Glucose-6-phosphate-mediated activation of liver glycogen synthase plays a key role in hepatic glycogen synthesis

Calebin A attenuated inflammation in RAW264.7 macrophages and adipose tissue to improve hepatic glucose metabolism and hyperglycemia in high-fat diet-fed obese mice

The increased incidence of obesity, which become a global health problem, requires more functional food products with minor side and excellent effects. Calebin A (CbA) is a non-curcuminoid compound, which is reported to be an effective treatment for lipid metabolism and thermogenesis. However, its ability and mechanism of action in improving obesity-associated hyperglycemia remain unclear. This study was designed to explore the effect and mechanism of CbA in hyperglycemia via improvement of inflammation and glucose metabolism in the adipose tissue and liver in high-fat diet (HFD)-fed mice. After 10 weeks fed HFD, obese mice supplemented with CbA (25 and 100 mg/kg) for another 10 weeks showed a remarkable reducing adiposity and blood glucose. CbA modulated M1/M2 macrophage polarization, ameliorated inflammatory cytokines, and restored adiponectin as well as Glut 4 expression in the adipose tissue. In the in vitro study, CbA attenuated pro-inflammatory markers while upregulated anti-inflammatory IL-10 in LPS + IFNγ-generated M1 phenotype macrophages. In the liver, CbA attenuated steatosis, inflammatory infiltration, and protein levels of inflammatory TNF-α and IL-6. Moreover, CbA markedly upregulated Adiponectin receptor 1, AMPK, and insulin downstream Akt signaling to improve glycogen content and increase Glut2 protein. These findings indicated that CbA may be a novel therapeutic approach to treat obesity and hyperglycemia phenotype targeting on adipose inflammation and hepatic insulin signaling.

Duration of Morning Hyperinsulinemia is Key to the Enhancement of Hepatic Glucose Uptake and Glycogen Storage Later in the Day

The second meal phenomenon refers to the improvement in glucose tolerance seen following a second identical meal. We previously showed that 4 hours of morning (AM) hyperinsulinemia, but not hyperglycemia, enhanced hepatic glucose uptake (HGU) and glycogen storage during an afternoon (PM) hyperinsulinemic hyperglycemic clamp (HIHG). Our current aim was to determine if the duration or pattern of morning hyperinsulinemia is important for the PM response to a HIHG clamp. To determine this, we administered the same total amount of insulin either over 2h in the first half of the morning (Ins2h-A), over 2h in the 2nd half of the morning (Ins2h-B), or over the entire 4h (Ins4h) of the morning. In the 4h PM period, all three groups had 4x basal insulin, 2x basal glycemia, and portal glucose infusion to simulate a meal. During the PM clamp, there was a marked increase in the mean hepatic glucose uptake and hepatic glycogen synthesis in the Ins4h group compared to the Ins2h-A and Ins2h-B groups, despite matched hepatic glucose and insulin loads. Thus, the longer duration (Ins4h) of mild hyperinsulinemia in the morning seems to be the key to much greater liver glucose uptake during the PM clamp.

Inflammatory liver diseases and susceptibility to sepsis

Patients with inflammatory liver diseases, particularly alcohol-associated liver disease and metabolic dysfunction-associated fatty liver disease (MAFLD), have higher incidence of infections and mortality rate due to sepsis. The current focus in the development of drugs for MAFLD is the resolution of non-alcoholic steatohepatitis and prevention of progression to cirrhosis. In patients with cirrhosis or alcoholic hepatitis, sepsis is a major cause of death. As the metabolic center and a key immune tissue, liver is the guardian, modifier, and target of sepsis. Septic patients with liver dysfunction have the highest mortality rate compared with other organ dysfunctions. In addition to maintaining metabolic homeostasis, the liver produces and secretes hepatokines and acute phase proteins (APPs) essential in tissue protection, immunomodulation, and coagulation. Inflammatory liver diseases cause profound metabolic disorder and impairment of energy metabolism, liver regeneration, and production/secretion of APPs and hepatokines. Herein, the author reviews the roles of (1) disorders in the metabolism of glucose, fatty acids, ketone bodies, and amino acids as well as the clearance of ammonia and lactate in the pathogenesis of inflammatory liver diseases and sepsis; (2) cytokines/chemokines in inflammatory liver diseases and sepsis; (3) APPs and hepatokines in the protection against tissue injury and infections; and (4) major nuclear receptors/signaling pathways underlying the metabolic disorders and tissue injuries as well as the major drug targets for inflammatory liver diseases and sepsis. Approaches that focus on the liver dysfunction and regeneration will not only treat inflammatory liver diseases but also prevent the development of severe infections and sepsis.

Integration of metabolic flux with hepatic glucagon signaling and gene expression profiles in the conscious dog

Glucagon rapidly and profoundly stimulates hepatic glucose production (HGP), but for reasons that are unclear, this effect normally wanes after a few hours, despite sustained plasma glucagon levels. This study characterized the time course of glucagon-mediated molecular events and their relevance to metabolic flux in the livers of conscious dogs. Glucagon was either infused into the hepato-portal vein at a sixfold basal rate in the presence of somatostatin and basal insulin, or it was maintained at a basal level in control studies. In one control group, glucose remained at basal, whereas in the other, glucose was infused to match the hyperglycemia that occurred in the hyperglucagonemic group. Elevated glucagon caused a rapid (30 min) and largely sustained increase in hepatic cAMP over 4 h, a continued elevation in glucose-6-phosphate (G6P), and activation and deactivation of glycogen phosphorylase and synthase activities, respectively. Net hepatic glycogenolysis increased rapidly, peaking at 15 min due to activation of the cAMP/PKA pathway, then slowly returned to baseline over the next 3 h in line with allosteric inhibition by glucose and G6P. Glucagon's stimulatory effect on HGP was sustained relative to the hyperglycemic control group due to continued PKA activation. Hepatic gluconeogenic flux did not increase due to the lack of glucagon's effect on substrate supply to the liver. Global gene expression profiling highlighted glucagon-regulated activation of genes involved in cellular respiration, metabolic processes, and signaling, as well as downregulation of genes involved in extracellular matrix assembly and development.NEW & NOTEWORTHY Glucagon rapidly stimulates hepatic glucose production, but these effects are transient. This study links the molecular and metabolic flux changes that occur in the liver over time in response to a rise in glucagon, demonstrating the strength of the dog as a translational model to couple findings in small animals and humans. In addition, this study clarifies why the rapid effects of glucagon on liver glycogen metabolism are not sustained.

The Role of Acyl-CoA Synthetase 1 in Bioactive Lipid Accumulation and the Development of Hepatic Insulin Resistance

The liver plays a crucial role in glucose metabolism. Obesity and a diet rich in fats (HFD) contribute to the accumulation of intracellular lipids. The aim of the study was to explore the involvement of acyl-CoA synthetase 1 (ACSL1) in bioactive lipid accumulation and the induction of liver insulin resistance (InsR) in animals fed an HFD. The experiments were performed on male C57BL/6 mice divided into the following experimental groups: 1. Animals fed a control diet; 2. animals fed HFD; and 3. HFD-fed animals with the hepatic ACSL1 gene silenced through a hydrodynamic gene delivery technique. Long-chain acyl-CoAs, sphingolipids, and diacylglycerols were measured by LC/MS/MS. Glycogen was measured by means of a commercially available kit. The protein expression and phosphorylation state of the insulin pathway was estimated by Western blot. HFD-fed mice developed InsR, manifested as an increase in fasting blood glucose levels (202.5 mg/dL vs. 130.5 mg/dL in the control group) and inhibition of the insulin pathway, which resulted in an increase in the rate of gluconeogenesis (0.420 vs. 0.208 in the control group) and a decrease in the hepatic glycogen content (1.17 μg/mg vs. 2.32 μg/mg in the control group). Hepatic ACSL1 silencing resulted in decreased lipid content and improved insulin sensitivity, accounting for the decreased rate of gluconeogenesis (0.348 vs. 0.420 in HFD(+/+)) and the increased glycogen content (4.3 μg/mg vs. 1.17 μg/mg in HFD(+/+)). The elevation of gluconeogenesis and the decrease in glycogenesis in the hepatic tissue of HFD-fed mice resulted from cellular lipid accumulation. Inhibition of lipid synthesis through silencing ACSL1 alleviated HFD-induced hepatic InsR.

