ER Stress Drives Lipogenesis and Steatohepatitis via Caspase-2 Activation of S1P.

Nonalcoholic fatty liver disease (NAFLD) progresses to nonalcoholic steatohepatitis (NASH) in response to elevated endoplasmic reticulum (ER) stress. Whereas the onset of simple steatosis requires elevated de novo lipogenesis, progression to NASH is triggered by accumulation of hepatocyte-free cholesterol. We now show that caspase-2, whose expression is ER-stress inducible and elevated in human and mouse NASH, controls the buildup of hepatic-free cholesterol and triglycerides by activating sterol regulatory element-binding proteins (SREBP) in a manner refractory to feedback inhibition. Caspase-2 colocalizes with site 1 protease (S1P) and cleaves it to generate a soluble active fragment that initiates SCAP-independent SREBP1/2 activation in the ER. Caspase-2 ablation or pharmacological inhibition prevents diet-induced steatosis and NASH progression in ER-stress-prone mice. Caspase-2 inhibition offers a specific and effective strategy for preventing or treating stress-driven fatty liver diseases, whereas caspase-2-generated S1P proteolytic fragments, which enter the secretory pathway, are potential NASH biomarkers.

TBK1 at the Crossroads of Inflammation and Energy Homeostasis in Adipose Tissue.

The noncanonical IKK family member TANK-binding kinase 1 (TBK1) is activated by pro-inflammatory cytokines, but its role in controlling metabolism remains unclear. Here, we report that the kinase uniquely controls energy metabolism. Tbk1 expression is increased in adipocytes of HFD-fed mice. Adipocyte-specific TBK1 knockout (ATKO) attenuates HFD-induced obesity by increasing energy expenditure; further studies show that TBK1 directly inhibits AMPK to repress respiration and increase energy storage. Conversely, activation of AMPK under catabolic conditions can increase TBK1 activity through phosphorylation, mediated by AMPK's downstream target ULK1. Surprisingly, ATKO also exaggerates adipose tissue inflammation and insulin resistance. TBK1 suppresses inflammation by phosphorylating and inducing the degradation of the IKK kinase NIK, thus attenuating NF-κB activity. Moreover, TBK1 mediates the negative impact of AMPK activity on NF-κB activation. These data implicate a unique role for TBK1 in mediating bidirectional crosstalk between energy sensing and inflammatory signaling pathways in both over- and undernutrition.

A futile approach to fighting obesity?

The current obesity epidemic has focused a great deal of attention on mechanisms controlling energy balance. While diet and nutrient absorption affect energy intake, on the other side of the equation, energy expenditure is determined by basal metabolism, physical activity, and adaptive thermogenesis. Given various challenges in modulating these energy balance mechanisms to combat human obesity, many efforts have concentrated on how it might be possible to achieve weight loss through increased thermogenesis. In this issue of Cell, Kazak et al. describe a previously unrecognized molecular pathway for thermogenesis in fat cells.

Glycogen metabolism links glucose homeostasis to thermogenesis in adipocytes.

Adipocytes increase energy expenditure in response to prolonged sympathetic activation via persistent expression of uncoupling protein 1 (UCP1)1,2. Here we report that the regulation of glycogen metabolism by catecholamines is critical for UCP1 expression. Chronic ß-adrenergic activation leads to increased glycogen accumulation in adipocytes expressing UCP1. Adipocyte-specific deletion of a scaffolding protein, protein targeting to glycogen (PTG), reduces glycogen levels in beige adipocytes, attenuating UCP1 expression and responsiveness to cold or ß-adrenergic receptor-stimulated weight loss in obese mice. Unexpectedly, we observed that glycogen synthesis and degradation are increased in response to catecholamines, and that glycogen turnover is required to produce reactive oxygen species leading to the activation of p38 MAPK, which drives UCP1 expression. Thus, glycogen has a key regulatory role in adipocytes, linking glucose metabolism to thermogenesis.

Derepressing nuclear receptors for metabolic adaptation.

