Mechanisms by which sheep milk consumption ameliorates insulin resistance in high-fat diet-fed mice

Sheep milk is rich in fat, protein, vitamins and minerals and is also one of the most important sources of natural bioactives. Several biopeptides in sheep milk have been reported to possess antibacterial, antiviral and anti-inflammatory properties, and they may prevent type 2 diabetes (T2D), disease and cancer. However, the precise mechanism(s) underlying the protective role of sheep milk against T2D development remains unclear. Therefore, in the current study, we investigated the effect of sheep milk on insulin resistance and glucose intolerance in high-fat diet (HFD)-fed mice, by conducting intraperitoneal glucose tolerance tests, metabolic cage studies, genomic sequencing, polymerase chain reaction, and biochemical assays. Hyperinsulinemic-euglycemic clamp-based experiments revealed that mice consuming sheep milk exhibited lower hepatic glucose production than mice in the control group. These findings further elucidate the mechanism by which dietary supplementation with sheep milk alleviates HFD-induced systemic glucose intolerance.

Regulation of plasma glucose levels by central dopamine D2 receptors is impaired in type 1 but not type 2 diabetic mouse models

Glucose metabolism is reported to be regulated by the central nervous system, but it is unclear whether this regulation is altered in diabetes. We investigated whether regulation of glucose metabolism by central dopamine D2 receptors is altered in type 1 and type 2 diabetic models. Intracerebroventricular injections of both the dopamine D2 receptor agonist quinpirole and the antagonist l-sulpiride induced hyperglycemia in control mice, but not in streptozotocin (STZ)-induced diabetic mice, a type 1 diabetic model. Hyperglycemia induced by quinpirole or l-sulpiride was diminished following fasting and these drugs did not affect hyperglycemia in the pyruvate tolerance test. In addition, both quinpirole and l-sulpiride increased hepatic glucose-6-phosphatase (G6Pase) mRNA. In STZ-induced diabetic mice, dopamine and dopamine D2 receptor mRNA in the hypothalamus, which regulates glucose homeostasis, were decreased. Hepatic glycogen and G6Pase mRNA were also decreased in STZ-induced diabetic mice. Neither quinpirole nor l-sulpiride increased hepatic G6Pase mRNA in STZ-induced diabetic mice. In diet-induced obesity mice, a type 2 diabetic model, both quinpirole and l-sulpiride induced hyperglycemia, and hypothalamic dopamine and dopamine D2 receptor mRNA were not altered. These results indicate that (i) stimulation or blockade of dopamine D2 receptors causes hyperglycemia by increasing hepatic glycogenolysis, and (ii) stimulation or blockade of dopamine D2 receptors does not affect glucose levels in type 1 but does so in type 2 diabetic models. Moreover, hypothalamic dopaminergic function and hepatic glycogenolysis are decreased in the type 1 diabetic model, which reduces hyperglycemia induced by stimulation or blockade of dopamine D2 receptors.

Hepatic p38α MAPK controls gluconeogenesis via FOXO1 phosphorylation at S273 during glucagon signalling in mice

AIMS/HYPOTHESIS

Hyperglucagonaemia-stimulated hepatic glucose production (HGP) contributes to hyperglycaemia during type 2 diabetes. A better understanding of glucagon action is important to enable efficient therapies to be developed for the treatment of diabetes. Here, we aimed to investigate the role of p38 MAPK family members in glucagon-induced HGP and determine the underlying mechanisms by which p38 MAPK regulates glucagon action.

METHODS

p38α, β, γ and δ MAPK siRNAs were transfected into primary hepatocytes, followed by measurement of glucagon-induced HGP. Adeno-associated virus serotype 8 carrying p38α MAPK short hairpin RNA (shRNA) was injected into liver-specific Foxo1 knockout, liver-specific Irs1/Irs2 double knockout and Foxo1^S273D knockin mice. Foxo1^S273A knockin mice were fed a high-fat diet for 10 weeks. Pyruvate tolerance tests, glucose tolerance tests, glucagon tolerance tests and insulin tolerance tests were carried out in mice, liver gene expression profiles were analysed and serum triglyceride, insulin and cholesterol levels were measured. Phosphorylation of forkhead box protein O1 (FOXO1) by p38α MAPK in vitro was analysed by LC-MS.

RESULTS

We found that p38α MAPK, but not the other p38 isoforms, stimulates FOXO1-S273 phosphorylation and increases FOXO1 protein stability, promoting HGP in response to glucagon stimulation. In hepatocytes and mouse models, inhibition of p38α MAPK blocked FOXO1-S273 phosphorylation, decreased FOXO1 levels and significantly impaired glucagon- and fasting-induced HGP. However, the effect of p38α MAPK inhibition on HGP was abolished by FOXO1 deficiency or a Foxo1 point mutation at position 273 from serine to aspartic acid (Foxo1^S273D) in both hepatocytes and mice. Moreover, an alanine mutation at position 273 (Foxo1^S273A) decreased glucose production, improved glucose tolerance and increased insulin sensitivity in diet-induced obese mice. Finally, we found that glucagon activates p38α through exchange protein activated by cAMP 2 (EPAC2) signalling in hepatocytes.

CONCLUSIONS/INTERPRETATION

This study found that p38α MAPK stimulates FOXO1-S273 phosphorylation to mediate the action of glucagon on glucose homeostasis in both health and disease. The glucagon-induced EPAC2-p38α MAPK-pFOXO1-S273 signalling pathway is a potential therapeutic target for the treatment of type 2 diabetes.

Sanhuang xiexin decoction synergizes insulin/PI3K-Akt/FoxO signaling pathway to inhibit hepatic glucose production and alleviate T2DM

ETHNOPHARMACOLOGICAL RELEVANCE

Sanhuang Xiexin Decoction (SHXXD) is a classic prescription for the treatment of diabetes. Excessive hepatic glucose production (HGP) is a major determinant of the occurrence and development of diabetes. Inhibition of HGP can significantly improve type 2 diabetes mellitus (T2DM).

AIM OF THE STUDY

To investigate the mechanism by which SHXXD inhibits HGP.

MATERIALS AND METHODS

First, a mouse model of T2DM was established through high-fat diet (HFD) feeding combined with streptozotocin (STZ) injection to determine the pharmacodynamic effect of SHXXD in T2DM mice. Then, the possible pathways induced by SHXXD in the treatment of T2DM were predicted by network pharmacology combined with transcriptomics (including target prediction, network analysis and enrichment analysis). Finally, the specific mechanism of SHXXD was elucidated by in vitro experiments.

