High insulin levels are required for FAT/CD36 plasma membrane translocation and enhanced fatty acid uptake in obese Zucker rat hepatocytes

Cholangiocarcinoma progression depends on the uptake and metabolization of extracellular lipids

BACKGROUND AND AIMS

Cholangiocarcinoma (CCA) includes a heterogeneous group of biliary cancers with a dismal prognosis. We investigated if lipid metabolism is disrupted in CCA and its role in tumor proliferation.

APPROACH AND RESULTS

The in vitro and in vivo tumorigenic capacity of five human CCA cell lines was analyzed. Proteome, lipid content, and metabolic fluxes were evaluated in CCA cells and compared with normal human cholangiocytes (NHC). The Akt1/NOTCH1 intracellular cytoplasmic domain (Nicd1)-driven CCA mouse model was also evaluated. The proteome of CCA cells was enriched in pathways involved in lipid and lipoprotein metabolism. The EGI1 CCA cell line presented the highest tumorigenic capacity. Metabolic studies in high (EGI1) versus low (HUCCT1) proliferative CCA cells in vitro showed that both EGI1 and HUCCT1 incorporated more fatty acids (FA) than NHC, leading to increased triglyceride storage, also observed in Akt1/Nicd1-driven CCA mouse model. The highly proliferative EGI1 CCA cells showed greater uptake of very-low-density and HDLs than NHC and HUCCT1 CCA cells and increased cholesteryl ester content. The FA oxidation (FAO) and related proteome enrichment were specifically up-regulated in EGI1, and consequently, pharmacological blockade of FAO induced more pronounced inhibition of their tumorigenic capacity compared with HUCCT1. The expression of acyl-CoA dehydrogenase ACADM, the first enzyme involved in FAO, was increased in human CCA tissues and correlated with the proliferation marker PCNA.

CONCLUSIONS

Highly proliferative human CCA cells rely on lipid and lipoprotein uptake to fuel FA catabolism, suggesting that inhibition of FAO and/or lipid uptake could represent a therapeutic strategy for this CCA subclass.

Fatty acid uptake in Trypanosoma brucei: Host resources and possible mechanisms

Trypanosoma brucei spp. causes African Sleeping Sickness in humans and nagana, a wasting disease, in cattle. As T. brucei goes through its life cycle in its mammalian and insect vector hosts, it is exposed to distinct environments that differ in their nutrient resources. One such nutrient resource is fatty acids, which T. brucei uses to build complex lipids or as a potential carbon source for oxidative metabolism. Of note, fatty acids are the membrane anchoring moiety of the glycosylphosphatidylinositol (GPI)-anchors of the major surface proteins, Variant Surface Glycoprotein (VSG) and the Procyclins, which are implicated in parasite survival in the host. While T. brucei can synthesize fatty acids de novo, it also readily acquires fatty acids from its surroundings. The relative contribution of parasite-derived vs. host-derived fatty acids to T. brucei growth and survival is not known, nor have the molecular mechanisms of fatty acid uptake been defined. To facilitate experimental inquiry into these important aspects of T. brucei biology, we addressed two questions in this review: (1) What is known about the availability of fatty acids in different host tissues where T. brucei can live? (2) What is known about the molecular mechanisms mediating fatty acid uptake in T. brucei? Finally, based on existing biochemical and genomic data, we suggest a model for T. brucei fatty acid uptake that proposes two major routes of fatty acid uptake: diffusion across membranes followed by intracellular trapping, and endocytosis of host lipoproteins.

Improving obesity and lipid metabolism using conjugated linoleic acid

BACKGROUND

Conjugated linoleic acid (CLA) can prevent fatty acid accumulation induced by a high-fructose diet and improve lipid metabolism disorders in patients.

OBJECTIVES

We aimed to investigate the effect of CLA on obesity and lipid metabolism and its possible mechanism.

METHODS

Eight-month-old male BKS.Cg-Dock7m +/+ Leprdb /JNju (db/db) mice (n = 12) were fed a CLA mix composed of equivalent c9, t11-CLA and t10, c12-CLA for 1 month. The effect of CLA on body weight, water and food intake, and triglyceride (TG) and total cholesterol (TC) levels was investigated. PPARα, PPARγ and CD36 expression was determined by quantitative PCR and western blotting. Additionally, the expression of these three genes was studied in HepG2 cells treated with CLA and linoleic acid.

RESULTS

CLA treatment notably reduced the dietary and water intake of db/db mice, effectively reduced body weight, and decreased serum TG and TC levels (p < 0.05). Increased expression of PPARα (p < 0.05) and decreased expression of CD36 (p < 0.001) were observed in the liver of mice that were fed CLA. CLA increased PPARα expression (p < 0.001) and decreased PPARγ (p < 0.001) and CD36 expression (p < 0.01) in HepG2 cells.

CONCLUSIONS

Our results showed that CLA can improve lipid metabolism in obese mice through upregulation of PPARα expression and downregulation of CD36 expression.

