Methionine adenosyltransferase 1A gene deletion disrupts hepatic very low-density lipoprotein assembly in mice

Holy Basil (Ocimum sanctum L.) Flower and Fenofibrate Improve Lipid Profiles in Rats with Metabolic Dysfunction Associated Steatotic Liver Disease (MASLD): The Role of Choline Metabolism

Metabolic dysfunction-associated steatotic liver disease (MASLD) is linked to choline metabolism. The present study investigated the effect of holy basil (Ocimum sanctum L.) flower water extract (OSLY) on MASLD with choline metabolism as an underlying mechanism. Rats with high-fat diet (HFD)-induced MASLD received 250-1000 mg/kg bw of OSLY, fenofibrate, or fenofibrate + 1000 mg/kg OSLY combination. Biochemical parameters, choline metabolites, and one-carbon gene transcription were analyzed. OSLY and fenofibrate independently reduced serum LDL cholesterol (p < 0.02), liver cholesterol (p < 0.001), and liver triglyceride levels (p < 0.001) in HFD-fed rats. Only OSLY reduced signs of liver injury and increased serum HDL. Fenofibrate influenced choline metabolism by decreasing liver glycerophosphocholine (GPC; p = 0.04), as well as increasing betaine (p < 0.001) and the betaine:choline ratio (p = 0.02) in HFD-fed rats. Fenofibrate (vs. HFD) increased the expression of one-carbon metabolism genes Mthfd1l, Pemt, Smpd3, and Chka (p < 0.04). The OSLY treatment decreased liver GPC (500 mg dose; p = 0.03) and increased Smpd3 expression (1000 mg dose; p = 0.04). OSLY and fenofibrate showed weak synergistic effects on lipid and choline metabolism. Collectively, OSLY and fenofibrate independently improve lipid profiles in MASLD rats. The benefits of fenofibrate are partially mediated by choline/one-carbon metabolism, while those of OSLY are not mediated by this pathway. Holy basil flower extract merits further development as an alternative medicine for MASLD.

Eggs, Dietary Choline, and Nonalcoholic Fatty Liver Disease in the Framingham Heart Study

BACKGROUND

Eggs are rich in bioactive compounds, including choline and carotenoids that may benefit cardiometabolic outcomes. However, little is known about their relationship with nonalcoholic fatty liver disease (NAFLD).

OBJECTIVES

We investigated the association between intakes of eggs and selected egg-rich nutrients (choline, lutein, and zeaxanthin) and NAFLD risk and changes in liver fat over ∼6 y of follow-up in the Framingham Offspring and Third Generation cohorts.

METHODS

On 2 separate occasions (2002-2005 and 2008-2011), liver fat was assessed using a computed tomography scan to estimate the average liver fat attenuation relative to a control phantom to create the liver phantom ratio (LPR). In 2008-2011, cases of incident NAFLD were identified as an LPR ≤0.33 in the absence of heavy alcohol use, after excluding prevalent NAFLD (LPR ≤0.33) in 2002-2005. Food frequency questionnaires were used to estimate egg intakes (classified as <1, 1, and ≥2 per week), dietary choline (adjusted for body weight using the residual method), and the combined intakes of lutein and zeaxanthin. Multivariable modified Poisson regression and general linear models were used to compute incident risk ratios (RR) of NAFLD and adjusted mean annualized liver fat change.

RESULTS

NAFLD cumulative incidence was 19% among a total of 1414 participants. We observed no associations between egg intake or the combined intakes of lutein and zeaxanthin with an incident NAFLD risk or liver fat change. Other diet and cardiometabolic risk factors did not modify the association between egg intake and NAFLD risk. However, dietary choline intakes were inversely associated with NAFLD risk (RR for tertile 3 compared with tertile 1: 0.69, 95% CI: 0.51, 0.94).

CONCLUSIONS

Although egg intake was not directly associated with NAFLD risk, eggs are a major source of dietary choline, which was strongly inversely associated with NAFLD risk in this community-based cohort.

Mechanistic safety assessment via multi-omic characterisation of systemic pathway perturbations following in vivo MAT2A inhibition

The tumour suppressor p16/CDKN2A and the metabolic gene, methyl-thio-adenosine phosphorylase (MTAP), are frequently co-deleted in some of the most aggressive and currently untreatable cancers. Cells with MTAP deletion are vulnerable to inhibition of the metabolic enzyme, methionine-adenosyl transferase 2A (MAT2A), and the protein arginine methyl transferase (PRMT5). This synthetic lethality has paved the way for the rapid development of drugs targeting the MAT2A/PRMT5 axis. MAT2A and its liver- and pancreas-specific isoform, MAT1A, generate the universal methyl donor S-adenosylmethionine (SAM) from ATP and methionine. Given the pleiotropic role SAM plays in methylation of diverse substrates, characterising the extent of SAM depletion and downstream perturbations following MAT2A/MAT1A inhibition (MATi) is critical for safety assessment. We have assessed in vivo target engagement and the resultant systemic phenotype using multi-omic tools to characterise response to a MAT2A inhibitor (AZ'9567). We observed significant SAM depletion and extensive methionine accumulation in the plasma, liver, brain and heart of treated rats, providing the first assessment of both global SAM depletion and evidence of hepatic MAT1A target engagement. An integrative analysis of multi-omic data from liver tissue identified broad perturbations in pathways covering one-carbon metabolism, trans-sulfuration and lipid metabolism. We infer that these pathway-wide perturbations represent adaptive responses to SAM depletion and confer a risk of oxidative stress, hepatic steatosis and an associated disturbance in plasma and cellular lipid homeostasis. The alterations also explain the dramatic increase in plasma and tissue methionine, which could be used as a safety and PD biomarker going forward to the clinic.

Dysfunctional VLDL metabolism in MASLD

Lipidomics has unveiled the intricate human lipidome, emphasizing the extensive diversity within lipid classes in mammalian tissues critical for cellular functions. This diversity poses a challenge in maintaining a delicate balance between adaptability to recurring physiological changes and overall stability. Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD), linked to factors such as obesity and diabetes, stems from a compromise in the structural and functional stability of the liver within the complexities of lipid metabolism. This compromise inaccurately senses an increase in energy status, such as during fasting-feeding cycles or an upsurge in lipogenesis. Serum lipidomic studies have delineated three distinct metabolic phenotypes, or "metabotypes" in MASLD. MASLD-A is characterized by lower very low-density lipoprotein (VLDL) secretion and triglyceride (TG) levels, associated with a reduced risk of cardiovascular disease (CVD). In contrast, MASLD-C exhibits increased VLDL secretion and TG levels, correlating with elevated CVD risk. An intermediate subtype, with a blend of features, is designated as the MASLD-B metabotype. In this perspective, we examine into recent findings that show the multifaceted regulation of VLDL secretion by S-adenosylmethionine, the primary cellular methyl donor. Furthermore, we explore the differential CVD and hepatic cancer risk across MASLD metabotypes and discuss the context and potential paths forward to gear the findings from genetic studies towards a better understanding of the observed heterogeneity in MASLD.

SHMT2 reduces fatty liver but is necessary for liver inflammation and fibrosis in mice

Non-alcoholic fatty liver disease is associated with an irregular serine metabolism. Serine hydroxymethyltransferase 2 (SHMT2) is a liver enzyme that breaks down serine into glycine and one-carbon (1C) units critical for liver methylation reactions and overall health. However, the contribution of SHMT2 to hepatic 1C homeostasis and biological functions has yet to be defined in genetically modified animal models. We created a mouse strain with targeted SHMT2 knockout in hepatocytes to investigate this. The absence of SHMT2 increased serine and glycine levels in circulation, decreased liver methylation potential, and increased susceptibility to fatty liver disease. Interestingly, SHMT2-deficient mice developed simultaneous fatty liver, but when fed a diet high in fat, fructose, and cholesterol, they had significantly less inflammation and fibrosis. This study highlights the critical role of SHMT2 in maintaining hepatic 1C homeostasis and its stage-specific functions in the pathogenesis of NAFLD.

