SREBP-1c Activation Research Articles

PKD1 mutant clones within cirrhotic livers inhibit steatohepatitis without promoting cancer

Somatic mutations in non-malignant tissues are selected for because they confer increased clonal fitness. However, it is uncertain whether these clones can benefit organ health. Here, ultra-deep targeted sequencing of 150 liver samples from 30 chronic liver disease patients revealed recurrent somatic mutations. PKD1 mutations were observed in 30% of patients, whereas they were only detected in 1.3% of hepatocellular carcinomas (HCCs). To interrogate tumor suppressor functionality, we perturbed PKD1 in two HCC cell lines and six invivo models, in some cases showing that PKD1 loss protected against HCC, but in most cases showing no impact. However, Pkd1 haploinsufficiency accelerated regeneration after partial hepatectomy. We tested Pkd1 in fatty liver disease, showing that Pkd1 loss was protective against steatosis and glucose intolerance. Mechanistically, Pkd1 loss selectively increased mTOR signaling without SREBP-1c activation. In summary, PKD1 mutations exert adaptive functionality on the organ level without increasing transformation risk.

Flindissone, a Limonoid Isolated from Trichilia prieuriana, Is an LXR Agonist.

In this study, the ability of six limonoids from Trichilia prieuriana (Meliaceae) to activate the liver X receptor (LXR) was assessed. One of these limonoids, flindissone, was shown to activate LXR by reporter-gene assays. Flindissone is a ring-intact limonoid, structurally similar to sterol-like LXR ligands. In endogenous cellular settings, flindissone showed an activity profile that is characteristic of LXR agonists. It induced cholesterol efflux in THP-1 macrophages by increasing the cholesterol transporter ABCA1 and ABCG1 gene expression. In HepG2 cells, flindissone induced the expression of IDOL, an LXR-target gene that is associated with the downregulation of the LDL receptor. However, unlike synthetic and similarly to sterol-based LXR agonists, flindissone did not induce the expression of the SREBP1c gene, a major transcription factor regulating de novo lipogenesis. Additionally, flindissone also appeared to be able to inhibit post-translational activation of SREBP1c. The results presented here reveal a natural product as a new LXR agonist and point to an additional property of T. prieuriana and other plant extracts containing flindissone.

Ergosterol fraction from Agaricus bisporus modulates adipogenesis and skeletal glucose uptake in high fat diet induced obese C57BL/6 mice

AimThe study aimed to optimize a method of extracting ergosterol rich concentrate (ECF) and to evaluate its significant impact on adipogenesis and associated complications in high-fat diet (HFD) induced obese mice. MethodsA comparative analysis (soxhlet and ultra sound assisted extraction) was done to obtain the highest yield of ergosterol from Agaricus bisporus. The ECF was evaluated for the biological effect on 3T3-L1 pre-adipocytes in-vitro and in male C57BL/6 mice model in-vivo. Key findingsUltra sound assisted extraction method using the solvent n-hexane resulted in highest ergosterol yield. ECF treatment significantly reduced the differentiation and lipid accumulation in pre-adipocyte cells without any cytotoxicity. In-vivo study illustrated beneficial impact on cholesterol metabolism by down regulating the hepatic gene expression of LXR-α, HMG-CoR and up-regulating LDL-R expression. Significant increase in fecal excretion of cholesterol and bile acids have also been observed among the ECF treated animals compared to high fat diet (HFD) fed mice. ECF had an anti-adipogenic activity in-vivo mainly by inhibiting the activity of PPAR-γ, C/EBP-α and SREBP-1c. The results also depicted the improvement of obesity associated insulin resistance by ECF treatment manly via decrease in plasma resistin and up-regulation in skeletal GLUT4 protein expression. SignificanceOur study illustrated diverse activity of ECF in the therapeutic management of obesity associated metabolic complications mainly by reducing adipogenesis and improving glucose uptake in skeletal muscle in conjunction with improved cholesterol metabolism.

