Neuronal pathway from the liver modulates energy expenditure and systemic insulin sensitivity

Chronic mild stress alters circadian expressions of molecular clock genes in the liver

Chronic stress is well known to affect metabolic regulation. However, molecular mechanisms interconnecting stress response systems and metabolic regulations have yet to be elucidated. Various physiological processes, including glucose/lipid metabolism, are regulated by the circadian clock, and core clock gene dysregulation reportedly leads to metabolic disorders. Glucocorticoids, acting as end-effectors of the hypothalamus-pituitary-adrenal (HPA) axis, entrain the circadian rhythms of peripheral organs, including the liver, by phase-shifting core clock gene expressions. Therefore, we examined whether chronic stress affects circadian expressions of core clock genes and metabolism-related genes in the liver using the chronic mild stress (CMS) procedure. In BALB/c mice, CMS elevated and phase-shifted serum corticosterone levels, indicating overactivation of the HPA axis. The rhythmic expressions of core clock genes, e.g., Clock, Npas2, Bmal1, Per1, and Cry1, were altered in the liver while being completely preserved in the hypothalamic suprachiasmatic nuculeus (SCN), suggesting that the SCN is not involved in alterations in hepatic core clock gene expressions. In addition, circadian patterns of glucose and lipid metabolism-related genes, e.g., peroxisome proliferator activated receptor (Ppar) α, Pparγ-1, Pparγ-coactivator-1α, and phosphoenolepyruvate carboxykinase, were also disturbed by CMS. In contrast, in C57BL/6 mice, the same CMS procedure altered neither serum corticosterone levels nor rhythmic expressions of hepatic core clock genes and metabolism-related genes. Thus, chronic stress can interfere with the circadian expressions of both core clock genes and metabolism-related genes in the liver possibly involving HPA axis overactivation. This mechanism might contribute to metabolic disorders in stressful modern societies.

Methionine-restricted C57BL/6J mice are resistant to diet-induced obesity and insulin resistance but have low bone density

Dietary methionine restriction (MR) extends lifespan, an effect associated with reduction of body weight gain, and improvement of insulin sensitivity in mice and rats as a result of metabolic adaptations in liver, adipose tissue and skeletal muscle. To test whether MR confers resistance to adiposity and insulin resistance, C57BL/6J mice were fed a high fat diet (HFD) containing either 0.86% methionine (control fed; CF) or 0.12% methionine (methionine-restricted; MR). MR mice on HFD had lower body weight gain despite increased food intake and absorption efficiency compared to their CF counterparts. MR mice on HFD were more glucose tolerant and insulin sensitive with reduced accumulation of hepatic triglycerides. In plasma, MR mice on HFD had higher levels of adiponectin and FGF21 while leptin and IGF-1 levels were reduced. Hepatic gene expression showed the downregulation of Scd1 while Pparg, Atgl, Cd36, Jak2 and Fgf21 were upregulated in MR mice on HFD. Restriction of growth rate in MR mice on HFD was also associated with lower bone mass and increased plasma levels of the collagen degradation marker C-terminal telopeptide of type 1 collagen (CTX-1). It is concluded that MR mice on HFD are metabolically healthy compared to CF mice on HFD but have decreased bone mass. These effects could be associated with the observed increase in FGF21 levels.

Hepatic glucokinase modulates obesity predisposition by regulating BAT thermogenesis via neural signals

Considering the explosive increase in obesity worldwide, there must be an unknown mechanism(s) promoting energy accumulation under conditions of overnutrition. We identified a feed-forward mechanism favoring energy storage, originating in hepatic glucokinase (GK) upregulation. High-fat feeding induced hepatic GK upregulation, and hepatic GK overexpression dose-dependently decreased adaptive thermogenesis by downregulating thermogenesis-related genes in brown adipose tissue (BAT). This intertissue (liver-to-BAT) system consists of the afferent vagus from the liver and sympathetic efferents from the medulla and antagonizes anti-obesity effects of leptin on thermogenesis. Furthermore, upregulation of endogenous GK in the liver by high-fat feeding was more marked in obesity-prone than in obesity-resistant strains and was inversely associated with BAT thermogenesis. Hepatic GK overexpression in obesity-resistant mice promoted weight gain, while hepatic GK knockdown in obesity-prone mice attenuated weight gain with increased adaptive thermogenesis. Thus, this intertissue energy-saving system may contribute to determining obesity predisposition.

Hepatocyte growth factor plays a key role in insulin resistance-associated compensatory mechanisms

Insulin resistance is present in obesity and in type 2 diabetes and is associated with islet cell hyperplasia and hyperinsulinemia, but the driving forces behind this compensatory mechanism are incompletely understood. Previous data have suggested the involvement of an unknown circulating insulin resistance-related β-cell growth factor. In this context, looking for candidates to be a circulating factor, we realized that hepatocyte growth factor (HGF) is a strong candidate as a link between insulin resistance and increased mass of islets/hyperinsulinemia. Our approach aimed to show a possible cause-effect relationship between increase in circulating HGF levels and compensatory islet hyperplasia/hyperinsulinemia by showing the strength of the association, whether or not is a dose-dependent response, the temporality, consistency, plausibility, and reversibility of the association. In this regard, our data showed: 1) a strong and consistent correlation between HGF and the compensatory mechanism in three animal models of insulin resistance; 2) HGF increases β-cell mass in a dose-dependent manner; 3) blocking HGF shuts down the compensatory mechanisms; and 4) an increase in HGF levels seems to precede the compensatory response associated with insulin resistance, indicating that these events occur in a sequential mode. Additionally, blockages of HGF receptor (Met) worsen the impaired insulin-induced insulin signaling in liver of diet-induced obesity rats. Overall, our data indicate that HGF is a growth factor playing a key role in islet mass increase and hyperinsulinemia in diet-induced obesity rats and suggest that the HGF-Met axis may have a role on insulin signaling in the liver.

S-adenosylmethionine in liver health, injury, and cancer

S-adenosylmethionine (AdoMet, also known as SAM and SAMe) is the principal biological methyl donor synthesized in all mammalian cells but most abundantly in the liver. Biosynthesis of AdoMet requires the enzyme methionine adenosyltransferase (MAT). In mammals, two genes, MAT1A that is largely expressed by normal liver and MAT2A that is expressed by all extrahepatic tissues, encode MAT. Patients with chronic liver disease have reduced MAT activity and AdoMet levels. Mice lacking Mat1a have reduced hepatic AdoMet levels and develop oxidative stress, steatohepatitis, and hepatocellular carcinoma (HCC). In these mice, several signaling pathways are abnormal that can contribute to HCC formation. However, injury and HCC also occur if hepatic AdoMet level is excessive chronically. This can result from inactive mutation of the enzyme glycine N-methyltransferase (GNMT). Children with GNMT mutation have elevated liver transaminases, and Gnmt knockout mice develop liver injury, fibrosis, and HCC. Thus a normal hepatic AdoMet level is necessary to maintain liver health and prevent injury and HCC. AdoMet is effective in cholestasis of pregnancy, and its role in other human liver diseases remains to be better defined. In experimental models, it is effective as a chemopreventive agent in HCC and perhaps other forms of cancer as well.

