ApoA5 knockdown improves whole-body insulin sensitivity in high-fat-fed mice by reducing ectopic lipid content

Liver lipid droplet cholesterol content is a key determinant of metabolic dysfunction-associated steatohepatitis

Metabolic dysfunction-associated steatohepatitis (MASH) represents a progressive form of steatotic liver disease which increases the risk for fibrosis and advanced liver disease. The accumulation of discrete species of bioactive lipids has been postulated to activate signaling pathways that promote inflammation and fibrosis. However, the key pathogenic lipid species is a matter of debate. We explored candidates using various dietary, molecular, and genetic models. Mice fed a choline-deficient L-amino acid-defined high-fat diet (CDAHFD) developed steatohepatitis and manifested early markers of liver fibrosis associated with increased cholesterol content in liver lipid droplets within 5 days without any changes in total liver cholesterol content. Treating mice with antisense oligonucleotides (ASOs) against Coenzyme A synthase (Cosay) or treatment with bempedoic acid or atorvastatin decreased liver lipid droplet cholesterol content and prevented CDAHFD-induced MASH and the fibrotic response. All these salutary effects were abrogated with dietary cholesterol supplementation. Analysis of human liver samples demonstrated that cholesterol in liver lipid droplets was increased in humans with MASH and liver fibrosis and was higher in PNPLA3 I148M (variants rs738409) than in HSD17B13 variants (rs72613567). Together, these data identify cholesterol in liver lipid droplets as a critical mediator of MASH and demonstrate that COASY knockdown and bempedoic acid are novel therapeutic approaches to reduce liver lipid droplet cholesterol content and thereby prevent the development of MASH and liver fibrosis.

Significance Statement

Metabolic dysfunction-associated steatohepatitis (MASH) is a progressive liver disease linked to fibrosis. The role of specific lipid species in its pathogenesis remains debated. Using dietary, molecular, and genetic models, we found that mice on a choline-deficient, high-fat diet (CDAHFD) developed steatohepatitis and early fibrosis, marked by increased cholesterol in liver lipid droplets within five days. Targeting COASY with antisense oligonucleotides or treating with bempedoic acid or atorvastatin reduced lipid droplet cholesterol and prevented MASH. However, dietary cholesterol supplementation negated these effects. Human liver samples confirmed elevated lipid droplet cholesterol in MASH and fibrosis, especially in PNPLA3 I148M carriers. These findings highlight cholesterol reduction as a potential MASH therapy.

APOA1/C3/A4/A5 Gene Cluster at 11q23.3 and Lipid Metabolism Disorders: From Epigenetic Mechanisms to Clinical Practices

The APOA1/C3/A4/A5 cluster is an essential component in regulating lipoprotein metabolism and maintaining plasma lipid homeostasis. A genome-wide association analysis and Mendelian randomization have revealed potential associations between genetic variants within this cluster and lipid metabolism disorders, including hyperlipidemia and cardiovascular events. An enhanced understanding of the complexity of gene regulation has led to growing recognition regarding the role of epigenetic variation in modulating APOA1/C3/A4/A5 gene expression. Intensive research into the epigenetic regulatory patterns of the APOA1/C3/A4/A5 cluster will help increase our understanding of the pathogenesis of lipid metabolism disorders and facilitate the development of new therapeutic approaches. This review discusses the biology of how the APOA1/C3/A4/A5 cluster affects circulating lipoproteins and the current progress in the epigenetic regulation of the APOA1/C3/A4/A5 cluster.

A short-term rodent model for non-alcoholic steatohepatitis induced by a high-fat diet and carbon tetrachloride

Several models of mice-fed high-fat diets have been used to trigger non-alcoholic steatohepatitis and some chemical substances, such as carbon tetrachloride. The present study aimed to evaluate the joint action of a high-fat diet and CCl4 in developing a short-term non-alcoholic steatohepatitis model. C57BL6/J mice were divided into two groups: standard diet-fed (SD), the high-fat diet-fed (HFD) and HFD + fructose-fed and carbon tetrachloride (HFD+CCl4). The animals fed with HFD+CCl4 presented increased lipid deposition compared with both SD and HFD mice. Plasma cholesterol was increased in animals from the HFD+CCl4 group compared with the SD and HFD groups, without significant differences between the SD and HFD groups. Plasma triglycerides showed no significant difference between the groups. The HFD+CCl4 animals had increased collagen deposition in the liver compared with both SD and HFD groups. Hydroxyproline was also increased in the HFD+CCl4 group. Liver enzymes, alanine aminotransferase and aspartate aminotransferase, were increased in the HFD+CCl4 group, compared with SD and HFD groups. Also, CCl4 was able to trigger an inflammatory process in the liver of HFD-fed animals by promoting an increase of ∼2 times in macrophage activity, ∼6 times in F4/80 gene expression, and pro-inflammatory cytokines (IL-1b and TNFa), in addition to an increase in inflammatory pathway protein phosphorylation (IKKbp). HFD e HFD+CCl4 animals increased glucose intolerance compared with SD mice, associated with reduced insulin-stimulated AKT activity in the liver. Therefore, our study has shown that short-term HFD feeding associated with fructose and CCl4 can trigger non-alcoholic steatohepatitis and cause damage to glucose metabolism.

Data-dependent and -independent acquisition lipidomics analysis reveals the tissue-dependent effect of metformin on lipid metabolism

INTRODUCTION

Despite the well-recognized health benefits, the mechanisms and site of action of metformin remains elusive. Metformin-induced global lipidomic changes in plasma of animal models and human subjects have been reported. However, there is a lack of systemic evaluation of metformin-induced lipidomic changes in different tissues. Metformin uptake requires active transporters such as organic cation transporters (OCTs), and hence, it is anticipated that metformin actions are tissue-dependent. In this study, we aim to characterize metformin effects in non-diabetic male mice with a special focus on lipidomics analysis. The findings from this study will help us to better understand the cell-autonomous (direct actions in target cells) or non-cell-autonomous (indirect actions in target cells) mechanisms of metformin and provide insights into the development of more potent yet safe drugs targeting a particular organ instead of systemic metabolism for metabolic regulations without major side effects.

OBJECTIVES

To characterize metformin-induced lipidomic alterations in different tissues of non-diabetic male mice and further identify lipids affected by metformin through cell-autonomous or systemic mechanisms based on the correlation between lipid alterations in tissues and the corresponding in-tissue metformin concentrations.

METHODS

A dual extraction method involving 80% methanol followed by MTBE (methyl tert-butyl ether) extraction enables the analysis of free fatty acids, polar metabolites, and lipids. Extracts from tissues and plasma of male mice treated with or without metformin in drinking water for 12 days were analyzed using HILIC chromatography coupled to Q Exactive Plus mass spectrometer or reversed-phase liquid chromatography coupled to MS/MS scan workflow (hybrid mode) on LC-Orbitrap Exploris 480 mass spectrometer using biologically relevant lipids-containing inclusion list for data-independent acquisition (DIA), named as BRI-DIA workflow followed by data-dependent acquisition (DDA), to maximum the coverage of lipids and minimize the negative effect of stochasticity of precursor selection on experimental consistency and reproducibility.

RESULTS

Lipidomics analysis of 6 mouse tissues and plasma allowed a systemic evaluation of lipidomic changes induced by metformin in different tissues. We observed that (1) the degrees of lipidomic changes induced by metformin treatment overly correlated with tissue concentrations of metformin; (2) the impact on lysophosphatidylcholine (lysoPC) and cardiolipins was positively correlated with tissue concentrations of metformin, while neutral lipids such as triglycerides did not correlate with the corresponding tissue metformin concentrations; (3) increase of intestinal tricarboxylic acid (TCA) cycle intermediates after metformin treatment.

CONCLUSION

The data collected in this study from non-diabetic mice with 12-day metformin treatment suggest that the overall metabolic effect of metformin is positively correlated with tissue concentrations and the effect on individual lipid subclass is via both cell-autonomous mechanisms (cardiolipins and lysoPC) and non-cell-autonomous mechanisms (triglycerides).

Strength Training Protects High-Fat-Fed Ovariectomized Mice against Insulin Resistance and Hepatic Steatosis

Menopause is characterized by a reduction in sex hormones in women and is associated with metabolic changes, including fatty liver and insulin resistance. Lifestyle changes, including a balanced diet and physical exercise, are necessary to prevent these undesirable changes. Strength training (ST) has been widely used because of the muscle and metabolic benefits it provides. Our study aims to evaluate the effects of ST on hepatic steatosis and insulin resistance in ovariectomized mice fed a high-fat diet (HFD) divided into four groups as follows: simulated sedentary surgery (SHAM-SED), trained simulated surgery (SHAM-EXE), sedentary ovariectomy (OVX-SED), and trained ovariectomy (OVX-EXE). They were fed an HFD for 9 weeks. ST was performed thrice a week. ST efficiently reduced body weight and fat percentage and increased lean mass in OVX mice. Furthermore, ST reduced the accumulation of ectopic hepatic lipids, increased AMPK phosphorylation, and inhibited the de novo lipogenesis pathway. OVX-EXE mice also showed a better glycemic profile, associated with greater insulin sensitivity identified by the euglycemic-hyperinsulinemic clamp, and reduced markers of hepatic oxidative stress compared with sedentary animals. Our data support the idea that ST can be indicated as a non-pharmacological treatment approach to mitigate metabolic changes resulting from menopause.

