflammation in obesity, thereby limiting the onset of obesity complications. Research has found nutraceuticals able to positively modulate the activity of adipocyte cell lines and further positive effects have been found in other aspects of pathogenic obesity. While their ability to affect weight loss is still controversial, it is clear that they have a great potential to reverse the development of overweight and obesity-related comorbidities; this, however, still requires much research especially that utilising well-structured randomised controlled trials.

Resveratrol upregulated SIRT1, FOXO1, and adiponectin and downregulated PPARγ1-3 mRNA expression in human visceral adipocytes

BACKGROUND

The SIRT1 enzyme is involved in adipose tissue (AT) lipolysis. FOXO1 is a protein that plays a significant role in regulating metabolism. Adiponectin is an adipokine, secreted by the AT, which has been considered to have an antiobesity function. PPARγ is one of the key actors in adipocytes differentiation. This study was undertaken to investigate whether resveratrol can regulate SIRT1, FOXO1, adiponectin, PPARγ1-3, and PPARβ/δ in human AT.

METHODS

The effects of resveratrol were analyzed in freshly isolated adipocytes prepared from visceral fat tissue samples obtained during bariatric surgery. Genes messenger ribonucleic acid (mRNA) levels were determined by qRT-PCR.

RESULTS

Ours results show that resveratrol modulates the studied genes, increasing SIRT1 (p = 0.021), FOXO1 (p = 0.001), and adiponectin (p = 0.025) mRNA expression and decreasing PPARγ1-3 (p = 0.003) mRNA in human visceral adipocytes.

CONCLUSIONS

Resveratrol, in vitro and at low concentration, modulates genes that are related to lipid metabolism, possibly preventing metabolic disease in human visceral adipose tissue (VAT).

The anti-inflammatory action of fermented soybean products in kidney of high-fat-fed rats

Soybean has many compounds with a variety of biological properties that potentially benefit human health; among them, isoflavones have inhibitory effects on lipid oxidation in adipose tissue. In this study, we examined two Korean traditional fermented soybean products--doenjang (DNJ) and cheonggukjang (CGJ)--for their ability to suppress redox-sensitive nuclear factor κB (NF-κB) activation in the kidney of rats fed a high-fat diet. Sprague-Dawley rats, 4 weeks old, were fed soybean, DNJ, or CGJ (1 g/kg/day) with a 20% fat diet for 6 weeks. Body weight and food intake were carefully monitored. NF-κB-related activities of genes for inflammatory proteins, such as inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), and vascular cell adhesion molecule-1 (VCAM-1), were determined. The soybean products exhibited antioxidative action by maintaining redox regulation, suppressing NF-κB activation, and modulating the expression of genes for NF-κB-induced inflammatory proteins such as COX-2, iNOS, and VCAM-1. Based on these results, we conclude that Korean traditional soybean fermented products, especially CGJ, suppress the generation of reactive species, NF-κB activity, and NF-κB-related inflammatory genes.

Winning a won game: caffeine panacea for obesity syndemic

Over the past decades, chronic sleep reduction and a concurrent development of obesity have been recognized as a common problem in the industrialized world. Among its numerous untoward effects, there is a possibility that insomnia is also a major contributor to obesity. This attribution poses a problem for caffeine, an inexpensive, “natural” agent that is purported to improve a number of conditions and is often indicated in a long-term pharmacotherapy in the context of weight management. The present study used the “common target” approach by exploring the tentative shared molecular networks of insomnia and adiposity. It discusses caffeine targets beyond those associated with adenosine signaling machinery, phosphodiesterases, and calcium release channels. Here, we provide a view suggesting that caffeine could exert some of its effects by acting on several signaling complexes composed of HIF-1α/VEGF/IL-8 along with NO, TNF-α, IL1, and GHRH, among others. Although the relevance of these targets to the reported therapeutic effects of caffeine has remained difficult to assess, the utilization of caffeine efficacies and potencies recommend its repurposing for development of novel therapeutic approaches. Among indications mentioned, are neuroprotective, nootropic, antioxidant, proliferative, anti-fibrotic, and anti-angiogenic that appear under a variety of dissimilar diagnostic labels comorbid with obesity. In the absence of safe and efficacious antiobesity agents, caffeine remains an attractive adjuvant.

Aberrant cytoplasm localization and protein stability of SIRT1 is regulated by PI3K/IGF-1R signaling in human cancer cells

SIRT1, an NAD-dependent histone/protein deacetylase, has classically been thought of as a nuclear protein. In this study, we demonstrate that SIRT1 is mainly localized in the nucleus of normal cells, but is predominantly localized in the cytoplasm of the cancer / transformed cells we tested. We found this predominant cytoplasmic localization of SIRT1 is regulated by elevated mitotic activity and PI3K/IGF-1R signaling in cancer cells. We show that aberrant cytoplasmic localization of SIRT1 is due to increased protein stability and is regulated by PI3K/IGF-1R signaling. In addition, we determined that SIRT1 is required for PI3K-mediated cancer cell growth. Our study represents the first identification that aberrant cytoplasm localization is one of the specific alternations to SIRT1 that occur in cancer cells, and PI3K/IGF-1R signaling plays an important role in the regulation of cytoplasmic SIRT1 stability. Our findings suggest that the over-expressed cytoplasmic SIRT1 in cancer cells may greatly contribute to its cancer-specific function by working downstream of the PI3K/IGF-1R signaling pathway.

Acute stimulation of white adipocyte respiration by PKA-induced lipolysis

OBJECTIVE

We examined the effect of β-adrenergic receptor (βAR) activation and cAMP-elevating agents on respiration and mitochondrial uncoupling in human adipocytes and probed the underlying molecular mechanisms.

