Hepatocyte Deletion of Triglyceride-Synthesis Enzyme Acyl CoA: Diacylglycerol Acyltransferase 2 Reduces Steatosis Without Increasing Inflammation or Fibrosis in Mice

Insights on post-translational modifications in fatty liver and fibrosis progression

Metabolic dysfunction-associated steatotic liver disease [MASLD] is a pervasive multifactorial health burden. Post-translational modifications [PTMs] of amino acid residues in protein domains demonstrate pivotal roles for imparting dynamic alterations in the cellular micro milieu. The crux of identifying novel druggable targets relies on comprehensively studying the etiology of metabolic disorders. This review article presents how different chemical moieties of various PTMs like phosphorylation, methylation, ubiquitination, glutathionylation, neddylation, acetylation, SUMOylation, lactylation, crotonylation, hydroxylation, glycosylation, citrullination, S-sulfhydration and succinylation presents the cause-effect contribution towards the MASLD spectra. Additionally, the therapeutic prospects in the management of liver steatosis and hepatic fibrosis via targeting PTMs and regulatory enzymes are also encapsulated. This review seeks to understand the function of protein modifications in progression and promote the markers discovery of diagnostic, prognostic and drug targets towards MASLD management which could also halt the progression of a catalogue of related diseases.

A computational model of the crosstalk between hepatocyte fatty acid metabolism and oxidative stress highlights the key enzymes, metabolites, and detoxification pathways in the context of MASLD

Metabolic dysfunction-associated steatotic liver disease (MASLD; formerly known as NAFLD) is a common liver disease worldwide and carries the risk of progressing to severe liver conditions, such as fibrosis and liver cancer. In the context of MASLD, evaluating fat accumulation in the liver and the subsequent production of oxidative stress is essential to understand the disease propagation. However, clinical studies using human patients to investigate the fat accumulation and the onset of oxidative stress in MASLD face ethical and technical challenges, highlighting the importance of alternative methods. To understand the relationship between fatty acid metabolism, lipid accumulation, oxidative stress generation, and antioxidant mechanisms in hepatocytes, we proposed a new mathematical model. The importance of this model lies in its ability to track the time-dependent changes in oxidative stress and glutathione concentration in response to the input of fatty acids. Furthermore, the model allows for the evaluation of the effects of altering the activity of the key enzymes involved in those mechanisms. Our model is anticipated to provide new insights into MASLD therapy strategies by identifying key pathways and predicting the effects of drug-induced changes in enzyme activity.

Silybin A from Silybum marianum reprograms lipid metabolism to induce a cell fate-dependent class switch from triglycerides to phospholipids

Rationale: Silybum marianum is used to protect against degenerative liver damage. The molecular mechanisms of its bioactive component, silybin, remained enigmatic, although membrane-stabilizing properties, modulation of membrane protein function, and metabolic regulation have been discussed for decades. Methods: Experiments were performed with hepatocyte cell lines and primary monocytes in vitro under both basal and stressed conditions, and in mice in vivo. Quantitative lipidomics was used to detect changes in phospholipids and triglycerides. Key findings were confirmed by Western blotting, quantitative PCR, microscopy, enzyme activity assays, metabolic flux studies, and functional relationships were investigated using selective inhibitors. Results: We show that specifically the stereoisomer silybin A decreases triglyceride levels and lipid droplet content, while enriching major phospholipid classes and maintaining a homeostatic phospholipid composition in human hepatocytes in vitro and in mouse liver in vivo under normal and pre-disease conditions. Conversely, in cell-based disease models of lipid overload and lipotoxic stress, silybin treatment primarily depletes triglycerides. Mechanistically, silymarin/silybin suppresses phospholipid-degrading enzymes, induces phospholipid biosynthesis to varying degrees depending on the conditions, and down-regulates triglyceride remodeling/biosynthesis, while inducing complex changes in sterol and fatty acid metabolism. Structure-activity relationship studies highlight the importance of the 1,4-benzodioxane ring configuration of silybin A in triglyceride reduction and the saturated 2,3-bond of the flavanonol moiety in phospholipid accumulation. Enrichment of hepatic phospholipids and intracellular membrane expansion are associated with a heightened biotransformation capacity. Conclusion: Our study deciphers the structural features of silybin contributing to hepatic lipid remodeling and suggests that silymarin/silybin protects the liver in individuals with mild metabolic dysregulation, involving a lipid class switch from triglycerides to phospholipids, whereas it may be less effective in disease states associated with severe metabolic dysregulation.

Circadian Influences on Brain Lipid Metabolism and Neurodegenerative Diseases

Circadian rhythms are intrinsic, 24 h cycles that regulate key physiological, mental, and behavioral processes, including sleep-wake cycles, hormone secretion, and metabolism. These rhythms are controlled by the brain's suprachiasmatic nucleus, which synchronizes with environmental signals, such as light and temperature, and consequently maintains alignment with the day-night cycle. Molecular feedback loops, driven by core circadian "clock genes", such as Clock, Bmal1, Per, and Cry, are essential for rhythmic gene expression; disruptions in these feedback loops are associated with various health issues. Dysregulated lipid metabolism in the brain has been implicated in the pathogenesis of neurological disorders by contributing to oxidative stress, neuroinflammation, and synaptic dysfunction, as observed in conditions such as Alzheimer's and Parkinson's diseases. Disruptions in circadian gene expression have been shown to perturb lipid regulatory mechanisms in the brain, thereby triggering neuroinflammatory responses and oxidative damage. This review synthesizes current insights into the interconnections between circadian rhythms and lipid metabolism, with a focus on their roles in neurological health and disease. It further examines how the desynchronization of circadian genes affects lipid metabolism and explores the potential mechanisms through which disrupted circadian signaling might contribute to the pathophysiology of neurodegenerative disorders.

Zebrafish are resilient to the loss of major diacylglycerol acyltransferase enzymes

In zebrafish, maternally deposited yolk is the source of nutrients for embryogenesis prior to digestive system maturation. Yolk nutrients are processed and secreted to the growing organism by an extra-embryonic tissue, the yolk syncytial layer (YSL). The export of lipids from the YSL occurs through the production of triacylglycerol-rich lipoproteins. Here we report that mutations in the triacylglycerol synthesis enzyme, diacylglycerol acyltransferase-2 (Dgat2), cause yolk sac opacity due to aberrant accumulation of cytoplasmic lipid droplets in the YSL. Although triacylglycerol synthesis continues, it is not properly coupled to lipoprotein production as dgat2 mutants produce fewer, smaller, ApoB-containing lipoproteins. Unlike DGAT2-null mice, which are lipopenic and die soon after birth, zebrafish dgat2 mutants are viable, fertile, and exhibit normal mass and adiposity. Residual Dgat activity cannot be explained by the activity of other known Dgat isoenzymes, as dgat1a;dgat1b;dgat2 triple mutants continue to produce YSL lipid droplets and remain viable as adults. Further, the newly identified diacylglycerol acyltransferase, Tmem68, is also not responsible for the residual triacylglycerol synthesis activity. Unlike overexpression of Dgat1a and Dgat1b, monoacylglycerol acyltransferase-3 (Mogat3b) overexpression does not rescue yolk opacity, suggesting it does not possess Dgat activity in the YSL. However, mogat3b;dgat2 double mutants exhibit increased yolk opacity and often have structural alterations of the yolk extension. Quadruple mogat3b;dgat1a;dgat1b;dgat2 mutants either have severely reduced viability and stunted growth or do not survive past 3 days post fertilization, depending on the dgat2 mutant allele present. Our study highlights the remarkable ability of vertebrates to synthesize triacylglycerol through multiple biosynthetic pathways.

Mechanisms coupling lipid droplets to MASLD pathophysiology

Hepatic steatosis, the buildup of neutral lipids in lipid droplets (LDs), is commonly referred to as metabolic dysfunction-associated steatotic liver disease when alcohol or viral infections are not involved. Metabolic dysfunction-associated steatotic liver disease encompasses simple steatosis and the more severe metabolic dysfunction-associated steatohepatitis, characterized by inflammation, hepatocyte injury, and fibrosis. Previously viewed as inert markers of disease, LDs are now understood to play active roles in disease etiology and have significant nonpathological and pathological functions in cell signaling and function. These dynamic properties of LDs are tightly regulated by hundreds of proteins that coat the LD surface, controlling lipid metabolism, trafficking, and signaling. The following review highlights various facets of LD biology with the primary goal of discussing key mechanisms through which LDs promote the development of advanced liver diseases, including metabolic dysfunction-associated steatohepatitis.

Disruption of the peripheral biological clock may play a role in sleep deprivation-induced dysregulation of lipid metabolism in both the daytime and nighttime phases

STUDY OBJECTIVES

This study aimed to examine the effect of sleep deprivation (SD) on lipid metabolism or lipid metabolism regulation in the liver and white adipose tissue (WAT) during the light and dark phases and explored the possible mechanisms underlying the diurnal effect of SD on lipid metabolism associated with clock genes.