Hepatocyte Kctd17 Inhibition Ameliorates Glucose Intolerance and Hepatic Steatosis Caused by Obesity-induced Chrebp Stabilization

Background & AimsObesity predisposes to type 2 diabetes (T2D) and nonalcoholic fatty liver disease (NAFLD), but underlying mechanisms are incompletely understood. Potassium channel tetramerization domain-containing protein 17 (Kctd17) levels are increased in livers from obese mice and humans. In this study, we investigated the mechanism of increased Kctd17 and whether it is causal to obesity-induced metabolic complications. MethodsWe transduced Rosa26-LSL-Cas9 knockin mice with AAV8-TBG-Cre (Control), AAV8-U6-Kctd17 sgRNA-TBG-Cre (L-Kctd17), AAV8-U6-Oga sgRNA-TBG-Cre (L-Oga), or AAV8-U6-Kctd17/Oga sgRNA-TBG-Cre (DKO). We fed mice a high-fat diet (HFD) and assessed for hepatic glucose and lipid homeostasis. We generated Kctd17, O-GlcNAcase (Oga), or Kctd17/Oga-knockout hepatoma cells by CRISPR-Cas9, and Kctd17-directed antisense oligonucleotide to test therapeutic potential in vivo. We analyzed transcriptomic data from patients with NAFLD. ResultsHepatocyte Kctd17 expression was increased in HFD-fed mice due to increased Srebp1c activity. HFD-fed L-Kctd17 or Kctd17 antisense oligonucleotide–treated mice show improved glucose tolerance and hepatic steatosis, whereas forced Kctd17 expression caused glucose intolerance and hepatic steatosis even in lean mice. Kctd17 induced Oga degradation, resulting in increasing carbohydrate response element-binding protein (Chrebp) protein, so concomitant Oga knockout negated metabolic benefits of hepatocyte Kctd17 deletion. In patients with NAFLD, KCTD17 messenger RNA was positively correlated with expression of Chrebp target and other lipogenic genes. ConclusionsSrebp1c-induced hepatocyte Kctd17 expression in obesity disrupted glucose and lipid metabolism by stabilizing Chrebp, and may represent a novel therapeutic target for obesity-induced T2D and NAFLD.

Chronotherapy with a glucokinase activator profoundly improves metabolism in obese Zucker rats.

Circadian rhythms play a critical role in regulating metabolism, including daily cycles of feeding/fasting. Glucokinase (GCK) is central for whole-body glucose homeostasis and oscillates according to a circadian clock. GCK activators (GKAs) effectively reduce hyperglycemia, but their use is also associated with hypoglycemia, hyperlipidemia, and hepatic steatosis. Given the circadian rhythmicity and natural postprandial activation of GCK, we hypothesized that GKA treatment would benefit from being timed specifically during feeding periods. Acute treatment of obese Zucker rats with the GKA AZD1656 robustly increased flux into all major metabolic pathways of glucose disposal, enhancing glucose elimination. Four weeks of continuous AZD1656 treatment of obese Zucker rats improved glycemic control; however, hepatic steatosis and inflammation manifested. In contrast, timing AZD1656 to feeding periods robustly reduced hepatic steatosis and inflammation in addition to improving glycemia, whereas treatment timed to fasting periods caused overall detrimental metabolic effects. Mechanistically, timing AZD1656 to feeding periods diverted newly synthesized lipid toward direct VLDL secretion rather than intrahepatic storage. In line with increased hepatic insulin signaling, timing AZD1656 to feeding resulted in robust activation of AKT, mTOR, and SREBP-1C after glucose loading, pathways known to regulate VLDL secretion and hepatic de novo lipogenesis. In conclusion, intermittent AZD1656 treatment timed to feeding periods promotes glucose disposal when needed the most, restores metabolic flexibility and hepatic insulin sensitivity, and thereby avoids hepatic steatosis. Thus, chronotherapeutic approaches may benefit the development of GKAs and other drugs acting on metabolic targets.

Integrated Omics analysis of pig muscle metabolism under the effects of dietary Chlorella vulgaris and exogenous enzymes