Update on pparγ and nonalcoholic Fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is the most common initial presentation of obesity and insulin resistance. Uninterrupted progression of hepatic lipid accumulation often leads to fatty liver disease and eventually cirrhosis. Insulin resistance is one of the characteristics of type 2 diabetes. Several types of treatment have been employed against type 2 diabetes some of which ameliorate NAFLD. The frequent line of treatment to improve insulin sensitivity is the use of thiazolidinediones (TZD) which activate the nuclear receptor, peroxisome proliferator activated receptor gamma (Pparγ). Although TZDs are proven to be very effective in promoting insulin sensitivity, its actions on Pparγ have been complicated, specifically on NAFLD. According to studies in different models, Pparγ manifests both beneficial and undesirable effects on NAFLD. This paper will focus on the current knowledge of Pparγ and its effect on NAFLD.

Blockade of the nuclear factor-κB pathway in the endothelium prevents insulin resistance and prolongs life spans

BACKGROUND

Nuclear factor-κB (NF-κB) signaling plays critical roles in physiological and pathological processes such as responses to inflammation and oxidative stress.

METHODS AND RESULTS

To examine the role of endothelial NF-κB signaling in vivo, we generated transgenic mice expressing dominant-negative IκB under the Tie2 promoter/enhancer (E-DNIκB mice). These mice exhibited functional inhibition of NF-κB signaling specifically in endothelial cells. Although E-DNIκB mice displayed no overt phenotypic changes when young and lean, they were protected from the development of insulin resistance associated with obesity, whether diet- or genetics-induced. Obesity-induced macrophage infiltration into adipose tissue and plasma oxidative stress markers were decreased and blood flow and mitochondrial content in muscle and active-phase locomotor activity were increased in E-DNIκB mice. In addition to inhibition of obesity-related metabolic deteriorations, blockade of endothelial NF-κB signaling prevented age-related insulin resistance and vascular senescence and, notably, prolonged life span. These antiaging phenotypes were also associated with decreased oxidative stress markers, increased muscle blood flow, enhanced active-phase locomotor activity, and aortic upregulation of mitochondrial sirtuin-related proteins.

CONCLUSIONS

The endothelium plays important roles in obesity- and age-related disorders through intracellular NF-κB signaling, thereby ultimately affecting life span. Endothelial NF-κB signaling is a potential target for treating the metabolic syndrome and for antiaging strategies.

Measurement of glucose homeostasis in vivo: combination of tracers and clamp techniques

A tracer technique referred to as "pancreatic-blood glucose clamp" allows assessment in response to a change in blood glucose, insulin, and/or glucagon of whole body glucose disposal, endogenous glucose production, specific tissue/organ glucose uptake and storage, and insulin secretion. This technique is currently considered the optimal method for measurement of insulin sensitivity and glucose effectiveness. We describe here, for use in conscious-unrestrained mice and rats, the pancreatic-blood glucose clamp technique and its associated methods; which include catheterization of blood vessels; a clamp of plasma insulin, glucagon, and glucose; analyses of metabolites and tracers; and calculations.

Impairment of central leptin-mediated PI3K signaling manifested as hepatic steatosis independent of hyperphagia and obesity

Hepatic steatosis is generally thought to develop via peripheral mechanisms associated with obesity. We show that chronic central infusion of leptin suppresses hepatic lipogenic gene expression and reduces triglyceride content via stimulation of hepatic sympathetic activity. This leptin function is independent of feeding and body weight but requires phosphatidylinositol 3-kinase (PI3K) signaling. Attenuation of leptin-induced PI3K signaling, brought about by transgenic expression of phosphatase and tensin homolog (PTEN) in leptin receptor neurons, leads to decreased hepatic sympathetic tone and increased triglyceride levels without affecting adiposity or hepatic insulin signaling. Central leptin's effects on hepatic norepinephrine levels and triglyceride content are blunted in these mutant mice. Simultaneous downregulation of PI3K and signal transducer and activator of transcription-3 (Stat3) in leptin receptor neurons does not exacerbate obesity but causes more severe hepatic steatosis. Together, our results indicate that central cellular leptin resistance in PI3K signaling manifests as hepatic steatosis without causing obesity.

Role of PPARg2 transcription factor in thiazolidinedione-induced insulin sensitization

OBJECTIVES

Adipose tissue is the key regulator of energy balance, playing an active role in lipid storage and metabolism and may be a dynamic buffer to control fatty acid flux. Peroxisome proliferator-activated receptor gamma isoform-2 (PPARg2), an isoform of the nuclear hormone receptor superfamily, has been implicated in almost all aspects of human metabolic alterations such as obesity, insulin resistance, type-2 diabetes and dyslipidaemia. The PPARg2 isoform is highly present in adipose tissue where it functions as a thrifty phenotype, which promotes adipocyte differentiation and triglyceride storage. Thiazolidinediones, antidiabetic drugs, induce insulin sensitivity by controlling adipokines. The thiazolidinediones bind with PPARg2 in adipocytes and exert an agonist effect by enhancing adipogenesis and fatty acid uptake. Thiazolidinediones stimulate PPARg2, by which they down-regulate tumour necrosis factor-α, leptin, interleukin-6 and plasminogen and also enhance insulin sensitivity. The aim of this work is to define role of PPARg2 transcription factor in thiazolidinedione-induced insulin sensitization.

KEY FINDINGS

The PPARg2 alters the transcription of the target gene. This altered gene transcription results in the up-regulation of insulin-sensitizing factors and down-regulation of insulin-resistant factors. The variant Pro12Ala of the PPARg2 gene is an important modulator in metabolic control in the body. Thiazolidinediones stimulate PPARg2 transcription factor by which PPARg2 binds to responsive elements located in the promoter regions of many genes and modulates their transcriptive activity. There is a strong mutual relationship between receptor binding and agonism, which is evidence of the insulin-sensitizing target of thiazolidinediones in PPARg2. This evidently increases the biological potency of the glucose-lowering effect of thiazolidinediones in vivo as well as their antidiabetic activity.

CONCLUSIONS

PPARg2 transcription factor plays an important role in treatment of type-2 diabetes with thiazolidindiones. The variant Pro12Ala of the PPARg2 gene promotes the activity of thiazolidinediones in minimizing insulin resistance. Transcriptional activity of Pro12Ala variant improves the activity of insulin. Thus thiazolidinediones promote the phosphorylation of PPARg2 to induce insulin sensitivity.

Inhibition of ubiquitin ligase F-box and WD repeat domain-containing 7α (Fbw7α) causes hepatosteatosis through Krüppel-like factor 5 (KLF5)/peroxisome proliferator-activated receptor γ2 (PPARγ2) pathway but not SREBP-1c protein in mice

F-box and WD repeat domain-containing 7α (Fbw7α) is the substrate recognition component of a ubiquitin ligase that controls the degradation of factors involved in cellular growth, including c-Myc, cyclin E, and c-Jun. In addition, Fbw7α degrades the nuclear form of sterol regulatory element-binding protein (SREBP)-1a, a global regulator of lipid synthesis, particularly during mitosis in cultured cells. This study investigated the in vivo role of Fbw7α in hepatic lipid metabolism. siRNA knockdown of Fbw7α in mice caused marked hepatosteatosis with the accumulation of triglycerides. However, inhibition of Fbw7α did not change the level of nuclear SREBP-1 protein or the expression of genes involved in fatty acid synthesis and oxidation. In vivo experiments on the gain and loss of Fbw7α function indicated that Fbw7α regulated the expression of peroxisome proliferator-activated receptor (PPAR) γ2 and its target genes involved in fatty acid uptake and triglyceride synthesis. These genes included fatty acid transporter Cd36, diacylglycerol acyltransferase 1 (Dgat1), and fat-specific protein 27 (Cidec). The regulation of PPARγ2 by Fbw7α was mediated, at least in part, by the direct degradation of the Krüppel-like factor 5 (KLF5) protein, upstream of PPARγ2 expression. Hepatic Fbw7α contributes to normal fatty acid and triglyceride metabolism, functions that represent novel aspects of this cell growth regulator.