METHODS

Male C57BL/6J mice aged 2 months were deprived of sleep daily for 20 h for ten consecutive days with weakly forced locomotion. The body weights and food consumption levels of the SD and control mice were recorded, and the mice were then sacrificed at ZT (zeitgeber time) 2 and ZT 14. The peripheral clock genes, enzymes involved in fat synthesis and catabolism in the WAT, and melatonin signalling pathway-mediated lipid metabolism in the liver were assessed. Untargeted metabolomics and tandem mass tag (TMT) proteomics were used to identify differential lipid metabolism pathways in the liver.

RESULTS

Bodyweight gain and daily food consumption were dramatically elevated after SD. Profound disruptions in the diurnal regulation of the hepatic peripheral clock and enzymes involved in fat synthesis and catabolism in the WAT were observed, with a strong emphasis on hepatic lipid metabolic pathways, while melatonin signalling pathway-mediated lipid metabolism exhibited moderate changes.

CONCLUSIONS

In mice, ten consecutive days of SD increased body weight gain and daily food consumption. In addition, SD profoundly disrupted lipid metabolism in the WAT and liver during the light and dark periods. These diurnal changes may be related to disorders of the peripheral biological clock.

Protective effect of alpha-tocopherol on lipogenesis and oxysterol production in hypercholesterolemia-induced nonalcoholic steatohepatitis

Despite limited number of studies, oxysterols are known to contribute to the progression of nonalcoholic steatohepatitis (NASH) by affecting lipid/cholesterol metabolism and elevating proinflammatory and profibrotic processes. Accordingly, we used a high cholesterol-mediated in vivo NASH model and aimed to determine alterations in fatty acid content and oxysterol levels together with their effects on cholesterol/lipid metabolism during the progression of the disease. We further investigated the beneficial role of α-tocopherol. To this end, in our hypercholesterolemic rabbit model, we determined fatty acid profile by GC-MS while 25-, 27-, 4β-, 7α, and 24(S)-Hydroxycholesterol levels by means of LC-MS/MS. Additionally, lipid (SREBP-1c, PPARα, PPARγ) and cholesterol metabolism-related proteins (LXRα, SREBP2 and ABCA1) were determined by immunoblotting. In conclusion, the present findings provide a complete analysis of the hepatic alterations in lipid and oxysterol profiles mediated by a high-cholesterol diet. In addition, this study explains the protective effect of α-tocopherol on lipogenesis and oxysterol production in hypercholesterolemia-induced NASH. We believe that present study will guide to novel theories in the progression and therapeutic targeting of fatty liver diseases.

DGAT2 Plays a Crucial Role to Control ESRRA-PROX1 Transcriptional Network to Maintain Hepatic Mitochondrial Sustainability

BACKGRUOUND

Diacylglycerol O-acyltransferase 2 (DGAT2) synthesizes triacylglycerol (TG) from diacylglycerol; therefore, DGAT2 is considered as a therapeutic target for steatosis. However, the consequence of inhibiting DGAT2 is not fully investigated due to side effects including lethality and lipotoxicity. In this article, we observed the role of DGAT2 in hepatocarcinoma.

METHODS

The role of DGAT2 is analyzed via loss-of-function assay. DGAT2 knockdown (KD) and inhibitor treatment on HepG2 cell line was analyzed. Cumulative analysis of cell metabolism with bioinformatic data were assessed, and further compared with different cohorts of liver cancer patients and non-alcoholic fatty liver disease (NAFLD) patients to elucidate how DGAT2 is regulating cancer metabolism.

RESULTS

Mitochondrial function is suppressed in DGAT2 KD HepG2 cell along with the decreased lipid droplets. In the aspect of the cancer, DGAT2 KD upregulates cell proliferation. Analyzing transcriptome of NAFLD and hepatocellular carcinoma (HCC) patients highlights negatively correlating expression patterns of 73 lipid-associated genes including DGAT2. Cancer patients with the lower DGAT2 expression face lower survival rate. DGAT2 KD cell and patients' transcriptome show downregulation in estrogen- related receptor alpha (ESRRA) via integrated system for motif activity response analysis (ISMARA), with increased dimerization with corepressor prospero homeobox 1 (PROX1).

CONCLUSION

DGAT2 sustains the stability of mitochondria in hepatoma via suppressing ESRRA-PROX1 transcriptional network and hinders HCC from shifting towards glycolytic metabolism, which lowers cell proliferation.

DGAT1 and DGAT2 Inhibitors for Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) Management: Benefits for Their Single or Combined Application

Inhibiting diacylglycerol acetyltransferase (DGAT1, DGAT2) enzymes (iDGAT1, iDGAT2), involved in triglyceride (TG) synthesis, improves hepatic steatosis in metabolic dysfunction-associated steatotic liver disease (MASLD) patients. However, their potential synergism in disease onset (SLD) and progression (metabolic dysfunction-associated steatohepatitis, fibrosis) has been poorly explored. We investigated iDGAT1 and iDGAT2 efficacy, alone or combined (iDGAT1/2) on fat accumulation and hepatocellular injury in hepatocytes (HepG2) and on fibrogenic processes in hepatic stellate cells (LX2). We further tested whether the addition of MitoQ antioxidant to iDGAT1/2 would enhance their effects. SLD and MASH conditions were reproduced in vitro by supplementing Dulbecco's Modified Eagle's Medium (DMEM) with palmitic/oleic acids (PAOA) alone (SLD-medium), or plus Lipopolisaccaride (LPS), fructose, and glucose (MASH-medium). In SLD-medium, iDGAT1 and iDGAT2 individually, and even more in combination, reduced TG synthesis in HepG2 cells. Markers of hepatocellular damage were slightly decreased after single iDGAT exposure. Conversely, iDGAT1/2 counteracted ER/oxidative stress and inflammation and enhanced mitochondrial Tricarboxylic acid cycle (TCA) and respiration. In HepG2 cells under a MASH-like condition, only iDGAT1/2 effectively ameliorated TG content and oxidative and inflammatory mediators, further improving bioenergetic balance. LX2 cells, challenged with SLD/MASH media, showed less proliferation and slower migration rates in response to iDGAT1/2 drugs. MitoQ combined with iDGAT1/2 improved cell viability and dampened free fatty acid release by stimulating β-oxidation. Dual DGAT inhibition combined with antioxidants open new perspectives for MASLD management.

Metabolic Dysfunction-Associated Steatotic Liver Disease: From Pathogenesis to Current Therapeutic Options

The epidemiological burden of liver steatosis associated with metabolic diseases is continuously growing worldwide and in all age classes. This condition generates possible progression of liver damage (i.e., inflammation, fibrosis, cirrhosis, hepatocellular carcinoma) but also independently increases the risk of cardio-metabolic diseases and cancer. In recent years, the terminological evolution from "nonalcoholic fatty liver disease" (NAFLD) to "metabolic dysfunction-associated fatty liver disease" (MAFLD) and, finally, "metabolic dysfunction-associated steatotic liver disease" (MASLD) has been paralleled by increased knowledge of mechanisms linking local (i.e., hepatic) and systemic pathogenic pathways. As a consequence, the need for an appropriate classification of individual phenotypes has been oriented to the investigation of innovative therapeutic tools. Besides the well-known role for lifestyle change, a number of pharmacological approaches have been explored, ranging from antidiabetic drugs to agonists acting on the gut-liver axis and at a systemic level (mainly farnesoid X receptor (FXR) agonists, PPAR agonists, thyroid hormone receptor agonists), anti-fibrotic and anti-inflammatory agents. The intrinsically complex pathophysiological history of MASLD makes the selection of a single effective treatment a major challenge, so far. In this evolving scenario, the cooperation between different stakeholders (including subjects at risk, health professionals, and pharmaceutical industries) could significantly improve the management of disease and the implementation of primary and secondary prevention measures. The high healthcare burden associated with MASLD makes the search for new, effective, and safe drugs a major pressing need, together with an accurate characterization of individual phenotypes. Recent and promising advances indicate that we may soon enter the era of precise and personalized therapy for MASLD/MASH.

