Hepatic deficiency in transcriptional cofactor TBL1 promotes liver steatosis and hypertriglyceridemia

Inositol-requiring enzyme 1α and c-Jun N-terminal kinase axis activation contributes to intracellular lipid accumulation in calf hepatocytes

During the perinatal period, dairy cows undergo negative energy balance, resulting in elevated circulating levels of nonesterified fatty acids (NEFA). Although increased blood NEFA concentrations are a physiological adaptation of early lactation, excessive NEFA in dairy cows is a major cause of fatty liver. Aberrant lipid metabolism leads to hepatic lipid accumulation and subsequently the development of fatty liver. Both inositol-requiring enzyme 1α (IRE1α) and c-Jun N-terminal kinase (JNK) have been validated for their association with hepatic lipid accumulation, including their regulatory functions in calf hepatocyte insulin resistance, oxidative stress, and apoptosis. Meanwhile, both IRE1α and JNK are involved in lipid metabolism in nonruminants. Therefore, the aim of this study was to investigate how IRE1α and JNK regulate lipid metabolism in bovine hepatocytes. An experiment was conducted on randomly selected 10 healthy cows (hepatic triglyceride [TG] content <1%) and 10 cows with fatty liver (hepatic TG content >5%). Liver tissue and blood samples were collected from experimental cows. Serum concentrations of NEFA and β-hydroxybutyrate (BHB) were greater, whereas serum concentrations of glucose and milk production were lower in cows with fatty liver. The western blot results revealed that dairy cows with fatty liver had higher phosphorylation levels of JNK, c-Jun, and IRE1α in the liver tissue. Three in vitro experiments were conducted using primary calf hepatocytes isolated from 5 healthy calves (body weight: 30-40 kg; 1 d old). First, hepatocytes were treated with NEFA (1.2 mM) for 0.5, 1, 2, 3, 5, 7, 9, or 12 h, which showed that the phosphorylated levels of JNK, c-Jun, and IRE1α increased in both linear and quadratic effects. In the second experiment, hepatocytes were treated with high concentrations of NEFA (1.2 mM) for 12 h with or without SP600125, a canonical inhibitor of JNK. Western blot results showed that SP600125 treatment could decrease the expression of lipogenesis-associated proteins (PPARγ and SREBP-1c) and increase the expression of fatty acid oxidation (FAO)-associated proteins (CPT1A and PPARα) in NEFA-treated hepatocytes. The perturbed expression of lipogenesis-associated genes (FASN, ACACA, and CD36) and FAO-associated gene ACOX1 were also recovered by JNK inhibition, indicating that JNK reduced excessive NEFA-induced lipogenesis and FAO dysregulation in calf hepatocytes. Third, short hairpin RNA targeting IRE1α (sh-IRE1α) was transfected into calf hepatocytes to silence IRE1α, and KIRA6 was used to inhibit the kinase activity of IRE1α. The blockage of IRE1α could at least partially suppressed NEFA-induced JNK activation. Moreover, the blockage of IRE1α downregulated the expression of lipogenesis genes and upregulated the expression of FAO genes in NEFA-treated hepatocytes. In conclusion, these findings indicate that targeting the IRE1α-JNK axis can reduce NEFA-induced lipid accumulation in bovine hepatocytes by modulating lipogenesis and FAO. This may offer a prospective therapeutic target for fatty liver in dairy cows.

Nuclear receptor corepressors non-canonically drive glucocorticoid receptor-dependent activation of hepatic gluconeogenesis

Nuclear receptor corepressors (NCoRs) function in multiprotein complexes containing histone deacetylase 3 (HDAC3) to alter transcriptional output primarily through repressive chromatin remodelling at target loci1-5. In the liver, loss of HDAC3 causes a marked hepatosteatosis largely because of de-repression of genes involved in lipid metabolism6,7; however, the individual roles and contribution of other complex members to hepatic and systemic metabolic regulation are unclear. Here we show that adult loss of both NCoR1 and NCoR2 (double knockout (KO)) in hepatocytes phenocopied the hepatomegalic fatty liver phenotype of HDAC3 KO. In addition, double KO livers exhibited a dramatic reduction in glycogen storage and gluconeogenic gene expression that was not observed with hepatic KO of individual NCoRs or HDAC3, resulting in profound fasting hypoglycaemia. This surprising HDAC3-independent activation function of NCoR1 and NCoR2 is due to an unexpected loss of chromatin accessibility on deletion of NCoRs that prevented glucocorticoid receptor binding and stimulatory effect on gluconeogenic genes. These studies reveal an unanticipated, non-canonical activation function of NCoRs that is required for metabolic health.

Embryonic expression patterns of TBL1 family in zebrafish

Except the addition of TBL1Y in human, transducing beta like 1 (TBL1) family mainly consists of two members TBL1X and TBL1XR1, taking part in multiple intracellular signaling pathways such as Wnt/β-catenin and NF-κB in cancer progression. However, the gene expression patterns of this family during embryonic development remain largely unknown. Here we took advantage of zebrafish model to characterize the spatial and temporal expression patterns of TBL1 family genes including tbl1x, tbl1xr1a and tbl1xr1b. The in situ hybridization studies of gene expression showed robust expressions of tbl1x and tbl1xr1b as maternal transcripts except tbl1xr1a. As the embryo develops, zygotic expressions of all TBL1 family members occur and have a redundant and broad pattern including in brain, neural retina, pharyngeal arches, otic vesicles, and pectoral fins. Ubiquitous expression of all family members were ranked from the strongest to the weakest: tbl1xr1a, tbl1x, and tbl1xr1b. In addition, one tbl1xr1a transcript tbl1xr1a202 showed unique and rich expression in the developing heart and lateral line neuromasts. Overall, all members of zebrafish TBL1 family shared numerous similarities and exhibited certain distinctions in the expression patterns, indicating that they might have redundant and exclusive functions to be further explored.

Subcellular effects and lipid metabolism alterations in the gilthead seabream Sparus aurata fed on ovatoxins-contaminated mussels

The marine microalgae Ostreopsis cf. ovata are a well-known producer of palytoxin (PlTXs) analogues, i.e. ovatoxins (OVTXs) among others, which arouse concern for animal and human health. Both in field and laboratory studies, presence of OVTXs, detected in species directly feeding on O. cf. ovata, was frequently correlated with impairment on organisms' physiology, development and behaviour, while similar knowledge is still lacking for animals feeding on contaminated preys. In this study, transfer and toxicity of OVTXs were evaluated in an exposure experiment, in which gilthead seabream Sparus aurata was fed with bivalve mussel Mytilus galloprovincialis, contaminated by a toxic strain of O. cf. ovata. Mussels exposed to O. cf. ovata for 21 days accumulated meanly 188 ± 13 μg/kg OVTXs in the whole tissues. Seabreams fed with OVTX-contaminated mussels started to reject the food after 6 days of contaminated diet. Although no detectable levels of OVTXs were measured in muscle, liver, gills and gastro-intestinal tracts, the OVTX-enriched diet induced alterations of lipid metabolism in seabreams livers, displaying a decreased content of total lipid and fatty acid, together with overexpression of fatty acid biosynthetic genes, downregulation of β-oxidation genes and modulation of several genes related to lipid transport and regulation. Results from this study would suggest the hypothesis that OVTXs produced by O. cf. ovata may not be subject to bioaccumulation in fish fed on contaminated preys, being however responsible of significant biological effects, with important implications for human consumption of seafood products.