Hepatic levels of S-adenosylmethionine regulate the adaptive response to fasting

There has been an intense focus to uncover the molecular mechanisms by which fasting triggers the adaptive cellular responses in the major organs of the body. Here, we show that in mice, hepatic S-adenosylmethionine (SAMe)-the principal methyl donor-acts as a metabolic sensor of nutrition to fine-tune the catabolic-fasting response by modulating phosphatidylethanolamine N-methyltransferase (PEMT) activity, endoplasmic reticulum-mitochondria contacts, β-oxidation, and ATP production in the liver, together with FGF21-mediated lipolysis and thermogenesis in adipose tissues. Notably, we show that glucagon induces the expression of the hepatic SAMe-synthesizing enzyme methionine adenosyltransferase α1 (MAT1A), which translocates to mitochondria-associated membranes. This leads to the production of this metabolite at these sites, which acts as a brake to prevent excessive β-oxidation and mitochondrial ATP synthesis and thereby endoplasmic reticulum stress and liver injury. This work provides important insights into the previously undescribed function of SAMe as a new arm of the metabolic adaptation to fasting.

The phospholipase A2 superfamily as a central hub of bioactive lipids and beyond

In essence, "phospholipase A₂" (PLA₂) means a group of enzymes that release fatty acids and lysophospholipids by hydrolyzing the sn-2 position of glycerophospholipids. To date, more than 50 enzymes possessing PLA₂ or related lipid-metabolizing activities have been identified in mammals, and these are subdivided into several families in terms of their structures, catalytic mechanisms, tissue/cellular localizations, and evolutionary relationships. From a general viewpoint, the PLA₂ superfamily has mainly been implicated in signal transduction, driving the production of a wide variety of bioactive lipid mediators. However, a growing body of evidence indicates that PLA₂s also contribute to phospholipid remodeling or recycling for membrane homeostasis, fatty acid β-oxidation for energy production, and barrier lipid formation on the body surface. Accordingly, PLA₂ enzymes are considered one of the key regulators of a broad range of lipid metabolism, and perturbation of specific PLA₂-driven lipid pathways often disrupts tissue and cellular homeostasis and may be associated with a variety of diseases. This review covers current understanding of the physiological functions of the PLA₂ superfamily, focusing particularly on the two major intracellular PLA₂ families (Ca²⁺-dependent cytosolic PLA₂s and Ca²⁺-independent patatin-like PLA₂s) as well as other PLA₂ families, based on studies using gene-manipulated mice and human diseases in combination with comprehensive lipidomics.

The Lysophospholipase PNPLA7 Controls Hepatic Choline and Methionine Metabolism

The in vivo roles of lysophospholipase, which cleaves a fatty acyl ester of lysophospholipid, remained unclear. Recently, we have unraveled a previously unrecognized physiological role of the lysophospholipase PNPLA7, a member of the Ca2+-independent phospholipase A2 (iPLA2) family, as a key regulator of the production of glycerophosphocholine (GPC), a precursor of endogenous choline, whose methyl groups are preferentially fluxed into the methionine cycle in the liver. PNPLA7 deficiency in mice markedly decreases hepatic GPC, choline, and several metabolites related to choline/methionine metabolism, leading to various symptoms reminiscent of methionine shortage. Overall metabolic alterations in the liver of Pnpla7-null mice in vivo largely recapitulate those in methionine-deprived hepatocytes in vitro. Reduction of the methyl donor S-adenosylmethionine (SAM) after methionine deprivation decreases the methylation of the PNPLA7 gene promoter, relieves PNPLA7 expression, and thereby increases GPC and choline levels, likely as a compensatory adaptation. In line with the view that SAM prevents the development of liver cancer, the expression of PNPLA7, as well as several enzymes in the choline/methionine metabolism, is reduced in human hepatocellular carcinoma. These findings uncover an unexplored role of a lysophospholipase in hepatic phospholipid catabolism coupled with choline/methionine metabolism.

Hepatic phosphatidylcholine catabolism driven by PNPLA7 and PNPLA8 supplies endogenous choline to replenish the methionine cycle with methyl groups

Choline supplies methyl groups for regeneration of methionine and the methyl donor S-adenosylmethionine in the liver. Here, we report that the catabolism of membrane phosphatidylcholine (PC) into water-soluble glycerophosphocholine (GPC) by the phospholipase/lysophospholipase PNPLA8-PNPLA7 axis enables endogenous choline stored in hepatic PC to be utilized in methyl metabolism. PNPLA7-deficient mice show marked decreases in hepatic GPC, choline, and several metabolites related to the methionine cycle, accompanied by various signs of methionine insufficiency, including growth retardation, hypoglycemia, hypolipidemia, increased energy consumption, reduced adiposity, increased fibroblast growth factor 21 (FGF21), and an altered histone/DNA methylation landscape. Moreover, PNPLA8-deficient mice recapitulate most of these phenotypes. In contrast to wild-type mice fed a methionine/choline-deficient diet, both knockout strains display decreased hepatic triglyceride, likely via reductions of lipogenesis and GPC-derived glycerol flux. Collectively, our findings highlight the biological importance of phospholipid catabolism driven by PNPLA8/PNPLA7 in methyl group flux and triglyceride synthesis in the liver.

Associations between plasma sulfur amino acids and specific fat depots in two independent cohorts: CODAM and The Maastricht Study

PURPOSE

Sulfur amino acids (SAAs) have been associated with obesity and obesity-related metabolic diseases. We investigated whether plasma SAAs (methionine, total cysteine (tCys), total homocysteine, cystathionine and total glutathione) are related to specific fat depots.

METHODS

We examined cross-sectional subsets from the CODAM cohort (n = 470, 61.3% men, median [IQR]: 67 [61, 71] years) and The Maastricht Study (DMS; n = 371, 53.4% men, 63 [55, 68] years), enriched with (pre)diabetic individuals. SAAs were measured in fasting EDTA plasma with LC-MS/MS. Outcomes comprised BMI, skinfolds, waist circumference (WC), dual-energy X-ray absorptiometry (DXA, DMS), body composition, abdominal subcutaneous and visceral adipose tissues (CODAM: ultrasound, DMS: MRI) and liver fat (estimated, in CODAM, or MRI-derived, in DMS, liver fat percentage and fatty liver disease). Associations were examined with linear or logistic regressions adjusted for relevant confounders with z-standardized primary exposures and outcomes.

RESULTS

Methionine was associated with all measures of liver fat, e.g., fatty liver disease [CODAM: OR = 1.49 (95% CI 1.19, 1.88); DMS: OR = 1.51 (1.09, 2.14)], but not with other fat depots. tCys was associated with overall obesity, e.g., BMI [CODAM: β = 0.19 (0.09, 0.28); DMS: β = 0.24 (0.14, 0.34)]; peripheral adiposity, e.g., biceps and triceps skinfolds [CODAM: β = 0.15 (0.08, 0.23); DMS: β = 0.20 (0.12, 0.29)]; and central adiposity, e.g., WC [CODAM: β = 0.16 (0.08, 0.25); DMS: β = 0.17 (0.08, 0.27)]. Associations of tCys with VAT and liver fat were inconsistent. Other SAAs were not associated with body fat.

CONCLUSION

Plasma concentrations of methionine and tCys showed distinct associations with different fat depots, with similar strengths in the two cohorts.

In Vivo Hepatic Triglyceride Secretion Rate in Antisense Oligonucleotide (ASO)-Treated Mice

The liver is a central organ in regulating the whole body metabolic homeostasis, and, among many other processes, it plays a crucial role in lipoprotein metabolism. The liver controls the secretion of very-low-density lipoproteins (VLDLs), particles specialized in the transport of liver lipids, mainly triglycerides (TGs), to the adipose tissue, heart, and muscle, among other tissues, providing fatty acids to be stored or to be used as an energy source. The analysis of this metabolic process provides relevant information about the crosstalk between the liver and other organs. It also helps to identify how the liver is able to secrete lipids to reduce its accumulation. This protocol shows how to analyze the liver TG secretion rate blocking the VLDL clearance from the blood by the administration of poloxamer 407. In addition, it shows how to isolate the VLDL produced by the liver at the end of the experiment, so that the apolipoprotein and lipid content and size can be measured. Using antisense oligonucleotides (ASOs) for silencing target proteins involved in metabolic diseases has emerged as a new promising therapeutic approach. Thus, the usage of ASOs has also been included in this protocol. As a conclusion, evaluation of TG secretion rate in mice provides key information to understand the organ crosstalk in metabolic diseases and the capacity of the liver to secrete lipids to blood.