RESEARCH DESIGN AND METHODS

Oxygen consumption rate (OCR, aerobic respiration) and extracellular acidification rate (ECAR, anaerobic respiration) were examined in response to isoproterenol (ISO), forskolin (FSK), and dibutyryl-cAMP (DB), coupled with measurements of mitochondrial depolarization, lipolysis, kinase activities, and gene targeting or knock-down approaches.

RESULTS

ISO, FSK, or DB rapidly increased oxidative and glycolytic respiration together with mitochondrial depolarization in human and mouse white adipocytes. The increase in OCR was oligomycin-insensitive and contingent on cAMP-dependent protein kinase A (PKA)-induced lipolysis. This increased respiration and the uncoupling were blocked by inhibiting the mitochondrial permeability transition pore (PTP) and its regulator, BAX. Interestingly, compared with lean individuals, adipocytes from obese subjects exhibited reduced OCR and uncoupling capacity in response to ISO.

CONCLUSIONS

Lipolysis stimulated by βAR activation or other maneuvers that increase cAMP levels in white adipocytes acutely induces mitochondrial uncoupling and cellular energetics, which are amplified in the absence of scavenging BSA. The increase in OCR is dependent on PKA-induced lipolysis and is mediated by the PTP and BAX. Because this effect is reduced with obesity, further exploration of this uncoupling mechanism will be needed to determine its cause and consequences.

KRIT1 regulates the homeostasis of intracellular reactive oxygen species

KRIT1 is a gene responsible for Cerebral Cavernous Malformations (CCM), a major cerebrovascular disease characterized by abnormally enlarged and leaky capillaries that predispose to seizures, focal neurological deficits, and fatal intracerebral hemorrhage. Comprehensive analysis of the KRIT1 gene in CCM patients has suggested that KRIT1 functions need to be severely impaired for pathogenesis. However, the molecular and cellular functions of KRIT1 as well as CCM pathogenesis mechanisms are still research challenges. We found that KRIT1 plays an important role in molecular mechanisms involved in the maintenance of the intracellular Reactive Oxygen Species (ROS) homeostasis to prevent oxidative cellular damage. In particular, we demonstrate that KRIT1 loss/down-regulation is associated with a significant increase in intracellular ROS levels. Conversely, ROS levels in KRIT1^−/− cells are significantly and dose-dependently reduced after restoration of KRIT1 expression. Moreover, we show that the modulation of intracellular ROS levels by KRIT1 loss/restoration is strictly correlated with the modulation of the expression of the antioxidant protein SOD2 as well as of the transcriptional factor FoxO1, a master regulator of cell responses to oxidative stress and a modulator of SOD2 levels. Furthermore, we show that the KRIT1-dependent maintenance of low ROS levels facilitates the downregulation of cyclin D1 expression required for cell transition from proliferative growth to quiescence. Finally, we demonstrate that the enhanced ROS levels in KRIT1^−/− cells are associated with an increased cell susceptibility to oxidative DNA damage and a marked induction of the DNA damage sensor and repair gene Gadd45α, as well as with a decline of mitochondrial energy metabolism. Taken together, our results point to a new model where KRIT1 limits the accumulation of intracellular oxidants and prevents oxidative stress-mediated cellular dysfunction and DNA damage by enhancing the cell capacity to scavenge intracellular ROS through an antioxidant pathway involving FoxO1 and SOD2, thus providing novel and useful insights into the understanding of KRIT1 molecular and cellular functions.

A role for 1-acylglycerol-3-phosphate-O-acyltransferase-1 in myoblast differentiation

AGPAT isoforms catalyze the acylation of lysophosphatidic acid (LPA) to form phosphatidic acid (PA). AGPAT2 mutations are associated with defective adipogenesis. Muscle and adipose tissue share common precursor cells. We investigated the role of AGPAT isoforms in skeletal muscle development. We demonstrate that small interference RNA-mediated knockdown of AGPAT1 expression prevents the induction of myogenin, a key transcriptional activator of the myogenic program, and inhibits the expression of myosin heavy chain. This effect is rescued by transfection with AGPAT1 but not AGPAT2. Knockdown of AGPAT2 has no effect. The regulation of myogenesis by AGPAT1 is associated with alterations on actin cytoskeleton. The role of AGPAT1 on actin cytoskeleton is further supported by colocalization of AGPAT1 to areas of active actin polymerization. AGPAT1 overexpression was not associated with an increase in PA levels. Our observations strongly implicate AGPAT1 in the development of skeletal muscle, specifically to terminal differentiation. These findings are linked to the regulation of actin cytoskeleton.

Involvement of inducible 6-phosphofructo-2-kinase in the anti-diabetic effect of peroxisome proliferator-activated receptor gamma activation in mice