Trans-omic analysis reveals opposite metabolic dysregulation between feeding and fasting in liver associated with obesity

Dysregulation of liver metabolism associated with obesity during feeding and fasting leads to the breakdown of metabolic homeostasis. However, the underlying mechanism remains unknown. Here, we measured multi-omics data in the liver of wild-type and leptin-deficient obese (ob/ob) mice at ad libitum feeding and constructed a differential regulatory trans-omic network of metabolic reactions. We compared the trans-omic network at feeding with that at 16 h fasting constructed in our previous study. Intermediate metabolites in glycolytic and nucleotide metabolism decreased in ob/ob mice at feeding but increased at fasting. Allosteric regulation reversely shifted between feeding and fasting, generally showing activation at feeding while inhibition at fasting in ob/ob mice. Transcriptional regulation was similar between feeding and fasting, generally showing inhibiting transcription factor regulations and activating enzyme protein regulations in ob/ob mice. The opposite metabolic dysregulation between feeding and fasting characterizes breakdown of metabolic homeostasis associated with obesity.

mTORC1 controls murine postprandial hepatic glycogen synthesis via Ppp1r3b

In response to a meal, insulin drives hepatic glycogen synthesis to help regulate systemic glucose homeostasis. The mechanistic target of rapamycin complex 1 (mTORC1) is a well-established insulin target and contributes to the postprandial control of liver lipid metabolism, autophagy, and protein synthesis. However, its role in hepatic glucose metabolism is less understood. Here, we used metabolomics, isotope tracing, and mouse genetics to define a role for liver mTORC1 signaling in the control of postprandial glycolytic intermediates and glycogen deposition. We show that mTORC1 is required for glycogen synthase activity and glycogenesis. Mechanistically, hepatic mTORC1 activity promotes the feeding-dependent induction of Ppp1r3b, a gene encoding a phosphatase important for glycogen synthase activity whose polymorphisms are linked to human diabetes. Reexpression of Ppp1r3b in livers lacking mTORC1 signaling enhances glycogen synthase activity and restores postprandial glycogen content. mTORC1-dependent transcriptional control of Ppp1r3b is facilitated by FOXO1, a well characterized transcriptional regulator involved in the hepatic response to nutrient intake. Collectively, we identify a role for mTORC1 signaling in the transcriptional regulation of Ppp1r3b and the subsequent induction of postprandial hepatic glycogen synthesis.

Hexokinase-linked glycolytic overload and unscheduled glycolysis in hyperglycemia-induced pathogenesis of insulin resistance, beta-cell glucotoxicity, and diabetic vascular complications

Hyperglycemia is a risk factor for the development of insulin resistance, beta-cell glucotoxicity, and vascular complications of diabetes. We propose the hypothesis, hexokinase-linked glycolytic overload and unscheduled glycolysis, in explanation. Hexokinases (HKs) catalyze the first step of glucose metabolism. Increased flux of glucose metabolism through glycolysis gated by HKs, when occurring without concomitant increased activity of glycolytic enzymes-unscheduled glycolysis-produces increased levels of glycolytic intermediates with overspill into effector pathways of cell dysfunction and pathogenesis. HK1 is saturated with glucose in euglycemia and, where it is the major HK, provides for basal glycolytic flux without glycolytic overload. HK2 has similar saturation characteristics, except that, in persistent hyperglycemia, it is stabilized to proteolysis by high intracellular glucose concentration, increasing HK activity and initiating glycolytic overload and unscheduled glycolysis. This drives the development of vascular complications of diabetes. Similar HK2-linked unscheduled glycolysis in skeletal muscle and adipose tissue in impaired fasting glucose drives the development of peripheral insulin resistance. Glucokinase (GCK or HK4)-linked glycolytic overload and unscheduled glycolysis occurs in persistent hyperglycemia in hepatocytes and beta-cells, contributing to hepatic insulin resistance and beta-cell glucotoxicity, leading to the development of type 2 diabetes. Downstream effector pathways of HK-linked unscheduled glycolysis are mitochondrial dysfunction and increased reactive oxygen species (ROS) formation; activation of hexosamine, protein kinase c, and dicarbonyl stress pathways; and increased Mlx/Mondo A signaling. Mitochondrial dysfunction and increased ROS was proposed as the initiator of metabolic dysfunction in hyperglycemia, but it is rather one of the multiple downstream effector pathways. Correction of HK2 dysregulation is proposed as a novel therapeutic target. Pharmacotherapy addressing it corrected insulin resistance in overweight and obese subjects in clinical trial. Overall, the damaging effects of hyperglycemia are a consequence of HK-gated increased flux of glucose metabolism without increased glycolytic enzyme activities to accommodate it.

Revving the engine: PKB/AKT as a key regulator of cellular glucose metabolism

Glucose metabolism is of critical importance for cell growth and proliferation, the disorders of which have been widely implicated in cancer progression. Glucose uptake is achieved differently by normal cells and cancer cells. Even in an aerobic environment, cancer cells tend to undergo metabolism through glycolysis rather than the oxidative phosphorylation pathway. Disordered metabolic syndrome is characterized by elevated levels of metabolites that can cause changes in the tumor microenvironment, thereby promoting tumor recurrence and metastasis. The activation of glycolysis-related proteins and transcription factors is involved in the regulation of cellular glucose metabolism. Changes in glucose metabolism activity are closely related to activation of protein kinase B (PKB/AKT). This review discusses recent findings on the regulation of glucose metabolism by AKT in tumors. Furthermore, the review summarizes the potential importance of AKT in the regulation of each process throughout glucose metabolism to provide a theoretical basis for AKT as a target for cancers.

Effects of monochromatic light on hepatic glycogen and lipid synthesis in broilers

Animal growth is closely related to glycolipid metabolism, and the liver is the main organ for glycogen storage and fat synthesis in birds, but whether monochromatic light affects glycogen and lipid synthesis in the liver is unclear. Therefore, in this study, a total of 96 Arbor Acre (AA) broilers at posthatching d 0 (P0) were raised under 4 kinds of light-emitting diode (LED) lights, white light (WL), red light (RL), green light (GL), and blue light (BL), to posthatching d 21 (P21) and 35 (P35). The results showed that the liver, abdominal fat, and abdominal fat indices gradually increased with increasing age under monochromatic light treatments. The liver glycogen and triglyceride (TG) contents also showed an increasing trend. Furthermore, compared with those at P21, the mRNA levels of glycogen synthase (GS), glycogen synthase kinase-3β (GSK-3β), and protein kinase B (AKT1) in the liver were increased in the WL and RL groups at P35, and the mRNA levels of acetyl-CoA carboxylase (ACC) and apolipoprotein B (APOB) increased in all groups at P35. At the same time, the total antioxidant capacity (T-AOC) and liver superoxide dismutase (SOD) contents increased in all groups at P35 compared with those at P21. In addition, at P21, compared with WL, GL and BL promoted the serum glucose (GLU) and TG contents by increasing the mRNA levels of GS, GSK-3β, glucose-6-phosphatase (G6PC), ACC, and fatty acid synthase (FAS), but no effect on the proliferative ability and damage of hepatocytes. At P35, RL promoted the hepatic glycogen and TG contents by increasing GSK-3β, AKT1, ACC, and APOB mRNA levels, and the serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels were increased than in the WL group. These results suggest that the effects of light color on liver glycogen and lipid synthesis in broilers changed with age, and also provide a theoretical guidance for scientific use of color of light information to improve productive performance in broilers.

Pathophysiology of diabetic hepatopathy and molecular mechanisms underlying the hepatoprotective effects of phytochemicals

Patients with diabetes are at risk for liver disorders including glycogen hepatopathy, non-alcoholic fatty liver disease, cirrhosis, and hepatic fibrosis. The pathophysiological mechanisms behind diabetic hepatopathy are complex, some of them include fatty acid accumulation, increased reactive oxygen species, increased advanced glycation end-products, hyperactivity of polyol pathways, increased apoptosis and necrosis, and promotion of fibrosis. A growing number of studies have shown that herbal extracts and their active phytochemicals have antihyperglycemic properties and beneficial effects on diabetic complications. The current review, for the first time, focused on herbal agents that showed beneficial effects on diabetic hepatopathy. For example, animal studies have shown that Moringa oleifera and Morus alba improve liver function in both type-1 and type-2 diabetes. Also, evidence from clinical trials suggests that Boswellia serrata, Juglans regia, Melissa officinalis, Portulaca oleracea, Silybum marianum, Talapotaka Churna, and Urtica dioica reduce serum liver enzymes in diabetic patients. The main active ingredient of these plants to protect the liver seems to be phenolic compounds such as niazirin, chlorogenic acid, resveratrol, etc. Mechanisms responsible for the hepatoprotective activity of herbal agents include improving glucose metabolism, restoring adipokines levels, antioxidant defense, and anti-inflammatory activity. Several signaling pathways are involved in hepatoprotective effects of herbal agents in diabetes, such as phosphoinositide 3-kinase, adenosine monophosphate-activated protein kinase, mitogen-activated protein kinase, and c-Jun NH2-terminal kinase.