Melanocortin 4 receptor-deficient mice as a novel mouse model of nonalcoholic steatohepatitis

Obesity may be viewed as a state of chronic low-grade inflammation that participates in the development of the metabolic syndrome. Nonalcoholic steatohepatitis (NASH) is considered a hepatic phenotype of the metabolic syndrome and a high risk for progression to cirrhosis and hepatocellular carcinoma. Although the "two hit" hypothesis suggests involvement of excessive hepatic lipid accumulation and chronic inflammation, the molecular mechanisms underlying the development of NASH remain unclear, in part because of lack of appropriate animal models. Herein we report that melanocortin 4 receptor-deficient mice (MC4R-KO) develop steatohepatitis when fed a high-fat diet, which is associated with obesity, insulin resistance, and dyslipidemia. Histologic analysis reveals inflammatory cell infiltration, hepatocyte ballooning, and pericellular fibrosis in the liver in MC4R-KO mice. Of note, all of the MC4R-KO mice examined developed well-differentiated hepatocellular carcinoma after being fed a high-fat diet for 1 year. They also demonstrated enhanced adipose tissue inflammation, ie, increased macrophage infiltration and fibrotic changes, which may contribute to excessive lipid accumulation and enhanced fibrosis in the liver. Thus, MC4R-KO mice provide a novel mouse model of NASH with which to investigate the sequence of events that make up diet-induced hepatic steatosis, liver fibrosis, and hepatocellular carcinoma and to aid in understanding the pathogenesis of NASH, pursuing specific biomarkers, and evaluating potential therapeutic strategies.

Hepatic peroxisome proliferator-activated receptor-γ-fat-specific protein 27 pathway contributes to obesity-related hypertension via afferent vagal signals

AIMS

Obesity is commonly associated with hypertension. Increased sympathetic tonus in obese subjects contributes to the underlying mechanism. However, the precise mechanisms whereby obesity induces this sympathetic activation remain unclear. Hepatic peroxisome proliferator-activated receptor (PPAR)-γ2 expression, which is reportedly upregulated during obesity development, affects sympathetic activation via hepatic vagal afferents. Herein, we report involvement of this neuronal relay in obesity-related hypertension.

METHODS AND RESULTS

Peroxisome proliferator-activated receptor-γ and a direct PPARγ target, fat-specific protein 27 (Fsp27), were adenovirally overexpressed or knocked down in the liver, in combination with surgical dissection or pharmacological deafferentation of the hepatic vagus. Adenoviral PPARγ2 expression in the liver raised blood pressure (BP) in wild-type but not in β1/β2/β3 adrenergic receptor-deficient mice. In addition, knockdown of endogenous PPARγ in the liver lowered BP in murine obesity models. Either surgical dissection or pharmacological deafferentation of the hepatic vagus markedly blunted BP elevation in mice with diet-induced and genetically-induced obesity. In contrast, BP was not elevated in other models of hepatic steatosis, DGAT1 and DGAT2 overexpressions, in which PPARγ is not upregulated in the liver. Thus, hepatic PPARγ upregulation associated with obesity is involved in BP elevation during obesity development. Furthermore, hepatic expression of Fsp27 raised BP and the effect was blocked by hepatic vagotomy. Hepatic Fsp27 is actually upregulated in murine obesity models and its knockdown reversed BP elevation.

CONCLUSION

The hepatic PPARγ-Fsp27 pathway plays important roles in the development of obesity-related hypertension via afferent vagal signals from the liver.

Association between coefficients of variation of the R-R intervals on electrocardiograms and post-challenge hyperglycemia in patients with newly diagnosed type 2 diabetes

The aim of the present study was to examine whether there is a relationship between autonomic function and post‐challenge hyperglycemia in patients with type 2 diabetes. Subjects included 122 Japanese patients newly diagnosed with type 2 diabetes. Autonomic nerve function was assessed using coefficients of variation of the R‐R intervals on electrocardiograms (CVRR). Unlike anthropometry, insulin secretion and insulin resistance, age (r = −0.209, P < 0.021) and post‐challenge plasma glucose at 120 min (PG120; r = −0.219, P < 0.015) were the only variables significantly correlated with CVRR. Age was not significantly correlated with PG120. In multiple regression analyses, CVRR Z‐score, but not age, was significantly correlated with PG120. The present results suggest that autonomic function affects post‐challenge blood glucose levels independently of age. (J Diabetes Invest,doi: 10.1111/j.2040‐1124.2010.00098.x, 2011)

The case for peripheral CB₁ receptor blockade in the treatment of visceral obesity and its cardiometabolic complications

In this review, we consider the role of endocannabinoids and cannabinoid-1 (CB(1)) cannabinoid receptors in metabolic regulation and as mediators of the thrifty phenotype that underlies the metabolic syndrome. We survey the actions of endocannabinoids on food intake and body weight, as well as on the metabolic complications of visceral obesity, including fatty liver, insulin resistance and dyslipidemias. Special emphasis is placed on weighing the relative importance of CB(1) receptors located in peripheral tissues versus the central nervous system in mediating the metabolic effects of endocannabinoids. Finally, we review recent observations that indicate that peripherally restricted CB(1) receptor antagonists retain efficacy in reducing weight and improving metabolic abnormalities in mouse models of obesity without causing behavioural effects predictive of neuropsychiatric side effects in humans.

Increased systemic glucose tolerance with increased muscle glucose uptake in transgenic mice overexpressing RXRγ in skeletal muscle

Background

Retinoid X receptor (RXR) γ is a nuclear receptor-type transcription factor expressed mostly in skeletal muscle, and regulated by nutritional conditions. Previously, we established transgenic mice overexpressing RXRγ in skeletal muscle (RXRγ mice), which showed lower blood glucose than the control mice. Here we investigated their glucose metabolism.

Methodology/Principal Findings

RXRγ mice were subjected to glucose and insulin tolerance tests, and glucose transporter expression levels, hyperinsulinemic-euglycemic clamp and glucose uptake were analyzed. Microarray and bioinformatics analyses were done. The glucose tolerance test revealed higher glucose disposal in RXRγ mice than in control mice, but insulin tolerance test revealed no difference in the insulin-induced hypoglycemic response. In the hyperinsulinemic-euglycemic clamp study, the basal glucose disposal rate was higher in RXRγ mice than in control mice, indicating an insulin-independent increase in glucose uptake. There was no difference in the rate of glucose infusion needed to maintain euglycemia (glucose infusion rate) between the RXRγ and control mice, which is consistent with the result of the insulin tolerance test. Skeletal muscle from RXRγ mice showed increased Glut1 expression, with increased glucose uptake, in an insulin-independent manner. Moreover, we performed in vivo luciferase reporter analysis using Glut1 promoter (Glut1-Luc). Combination of RXRγ and PPARδ resulted in an increase in Glut1-Luc activity in skeletal muscle in vivo. Microarray data showed that RXRγ overexpression increased a diverse set of genes, including glucose metabolism genes, whose promoter contained putative PPAR-binding motifs.

