Hepatic Very Low Density Lipoprotein Research Articles

α-Tocopherol reduces VLDL secretion through modulation of intracellular ER-to-Golgi transport of VLDL.

Avoiding hepatic steatosis is crucial for preventing liver dysfunction, and one mechanism by which this is accomplished is through synchronization of the rate of very low density lipoprotein (VLDL) synthesis with its secretion. Endoplasmic reticulum (ER)-to-Golgi transport of nascent VLDL is the rate-limiting step in its secretion and is mediated by the VLDL transport vesicle (VTV). Recent in vivo studies have indicated that α-tocopherol (α-T) supplementation can reverse steatosis in nonalcoholic fatty liver disease, but its effects on hepatic lipoprotein metabolism are poorly understood. Here, we investigated the impact of α-T on hepatic VLDL synthesis, secretion, and intracellular ER-to-Golgi VLDL trafficking using an in vitro model. Pulse-chase assays using [3H]-oleic acid and 100 µmol/L α-T demonstrated a disruption of early VLDL synthesis, resulting in enhanced apolipoprotein B-100 expression, decreased expression in markers for VTV budding, ER-to-Golgi VLDL transport, and reduced VLDL secretion. Additionally, an in vitro VTV budding assay indicated a significant decrease in VTV production and VTV-Golgi fusion. Confocal imaging of lipid droplet (LD) localization revealed a decrease in overall LD retention, diminished presence of ER-associated LDs, and an increase in Golgi-level LD retention. We conclude that α-T disrupts ER-to-Golgi VLDL transport by modulating the expression of specific proteins and thus reduces VLDL secretion.

Inadequate Hepatic Biotransformation of Choline to Phospholipids Contributes to Lipid Disorders in Early Lactation of Contemporary Holstein Cows

ObjectivesContemporary Holsteins (CH) producing more milk than unselected Holsteins (UH), but they are also more susceptible to lipid disorders, especially hepatic steatosis in early lactation. The objective of this study is to investigate the metabolic events underlying the susceptibility of genetically selected cows to lipid disorders.MethodsSerum and liver samples from periparturient CH and UH cows (n = 12/genotype) were examined by biochemical, metabolomics, and transcriptomic analyses. Serum samples were collected weekly from −5 to + 10 weeks of lactation, and the liver samples were collected on day −14, +3, +14, and + 42 of lactation. Data were analyzed by two-way ANOVA with genotype and time as two factors.ResultsThe postpartum increases of blood serum non-esterified fatty acids (NEFAs) was greater in CH cow. Serum triacylglycerols (TAGs) concentration remained comparable to those of the UH cows but the postpartum accumulation of hepatic TAG was greater in CH than in UH cows. This imbalanced lipidome in CH cows was further reflected by a greater decrease of hepatic phosphatidylcholines (PCs), which are responsible for packaging TAGs into the very-low-density lipoproteins (VLDLs). Hepatic choline content did not differ between UH and CH cows, but less phosphocholine and glycerophosphocholine were present in the liver of CH cows. Transcriptomic analysis indicated that, in the CH cows, the expression of CPT and CEPT genes in the CDP-choline pathway, the major PC biosynthesis pathway, were decreased, while the expression of BHMT and PEMT genes in the minor PC biosynthesis pathway, were increased in early lactation.ConclusionsInadequate hepatic biotransformation of choline to phospholipids contributes to PC deficiency and hepatic steatosis in the CH cow. Efforts to elevate PC synthesis could enhance hepatic VLDL export, reduce hepatic steatosis, and improve metabolic health of high-producing dairy cows in early lactation.Funding SourcesUSDA grant.

Lipid overload impairs hepatic VLDL secretion via oxidative stress-mediated PKCδ-HNF4α-MTP pathway in large yellow croaker (Larimichthys crocea)

Lipid overload-induced hepatic steatosis is a major public health problem worldwide. However, the potential molecular mechanism is not completely understood. Herein, we found that high-fat diet (HFD) or oleic acid (OA) treatment induced oxidative stress which prevented the entry of hepatocyte nuclear factor 4 alpha (HNF4α) into the nucleus by activating protein kinase C delta (PKCδ) in vivo and in vitro in large yellow croaker (Larimichthys crocea). This reduced the level of microsomal triglyceride transfer protein (MTP) transcription, resulting in the impaired secretion of very-low-density lipoprotein (VLDL) and the abnormal accumulation of triglyceride (TG) in hepatocytes. Meanwhile, the detrimental effects induced by lipid overload could be partly alleviated by pretreating hepatocytes with Go6983 (PKCδ inhibitor) or N-acetylcysteine (NAC, reactive oxygen species (ROS) scavenger). In conclusion, for the first time, we revealed that lipid overload impaired hepatic VLDL secretion via oxidative stress-mediated PKCδ-HNF4α-MTP pathway in fish. This study may provide critical insights into potential intervention strategies against lipid overload-induced hepatic steatosis of fish and human beings.

