ATGL-dependent white adipose tissue lipolysis controls hepatocyte PPARα activity

Rab30 facilitates lipid homeostasis during fasting

To facilitate inter-tissue communication and the exchange of proteins, lipoproteins, and metabolites with the circulation, hepatocytes have an intricate and efficient intracellular trafficking system regulated by small Rab GTPases. Here, we show that Rab30 is induced in the mouse liver by fasting, which is amplified in liver-specific carnitine palmitoyltransferase 2 knockout mice (Cpt2L-/-) lacking the ability to oxidize fatty acids, in a Pparα-dependent manner. Live-cell super-resolution imaging and in vivo proximity labeling demonstrates that Rab30-marked vesicles are highly dynamic and interact with proteins throughout the secretory pathway. Rab30 whole-body, liver-specific, and Rab30; Cpt2 liver-specific double knockout (DKO) mice are viable with intact Golgi ultrastructure, although Rab30 deficiency in DKO mice suppresses the serum dyslipidemia observed in Cpt2L-/- mice. Corresponding with decreased serum triglyceride and cholesterol levels, DKO mice exhibit decreased circulating but not hepatic ApoA4 protein, indicative of a trafficking defect. Together, these data suggest a role for Rab30 in the selective sorting of lipoproteins to influence hepatocyte and circulating triglyceride levels, particularly during times of excessive lipid burden.

Adipocyte HSL is required for maintaining circulating vitamin A and RBP4 levels during fasting

Vitamin A (retinol) is distributed via the blood bound to its specific carrier protein, retinol-binding protein 4 (RBP4). Retinol-loaded RBP4 is secreted into the circulation exclusively from hepatocytes, thereby mobilizing hepatic retinoid stores that represent the major vitamin A reserves in the body. The relevance of extrahepatic retinoid stores for circulating retinol and RBP4 levels that are usually kept within narrow physiological limits is unknown. Here, we show that fasting affects retinoid mobilization in a tissue-specific manner, and that hormone-sensitive lipase (HSL) in adipose tissue is required to maintain serum concentrations of retinol and RBP4 during fasting in mice. We found that extracellular retinol-free apo-RBP4 induces retinol release by adipocytes in an HSL-dependent manner. Consistently, global or adipocyte-specific HSL deficiency leads to an accumulation of retinoids in adipose tissue and a drop of serum retinol and RBP4 during fasting, which affects retinoid-responsive gene expression in eye and kidney and lowers renal retinoid content. These findings establish a novel crosstalk between liver and adipose tissue retinoid stores for the maintenance of systemic vitamin A homeostasis during fasting.

Is stimulation of browning of human adipose tissue a relevant therapeutic target?

Brown adipose tissue (BAT) and beige adipose tissues are important contributors to cold-induced whole body thermogenesis in rodents. The documentation in humans of cold- and ß-adrenergic receptor agonist-stimulated BAT glucose uptake using positron emission tomography (PET) and of a decrease of this response in individuals with cardiometabolic disorders led to the suggestion that BAT/beige adipose tissues could be relevant targets for prevention and treatment of these conditions. In this brief review, we will critically assess this question by first describing the basic rationale for this affirmation, second by examining the evidence in human studies, and third by discussing the possible means to activate the thermogenic response of these tissues in humans.

Physiologic disruption and metabolic reprogramming in infection and sepsis

Effective responses against severe systemic infection require coordination between two complementary defense strategies that minimize the negative impact of infection on the host: resistance, aimed at pathogen elimination, and disease tolerance, which limits tissue damage and preserves organ function. Resistance and disease tolerance mostly rely on divergent metabolic programs that may not operate simultaneously in time and space. Due to evolutionary reasons, the host initially prioritizes the elimination of the pathogen, leading to dominant resistance mechanisms at the potential expense of disease tolerance, which can contribute to organ failure. Here, we summarize our current understanding of the role of physiological perturbations resulting from infection in immune response dynamics and the metabolic program requirements associated with resistance and disease tolerance mechanisms. We then discuss how insight into the interplay of these mechanisms could inform future research aimed at improving sepsis outcomes and the potential for therapeutic interventions.

Increased FGF-21 Improves Ectopic Lipid Deposition in the Liver and Skeletal Muscle

Obesity can lead to excessive lipid accumulation in non-adipose tissues, such as the liver and skeletal muscles, leading to ectopic lipid deposition and damaging target organ function through lipotoxicity. FGF-21 is a key factor in regulating lipid metabolism, so we aim to explore whether FGF-21 is involved in improving ectopic lipid deposition. We observed the characteristics of ectopic lipid deposition in the liver and skeletal muscles of obesity-resistant mice, detected the expression of FGF-21 and perilipin, and found that obesity-resistant mice showed a decrease in ectopic lipid deposition in the liver and skeletal muscles and increased expression of FGF-21. After inhibiting the expression of FGF-21, a more severe lipid deposition in liver cells and skeletal muscle cells was found. The results indicate that inhibiting FGF-21 can exacerbate ectopic lipid deposition via regulating lipid droplet synthesis and decomposition, as well as free fatty acid translocation and oxidation. In conclusion, FGF-21 is involved in improving ectopic lipid deposition caused by obesity in the liver and skeletal muscles.