Treating inflammation to combat non-alcoholic fatty liver disease

Non-alcoholic fatty liver disease (NAFLD) with its more progressive form non-alcoholic steatohepatitis (NASH) has become the most common chronic liver disease, thereby representing a great burden for patients and healthcare systems. Specific pharmacological therapies for NAFLD are still missing. Inflammation is an important driver in the pathogenesis of NASH, and the mechanisms underlying inflammation in NAFLD represent possible therapeutic targets. In NASH, various intra- and extrahepatic triggers involved in the metabolic injury typically lead to the activation of different immune cells. This includes hepatic Kupffer cells, i.e. liver-resident macrophages, which can adopt an inflammatory phenotype and activate other immune cells by releasing inflammatory cytokines. As inflammation progresses, Kupffer cells are increasingly replaced by monocyte-derived macrophages with a distinct lipid-associated and scar-associated phenotype. Many other immune cells, including neutrophils, T lymphocytes - such as auto-aggressive cytotoxic as well as regulatory T cells - and innate lymphoid cells balance the progression and regression of inflammation and subsequent fibrosis. The detailed understanding of inflammatory cell subsets and their activation pathways prompted preclinical and clinical exploration of potential targets in NAFLD/NASH. These approaches to target inflammation in NASH include inhibition of immune cell recruitment via chemokine receptors (e.g. cenicriviroc), neutralization of CD44 or galectin-3 as well as agonism to nuclear factors like peroxisome proliferator-activated receptors and farnesoid X receptor that interfere with the activation of immune cells. As some of these approaches did not demonstrate convincing efficacy as monotherapies, a rational and personalized combination of therapeutic interventions may be needed for the near future.

Genetic and Diet-Induced Animal Models for Non-Alcoholic Fatty Liver Disease (NAFLD) Research

A rapidly increasing incidence of non-alcoholic fatty liver disease (NAFLD) is noted worldwide due to the adoption of western-type lifestyles and eating habits. This makes the understanding of the molecular mechanisms that drive the pathogenesis of this chronic disease and the development of newly approved treatments of utmost necessity. Animal models are indispensable tools for achieving these ends. Although the ideal mouse model for human NAFLD does not exist yet, several models have arisen with the combination of dietary interventions, genetic manipulations and/or administration of chemical substances. Herein, we present the most common mouse models used in the research of NAFLD, either for the whole disease spectrum or for a particular disease stage (e.g., non-alcoholic steatohepatitis). We also discuss the advantages and disadvantages of each model, along with the challenges facing the researchers who aim to develop and use animal models for translational research in NAFLD. Based on these characteristics and the specific study aims/needs, researchers should select the most appropriate model with caution when translating results from animal to human.

One Carbon Metabolism and S-Adenosylmethionine in Non-Alcoholic Fatty Liver Disease Pathogenesis and Subtypes

One carbon metabolism (1CM) can be defined as the transfer of a carbon unit from one metabolite to another and its replenishment by different sources of labile methyl-group nutrients: primarily choline, methionine, betaine, and serine. This flow of carbon units allows the biosynthesis of nucleotides, amino acids, formylated methionyl-tRNA, polyamines, glutathione, phospholipids, detoxification reactions, maintenance of the redox status and the concentration of NAD, and methylation reactions including epigenetic modifications. That is, 1CM functions as a nutrient sensor and integrator of cellular metabolism. A critical process in 1CM is the synthesis of S-adenosylmethionine (SAMe), the source of essentially all the hundreds of millions of daily methyl transfer reactions in a cell. This versatility of SAMe imposes a tight control in its synthesis and catabolism. Much of our knowledge concerning 1CM has been gained from studies in the production and prevention of nonalcoholic fatty liver disease (NAFLD). Here, we discuss in detail the function of the most important enzymes for their quantitative contribution to maintaining the flux of carbon units through 1CM in the liver and discuss how alterations in their enzymatic activity contribute to the development of NAFLD. Next, we discuss NAFLD subtypes based on serum lipidomic profiles with different risk of cardiovascular disease. Among the latter, we highlight the so-called subtype A for its serum lipidomic profile phenocopying that of mice deficient in SAMe synthesis and because its high frequency (about 50% of the NAFLD patients).

TFEB regulates sulfur amino acid and coenzyme A metabolism to support hepatic metabolic adaptation and redox homeostasis

Fatty liver is a highly heterogenous condition driven by various pathogenic factors in addition to the severity of steatosis. Protein insufficiency has been causally linked to fatty liver with incompletely defined mechanisms. Here we report that fatty liver is a sulfur amino acid insufficient state that promotes metabolic inflexibility via limiting coenzyme A availability. We demonstrate that the nutrient-sensing transcriptional factor EB synergistically stimulates lysosome proteolysis and methionine adenosyltransferase to increase cysteine pool that drives the production of coenzyme A and glutathione, which support metabolic adaptation and antioxidant defense during increased lipid influx. Intriguingly, mice consuming an isocaloric protein-deficient Western diet exhibit selective hepatic cysteine, coenzyme A and glutathione deficiency and acylcarnitine accumulation, which are reversed by cystine supplementation without normalizing dietary protein intake. These findings support a pathogenic link of dysregulated sulfur amino acid metabolism to metabolic inflexibility that underlies both overnutrition and protein malnutrition-associated fatty liver development.

Effects of Amino Acids Supplementation on Lipid and Glucose Metabolism in HepG2 Cells

Non-alcoholic fatty liver disease and type 2 diabetes are representing symptoms of metabolic syndrome, which is often accompanied with hepatic fat accumulation and insulin resistance. Since liver is the major site of glucose and lipid metabolism, this study aimed to understand the effects of SCAAs and BCAAs supplementations on glucose and lipid metabolism in HepG2 cells. These cells were pretreated with SAMe, betaine, taurine, and BCAA for 24 h, followed by treatments of a high concentration of glucose (50 mM) or palmitic acid (PA, 0.5 mM) for 48 h to simulate high-glucose and high-fat environments. Pretreatment of BCAA and SCAAs inhibited the fat accumulation. At the transcriptional level, glucose and PA treatment led to significant increase of mRNA gluconeogenic enzyme. The mRNA expression level of GLUT2 was decreased by 20% in the SAMe-treated group and inhibited glucose synthesis by reducing the level of gluconeogenic enzyme. After SAMe or BCAA pretreatment, the mRNA expression of lipogenic enzymes was decreased. The PPAR-γ expression was increased after BCAA pretreatment, but SAMe not only downregulated the expression of PPAR-γ, but also inhibited the expression of ChREBP approximately 20% and SREBP-1c decreased by about 15%. Taken together, the effect of SAMe on glucose and lipid metabolism is significant especially on inhibiting hepatic lipogenesis and gluconeogenesis under the metabolic syndrome environment.

Methionine cycle in nonalcoholic fatty liver disease and its potential applications

As a chronic metabolic disease affecting epidemic proportions worldwide, the pathogenesis of Nonalcoholic Fatty Liver Disease (NAFLD) is not clear yet. There is also a lack of precise biomarkers and specific medicine for the diagnosis and treatment of NAFLD. Methionine metabolic cycle, which is critical for the maintaining of cellular methylation and redox state, is involved in the pathophysiology of NAFLD. However, the molecular basis and mechanism of methionine metabolism in NAFLD are not completely understood. Here, we mainly focus on specific enzymes that participates in methionine cycle, to reveal their interconnections with NAFLD, in order to recognize the pathogenesis of NAFLD from a new angle and at the same time, explore the clinical characteristics and therapeutic strategies.