LZP is required for hepatic triacylglycerol transportation through maintaining apolipoprotein B stability.

The conserved zona pellucida (ZP) domain is found in hundreds of extracellular proteins that are expressed in various organs and play a variety of roles as structural components, receptors and tumor suppressors. A liver-specific zona pellucida domain-containing protein (LZP), also named OIT3, has been shown to be mainly expressed in human and mouse hepatocytes; however, the physiological function of LZP in the liver remains unclear. Here, we show that Lzp deletion inhibited very low-density lipoprotein (VLDL) secretion, leading to hepatic TG accumulation and lower serum TG levels in mice. The apolipoprotein B (apoB) levels were significantly decreased in the liver, serum, and VLDL particles of LZP-deficient mice. In the presence of LZP, which is localized to the endoplasmic reticulum (ER) and Golgi apparatus, the ER-associated degradation (ERAD) of apoB was attenuated; in contrast, in the absence of LZP, apoB was ubiquitinated by AMFR, a known E3 ubiquitin ligase specific for apoB, and was subsequently degraded, leading to lower hepatic apoB levels and inhibited VLDL secretion. Interestingly, hepatic LZP levels were elevated in mice challenged with a high-fat diet and humans with simple hepatic steatosis, suggesting that LZP contributes to the physiological regulation of hepatic TG homeostasis. In general, our data establish an essential role for LZP in hepatic TG transportation and VLDL secretion by preventing the AMFR-mediated ubiquitination and degradation of apoB and therefore provide insight into the molecular function of LZP in hepatic lipid metabolism.

Hepatic Carbohydrate Response Element Binding Protein Activation Limits Nonalcoholic Fatty Liver Disease Development in a Mouse Model for Glycogen Storage Disease Type 1a.

Background and AimsGlycogen storage disease (GSD) type 1a is an inborn error of metabolism caused by defective glucose‐6‐phosphatase catalytic subunit (G6PC) activity. Patients with GSD 1a exhibit severe hepatomegaly due to glycogen and triglyceride (TG) accumulation in the liver. We have shown that the activity of carbohydrate response element binding protein (ChREBP), a key regulator of glycolysis and de novo lipogenesis, is increased in GSD 1a. In the current study, we assessed the contribution of ChREBP to nonalcoholic fatty liver disease (NAFLD) development in a mouse model for hepatic GSD 1a.Approach and ResultsLiver‐specific G6pc–knockout (L‐G6pc −/−) mice were treated with adeno‐associated viruses (AAVs) 2 or 8 directed against short hairpin ChREBP to normalize hepatic ChREBP activity to levels observed in wild‐type mice receiving AAV8–scrambled short hairpin RNA (shSCR). Hepatic ChREBP knockdown markedly increased liver weight and hepatocyte size in L‐G6pc −/− mice. This was associated with hepatic accumulation of G6P, glycogen, and lipids, whereas the expression of glycolytic and lipogenic genes was reduced. Enzyme activities, flux measurements, hepatic metabolite analysis and very low density lipoprotein (VLDL)‐TG secretion assays revealed that hepatic ChREBP knockdown reduced downstream glycolysis and de novo lipogenesis but also strongly suppressed hepatic VLDL lipidation, hence promoting the storage of “old fat.” Interestingly, enhanced VLDL‐TG secretion in shSCR‐treated L‐G6pc −/− mice associated with a ChREBP‐dependent induction of the VLDL lipidation proteins microsomal TG transfer protein and transmembrane 6 superfamily member 2 (TM6SF2), the latter being confirmed by ChIP‐qPCR.ConclusionsAttenuation of hepatic ChREBP induction in GSD 1a liver aggravates hepatomegaly because of further accumulation of glycogen and lipids as a result of reduced glycolysis and suppressed VLDL‐TG secretion. TM6SF2, critical for VLDL formation, was identified as a ChREBP target in mouse liver. Altogether, our data show that enhanced ChREBP activity limits NAFLD development in GSD 1a by balancing hepatic TG production and secretion.