RESULTS

In vivo experiments showed that SHXXD reduced fasting blood glucose and alleviated weight loss in T2DM mice. Improved glucose clearance rates and insulin sensitivity improve dyslipidemia, liver tissue structural abnormalities and inflammatory cell infiltration as well as increase glycogen storage in T2DM mice. The results of network pharmacology and transcriptome analysis showed that SHXXD contained 378 compounds and 2625 targets. In total, 292 intersection targets were identified between the differentially expressed genes (DEGs) of the liver tissue insulin resistance (IR) related dataset GSE23343. KEGG enrichment analysis showed that the insulin/PI3K-Akt/FoxO signaling pathway may be related to SHXXD-mediated improvements in T2DM. In vitro experimental results showed that SHXXD increased glucose consumption by HepG2-IR cells and improved their insulin sensitivity. RT‒qPCR and Western blotting results showed that SHXXD inhibited hepatic gluconeogenesis through the insulin/PI3K-Akt/FoxO signaling pathway by promoting IGFIR, PIK3R1 and AKT2 expression and subsequently inhibiting PEPCK and FBP1 expression via phosphorylation of Foxo1. In addition, PI3K/Akt deactivated p-GSK3β through phosphorylation, thereby promoting GS expression and increasing glycogen synthesis.

CONCLUSIONS

SHXXD can target the liver to cooperate with the insulin/PI3K-Akt/FoxO signaling pathway to inhibit HGP to alleviate T2DM.

Pancreastatin inhibitor PSTi8 ameliorates streptozotocin-induced diabetes by suppressing hepatic glucose production

Elevated plasma glucose concentration, as a consequence of excessive hepatic glucose production, plays a pivotal role in the development of diabetes. A chromogranin A-derived diabetogenic peptide Pancreastatin (PST) enhances hepatic glucose output leading to diabetes. Therefore, here we probed the role of PSTi8, a PST inhibitor in ameliorating diabetes by investigating the effect of high glucose (HG) or PST on glucose metabolism. Further, we also explored the action mechanism of the underlying anti-hyperglycemic effect of PSTi8. PSTi8 treatment rescue cultured L6 and HepG2 cells from HG and PST-induced insulin resistance, respectively. It also enhances insulin receptor kinase activity by interacting with the insulin receptor and enhancing GLUT4 translocation and glucose uptake. Thus, our in-silico and in-vitro data support the PST-dependent and independent activity of PSTi8. Additionally, PSTi8 treatment in streptozotocin-induced diabetic rats improved glucose tolerance by lowering blood glucose and plasma PST levels. Concomitantly, the treated animals exhibited reduced hepatic glucose production accompanied by downregulation of hepatic gluconeogenic genes PEPCK and G6Pase. PSTi8-treated rats also exhibited enhanced hepatic glycogen in line with reduced plasma glucagon concentrations. Consistently, improved plasma insulin levels in PSTi8-treated rats enhanced skeletal muscle glucose disposal via enhanced P-Akt expression. In summary, these findings suggest PSTi8 has anti-hyperglycemic properties with enhanced skeletal muscle glucose disposal and reduced hepatic gluconeogenesis both PST dependent as well as independent.

HM-chromanone suppresses hepatic glucose production via activation of AMP-activated protein kinase in HepG2 cell

We investigated whether (E)-5-hydroxy-7-methoxy-3-(2-hydroxybenzyl)-4-chromanone (HM-chromanone) could suppress the transcription factors expression and enzymes involved in glucose production by activating AMPK in hepatocytes. HepG2 cells were treated with a medium containing HM-chromanone (5-100 μM), compound C (10 μM) and insulin (100 nM). Glucose production and glycogen synthesis assay were determined using a glucose assay kit and glycogen assay kit, respectively. Activities of AMP-activated protein kinase (AMPK), acetyl CoA carboxylase (ACC), cAMP response element-binding protein (CREB), PPAR coactivator-1α (PGC1α), CREB-regulated transcription coactivator 2 (CRTC2), Glycogen synthase kinase (GSK3β), Phosphoenolpyruvate carboxykinase (PEPCK), glycogen synthase (GS), Glucose 6-phosphatase (G6pase) and β-actin were determined by Western blot analysis. HM-chromanone significantly inhibited hepatic glucose production and increased glycogen synthesis by activating glycogen synthase. HM-chromanone induced the phosphorylation of CRTC2 and GSK-3β by phosphorylating AMPK in HepG2 cells, which was confirmed by compound C. Furthermore, it significantly decreased the phosphorylation of CREB in a time- and concentration-dependent manner, and the effect was reversed in the presence of compound C. Therefore, the complex formation of CRTC2 and CREB was inhibited. HM-chromanone inhibited the expression of PGC-1α, PEPCK, and G6Pase genes involved in production of hepatic glucose. The results showed that HM-chromanone activates AMPK in a time and concentration dependent manner, thus suppressing hepatic glucose production and increasing glycogen synthesis in HepG2 cells.

Prandial hepatic glucose production during hypoglycemia is altered after gastric bypass surgery and sleeve gastrectomy

AIMS/HYPOTHESIS

Roux-en Y gastric bypass surgery (GB) and sleeve gastrectomy (SG) alter prandial glucose metabolism, producing lower nadir glucose values and predisposing susceptible individuals to prandial hypoglycemia. The glycemic phenotype of GB or SG is associated with prandial hyperinsulinemia and hyperglucagonemia along with an increased influx of ingested glucose. Following insulin-induced hypoglycemia, glucagon is the most important stimulus for hepatic glucose production (HGP). It is unclear whether prandial hyperglucagonemia after GB or SG changes HGP under hyperinsulinemic hypoglycemia conditions. This study examined the hypothesis that prandial glucose production is reduced after GB and SG during hypoglycemia.

METHODS

Glucose kinetics and islet-cell and gut hormone secretion during hyperinsulinemic (120 mU.m^-2.min^-1) hypoglycemic clamp (~3.2 mM) were measured before and after mixed meal ingestion in 9 non-diabetic subjects with GB, 7 with SG, and 5 matched non-operated controls (CN).

RESULTS

Systemic appearance of ingested glucose was faster in GB compared to SG, and in SG compared to CN (p < 0.05). Subjects with GB and SG had greater plasma glucagon levels after eating (AUC_Glucagon) compared to CN (p < 0.05). But prandial HGP response during insulin-induced hypoglycemia (AUC_HGP) was smaller and shorter in duration in surgical groups (p < 0.05). In the absence of meal stimuli, however, glucose counterregulatory response to hypoglycemia was comparable among the 3 groups during hyperinsulinemic clamp.