Methionine adenosyltransferase 1a antisense oligonucleotides activate the liver-brown adipose tissue axis preventing obesity and associated hepatosteatosis

Altered methionine metabolism is associated with weight gain in obesity. The methionine adenosyltransferase (MAT), catalyzing the first reaction of the methionine cycle, plays an important role regulating lipid metabolism. However, its role in obesity, when a plethora of metabolic diseases occurs, is still unknown. By using antisense oligonucleotides (ASO) and genetic depletion of Mat1a, here, we demonstrate that Mat1a deficiency in diet-induce obese or genetically obese mice prevented and reversed obesity and obesity-associated insulin resistance and hepatosteatosis by increasing energy expenditure in a hepatocyte FGF21 dependent fashion. The increased NRF2-mediated FGF21 secretion induced by targeting Mat1a, mobilized plasma lipids towards the BAT to be catabolized, induced thermogenesis and reduced body weight, inhibiting hepatic de novo lipogenesis. The beneficial effects of Mat1a ASO were abolished following FGF21 depletion in hepatocytes. Thus, targeting Mat1a activates the liver-BAT axis by increasing NRF2-mediated FGF21 secretion, which prevents obesity, insulin resistance and hepatosteatosis.

Role of diacylglycerol O-acyltransferase (DGAT) isoforms in bovine hepatic fatty acid metabolism

Fatty acid accumulation in hepatocytes induced by high concentrations of fatty acids due to lipolysis and the associated oxidative damage they cause occur most frequently after calving. Because of their role in esterification of fatty acids, diacylglycerol acyltransferase isoforms (DGAT1 and DGAT2) could play a role in the susceptibility of dairy cows to develop fatty liver. To gain mechanistic insights, we performed in vivo and in vitro analyses using liver biopsies or isolated primary hepatocytes. The in vivo study (n = 5 cows/group) involved healthy cows [average liver triacylglycerol (TAG) = 0.78%; 0.58 to 0.93%, ratio of triglyceride weight to wet liver weight] or cows diagnosed with fatty liver (average TAG = 7.60%; 5.31 to 10.54%). In vitro, hepatocytes isolated from 3 healthy female calves (1 d old, 44 to 53 kg) were challenged with (fatty acids) or without (control) a 1.2 mM mixture of fatty acids in an attempt to induce metabolic stress. Furthermore, hepatocytes were treated with DGAT1 inhibitor or DGAT2 inhibitor for 2 h followed by a challenge with (DGAT1 inhibitor + fatty acids or DGAT2 inhibitor + fatty acids) or without (DGAT1 inhibitor or DGAT2 inhibitor) the 1.2 mM mixture of fatty acids for 12 h. Data analysis of liver biopsies was compared using a 2-tailed unpaired Student's t-test. Data from calf hepatocyte treatment comparisons were assessed by one-way ANOVA, and multiplicity for each experiment was adjusted by the Holm's procedure. Data indicated that both fatty liver and in vitro challenge with fatty acids were associated with greater mRNA and protein abundance of SREBF1, FASN, DGAT1, and DGAT2. In contrast, mRNA and protein abundance of CPT1A and very low-density lipoprotein synthesis-related proteins MTTP and APOB were markedly lower. However, compared with fatty acid challenge alone, DGAT1 inhibitor + fatty acids led to greater mRNA and protein abundance of CPT1A and APOB, and greater mRNA abundance of SREBF1 and MTTP. Furthermore, this treatment led to lower mRNA abundance of FASN and DGAT2 and TAG concentrations. Compared with fatty acid challenge alone, DGAT2 inhibitor + fatty acids led to greater mRNA and protein abundance of CPT1A, MTTP, and APOB, and lower mRNA and protein abundance of SREBF1 and FASN. In addition, compared with control and fatty acids, there was greater protein abundance of GRP78 and PERK in both DGAT1 and DGAT2 inhibitor with or without fatty acids. Furthermore, compared with control and fatty acids, reactive oxygen species concentrations in the DGAT1 inhibitor with or without fatty acid group was greater. Overall, data suggested that DGAT1 is particularly relevant in the context of hepatocyte TAG synthesis from exogenous fatty acids. Disruption of both DGAT1 and DGAT2 altered lipid homeostasis, channeling fatty acids toward oxidation and generation of reactive oxygen species. Both DGAT isoforms play a role in promoting fatty acid storage into TAG and lipid droplets to protect hepatocytes from oxidative damage.

Dysregulation of S-adenosylmethionine Metabolism in Nonalcoholic Steatohepatitis Leads to Polyamine Flux and Oxidative Stress

Nonalcoholic fatty liver disease (NAFLD) is the number one cause of chronic liver disease worldwide, with 25% of these patients developing nonalcoholic steatohepatitis (NASH). NASH significantly increases the risk of cirrhosis and decompensated liver failure. Past studies in rodent models have shown that glycine-N-methyltransferase (GNMT) knockout results in rapid steatosis, fibrosis, and hepatocellular carcinoma progression. However, the attenuation of GNMT in subjects with NASH and the molecular basis for its impact on the disease process is still unclear. To address this knowledge gap, we show the reduction of GNMT protein levels in the liver of NASH subjects compared to healthy controls. To gain insight into the impact of decreased GNMT in the disease process, we performed global label-free proteome studies on the livers from a murine modified amylin diet-based model of NASH. Histological and molecular characterization of the animal model demonstrate a high resemblance to human disease. We found that a reduction of GNMT leads to a significant increase in S-adenosylmethionine (AdoMet), an essential metabolite for transmethylation reactions and a substrate for polyamine synthesis. Further targeted proteomic and metabolomic studies demonstrated a decrease in GNMT transmethylation, increased flux through the polyamine pathway, and increased oxidative stress production contributing to NASH pathogenesis.

Mechanisms and Active Compounds Polysaccharides and Bibenzyls of Medicinal Dendrobiums for Diabetes Management

BACKGROUND

Medicinal dendrobiums are used popularly in traditional Chinese medicine for the treatment of diabetes, while their active compounds and mechanism remain unclear. This review aimed to evaluate the mechanism and active compounds of medicinal dendrobiums in diabetes management through a systematic approach.

METHODS

A systematic approach was conducted to search for the mechanism and active phytochemicals in Dendrobium responsible for anti-diabetic actions using databases PubMed, Embase, and SciFinder.

RESULTS

Current literature indicates polysaccharides, bibenzyls, phenanthrene, and alkaloids are commonly isolated in Dendrobium genusin which polysaccharides and bibenzyls are most aboundant. Many animal studies have shown that polysaccharides from the species of Dendrobium provide with antidiabetic effects by lowering glucose level and reversing chronic inflammation of T2DM taken orally at 200 mg/kg. Dendrobium polysaccharides protect pancreatic β-cell dysfunction and insulin resistance in liver. Dendrobium polysaccharides up-regulate the abundance of short-chain fatty acid to stimulate GLP-1 secretion through gut microbiota. Bibenzyls also have great potency to inhibit the progression of the chronic inflammation in cellular studies.

CONCLUSION

Polysaccharides and bibenzyls are the major active compounds in medicinal dendrobiums for diabetic management through the mechanisms of lowering glucose level and reversing chronic inflammation of T2DM by modulating pancreatic β-cell dysfunction and insulin resistance in liver as a result from gut microbita regulation.

S-ademetionine in the treatment of non-alcoholic fatty liver disease (NAFLD)

The experimental and clinic data supporting S- ademetionin application in NAFLD complex therapy were presented. The therapy corrects an oxidative stress in hepatocytes and transforms the nutrition behavior in patients with excessive body weight when depressive syndrome is developed.

E2F1 and E2F2-Mediated Repression of CPT2 Establishes a Lipid-Rich Tumor-Promoting Environment

Lipid metabolism rearrangements in nonalcoholic fatty liver disease (NAFLD) contribute to disease progression. NAFLD has emerged as a major risk for hepatocellular carcinoma (HCC), where metabolic reprogramming is a hallmark. Identification of metabolic drivers might reveal therapeutic targets to improve HCC treatment. Here, we investigated the contribution of transcription factors E2F1 and E2F2 to NAFLD-related HCC and their involvement in metabolic rewiring during disease progression. In mice receiving a high-fat diet (HFD) and diethylnitrosamine (DEN) administration, E2f1 and E2f2 expressions were increased in NAFLD-related HCC. In human NAFLD, E2F1 and E2F2 levels were also increased and positively correlated. E2f1 ^-/- and E2f2 ^-/- mice were resistant to DEN-HFD-induced hepatocarcinogenesis and associated lipid accumulation. Administration of DEN-HFD in E2f1 ^-/- and E2f2 ^-/- mice enhanced fatty acid oxidation (FAO) and increased expression of Cpt2, an enzyme essential for FAO, whose downregulation is linked to NAFLD-related hepatocarcinogenesis. These results were recapitulated following E2f2 knockdown in liver, and overexpression of E2f2 elicited opposing effects. E2F2 binding to the Cpt2 promoter was enhanced in DEN-HFD-administered mouse livers compared with controls, implying a direct role for E2F2 in transcriptional repression. In human HCC, E2F1 and E2F2 expressions inversely correlated with CPT2 expression. Collectively, these results indicate that activation of the E2F1-E2F2-CPT2 axis provides a lipid-rich environment required for hepatocarcinogenesis. SIGNIFICANCE: These findings identify E2F1 and E2F2 transcription factors as metabolic drivers of hepatocellular carcinoma, where deletion of just one is sufficient to prevent disease. GRAPHICAL ABSTRACT: http://cancerres.aacrjournals.org/content/canres/81/11/2874/F1.large.jpg.