The Patatin-Like Phospholipase Domain Containing Protein 7 Facilitates VLDL Secretion by Modulating ApoE Stability.

The regulation of hepatic very-low-density lipoprotein (VLDL) secretion is vital for lipid metabolism whose pathogenetic status is involved in fatty liver disease and dyslipidemia seen in hepatic steatosis. Accumulated evidence suggest that apolipoprotein E (ApoE) is closely related to hepatic VLDL secretion. Here, we report that the expression of patatin-like phospholipase domain containing protein 7 (PNPLA7) is strongly induced by hepatic steatosis and positively correlates with plasma triacylglycerol (TAG) levels in the human subjects, whereas the role of PNPLA7 in hepatic VLDL secretion is unknown. Herein, with genetic manipulation in the mice, the deficiency of hepatic PNPLA7 expression resulted in reduced VLDL secretion accompanied by enhanced hepatic lipid accumulation and decreased hepatic ApoE expression. Furthermore, knockdown of PNPLA7 in the livers of the db/db mice also resulted in significant reduction in plasma TAG level but aggravated hepatic steatosis. Importantly, we observed that PNPLA7 interacted with ApoE and presumably at the site of endoplasmic reticulum. Mechanistically, we have shown that PNPLA7 could modulate polyubiquitination and proteasomal-mediated degradation of ApoE. Overexpressed ApoE restored the impaired VLDL-TAG metabolism in PNPLA7-knockdown primary hepatocytes. PNPLA7 plays a critical role in regulating hepatic VLDL secretion by modulating ApoE stability through its interaction with ApoE.

Andrographolide modulates HNF4α activity imparting on hepatic metabolism

Hepatic nuclear factor 4 alpha (HNF4α) drives the expression of apolipoprotein B (ApoB), microsomal triglyceride transfer protein (MTP) and phospholipase A2 G12B (PLA2G12B), governing hepatic very-low-density lipoprotein (VLDL) production and secretion. Andrographolide (AP) is a major constituent isolated from Andrographis paniculata. We found that AP can disrupt the interaction between HNF4α and its coactivator peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC1α). Virtual docking and mutational analysis indicated that arginine 235 of HNF4α is essential for binding to AP. As a consequence of antagonizing the activity of HNF4α, AP suppresses the expression of ApoB, MTP and PLA2G12B and reduces the rate of hepatic VLDL secretion in vivo. AP additionally reduced gluconeogenesis via down-regulating the expression of HNF4α target genes phosphoenolpyruvate carboxykinase (Pepck) and glucose-6-phosphatase (G6pc). Collectively, our results suggest that AP affects liver function via modulating the transcriptional activity of HNF4α.

Review article: the emerging role of genetics in precision medicine for patients with non-alcoholic steatohepatitis.

SummaryBackgroundNon‐alcoholic steatohepatitis (NASH) is a severe form of non‐alcoholic fatty liver disease (NAFLD) characterised by liver fat accumulation, inflammation and progressive fibrosis. Emerging data indicate that genetic susceptibility increases risks of NAFLD, NASH and NASH‐related cirrhosis.AimsTo review NASH genetics and discuss the potential for precision medicine approaches to treatment.MethodPubMed search and inclusion of relevant literature.ResultsSingle‐nucleotide polymorphisms in PNPLA3, TM6SF2, GCKR, MBOAT7 and HSD17B13 are clearly associated with NASH development or progression. These genetic variants are common and have moderate‐to‐large effect sizes for development of NAFLD, NASH and hepatocellular carcinoma (HCC). The genes play roles in lipid remodelling in lipid droplets, hepatic very low‐density lipoprotein (VLDL) secretion and de novo lipogenesis. The PNPLA3 I148M variant (rs738409) has large effects, with approximately twofold increased odds of NAFLD and threefold increased odds of NASH and HCC per allele. Obesity interacts with PNPLA3 I148M to elevate liver fat content and increase rates of NASH. Although the isoleucine‐to‐methionine substitution at amino acid position 148 of the PNPLA3 enzyme inactivates its lipid remodelling activity, the effect of PNPLA3 I148M results from trans‐repression of another lipase (ATGL/PNPLA2) by sequestration of a shared cofactor (CGI‐58/ABHD5), leading to decreased hepatic lipolysis and VLDL secretion. In homozygous Pnpla3 I148M knock‐in rodent models of NAFLD, targeted PNPLA3 mRNA knockdown reduces hepatic steatosis, inflammation and fibrosis.ConclusionThe emerging genetic and molecular understanding of NASH paves the way for novel interventions, including precision medicines that can modulate the activity of specific genes associated with NASH.