CONCLUSION

After bariatric surgery, prandial glucose counterregulatory response to hypoglycemia is impaired. Considering post-meal hyperglucagonemia after GB or SG the blunted HGP response suggests a lower sensitivity of liver to glucagon that can predispose to hypoglycemia in this population.

Low-level lead exposure promotes hepatic gluconeogenesis and contributes to the elevation of fasting glucose level

BACKGROUND

Lead (Pb) is considered an endocrine-disrupting chemical. However, few studies have investigated the effects of low-level Pb exposure on plasma glucose levels. Herein, we aimed to investigate whether low-level Pb exposure causes elevated plasma glucose levels and the possible mechanisms involved.

METHODS

We conducted a cross-sectional study of 5747 participants from 16 sites in China. The participants underwent measurements of anthropometric factors, blood lead level (BLL) and fasting plasma glucose (FPG). Wistar rats were exposed to 0.05% Pb through drinking water or fed with a high-fat diet (HFD) for 28 weeks. The relevant parameters of glucose homeostasis, hepatic glucose production (HGP) and gene expression levels of hepatic gluconeogenesis enzymes, including phosphoenolpyruvate carboxy kinase (PEPCK), glucose-6-phosphatase (G6PC) and fructose-1,6-bisphosphatase (FBP1), were measured. In addition, gene expression levels of gluconeogenesis enzymes were also measured in HepG2 cells administered with different concentrations of lead acetate for 24 h.

RESULTS

In humans, after adjusting for confounders, the odds of having High_FPG (≥5.6 mmol/L) were significantly increased by 25% in the participants in the fourth BLL quartile (OR 1.25, 95% CI 1.05, 1.49). In the animals exposed to 0.05% Pb, FPG, HGP and hepatic gene expression levels of PEPCK, G6PC and FBP1 were increased. In addition, the mRNA expression levels of PEPCK, G6PC and FBP1 in HepG2 cells were also increased in response to Pb exposure.

CONCLUSIONS

These findings support the possibility that low-level Pb exposure may increase HGP by affecting key enzymes of hepatic gluconeogenesis, eventually resulting in impaired FPG and hyperglycemia.

FOXO1 inhibition synergizes with FGF21 to normalize glucose control in diabetic mice

OBJECTIVE

Forkhead box protein O1 (FOXO1) plays a key role in regulating hepatic glucose production, but investigations of FOXO1 inhibition as a potential therapeutic approach have been hampered by a lack of selective chemical inhibitors. By profiling structurally diverse FOXO1 inhibitors, the current study validates FOXO1 as a viable target for the treatment of diabetes.

METHODS

Using reporter gene assays, hepatocyte gene expression studies, and in vivo studies in mice, we profiled our leading tool compound 10 and a previously characterized FOXO1 inhibitor, AS1842856 (AS).

RESULTS

We show that AS has significant FOXO1-independent effects, as demonstrated by testing in FOXO1-deficient cell lines and animals, while compound 10 is highly selective for FOXO1 both in vitro and in vivo and fails to elicit any effect in genetic models of FOXO1 ablation. Chronic administration of compound 10 improved insulin sensitivity and glucose control in db/db mice without causing weight gain. Furthermore, chronic compound 10 treatment combined with FGF21 led to synergistic glucose lowering in lean, streptozotocin-induced diabetic mice.

CONCLUSIONS

We show that the widely used AS compound has substantial off-target activities and that compound 10 is a superior tool molecule for the investigation of FOXO1 function. In addition, we provide preclinical evidence that selective FOXO1 inhibition has potential therapeutic benefits for diabetes as a monotherapy or in combination with FGF21.

A bioinformatics and transcriptomics based investigation reveals an inhibitory role of Huanglian-Renshen-Decoction on hepatic glucose production of T2DM mice via PI3K/Akt/FoxO1 signaling pathway

BACKGROUND

Excessive hepatic glucose production (HGP) largely promotes the development of type 2 diabetes mellitus (T2DM), and the inhibition of HGP significantly ameliorates T2DM. Huanglian-Renshen-Decoction (HRD), a classic traditional Chinese herb medicine, is widely used for the treatment of diabetes in clinic for centuries and proved effective. However, the relevant mechanisms of HRD are not fully understood.

PURPOSE

Based on that, this study was designed to identify the potential effects and underlying mechanisms of HRD on HGP by a comprehensive investigation that integrated in vivo functional experiments, network pharmacology, molecular docking, transcriptomics and molecular biology.

METHODS

After confirming the therapeutic effects of HRD on T2DM mice, the inhibitory role of HRD on HGP was evaluated by pyruvate and glucagon tolerance tests, liver positron emission tomography (PET) imaging and the detection of gluconeogenic key enzymes. Then, network pharmacology and transcriptomics approaches were used to clarify the underlying mechanisms. Molecular biology, computational docking analysis and in vitro experiments were applied for final mechanism verification.

RESULTS

Here, our results showed that HRD can decrease weight gain and blood glucose, increase fasting insulin, glucose clearance and insulin sensitivity in T2DM mice. Dysregulated lipid profile was also corrected by HRD administration. Pyruvate, glucagon tolerance tests and liver PET imaging all indicated that HRD inhibited the abnormal HGP of T2DM, and the expressions of phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase) were significantly suppressed by HRD as expected. Network pharmacology and transcriptomics approaches illustrated that PI3K/Akt/FoxO1 signaling pathway may be responsible for the inhibitory effect of HRD on HGP. Afterward, further western blot and immunoprecipitation found that HRD did activate PI3K/Akt/FoxO1 signaling pathway in T2DM mice, which confirmed previous results. Additionally, the conclusion was further supported by molecular docking and in vitro experiments, in which identified HRD compound, oxyberberine, was proven to exert an obvious effect on Akt.

CONCLUSION

Our data demonstrated that HRD can treat T2DM by inhibiting hepatic glucose production, the underlying mechanisms were associated with the activation of PI3K/Akt/FoxO1 signaling pathway.

Interaction of glucose sensing and leptin action in the brain

Background

In response to energy abundant or deprived conditions, nutrients and hormones activate hypothalamic pathways to maintain energy and glucose homeostasis. The underlying CNS mechanisms, however, remain elusive in rodents and humans.

Scope of review

Here, we first discuss brain glucose sensing mechanisms in the presence of a rise or fall of plasma glucose levels, and highlight defects in hypothalamic glucose sensing disrupt in vivo glucose homeostasis in high-fat fed, obese, and/or diabetic conditions. Second, we discuss brain leptin signalling pathways that impact glucose homeostasis in glucose-deprived and excessed conditions, and propose that leptin enhances hypothalamic glucose sensing and restores glucose homeostasis in short-term high-fat fed and/or uncontrolled diabetic conditions.