Hepatokine ITIH3 protects against hepatic steatosis by downregulating mitochondrial bioenergetics and de novo lipogenesis

Recent studies demonstrate that liver secretory proteins, also known as hepatokines, regulate normal development, obesity, and simple steatosis to non-alcoholic steatohepatitis (NASH) progression. Using a panel of ∼100 diverse inbred strains of mice and a cohort of bariatric surgery patients, we found that one such hepatokine, inter-trypsin inhibitor heavy chain 3 (ITIH3), was progressively lower in severe non-alcoholic fatty liver disease (NAFLD) disease states highlighting an inverse relationship between Itih3/ITIH3 expression and NAFLD severity. Follow-up animal and cell culture models demonstrated that hepatic ITIH3 overexpression lowered liver triglyceride and lipid droplet accumulation, respectively. Conversely, ITIH3 knockdown in mice increased the liver triglyceride in two independent NAFLD models. Mechanistically, ITIH3 reduced mitochondrial respiration and this, in turn, reduced liver triglycerides, via downregulated de novo lipogenesis. This was accompanied by increased STAT1 signaling and Stat3 expression, both of which are known to protect against NAFLD/NASH. Our findings indicate hepatokine ITIH3 as a potential biomarker and/or treatment for NAFLD.

Upregulation of Hepatic Glutathione S-Transferase Alpha 1 Ameliorates Metabolic Dysfunction-Associated Steatosis by Degrading Fatty Acid Binding Protein 1

Metabolic dysfunction-associated steatotic liver disease (MASLD) is the most common metabolic disease of the liver, characterized by hepatic steatosis in more than 5% of hepatocytes. However, despite the recent approval of the first drug, resmetirom, for the management of metabolic dysfunction-associated steatohepatitis, decades of target exploration and hundreds of clinical trials have failed, highlighting the urgent need to find new druggable targets for the discovery of innovative drug candidates against MASLD. Here, we found that glutathione S-transferase alpha 1 (GSTA1) expression was negatively associated with lipid droplet accumulation in vitro and in vivo. Overexpression of GSTA1 significantly attenuated oleic acid-induced steatosis in hepatocytes or high-fat diet-induced steatosis in the mouse liver. The hepatoprotective and anti-inflammatory drug bicyclol also attenuated steatosis by upregulating GSTA1 expression. A detailed mechanism showed that GSTA1 directly interacts with fatty acid binding protein 1 (FABP1) and facilitates the degradation of FABP1, thereby inhibiting intracellular triglyceride synthesis by impeding the uptake and transportation of free fatty acids. Conclusion: GSTA1 may be a good target for the discovery of innovative drug candidates as GSTA1 stabilizers or enhancers against MASLD.

Examining the Pathogenesis of MAFLD and the Medicinal Properties of Natural Products from a Metabolic Perspective

Metabolic-associated fatty liver disease (MAFLD), characterized primarily by hepatic steatosis, has become the most prevalent liver disease worldwide, affecting approximately two-fifths of the global population. The pathogenesis of MAFLD is extremely complex, and to date, there are no approved therapeutic drugs for clinical use. Considerable evidence indicates that various metabolic disorders play a pivotal role in the progression of MAFLD, including lipids, carbohydrates, amino acids, and micronutrients. In recent years, the medicinal properties of natural products have attracted widespread attention, and numerous studies have reported their efficacy in ameliorating metabolic disorders and subsequently alleviating MAFLD. This review aims to summarize the metabolic-associated pathological mechanisms of MAFLD, as well as the natural products that regulate metabolic pathways to alleviate MAFLD.

The role of DGAT1 and DGAT2 in regulating tumor cell growth and their potential clinical implications

Lipid metabolism is widely reprogrammed in tumor cells. Lipid droplet is a common organelle existing in most mammal cells, and its complex and dynamic functions in maintaining redox and metabolic balance, regulating endoplasmic reticulum stress, modulating chemoresistance, and providing essential biomolecules and ATP have been well established in tumor cells. The balance between lipid droplet formation and catabolism is critical to maintaining energy metabolism in tumor cells, while the process of energy metabolism affects various functions essential for tumor growth. The imbalance of synthesis and catabolism of fatty acids in tumor cells leads to the alteration of lipid droplet content in tumor cells. Diacylglycerol acyltransferase 1 and diacylglycerol acyltransferase 2, the enzymes that catalyze the final step of triglyceride synthesis, participate in the formation of lipid droplets in tumor cells and in the regulation of cell proliferation, migration and invasion, chemoresistance, and prognosis in tumor. Several diacylglycerol acyltransferase 1 and diacylglycerol acyltransferase 2 inhibitors have been developed over the past decade and have shown anti-tumor effects in preclinical tumor models and improvement of metabolism in clinical trials. In this review, we highlight key features of fatty acid metabolism and different paradigms of diacylglycerol acyltransferase 1 and diacylglycerol acyltransferase 2 activities on cell proliferation, migration, chemoresistance, and prognosis in tumor, with the hope that these scientific findings will have potential clinical implications.

DGAT2 inhibition blocks SREBP-1 cleavage and improves hepatic steatosis by increasing phosphatidylethanolamine in the ER

Diacylglycerol acyltransferase 2 (DGAT2) catalyzes the final step of triglyceride (TG) synthesis. DGAT2 deletion in mice lowers liver TGs, and DGAT2 inhibitors are under investigation for the treatment of fatty liver disease. Here, we show that DGAT2 inhibition also suppressed SREBP-1 cleavage, reduced fatty acid synthesis, and lowered TG accumulation and secretion from liver. DGAT2 inhibition increased phosphatidylethanolamine (PE) levels in the endoplasmic reticulum (ER) and inhibited SREBP-1 cleavage, while DGAT2 overexpression lowered ER PE concentrations and increased SREBP-1 cleavage in vivo. ER enrichment with PE blocked SREBP-1 cleavage independent of Insigs, which are ER proteins that normally retain SREBPs in the ER. Thus, inhibition of DGAT2 shunted diacylglycerol into phospholipid synthesis, increasing the PE content of the ER, resulting in reduced SREBP-1 cleavage and less hepatic steatosis. This study reveals a new mechanism that regulates SREBP-1 activation and lipogenesis that is independent of sterols and SREBP-2 in liver.

Lipid droplets in steatotic liver disease

PURPOSE OF REVIEW

This review aims to discuss the most recent evidence exploring the role of lipid droplets in steatotic liver disease (SLD). We highlight the breadth of mechanisms by which lipid droplets may contribute to the progression of SLD with a particular focus on the role of lipid droplets as inducers of mechanical stress within hepatocytes and genetic mutations in lipid droplet associated proteins. Finally, this review provides an update on clinical trials exploring the therapeutic potential and strategies targeting lipid droplets.

RECENT FINDINGS

The size, composition and location of hepatic lipid droplets strongly influence the pathological role of these organelles in SLD. Emerging studies are beginning to elucidate the importance of lipid droplet induced hepatocyte mechanical stress. Novel strategies targeting lipid droplets, including the effects of lipid droplet associated protein mutations, show promising therapeutic potential.

SUMMARY

Much more than a histological feature, lipid droplets are complex heterogenous organelles crucial to cellular metabolism with important causative roles in the development and progression of SLD. Lipid droplet induced mechanical stress may exacerbate hepatic inflammation and fibrogenesis and potentially contribute to the development of a pro-carcinogenic hepatic environment. The integration of advancements in genetics and molecular biology in upcoming treatments aspires to transcend symptomatic alleviation and address the fundamental causes and pathological development of SLD.

Investigation of pharmacokinetic drug interaction between clesacostat and DGAT2 inhibitor ervogastat in healthy adult participants

Co-administration of clesacostat (acetyl-CoA carboxylase inhibitor, PF-05221304) and ervogastat (diacylglycerol O-acyltransferase inhibitor, PF-06865571) in laboratory models improved non-alcoholic fatty liver disease (NAFLD)/non-alcoholic steatohepatitis (NASH) end points and mitigated clesacostat-induced elevations in circulating triglycerides. Clesacostat is cleared via organic anion-transporting polypeptide-mediated hepatic uptake and cytochrome P450 family 3A (CYP3A); in vitro clesacostat is identified as a potential CYP3A time-dependent inactivator. In vitro ervogastat is identified as a substrate and potential inducer of CYP3A. Prior to longer-term efficacy trials in participants with NAFLD, safety and pharmacokinetics (PK) were evaluated in a phase I, non-randomized, open-label, fixed-sequence trial in healthy participants. In Cohort 1, participants (n = 7) received clesacostat 15 mg twice daily (b.i.d.) alone (Days 1-7) and co-administered with ervogastat 300 mg b.i.d. (Days 8-14). Mean systemic clesacostat exposures, when co-administered with ervogastat, decreased by 12% and 19%, based on maximum plasma drug concentration and area under the plasma drug concentration-time curve during the dosing interval, respectively. In Cohort 2, participants (n = 9) received ervogastat 300 mg b.i.d. alone (Days 1-7) and co-administered with clesacostat 15 mg b.i.d. (Days 8-14). There were no meaningful differences in systemic ervogastat exposures when administered alone or with clesacostat. Clesacostat 15 mg b.i.d. and ervogastat 300 mg b.i.d. co-administration was overall safe and well tolerated in healthy participants. Cumulative safety and no clinically meaningful PK drug interactions observed in this study supported co-administration of these two novel agents in additional studies exploring efficacy and safety in the management of NAFLD.