Monogastric feeding is dependent on costly conventional feedstuffs. Microalgae such as Chlorella vulgaris are a sustainable alternative; however, its recalcitrant cell wall hinders monogastric digestion. Carbohydrate Active Enzyme (CAZyme) supplementation is a possible solution. The objective of this work was to evaluate the effect of 5% dietary C. vulgaris (CV) and enzymatic supplementation (CV + R—Rovabio® Excel AP; CV + M—four CAZyme mix) on muscle transcriptome and proteome of finishing pigs, in an integrated approach. Control pigs increased the abundance of contractile apparatus (MYH1, MYH2, MYH4) and energy metabolism (CKMT1, NDUFS3) proteins, demonstrating increased nutrient availability. They had increased expression of SCD, characteristic of increased glucose availability, via the activation of SREBP-1c and ChREBP. CV and CV + R pigs upregulated proteolytic and apoptotic genes (BAX, DDA1), whilst increasing the abundance of glucose (UQCRFS1) and fatty acid catabolism (ACADS) proteins. CV + R pigs upregulated ACOT8 and SIRT3 genes as a response to reduced nutrient availability, maintaining energy homeostasis. The cell wall specific CAZyme mix, CV + M, was able to comparatively reduce Omics alterations in the muscle, thereby reducing endogenous nutrient catabolism compared to the CV + R and CV.

Paradoxical activation of transcription factor SREBP1c and de novo lipogenesis by hepatocyte-selective ATP-citrate lyase depletion in obese mice

Hepatic steatosis associated with high-fat diet, obesity, and type 2 diabetes is thought to be the major driver of severe liver inflammation, fibrosis, and cirrhosis. Cytosolic acetyl CoA (AcCoA), a central metabolite and substrate for de novo lipogenesis (DNL), is produced from citrate by ATP-citrate lyase (ACLY) and from acetate through AcCoA synthase short chain family member 2 (ACSS2). However, the relative contributions of these two enzymes to hepatic AcCoA pools and DNL rates in response to high-fat feeding are unknown. We report here that hepatocyte-selective depletion of either ACSS2 or ACLY caused similar 50% decreases in liver AcCoA levels in obese mice, showing that both pathways contribute to the generation of this DNL substrate. Unexpectedly however, the hepatocyte ACLY depletion in obese mice paradoxically increased total DNL flux measured by D2O incorporation into palmitate, whereas in contrast, ACSS2 depletion had no effect. The increase in liver DNL upon ACLY depletion was associated with increased expression of nuclear sterol regulatory element–binding protein 1c and of its target DNL enzymes. This upregulated DNL enzyme expression explains the increased rate of palmitate synthesis in ACLY-depleted livers. Furthermore, this increased flux through DNL may also contribute to the observed depletion of AcCoA levels because of its increased conversion to malonyl CoA and palmitate. Together, these data indicate that in fat diet–fed obese mice, hepatic DNL is not limited by its immediate substrates AcCoA or malonyl CoA but rather by activities of DNL enzymes.

LncNONMMUG027912 alleviates lipid accumulation through AMPKα/mTOR/SREBP1C axis in nonalcoholic fatty liver

Various metabolic diseases are closely related to lipid metabolism disorders, but the regulatory effect of long noncoding RNAs (lncRNAs) on the function of lipids has been poorly elucidated. Previous our work has found that lncNONMMUG027912 (abbreviated as lnc027912) involved in cholesterol metabolism. Here, we further explored the novel function of lipid metabolism-associated lnc027912. We found that upregulated lnc027912 in AML12 cells treated with oleic acid (OA) and palmitic acid (PA) showed a significant decrease in lipid accumulation, triglyceride (TG) levels, and lipid biosynthesis genes. In terms of regulatory mechanisms, lnc027912 increased the expression of p-AMPKα, inhibited p-mTOR levels, decreased the expression of SREBP1C in nuclei, decreased the promoter activity of SREBP1C, and inhibited the expression of lipid synthesis genes. Most importantly, lnc027912 could reduce lipid accumulation and liver inflammation through AMPKα/mTOR signal axis in nonalcoholic fatty liver disease (NAFLD) mice model. Altogether, our study revealed a novel molecular mechanism of lnc027912 in lipid metabolism through the AMPKα/mTOR/SREBP1C signaling axis and highlights the potential of lnc027912 as a new treatment target for lipid disorder diseases (such as NAFLD).

Hepatic lipid accumulation induced by a high-fat diet is regulated by Nrf2 through multiple pathways.