Plasticity of Adipose Tissues: Interconversion among White, Brown, and Beige Fat and Its Role in Energy Homeostasis

Obesity, characterized by the excessive accumulation of adipose tissue, has emerged as a major public health concern worldwide. To develop effective strategies for treating obesity, it is essential to comprehend the biological properties of different adipose tissue types and their respective roles in maintaining energy balance. Adipose tissue serves as a crucial organ for energy storage and metabolism in the human body, with functions extending beyond simple fat storage to encompass the regulation of energy homeostasis and the secretion of endocrine factors. This review provides an overview of the key characteristics, functional differences, and interconversion processes among white adipose tissue (WAT), brown adipose tissue (BAT), and beige adipose tissue. Moreover, it delves into the molecular mechanisms and recent research advancements concerning the browning of WAT, activation of BAT, and whitening of BAT. Although targeting adipose tissue metabolism holds promise as a potential approach for obesity treatment, further investigations are necessary to unravel the intricate biological features of various adipose tissue types and elucidate the molecular pathways governing their interconversion. Such research endeavors will pave the way for the development of more efficient and targeted therapeutic interventions in the fight against obesity.

Glucose controls lipolysis through Golgi PtdIns4P-mediated regulation of ATGL

Metabolic crosstalk of the major nutrients glucose, amino acids and fatty acids (FAs) ensures systemic metabolic homeostasis. The coordination between the supply of glucose and FAs to meet various physiological demands is especially important as improper nutrient levels lead to metabolic disorders, such as diabetes and metabolic dysfunction-associated steatohepatitis (MASH). In response to the oscillations in blood glucose levels, lipolysis is thought to be mainly regulated hormonally to control FA liberation from lipid droplets by insulin, catecholamine and glucagon. However, whether general cell-intrinsic mechanisms exist to directly modulate lipolysis via glucose sensing remains largely unknown. Here we report the identification of such an intrinsic mechanism, which involves Golgi PtdIns4P-mediated regulation of adipose triglyceride lipase (ATGL)-driven lipolysis via intracellular glucose sensing. Mechanistically, depletion of intracellular glucose results in lower Golgi PtdIns4P levels, and thus reduced assembly of the E3 ligase complex CUL7FBXW8 in the Golgi apparatus. Decreased levels of the E3 ligase complex lead to reduced polyubiquitylation of ATGL in the Golgi and enhancement of ATGL-driven lipolysis. This cell-intrinsic mechanism regulates both the pool of intracellular FAs and their extracellular release to meet physiological demands during fasting and glucose deprivation. Moreover, genetic and pharmacological manipulation of the Golgi PtdIns4P-CUL7FBXW8-ATGL axis in mouse models of simple hepatic steatosis and MASH, as well as during ex vivo perfusion of a human steatotic liver graft leads to the amelioration of steatosis, suggesting that this pathway might be a promising target for metabolic dysfunction-associated steatotic liver disease and possibly MASH.

Characterization of sexual dimorphism in ANGPTL4 levels and function

ANGPTL4 is an attractive pharmacological target for lowering plasma triglycerides and cardiovascular risk. Since most preclinical studies on ANGPTL4 were performed in male mice, little is known about sexual dimorphism in ANGPTL4 regulation and function. Here, we aimed to study potential sexual dimorphism in ANGPTL4 mRNA and protein levels and ANGPTL4 function. Additionally, we performed exploratory studies on the function of ANGPTL4 in the liver during fasting using Angptl4-transgenic and Angptl4-/- mice. Compared to female mice, male mice showed higher hepatic and adipose ANGPTL4 mRNA and protein levels, as well as a more pronounced effect of genetic ANGPTL4 modulation on plasma lipids. By contrast, very limited sexual dimorphism in ANGPTL4 levels was observed in human liver and adipose tissue. In human and mouse adipose tissue, ANGPTL8 mRNA and/or protein levels were significantly higher in females than males. Adipose LPL protein levels were higher in female than male Angptl4-/- mice, which was abolished by ANGPTL4 (over) expression. At the human genetic level, the ANGPTL4 E40K loss-of-function variant was associated with similar plasma triglyceride reductions in women and men. Finally, ANGPTL4 ablation in fasted mice was associated with changes in hepatic gene expression consistent with PPARα activation. In conclusion, the levels of ANGPTL4 and the magnitude of the effect of ANGPTL4 on plasma lipids exhibit sexual dimorphism. Nonetheless, inactivation of ANGPTL4 should confer a similar metabolic benefit in women and men. Expression levels of ANGPTL8 in human and mouse adipose tissue are highly sexually dimorphic, showing higher levels in females than males.

Gypenoside XIII regulates lipid metabolism in HepG2 hepatocytes and ameliorates nonalcoholic steatohepatitis in mice

Gypenoside XIII is isolated from Gynostemma pentaphyllum (Thunb.) Makino. In mice, G. pentaphyllum extract and gypenoside LXXV have been shown to improve non-alcoholic steatohepatitis (NASH). This study investigated whether gypenoside XIII can regulate lipid accumulation in fatty liver cells or attenuate NASH in mice. We used HepG2 hepatocytes to establish a fatty liver cell model using 0.5 mM oleic acid. Fatty liver cells were treated with different concentrations of gypenoside XIII to evaluate the molecular mechanisms of lipid metabolism. In addition, a methionine/choline-deficient diet induced NASH in C57BL/6 mice, which were given 10 mg/kg gypenoside XIII by intraperitoneal injection. In fatty liver cells, gypenoside XIII effectively suppressed lipid accumulation and lipid peroxidation. Furthermore, gypenoside XIII significantly increased SIRT1 and AMPK phosphorylation to decrease acetyl-CoA carboxylase phosphorylation, reducing fatty acid synthesis activity. Gypenoside XIII also decreased lipogenesis by suppressing sterol regulatory element-binding protein 1c and fatty acid synthase production. Gypenoside XIII also increased lipolysis and fatty acid β-oxidation by promoting adipose triglyceride lipase and carnitine palmitoyltransferase 1, respectively. In an animal model of NASH, gypenoside XIII effectively decreased the lipid vacuole size and number and reduced liver fibrosis and inflammation. These findings suggest that gypenoside XIII can regulate lipid metabolism in fatty liver cells and improve liver fibrosis in NASH mice. Therefore, gypenoside XIII has potential as a novel agent for the treatment of NASH.

