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Monoacylglycerol O-acyltransferase 1 (Mogat1), a lipogenic enzyme that converts monoacylglycerol to diacylglycerol, is highly expressed in adipocytes and may regulate lipolysis by re-esterifying fatty acids released during times when lipolytic rates are low. However, the role of Mogat1 in regulating adipocyte fat storage during differentiation and diet-induced obesity is relatively understudied.Here, adipocyte-specific Mogat1 knockout mice were generated and subjected to a high-fat diet to determine the effects of Mogat1 deficiency on diet-induced obesity. Mogat1 floxed mice were also used to develop preadipocyte cell lines wherein Mogat1 could be conditionally knocked out to study adipocyte differentiation in vitro.In preadipocytes, it was found that Mogat1 knockout at the onset of preadipocyte differentiation prevented the accumulation of glycerolipids and reduced the differentiation capacity of preadipocytes. However, the loss of adipocyte Mogat1 did not affect weight gain or fat mass induced by a high-fat diet in mice. Furthermore, loss of Mogat1 in adipocytes did not affect plasma lipid or glucose concentrations or insulin tolerance.These data suggest Mogat1 may play a role in adipocyte differentiation in vitro but not adipose tissue expansion in response to nutrient overload in mice.

Pyruvate metabolism, a central nexus of carbon homeostasis, is an evolutionarily conserved process and aberrant pyruvate metabolism is associated with and contributes to numerous human metabolic disorders including diabetes, cancer, and heart disease. As a product of glycolysis, pyruvate is primarily generated in the cytosol before being transported into the mitochondrion for further metabolism. Pyruvate entry into the mitochondrial matrix is a critical step for efficient generation of reducing equivalents and ATP and for the biosynthesis of glucose, fatty acids, and amino acids from pyruvate. However, for many years, the identity of the carrier protein(s) that transported pyruvate into the mitochondrial matrix remained a mystery. In 2012, the molecular-genetic identification of the mitochondrial pyruvate carrier (MPC), a heterodimeric complex composed of protein subunits MPC1 and MPC2, enabled studies that shed light on the many metabolic and physiological processes regulated by pyruvate metabolism. A better understanding of the mechanisms regulating pyruvate transport and the processes affected by pyruvate metabolism may enable novel therapeutics to modulate mitochondrial pyruvate flux to treat a variety of disorders. Herein, we review our current knowledge of the MPC, discuss recent advances in the understanding of mitochondrial pyruvate metabolism in various tissue and cell types, and address some of the outstanding questions relevant to this field.

The geroscience hypothesis states that a therapy that prevents the underlying aging process should prevent multiple aging related diseases. The mTOR (mechanistic target of rapamycin)/insulin and NAD+ (nicotinamide adenine dinucleotide) pathways are two of the most validated aging pathways. Yet, it’s largely unclear how they might talk to each other in aging. In genome-wide CRISPRa screening with a novel class of N-O-Methyl-propanamide-containing compounds we named BIOIO-1001, we identified lipid metabolism centering on SIRT3 as a point of intersection of the mTOR/insulin and NAD+ pathways. In vivo testing indicated that BIOIO-1001 reduced high fat, high sugar diet-induced metabolic derangements, inflammation, and fibrosis, each being characteristic of non-alcoholic steatohepatitis (NASH). An unbiased screen of patient datasets suggested a potential link between the anti-inflammatory and anti-fibrotic effects of BIOIO-1001 in NASH models to those in amyotrophic lateral sclerosis (ALS). Directed experiments subsequently determined that BIOIO-1001 was protective in both sporadic and familial ALS models. Both NASH and ALS have no treatments and suffer from a lack of convenient biomarkers to monitor therapeutic efficacy. A potential strength in considering BIOIO-1001 as a therapy is that the blood biomarker that it modulates, namely plasma triglycerides, can be conveniently used to screen patients for responders. More conceptually, to our knowledge BIOIO-1001 is a first therapy that fits the geroscience hypothesis by acting on multiple core aging pathways and that can alleviate multiple conditions after they have set in.Brief SummaryThese studies characterize a novel gerotherapy, BIOIO-1001, that identifies lipid metabolism as an intersection of the mTOR and NAD+ pathways.

Farnesoid X receptor (FXR) is an important regulator of bile acid, lipid, amino acid, and glucose homeostasis, hepatic inflammation, regeneration, and fibrosis. FXR has been recognized as a promising drug target for various metabolic diseases such as lipid disorders, nonalcoholic fatty liver disease (NAFLD), nonalcoholic steatohepatitis (NASH), and chronic kidney disease. A large number of FXR ligands have been developed by pharmaceutical companies and academic institutions, and several candidates have progressed into clinical trials in the past decade. However, it is continually a challenge to discover drugs targeting FXR due to side effects associated with long-term administration. In this perspective, we summarize the research progress on medicinal chemistry of FXR modulators from 2018 to the present by discussing the diverse structures of synthetic FXR modulators including steroidal and non-steroidal ligands, their structure-activity relationships (SARs), and their therapeutic applications.

SUMMARYThe mitochondrial response to changes in cellular energy demand is necessary for cellular adaptation and organ function. Many genes are essential in orchestrating this response, including the transforming growth factor (TGF)-β1 target gene MSS51, which is an inhibitor of skeletal muscle mitochondrial metabolism. Despite the potential importance of MSS51 in the pathophysiology of obesity and musculoskeletal disease, how MSS51 is regulated is not entirely understood. Site-1 Protease (S1P) is a Golgi-resident protease that is a key activator of several transcription factors required for cellular adaptation. However, the role of S1P in muscle and mitochondrial function are unknown. Here, we identify S1P as a negative regulator of muscle mass and mitochondrial metabolism. Disruption of S1P in mouse skeletal muscle and cultured myofibers leads to a reduction in MSS51 expression, increased muscle mass, and increased mitochondrial oxygen consumption. The effects of S1P deficiency on mitochondrial activity are counteracted by overexpressing MSS51, suggesting that S1P inhibits mitochondrial metabolism by regulating the expression of MSS51. Furthermore, S1P suppression enhances TGF-β signaling via the AKT pathway, potentially explaining muscle hypertrophy in S1P deficient mice. The discovery of S1P as a regulator of mitochondrial metabolism and muscle mass expands our understanding of TGF-β signaling and suggests this protease could be a target for therapeutic intervention in muscle.

Nonalcoholic steatohepatitis (NASH) is a multisystem disease that affects not only the liver but also heart, pancreas, and kidney. We currently lack a comprehensive understanding of mechanisms responsible for the development of NASH-associated cardiomyopathy or the influence of sex on pathophysiology. There is a critical need to address these gaps in knowledge in order to accelerate translation of knowledge into clinical practice.NASH and cardiovascular disease share common risk factors such as chronic inflammation, hyperlipidemia, and insulin resistance. Early cardiac dysfunction in NASH that is independent of obesity or other cardiometabolic risk factors suggests roles for liver-heart crosstalk in disease pathogenesis. Inflammation is a driving force in the pathogenesis of NASH, and it is likely that 'spill over' of NASH inflammation contributes to the development of cardiomyopathy. However, molecular and cellular mechanisms that mediate NASH-associated cardiomyopathy remain unclear because of inherent limitations of experimental models. Even so, recent studies implicate inflammatory, metabolic, and physiologic mechanisms that enhance our understanding of NASH-associated cardiomyopathy and the role of liver-heart crosstalk.An innovative, detailed, and mechanistic understanding of NASH-associated cardiomyopathy is relevant to public health and will be fundamental for the comprehensive care of these patients.