Pharmacologic inhibition of lipogenesis for the treatment of NAFLD

The hepatic accumulation of excess triglycerides is a seminal event in the initiation and progression of non-alcoholic fatty liver disease (NAFLD). Hepatic steatosis occurs when the hepatic accrual of fatty acids from the plasma and de novo lipogenesis (DNL) is no longer balanced by rates of fatty acid oxidation and secretion of very low-density lipoprotein-triglycerides. Accumulating data indicate that increased rates of DNL are central to the development of hepatic steatosis in NAFLD. Whereas the main drivers in NAFLD are transcriptional, owing to both hyperinsulinemia and hyperglycaemia, the effectors of DNL are a series of well-characterised enzymes. Several have proven amenable to pharmacologic inhibition or oligonucleotide-mediated knockdown, with lead compounds showing liver fat-lowering efficacy in phase II clinical trials. In humans with NAFLD, percent reductions in liver fat have closely mirrored percent inhibition of DNL, thereby affirming the critical contributions of DNL to NAFLD pathogenesis. The safety profiles of these compounds have so far been encouraging. It is anticipated that inhibitors of DNL, when administered alone or in combination with other therapeutic agents, will become important agents in the management of human NAFLD.

VLDL Biogenesis and Secretion: It Takes a Village

The production and secretion of VLDLs (very-low-density lipoproteins) by hepatocytes has a direct impact on liver fat content, as well as the concentrations of cholesterol and triglycerides in the circulation and thus affects both liver and cardiovascular health, respectively. Importantly, insulin resistance, excess caloric intake, and lack of physical activity are associated with overproduction of VLDL, hepatic steatosis, and increased plasma levels of atherogenic lipoproteins. Cholesterol and triglycerides in remnant particles generated by VLDL lipolysis are risk factors for atherosclerotic cardiovascular disease and have garnered increasing attention over the last few decades. Presently, however, increased risk of atherosclerosis is not the only concern when considering today's cardiometabolic patients, as they often also experience hepatic steatosis, a prevalent disorder that can progress to steatohepatitis and cirrhosis. This duality of metabolic risk highlights the importance of understanding the molecular regulation of the biogenesis of VLDL, the lipoprotein that transports triglycerides and cholesterol out of the liver. Fortunately, there has been a resurgence of interest in the intracellular assembly, trafficking, degradation, and secretion of VLDL by hepatocytes, which has led to many exciting new molecular insights that are the topic of this review. Increasing our understanding of the biology of this pathway will aid to the identification of novel therapeutic targets to improve both the cardiovascular and the hepatic health of cardiometabolic patients. This review focuses, for the first time, on this duality.

A missense variant in human perilipin 2 (PLIN2 Ser251Pro) reduces hepatic steatosis in mice

Background & Aims

Non-alcoholic fatty liver disease (NAFLD) is characterised by the accumulation of lipid droplets (LDs) within hepatocytes. Perilipin 2 (PLIN2) is the most abundant protein in hepatic LDs and its expression correlates with intracellular lipid accumulation. A recently discovered PLIN2 coding variant, Ser251Pro (rs35568725), was found to promote the accumulation of small LDs in embryonic kidney cells. In this study, we investigate the role of PLIN2-Ser251Pro (PLIN2-Pro251) on hepatic LD metabolism in vivo and research the metabolic phenotypes associated with this variant in humans.

Methods

For our animal model, we used Plin2 knockout mice in which we expressed either human PLIN2-Pro251 (Pro251 mice) or wild-type human PLIN2-Ser251 (Ser251 mice) in a hepatocyte-specific manner. We fed both cohorts a lipogenic high-fat, high-cholesterol, high-fructose diet for 12 weeks.

Results

Pro251 mice were associated with reduced liver triglycerides (TGs) and had lower mRNA expression of fatty acid synthase and diacylglycerol O-acyltransferase-2 compared with Ser251 mice. Moreover, Pro251 mice had a reduction of polyunsaturated fatty acids-TGs and reduced expression of epoxygenase genes. For our human study, we analysed the Penn Medicine BioBank, the Million Veteran Program, and UK Biobank. Across these databases, the minor allele frequency of PLIN2-Pro251 was approximately 5%. There was no association with the clinical diagnosis of NAFLD, however, there was a trend toward reduced liver fat in PLIN2-Pro251 carriers by MRI-spectroscopy in UK Biobank subjects.

Conclusions

In mice lacking endogenous Plin2, expression of human PLIN2-Pro251 attenuated high-fat, high-fructose, high-cholesterol, diet-induced hepatic steatosis compared with human wild-type PLIN2-Ser251. Moreover, Pro251 mice had lower polyunsaturated fatty acids-TGs and epoxygenase genes expression, suggesting less liver oxidative stress. In humans, PLIN2-Pro251 is not associated with NAFLD.

Impact and Implications

Lipid droplet accumulation in hepatocytes is the distinctive characteristic of non-alcoholic fatty liver disease. Perilipin 2 (PLIN2) is the most abundant protein in hepatic lipid droplets; however, little is known on the role of a specific polymorphism PLIN2-Pro251 on hepatic lipid droplet metabolism. PLIN2-Pro251 attenuates liver triglycerides accumulation after a high-fat-high-glucose-diet. PLIN2-Pro251 may be a novel lipid droplet protein target for the treatment of liver steatosis.

Multiomics analyses reveal dose-dependent effects of dicofol exposure on host metabolic homeostasis and the gut microbiota in mice

BACKGROUND

Environmental exposure to dicofol (DCF), one of common organochlorine pesticides (OCPs) widely used for controlling agricultural pests, elicits a potential risk for human health due to its toxicity. However, potential physiological hazards of oral DCF exposure remain largely unknown.

METHODS

Mice were exposed to relatively chronic and subacute DCF at different doses (5, 20 and 100 mg/kg) by gavage for 2 weeks. 1H NMR-based metabolomics was used to explore alterations of metabolic profiling induced by DCF exposure. Targeted metabolomics was subsequently employed to investigate the dose-dependent effects of oral DCF exposure on lipid metabolism and the gut microbiota-derived metabolites of mice. 16S rRNA gene sequencing was further employed to evaluate the changes of gut community of mice exposed to DCF.

RESULTS

Oral exposure to DCF dose-dependently induced liver injury, manifested by hepatic lipogenesis, inflammation and liver dysfunction of mice. Typically, DCF exposure disrupted host fatty acids metabolism that were confirmed by marked alteration in the levels of related genes. DCF exposure also dose-dependently caused dysbiosis of the gut bacteria and its metabolites including altered microbial composition accompanied by inhibition of bacterial fermentation.

CONCLUSION

These results provide metabolic evidence that DCF exposure dose-dependently induces liver lipidosis and disruption of the gut microbiota in mice, which enrich our views of molecular mechanism of DCF hepatoxicity.

The roles of DGAT1 and DGAT2 in human myotubes are dependent on donor patho-physiological background

The roles of DGAT1 and DGAT2 in lipid metabolism and insulin responsiveness of human skeletal muscle were studied using cryosections and myotubes prepared from muscle biopsies from control, athlete, and impaired glucose regulation (IGR) cohorts of men. The previously observed increases in intramuscular triacylglycerol (IMTG) in athletes and IGR were shown to be related to an increase in lipid droplet (LD) area in type I fibers in athletes but, conversely, in type II fibers in IGR subjects. Specific inhibition of both diacylglycerol acyltransferase (DGAT) 1 and 2 decreased fatty acid (FA) uptake by myotubes, whereas only DGAT2 inhibition also decreased fatty acid oxidation. Fatty acid uptake in myotubes was negatively correlated with the lactate thresholds of the respective donors. DGAT2 inhibition lowered acetate uptake and oxidation in myotubes from all cohorts whereas DGAT1 inhibition had no effect. A positive correlation between acetate oxidation in myotubes and resting metabolic rate (RMR) from fatty acid oxidation in vivo was observed. Myotubes from athletes and IGR had higher rates of de novo lipogenesis from acetate that were normalized by DGAT2 inhibition. Moreover, DGAT2 inhibition in myotubes also resulted in increased insulin-induced Akt phosphorylation. The differential effects of DGAT1 and DGAT2 inhibition suggest that the specialized role of DGAT2 in esterifying nascent diacylglycerols and de novo synthesized FA is associated with synthesis of a pool of triacylglycerol, which upon hydrolysis results in effectors that promote mitochondrial fatty acid oxidation but decrease insulin signaling in skeletal muscle cells.