Metabolic perturbations in pregnant rats exposed to low-dose perfluorooctanesulfonic acid: An integrated multi-omics analysis

Emerging epidemiological evidence has linked per- and polyfluoroalkyl substances (PFAS) exposure could be linked to the disturbance of gestational glucolipid metabolism, but the toxicological mechanism is unclear, especially when the exposure is at a low level. This study examined the glucolipid metabolic changes in pregnant rats treated with relatively low dose perfluorooctanesulfonic acid (PFOS) through oral gavage during pregnancy [gestational day (GD): 1-18]. We explored the molecular mechanisms underlying the metabolic perturbation. Oral glucose tolerance test (OGTT) and biochemical tests were performed to assess the glucose homeostasis and serum lipid profiles in pregnant Sprague-Dawley (SD) rats randomly assigned to starch, 0.03 and 0.3 mg/kg·bw·d groups. Transcriptome sequencing combined with non-targeted metabolomic assays were further performed to identify differentially altered genes and metabolites in the liver of maternal rats, and to determine their correlation with the maternal metabolic phenotypes. Results of transcriptome showed that differentially expressed genes at 0.03 and 0.3 mg/kg·bw·d PFOS exposure were related to several metabolic pathways, such as peroxisome proliferator-activated receptors (PPARs) signaling, ovarian steroid synthesis, arachidonic acid metabolism, insulin resistance, cholesterol metabolism, unsaturated fatty acid synthesis, bile acid secretion. The untargeted metabolomics identified 164 and 158 differential metabolites in 0.03 and 0.3 mg/kg·bw·d exposure groups, respectively under negative ion mode of Electrospray Ionization (ESI-), which could be enriched in metabolic pathways such as α-linolenic acid metabolism, glycolysis/gluconeogenesis, glycerolipid metabolism, glucagon signaling pathway, glycine, serine and threonine metabolism. Co-enrichment analysis indicated that PFOS exposure may disturb the metabolism pathways of glycerolipid, glycolysis/gluconeogenesis, linoleic acid, steroid biosynthesis, glycine, serine and threonine. The key involved genes included down-regulated Ppp1r3c and Abcd2, and up-regulated Ogdhland Ppp1r3g, and the key metabolites such as increased glycerol 3-phosphate and lactosylceramide were further identified. Both of them were significantly associated with maternal fasting blood glucose (FBG) level. Our findings may provide mechanistic clues for clarifying metabolic toxicity of PFOS in human, especially for susceptible population such as pregnant women.

In vivo transomic analyses of glucose-responsive metabolism in skeletal muscle reveal core differences between the healthy and obese states

Metabolic regulation in skeletal muscle is essential for blood glucose homeostasis. Obesity causes insulin resistance in skeletal muscle, leading to hyperglycemia and type 2 diabetes. In this study, we performed multiomic analysis of the skeletal muscle of wild-type (WT) and leptin-deficient obese (ob/ob) mice, and constructed regulatory transomic networks for metabolism after oral glucose administration. Our network revealed that metabolic regulation by glucose-responsive metabolites had a major effect on WT mice, especially carbohydrate metabolic pathways. By contrast, in ob/ob mice, much of the metabolic regulation by glucose-responsive metabolites was lost and metabolic regulation by glucose-responsive genes was largely increased, especially in carbohydrate and lipid metabolic pathways. We present some characteristic metabolic regulatory pathways found in central carbon, branched amino acids, and ketone body metabolism. Our transomic analysis will provide insights into how skeletal muscle responds to changes in blood glucose and how it fails to respond in obesity.

Mechanism of glycogen synthase inactivation and interaction with glycogenin

Glycogen is the major glucose reserve in eukaryotes, and defects in glycogen metabolism and structure lead to disease. Glycogenesis involves interaction of glycogenin (GN) with glycogen synthase (GS), where GS is activated by glucose-6-phosphate (G6P) and inactivated by phosphorylation. We describe the 2.6 Å resolution cryo-EM structure of phosphorylated human GS revealing an autoinhibited GS tetramer flanked by two GN dimers. Phosphorylated N- and C-termini from two GS protomers converge near the G6P-binding pocket and buttress against GS regulatory helices. This keeps GS in an inactive conformation mediated by phospho-Ser641 interactions with a composite “arginine cradle”. Structure-guided mutagenesis perturbing interactions with phosphorylated tails led to increased basal/unstimulated GS activity. We propose that multivalent phosphorylation supports GS autoinhibition through interactions from a dynamic “spike” region, allowing a tuneable rheostat for regulating GS activity. This work therefore provides insights into glycogen synthesis regulation and facilitates studies of glycogen-related diseases.

Glycogen is a major energy reserve in eukaryotes and is synthesised in part by glycogenin (GN) and glycogen synthase (GS). Here, authors describe the structural basis of GS regulation, specifically the mechanism of inactivation by phosphorylation.

Hepatic Mitochondrial Dysfunction and Risk of Liver Disease in an Ovine Model of “PCOS Males”

First-degree male relatives of polycystic ovary syndrome (PCOS) sufferers can develop metabolic abnormalities evidenced by elevated circulating cholesterol and triglycerides, suggestive of a male PCOS equivalent. Similarly, male sheep overexposed to excess androgens in fetal life develop dyslipidaemia in adolescence. Dyslipidaemia, altered lipid metabolism, and dysfunctional hepatic mitochondria are associated with the development of non-alcoholic liver disease (NAFLD). We therefore dissected hepatic mitochondrial function and lipid metabolism in adolescent prenatally androgenized (PA) males from an ovine model of PCOS. Testosterone was directly administered to male ovine fetuses to create prenatal androgenic overexposure. Liver RNA sequencing and proteomics occurred at 6 months of age. Hepatic lipids, glycogen, ATP, reactive oxygen species (ROS), DNA damage, and collagen were assessed. Adolescent PA males had an increased accumulation of hepatic cholesterol and glycogen, together with perturbed glucose and fatty acid metabolism, mitochondrial dysfunction, with altered mitochondrial transport, decreased oxidative phosphorylation and ATP synthesis, and impaired mitophagy. Mitochondrial dysfunction in PA males was associated with increased hepatic ROS level and signs of early liver fibrosis, with clinical relevance to NAFLD progression. We conclude that excess in utero androgen exposure in male fetuses leads to a PCOS-like metabolic phenotype with dysregulated mitochondrial function and likely lifelong health sequelae.

Dietary Succinate Impacts the Nutritional Metabolism, Protein Succinylation and Gut Microbiota of Zebrafish

Succinate is widely used in the food and feed industry as an acidulant, flavoring additive, and antimicrobial agent. This study investigated the effects of dietary succinate on growth, energy budget, nutritional metabolism, protein succinylation, and gut microbiota composition of zebrafish. Zebrafish were fed a control-check (0% succinate) or four succinate-supplemented diets (0.05, 0.10, 0.15, and 0.2%) for 4 weeks. The results showed that dietary succinate at the 0.15% additive amount (S0.15) can optimally promote weight gain and feed intake. Whole body protein, fat, and energy deposition increased in the S0.15 group. Fasting plasma glucose level decreased in fish fed the S0.15 diet, along with improved glucose tolerance. Lipid synthesis in the intestine, liver, and muscle increased with S0.15 feeding. Diet with 0.15% succinate inhibited intestinal gluconeogenesis but promoted hepatic gluconeogenesis. Glycogen synthesis increased in the liver and muscle of S0.15-fed fish. Glycolysis was increased in the muscle of S0.15-fed fish. In addition, 0.15% succinate-supplemented diet inhibited protein degradation in the intestine, liver, and muscle. Interestingly, different protein succinylation patterns in the intestine and liver were observed in fish fed the S0.15 diet. Intestinal proteins with increased succinylation levels were enriched in the tricarboxylic acid cycle while proteins with decreased succinylation levels were enriched in pathways related to fatty acid and amino acid degradation. Hepatic proteins with increased succinylation levels were enriched in oxidative phosphorylation while proteins with decreased succinylation levels were enriched in the processes of protein processing and transport in the endoplasmic reticulum. Finally, fish fed the S0.15 diet had a higher abundance of Proteobacteria but a lower abundance of Fusobacteria and Cetobacterium. In conclusion, dietary succinate could promote growth and feed intake, promote lipid anabolism, improve glucose homeostasis, and spare protein. The effects of succinate on nutritional metabolism are associated with alterations in the levels of metabolic intermediates, transcriptional regulation, and protein succinylation levels. However, hepatic fat accumulation and gut microbiota dysbiosis induced by dietary succinate suggest potential risks of succinate application as a feed additive for fish. This study would be beneficial in understanding the application of succinate as an aquatic feed additive.