PFKFB3 is the gene that codes for the inducible isoform of 6-phosphofructo-2-kinase (iPFK2), a key regulatory enzyme of glycolysis. As one of the targets of peroxisome proliferator-activated receptor gamma (PPARgamma), PFKFB3/iPFK2 is up-regulated by thiazolidinediones. In the present study, using PFKFB3/iPFK2-disrupted mice, the role of PFKFB3/iPFK2 in the anti-diabetic effect of PPARgamma activation was determined. In wild-type littermate mice, PPARgamma activation (i.e. treatment with rosiglitazone) restored euglycemia and reversed high fat diet-induced insulin resistance and glucose intolerance. In contrast, PPARgamma activation did not reduce high fat diet-induced hyperglycemia and failed to reverse insulin resistance and glucose intolerance in PFKFB3(+/-) mice. The lack of anti-diabetic effect in PFKFB3(+/-) mice was associated with the inability of PPARgamma activation to suppress adipose tissue lipolysis and proinflammatory cytokine production, stimulate visceral fat accumulation, enhance adipose tissue insulin signaling, and appropriately regulate adipokine expression. Similarly, in cultured 3T3-L1 adipocytes, knockdown of PFKFB3/iPFK2 lessened the effect of PPARgamma activation on stimulating lipid accumulation. Furthermore, PPARgamma activation did not suppress inflammatory signaling in PFKFB3/iPFK2-knockdown adipocytes as it did in control adipocytes. Upon inhibition of excessive fatty acid oxidation in PFKFB3/iPFK2-knockdown adipocytes, PPARgamma activation was able to significantly reverse inflammatory signaling and proinflammatory cytokine expression and restore insulin signaling. Together, these data demonstrate that PFKFB3/iPFK2 is critically involved in the anti-diabetic effect of PPARgamma activation.

Mitochondrial dysfunction is induced by high levels of glucose and free fatty acids in 3T3-L1 adipocytes

Hyperglycemia and high free fatty acids (FFAs) are two well-known characteristics of type 2 diabetes, and are also implicated in the etiology of insulin resistance. However, their roles in mitochondrial dysfunction of white adipocytes are not well-studied. In this study, we investigated the effects of high glucose (25 mM), high free fatty acids (FFAs, 1mM), or a combination of both high glucose+high FFAs on mitochondrial function in differentiated 3T3-L1 adipocytes after 48 h of treatment. We found that high glucose, high FFAs, or high glucose+high FFAs reduced insulin-stimulated glucose uptake in differentiated 3T3-L1 adipocytes. In addition, the mitochondria became smaller and more compact. Levels of the mitofusion protein mfn1 decreased and levels of the mitofission protein Drp1 increased as compared to controls. NRF1 was downregulated, and PGC-1 beta levels were diminished in the high glucose and high glucose+high FFAs conditions. Levels of PGC-1 alpha and mtTFA mRNA were greatly downregulated. No difference was found in the mitochondrial DNA (mtDNA) and intracellular ATP levels of treated cells compared to control cells. Cells treated with high glucose or high FFAs accumulated significant amounts of reactive oxygen species (ROS) and displayed a loss of the mitochondrial membrane potential. High glucose and high glucose+high FFAs led to similar decreases in intramitochondrial calcium concentration, although high FFAs had no effect. Therefore, high glucose and high FFAs can regulate insulin sensitivity, and mitochondrial dysfunction may occur in this process.

AMPK and SIRT1: a long-standing partnership?

AMP-activated protein kinase (AMPK) and the histone/protein deacetylase SIRT1 are fuel-sensing molecules that have coexisted in cells throughout evolution. When a cell's energy state is diminished, AMPK activation restores energy balance by stimulating catabolic processes that generate ATP and downregulating anabolic processes that consume ATP but are not acutely needed for survival. SIRT1 in turn is best known historically for producing genetic changes that mediate the increase in longevity caused by calorie restriction. Although the two molecules have been studied intensively for many years, only recently has it become apparent that they have similar effects on diverse processes such as cellular fuel metabolism, inflammation, and mitochondrial function. In this review we will examine the evidence that these similarities occur because AMPK and SIRT1 both regulate each other and share many common target molecules. In addition, we will discuss the clinical relevance of these interactions and in particular the possibility that their dysregulation predisposes to disorders such as type 2 diabetes and atherosclerotic cardiovascular disease and is a target for their therapy.

Saturated fatty acids and insulin resistance

Insulin resistance is one of the pathophysiological features of obesity and type 2 diabetes. Recent findings have linked insulin resistance to chronic low-grade inflammation in white adipose tissue. Excess storage of saturated fat in white adipose tissue due to a modern life style causes hypertrophy and hyperplasia of adipocytes, which exhibit attenuated insulin signaling due to their production and release of saturated fatty acids. These adipocytes recruit macrophages to white adipose tissue and, together with them, initiate a proinflammatory response. Proinflammatory factors and saturated fatty acids secreted into the bloodstream from white adipose tissue impair insulin signaling in non-adipose tissues, which causes whole-body insulin resistance.

Berberine improves free-fatty-acid-induced insulin resistance in L6 myotubes through inhibiting peroxisome proliferator-activated receptor gamma and fatty acid transferase expressions

The plant alkaloid berberine (BBR) has been reported to have antidiabetic effect in humans and animals. However, the mechanism of action is not well understood. The present study was conducted to determine the effect and mechanism of action of BBR on the free-fatty-acid (FFA)-induced insulin resistance in muscle cells. The FFA-induced insulin-resistant cell model was established in L6 myotubes by treating them with 250 mumol/L of palmitic acid. The inclusion of FFA in the medium increased peroxisome proliferator-activated receptor gamma (PPARgamma) and fatty acid transferase (FAT/CD36) expressions by 26% and 50% and decreased glucose consumption by 43% and insulin-mediated glucose uptake by 63%, respectively. Berberine treatment increased the glucose consumption and insulin-stimulated glucose uptake in normal cells and improved glucose uptake in the FFA-induced insulin-resistant cells. The improved glucose uptake by BBR was accompanied with a dose-dependent decrease in PPARgamma and FAT/CD36 protein expressions. In insulin-resistant myotubes, BBR (5 micromol/L) decreased PPARgamma and FAT/CD36 proteins by 31% and 24%, whereas PPARgamma antagonist GW9662 reduced both proteins by 56% and 46%, respectively. In contrast, PPARgamma agonist rosiglitazone increased the expression of PPARgamma and FAT/CD36 by 34% and 21%, respectively. Our results suggest that BBR improves the FFA-induced insulin resistance in myotubes through inhibiting fatty acid uptake at least in part by reducing PPARgamma and FAT/CD36 expressions.