Depletion of ApoA5 aggravates spontaneous and diet-induced nonalcoholic fatty liver disease by reducing hepatic NR1D1 in hamsters

Background: ApoA5 mainly synthesized and secreted by liver is a key modulator of lipoprotein lipase (LPL) activity and triglyceride-rich lipoproteins (TRLs). Although the role of ApoA5 in extrahepatic triglyceride (TG) metabolism in circulation has been well documented, the relationship between ApoA5 and nonalcoholic fatty liver disease (NAFLD) remains incompletely understood and the underlying molecular mechanism still needs to be elucidated. Methods: We used CRISPR/Cas9 gene editing to delete Apoa5 gene from Syrian golden hamster, a small rodent model replicating human metabolic features. Then, the ApoA5-deficient (ApoA5-/-) hamsters were used to investigate NAFLD with or without challenging a high fat diet (HFD). Results: ApoA5-/- hamsters exhibited hypertriglyceridemia (HTG) with markedly elevated TG levels at 2300 mg/dL and hepatic steatosis on a regular chow diet, accompanied with an increase in the expression levels of genes regulating lipolysis and small adipocytes in the adipose tissue. An HFD challenge predisposed ApoA5-/- hamsters to severe HTG (sHTG) and nonalcoholic steatohepatitis (NASH). Mechanistic studies in vitro and in vivo revealed that targeting ApoA5 disrupted NR1D1 mRNA stability in the HepG2 cells and the liver to reduce both mRNA and protein levels of NR1D1, respectively. Overexpression of human NR1D1 by adeno-associated virus 8 (AAV8) in the livers of ApoA5-/- hamsters significantly ameliorated fatty liver without affecting plasma lipid levels. Moreover, restoration of hepatic ApoA5 or activation of UCP1 in brown adipose tissue (BAT) by cold exposure or CL316243 administration could significantly correct sHTG and hepatic steatosis in ApoA5-/- hamsters. Conclusions: Our data demonstrate that HTG caused by ApoA5 deficiency in hamsters is sufficient to elicit hepatic steatosis and HFD aggravates NAFLD by reducing hepatic NR1D1 mRNA and protein levels, which provides a mechanistic link between ApoA5 and NAFLD and suggests the new insights into the potential therapeutic approaches for the treatment of HTG and the related disorders due to ApoA5 deficiency in the clinical trials in future.

Ethnic Disparities in Lipid Metabolism and Clinical Outcomes between Dutch South Asians and Dutch White Caucasians with Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) poses a higher risk for complications in South Asian individuals compared to other ethnic groups. To shed light on potential mediating factors, we investigated lipidomic changes in plasma of Dutch South Asians (DSA) and Dutch white Caucasians (DwC) with and without T2DM and explore their associations with clinical features. Using a targeted quantitative lipidomics platform, monitoring over 1000 lipids across 17 classes, along with 1H NMR based lipoprotein analysis, we studied 51 healthy participants (21 DSA, 30 DwC) and 92 T2DM patients (47 DSA, 45 DwC) from the MAGNetic resonance Assessment of VICTOza efficacy in the Regression of cardiovascular dysfunction in type 2 dIAbetes mellitus (MAGNA VICTORIA) study. This comprehensive mapping of the circulating lipidome allowed us to identify relevant lipid modules through unbiased weighted correlation network analysis, as well as disease and ethnicity related key mediatory lipids. Significant differences in lipidomic profiles, encompassing various lipid classes and species, were observed between T2DM patients and healthy controls in both the DSA and DwC populations. Our analyses revealed that healthy DSA, but not DwC, controls already exhibited a lipid profile prone to develop T2DM. Particularly, in DSA-T2DM patients, specific lipid changes correlated with clinical features, particularly diacylglycerols (DGs), showing significant associations with glycemic control and renal function. Our findings highlight an ethnic distinction in lipid modules influencing clinical outcomes in renal health. We discover distinctive ethnic disparities of the circulating lipidome and identify ethnicity-specific lipid markers. Jointly, our discoveries show great potential as personalized biomarkers for the assessment of glycemic control and renal function in DSA-T2DM individuals.

Estradiol Protects Female ApoE KO Mice against Western-Diet-Induced Non-Alcoholic Steatohepatitis

The prevalence of non-alcoholic fatty liver disease (NAFLD) and its severe form, non-alcoholic steatohepatitis (NASH), is higher in men than in women of reproductive age, and postmenopausal women are especially susceptible to developing the disease.

AIM

we evaluated if female apolipoprotein E (ApoE) KO mice were protected against Western-diet (WD)-induced NASH.

METHODS

Female ovariectomized (OVX) ApoE KO mice or sham-operated (SHAM) mice were fed either a WD or a regular chow (RC) for 7 weeks. Additionally, OVX mice fed a WD were treated with either estradiol (OVX + E2) or vehicle (OVX).

RESULTS

Whole-body fat, plasma glucose, and plasma insulin were increased and associated with increased glucose intolerance in OVX mice fed a WD (OVX + WD). Plasma and hepatic triglycerides, alanine aminotransferase (ALT), and aspartate aminotransferase (AST) hepatic enzymes were also increased in the plasma of OVX + WD group, which was associated with hepatic fibrosis and inflammation. Estradiol replacement in OVX mice reduced body weight, body fat, glycemia, and plasma insulin associated with reduced glucose intolerance. Treatment also reduced hepatic triglycerides, ALT, AST, hepatic fibrosis, and inflammation in OVX mice.

CONCLUSIONS

These data support the hypothesis that estradiol protects OVX ApoE KO mice from NASH and glucose intolerance.

Carboxylesterase 2a deletion provokes hepatic steatosis and insulin resistance in mice involving impaired diacylglycerol and lysophosphatidylcholine catabolism

OBJECTIVE

Hepatic triacylglycerol accumulation and insulin resistance are key features of NAFLD. However, NAFLD development and progression are rather triggered by the aberrant generation of lipid metabolites and signaling molecules including diacylglycerol (DAG) and lysophosphatidylcholine (lysoPC). Recent studies showed decreased expression of carboxylesterase 2 (CES2) in the liver of NASH patients and hepatic DAG accumulation was linked to low CES2 activity in obese individuals. The mouse genome encodes several Ces2 genes with Ces2a showing highest expression in the liver. Herein we investigated the role of mouse Ces2a and human CES2 in lipid metabolism in vivo and in vitro.

METHODS

Lipid metabolism and insulin signaling were investigated in mice lacking Ces2a and in a human liver cell line upon pharmacological CES2 inhibition. Lipid hydrolytic activities were determined in vivo and from recombinant proteins.

RESULTS

Ces2a deficient mice (Ces2a-ko) are obese and feeding a high-fat diet (HFD) provokes severe hepatic steatosis and insulin resistance together with elevated inflammatory and fibrotic gene expression. Lipidomic analysis revealed a marked rise in DAG and lysoPC levels in the liver of Ces2a-ko mice fed HFD. Hepatic lipid accumulation in Ces2a deficiency is linked to lower DAG and lysoPC hydrolytic activities in liver microsomal preparations. Moreover, Ces2a deficiency significantly increases hepatic expression and activity of MGAT1, a PPAR gamma target gene, suggesting aberrant lipid signaling upon Ces2a deficiency. Mechanistically, we found that recombinant Ces2a and CES2 show significant hydrolytic activity towards lysoPC (and DAG) and pharmacological inhibition of CES2 in human HepG2 cells largely phenocopies the lipid metabolic changes present in Ces2a-ko mice including reduced lysoPC and DAG hydrolysis, DAG accumulation and impaired insulin signaling.

CONCLUSIONS

Ces2a and CES2 are critical players in hepatic lipid signaling likely via the hydrolysis of DAG and lysoPC at the ER.

Progesterone Has No Impact on the Beneficial Effects of Estradiol Treatment in High-Fat-Fed Ovariectomized Mice

In recent decades, clinical and experimental studies have revealed that estradiol contributes enormously to glycemic homeostasis. However, the same consensus does not exist in women during menopause who undergo replacement with progesterone or conjugated estradiol and progesterone. Since most hormone replacement treatments in menopausal women are performed with estradiol (E2) and progesterone (P4) combined, this work aimed to investigate the effects of progesterone on energy metabolism and insulin resistance in an experimental model of menopause (ovariectomized female mice-OVX mice) fed a high-fat diet (HFD). OVX mice were treated with E2 or P4 (or both combined). OVX mice treated with E2 alone or combined with P4 displayed reduced body weight after six weeks of HFD feeding compared to OVX mice and OVX mice treated with P4 alone. These data were associated with improved glucose tolerance and insulin sensitivity in OVX mice treated with E2 (alone or combined with P4) compared to OVX and P4-treated mice. Additionally, E2 treatment (alone or combined with P4) reduced both hepatic and muscle triglyceride content compared with OVX control mice and OVX + P4 mice. There were no differences between groups regarding hepatic enzymes in plasma and inflammatory markers. Therefore, our results revealed that progesterone replacement alone does not seem to influence glucose homeostasis and ectopic lipid accumulation in OVX mice. These results will help expand knowledge about hormone replacement in postmenopausal women associated with metabolic syndrome and non-alcoholic fatty liver disease.