Preferential lipolysis of DGAT1 over DGAT2 generated triacylglycerol in Huh7 hepatocytes

Two distinct diacylglycerol acyltransferases (DGAT1 and DGAT2) catalyze the final committed step of triacylglycerol (TG) synthesis in hepatocytes. After its synthesis in the endoplasmic reticulum (ER) TG is either stored in cytosolic lipid droplets (LDs) or is assembled into very low-density lipoproteins in the ER lumen. TG stored in cytosolic LDs is hydrolyzed by adipose triglyceride lipase (ATGL) and the released fatty acids are converted to energy by oxidation in mitochondria. We hypothesized that targeting/association of ATGL to LDs would differ depending on whether the TG stores were generated through DGAT1 or DGAT2 activities. Individual inhibition of DGAT1 or DGAT2 in Huh7 hepatocytes incubated with oleic acid did not yield differences in TG accretion while combined inhibition of both DGATs completely prevented TG synthesis suggesting that either DGAT can efficiently esterify exogenously supplied fatty acid. DGAT2-made TG was stored in larger LDs, whereas TG formed by DGAT1 accumulated in smaller LDs. Inactivation of DGAT1 or DGAT2 did not alter expression (mRNA or protein) of ATGL, the ATGL activator ABHD5/CGI-58, or LD coat proteins PLIN2 or PLIN5, but inactivation of both DGATs increased PLIN2 abundance despite a dramatic reduction in the number of LDs. ATGL was found to preferentially target to LDs generated by DGAT1 and fatty acids released from TG in these LDs were also preferentially used for fatty acid oxidation. Combined inhibition of DGAT2 and ATGL resulted in larger LDs, suggesting that the smaller size of DGAT1-generated LDs is the result of increased lipolysis of TG in these LDs.

Protective Effects of Gnetin C from Melinjo Seed Extract against High-Fat Diet-Induced Hepatic Steatosis and Liver Fibrosis in NAFLD Mice Model

Nonalcoholic fatty liver disease (NAFLD), the most common form of chronic liver disease, can progress to hepatic steatosis, inflammation, and advanced fibrosis, increasing the risk of cirrhosis. Resveratrol, a natural polyphenol with antioxidant and anti-inflammatory properties, is beneficial in treating multiple metabolic diseases. Gnetin C, a resveratrol derivative obtained from Melinjo seed extract (MSE), shares similar health-promoting properties. We investigated the role of gnetin C in preventing NAFLD in a mouse model and compared it with resveratrol. Male C57BL/6J mice were fed a control diet (10% calories from fat), a high-fat choline-deficient (HFCD) diet (46% calories from fat) and HFCD diet supplemented with gnetin C (150 mg/kg BW·day-1) or resveratrol (150 mg/kg BW·day-1) for 12 weeks. Gnetin C supplementation reduced body and liver weight, and improved blood glucose levels and insulin sensitivity. Both gnetin C- and resveratrol reduced hepatic steatosis, with gnetin C also decreasing liver lipid content. Gnetin C and resveratrol ameliorated HFCD diet-induced hepatic fibrosis. The mRNA expression results, and western blot analyses showed that gnetin C and, to some extent, resveratrol downregulated fibrosis markers in the TGF-β1 signaling pathway, indicating a possible safeguarding mechanism against NAFLD. These results suggest that gnetin C supplementation may protect against lipid deposition and hepatic fibrosis.

Identification of an alternative triglyceride biosynthesis pathway

Triacylglycerols (TAGs) are the main source of stored energy in the body, providing an important substrate pool for mitochondrial beta-oxidation. Imbalances in the amount of TAGs are associated with obesity, cardiac disease and various other pathologies1,2. In humans, TAGs are synthesized from excess, coenzyme A-conjugated fatty acids by diacylglycerol O-acyltransferases (DGAT1 and DGAT2)3. In other organisms, this activity is complemented by additional enzymes4, but whether such alternative pathways exist in humans remains unknown. Here we disrupt the DGAT pathway in haploid human cells and use iterative genetics to reveal an unrelated TAG-synthesizing system composed of a protein we called DIESL (also known as TMEM68, an acyltransferase of previously unknown function) and its regulator TMX1. Mechanistically, TMX1 binds to and controls DIESL at the endoplasmic reticulum, and loss of TMX1 leads to the unconstrained formation of DIESL-dependent lipid droplets. DIESL is an autonomous TAG synthase, and expression of human DIESL in Escherichia coli endows this organism with the ability to synthesize TAG. Although both DIESL and the DGATs function as diacylglycerol acyltransferases, they contribute to the cellular TAG pool under specific conditions. Functionally, DIESL synthesizes TAG at the expense of membrane phospholipids and maintains mitochondrial function during periods of extracellular lipid starvation. In mice, DIESL deficiency impedes rapid postnatal growth and affects energy homeostasis during changes in nutrient availability. We have therefore identified an alternative TAG biosynthetic pathway driven by DIESL under potent control by TMX1.

Liver lipophagy ameliorates nonalcoholic steatohepatitis through extracellular lipid secretion

Nonalcoholic steatohepatitis (NASH) is a progressive disorder with aberrant lipid accumulation and subsequent inflammatory and profibrotic response. Therapeutic efforts at lipid reduction via increasing cytoplasmic lipolysis unfortunately worsens hepatitis due to toxicity of liberated fatty acid. An alternative approach could be lipid reduction through autophagic disposal, i.e., lipophagy. We engineered a synthetic adaptor protein to induce lipophagy, combining a lipid droplet-targeting signal with optimized LC3-interacting domain. Activating hepatocyte lipophagy in vivo strongly mitigated both steatosis and hepatitis in a diet-induced mouse NASH model. Mechanistically, activated lipophagy promoted the excretion of lipid from hepatocytes, thereby suppressing harmful intracellular accumulation of nonesterified fatty acid. A high-content compound screen identified alpelisib and digoxin, clinically-approved compounds, as effective activators of lipophagy. Administration of alpelisib or digoxin in vivo strongly inhibited the transition to steatohepatitis. These data thus identify lipophagy as a promising therapeutic approach to prevent NASH progression.

Pharmacotherapies of NAFLD: updated opportunities based on metabolic intervention

Non-alcoholic fatty liver disease (NAFLD) is a chronic liver disease that is becoming increasingly prevalent, and it ranges from simple steatosis to cirrhosis. However, there is still a lack of pharmacotherapeutic strategies approved by the Food and Drug Administration, which results in a higher risk of death related to carcinoma and cardiovascular complications. Of note, it is well established that the pathogenesis of NAFLD is tightly associated with whole metabolic dysfunction. Thus, targeting interconnected metabolic conditions could present promising benefits to NAFLD, according to a number of clinical studies. Here, we summarize the metabolic characteristics of the development of NAFLD, including glucose metabolism, lipid metabolism and intestinal metabolism, and provide insight into pharmacological targets. In addition, we present updates on the progresses in the development of pharmacotherapeutic strategies based on metabolic intervention globally, which could lead to new opportunities for NAFLD drug development.

Insulin Regulation of Hepatic Lipid Homeostasis

The incidence of obesity, insulin resistance, and type II diabetes (T2DM) continues to rise worldwide. The liver is a central insulin-responsive metabolic organ that governs whole-body metabolic homeostasis. Therefore, defining the mechanisms underlying insulin action in the liver is essential to our understanding of the pathogenesis of insulin resistance. During periods of fasting, the liver catabolizes fatty acids and stored glycogen to meet the metabolic demands of the body. In postprandial conditions, insulin signals to the liver to store excess nutrients into triglycerides, cholesterol, and glycogen. In insulin-resistant states, such as T2DM, hepatic insulin signaling continues to promote lipid synthesis but fails to suppress glucose production, leading to hypertriglyceridemia and hyperglycemia. Insulin resistance is associated with the development of metabolic disorders such as cardiovascular and kidney disease, atherosclerosis, stroke, and cancer. Of note, nonalcoholic fatty liver disease (NAFLD), a spectrum of diseases encompassing fatty liver, inflammation, fibrosis, and cirrhosis, is linked to abnormalities in insulin-mediated lipid metabolism. Therefore, understanding the role of insulin signaling under normal and pathologic states may provide insights into preventative and therapeutic opportunities for the treatment of metabolic diseases. Here, we provide a review of the field of hepatic insulin signaling and lipid regulation, including providing historical context, detailed molecular mechanisms, and address gaps in our understanding of hepatic lipid regulation and the derangements under insulin-resistant conditions. © 2023 American Physiological Society. Compr Physiol 13:4785-4809, 2023.