NCoR is a corepressor of several transcription factors, including the PPAR family of nuclear receptors in fat and muscle. By specifically deleting NCoR in these tissues, Li et al. and Yamamoto et al. now uncover an important role for NCoR in regulating lipid homeostasis through the coordinated control of different nuclear receptors.

Fishing out a sensor for anti-inflammatory oils.

The omega-3 fatty acids have anti-inflammatory and antidiabetic effects in humans. Now, Oh et al. (2010) demonstrate that the G protein-coupled receptor GPR120 is a receptor for omega-3 fatty acids on macrophages and fat cells. Activation of GPR120 by omega-3 fatty acids inhibits multiple inflammation cascades in macrophages and reverses insulin resistance in obese mice.

White, brown, and beige; type 2 immunity gets hot.

The biogenesis of beige fat is poorly understood. In recent issues of Nature and Cell, Brestoff et al. (2014) and Lee et al. (2015) demonstrate that resident innate lymphoid cells in subcutaneous fat generate and activate beige adipocytes, producing thermogenesis.

Regulation of glucose transport by insulin: traffic control of GLUT4.

Despite daily fasting and feeding, plasma glucose levels are normally maintained within a narrow range owing to the hormones insulin and glucagon. Insulin increases glucose uptake into fat and muscle cells through the regulated trafficking of vesicles that contain glucose transporter type 4 (GLUT4). New insights into insulin signalling reveal that phosphorylation events initiated by the insulin receptor regulate key GLUT4 trafficking proteins, including small GTPases, tethering complexes and the vesicle fusion machinery. These proteins, in turn, control GLUT4 movement through the endosomal system, formation and retention of specialized GLUT4 storage vesicles and targeted exocytosis of these vesicles. Understanding these processes may help to explain the development of insulin resistance in type 2 diabetes and provide new potential therapeutic targets.

The protein kinase IKKepsilon regulates energy balance in obese mice.

Obesity is associated with chronic low-grade inflammation that negatively impacts insulin sensitivity. Here, we show that high-fat diet can increase NF-kappaB activation in mice, which leads to a sustained elevation in level of IkappaB kinase epsilon (IKKepsilon) in liver, adipocytes, and adipose tissue macrophages. IKKepsilon knockout mice are protected from high-fat diet-induced obesity, chronic inflammation in liver and fat, hepatic steatosis, and whole-body insulin resistance. These mice show increased energy expenditure and thermogenesis via enhanced expression of the uncoupling protein UCP1. They maintain insulin sensitivity in liver and fat, without activation of the proinflammatory JNK pathway. Gene expression analyses indicate that IKKepsilon knockout reduces expression of inflammatory cytokines, and changes expression of certain regulatory proteins and enzymes involved in glucose and lipid metabolism. Thus, IKKepsilon may represent an attractive therapeutic target for obesity, insulin resistance, diabetes, and other complications associated with these disorders.

Bi-allelic Variants in RALGAPA1 Cause Profound Neurodevelopmental Disability, Muscular Hypotonia, Infantile Spasms, and Feeding Abnormalities.

Ral (Ras-like) GTPases play an important role in the control of cell migration and have been implicated in Ras-mediated tumorigenicity. Recently, variants in RALA were also described as a cause of intellectual disability and developmental delay, indicating the relevance of this pathway to neuropediatric diseases. Here, we report the identification of bi-allelic variants in RALGAPA1 (encoding Ral GTPase activating protein catalytic alpha subunit 1) in four unrelated individuals with profound neurodevelopmental disability, muscular hypotonia, feeding abnormalities, recurrent fever episodes, and infantile spasms . Dysplasia of corpus callosum with focal thinning of the posterior part and characteristic facial features appeared to be unifying findings. RalGAPA1 was absent in the fibroblasts derived from two affected individuals suggesting a loss-of-function effect of the RALGAPA1 variants. Consequently, RalA activity was increased in these cell lines, which is in keeping with the idea that RalGAPA1 deficiency causes a constitutive activation of RalA. Additionally, levels of RalGAPB, a scaffolding subunit of the RalGAP complex, were dramatically reduced, indicating a dysfunctional RalGAP complex. Moreover, RalGAPA1 deficiency clearly increased cell-surface levels of lipid raft components in detached fibroblasts, which might indicate that anchorage-dependence of cell growth signaling is disturbed. Our findings indicate that the dysregulation of the RalA pathway has an important impact on neuronal function and brain development. In light of the partially overlapping phenotype between RALA- and RALGAPA1-associated diseases, it appears likely that dysregulation of the RalA signaling pathway leads to a distinct group of genetic syndromes that we suggest could be named RALopathies.