The role of ChREBP in carbohydrate sensing and NAFLD development

Excessive sugar consumption and defective glucose sensing by hepatocytes contribute to the development of metabolic diseases including type 2 diabetes mellitus (T2DM) and nonalcoholic fatty liver disease (NAFLD). Hepatic metabolism of carbohydrates into lipids is largely dependent on the carbohydrate-responsive element binding protein (ChREBP), a transcription factor that senses intracellular carbohydrates and activates many different target genes, through the activation of de novo lipogenesis (DNL). This process is crucial for the storage of energy as triglycerides in hepatocytes. Furthermore, ChREBP and its downstream targets represent promising targets for the development of therapies for the treatment of NAFLD and T2DM. Although lipogenic inhibitors (for example, inhibitors of fatty acid synthase, acetyl-CoA carboxylase or ATP citrate lyase) are currently under investigation, targeting lipogenesis remains a topic of discussion for NAFLD treatment. In this Review, we discuss mechanisms that regulate ChREBP activity in a tissue-specific manner and their respective roles in controlling DNL and beyond. We also provide in-depth discussion of the roles of ChREBP in the onset and progression of NAFLD and consider emerging targets for NAFLD therapeutics.

TBL1X: At the crossroads of transcriptional and posttranscriptional regulation

Over the past 2 decades, the adaptor protein transducin β-like 1 (TBL1X) and its homolog TBL1XR1 have been shown to be upregulated in solid tumors and hematologic malignancies, and their overexpression is associated with poor clinical outcomes. Moreover, dysregulation of the TBL1 family of proteins has been implicated as a key component of oncogenic prosurvival signaling, cancer progression, and metastasis. Herein, we discuss how TBL1X and TBL1XR1 are required for the regulation of major transcriptional programs through the silencing mediator for tetanoid and thyroid hormone receptor (SMRT)/nuclear receptor corepressor (NCOR)/ B cell lymphoma 6 (BCL6) complex, Wnt/β catenin, and NF-κB signaling. We outline the utilization of tegavivint (Iterion Therapeutics), a first-in-class small molecule targeting the N-terminus domain of TBL1, as a novel therapeutic strategy in preclinical models of cancer and clinically. Although most published work has focused on the transcriptional role of TBL1X, we recently showed that in diffuse large B-cell lymphoma (DLBCL), the most common lymphoma subtype, genetic knockdown of TBL1X and treatment with tegavivint resulted in decreased expression of critical (onco)-proteins in a posttranscriptional/β-catenin-independent manner by promoting their proteasomal degradation through a Skp1/Cul1/F-box (SCF)/TBL1X supercomplex and potentially through the regulation of protein synthesis. However, given that TBL1X controls multiple oncogenic signaling pathways in cancer, treatment with tegavivint may ultimately result in drug resistance, providing the rationale for combination strategies. Although many questions related to TBL1X function remain to be answered in lymphoma and other diseases, these data provide a growing body of evidence that TBL1X is a promising therapeutic target in oncology.

FGF4 protects the liver from nonalcoholic fatty liver disease by activating the AMP-activated protein kinase-Caspase 6 signal axis

BACKGROUND AND AIMS

NAFLD represents an increasing health problem in association with obesity and diabetes with no effective pharmacotherapies. Growing evidence suggests that several FGFs play important roles in diverse aspects of liver pathophysiology. Here, we report a previously unappreciated role of FGF4 in the liver.

APPROACH AND RESULTS

Expression of hepatic FGF4 is inversely associated with NAFLD pathological grades in both human patients and mouse models. Loss of hepatic Fgf4 aggravates hepatic steatosis and liver damage resulted from an obesogenic high-fat diet. By contrast, pharmacological administration of recombinant FGF4 mitigates hepatic steatosis, inflammation, liver damage, and fibrogenic markers in mouse livers induced to develop NAFLD and NASH under dietary challenges. Such beneficial effects of FGF4 are mediated predominantly by activating hepatic FGF receptor (FGFR) 4, which activates a downstream Ca²⁺ -Ca²⁺ /calmodulin-dependent protein kinase kinase beta-dependent AMP-activated protein kinase (AMPK)-Caspase 6 signal axis, leading to enhanced fatty acid oxidation, reduced hepatocellular apoptosis, and mitigation of liver damage.

CONCLUSIONS

Our study identifies FGF4 as a stress-responsive regulator of liver pathophysiology that acts through an FGFR4-AMPK-Caspase 6 signal pathway, shedding light on strategies for treating NAFLD and associated liver pathologies.

Balanced control of thermogenesis by nuclear receptor corepressors in brown adipose tissue

Brown adipose tissue (BAT) is a key thermogenic organ whose expression of uncoupling protein 1 (UCP1) and ability to maintain body temperature in response to acute cold exposure require histone deacetylase 3 (HDAC3). HDAC3 exists in tight association with nuclear receptor corepressors (NCoRs) NCoR1 and NCoR2 (also known as silencing mediator of retinoid and thyroid receptors [SMRT]), but the functions of NCoR1/2 in BAT have not been established. Here we report that as expected, genetic loss of NCoR1/2 in BAT (NCoR1/2 BAT-dKO) leads to loss of HDAC3 activity. In addition, HDAC3 is no longer bound at its physiological genomic sites in the absence of NCoR1/2, leading to a shared deregulation of BAT lipid metabolism between NCoR1/2 BAT-dKO and HDAC3 BAT-KO mice. Despite these commonalities, loss of NCoR1/2 in BAT does not phenocopy the cold sensitivity observed in HDAC3 BAT-KO, nor does loss of either corepressor alone. Instead, BAT lacking NCoR1/2 is inflamed, particularly with respect to the interleukin-17 axis that increases thermogenic capacity by enhancing innervation. Integration of BAT RNA sequencing and chromatin immunoprecipitation sequencing data revealed that NCoR1/2 directly regulate Mmp9, which integrates extracellular matrix remodeling and inflammation. These findings reveal pleiotropic functions of the NCoR/HDAC3 corepressor complex in BAT, such that HDAC3-independent suppression of BAT inflammation counterbalances stimulation of HDAC3 activity in the control of thermogenesis.

Modeling a Novel Variant of Glycogenosis IXa Using a Clonal Inducible Reprogramming System to Generate "Diseased" Hepatocytes for Accurate Diagnosis

The diagnosis of inherited metabolic disorders is a long and tedious process. The matching of clinical data with a genomic variant in a specific metabolic pathway is an essential step, but the link between a genome and the clinical data is normally difficult, primarily for new missense variants or alterations in intron sequences. Notwithstanding, elucidation of the pathogenicity of a specific variant might be critical for an accurate diagnosis. In this study, we described a novel intronic variant c.2597 + 5G > T in the donor splice sequence of the PHKA2 gene. To investigate PHKA2 mRNA splicing, as well as the functional consequences on glycogen metabolism, we generated hepatocyte-like cells from a proband’s fibroblasts by direct reprogramming. We demonstrated an aberrant splicing of PHKA2, resulting in the incorporation of a 27 bp upstream of intron 23 into exon 23, which leads to an immediate premature STOP codon. The truncated protein was unable to phosphorylate the PYGL protein, causing a 4-fold increase in the accumulation of glycogen in hepatocyte-like cells. Collectively, the generation of personalized hepatocyte-like cells enabled an unequivocal molecular diagnosis and qualified the sister’s proband, a carrier of the same mutation, as a candidate for a preimplantation genetic diagnosis. Additionally, our direct reprogramming strategy allows for an unlimited source of “diseased” hepatocyte-like cells compatible with high-throughput platforms.