Molecular Mechanisms and Mediators of Hepatotoxicity Resulting from an Excess of Lipids and Non-Alcoholic Fatty Liver Disease

The paper reviews some of the mechanisms implicated in hepatotoxicity, which is induced by an excess of lipids. The paper spans a wide variety of topics: from the molecular mechanisms of excess lipids, to the therapy of hyperlipidemia, to the hepatotoxicity of lipid-lowering drugs. NAFLD is currently the leading cause of chronic liver disease in Western countries; the molecular mechanisms leading to NAFLD are only partially understood and there are no effective therapeutic interventions. The prevalence of liver disease is constantly increasing in industrialized countries due to a number of lifestyle variables, including excessive caloric intake, unbalanced diet, lack of physical activity, and abuse of hepatotoxic medicines. Considering the important functions of cell death and inflammation in the etiology of the majority, if not all, liver diseases, one efficient therapeutic treatment may include the administration of hepatoprotective and anti-inflammatory drugs, either alone or in combination. Clinical trials are currently being conducted in cohorts of patients with different liver diseases in order to explore this theory.

Lipid droplet biogenesis and functions in health and disease

Ubiquitous yet unique, lipid droplets are intracellular organelles that are increasingly being recognized for their versatility beyond energy storage. Advances uncovering the intricacies of their biogenesis and the diversity of their physiological and pathological roles have yielded new insights into lipid droplet biology. Despite these insights, the mechanisms governing the biogenesis and functions of lipid droplets remain incompletely understood. Moreover, the causal relationship between the biogenesis and function of lipid droplets and human diseases is poorly resolved. Here, we provide an update on the current understanding of the biogenesis and functions of lipid droplets in health and disease, highlighting a key role for lipid droplet biogenesis in alleviating cellular stresses. We also discuss therapeutic strategies of targeting lipid droplet biogenesis, growth or degradation that could be applied in the future to common diseases, such as cancer, hepatic steatosis and viral infection.

Glycerolipid Synthesis and Lipid Droplet Formation in the Endoplasmic Reticulum

More than 60 years ago, Eugene Kennedy and coworkers elucidated the endoplasmic reticulum (ER)-based pathways of glycerolipid synthesis, including the synthesis of phospholipids and triacylglycerols (TGs). The reactions of the Kennedy pathway were identified by studying the conversion of lipid intermediates and the isolation of biochemical enzymatic activities, but the molecular basis for most of these reactions was unknown. With recent progress in the cell biology, biochemistry, and structural biology in this area, we have a much more mechanistic understanding of this pathway and its reactions. In this review, we provide an overview of molecular aspects of glycerolipid synthesis, focusing on recent insights into the synthesis of TGs. Further, we go beyond the Kennedy pathway to describe the mechanisms for storage of TG in cytosolic lipid droplets and discuss how overwhelming these pathways leads to ER stress and cellular toxicity, as seen in diseases linked to lipid overload and obesity.

From Non-Alcoholic Fatty Liver to Hepatocellular Carcinoma: A Story of (Mal)Adapted Mitochondria

Non-alcoholic fatty liver disease (NAFLD) is a global pandemic affecting 25% of the world's population and is a serious health and economic concern worldwide. NAFLD is mainly the result of unhealthy dietary habits combined with sedentary lifestyle, although some genetic contributions to NAFLD have been documented. NAFLD is characterized by the excessive accumulation of triglycerides (TGs) in hepatocytes and encompasses a spectrum of chronic liver abnormalities, ranging from simple steatosis (NAFL) to steatohepatitis (NASH), significant liver fibrosis, cirrhosis, and hepatocellular carcinoma. Although the molecular mechanisms that cause the progression of steatosis to severe liver damage are not fully understood, metabolic-dysfunction-associated fatty liver disease is strong evidence that mitochondrial dysfunction plays a significant role in the development and progression of NAFLD. Mitochondria are highly dynamic organelles that undergo functional and structural adaptations to meet the metabolic requirements of the cell. Alterations in nutrient availability or cellular energy needs can modify mitochondria formation through biogenesis or the opposite processes of fission and fusion and fragmentation. In NAFL, simple steatosis can be seen as an adaptive response to storing lipotoxic free fatty acids (FFAs) as inert TGs due to chronic perturbation in lipid metabolism and lipotoxic insults. However, when liver hepatocytes' adaptive mechanisms are overburdened, lipotoxicity occurs, contributing to reactive oxygen species (ROS) formation, mitochondrial dysfunction, and endoplasmic reticulum (ER) stress. Impaired mitochondrial fatty acid oxidation, reduction in mitochondrial quality, and disrupted mitochondrial function are associated with a decrease in the energy levels and impaired redox balance and negatively affect mitochondria hepatocyte tolerance towards damaging hits. However, the sequence of events underlying mitochondrial failure from steatosis to hepatocarcinoma is still yet to be fully clarified. This review provides an overview of our understanding of mitochondrial adaptation in initial NAFLD stages and highlights how hepatic mitochondrial dysfunction and heterogeneity contribute to disease pathophysiology progression, from steatosis to hepatocellular carcinoma. Improving our understanding of different aspects of hepatocytes' mitochondrial physiology in the context of disease development and progression is crucial to improving diagnosis, management, and therapy of NAFLD/NASH.

The role of ChREBP in carbohydrate sensing and NAFLD development

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

Fitm2 is required for ER homeostasis and normal function of murine liver

The endoplasmic reticulum (ER)-resident protein fat storage-inducing transmembrane protein 2 (FIT2) catalyzes acyl-CoA cleavage in vitro and is required for ER homeostasis and normal lipid storage in cells. The gene encoding FIT2 is essential for the viability of mice and worms. Whether FIT2 acts as an acyl-CoA diphosphatase in vivo and how this activity affects the liver, where the protein was discovered, are unknown. Here, we report that hepatocyte-specific Fitm2 knockout (FIT2-LKO) mice fed a chow diet exhibited elevated acyl-CoA levels, ER stress, and signs of liver injury. These mice also had more triglycerides in their livers than control littermates due, in part, to impaired secretion of triglyceride-rich lipoproteins and reduced capacity for fatty acid oxidation. We found that challenging FIT2-LKO mice with a high-fat diet worsened hepatic ER stress and liver injury but unexpectedly reversed the steatosis phenotype, similar to what is observed in FIT2-deficient cells loaded with fatty acids. Our findings support the model that FIT2 acts as an acyl-CoA diphosphatase in vivo and is crucial for normal hepatocyte function and ER homeostasis in the murine liver.

Recent updates on targeting the molecular mediators of NAFLD

Nonalcoholic fatty liver disease (NAFLD) is rapidly becoming the most common disease worldwide in an era of rapid economic growth. NAFLD is a multifactorial disease, involving multiple genetic, metabolic, and environmental factors, and is closely associated with metabolic syndrome, obesity, and cardiovascular disease. NAFLD can be classified into nonalcoholic fatty liver disease (NAFL) and nonalcoholic steatohepatitis (NASH), which can both progress to cirrhosis and even hepatocellular carcinoma (HCC). Due to the enormous burden of NAFLD and its complications, no FDA-approved drugs for the treatment of NAFLD are on the market, and therapeutic targets and drug therapies are being actively investigated. In view of the various pathological mechanisms of NAFLD, numbers of preclinical studies and clinical trials have made rapid progress. This review mainly summarizes the most recently characterized mechanisms and therapeutic targets in each mechanism of NAFLD, focusing on the mechanism and application potential.

Inhibition of Diacylglycerol Acyltransferase 2 Versus Diacylglycerol Acyltransferase 1: Potential Therapeutic Implications of Pharmacology

PURPOSE

Hepatic steatosis due to altered lipid metabolism and accumulation of hepatic triglycerides is a hallmark of nonalcoholic fatty liver disease (NAFLD). Diacylglycerol acyltransferase (DGAT) enzymes, DGAT1 and DGAT2, catalyze the terminal reaction in triglyceride synthesis, making them attractive targets for pharmacologic intervention. There is a common misconception that these enzymes are related; however, despite their similar names, DGAT1 and DGAT2 differ significantly on multiple levels. As we look ahead to future clinical studies of DGAT2 inhibitors in patients with NAFLD and nonalcoholic steatohepatitis (NASH), we review key differences and include evidence to highlight and support DGAT2 inhibitor (DGAT2i) pharmacology.

METHODS

Three Phase I, randomized, double-blind, placebo-controlled trials assessed the safety, tolerability, and pharmacokinetic properties of the DGAT2i ervogastat (PF-06865571) in healthy adult participants (Single Dose Study to Assess the Safety, Tolerability and Pharmacokinetics of PF-06865571 [study C2541001] and Study to Assess the Safety, Tolerability, and Pharmacokinetics of Multiple Doses of PF-06865571 in Healthy, Including Overweight and Obese, Adult Subjects [study C2541002]) or participants with NAFLD (2-Week Study in People With Nonalcoholic Fatty Liver Disease [study C2541005]). Data from 2 Phase I, randomized, double-blind, placebo-controlled trials of the DGAT1i PF-04620110 in healthy participants (A Single Dose Study of PF-04620110 in Overweight and Obese, Otherwise Healthy Volunteers [study B0961001] and A Multiple Dose Study of PF-04620110 in Overweight and Obese, Otherwise Healthy Volunteers [study B0961002]) were included for comparison. Safety outcomes were the primary end point in all studies, except in study C2541005, in which safety was the secondary end point, with relative change from baseline in whole liver fat at day 15 assessed as the primary end point. Safety data were analyzed across studies by total daily dose of ervogastat (5, 15, 50, 100, 150, 500, 600, 1000, and 1500 mg) or PF-04620110 (0.3, 1, 3, 5, 7, 10, 14, and 21 mg), with placebo data pooled separately across ervogastat and PF-04620110 studies.