18β-Glycyrrhetinic acid acts through hepatocyte nuclear factor 4 alpha to modulate lipid and carbohydrate metabolism

Hepatocyte nuclear factor 4 alpha (HNF4α) regulates the expression of essential genes involved in very-low-density lipoprotein (VLDL) homeostasis and gluconeogenesis. 18β-glycyrrhetinic acid (GA) is an active ingredient of Glycyrrhiza uralensis an herbal medicine used for treating liver aliments. In this study, we established that GA functions as a partial antagonist of HNF4α through HNF4α-driven reporter luciferase assay and co-immunoprecipitation experiments with co-activator PGC1α. By virtual docking and site-directed mutagenesis analysis, we confirmed that serine 190 and arginine 235 of HNF4α are both essential for GA to exert its antagonistic action on HNF4α. Importantly, GA suppressed the expression of HNF4α target genes such as apolipoprotein B (ApoB), microsomal triglyceride transfer protein (MTP) and phospholipase A2 G12B (PLA2G12B) modulating hepatic VLDL secretion in mice fed on a high fat diet. In addition, GA also suppressed gluconeogenesis and ameliorated glucose intolerance via down-regulating the expression of HNF4α target genes glucose-6-phosphatase (G6pc) and phosphoenolpyruvate carboxykinase (Pepck). Furthermore, GA significantly lowered blood glucose and improved insulin resistance in db/db mice. In all, we established that GA acts as a partial HNF4α antagonist modulating lipid and carbohydrate metabolism.

Dysfunction of estrogen-related receptor alpha-dependent hepatic VLDL secretion contributes to sex disparity in NAFLD/NASH development.

Rationale: Men and postmenopausal women are more prone to developing non-alcoholic fatty liver disease/steatohepatitis (NAFLD/NASH) than premenopausal women. However, the pathological links and underlying mechanisms of this disparity are still elusive. The sex-difference in hepatic very low-density lipoprotein (VLDL) assembly and secretion may contribute to NAFLD development. Estrogen-related receptor alpha (ERRα) is a key regulator of several metabolic processes. We hypothesized that ERRα plays a role contributing to the sex-difference in hepatic VLDL assembly and secretion.Methods: VLDL secretion and essential genes governing said process were assessed in male and female mice. Liver-specific ERRα-deficient (ERRαLKO) mice were generated to assess the rate of hepatic VLDL secretion and alteration in target gene expression. Overexpression of either microsomal triglyceride transfer protein (Mttp) or phospholipase A2 G12B (Pla2g12b) by adenovirus was performed to test if the fatty liver phenotype in male ERRαLKO mice was due to defects in hepatic VLDL secretion. Female ERRαLKO mice were put on a diet high in saturated fat, fructose and cholesterol (HFHC) to promote NASH development. Wild type female mice were either ovariectomized or treated with tamoxifen to induce a state of estrogen deficiency or disruption in estrogen signaling. Adenovirus was used to overexpress ERRα in these mice to test if ERRα was sufficient to rescue the suppressed VLDL secretion due to estrogen dysfunction. Finally, wild type male mice on a high-fat diet (HFD) were treated with an ERRα inverse agonist to assess if suppressing ERRα activity pharmacologically would lead to fatty liver development.Results: ERRα is an indispensable mediator modulating hepatic triglyceride-rich very low-density lipoprotein (VLDL-TG) assembly and secretion through coordinately controlling target genes apolipoprotein B (Apob), Mttp and Pla2g12b in a sex-different manner. Hepatic VLDL-TG secretion is blunted in ERRαLKO mice, leading to hepatosteatosis which exacerbates endoplasmic reticulum stress and inflammation paving ways for NASH development. Importantly, ERRα acts downstream of estrogen/ERα signaling in contributing to the sex-difference in hepatic VLDL secretion effecting hepatic lipid homeostasis.Conclusions: Our results highlight ERRα as a key mediator which contributes to the sex disparity in NAFLD development, suggesting that selectively restoring ERRα activity in the liver may be a novel strategy for treating NAFLD/NASH.

