A subcutaneous adipose tissue-liver signalling axis controls hepatic gluconeogenesis

Loss of Tbk1 kinase activity protects mice from diet-induced metabolic dysfunction

OBJECTIVE

TANK Binding Kinase 1 (TBK1) has been implicated in the regulation of metabolism through studies with the drug amlexanox, an inhibitor of the IκB kinase (IKK)-related kinases. Amlexanox induced weight loss, reduced fatty liver and insulin resistance in high fat diet (HFD) fed mice and has now progressed into clinical testing for the treatment and prevention of obesity and type 2 diabetes. However, since amlexanox is a dual IKKε/TBK1 inhibitor, the specific metabolic contribution of TBK1 is not clear.

METHODS

To distinguish metabolic functions unique to TBK1, we examined the metabolic profile of global Tbk1 mutant mice challenged with an obesogenic diet and investigated potential mechanisms for the improved metabolic phenotype.

RESULTS AND CONCLUSION

We report that systemic loss of TBK1 kinase function has an overall protective effect on metabolic readouts in mice on an obesogenic diet, which is mediated by loss of an inhibitory interaction between TBK1 and the insulin receptor.

Rosiglitazone Elicits an Adiponectin-Mediated Insulin-Sensitizing Action at the Adipose Tissue-Liver Axis in Otsuka Long-Evans Tokushima Fatty Rats

Rosiglitazone is an agonist of peroxisome proliferator-activated receptor- (PPAR-) γ that is principally associated with insulin action. The exact mechanisms underlying its insulin-sensitizing action are still not fully elucidated. It is well known that adiponectin mostly secreted in adipose tissue is an insulin sensitizer. Here, we found that treatment of Otsuka Long-Evans Tokushima Fatty (OLETF) rats with rosiglitazone (3 mg/kg, once daily, by oral gavage for 33 weeks) attenuated the increase in fasting plasma insulin concentrations and the index of the homeostasis model assessment of insulin resistance along with the age growth and glucose concentrations during an oral glucose tolerance test. In addition, the increase in plasma alanine aminotransferase activity, concentrations of fasting plasma nonesterified fatty acids and triglyceride, and hepatic triglyceride content was also suppressed. The hepatic protein expression profile revealed that rosiglitazone increased the downregulated total protein expression of insulin receptor substrate 1 (IRS-1) and IRS-2. Furthermore, the treatment suppressed the upregulated phosphorylation of IRS-1 at Ser³⁰⁷ and IRS-2 at Ser⁷³¹. The results indicate that rosiglitazone ameliorates hepatic and systemic insulin resistance, hepatic inflammation, and fatty liver. Mechanistically, rosiglitazone suppressed hepatic protein overexpression of both phosphorylated nuclear factor- (NF-) κBp65 and inhibitory-κB kinase-α/β, a transcription factor that primarily regulates chronic inflammatory responses and the upstream NF-κB signal transduction cascades which are necessary for activating NF-κB, respectively. More importantly, rosiglitazone attenuated the decreases in adipose adiponectin mRNA level, plasma adiponectin concentrations, and hepatic protein expression of adiponectin receptor-1 and receptor-2. Thus, we can draw the conclusion that rosiglitazone elicits an adiponectin-mediated insulin-sensitizing action at the adipose tissue-liver axis in obese rats. Our findings may provide new insights into the mechanisms of action of rosiglitazone.

Amlexanox provides a potential therapy for nonsense mutations in the lysosomal storage disorder Aspartylglucosaminuria

Aspartylglucosaminuria (AGU) is a lysosomal storage disorder caused by mutations in the gene for aspartylglucosaminidase (AGA). This enzyme participates in glycoprotein degradation in lysosomes. AGU results in progressive mental retardation, and no curative therapy is currently available. We have here characterized the consequences of AGA gene mutations in a compound heterozygous patient who exhibits a missense mutation producing a Ser72Pro substitution in one allele, and a nonsense mutation Trp168X in the other. Ser72 is not a catalytic residue, but is required for the stabilization of the active site conformation. Thus, Ser72Pro exchange impairs the autocatalytic activation of the AGA precursor, and results in a considerable reduction of the enzyme activity and in altered AGA precursor processing. Betaine, which can partially rescue the AGA activity in AGU patients carrying certain missense mutations, turned out to be ineffective in the case of Ser72Pro substitution. The Trp168X nonsense allele results in complete lack of AGA polypeptide due to nonsense-mediated decay (NMD) of the mRNA. Amlexanox, which inhibits NMD and causes a translational read-through, facilitated the synthesis of a full-length, functional AGA protein from the nonsense allele. This could be demonstrated as presence of the AGA polypeptide and increased enzyme activity upon Amlexanox treatment. Furthermore, in the Ser72Pro/Trp168X expressing cells, Amlexanox induced a synergistic increase in AGA activity and polypeptide processing due to enhanced processing of the Ser72Pro polypeptide. Our data show for the first time that Amlexanox might provide a valid therapy for AGU.