Major conclusions

In conclusion, we believe basic studies that investigate the interaction of glucose sensing and leptin action in the brain will address the translational impact of hypothalamic glucose sensing in diabetes and obesity.

Telmisartan increases hepatic glucose production via protein kinase C ζ-dependent insulin receptor substrate-1 phosphorylation in HepG2 cells and mouse liver

Background

Dysregulation of hepatic glucose production (HGP) contributes to the development of type 2 diabetes mellitus. Telmisartan, an angiotensin II type 1 receptor blocker (ARB), has various ancillary effects in addition to common blood pressure-lowering effects. The effects and mechanism of telmisartan on HGP have not been fully elucidated and, therefore, we investigated these phenomena in hyperglycemic HepG2 cells and high-fat diet (HFD)-fed mice.

Methods

Glucose production and glucose uptake were measured in HepG2 cells. Expression levels of phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase α (G6Pase-α), and phosphorylation levels of insulin receptor substrate-1 (IRS-1) and protein kinase C ζ (PKCζ) were assessed by western blot analysis. Animal studies were performed using HFD-fed mice.

Results

Telmisartan dose-dependently increased HGP, and PEPCK expression was minimally increased at a 40 μM concentration without a change in G6Pase-α expression. In contrast, telmisartan increased phosphorylation of IRS-1 at Ser302 (p-IRS-1-Ser³⁰²) and decreased p-IRS-1-Tyr⁶³² dose-dependently. Telmisartan dose-dependently increased p-PKCζ-Thr⁴¹⁰ which is known to reduce insulin action by inducing IRS-1 serine phosphorylation. Ectopic expression of dominant-negative PKCζ significantly attenuated telmisartan-induced HGP and p-IRS-1-Ser³⁰² and -inhibited p-IRS-1-Tyr⁶³². Among ARBs, including losartan and fimasartan, only telmisartan changed IRS-1 phosphorylation and pretreatment with GW9662, a specific and irreversible peroxisome proliferator-activated receptor γ (PPARγ) antagonist, did not alter this effect. Finally, in the livers from HFD-fed mice, telmisartan increased p-IRS-1-Ser³⁰² and decreased p-IRS-1-Tyr⁶³², which was accompanied by an increase in p-PKCζ-Thr⁴¹⁰.

Conclusion

These results suggest that telmisartan increases HGP by inducing p-PKCζ-Thr⁴¹⁰ that increases p-IRS-1-Ser³⁰² and decreases p-IRS-1-Tyr⁶³² in a PPARγ-independent manner.

The Production of a Rat Model That Inhibits Phosphoenolpyruvate Carboxykinase (PEPCK), a Rate-Limiting Enzyme of Hepatic Gluconeogenesis

We previously showed that taurine administration contributed to the extension of time to exhaustion through exercise-induced hypoglycemia restraint, and we suggested that the activation of hepatic gluconeogenesis was initiated before the exercise with the taurine administration. We hypothesize that the extension effect of exercise duration with the taurine administration is restrained in the rats which inhibited hepatic gluconeogenesis. In this study, we aimed to produce a rat model that inhibited hepatic gluconeogenesis as a first step in testing our hypothesis.F344 male rats of 8 weeks after birth were purchased. The blood samples were collected via jugular vein catheter to perform the pyruvate tolerance test (PTT) with the intraperitoneal administration, and to determine the optimal time point of blood glucose measurement. 3-mercaptopicolinic acids (3MPA) was used as an inhibitor of PEPCK. The rats were divided into three groups, Non-dosage control (CON) group, 30 mg/kg・BW 3MPA (3MPA 30) group, and 300 mg/kg・BW 3MPA (3MPA 300) group.The blood glucose level showed a significant peak 15 min after pyruvate administration. The change of the blood glucose level after the PTT in 3MPA 300 group was significantly smaller than that of the CON group at this time point. These results show we could prepare the rat model that inhibited hepatic gluconeogenesis.

Decrease of FGF19 contributes to the increase of fasting glucose in human in an insulin-independent manner

PURPOSE

The ileum-derived fibroblast growth factor 19 (FGF19) plays key roles in hepatic glucose homeostasis in animals in an insulin-independent manner. Here, we analyzed the association of FGF19 with glucose effectiveness (GE, the insulin-independent glucose regulation), as well as hepatic glucose production (HGP) in Chinese subjects.

METHODS

GE was measured by frequently sampled intravenous glucose tolerance test (FSIVGTT) in normal glucose tolerance (NGT), isolated-impaired glucose tolerance (I-IGT), and isolated-impaired fasting glucose (I-IFG) subjects. The oral glucose tolerance test-derived surrogate of GE (oGE) was determined in NGT, I-IFG, combined glucose intolerance (CGI), and type 2 diabetes (T2DM) subjects. HGP was assessed by labeled ([3-³H]-glucose) hyperinsulinemic-euglycemic clamp in NGT subjects. Insulin secretion and sensitivity were calculated by the hyperglycemic and hyperinsulinemic-euglycemic clamps in a subgroup of NGT, I-IGT, and I-IFG subjects. Serum FGF19 levels were determined by ELISA.

RESULTS

FGF19 positively correlated with GE (r = 0.29, P = 0.004) as determined by FSIVGTT. The result was further confirmed by oGE (r = 0.261, P < 0.001). FGF19 was negatively associated with FPG (r = - 0.228, P = 0.025), but the association no longer existed after adjusting for GE (r = - 0.177, P = 0.086). FGF19 was negatively associated with basal HGP (r = - 0.697, P = 0.006). However, the correlation between FGF19 and insulin secretion and sensitivity were not found.

CONCLUSIONS

FGF19 levels are associated positively with GE and negatively with HGP. The increase of FPG in human is at least partially due to the decrease of FGF19 in an insulin-independent manner.

Roles for gut vagal sensory signals in determining energy availability and energy expenditure

The gut sensory vagus transmits a wide range of meal-related mechanical, chemical and gut peptide signals from gastrointestinal and hepatic tissues to the central nervous system at the level of the caudal brainstem. Results from studies using neurophysiological, behavioral physiological and metabolic approaches that challenge the integrity of this gut-brain axis support an important role for these gut signals in the negative feedback control of energy availability by limiting food intake during a meal. These experimental approaches have now been applied to identify important and unanticipated contributions of the vagal sensory gut-brain axis to the control of two additional effectors of overall energy balance: the feedback control of endogenous energy availability through hepatic glucose production and metabolism, and the control of energy expenditure through brown adipose tissue thermogenesis. Taken together, these studies reveal the pleiotropic influences of gut vagal meal-related signals on energy balance, and encourage experimental efforts aimed at understanding how the brainstem represents, organizes and coordinates gut vagal sensory signals with these three determinants of energy homeostasis.