Stable Isotopic Tracer Phospholipidomics Reveals Contributions of Key Phospholipid Biosynthetic Pathways to Low Hepatocyte Phosphatidylcholine to Phosphatidylethanolamine Ratio Induced by Free Fatty Acids

There is a strong association between hepatocyte phospholipid homeostasis and non-alcoholic fatty liver disease (NAFLD). The phosphatidylcholine to phosphatidylethanolamine ratio (PC/PE) often draws special attention as genetic and dietary disruptions to this ratio can provoke steatohepatitis and other signs of NAFLD. Here we demonstrated that excessive free fatty acid (1:2 mixture of palmitic and oleic acid) alone was able to significantly lower the phosphatidylcholine to phosphatidylethanolamine ratio, along with substantial alterations to phospholipid composition in rat hepatocytes. This involved both a decrease in hepatocyte phosphatidylcholine (less prominent) and an increase in phosphatidylethanolamine, with the latter contributing more to the lowered ratio. Stable isotopic tracer phospholipidomic analysis revealed several previously unidentified changes that were triggered by excessive free fatty acid. Importantly, the enhanced cytidine diphosphate (CDP)-ethanolamine pathway activity appeared to be driven by the increased supply of preferred fatty acid substrates. By contrast, the phosphatidylethanolamine N-methyl transferase (PEMT) pathway was restricted by low endogenous methionine and consequently low S-adenosylmethionine, which resulted in a concomitant decrease in phosphatidylcholine and accumulation of phosphatidylethanolamine. Overall, our study identified several previously unreported links in the relationship between hepatocyte free fatty acid overload, phospholipid homeostasis, and the development of NAFLD.

Postnatal exposure to DINP was associated with greater alterations of lipidomic markers for hepatic steatosis than DEHP in postweaning mice

The toxicity of the endocrine disruptor di(2-ethylhexyl) phthalate (DEHP) has been extensively studied for its hormonal dysregulation, obesogenic effect and associated metabolic diseases. DEHP's primary substitute di-isononyl phthalate (DINP), however, although increased in annual production globally, requires better understanding of its health effect. Our previous work reported disruptions in plasma lipid profiles, but the metabolic responses following phthalate exposure in the liver, particularly the entire hepatic lipidome, have been lacking. A targeted lipidomic technique was applied to accurately quantify a total of 363 lipid species in the liver of neonatal mice after exposure to a daily dose of 4.8 mg/kg body weight/day from birth throughout lactation. Distinct patterns of disruption for each sum of lipid classes or sub-classes between the genders were the most noticeable. Following DINP administration, female pups were subject to greater changes in phosphatidylethanolamines, bis(monoacylglycero)phosphate and ceramides. In contrast, the males exhibited less changes in the phosphoglycerol backbone-based molecules, whereas glycerol and cholesterol esters were more disrupted by DINP. DEHP, however, induced less changes overall compared to DINP. These findings highlighted the predominant lipidomic disruption of DINP on glycerol (diacylglycerides and triacylglycerides) and/or cholesterol (in ester or free form) molecules in neonatal mice across genders, suggesting the genesis of hepatic steatosis occurring at as early as post weaning. Collectively, these findings question the suitability of DINP as a safe DEHP substitute and warrant further investigation on longer-term exposure to elucidate its effect on chronic liver diseases.

Duodenal-jejunal bypass maintains hepatic S-adenosylmethionine/S-homocysteine ratio in diet-induced obese rats

We previously reported that the duodenal-jejunal bypass (DJB) surgery altered transsulfuration and purine metabolism via flux changes in 1-carbon metabolism in the liver. In this study, we extended our study to gain further insight into mechanistic details of how the DJB-induced flux changes in 1-carbon metabolism contributes to the improvement of diet-induced nonalcoholic fatty liver disease. Rodents were subjected to surgical (sham operation and DJB) or dietary (reduced food supply to follow the weight changes in the DJB group) interventions. The microscopic features of the liver were examined by immunohistochemistry. The expressions of genes in lipid synthesis and in 1-carbon cycle in the liver were analyzed by real-time polymerase chain reaction and western blotting. Metabolic changes in the liver were determined. We observed that DJB reduces hepatic steatosis and improves insulin sensitivity in both high-fat diet-fed rats and mice. Metabolic analyses revealed that the possible underlying mechanism may involve decreased S-adenosylmethionine (SAM)-to-S-adenosylhomocysteine ratio via downregulation of SAM synthesizing enzyme and upregulation of SAM catabolizing enzyme. We also found in mice that DJB-mediated attenuation of hepatic steatosis is independent of weight loss. DJB also increased hepatic expression levels of GNMT while decreasing those of PEMT and BHMT, a change in 1-carbon metabolism that may decrease the ratio of SAM to S-adenosylhomocysteine, thereby resulting in the prevention of fat accumulation in the liver. Thus, we suggest that the change in 1-carbon metabolism, especially the SAM metabolism, may contribute to the improvement of diet-induced fatty liver disease after DJB surgery.

Juice from Fructus Rosae Roxburghii normalizes blood lipids in mice with diet-induced hyperlipidemia*†

Fructus Rosae Roxburghii (FRR) as a dietary supplement is considered to possess anti-atherosclerosis (AS), and hyperlipidemia (HLP) is material basis for AS formation, so the effects and molecular mechanism of FRR on diet-induced hyperlipidemic mice were explored. In Diet IV2 group, hepatic steatosis was significantly relieved; meanwhile, TC, TG, LDL-C, HDL-C, and ASI in serum were regulated to control level. Thirty-seven DCEG in Diet I, Diet II, and Diet IV2 groups were obtained by RNA-seq analysis. Relative mRNA levels were further determined by qRT-PCR, of which 28 genes were matched with those detected by RNA-seq. Ten DCEP were verified by targeted quantitative proteomic analysis, but expressive patterns of only six proteins were correlated with qRT-PCR data. These DCEG and DCEP played important roles in regulating the biosynthesis of BAs and steroids, fatty acid metabolism, and LPO production. They might cooperatively regulate the function of HDL or RCT by PPAR signaling pathway under the FRR action. As we know, it is the first time the potential anti-atherosclerotic mechanism of FRR regulating the blood lipids was explored.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) dysregulates hepatic one carbon metabolism during the progression of steatosis to steatohepatitis with fibrosis in mice

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a persistent environmental contaminant, induces steatosis that can progress to steatohepatitis with fibrosis, pathologies that parallel stages in the development of non-alcoholic fatty liver disease (NAFLD). Coincidently, one carbon metabolism (OCM) gene expression and metabolites are often altered during NAFLD progression. In this study, the time- and dose-dependent effects of TCDD were examined on hepatic OCM in mice. Despite AhR ChIP-seq enrichment at 2 h, OCM gene expression was not changed within 72 h following a bolus dose of TCDD. Dose-dependent repression of methionine adenosyltransferase 1A (Mat1a), adenosylhomocysteinase (Achy) and betaine-homocysteine S-methyltransferase (Bhmt) mRNA and protein levels following repeated treatments were greater at 28 days compared to 8 days. Accordingly, levels of methionine, betaine, and homocysteic acid were dose-dependently increased, while S-adenosylmethionine, S-adenosylhomocysteine, and cystathionine exhibited non-monotonic dose-dependent responses consistent with regulation by OCM intermediates and repression of glycine N-methyltransferase (Gnmt). However, the dose-dependent effects on SAM-dependent metabolism of polyamines and creatine could not be directly attributed to alterations in SAM levels. Collectively, these results demonstrate persistent AhR activation disrupts hepatic OCM metabolism at the transcript, protein and metabolite levels within context of TCDD-elicited progression of steatosis to steatohepatitis with fibrosis.