Nuclear factor erythroid 2-related factor 2 (Nrf2) is reportedly involved in hepatic lipid metabolism, but the results are contradictory, and the underlying mechanism remains unclear. Here, we focused on elucidating the effects of Nrf2 on hepatic adipogenesis and on determining the possible underlying mechanism. We established a non-alcoholic fatty liver disease (NAFLD) model in a high-fat diet (HFD)-fed Nrf2 knockout (Nrf2KO) mice; further, a cell model of lipid accumulation was established using mouse primary hepatocytes (MPHs) treated with free fatty acids (FAs). Using these models, we investigated the relationship between Nrf2 and autophagy and its role in the development of NAFLD. We observed that Nrf2 expression levels were upregulated in patients with NAFLD and diet-induced obese mice. Nrf2 deficiency led to hepatic lipid accumulation in vivo and in vitro, in addition to, promoting lipogenesis mainly by increasing SREBP-1c activity. Moreover, Nrf2 deficiency attenuated autophagic flux and inhibited the fusion of autophagosomes and lysosomes in vivo and in vitro. Decreased autophagy caused reduced lipolysis in the liver. Importantly, chromatin immunoprecipitation-qPCR (ChIP-qPCR) and dual-luciferase assay results proved that Nrf2 bound to the LAMP1 promoter and regulated its transcriptional activity. Accordingly, we report that Nrf2-LAMP1 interaction plays an indispensable role in Nrf2-regulated hepatosteatosis. Our data collectively confirm that Nrf2 deficiency promotes hepatosteatosis by enhancing SREBP-1c activity and attenuating autophagy. Our findings provide a novel multi-pathway effect of Nrf2 on lipid metabolism in the liver. We believe that multi-target intervention of Nrf2 is a novel strategy for the treatment of NAFLD.

The COVID-19-diabetes mellitus molecular tetrahedron.

Accumulating molecular evidence suggests that insulin resistance, rather than SARS-CoV-2- provoked beta-cell impairment, plays a major role in the observed rapid metabolic deterioration in diabetes, or new-onset hyperglycemia, during the COVID-19 clinical course. In order to clarify the underlying complexity of COVID-19 and diabetes mellitus interactions, we propose the imaginary diabetes-COVID-19 molecular tetrahedron with four lateral faces consisting of SARS-CoV-2 entry via ACE2 (lateral face 1), the viral hijacking and replication (lateral face 2), acute inflammatory responses (lateral face 3), and the resulting insulin resistance (lateral face 4). The entrance of SARS-CoV-2 using ACE2 receptor triggers an array of multiple molecular signaling beyond that of the angiotensin II/ACE2-Ang-(1–7) axis, such as down-regulation of PGC-1 α/irisin, increased SREBP-1c activity, upregulation of CD36 and Sirt1 inhibition leading to insulin resistance. In another arm of the molecular cascade, the SARS-CoV-2 hijacking and replication induces a series of molecular events in the host cell metabolic machinery, including upregulation of SREBP-2, decrement in Sirt1 expression, dysregulation in PPAR-ɣ, and LPI resulting in insulin resistance. The COVID-19-diabetes molecular tetrahedron may suggest novel targets for therapeutic interventions to overcome insulin resistance that underlies the pathophysiology of worsening metabolic control in patients with diabetes mellitus or the new-onset of hyperglycemia in COVID-19.

Health effects of omega-3 polyunsaturated fatty acids in common diseases

Omega-3 polyunsaturated fatty acids (n-3 PUFAs), such as alpha-linolenic, eicosapentaenoic, and docosahexaenoic acids mostly exist in marine-derived foods, and have shown beneficial effects for hypertriglyceridemia, endothelial function, inflammation, and oxidative stress. Studies suggest that n-3 PUFAs can regulate the activity of NF-κB, Nrf2, SREBP-1c, and PPARα, which are linked to inflammations, ROS homeostasis, and lipid metabolism. Several epidemiological trials and physiological studies indicated protective effect of n-3 PUFAs against various common diseases such as cardiovascular diseases, diabetes mellitus, and non-alcoholic fatty liver disease. This review summarises the findings of many such studies highlighting the beneficial effects of n-3 PUFAs.