Impact of skeletal muscle IL-6 on regulation of liver and adipose tissue metabolism during fasting

The liver and adipose tissue are important tissues in whole-body metabolic regulation during fasting. Interleukin 6 (IL-6) is a cytokine shown to be secreted from contracting muscle in humans and suggested to signal to the liver and adipose tissue. Furthermore, skeletal muscle IL-6 has been proposed to play a role during fasting. Therefore the aim of the present study was to investigate the role of skeletal muscle IL-6 in the regulation of substrate production in the liver and adipose tissue during fasting. Male skeletal muscle-specific IL-6 knockout (IL-6 MKO) mice and littermate floxed (control) mice fasted for 6 or 18 h (6 h fasting or 18 h fasting) with corresponding fed control groups (6 h fed or 18 h fed) and liver and adipose tissue were quickly obtained. Plasma β-hydroxybutyrate increased and hepatic glucose, lactate and glycogen decreased with fasting. In addition, fasting increased phosphoenolpyruvate carboxykinase protein and phosphorylation of pyruvate dehydrogenase (PDH) in the liver as well as hormone-sensitive lipase (HSL)^Ser660 and HSL^Ser563 phosphorylation, PDH phosphorylation, adipose triglyceride lipase phosphorylation and perilipin phosphorylation and protein content in adipose tissue without any effect of lack of skeletal muscle IL-6. In conclusion, fasting induced regulation of enzymes in adipose tissue lipolysis and glyceroneogenesis as well as regulation of hepatic gluconeogenic capacity and hepatic substrate utilization in mice. However, skeletal muscle IL-6 was not required for these fasting-induced effects, but had minor effects on markers of lipolysis and glyceroneogenesis in adipose tissue as well as markers of hepatic gluconeogenesis in the fed state.

Deletion of hepatic carbohydrate response element binding protein (ChREBP) impairs glucose homeostasis and hepatic insulin sensitivity in mice

OBJECTIVE

Carbohydrate response element binding protein (ChREBP) is a transcription factor that responds to glucose and activates genes involved in the glycolytic and lipogenic pathways. Recent studies have linked adipose ChREBP to insulin sensitivity in mice. However, while ChREBP is most highly expressed in the liver, the effect of hepatic ChREBP on insulin sensitivity remains unknown. To clarify the importance of hepatic ChREBP on glucose homeostasis, we have generated a knockout mouse model that lacks this protein specifically in the liver (Liver-ChREBP KO).

METHODS

Using Liver-ChREBP KO mice, we investigated whether hepatic ChREBP deletion influences insulin sensitivity, glucose homeostasis and the development of hepatic steatosis utilizing various dietary stressors. Furthermore, we determined gene expression changes in response to fasted and fed states in liver, white, and brown adipose tissues.

RESULTS

Liver-ChREBP KO mice had impaired insulin sensitivity as indicated by reduced glucose infusion to maintain euglycemia during hyperinsulinemic-euglycemic clamps on both chow (25% lower) and high-fat diet (33% lower) (p < 0.05). This corresponded with attenuated suppression of hepatic glucose production. Although Liver-ChREBP KO mice were protected against carbohydrate-induced hepatic steatosis, they displayed worsened glucose tolerance. Liver-ChREBP KO mice did not show the expected gene expression changes in liver in response to fasted and fed states. Interestingly, hepatic ChREBP deletion also resulted in gene expression changes in white and brown adipose tissues, suggesting inter-tissue communication. This included an almost complete abolition of BAT ChREBPβ induction in the fed state (0.15-fold) (p = 0.015) along with reduced lipogenic genes. In contrast, WAT showed inappropriate increases in lipogenic genes in the fasted state along with increased PEPCK1 in both fasted (3.4-fold) and fed (5.1-fold) states (p < 0.0001).

CONCLUSIONS

Overall, hepatic ChREBP is protective in regards to hepatic insulin sensitivity and whole body glucose homeostasis. Hepatic ChREBP action can influence other peripheral tissues and is likely essential in coordinating the body's response to different feeding states.

Glucagon, a key factor in the pathophysiology of type 2 diabetes

Excessive circulating glucagon levels have been reported in all forms of diabetes, clinical or experimental. The hyperglucagonemia of diabetes results from an excessive secretion of the hormone secondary from a deficit in insulin secretion and/or a dysfunction of various cells within the islets of Langerhans (somatostatin) leading to the notion of "paracrinopathy". Hyperglucagonemia contributes to the fasting and postprandial hyperglycemia in diabetic patients through an increased hepatic glucose production (mainly gluconeogenesis). The aim of the present review is to summarize the clinical and experimental arguments suggesting that glucagon is essential for the development of glucose dysregulation in diabetes.

Cannabinoid CB1 receptor inverse agonist MJ08 stimulates glucose production via hepatic sympathetic innervation in rats

As a key insulin target tissue for maintaining systemic glucose homeostasis, the liver plays important roles in improving obesity-associated insulin intolerance via selective cannabinoid CB₁ receptor antagonism/inverse agonism. However, it is unclear whether this receptor inverse agonism affects hepatic glucose metabolism. MJ08 is a novel cannabinoid CB₁ receptor antagonist/inverse agonist that has superior inverse agonism over the well-known antagonist/inverse agonist, SR141716 (rimonabant). MJ08 remarkably elevates fasting blood glucose independent of inhibition of insulin release in mice. In the current study, MJ08 was used to investigate the mechanism by which liver cannabinoid CB₁ receptor inverse activation regulates hepatic glucose metabolism. MJ08 stimulated hepatic glucose production (HGP) in a dose-dependent manner and promoted gluconeogenic gene expression in perfused rat liver. SR141716 exhibited similar but weaker effects. The cannabinoid CB₁ receptor agonist (WIN 55,212-2), Gs protein-cyclic AMP (cAMP)-dependent pathway inhibitors (NF449 and H89), β-adrenoceptor antagonist (propranolol), and peripheral sympathetic inhibitor (reserpine) could antagonize MJ08-induced HGP. Furthermore, MJ08 and SR141716 induced monoamine neurotransmitter (noradrenaline) release and increased cAMP content significantly in perfused liver, although only a slight increase was observed in primary cultured hepatocytes. These results indicate that local liver cannabinoid CB₁ receptor inverse agonism via hepatic sympathetic innervation is responsible for the HGP induced by MJ08. Thus, high inverse agonistic activity could increase fasting blood glucose levels and should be avoided in the development of peripheral cannabinoid CB₁ receptor-targeted weight-loss drugs.