Targeting Gut Microbiota and Host Metabolism with Dendrobium officinale Dietary Fiber to Prevent Obesity and Improve Glucose Homeostasis in Diet-Induced Obese Mice

SCOPE

Obesity is becoming a major public health problem due to excess dietary fat intake. Dendrobium officinale (D. officinale) is a medicine food homology plant and exerts multiple health-promoting effects. However, its antiobesity effects and the potential mechanisms remain unclear.

METHODS AND RESULTS

High-fat diet (HFD)-fed mice are administered D. officinale dietary fiber (DODF) daily by gavage for 11 weeks. The results show that treatment with DODF alleviates obesity, liver steatosis, inflammation, and oxidant stress in HFD-induced obese mice. Improved glucose homeostasis in obese mice after DODF treatment is achieved by enhancing insulin pathway and hepatic glycogen synthesis. DODF restructures the gut microbiota in obese mice by decreasing the relative abundance of Bilophila and increasing the relative abundances of Akkermansia, Bifidobacterium, and Muribaculum. Also, DODF reshapes the metabolic phenotype of obese mice as indicated by up-regulating energy metabolism, increasing acetate and taurine, and reducing serum low density/very low density lipoproteins (LDL/VLDL). These beneficial effects are partly transferred by FMT, implying the gut microbiota as a target for the protective effect of DODF on obesity-related symptoms.

CONCLUSION

The results suggest that DODF can be used as a novel prebiotics to maintain the gut microbial homeostasis and improve metabolic health, preventing obesity and related metabolic syndrome.

AMPK signaling in diabetes mellitus, insulin resistance and diabetic complications: A pre-clinical and clinical investigation

Diabetes mellitus (DM) is considered as a main challenge in both developing and developed countries, as lifestyle has changed and its management seems to be vital. Type I and type II diabetes are the main kinds and they result in hyperglycemia in patients and related complications. The gene expression alteration can lead to development of DM and related complications. The AMP-activated protein kinase (AMPK) is an energy sensor with aberrant expression in various diseases including cancer, cardiovascular diseases and DM. The present review focuses on understanding AMPK role in DM. Inducing AMPK signaling promotes glucose in DM that is of importance for ameliorating hyperglycemia. Further investigation reveals the role of AMPK signaling in enhancing insulin sensitivity for treatment of diabetic patients. Furthermore, AMPK upregulation inhibits stress and cell death in β cells that is of importance for preventing type I diabetes development. The clinical studies on diabetic patients have shown the role of AMPK signaling in improving diabetic complications such as brain disorders. Furthermore, AMPK can improve neuropathy, nephropathy, liver diseases and reproductive alterations occurring during DM. For exerting such protective impacts, AMPK signaling interacts with other molecular pathways such as PGC-1α, PI3K/Akt, NOX4 and NF-κB among others. Therefore, providing therapeutics based on AMPK targeting can be beneficial for amelioration of DM.

Heart failure in diabetes

Heart failure and cardiovascular disorders represent the leading cause of death in diabetic patients. Here we present a systematic review of the main mechanisms underlying the development of diabetic cardiomyopathy. We also provide an excursus on the relative contribution of cardiomyocytes, fibroblasts, endothelial and smooth muscle cells to the pathophysiology of heart failure in diabetes. After having described the preclinical tools currently available to dissect the mechanisms of this complex disease, we conclude with a section on the most recent updates of the literature on clinical management.

Role of hepatic PKCβ in nutritional regulation of hepatic glycogen synthesis

The signaling mechanisms by which dietary fat and cholesterol signals regulate central pathways of glucose homeostasis are not completely understood. By using a hepatocyte-specific PKCβ-deficient (PKCβHep-/-) mouse model, we demonstrated the role of hepatic PKCβ in slowing disposal of glucose overload by suppressing glycogenesis and increasing hepatic glucose output. PKCβHep-/- mice exhibited lower plasma glucose under the fed condition, modestly improved systemic glucose tolerance and mildly suppressed gluconeogenesis, increased hepatic glycogen accumulation and synthesis due to elevated glucokinase expression and activated glycogen synthase (GS), and suppressed glucose-6-phosphatase expression compared with controls. These events were independent of hepatic AKT/GSK-3α/β signaling and were accompanied by increased HNF-4α transactivation, reduced FoxO1 protein abundance, and elevated expression of GS targeting protein phosphatase 1 regulatory subunit 3C in the PKCβHep-/- liver compared with controls. The above data strongly imply that hepatic PKCβ deficiency causes hypoglycemia postprandially by promoting glucose phosphorylation via upregulating glucokinase and subsequently redirecting more glucose-6-phosphate to glycogen via activating GS. In summary, hepatic PKCβ has a unique and essential ability to induce a coordinated response that negatively affects glycogenesis at multiple levels under physiological postprandial conditions, thereby integrating nutritional fat intake with dysregulation of glucose homeostasis.

Insulin signaling in health and disease

The molecular mechanisms of cellular insulin action have been the focus of much investigation since the discovery of the hormone 100 years ago. Insulin action is impaired in metabolic syndrome, a condition known as insulin resistance. The actions of the hormone are initiated by binding to its receptor on the surface of target cells. The receptor is an α2β2 heterodimer that binds to insulin with high affinity, resulting in the activation of its tyrosine kinase activity. Once activated, the receptor can phosphorylate a number of intracellular substrates that initiate discrete signaling pathways. The tyrosine phosphorylation of some substrates activates phosphatidylinositol-3-kinase (PI3K), which produces polyphosphoinositides that interact with protein kinases, leading to activation of the kinase Akt. Phosphorylation of Shc leads to activation of the Ras/MAP kinase pathway. Phosphorylation of SH2B2 and of Cbl initiates activation of G proteins such as TC10. Activation of Akt and other protein kinases produces phosphorylation of a variety of substrates, including transcription factors, GTPase-activating proteins, and other kinases that control key metabolic events. Among the cellular processes controlled by insulin are vesicle trafficking, activities of metabolic enzymes, transcriptional factors, and degradation of insulin itself. Together these complex processes are coordinated to ensure glucose homeostasis.

Increased liver glycogen levels enhance exercise capacity in mice

Muscle glycogen depletion has been proposed as one of the main causes of fatigue during exercise. However, few studies have addressed the contribution of liver glycogen to exercise performance. Using a low-intensity running protocol, here, we analyzed exercise capacity in mice overexpressing protein targeting to glycogen (PTG) specifically in the liver (PTG^OE mice), which show a high concentration of glycogen in this organ. PTG^OE mice showed improved exercise capacity, as determined by the distance covered and time ran in an extenuating endurance exercise, compared with control mice. Moreover, fasting decreased exercise capacity in control mice but not in PTG^OE mice. After exercise, liver glycogen stores were totally depleted in control mice, but PTG^OE mice maintained significant glycogen levels even in fasting conditions. In addition, PTG^OE mice displayed an increased hepatic energy state after exercise compared with control mice. Exercise caused a reduction in the blood glucose concentration in control mice that was less pronounced in PTG^OE mice. No changes were found in the levels of blood lactate, plasma free fatty acids, or β-hydroxybutyrate. Plasma glucagon was elevated after exercise in control mice, but not in PTG^OE mice. Exercise-induced changes in skeletal muscle were similar in both genotypes. These results identify hepatic glycogen as a key regulator of endurance capacity in mice, an effect that may be exerted through the maintenance of blood glucose levels.

Adaptive and maladaptive roles for ChREBP in the liver and pancreatic islets

Excessive sugar consumption is a contributor to the worldwide epidemic of cardiometabolic disease. Understanding mechanisms by which sugar is sensed and regulates metabolic processes may provide new opportunities to prevent and treat these epidemics. Carbohydrate Responsive-Element Binding Protein (ChREBP) is a sugar-sensing transcription factor that mediates genomic responses to changes in carbohydrate abundance in key metabolic tissues. Carbohydrate metabolites activate the canonical form of ChREBP, ChREBP-alpha, which stimulates production of a potent, constitutively active ChREBP isoform called ChREBP-beta. Carbohydrate metabolites and other metabolic signals may also regulate ChREBP activity via posttranslational modifications including phosphorylation, acetylation, and O-GlcNAcylation that can affect ChREBP’s cellular localization, stability, binding to cofactors, and transcriptional activity. In this review, we discuss mechanisms regulating ChREBP activity and highlight phenotypes and controversies in ChREBP gain- and loss-of-function genetic rodent models focused on the liver and pancreatic islets.