Deletion of RAGE fails to prevent hepatosteatosis in obese mice due to impairment of other AGEs receptors and detoxifying systems

Advanced glycation endproducts (AGEs) are involved in several diseases, including NAFLD and NASH. RAGE is the main receptor mediating the pro-inflammatory signalling induced by AGEs. Therefore, targeting of RAGE has been proposed for prevention of chronic inflammatory diseases. However, the role of RAGE in the development of NAFLD and NASH remains poorly understood. We thus aimed to analyse the effect of obesity on AGEs accumulation, AGE-receptors and AGE-detoxification, and whether the absence of RAGE might improve hepatosteatosis and inflammation, by comparing the liver of lean control, obese (LeptrDb−/−) and obese RAGE-deficient (RAGE−/− LeptrDb−/−) mice. Obesity induced AGEs accumulation and RAGE expression with hepatosteatosis and inflammation in LeptrDb−/−, compared to lean controls. Despite the genetic deletion of RAGE in the LeptrDb−/− mice, high levels of intrahepatic AGEs were maintained accompanied by decreased expression of the protective AGE-receptor-1, impaired AGE-detoxifying system glyoxalase-1, and increased expression of the alternative AGE-receptor galectin-3. We also found sustained hepatosteatosis and inflammation as determined by persistent activation of the lipogenic SREBP1c and proinflammatory NLRP3 signalling pathways. Thus, RAGE targeting is not effective in the prevention of NAFLD in conditions of obesity, likely due to the direct liver specific crosstalk of RAGE with other AGE-receptors and AGE-detoxifying systems.

Missense variant in insulin receptor (Y1355H) segregates in family with fatty liver disease

A missense variant in the cytoplasmic domain of the insulin receptor (INSR) was identified by exome sequencing in affected members of a four-generation family with fatty liver disease (FLD). The variant (rs766457461, c.4063T>C, p.Y1355H) results in the substitution of histidine for a tyrosine that undergoes autophosphorylation in response to insulin stimulation in vitro. Because insulin promotes lipogenesis in hepatocytes, we hypothesized that the variant was causally linked to FLD in the family. To test this hypothesis, we used CRISPR/Cas9 technology to replace the corresponding tyrosine in mouse INSR with histidine (Y1345H). No significant differences were found in hepatic insulin signaling, as assessed by phosphorylation of INSR or AKT levels or in activation of the insulin-responsive transcription factor SREBP-1c. Glucose tolerance and hepatic triglyceride (TG) content in Insr1345H/H mice fed a chow diet or diets rich in fat, sucrose or fructose did not differ significantly from WT littermates. Thus, our studies in mice failed to support the notion that INSR (Y1355H) is causally related to FLD in the family or that phosphorylation of this residue alters hepatic TG metabolism.

Berteroin ameliorates lipid accumulation through AMPK-mediated regulation of hepatic lipid metabolism and inhibition of adipocyte differentiation

AimsBerteroin (5-methylthiopentyl isothiocyanate) is a naturally occurring sulforaphane analog containing a non-oxidized sulfur atom in cruciferous vegetables. The objectives of the present study were to determine the effects of berteroin on lipid metabolism in hepatocytes and adipocytes and to elucidate the mechanisms involved. Main methodsThe effect of berteroin on lipid metabolism were evaluated in liver X receptor α agonist-stimulated HepG2 cells and adipocyte differentiation-induced 3T3-L1 cells using MTT assay, western blot, real time polymerase chain reaction, oil red O staining, and triglyceride assay. Key findingsT0901317 treatment increased the expression of sterol regulatory element binding protein (SREBP)-1c, a major transcription factor that mediates lipogenesis, and berteroin pretreatment significantly inhibited the expressions of T0901317-induced SREBP-1c and lipogenic genes. Especially, berteroin had a greater inhibitory effect on T0901317-induced SREBP-1c activation than sulforaphane, AICAR, or metformin. Berteroin also markedly suppressed lipid droplet formations and triglyceride accumulations caused by both T0901317 stimulation in HepG2 hepatocytes and differentiation induction in 3T3-L1 preadipocytes. However, berteroin significantly increased the expression of mitochondrial fatty acid oxidation-related genes (carnitine palmitoyltransferase 1 (CPT-1) and peroxisome proliferator-activated receptor gamma coactivator-1α) and the phosphorylation of adenosine monophosphate-activated protein kinase (AMPK) in HepG2 cells. Interestingly, effects of berteroin on the expressions of SREBP-1c protein and CPT-1 mRNA were remarkably prevented by compound C (an AMPK inhibitor). SignificanceOur results suggest berteroin-inhibited hepatic lipid accumulation and adipocyte differentiation might be mediated by AMPK activation and that berteroin might be useful for the prevention, amelioration, and treatment of metabolic diseases, including hepatic steatosis.