Disruption of inducible 6-phosphofructo-2-kinase ameliorates diet-induced adiposity but exacerbates systemic insulin resistance and adipose tissue inflammatory response

Adiposity is commonly associated with adipose tissue dysfunction and many overnutrition-related metabolic diseases including type 2 diabetes. Much attention has been paid to reducing adiposity as a way to improve adipose tissue function and systemic insulin sensitivity. PFKFB3/iPFK2 is a master regulator of adipocyte nutrient metabolism. Using PFKFB3(+/-) mice, the present study investigated the role of PFKFB3/iPFK2 in regulating diet-induced adiposity and systemic insulin resistance. On a high-fat diet (HFD), PFKFB3(+/-) mice gained much less body weight than did wild-type littermates. This was attributed to a smaller increase in adiposity in PFKFB3(+/-) mice than in wild-type controls. However, HFD-induced systemic insulin resistance was more severe in PFKFB3(+/-) mice than in wild-type littermates. Compared with wild-type littermates, PFKFB3(+/-) mice exhibited increased severity of HFD-induced adipose tissue dysfunction, as evidenced by increased adipose tissue lipolysis, inappropriate adipokine expression, and decreased insulin signaling, as well as increased levels of proinflammatory cytokines in both isolated adipose tissue macrophages and adipocytes. In an in vitro system, knockdown of PFKFB3/iPFK2 in 3T3-L1 adipocytes caused a decrease in the rate of glucose incorporation into lipid but an increase in the production of reactive oxygen species. Furthermore, knockdown of PFKFB3/iPFK2 in 3T3-L1 adipocytes inappropriately altered the expression of adipokines, decreased insulin signaling, increased the phosphorylation states of JNK and NFkappaB p65, and enhanced the production of proinflammatory cytokines. Together, these data suggest that PFKFB3/iPFK2, although contributing to adiposity, protects against diet-induced insulin resistance and adipose tissue inflammatory response.

Dietary antioxidant capacity and concentration of adiponectin in apparently healthy adults: the ATTICA study

BACKGROUND/OBJECTIVES

This study aimed at evaluating the relationship of adiponectin concentration with total dietary antioxidant capacity in free-living, apparently healthy adults from the ATTICA study.

SUBJECTS/METHODS

A random subsample from the ATTICA study, consisting of 310 men (40+/-11 years) and 222 women (38+/-12 years), was selected. Adiponectin, along with other inflammatory markers, was measured in fasting participants. Dietary habits were evaluated using a food frequency questionnaire and the dietary antioxidant capacity was based on published values of Italian foods measured by three different assays: ferric-reducing antioxidant power (FRAP), total radical-trapping antioxidant parameters (TRAP) and trolox-equivalent antioxidant capacity (TEAC).

RESULTS

Positive associations were observed between dietary antioxidant capacity and adiponectin concentration, as assessed with FRAP (b+/-s.e.=0.012+/-0.005, P=0.018 per 1 mmol Fe (II)/day), TRAP (b+/-s.e.=0.030+/-0.013, P=0.017 per 1 mmol trolox equivalent/day) and TEAC (b+/-s.e.=0.025+/-0.012, P=0.042, per 1 mmol trolox equivalent/day) in multiadjusted analysis. Moreover, a negative relation of dietary antioxidant indices with inflammatory markers was revealed.

CONCLUSIONS

Diets with high antioxidant capacity are related to increased adiponectin levels. An adiponectin-mediated route through which antioxidant-rich foods exert beneficial effects against inflammation and cardiovascular diseases can be thus hypothesized.

Downregulation of adiponectin/AdipoR2 is associated with steatohepatitis in obese mice

BACKGROUND

Recent evidence suggests that obesity is associated with hypo-adiponectinmia and chronic inflammation. Adiponectin regulates fat storage, energy expenditure, and inflammation. We propose that high fat diet induces steatohepatitis, reduces serum adiponectin, and liver adiponectin receptors.

METHODS

A 4-week-old C57BL male mice were fed high fat diet (n = 8) or regular chow (control; n = 6) for 7 weeks. Body weight, liver weight, and serum adiponectin were measured. Liver sections were stained with hematoxylin and eosin and oil red for fat content. Liver homogenates were used for protein (immunoblotting) and mRNA (reverse transcription PCR) of Toll-like receptor 4 (TLR4), tumor necrosis factor alpha (TNF-alpha), interleukin (IL)-6, sterol regulatory element-binding proteins (SREBP)-1c, and adiponectin receptors (AdipoR1/AdipoR2) in addition to nuclear phorsphorylated p65NF-kappaB. Gels were quantified using densitometry; t test was used, and p < 0.05 was significant.

RESULTS

High fat diet increased body (50%) and liver weight (33%), as well as hepatocyte fat content and ballooning. Mice fed high fat diet exhibited reduced serum adiponectin and liver AdipoR2. High fat diet increased hepatic levels of SREBP-1c, TLR4, TNF-alpha, and IL-6 protein and mRNA and increased activation of p65NF-kappaB.

CONCLUSIONS

Diet-induced liver steatosis is associated with increased lipogensis, upregulation of pro-inflammatory cytokines, and transcription factors as well as downregulation of AdipoR2. Reduction in serum adiponectin suggests that adiponectin signaling may be the crosslink between high fat diet, hepatic inflammation, and nonalcoholic fatty liver disease.