A mouse model of Bardet-Biedl Syndrome has impaired fear memory, which is rescued by lithium treatment

Primary cilia are microtubule-based organelles present on most cells that regulate many physiological processes, ranging from maintaining energy homeostasis to renal function. However, the role of these structures in the regulation of behavior remains unknown. To study the role of cilia in behavior, we employ mouse models of the human ciliopathy, Bardet-Biedl Syndrome (BBS). Here, we demonstrate that BBS mice have significant impairments in context fear conditioning, a form of associative learning. Moreover, we show that postnatal deletion of BBS gene function, as well as congenital deletion, specifically in the forebrain, impairs context fear conditioning. Analyses indicated that these behavioral impairments are not the result of impaired hippocampal long-term potentiation. However, our results indicate that these behavioral impairments are the result of impaired hippocampal neurogenesis. Two-week treatment with lithium chloride partially restores the proliferation of hippocampal neurons which leads to a rescue of context fear conditioning. Overall, our results identify a novel role of cilia genes in hippocampal neurogenesis and long-term context fear conditioning.

Transomics analysis reveals allosteric and gene regulation axes for altered hepatic glucose-responsive metabolism in obesity

Impaired glucose tolerance associated with obesity causes postprandial hyperglycemia and can lead to type 2 diabetes. To study the differences in liver metabolism in healthy and obese states, we constructed and analyzed transomics glucose-responsive metabolic networks with layers for metabolites, expression data for metabolic enzyme genes, transcription factors, and insulin signaling proteins from the livers of healthy and obese mice. We integrated multiomics time course data from wild-type and leptin-deficient obese (ob/ob) mice after orally administered glucose. In wild-type mice, metabolic reactions were rapidly regulated within 10 min of oral glucose administration by glucose-responsive metabolites, which functioned as allosteric regulators and substrates of metabolic enzymes, and by Akt-induced changes in the expression of glucose-responsive genes encoding metabolic enzymes. In ob/ob mice, the majority of rapid regulation by glucose-responsive metabolites was absent. Instead, glucose administration produced slow changes in the expression of carbohydrate, lipid, and amino acid metabolic enzyme-encoding genes to alter metabolic reactions on a time scale of hours. Few regulatory events occurred in both healthy and obese mice. Thus, our transomics network analysis revealed that regulation of glucose-responsive liver metabolism is mediated through different mechanisms in healthy and obese states. Rapid changes in allosteric regulators and substrates and in gene expression dominate the healthy state, whereas slow changes in gene expression dominate the obese state.

Iodine Deficiency Increases Fat Contribution to Energy Expenditure in Male Mice

More than a billion people worldwide are at risk of iodine deficiency (ID), with well-known consequences for development of the central nervous system. Furthermore, ID has also been associated with dyslipidemia and obesity in humans. To further understand the metabolic consequences of ID, here we kept 8-week-old C57/Bl6 mice at thermoneutrality (~28°C) while feeding them on a low iodine diet (LID). When compared with mice kept on control diet (LID + 0.71 μg/g iodine), the LID mice exhibited marked reduction in T4 and elevated plasma TSH, without changes in plasma T3 levels. LID mice grew normally, and had normal oxygen consumption, ambulatory activity, and heart expression of T3-responsive gene, confirming systemic euthyroidism. However, LID mice exhibited ~5% lower respiratory quotient (RQ), which reflected a ~2.3-fold higher contribution of fat to energy expenditure. LID mice also presented increased circulating levels of nonesterified fatty acids, ~60% smaller fat depots, and increased hepatic glycogen content, all indicative of accelerated lipolysis. LID mice responded much less to forced mobilization of energy substrates (50% food restriction for 3 days or starvation during 36 hours) because of limited size of the adipose depots. A 4-day treatment with T4 restored plasma T4 and TSH levels in LID mice and normalized RQ. We conclude that ID accelerates lipolysis and fatty acid oxidation, without affecting systemic thyroid hormone signaling. It is conceivable that the elevated plasma TSH levels trigger these changes by directly activating lipolysis in the adipose tissues.

Maternal organophosphate flame-retardant exposure alters offspring energy and glucose homeostasis in a sexually dimorphic manner in mice

Persistent organic pollutants such as organophosphate flame retardants (OPFRs) can accumulate in the body and interact with nuclear receptors that control energy homeostasis. One sensitive window of exposure is during development, either in utero or neonatal. Therefore, we investigated if maternal exposure to a mixture of OPFRs alters metabolism on a low-fat diet (LFD) or a high-fat diet (HFD) in both male and female offspring. Wild-type C57Bl/6J dams were orally dosed with vehicle (sesame oil) or an OPFR mixture (1 mg/kg each of tris(1,3-dichloro-2-propyl)phosphate, triphenyl phosphate, and tricresyl phosphate) from gestation day 7 to postnatal day 14. After weaning, pups were fed LFD or HFD. To assess metabolism, we measured body weight and food intake weekly and determined body composition, metabolism, activity, and glucose homeostasis at 6 months of age. Although maternal OPFR exposure did not alter body weight or adiposity, OPFR exposure altered substrate utilization and energy expenditure depending on diet in both sexes. Systolic and diastolic blood pressure was increased by OPFR in male offspring. OPFR exposure interacted with HFD to increase fasting glucose in females and alter glucose and insulin tolerance in male offspring. Plasma leptin was reduced in male and female offspring when fed HFD, whereas liver expression of Pepck was increased in females and Esr1 (estrogen receptor α) was increased in both sex. The physiological implications indicate maternal exposure to OPFRs programs peripheral organs including the liver and adipose tissue, in a sex-dependent manner, thus changing the response to an obesogenic diet and altering adult offspring energy homeostasis.

Role of nutraceutical starch and proanthocyanidins of pigmented rice in regulating hyperglycemia: Enzyme inhibition, enhanced glucose uptake and hepatic glucose homeostasis using in vitro model

Dysregulation of glucose homeostasis result in hyperglycemia and pigmented rice, unique combination of high quality starch and phenolics has the potential in regulating it. In this study, pigmented rice was characterized in terms of nutraceutical starch (NS) and phenolic content. Further the effect of rice phenolics on carbolytic enzyme inhibition, glucose uptake, hepatic glucose homeostasis and anti-glycation ability was analyzed in vitro. The most relevant effect on enzyme inhibition (α-amylase: IC₅₀-42.34 µg/mL; α-glucosidase: IC₅₀:63.89 µg/mL), basal uptake of glucose (>39.5%) and anti-glycation ability (92%) was found in red rice (RR), than black rice (BR). The role of RR phenolics in regulating glucose homeostasis was deciphered using hepatic cell line system, which found up-regulation of glucose transporter 2 (GLUT2) and glycogen synthase 2 (GYS2); while expression of gluconeogenic genes were found down regulated. To our knowledge this study is the first report validating the role of starch-phenolic quality towards anti-hyperglycemic effect of RR.

Effects of exogenous lactate administration on fat metabolism and glycogen synthesis factors in rats

PURPOSE

Lactate has several beneficial roles as an energy resource and in metabolism. However, studies on the effects of oral administration of lactate on fat metabolism and glycogen synthesis are limited. Therefore, the purpose of the present study was to investigate how oral administration of lactate affects fat metabolism and glycogen synthesis factors at specific times (0, 30, 60, 120 min) after intake.

METHODS

Male Sprague Dawley (SD) rats (n = 24) were divided into four groups as follows: the control group (0 min) was sacrificed immediately after oral lactate administration; the test groups were administered lactate (2 g/kg) and sacrificed after 30, 60, and 120 min. Skeletal muscle and liver mRNA expression of GLUT4, FAT/CD36, PDH, CS, PC and GYS2 was assessed using reverse transcription-polymerase chain reaction.

RESULTS

GLUT4 and FAT/CD36 expression was significantly increased in skeletal muscle 120 min after lactate administration. PDH expression in skeletal muscle was altered at 30 and 120 min after lactate consumption, but was not significantly different compared to the control. CS, PC and GYS2 expression in liver was increased 60 min after lactate administration.

CONCLUSION

Our results indicate that exogenous lactate administration increases GLUT4 and FAT/CD36 expression in the muscle as well as glycogen synthase factors (PC, GYS2) in the liver after 60 min. Therefore, lactate supplementation may increase fat utilization as well as induce positive effects on glycogen synthesis in athletes.