Mechanisms of hepatic fatty acid oxidation and ketogenesis during fasting

Fasting is part of many weight management and health-boosting regimens. Fasting causes substantial metabolic adaptations in the liver that include the stimulation of fatty acid oxidation and ketogenesis. The induction of fatty acid oxidation and ketogenesis during fasting is mainly driven by interrelated changes in plasma levels of various hormones and an increase in plasma nonesterified fatty acid (NEFA) levels and is mediated transcriptionally by the peroxisome proliferator-activated receptor (PPAR)α, supported by CREB3L3 (cyclic AMP-responsive element-binding protein 3 like 3). Compared with men, women exhibit higher ketone levels during fasting, likely due to higher NEFA availability, suggesting that the metabolic response to fasting shows sexual dimorphism. Here, we synthesize the current molecular knowledge on the impact of fasting on hepatic fatty acid oxidation and ketogenesis.

Peroxisome proliferator-activated receptors: A key link between lipid metabolism and cancer progression

Lipids represent the essential components of membranes, serve as fuels for high-energy processes, and play crucial roles in signaling and cellular function. One of the key hallmarks of cancer is the reprogramming of metabolic pathways, especially abnormal lipid metabolism. Alterations in lipid uptake, lipid desaturation, de novo lipogenesis, lipid droplets, and fatty acid oxidation in cancer cells all contribute to cell survival in a changing microenvironment by regulating feedforward oncogenic signals, key oncogenic functions, oxidative and other stresses, immune responses, or intercellular communication. Peroxisome proliferator-activated receptors (PPARs) are transcription factors activated by fatty acids and act as core lipid sensors involved in the regulation of lipid homeostasis and cell fate. In addition to regulating whole-body energy homeostasis in physiological states, PPARs play a key role in lipid metabolism in cancer, which is receiving increasing research attention, especially the fundamental molecular mechanisms and cancer therapies targeting PPARs. In this review, we discuss how cancer cells alter metabolic patterns and regulate lipid metabolism to promote their own survival and progression through PPARs. Finally, we discuss potential therapeutic strategies for targeting PPARs in cancer based on recent studies from the last five years.

An Overview of the Role of Peroxisome Proliferator-activated Receptors in Liver Diseases

Peroxisome proliferator-activated receptors (PPARs) are a superfamily of nuclear transcription receptors, consisting of PPARα, PPARγ, and PPARβ/δ, which are highly expressed in the liver. They control and modulate the expression of a large number of genes involved in metabolism and energy homeostasis, oxidative stress, inflammation, and even apoptosis in the liver. Therefore, they have critical roles in the pathophysiology of hepatic diseases. This review provides a general insight into the role of PPARs in liver diseases and some of their agonists in the clinic.

Adipose tissue lipid metabolism: lipolysis

White adipose tissue stores fatty acid (FA) as triglyceride in the lipid droplet organelle of highly specialized cells known as fat cells or adipocytes. Depending on the nutritional state and energy demand, hormonal and biochemical signals converge on activating an elegant and fundamental process known as lipolysis, which involves triglyceride hydrolysis to FAs. Almost six decades of work have vastly expanded our knowledge of lipolysis from enzymatic processes to complex protein assembly, disassembly, and post-translational modification. Research in recent decades ushered in the discovery of new lipolytic enzymes and coregulators and the characterization of numerous factors and signaling pathways that regulate lipid hydrolysis on transcriptional and post-transcriptional levels. This review will discuss recent developments with particular emphasis on the past two years in enzymatic lipolytic pathways and transcriptional regulation of lipolysis. We will summarize the positive and negative regulators of lipolysis, the adipose tissue microenvironment in lipolysis, and the systemic effects of lipolysis. The dynamic nature of adipocyte lipolysis is emerging as an essential regulator of metabolism and energy balance, and we will discuss recent developments in this area.

Hepatic Activin E mediates liver-adipose inter-organ communication, suppressing adipose lipolysis in response to elevated serum fatty acids

OBJECTIVE

The liver is a central regulator of energy metabolism exerting its influence both through intrinsic processing of substrates such as glucose and fatty acid as well as by secreting endocrine factors, known as hepatokines, which influence metabolism in peripheral tissues. Human genome wide association studies indicate that a predicted loss-of-function variant in the Inhibin βE gene (INHBE), encoding the putative hepatokine Activin E, is associated with reduced abdominal fat mass and cardiometabolic disease risk. However, the regulation of hepatic Activin E and the influence of Activin E on adiposity and metabolic disease are not well understood. Here, we examine the relationship between hepatic Activin E and adipose metabolism, testing the hypothesis that Activin E functions as part of a liver-adipose, inter-organ feedback loop to suppress adipose tissue lipolysis in response to elevated serum fatty acids and hepatic fatty acid exposure.

METHODS

The relationship between hepatic Activin E and non-esterified fatty acids (NEFA) released from adipose lipolysis was assessed in vivo using fasted CL 316,243 treated mice and in vitro using Huh7 hepatocytes treated with fatty acids. The influence of Activin E on adipose lipolysis was examined using a combination of Inhbe knockout mice, a mouse model of hepatocyte-specific overexpression of Activin E, and mouse brown adipocytes treated with Activin E enriched media.