Resveratrol: its biologic targets and functional activity

The polyphenolic phytoalexin resveratrol (RSV) and its analogues have received tremendous attention over the past couple of decades because of a number of reports highlighting their benefits in vitro and in vivo in a variety of human disease models, including cardio- and neuroprotection, immune regulation, and cancer chemoprevention. These studies have underscored the high degree of diversity in terms of the signaling networks and cellular effector mechanisms that are affected by RSV. The activity of RSV has been linked to cell-surface receptors, membrane signaling pathways, intracellular signal-transduction machinery, nuclear receptors, gene transcription, and metabolic pathways. The promise shown by RSV has prompted heightened interest in studies aimed at translating these observations to clinical settings. In this review, we present a comprehensive account of the basic chemistry of RSV, its bioavailability, and its multiple intracellular target proteins and signaling pathways.

Aging alters PPARgamma in rodent and human adipose tissue by modulating the balance in steroid receptor coactivator-1

Age is an important risk factor for the development of metabolic diseases (e.g. obesity, diabetes and atherosclerosis). Yet, little is known about the molecular mechanisms occurring upon aging that affect energy metabolism. Although visceral white adipose tissue (vWAT) is known for its key impact on metabolism, recent studies have indicated it could also be a key regulator of lifespan, suggesting that it can serve as a node for age-associated fat accretion. Here we show that aging triggers changes in the transcriptional milieu of the nuclear receptor peroxisome proliferator-activated receptor gamma (PPARgamma) in vWAT, which leads to a modified potential for transactivation of target genes upon ligand treatment. We found that in vWAT of mice, rats and men, aging induced a specific decrease in the expression of steroid receptor coactivator-1 (SRC-1), whose recruitment to PPARgamma is associated with improved insulin sensitivity and low adipogenic activity. In contrast, aging and oxidative stress did not impact on PPARgamma expression and PPARgamma ligand production. Age-induced loss of PPARgamma/SRC-1 interactions increased the binding of PPARgamma to the promoter of the adipogenic gene aP2. These findings suggest that strategies aimed at increasing SRC-1 expression and recruitment to PPARgamma upon aging might help improve age-associated metabolic disorders.

SIRT1 and insulin resistance

Sirtuin 1 (SIRT1), the mammalian homolog of SIR2, was originally identified as a NAD-dependent histone deacetylase, the activity of which is closely associated with lifespan under calorie restriction. Growing evidence suggests that SIRT1 regulates glucose or lipid metabolism through its deacetylase activity for over two dozen known substrates, and has a positive role in the metabolic pathway through its direct or indirect involvement in insulin signaling. SIRT1 stimulates a glucose-dependent insulin secretion from pancreatic beta cells, and directly stimulates insulin signaling pathways in insulin-sensitive organs. Furthermore, SIRT1 regulates adiponectin secretion, inflammatory responses, gluconeogenesis, and levels of reactive oxygen species, which together contribute to the development of insulin resistance. Moreover, overexpression of SIRT1 and several SIRT1 activators has beneficial effects on glucose homeostasis and insulin sensitivity in obese mice models. These findings suggest that SIRT1 might be a new therapeutic target for the prevention of disease related to insulin resistance, such as metabolic syndrome and diabetes mellitus, although direct evidence from clinical studies in humans is needed to prove this possibility. In this Review, we discuss the potential role and therapeutic promise of SIRT1 in insulin resistance on the basis of the latest experimental studies.

Adiponectin: a key adipokine in alcoholic fatty liver

Alcoholic fatty liver is a major risk factor for advanced liver injuries such as steatohepatitis, fibrosis, and cirrhosis. While the underlying mechanisms are multiple, the development of alcoholic fatty liver has been attributed to a combined increase in the rate of de novo lipogenesis and a decrease in the rate of fatty acid oxidation in animal liver. Among various transcriptional regulators, the hepatic SIRT1 (sirtuin 1)-AMPK (AMPK-activated kinase) signaling system represents a central target for the action of ethanol in the liver. Adiponectin is one of the adipocyte-derived adipokines with potent lipid-lowering properties. Growing evidence has demonstrated that the development of alcoholic fatty liver is associated with reduced circulating adiponectin levels, decreased hepatic adiponectin receptor expression, and impaired hepatic adiponectin signaling. Adiponectin confers protection against alcoholic fatty liver via modulation of complex hepatic signaling pathways largely controlled by the central regulatory system, SIRT1-AMPK axis. This review aims to integrate the current research findings of ethanol-mediated dysregulation of adiponectin and its receptors and to provide a comprehensive point of view for understanding the role of adiponectin signaling in the development of alcoholic fatty liver.

The role of FOXO in the regulation of metabolism

Forkhead box O (FOXO) transcription factors play an important role in modulating metabolic functions. FOXO is regulated by several modifications, but one of the most critical is phosphorylation and nuclear exclusion by Akt. Given the impact of insulin signaling on Akt-mediated phosphorylation of FOXO and the relatively high expression of Foxo1 in insulin-responsive tissues, this transcription factor is highly poised to regulate energy metabolism. When nutrient and insulin levels are low, Foxo1 promotes expression of gluconeogenic enzymes. Conversely, in the fed state, insulin levels rise and stimulate uptake of glucose primarily into skeletal muscle and other organs, including adipose tissue. Under certain pathophysiologic conditions, including insulin resistance, negative signaling to Foxo1 is compromised. Further clarification of the role of Foxo1 in insulin-responsive tissues will strengthen our understanding and allow us to better combat insulin resistance and diabetes mellitus.