Albumin Deficiency Reduces Hepatic Steatosis and Improves Glucose Metabolism in a Mouse Model of Diet-Induced Obesity

Serum albumin facilitates the transport of free fatty acids (FFAs) from adipose tissue to other organs. It was not known if impeding this process could protect from hepatic steatosis and metabolic dysfunction in obesity. We tested whether albumin knockout (Alb-/-) mice would exhibit a reduction in plasma FFA concentration, reduced hepatic lipid accumulation, and improved glucoregulation as compared to wild-type (WT) mice. Male homozygous albumin knockout mice (Alb-/-) and WT controls were fed a low-fat diet (LFD) or high-fat diet (HFD). Alb-/- mice exhibited a similar body weight gain and body composition as WT on both diets. Despite HFD-induced obesity, Alb-/- mice were protected from various comorbidities. Compared to WT mice on the HFD, Alb-/- exhibited lower plasma FFA levels, lower blood glucose levels during glucose tolerance and insulin tolerance tests, and lower hepatic steatosis and inflammation. Alb-/- mice on HFD also exhibited elevated expression of multiple genes in the liver and adipose tissues, such as peroxisome proliferator-activated receptor α in both tissues, as well as glucose transporter-4 and adiponectin in adipose tissues. The results indicate that albumin's FFA transport function may be involved in the development of hepatic lipid accumulation and dysregulated glucose metabolism in obesity.

Data-dependent and -independent acquisition lipidomics analysis reveals the tissue-dependent effect of metformin on lipid metabolism

Introduction

Despite the well-recognized health benefits, the mechanisms and site of action of metformin remains elusive. Metformin-induced global lipidomic changes in plasma of animal models and human subjects have been reported. However, there is a lack of systemic evaluation of metformin-induced lipidomic changes in different tissues. Metformin uptake requires active transporters such as organic cation transporters (OCTs), and hence, it is anticipated that metformin actions are tissue-dependent. In this study, we aim to characterize metformin effects in non-diabetic male mice with a special focus on lipidomics analysis. The findings from this study will help us to better understand the cell-autonomous (direct actions in target cells) or non-cell-autonomous (indirect actions in target cells) mechanisms of metformin and provide insights into the development of more potent yet safe drugs targeting a particular organ instead of systemic metabolism for metabolic regulations without major side effects.

Objectives

To characterize metformin-induced lipidomic alterations in different tissues of non-diabetic male mice and further identify lipids affected by metformin through cell-autonomous or systemic mechanisms based on the correlation between lipid alterations in tissues and the corresponding in-tissue metformin concentrations.

Methods

Lipids were extracted from tissues and plasma of male mice treated with or without metformin in drinking water for 12 days and analyzed using MS/MS scan workflow (hybrid mode) on LC-Orbitrap Exploris 480 mass spectrometer using biologically relevant lipids-containing inclusion list for data-independent acquisition (DIA), named as BRI-DIA workflow followed by data-dependent acquisition (DDA), to maximum the coverage of lipids and minimize the negative effect of stochasticity of precursor selection on experimental consistency and reproducibility.

Results

Lipidomics analysis of 6 mouse tissues and plasma using MS/MS combining BRI-DIA and DDA allowed a systemic evaluation of lipidomic changes induced by metformin in different tissues. We observed that 1) the degrees of lipidomic changes induced by metformin treatment overly correlated with tissue concentrations of metformin; 2) the impact on lysophosphorylcholine and cardiolipins was positively correlated with tissue concentrations of metformin, while neutral lipids such as triglycerides did not correlate with the corresponding tissue metformin concentrations.

Conclusion

The data collected in this study from non-diabetic mice with 12-day metformin treatment suggest that the overall metabolic effect of metformin is positively correlated with tissue concentrations and the effect on individual lipid subclass is via both cell-autonomous mechanisms (cardiolipins and lysoPC) and non-cell-autonomous mechanisms (triglycerides).

Pep19 Has a Positive Effect on Insulin Sensitivity and Ameliorates Both Hepatic and Adipose Tissue Phenotype of Diet-Induced Obese Mice

Peptide DIIADDEPLT (Pep19) has been previously suggested to improve metabolic parameters, without adverse central nervous system effects, in a murine model of diet-induced obesity. Here, we aimed to further evaluate whether Pep19 oral administration has anti-obesogenic effects, in a well-established high-fat diet-induced obesity model. Male Swiss mice, fed either a standard diet (SD) or high-fat diet (HFD), were orally administrated for 30 consecutive days, once a day, with saline vehicle or Pep19 (1 mg/kg). Next, several metabolic, morphological, and behavioral parameters were evaluated. Oral administration of Pep19 attenuated HFD body-weight gain, reduced in approximately 40% the absolute mass of the endocrine pancreas, and improved the relationship between circulating insulin and peripheral insulin sensitivity. Pep19 treatment of HFD-fed mice attenuated liver inflammation, hepatic fat distribution and accumulation, and lowered plasma alanine aminotransferase activity. The inguinal fat depot from the SD group treated with Pep19 showed multilocular brown-fat-like cells and increased mRNA expression of uncoupling protein 1 (UCP1), suggesting browning on inguinal white adipose cells. Morphological analysis of brown adipose tissue (BAT) from HFD mice showed the presence of larger white-like unilocular cells, compared to BAT from SD, Pep19-treated SD or HFD mice. Pep19 treatment produced no alterations in mice behavior. Oral administration of Pep19 ameliorates some metabolic traits altered by diet-induced obesity in a Swiss mice model.

Epigenetic changes induced in mice liver by methionine-supplemented and methionine-deficient diets

A diet deficient in donors of methyl group, such as methionine, affects DNA methylation and hepatic lipid metabolism. Methionine also affects other epigenetic mechanisms, such as microRNAs. We investigated the effects of methionine-supplemented or methionine-deficient diets on the expression of chromatin-modifying genes, global DNA methylation, the expression and methylation of genes related to lipid metabolism, and the expression of microRNAs in mouse liver. Female Swiss albino mice were fed a control diet (0.3% methionine), a methionine-supplemented diet (2% methionine), and a methionine-deficient diet (0% methionine) for 10 weeks. The genes most affected by the methionine-supplemented diet were associated with histone and DNA methyltransferases activity, while the methionine-deficient diet mostly altered the expression of histone methyltransferases genes. Both diets altered the global DNA methylation and the expression and gene-specific methylation of the lipid metabolism gene Apoa5. Both diets altered the expression of several liver homeostasis-related microRNAs, including miR-190b-5p, miR-130b-3p, miR-376c-3p, miR-411-5p, miR-29c-3p, miR-295-3p, and miR-467d-5p, with the methionine-deficient diet causing a more substantial effect. The effects of improper amounts of methionine in the diet on liver pathologies may involve a cooperative action of chromatin-modifying genes, which results in an aberrant pattern of global and gene-specific methylation, and microRNAs responsible for liver homeostasis.

Albumin knockout mice exhibit reduced plasma free fatty acid concentration and enhanced insulin sensitivity

Circulating albumin is expected to play a significant role in the trafficking of plasma free fatty acids (FFA) between tissues, such as FFA transfer from adipose tissue to the liver. However, it was not yet known how disrupting FFA binding to albumin in circulation would alter lipid metabolism and any resulting impacts upon control of glycemia. To improve understanding of metabolic control, we aimed to determine whether lack of serum albumin would decrease plasma FFA, hepatic lipid storage, whole body substrate oxidation, and glucose metabolism. Male and female homozygous albumin knockout mice and C57BL/6J wild type controls, each on a standard diet containing a moderate fat content, were studied at 6-8 weeks of age. Indirect calorimetry, glucose tolerance testing, insulin tolerance testing, exercise performance, plasma proteome, and tissue analyses were performed. In both sexes of albumin knockout mice compared to the wild type mice, significant reductions (p < 0.05) were observed for plasma FFA concentration, hepatic triacylglycerol and diacylglycerol content, blood glucose during the glucose tolerance test, and blood glucose during the insulin tolerance test. Albumin deficiency did not reduce whole body fat oxidation over a 24-h period and did not alter exercise performance in an incremental treadmill test. The system-level phenotypic changes in lipid and glucose metabolism were accompanied by reduced hepatic perilipin-2 expression (p < 0.05), as well as increased expression of adiponectin (p < 0.05) and glucose transporter-4 (p < 0.05) in adipose tissue. The results indicate an important role of albumin and plasma FFA concentration in lipid metabolism and glucoregulation.