RESULTS

Increasing hepatocyte NEFA exposure in vivo by inducing adipose lipolysis through fasting or CL 316,243 treatment increased hepatic Inhbe expression. Similarly, incubation of Huh7 human hepatocytes with fatty acids increased expression of INHBE. Genetic ablation of Inhbe in mice increased fasting circulating NEFA and hepatic triglyceride accumulation. Treatment of mouse brown adipocytes with Activin E conditioned media and overexpression of Activin E in mice suppressed adipose lipolysis and reduced serum FFA levels, respectively. The suppressive effects of Activin E on lipolysis were lost in CRISPR-mediated ALK7 deficient cells and ALK7 kinase deficient mice. Disruption of the Activin E-ALK7 signaling axis in Inhbe KO mice reduced adiposity upon HFD feeding, but caused hepatic steatosis and insulin resistance.

CONCLUSIONS

Taken together, our data suggest that Activin E functions as part of a liver-adipose feedback loop, such that in response to increased serum free fatty acids and elevated hepatic triglyceride, Activin E is released from hepatocytes and signals in adipose through ALK7 to suppress lipolysis, thereby reducing free fatty acid efflux to the liver and preventing excessive hepatic lipid accumulation. We find that disrupting this Activin E-ALK7 inter-organ communication network by ablation of Inhbe in mice increases lipolysis and reduces adiposity, but results in elevated hepatic triglyceride and impaired insulin sensitivity. These results highlight the liver-adipose, Activin E-ALK7 signaling axis as a critical regulator of metabolic homeostasis.

Intestinal Pgc1α ablation protects from liver steatosis and fibrosis

Background & Aims

The gut-liver axis modulates the progression of metabolic dysfunction-associated steatotic liver disease (MASLD), a spectrum of conditions characterised by hepatic steatosis and a progressive increase of inflammation and fibrosis, culminating in metabolic dysfunction-associated steatohepatitis. Peroxisome proliferator-activated receptor-gamma coactivator 1α (Pgc1α) is a transcriptional co-regulator of mitochondrial activity and lipid metabolism. Here, the intestinal-specific role of Pgc1α was analysed in liver steatosis and fibrosis.

Methods

We used a mouse model in which Pgc1α was selectively deleted from the intestinal epithelium. We fed these mice and their wild-type littermates a Western diet to recapitulate the major features of liver steatosis (after 2 months of diet) and metabolic dysfunction-associated steatohepatitis (after 4 months of diet). The chow diet was administered as a control diet.

Results

In humans and mice, low expression of intestinal Pgc1α is inversely associated with liver steatosis, inflammation, and fibrosis. Intestinal disruption of Pgc1α impairs the transcription of a wide number of genes, including the cholesterol transporter Niemann-Pick C1-like 1 (Npc1l1), thus limiting the uptake of cholesterol from the gut. This results in a lower cholesterol accretion in the liver and a decreased production of new fatty acids, which protect the liver from lipotoxic lipid species accumulation, inflammation, and related fibrotic processes.

Conclusions

In humans and mice, intestinal Pgc1α induction during Western diet may be another culprit driving hepatic steatosis and fibrosis. Here, we show that enterocyte-specific Pgc1α ablation protects the liver from steatosis and fibrosis by reducing intestinal cholesterol absorption, with subsequent decrease of cholesterol and de novo fatty acid accumulation in the liver.

Impact and implications

Liver diseases result from several insults, including signals from the gut. Although the incidence of liver diseases is continuously increasing worldwide, effective drug therapy is still lacking. Here, we showed that the modulation of an intestinal coactivator regulates the liver response to a Western diet, by limiting the uptake of dietary cholesterol. This results in a lower accumulation of hepatic lipids together with decreased inflammation and fibrosis, thus limiting the progression of liver steatosis and fibrosis towards severe end-stage diseases.

Hepatocyte TIPE2 is a fasting-induced Raf-1 inactivator that drives hepatic gluconeogenesis to maintain glucose homeostasis

BACKGROUND

The liver regulates metabolic balance during fasting-feeding cycle. Hepatic adaptation to fasting is precisely modulated on multiple levels. Tumor necrosis factor-α-induced protein 8-like 2 (TIPE2) is a negative regulator of immunity that reduces several liver pathologies, but its physiological roles in hepatic metabolism are largely unknown.

METHODS

TIPE2 expression was examined in mouse liver during fasting-feeding cycle. TIPE2-knockout mice, liver-specific TIPE2-knockout mice, liver-specific TIPE2-overexpressed mice were examined for fasting blood glucose and pyruvate tolerance test. Primary hepatocytes or liver tissues from these mice were evaluated for glucose production, lipid accumulation, gene expression and regulatory pathways. TIPE2 interaction with Raf-1 and TIPE2 transcription regulated by PPAR-α were examined using gene overexpression or knockdown, co-immunoprecipitation, western blot, luciferase reporter assay and DNA-protein binding assay.

RESULTS

TIPE2 expression was upregulated in fasted mouse liver and starved hepatocytes, which was positively correlated with gluconeogenic genes. Liver-specific TIPE2 deficiency impaired blood glucose homeostasis and gluconeogenic capacity in mice upon fasting, while liver-specific TIPE2 overexpression elevated fasting blood glucose and hepatic gluconeogenesis in mice. In primary hepatocytes upon starvation, TIPE2 interacted with Raf-1 to accelerate its ubiquitination and degradation, resulting in ERK deactivation and FOXO1 maintenance to sustain gluconeogenesis. During prolonged fasting, hepatic TIPE2 deficiency caused aberrant activation of ERK-mTORC1 axis that increased hepatic lipid accumulation via lipogenesis. In hepatocytes upon starvation, PPAR-α bound with TIPE2 promoter and triggered its transcriptional expression.