Small molecule activators of SIRT1 replicate signaling pathways triggered by calorie restriction in vivo

Background

Calorie restriction (CR) produces a number of health benefits and ameliorates diseases of aging such as type 2 diabetes. The components of the pathways downstream of CR may provide intervention points for developing therapeutics for treating diseases of aging. The NAD⁺-dependent protein deacetylase SIRT1 has been implicated as one of the key downstream regulators of CR in yeast, rodents, and humans. Small molecule activators of SIRT1 have been identified that exhibit efficacy in animal models of diseases typically associated with aging including type 2 diabetes. To identify molecular processes induced in the liver of mice treated with two structurally distinct SIRT1 activators, SIRT501 (formulated resveratrol) and SRT1720, for three days, we utilized a systems biology approach and applied Causal Network Modeling (CNM) on gene expression data to elucidate downstream effects of SIRT1 activation.

Results

Here we demonstrate that SIRT1 activators recapitulate many of the molecular events downstream of CR in vivo, such as enhancing mitochondrial biogenesis, improving metabolic signaling pathways, and blunting pro-inflammatory pathways in mice fed a high fat, high calorie diet.

Conclusion

CNM of gene expression data from mice treated with SRT501 or SRT1720 in combination with supporting in vitro and in vivo data demonstrates that SRT501 and SRT1720 produce a signaling profile that mirrors CR, improves glucose and insulin homeostasis, and acts via SIRT1 activation in vivo. Taken together these results are encouraging regarding the use of small molecule activators of SIRT1 for therapeutic intervention into type 2 diabetes, a strategy which is currently being investigated in multiple clinical trials.

Positive and negative regulation of insulin signaling by reactive oxygen and nitrogen species

Regulated production of reactive oxygen species (ROS)/reactive nitrogen species (RNS) adequately balanced by antioxidant systems is a prerequisite for the participation of these active substances in physiological processes, including insulin action. Yet, increasing evidence implicates ROS and RNS as negative regulators of insulin signaling, rendering them putative mediators in the development of insulin resistance, a common endocrine abnormality that accompanies obesity and is a risk factor of type 2 diabetes. This review deals with this dual, seemingly contradictory, function of ROS and RNS in regulating insulin action: the major processes for ROS and RNS generation and detoxification are presented, and a critical review of the evidence that they participate in the positive and negative regulation of insulin action is provided. The cellular and molecular mechanisms by which ROS and RNS are thought to participate in normal insulin action and in the induction of insulin resistance are then described. Finally, we explore the potential usefulness and the challenges in modulating the oxidant-antioxidant balance as a potentially promising, but currently disappointing, means of improving insulin action in insulin resistance-associated conditions, leading causes of human morbidity and mortality of our era.

It's not how fat you are, it's what you do with it that counts

Mechanisms underlying obesity-related metabolic disorders are poorly understood. Samuel Virtue and Antonio Vidal-Puig explore the evidence for an emerging hypothesis that attributes metabolic complications not to obesity per se, but to an individual's capacity for adipose tissue expandability.

Resveratrol alleviates alcoholic fatty liver in mice

Alcoholic fatty liver is associated with inhibition of sirtuin 1 (SIRT1) and AMP-activated kinase (AMPK), two critical signaling molecules regulating the pathways of hepatic lipid metabolism in animals. Resveratrol, a dietary polyphenol, has been identified as a potent activator for both SIRT1 and AMPK. In the present study, we have carried out in vivo animal experiments that test the ability of resveratrol to reverse the inhibitory effects of chronic ethanol feeding on hepatic SIRT1-AMPK signaling system and to prevent the development of alcoholic liver steatosis. Resveratrol treatment increased SIRT1 expression levels and stimulated AMPK activity in livers of ethanol-fed mice. The resveratrol-mediated increase in activities of SIRT1 and AMPK was associated with suppression of sterol regulatory element binding protein 1 (SREBP-1) and activation of peroxisome proliferator-activated receptor gamma coactivator alpha (PGC-1alpha). In parallel, in ethanol-fed mice, resveratrol administration markedly increased circulating adiponectin levels and enhanced mRNA expression of hepatic adiponectin receptors (AdipoR1/R2). In conclusion, resveratrol treatment led to reduced lipid synthesis and increased rates of fatty acid oxidation and prevented alcoholic liver steatosis. The protective action of resveratrol is in whole or in part mediated through the upregulation of a SIRT1-AMPK signaling system in the livers of ethanol-fed mice. Our study suggests that resveratrol may serve as a promising agent for preventing or treating human alcoholic fatty liver disease.

1,25-Dihydroxyvitamin D improved the free fatty-acid-induced insulin resistance in cultured C2C12 cells

BACKGROUND

Epidemiological evidence has indicated that vitamin D deficiency increased the risk of insulin resistance in metabolic syndrome. The present study was conducted to test the hypothesis that 1,25-dihydroxyvitamin D may improve the free fatty-acid (FFA)-induced insulin resistance in muscle cells.

METHOD

The insulin resistance of muscle cell model was established by treatment of FFA in differentiated C2C12 cells. Glucose uptake of C2C12 myotubes was analysed by the 3H-labelled 2-deoxyglucose uptake assay. The diameter of myotubes was measured under the condition of glutaraldehyde-induced autofluorescense. Tyrosine phosphorylated insulin receptor substrate 1 (IRS-1) was measured by immunoprecipitation. Serine phosphorylated IRS-1 and protein kinase B (Akt), extracellular signal-related kinase (ERK), c-Jun amino-terminal kinases (JNK) as well as their phosphorylated form were analysed by Western blots.