Conclusions/Significance

Systemic glucose metabolism was increased in transgenic mice overexpressing RXRγ. The enhanced glucose tolerance in RXRγ mice may be mediated at least in part by increased Glut1 in skeletal muscle. These results show the importance of skeletal muscle gene regulation in systemic glucose metabolism. Increasing RXRγ expression may be a novel therapeutic strategy against type 2 diabetes.

Cyclophilin D deficiency prevents diet-induced obesity in mice

Mitochondrial coupling efficiency is pivotal in thermogenesis and energy homeostasis. Here we show that deletion of cyclophilin D (CypD), a key modulator of the mitochondrial permeability transition pore, demonstrated resistance to diet-induced obesity (DIO) in both male and female mice, due to increased basal metabolic rate, heat production, total energy expenditure and expenditure of fat energy, despite increased food consumption. Absorption of fatty acids is not altered between CypD(-/-) and wild-type mice. Adult CypD(-/-) developed hyperglycemia, insulin resistance and glucose intolerance albeit resistant to DIO. These data demonstrate that inhibition of CypD function could protect from HFD-IO by increasing energy expenditure in both male and female mice. Inhibition of CypD may offer a novel target to modulate metabolism.

Interleukin-6 enhances glucose-stimulated insulin secretion from pancreatic beta-cells: potential involvement of the PLC-IP3-dependent pathway

OBJECTIVE

Interleukin-6 (IL-6) has a significant impact on glucose metabolism. However, the effects of IL-6 on insulin secretion from pancreatic β-cells are controversial. Therefore, we analyzed IL-6 effects on pancreatic β-cell functions both in vivo and in vitro.

RESEARCH DESIGN AND METHODS

First, to examine the effects of IL-6 on in vivo insulin secretion, we expressed IL-6 in the livers of mice using the adenoviral gene transfer system. In addition, using both MIN-6 cells, a murine β-cell line, and pancreatic islets isolated from mice, we analyzed the in vitro effects of IL-6 pretreatment on insulin secretion. Furthermore, using pharmacological inhibitors and small interfering RNAs, we studied the intracellular signaling pathway through which IL-6 may affect insulin secretion from MIN-6 cells.

RESULTS

Hepatic IL-6 expression raised circulating IL-6 and improved glucose tolerance due to enhancement of glucose stimulated-insulin secretion (GSIS). In addition, in both isolated pancreatic islets and MIN-6 cells, 24-h pretreatment with IL-6 significantly enhanced GSIS. Furthermore, pretreatment of MIN-6 cells with phospholipase C (PLC) inhibitors with different mechanisms of action, U-73122 and neomycin, and knockdowns of the IL-6 receptor and PLC-β(1), but not with a protein kinase A inhibitor, H-89, inhibited IL-6-induced enhancement of GSIS. An inositol triphosphate (IP(3)) receptor antagonist, Xestospondin C, also abrogated the GSIS enhancement induced by IL-6.

CONCLUSIONS

The results obtained from both in vivo and in vitro experiments strongly suggest that IL-6 acts directly on pancreatic β-cells and enhances GSIS. The PLC-IP(3)-dependent pathway is likely to be involved in IL-6-mediated enhancements of GSIS.

Endocannabinoids in liver disease

Endocannabinoids are lipid mediators of the same cannabinoid (CB) receptors that mediate the effects of marijuana. The endocannabinoid system (ECS) consists of CB receptors, endocannabinoids, and the enzymes involved in their biosynthesis and degradation, and it is present in both brain and peripheral tissues, including the liver. The hepatic ECS is activated in various liver diseases and contributes to the underlying pathologies. In patients with cirrhosis of various etiologies, the activation of vascular and cardiac CB(1) receptors by macrophage-derived and platelet-derived endocannabinoids contributes to the vasodilated state and cardiomyopathy, which can be reversed by CB(1) blockade. In mouse models of liver fibrosis, the activation of CB(1) receptors on hepatic stellate cells is fibrogenic, and CB(1) blockade slows the progression of fibrosis. Fatty liver induced by a high-fat diet or chronic alcohol feeding depends on the activation of peripheral receptors, including hepatic CB(1) receptors, which also contribute to insulin resistance and dyslipidemias. Although the documented therapeutic potential of CB(1) blockade is limited by neuropsychiatric side effects, these may be mitigated by using novel, peripherally restricted CB(1) antagonists.

Insulin resistance in tetraplegia but not in mid-thoracic paraplegia: is the mid-thoracic spinal cord involved in glucose regulation?

STUDY DESIGN

Controlled experimental human study.

OBJECTIVES

To assess insulin resistance (IR) in tetraplegia and paraplegia, and the role of the spinal cord (SC) in glucose regulation.

SETTING

Laboratory of Spinal Research, Loewenstein Rehabilitation Hospital.

METHODS

Glucose and insulin levels and the heart rate variation spectral components LF (low frequency), HF (high frequency) and LF/HF were studied at supine rest, head-up tilt and after a standard meal in three groups: 13 healthy subjects, 7 patients with T(4)-T(6) paraplegia and 11 patients with C(4)-C(7) tetraplegia.

RESULTS

Glucose and insulin increased significantly after the meal in all groups (P<0.001). Glucose increased significantly more in the tetraplegia than in the other groups (P<0.01). Increases in insulin level tended to accompany increases in LF/HF after the meal in the tetraplegia and control groups but not in the paraplegia group.

CONCLUSION

Post-prandial IR appears in C(4)-C(7) but not in T(4)-T(6) SC injury. The results of the study, combined with previously published findings, are consistent with the hypotheses that IR is related to activation of the sympathetic nervous system, and that below T(4) the mid-thoracic SC is involved in the regulation of glucose and insulin levels.

A physiological role for fat specific protein 27/cell death-inducing DFF45-like effector C in adipose and liver

Fat specific protein 27 (FSP27) was originally isolated by screen for genes specifically expressed in fully differentiated mouse adipocytes. FSP27 and cell death-inducing DFF45-like effector C (CIDEC), the human homologue of FSP27, belong to the CIDE family. The FSP27 in adipocytes was recently reported to be a lipid droplet (LD)-associated protein, that promotes the formation of unilocular LDs. An FSP27 knockout mouse demonstrated lean phenotypes with atrophic adipose tissue as a result of high-energy expenditure; this mouse line was also resistant to diet-induced obesity and insulin resistance. Interestingly, FSP27 was also expressed in the steatoic liver of a type II diabetes model mouse. The expression of FSP27 was markedly decreased in livers lacking the nuclear receptor peroxisome proliferator-activated receptor gamma. Forced expression of FSP27 in hepatocytes in vitro or in vivo led to an increase of LD through increased triglyceride levels. The current status of the physiological roles of FSP27/CIDEC in adipose tissue and liver are discussed along with its significance as a factor involved in the development of metabolic disorders.

Fatty liver and fibrosis in glycine N-methyltransferase knockout mice is prevented by nicotinamide

Deletion of glycine N-methyltransferase (GNMT), the main gene involved in liver S-adenosylmethionine (SAM) catabolism, leads to the hepatic accumulation of this molecule and the development of fatty liver and fibrosis in mice. To demonstrate that the excess of hepatic SAM is the main agent contributing to liver disease in GNMT knockout (KO) mice, we treated 1.5-month-old GNMT-KO mice for 6 weeks with nicotinamide (NAM), a substrate of the enzyme NAM N-methyltransferase. NAM administration markedly reduced hepatic SAM content, prevented DNA hypermethylation, and normalized the expression of critical genes involved in fatty acid metabolism, oxidative stress, inflammation, cell proliferation, and apoptosis. More importantly, NAM treatment prevented the development of fatty liver and fibrosis in GNMT-KO mice. Because GNMT expression is down-regulated in patients with cirrhosis, and because some subjects with GNMT mutations have spontaneous liver disease, the clinical implications of the present findings are obvious, at least with respect to these latter individuals. Because NAM has been used for many years to treat a broad spectrum of diseases (including pellagra and diabetes) without significant side effects, it should be considered in subjects with GNMT mutations.