Adapting to obesity with adipose tissue inflammation

Adipose tissue not only has an important role in the storage of excess nutrients but also senses nutrient status and regulates energy mobilization. An overall positive energy balance is associated with overnutrition and leads to excessive accumulation of fat in adipocytes. These cells respond by initiating an inflammatory response that, although maladaptive in the long run, might initially be a physiological response to the stresses obesity places on adipose tissue. In this Review, we characterize adipose tissue inflammation and review the current knowledge of what triggers obesity-associated inflammation in adipose tissue. We examine the connection between adipose tissue inflammation and the development of insulin resistance and catecholamine resistance and discuss the ensuing state of metabolic inflexibility. Finally, we review the current and potential new anti-inflammatory treatments for obesity-associated metabolic disease.

Amlexanox, a selective inhibitor of IKBKE, generates anti-tumoral effects by disrupting the Hippo pathway in human glioblastoma cell lines

Glioblastoma multiforme (GBM) is the most prevalent form of malignant brain tumor. Amlexanox, a novel compound, has been shown to have anti-cancer potential. In this study, the anti-tumoral effects and the underlying mechanisms of amlexanox were investigated. Amlexanox significantly suppressed proliferation and invasion and induced apoptosis in glioblastoma cells. Furthermore, we found that amlexanox altered the protein expression of the Hippo pathway by downregulating IKBKE. Our data indicates that IKBKE directly targets LATS1/2 and induces degradation of LATS1/2, thereby inhibiting the activity of the Hippo pathway. In vivo results further confirmed the tumor inhibitory effect of amlexanox via the downregulation of IKBKE, and amlexanox induced no apparent toxicity. Collectively, our studies suggest that amlexanox is a promising therapeutic agent for the treatment of GBM.

Inhibition of IKKɛ and TBK1 Improves Glucose Control in a Subset of Patients with Type 2 Diabetes

Numerous studies indicate an inflammatory link between obesity and type 2 diabetes. The inflammatory kinases IKKɛ and TBK1 are elevated in obesity; their inhibition in obese mice reduces weight, insulin resistance, fatty liver and inflammation. Here we studied amlexanox, an inhibitor of IKKɛ and TBK1, in a proof-of-concept randomized, double-blind, placebo-controlled study of 42 obese patients with type 2 diabetes and nonalcoholic fatty liver disease. Treatment of patients with amlexanox produced a statistically significant reduction in Hemoglobin A1c and fructosamine. Interestingly, a subset of drug responders also exhibited improvements in insulin sensitivity and hepatic steatosis. This subgroup was characterized by a distinct inflammatory gene expression signature from biopsied subcutaneous fat at baseline. They also exhibited a unique pattern of gene expression changes in response to amlexanox, consistent with increased energy expenditure. Together, these data suggest that dual-specificity inhibitors of IKKɛ and TBK1 may be effective therapies for metabolic disease in an identifiable subset of patients.

Adipose tissue autophagy and homeostasis in alcohol-induced liver injury

Alcohol consumption leads to injury in multiple organs and systems, including the liver, brain, heart, skeletal muscle, pancreas, bone, immune system, and endocrine system. Emerging evidence indicates that alcohol also promotes adipose tissue dysfunction, which may contribute to injury progression in other organs and systems. Autophagy is a lysosomal degradation pathway that has been shown to regulate adipose tissue homeostasis and adipogenesis. Increasing evidence also demonstrates that alcohol consumption affects autophagy in multiple tissues. This review summarizes current knowledge regarding the effect of autophagy on adipose tissue and its potential roles in alcohol-induced adipose tissue atrophy as well as its contribution to alcohol-induced liver injury.