Liver failure secondary to nonalcoholic fatty liver disease (NAFLD) has become the most common cause for liver transplantation in many parts of the world. Moreover, the prevalence of NAFLD not only increases the demand for liver transplantation, but also limits the supply of suitable donor organs because steatosis predisposes grafts to ischemia-reperfusion injury (IRI). There are currently no pharmacological interventions to limit hepatic IR injury because the mechanisms by which steatosis leads to increased injury are unclear. To identify potential novel mediators of IR injury, we used liquid chromatography and mass spectrometry to assess temporal changes in the hepatic lipidome in steatotic and non-steatotic livers after warm IRI in mice. Our untargeted analyses revealed distinct differences between the steatotic and non-steatotic response to IRI and highlighted dynamic changes in lipid composition with marked changes in glycerolipids and glycerophospholipids. These findings enhance our knowledge of the lipidomic changes that occur following IRI and provide a foundation for future mechanistic studies. A better understanding of the mechanisms underlying such changes will lead to novel therapeutic strategies to combat IR injury.

Non-alcoholic fatty liver disease (NAFLD) has emerged as the most prevalent liver disease in the world, yet there are still no approved pharmacological therapies to prevent or treat this condition. NAFLD encompasses a spectrum of severity, ranging from simple steatosis to non-alcoholic steatohepatitis (NASH). Although NASH is linked to an increased risk of hepatocellular carcinoma and cirrhosis and has now become the leading cause of liver failure-related transplantation, the majority of patients with NASH will ultimately die as a result of complications of type 2 diabetes mellitus (T2DM) and cardiometabolic diseases. Importantly, NAFLD is closely linked to obesity and tightly interrelated with insulin resistance and T2DM. Thus, targeting these interconnected conditions and taking a holistic attitude to the treatment of metabolic disease could prove to be a very beneficial approach. This Review will explore the latest relevant literature and discuss the ongoing therapeutic options for NAFLD focused on targeting intermediary metabolism, insulin resistance and T2DM to remedy the global health burden of these diseases.

Pyruvate is a critical intermediary metabolite in gluconeogenesis, lipogenesis, as well as NADH production. As a result, there is growing interest in targeting the mitochondrial pyruvate carrier (MPC) complex in liver and metabolic diseases. However, recent in vitro data indicate that MPC inhibition diverts glutamine/glutamate away from glutathione synthesis and toward glutaminolysis to compensate for loss of pyruvate oxidation, possibly sensitizing cells to oxidative insult. Here, we explored this using the clinically relevant acetaminophen (APAP) overdose model of acute liver injury, which is driven by oxidative stress. We report that MPC inhibition does indeed sensitize the liver to APAP-induced injury in vivo, but only with concomitant loss of alanine aminotransferase 2 (ALT2). Pharmacologic and genetic manipulation of neither MPC2 nor ALT2 alone affected APAP toxicity, but liver-specific double knockout (DKO) of these proteins significantly worsened the liver damage. Further investigation confirmed that DKO impaired glutathione synthesis and increased urea cycle flux, consistent with increased glutaminolysis. Furthermore, APAP toxicity was exacerbated by inhibition of both the MPC and ALT in vitro. Thus, increased glutaminolysis and susceptibility to oxidative stress requires loss of both the MPC and ALT2 in vivo and exacerbates them in vitro. Finally, induction of ALT2 reduced APAP-induced injury.

Alzheimer’s (AD) and Parkinson’s Diseases (PD) are common neurodegenerative disorders growing in incidence and prevalence and for which there are no disease-modifying treatments. While there are considerable complexities in the presentations of these diseases, the histological pictures of these pathologies, as well as several rare genetic predispositions for each, point to the involvement of maladaptive protein processing and inflammation. Importantly, the common presentations of AD and PD are connected to aging and to dysmetabolism, including common co-diagnosis of metabolic syndrome or diabetes. Examination of anti-diabetic therapies in preclinical models and in some observational clinical studies have suggested effectiveness of the first generation insulin sensitizer pioglitazone in both AD and PD. Recently, the mitochondrial pyruvate carrier (MPC) was shown to be a previously unrecognized target of pioglitazone. New insulin sensitizers are in development that can be dosed to full engagement of this previously unappreciated mitochondrial target. Here we review molecular mechanisms that connect modification of pyruvate metabolism with known liabilities of AD and PD. The mechanisms involve modification of autophagy, inflammation, and cell differentiation in various cell types including neurons, glia, macrophages, and endothelium. These observations have implications for the understanding of the general pathology of neurodegeneration and suggest general therapeutic approaches to disease modification.

ObjectiveThe mitochondrial pyruvate carrier (MPC) has emerged as a therapeutic target for treating insulin resistance, type 2 diabetes, and nonalcoholic steatohepatitis (NASH). We evaluated whether MPC inhibitors (MPCi) might correct impairments in branched chain amino acid (BCAA) catabolism, which are predictive of developing diabetes and NASH.MethodsCirculating BCAA concentrations were measured in people with NASH and type 2 diabetes, who participated in a recent randomized, placebo-controlled Phase IIB clinical trial to test the efficacy and safety of the MPCi MSDC-0602K (EMMINENCE;NCT02784444). In this 52-week trial, patients were randomly assigned to placebo (n = 94) or 250 mg MSDC-0602K (n = 101). Human hepatoma cell lines were used to test the direct effects of various MPCi on BCAA catabolism in vitro. Lastly, we investigated how hepatocyte-specific deletion of MPC2 affects BCAA catabolism in the liver of obese mice.ResultsIn patients with NASH, MSDC-0602K treatment, which led to marked improvements in insulin sensitivity and diabetes, decreased plasma concentrations of BCAAs compared to baseline while placebo had no effect. The rate-limiting enzyme in BCAA catabolism is the mitochondrial branched chain ketoacid dehydrogenase (BCKDH), which is deactivated by phosphorylation. In multiple human hepatoma cell lines, MPCi markedly reduced BCKDH phosphorylation and stimulated branched chain keto acid catabolism; an effect that required the BCKDH phosphatase PPM1K. Mechanistically, the effects of MPCi were linked to activation of the energy sensing AMP-dependent protein kinase (AMPK) and mechanistic target of rapamycin (mTOR) kinase signaling cascades. Finally, BCKDH phosphorylation was reduced in liver of obese, hepatocyte-specific MPC2 knockout (LS-Mpc2-/-) mice compared to wild-type controls concomitant with activation of mTOR signaling in vivo.ConclusionsThese data demonstrate novel cross talk between mitochondrial pyruvate and BCAA metabolism and suggest that MPC inhibition leads to lower plasma BCAA concentrations and BCKDH phosphorylation by activating the AMPK/mTOR axis.

The pervasion of three daily meals and snacks is a relatively new introduction to our shared experience, and is coincident with an epidemic rise in obesity and cardiometabolic disorders of overnutrition. The past two decades have yielded convincing evidence regarding the adaptive, protective effects of calorie restriction (CR) and intermittent fasting (IF) against cardiometabolic, neurodegenerative, proteostatic and inflammatory diseases. Yet, durable adherence to intensive lifestyle changes is rarely attainable. New evidence now demonstrates that restricting carbohydrate entry into the hepatocyte by itself mimics several key signaling responses and physiological outcomes of IF and CR. This discovery raises the intriguing proposition that targeting hepatocyte carbohydrate transport to mimic fasting and caloric restriction can abate cardiometabolic and perhaps other fasting-treatable diseases. Here, we review the metabolic and signaling fates of a hepatocyte carbohydrate, identify evidence to target the key mediators within these pathways, and provide rationale and data to highlight carbohydrate transport as a broad, proximal intervention to block the deleterious sequelae of hepatic glucose and fructose metabolism.