Coronary Heart Disease in Type 2 Diabetes Mellitus: Genetic Factors and Their Mechanisms, Gene-Gene, and Gene-Environment Interactions in the Asian Populations

Asians are more susceptible to type 2 diabetes mellitus (T2D) and its coronary heart disease (CHD) complications than the Western populations, possibly due to genetic factors, higher degrees of obesity, insulin resistance, and endothelial dysfunction that could occur even in healthy individuals. The genetic factors and their mechanisms, along with gene-gene and gene-environment interactions associated with CHD in T2D Asians, are yet to be explored. Therefore, the objectives of this paper were to review the current evidence of genetic factors for CHD, summarize the proposed mechanisms of these genes and how they may associate with CHD risk, and review the gene-gene and gene-environment interactions in T2D Asians with CHD. The genetic factors can be grouped according to their involvement in the energy and lipoprotein metabolism, vascular and endothelial pathology, antioxidation, cell cycle regulation, DNA damage repair, hormonal regulation of glucose metabolism, as well as cytoskeletal function and intracellular transport. Meanwhile, interactions between single nucleotide polymorphisms (SNPs) from different genes, SNPs within a single gene, and genetic interaction with environmental factors including obesity, smoking habit, and hyperlipidemia could modify the gene's effect on the disease risk. Collectively, these factors illustrate the complexities of CHD in T2D, specifically among Asians.

Hypothyroidism-Associated Dyslipidemia: Potential Molecular Mechanisms Leading to NAFLD

Thyroid hormones control lipid metabolism by exhibiting specific effects on the liver and adipose tissue in a coordinated manner. Different diseases of the thyroid gland can result in hypothyroidism. Hypothyroidism is frequently associated with dyslipidemia. Hypothyroidism-associated dyslipidemia subsequently results in intrahepatic accumulation of fat, leading to nonalcoholic fatty liver disease (NAFLD), which leads to the development of hepatic insulin resistance. The prevalence of NAFLD in the western world is increasing, and evidence of its association with hypothyroidism is accumulating. Since hypothyroidism has been identified as a modifiable risk factor of NAFLD and recent data provides evidence that selective thyroid hormone receptor β (THR-β) agonists are effective in the treatment of dyslipidemia and NAFLD, interest in potential therapeutic options for NAFLD targeting these receptors is growing. In this review, we summarize current knowledge regarding clinical and molecular data exploring the association of hypothyroidism, dyslipidemia and NAFLD.

Genetically-engineered hamster models: applications and perspective in dyslipidemia and atherosclerosis-related cardiovascular disease

Cardiovascular disease is the leading cause of morbidity and mortality in both developed and developing countries, in which atherosclerosis triggered by dyslipidemia is the major pathological basis. Over the past 40 years, small rodent animals, such as mice, have been widely used for understanding of human atherosclerosis-related cardiovascular disease (ASCVD) with the advantages of low cost and ease of maintenance and manipulation. However, based on the concept of precision medicine and high demand of translational research, the applications of mouse models for human ASCVD study would be limited due to the natural differences in metabolic features between mice and humans even though they are still the most powerful tools in this research field, indicating that other species with biological similarity to humans need to be considered for studying ASCVD in future. With the development and breakthrough of novel gene editing technology, Syrian golden hamster, a small rodent animal replicating the metabolic characteristics of humans, has been genetically modified, suggesting that gene-targeted hamster models will provide new insights into the precision medicine and translational research of ASCVD. The purpose of this review was to summarize the genetically-modified hamster models with dyslipidemia to date, and their potential applications and perspective for ASCVD.

Metabolic-associated fatty liver disease and lipoprotein metabolism

BACKGROUND

Non-alcoholic fatty liver disease, or as recently proposed 'metabolic-associated fatty liver disease' (MAFLD), is characterized by pathological accumulation of triglycerides and other lipids in hepatocytes. This common disease can progress from simple steatosis to steatohepatitis, and eventually end-stage liver diseases. MAFLD is closely related to disturbances in systemic energy metabolism, including insulin resistance and atherogenic dyslipidemia.

SCOPE OF REVIEW

The liver is the central organ in lipid metabolism by secreting very low density lipoproteins (VLDL) and, on the other hand, by internalizing fatty acids and lipoproteins. This review article discusses recent research addressing hepatic lipid synthesis, VLDL production, and lipoprotein internalization as well as the lipid exchange between adipose tissue and the liver in the context of MAFLD.

MAJOR CONCLUSIONS

Liver steatosis in MAFLD is triggered by excessive hepatic triglyceride synthesis utilizing fatty acids derived from white adipose tissue (WAT), de novo lipogenesis (DNL) and endocytosed remnants of triglyceride-rich lipoproteins. In consequence of high hepatic lipid content, VLDL secretion is enhanced, which is the primary cause of complex dyslipidemia typical for subjects with MAFLD. Interventions reducing VLDL secretory capacity attenuate dyslipidemia while they exacerbate MAFLD, indicating that the balance of lipid storage versus secretion in hepatocytes is a critical parameter determining disease outcome. Proof of concept studies have shown that promoting lipid storage and energy combustion in adipose tissues reduces hepatic lipid load and thus ameliorates MAFLD. Moreover, hepatocellular triglyceride synthesis from DNL and WAT-derived fatty acids can be targeted to treat MAFLD. However, more research is needed to understand how individual transporters, enzymes, and their isoforms affect steatosis and dyslipidemia in vivo, and whether these two aspects of MAFLD can be selectively treated. Processing of cholesterol-enriched lipoproteins appears less important for steatosis. It may, however, modulate inflammation and consequently MAFLD progression.

Plasma Free Fatty Acid Concentration as a Modifiable Risk Factor for Metabolic Disease

Plasma free fatty acid (FFA) concentration is elevated in obesity, insulin resistance (IR), non-alcoholic fatty liver disease (NAFLD), type 2 diabetes (T2D), and related comorbidities such as cardiovascular disease (CVD). Furthermore, experimentally manipulating plasma FFA in the laboratory setting modulates metabolic markers of these disease processes. In this article, evidence is presented indicating that plasma FFA is a disease risk factor. Elevations of plasma FFA can promote ectopic lipid deposition, IR, as well as vascular and cardiac dysfunction. Typically, elevated plasma FFA results from accelerated adipose tissue lipolysis, caused by a high adipose tissue mass, adrenal hormones, or other physiological stressors. Reducing an individual’s postabsorptive and postprandial plasma FFA concentration is expected to improve health. Lifestyle change could provide a significant opportunity for plasma FFA reduction. Various factors can impact plasma FFA concentration, such as chronic restriction of dietary energy intake and weight loss, as well as exercise, sleep quality and quantity, and cigarette smoking. In this review, consideration is given to multiple factors which lead to plasma FFA elevation and subsequent disruption of metabolic health. From considering a variety of medical conditions and lifestyle factors, it becomes clear that plasma FFA concentration is a modifiable risk factor for metabolic disease.

Transient elevation of triacylglycerol content in the liver: a fundamental component of the acute response to exercise

Exercise is well appreciated as a therapeutic approach to improve health. Although chronic exercise training can change metabolism, even a single exercise session can have significant effects upon metabolism. Responses of adipose tissue lipolysis and skeletal muscle triacylglycerol (TAG) utilization have been well appreciated as components of the acute exercise response. However, there are other central components of the physiological response to be considered, as well. A robust and growing body of literature depicts a rapid responsiveness of hepatic TAG content to single bouts of exercise, and there is a remaining need to incorporate this information into our overall understanding of how exercise affects the liver. TAG content in the liver increases during an exercise session and can continue to rise for a few hours afterwards, followed by a fairly rapid return to baseline. Here, we summarize evidence that rapid responsiveness of hepatic TAG content to metabolic stress is a fundamental component of the exercise response. Adipose tissue lipolysis and plasma free fatty acid concentration are likely the major metabolic controllers of enhanced lipid storage in the liver after each exercise bout, and we discuss nutritional impacts as well as health implications. Although traditionally clinicians would be merely concerned with hepatic lipids in overnight-fasted, rested individuals, it is now apparent that the content of hepatic TAG fluctuates in response to metabolic challenges such as exercise, and these responses likely exert significant impacts on health and cellular homeostasis.

Apolipoprotein A5 controls fructose-induced metabolic dysregulation in mice

BACKGROUND AND AIMS

Western dietary habits are partially characterized by increased uptake of fructose, which contributes to metabolic dysregulation and associated liver diseases. For example, a diet enriched with fructose drives insulin resistance and non-alcoholic fatty liver disease (NAFLD). The molecular hubs that control fructose-induced metabolic dysregulation are poorly understood. Apolipoprotein A5 (apoA5) controls triglyceride metabolism with a putative role in hepatic lipid deposition. We explored apoA5 as a rheostat for fructose-induced hepatic and metabolic disease in mammals.

METHODS AND RESULTS

ApoA5 knock out (-/-) and wildtype (wt) mice were fed with high fructose diet or standard diet for 10 weeks. Afterwards, we conducted a metabolic characterization by insulin tolerance test as well as oral glucose tolerance test. Additionally, hepatic lipid content as well as transcription patterns of key enzymes and transcription factors in glucose and lipid metabolism were evaluated. Despite comparable body weight, insulin sensitivity was significantly improved in high fructose diet fed apoA5 (-/-) when compared to wildtype mice on the same diet. In parallel, hepatic triglyceride content was significantly lower in apoA5 (-/-) mice than in wt mice. No difference was seen between apoA5 (-/-) and wt mice on a standard diet.

CONCLUSION

ApoA5 is involved in fructose-induced metabolic dysregulation and associated hepatic steatosis suggesting that apoA5 may be a novel target to treat metabolic diseases.

