Control Of Hepatic Lipid Metabolism Research Articles

Perilipin 5 S155 phosphorylation by PKA is required for the control of hepatic lipid metabolism and glycemic control

Perilipin 5 (PLIN5) is a lipid-droplet-associated protein that coordinates intracellular lipolysis in highly oxidative tissues and is thought to regulate lipid metabolism in response to phosphorylation by protein kinase A (PKA). We sought to identify PKA phosphorylation sites in PLIN5 and assess their functional relevance in cultured cells and the livers of mice. We detected phosphorylation on S155 and identified S155 as a functionally important site for lipid metabolism. Expression of phosphorylation-defective PLIN5 S155A in Plin5 null cells resulted in decreased rates of lipolysis and triglyceride-derived fatty acid oxidation. FLIM-FRET analysis of protein-protein interactions showed that PLIN5 S155 phosphorylation regulates PLIN5 interaction with adipose triglyceride lipase at the lipid droplet, but not with α-β hydrolase domain-containing 5. Re-expression of PLIN5 S155A in the liver of Plin5 liver-specific null mice reduced lipolysis compared with wild-type PLIN5 re-expression, but was not associated with other changes in hepatic lipid metabolism. Furthermore, glycemic control was impaired in mice with expression of PLIN5 S155A compared with mice expressing PLIN5. Together, these studies demonstrate that PLIN5 S155 is required for PKA-mediated lipolysis and builds on the body of evidence demonstrating a critical role for PLIN5 in coordinating lipid and glucose metabolism.

Alcohol and Liver Clock Disruption Increase Small Droplet Macrosteatosis, Alter Lipid Metabolism and Clock Gene mRNA Rhythms, and Remodel the Triglyceride Lipidome in Mouse Liver.

Heavy alcohol drinking dysregulates lipid metabolism, promoting hepatic steatosis – the first stage of alcohol-related liver disease (ALD). The molecular circadian clock plays a major role in synchronizing daily rhythms in behavior and metabolism and clock disruption can cause pathology, including liver disease. Previous studies indicate that alcohol consumption alters liver clock function, but the impact alcohol or clock disruption, or both have on the temporal control of hepatic lipid metabolism and injury remains unclear. Here, we undertook studies to determine whether genetic disruption of the liver clock exacerbates alterations in lipid metabolism and worsens steatosis in alcohol-fed mice. To address this question, male liver-specific Bmal1 knockout (LKO) and flox/flox (Fl/Fl) control mice were fed a control or alcohol-containing diet for 5 weeks. Alcohol significantly dampened diurnal rhythms of mRNA levels in clock genes Bmal1 and Dbp, phase advanced Nr1d1/REV-ERBα, and induced arrhythmicity in Clock, Noct, and Nfil3/E4BP4, with further disruption in livers of LKO mice. Alcohol-fed LKO mice exhibited higher plasma triglyceride (TG) and different time-of-day patterns of hepatic TG and macrosteatosis, with elevated levels of small droplet macrosteatosis compared to alcohol-fed Fl/Fl mice. Diurnal rhythms in mRNA levels of lipid metabolism transcription factors (Srebf1, Nr1h2, and Ppara) were significantly altered by alcohol and clock disruption. Alcohol and/or clock disruption significantly altered diurnal rhythms in mRNA levels of fatty acid (FA) synthesis and oxidation (Acaca/b, Mlycd, Cpt1a, Fasn, Elovl5/6, and Fads1/2), TG turnover (Gpat1, Agpat1/2, Lpin1/2, Dgat2, and Pnpla2/3), and lipid droplet (Plin2/5, Lipe, Mgll, and Abdh5) genes, along with protein abundances of p-ACC, MCD, and FASN. Lipidomics analyses showed that alcohol, clock disruption, or both significantly altered FA saturation and remodeled the FA composition of the hepatic TG pool, with higher percentages of several long and very long chain FA in livers of alcohol-fed LKO mice. In conclusion, these results show that the liver clock is important for maintaining temporal control of hepatic lipid metabolism and that disrupting the liver clock exacerbates alcohol-related hepatic steatosis.