CONCLUSIONS

Hepatocyte TIPE2 is a PPAR-α-induced Raf-1 inactivator that sustains hepatic gluconeogenesis and prevents excessive hepatic lipid accumulation, playing beneficial roles in hepatocyte adaptation to fasting.

Association between Impaired Ketogenesis and Metabolic-Associated Fatty Liver Disease

Metabolic (dysfunction) associated fatty liver disease (MAFLD) is generally developed with excessive accumulation of lipids in the liver. Ketogenesis is an efficient pathway for the disposal of fatty acids in the liver and its metabolic benefits have been reported. In this review, we examined previous studies on the association between ketogenesis and MAFLD and reviewed the candidate mechanisms that can explain this association.

Adipose Tissue Protects against Hepatic Steatosis in Male Rats Fed a High-Fat Diet plus Liquid Fructose: Sex-Related Differences

Non-alcoholic fatty liver disease is a sexual dimorphic disease, with adipose tissue playing an essential role. Our previous work showed that female rats fed a high-fat high-fructose diet devoid of cholesterol (HFHFr) developed simple hepatic steatosis dissociated from obesity. This study assessed the impact of the HFHFr diet on the male rat metabolism compared with data obtained for female rats. A total of 16 Sprague Dawley (SD) male rats were fed either a control (standard rodent chow and water) or HFHFr (high-fat diet devoid of cholesterol, plus 10% fructose in drinking water) diet for 3 months. Unlike female rats, and despite similar increases in energy consumption, HFHFr males showed increased adiposity and hyperleptinemia. The expression of hormone-sensitive lipase in the subcutaneous white adipose tissue was enhanced, leading to high free fatty acid and glycerol serum levels. HFHFr males presented hypertriglyceridemia, but not hepatic steatosis, partially due to enhanced liver PPARα-related fatty acid β-oxidation and the VLDL-promoting effect of leptin. In conclusion, the SD rats showed a sex-related dimorphic response to the HFHFr diet. Contrary to previous results for HFHFr female rats, the male rats were able to expand the adipose tissue, increase fatty acid catabolism, or export it as VLDL, avoiding liver lipid deposition.

Hepatic ketogenesis regulates lipid homeostasis via ACSL1-mediated fatty acid partitioning

Liver-derived ketone bodies play a crucial role in fasting energy homeostasis by fueling the brain and peripheral tissues. Ketogenesis also acts as a conduit to remove excess acetyl-CoA generated from fatty acid oxidation and protects against diet-induced hepatic steatosis. Surprisingly, no study has examined the role of ketogenesis in fasting-associated hepatocellular lipid metabolism. Ketogenesis is driven by the rate-limiting mitochondrial enzyme 3-hydroxymethylglutaryl CoA synthase (HMGCS2) abundantly expressed in the liver. Here, we show that ketogenic insufficiency via disruption of hepatic HMGCS2 exacerbates liver steatosis in fasted chow and high-fat-fed mice. We found that the hepatic steatosis is driven by increased fatty acid partitioning to the endoplasmic reticulum (ER) for re-esterification via acyl-CoA synthetase long-chain family member 1 (ACSL1). Mechanistically, acetyl-CoA accumulation from impaired hepatic ketogenesis is responsible for the elevated translocation of ACSL1 to the ER. Moreover, we show increased ER-localized ACSL1 and re-esterification of lipids in human NASH displaying impaired hepatic ketogenesis. Finally, we show that L-carnitine, which buffers excess acetyl-CoA, decreases the ER-associated ACSL1 and alleviates hepatic steatosis. Thus, ketogenesis via controlling hepatocellular acetyl-CoA homeostasis regulates lipid partitioning and protects against hepatic steatosis.

Exercise Equals the Mobilization of Visceral versus Subcutaneous Adipose Fatty Acid Molecules in Fasted Rats Associated with the Modulation of the AMPK/ATGL/HSL Axis

Combining exercise with fasting is known to boost fat mass-loss, but detailed analysis on the consequential mobilization of visceral and subcutaneous WAT-derived fatty acids has not been performed. In this study, a subset of fasted male rats (66 h) was submitted to daily bouts of mild exercise. Subsequently, by using gas chromatography-flame ionization detection, the content of 22 fatty acids (FA) in visceral (v) versus subcutaneous (sc) white adipose tissue (WAT) depots was compared to those found in response to the separate events. Findings were related to those obtained in serum and liver samples, the latter taking up FA to increase gluconeogenesis and ketogenesis. Each separate intervention reduced scWAT FA content, associated with increased levels of adipose triglyceride lipase (ATGL) protein despite unaltered AMP-activated protein kinase (AMPK) Thr172 phosphorylation, known to induce ATGL expression. The mobility of FAs from vWAT during fasting was absent with the exception of the MUFA 16:1 n-7 and only induced by combining fasting with exercise which was accompanied with reduced hormone sensitive lipase (HSL) Ser563 and increased Ser565 phosphorylation, whereas ATGL protein levels were elevated during fasting in association with the persistently increased phosphorylation of AMPK at Thr172 both during fasting and in response to the combined intervention. As expected, liver FA content increased during fasting, and was not further affected by exercise, despite additional FA release from vWAT in this condition, underlining increased hepatic FA metabolism. Both fasting and its combination with exercise showed preferential hepatic metabolism of the prominent saturated FAs C:16 and C:18 compared to the unsaturated FAs 18:1 n-9 and 18:2 n-6:1. In conclusion, depot-specific differences in WAT fatty acid molecule release during fasting, irrelevant to their degree of saturation or chain length, are mitigated when combined with exercise, to provide fuel to surrounding organs such as the liver which is correlated with increased ATGL/ HSL ratios, involving AMPK only in vWAT.