Metabolomic Analysis Identifies Glycometabolism Pathways as Potential Targets of Qianggan Extract in Hyperglycemia Rats

Qianggan formula, a designed prescription according to the Traditional Chinese Medicine (TCM) theory, is widely used in treating chronic liver diseases, and indicated to prevent blood glucose increase in patients via unknown mechanisms. To unravel the effects and underlying mechanisms of Qianggan formula on hyperglycemia, we administrated Qianggan extract to high fat and high sucrose (HFHS) diet rats. Results showed that four-week Qianggan extract intervention significantly decreased serum fasting blood glucose, hemoglobin A1c, and liver glycogen levels. Gas chromatography-mass spectrometry (GC-MS) approach was employed to explore metabolomic profiles in liver and fecal samples. By multivariate and univariate statistical analysis (variable importance of projection value > 1 and p value < 0.05), 44 metabolites (18 in liver and 30 in feces) were identified as significantly different. Hierarchical cluster analysis revealed that most differential metabolites had opposite patterns between pair-wise groups. Qianggan extract restored the diet induced metabolite perturbations. Metabolite sets enrichment and pathway enrichment analysis revealed that the affected metabolites were mainly enriched in glycometabolism pathways such as glycolysis/gluconeogenesis, pentose phosphate pathway, fructose, and mannose metabolism. By compound-reaction-enzyme-gene network analysis, batches of genes (e.g. Hk1, Gck, Rpia, etc) or enzymes (e.g. hexokinase and glucokinase) related to metabolites in enriched pathways were obtained. Our findings demonstrated that Qianggan extract alleviated hyperglycemia, and the effects might be partially due to the regulation of glycometabolism related pathways.

Oleoylethanolamide Increases Glycogen Synthesis and Inhibits Hepatic Gluconeogenesis via the LKB1/AMPK Pathway in Type 2 Diabetic Model

Oleoylethanolamide (OEA) is an endogenous peroxisome proliferator-activated receptor α (PPARα) agonist that acts on the peripheral control of energy metabolism. However, its therapeutic potential and related mechanisms in hepatic glucose metabolism under type 2 diabetes mellitus (T2DM) are not clear. Here, OEA treatment markedly improved glucose homeostasis in a PPARα-independent manner. OEA efficiently promoted glycogen synthesis and suppressed gluconeogenesis in mouse primary hepatocytes and liver tissue. OEA enhanced hepatic glycogen synthesis and inhibited gluconeogenesis via liver kinase B1 (LKB1)/5' AMP-activated protein kinase (AMPK) signaling pathways. PPARα was not involved in the roles of OEA in the LKB1/AMPK pathways. We found that OEA exerts its antidiabetic effect by increasing glycogenesis and decreasing gluconeogenesis via the LKB1/AMPK pathway. The ability of OEA to control hepatic LKB1/AMPK pathways may serve as a novel therapeutic approach for the treatment of T2DM. SIGNIFICANCE STATEMENT: Oleoylethanolamide (OEA) exerted a potent antihyperglycemic effect in a peroxisome proliferator-activated receptor α-independent manner. OEA played an antihyperglycemic role primarily via regulation of hepatic glycogen synthesis and gluconeogenesis. The main molecular mechanism of OEA in regulating liver glycometabolism is activating the liver kinase B1/5' AMP-activated protein kinase signaling pathways.

High-Intensity Interval Training Restores Glycolipid Metabolism and Mitochondrial Function in Skeletal Muscle of Mice With Type 2 Diabetes

High-intensity interval training has been reported to lower fasting blood glucose and improve insulin resistance of type 2 diabetes without clear underlying mechanisms. The purpose of this study was to investigate the effect of high-intensity interval training on the glycolipid metabolism and mitochondrial dynamics in skeletal muscle of high-fat diet (HFD) and one-time 100 mg/kg streptozocin intraperitoneal injection-induced type 2 diabetes mellitus (T2DM) mice. Our results confirmed that high-intensity interval training reduced the body weight, fat mass, fasting blood glucose, and serum insulin of the T2DM mice. High-intensity interval training also improved glucose tolerance and insulin tolerance of the T2DM mice. Moreover, we found that high-intensity interval training also decreased lipid accumulation and increased glycogen synthesis in skeletal muscle of the T2DM mice. Ultrastructural analysis of the mitochondria showed that mitochondrial morphology and quantity were improved after 8 weeks of high-intensity interval training. Western blot analysis showed that the expression of mitochondrial biosynthesis related proteins and mitochondrial dynamics related proteins in high-intensity interval trained mice in skeletal muscle were enhanced. Taken together, these data suggest high-intensity interval training improved fasting blood glucose and glucose homeostasis possibly by ameliorating glycolipid metabolism and mitochondrial dynamics in skeletal muscle of the T2DM mice.

Glycogen in Astrocytes and Neurons: Physiological and Pathological Aspects

Brain glycogen is stored mainly in astrocytes, although neurons also have an active glycogen metabolism. Glycogen has gained relevance as a key player in brain function. In this regard, genetically modified animals have allowed researchers to unravel new roles of this polysaccharide in the brain. Remarkably, mice in which glycogen synthase is abolished in the brain, and thus devoid of brain glycogen, are viable, thereby indicating that the polysaccharide in this organ is not a requirement for survival. While there was growing evidence supporting a role of glycogen in learning and memory, these animals have now confirmed that glycogen participates in these two processes.The association of epilepsy with brain glycogen has also attracted attention. Analysis of genetically modified mice indicates that the relation between brain glycogen and epilepsy is complex. While the formation of glycogen aggregates clearly underlies epilepsy, as in Lafora Disease (LD), the absence of glycogen also favors the occurrence of seizures.LD is a rare genetic condition that affects children. It is characterized by epileptic seizures and neurodegeneration, and it develops rapidly until finally causing death. Research into this disease has unveiled new aspects of glycogen metabolism. Animal models of LD accumulate polyglucosan bodies formed by aberrant glycogen aggregates, called Lafora bodies (LBs). The abolition of glycogen synthase (GS) prevents the formation of LBs and the development of LD, thereby indicating that glycogen accumulation underlies this disease and the associated symptoms, and thus establishing a clear relation between the accumulation of glycogen aggregates and the incidence of seizures.Although it was initially accepted that LBs were essentially neuronal, it is now evident that astrocytes also accumulate polyglucosan aggregates in LD. However, the appearance and composition of these deposits differs from that observed in neurons. Of note, the astrocytic aggregates in LD models show remarkable similarities with corpora amylacea (CA), a type of polyglucosan aggregate observed in the brains of aged mice and humans. The abolition of GS in mice also impedes the formation of CA with age and at the same time prevents the formation of a number of protein aggregates associated with aging. Therefore CA may play a role in age-related neurological decline.

The integrative biology of type 2 diabetes

Obesity and type 2 diabetes are the most frequent metabolic disorders, but their causes remain largely unclear. Insulin resistance, the common underlying abnormality, results from imbalance between energy intake and expenditure favouring nutrient-storage pathways, which evolved to maximize energy utilization and preserve adequate substrate supply to the brain. Initially, dysfunction of white adipose tissue and circulating metabolites modulate tissue communication and insulin signalling. However, when the energy imbalance is chronic, mechanisms such as inflammatory pathways accelerate these abnormalities. Here we summarize recent studies providing insights into insulin resistance and increased hepatic gluconeogenesis associated with obesity and type 2 diabetes, focusing on data from humans and relevant animal models.

Anti-diabetic activities of agaropectin-derived oligosaccharides from Gloiopeltis furcata via regulation of mitochondrial function

The aim of the present study was to investigate whether agaropectin-derived oligosaccharides from Gloiopeltis furcata (SAOs) exert an anti-diabetic effect in sodium palmitate (PA)-induced insulin resistant HepG2 cells. We found that SAOs were co-localized with mitochondria and regulated mitochondrial function. SAOs reduced respiratory chain activities, which led to reduced respiratory oxygen consumption and increased the cellular ADP/ATP ratio in a certain degree of dose-dependent manner. Thus, SAOs alleviated the oxidative stress state in PA-treated cells and, moreover, concurrently regulated the ROS-JNK-IRS-1 pathway. As a result, SAOs enhanced insulin sensitivity and glucose metabolism by activating the IRS-1-AKT-GSK-3β-GS pathway. Additionally, SAOs activated AMPK through both PKA-LKB1 and mitochondrial-regulated energy metabolism pathways. Therefore, SAOs decreased accumulation of lipids and improved lipid metabolism via regulating HMGCR, ACC and SREBP-1 proteins in HepG2 cells. Taken together, we conclude that SAOs could significantly ameliorate diabetic states in vitro via regulating mitochondria and their downstream signaling pathways.