Reprogramming of Hepatic Metabolism and Microenvironment in Nonalcoholic Steatohepatitis

Nonalcoholic fatty liver disease (NAFLD), a spectrum of metabolic liver disease associated with obesity, ranges from relatively benign hepatic steatosis to nonalcoholic steatohepatitis (NASH). The latter is characterized by persistent liver injury, inflammation, and liver fibrosis, which collectively increase the risk for end-stage liver diseases such as cirrhosis and hepatocellular carcinoma. Recent work has shed new light on the pathophysiology of NAFLD/NASH, particularly the role of genetic, epigenetic, and dietary factors and metabolic dysfunctions in other tissues in driving excess hepatic fat accumulation and liver injury. In parallel, single-cell RNA sequencing studies have revealed unprecedented details of the molecular nature of liver cell heterogeneity, intrahepatic cross talk, and disease-associated reprogramming of the liver immune and stromal vascular microenvironment. This review covers the recent advances in these areas, the emerging concepts of NASH pathogenesis, and potential new therapeutic opportunities. Expected final online publication date for the Annual Review of Nutrition, Volume 42 is August 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

A macrophage-hepatocyte glucocorticoid receptor axis coordinates fasting ketogenesis

Fasting metabolism and immunity are tightly linked; however, it is largely unknown how immune cells contribute to metabolic homeostasis during fasting in healthy subjects. Here, we combined cell-type-resolved genomics and computational approaches to map crosstalk between hepatocytes and liver macrophages during fasting. We identified the glucocorticoid receptor (GR) as a key driver of fasting-induced reprogramming of the macrophage secretome including fasting-suppressed cytokines and showed that lack of macrophage GR impaired induction of ketogenesis during fasting as well as endotoxemia. Mechanistically, macrophage GR suppressed the expression of tumor necrosis factor (TNF) and promoted nuclear translocation of hepatocyte GR to activate a fat oxidation/ketogenesis-related gene program, cooperatively induced by GR and peroxisome proliferator-activated receptor alpha (PPARα) in hepatocytes. Together, our results demonstrate how resident liver macrophages directly influence ketogenesis in hepatocytes, thereby also outlining a strategy by which the immune system can set the metabolic tone during inflammatory disease and infection.

Defective Lipid Droplet-Lysosome Interaction Causes Fatty Liver Disease as Evidenced by Human Mutations in TMEM199 and CCDC115

BACKGROUND & AIMS

Recently, novel inborn errors of metabolism were identified because of mutations in V-ATPase assembly factors TMEM199 and CCDC115. Patients are characterized by generalized protein glycosylation defects, hypercholesterolemia, and fatty liver disease. Here, we set out to characterize the lipid and fatty liver phenotype in human plasma, cell models, and a mouse model.

METHODS AND RESULTS

Patients with TMEM199 and CCDC115 mutations displayed hyperlipidemia, characterized by increased levels of lipoproteins in the very low density lipoprotein range. HepG2 hepatoma cells, in which the expression of TMEM199 and CCDC115 was silenced, and induced pluripotent stem cell (iPSC)-derived hepatocyte-like cells from patients with TMEM199 mutations showed markedly increased secretion of apolipoprotein B (apoB) compared with controls. A mouse model for TMEM199 deficiency with a CRISPR/Cas9-mediated knock-in of the human A7E mutation had marked hepatic steatosis on chow diet. Plasma N-glycans were hypogalactosylated, consistent with the patient phenotype, but no clear plasma lipid abnormalities were observed in the mouse model. In the siTMEM199 and siCCDC115 HepG2 hepatocyte models, increased numbers and size of lipid droplets were observed, including abnormally large lipid droplets, which colocalized with lysosomes. Excessive de novo lipogenesis, failing oxidative capacity, and elevated lipid uptake were not observed. Further investigation of lysosomal function revealed impaired acidification combined with impaired autophagic capacity.

CONCLUSIONS

Our data suggest that the hypercholesterolemia in TMEM199 and CCDC115 deficiency is due to increased secretion of apoB-containing particles. This may in turn be secondary to the hepatic steatosis observed in these patients as well as in the mouse model. Mechanistically, we observed impaired lysosomal function characterized by reduced acidification, autophagy, and increased lysosomal lipid accumulation. These findings could explain the hepatic steatosis seen in patients and highlight the importance of lipophagy in fatty liver disease. Because this pathway remains understudied and its regulation is largely untargeted, further exploration of this pathway may offer novel strategies for therapeutic interventions to reduce lipotoxicity in fatty liver disease.

Liver-fibrosis-activated transcriptional networks govern hepatocyte reprogramming and intra-hepatic communication

Liver fibrosis is a strong predictor of long-term mortality in individuals with metabolic-associated fatty liver disease; yet, the mechanisms underlying the progression from the comparatively benign fatty liver state to advanced non-alcoholic steatohepatitis (NASH) and liver fibrosis are incompletely understood. Using cell-type-resolved genomics, we show that comprehensive alterations in hepatocyte genomic and transcriptional settings during NASH progression, led to a loss of hepatocyte identity. The hepatocyte reprogramming was under tight cooperative control of a network of fibrosis-activated transcription factors, as exemplified by the transcription factor Elf-3 (ELF3) and zinc finger protein GLIS2 (GLIS2). Indeed, ELF3- and GLIS2-controlled fibrosis-dependent hepatokine genes targeting disease-associated hepatic stellate cell gene programs. Thus, interconnected transcription factor networks not only promoted hepatocyte dysfunction but also directed the intra-hepatic crosstalk necessary for NASH and fibrosis progression, implying that molecular "hub-centered" targeting strategies are superior to existing mono-target approaches as currently used in NASH therapy.

Role of the Nuclear Receptor Corepressor 1 (NCOR1) in Atherosclerosis and Associated Immunometabolic Diseases

Atherosclerotic cardiovascular disease is part of chronic immunometabolic disorders such as type 2 diabetes and nonalcoholic fatty liver disease. Their common risk factors comprise hypertension, insulin resistance, visceral obesity, and dyslipidemias, such as hypercholesterolemia and hypertriglyceridemia, which are part of the metabolic syndrome. Immunometabolic diseases include chronic pathologies that are affected by both metabolic and inflammatory triggers and mediators. Important and challenging questions in this context are to reveal how metabolic triggers and their downstream signaling affect inflammatory processes and vice-versa. Along these lines, specific nuclear receptors sense changes in lipid metabolism and in turn induce downstream inflammatory and metabolic processes. The transcriptional activity of these nuclear receptors is regulated by the nuclear receptor corepressors (NCORs), including NCOR1. In this review we describe the function of NCOR1 as a central immunometabolic regulator and focus on its role in atherosclerosis and associated immunometabolic diseases.

Hepatocyte-specific loss of GPS2 in mice reduces non-alcoholic steatohepatitis via activation of PPARα

Obesity triggers the development of non-alcoholic fatty liver disease (NAFLD), which involves alterations of regulatory transcription networks and epigenomes in hepatocytes. Here we demonstrate that G protein pathway suppressor 2 (GPS2), a subunit of the nuclear receptor corepressor (NCOR) and histone deacetylase 3 (HDAC3) complex, has a central role in these alterations and accelerates the progression of NAFLD towards non-alcoholic steatohepatitis (NASH). Hepatocyte-specific Gps2 knockout in mice alleviates the development of diet-induced steatosis and fibrosis and causes activation of lipid catabolic genes. Integrative cistrome, epigenome and transcriptome analysis identifies the lipid-sensing peroxisome proliferator-activated receptor α (PPARα, NR1C1) as a direct GPS2 target. Liver gene expression data from human patients reveal that Gps2 expression positively correlates with a NASH/fibrosis gene signature. Collectively, our data suggest that the GPS2-PPARα partnership in hepatocytes coordinates the progression of NAFLD in mice and in humans and thus might be of therapeutic interest.