Inhibition of VEGF-B signaling prevents non-alcoholic fatty liver disease development by targeting lipolysis in the white adipose tissue

BACKGROUND & AIMS

Hepatic steatosis is a hallmark of non-alcoholic fatty liver disease (NAFLD), a common comorbidity in type 2 diabetes mellitus (T2DM). The pathogenesis of NAFLD is complex and involves the crosstalk between the liver and the white adipose tissue (WAT). Vascular endothelial growth factor B (VEGF-B) has been shown to control tissue lipid accumulation by regulating the transport properties of the vasculature. The role of VEGF-B signaling and the contribution to hepatic steatosis and NAFLD in T2DM is currently not understood.

METHODS

C57BL/6 J mice treated with a neutralizing antibody against VEGF-B, or mice with adipocyte-specific overexpression or under-expression of VEGF-B (AdipoqCre+/VEGF-BTG/+ mice and AdipoqCre+/Vegfbfl/+mice) were subjected to a 6-month high-fat diet (HFD), or chow-diet, whereafter NAFLD development was assessed. VEGF-B expression was analysed in WAT biopsies from patients with obesity and NAFLD in a pre-existing clinical cohort (n = 24 patients with NAFLD and n = 24 without NAFLD) and correlated to clinicopathological features.

RESULTS

Pharmacological inhibition of VEGF-B signaling in diabetic mice reduced hepatic steatosis and NAFLD by blocking WAT lipolysis. Mechanistically we show, by using HFD-fed AdipoqCre+/VEGF-BTG/+ mice and HFD-fed AdipoqCre+/Vegfbfl/+mice, that inhibition of VEGF-B signaling targets lipolysis in adipocytes. Reducing VEGF-B signaling ameliorated NAFLD by decreasing WAT inflammation, resolving WAT insulin resistance, and lowering the activity of the hormone sensitive lipase. Analyses of human WAT biopsies from individuals with NAFLD provided evidence supporting the contribution of VEGF-B signaling to NAFLD development. VEGF-B expression levels in adipocytes from two WAT depots correlated with development of dysfunctional WAT and NAFLD in humans.

CONCLUSIONS

Taken together, our data from mouse models and humans suggest that VEGF-B antagonism may represent an approach to combat NAFLD by targeting hepatic steatosis through suppression of lipolysis.

IMPACT AND IMPLICATIONS

Non-alcoholic fatty liver disease (NAFLD) is a common comorbidity in type 2 diabetes mellitus (T2DM) and has a global prevalence of between 25-29%. There are currently no approved drugs for NAFLD, and given the scale of the ongoing diabetes epidemics, there is an urgent need to identify new treatment options. Our work suggests that VEGF-B antagonism may represent an approach to combat NAFLD by targeting hepatic steatosis through suppression of lipolysis. The neutralizing anti-VEGF-B antibody, which was used in this study, has already entered clinical trials for patients with diabetes. Therefore, we believe that our results are of great general interest to a broad audience, including patients and patient organizations, the medical community, academia, the life science industry and the public.

Steatosis in metabolic diseases: A focus on lipolysis and lipophagy

Fatty acids (FAs), as part of lipids, are involved in cell membrane composition, cellular energy storage, and cell signaling. FAs can also be toxic when their concentrations inside and/or outside the cell exceed physiological levels, which is called "lipotoxicity", and steatosis is a form of lipotoxity. To facilitate the storage of large quantities of FAs in cells, they undergo a process called lipolysis or lipophagy. This review focuses on the effects of lipolytic enzymes including cytoplasmic "neutral" lipolysis, lysosomal "acid" lipolysis, and lipophagy. Moreover, the impact of related lipolytic enzymes on lipid metabolism homeostasis and energy conservation, as well as their role in lipid-related metabolic diseases. In addition, we describe how they affect lipid metabolism homeostasis and energy conservation in lipid-related metabolic diseases with a focus on hepatic steatosis and cancer and the pathogenesis and therapeutic targets of AMPK/SIRTs/FOXOs, PI3K/Akt, PPARs/PGC-1α, MAPK/ERK1/2, TLR4/NF-κB, AMPK/mTOR/TFEB, Wnt/β-catenin through immune inflammation, oxidative stress and autophagy-related pathways. As well as the current application of lipolytic enzyme inhibitors (especially Monoacylglycerol lipase (MGL) inhibitors) to provide new strategies for future exploration of metabolic programming in metabolic diseases.

Adipocyte-hepatocyte crosstalk in cellular models of obesity: Role of soluble factors

Hepatic steatosis is often a consequence of obesity. Adipose tissue is an important endocrine regulator of metabolic homeostasis in the body. In obesity, adipocytes become hypertrophic and develop an inflammatory phenotype, altering the panel of secreted adipokines. Moreover, excess fatty acids are, in part, released by adipocytes and delivered to the liver. These multiple pathways of adipose-liver crosstalk contribute to the development and progression of liver disease: TNFα induces hepatocyte dysfunction, excess of circulating fatty acids promotes hepatic steatosis and inflammation, whilst adipokines mediate and exacerbate liver injury. In this study, we investigated in vitro the effects and mechanisms of the crosstalk between adipocytes and hepatocytes, as a function of the different adipocyte status (mature vs hypertrophic) being mediated by soluble factors. We employed the conditioned medium method to test how mature and hypertrophic adipocytes distinctively affect the liver, leading to metabolic dysfunction. The media collected from adipocytes were characterized by high triglyceride content and led to lipid accumulation and fat-dependent dysfunction in hepatocytes. The present findings seem to suggest that, in addition to triglycerides, other soluble mediators, cytokines, are released by mature and hypertrophic adipocytes and influence the metabolic status of liver cells. Understanding the precise factors involved in the pathogenesis and pathophysiology of NAFLD in obesity will provide important insights into the mechanisms responsible for the metabolic complications of obesity, paving the way for new possible approaches.