CONCLUSION

The findings of this study indicate that the anomalous accumulation of SAM in GNMT-KO mice can be corrected by NAM treatment leading to the normalization of the expression of many genes involved in fatty acid metabolism, oxidative stress, inflammation, cell proliferation, and apoptosis, as well as reversion of the appearance of the pathologic phenotype.

Metabolic syndrome is a low-grade systemic inflammatory condition

An increase in proinflammatory cytokines, a decrease in endothelial nitric oxide and adiponectin levels and an alteration in hypothalamic peptides and gastrointestinal hormones that regulate satiety, hunger and food intake all occur in metabolic syndrome. Consumption of a diet that is energy dense and rich in saturated and trans-fats by pregnant women and lactating mothers, in childhood and adult life may trigger changes in the hypothalamic and gut peptides and hormones. Such changes modulate immune response and inflammation and lead to alterations in the hypothalamic 'bodyweight/appetite/satiety set point' and result in the initiation and development of the metabolic syndrome. Roux-en-gastric bypass induces weight loss, decreases the levels of cytokines and restores hypothalamic neuropeptides and gut hormones and the hypothalamic bodyweight/appetite/satiety set point to normal. Thus, metabolic syndrome is a low-grade systemic inflammatory condition with its origins in the perinatal period and childhood.

Alterations in skeletal muscle fatty acid handling predisposes middle-aged mice to diet-induced insulin resistance

OBJECTIVE

Although advanced age is a risk factor for type 2 diabetes, a clear understanding of the changes that occur during middle age that contribute to the development of skeletal muscle insulin resistance is currently lacking. Therefore, we sought to investigate how middle age impacts skeletal muscle fatty acid handling and to determine how this contributes to the development of diet-induced insulin resistance.

RESEARCH DESIGN AND METHODS

Whole-body and skeletal muscle insulin resistance were studied in young and middle-aged wild-type and CD36 knockout (KO) mice fed either a standard or a high-fat diet for 12 weeks. Molecular signaling pathways, intramuscular triglycerides accumulation, and targeted metabolomics of in vivo mitochondrial substrate flux were also analyzed in the skeletal muscle of mice of all ages.

RESULTS

Middle-aged mice fed a standard diet demonstrated an increase in intramuscular triglycerides without a concomitant increase in insulin resistance. However, middle-aged mice fed a high-fat diet were more susceptible to the development of insulin resistance-a condition that could be prevented by limiting skeletal muscle fatty acid transport and excessive lipid accumulation in middle-aged CD36 KO mice.

CONCLUSION

Our data provide insight into the mechanisms by which aging becomes a risk factor for the development of insulin resistance. Our data also demonstrate that limiting skeletal muscle fatty acid transport is an effective approach for delaying the development of age-associated insulin resistance and metabolic disease during exposure to a high-fat diet.

Neural relay from the liver induces proliferation of pancreatic beta cells: a path to regenerative medicine using the self-renewal capabilities

Systemic homeostasis requires coordinated metabolic regulation among multiple tissues/organs via inter-organ communication. We have reported that neuronal signaling plays important roles in this inter-organ metabolic communication. First, we found that liver-selective extracellular signal-regulated kinase (ERK) activation induces insulin hypersecretion and pancreatic beta cell proliferation. Denervation experiments revealed that these inter- organ (liver-to-pancreas) effects are mediated by a neural relay consisting of splanchnic afferents (from the liver) and vagal efferents (to the pancreas). The central nervous system also participates in this inter-organ communication. This neural relay system originating in the liver is physiologically involved in the anti-diabetes mechanism whereby, during obesity development, insulin hypersecretion and pancreatic beta cell hyperplasia occur in response to insulin resistance. This indicates the pathophysiological importance of this system in diabetes prevention and hyperinsulinemia development. Furthermore, when applied to mouse models of insulin-deficient diabetes, both type 1 and type 2, hepatic activation of ERK signaling increased pancreatic beta cell mass and normalized blood glucose. Thus, this inter-organ system may serve as a valuable therapeutic target for diabetes by regenerating pancreatic beta cells. The concept that manipulation of an endogenous mechanism can regenerate a damaged tissue in vivo may open a new paradigm for regenerative trreatments for degenerative disorders.

Impaired de novo choline synthesis explains why phosphatidylethanolamine N-methyltransferase-deficient mice are protected from diet-induced obesity

Phosphatidylcholine (PC) is synthesized from choline via the CDP-choline pathway. Liver cells can also synthesize PC via the sequential methylation of phosphatidylethanolamine, catalyzed by phosphatidylethanolamine N-methyltransferase (PEMT). The current study investigates whether or not hepatic PC biosynthesis is linked to diet-induced obesity. Pemt(+/+) mice fed a high fat diet for 10 weeks increased in body mass by 60% and displayed insulin resistance, whereas Pemt(-/-) mice did not. Compared with Pemt(+/+) mice, Pemt(-/-) mice had increased energy expenditure and maintained normal peripheral insulin sensitivity; however, they developed hepatomegaly and steatosis. In contrast, mice with impaired biosynthesis of PC via the CDP-choline pathway in liver became obese when fed a high fat diet. We, therefore, hypothesized that insufficient choline, rather than decreased hepatic phosphatidylcholine, was responsible for the lack of weight gain in Pemt(-/-) mice despite the presence of 1.3 g of choline/kg high fat diet. Supplementation with an additional 2.7 g of choline (but not betaine)/kg of diet normalized energy metabolism, weight gain, and insulin resistance in high fat diet-fed Pemt(-/-) mice. Furthermore, Pemt(+/+) mice that were fed a choline-deficient diet had increased oxygen consumption, had improved glucose tolerance, and gained less weight. Thus, de novo synthesis of choline via PEMT has a previously unappreciated role in regulating whole body energy metabolism.

What can bariatric surgery teach us about the pathophysiology of type 2 diabetes?

Bariatric surgery is indicated in cases of severe obesity. However, malabsorption-based techniques (gastric bypass and biliopancreatic diversion, both of which exclude the duodenum and jejunum from the alimentary circuit), but not restrictive techniques, can abolish type 2 diabetes within days of surgery, even before any significant weight loss has occurred. This means that calorie restriction alone cannot entirely account for this effect. In Goto-Kakizaki rats, a type 2 diabetes model, glycaemic equilibrium is improved by surgical exclusion of the proximal intestine, but deteriorates again when the proximal intestine is reconnected to the circuit in the same animals. This effect is independent of weight, suggesting that the intestine is itself involved in the immediate regulation of carbohydrate homoeostasis. In humans, the rapid improvement in carbohydrate homoeostasis observed after bypass surgery is secondary to an increase in insulin sensitivity rather than an increase in insulin secretion, which occurs later. Several mechanisms are involved--disappearance of hypertriglyceridaemia and decrease in levels of circulating fatty acids, disappearance of the mechanisms of lipotoxicity in the liver and skeletal muscle, and increases in secretion of GLP-1 and PYY--and may be intricately linked. In the medium term and in parallel with weight loss, a decrease in fatty tissue inflammation (which is also seen with restrictive techniques) may also be involved in metabolic improvement. Other mechanisms specific to malabsorption-based techniques (due to the required exclusion of part of the intestine), such as changes in the activity of digestive vagal afferents, changes in intestinal flora and stimulation of intestinal neoglucogenesis, also need to be studied in greater detail. The intestine is, thus, a key organ in the regulation of glycaemic equilibrium and may even be involved in the pathophysiology of type 2 diabetes.