ObjectiveMonoacylglycerol O-acyltransferase 1 (Mogat1), a lipogenic enzyme that converts monoacylglycerol to diacylglycerol, is highly expressed in adipocytes and may regulate lipolysis by re-esterifying fatty acids released during times when lipolytic rates are low. However, the role of Mogat1 in regulating adipocyte fat storage during differentiation and diet-induced obesity is relatively understudied.MethodsHere we generated adipocyte-specific Mogat1 knockout mice and subjected them to a high-fat diet to determine the effects of Mogat1 deficiency on diet-induced obesity. We also used Mogat1 floxed mice to develop preadipocyte cell lines wherein Mogat1 could be conditionally knocked out to study adipocyte differentiation in vitro.ResultsIn preadipocytes, we found that Mogat1 knockout at the onset of preadipocyte differentiation prevented the accumulation of glycerolipids and reduced the differentiation capacity of preadipocytes. However, the loss of adipocyte Mogat1 did not affect weight gain or fat mass induced by high-fat diet in mice. Furthermore, loss of Mogat1 in adipocytes did not affect plasma lipid or glucose concentrations or insulin tolerance.ConclusionsThese data suggest Mogat1 may play a role in adipocyte differentiation in vitro but not adipose tissue expansion in response to nutrient overload in mice.STUDY IMPORTANCEWhat is already known?Adipose tissue expansion through adipocyte precursor cell differentiation is critical for proper lipid storage during nutrient overload.Monoacylglycerol O-acyltransferase 1 (Mogat1), a lipogenic enzyme, is highly induced during adipocyte differentiation of human and mouse precursor cells and is reduced in patients with obesity and metabolic dysfunction.What does this study add?Mogat1 deletion during early adipocyte differentiation reduces differentiation capacity, adipogenic gene expression and lowers glycerolipid content of differentiated adipocytes.Adipocyte Mogat1 expression is dispensable for adiposity and metabolic outcomes high-fat fed mice and suggests compensation from other glycerolipid synthesis enzymes.How might these results change the direction of research?Understanding the molecular mechanisms of glycerolipid metabolism healthy adipose tissue expansion.

Hepatic gluconeogenesis from amino acids contributes significantly to diabetic hyperglycemia, but the molecular mechanisms involved are incompletely understood. Alanine transaminases (ALT1 and ALT2) catalyze the interconversion of alanine and pyruvate, which is required for gluconeogenesis from alanine. We find that ALT2 is overexpressed in the liver of diet-induced obese and db/db mice and that the expression of the gene encoding ALT2 (GPT2) is downregulated following bariatric surgery in people with obesity. The increased hepatic expression of Gpt2 in db/db liver is mediated by activating transcription factor 4, an endoplasmic reticulum stress-activated transcription factor. Hepatocyte-specific knockout of Gpt2 attenuates incorporation of

SUMMARYHepatic gluconeogenesis from amino acids contributes significantly to diabetic hyperglycemia, but the molecular mechanisms involved are incompletely understood. Alanine transaminases (ALT1 and ALT2) catalyze the interconversion of alanine and pyruvate, which is required for gluconeogenesis from alanine. We found that ALT2 was overexpressed in liver of diet-induced obese and db/db mice and that the expression of the gene encoding ALT2 (GPT2) was downregulated following bariatric surgery in people with obesity. The increased hepatic expression of Gpt2 in db/db liver was mediated by activating transcription factor 4; an endoplasmic reticulum stress-activated transcription factor. Hepatocyte-specific knockout of Gpt2 attenuated incorporation of 13C-alanine into newly synthesized glucose by hepatocytes. In vivo Gpt2 knockdown or knockout in liver had no effect on glucose concentrations in lean mice, but Gpt2 suppression alleviated hyperglycemia in db/db mice. These data suggest that ALT2 plays a significant role in hepatic gluconeogenesis from amino acids in diabetes.

SummaryHematopoietic homeostasis is maintained by stem and progenitor cells in part by extrinsic feedback cues triggered by mature cell loss. We demonstrate a different mechanism by which hematopoietic progenitors intrinsically anticipate and prevent the loss of mature progeny through metabolic switches. We examined hematopoiesis in mice conditionally deficient in long-chain fatty acid oxidation (carnitine palmitoyltransferase 2,Cpt2), glutaminolysis (glutaminase,Gls), or mitochondrial pyruvate import (mitochondrial pyruvate carrier 2,Mpc2). While genetic ablation ofCpt2orGlsminimally impacted most blood lineages, deletion ofMpc2led to a sharp decline in mature myeloid cells. However, MPC2-deficient myeloid cells rapidly recovered due to a transient increase in myeloid progenitor proliferation. Competitive bone marrow chimera and stable isotope tracing experiments demonstrated that this proliferative burst was intrinsic to MPC2-deficient progenitors and accompanied by a metabolic switch to glutaminolysis. Thus, hematopoietic progenitors intrinsically adjust to metabolic perturbations independently of feedback from downstream mature cells to maintain homeostasis.

Insulin signaling is known to regulate essential proteostasis mechanisms.The analyses here examined effects of insulin signaling in the PiZ mouse model of α1-antitrypsin deficiency in which hepatocellular accumulation and proteotoxicity of the misfolded α1-antitrypsin Z variant (ATZ) causes liver fibrosis and cancer.We first studied the effects of breeding PiZ mice to liver-insulin-receptor knockout (LIRKO) mice (with hepatocyte-specific insulin-receptor gene disruption). The results showed decreased hepatic ATZ accumulation and liver fibrosis in PiZ x LIRKO vs PiZ mice, with reversal of those effects when we bred PiZ x LIRKO mice onto a FOXO1-deficient background. Increased intracellular degradation of ATZ mediated by autophagy was identified as the likely mechanism for diminished hepatic proteotoxicity in PiZ x LIRKO mice and the converse was responsible for enhanced toxicity in PiZ x LIRKO x FOXO1-KO animals. Transcriptomic studies showed major effects on oxidative phosphorylation and autophagy genes, and significant induction of peroxisome proliferator-activated-receptor-γ-coactivator-1α (PGC1α) expression in PiZ-LIRKO mice. Because PGC1α plays a key role in oxidative phosphorylation, we further investigated its effects on ATZ proteostasis in our ATZ-expressing mammalian cell model. The results showed PGC1α overexpression or activation enhances autophagic ATZ degradation.These data implicate suppression of autophagic ATZ degradation by down-regulation of PGC1α as one mechanism by which insulin signaling exacerbates hepatic proteotoxicity in PiZ mice, and identify PGC1α as a novel target for development of new human α1-antitrypsin deficiency liver disease therapies.