Sphingosine 1-phosphate metabolism and insulin signaling

Insulin is the main anabolic hormone secreted by β-cells of the pancreas stimulating the assimilation and storage of glucose in muscle and fat cells. It modulates the postprandial balance of carbohydrates, lipids and proteins via enhancing lipogenesis, glycogen and protein synthesis and suppressing glucose generation and its release from the liver. Resistance to insulin is a severe metabolic disorder related to a diminished response of peripheral tissues to the insulin action and signaling. This leads to a disturbed glucose homeostasis that precedes the onset of type 2 diabetes (T2D), a disease reaching epidemic proportions. A large number of studies reported an association between elevated circulating fatty acids and the development of insulin resistance. The increased fatty acid lipid flux results in the accumulation of lipid droplets in a variety of tissues. However, lipid intermediates such as diacylglycerols and ceramides are also formed in response to elevated fatty acid levels. These bioactive lipids have been associated with the pathogenesis of insulin resistance. More recently, sphingosine 1-phosphate (S1P), another bioactive sphingolipid derivative, has also been shown to increase in T2D and obesity. Although many studies propose a protective role of S1P metabolism on insulin signaling in peripheral tissues, other studies suggest a causal role of S1P on insulin resistance. In this review, we critically summarize the current state of knowledge of S1P metabolism and its modulating role on insulin resistance. A particular emphasis is placed on S1P and insulin signaling in hepatocytes, skeletal muscle cells, adipocytes and pancreatic β-cells. In particular, modulation of receptors and enzymes that regulate S1P metabolism can be considered as a new therapeutic option for the treatment of insulin resistance and T2D.

Pathophysiology of NASH in endocrine diseases

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease in the industrialized world. NAFLD encompasses a whole spectrum ranging from simple steatosis to nonalcoholic steatohepatitis (NASH) and cirrhosis. The latter can lead to hepatocellular carcinoma. Furthermore, NASH is the most rapidly increasing indication for liver transplantation in western countries and therefore represents a global health issue. The pathophysiology of NASH is complex and includes multiple parallel hits. NASH is notably characterized by steatosis as well as evidence of hepatocyte injury and inflammation, with or without fibrosis. NASH is frequently associated with type 2 diabetes and conditions associated with insulin resistance. Moreover, NASH may also be found in many other endocrine diseases such as polycystic ovary syndrome, hypothyroidism, male hypogonadism, growth hormone deficiency or glucocorticoid excess, for example. In this review, we will discuss the pathophysiology of NASH associated with different endocrinopathies.

Lipoprotein Lipase and Its Regulators: An Unfolding Story

Lipoprotein lipase (LPL) is one of the most important factors in systemic lipid partitioning and metabolism. It mediates intravascular hydrolysis of triglycerides packed in lipoproteins such as chylomicrons and very-low-density lipoprotein (VLDL). Since its initial discovery in the 1940s, its biology and pathophysiological significance have been well characterized. Nonetheless, several studies in the past decade, with recent delineation of LPL crystal structure and the discovery of several new regulators such as angiopoietin-like proteins (ANGPTLs), glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1), lipase maturation factor 1 (LMF1) and Sel-1 suppressor of Lin-12-like 1 (SEL1L), have completely transformed our understanding of LPL biology.

Mechanisms by which adiponectin reverses high fat diet-induced insulin resistance in mice

Adiponectin has emerged as a potential therapy for type 2 diabetes mellitus, but the molecular mechanism by which adiponectin reverses insulin resistance remains unclear. Two weeks of globular adiponectin (gAcrp30) treatment reduced fasting plasma glucose, triglyceride (TAG), and insulin concentrations and reversed whole-body insulin resistance, which could be attributed to both improved insulin-mediated suppression of endogenous glucose production and increased insulin-stimulated glucose uptake in muscle and adipose tissues. These improvements in liver and muscle sensitivity were associated with ∼50% reductions in liver and muscle TAG and plasma membrane (PM)-associated diacylglycerol (DAG) content and occurred independent of reductions in total ceramide content. Reductions of PM DAG content in liver and skeletal muscle were associated with reduced PKCε translocation in liver and reduced PKCθ and PKCε translocation in skeletal muscle resulting in increased insulin-stimulated insulin receptor tyrosine1162 phosphorylation, IRS-1/IRS-2-associated PI3-kinase activity, and Akt-serine phosphorylation. Both gAcrp30 and full-length adiponectin (Acrp30) treatment increased eNOS/AMPK activation in muscle and muscle fatty acid oxidation. gAcrp30 and Acrp30 infusions also increased TAG uptake in epididymal white adipose tissue (eWAT), which could be attributed to increased lipoprotein lipase (LPL) activity. These data suggest that adiponectin and adiponectin-related molecules reverse lipid-induced liver and muscle insulin resistance by reducing ectopic lipid storage in these organs, resulting in decreased plasma membrane sn-1,2-DAG-induced nPKC activity and increased insulin signaling. Adiponectin mediates these effects by both promoting the storage of TAG in eWAT likely through stimulation of LPL as well as by stimulation of AMPK in muscle resulting in increased muscle fat oxidation.

Acute exercise in mice transiently remodels the hepatic lipidome in an intensity-dependent manner

Background

The content of triacylglycerol (TAG) in the liver is known to rapidly increase after a single bout of exercise followed by recovery to sedentary levels. The response of other hepatic lipids, and acyl chain composition of lipid classes, would provide a deeper understanding of the response of hepatic lipid metabolism to acute exercise.

Methods

Female mice performed a single bout of continuous exercise (CE), high-intensity interval exercise (HIIE), or no exercise (CON). The total content of various lipids in the liver, and fatty acids within lipid classes, were measured in tissues collected 3 h after exercise (Day 1) and the day following exercise (Day 2).

Results

The total concentration of TAG rose on Day 1 after exercise (P < 0.05), with a greater elevation in HIIE than CE (P < 0.05), followed by a decline toward CON levels on Day 2. The total concentration of other measured lipid classes was not significantly altered by exercise. However, n-6 polyunsaturated fatty acid relative abundance in diacylglycerol (DAG) was increased by HIIE (P < 0.05). In CON liver, TAG content was positively correlated with DAG and phosphatidylethanolamine (P < 0.05), while these statistical associations were disrupted in exercised mice on Day 1.

Conclusions

The response of lipid metabolism to exercise involves the coordination of metabolism between various tissues, and the lipid metabolism response to acute exercise places a metabolic burden upon the liver. The present findings describe how the liver copes with this metabolic challenge. The flexibility of the TAG pool size in the liver, and other remodeling of the hepatic lipidome, may be fundamental components of the physiological response to intense exercise.

Assessment of the role of FGF15 in mediating the metabolic outcomes of murine Vertical Sleeve Gastrectomy (VSG)

Vertical sleeve gastrectomy (VSG) is the best current therapy for remission of obesity and its co-morbidities. It is understood to alter the enterohepatic circulation of bile acids in vivo. Fibroblast growth factor 19 (FGF19) in human and its murine orthologue Fgf15 plays a pivotal role in this bile acid driven enterohepatic signaling. The present study evaluated the metabolic outcomes of VSG in Fgf15 deficient mice. 6-8 weeks old male wildtype mice (WT) and Fgf15 deficient mice (KO) were fed a high fat diet (HFD) for 8 weeks. At 8th week of diet, both WT and KO mice were randomly distributed to VSG or sham surgery. Post-surgery, mice were observed for 8 weeks while fed a HFD and then euthanized to collect tissues for experimental analysis. Fgf15 deficient (KO) mice lost weight post VSG, but glucose tolerance in KO mice did not improve post VSG compared to WT mice. Enteroids derived from WT and KO mice proliferated with bile acid exposure in vitro. Post VSG both WT and KO mice had similarly altered bile acid enterohepatic flux, however Fgf15 deficient mice post VSG had increased hepatic accumulation of free and esterified cholesterol leading to lipotoxicity related ER stress, inflammasome activation, and increased Fgf21 expression. Intact Fgf15 mediated enterohepatic bile acid signaling, but not changes in bile acid flux, appear to be important for the metabolic improvements post-murine bariatric surgery. These novel data introduce a potential point of distinction between bile acids acting as ligands compared to their canonical downstream signaling pathways.

Emerging Pharmacological Targets for the Treatment of Nonalcoholic Fatty Liver Disease, Insulin Resistance, and Type 2 Diabetes

Type 2 diabetes (T2D) is characterized by persistent hyperglycemia despite hyperinsulinemia, affects more than 400 million people worldwide, and is a major cause of morbidity and mortality. Insulin resistance, of which ectopic lipid accumulation in the liver [nonalcoholic fatty liver disease (NAFLD)] and skeletal muscle is the root cause, plays a major role in the development of T2D. Although lifestyle interventions and weight loss are highly effective at reversing NAFLD and T2D, weight loss is difficult to sustain, and newer approaches aimed at treating the root cause of T2D are urgently needed. In this review, we highlight emerging pharmacological strategies aimed at improving insulin sensitivity and T2D by altering hepatic energy balance or inhibiting key enzymes involved in hepatic lipid synthesis. We also summarize recent research suggesting that liver-targeted mitochondrial uncoupling may be an attractive therapeutic approach to treat NAFLD, nonalcoholic steatohepatitis, and T2D.