Decrease of food intake in rats after ingestion of medium-chain triacylglycerol

Previous studies have demonstrated that fatty acid oxidation in the liver may affect food intake. This study examined the influence of preloading of medium-chain triacylglycerol (MCT) on food intake in comparison with long-chain triacylglycerol (LCT). Male rats were fasted for 18 h and then administered LCT or MCT emulsion orally. Each group of rats was allowed to rest for 30 min, and then food intake during 1 h was measured. Food intake in the MCT group was significantly lower than that in the LCT group. To examine the influence of hepatic oxidation, the MCT+MA group was injected intraperitoneally with mercaptoacetate (MA), an inhibitor of fatty acid oxidation, 2 h before ingestion of MCT emulsion. Then, 30 min after ingestion of LCT or MCT emulsion, food intake was measured for 1 h. Food intake in the MCT group was significantly lower than that in the LCT group, but there was no significant difference between the MCT+MA group and the LCT group. Food intake in the MCT+MA group was significantly higher than that in the MCT group. The hepatic ATP content after MCT ingestion was significantly higher than that after LCT ingestion, but there was no significant difference between the MCT+MA group and the LCT group. The hepatic ATP content after MCT+MA ingestion was significantly lower than that after MCT ingestion. These results suggest that ingestion of medium-chain fatty acid (MCFA) increases the liver ATP content in fasted rats, consequently decreasing food intake.

Diabetes-accelerated memory dysfunction via cerebrovascular inflammation and Abeta deposition in an Alzheimer mouse model with diabetes

Recent epidemiological studies suggest that diabetes mellitus is a strong risk factor for Alzheimer disease. However, the underlying mechanisms remain largely unknown. In this study, to investigate the pathophysiological interaction between these diseases, we generated animal models that reflect the pathologic conditions of both diseases. We crossed Alzheimer transgenic mice (APP23) with two types of diabetic mice (ob/ob and NSY mice), and analyzed their metabolic and brain pathology. The onset of diabetes exacerbated Alzheimer-like cognitive dysfunction without an increase in brain amyloid-beta burden in double-mutant (APP(+)-ob/ob) mice. Notably, APP(+)-ob/ob mice showed cerebrovascular inflammation and severe amyloid angiopathy. Conversely, the cross-bred mice showed an accelerated diabetic phenotype compared with ob/ob mice, suggesting that Alzheimer amyloid pathology could aggravate diabetes. Similarly, APP(+)-NSY fusion mice showed more severe glucose intolerance compared with diabetic NSY mice. Furthermore, high-fat diet feeding induced severe memory deficits in APP(+)-NSY mice without an increase in brain amyloid-beta load. Here, we created Alzheimer mouse models with early onset of cognitive dysfunction. Cerebrovascular changes and alteration in brain insulin signaling might play a pivotal role in this relationship. These findings could provide insights into this intensely debated association.

Molecular Characterization of the Tumor Suppressor Candidate 5 Gene: Regulation by PPARgamma and Identification of TUSC5 Coding Variants in Lean and Obese Humans

Tumor suppressor candidate 5 (TUSC5) is a gene expressed abundantly in white adipose tissue (WAT), brown adipose tissue (BAT), and peripheral afferent neurons. Strong adipocyte expression and increased expression following peroxisome proliferator activated receptor gamma (PPARgamma) agonist treatment of 3T3-L1 adipocytes suggested a role for Tusc5 in fat cell proliferation and/or metabolism. However, the regulation of Tusc5 in WAT and its potential association with obesity phenotypes remain unclear. We tested the hypothesis that the TUSC5 gene is a bona fide PPARgamma target and evaluated whether its WAT expression or single-nucleotide polymorphisms (SNPs) in the TUSC5 coding region are associated with human obesity. Induction of Tusc5 mRNA levels in 3T3-L1 adipocytes by troglitazone and GW1929 followed a dose-response consistent with these agents' binding affinities for PPARgamma. Chromatin immunoprecipitation (ChIP) experiments confirmed that PPARgamma protein binds a approximately -1.1 kb promotor sequence of murine TUSC5 transiently during 3T3-L1 adipogenesis, concurrent with histone H3 acetylation. No change in Tusc5 mRNA or protein levels was evident in type 2 diabetic patients treated with pioglitazone. Tusc5 expression was not induced appreciably in liver preparations overexpressing PPARs, suggesting that tissue-specific factors regulate PPARgamma responsiveness of the TUSC5 gene. Finally, we observed no differences in Tusc5 WAT expression or prevalence of coding region SNPs in lean versus obese human subjects. These studies firmly establish the murine TUSC5 gene locus as a PPARgamma target, but the significance of Tusc5 in obesity phenotypes or in the pharmacologic actions of PPARgamma agonists in humans remains equivocal.

Afferent mechanisms of sodium retention in cirrhosis and hepatorenal syndrome

Cirrhosis induces extra-cellular fluid volume expansion, which when the disease is advanced can be severe and poorly responsive to therapy. Prevention and/or effective therapy for cirrhotic edema requires understanding the stimulus that initiates and maintains sodium retention. Despite much study, this stimulus remains unknown. Work over the last several years has shown that signals originating in the liver can influence a variety of systemic functions, including extra-cellular fluid volume control. We review work on the afferent mechanisms triggering sodium retention in cirrhosis and suggest that the data are most consistent with the existence of a sensor in the hepatic circulation that contributes to normal extra-cellular fluid volume control (that is, a 'volume' sensor) and that in cirrhosis, the sensor is pathologically activated by the hepatic circulatory abnormalities caused by the disease. Detailed analysis of the hepatic circulation in normal conditions and cirrhosis is needed.

Molecular mechanism by which pioglitazone preserves pancreatic beta-cells in obese diabetic mice: evidence for acute and chronic actions as a PPARgamma agonist

Pioglitazone preserves pancreatic beta-cell morphology and function in diabetic animal models. In this study, we investigated the molecular mechanisms by which pioglitazone protects beta-cells in diabetic db/db mice. In addition to the morphological analysis of the islets, gene expression profiles of the pancreatic islet were analyzed using laser capture microdissection and were compared with real-time RT-PCR of db/db and nondiabetic m/m mice treated with or without pioglitazone for 2 wk or 2 days. Pioglitazone treatment (2 wk) ameliorated dysmetabolism, increased islet insulin content, restored glucose-stimulated insulin secretion, and preserved beta-cell mass in db/db mice but had no significant effects in m/m mice. Pioglitazone upregulated genes that promote cell differentiation/proliferation in diabetic and nondiabetic mice. In db/db mice, pioglitazone downregulated the apoptosis-promoting caspase-activated DNase gene and upregulated anti-apoptosis-related genes. The above-mentioned effects of pioglitazone treatment were also observed after 2 days of treatment. By contrast, the oxidative stress-promoting NADPH oxidase gene was downregulated, and antioxidative stress-related genes were upregulated, in db/db mice treated with pioglitazone for 2 wk, rather than 2 days. Morphometric results for proliferative cell number antigen and 4-hydroxy-2-noneal modified protein were consistent with the results of gene expression analysis. The present results strongly suggest that pioglitazone preserves beta-cell mass in diabetic mice mostly by two ways; directly, by acceleration of cell differentiation/proliferation and suppression of apoptosis (acute effect); and indirectly, by deceleration of oxidative stress because of amelioration of the underlying metabolic disorder (chronic effect).