Decreased basal hepatic glucose uptake in impaired fasting glucose

AIMS/HYPOTHESIS

This research aimed to define the pathophysiological defects responsible for the elevated fasting plasma glucose (FPG) concentration and excessive rise in post-load plasma glucose observed in individuals with impaired fasting glucose (IFG).

METHODS

We used tracer techniques to quantify basal splanchnic (primarily hepatic) glucose uptake and glucose fluxes following glucose ingestion in individuals with normal glucose tolerance (NGT; n = 10) and IFG (n = 10).

RESULTS

Individuals with IFG had a comparable basal rate of hepatic glucose production to those with NGT (15.2 ± 0.2 vs 18.0 ± 0.8 μmol min^-1 [kg lean body mass (LBM)]^-1; p = 0.09). However, they had a significantly reduced glucose clearance rate during the fasting state compared with NGT (2.64 ± 0.11 vs 3.62 ± 0.20 ml min^-1 [kg LBM]^-1; p < 0.01). The difference between the basal rate of glucose appearance measured with [3-³H]glucose and [1-¹⁴C]glucose, which represent basal splanchnic glucose uptake, was significantly reduced in IFG compared with NGT (1.39 ± 0.28 vs 3.16 ± 0.44 μmol min^-1 [kg LBM]^-1; p = 0.02). Following glucose ingestion, the total amount of exogenous glucose that appeared in the systemic circulation was not significantly different between groups. However, suppression of endogenous glucose production (EGP) was markedly impaired in individuals with IFG.

CONCLUSIONS/INTERPRETATION

These results demonstrate that decreased tissue (liver) glucose uptake, not enhanced EGP, is the cause for elevated FPG concentration in individuals with IFG, while the excessive rise in plasma glucose concentration following a glucose load in these individuals is the result of impaired suppression of hepatic glucose production.

Lupinus albus Conglutin Gamma Modifies the Gene Expressions of Enzymes Involved in Glucose Hepatic Production In Vivo

Lupinus albus seeds contain conglutin gamma (Cγ) protein, which exerts a hypoglycemic effect and positively modifies proteins involved in glucose homeostasis. Cγ could potentially be used to manage patients with impaired glucose metabolism, but there remains a need to evaluate its effects on hepatic glucose production. The present study aimed to analyze G6pc, Fbp1, and Pck1 gene expressions in two experimental animal models of impaired glucose metabolism. We also evaluated hepatic and renal tissue integrity following Cγ treatment. To generate an insulin resistance model, male Wistar rats were provided 30% sucrose solution ad libitum for 20 weeks. To generate a type 2 diabetes model (STZ), five-day-old rats were intraperitoneally injected with streptozotocin (150 mg/kg). Each animal model was randomized into three subgroups that received the following oral treatments daily for one week: 0.9% w/v NaCl (vehicle; IR-Ctrl and STZ-Ctrl); metformin 300 mg/kg (IR-Met and STZ-Met); and Cγ 150 mg/kg (IR-Cγ and STZ-Cγ). Biochemical parameters were assessed pre- and post-treatment using colorimetric or enzymatic methods. We also performed histological analysis of hepatic and renal tissue. G6pc, Fbp1, and Pck1 gene expressions were quantified using real-time PCR. No histological changes were observed in any group. Post-treatment G6pc gene expression was decreased in the IR-Cγ and STZ-Cγ groups. Post-treatment Fbp1 and Pck1 gene expressions were reduced in the IR-Cγ group but increased in STZ-Cγ animals. Overall, these findings suggest that Cγ is involved in reducing hepatic glucose production, mainly through G6pc inhibition in impaired glucose metabolism disorders.

The suppression of hepatic glucose production improves metabolism and insulin sensitivity in subcutaneous adipose tissue in mice

AIMS/HYPOTHESIS

Despite the strong correlation between non-alcoholic fatty liver disease and insulin resistance, hepatic steatosis is associated with greater whole-body insulin sensitivity in several models. We previously reported that the inhibition of hepatic glucose production (HGP) protects against the development of obesity and diabetes despite severe steatosis, thanks to the secretion of specific hepatokines such as fibroblast growth factor 21 (FGF21) and angiopoietin-related growth factor. In this work, we focused on adipose tissue to assess whether liver metabolic fluxes might, by interorgan communication, control insulin signalling in lean animals.

METHODS

Insulin signalling was studied in the adipose tissue of mice lacking the catalytic subunit of glucose 6-phosphatase, the key enzyme in endogenous glucose production, in the liver (L-G6pc ^-/- mice). Morphological and metabolic changes in the adipose tissues were characterised by histological analyses, gene expression and protein content.

RESULTS

Mice lacking HGP exhibited improved insulin sensitivity of the phosphoinositide 3-kinase/Akt pathway in the subcutaneous adipose tissue associated with a browning of adipocytes. The suppression of HGP increased FGF21 levels in lean animals, and increased FGF21 was responsible for the metabolic changes in the subcutaneous adipose tissue but not for its greater insulin sensitivity. The latter might be linked to an increase in the ratio of monounsaturated to saturated fatty acids released by the liver.

CONCLUSIONS

Our work provides evidence that HGP controls subcutaneous adipose tissue browning and insulin sensitivity through two pathways: the release of beneficial hepatokines and changes in hepatic fatty acids profile.

The role of leptin in diabetes: metabolic effects

While it is well established that the adiposity hormone leptin plays a key role in the regulation of energy homeostasis, growing evidence suggests that leptin is also critical for glycaemic control. In this review we examine the role of the brain in the glucose-lowering actions of leptin and the potential mediators responsible for driving hyperglycaemia in states of uncontrolled insulin-deficient diabetes (uDM). These considerations highlight the possibility of targeting leptin-sensitive pathways as a therapeutic option for the treatment of diabetes. This review summarises a presentation given at the 'Is leptin coming back?' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Christoffer Clemmensen and colleagues, DOI: 10.1007/s00125-016-3906-7 , and by Gerald Shulman and colleagues, DOI: 10.1007/s00125-016-3909-4 ) and an overview by the Session Chair, Ulf Smith (DOI: 10.1007/s00125-016-3894-7 ).