Role of Inflammation in Control of Hepatic Lipid Metabolism—New Insights to the Pathogenesis of NASH

Non‐alcoholic fatty liver disease (NAFLD) is a common metabolic disorder whose incidence among US adults exceeds 30%. While NAFLD manifests as benign hepatic steatosis in most patients, in about 20% of patients it assumes a much more aggressive form – non‐alcoholic steatohepatitis (NASH). The mechanisms that control the switch from benign steatosis to NASH are poorly understood, but were suggested to depend on ER stress. To query the role of ER stress in NASH development and establish a proper mouse model for studying the disease, we fed high‐fat diet (HFD) to MUP‐uPA mice, which are prone to liver ER stress due to hepatocyte‐specific urokinase plasminogen activator (uPA) expression. Placing these mice on HFD triggered full‐blown NASH within 3–4 months and most of the NASH‐afflicted mice progressed to develop hepatocellular carcinoma (HCC), the most common form of liver cancer whose incidence and development rates greatly increase with steatohepatitis. Both NASH and HCC development in MUP‐uPA mice depend on ER stress and are associated with persistent activation of sterol response element binding proteins (SREBP) 1 and 2. SREBP1 activation drives de novo lipogenesis (DNL) and cholesterol biosynthesis in HFD‐fed MUP‐uPA mice remains elevated without any sign of feedback inhibition mechanisms that control SREBP2 activation in tissue culture cells. DNL and cholesterol synthesis are also chronically elevated in NASH patients and the hepatic accumulation of free cholesterol was suggested to be a key switch from simple steatosis to NASH. While investigating the mechanism by which ER stress leads to persistent SREBP activation, we uncovered a previously unknown pathway in which caspase‐2 (Casp2) leads to constitutive activation of site 1 protease (S1P), which initiates SREBP cleavage activating protein (SCAP)‐independent SREBP activation. Of note, genetic ablation or pharmacological inhibition of Casp2 in MUP‐uPA mice blocks NASH development by preventing hepatic steatosis. Casp2 ablation in both MUP‐uPA and BL6 mice also prevents HFD‐induced adipocyte hypertrophy and increases energy consumption. None of the effects of Casp2 on liver lipid metabolism are related to cell death. The NAFLD/NASH epidemic was also suggested to be associated with increased consumption of fructose, which is a major constituent of high‐fructose corn syrup (HFCS), the main sweetener and a general food additive in the US. Increased consumption of HFCS has been linked to elevated risk of developing HCC, pancreatic and colon cancer. Indeed, feeding MUP‐uPA mice a high‐fructose diet (HFrD) results in NASH and HCC development even without the peripheral obesity that accompanies HFD consumption. Although fructose was believed to give rise to hepatic steatosis through preferential uptake and direct incorporation into the glycolytic pathway in hepatocytes, recent results indicate that the first site of fructose metabolism is the small intestine. Correspondingly, we found that fructose gives rise to NASH and HCC by first disrupting the intestinal epithelial barrier, evoking an inflammatory response that drives both NASH and HCC development.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Hepatocyte Specific Shp1 Deletion and Low Dose Rosiglitazone Treatment Act in Concert to Improve Liver Glucose Homeostasis in Diet‐induced Obese Mice