Microglia activation due to obesity programs metabolic failure leading to type two diabetes

Obesity is an energy metabolism disorder that increases susceptibility to the development of metabolic diseases. Recently, it has been described that obese subjects have a phenotype of chronic inflammation in organs that are metabolically relevant for glucose homeostasis and energy. Altered expression of immune system molecules such as interleukins IL-1, IL-6, IL-18, tumor necrosis factor alpha (TNF-α), serum amyloid A (SAA), and plasminogen activator inhibitor-1 (PAI-1), among others, has been associated with the development of chronic inflammation in obesity. Chronic inflammation modulates the development of metabolic-related comorbidities like metabolic syndrome (insulin resistance, glucose tolerance, hypertension and hyperlipidemia). Recent evidence suggests that microglia activation in the central nervous system (CNS) is a priority in the deregulation of energy homeostasis and promotes increased glucose levels. This review will cover the most significant advances that explore the molecular signals during microglia activation and inflammatory stage in the brain in the context of obesity, and its influence on the development of metabolic syndrome and type two diabetes.

Lysophosphatidic acid counteracts glucagon-induced hepatocyte glucose production via STAT3

Hepatic glucose production (HGP) is required to maintain normoglycemia during fasting. Glucagon is the primary hormone responsible for increasing HGP; however, there are many additional hormone and metabolic factors that influence glucagon sensitivity. In this study we report that the bioactive lipid lysophosphatidic acid (LPA) regulates hepatocyte glucose production by antagonizing glucagon-induced expression of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK). Treatment of primary hepatocytes with exogenous LPA blunted glucagon-induced PEPCK expression and glucose production. Similarly, knockout mice lacking the LPA-degrading enzyme phospholipid phosphate phosphatase type 1 (PLPP1) had a 2-fold increase in endogenous LPA levels, reduced PEPCK levels during fasting, and decreased hepatic gluconeogenesis in response to a pyruvate challenge. Mechanistically, LPA antagonized glucagon-mediated inhibition of STAT3, a transcriptional repressor of PEPCK. Importantly, LPA did not blunt glucagon-stimulated glucose production or PEPCK expression in hepatocytes lacking STAT3. These data identify a novel role for PLPP1 activity and hepatocyte LPA levels in glucagon sensitivity via a mechanism involving STAT3.

Macrophage function in obesity-induced inflammation and insulin resistance

The steadily increasing obesity epidemic affects currently 30% of western populations and is causative for numerous disorders. It has been demonstrated that immune cells such as macrophages reside in or infiltrate metabolic organs under obese conditions and cause the so-called low-grade inflammation or metaflammation that impairs insulin action thus leading to the development of insulin resistance. Here, we report on data that specifically address macrophage biology/physiology in obesity-induced inflammation and insulin resistance.

APPL1-mediated activation of STAT3 contributes to inhibitory effect of adiponectin on hepatic gluconeogenesis

Adiponectin has been shown to suppress hepatic gluconeogenesis. However, the signaling pathways underlying its action remain ill-defined. The purpose of this study was to examine the potential role of APPL1 in mediating anti-gluconeogenic ability of adiponectin. Primary hepatocytes were isolated from male C57BL/6 mice. Western blot and RT-PCR were performed to detect protein expression and mRNA level, respectively. The protein-protein association was determined by immunoprecipitation and GST pull-down assay. We found that APPL1 protein levels were negatively associated with expressions of proteins and mRNAs of gluconeogenesis enzymes under stimulation with adiponectin. In addition, adiponectin-stimulated STAT3 phosphorylation and acetylation were positively regulated by APPL1 and negative regulated by SirT1. Pharmacological and genetic inhibition of STAT3 mitigated impact of adiponectin on hepatic gluconeogenesis. Furthermore, adiponectin administration facilitated the binding of APPL1 to SirT1 and suppressed the association of SirT1 with STAT3. Taken together, our study showed that APPL1-SirT1-STAT3 pathway mediated adiponectin signaling in primary hepatocytes. This new finding provides a novel mechanism by which adiponectin suppresses hepatic gluconeogenesis.

Lack of Skeletal Muscle IL-6 Affects Pyruvate Dehydrogenase Activity at Rest and during Prolonged Exercise

Pyruvate dehydrogenase (PDH) plays a key role in the regulation of skeletal muscle substrate utilization. IL-6 is produced in skeletal muscle during exercise in a duration dependent manner and has been reported to increase whole body fatty acid oxidation, muscle glucose uptake and decrease PDHa activity in skeletal muscle of fed mice. The aim of the present study was to examine whether muscle IL-6 contributes to exercise-induced PDH regulation in skeletal muscle. Skeletal muscle-specific IL-6 knockout (IL-6 MKO) mice and floxed littermate controls (control) completed a single bout of treadmill exercise for 10, 60 or 120 min, with rested mice of each genotype serving as basal controls. The respiratory exchange ratio (RER) was overall higher (P<0.05) in IL-6 MKO than control mice during the 120 min of treadmill exercise, while RER decreased during exercise independent of genotype. AMPK and ACC phosphorylation also increased with exercise independent of genotype. PDHa activity was in control mice higher (P<0.05) at 10 and 60 min of exercise than at rest but remained unchanged in IL-6 MKO mice. In addition, PDHa activity was higher (P<0.05) in IL-6 MKO than control mice at rest and 60 min of exercise. Neither PDH phosphorylation nor acetylation could explain the genotype differences in PDHa activity. Together, this provides evidence that skeletal muscle IL-6 contributes to the regulation of PDH at rest and during prolonged exercise and suggests that muscle IL-6 normally dampens carbohydrate utilization during prolonged exercise via effects on PDH.