The PARsylation activity of tankyrase in adipose tissue modulates systemic glucose metabolism in mice

AIMS/HYPOTHESIS

Tankyrase (TNKS) is a ubiquitously expressed molecular scaffold that is implicated in diverse processes. The catalytic activity of TNKS modifies substrate proteins through poly-ADP-ribosylation (PARsylation) and is responsive to cellular energetic state. Global deficiency of the TNKS protein in mice accelerates glucose utilisation and raises plasma adiponectin levels. The aim of this study was to investigate whether the PARsylation activity of TNKS in adipocytes plays a role in systemic glucose homeostasis.

METHODS

To inhibit TNKS-mediated PARsylation, we fed mice with a diet containing the TNKS-specific inhibitor G007-LK. To genetically inactivate TNKS catalysis in adipocytes while preserving its function as a molecular scaffold, we used an adipocyte-selective Cre transgene to delete TNKS exons that encoded the catalytic domain at the C-terminus. Tissue-specific insulin sensitivity in mice was investigated using hyperinsulinaemic-euglycaemic clamps. To model adipose-liver crosstalk ex vivo, we applied adipocyte-conditioned media to hepatocytes and assessed the effect on gluconeogenesis.

RESULTS

The TNKS inhibitor G007-LK improved glucose tolerance and insulin sensitivity and promptly increased plasma adiponectin levels. In female mice, but not in male mice, adipocyte-selective genetic inactivation of TNKS catalysis improved hepatic insulin sensitivity and post-transcriptionally increased plasma adiponectin levels. Both pharmacological and genetic TNKS inhibition in female mouse-derived adipocytes induced a change in secreted factors to decrease gluconeogenesis in primary hepatocytes.

CONCLUSIONS/INTERPRETATION

Systemic glucose homeostasis is regulated by the PARsylation activity of TNKS in adipocytes. This regulation is mediated in part by adipocyte-secreted factors that modulate hepatic glucose production. Pharmacological TNKS inhibition could potentially be used to improve glucose tolerance.

Pharmacological inhibition of S6K1 increases glucose metabolism and Akt signalling in vitro and in diet-induced obese mice

AIMS/HYPOTHESIS

The mammalian target of rapamycin complex 1 (mTORC1)/p70 ribosomal S6 kinase (S6K)1 pathway is overactivated in obesity, leading to inhibition of phosphoinositide 3-kinase (PI3K)/Akt signalling and insulin resistance. However, chronic mTORC1 inhibition by rapamycin impairs glucose homeostasis because of robust induction of liver gluconeogenesis. Here, we compared the effect of rapamycin with that of the selective S6K1 inhibitor, PF-4708671, on glucose metabolism in vitro and in vivo.

METHODS

We used L6 myocytes and FAO hepatocytes to explore the effect of PF-4708671 on the regulation of glucose uptake, glucose production and insulin signalling. We also treated high-fat (HF)-fed obese mice for 7 days with PF-4708671 in comparison with rapamycin to assess glucose tolerance, insulin resistance and insulin signalling in vivo.

RESULTS

Chronic rapamycin treatment induced insulin resistance and impaired glucose metabolism in hepatic and muscle cells. Conversely, chronic S6K1 inhibition with PF-4708671 reduced glucose production in hepatocytes and enhanced glucose uptake in myocytes. Whereas rapamycin treatment inhibited Akt phosphorylation, PF-4708671 increased Akt phosphorylation in both cell lines. These opposite effects of the mTORC1 and S6K1 inhibitors were also observed in vivo. Indeed, while rapamycin treatment induced glucose intolerance and failed to improve Akt phosphorylation in liver and muscle of HF-fed mice, PF-4708671 treatment improved glucose tolerance and increased Akt phosphorylation in metabolic tissues of these obese mice.

CONCLUSIONS/INTERPRETATION

Chronic S6K1 inhibition by PF-4708671 improves glucose homeostasis in obese mice through enhanced Akt activation in liver and muscle. Our results suggest that specific S6K1 blockade is a valid pharmacological approach to improve glucose disposal in obese diabetic individuals.

The new biology of diabetes

Until recently, type 2 diabetes was seen as a disease caused by an impaired ability of insulin to promote the uptake and utilisation of glucose. Work on forkhead box protein O (FOXO) transcription factors revealed new aspects of insulin action that have led us to articulate a liver- and beta cell-centric narrative of diabetes pathophysiology and treatment. FOXO integrate a surprisingly diverse subset of biological functions to promote metabolic flexibility. In the liver, they controls the glucokinase/glucose-6-phosphatase switch and bile acid pool composition, directing carbons to glucose or lipid utilisation, thus providing a unifying mechanism for the two abnormalities of the diabetic liver: excessive glucose production and increased lipid synthesis and secretion. Moreover, FOXO are necessary to maintain beta cell differentiation, and diabetes development is associated with a gradual loss of FOXO function that brings about beta cell dedifferentiation. We proposed that dedifferentiation is the main cause of beta cell failure and conversion into non-beta endocrine cells, and that treatment should restore beta cell differentiation. Our studies investigating these proposals have revealed new dimensions to the pathophysiology of diabetes that can be leveraged to design new therapies.

Mechanisms through which a small protein and lipid preload improves glucose tolerance

AIMS/HYPOTHESIS

Small protein or lipid preloads are able to improve glucose tolerance to a different extent and through different and poorly defined mechanisms. We aimed at quantifying the effect of a mixed protein and lipid preload and at evaluating the underlying mechanisms.

METHODS

Volunteers with normal (NGT, n = 12) or impaired (IGT, n = 13) glucose tolerance and patients with type 2 diabetes (n = 10) underwent two OGTTs coupled to the double glucose tracer protocol, preceded by either 50 g of parmesan cheese, a boiled egg and 300 ml of water, or 500 ml of water. We measured plasma glucose, insulin, C-peptide, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP), pancreatic polypeptide (PP), NEFA and glucose tracers, and calculated glucose fluxes, beta cell function variables, insulin sensitivity and clearance.

RESULTS

After the nutrient preload, the OGTT-induced rise of plasma glucose was lower than after water alone in each study group. This reduction—more pronounced across classes of glucose tolerance (NGT -32%, IGT -37%, type 2 diabetes -49%; p < 0.002)—was the result of different combinations of slower exogenous glucose rate of appearance, improved beta cell function and reduced insulin clearance, in this order of relevance, which were associated with an only mild stimulation of GIP and GLP-1.

CONCLUSIONS/INTERPRETATION

After a non-glucidic nutrient preload, glucose tolerance improved in proportion to the degree of its baseline deterioration through mechanisms that appear particularly effective in type 2 diabetes. Exploiting the physiological responses to nutrient ingestion might reveal, at least in the first stages of the diabetic disease, a potent tool to improve daily life glycaemic control.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02342834 FUNDING: This work was supported by grants from the University of Pisa (Fondi di Ateneo) and by FCT grant (PIC/IC/82956/2007).