Targeting activin receptor-like kinase 7 ameliorates adiposity and associated metabolic disorders

Activin receptor-like kinase 7 (ALK7) is a type I receptor in the TGF-β superfamily preferentially expressed in adipose tissue and associated with lipid metabolism. Inactivation of ALK7 signaling in mice results in increased lipolysis and resistance to both genetic and diet-induced obesity. Human genetic studies have recently revealed an association between ALK7 variants and both reduced waist to hip ratios and resistance to development of diabetes. In the present study, treatment with a neutralizing mAb against ALK7 caused a substantial loss of adipose mass and improved glucose intolerance and insulin resistance in both genetic and diet-induced mouse obesity models. The enhanced lipolysis increased fatty acid supply from adipocytes to promote fatty acid oxidation in muscle and oxygen consumption at the whole-body level. The treatment temporarily increased hepatic triglyceride levels, which resolved with long-term Ab treatment. Blocking of ALK7 signals also decreased production of its ligand, growth differentiation factor 3, by downregulating S100A8/A9 release from adipocytes and, subsequently, IL-1β release from adipose tissue macrophages. These findings support the feasibility of potential therapeutics targeting ALK7 as a treatment for obesity and diabetes.

Effects of Dietary Steroid Saponins on Growth Performance, Serum and Liver Glucose, Lipid Metabolism and Immune Molecules of Hybrid Groupers (♀Epinephelus fuscoguttatus × ♂Epinephelus lanceolatu) Fed High-Lipid Diets

High-lipid diets are attributed to excessive lipid deposition and metabolic disturbances in fish. The aim of this experiment was to investigate the effects of steroidal saponins on growth performance, immune molecules and metabolism of glucose and lipids in hybrid groupers (initial weight 22.71 ± 0.12 g) fed high-lipid diets. steroidal saponins (0%, 0.1% and 0.2%) were added to the basal diet (crude lipid, 14%) to produce three experimental diets, designated S₀, S_0.1 and S_0.2, respectively. After an 8-week feeding trial, no significant differences were found between the S₀ and S_0.1 groups in percent weight gain, specific growth rate, feed conversion ratio, protein efficiency ratio and protein deposition rate (p > 0.05). All those in the S_0.2 group were significantly decreased (p < 0.05). Compared to the S₀ group, fish in the S_0.1 group had lower contents of serum triglyceride and low-density lipoprotein cholesterol and higher high-density lipoprotein cholesterol and glucose (p < 0.05). The activities of superoxide dismutase, catalase and glutathione peroxidase were significantly higher, and malondialdehyde contents were significantly lower in the S_0.1 group than in the S₀ group (p < 0.05). Hepatic triglyceride, total cholesterol and glycogen were significantly lower in the S_0.1 group than in the S₀ group (p < 0.05). Activities of lipoprotein lipase, total lipase, glucokinase and pyruvate kinase, and gene expression of lipoprotein lipase, triglyceride lipase and glucokinase, were significantly higher in the S_0.1 group than in the S₀ group. Interleukin-10 mRNA expression in the S_0.1 group was significantly higher than that in the S₀ group, while the expression of interleukin-6 and tumor necrosis factor-α genes were significantly lower than those in the S₀ group. In summary, adding 0.1% steroidal saponins to a high-lipid diet not only promoted lipolysis in fish livers, but also activated glycolysis pathways, thus enhancing the utilization of the dietary energy of the groupers, as well as supporting the fish's nonspecial immune-defense mechanism.

Peroxisome Proliferator-Activated Receptor-Targeted Therapies: Challenges upon Infectious Diseases

Peroxisome proliferator-activated receptors (PPARs) α, β, and γ are nuclear receptors that orchestrate the transcriptional regulation of genes involved in a variety of biological responses, such as energy metabolism and homeostasis, regulation of inflammation, cellular development, and differentiation. The many roles played by the PPAR signaling pathways indicate that PPARs may be useful targets for various human diseases, including metabolic and inflammatory conditions and tumors. Accumulating evidence suggests that each PPAR plays prominent but different roles in viral, bacterial, and parasitic infectious disease development. In this review, we discuss recent PPAR research works that are focused on how PPARs control various infections and immune responses. In addition, we describe the current and potential therapeutic uses of PPAR agonists/antagonists in the context of infectious diseases. A more comprehensive understanding of the roles played by PPARs in terms of host-pathogen interactions will yield potential adjunctive personalized therapies employing PPAR-modulating agents.

Altered Adipocyte Cell Size Distribution Prior to Weight Loss in the R6/2 Model of Huntington's Disease

BACKGROUND

Metabolic alterations contribute to disease onset and prognosis of Huntington's disease (HD). Weight loss in the R6/2 mouse model of HD is a consistent feature, with onset in mid-to-late stage of disease.

OBJECTIVE

In the present study, we aimed to investigate molecular and functional changes in white adipose tissue (WAT) that occur at weight loss in R6/2 mice. We further elaborated on the effect of leptin-deficiency and early obesity in R6/2 mice.