The Nuclear Receptor-Co-repressor Complex in Control of Liver Metabolism and Disease

Hepatocytes are the major cell-type in the liver responsible for the coordination of metabolism in response to multiple signaling inputs. Coordination occurs primarily at the level of gene expression via transcriptional networks composed of transcription factors, in particular nuclear receptors (NRs), and associated co-regulators, including chromatin-modifying complexes. Disturbance of these networks by genetic, environmental or nutritional factors can lead to metabolic dysregulation and has been linked to the progression of non-alcoholic fatty liver disease (NAFLD) toward steatohepatitis and even liver cancer. Since there are currently no approved therapies, major efforts are dedicated to identify the critical factors that can be employed for drug development. Amongst the identified factors with clinical significance are currently lipid-sensing NRs including PPARs, LXRs, and FXR. However, major obstacles of NR-targeting are the undesired side effects associated with the genome-wide NR activities in multiple cell-types. Thus, of particular interest are co-regulators that determine NR activities, context-selectivity, and associated chromatin states. Current research on the role of co-regulators in hepatocytes is still premature due to the large number of candidates, the limited number of available mouse models, and the technical challenges in studying their chromatin occupancy. As a result, how NR-co-regulator networks in hepatocytes are coordinated by extracellular signals, and how NR-pathway selectivity is achieved, remains currently poorly understood. We will here review a notable exception, namely a fundamental transcriptional co-repressor complex that during the past decade has become the probably most-studied and best-understood physiological relevant co-regulator in hepatocytes. This multiprotein complex contains the core subunits HDAC3, NCOR, SMRT, TBL1, TBLR1, and GPS2 and is referred to as the "NR-co-repressor complex." We will particularly discuss recent advances in characterizing hepatocyte-specific loss-of-function mouse models and in applying genome-wide sequencing approaches including ChIP-seq. Both have been instrumental to uncover the role of each of the subunits under physiological conditions and in disease models, but they also revealed insights into the NR target range and genomic mechanisms of action of the co-repressor complex. We will integrate a discussion of translational aspects about the role of the complex in NAFLD pathways and in particular about the hypothesis that patient-specific alterations of specific subunits may determine NAFLD susceptibility and the therapeutic outcomes of NR-directed treatments.

Role of NCoR1 in mitochondrial function and energy metabolism

Mitochondrial number and shape are constantly changing in response to increased energy demands. The ability to synchronize mitochondrial pathways to respond to energy fluctuations within the cell is a central aspect of mammalian homeostasis. This dynamic process depends on the coordinated activation of transcriptional complexes to promote the expression of genes encoding for mitochondrial proteins. Recent evidence has shown that the nuclear corepressor NCoR1 is an essential metabolic switch which acts on oxidative metabolism signaling. Here, we provide an overview of the emerging role of NCoR1 in the transcriptional control of energy metabolism. The identification and characterization of NCoR1 as a central, evolutionary conserved player in mitochondrial function have revealed a novel layer of metabolic control. Defining the precise mechanisms by which NCoR1 acts on energy homeostasis will ultimately contribute towards the development of novel therapies for the treatment of metabolic diseases such as obesity and type 2 diabetes.

Treatment of Diabetic Mice with a Combination of Ketogenic Diet and Aerobic Exercise via Modulations of PPARs Gene Programs

Type 2 diabetes is a prevalent chronic disease arising as a serious public health problem worldwide. Diet intervention is considered to be a critical strategy in glycemic control of diabetic patients. Recently, the low-carbohydrate ketogenic diet is shown to be effective in glycemic control and weight loss. However, hepatic lipid accumulation could be observed in mice treated with ketogenic diet. On the other hand, exercise is a well-known approach for treating nonalcoholic fatty liver disease. We thus hypothesize that the combination of ketogenic diet and exercise could improve insulin sensitivity, while minimizing adverse effect of hepatic steatosis. In order to test this hypothesis, we established diabetic mice model with streptozotocin (STZ) and divided them into control group, ketogenic diet group, and ketogenic diet with aerobic exercise group. We found that after six weeks of intervention, mice treated with ketogenic diet and ketogenic diet combined with exercise both have lower body weights, HbAlc level, HOMA index, and improvements in insulin sensitivity, compared with diabetes group. In addition, mice in ketogenic diet intervention exhibited hepatic steatosis shown by serum and hepatic parameters, as well as histochemistry staining in the liver, which could be largely relieved by exercise. Furthermore, gene analysis revealed that ketogenic diet in combination with exercise reduced PPARγ and lipid synthetic genes, as well as enhancing PPARα and lipid β-oxidation gene program in the liver compared to those in ketogenic diet without exercise. Overall, the present study demonstrated that the combination of ketogenic diet and a moderate-intensity aerobic exercise intervention improved insulin sensitivity in diabetic mice, while avoiding hepatic steatosis, which provided a novel strategy in the combat of diabetes.

An HDAC3-PROX1 corepressor module acts on HNF4α to control hepatic triglycerides

The histone deacetylase HDAC3 is a critical mediator of hepatic lipid metabolism, and liver-specific deletion of HDAC3 leads to fatty liver. To elucidate the underlying mechanism, here we report a method of cross-linking followed by mass spectrometry to define a high-confidence HDAC3 interactome in vivo that includes the canonical NCoR-HDAC3 complex as well as Prospero-related homeobox 1 protein (PROX1). HDAC3 and PROX1 co-localize extensively on the mouse liver genome, and are co-recruited by hepatocyte nuclear factor 4α (HNF4α). The HDAC3-PROX1 module controls the expression of a gene program regulating lipid homeostasis, and hepatic-specific ablation of either component increases triglyceride content in liver. These findings underscore the importance of specific combinations of transcription factors and coregulators in the fine tuning of organismal metabolism.HDAC3 is a critical mediator of hepatic lipid metabolism and its loss leads to fatty liver. Here, the authors characterize the liver HDAC3 interactome in vivo, provide evidence that HDAC3 interacts with PROX1, and show that HDAC3 and PROX1 control expression of genes regulating lipid homeostasis.

A Hepatic GAbp-AMPK Axis Links Inflammatory Signaling to Systemic Vascular Damage

Increased pro-inflammatory signaling is a hallmark of metabolic dysfunction in obesity and diabetes. Although both inflammatory and energy substrate handling processes represent critical layers of metabolic control, their molecular integration sites remain largely unknown. Here, we identify the heterodimerization interface between the α and β subunits of transcription factor GA-binding protein (GAbp) as a negative target of tumor necrosis factor alpha (TNF-α) signaling. TNF-α prevented GAbpα and β complex formation via reactive oxygen species (ROS), leading to the non-energy-dependent transcriptional inactivation of AMP-activated kinase (AMPK) β1, which was identified as a direct hepatic GAbp target. Impairment of AMPKβ1, in turn, elevated downstream cellular cholesterol biosynthesis, and hepatocyte-specific ablation of GAbpα induced systemic hypercholesterolemia and early macro-vascular lesion formation in mice. As GAbpα and AMPKβ1 levels were also found to correlate in obese human patients, the ROS-GAbp-AMPK pathway may represent a key component of a hepato-vascular axis in diabetic long-term complications.

Mutations in TBL1X Are Associated With Central Hypothyroidism

CONTEXT

Isolated congenital central hypothyroidism (CeH) can result from mutations in TRHR, TSHB, and IGSF1, but its etiology often remains unexplained. We identified a missense mutation in the transducin β-like protein 1, X-linked (TBL1X) gene in three relatives diagnosed with isolated CeH. TBL1X is part of the thyroid hormone receptor-corepressor complex.

OBJECTIVE

The objectives of the study were the identification of TBL1X mutations in patients with unexplained isolated CeH, Sanger sequencing of relatives of affected individuals, and clinical and biochemical characterization; in vitro investigation of functional consequences of mutations; and mRNA expression in, and immunostaining of, human hypothalami and pituitary glands.

DESIGN

This was an observational study.

SETTING

The study was conducted at university medical centers.

PATIENTS

Nineteen individuals with and seven without a mutation participated in the study.

MAIN OUTCOME MEASURES

Outcome measures included sequencing results, clinical and biochemical characteristics of mutation carriers, and results of in vitro functional and expression studies.

RESULTS

Sanger sequencing yielded five additional mutations. All patients (n = 8; six males) were previously diagnosed with CeH (free T₄ [FT4] concentration below the reference interval, normal thyrotropin). Eleven relatives (two males) also carried mutations. One female had CeH, whereas 10 others had low-normal FT4 concentrations. As a group, adult mutation carriers had 20%-25% lower FT4 concentrations than controls. Twelve of 19 evaluated carriers had hearing loss. Mutations are located in the highly conserved WD40-repeat domain of the protein, influencing its expression and thermal stability. TBL1X mRNA and protein are expressed in the human hypothalamus and pituitary.