The role of the autonomic nervous liver innervation in the control of energy metabolism

Despite a longstanding research interest ever since the early work by Claude Bernard, the functional significance of autonomic liver innervation, either sympathetic or parasympathetic, is still ill defined. This scarcity of information not only holds for the brain control of hepatic metabolism, but also for the metabolic sensing function of the liver and the way in which this metabolic information from the liver affects the brain. Clinical information from the bedside suggests that successful human liver transplantation (implying a complete autonomic liver denervation) causes no life threatening metabolic derangements, at least in the absence of severe metabolic challenges such as hypoglycemia. However, from the benchside, data are accumulating that interference with the neuronal brain-liver connection does cause pronounced changes in liver metabolism. This review provides an extensive overview on how metabolic information is sensed by the liver, and how this information is processed via neuronal pathways to the brain. With this information the brain controls liver metabolism and that of other organs and tissues. We will pay special attention to the hypothalamic pathways involved in these liver-brain-liver circuits. At this stage, we still do not know the final destination and processing of the metabolic information that is transferred from the liver to the brain. On the other hand, in recent years, there has been a considerable increase in the understanding which brain areas are involved in the control of liver metabolism via its autonomic innervation. However, in view of the ever rising prevalence of type 2 diabetes, this potentially highly relevant knowledge is still by far too limited. Thus the autonomic innervation of the liver and its role in the control of metabolism needs our continued and devoted attention.

The metabolic phenotype of SCD1-deficient mice is independent of melanin-concentrating hormone

We propose that deletion of pro-melanin-concentrating hormone (pMCH) would increase energy expenditure and further improve glucose tolerance in mice lacking stearoyl-coA desaturase-1 (SCD1). To test our hypothesis, we bred and metabolically challenged Pmch-/-; Scd1-/- double-knockout mice, with comparison to Pmch-/- mice; Scd1-/- mice and C57Bl/6J controls. Deletion of both Pmch and Scd1 increased both food intake and energy expenditure relative to control mice. Pmch-/-; Scd1-/- double-knockout mice had improved glucose tolerance relative to control mice. The majority of the metabolic effects were contributed by inactivation of the Scd1 gene. We conclude that the increased food intake and increased energy expenditure of Scd1-/- mice are independent of the neuropeptide melanin-concentrating hormone.

Role of central leptin signaling in renal macrophage infiltration

Monocytes/macrophages are key mediators of wound repair, tissue remodeling, and inflammation. However, the molecular mechanisms underlying macrophage recruitment to the site of inflammation is not fully understood. Leptin acts directly on the hypothalamus, thereby regulating food intake and energy expenditure. The leptin receptor, a single transmembrane protein that belongs to the gp130 family of cytokine receptor superfamily, is expressed not only in the hypothalamus but in a variety of peripheral tissues, suggesting the role of leptin as a pro-inflammatory adipocytokine in peripheral tissues. Here, we show that deficiency of leptin signaling reduces renal macrophage infiltration after unilateral ureteral obstruction (UUO). Bone marrow transplantation studies using leptin signaling-deficient db/db mice revealed that leptin signaling in bone marrow cells may not play a major role in the UUO-induced renal macrophage infiltration. Interestingly, central leptin administration reverses the otherwise reduced UUO-induced renal macrophage infiltration in leptin-deficient ob/ob mice. This is effectively abolished by central co-administration of SHU9119, a melanocortin-3 receptor/melanocortin-4 receptor antagonist. This study demonstrates that central leptin administration in ob/ ob mice accelerates renal macrophage infiltration through the melanocortin system, thereby suggesting that the central nervous system, which is inherent to integrate information from throughout the organism, is able to control peripheral inflammation.

Obesity: genes, brain, gut, and environment

Obesity, which is assuming alarming proportions, has been attributed to genetic factors, hypothalamic dysfunction, and intestinal gut bacteria and an increase in the consumption of energy-dense food. Obesity predisposes to the development of type 2 diabetes mellitus, hypertension, coronary heart disease, and certain forms of cancer. Recent studies have shown that the intestinal bacteria in obese humans and mice differ from those in lean that could trigger a low-grade systemic inflammation. Consumption of a calorie-dense diet that initiates and perpetuates obesity could be due to failure of homeostatic mechanisms that regulate appetite, food consumption, and energy balance. Hypothalamic factors that regulate energy needs of the body, control appetite and satiety, and gut bacteria that participate in food digestion play a critical role in the onset of obesity. Incretins, cholecystokinin, brain-derived neurotrophic factor, leptin, long-chain fatty acid coenzyme A, endocannabinoids and vagal neurotransmitter acetylcholine play a role in the regulation of energy intake, glucose homeostasis, insulin secretion, and pathobiology of obesity and type 2 diabetes mellitus. Thus, there is a cross-talk among the gut, liver, pancreas, adipose tissue, and hypothalamus. Based on these evidences, it is clear that management of obesity needs a multifactorial approach.

Peripheral oscillators: the driving force for food-anticipatory activity

Food-anticipatory activity (FAA) and especially the food-entrained oscillator (FEO) have driven many scientists to seek their mechanisms and locations. Starting our research on FAA we, possibly like many other scientists, were convinced that clock genes held the key to the location and the underlying mechanisms for FAA. In this review, which is aimed especially at discussing the contribution of the peripheral oscillators, we have put together the accumulating evidence that the clock gene machinery as we know it today is not sufficient to explain food entrainment. We discuss the contribution of three types of oscillating processes: (i) within the suprachiasmatic nucleus (SCN), neurons capable of maintaining a 24-h oscillation in electrical activity driven by a set of clock genes; (ii) oscillations in metabolic genes and clock genes in other parts of the brain and in peripheral organs driven by the SCN or by food, which damp out after a few cycles; (iii) an FEO which, we propose, is a system built up of different oscillatory processes and consisting of an as-yet-unidentified network of central and peripheral structures. In view of the evidence that clock genes and metabolic oscillations are not essential for the persistence of FAA we propose that food entrainment is initiated by a repeated metabolic state of scarcity that drives an oscillating network of brain nuclei in interaction with peripheral oscillators. This complex may constitute the proposed FEO and is distributed in our peripheral organs as well as within the central nervous system.

Sites and mechanisms of insulin resistance in nonobese, nondiabetic patients with chronic hepatitis C

Chronic hepatitis C (CHC) has been associated with type 2 diabetes and insulin resistance, but the extent of impairment in insulin action, the target pathways involved, and the role of the virus per se have not been defined. In this study, we performed a euglycemic hyperinsulinemic clamp (1 mU x minute(-1) x kg(-1)) coupled with infusion of tracers ([6,6-(2)H(2)]glucose, [(2)H(5)]glycerol) and indirect calorimetry in 14 patients with biopsy-proven CHC, who were selected not to have any features of the metabolic syndrome, and in seven healthy controls. We also measured liver expression of inflammatory cytokines/mediators and tested their association with the metabolic parameters. Compared to controls, in patients with CHC: (1) total glucose disposal (TGD) during the clamp was 25% lower (P = 0.003) due to impaired glucose oxidation (P = 0.0002), (2) basal endogenous glucose production (EGP) was 20% higher (P = 0.011) and its suppression during the clamp was markedly reduced (P = 0.007), and (3) glycerol appearance was not different in the basal state or during the clamp, but lipid oxidation was less suppressed by insulin (P = 0.004). Lipid oxidation was higher in patients with CHC who had more steatosis and was directly related to EGP, TGD, and glucose oxidation. The decreased insulin-stimulated suppression of EGP was associated with increased hepatic suppressor of cytokine signaling 3 (SOCS3; P < 0.05) and interleukin-18 (P < 0.05) expression.