MiR-130b is a potent stimulator of hepatic very-low-density lipoprotein assembly and secretion via marked induction of microsomal triglyceride transfer protein

miR-130b is a microRNA whose expression is particularly elevated within adipose tissue and in the circulation in diabetic states. Hepatic miR-130b expression has been linked to hepatocellular carcinoma and changes in lipid metabolism. Here, we investigated the role of miR-130b in hepatic lipid homeostasis and lipoprotein export. We observed that overexpression of miR-130b-3p or -5p in HepG2 cells markedly enhanced the secretion of very-low-density lipoprotein (VLDL) particles, enhanced the secretion of [3H]glycerol metabolically labeled triglyceride (TG), and significantly increased the number or the average size of lipid droplets (LDs), respectively. Overexpression of miR-130b also altered the expression of key genes involved in lipid metabolism and in particular markedly increased both mRNA and protein expression levels of microsomal triglyceride transfer protein (MTP). Conversely, the miR-130b inhibitor decreased mRNA levels of MTP and fatty acid synthase (FAS) in HepG2 cells. However, dual-luciferase reporter assays indicated that MTP is not a direct target of miR-130b-3p. miR-130b overexpression did not alter de novo synthesized TG or the stability and secretion of apolipoprotein B 100. Interestingly, knockdown of phosphatase and tensin homolog (PTEN) blocked the upregulation of MTP mRNA induced by miR-130b. Finally, miR-130b-induced stimulation of VLDL secretion was also observed in a second hepatocyte cell culture model, immortalized human hepatocytes, confirming the effects observed in HepG2 cells. Overall, these data suggest a potential role for miR-130b in promoting hepatic VLDL assembly and secretion mediated by marked stimulation of MTP expression and TG mobilization. Thus miR-130b overexpression corrects the defect in VLDL production in HepG2 cells.

Protective Effect of Patchouli Alcohol Against High-Fat Diet Induced Hepatic Steatosis by Alleviating Endoplasmic Reticulum Stress and Regulating VLDL Metabolism in Rats.

Nonalcoholic fatty liver disease (NAFLD) is currently the most common chronic hepatic disorder worldwide. The earliest stage of NAFLD is simple steatosis, which is characterized by the accumulation of triglycerides in hepatocytes. Inhibition of steatosis is a potential treatment for NAFLD. Patchouli alcohol (PA) is an active component of Pogostemon cablin (Blanco) Benth. (Labiatae), which is a medicinal food in Asia countries and proved to possess hepatoprotective effect. This research aimed to investigate the effectiveness of PA against high fat diet (HFD)-induced hepatic steatosis in rats. In this study, male Sprague Dawley rats were fed a HFD for 4 weeks to induce NAFLD. Oral administration with PA significantly reduced pathological severity of steatosis in HFD-fed rats. It was associated with suppressing endoplasmic reticulum (ER) stress and regulating very low-density lipoprotein (VLDL) metabolism. Our data showed that PA treatment effectively attenuated ER stress by inhibiting the activation of protein kinase-like ER kinase (PERK), inositol-requiring transmembrane kinase/endoribonuclease 1 (IRE1), and activating transcription factor 6 (ATF6). Moreover, PA decreased hepatic VLDL uptake by suppressing very low-density lipoprotein receptor (VLDLR) expression. It also restored VLDL synthesis and export by increasing apolipoprotein B100 (apoB 100) secretion and microsomal triglyceride-transfer protein (MTP) activity. Taken together, PA exerted a protective effect on the treatment of NAFLD in HFD-fed rats and may be potential therapeutic agent acting on hepatic steatosis.

Thioesterase Superfamily Member 2 Promotes Hepatic VLDL Secretion by Channeling Fatty Acids Into Triglyceride Biosynthesis.