Ral and Rheb GTPase activating proteins integrate mTOR and GTPase signaling in aging, autophagy, and tumor cell invasion.

Diverse environmental cues converge on and are integrated by the mTOR signaling network to control cellular growth and homeostasis. The mammalian Tsc1-Tsc2 GTPase activating protein (GAP) heterodimer is a critical negative regulator of Rheb and mTOR activation. The RalGAPα-RalGAPß heterodimer shares sequence and structural similarity with Tsc1-Tsc2. Unexpectedly, we observed that C. elegans expresses orthologs for the Rheb and RalA/B GTPases and for RalGAPα/ß, but not Tsc1/2. This prompted our investigation to determine whether RalGAPs additionally modulate mTOR signaling. We determined that C. elegans RalGAP loss decreased lifespan, consistent with a Tsc-like function. Additionally, RalGAP suppression in mammalian cells caused RalB-selective activation and Sec5- and exocyst-dependent engagement of mTORC1 and suppression of autophagy. Unexpectedly, we also found that Tsc1-Tsc2 loss activated RalA/B independently of Rheb-mTOR signaling. Finally, RalGAP suppression caused mTORC1-dependent pancreatic tumor cell invasion. Our findings identify an unexpected crosstalk and integration of the Ral and mTOR signaling networks.

YIPF6 controls sorting of FGF21 into COPII vesicles and promotes obesity.

Fibroblast growth factor 21 (FGF21) is an endocrine hormone that regulates glucose, lipid, and energy homeostasis. While gene expression of FGF21 is regulated by the nuclear hormone receptor peroxisome proliferator-activated receptor alpha in the fasted state, little is known about the regulation of trafficking and secretion of FGF21. We show that mice with a mutation in the Yip1 domain family, member 6 gene (Klein-Zschocher [KLZ]; Yipf6KLZ/Y ) on a high-fat diet (HFD) have higher plasma levels of FGF21 than mice that do not carry this mutation (controls) and hepatocytes from Yipf6KLZ/Y mice secrete more FGF21 than hepatocytes from wild-type mice. Consequently, Yipf6KLZ/Y mice are resistant to HFD-induced features of the metabolic syndrome and have increased lipolysis, energy expenditure, and thermogenesis, with an increase in core body temperature. Yipf6KLZ/Y mice with hepatocyte-specific deletion of FGF21 were no longer protected from diet-induced obesity. We show that YIPF6 binds FGF21 in the endoplasmic reticulum to limit its secretion and specifies packaging of FGF21 into coat protein complex II (COPII) vesicles during development of obesity in mice. Levels of YIPF6 protein in human liver correlate with hepatic steatosis and correlate inversely with levels of FGF21 in serum from patients with nonalcoholic fatty liver disease (NAFLD). YIPF6 is therefore a newly identified regulator of FGF21 secretion during development of obesity and could be a target for treatment of obesity and NAFLD.

From overnutrition to liver injury: AMP-activated protein kinase in nonalcoholic fatty liver diseases.