The protein‐tyrosine phosphatase Shp1 plays a key role in high‐fat diet (HFD)‐induced insulin resistance by negatively regulating insulin action on glucose homeostasis. Hepatocyte‐specific Shp1 knockout mice (Ptpn6H‐KO) on HFD show an improvement of hepatic insulin resistance and glucose homeostasis but also develop increased hepatic steatosis compared to their wild‐type littermates (Ptpn6fl/fl). Nevertheless, livers of Ptpn6H‐KO mice on HFD have reduced hepatocellular damage and a reduced inflammatory profile alongside with increased peroxisome proliferator‐activated receptor gamma (PPARγ) activity, suggesting a new role for Shp1 in the regulation of PPARγ and the control of hepatic lipid metabolism. Therefore, the aim of the present study was to determine whether activation of PPARγ activity using rosiglitazone could further ameliorate hepatic glucose and lipid homeostasis as well as the inflammatory profile in Ptpn6H‐KO mice. Male Ptpn6H‐KO and Ptpn6fl/fl mice were fed a HFD for 18 weeks. During the last 4 weeks the animals were treated with a very low dose of rosiglitazone (0.3 mg/kg/day). Low‐dose rosiglitazone treatment did not affect total body or adipose tissue weight, plasma levels of adiponectin or expression of PPARγ target genes in adipose tissue whereas PPARγ target genes expression was highly increased in liver. However, liver weight and liver triglycerides were increased in both rosiglitazone treated groups while plasma triglycerides were reduced by rosiglitazone. Furthermore, treatment with rosiglitazone led to an improvement in whole‐body glucose homeostasis and hepatic Akt activation both of which were even more ameliorated in HFD‐fed Ptpn6H‐KO compared to Ptpn6fl/fl mice. These results show that a low‐dose rosiglitazone treatment selectively target liver metabolism without detectable effects on adipose tissue, and our data further suggest that hepatic invalidation of Shp1 acts in concert with rosiglitazone to improve glucose metabolism in liver of HFD‐fed obese mice.Support or Funding InformationCIHRThis abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Circadian clock control of hepatic lipid metabolism: role of small heterodimer partner (Shp).

Hepatic steatosis, the accumulation of triglyceride droplets in the hepatocytes, is a common hepatic pathology seen in subjects with obesity/metabolic syndrome and those with excessive alcohol use. The pathogenesis underlying hepatic steatosis is complex. Recent studies have shown the specific role played by the molecular clock mechanism in the control of lipid metabolism and that the disruption of these tissue clocks may lead to the disturbances in lipid homeostasis. This review reports a novel role of small heterodimer partner in maintaining triglyceride and lipoprotein homeostasis through neuronal PAS domain protein 2.

Ubiquitin Ligase COP1 Controls Hepatic Fat Metabolism by Targeting ATGL for Degradation

Optimal control of hepatic lipid metabolism is critical for organismal metabolic fitness. In liver, adipose triglyceride lipase (ATGL) serves as a major triacylglycerol (TAG) lipase and controls the bulk of intracellular lipid turnover. However, regulation of ATGL expression and its functional implications in hepatic lipid metabolism, particularly in the context of fatty liver disease, is unclear. We show that E3 ubiquitin ligase COP1 (also known as RFWD2) binds to the consensus VP motif of ATGL and targets it for proteasomal degradation by K-48 linked polyubiquitination, predominantly at the lysine 100 residue. COP1 thus serves as a critical regulator of hepatocyte TAG content, fatty acid mobilization, and oxidation. Moreover, COP1-mediated regulation of hepatic lipid metabolism requires optimum ATGL expression for its metabolic outcome. In vivo, adenovirus-mediated depletion of COP1 ameliorates high-fat diet-induced steatosis in mouse liver and improves liver function. Our study thus provides new insights into the regulation of hepatic lipid metabolism by the ubiquitin-proteasome system and suggests COP1 as a potential therapeutic target for nonalcoholic fatty liver disease.

Metabolic consequences of microRNA-122 inhibition in rainbow trout, Oncorhynchus mykiss

BackgroundMicroRNAs (miRNAs) are small regulatory molecules which post-transcriptionally regulate mRNA stability and translation. Several microRNAs have received attention due to their role as key metabolic regulators. In spite of the high evolutionary conservation of several miRNAs, the role of miRNAs in lower taxa of vertebrates has not been studied with regard to metabolism. The liver-specific and highly abundant miRNA-122 is one of the most widely studied miRNA in mammals, where it has been implicated in the control of hepatic lipid metabolism. Following our identification of acute postprandial, nutritional and endocrine regulation of hepatic miRNA-122 isomiRNA expression in rainbow trout, we used complementary in silico and in vivo approaches to study the role of miRNA-122 in rainbow trout metabolism. We hypothesized that the role of miRNA-122 in regulating lipid metabolism in rainbow trout is conserved to that in mammals and that modulation of miRNA-122 function would result in altered lipid homeostasis and secondarily altered glucose homeostasis, since lipogenesis has been suggested to act as glucose sink in trout.ResultsOur results show that miRNA-122 was functionally inhibited in vivo in the liver. Postprandial glucose concentrations increased significantly in rainbow trout injected with a miRNA-122 inhibitor, and this effect correlated with decreases in hepatic FAS protein abundance, indicative of altered lipogenic potential. Additionally, miRNA-122 inhibition resulted in a 20% decrease in plasma cholesterol concentration, an effect associated with increased expression of genes involved in cholesterol degradation and excretion.ConclusionsOverall evidence suggests that miRNA-122 may have evolved in early vertebrates to support liver-specific metabolic functions. Nevertheless, our data also indicate that metabolic consequences of miRNA-122 inhibition may differ quantitatively between vertebrate species and that distinct direct molecular targets of miRNA-122 may mediate metabolic effects between vertebrate species, indicating that miRNA-122 - mRNA target relationships may have undergone species-specific evolutionary changes.