In nonalcoholic fatty liver disease (NAFLD), triglycerides accumulate within the liver because the rates of fatty acid accrual by uptake from plasma and de novo synthesis exceed elimination by mitochondrial oxidation and secretion as very low-density lipoprotein (VLDL) triglycerides. Thioesterase superfamily member 2 (Them2) is an acyl-coenzyme A (CoA) thioesterase that catalyzes the hydrolysis of fatty acyl-CoAs into free fatty acids plus CoASH. Them2 is highly expressed in the liver, as well as other oxidative tissues. Mice globally lacking Them2 are resistant to diet-induced obesity and hepatic steatosis, and exhibit improved glucose homeostasis. These phenotypes are attributable, at least in part, to roles of Them2 in the suppression of thermogenesis in brown adipose tissue and insulin signaling in skeletal muscle. To elucidate the hepatic function of Them2, we created mice with liver-specific deletion of Them2 (L-Them2-/- ). Although L-Them2-/- mice were not protected against excess weight gain, hepatic steatosis or glucose intolerance, they exhibited marked decreases in plasma triglyceride and apolipoprotein B100 concentrations. These were attributable to reduced rates of VLDL secretion owing to decreased incorporation of plasma-derived fatty acids into triglycerides. The absence of hepatic steatosis in L-Them2-/- mice fed chow was explained by compensatory increases in rates of fatty acid oxidation and by decreased de novo lipogenesis in high fat-fed mice. Consistent with a role for Them2 in hepatic VLDL secretion, THEM2 levels were increased in livers of obese patients with NAFLD characterized by simple steatosis. Conclusion: Them2 functions in the liver to direct fatty acids toward triglyceride synthesis for incorporation into VLDL particles. When taken together with its functions in brown adipose and muscle, these findings suggest that Them2 is a target for the management of NAFLD and dyslipidemia.

Targeting Hepatic Glutaminase 1 Ameliorates Non-Alcoholic Steatohepatitis by Restoring Disrupted Hepatic Very-Low Density Lipoproteins Triglyceride Assembly

Non-Alcoholic Steatohepatitis (NASH) is characterized by the accumulation of hepatic fat in an inflammatory/fibrotic background. Herein, we show that the hepatic high-activity glutaminase 1 isoform (GLS1), the first enzyme of glutamine catabolism, is overexpressed both in clinical and pre-clinical NASH models. After methionine and choline deprivation-induced NASH, GLS1 inhibition restored hepatic phosphatidylcholine content and Very-Low Density Lipoprotein (VLDL) triglyceride export, resulting in reduced hepatic steatosis in mouse primary hepatocytes and in mouse livers. We suggest that, under these circumstances, defective glutamine fueling of anaplerotic mitochondrial metabolism and concomitant reduction of oxidative stress promotes a reprograming of serine metabolism, where serine is shifted from the generation of the antioxidant glutathione and channeled to provide one-carbon units to regenerate the methionine cycle. Restored methionine cycle induces phosphatidylcholine synthesis both from the CDP-choline and the phosphatidylethanolamine N-methyltransferase-mediated pathways. Overall, we provide evidence that hepatic GLS1 targeting is a valuable therapeutic approach in NASH.

GLP-2 Dysregulates Hepatic Lipoprotein Metabolism, Inducing Fatty Liver and VLDL Overproduction in Male Hamsters and Mice.

Fundamental complications of insulin resistance and type 2 diabetes include the development of nonalcoholic fatty liver disease and an atherogenic fasting dyslipidemic profile, primarily due to increases in hepatic very-low-density lipoprotein (VLDL) production. Recently, central glucagon-like peptide-2 receptor (GLP2R) signaling has been implicated in regulating hepatic insulin sensitivity; however, its role in hepatic lipid and lipoprotein metabolism is unknown. We investigated the role of glucagon-like peptide-2 (GLP-2) in regulating hepatic lipid and lipoprotein metabolism in Syrian golden hamsters, C57BL/6J mice, and Glp2r-/- mice consuming either a normal chow or high-fat diet (HFD). In the chow-fed hamsters, IP GLP-2 administration significantly increased fasting dyslipidemia, hepatic VLDL production, and the expression of key genes involved in hepatic de novo lipogenesis. In HFD-fed hamsters and chow-fed mice, GLP-2 administration exacerbated or induced hepatic lipid accumulation. HFD-fed Glp2r-/- mice displayed reduced glucose tolerance, VLDL secretion, and microsomal transfer protein lipid transfer activity, as well as exacerbated fatty liver. Thus, we conclude that GLP-2 plays a lipogenic role in the liver by increasing lipogenic gene expression and inducing hepatic steatosis, fasting dyslipidemia, and VLDL overproduction. In contrast, the lack of Glp2r appears to interfere with VLDL secretion, resulting in enhanced hepatic lipid accumulation. These studies have uncovered a role for GLP-2 in maintaining hepatic lipid and lipoprotein homeostasis.