The loss of ERE-dependent ERα signaling potentiates the effects of maternal high-fat diet on energy homeostasis in female offspring fed an obesogenic diet

Maternal high-fat diet (HFD) alters hypothalamic programming and disrupts offspring energy homeostasis in rodents. We previously reported that the loss of ERα signaling partially blocks the effects of maternal HFD in female offspring fed a standard chow diet. In a companion study, we determined if the effects of maternal HFD were magnified by an adult obesogenic diet in our transgenic mouse models. Heterozygous ERα knockout (wild-type (WT)/KO) dams were fed a control breeder chow diet (25% fat) or a semipurified HFD (45% fat) 4 weeks prior to mating with heterozygous males (WT/KO or WT/ knockin) to produce WT, ERα KO, or ERα knockin/knockout (KIKO) (no estrogen response element (ERE) binding) female offspring, which were fed HFD for 20 weeks. Maternal HFD potentiated the effects of adult HFD on KIKO and KO body weight due to increased adiposity and decreased activity. Maternal HFD also produced KIKO females that exhibit KO-like insulin intolerance and impaired glucose homeostasis. Maternal HFD increased plasma interleukin 6 and monocyte chemoattractant protein 1 levels and G6pc and Pepck liver expression only in WT mice. Insulin and tumor necrosis factor α levels were higher in KO offspring from HFD-fed dams. Arcuate and liver expression of Esr1 was altered in KIKO and WT, respectively. These data suggest that loss of ERE-dependent ERα signaling, and not total ERα signaling, sensitizes females to the deleterious influence of maternal HFD on offspring energy and glucose potentially through the control of peripheral inflammation and hypothalamic and liver gene expression. Future studies will interrogate the tissue-specific mechanisms of maternal HFD programming through ERα signaling.

Regulation of Glucose Production in the Pathogenesis of Type 2 Diabetes

PURPOSE OF REVIEW

Increased glucose production associated with hepatic insulin resistance contributes to the development of hyperglycemia in T2D. The molecular mechanisms accounting for increased glucose production remain controversial. Our aims were to review recent literature concerning molecular mechanisms regulating glucose production and to discuss these mechanisms in the context of physiological experiments and observations in humans and large animal models.

RECENT FINDINGS

Genetic intervention studies in rodents demonstrate that insulin can control hepatic glucose production through both direct effects on the liver, and through indirect effects to inhibit adipose tissue lipolysis and limit gluconeogenic substrate delivery. However, recent experiments in canine models indicate that the direct effects of insulin on the liver are dominant over the indirect effects to regulate glucose production. Recent molecular studies have also identified insulin-independent mechanisms by which hepatocytes sense intrahepatic carbohydrate levels to regulate carbohydrate disposal. Dysregulation of hepatic carbohydrate sensing systems may participate in increased glucose production in the development of diabetes.

In vivo stabilization of OPA1 in hepatocytes potentiates mitochondrial respiration and gluconeogenesis in a prohibitin-dependent way

Patients with fatty liver diseases present altered mitochondrial morphology and impaired metabolic function. Mitochondrial dynamics and related cell function require the uncleaved form of the dynamin-like GTPase OPA1. Stabilization of OPA1 might then confer a protective mechanism against stress-induced tissue damages. To study the putative role of hepatic mitochondrial morphology in a sick liver, we expressed a cleavage-resistant long form of OPA1 (L-OPA1Δ) in the liver of a mouse model with mitochondrial liver dysfunction (i.e. the hepatocyte-specific prohibitin-2 knockout (Hep-Phb2^-/-) mice). Liver prohibitin-2 deficiency caused excessive proteolytic cleavage of L-OPA1, mitochondrial fragmentation, and increased apoptosis. These molecular alterations were associated with lipid accumulation, abolished gluconeogenesis, and extensive liver damage. Such liver dysfunction was associated with severe hypoglycemia. In prohibitin-2 knockout mice, expression of L-OPA1Δ by in vivo adenovirus delivery restored the morphology but not the function of mitochondria in hepatocytes. In prohibitin-competent mice, elongation of liver mitochondria by expression of L-OPA1Δ resulted in excessive glucose production associated with increased mitochondrial respiration. In conclusion, mitochondrial dynamics participates in the control of hepatic glucose production.

Reactive oxygen species-induced changes in glucose and lipid metabolism contribute to the accumulation of cholesterol in the liver during aging

Aging is a major risk factor for many chronic diseases due to increased vulnerability to external stress and susceptibility to disease. Aging is associated with metabolic liver disease such as nonalcoholic fatty liver. In this study, we investigated changes in lipid metabolism during aging in mice and the mechanisms involved. Lipid accumulation was increased in liver tissues of aged mice, particularly cholesterol. Increased uptake of both cholesterol and glucose was observed in hepatocytes of aged mice as compared with younger mice. The mRNA expression of GLUT2, GK, SREBP2, HMGCR, and HMGCS, genes for cholesterol synthesis, was gradually increased in liver tissues during aging. Reactive oxygen species (ROS) increase with aging and are closely related to various aging‐related diseases. When we treated HepG2 cells and primary hepatocytes with the ROS inducer, H₂O₂, lipid accumulation increased significantly compared to the case for untreated HepG2 cells. H₂O₂ treatment significantly increased glucose uptake and acetyl‐CoA production, which results in glycolysis and lipid synthesis. Treatment with H₂O₂ significantly increased the expression of mRNA for genes related to cholesterol synthesis and uptake. These results suggest that ROS play an important role in altering cholesterol metabolism and consequently contribute to the accumulation of cholesterol in the liver during the aging process.

Liver-specific insulin receptor isoform A expression enhances hepatic glucose uptake and ameliorates liver steatosis in a mouse model of diet-induced obesity

Among the main complications associated with obesity are insulin resistance and altered glucose and lipid metabolism within the liver. It has previously been described that insulin receptor isoform A (IRA) favors glucose uptake and glycogen storage in hepatocytes compared with isoform B (IRB), improving glucose homeostasis in mice lacking liver insulin receptor. Thus, we hypothesized that IRA could also improve glucose and lipid metabolism in a mouse model of high-fat-diet-induced obesity. We addressed the role of insulin receptor isoforms in glucose and lipid metabolism in vivo. We expressed IRA or IRB specifically in the liver by using adeno-associated viruses (AAVs) in a mouse model of diet-induced insulin resistance and obesity. IRA, but not IRB, expression induced increased glucose uptake in the liver and muscle, improving insulin tolerance. Regarding lipid metabolism, we found that AAV-mediated IRA expression also ameliorated hepatic steatosis by decreasing the expression of Fasn, Pgc1a, Acaca and Dgat2 and increasing Scd-1 expression. Taken together, our results further unravel the role of insulin receptor isoforms in hepatic glucose and lipid metabolism in an insulin-resistant scenario. Our data strongly suggest that IRA is more efficient than IRB at favoring hepatic glucose uptake, improving insulin tolerance and ameliorating hepatic steatosis. Therefore, we conclude that a gene therapy approach for hepatic IRA expression could be a safe and promising tool for the regulation of hepatic glucose consumption and lipid metabolism, two key processes in the development of non-alcoholic fatty liver disease associated with obesity.

This article has an associated First Person interview with the first author of the paper.

Summary: Adeno-associated-virus-mediated gene therapy for insulin receptor isoform A expression in the liver improves glucose disposal and alleviates lipid accumulation in wild-type mice under a high-fat diet.

Hydrogen Sulfide as a Novel Regulatory Factor in Liver Health and Disease

Hydrogen sulfide (H₂S), a colorless gas smelling of rotten egg, has long been recognized as a toxic gas and environment pollutant. However, increasing evidence suggests that H₂S acts as a novel gasotransmitter and plays important roles in a variety of physiological and pathological processes in mammals. H₂S is involved in many hepatic functions, including the regulation of oxidative stress, glucose and lipid metabolism, vasculature, mitochondrial function, differentiation, and circadian rhythm. In addition, H₂S contributes to the pathogenesis and treatment of a number of liver diseases, such as hepatic fibrosis, liver cirrhosis, liver cancer, hepatic ischemia/reperfusion injury, nonalcoholic fatty liver disease/nonalcoholic steatohepatitis, hepatotoxicity, and acute liver failure. In this review, the biosynthesis and metabolism of H₂S in the liver are summarized and the role and mechanism of H₂S in liver health and disease are further discussed.