FINDINGS

Published data indicate that DGAT1 and DGAT2 differ in multiple dimensions, including gene family, subcellular localization, substrate preference, and specificity, with unrelated pharmacologic inhibition properties and differing safety profiles. Although initial nonclinical studies suggested a potentially attractive therapeutic profile with DGAT1 inhibition, genetic and pharmacologic data suggest otherwise, with common gastrointestinal adverse events, including nausea, vomiting, and diarrhea, limiting further clinical development. Conversely, DGAT2 inhibition, although initially not pursued as aggressively as a potential target for pharmacologic intervention, has consistent efficacy in nonclinical studies, with reduced triglyceride synthesis accompanied by reduced expression of genes essential for de novo lipogenesis. In addition, early clinical data indicate antisteatotic effects with DGAT2i ervogastat, in participants with NAFLD, accompanied by a well-tolerated safety profile.

IMPLICATIONS

Although pharmacologic DGAT1is are limited by an adverse safety profile, data support use of DGAT2i as an effective and well-tolerated therapeutic strategy for patients with NAFLD, NASH, and NASH with liver fibrosis.

CLINICALTRIALS

gov identifiers: NCT03092232, NCT03230383, NCT03513588, NCT00799006, and NCT00959426.

Naringin attenuates fructose-induced NAFLD progression in rats through reducing endogenous triglyceride synthesis and activating the Nrf2/HO-1 pathway

Background: Excessive fructose consumption causes hepatic lipid accumulation via increased triglyceride (TG) synthesis, leading to the development and progression of non-alcoholic fatty liver disease (NALFD). Naringin, a flavanone glycoside found in citrus fruit, has antioxidant and hypolipidemic properties. Therefore, the aim of this study was to investigate the effect of naringin on fructose-induced NAFLD in rats and the possible underlying mechanism. Methods: Male Sprague Dawley rats were given 10% (w/v) fructose in drinking water for 12 weeks. Naringin (100 mg/kg/day) was administered orally to rats for the last 4 weeks of fructose overload. After 12 weeks of treatment, the hepatic lipid content was determined. In addition, the expression of proteins involved in de novo lipogenesis (DNL) and TG synthesis as well as antioxidant and inflammatory mediators in the liver were examined by western blot analysis. Results: Treatment of fructose-fed rats with naringin significantly decreased the hepatic TG and cholesterol content as well as serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities. Naringin treatment also decreased the hepatic expression of carbohydrate response element binding protein (ChREBP), sterol regulatory element-binding protein-1c (SREBP-1c) and nuclear SREBP-1c (nSREBP-1c) as well as enzymes involved in DNL (acetyl CoA carboxylase [ACC] and fatty acid synthase [FAS]) and an enzyme involved in TG synthesis (glycerol-3-phosphate acyltransferase 1 [GPAT-1] and diacylglycerol acyltransferase2 [DGAT2]) in fructose-fed rats. In addition, naringin induced a significant decrease in the hepatic expression of nuclear factor kappa B (NF-κB) and tumor necrosis factor α (TNF-α). Furthermore, naringin administration restored the expression of the antioxidant mediators nuclear factor (erythroid-derived 2)-like 2 (Nrf2) and heme oxygenase-1 (HO-1) in the liver of fructose-fed rats. Conclusion: These results demonstrate that oral administration of naringin protects against fructose-induced hepatic steatosis by decreasing DNL and TG synthesis. In addition, naringin could prevent NAFLD progression via targeting the Nrf2/HO-1 and the NF-κB/TNF-α pathways.

Discovery of Ervogastat (PF-06865571): A Potent and Selective Inhibitor of Diacylglycerol Acyltransferase 2 for the Treatment of Non-alcoholic Steatohepatitis

Discovery efforts leading to the identification of ervogastat (PF-06865571), a systemically acting diacylglycerol acyltransferase (DGAT2) inhibitor that has advanced into clinical trials for the treatment of non-alcoholic steatohepatitis (NASH) with liver fibrosis, are described herein. Ervogastat is a first-in-class DGAT2 inhibitor that addressed potential development risks of the prototype liver-targeted DGAT2 inhibitor PF-06427878. Key design elements that culminated in the discovery of ervogastat are (1) replacement of the metabolically labile motif with a 3,5-disubstituted pyridine system, which addressed potential safety risks arising from a cytochrome P450-mediated O-dearylation of PF-06427878 to a reactive quinone metabolite precursor, and (2) modifications of the amide group to a 3-THF group, guided by metabolite identification studies coupled with property-based drug design.

Moderate Treadmill Exercise Alleviates NAFLD by Regulating the Biogenesis and Autophagy of Lipid Droplet

Lipid droplet is a dynamic organelle that undergoes periods of biogenesis and degradation under environmental stimuli. The excessive accumulation of lipid droplets is the major characteristic of non-alcoholic fatty liver disease (NAFLD). Moderate aerobic exercise is a powerful intervention protecting against the progress of NAFLD. However, its impact on lipid droplet dynamics remains ambiguous. Mice were fed with 15 weeks of high-fat diet in order to induce NAFLD. Meanwhile, the mice performed 15 weeks of treadmill exercise. Our results showed that 15 weeks of regular moderate treadmill exercise alleviated obesity, insulin intolerance, hyperlipidemia, and hyperglycemia induced by HFD. Importantly, exercise improved histological phenotypes of NAFLD, including hepatic steatosis, inflammation, and locular ballooning, as well as prevented liver fat deposition and liver injury induced by HFD. Exercise reduced hepatic lipid droplet size, and moreover, it reduced PLIN2 protein level and increased PLIN3 protein level in the liver of HFD mice. Interestingly, our results showed that exercise did not significantly affect the gene expressions of DGAT1, DGAT2, or SEIPIN, which were involved in TG synthesis. However, it did reduce the expressions of FITM2, CIDEA, and FSP27, which were major involved in lipid droplet growth and budding, and lipid droplet expansion. In addition, exercise reduced ATGL protein level in HFD mice, and regulated lipophagy-related markers, including increasing ATG5, LAMP1, LAMP2, LAL, and CTSD, decreasing LC3II/I and p62, and promoting colocalization of LAMP1 with LDs. In summary, our data suggested that 15 weeks of moderate treadmill exercise was beneficial for regulating liver lipid droplet dynamics in HFD mice by inhibiting abnormal lipid droplets expansion and enhancing clearance of lipid droplets by lysosomes during the lipophagic process, which might provide highly flexible turnover for lipid mobilization and metabolism. Abbreviations: β-actin: actin beta; ATG5: autophagy related 5; LAMP2: lysosomal-associated membrane protein 2; LAMP1: lysosomal-associated membrane protein 1; SQSTM1/p62: sequestosome 1; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; ATGL: adipose triglyceride lipase; CSTD: cathepsin D; LAL: lysosomal acid lipase; DGAT1: diacylglycerol-o-acyltransferase 1; DGAT2: diacylglycerol-o-acyltransferase 2; CIDEA: cell death inducing dffa-like effector a; CIDEC/FSP27: cell death inducing dffa-like effector c; FITM2: fat storage-inducing transmembrane protein 2; PLIN2: adipose differentiation related protein; PLN3: tail-interacting protein 47; HSP90: heat shock protein 90; SREBP1c: sterol regulatory element binding protein-1c; chREBP: carbohydrate response element binding protein.

Probiotics suppress nonalcoholic steatohepatitis and carcinogenesis progression in hepatocyte-specific PTEN knockout mice

Nonalcoholic fatty liver disease (NAFLD), a hepatic characteristic of metabolic syndrome, received significant attention in clinical settings. The multiple-hit theory is one of the proposed mechanisms of NAFLD, and gut dysbiosis is considered a hit. Thus, controlling gut microbiota is a potential target in the management of NAFLD, and probiotics can be used as a treatment agent for NAFLD. The current study aimed to investigate the efficacy of probiotics against nonalcoholic steatohepatitis in a hepatocyte-specific PTEN knockout mouse model that mimics the characteristics of human NAFLD. Probiotics were administered to male knockout mice for 8 or 40 weeks. Next, we assessed hepatic inflammation, fibrosis, carcinogenesis, and oxidative stress. Probiotics were found to reduce serum transaminase levels, NAFLD activity score, and the gene expression of pro-inflammatory cytokines. In addition, they decreased liver fibrosis grade, which was examined via Sirius red staining, gene expression of fibrotic markers, and hydroxyproline. Furthermore, probiotics suppressed the number of liver tumors, particular in HCC. Probiotics reduced oxidative stresses, including glutathione levels, and anti-oxidative stress marker, which may be an underlying mechanism for their beneficial effects. In conclusion, probiotics treatment had beneficial effects against NAFLD and carcinogenesis in hepatocyte-specific PTEN knockout mice.