The peptide hormone adropin regulates energy metabolism in skeletal muscle and plays important roles in the regulation of metabolic homeostasis. Besides muscle, the liver has an essential role in regulating glucose homeostasis. Previous studies have reported that treatment of diet-induced obese (DIO) male mice with adropin(34–76) (the putative secreted domain) reduces fasting blood glucose independently of body weight changes, suggesting that adropin suppresses glucose production in the liver. Here, we explored the molecular mechanisms underlying adropin's effects on hepatic glucose metabolism in DIO mice. Male DIO B6 mice maintained on a high-fat diet received five intraperitoneal injections of adropin(34–76) (450 nmol/kg/injection) over a 48-h period. We found that adropin(34–76) enhances major intracellular signaling activities in the liver that are involved in insulin-mediated regulation of glucose homeostasis. Moreover, treatment with adropin(34–76) alleviated endoplasmic reticulum stress responses and reduced activity of c-Jun N-terminal kinase in the liver, explaining the enhanced activities of hepatic insulin signaling pathways observed with adropin(34–76) treatment. Furthermore, adropin(34–76) suppressed cAMP activated protein kinase A (PKA) activities, resulting in reduced phosphorylation of inositol trisphosphate receptor, which mediates endoplasmic reticulum calcium efflux, and of cAMP-responsive element–binding protein, a key transcription factor in hepatic regulation of glucose metabolism. Adropin(34–76) directly affected liver metabolism, decreasing glucose production and reducing PKA-mediated phosphorylation in primary mouse hepatocytes in vitro. Our findings indicate that major hepatic signaling pathways contribute to the improved glycemic control achieved with adropin(34–76) treatment in situations of obesity.

A hepatic comorbidity of metabolic syndrome, known as nonalcoholic fatty liver disease (NAFLD), is increasing in prevalence in conjunction with the pandemics of obesity and diabetes. The spectrum of NAFLD ranges from simple hepatic fat accumulation to a more severe disease termed nonalcoholic steatohepatitis (NASH), involving inflammation, hepatocyte death, and fibrosis. Importantly, NASH is linked to a much higher risk of cirrhosis, liver failure, and hepatocellular carcinoma, as well as an increased risk for nonhepatic malignancies and cardiovascular disease. Interest in the understanding of the disease processes and search for treatments for the spectrum of NAFLD-NASH has increased exponentially, but there are no approved pharmacologic therapies. In this review, we discuss the existing literature supporting insulin-sensitizing thiazolidinedione compounds as potential drug candidates for the treatment of NASH. In addition, we put these results into new context by summarizing recent studies suggesting these compounds alter mitochondrial metabolism by binding and inhibiting the mitochondrial pyruvate carrier. Keywords: Nonalcoholic Steatohepatitis, Mitochondria, Pyruvate, Thiazolidinedione

The mitochondrial pyruvate carrier (MPC) has emerged as a therapeutic target for treating insulin resistance, type 2 diabetes, and nonalcoholic steatohepatitis (NASH). However, the molecular mechanisms by which MPC inhibition leads to metabolic improvements are incompletely understood. We evaluated whether MPC inhibitors might correct impairments in branched chain amino acid (BCAA) catabolism, which are predictive of developing diabetes and NASH. BCAA concentrations were assessed in plasma collected in EMMINENCE (NCT02784444), a randomized, placebo-controlled trial of the MPC inhibitor MSDC-0602K in people with NASH. MSDC-0602K treatment, which led to marked improvements in insulin sensitivity and diabetes endpoints, decreased plasma concentrations of BCAAs compared to baseline while placebo had no effect. The rate-limiting enzyme in BCAA catabolism is the mitochondrial branched chain ketoacid dehydrogenase (BCKDH). BCKDH activity is suppressed by phosphorylation mediated by a kinase (BDK) and activated by the phosphatase (PPM1K). In Huh7 or HepG2 cells, MPC inhibitors markedly reduced BCKDH phosphorylation, suggesting increased BCKDH activity. Chemical inhibition or siRNA-mediated silencing of BDK had no effect on suppression of BCKDH phosphorylation by MPC inhibitors. In contrast, PPM1K knockdown prevented the effects of MPC inhibition on phospho-BCKDH. Lastly, we quantified BCKDH phosphorylation in liver of wild-type and hepatocyte-specific MPC2 knockout (LS-Mpc2-/-) mice after 20 weeks of high fat diet. LS-Mpc2-/- mice exhibited reduced phosphorylation of BCKDH compared to wild-type mice. Lean LS-Mpc2-/- mice also exhibited reduced plasma BCAA concentrations after an overnight fast. These data demonstrate novel cross talk between mitochondrial pyruvate and BCAA metabolism and suggest that MPC inhibition leads to lower plasma BCAA concentrations and BCKDH phosphorylation via the phosphatase PPM1K. Disclosure D. Ferguson: None. N. K. H. Yiew: None. K. S. Mccommis: None. J. R. Colca: Board Member; Self; Metabolic Solutions Development Company, LLC, Employee; Self; Cirius Therapeutics. B. N. Finck: Advisory Panel; Self; Cirius Therapeutics, Stock/Shareholder; Self; Cirius Therapeutics. Funding National Institutes of Health (R01DK104735, T32DK007120)

We previously demonstrated that endogenous phosphatidic acid (PA) promotes liver regeneration after acetaminophen (APAP) hepatotoxicity. Here, we hypothesized that exogenous PA is also beneficial. To test that, we treated mice with a toxic APAP dose at 0 h, followed by PA or vehicle (Veh) post-treatment. We then collected blood and liver at 6, 24, and 52 h. Post-treatment with PA 2 h after APAP protected against liver injury at 6 h, and the combination of PA and N-acetyl-l-cysteine (NAC) reduced injury more than NAC alone. Interestingly, PA did not affect canonical mechanisms of APAP toxicity. Instead, transcriptomics revealed that PA activated interleukin-6 (IL-6) signaling in the liver. Consistent with that, serum IL-6 and hepatic signal transducer and activator of transcription 3 (Stat3) phosphorylation increased in PA-treated mice. Furthermore, PA failed to protect against APAP in IL-6-deficient animals. Interestingly, IL-6 expression increased 18-fold in adipose tissue after PA, indicating that adipose is a source of PA-induced circulating IL-6. Surprisingly, however, exogenous PA did not alter regeneration, despite the importance of endogenous PA in liver repair, possibly due to its short half-life. These data demonstrate that exogenous PA is also beneficial in APAP toxicity and reinforce the protective effects of IL-6 in this model., Graphical abstract Exogenous phosphatidic acid (PA) increases interleukin-6 (IL-6) production in peripheral adipose tissue. The IL-6 then enters circulation and signals to the liver to reduce early acetaminophen (APAP)-induced liver injury.Image 1

Objective Monoacylglycerol acyltransferase (MGAT) enzymes catalyze the synthesis of diacylglycerol from monoacylglycerol. Previous work has suggested the importance of MGAT activity in the development of obesity-related hepatic insulin resistance. Indeed, antisense oligonucleotide (ASO)-mediated knockdown of Mogat1 mRNA, which encodes MGAT1, reduced hepatic MGAT activity and improved glucose tolerance and insulin resistance in high-fat diet (HFD)-fed mice. However, recent work has suggested that some ASOs may have off-target effects on body weight and metabolic parameters via activation of the interferon alpha/beta receptor 1 (IFNAR-1) pathway. Methods Mice with whole-body Mogat1 knockout or a floxed allele for Mogat1 to allow for liver-specific Mogat1-knockout (by either a liver-specific transgenic or adeno-associated virus-driven Cre recombinase) were generated. These mice were placed on an HFD, and glucose metabolism and insulin sensitivity were assessed after 16 weeks on diet. In some experiments, mice were treated with control scramble or Mogat1 ASOs in the presence or absence of IFNAR-1 neutralizing antibody. Results Genetic deletion of hepatic Mogat1, either acutely or chronically, did not improve hepatic steatosis, glucose tolerance, or insulin sensitivity in HFD-fed mice. Furthermore, constitutive Mogat1 knockout in all tissues actually exacerbated HFD-induced obesity, insulin sensitivity, and glucose intolerance on an HFD. Despite markedly reduced Mogat1 expression, liver MGAT activity was unaffected in all knockout mouse models. Mogat1 overexpression in hepatocytes increased liver MGAT activity and TAG content in low-fat-fed mice but did not cause insulin resistance. Multiple Mogat1 ASO sequences improved glucose tolerance in both wild-type and Mogat1 null mice, suggesting an off-target effect. Hepatic IFNAR-1 signaling was activated by multiple Mogat1 ASOs, but its blockade did not prevent the effects of either Mogat1 ASO on glucose homeostasis. Conclusion These results indicate that genetic loss of Mogat1 does not affect hepatic MGAT activity or metabolic homeostasis on HFD and show that multiple Mogat1 ASOs improve glucose metabolism through effects independent of targeting Mogat1 or activation of IFNAR-1 signaling., Graphical abstract Image 1, Highlights • Mogat1 liver-specific KO or KD does not improve metabolism in HFD-fed mice. • Whole-body Mogat1 deletion impairs insulin tolerance in HFD-fed mice. • Mogat1 ASOs improve whole-body metabolism independently of gene knockdown. • Blockade of the INFAR1 response does not prevent off-target effects of Mogat1 ASOs.