Folic acid supplementation prevents high fructose-induced non-alcoholic fatty liver disease by activating the AMPK and LKB1 signaling pathways

BACKGROUND/OBJECTIVES

The present study aimed to evaluate the effects of folic acid supplementation in high-fructose-induced hepatic steatosis and clarify the underlying mechanism of folic acid supplementation.

MATERIALS/METHODS

Male SD rats were fed control, 64% high-fructose diet, or 64% high-fructose diet with folic acid for eight weeks. Plasma glutamate-pyruvate transaminase, glutamate-oxaloacetate transaminase, lipid profiles, hepatic lipid content, S-adenosylmethionine (SAM), and S-adenosylhomocysteine (SAH) were measured.

RESULTS

The HF diet significantly increased hepatic total lipid and triglyceride (TG) and decreased hepatic SAM, SAH, and SAM:SAH ratio. In rats fed a high fructose diet, folic acid supplementation significantly reduced hepatic TG, increased hepatic SAM, and alleviated hepatic steatosis. Moreover, folic acid supplementation in rats fed high fructose enhanced the levels of phosphorylated AMP-activated protein kinase (AMPK) and liver kinase B (LKB1) and inhibited phosphorylation of acetyl coenzyme A carboxylase (ACC) in the liver.

CONCLUSIONS

These results suggest that the protective effect of folic acid supplementation in rats fed high fructose may include the activation of LKB1/AMPK/ACC and increased SAM in the liver, which inhibit hepatic lipogenesis, thus ameliorating hepatic steatosis. The present study may provide evidence for the beneficial effects of folic acid supplementation in the treatment of non-alcoholic fatty liver disease.

Molecular Mechanisms Regulating Obesity-Associated Hepatocellular Carcinoma

Obesity is a global, intractable issue, altering inflammatory and stress response pathways, and promoting tissue adiposity and tumorigenesis. Visceral fat accumulation is correlated with primary tumor recurrence, poor prognosis and chemotherapeutic resistance. Accumulating evidence highlights a close association between obesity and an increased incidence of hepatocellular carcinoma (HCC). Obesity drives HCC, and obesity-associated tumorigenesis develops via nonalcoholic fatty liver (NAFL), progressing to nonalcoholic steatohepatitis (NASH) and ultimately to HCC. The better molecular elucidation and proteogenomic characterization of obesity-associated HCC might eventually open up potential therapeutic avenues. The mechanisms relating obesity and HCC are correlated with adipose tissue remodeling, alteration in the gut microbiome, genetic factors, ER stress, oxidative stress and epigenetic changes. During obesity-related hepatocarcinogenesis, adipokine secretion is dysregulated and the nuclear factor erythroid 2 related factor 1 (Nrf-1), nuclear factor kappa B (NF-κB), mammalian target of rapamycin (mTOR), phosphatidylinositol-3-kinase (PI3K)/phosphatase and tensin homolog (PTEN)/Akt, and Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathways are activated. This review captures the present trends allied with the molecular mechanisms involved in obesity-associated hepatic tumorigenesis, showcasing next generation molecular therapeutic strategies and their mechanisms for the successful treatment of HCC.

Folic acid attenuates high-fat diet-induced steatohepatitis via deacetylase SIRT1-dependent restoration of PPARα

BACKGROUND

Folic acid has been shown to improve non-alcoholic steatohepatitis (NASH), but its roles in hepatic lipid metabolism, hepatic one-carbon metabolism, and gut microbiota are still unknown.

AIM

To demonstrate the role of folic acid in lipid metabolism and gut microbiota in NASH.

METHODS

Twenty-four Sprague-Dawley rats were assigned into three groups: Chow diet, high-fat diet (HFD), and HFD with folic acid administration. At the end of 16 wk, the liver histology, the expression of hepatic genes related to lipid metabolism, one-carbon metabolism, and gut microbiota structure analysis of fecal samples based on 16S rRNA sequencing were measured to evaluate the effect of folic acid. Palmitic acid-exposed Huh7 cell line was used to evaluate the role of folic acid in hepatic lipid metabolism.

RESULTS

Folic acid treatment attenuated steatosis, lobular inflammation, and hepatocellular ballooning in rats with HFD-induced steatohepatitis. Genes related to lipid de novo lipogenesis, β-oxidation, and lipid uptake were improved in HFD-fed folic acid-treated rats. Furthermore, peroxisome proliferator-activated receptor alpha (PPARα) and silence information regulation factor 1 (SIRT1) were restored by folic acid in HFD-fed rats and palmitic acid-exposed Huh7 cell line. The restoration of PPARα by folic acid was blocked after transfection with SIRT1 siRNA in the Huh7 cell line. Additionally, folic acid administration ameliorated depleted hepatic one-carbon metabolism and restored the diversity of the gut microbiota in rats with HFD-induced steatohepatitis.

CONCLUSION

Folic acid improves hepatic lipid metabolism by upregulating PPARα levels via a SIRT1-dependent mechanism and restores hepatic one-carbon metabolism and diversity of gut microbiota, thereby attenuating HFD-induced NASH in rats.

One-Carbon Metabolism in Fatty Liver Disease and Fibrosis: One-Carbon to Rule Them All

Nonalcoholic fatty liver disease (NAFLD) is a term used to characterize a range of disease states that involve the accumulation of fat in the liver but are not associated with excessive alcohol consumption. NAFLD is a prevalent disease that can progress to organ damage like liver cirrhosis and hepatocellular carcinoma. Many animal models have demonstrated that one-carbon metabolism is strongly associated with NAFLD. Phosphatidylcholine is an important phospholipid that affects hepatic lipid homeostasis and de novo synthesis of this phospholipid is associated with NAFLD. However, one-carbon metabolism serves to support all cellular methylation reactions and catabolism of methionine, serine, glycine, choline, betaine, tryptophan, and histidine. Several different pathways within one-carbon metabolism that play important roles in regulating energy metabolism and immune function have received less attention in the study of fatty liver disease and fibrosis. This review examines what we have learned about hepatic lipid metabolism and liver damage from the study of one-carbon metabolism thus far and highlights unexplored opportunities for future research.

Targeting Hepatic Glutaminase 1 Ameliorates Non-alcoholic Steatohepatitis by Restoring Very-Low-Density Lipoprotein Triglyceride Assembly

Non-alcoholic steatohepatitis (NASH) is characterized by the accumulation of hepatic fat in an inflammatory/fibrotic background. Herein, we show that the hepatic high-activity glutaminase 1 isoform (GLS1) is overexpressed in NASH. Importantly, GLS1 inhibition reduces lipid content in choline and/or methionine deprivation-induced steatotic mouse primary hepatocytes, in human hepatocyte cell lines, and in NASH mouse livers. We suggest that under these circumstances, defective glutamine fueling of anaplerotic mitochondrial metabolism and concomitant reduction of oxidative stress promotes a reprogramming of serine metabolism, wherein serine is shifted from the generation of the antioxidant glutathione and channeled to provide one-carbon units to regenerate the methionine cycle. The restored methionine cycle can induce phosphatidylcholine synthesis from the phosphatidylethanolamine N-methyltransferase-mediated and CDP-choline pathways as well as by base-exchange reactions between phospholipids, thereby restoring hepatic phosphatidylcholine content and very-low-density lipoprotein export. Overall, we provide evidence that hepatic GLS1 targeting is a valuable therapeutic approach in NASH.

Methyl-Sensing Nuclear Receptor Liver Receptor Homolog-1 Regulates Mitochondrial Function in Mouse Hepatocytes

BACKGROUND AND AIMS

Liver receptor homolog-1 (LRH-1; NR5A2) is a nuclear receptor that regulates metabolic homeostasis in the liver. Previous studies identified phosphatidylcholines as potential endogenous agonist ligands for LRH-1. In the liver, distinct subsets of phosphatidylcholine species are generated by two different pathways: choline addition to phosphatidic acid through the Kennedy pathway and trimethylation of phosphatidylethanolamine through phosphatidylethanolamine N-methyl transferase (PEMT).