Influence of Genistein on Hepatic Lipid Metabolism in an In Vitro Model of Hepatic Steatosis.

Nonalcoholic fatty liver disease (NAFLD) is among the leading causes of end-stage liver disease. The impaired hepatic lipid metabolism in NAFLD is exhibited by dysregulated PPARα and SREBP-1c signaling pathways, which are central transcription factors associated with lipid degradation and de novo lipogenesis. Despite the growing prevalence of this disease, current pharmacological treatment options are unsatisfactory. Genistein, a soy isoflavone, has beneficial effects on lipid metabolism and may be a candidate for NAFLD treatment. In an in vitro model of hepatic steatosis, primary human hepatocytes (PHHs) were incubated with free fatty acids (FFAs) and different doses of genistein. Lipid accumulation and the cytotoxic effects of FFAs and genistein treatment were evaluated by colorimetric and enzymatic assays. Changes in lipid homeostasis were examined by RT-qPCR and Western blot analyses. PPARα protein expression was induced in steatotic PHHs, accompanied by an increase in CPT1L and ACSL1 mRNA. Genistein treatment increased PPARα protein expression only in control PHHs, while CPTL1 and ACSL1 were unchanged and PPARα mRNA was reduced. In steatotic PHHs, genistein reversed the increase in activated SREBP-1c protein. The model realistically reflected the molecular changes in hepatic steatosis. Genistein suppressed the activation of SREBP-1c in steatotic hepatocytes, but the genistein-mediated effects on PPARα were abolished by high hepatic lipid levels.

Hepatic deletion of Mboat7 (LPIAT1) causes activation of SREBP-1c and fatty liver

AbstractGenetic variants that increase the risk of fatty liver disease and cirrhosis have recently been identified in the proximity of membrane-bound O-acyltransferase domain-containing 7 (MBOAT7). To elucidate the link between these variants and fatty liver disease, we characterized Mboat7 liver-specific KO mice (Mboat7 LSKO). Chow-fed Mboat7 LSKO mice developed fatty livers and associated liver injury. Lipidomic analysis of liver using MS revealed a pronounced reduction in 20-carbon PUFA content in phosphatidylinositols (PIs) but not in other phospholipids. The change in fatty acid composition of PIs in these mice was associated with a marked increase in de novo lipogenesis because of activation of SREBP-1c, a transcription factor that coordinates the activation of genes encoding enzymes in the fatty acid biosynthesis pathway. Hepatic removal of both SREBP cleavage-activating protein (Scap) and Mboat7 normalized hepatic triglycerides relative to Scap-only hepatic KO, showing that increased SREBP-1c processing is required for Mboat7-induced steatosis. This study reveals a clear relationship between PI fatty acid composition and regulation of hepatic fat synthesis and delineates the mechanism by which mutations in MBOAT7 cause hepatic steatosis.

4β-Hydroxycholesterol is a prolipogenic factor that promotes SREBP1c expression and activity through the liver X receptor

Oxysterols are oxidized derivatives of cholesterol that play regulatory roles in lipid biosynthesis and homeostasis. How oxysterol signaling coordinates different lipid classes such as sterols and triglycerides remains incompletely understood. Here, we show that 4β-hydroxycholesterol (HC) (4β-HC), a liver and serum abundant oxysterol of poorly defined functions, is a potent and selective inducer of the master lipogenic transcription factor, SREBP1c, but not the related steroidogenic transcription factor SREBP2. By correlating tracing of lipid synthesis with lipogenic gene expression profiling, we found that 4β-HC acts as a putative agonist for the liver X receptor (LXR), a sterol sensor and transcriptional regulator previously linked to SREBP1c activation. Unique among the oxysterol agonists of the LXR, 4β-HC induced expression of the lipogenic program downstream of SREBP1c and triggered de novo lipogenesis both in primary hepatocytes and in the mouse liver. In addition, 4β-HC acted in parallel to insulin-PI3K–dependent signaling to stimulate triglyceride synthesis and lipid-droplet accumulation. Thus, 4β-HC is an endogenous regulator of de novo lipogenesis through the LXR-SREBP1c axis.

Hepatic Levels of DHA-Containing Phospholipids Instruct SREBP1-Mediated Synthesis and Systemic Delivery of Polyunsaturated Fatty Acids.