Nicotinamide adenine dinucleotide (NAD+) is an essential coenzyme that regulates cellular energy metabolism in many cell types. The major purpose of the present study was to test the hypothesis that NAD+ in white adipose tissue (WAT) is a regulator of whole-body metabolic flexibility in response to changes in insulin sensitivity and with respect to substrate availability and use during feeding and fasting conditions. To this end, we first evaluated the relationship between WAT NAD+ concentration and metabolic flexibility in mice and humans. We found that WAT NAD+ concentration was increased in mice after calorie restriction and exercise, 2 enhancers of metabolic flexibility. Bariatric surgery-induced 20% weight loss increased plasma adiponectin concentration, skeletal muscle insulin sensitivity, and WAT NAD+ concentration in people with obesity. We next analyzed adipocyte-specific nicotinamide phosphoribosyltransferase (Nampt) knockout (ANKO) mice, which have markedly decreased NAD+ concentrations in WAT. ANKO mice oxidized more glucose during the light period and after fasting than control mice. In contrast, the normal postprandial stimulation of glucose oxidation and suppression of fat oxidation were impaired in ANKO mice. Data obtained from RNA-sequencing of WAT suggest that loss of NAMPT increases inflammation, and impairs insulin sensitivity, glucose oxidation, lipolysis, branched-chain amino acid catabolism, and mitochondrial function in WAT, which are features of metabolic inflexibility. These results demonstrate a novel function of WAT NAMPT-mediated NAD+ biosynthesis in regulating whole-body metabolic flexibility, and provide new insights into the role of adipose tissue NAD+ biology in metabolic health.

ObjectiveMonoacylglycerol acyltransferase (MGAT) enzymes catalyze the synthesis of diacylglycerol from monoacylglycerol. Previous work has suggested the importance of MGAT activity in the development of obesity-related hepatic insulin resistance. Indeed, antisense oligonucleotide (ASO)-mediated knockdown of the gene encoding MGAT1, Mogat1, reduced hepatic MGAT activity and improved glucose tolerance and insulin resistance in high fat diet (HFD) fed mice. However, recent work has suggested that some ASOs may have off-target effects on body weight and metabolic parameters via activation of the interferon alpha/beta receptor 1 (IFNAR-1) pathway.MethodsMice with whole-body Mogat1 knockout or a floxed allele for Mogat1 to allow for liver-specific Mogat1-knockout (by either a liver-specific transgenic or adeno-associated virus-driven Cre recombinase) were generated. These mice were placed on a high fat diet and glucose metabolism and insulin sensitivity was assessed after 16 weeks on diet. In some experiments, mice were treated with control or Mogat1 or control ASOs in the presence or absence of IFNAR-1 neutralizing antibody.ResultsGenetic deletion of hepatic Mogat1, either acutely or chronically, did not improve hepatic steatosis, glucose tolerance, or insulin sensitivity in HFD-fed mice. Furthermore, constitutive Mogat1 knockout in all tissues actually exacerbated HFD-induced weight gain, insulin resistance, and glucose intolerance on a HFD. Despite markedly reduced Mogat1 expression, liver MGAT activity was unaffected in all knockout mouse models. Mogat1 overexpression hepatocytes increased liver MGAT activity and TAG content in low-fat fed mice, but did not cause insulin resistance. Interestingly, Mogat1 ASO treatment improved glucose tolerance in both wild-type and Mogat1 null mice, suggesting an off target effect. Inhibition of IFNAR-1 did not block the effect of Mogat1 ASO on glucose homeostasis.ConclusionThese results indicate that genetic loss of Mogat1 does not affect hepatic MGAT activity or metabolic homeostasis on HFD and show that Mogat1 ASOs improve glucose metabolism through effects independent of targeting Mogat1 or activation of IFNAR-1 signaling.Abstract FigureHighlightsMogat1 liver-specific KO or KD does not improve metabolism in HFD fed mice.Whole-body Mogat1-deletion impairs insulin tolerance in HFD fed mice.Mogat1 ASOs improves whole body metabolism independently of gene knockdown.Blockade of the INFR response does not prevent off-target effects of Mogat1 ASOs.

Macrophages reprogram their metabolism to promote appropriate responses. Pro-resolving macrophages primarily utilize fatty acid oxidation as an energy source. Metabolites generated during the catabolism of fatty acids aid in the resolution of inflammation and tissue repair, but the regulatory mechanisms that control lipid metabolism in macrophages is not fully elucidated. In this current study we show that lipin-1, a phosphatidic acid phosphatase and regulator of lipid metabolism, is required for increased oxidative phosphorylation during IL-4 mediated responses. We also show that the transcriptional coregulatory function of lipin-1 is required for β-oxidation in response to palmitate (free fatty acid) and apoptotic cell derived lipids. BMDMs lacking lipin-1 have a reduction in critical TCA cycle metabolites following IL-4 stimulation, suggesting a break in the TCA cycle that is supportive of lipid synthesis rather than lipid catabolism. Together, our data demonstrate that lipin-1 regulates intermediary metabolism within pro-resolving macrophages and highlights the importance of aligning macrophage metabolism with proper responses to stimuli.

During atherosclerosis, macrophages engulf and break down deposited modified low-density lipoproteins (modLDLs) into lipids and free fatty acids. The lipids and free fatty acids from these modLDLs either need to be stored during a process called glycerolipid synthesis or broken down during β-oxidation. In addition, free fatty acids can activate transcription factors to promote a pro-resolving macrophage phenotype. The protein lipin-1 is involved in both glycerolipid synthesis and β-oxidation. Lipin-1 enzymatic activity is a key step in the glycerolipid synthesis pathway; lipin-1 transcriptional co-regulatory activity either augments or represses various transcription factors that are activated via free fatty acids that promote β-oxidation and inhibit inflammation. Lipin-1 enzymatic activity increases pro-inflammatory macrophage phenotypes and is atherogenic. In contrast, we have also demonstrated that lipin-1 transcriptional co-regulatory activity promotes pro-resolving macrophage phenotypes leading us to the hypothesis that lipin-1 transcriptional co-regulatory activity is atheroprotective. Using a mouse model to delete lipin-1 in myeloid cells, we have demonstrated that loss of lipin-1 increases plaque size and pro-inflammatory gene expression. We have also shown mice lacking lipin-1 in myeloid cells have increased plaque collagen deposition and larger necrotic core formation. Combined, these data suggest that though lipin-1 enzymatic activity is atherogenic, lipin-1 transcriptional co-regulatory activity is atheroprotective. Overall, the results suggest that the dual activities of lipin-1 contribute to atherosclerosis progression in opposite ways.