Glucagon stimulates gluconeogenesis by INSP3R1-mediated hepatic lipolysis

While it is well-established that alterations in the portal vein insulin/glucagon ratio play a major role in causing dysregulated hepatic glucose metabolism in type 2 diabetes (T2D)^1–3, the mechanisms by which glucagon alters hepatic glucose production and mitochondrial oxidation remain poorly understood. Here we show that glucagon stimulates hepatic gluconeogenesis by increasing hepatic adipose triglyceride lipase activity, intrahepatic lipolysis, hepatic acetyl-CoA content, and pyruvate carboxylase flux, while also increasing mitochondrial fat oxidation, mediated by stimulation of the inositol triphosphate receptor-1 (InsP₃R-I). Chronic physiological increases in plasma glucagon concentrations increased mitochondrial hepatic fat oxidation and reversed diet-induced hepatic steatosis and insulin resistance in rats and mice; however, the effect of chronic glucagon treatment to reverse hepatic steatosis and glucose intolerance was abrogated in InsP₃R-I knockout mice. These results provide new insights into glucagon biology and suggest that InsP₃R-I may be a novel therapeutic target to reverse nonalcoholic fatty liver disease and T2D.

Hypertriglyceridemia and Atherosclerosis: Using Human Research to Guide Mechanistic Studies in Animal Models

Human studies support a strong association between hypertriglyceridemia and atherosclerotic cardiovascular disease (CVD). However, whether a causal relationship exists between hypertriglyceridemia and increased CVD risk is still unclear. One plausible explanation for the difficulty establishing a clear causal role for hypertriglyceridemia in CVD risk is that lipolysis products of triglyceride-rich lipoproteins (TRLs), rather than the TRLs themselves, are the likely mediators of increased CVD risk. This hypothesis is supported by studies of rare mutations in humans resulting in impaired clearance of such lipolysis products (remnant lipoprotein particles; RLPs). Several animal models of hypertriglyceridemia support this hypothesis and have provided additional mechanistic understanding. Mice deficient in lipoprotein lipase (LPL), the major vascular enzyme responsible for TRL lipolysis and generation of RLPs, or its endothelial anchor GPIHBP1, are severely hypertriglyceridemic but develop only minimal atherosclerosis as compared with animal models deficient in apolipoprotein (APO) E, which is required to clear TRLs and RLPs. Likewise, animal models convincingly show that increased clearance of TRLs and RLPs by LPL activation (achieved by inhibition of APOC3, ANGPTL3, or ANGPTL4 action, or increased APOA5) results in protection from atherosclerosis. Mechanistic studies suggest that RLPs are more atherogenic than large TRLs because they more readily enter the artery wall, and because they are enriched in cholesterol relative to triglycerides, which promotes pro-atherogenic effects in lesional cells. Other mechanistic studies show that hepatic receptors (LDLR and LRP1) and APOE are critical for RLP clearance. Thus, studies in animal models have provided additional mechanistic insight and generally agree with the hypothesis that RLPs derived from TRLs are highly atherogenic whereas hypertriglyceridemia due to accumulation of very large TRLs in plasma is not markedly atherogenic in the absence of TRL lipolysis products.

Green Tea Prevents NAFLD by Modulation of miR-34a and miR-194 Expression in a High-Fat Diet Mouse Model

Background/Aims

Nonalcoholic fatty liver disease (NAFLD) is considered the hepatic manifestation of metabolic syndrome. It is currently the most common chronic liver disease with complex pathogenesis and challenging treatment. Here, we investigated the hepatoprotective role of green tea (GT) and determined the involvement of miRNAs and its mechanism of action.

Methods

Male C57Bl/6 mice were fed with a high-fat diet for 4 weeks. After this period, the animals received gavage with GT (500 mg/kg body weight) over 12 weeks (5 days/week). HepG2 cell lines were transfected with miR-34a or miR-194 mimetics and inhibitors to validate the in vivo results or were treated with TNF-α to evaluate miRNA regulation.

Results

GT supplementation protects against NAFLD development by altering lipid metabolism, increasing gene expression involved in triglycerides and fatty acid catabolism, and decreasing uptake and lipid accumulation. This phenotype was accompanied by miR-34a downregulation and an increase in their mRNA targets Sirt1, Pparα, and Insig2. GT upregulated hepatic miR-194 by inhibiting TNF-α action leading to a decrease in miR-194 target genes Hmgcs/Apoa5.

Conclusion

Our study identified for the first time that the beneficial effects of GT in the liver can be due to the modulation of miRNAs, opening new perspectives for the treatment of NAFLD focusing on epigenetic regulation of miR-34a and miR-194 as green tea targets.

Evaluation of Hepatic Steatosis in Rodents by Time-Domain Nuclear Magnetic Resonance

Devices that analyze body composition of rodents by time-domain nuclear magnetic resonance (TD-NMR) are becoming popular in research centers that study metabolism. Theoretically, TD-NMR devices can also evaluate lipid content in isolated tissues. However, the accuracy of TD-NMR to determine hepatic steatosis in the liver of small laboratory animals has not been evaluated in detail. We observed that TD-NMR was able to detect increased lipid content in the liver of rats consuming high-fat diet (HFD) for 12 weeks and in genetically obese (Lep^ob/ob and Lepr^db/db) mice. The lipid content determined by TD-NMR showed a positive correlation with triglyceride content measured by colorimetric assays. In contrast, TD-NMR did not detect hepatic steatosis in C57BL/6 mice consuming HFD for 4 or 12 weeks, despite their obesity and increased liver triglyceride content. These findings indicate that tissue mass and the severity of hepatic steatosis affect the sensitivity of TD-NMR to detect liver lipid content.

Anti-inflammatory effects of oestrogen mediate the sexual dimorphic response to lipid-induced insulin resistance

KEY POINTS

Oestrogen has been shown to play an important role in the regulation of metabolic homeostasis and insulin sensitivity in both human and rodent studies. Insulin sensitivity is greater in premenopausal women compared with age-matched men, and metabolism-related cardiovascular diseases and type 2 diabetes are less frequent in these same women. Both female and male mice treated with oestradiol are protected against obesity-induced insulin resistance. The protection against obesity-induced insulin resistance is associated with reduced ectopic lipid content in liver and skeletal muscle. These results were associated with increased insulin-stimulated suppression of white adipose tissue lipolysis and reduced inflammation.

ABSTRACT

Oestrogen has been shown to play an important role in the regulation of metabolic homeostasis and insulin sensitivity in both human and rodent studies. Overall, females are protected against obesity-induced insulin resistance; yet, the mechanisms responsible for this protection are not well understood. Therefore, the aim of the present work was to evaluate the underlying mechanism(s) by which female mice are protected against obesity-induced insulin resistance compared with male mice. We studied male and female mice in age-matched or body weight-matched conditions. They were fed a high-fat diet (HFD) or regular chow for 4 weeks. We also studied HFD male mice treated with oestradiol or vehicle. Both HFD female and HFD male mice treated with oestradiol displayed increased whole-body insulin sensitivity, associated with reduction in ectopic hepatic and muscle lipid content compared to HFD male mice. Reductions in ectopic lipid content in these mice were associated with increased insulin-stimulated suppression of white adipose tissue (WAT) lipolysis. Both HFD female and HFD male mice treated with oestradiol also displayed striking reductions in WAT inflammation, represented by reductions in plasma and adipose tissue tumour necrosis factor α and interleukin 6 concentrations. Taken together these data support the hypothesis that HFD female mice are protected from obesity-induced insulin resistance due to oestradiol-mediated reductions in WAT inflammation, leading to improved insulin-mediated suppression of WAT lipolysis and reduced ectopic lipid content in liver and skeletal muscle.

Acetyl-CoA Carboxylase Inhibition Reverses NAFLD and Hepatic Insulin Resistance but Promotes Hypertriglyceridemia in Rodents

Pharmacologic inhibition of acetyl-CoA carboxylase (ACC) enzymes, ACC1 and ACC2, offers an attractive therapeutic strategy for nonalcoholic fatty liver disease (NAFLD) through simultaneous inhibition of fatty acid synthesis and stimulation of fatty acid oxidation. However, the effects of ACC inhibition on hepatic mitochondrial oxidation, anaplerosis, and ketogenesis in vivo are unknown. Here, we evaluated the effect of a liver-directed allosteric inhibitor of ACC1 and ACC2 (Compound 1) on these parameters, as well as glucose and lipid metabolism, in control and diet-induced rodent models of NAFLD. Oral administration of Compound 1 preferentially inhibited ACC enzymatic activity in the liver, reduced hepatic malonyl-CoA levels, and enhanced hepatic ketogenesis by 50%. Furthermore, administration for 6 days to high-fructose-fed rats resulted in a 20% reduction in hepatic de novo lipogenesis. Importantly, long-term treatment (21 days) significantly reduced high-fat sucrose diet-induced hepatic steatosis, protein kinase C epsilon activation, and hepatic insulin resistance. ACCi treatment was associated with a significant increase in plasma triglycerides (approximately 30% to 130%, depending on the length of fasting). ACCi-mediated hypertriglyceridemia could be attributed to approximately a 15% increase in hepatic very low-density lipoprotein production and approximately a 20% reduction in triglyceride clearance by lipoprotein lipase (P ≤ 0.05). At the molecular level, these changes were associated with increases in liver X receptor/sterol response element-binding protein-1 and decreases in peroxisome proliferator-activated receptor-α target activation and could be reversed with fenofibrate co-treatment in a high-fat diet mouse model. Conclusion: Collectively, these studies warrant further investigation into the therapeutic utility of liver-directed ACC inhibition for the treatment of NAFLD and hepatic insulin resistance.