CONCLUSION

Hepatitis C infection per se is associated with peripheral and hepatic insulin resistance. Substrate competition by increased lipid oxidation and possibly enhanced hepatic expression of inflammatory cytokines/mediators could be involved in the defective glucose regulation.

Application of serial analysis of gene expression to the study of human genetic disease

Sequence tag analysis using serial analysis of gene expression (SAGE) is a powerful strategy for the quantitative analysis of gene expression in human genetic disorders. SAGE facilitates the measurement of mRNA transcripts and generates a non-biased gene expression profile of normal and pathological disease tissue. In addition, the SAGE technique has the capacity of detecting the expression of novel transcripts allowing for the identification of previously uncharacterised genes, thus providing a unique advantage over the traditional microarray-based approach for expression profiling. The technique has been successful in providing pathological gene expression profiles in a number of common genetic disorders including diabetes, cardiovascular disease, Parkinson disease and Down syndrome. When combined with next generation sequencing platforms, SAGE has the potential to become a more powerful and sensitive technique making it more amenable for diagnostic use. This review will therefore discuss the application of SAGE to several common genetic disorders and will further evaluate its potential use in diagnosing human genetic disease.

Effects of nocturnal light on (clock) gene expression in peripheral organs: a role for the autonomic innervation of the liver

Background

The biological clock, located in the hypothalamic suprachiasmatic nucleus (SCN), controls the daily rhythms in physiology and behavior. Early studies demonstrated that light exposure not only affects the phase of the SCN but also the functional activity of peripheral organs. More recently it was shown that the same light stimulus induces immediate changes in clock gene expression in the pineal and adrenal, suggesting a role of peripheral clocks in the organ-specific output. In the present study, we further investigated the immediate effect of nocturnal light exposure on clock genes and metabolism-related genes in different organs of the rat. In addition, we investigated the role of the autonomic nervous system as a possible output pathway of the SCN to modify the activity of the liver after light exposure.

Methodology and Principal Findings

First, we demonstrated that light, applied at different circadian times, affects clock gene expression in a different manner, depending on the time of day and the organ. However, the changes in clock gene expression did not correlate in a consistent manner with those of the output genes (i.e., genes involved in the functional output of an organ). Then, by selectively removing the autonomic innervation to the liver, we demonstrated that light affects liver gene expression not only via the hormonal pathway but also via the autonomic input.

Conclusion

Nocturnal light immediately affects peripheral clock gene expression but without a clear correlation with organ-specific output genes, raising the question whether the peripheral clock plays a “decisive” role in the immediate (functional) response of an organ to nocturnal light exposure. Interestingly, the autonomic innervation of the liver is essential to transmit the light information from the SCN, indicating that the autonomic nervous system is an important gateway for the SCN to cause an immediate resetting of peripheral physiology after phase-shift inducing light exposures.

Visceral brain-body information transfer

Organisms interact with their environments through various afferent (sensory) and efferent (motor) mechanisms. While the usual environment of interest has been external to the organism, the internal environment is also of fundamental importance. This article briefly reviews many of the interactive mechanisms between the brain and the visceral environment, along with identification of relevant brain structures and linkages related to these peripheral functions (particularly the hypothalamus). Afferent and efferent neural (autonomic nervous system) and chemical (endocrine, immune, and blood-brain barrier and circumventricular organs) pathways are described, and potential unifying principles (emotion and, especially, homeostasis, including allostasis and stress) are identified. The importance of bidirectional (afferent, efferent) communication is emphasized. These systems of visceral brain-body information transfer are major connections between the central nervous system and the body through which and by which many psychosomatic processes occur.

Narrative review: evolving concepts in potassium homeostasis and hypokalemia

Humans are intermittently exposed to large variations in potassium intake, which range from periods of fasting to ingestion of potassium-rich meals. These fluctuations would abruptly alter plasma potassium concentration if not for rapid mechanisms, primarily in skeletal muscle and the liver, that buffer the changes in plasma potassium concentration by means of transcellular potassium redistribution and feedback control of renal potassium excretion. However, buffers have capacity limits, and even robust feedback control mechanisms require that the perturbation occur before feedback can initiate corrective action. In contrast, feedforward control mechanisms sense the effect of disturbances on the system's homeostasis. This review highlights recent experimental insights into the participation of feedback and feedforward control mechanisms in potassium homeostasis. New data make clear that feedforward homeostatic responses activate when decreased potassium intake is sensed, even when plasma potassium concentration is still within the normal range and before frank hypokalemia ensues, in addition to the classic feedback activation of renal potassium conservation when plasma potassium concentration decreases. Given the clinical importance of dyskalemias in patients, these novel experimental paradigms invite renewed clinical inquiry into this important area.

Role of the liver in glucose homeostasis in PI 3-kinase p85alpha-deficient mice

Phosphoinositide 3-kinase (PI3K) p85alpha-deficient mice exhibit hypoglycemia as a result of increased insulin sensitivity and glucose uptake in peripheral tissues. Although PI3K is central to the metabolic actions of insulin, its mechanism of action in liver is not well understood. In the present study, we investigated hepatic insulin signaling and glucose homeostasis in p85alpha-deficient and wild-type mice. In the livers of p85alpha-deficient mice, p50alpha played a compensatory role in insulin-stimulated PI3K activation by binding to insulin receptor substrate (IRS)-1/2. In p85alpha-deficient mice, the ratio of p50alpha over p110 catalytic subunit of PI3K in the liver was higher than in the muscles. PI3K activity associated with IRS-1/2 was not affected by the lack of p85alpha in the liver. Insulin-stimulated Akt and phosphatase and tensin homologue deleted on chromosome 10 (PTEN) activities in the liver were similar in p85alpha-deficient and wild-type mice. A hyperinsulinemic-euglycemic clamp study revealed that the glucose infusion rate and the rate of disappearance were higher in p85alpha-deficient mice than in wild-type mice but that endogenous glucose production tended to be higher in p85alpha-deficient mice than in wild-type mice. Consistent with this finding, the expression of glucose-6-phosphatase and phosphoenolpyruvate carboxykinase in livers after fasting was higher in p85alpha-deficient mice than in wild-type mice. After mice were fasted, the intrahepatic glucose-6-phosphate level was almost completely depleted in p85alpha-deficient mice. The glycogen content fell to nearly zero as a result of glycogenolysis shortly after the initiation of fasting in p85alpha-deficient mice. The absence of an increase in insulin-stimulated PI3K activation in the liver of p85alpha-deficient mice, unlike the muscles, may be associated with the molecular balance between the regulatory subunit and the catalytic subunit of PI3K. Gluconeogenesis was rather elevated in p85alpha-deficient mice, compared with in wild-type mice, and the liver seemed to partially compensate for the increase in glucose uptake in peripheral tissues.