Emerging evidence suggests that the capacity of adipocytes to appropriately store lipids and prevent their ectopic accumulation in liver and skeletal muscle may be an important regulator of systemic insulin sensitivity. Lipin 1 is a lipid phosphatase that is highly expressed in adipocytes and catalyzes the conversion of phosphatidic acid (PA) to diacylglycerol (DAG); the penultimate step in triglyceride synthesis. Adipose tissue lipin 1 expression positively correlates with insulin sensitivity in obese humans and mouse models. To test the role of adipose tissue lipin 1 in systemic insulin sensitivity, we generated mice lacking lipin 1 in adipocytes (Adn-Lpin1-/- mice). On normal chow, loss of lipin 1 in adipocytes modestly reduced white and brown adipose tissue mass, but did not affect glucose tolerance or insulin sensitivity. However, in response to 2 weeks high fat diet (HFD; 60% fat) feeding, Adn-Lpin1-/- mice remained lean, but developed hyperglycemia compared to littermate wild type (WT) mice. Lipidomic analyses demonstrated an increase in liver lipids (PA, DAG, and triglyceride) in HFD-fed Adn-Lpin1-/- mice compared to WT controls, whereas only PA accumulated in skeletal muscle. Plasma triglyceride concentration was not different, but free fatty acids and glycerol were significantly reduced in Adn-Lpin1-/- vs. WT mice fed HFD, suggesting that lipolysis may be decreased. Plasma adiponectin and resistin were significantly reduced in HFD-fed Adn-Lpin1-/- vs. WT control mice while leptin concentrations were not different. Lastly, with 5 weeks of HFD feeding, despite being leaner, Adn-Lpin1-/- mice became markedly hyperglycemic and glucose intolerant compared to littermate WT controls. Collectively, these data suggest that loss of lipin 1 in adipocytes leads to insulin resistance on HFD, likely due to an impaired capacity to store lipids appropriately in adipose tissue. Disclosure A.J. Lutkewitte: None. B.N. Finck: Advisory Panel; Self; Cirius Therapeutics. Stock/Shareholder; Self; Cirius Therapeutics.

Gluconeogenesis from amino acids contributes significantly to hepatic glucose production and hyperglycemia in diabetes. Alanine transaminases (ALT) catalyze the bi-directional interconversion of alanine to pyruvate with concomitant interconversion of α-ketoglutarate to glutamate. ALT1 is cytosolic while ALT2 is localized to the mitochondrial matrix. The mRNA expression and protein abundance of ALT1 and 2 was significantly increased in liver of db/db or diet induced obese mice compared to lean controls. The expression of ALT2 was also increased in obese humans compared to lean controls and was downregulated following bariatric surgery. The increase in hepatic ALT2 expression in diabetic mice was caused by endoplasmic reticulum stress and mediated by activating transcription factor 4 (ATF4). Indeed, inhibition of ATF4 reduced ALT2 expression in liver of db/db mice. Consistent with enhanced use of amino acids in gluconeogenesis, db/db and diet-induced obese mice had elevated blood glucose concentrations following IP administration of L-alanine or glutamine. ALT2 silencing in liver had no effect on glucose concentrations in lean mice, but alleviated alanine-induced hyperglycemia in db/db mice; likely by reducing the incorporation of alanine and/or glutamine into newly synthesized glucose. Plasma alanine tended to rise and glutamate significantly increased in db/db mice with ALT2 silencing whereas concentrations of the branched chain amino acids isoleucine, leucine, and valine decreased or tended to decrease. In summary, our results are consistent with a significant role for ALT2 in hepatic gluconeogenesis from amino acids and in the regulation of blood glucose levels in obesity and diabetes. Disclosure M.R. Martino: None. M. Gutierrez Aguilar: None. K.S. McCommis: None. J. Yoshino: None. B.N. Finck: Advisory Panel; Self; Cirius Therapeutics. Stock/Shareholder; Self; Cirius Therapeutics. Funding National Institutes of Health (R01DK117657)

The mitochondrial pyruvate carrier (MPC) is an inner-mitochondrial membrane protein complex that has emerged as a drug target for treating a variety of human conditions. A heterodimer of two proteins, MPC1 and MPC2, comprises the functional MPC complex in higher organisms; however, the structure of this complex, including the critical residues that mediate binding of pyruvate and inhibitors, remain to be determined. Using homology modeling, we identified a putative substrate-binding cavity in the MPC dimer. Three amino acid residues (Phe66 (MPC1) and Asn100 and Lys49 (MPC2)) were validated by mutagenesis experiments to be important for substrate and inhibitor binding. Using this information, we developed a pharmacophore model and then performed a virtual screen of a chemical library. We identified five new non-indole MPC inhibitors, four with IC50 values in the nanomolar range that were up to 7-fold more potent than the canonical inhibitor UK-5099. These novel compounds possess drug-like properties and complied with Lipinski’s Rule of Five. They are predicted to have good aqueous solubility, oral bioavailability, and metabolic stability. Collectively, these studies provide important information about the structure-function relationships of the MPC complex and for future drug discovery efforts targeting the MPC.

The mitochondrial pyruvate carrier (MPC) is an inner mitochondrial membrane complex that plays a critical role in intermediary metabolism. Inhibition of the MPC, especially in liver, may have efficacy for treating type 2 diabetes mellitus. Herein, we examined the antidiabetic effects of zaprinast and 7ACC2, small molecules which have been reported to act as MPC inhibitors. Both compounds activated a bioluminescence resonance energy transfer-based MPC reporter assay (reporter sensitive to pyruvate) and potently inhibited pyruvate-mediated respiration in isolated mitochondria. Furthermore, zaprinast and 7ACC2 acutely improved glucose tolerance in diet-induced obese mice in vivo. Although some findings were suggestive of improved insulin sensitivity, hyperinsulinemic-euglycemic clamp studies did not detect enhanced insulin action in response to 7ACC2 treatment. Rather, our data suggest acute glucose-lowering effects of MPC inhibition may be due to suppressed hepatic gluconeogenesis. Finally, we used reporter sensitive to pyruvate to screen a chemical library of drugs and identified 35 potentially novel MPC modulators. Using available evidence, we generated a pharmacophore model to prioritize which hits to pursue. Our analysis revealed carsalam and six quinolone antibiotics, as well as 7ACC1, share a common pharmacophore with 7ACC2. We validated that these compounds are novel inhibitors of the MPC and suppress hepatocyte glucose production and demonstrated that one quinolone (nalidixic acid) improved glucose tolerance in obese mice. In conclusion, these data demonstrate the feasibility of therapeutic targeting of the MPC for treating diabetes and provide scaffolds that can be used to develop potent and novel classes of MPC inhibitors.

Adipocyte triglyceride storage provides a reservoir of energy that allows the organism to survive times of nutrient scarcity, but excessive adiposity has emerged as a health problem in many areas of the world. Monoacylglycerol acyltransferase (MGAT) acylates monoacylglycerol to produce diacylglycerol; the penultimate step in triglyceride synthesis. However, little is known about MGAT activity in adipocytes, which are believed to rely primarily on another pathway for triglyceride synthesis. We show that expression of the gene that encodes MGAT1 is robustly induced during adipocyte differentiation and that its expression is suppressed in fat of genetically-obese mice and metabolically-abnormal obese human subjects. Interestingly, MGAT1 expression is also reduced in physiologic contexts where lipolysis is high. Moreover, knockdown or knockout of MGAT1 in adipocytes leads to higher rates of basal adipocyte lipolysis. Collectively, these data suggest that MGAT1 activity may play a role in regulating basal adipocyte FFA retention.