CONCLUSIONS

TBL1X mutations are associated with CeH and hearing loss. FT4 concentrations in mutation carriers vary from low-normal to values compatible with CeH.

Control of diabetic hyperglycaemia and insulin resistance through TSC22D4

Obesity-related insulin resistance represents the core component of the metabolic syndrome, promoting glucose intolerance, pancreatic beta cell failure and type 2 diabetes. Efficient and safe insulin sensitization and glucose control remain critical therapeutic aims to prevent diabetic late complications Here, we identify transforming growth factor beta-like stimulated clone (TSC) 22 D4 as a molecular determinant of insulin signalling and glucose handling. Hepatic TSC22D4 inhibition both prevents and reverses hyperglycaemia, glucose intolerance and insulin resistance in diabetes mouse models. TSC22D4 exerts its effects on systemic glucose homeostasis—at least in part—through the direct transcriptional regulation of the small secretory protein lipocalin 13 (LCN13). Human diabetic patients display elevated hepatic TSC22D4 expression, which correlates with decreased insulin sensitivity, hyperglycaemia and LCN13 serum levels. Our results establish TSC22D4 as a checkpoint in systemic glucose metabolism in both mice and humans, and propose TSC22D4 inhibition as an insulin sensitizing option in diabetes therapy.

TSC22D4 regulates hepatic lipoprotein production, but has so far mainly been studied in the context of cancer cachexia. Here, the authors show TSC22D4 inhibition improves insulin sensitivity in several mouse models of diabetes, which they attribute at least in part to the induction of secreted LCN13.

Transcriptional co-factor Transducin beta-like (TBL) 1 acts as a checkpoint in pancreatic cancer malignancy

Pancreatic ductal adenocarcinoma (PDAC) is the fourth leading cause of cancer fatalities in Western societies, characterized by high metastatic potential and resistance to chemotherapy. Critical molecular mechanisms of these phenotypical features still remain unknown, thus hampering the development of effective prognostic and therapeutic measures in PDAC. Here, we show that transcriptional co-factor Transducin beta-like (TBL) 1 was over-expressed in both human and murine PDAC. Inactivation of TBL1 in human and mouse pancreatic cancer cells reduced cellular proliferation and invasiveness, correlating with diminished glucose uptake, glycolytic flux, and oncogenic PI3 kinase signaling which in turn could rescue TBL1 deficiency-dependent phenotypes. TBL1 deficiency both prevented and reversed pancreatic tumor growth, mediated transcriptional PI3 kinase inhibition, and increased chemosensitivity of PDAC cells in vivo. As TBL1 mRNA levels were also found to correlate with PI3 kinase levels and overall survival in a cohort of human PDAC patients, TBL1 was identified as a checkpoint in the malignant behavior of pancreatic cancer and its expression may serve as a novel molecular target in the treatment of human PDAC.

Epigenetic targets in hepatocellular carcinoma cells: identification of chaperone protein complexes with histone deacetylases

AIMS

The study was designed to find out the protein complex(s) associated with HDAC3 in liver cancer using a modified form of affinity purification coupled with a mass spectrometry technique in HepG2 cells. The organ-specific requirement for HDAC1 and HDAC3 during liver formation in zebrafish and their altered expression in liver cancer tissues indicates they are indispensible for hepato-organogenesis and hepatocarcinogenesis. However, how they exert their function is unknown.

MATERIAL & METHODS

HepG2 cells were transfected with either mock or construct-containing HDAC3 using a C-terminal strepIII-HA tag as bait. The bait proteins were purified by double affinity columns and were analyzed on a Thermo LTQ Orbitrap™ (Thermo Scientific, MA, USA) chromatography system.

RESULTS

Affinity purification coupled with mass spectrometry resulted in the identification of 24 putative binders of HDAC3 in HepG2 cells. The majority (83%) of these are novel interactions are reported for the first time in this study.

CONCLUSION

This is the first study reporting the affinity purification and identification of protein complexes with two closely related proteins in one cell line. The novel HDAC1 and HDAC3 complexes identified in HepG2 cells could serve as a platform for the design of future therapeutic medicine for the treatment of liver cancer.

Theranostical nanosystem-mediated identification of an oncogene and highly effective therapy in hepatocellular carcinoma

Because the primary surgical treatment options for hepatocellular carcinoma (HCC)-including hepatic resection and liver transplantation-often fail due to recurrence and metastasis, identifying early prognostic biomarkers and therapeutic targets for HCC is of great importance. This study shows that transducin β-like protein 1-related protein (TBLR1) is a key HCC oncogene that plays important roles in HCC proliferation, antiapoptosis, and angiogenesis by regulating the Wnt/β-catenin pathway. The folate-targeted theranostic small interfering RNA (siRNA) nanomedicine Fa-PEG-g-PEI-SPION/psiRNA-TBLR1 effectively silences the TBLR1 gene in different human HCC cell lines in vitro and in human HCC samples in vivo, resulting in the simultaneous suppression of HCC cell proliferation, antiapoptosis, and angiogenesis. Because of its multi-anticancer functions against HCC, intravenous injection of the folate-targeted siRNA nanomedicine into nude mice bearing intrahepatic or subcutaneous xenografts of human HCC has a significant therapeutic effect. Tumor growth in those animals was almost completely inhibited by treatment with Fa-PEG-g-PEI-SPION/psiRNA-TBLR1. Moreover, the SPION-encapsulated polyplexes possess high magnetic resonance imaging (MRI) detection sensitivity, which makes tumor-targeted siRNA delivery easily trackable using the clinical MRI technique.

CONCLUSION

The theranostic siRNA nanomedicine examined here possesses great theranostic potential for combined gene therapy and MRI diagnosis of HCC.

Fetuin A is a Predictor of Liver Fat in Preoperative Patients with Nonalcoholic Fatty Liver Disease

BACKGROUND

Nonalcoholic fatty liver disease (NAFLD) and steatohepatitis (NASH) are frequent comorbidities in perioperative patients. However, the predictive role of the hepatokine fetuin A was not evaluated in this collective.

OBJECTIVE

To study fetuin A as predictor of NAFLD/NASH in preoperative patients.

METHODS

58 subjects were included. Fetuin A was studied in patients undergoing open abdominal surgery and in a subset with acute liver failure. Blood and liver specimens were sampled. NAFLD was histologically evaluated. Liver fat was additionally analyzed by an enzymatic approach, circulating fetuin A by enzyme linked-immunosorbent assay, fetuin A mRNA by reverse-transcription PCR.

RESULTS

Univariate correlation studies linked fetuin A to liver steatosis (r = 0.40, p = .029) and hepatocellular ballooning degeneration (r = 0.34, p = .026). Compared to non-NAFLD subjects fetuin A was increased in NAFLD (p = .009) and in NASH (p = .029). However, when corrected for main confounders by linear modeling, fetuin A remained related to hepatic steatosis, but not to ballooning degeneration or other NAFLD features. In support of this, biochemically analyzed liver lipids correlated with fetuin A in plasma (r = 0.34, p = .033) and with hepatic fetuin A mRNA (r = 0.54, p < .001). In addition, plasma fetuin A was related to hepatic mRNA (r = 0.32, p = .036), while circulating levels were reduced by 64% with acute liver failure (p < .001), confirming the liver as main fetuin A source.

CONCLUSION

Fetuin A is suggested as noninvasive biomarker of hepatic steatosis in preoperative settings.