APPROACH AND RESULTS

Here, we report that a PEMT-LRH-1 pathway specifically couples methyl metabolism and mitochondrial activities in hepatocytes. We show that the loss of Lrh-1 reduces mitochondrial number, basal respiration, beta-oxidation, and adenosine triphosphate production in hepatocytes and decreases expression of mitochondrial biogenesis and beta-oxidation genes. In contrast, activation of LRH-1 by its phosphatidylcholine agonists exerts opposite effects. While disruption of the Kennedy pathway does not affect the LRH-1-mediated regulation of mitochondrial activities, genetic or pharmaceutical inhibition of the PEMT pathway recapitulates the effects of Lrh-1 knockdown on mitochondria. Furthermore, we show that S-adenosyl methionine, a cofactor required for PEMT, is sufficient to induce Lrh-1 transactivation and consequently mitochondrial biogenesis.

CONCLUSIONS

A PEMT-LRH-1 axis regulates mitochondrial biogenesis and beta-oxidation in hepatocytes.

NAFLD Preclinical Models: More than a Handful, Less of a Concern?

Non-alcoholic fatty liver disease (NAFLD) is a spectrum of liver diseases ranging from simple steatosis to non-alcoholic steatohepatitis, fibrosis, cirrhosis, and/or hepatocellular carcinoma. Due to its increasing prevalence, NAFLD is currently a major public health concern. Although a wide variety of preclinical models have contributed to better understanding the pathophysiology of NAFLD, it is not always obvious which model is best suitable for addressing a specific research question. This review provides insights into currently existing models, mainly focusing on murine models, which is of great importance to aid in the identification of novel therapeutic options for human NAFLD.

Impact of Phospholipid Transfer Protein in Lipid Metabolism and Cardiovascular Diseases

PLTP plays an important role in lipoprotein metabolism and cardiovascular disease development in humans; however, the mechanisms are still not completely understood. In mouse models, PLTP deficiency reduces cardiovascular disease, while its overexpression induces it. Therefore, we used mouse models to investigate the involved mechanisms. In this chapter, the recent main progresses in the field of PLTP research are summarized, and our focus is on the relationship between PLTP and lipoprotein metabolism, as well as PLTP and cardiovascular diseases.

Sphingolipids in Non-Alcoholic Fatty Liver Disease and Hepatocellular Carcinoma: Ceramide Turnover

Non-alcoholic fatty liver disease (NAFLD) has emerged as one of the main causes of chronic liver disease worldwide. NAFLD comprises a group of conditions characterized by the accumulation of hepatic lipids that can eventually lead to non-alcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and hepatocellular carcinoma (HCC), the fifth most common cancer type with a poor survival rate. In this context, several works have pointed out perturbations in lipid metabolism and, particularly, changes in bioactive sphingolipids, as a hallmark of NAFLD and derived HCC. In the present work, we have reviewed existing literature about sphingolipids and the development of NAFLD and NAFLD-derived HCC. During metabolic syndrome, considered a risk factor for steatosis development, an increase in ceramide and sphigosine-1-phosphate (S1P) have been reported. Likewise, other reports have highlighted that increased sphingomyelin and ceramide content is observed during steatosis and NASH. Ceramide also plays a role in liver fibrosis and cirrhosis, acting synergistically with S1P. Finally, during HCC, metabolic fluxes are redirected to reduce cellular ceramide levels whilst increasing S1P to support tumor growth.

DEHP and DINP Induce Tissue- and Gender-Specific Disturbances in Fatty Acid and Lipidomic Profiles in Neonatal Mice: A Comparative Study

Di-isononyl phthalate (DINP) is considered one of the main industrial alternatives to di(2-ethylhexyl)phthalate (DEHP), a well-known chemical with various toxic effects including the disruption with lipid metabolism. However, the potential effects of DINP on lipid metabolism have rarely been investigated in mammals. Our study demonstrated that exposure of neonatal mice to DEHP and DINP at a daily dose of 0.048 or 4.8 mg/kg from postnatal day 0 (PND0) to PND21 caused nonmonotonic as well as tissue- and gender-specific alterations of total fatty acid (FA) compositions in plasma, heart, and adipose tissues. However, the patterns of disruption differed between DEHP- and DINP-treated groups. On the basis of targeted lipidomic analyses, we further identified gender-specific alterations of eight lipid classes in plasma following DEHP or DINP exposure. At the higher dose, DEHP induced decreases in total phosphatidylcholines and phosphatidylinositol (PI) in females and increases in phosphatidylethanolamines (PEs) and triglycerides in males. By contrast, DINP at the higher dose caused alterations of PEs, PIs, phosphatidylserines, and cholesterols exclusively in male mice, but no changes were observed in female pups. Although the most significant dysregulation of lipid metabolism was often observed for the higher dose, the lower one could also disrupt lipid profiles and sometimes its effects may even be more significant than those induced by the higher dose. Our study for the first time identified tissue- and gender-specific disruptions of FA compositions and lipidomic profiles in mice neonatally exposed to DINP. These findings question the suitability of DINP as a safe DEHP substitute and lay a solid foundation for further elucidation of its effects on lipid metabolism and underlying mechanisms.

Metabolomics and Lipidomics Profiling Reveals Hypocholesterolemic and Hypolipidemic Effects of Arabinoxylan on Type 2 Diabetic Rats

Type 2 diabetes (T2D) is a pandemic disease chiefly characterized by hyperglycemia. In this study, the combination of serum lipidomic and metabolomic approach was employed to investigate the effect of arabinoxylan on type 2 diabetic rats and identify the critical biomarkers of T2D. Metabolomics analysis revealed that branched-chain amino acids, 12α-hydroxylated bile acids, ketone bodies, and several short- and long-chain acylcarnitines were significantly increased in T2D, whereas lysophosphatidylcholines (LPCs) were significantly decreased. Lipidomics analysis indicated T2D-related dyslipidemia was mainly associated with the increased levels of acetylcarnitine, free fatty acids (FFA), diacylglycerols, triacylglycerols, and cholesteryl esters and the decreased levels of some unsaturated phosphatidylcholines (less than 22 carbons). These variations indicated the disturbed amino acid and lipid metabolism in T2D, and the accumulation of incompletely oxidized lipid species might eventually contribute to impaired insulin action and glucose homeostasis. Arabinoxylan treatment decreased the concentrations of 12α-hydroxylated bile acids, carnitines, and FFAs and increased the levels of LPCs. The improved bile acid and lipid metabolism by arabinoxylan might be involved in the alleviation of hypercholesterolemia and hyperlipidemia in T2D.

Review article: drug-induced liver injury in the context of nonalcoholic fatty liver disease - a physiopathological and clinical integrated view

BACKGROUND

Nonalcoholic fatty disease (NAFLD) is the most common liver disease, since it is strongly associated with obesity and metabolic syndrome pandemics. NAFLD may affect drug disposal and has common pathophysiological mechanisms with drug-induced liver injury (DILI); this may predispose to hepatoxicity induced by certain drugs that share these pathophysiological mechanisms. In addition, drugs may trigger fatty liver and inflammation per se by mimicking NAFLD pathophysiological mechanisms.

AIMS

To provide a comprehensive update on (a) potential mechanisms whereby certain drugs can be more hepatotoxic in NAFLD patients, (b) the steatogenic effects of drugs, and (c) the mechanism involved in drug-induced steatohepatitis (DISH).

METHODS

A language- and date-unrestricted Medline literature search was conducted to identify pertinent basic and clinical studies on the topic.

RESULTS

Drugs can induce macrovesicular steatosis by mimicking NAFLD pathogenic factors, including insulin resistance and imbalance between fat gain and loss. Other forms of hepatic fat accumulation exist, such as microvesicular steatosis and phospholipidosis, and are mostly associated with acute mitochondrial dysfunction and defective lipophagy, respectively. Drug-induced mitochondrial dysfunction is also commonly involved in DISH. Patients with pre-existing NAFLD may be at higher risk of DILI induced by certain drugs, and polypharmacy in obese individuals to treat their comorbidities may be a contributing factor.