RESULTS

Compared with a vehicle-treated group, FFA treatment in myotubes was associated with 70.6% reduction in insulin-mediated uptake of glucose, a five-fold increase in serine phosphorylation of IRS-1, 76.9% decrease in tyrosine phosphorylation of IRS-1 and 81.8% decrease in phosphorylation of Akt. Supplement of 1,25-dihydroxyvitamin D improved the FFA-induced inhibition of glucose uptake in a dose- dependent (p < 0.001) and time-dependent manner (p < 0.01). This was accompanied by increase in tyrosine phosphorylation of IRS-1 and phosphorylated Akt and decrease in serine phosphorylation of IRS-1 (p < 0.001). 1,25-Dihydroxyvitamin D also inhibited the FFA-induced reduction in myotube diameter by 35.3% (p < 0.001). JNK phosphorylation was reduced by 126.7% with treatment of 1,25-dihydroxyvitamin D (p < 0.001). 1,25-Dihydroxyvitamin D had no effect on FFA-induced ERK phosphorylation (p = 0.84).

CONCLUSION

1,25-Dihydroxyvitamin D improved the FFA-induced insulin resistance in muscle cells.

Dietary curcumin significantly improves obesity-associated inflammation and diabetes in mouse models of diabesity

Obesity is a major risk factor for the development of type 2 diabetes, and both conditions are now recognized to possess significant inflammatory components underlying their pathophysiologies. We tested the hypothesis that the plant polyphenolic compound curcumin, which is known to exert potent antiinflammatory and antioxidant effects, would ameliorate diabetes and inflammation in murine models of insulin-resistant obesity. We found that dietary curcumin admixture ameliorated diabetes in high-fat diet-induced obese and leptin-deficient ob/ob male C57BL/6J mice as determined by glucose and insulin tolerance testing and hemoglobin A1c percentages. Curcumin treatment also significantly reduced macrophage infiltration of white adipose tissue, increased adipose tissue adiponectin production, and decreased hepatic nuclear factor-kappaB activity, hepatomegaly, and markers of hepatic inflammation. We therefore conclude that orally ingested curcumin reverses many of the inflammatory and metabolic derangements associated with obesity and improves glycemic control in mouse models of type 2 diabetes. This or related compounds warrant further investigation as novel adjunctive therapies for type 2 diabetes in man.

JNK- and IkappaB-dependent pathways regulate MCP-1 but not adiponectin release from artificially hypertrophied 3T3-L1 adipocytes preloaded with palmitate in vitro

Obese conditions increase the expression of adipocytokine monocyte chemoattractant protein-1 (MCP-1) in adipose tissue as well as MCP-1 plasma levels. To investigate the mechanism behind increased MCP-1, we used a model in which 3T3-L1 adipocytes were artificially hypertrophied by preloading with palmitate in vitro. As observed in obesity, under our model conditions, palmitate-preloaded cells showed significantly increased oxidative stress and increased MCP-1 expression relative to control cells. This increased MCP-1 expression was enhanced by adding exogenous tumor necrosis factor-alpha (TNF-alpha; 17.8-fold vs. control cells, P < 0.01) rather than interleukin-1beta (IL-1beta; 2.6-fold vs. control cells, P < 0.01). However, endogenous TNF-alpha and IL-1beta release was not affected in hypertrophied cells, suggesting that these endogenous cytokines do not mediate hypertrophy-induced increase in MCP-1. MCP-1 secretion from hypertrophied cells was significantly decreased by treatment with antioxidant N-acetyl-cysteine, JNK inhibitors SP600125 and JIP-1 peptide, and IkappaB phosphorylation inhibitors BAY 11-7085 and BMS-345541 (P < 0.01). MCP-1 secretion was not affected by peroxisome proliferator-activated receptor-gamma (PPARgamma) antagonists assayed. Adiponectin, another adipocytokine studied in parallel, also showed increased release in hypertrophy relative to control cells. But in contrast to MCP-1, adiponectin release was significantly suppressed by both exogenous TNF-alpha and IL-1beta as well as by PPARgamma antagonists bisphenol A diglycidyl ether and T0070907 (P < 0.01). JNK inhibitors and IkappaB phosphorylation inhibitors showed no significant effect on adiponectin. We conclude that adipocyte hypertrophy through palmitate loading causes oxidative stress, which in turn increases MCP-1 expression and secretion through JNK and IkappaB signaling. In contrast, the parallel increase in adiponectin expression appears to be related to the PPARgamma ligand properties of palmitate.

The role of FoxO in the regulation of metabolism

Forkhead proteins, and FoxO1 in particular, play a significant role in regulating whole body energy metabolism. Glucose homeostasis is achieved by adjusting endogenous glucose production as well as glucose uptake by peripheral tissues in response to insulin. In the fasted state, the liver is primarily responsible for maintaining glucose levels, with FoxO1 playing a key role in promoting the expression of gluconeogenic enzymes. Following feeding, pancreatic beta cells secrete insulin, which promotes the uptake of glucose by peripheral tissues including skeletal muscle and adipose tissue, and can in part suppress gluconeogenic enzyme expression in the liver. In addition to directly regulating metabolism, FoxO1 also plays a role in the formation of both adipose tissue and skeletal muscle, two major organs that are critical for maintaining energy homeostasis. The importance of FoxO1 in energy homeostasis is particularly striking under conditions of metabolic dysfunction or insulin resistance. In obese or diabetic states, FoxO1-dependent gene expression promotes some of the deleterious characteristics associated with these conditions, including hyperglycemia and glucose intolerance. In addition, the increase in pancreatic beta cell mass that normally occurs in response to a rise in insulin demand is blunted by nuclear FoxO1 expression. However, under these same pathophysiological conditions, FoxO1 expression may help drive the expression of genes involved in combating oxidative stress, thereby preserving cellular function. FoxO1 may also be involved in promoting the switch from carbohydrate to fatty acid as the major energy source during starvation.