Effect of Phaseolus angularis Seed on Experimental Cellular Model of Nonalcoholic Fatty Liver Disease

Here we tried to uncover the potential anti-lipogenic effect and the underlying mechanism of Phaseolus angularis seed in a cellular model of nonalcoholic fatty liver disease (NAFLD) induced in HepG2 cells. Ethanol extract of Phaseolus angularis seed (JSD) was prepared. HepG2 cells were incubated in palmitate containing media to induce intracellular lipid accumulation, and co-treated with JSD for 16 hrs before examine intracellular lipid content. In control group, the cells were not co-treated with JSD. We measured the effects of JSD on liver X receptor α (LXRα) and sterol regulatory element-binding transcription factor-1c (SREBP-1c) expression, transcription level of lipogenic genes, including acetyl-CoA carboxylase (ACC), fatty acid synthase (FAS), stearoyl-CoA desaturase-1 (SCD-1), and AMP-activated protein kinase (AMPK) activation in HepG2 cells. JSD markedly reduced palmitate-induced intracellular lipid accumulation in HepG2 cells. JSD suppressed LXRα/SREBP-1c expression, and SREBP-1c mediated induction of ACC, FAS, and SCD-1. Furthermore, JSD activated AMPK, which plays a major role in the control of hepatic lipid metabolism. Taken together, it is suggested that JSD has a potential to alleviate hepatic steatosis, at least in part, by suppressing LXRα/SREBP-1c mediated induction of lipogenic genes. In addtion, the anti-lipogenic potential may be associated with activation of AMPK. Therefore, the Phaseolus angularis seed could be applied as a potential therapeutics for NAFLD with additional clinical studies.

Transcriptional Control of Hepatic Lipid Metabolism by SREBP and ChREBP

The liver is a central organ that controls systemic energy homeostasis and nutrient metabolism. Dietary carbohydrates and lipids, and fatty acids derived from adipose tissue are delivered to the liver, and utilized for gluconeogenesis, lipogenesis, and ketogenesis, which are tightly regulated by hormonal and neural signals. Hepatic lipogenesis is activated primarily by insulin that is secreted from the pancreas after a high-carbohydrate meal. Sterol regulatory element binding protein-1c (SREBP-1c) and carbohydrate-responsive element-binding protein (ChREBP) are major transcriptional regulators that induce key lipogenic enzymes to promote lipogenesis in the liver. Sterol regulatory element binding protein-1c is activated by insulin through complex signaling cascades that control SREBP-1c at both transcriptional and posttranslational levels. Carbohydrate-responsive element-binding protein is activated by glucose independently of insulin. Here, the authors attempt to summarize the current understanding of the molecular mechanism for the transcriptional regulation of hepatic lipogenesis, focusing on recent studies that explore the signaling pathways controlling SREBPs and ChREBP.

Non-classic thyroid hormone signalling involved in hepatic lipid metabolism

Thyroid hormones are important modulators of lipid metabolism because the liver is a primary hormonal target. The hypolipidaemic effects of thyroid hormones result from the balance between direct and indirect actions resulting in stimulation of lipid synthesis and lipid oxidation, which favours degradation pathways. Originally, it was believed that thyroid hormone activity was only transduced by alteration of gene transcription mediated by the nuclear receptor thyroid hormone receptors, comprising the classic action of thyroid hormone. However, the discovery of other effects independent of this classic mechanism characterised a new model of thyroid hormone action, the non-classic mechanism that involves other signalling pathways. To date, this mechanism and its relevance have been intensively described. Considering the increasing evidence for non-classic signalling of thyroid hormones and the major influence of these hormones in the regulation of lipid metabolism, we reviewed the role of thyroid hormone in cytosolic signalling cascades, focusing on the regulation of second messengers, and the activity of effector proteins and the implication of these mechanisms on the control of hepatic lipid metabolism.