Mechanisms of Insulin Action and Insulin Resistance

The 1921 discovery of insulin was a Big Bang from which a vast and expanding universe of research into insulin action and resistance has issued. In the intervening century, some discoveries have matured, coalescing into solid and fertile ground for clinical application; others remain incompletely investigated and scientifically controversial. Here, we attempt to synthesize this work to guide further mechanistic investigation and to inform the development of novel therapies for type 2 diabetes (T2D). The rational development of such therapies necessitates detailed knowledge of one of the key pathophysiological processes involved in T2D: insulin resistance. Understanding insulin resistance, in turn, requires knowledge of normal insulin action. In this review, both the physiology of insulin action and the pathophysiology of insulin resistance are described, focusing on three key insulin target tissues: skeletal muscle, liver, and white adipose tissue. We aim to develop an integrated physiological perspective, placing the intricate signaling effectors that carry out the cell-autonomous response to insulin in the context of the tissue-specific functions that generate the coordinated organismal response. First, in section II, the effectors and effects of direct, cell-autonomous insulin action in muscle, liver, and white adipose tissue are reviewed, beginning at the insulin receptor and working downstream. Section III considers the critical and underappreciated role of tissue crosstalk in whole body insulin action, especially the essential interaction between adipose lipolysis and hepatic gluconeogenesis. The pathophysiology of insulin resistance is then described in section IV. Special attention is given to which signaling pathways and functions become insulin resistant in the setting of chronic overnutrition, and an alternative explanation for the phenomenon of ‟selective hepatic insulin resistanceˮ is presented. Sections V, VI, and VII critically examine the evidence for and against several putative mediators of insulin resistance. Section V reviews work linking the bioactive lipids diacylglycerol, ceramide, and acylcarnitine to insulin resistance; section VI considers the impact of nutrient stresses in the endoplasmic reticulum and mitochondria on insulin resistance; and section VII discusses non-cell autonomous factors proposed to induce insulin resistance, including inflammatory mediators, branched-chain amino acids, adipokines, and hepatokines. Finally, in section VIII, we propose an integrated model of insulin resistance that links these mediators to final common pathways of metabolite-driven gluconeogenesis and ectopic lipid accumulation.

Uncaria tomentosa improves insulin sensitivity and inflammation in experimental NAFLD

We investigated the effect of the crude herbal extract from Uncaria tomentosa (UT) on non-alcoholic fatty liver disease (NAFLD) in two models of obesity: high fat diet (HFD) and genetically obese (ob/ob) mice. Both obese mouse models were insulin resistant and exhibited an abundance of lipid droplets in the hepatocytes and inflammatory cell infiltration in the liver, while only the HFD group had collagen deposition in the perivascular space of the liver. UT treatment significantly reduced liver steatosis and inflammation in both obese mouse models. Furthermore, serine phosphorylation of IRS-1 was reduced by 25% in the HFD mice treated with UT. Overall, UT treated animals exhibited higher insulin sensitivity as compared to vehicle administration. In conclusion, Uncaria tomentosa extract improved glucose homeostasis and reverted NAFLD to a benign hepatic steatosis condition and these effects were associated with the attenuation of liver inflammation in obese mice.

Brown adipose tissue derived ANGPTL4 controls glucose and lipid metabolism and regulates thermogenesis

Objectives

Brown adipose tissue (BAT) controls triglyceride-rich lipoprotein (TRL) catabolism. This process is mediated by the lipoprotein lipase (LPL), an enzyme that catalyzed the hydrolysis of triglyceride (TAG) in glycerol and fatty acids (FA), which are burned to generate heat. LPL activity is regulated by angiopoietin-like 4 (ANGPTL4), a secretory protein produced in adipose tissues (AT), liver, kidney, and muscle. While the role of ANGPTL4 in regulating lipoprotein metabolism is well established, the specific contribution of BAT derived ANGPTL4 in controlling lipid and glucose homeostasis is not well understood.

Methods and results

We generated a novel mouse model lacking ANGPTL4 specifically in brown adipose tissue (BAT-KO). Here, we report that specific deletion of ANGPTL4 in BAT results in enhanced LPL activity, circulating TAG clearance and thermogenesis. Absence of ANGPTL4 in BAT increased FA oxidation and reduced FA synthesis. Importantly, we observed that absence of ANGPTL4 in BAT leads to a remarkable improvement in glucose tolerance in short-term HFD feeding.

Conclusion

Our findings demonstrate an important role of BAT derived ANGPTL4 in regulating lipoprotein metabolism, whole-body lipid and glucose metabolism, and thermogenesis during acute cold exposure.

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Absence of ANGPTL4 in brown adipose tissue reduces circulating triglycerides.

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Loss of ANGPTL4 in brown adipose tissue enhances glucose tolerance and insulin sensitivity.

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Lack of ANGPTL4 in brown adipose tissue improves thermogenesis in response to acute cold exposure.

Absence of ANGPTL4 in adipose tissue improves glucose tolerance and attenuates atherogenesis

Alterations in ectopic lipid deposition and circulating lipids are major risk factors for developing cardiometabolic diseases. Angiopoietin-like protein 4 (ANGPTL4), a protein that inhibits lipoprotein lipase (LPL), controls fatty acid (FA) uptake in adipose and oxidative tissues and regulates circulating triacylglycerol-rich (TAG-rich) lipoproteins. Unfortunately, global depletion of ANGPTL4 results in severe metabolic abnormalities, inflammation, and fibrosis when mice are fed a high-fat diet (HFD), limiting our understanding of the contribution of ANGPTL4 in metabolic disorders. Here, we demonstrate that genetic ablation of ANGPTL4 in adipose tissue (AT) results in enhanced LPL activity, rapid clearance of circulating TAGs, increased AT lipolysis and FA oxidation, and decreased FA synthesis in AT. Most importantly, we found that absence of ANGPTL4 in AT prevents excessive ectopic lipid deposition in the liver and muscle, reducing novel PKC (nPKC) membrane translocation and enhancing insulin signaling. As a result, we observed a remarkable improvement in glucose tolerance in short-term HFD-fed AT-specific Angptl4-KO mice. Finally, lack of ANGPTL4 in AT enhances the clearance of proatherogenic lipoproteins, attenuates inflammation, and reduces atherosclerosis. Together, these findings uncovered an essential role of AT ANGPTL4 in regulating peripheral lipid deposition, influencing whole-body lipid and glucose metabolism and the progression of atherosclerosis.

Mechanism by which arylamine N-acetyltransferase 1 ablation causes insulin resistance in mice

Insulin resistance in liver and skeletal muscle are major factors in the pathogenesis of type 2 diabetes; however, the molecular mechanism or mechanisms responsible for this phenomenon have not been established. Recently, an association of a single-nucleotide polymorphism in the human N-acetyltransferase 2 (Nat2) gene with insulin resistance in humans was found. Here, we show that the murine ortholog Nat1 knockout (KO) mice manifested whole-body insulin resistance associated with marked increases in liver and muscle lipid content. Nat1 KO mice also displayed reduced whole-body energy expenditure and reduced mitochondrial activity. Taken together, these studies demonstrate that Nat1 deletion promotes reduced mitochondrial activity and is associated with ectopic lipid-induced liver and muscle insulin resistance.

A single-nucleotide polymorphism in the human arylamine N-acetyltransferase 2 (Nat2) gene has recently been identified as associated with insulin resistance in humans. To understand the cellular and molecular mechanisms by which alterations in Nat2 activity might cause insulin resistance, we examined murine ortholog Nat1 knockout (KO) mice. Nat1 KO mice manifested whole-body insulin resistance, which could be attributed to reduced muscle, liver, and adipose tissue insulin sensitivity. Hepatic and muscle insulin resistance were associated with marked increases in both liver and muscle triglyceride (TAG) and diacylglycerol (DAG) content, which was associated with increased PKCε activation in liver and increased PKCθ activation in skeletal muscle. Nat1 KO mice also displayed reduced whole-body energy expenditure and reduced mitochondrial oxygen consumption in white adipose tissue, brown adipose tissue, and hepatocytes. Taken together, these studies demonstrate that Nat1 deletion promotes reduced mitochondrial activity and is associated with ectopic lipid-induced insulin resistance. These results provide a potential genetic link among mitochondrial dysfunction with increased ectopic lipid deposition, insulin resistance, and type 2 diabetes.