Nonalcoholic fatty liver diseases (NAFLDs), especially nonalcoholic steatohepatitis (NASH), have become a major cause of liver transplant and liver-associated death. However, the pathogenesis of NASH is still unclear. Currently, there is no FDA-approved medication to treat this devastating disease. AMP-activated protein kinase (AMPK) senses energy status and regulates metabolic processes to maintain homeostasis. The activity of AMPK is regulated by the availability of nutrients, such as carbohydrates, lipids, and amino acids. AMPK activity is increased by nutrient deprivation and inhibited by overnutrition, inflammation, and hypersecretion of certain anabolic hormones, such as insulin, during obesity. The repression of hepatic AMPK activity permits the transition from simple steatosis to hepatocellular death; thus, activation might ameliorate multiple aspects of NASH. Here we review the pathogenesis of NAFLD and the impact of AMPK activity state on hepatic steatosis, inflammation, liver injury, and fibrosis during the transition of NAFL to NASH and liver failure.

Inhibition of AMPK catabolic action by GSK3.

AMP-activated protein kinase (AMPK) regulates cellular energy homeostasis by inhibiting anabolic and activating catabolic processes. While AMPK activation has been extensively studied, mechanisms that inhibit AMPK remain elusive. Here we report that glycogen synthase kinase 3 (GSK3) inhibits AMPK function. GSK3 forms a stable complex with AMPK through interactions with the AMPK ß regulatory subunit and phosphorylates the AMPK α catalytic subunit. This phosphorylation enhances the accessibility of the activation loop of the α subunit to phosphatases, thereby inhibiting AMPK kinase activity. Surprisingly, PI3K-Akt signaling, which is a major anabolic signaling and normally inhibits GSK3 activity, promotes GSK3 phosphorylation and inhibition of AMPK, thus revealing how AMPK senses anabolic environments in addition to cellular energy levels. Consistently, disrupting GSK3 function within the AMPK complex sustains higher AMPK activity and cellular catabolic processes even under anabolic conditions, indicating that GSK3 acts as a critical sensor for anabolic signaling to regulate AMPK.

RalA controls glucose homeostasis by regulating glucose uptake in brown fat.

Insulin increases glucose uptake into adipose tissue and muscle by increasing trafficking of the glucose transporter Glut4. In cultured adipocytes, the exocytosis of Glut4 relies on activation of the small G protein RalA by insulin, via inhibition of its GTPase activating complex RalGAP. Here, we evaluate the role of RalA in glucose uptake in vivo with specific chemical inhibitors and by generation of mice with adipocyte-specific knockout of RalGAPB. RalA was profoundly activated in brown adipose tissue after feeding, and its inhibition prevented Glut4 exocytosis. RalGAPB knockout mice with diet-induced obesity were protected from the development of metabolic disease due to increased glucose uptake into brown fat. Thus, RalA plays a crucial role in glucose transport in adipose tissue in vivo.

Vinexin family (SORBS) proteins play different roles in stiffness-sensing and contractile force generation.

Vinexin, c-Cbl associated protein (CAP) and Arg-binding protein 2 (ArgBP2) constitute an adaptor protein family called the vinexin (SORBS) family that is targeted to focal adhesions (FAs). Although numerous studies have focused on each of the SORBS proteins and partially elucidated their involvement in mechanotransduction, a comparative analysis of their function has not been well addressed. Here, we established mouse embryonic fibroblasts that individually expressed SORBS proteins and analysed their functions in an identical cell context. Both vinexin-α and CAP co-localized with vinculin at FAs and promoted the appearance of vinculin-rich FAs, whereas ArgBP2 co-localized with α-actinin at the proximal end of FAs and punctate structures on actin stress fibers (SFs), and induced paxillin-rich FAs. Furthermore, both vinexin-α and CAP contributed to extracellular matrix stiffness-dependent vinculin behaviors, while ArgBP2 stabilized α-actinin on SFs and enhanced intracellular contractile forces. These results demonstrate the differential roles of SORBS proteins in mechanotransduction.

Lipotoxicity induces hepatic protein inclusions through TANK binding kinase 1-mediated p62/sequestosome 1 phosphorylation.