SummaryPolyunsaturated fatty acids (PUFAs), such as docosahexaenoic acid (DHA) and arachidonic acid (ARA), play fundamental roles in mammalian physiology. Although PUFA imbalance causes various disorders, mechanisms of the regulation of their systemic levels are poorly understood. Here, we report that hepatic DHA-containing phospholipids (DHA-PLs) determine the systemic levels of PUFAs through the SREBP1-mediated transcriptional program. We demonstrated that liver-specific deletion of Agpat3 leads to a decrease of DHA-PLs and a compensatory increase of ARA-PLs not only in the liver but also in other tissues including the brain. Together with recent findings that plasma lysophosphatidylcholine (lysoPC) is the major source of brain DHA, our results indicate that hepatic AGPAT3 contributes to brain DHA accumulation by supplying DHA-PLs as precursors of DHA-lysoPC. Furthermore, dietary fish oil-mediated suppression of hepatic PUFA biosynthetic program was blunted in liver-specific Agpat3 deletion. Our findings highlight the central role of hepatic DHA-PLs as the molecular rheostat for systemic homeostasis of PUFAs.

Hyperuricemia induces lipid disturbances mediated by LPCAT3 upregulation in the liver.

Potential underlying molecular mechanisms for uric acid-induced lipid metabolic disturbances had not been elucidated clearly. This study investigated the effects and underlying mechanisms of uric acid on the development of lipid metabolic disorders. We collected blood samples from 100 healthy people and 100 patients with hyperuricemia for whom serum lipid analysis was performed. Meanwhile, a mouse model of hyperuricemia was generated, and lipidomics was performed on liver tissues, comparing control and hyperuricemia groups, to analyze lipid profiles and key metabolic enzymes. Uric acid directly induced serum lipid metabolic disorders in both humans and mice based on triglycerides, total cholesterol, and low-density lipoprotein cholesterol. Through lipidomic analysis, 46 lipids were differentially expressed in hyperuricemic mouse livers, and the phosphatidylcholine composition was altered, which was mediated by LPCAT3 upregulation. High-uric acid levels-induced p-STAT3 inhibition and SREBP-1c activation in vivo and in vitro. Moreover, LPCAT3-knockdown significantly attenuated uric acid-induced p-STAT3 inhibition, SREBP-1c activation, and lipid metabolic disorders in L02 cells. In conclusion, uric acid induces lipid metabolic disturbances through LPCAT3-mediated p-STAT3 inhibition and SREBP-1c activation. LPCAT3 could be a key regulatory factor linking hyperuricemia and lipid metabolic disorders. These results might provide novel insights into the clinical treatment of hyperuricemia.

578-P: A Loss-of-Function Mutation of NRG4 Contributes to the Pathogenesis of NAFLD and Insulin Resistance

Neuregulin 4 (Nrg4), as an adipose tissue-enriched endocrine factor, participates in adipocyte-to-hepatocyte communication which elicits beneficial metabolic effects in NAFLD. We identified a mutation of NRG4 by whole exome sequencing in 151 severe obese subjects and exome array in a community-based population of 2388 subjects. Nrg4 R44H mutation carriers have increased visceral fat area, abnormal liver function and hyperlipidemia. Then we hypothesized that the mutation would affect the function of Nrg4 and accelerated the development of NAFLD. Compared with Nrg4 WT mice, Nrg4 R44H mice gained more weight, accompanied by significantly increased fat mass and lipid accumulation in the liver upon HFD feeding. In vitro, we found that Nrg4 WT activated ErbB4 and stimulated the STAT5 signaling pathway and decreased transcriptional activation of srebp1c. As we hypothesized, the primary hepatocytes treated with Nrg4 R44H purified protein didn’t showed the same activation of cell signaling as Nrg4 WT. In order to reveal the potential mechanisms of loss-of-function of Nrg4 R44H, we performed surface plasmon resonance (SPR) experiments to test the binding affinity of purified Nrg4 and ErbB4 proteins. The SPR results showed Nrg4 WT bind to ErbB4 with an affinity of 2.20uM, while Nrg4 R44H showed no binding. Taken together, these results suggested Nrg4 WT prevented HFD-induced NAFLD and alleviated insulin resistance, and demonstrated Nrg4 R44H lost its ability to bind to the receptor ErbB4, thus losing its physiological function. Disclosure C. Hu: None. Y. Li: None.