PPP2R5C Couples Hepatic Glucose and Lipid Homeostasis

In mammals, the liver plays a central role in maintaining carbohydrate and lipid homeostasis by acting both as a major source and a major sink of glucose and lipids. In particular, when dietary carbohydrates are in excess, the liver converts them to lipids via de novo lipogenesis. The molecular checkpoints regulating the balance between carbohydrate and lipid homeostasis, however, are not fully understood. Here we identify PPP2R5C, a regulatory subunit of PP2A, as a novel modulator of liver metabolism in postprandial physiology. Inactivation of PPP2R5C in isolated hepatocytes leads to increased glucose uptake and increased de novo lipogenesis. These phenotypes are reiterated in vivo, where hepatocyte specific PPP2R5C knockdown yields mice with improved systemic glucose tolerance and insulin sensitivity, but elevated circulating triglyceride levels. We show that modulation of PPP2R5C levels leads to alterations in AMPK and SREBP-1 activity. We find that hepatic levels of PPP2R5C are elevated in human diabetic patients, and correlate with obesity and insulin resistance in these subjects. In sum, our data suggest that hepatic PPP2R5C represents an important factor in the functional wiring of energy metabolism and the maintenance of a metabolically healthy state.

After a meal, dietary glucose travels through the hepatic portal vein to the liver. A substantial part of this glucose is taken up by liver, which converts it to glycogen which is stored, and lipids which are in part stored and in part secreted as VLDL particles. The rest of the organs receive whatever glucose the liver leaves in circulation, plus the secreted lipids. Hence the liver plays a crucial role in determining the balance of sugar versus lipids in the body after a meal. This balance is very important, because too much glucose in circulation leads to diabetic complications whereas too much VLDL increases risk of atherosclerosis. Little is known about how the liver strikes this balance. We identify here a phosphatase—the PP2A holoenzyme containing the PPP2R5C regulatory subunit—as a regulator of this process. We find that knockdown of PPP2R5C in mouse liver specifically causes it to uptake elevated levels of glucose, and secrete elevated levels of VLDL into circulation. This leads to a phenotype of improved glucose tolerance and insulin sensitivity. The prediction from these functional studies in mice is that elevated levels of PPP2R5C expression should lead to insulin resistance. Indeed, we find that PPP2R5C expression levels are elevated in diabetic patients, or healthy controls with visceral obesity, raising the possibility that dysregulation of PPP2R5C expression in humans may contribute towards metabolic dysfunction.

SENP2 regulates MMP13 expression in a bladder cancer cell line through SUMOylation of TBL1/TBLR1

Bladder cancer (BC) is the most popular malignant urinary cancer in China. BC has the highest incidence and mortality among all genitourinary system tumors. Although the early-stage BC could be treated with advanced electron flexible systourethroscope, early metastasis of the BC occur frequently, and often results in poor prognosis. Recently, we reported that small ubiquitin related modifier (SUMO)-specific protease 2 (SENP2) was downregulated in BC specimen. SENP2 appeared to inhibit migration and invasion of bladder cancer cells in vitro, through suppressing MMP13 in BC cells. However, the exact underlying mechanisms remain unknown. Here, we reported that SENP2 inhibited nuclear translocation of β-catenin, which targeted the promotor of MMP13 to activate MMP13 to enhance BC cell metastasis. WNT ligands induced TBL1/TBLR1 SUMOylation to form complexes with β-catenin to facilitate β-catenin nuclear translocation, which could be efficiently inhibited through suppression of SUMOylation of TBL1/TBLR1. Together, our data suggest that SENP2 inhibits MMP13 expression in BC cells through de-SUMOylation of TBL1/TBLR1, which inhibits nuclear translocation of β-catenin. Thus, SENP2 may be a promising therapeutic target for BC.

Protective effects of aerobic swimming training on high-fat diet induced nonalcoholic fatty liver disease: regulation of lipid metabolism via PANDER-AKT pathway

This study aimed to investigate the mechanism by which aerobic swimming training prevents high-fat-diet-induced nonalcoholic fatty liver disease (NAFLD). Forty-two male C57BL/6 mice were randomized into normal-diet sedentary (ND; n = 8), ND exercised (n = 8), high-fat diet sedentary (HFD; n = 13), and HFD exercised groups (n = 13). After 2 weeks of training adaptation, the mice were subjected to an aerobic swimming protocol (60 min/day) 5 days/week for 10 weeks. The HFD group exhibited significantly higher mRNA levels of fatty acid transport-, lipogenesis-, and β-oxidation-associated gene expressions than the ND group. PANDER and FOXO1 expressions increased, whereas AKT expression decreased in the HFD group. The aerobic swimming program with the HFD reversed the effects of the HFD on the expressions of thrombospondin-1 receptor, liver fatty acid-binding protein, long-chain fatty-acid elongase-6, Fas cell surface death receptor, and stearoyl-coenzyme A desaturase-1, as well as PANDER, FOXO1, and AKT. In the HFD exercised group, PPARα and AOX expressions were much higher. Our findings suggest that aerobic swimming training can prevent NAFLD via the regulation of fatty acid transport-, lipogenesis-, and β-oxidation-associated genes. In addition, the benefits from aerobic swimming training were achieved partly through the PANDER-AKT-FOXO1 pathway.

microRNA-379 couples glucocorticoid hormones to dysfunctional lipid homeostasis

In mammals, glucocorticoids (GCs) and their intracellular receptor, the glucocorticoid receptor (GR), represent critical checkpoints in the endocrine control of energy homeostasis. Indeed, aberrant GC action is linked to severe metabolic stress conditions as seen in Cushing's syndrome, GC therapy and certain components of the Metabolic Syndrome, including obesity and insulin resistance. Here, we identify the hepatic induction of the mammalian conserved microRNA (miR)-379/410 genomic cluster as a key component of GC/GR-driven metabolic dysfunction. Particularly, miR-379 was up-regulated in mouse models of hyperglucocorticoidemia and obesity as well as human liver in a GC/GR-dependent manner. Hepatocyte-specific silencing of miR-379 substantially reduced circulating very-low-density lipoprotein (VLDL)-associated triglyceride (TG) levels in healthy mice and normalized aberrant lipid profiles in metabolically challenged animals, mediated through miR-379 effects on key receptors in hepatic TG re-uptake. As hepatic miR-379 levels were also correlated with GC and TG levels in human obese patients, the identification of a GC/GR-controlled miRNA cluster not only defines a novel layer of hormone-dependent metabolic control but also paves the way to alternative miRNA-based therapeutic approaches in metabolic dysfunction.

Energy metabolism in the liver

The liver is an essential metabolic organ, and its metabolic function is controlled by insulin and other metabolic hormones. Glucose is converted into pyruvate through glycolysis in the cytoplasm, and pyruvate is subsequently oxidized in the mitochondria to generate ATP through the TCA cycle and oxidative phosphorylation. In the fed state, glycolytic products are used to synthesize fatty acids through de novo lipogenesis. Long-chain fatty acids are incorporated into triacylglycerol, phospholipids, and/or cholesterol esters in hepatocytes. These complex lipids are stored in lipid droplets and membrane structures, or secreted into the circulation as very low-density lipoprotein particles. In the fasted state, the liver secretes glucose through both glycogenolysis and gluconeogenesis. During pronged fasting, hepatic gluconeogenesis is the primary source for endogenous glucose production. Fasting also promotes lipolysis in adipose tissue, resulting in release of nonesterified fatty acids which are converted into ketone bodies in hepatic mitochondria though β-oxidation and ketogenesis. Ketone bodies provide a metabolic fuel for extrahepatic tissues. Liver energy metabolism is tightly regulated by neuronal and hormonal signals. The sympathetic system stimulates, whereas the parasympathetic system suppresses, hepatic gluconeogenesis. Insulin stimulates glycolysis and lipogenesis but suppresses gluconeogenesis, and glucagon counteracts insulin action. Numerous transcription factors and coactivators, including CREB, FOXO1, ChREBP, SREBP, PGC-1α, and CRTC2, control the expression of the enzymes which catalyze key steps of metabolic pathways, thus controlling liver energy metabolism. Aberrant energy metabolism in the liver promotes insulin resistance, diabetes, and nonalcoholic fatty liver diseases.