Myeloid TM6SF2 Deficiency Inhibits Atherosclerosis

Genetic variants in transmembrane 6 superfamily member 2 (TM6SF2), such as E167K, are associated with atherosclerotic cardiovascular disease (ASCVD). Chronic inflammation and lipid-laden macrophage foam cell formation are the central pathogeneses in the development of atherosclerosis. This study was undertaken to illustrate the biological function of TM6SF2 in macrophages and its role during atherosclerosis development. We generated myeloid cell-specific Tm6sf2 knockout mice on ApoE-deficient background (LysM Cre+/Tm6sf2fl/fl/ApoE-/-, TM6 mKO) with littermate LysM Cre-/Tm6sf2fl/fl/ApoE-/- (Control) mice as controls. Mice were fed a Western diet for 12 weeks to induce atherosclerosis. Myeloid Tm6sf2 deficiency inhibited atherosclerosis and decreased foam cells in the plaques without changing the plasma lipid profile. RNA sequencing of bone marrow-derived macrophages (BMDMs) from TM6 mKO mice demonstrated the downregulation of genes associated with inflammation, cholesterol uptake, and endoplasmic reticulum (ER) stress. TM6SF2 was upregulated by oxidized low-density lipoprotein (oxLDL) in macrophages. Silencing TM6SF2 in THP-1-derived macrophages and Tm6sf2 deficiency in BMDMs reduced inflammatory responses and ER stress and attenuated cholesterol uptake and foam cell formation, while the overexpression of TM6SF2 showed opposite effects. In conclusion, myeloid TM6SF2 deficiency inhibits atherosclerosis development and is a potential therapeutic target for the treatment of atherogenesis.

Acyl-CoA:diacylglycerol acyltransferase: Properties, physiological roles, metabolic engineering and intentional control

Acyl-CoA:diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) catalyzes the last reaction in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG). DGAT activity resides mainly in membrane-bound DGAT1 and DGAT2 in eukaryotes and bifunctional wax ester synthase-diacylglycerol acyltransferase (WSD) in bacteria, which are all membrane-bound proteins but exhibit no sequence homology to each other. Recent studies also identified other DGAT enzymes such as the soluble DGAT3 and diacylglycerol acetyltransferase (EaDAcT), as well as enzymes with DGAT activities including defective in cuticular ridges (DCR) and steryl and phytyl ester synthases (PESs). This review comprehensively discusses research advances on DGATs in prokaryotes and eukaryotes with a focus on their biochemical properties, physiological roles, and biotechnological and therapeutic applications. The review begins with a discussion of DGAT assay methods, followed by a systematic discussion of TAG biosynthesis and the properties and physiological role of DGATs. Thereafter, the review discusses the three-dimensional structure and insights into mechanism of action of human DGAT1, and the modeled DGAT1 from Brassica napus. The review then examines metabolic engineering strategies involving manipulation of DGAT, followed by a discussion of its therapeutic applications. DGAT in relation to improvement of livestock traits is also discussed along with DGATs in various other eukaryotic organisms.

Endurance exercise-mediated metabolic reshuffle attenuates high-caloric diet-induced non-alcoholic fatty liver disease

INTRODUCTION AND AIM

Non-alcoholic fatty liver disease (NAFLD) is one of the most common diseases in the United States. Metabolic distress (obese diabetes) is the main causative element of NAFLD. While there is no cure for NAFLD, endurance exercise (EEx) has emerged as a therapeutic strategy against NAFLD. However, mechanisms of EXE-induced hepatic protection especially in female subjects remain unidentified. Thus, the aim of the study is to examine molecular mechanisms of EXE-induced hepatic protection against diet-induced NAFLD in female mice.

MATERIAL AND METHODS

Nine-week-old female C57BL/6J mice were randomly divided into three groups: normal-diet control group (CON, n=11); high-fat diet/high-fructose group (HFD/HF, n=11); and HFD/HF+EEx group (HFD/HF+EEx, n=11). The mice assigned to HFD/HF and HFD/HF+EEx groups were fed with HFD/HF for 12 weeks, after which the mice assigned to the EEx group began treadmill exercise for 12 weeks, with HFD/HF continued.

RESULTS

EEx attenuated hepatic steatosis, reduced de novo lipogenesis (reduction in ATP-Citrate- Lyase and diacylglycerol-O-acyltransferase 1), and enhanced mitochondrial biogenesis and fatty-acid activation (oxidative phosphorylation enzymes and Acyl-CoA synthetase1). Also, EEx prevented upregulation of gluconeogenic proteins (glyceraldehyde-3-phosphate dehydrogenase, glucose-6-phosphatase, and phosphoenolpyruvate-carboxykinase1), premature senescence (suppression of p53, p22, and p16, tumor-necrosis-factor-α, and interleukin-1β, and oxidative stress), and autophagy deficiency. Furthermore, EXE reversed apoptosis arrest (cleaved cysteine-dependent-aspartate-directed protease3 and Poly-(ADP-ribose)-polymerase1).

CONCLUSION

EEx-mediated reparations of metabolic and redox imbalance (utilization of pentose phosphate pathway), and autophagy deficiency caused by metabolic distress critically contribute to preventing/delaying severe progression of NAFLD. Also, EEx-induced anti-senescence and cell turnover are crucial protective mechanisms against NAFLD.

Pathogenesis of Alcohol-Associated Fatty Liver: Lessons From Transgenic Mice

Alcohol-associated liver disease (ALD) is a major public health issue that significantly contributes to human morbidity and mortality, with no FDA-approved therapeutic intervention available. The health burden of ALD has worsened during the COVID-19 pandemic, which has been associated with a spike in alcohol abuse, and a subsequent increase in hospitalization rates for ALD. A key knowledge gap that underlies the lack of novel therapies for ALD is a need to better understand the pathogenic mechanisms that contribute to ALD initiation, particularly with respect to hepatic lipid accumulation and the development of fatty liver, which is the first step in the ALD spectrum. The goal of this review is to evaluate the existing literature to gain insight into the pathogenesis of alcohol-associated fatty liver, and to synthesize alcohol’s known effects on hepatic lipid metabolism. To achieve this goal, we specifically focus on studies from transgenic mouse models of ALD, allowing for a genetic dissection of alcohol’s effects, and integrate these findings with our current understanding of ALD pathogenesis. Existing studies using transgenic mouse models of ALD have revealed roles for specific genes involved in hepatic lipid metabolic pathways including fatty acid uptake, mitochondrial β-oxidation, de novo lipogenesis, triglyceride metabolism, and lipid droplet formation. In addition to reviewing this literature, we conclude by identifying current gaps in our understanding of how alcohol abuse impairs hepatic lipid metabolism and identify future directions to address these gaps. In summary, transgenic mice provide a powerful tool to understand alcohol’s effect on hepatic lipid metabolism and highlight that alcohol abuse has diverse effects that contribute to the development of alcohol-associated fatty liver disease.

Update on Non-Alcoholic Fatty Liver Disease-Associated Single Nucleotide Polymorphisms and Their Involvement in Liver Steatosis, Inflammation, and Fibrosis: A Narrative Review

Genetic factors are involved in the development, progression, and severity of non-alcoholic fatty liver disease (NAFLD). Polymorphisms in genes regulating liver functions may increase liver susceptibility to NAFLD. Therefore, we conducted this literature study to present recent findings on NAFLD-associated polymorphisms from published articles in PubMed from 2016 to 2021. From 69 selected research articles, 20 genes and 34 SNPs were reported to be associated with NAFLD. These mutated genes affect NAFLD by promoting liver steatosis (PNPLA3, MBOAT7, TM2SF6, PTPRD, FNDC5, IL-1B, PPARGC1A, UCP2, TCF7L2, SAMM50, IL-6, AGTR1, and NNMT), inflammation (PNPLA3, TNF-α, AGTR1, IL-17A, IL-1B, PTPRD, and GATAD2A), and fibrosis (IL-1B, PNPLA3, MBOAT7, TCF7L2, GATAD2A, IL-6, NNMT, UCP, AGTR1, and TM2SF6). The identification of these genetic factors helps to better understand the pathogenesis pathways of NAFLD.