IKBKE Is a Substrate of EGFR and a Therapeutic Target in Non-Small Cell Lung Cancer with Activating Mutations of EGFR

Non-small cell lung cancers (NSCLC) marked by EGFR mutations tend to develop resistance to therapeutic EGFR inhibitors, often due to secondary mutation EGFR(T790M) but also other mechanisms. Here we report support for a rationale to target IKBKE, an IκB kinase family member that activates the AKT and NF-κB pathways, as one strategy to address NSCLC resistant to EGFR inhibitors. While wild-type and mutant EGFR directly interacted with IKBKE, only mutant EGFR phosphorylated IKBKE on residues Y153 and Y179. The unphosphorylatable mutant IKBKE-Y153F/Y179-F that lost kinase activity failed to activate AKT and inhibited EGFR signaling. In clinical specimens of NSCLC with activating mutations of EGFR, we observed elevated levels of phospho-Y153 IKBKE. IKBKE ablation with shRNA or small-molecule inhibitor amlexanox selectively inhibited the viability of NSCLC cells with EGFR mutations in vitro In parallel, we found that these treatments activated the MAPK pathway due to attenuation of an IKBKE feedback mechanism. In vivo studies revealed that combining amlexanox with MEK inhibitor AZD6244 significantly inhibited the xenograft tumor growth of NSCLC cells harboring activating EGFR mutations, including EGFR(T790M) Overall, our findings define IKBKE as a direct effector target of EGFR and provide a therapeutic rationale to target IKBKE as a strategy to eradicate EGFR-TKI-resistant NSCLC cells. Cancer Res; 76(15); 4418-29. ©2016 AACR.

Targeting adipose tissue in the treatment of obesity-associated diabetes

Adipose tissue regulates numerous physiological processes, and its dysfunction in obese humans is associated with disrupted metabolic homeostasis, insulin resistance and type 2 diabetes mellitus (T2DM). Although several US-approved treatments for obesity and T2DM exist, these are limited by adverse effects and a lack of effective long-term glucose control. In this Review, we provide an overview of the role of adipose tissue in metabolic homeostasis and assess emerging novel therapeutic strategies targeting adipose tissue, including adipokine-based strategies, promotion of white adipose tissue beiging as well as reduction of inflammation and fibrosis.

Metabolic interplay between white, beige, brown adipocytes and the liver

In mammalian evolution, three types of adipocytes have developed, white, brown and beige adipocytes. White adipocytes are the major constituents of white adipose tissue (WAT), the predominant store for energy-dense triglycerides in the body that are released as fatty acids during catabolic conditions. The less abundant brown adipocytes, the defining parenchymal cells of brown adipose tissue (BAT), internalize triglycerides that are stored intracellularly in multilocular lipid droplets. Beige adipocytes (also known as brite or inducible brown adipocytes) are functionally very similar to brown adipocytes and emerge in specific WAT depots in response to various stimuli including sustained cold exposure. The activation of brown and beige adipocytes (together referred to as thermogenic adipocytes) causes both the hydrolysis of stored triglycerides as well as the uptake of lipids and glucose from the circulation. Together, these fuels are combusted for heat production to maintain body temperature in mammals including adult humans. Given that heating by brown and beige adipocytes is a very-well controlled and energy-demanding process which entails pronounced shifts in energy fluxes, it is not surprising that an intensive interplay exists between the various adipocyte types and parenchymal liver cells, and that this influences systemic metabolic fluxes and endocrine networks. In this review we will emphasize the role of hepatic factors that regulate the metabolic activity of white and thermogenic adipocytes. In addition, we will discuss the relevance of lipids and hormones that are secreted by white, brown and beige adipocytes regulating liver metabolism in order to maintain systemic energy metabolism in health and disease.