Aims To examine the effects of a high-fat-diet (HFD) on monogenic neonatal diabetes, without the confounding effects of compensatory hyperinsulinaemia. Methods Mice expressing KATP channel gain-of-function (KATP -GOF) mutations, which models human neonatal diabetes, were fed an HFD. Results Surprisingly, KATP -GOF mice exhibited resistance to HFD-induced obesity, accompanied by markedly divergent blood glucose control, with some KATP -GOF mice showing persistent diabetes (KATP -GOF-non-remitter [NR] mice) and others showing remission of diabetes (KATP -GOF-remitter [R] mice). Compared with the severely diabetic and insulin-resistant KATP -GOF-NR mice, HFD-fed KATP -GOF-R mice had lower blood glucose, improved insulin sensitivity, and increased circulating plasma insulin and glucagon-like peptide-1 concentrations. Strikingly, while HFD-fed KATP -GOF-NR mice showed increased food intake and decreased physical activity, reduced whole body fat mass and increased plasma lipids, KATP -GOF-R mice showed similar features to those of control littermates. Importantly, KATP -GOF-R mice had restored insulin content and β-cell mass compared with the marked loss observed in both HFD-fed KATP -GOF-NR and chow-fed KATP -GOF mice. Conclusion Together, our results suggest that restriction of dietary carbohydrates and caloric replacement by fat can induce metabolic changes that are beneficial in reducing glucotoxicity and secondary consequences of diabetes in a mouse model of insulin-secretory deficiency.

The prevalence of obesity-associated nonalcoholic fatty liver disease has significantly increased over the past decade, and end-stage liver disease secondary to nonalcoholic steatohepatitis has become 1 of the most common indications for liver transplantation. This both increases the demand for organs and decreases the availability of donor livers deemed suitable for transplantation. Although in the past many steatotic livers were discarded due to concerns over enhanced susceptibility to ischemia/reperfusion injury (IRI) and organ failure, the discrepancy between supply and demand has resulted in increasing use of expanded criteria donor organs including steatotic livers. However, it remains controversial whether steatotic livers can be safely used for transplantation and how best to improve the performance of steatotic grafts. We aimed to evaluate the impact of diet-induced hepatic steatosis in a murine model of IRI. Using a diet of high trans-fat, fructose, and cholesterol (HTF-C) and a diet high in saturated fats, sucrose, and cholesterol (Western diet), we were able to establish models of mixed macrovesicular and microvesicular steatosis (HTF-C) and microvesicular steatosis (Western). We found that the presence of hepatic steatosis, whether it is predominantly macrovesicular or microvesicular, significantly worsens IRI as measured by plasma alanine aminotransferase levels and inflammatory cytokine concentration, and histological evaluation for necrosis. Additionally, we report on a novel finding in which hepatic IRI in the setting of steatosis results in the induction of the necroptosis factors, receptor interacting protein kinase (RIPK) 3, RIPK1, and mixed-lineage kinase domain-like. These data lay the groundwork for additional experimentation to test potential therapeutic approaches to limit IRI in steatotic livers by using a genetically tractable system. Liver Transplantation 24 908-921 2018 AASLD.

delta-5 desaturase and delta-6 desaturase are enzymes known to be involved in the synthesis of highly unsaturated fatty acids. In this issue, Kim et al. (2019) show that production of NAD(+) by this desaturase reaction is an adaptive response to NAD(+) depletion that may regulate cellular REDOX status.

Rosiglitazone and pioglitazone are thiazolidinedione (TZD) compounds that have been used clinically as insulin-sensitizing drugs and are generally believed to mediate their effects via activation of the peroxisome proliferator-activated receptor γ (PPARγ). Recent work has shown that it is possible to synthesize TZD compounds with potent insulin-sensitizing effects and markedly-diminished affinity for PPARγ. Both clinically-used TZDs and investigational PPARγ-sparing TZDs, such as MSDC-0602, interact with the mitochondrial pyruvate carrier (MPC) and inhibit its activity. Two proteins, MPC1 and MPC2, comprise the MPC complex. Herein, we used mice expressing a hypomorphic MPC2 protein missing 16 amino acids in the N terminus (Mpc2Δ16 mice) to determine the effects of these residues in mediating the insulin-sensitizing effects of TZDs in diet-induced obese mice. We found that both pioglitazone and MSDC-0602 elicited their beneficial metabolic effects, including improving glucose tolerance, attenuating hepatic steatosis, reducing adipose tissue inflammation, and stimulating adipocyte browning, in both WT and Mpc2Δ16 mice after high fat diet feeding. In addition, truncation of MPC2 failed to attenuate the interaction between TZDs and the MPC in a BRET-based assay or to affect the suppression of pyruvate-stimulated respiration in cells. Collectively, these data suggest that the interaction between TZDs and MPC2 is not affected by loss of the N-terminal 16 amino acids nor are these residues required for the insulin-sensitizing effects of these compounds. This article is protected by copyright. All rights reserved

Nonalcoholic fatty liver disease (NAFLD) encompasses a range of liver pathology ranging from simple steatosis to varying degrees of inflammation, hepatocyte injury and fibrosis. Without intervention it can progress to end-stage liver disease and hepatocellular carcinoma. Given its close association with obesity, the prevalence of NAFLD has increased dramatically worldwide. Currently, there are no FDA-approved medications for the treatment of NAFLD and although lifestyle modifications with appropriate diet and exercise have been shown to be beneficial, this has been difficult to achieve and sustain for the majority of patients. As such, the search for effective therapeutic agents is an active area of research. Peroxisome proliferator-activated receptors (PPARs) belong to a class of nuclear receptors. Because of their key role in the transcriptional regulation of glucose and lipid metabolism, PPAR ligands have been investigated as possible therapeutic agents for NAFLD. Here we review the current evidence from preclinical and clinical studies investigating the therapeutic potential of PPAR ligands for the treatment of this spectrum of liver disorders.

Modifying the entry of pyruvate into mitochondria may provide a unique approach to treat metabolic disease. The pharmacology of a new class of insulin sensitizers directed against a newly identified mitochondrial target may treat many aspects of nonalcoholic steatohepatitis, including fibrosis. This commentary suggests treating nonalcoholic steatohepatitis through a newly identified mechanism consistent with pathophysiology. (Hepatology Communications 2017;1:193-197).

Diseases of the liver related to metabolic syndrome have emerged as the most common and undertreated hepatic ailments. The cause of nonalcoholic fatty liver disease is the aberrant accumulation of lipid in hepatocytes, though the mechanisms whereby this leads to hepatocyte dysfunction, death, and hepatic fibrosis are still unclear. Insulin-sensitizing thiazolidinediones have shown efficacy in treating nonalcoholic steatohepatitis (NASH), but their widespread use is constrained by dose-limiting side effects thought to be due to activation of the peroxisome proliferator–activated receptor γ. We sought to determine whether a next-generation thiazolidinedione with markedly diminished ability to activate peroxisome proliferator–activated receptor γ (MSDC-0602) would retain its efficacy for treating NASH in a rodent model. We also determined whether some or all of these beneficial effects would be mediated through an inhibitory interaction with the mitochondrial pyruvate carrier 2 (MPC2), which was recently identified as a mitochondrial binding site for thiazolidinediones, including MSDC-0602. We found that MSDC-0602 prevented and reversed liver fibrosis and suppressed expression of markers of stellate cell activation in livers of mice fed a diet rich in trans-fatty acids, fructose, and cholesterol. Moreover, mice with liver-specific deletion of MPC2 were protected from development of NASH on this diet. Finally, MSDC-0602 directly reduced hepatic stellate cell activation in vitro, and MSDC-0602 treatment or hepatocyte MPC2 deletion also limited stellate cell activation indirectly by affecting secretion of exosomes from hepatocytes. Conclusion: Collectively, these data demonstrate the effectiveness of MSDC-0602 for attenuating NASH in a rodent model and suggest that targeting hepatic MPC2 may be an effective strategy for pharmacologic development. (Hepatology 2017;65:1543-1556).