Hormonal and cellular control of hepatic lipid metabolism

We are interested in the mechanisms whereby the body senses its nutrient environment and responds to adapt cellular and organismal metabolism to this changing environment. One maladaptive response to excess feeding is excess fat storage driven by the Sterol Response Element Binding Protein‐1c (SREBP‐1c) transcription factor. SREBP‐1c is activated at multiple levels in response to insulin signaling and stimulates the expression of the entire de novo lipogenesis and triglyceride biosynthesis program. The signaling pathway whereby insulin stimulates SREBP‐1c activity includes the canonical PI3K‐Akt module, but has recently been shown to also depend on the Akt‐dependent activation of the mTOR Complex 1 (mTORC1). We have recently characterized a novel component of this pathway that sheds additional light on how feeding‐induced lipogenesis is controlled.PAS kinase (PASK) is a highly evolutionarily conserved protein kinase that we have shown to be metabolically regulated and to control glucose‐responsive processes in pancreatic beta‐cells. A mouse lacking PASK, in addition to modest beta‐cell defects, has a profound resistance to the pathological effects of a high‐fat diet. Specifically, PASK−/− mice are protected from the obesity, insulin resistance and dyslipidemia observed in wild‐type mice on a high‐fat diet. The most striking effect that we observed was a loss of the massive hepatic lipid accumulation that occurs on a high‐fat diet. This was accompanied by decreased expression of many of the genes that promote lipid uptake and synthesis in the liver. We demonstrated that PASK strongly regulates SREBP‐1c in cultured cells using a combination of loss‐of‐function and gain‐of‐function approaches and genetics, cell biology and biochemistry. Based on these observations in vitro, we went back to the mouse model and showed that, indeed, the entirety of the SREBP‐1c transcriptional program was decreased in PASK−/− mice. Our preliminary data suggests that PASK regulates SREBP‐1c at the level of proteolytic maturation.In parallel, we observed that PASK was strongly activated post‐translationally by insulin and that mTORC1 activation was necessary and sufficient for this effect. The current evidence suggests that mTORC1 activates PASK through direct interaction with and phosphorylation. Having found that insulin/mTORC1 activates PASK and PASK activates SREBP‐1c, we then tested whether PASK is a required mediator of the acute insulin/mTORC1 effects on SREBP‐1c and found that it is. My laboratory is now focused on understanding the mTORC1‐PASK‐SREBP‐1c pathway, both the mechanisms of signal transduction and the physiological importance of this pathway.

Akt Stimulates Hepatic SREBP1c and Lipogenesis through Parallel mTORC1-Dependent and Independent Pathways

Through unknown mechanisms, insulin activates the sterol regulatory element-binding protein (SREBP1c) transcription factor to promote hepatic lipogenesis. We find that this induction is dependent on the mammalian target of rapamycin (mTOR) complex 1 (mTORC1). To further define the role of mTORC1 in the regulation of SREBP1c in the liver, we generated mice with liver-specific deletion of TSC1 (LTsc1KO), which results in insulin-independent activation of mTORC1. Surprisingly, the LTsc1KO mice are protected from age- and diet-induced hepatic steatosis and display hepatocyte-intrinsic defects in SREBP1c activation and de novo lipogenesis. These phenotypes result from attenuation of Akt signaling driven by mTORC1-dependent insulin resistance. Therefore, mTORC1 activation is not sufficient to stimulate hepatic SREBP1c in the absence of Akt signaling, revealing the existence of an additional downstream pathway also required for this induction. We provide evidence that this mTORC1-independent pathway involves Akt-mediated suppression of Insig2a, a liver-specific transcript encoding the SREBP1c inhibitor INSIG2.