Convertible visceral fat as a therapeutic target to curb obesity

New therapeutic and preventative strategies are needed to address the growing obesity epidemic. In animal models, brown adipose tissue activation and the associated heat produced contribute to countering obesity and the accompanying metabolic abnormalities. Adult humans also have functional brown fat. Here, we present and discuss the concepts of murine and human white adipose tissue plasticity and the transdifferentiation of white adipocytes into brown adipocytes. Human visceral adipocytes - which are crucial contributors to the burden of obesity and its complications - are particularly susceptible to such transdifferentiation. Therefore, we propose that this process should be a focus of anti-obesity research. Approved drugs that have browning properties as well as future drugs that target molecular pathways involved in white-to-brown visceral adipocyte transdifferentiation may provide new avenues for obesity therapy.

Regulation of metabolism by the innate immune system

Low-grade tissue inflammation induced by obesity can result in insulin resistance, which in turn is a key cause of type 2 diabetes mellitus. Cells of the innate immune system produce cytokines and other factors that impair insulin signalling, which contributes to the connection between obesity and the onset of type 2 diabetes mellitus. Here, we review the innate immune cells involved in secreting inflammatory factors in the obese state. In the adipose tissue, these cells include proinflammatory adipose tissue macrophages and natural killer cells. We also discuss the role of innate immune cells, such as anti-inflammatory adipose tissue macrophages, eosinophils, group 2 innate lymphoid cells and invariant natural killer T cells, in maintaining an anti-inflammatory and insulin-sensitive environment in the lean state. In the liver, both Kupffer cells and recruited hepatic macrophages can contribute to decreased hepatic insulin sensitivity. Proinflammatory macrophages might also adversely affect insulin sensitivity in the skeletal muscle and pancreatic β-cell function. Finally, this Review provides an overview of the mechanisms for regulating proinflammatory immune responses that could lead to future therapeutic opportunities to improve insulin sensitivity.

Knockout of STAT3 in skeletal muscle does not prevent high-fat diet-induced insulin resistance

Objective

Increased signal transducer and activator of transcription 3 (STAT3) signaling has been implicated in the development of skeletal muscle insulin resistance, though its contribution, in vivo, remains to be fully defined. Therefore, the aim of this study was to determine whether knockout of skeletal muscle STAT3 would prevent high-fat diet (HFD)-induced insulin resistance.

Methods

We used Cre-LoxP methodology to generate mice with muscle-specific knockout (KO) of STAT3 (mKO). Beginning at 10 weeks of age, mKO mice and their wildtype/floxed (WT) littermates either continued consuming a low fat, control diet (CON; 10% of calories from fat) or were switched to a HFD (60% of calories from fat) for 20 days. We measured body composition, energy expenditure, oral glucose tolerance and in vivo insulin action using hyperinsulinemic-euglycemic clamps. We also measured insulin sensitivity in isolated soleus and extensor digitorum longus muscles using the 2-deoxy-glucose (2DOG) uptake technique.

Results

STAT3 protein expression was reduced ∼75–100% in muscle from mKO vs. WT mice. Fat mass and body fat percentage did not differ between WT and mKO mice on CON and were increased equally by HFD. There were also no genotype differences in energy expenditure or whole-body fat oxidation. As determined, in vivo (hyperinsulinemic-euglycemic clamps) and ex vivo (2DOG uptake), skeletal muscle insulin sensitivity did not differ between CON-fed mice, and was impaired similarly by HFD.

Conclusions

These results demonstrate that STAT3 activation does not underlie the development of HFD-induced skeletal muscle insulin resistance.

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Loss of STAT3 in skeletal muscle does not effect whole body energy expenditure in mice.

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Mice with knockout of STAT3 in skeletal muscle (mKO) develop glucose intolerance with HFD feeding similar to littermate controls.

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HFD-induced insulin resistance in skeletal muscle is not prevented by knockout of STAT3.

The brown fat secretome: metabolic functions beyond thermogenesis

Brown fat is highly active in fuel oxidation and dissipates chemical energy through uncoupling protein (UCP)1-mediated heat production. Activation of brown fat leads to increased energy expenditure, reduced adiposity, and lower plasma glucose and lipid levels, thus contributing to better homeostasis. Uncoupled respiration and thermogenesis have been considered to be responsible for the metabolic benefits of brown adipose tissue. Recent studies have demonstrated that brown adipocytes also secrete factors that act locally and systemically to influence fuel and energy metabolism. This review discusses the evidence supporting a thermogenesis-independent role of brown fat, particularly through its release of secreted factors, and their implications in physiology and therapeutic development.