Macrophage responses contribute to a diverse array of pathologies ranging from infectious disease to sterile inflammation. Polarization of macrophages determines their cellular function within biological processes. Lipin-1 is a phosphatidic acid phosphatase in which its enzymatic activity contributes to macrophage pro-inflammatory responses. Lipin-1 also possesses transcriptional co-regulator activity and whether this activity is required for macrophage polarization is unknown. Using mice that lack only lipin-1 enzymatic activity or both enzymatic and transcriptional coregulator activities from myeloid cells, we investigated the contribution of lipin-1 transcriptional co-regulator function towards macrophage wound healing polarization. Macrophages lacking both lipin-1 activities did elicit IL-4 mediated gene expression to levels seen in either wild-type or lipin-1 enzymatically deficient macrophages. Furthermore, we provide evidence that the lack of myeloid-associated lipin-1 transcriptional co-regulator activity leads to impaired full thickness excisional wound healing. Our study demonstrates that lipin-1 transcriptional co-regulatory activity contributes to macrophage polarization and the macrophage contribution to wound healing in vivo.

Endoplasmic reticulum (ER) stress is associated with dysregulated metabolism, but little is known about how the ER responds to metabolic activity. Here, working primarily in mouse hepatocytes, we show that decreasing the availability of substrate for the TCA cycle diminished NADPH production and attenuated ER stress in a manner that depended on glutathione oxidation. ER stress was also alleviated by impairing either TCA-dependent NADPH production or Glutathione Reductase. Conversely, stimulating TCA activity favored NADPH production, glutathione reduction, and ER stress. Validating these findings, we show that deletion of the mitochondrial pyruvate carrier, which is known to decrease TCA cycle activity and protect the liver from diet-induced injury, also diminished NADPH, elevated glutathione oxidation, and alleviated ER stress. These results provide independent genetic evidence that mitochondrial oxidative metabolism is linked to ER homeostasis. Our results demonstrate a novel pathway of communication between mitochondria and the ER, through relay of redox metabolites.

Summary: Macrophage-mediated inflammation is critical in the pathogenesis of non-alcoholic steatohepatitis (NASH). Here, we describe that, with high-fat, high-sucrose-diet feeding, mature TIM4pos Kupffer cells (KCs) decrease in number, while monocyte-derived Tim4neg macrophages accumulate. In concert, monocyte-derived infiltrating macrophages enter the liver and consist of a transitional subset that expresses Cx3cr1/Ccr2 and a second subset characterized by expression of Trem2, Cd63, Cd9, and Gpmnb; markers ascribed to lipid-associated macrophages (LAMs). The Cx3cr1/Ccr2-expressing macrophages, referred to as C-LAMs, localize to macrophage aggregates and hepatic crown-like structures (hCLSs) in the steatotic liver. In C-motif chemokine receptor 2 (Ccr2)-deficient mice, C-LAMs fail to appear in the liver, and this prevents hCLS formation, reduces LAM numbers, and increases liver fibrosis. Taken together, our data reveal dynamic changes in liver macrophage subsets during the pathogenesis of NASH and link these shifts to pathologic tissue remodeling.

Objective Transport of pyruvate into the mitochondrial matrix by the Mitochondrial Pyruvate Carrier (MPC) is an important and rate-limiting step in its metabolism. In pancreatic β-cells, mitochondrial pyruvate metabolism is thought to be important for glucose sensing and glucose-stimulated insulin secretion. Methods To evaluate the role that the MPC plays in maintaining systemic glucose homeostasis, we used genetically-engineered Drosophila and mice with loss of MPC activity in insulin-producing cells. Results In both species, MPC deficiency results in elevated blood sugar concentrations and glucose intolerance accompanied by impaired glucose-stimulated insulin secretion. In mouse islets, β-cell MPC-deficiency resulted in decreased respiration with glucose, ATP-sensitive potassium (KATP) channel hyperactivity, and impaired insulin release. Moreover, treatment of pancreas-specific MPC knockout mice with glibenclamide, a sulfonylurea KATP channel inhibitor, improved defects in islet insulin secretion and abnormalities in glucose homeostasis in vivo. Finally, using a recently-developed biosensor for MPC activity, we show that the MPC is rapidly stimulated by glucose treatment in INS-1 insulinoma cells suggesting that glucose sensing is coupled to mitochondrial pyruvate carrier activity. Conclusions Altogether, these studies suggest that the MPC plays an important and ancestral role in insulin-secreting cells in mediating glucose sensing, regulating insulin secretion, and controlling systemic glycemia., Highlights • Loss of MPC activity in Drosophila leads to hyperglycemia. • Glucose-stimulated insulin secretion is impaired in Drosophila lacking MPC. • β-cell-specific MPC deletion in mice also results in impaired insulin secretion. • The MPC may be a sensor for glucose concentration in β-cells and couple pyruvate production to insulin secretion.

Overconsumption of fructose and other sugars has been linked to nonalcoholic fatty liver disease (NAFLD); however, the sugar-associated effects that lead to disease are poorly defined. In this issue of the JCI, Zhang and colleagues show that the carbohydrate response element-binding protein (ChREBP) coordinates an adaptive response to a high-fructose diet in mice and that loss of this transcription factor leads to hepatic inflammation and early signs of fibrosis. Intriguingly, ChREBP-dependent effects were due to an exaggerated activation of the proapoptotic arms of the endoplasmic reticulum stress response that is probably secondary to inappropriate derepression of cholesterol biosynthesis. These findings suggest that a previously unknown link exists between ChREBP and the regulation of cholesterol synthesis that affects liver injury.

Due to the rarity of hematopoietic stem cells, their differentiation into progenitors then on to mature cells must occur with proliferation, leading to an expansion of each downstream population. This process, called transit amplification, is required to produce a sufficient mature cell population and must be balanced with survival to maintain homeostasis and prevent hematological malignancies. How transit amplification and subsequent homeostasis is controlled is not well understood but may involve metabolic switches. Various groups have demonstrated that hematopoietic expansion requires a switch from glycolysis to oxidative phosphorylation, but what induces this switch and the specific carbon sources that fuel it remain unclear. To address these questions, we employed mice conditionally deficient in mitochondrial pyruvate import, fatty acid oxidation, or glutamine hydrolysis and examined the requirements of these pathways in hematopoietic homeostasis. Ablation of carnitine palmitoyltransferase II (Cpt2) or glutaminase (Gls) had no effect on lineages examined, suggesting fatty acid oxidation and glutaminolysis are not required for the production or survival of various immune cells. In contrast, upon deletion of mitochondrial pyruvate carrier 2 (Mpc2), peripheral myeloid cell numbers declined dramatically. Over time after Mpc2 ablation, a recovery of myeloid cells was observed. This recovery was associated with increased proliferation of Mpc2-deficient progenitors relative to wild type counterparts immediately following genetic deletion. Further studies are being done to elucidate what metabolic pathway may allow myeloid progenitors to rapidly proliferate and recover after mitochondrial pyruvate ablation.