AMP-activated protein kinase is activated as a consequence of lipolysis in the adipocyte: potential mechanism and physiological relevance

AMP-activated protein kinase (AMPK) is activated in adipocytes during exercise and other states in which lipolysis is stimulated. However, the mechanism(s) responsible for this effect and its physiological relevance are unclear. To examine these questions, 3T3-L1 adipocytes were treated with cAMP-inducing agents (isoproterenol, forskolin, and isobutylmethylxanthine), which stimulate lipolysis and activate AMPK. When lipolysis was partially inhibited with the general lipase inhibitor orlistat, AMPK activation by these agents was also partially reduced, but the increases in cAMP levels and cAMP-dependent protein kinase (PKA) activity were unaffected. Likewise, small hairpin RNA-mediated silencing of adipose tissue triglyceride lipase inhibited both forskolin-stimulated lipolysis and AMPK activation but not that of PKA. Forskolin treatment increased the AMP:ATP ratio, and this too was reduced by orlistat. When acyl-CoA synthetase, which catalyzes the conversion of fatty acids to fatty acyl-CoA, was inhibited with triacsin C, the increases in both AMPK activity and AMP:ATP ratio were blunted. Isoproterenol-stimulated lipolysis was accompanied by an increase in oxidative stress, an effect that was quintupled in cells incubated with the AMPK inhibitor compound C. The isoproterenol-induced increase in the AMP:ATP ratio was also much greater in these cells. In conclusion, the results indicate that activation of AMPK in adipocytes by cAMP-inducing agents is a consequence of lipolysis and not of PKA activation. They suggest that AMPK activation in this setting is caused by an increase in the AMP:ATP ratio that appears to be due, at least in part, to the acylation of fatty acids. Finally, this AMPK activation appears to restrain the energy depletion and oxidative stress caused by lipolysis.

Inflammation and insulin resistance in uremia

Low-grade systemic inflammation is an important potential factor in the pathogenesis of insulin resistance in end-stage renal disease (ESRD). Insulin resistance and diabetes, characterized by impaired skeletal muscle glucose uptake or excess hepatic glucose production, are in turn relevant contributors to morbidity and mortality in ESRD patients. Oxidative stress is increased in ESRD, in conservative therapy as well as hemodialysis treatment. Recent evidence suggests that oxidative stress contributes, at least in part, to both inflammation and insulin resistance by modulating the production of proinflammatory cytokines and adipokines in monocytes and in adipose tissue. This review focuses on the pathogenesis of inflammation and oxidative stress, and the effects of their interplay on insulin resistance in ESRD.

Obesity-induced inflammation in white adipose tissue is attenuated by loss of melanocortin-3 receptor signaling

Metabolic syndrome, a complex of highly debilitating disorders that includes insulin resistance, hypertension, and dyslipidemia, is associated with the development of obesity in humans as well as rodent models. White adipose tissue (WAT) inflammation, caused in part by macrophage infiltration, and fat accumulation in the liver are both linked to development of the metabolic syndrome. Despite large increases in body fat, melanocortin 3-receptor (MC3-R)-deficient mice do not get fatty liver disease or severe insulin resistance. This is in contrast to obese melanocortin 4-receptor (MC4-R)-deficient mice and diet-induced obese (DIO) mice, which show increased adiposity, fatty liver disease, and insulin resistance. We hypothesized that defects in the inflammatory response to obesity may underlie the protection from metabolic syndrome seen in MC3-R null mice. MC4-R mice fed a chow diet show increased proinflammatory gene expression and macrophage infiltration in WAT, as do wild-type (WT) DIO mice. In contrast, MC3-R-deficient mice fed a normal chow diet show neither of these inflammatory changes, despite their elevated adiposity and a comparable degree of adipocyte hypertrophy to the MC4-R null and DIO mice. Furthermore, even when challenged with high-fat chow for 4 wk, a period of time shown to induce an inflammatory response in WAT of WT animals, MC3-R nulls showed an attenuated up-regulation in both monocyte chemoattractant protein-1 (MCP-1) and TNFalpha mRNA in WAT compared with WT high-fat-fed animals.

Resveratrol inhibits TNF-alpha-induced changes of adipokines in 3T3-L1 adipocytes

Tumor necrosis factor-alpha (TNF-alpha) is chronically elevated in adipose tissues of obese rodents and humans. Increased levels of TNF-alpha are implicated in the induction of atherogenic adipokines, such as plasminogen activator inhibitor -1 (PAI-1) and IL-6, and the inhibition of the anti-atherogenic adipokine, adiponectin. In this study, we investigated the effects of resveratrol on TNF-alpha-induced atherogenic changes of the adipokines in 3T3-L1 cells. Exposure to TNF-alpha for 24 h increased PAI-1 and IL-6 secretion and decreased adiponectin secretion. The mRNA expression of adipokines changed in parallel with mRNA expression. Resveratrol effectively reversed the secretion and mRNA expression of the atherogenic adipokines, PAI-1 and IL-6, induced by TNF-alpha. Decreased secretion levels and mRNA expression of adiponectin by TNF-alpha were also recovered by resveratrol treatment. Our results suggest that resveratrol may improve obesity-induced cardiovascular disease, particularly atherosclerosis, by attenuating the TNF-alpha-induced changes of adipokines.