Pathophysiology of Diabetic Dyslipidemia

Accumulating clinical evidence has suggested serum triglyceride (TG) is a leading predictor of atherosclerotic cardiovascular disease, comparable to low-density lipoprotein (LDL)-cholesterol (C) in populations with type 2 diabetes, which exceeds the predictive power of hemoglobinA1c. Atherogenic dyslipidemia in diabetes consists of elevated serum concentrations of TG-rich lipoproteins (TRLs), a high prevalence of small dense low-density lipoprotein (LDL), and low concentrations of cholesterol-rich high-density lipoprotein (HDL)2-C. A central lipoprotein abnormality is an increase in large TG-rich very-low-density lipoprotein (VLDL)1, and other lipoprotein abnormalities are metabolically linked to increased TRLs. Insulin critically regulates serum VLDL concentrations by suppressing hepatic VLDL production and stimulating VLDL removal by activation of lipoprotein lipase. It is still debated whether hyperinsulinemia compensatory for insulin resistance is causally associated with the overproduction of VLDL. This review introduces experimental and clinical observations revealing that insulin resistance, but not hyperinsulinemia stimulates hepatic VLDL production. LDL and HDL consist of heterogeneous particles with different size and density. Cholesterol-depleted small dense LDL and cholesterol-rich HDL2 subspecies are particularly affected by insulin resistance and can be named “Metabolic LDL and HDL,” respectively. We established the direct assays for quantifying small dense LDL-C and small dense HDL(HDL3)-C, respectively. Subtracting HDL3-C from HDL-C gives HDL2-C. I will explain clinical relevance of measurements of LDL and HDL subspecies determined by our assays. Diabetic kidney disease (DKD) substantially worsens plasma lipid profile thereby potentiated atherogenic risk. Finally, I briefly overview pathophysiology of dyslipidemia associated with DKD, which has not been so much taken up by other review articles.

Phytoceuticals in Fenugreek Ameliorate VLDL Overproduction and Insulin Resistance via the Insig Signaling Pathway.

This study aims to characterize the effect of fenugreek (Trigonella foenum-graecum) seed and its phytoceutical trigonelline in antimetabolic inflammation and ameliorating overproduction of very low density lipoprotein (VLDL) in insulin resistance. Two groups of genetic hyperlipidemic mice generated by depletion of cAMP responsive element binding protein H (CREBH) are fed either a chow containing 2% fenugreek seed or vehicle for 7 weeks. Q-RT-PCR and immunoblotting analysis demonstrated that fenugreek seed containing diet inhibits hepatic SREBP-1c activation and the subsequent de novo lipogenesis by enhancing expression of insulin-inducible gene-1 (Insig-1) and gene-2 (Insig-2). mRNA expression of PPARαand its target genes that are involved in fatty acid β-oxidation are also upregulated in the fenugreek seed fed-mice which is accompanied by significantly reduced hepatic lipid accumulation and VLDL secretion, improved endoplasmic reticulum (ER) stress, and ameliorated metabolic inflammation. These actions enhance insulin sensitivity and improve hyperlipidemia. In vitro, treating a rat hepatoma cell line, McA-RH7777 (McA), with trigonelline is able to recapitulate the results observed in vivo. This study unveils a novel mechanism of fenugreek seed and trigonelline in countering hepatic VLDL overproduction and insulin resistance by enhancing the Insig signaling pathways and ameliorating metabolic inflammatory stress in the liver.

Quercetin ameliorates HFD-induced NAFLD by promoting hepatic VLDL assembly and lipophagy via the IRE1a/XBP1s pathway