Regulation of hepatic TRB3/Akt interaction induced by physical exercise and its effect on the hepatic glucose production in an insulin resistance state

To maintain euglycemia in healthy organisms, hepatic glucose production is increased during fasting and decreased during the postprandial period. This whole process is supported by insulin levels. These responses are associated with the insulin signaling pathway and the reduction in the activity of key gluconeogenic enzymes, resulting in a decrease of hepatic glucose production. On the other hand, defects in the liver insulin signaling pathway might promote inadequate suppression of gluconeogenesis, leading to hyperglycemia during fasting and after meals. The hepatocyte nuclear factor 4, the transcription cofactor PGC1-α, and the transcription factor Foxo1 have fundamental roles in regulating gluconeogenesis. The loss of insulin action is associated with the production of pro-inflammatory biomolecules in obesity conditions. Among the molecular mechanisms involved, we emphasize in this review the participation of TRB3 protein (a mammalian homolog of Drosophila tribbles), which is able to inhibit Akt activity and, thereby, maintain Foxo1 activity in the nucleus of hepatocytes, inducing hyperglycemia. In contrast, physical exercise has been shown as an important tool to reduce insulin resistance in the liver by reducing the inflammatory process, including the inhibition of TRB3 and, therefore, suppressing gluconeogenesis. The understanding of these new mechanisms by which physical exercise regulates glucose homeostasis has critical importance for the understanding and prevention of diabetes.

The Polycomb protein, Bmi1, regulates insulin sensitivity

Objective

The Polycomb Repressive Complexes (PRC) 1 and 2 function to epigenetically repress target genes. The PRC1 component, Bmi1, plays a crucial role in maintenance of glucose homeostasis and beta cell mass through repression of the Ink4a/Arf locus. Here we have explored the role of Bmi1 in regulating glucose homeostasis in the adult animal, which had not been previously reported due to poor postnatal survival of Bmi1^−/− mice.

Methods

The metabolic phenotype of Bmi1^+/− mice was characterized, both in vivo and ex vivo. Glucose and insulin tolerance tests and hyperinsulinemic-euglycemic clamps were performed. The insulin signaling pathway was assessed at the protein and transcript level.

Results

Here we report a negative correlation between Bmi1 levels and insulin sensitivity in two models of insulin resistance, aging and liver-specific insulin receptor deficiency. Further, heterozygous loss of Bmi1 results in increased insulin sensitivity in adult mice, with no impact on body weight or composition. Hyperinsulinemic-euglycemic clamp reveals increased suppression of hepatic glucose production and increased glucose disposal rate, indicating elevated glucose uptake to peripheral tissues, in Bmi1^+/− mice. Enhancement of insulin signaling, specifically an increase in Akt phosphorylation, in liver and, to a lesser extent, in muscle appears to contribute to this phenotype.

Conclusions

Together, these data define a new role for Bmi1 in regulating insulin sensitivity via enhancement of Akt phosphorylation.

Tumor necrosis factor alpha abolished the suppressive effect of insulin on hepatic glucose production and glycogenolysis stimulated by cAMP

BACKGROUND

Tumor necrosis factor alpha (TNFα) is implicated in the development of insulin resistance in obesity, type 2 diabetes and cancer. However, its ability to modulate the action of insulin on glycogen catabolism in the liver is controversial. The aim of the present study was to investigate whether TNFα acutely affects the suppression by insulin of hepatic glucose production (HGP) and glycogenolysis stimulated by cyclic adenosine monophosphate (cAMP).

METHODS

TNFα (10 μg/kg) was injected intravenously to rats and, 1 or 6h later, their livers were subjected to in situ perfusion with cAMP (3 μM), in the presence or absence of physiological (20 μU/mL) or supraphysiological (500 μU/mL) concentrations of insulin.

RESULTS

The injection of TNFα, 1 or 6h before liver perfusion, had no direct effect on the action of cAMP in stimulating HGP and glycogenolysis. However, when TNFα was injected 1h, but not 6h, before liver perfusion it completely abolished (p<0.05) the suppressive effect of 20 μU/mL insulin on HGP and glycogenolysis stimulated by cAMP. Furthermore, the injection of TNFα 1h or 6h before liver perfusion did not influence the suppression of cAMP-stimulated HGP and glycogenolysis by 500 μU/mL insulin.

CONCLUSION

TNFα acutely abolished the suppressive effect of physiological, but not supraphysiological, levels of insulin on HGP and glycogenolysis stimulated by cAMP, suggesting an important role of this mechanism to the increased HGP in several pathological states.

The effect of LXRα, ChREBP and Elovl6 in liver and white adipose tissue on medium- and long-chain fatty acid diet-induced insulin resistance

AIMS

We aimed to investigate the effects of LXRα, ChREBP and Elovl6 in the development of insulin resistance-induced by medium- and long-chain fatty acids.

METHODS

Sprague Dawley rats were fed a standard chow diet (Control group) or a high-fat, high sucrose diet with different fat sources (coconut oil, lard, sunflower and fish oil) for 8 weeks. These oils were rich in medium-chain saturated fatty acids (MCFA group), long-chain saturated fatty acids (LCFA group), n-6 and n-3 long-chain polyunsaturated fatty acids (n-6 PUFA and n-3 PUFA groups), respectively, which had different chain lengths and degrees of unsaturation. Hyperinsulinemic-euglycemic clamp with [6-(3)H] glucose infusion was performed in conscious rats to assess hepatic insulin sensitivity.

RESULTS

LCFA and n-6 PUFA groups induced hepatic insulin resistance and increased liver X receptor α (LXRα), carbohydrate response element binding protein (ChREBP) and long-chain fatty acid elongase 6 (Elovl6) expression in liver and white adipose tissue (WAT). Furthermore, LCFA and n-6 PUFA groups suppressed Akt serine 473 phosphorylation in liver and WAT. By contrast, in liver and WAT, MCFA and n-3 PUFA groups decreased LXRα, ChREBP and Elovl6 expression and improved insulin signaling and insulin resistance, but Akt serine 473 phosphorylation was not restored by MCFA group in WAT.

CONCLUSIONS

This study demonstrated that the mechanism of the different effects of medium- and long-chain fatty acids on hepatic insulin resistance involves LXRα, ChREBP and Elovl6 alternations in liver and WAT. It points to a new strategy for ameliorating insulin resistance and diabetes through intervention on Elovl6 or its control genes.