Transducin β-like 1 X-linked receptor 1 suppresses cisplatin sensitivity in nasopharyngeal carcinoma via activation of NF-κB pathway

Background

Transducin β-like 1 X-linked receptor 1 (TBL1XR1) is an important transcriptional cofactor involved in the regulation of many signaling pathways, and is associated with carcinogenesis and tumor progression. However, the precise role of TBL1XR1 in these processes is not well understood.

Methods

We detected the expression of TBL1XR1 protein and mRNA in nasopharyngeal carcinoma (NPC) cell lines and biopsies by western blotting, real-time PCR and immunohistochemical staining (IHC). Overexpression of TBL1XR1 in NPC enhanced chemoresistance to cisplatin using two NPC cell lines in vitro and in vivo.

Results

TBL1XR1 was upregulated in NPC cell lines and clinical samples. The expression of TBL1XR1 was correlated with several clinicopathological factors including clinical stage, T classification, N classification and patient survival. Univariate and multivariate analysis revealed that TBL1XR1 was an independent prognostic factor for patient survival. In vitro and in vivo studies demonstrated that TBL1XR1 high expression induced resistance to cisplatin-induced apoptosis in NPC cells. Furthermore, we found that TBL1XR1 activated the NF-κB pathway and promoted transcription of genes downstream of NF-κB, especially anti-apoptotic genes.

Conclusions

Upregulation of TBL1XR1 induces NPC cells resistance to cisplatin by activating the NF-κB pathway, and correlates with poor overall survival of NPC patients. TBL1XR1 has a pivotal role in NPC and could be a valuable prognostic factor as well as a novel biomarker for tailoring appropriate therapeutic regimes.

Electronic supplementary material

The online version of this article (doi:10.1186/1476-4598-13-195) contains supplementary material, which is available to authorized users.

Periostin promotes liver steatosis and hypertriglyceridemia through downregulation of PPARα

Hepatosteatosis is characterized by an aberrant accumulation of triglycerides in the liver; however, the factors that drive obesity-induced fatty liver remain largely unknown. Here, we demonstrated that the secreted cell adhesion protein periostin is markedly upregulated in livers of obese rodents and humans. Notably, overexpression of periostin in the livers of WT mice promoted hepatic steatosis and hypertriglyceridemia. Conversely, both genetic ablation of periostin and administration of a periostin-neutralizing antibody dramatically improved hepatosteatosis and hypertriglyceridemia in obese mice. Overexpression of periostin resulted in reduced expression of peroxisome proliferator-activated receptor α (PPARα), a master regulator of fatty acid oxidation, and activation of the JNK signaling pathway. In mouse primary hepatocytes, inhibition of α6β4 integrin prevented activation of JNK and suppression of PPARα in response to periostin. Periostin-dependent activation of JNK resulted in activation of c-Jun, which prevented RORα binding and transactional activation at the Ppara promoter. Together, these results identify a periostin-dependent pathway that mediates obesity-induced hepatosteatosis.

Hepatic transforming growth factor-β 1 stimulated clone-22 D1 controls systemic cholesterol metabolism

Disturbances in lipid homeostasis are hallmarks of severe metabolic disorders and their long-term complications, including obesity, diabetes, and atherosclerosis. Whereas elevation of triglyceride (TG)-rich very-low-density lipoproteins (VLDL) has been identified as a risk factor for cardiovascular complications, high-density lipoprotein (HDL)-associated cholesterol confers atheroprotection under obese and/or diabetic conditions. Here we show that hepatocyte-specific deficiency of transcription factor transforming growth factor β 1-stimulated clone (TSC) 22 D1 led to a substantial reduction in HDL levels in both wild-type and obese mice, mediated through the transcriptional down-regulation of the HDL formation pathway in liver. Indeed, overexpression of TSC22D1 promoted high levels of HDL cholesterol in healthy animals, and hepatic expression of TSC22D1 was found to be aberrantly regulated in disease models of opposing energy availability. The hepatic TSC22D1 transcription factor complex may thus represent an attractive target in HDL raising strategies in obesity/diabetes-related dyslipidemia and atheroprotection.

Neuroendocrine and metabolic components of dopamine agonist amelioration of metabolic syndrome in SHR rats

BACKGROUND

The hypertensive, pro-inflammatory, obese state is strongly coupled to peripheral and hepatic insulin resistance (in composite termed metabolic syndrome [MS]). Hepatic pro-inflammatory pathways have been demonstrated to initiate or exacerbate hepatic insulin resistance and contribute to fatty liver, a correlate of MS. Previous studies in seasonally obese animals have implicated an important role for circadian phase-dependent increases in hypothalamic dopaminergic tone in the maintenance of the lean, insulin sensitive condition. However, mechanisms driving this dopaminergic effect have not been fully delineated and the impact of such dopaminergic function upon the above mentioned parameters of MS, particularly upon key intra-hepatic regulators of liver inflammation and lipid and glucose metabolism have never been investigated.

OBJECTIVE

This study therefore investigated the effects of timed daily administration of bromocriptine, a potent dopamine D2 receptor agonist, on a) ventromedial hypothalamic catecholamine activity, b) MS and c) hepatic protein levels of key regulators of liver inflammation and glucose and lipid metabolism in a non-seasonal model of MS - the hypertensive, obese SHR rat.

METHODS

Sixteen week old SHR rats maintained on 14 hour daily photoperiods were treated daily for 16 days with bromocriptine (10 mg/kg, i.p.) or vehicle at 1 hour before light offset and, subsequent to blood pressure recordings on day 14, were then utilized for in vivo microdialysis of ventromedial hypothalamic catecholamine activity or sacrificed for the analyses of MS factors and regulators of hepatic metabolism. Normal Wistar rats served as wild-type controls for hypothalamic activity, body fat levels, and insulin sensitivity.

RESULTS

Bromocriptine treatment significantly reduced ventromedial hypothalamic norepinephrine and serotonin levels to the normal range and systolic and diastolic blood pressures, retroperitoneal body fat level, plasma insulin and glucose levels and HOMA-IR relative to vehicle treated SHR controls. Such treatment also reduced plasma levels of C-reactive protein, leptin, and norepinephrine and increased that of plasma adiponectin significantly relative to SHR controls. Finally, bromocriptine treatment significantly reduced hepatic levels of several pro-inflammatory pathway proteins and of the master transcriptional activators of lipogenesis, gluconeogenesis, and free fatty acid oxidation versus control SHR rats.

CONCLUSION

These findings indicate that in SHR rats, timed daily dopamine agonist treatment improves hypothalamic and neuroendocrine pathologies associated with MS and such neuroendocrine events are coupled to a transformation of liver metabolism potentiating a reduction of elevated lipogenic and gluconeogenic capacity. This liver effect may be driven in part by concurrent reductions in hyperinsulinemia and sympathetic tone as well as by reductions in intra-hepatic inflammation.

Immune cells and metabolic dysfunction

Throughout evolution, effective nutrient sensing and control of systemic energy homeostasis have relied on a close physical and functional interaction between immune and metabolically active cells. However, in today's obesogenic environment, this fine-tuned immunometabolic interface is perturbed. As a consequence, chronic inflammatory conditions and aberrant activation of immune cells have emerged as key features of obesity-related metabolic disorders, including insulin resistance, cardiovascular complications, and type 2 diabetes, whereas a major research focus has been placed on the adipocyte-macrophage interaction in the context of metabolic dysfunction; recent studies have not only expanded the scope of relevant immune cells in this setting but also highlight the impact of distinct metabolic organs, including the liver, on immunometabolic control, metabolic disease development, and potential anti-inflammatory therapeutic options in obesity-driven pathologies. This review will thus summarize recent progress in this emerging area of metabolic research.