CONCLUSIONS

The relationship between DILI and NAFLD may be reciprocal: drugs can cause NAFLD by acting as steatogenic factors, and pre-existing NAFLD could be a predisposing condition for certain drugs to cause DILI. Polypharmacy associated with obesity might potentiate the association between this condition and DILI.

Chronic liver diseases and the potential use of S-adenosyl-L-methionine as a hepatoprotector

Chronic liver diseases result in overall deterioration of health status and changes in metabolism. The search for strategies to control and combat these hepatic diseases has witnessed a great boom in the last decades. Nutritional therapy for controlling and managing liver diseases may be a positive influence as it improves the function of the liver. In this review, we focus mainly on describing liver conditions such as nonalcoholic fatty liver disease, and intrahepatic cholestasis as well as using S-adenosyl-L-methionine as a dietary supplement and its potential alternative therapeutic effect to correct the hepatic dysfunction associated with these conditions.

Metabolomic Identification of Subtypes of Nonalcoholic Steatohepatitis

BACKGROUND & AIMS

Nonalcoholic fatty liver disease (NAFLD) is a consequence of defects in diverse metabolic pathways that involve hepatic accumulation of triglycerides. Features of these aberrations might determine whether NAFLD progresses to nonalcoholic steatohepatitis (NASH). We investigated whether the diverse defects observed in patients with NAFLD are caused by different NAFLD subtypes with specific serum metabolomic profiles, and whether these can distinguish patients with NASH from patients with simple steatosis.

METHODS

We collected liver and serum from methionine adenosyltransferase 1a knockout (MAT1A-KO) mice, which have chronically low levels of hepatic S-adenosylmethionine (SAMe) and spontaneously develop steatohepatitis, as well as C57Bl/6 mice (controls); the metabolomes of all samples were determined. We also analyzed serum metabolomes of 535 patients with biopsy-proven NAFLD (353 with simple steatosis and 182 with NASH) and compared them with serum metabolomes of mice. MAT1A-KO mice were also given SAMe (30 mg/kg/day for 8 weeks); liver samples were collected and analyzed histologically for steatohepatitis.

RESULTS

Livers of MAT1A-KO mice were characterized by high levels of triglycerides, diglycerides, fatty acids, ceramides, and oxidized fatty acids, as well as low levels of SAMe and downstream metabolites. There was a correlation between liver and serum metabolomes. We identified a serum metabolomic signature associated with MAT1A-KO mice that also was present in 49% of the patients; based on this signature, we identified 2 NAFLD subtypes. We identified specific panels of markers that could distinguish patients with NASH from patients with simple steatosis for each subtype of NAFLD. Administration of SAMe reduced features of steatohepatitis in MAT1A-KO mice.

CONCLUSIONS

In an analysis of serum metabolomes of patients with NAFLD and MAT1A-KO mice with steatohepatitis, we identified 2 major subtypes of NAFLD and markers that differentiate steatosis from NASH in each subtype. These might be used to monitor disease progression and identify therapeutic targets for patients.

The critical role of phosphatidylcholine and phosphatidylethanolamine metabolism in health and disease

Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are the most abundant phospholipids in all mammalian cell membranes. In the 1950s, Eugene Kennedy and co-workers performed groundbreaking research that established the general outline of many of the pathways of phospholipid biosynthesis. In recent years, the importance of phospholipid metabolism in regulating lipid, lipoprotein and whole-body energy metabolism has been demonstrated in numerous dietary studies and knockout animal models. The purpose of this review is to highlight the unappreciated impact of phospholipid metabolism on health and disease. Abnormally high, and abnormally low, cellular PC/PE molar ratios in various tissues can influence energy metabolism and have been linked to disease progression. For example, inhibition of hepatic PC synthesis impairs very low density lipoprotein secretion and changes in hepatic phospholipid composition have been linked to fatty liver disease and impaired liver regeneration after surgery. The relative abundance of PC and PE regulates the size and dynamics of lipid droplets. In mitochondria, changes in the PC/PE molar ratio affect energy production. We highlight data showing that changes in the PC and/or PE content of various tissues are implicated in metabolic disorders such as atherosclerosis, insulin resistance and obesity. This article is part of a Special Issue entitled: Membrane Lipid Therapy: Drugs Targeting Biomembranes edited by Pablo V. Escribá.

Sphingolipids and phospholipids in insulin resistance and related metabolic disorders

Obesity, insulin resistance, type 2 diabetes mellitus and cardiovascular disease form a metabolic disease continuum that has seen a dramatic increase in prevalence in developed and developing countries over the past two decades. Dyslipidaemia resulting from hypercaloric diets is a major contributor to the pathogenesis of metabolic disease, and lipid-lowering therapies are the main therapeutic option for this group of disorders. However, the fact that dysfunctional lipid metabolism extends far beyond cholesterol and triglycerides is becoming increasingly clear. Lipidomic studies and mouse models are helping to explain the complex interactions between diet, lipid metabolism and metabolic disease. These studies are not only improving our understanding of this complex biology, but are also identifying potential therapeutic avenues to combat this growing epidemic. This Review examines what is currently known about phospholipid and sphingolipid metabolism in the setting of obesity and how metabolic pathways are being modulated for therapeutic effect.

1-Carbon Cycle Metabolites Methylate Their Way to Fatty Liver

Fatty liver is a complex disease often accompanying metabolic syndrome and Type 2 diabetes mellitus (T2DM). Hepatosteatosis may have roots in multiple metabolic abnormalities. However, metabolic dysfunction in the 1-carbon cycle (1CC), which produces the methyl donor S-adenosylmethionine (SAM) and phosphatidylcholine (PC), induces hepatic lipogenesis in model systems. Human diseases where 1CC or PC synthesis is disrupted, such as alcoholism, congenital lipodystrophy, or cystic fibrosis, often present with fatty liver. Given that the 1CC is clearly linked to this disease, it is critical to understand how the individual metabolites drive mechanisms increasing stored hepatic lipids. In this review, I summarize evidence that ties the 1CC to fatty liver disease along with data proposing mechanisms for increased lipogenesis or decreased lipid export by phosphatidylcholine.

Gut Microbiota and Nonalcoholic Fatty Liver Disease: Insights on Mechanism and Application of Metabolomics

Gut microbiota are intricately involved in the development of obesity-related metabolic diseases such as nonalcoholic fatty liver disease (NAFLD), type 2 diabetes, and insulin resistance. In the current review, we discuss the role of gut microbiota in the development of NAFLD by focusing on the mechanisms of gut microbiota-mediated host energy metabolism, insulin resistance, regulation of bile acids and choline metabolism, as well as gut microbiota-targeted therapy. We also discuss the application of a metabolomic approach to characterize gut microbial metabotypes in NAFLD.

Serum S-adenosylmethionine, but not methionine, increases in response to overfeeding in humans

BACKGROUND

Plasma concentration of the methyl donor S-adenosylmethionine (SAM) is linearly associated with body mass index (BMI) and fat mass. As SAM is a high-energy compound and a sensor of cellular nutrient status, we hypothesized that SAM would increase with overfeeding.

METHODS

Forty normal to overweight men and women were overfed by 1250 kcal per day for 28 days.

RESULTS

Serum SAM increased from 106 to 130 nmol/l (P=0.006). In stratified analysis, only those with weight gain above the median (high-weight gainers; average weight gain 3.9±0.3 kg) had increased SAM (+42%, P=0.001), whereas low-weight gainers (weight gain 1.5±0.2 kg) did not (Pinteraction=0.018). Overfeeding did not alter serum concentrations of the SAM precursor, methionine or the products, S-adenosyl-homocysteine and homocysteine. The SAM/SAH (S-adenosylhomocysteine) ratio was unchanged in the total population, but increased in high-weight gainers (+52%, P=0.006, Pinteraction =0.005). Change in SAM correlated positively with change in weight (r=0.33, P=0.041) and fat mass (r=0.44, P=0.009), but not with change in protein intake or plasma methionine, glucose, insulin or low-density lipoprotein (LDL)-cholesterol.

CONCLUSION

Overfeeding raised serum SAM in proportion to the fat mass gained. The increase in SAM may help stabilize methionine levels, and denotes a responsiveness of SAM to nutrient state in humans. The role of SAM in human energy metabolism deserves further attention.