The consumption of a quercetin-rich diet has been well-established as a feasible method against non-alcoholic fatty liver disease (NAFLD); however, the molecular mechanisms underlying the progression of NAFLD and its intervention by quercetin remain largely obscure. Male Sprague-Dawley rats fed high-fat diet (HFD), and HepG2 cells stimulated with free fatty acid, were treated with quercetin and various pharmacological reagents to explore the effect of signaling pathways involved in endoplasmic reticulum stress on very low-density lipoprotein (VLDL) assembly and lipophagy. Quercetin intake decreased hepatic TG content by 39%, with a 1.5-fold increase in VLDL, and up-regulated spliced X-box binding protein 1 (XBP1s) expression compared with the HFD group. Thapsigargin or STF-083010 (an IRE1α endonuclease inhibitor) decreased VLDL content in a dose-dependent manner, partially counteracting the protective effects of quercetin, 4-PBA or APY-29 (an IRE1α endonuclease activator). Additionally, microsomal TG-transfer protein complex expression was increased with quercetin-treated and down-regulated by STF-083010. Moreover, quercetin increased co-localization of lysosomes with lipid droplets (LDs) accompanied by decreased p62 accumulation. STF-083010 partially abolished the effect of quercetin on LDs autophagy in an mTOR-independent manner. Collectively, these findings demonstrate that hepatic VLDL assembly and lipophagy are the main targets of quercetin against NAFLD via the IRE1a/XBP1s pathway.

Targeting hepatic heparin-binding EGF-like growth factor (HB-EGF) induces anti-hyperlipidemia leading to reduction of angiotensin II-induced aneurysm development.

ObjectiveThe upregulated expression of heparin binding EGF-like growth factor (HB-EGF) in the vessel and circulation is associated with risk of cardiovascular disease. In this study, we tested the effects of HB-EGF targeting using HB-EGF-specific antisense oligonucleotide (ASO) on the development of aortic aneurysm in a mouse aneurysm model.Approach and resultsLow-density lipoprotein receptor (LDLR) deficient mice (male, 16 weeks of age) were injected with control and HB-EGF ASOs for 10 weeks. To induce aneurysm, the mice were fed a high fat diet (22% fat, 0.2% cholesterol; w/w) at 5 week point of ASO administration and infused with angiotensin II (AngII, 1,000ng/kg/min) for the last 4 weeks of ASO administration. We confirmed that the HB-EGF ASO administration significantly downregulated HB-EGF expression in multiple tissues including the liver. Importantly, the HB-EGF ASO administration significantly suppressed development of aortic aneurysms including thoracic and abdominal types. Interestingly, the HB-EGF ASO administration induced a remarkable anti-hyperlipidemic effect by suppressing very low density lipoprotein (VLDL) level in the blood. Mechanistically, the HB-EGF targeting suppressed hepatic VLDL secretion rate without changing heparin-releasable plasma triglyceride (TG) hydrolytic activity or fecal neutral cholesterol excretion rate.ConclusionThis result suggested that the HB-EGF targeting induced protection against aneurysm development through anti-hyperlipidemic effects. Suppression of hepatic VLDL production process appears to be a key mechanism for the anti-hyperlipidemic effects by the HB-EGF targeting.

Tamoxifen reduces hepatic VLDL production and GH secretion in women: a possible mechanism for steatosis development.

Growth hormone (GH) stimulates hepatic synthesis of very-low-density lipoproteins (VLDL), whereas hepatic steatosis develops as a result of GH deficiency. Steatosis is also a complication of tamoxifen treatment, the cause of which is not known. As tamoxifen inhibits the secretion and action of GH, we hypothesize that it induces steatosis by inhibiting hepatic VLDL export. To investigate whether tamoxifen reduces hepatic VLDL secretion. Eight healthy, normolipidemic women (age: 64.4 ± 2.1 years) were studied in random sequence at baseline, after 2 weeks of tamoxifen (20 mg/day) and after 2 weeks of estradiol valerate (EV; 2 mg/day) treatments, separated by a 4-week washout period. The kinetics of apolipoprotein B (apoB), the structural protein of VLDL particles, were measured using a stable isotope 2H3-leucine turnover technique. VLDL-apoB fractional catabolic rate (FCR) was determined using a multicompartment model. VLDL-apoB secretion was estimated as the product of FCR and VLDL-apoB concentration. GH response to arginine stimulation, circulating levels of IGF-1, FFA, and TG, along with TG content in VLDL were measured. Tamoxifen significantly (P < 0.05) reduced VLDL-apoB concentration and secretion by 27.3 ± 7.8% and 29.8 ± 10.2%, respectively. In contrast, EV did not significantly change VLDL-apoB concentration or secretion. Tamoxifen but not EV significantly reduced (P < 0.05) GH response to arginine stimulation. Both treatments significantly lowered (P < 0.05) circulating IGF-1. Inhibition of VLDL secretion may contribute to the development of fatty liver during tamoxifen therapy. As GH stimulates VLDL secretion, the development of steatosis may arise secondarily from GH insufficiency induced by tamoxifen.