Metformin effects on the heart and the cardiovascular system: A review of experimental and clinical data

BACKGROUND

Metformin, the eldest and most widely used glucose lowering drug, is likely to be effective also on cardiac and vascular disease prevention. Nonetheless, uncertainty still exists with regard to its effects on the cardiovascular system as a whole and specifically on the myocardium, both at the organ and cellular levels.

METHODS

We reviewed the available data on the cardiac and vascular effects of metformin, encompassing both in vitro, either tissue or isolated organ, and in vivo studies in experimental animals and humans, as well as the evidence generated by major clinical trials.

RESULTS

At the cellular level metformin's produces both AMP-activated kinase (AMPK) dependent and independent effects. At the systemic level, possibly also through other pathways, this drug improves endothelial function, protects from oxidative stress and inflammation, and from the negative effects of angiotensin II. On the myocardium it attenuates ischemia-reperfusion injury and prevents adverse remodeling induced by humoral and hemodynamic factors. The effects on myocardial cell metabolism and contractile function being not evident at rest or in more advanced stages of cardiac dysfunction, could be relevant during transient ischemia, during an acute increase in workload and in the early stages of diabetic/hypertensive cardiomyopathy as confirmed by few small clinical trials and some observational studies. The overall evidence emerging from both clinical trials and real world registry is in favor of a protective effect of metformin with respect to both coronary events and progression to heart failure.

CONCLUSIONS

Given this potential, its efficacy and its safety (and also its low cost) metformin remains the central pillar of the therapy of diabetes.

Roles of Diacylglycerols and Ceramides in Hepatic Insulin Resistance

Although ample evidence links hepatic lipid accumulation with hepatic insulin resistance, the mechanistic basis of this association is incompletely understood and controversial. Diacylglycerols (DAGs) and ceramides have emerged as the two best-studied putative mediators of lipid-induced hepatic insulin resistance. Both lipids were first associated with insulin resistance in skeletal muscle and were subsequently hypothesized to mediate insulin resistance in the liver. However, the putative roles for DAGs and ceramides in hepatic insulin resistance have proved more complex than originally imagined, with various genetic and pharmacologic manipulations yielding a vast and occasionally contradictory trove of data to sort. In this review we examine the state of this field, turning a critical eye toward both DAGs and ceramides as putative mediators of lipid-induced hepatic insulin resistance.

Insulin Production and Resistance in Different Models of Diet-Induced Obesity and Metabolic Syndrome

The role of the liver and the endocrine pancreas in development of hyperinsulinemia in different types of obesity remains unclear. Sedentary rats (160 g) were fed a low-fat-diet (LFD, chow 13% kcal fat), high-fat-diet (HFD, 35% fat), or HFD+ 30% ethanol+ 30% fructose (HF-EFr, 22% fat). Overnight-fasted rats were culled after one, four or eight weeks. Pancreatic and hepatic mRNAs were isolated for subsequent RT-PCR analysis. After eight weeks, body weights increased three-fold in the LFD group, 2.8-fold in the HFD group, and 2.4-fold in the HF-EFr (p < 0.01). HF-EFr-fed rats had the greatest liver weights and consumed less food during Weeks 4–8 (p < 0.05). Hepatic-triglyceride content increased progressively in all groups. At Week 8, HOMA-IR values, fasting serum glucose, C-peptide, and triglycerides levels were significantly increased in LFD-fed rats compared to that at earlier time points. The greatest plasma levels of glucose, triglycerides and leptin were observed in the HF-EFr at Week 8. Gene expression of pancreatic-insulin was significantly greater in the HFD and HF-EFr groups versus the LFD. Nevertheless, insulin: C-peptide ratios and HOMA-IR values were substantially higher in HF-EFr. Hepatic gene-expression of insulin-receptor-substrate-1/2 was downregulated in the HF-EFr. The expression of phospho-ERK-1/2 and inflammatory-mediators were greatest in the HF-EFr-fed rats. Chronic intake of both LFD and HFD induced obesity, MetS, and intrahepatic-fat accumulation. The hyperinsulinemia is the strongest in rats with the lowest body weights, but having the highest liver weights. This accompanies the strongest increase of pancreatic insulin production and the maximal decrease of hepatic insulin signaling, which is possibly secondary to hepatic fat deposition, inflammation and other factors.

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

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

FGF21 Lowers Plasma Triglycerides by Accelerating Lipoprotein Catabolism in White and Brown Adipose Tissues

FGF21 decreases plasma triglycerides (TGs) in rodents and humans; however, the underlying mechanism or mechanisms are unclear. In the present study, we examined the role of FGF21 in production and disposal of TG-rich lipoproteins (TRLs) in mice. Treatment with pharmacological doses of FGF21 acutely reduced plasma non-esterified fatty acids (NEFAs), liver TG content, and VLDL-TG secretion. In addition, metabolic turnover studies revealed that FGF21 facilitated the catabolism of TRL in white adipose tissue (WAT) and brown adipose tissue (BAT). FGF21-dependent TRL processing was strongly attenuated in CD36-deficient mice and transgenic mice lacking lipoprotein lipase in adipose tissues. Insulin resistance in diet-induced obese and ob/ob mice shifted FGF21 responses from WAT toward energy-combusting BAT. In conclusion, FGF21 lowers plasma TGs through a dual mechanism: first, by reducing NEFA plasma levels and consequently hepatic VLDL lipidation and, second, by increasing CD36 and LPL-dependent TRL disposal in WAT and BAT.

Anti-myostatin antibody increases muscle mass and strength and improves insulin sensitivity in old mice

Sarcopenia, or skeletal muscle atrophy, is a debilitating comorbidity of many physiological and pathophysiological processes, including normal aging. There are no approved therapies for sarcopenia, but the antihypertrophic myokine myostatin is a potential therapeutic target. Here, we show that treatment of young and old mice with an anti-myostatin antibody (ATA 842) for 4 wk increased muscle mass and muscle strength in both groups. Furthermore, ATA 842 treatment also increased insulin-stimulated whole body glucose metabolism in old mice, which could be attributed to increased insulin-stimulated skeletal muscle glucose uptake as measured by a hyperinsulinemic-euglycemic clamp. Taken together, these studies provide support for pharmacological inhibition of myostatin as a potential therapeutic approach for age-related sarcopenia and metabolic disease.

Disparate metabolic response to fructose feeding between different mouse strains

Diets enriched in fructose (FR) increase lipogenesis in the liver, leading to hepatic lipid accumulation and the development of insulin resistance. Previously, we have shown that in contrast to other mouse strains, BALB/c mice are resistant to high fat diet-induced metabolic deterioration, potentially due to a lack of ectopic lipid accumulation in the liver. In this study we have compared the metabolic response of BALB/c and C57BL/6 (BL6) mice to a fructose-enriched diet. Both strains of mice increased adiposity in response to FR-feeding, while only BL6 mice displayed elevated hepatic triglyceride (TAG) accumulation and glucose intolerance. The lack of hepatic TAG accumulation in BALB/c mice appeared to be linked to an altered balance between lipogenic and lipolytic pathways, while the protection from fructose-induced glucose intolerance in this strain was likely related to low levels of ER stress, a slight elevation in insulin levels and an altered profile of diacylglycerol species in the liver. Collectively these findings highlight the multifactorial nature of metabolic defects that develop in response to changes in the intake of specific nutrients and the divergent response of different mouse strains to dietary challenges.

Multigenerational epigenetic inheritance in humans: DNA methylation changes associated with maternal exposure to lead can be transmitted to the grandchildren

We report that the DNA methylation profile of a child’s neonatal whole blood can be significantly influenced by his or her mother’s neonatal blood lead levels (BLL). We recruited 35 mother-infant pairs in Detroit and measured the whole blood lead (Pb) levels and DNA methylation levels at over 450,000 loci from current blood and neonatal blood from both the mother and the child. We found that mothers with high neonatal BLL correlate with altered DNA methylation at 564 loci in their children’s neonatal blood. Our results suggest that Pb exposure during pregnancy affects the DNA methylation status of the fetal germ cells, which leads to altered DNA methylation in grandchildren’s neonatal dried blood spots. This is the first demonstration that an environmental exposure in pregnant mothers can have an epigenetic effect on the DNA methylation pattern in the grandchildren.

Apolipoprotein A5: Extracellular and Intracellular Roles in Triglyceride Metabolism

This review addresses two major functions of apolipoprotein (apo) A5 including (1) its role in maintaining normal plasma levels of circulating triglyceride (TG) and (2) its role as a component of hepatic lipid droplets. ApoA5 is synthesized solely in the liver and circulating concentrations are extremely low. In the plasma, ApoA5 associates with TG-rich lipoproteins and enhances TG hydrolysis and remnant lipoprotein clearance. ApoA5 loss-of-function single nucleotide polymorphisms are associated with reduced lipolysis, poor remnant clearance and concomitantly, hypertriglyceridemia. Although there have been substantial breakthroughs in understanding pathophysiology associated with secreted ApoA5, there is a paucity of knowledge on the functionality of intracellular ApoA5. However, recent studies indicate that overexpression of intracellular ApoA5 is positively associated with accumulation of TG-rich lipid droplets in hepatocytes. It is thought that ApoA5 may have a causal role in non-alcoholic fatty liver disease (NAFLD) and thus, may serve as a target for developing therapeutics for NAFLD.