Obesity commonly leads to hepatic steatosis, which often provokes lipotoxic injuries to hepatocytes that cause nonalcoholic steatohepatitis (NASH). NASH, in turn, is associated with the accumulation of insoluble protein aggregates that are composed of ubiquitinated proteins and ubiquitin adaptor p62/sequestosome 1 (SQSTM1). Formation of p62 inclusions in hepatocytes is the critical marker that distinguishes simple fatty liver from NASH and predicts a poor prognostic outcome for subsequent liver carcinogenesis. However, the molecular mechanism by which lipotoxicity induces protein aggregation is currently unknown. Here, we show that, upon saturated fatty acid-induced lipotoxicity, TANK binding kinase 1 (TBK1) is activated and phosphorylates p62. TBK1-mediated p62 phosphorylation is important for lipotoxicity-induced aggregation of ubiquitinated proteins and formation of large protein inclusions in hepatocytes. In addition, cyclic GMP-AMP synthase (cGAS) and stimulator of interferon genes (STING), upstream regulators of TBK1, are involved in lipotoxic activation of TBK1 and subsequent p62 phosphorylation in hepatocytes. Furthermore, TBK1 inhibition prevented formation of ubiquitin-p62 aggregates not only in cultured hepatocytes, but also in mouse models of obesity and NASH. CONCLUSION: These results suggest that lipotoxic activation of TBK1 and subsequent p62 phosphorylation are critical steps in the NASH pathology of protein inclusion accumulation in hepatocytes. This mechanism can provide an explanation for how hypernutrition and obesity promote the development of severe liver pathologies, such as steatohepatitis and liver cancer, by facilitating the formation of p62 inclusions. (Hepatology 2018).

Synthesis of deuterium-labelled amlexanox and its metabolic stability against mouse, rat, and human microsomes.

As part of a program toward making analogues of amlexanox (1), currently under clinical investigation for the treatment of type 2 diabetes and obesity, we have synthesized derivative 5 in which deuterium has been introduced into two sites of metabolism on the C-7 isopropyl function of amlexanox. The synthesis of 5 was completed in an efficient three-step process utilizing reduction of key olefin 7b to 8 by Wilkinson's catalyst to provide specific incorporation of di-deuterium across the double bond. Compound 5 displayed nearly equivalent potency to amlexanox (IC50 , 1.1µM vs 0.6µM, respectively) against recombinant human TBK1. When incubated with human, rat, and mouse liver microsomes, amlexanox (1) and d2 -amlexanox (5) were stable (t1/2  > 60 minutes) with 1 showing marginally greater stability relative to 5 except for rat liver microsomes. These data show that incorporating deuterium into two sites of metabolism does not majorly suppress Cyp-mediated metabolism relative to amlexanox.

Carboxylic Acid Derivatives of Amlexanox Display Enhanced Potency toward TBK1 and IKKε and Reveal Mechanisms for Selective Inhibition.

Chronic low-grade inflammation is a hallmark of obesity, which is a risk factor for the development of type 2 diabetes. The drug amlexanox inhibits IκB kinase ε (IKKε) and TANK binding kinase 1 (TBK1) to promote energy expenditure and improve insulin sensitivity. Clinical studies have demonstrated efficacy in a subset of diabetic patients with underlying adipose tissue inflammation, albeit with moderate potency, necessitating the need for improved analogs. Herein we report crystal structures of TBK1 in complex with amlexanox and a series of analogs that modify its carboxylic acid moiety. Removal of the carboxylic acid or mutation of the adjacent Thr156 residue significantly reduces potency toward TBK1, whereas conversion to a short amide or ester nearly abolishes the inhibitory effects. IKKε is less affected by these modifications, possibly due to variation in its hinge that allows for increased conformational plasticity. Installation of a tetrazole carboxylic acid bioisostere improved potency to 200 and 400 nM toward IKKε and TBK1, respectively. Despite improvements in the in vitro potency, no analog produced a greater response in adipocytes than amlexanox, perhaps because of altered absorption and distribution. The structure-activity relationships and cocrystal structures described herein will aid in future structure-guided inhibitor development using the amlexanox pharmacophore for the treatment of obesity and type 2 diabetes.