κ‐Opioid receptors control the metabolic response to a high‐energy diet in mice

General opioid receptor antagonists reduce food intake and body weight in rodents, but the contributions of specific receptor subtypes are unknown. We examined whether genetic deletion of the kappa-opioid receptor (KOR) in mice alters metabolic physiology. KOR-knockout (KO) and wild-type (WT) mice were fed a high-energy diet (HED) for 16 wk. KO mice had 28% lower body weight and 45% lower fat mass when compared to WT mice fed an HED. No differences in caloric intake were found. An HED reduced energy expenditure in WT mice, but not in KO mice. KOR deficiency led to an attenuation of triglyceride synthesis in the liver. Malonyl CoA levels were also reduced in response to an HED, thereby promoting hepatic beta-oxidation. Glycemic control was also found to be improved in KO mice. These data suggest a key role for KORs in the central nervous system regulation of the metabolic adaptation to an HED, as we were unable to detect expression of KOR in liver, white adipose tissue, or skeletal muscle in WT mice. This study provides the first evidence that KORs play an essential physiological role in the control of hepatic lipid metabolism, and KOR activation is a permissive signal toward fat storage.-Czyzyk, T. A., Nogueiras, R., Lockwood, J. F., McKinzie, J. H., Coskun, T., Pintar, J. E., Hammond, C., Tschöp, M. H., Statnick, M. A. kappa-Opioid receptors control the metabolic response to a high-energy diet in mice.

Lipin 1 is an inducible amplifier of the hepatic PGC-1α/PPARα regulatory pathway

Perturbations in hepatic lipid homeostasis are linked to the development of obesity-related steatohepatitis. Mutations in the gene encoding lipin 1 cause hepatic steatosis in fld mice, a genetic model of lipodystrophy. However, the molecular function of lipin 1 is unclear. Herein, we demonstrate that the expression of lipin 1 is induced by peroxisome proliferator-activated receptor gamma (PPARgamma) coactivator 1alpha (PGC-1alpha), a transcriptional coactivator controlling several key hepatic metabolic pathways. Gain-of-function and loss-of-function strategies demonstrated that lipin selectively activates a subset of PGC-1alpha target pathways, including fatty acid oxidation and mitochondrial oxidative phosphorylation, while suppressing the lipogenic program and lowering circulating lipid levels. Lipin activates mitochondrial fatty acid oxidative metabolism by inducing expression of the nuclear receptor PPARalpha, a known PGC-1alpha target, and via direct physical interactions with PPARalpha and PGC-1alpha. These results identify lipin 1 as a selective physiological amplifier of the PGC-1alpha/PPARalpha-mediated control of hepatic lipid metabolism.

Polyunsaturated Fatty Acid Suppression of Hepatic Fatty Acid Synthase and S14 Gene Expression Does Not Require Peroxisome Proliferator-activated Receptor α

Dietary polyunsaturated fatty acids (PUFA) induce hepatic peroxisomal and microsomal fatty acid oxidation and suppress lipogenic gene expression. The peroxisome proliferator-activated receptor alpha (PPARalpha) has been implicated as a mediator of fatty acid effects on gene transcription. This report uses the PPARalpha-deficient mouse to examine the role of PPARalpha in the PUFA regulation of mRNAs encoding hepatic lipogenic (fatty acid synthase (FAS) and the S14 protein (S14)), microsomal (cytochrome P450 4A2 (CYP4A2)), and peroxisomal (acyl-CoA oxidase (AOX)) enzymes. PUFA ingestion induced mRNAAOX (2.3-fold) and mRNACYP4A2 (8-fold) and suppressed mRNAFAS and mRNAS14 by >/=80% in wild type mice. In PPARalpha-deficient mice, PUFA did not induce mRNAAOX or mRNACYP4A2, indicating a requirement for PPARalpha in the PUFA-mediated induction of these enzymes. However, PUFA still suppressed mRNAFAS and mRNAS14 in the PPARalpha-deficient mice. Studies in rats provided additional support for the differential regulation of lipogenic and peroxisomal enzymes by PUFA. These studies provide evidence for two distinct pathways for PUFA control of hepatic lipid metabolism. One requires PPARalpha and is involved in regulating peroxisomal and microsomal enzymes. The other pathway does not require PPARalpha and is involved in the PUFA-mediated suppression of lipogenic gene expression.