METHODS

We performed analyses at 12 weeks of age; a time point that coincides with the start of weight loss in our R6/2 mouse colony. Gonadal (visceral) and inguinal (subcutaneous) WAT depot weights were monitored, as well as adipocyte size distribution. Response to isoprenaline-stimulated glycerol release and insulin-stimulated glucose uptake in adipocytes from gonadal WAT was assessed.

RESULTS

In R6/2 mice, WAT depot weights were comparable to wildtype (WT) mice, and the response to insulin and isoprenaline in gonadal adipocytes was unaltered. Leptin-deficient R6/2 mice exhibited distinct changes compared to leptin-deficient WT mice. At 12 weeks, female leptin-deficient R6/2 mice had reduced body weight accompanied by an increased proportion of smaller adipocytes, while in contrast; male mice displayed a shift towards larger adipocyte sizes without a significant body weight reduction at this timepoint.

CONCLUSIONS

We here show that there are early sex-specific changes in adipocyte cell size distribution in WAT of R6/2 mice and leptin-deficient R6/2 mice.

Combinatorial pathway disruption is a powerful approach to delineate metabolic impacts of endocrine disruptors

The prevalence of metabolic diseases, such as obesity, diabetes, metabolic syndrome and chronic liver diseases among others, has been rising for several years. Epidemiology and mechanistic (in vivo, in vitro and in silico) toxicology have recently provided compelling evidence implicating the chemical environment in the pathogenesis of these diseases. In this review, we will describe the biological processes that contribute to the development of metabolic diseases targeted by metabolic disruptors, and will propose an integrated pathophysiological vision of their effects on several organs. With regard to these pathomechanisms, we will discuss the needs, and the stakes of evolving the testing and assessment of endocrine disruptors to improve the prevention and management of metabolic diseases that have become a global epidemic since the end of last century.

Acute retinol mobilization by retinol-binding protein 4 in mouse liver induces fibroblast growth factor 21 expression

Hepatocytes secrete retinol-binding protein 4 (RBP4) into circulation, thereby mobilizing vitamin A from the liver to provide retinol for extrahepatic tissues. Obesity and insulin resistance are associated with elevated RBP4 levels in the blood. However, in a previous study, we observed that chronically increased RBP4 by forced Rbp4 expression in the liver does not impair glucose homeostasis in mice. Here, we investigated the effects of an acute mobilization of hepatic vitamin A stores by hepatic overexpression of RBP4 in mice. We show that hepatic retinol mobilization decreases body fat content and enhances fat turnover. Mechanistically, we found that acute retinol mobilization increases hepatic expression and serum levels of fibroblast growth factor 21 (FGF21), which is regulated by retinol mobilization and retinoic acid in primary hepatocytes. Moreover, we provide evidence that the insulin-sensitizing effect of FGF21 is associated with organ-specific adaptations in retinoid homeostasis. Taken together, our findings identify a novel crosstalk between retinoid homeostasis and FGF21 in mice with acute RBP4-mediated retinol mobilization from the liver.

CircRNA-mediated regulation of brown adipose tissue adipogenesis

Adipose tissue represents a candidate target for the treatment of metabolic illnesses, such as obesity. Brown adipose tissue (BAT), an important heat source within the body, promotes metabolic health through fat consumption. Therefore, the induction of white fat browning may improve lipid metabolism. Currently, the specific roles of circRNA in BAT and white adipose tissue (WAT) remain elusive. Herein, we conducted circRNA expression profiling of mouse BAT and WAT using RNA-seq. We identified a total of 12,183 circRNAs, including 165 upregulated and 79 downregulated circRNAs between BAT and WAT. Differentially expressed (DE) circRNAs were associated with the mitochondrion, mitochondrial part, mitochondrial inner membrane, mitochondrial envelope, therefore, these circRNAs may affect the thermogenesis and lipid metabolism of BAT. Moreover, DE circRNAs were enriched in browning- and thermogenesis-related pathways, including AMPK and HIF-1 signaling. In addition, a novel circRNA, circOgdh, was found to be highly expressed in BAT, formed by back-splicing of the third and fourth exons of the Ogdh gene, and exhibited higher stability than linear Ogdh. circOgdh was mainly distributed in the cytoplasm and could sponge miR-34a-5p, upregulating the expression of Atgl, a key lipolysis gene, which enhanced brown adipocyte lipolysis and suppressed lipid droplet accumulation. Our findings offer in-depth knowledge of the modulatory functions of circRNAs in BAT adipogenesis.

Emerging Role of Hepatic Ketogenesis in Fatty Liver Disease

Non-alcoholic fatty liver disease (NAFLD), the most common chronic liver diseases, arise from non-alcoholic fatty liver (NAFL) characterized by excessive fat accumulation as triglycerides. Although NAFL is benign, it could progress to non-alcoholic steatohepatitis (NASH) manifested with inflammation, hepatocyte damage and fibrosis. A subset of NASH patients develops end-stage liver diseases such as cirrhosis and hepatocellular carcinoma. The pathogenesis of NAFLD is highly complex and strongly associated with perturbations in lipid and glucose metabolism. Lipid disposal pathways, in particular, impairment in condensation of acetyl-CoA derived from β-oxidation into ketogenic pathway strongly influence the hepatic lipid loads and glucose metabolism. Current evidence suggests that ketogenesis dispose up to two-thirds of the lipids entering the liver, and its dysregulation significantly contribute to the NAFLD pathogenesis. Moreover, ketone body administration in mice and humans shows a significant improvement in NAFLD. This review focuses on hepatic ketogenesis and its role in NAFLD pathogenesis. We review the possible mechanisms through which impaired hepatic ketogenesis may promote NAFLD progression. Finally, the review sheds light on the therapeutic implications of a ketogenic diet in NAFLD.