PEPCK1 Antisense Oligonucleotide Prevents Adiposity and Impairs Hepatic Glycogen Synthesis in High-Fat Male Fed Rats

The increased hepatic gluconeogenesis in type 2 diabetes mellitus has often been ascribed to increased transcription of phosphoenolpyruvate carboxykinase 1, cystolic form (PEPCK1), although recent evidence has questioned this attribution. To assess the metabolic role of PEPCK1, we treated regular chow fed and high-fat fed (HFF) male Sprague-Dawley rats with a 2'-O-methoxyethyl chimeric antisense oligonucleotide (ASO) against PEPCK1 and compared them with control ASO-treated rats. PEPCK1 ASO effectively decreased PEPCK1 expression in the liver and white adipose tissue. In chow fed rats, PEPCK1 ASO did not alter adiposity, plasma glucose, or insulin. In contrast, PEPCK1 ASO decreased the white adipose tissue mass in HFF rats but without altering basal rates of lipolysis, de novo lipogenesis, or glyceroneogenesis in vivo. Despite the protection from adiposity, hepatic insulin sensitivity was impaired in HFF PEPCK1 ASO-treated rats. PEPCK1 ASO worsened hepatic steatosis, although without additional impairments in hepatic insulin signaling or activation of inflammatory signals in the liver. Instead, the development of hepatic insulin resistance and the decrease in hepatic glycogen synthesis during a hyperglycemic clamp was attributed to a decrease in hepatic glucokinase (GCK) expression and decreased synthesis of glycogen via the direct pathway. The decrease in GCK expression was associated with increased expression of activating transcription factor 3, a negative regulator of GCK transcription. These studies have demonstrated that PEPCK1 is integral to coordinating cellular metabolism in the liver and adipose tissue, although it does not directly effect hepatic glucose production or adipose glyceroneogenesis.

A GYS2/p53 Negative Feedback Loop Restricts Tumor Growth in HBV-Related Hepatocellular Carcinoma

Hepatocellular carcinogenesis is attributed to the reprogramming of cellular metabolism as a consequence of the alteration in metabolite-related gene regulation. Identifying the mechanism of aberrant metabolism is of great potential to provide novel targets for the treatment of hepatocellular carcinoma (HCC). Here, we demonstrated that glycogen synthase 2 (GYS2) restricted tumor growth in hepatitis B virus-related HCC via a negative feedback loop with p53. Expression of GYS2 was significantly downregulated in HCC and correlated with decreased glycogen content and unfavorable patient outcomes. GYS2 overexpression suppressed, whereas GYS2 knockdown facilitated cell proliferation in vitro and tumor growth in vivo via modulating p53 expression. GYS2 competitively bound to MDM2 to prevent p53 from MDM2-mediated ubiquitination and degradation. Furthermore, GYS2 enhanced the p300-induced acetylation of p53 at K373/382, which in turn inhibited the transcription of GYS2 in the support of HBx/HDAC1 complex. In summary, our findings suggest that GYS2 serves as a prognostic factor and functions as a tumor suppressor in HCC. The newly identified HBx/GYS2/p53 axis is responsible for the deregulation of glycogen metabolism and represents a promising therapeutic target for the clinical management of HCC. SIGNIFICANCE: We elucidated the clinical significance, biological function, and regulation of the HBx/GYS2/p53 axis, which supplement the understanding of tumor glycogen metabolism and provide potential prognostic and therapeutic targets for HCC treatment.Graphical Abstract: http://cancerres.aacrjournals.org/content/canres/79/3/534/F1.large.jpg.

Resolving the Paradox of Hepatic Insulin Resistance

Insulin resistance is associated with numerous metabolic disorders, such as obesity and type II diabetes, that currently plague our society. Although insulin normally promotes anabolic metabolism in the liver by increasing glucose consumption and lipid synthesis, insulin-resistant individuals fail to inhibit hepatic glucose production and paradoxically have increased liver lipid synthesis, leading to hyperglycemia and hypertriglyceridemia. Here, we detail the intrahepatic and extrahepatic pathways mediating insulin's control of glucose and lipid metabolism. We propose that the interplay between both of these pathways controls insulin signaling and that mis-regulation between the 2 results in the paradoxic effects seen in the insulin-resistant liver instead of the commonly proposed deficiencies in particular branches of only the direct hepatic pathway.

Liver-specific ablation of insulin-degrading enzyme causes hepatic insulin resistance and glucose intolerance, without affecting insulin clearance in mice

The role of insulin-degrading enzyme (IDE), a metalloprotease with high affinity for insulin, in insulin clearance remains poorly understood.

OBJECTIVE

This study aimed to clarify whether IDE is a major mediator of insulin clearance, and to define its role in the etiology of hepatic insulin resistance.

METHODS

We generated mice with liver-specific deletion of Ide (L-IDE-KO) and assessed insulin clearance and action.

RESULTS

L-IDE-KO mice exhibited higher (~20%) fasting and non-fasting plasma glucose levels, glucose intolerance and insulin resistance. This phenotype was associated with ~30% lower plasma membrane insulin receptor levels in liver, as well as ~55% reduction in insulin-stimulated phosphorylation of the insulin receptor, and its downstream signaling molecules, AKT1 and AKT2 (reduced by ~40%). In addition, FoxO1 was aberrantly distributed in cellular nuclei, in parallel with up-regulation of the gluconeogenic genes Pck1 and G6pc. Surprisingly, L-IDE-KO mice showed similar plasma insulin levels and hepatic insulin clearance as control mice, despite reduced phosphorylation of the carcinoembryonic antigen-related cell adhesion molecule 1, which upon its insulin-stimulated phosphorylation, promotes receptor-mediated insulin uptake to be degraded.

CONCLUSION

IDE is not a rate-limiting regulator of plasma insulin levels in vivo.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Organic cation transporter 1 (OCT1) modulates multiple cardiometabolic traits through effects on hepatic thiamine content

A constellation of metabolic disorders, including obesity, dysregulated lipids, and elevations in blood glucose levels, has been associated with cardiovascular disease and diabetes. Analysis of data from recently published genome-wide association studies (GWAS) demonstrated that reduced-function polymorphisms in the organic cation transporter, OCT1 (SLC22A1), are significantly associated with higher total cholesterol, low-density lipoprotein (LDL) cholesterol, and triglyceride (TG) levels and an increased risk for type 2 diabetes mellitus, yet the mechanism linking OCT1 to these metabolic traits remains puzzling. Here, we show that OCT1, widely characterized as a drug transporter, plays a key role in modulating hepatic glucose and lipid metabolism, potentially by mediating thiamine (vitamin B1) uptake and hence its levels in the liver. Deletion of Oct1 in mice resulted in reduced activity of thiamine-dependent enzymes, including pyruvate dehydrogenase (PDH), which disrupted the hepatic glucose–fatty acid cycle and shifted the source of energy production from glucose to fatty acids, leading to a reduction in glucose utilization, increased gluconeogenesis, and altered lipid metabolism. In turn, these effects resulted in increased total body adiposity and systemic levels of glucose and lipids. Importantly, wild-type mice on thiamine deficient diets (TDs) exhibited impaired glucose metabolism that phenocopied Oct1 deficient mice. Collectively, our study reveals a critical role of hepatic thiamine deficiency through OCT1 deficiency in promoting the metabolic inflexibility that leads to the pathogenesis of cardiometabolic disease.

The liver is the major organ for glucose and lipid metabolism; impairment in liver energy metabolism is often found in metabolic disorders. Traditionally, excesses in macronutrients (fat and glucose) are linked to the development of metabolic disorders. Our study provides evidence that imbalances in a micronutrient, vitamin B1 (thiamine), can serve as an etiological cause of lipid and glucose disorders and implicates the organic cation transporter, OCT1, in these disorders. OCT1 is a key determinant of thiamine levels in the liver. In humans, reduced-function polymorphisms of OCT1 significantly associate with high LDL cholesterol levels. Using Oct1 knockout mice, we show that reduced OCT1-mediated thiamine uptake in the liver leads to reduced levels of TPP—the active metabolite of thiamine—and decreased activity of key TPP-dependent enzymes. As a result, a shift from glucose to fatty acid oxidation occurs, leading to imbalances in key metabolic intermediates, alterations in metabolic flux pathways, and disruptions of various metabolic regulatory mechanisms. The extensive characterization of Oct1 knockout mice provides evidence for the molecular mechanisms responsible for various metabolic traits and indicates an important role for imbalances in micronutrients in cardiometabolic disorders.