Transcriptional cofactor TBLR1 controls lipid mobilization in white adipose tissue

Lipid mobilization (lipolysis) in white adipose tissue (WAT) critically controls lipid turnover and adiposity in humans. While the acute regulation of lipolysis has been studied in detail, the transcriptional determinants of WAT lipolytic activity remain still largely unexplored. Here we show that the genetic inactivation of transcriptional cofactor transducin beta-like-related 1(TBLR1) blunts the lipolytic response of white adipocytes through the impairment of cAMP-dependent signal transduction. Indeed, mice lacking TBLR1 in adipocytes are defective in fasting-induced lipid mobilization and, when placed on a high-fat-diet, show aggravated adiposity, glucose intolerance, and insulin resistance. TBLR1 levels are found to increase under lipolytic conditions in WAT of both human patients and mice, correlating with serum free fatty acids (FFAs). As a critical regulator of WAT cAMP signaling and lipid mobilization, proper activity of TBLR1 in adipocytes might thus represent a critical molecular checkpoint for the prevention of metabolic dysfunction in subjects with obesity-related disorders.

TSC22D4 is a molecular output of hepatic wasting metabolism

In mammals, proper storage and distribution of lipids in and between tissues is essential for the maintenance of energy homeostasis. Here, we show that tumour growth triggers hepatic metabolic dysfunction as part of the cancer cachectic phenotype, particularly by reduced hepatic very-low-density-lipoprotein (VLDL) secretion and hypobetalipoproteinemia. As a molecular cachexia output pathway, hepatic levels of the transcription factor transforming growth factor beta 1-stimulated clone (TSC) 22 D4 were increased in cancer cachexia. Mimicking high cachectic levels of TSC22D4 in healthy livers led to the inhibition of hepatic VLDL release and lipogenic genes, and diminished systemic VLDL levels under both normal and high fat dietary conditions. Liver-specific ablation of TSC22D4 triggered hypertriglyceridemia through the induction of hepatic VLDL secretion. Furthermore, hepatic TSC22D4 expression levels were correlated with the degree of body weight loss and VLDL hypo-secretion in cancer cachexia, and TSC22D4 deficiency rescued tumour cell-induced metabolic dysfunction in hepatocytes. Therefore, hepatic TSC22D4 activity may represent a molecular rationale for peripheral energy deprivation in subjects with metabolic wasting diseases, including cancer cachexia.

Dissociating fatty liver and diabetes

Fatty liver disease is epidemiologically associated with type 2 diabetes (T2D), leading to a speculation of a reciprocal cause-effect relationship and a vicious cycle of pathology. Here, we summarize recent literature reporting dissociation of hepatosteatosis from insulin resistance in genetic mouse models and clinical studies. We highlight rhythmic flows of metabolic intermediates between hepatic lipid synthesis and glucose production in normal circadian physiology. Blocking triglyceride (TG) secretion, subcellular lipid sequestration, lipolysis deficiency, enhanced lipogenesis, gluconeogenesis defects, or inhibition of fatty acid oxidation all result in hepatosteatosis without causing hyperglycemia or insulin resistance, suggesting that the cause-effect relationship between hepatosteatosis and diabetes does not exist in all situations.

Peroxisomal localization and circadian regulation of ubiquitin-specific protease 2

Temporal regulation of nutrient and energy metabolism is emerging as an important aspect of metabolic homeostasis. The regulatory network that integrates the timing cues and nutritional signals to drive diurnal metabolic rhythms remains poorly defined. The 45-kDa isoform of ubiquitin-specific protease 2 (USP2-45) is a deubiquitinase that regulates hepatic gluconeogenesis and glucose metabolism. In this study, we found that USP2-45 is localized to peroxisomes in hepatocytes through a canonical peroxisome-targeting motif at its C-terminus. Clustering analysis indicates that the expression of a subset of peroxisomal genes exhibits robust diurnal rhythm in the liver. Despite this, nuclear hormone receptor PPARα, a known regulator of peroxisome gene expression, does not induce USP2-45 in hepatocytes and is dispensible for its expression during starvation. In contrast, a functional liver clock is required for the proper nutritional and circadian regulation of USP2-45 expression. At the molecular level, transcriptional coactivators PGC-1α and PGC-1β and repressor E4BP4 exert opposing effects on USP2-45 promoter activity. These studies provide insights into the subcellular localization and transcriptional regulation of a clock-controlled deubiquitinase that regulates glucose metabolism.

Molecular cloning and characterization of the anti-obesity gene adipose in pig

Obesity has become an epidemic health problem characterized by aberrant energy metabolism. As the major player in energy homeostasis, adipose tissue has a decisive role in the development of obesity. Many genes involved in adipogenesis are also correlated with obesity. Adipose (Adp) has been established as an anti-obesity gene to repress adipogenesis and fat accumulation in mice, which inhibits the transcriptional activity of PPARγ by forming a chromatin remodeling complex with histones and HDAC3. Here, we reported the cloning and characterization of the pig Adp gene. Pig Adp cDNA had an ORF of 2034 nucleotides and was highly conserved among various species. Genomic sequence analysis indicated that pig Adp gene contains 16 exons and 15 introns, spanning more than 60kb on chromosome 6q21-24. The expression of pig Adp was high in testis, lung, kidney and adipose tissues, and relatively low in skeletal muscle. Bioinformatic analysis of 5'-flanking region of Adp has identified several potential binding sites for pivotal transcriptional factors related to both adipocyte differentiation and inflammation, highlighting the significance of Adp in energy metabolism. We have confirmed that KLF6, a positive regulator of adipogenesis, can enhance the promoter activity of Adp and up-regulate its mRNA expression. Taken together, our results would be helpful for further study of Adp regulation in the process of fat accumulation.

Amorfrutins are potent antidiabetic dietary natural products

Given worldwide increases in the incidence of obesity and type 2 diabetes, new strategies for preventing and treating metabolic diseases are needed. The nuclear receptor PPARγ (peroxisome proliferator-activated receptor gamma) plays a central role in lipid and glucose metabolism; however, current PPARγ-targeting drugs are characterized by undesirable side effects. Natural products from edible biomaterial provide a structurally diverse resource to alleviate complex disorders via tailored nutritional intervention. We identified a family of natural products, the amorfrutins, from edible parts of two legumes, Glycyrrhiza foetida and Amorpha fruticosa, as structurally new and powerful antidiabetics with unprecedented effects for a dietary molecule. Amorfrutins bind to and activate PPARγ, which results in selective gene expression and physiological profiles markedly different from activation by current synthetic PPARγ drugs. In diet-induced obese and db/db mice, amorfrutin treatment strongly improves insulin resistance and other metabolic and inflammatory parameters without concomitant increase of fat storage or other unwanted side effects such as hepatoxicity. These results show that selective PPARγ-activation by diet-derived ligands may constitute a promising approach to combat metabolic disease.

A monocarboxylate transporter required for hepatocyte secretion of ketone bodies during fasting

To find new genes that influence liver lipid mass, we performed a genetic screen for zebrafish mutants with hepatic steatosis, a pathological accumulation of fat. The red moon (rmn) mutant develops hepatic steatosis as maternally deposited yolk is depleted. Conversely, hepatic steatosis is suppressed in rmn mutants by adequate nutrition. Adult rmn mutants show increased liver neutral lipids and induction of hepatic lipid biosynthetic genes when fasted. Positional cloning of the rmn locus reveals a loss-of-function mutation in slc16a6a (solute carrier family 16a, member 6a), a gene that we show encodes a transporter of the major ketone body β-hydroxybutyrate. Restoring wild-type zebrafish slc16a6a expression or introducing human SLC16A6 in rmn mutant livers rescues the mutant phenotype. Radiotracer analysis confirms that loss of Slc16a6a function causes diversion of liver-trapped ketogenic precursors into triacylglycerol. Underscoring the importance of Slc16a6a to normal fasting physiology, previously fed rmn mutants are more sensitive to death by starvation than are wild-type larvae. Our unbiased, forward genetic approach has found a heretofore unrecognized critical step in fasting energy metabolism: hepatic ketone body transport. Since β-hydroxybutyrate is both a major fuel and a signaling molecule in fasting, the discovery of this transporter provides a new direction for modulating circulating levels of ketone bodies in metabolic diseases.