Differences in lipid metabolism in acquired versus preexisting glucose intolerance during gestation: role of free fatty acids and sphingosine-1-phosphate

BACKGROUND

The prevalence of gestational diabetes mellitus (GDM) is increasing worldwide. There is increasing evidence that GDM is a heterogeneous disease with different subtypes. An important question in this context is whether impaired glucose tolerance (IGT), which is a typical feature of the disease, may already be present before pregnancy and manifestation of the disease. The latter type resembles in its clinical manifestation prediabetes that has not yet manifested as type 2 diabetes (T2DM). Altered lipid metabolism plays a crucial role in the disorder's pathophysiology. The aim was to investigate the role of lipids which are relevant in diabetes-like phenotypes in these both models with different time of initial onset of IGT.

METHODS

Two rodent models reflecting different characteristics of human GDM were used to characterize changes in lipid metabolism occurring during gestation. Since the New Zealand obese (NZO)-mice already exhibit IGT before and during gestation, they served as a subtype model for GDM with preexisting IGT (preIGT) and were compared with C57BL/6 N mice with transient IGT acquired during gestation (aqIGT). While the latter model does not develop manifest diabetes even under metabolic stress conditions, the NZO mouse is prone to severe disease progression later in life. Metabolically healthy Naval Medical Research Institute (NMRI) mice served as controls.

RESULTS

In contrast to the aqIGT model, preIGT mice showed hyperlipidemia during gestation with elevated free fatty acids (FFA), triglycerides (TG), and increased atherogenic index. Interestingly, sphingomyelin (SM) concentrations in the liver decreased during gestation concomitantly with an increase in the sphingosine-1-phosphate (S1P) concentration in plasma. Further, preIGT mice showed impaired hepatic weight adjustment and alterations in hepatic FFA metabolism during gestation. This was accompanied by decreased expression of peroxisome proliferator-activated receptor alpha (PPARα) and lack of translocation of fatty acid translocase (FAT/CD36) to the hepatocellular plasma membrane.

CONCLUSION

The preIGT model showed impaired lipid metabolism both in plasma and liver, as well as features of insulin resistance consistent with increased S1P concentrations, and in these characteristics, the preIGT model differs from the common GDM subtype with aqIGT. Thus, concomitantly elevated plasma FFA and S1P concentrations, in addition to general shifts in sphingolipid fractions, could be an interesting signal that the metabolic disorder existed before gestation and that future pregnancies require more intensive monitoring to avoid complications. This graphical abstract was created with BioRender.com .

High Hydrostatic Pressure Extract of Mulberry Leaf Attenuated Obesity-Induced Inflammation in Rats

Low-grade inflammation might be a link between obesity and obesity-associated metabolic dysfunction, including diabetes, hepatic steatosis, and other health complications. This study investigated whether the supplementation of high hydrostatic pressure extract of mulberry (Morus alba L.) leaves (HML) to obese rats could counteract obesity-related inflammation. Three-week-old male Sprague-Dawley rats were separated into three groups as follows: (a) a normal diet, (b) 45% high-fat (HF) diet, and HF diet containing 0.4% HML (c) or 0.8% HML (d) (IACUC No. 17-033). After 14 weeks of HML supplementation, adipose tissue mass, mRNA expression of adipogenic genes, such as aP2, peroxisome proliferator-activated receptor γ (PPARγ), and sterol regulatory element binding protein 1c (SREBP1c), and macrophage recruitment were significantly decreased in HF-fed obese rats. Serum concentrations of nitric oxide and mRNA levels of arginase1 (Arg1), CD11c, and inducible nitric oxide synthase (iNOS) involved in adipose tissue macrophage M1 polarization were also significantly reduced by HML. Moreover, HML alleviated the serum and hepatic lipid profiles and reduced hepatic lipogenic gene expression of acetyl-CoA carboxylase (ACC), cluster of differentiation 36 (CD36), CPT1, fatty acid synthase (FAS), stearoyl-CoA desaturase (SCD1), and SREBP1c, and inflammation-associated genes, including IL1β, interleukin 6 (IL6), and tumor necrosis factor α (TNFα). Serum IL6 and TNFα levels were remarkedly suppressed in the 0.8% HML group. These results suggested that the favorable effect of HML on obesity-associated inflammation might be related in part to the decrease in adipose tissue and hepatic fat deposition and inflammation.

Overexpression of PDE4D in mouse liver is sufficient to trigger NAFLD and hypertension in a CD36-TGF-β1 pathway: therapeutic role of roflumilast

Emerging evidence has shown that nonalcoholic fatty liver disease (NAFLD) may be both a consequence and a cause of hypertension. Recent studies have demonstrated that phosphodiesterase 4 (PDE4)-cAMP signaling represents a pathway relevant to the pathophysiology of metabolic disorders. This study aims to investigate the impact and the underlying mechanism of PDE4 in the pathogenesis of NAFLD and its associated hypertension. Here we demonstrated that high-fat-diet (HFD) fed mice developed NAFLD and hypertension, with an associated increase in hepatic PDE4D expression, which can be prevented and even reversed by PDE4 inhibitor roflumilast. Furthermore, we demonstrated that hepatic overexpression of PDE4D drove significant hepatic steatosis and elevation of blood pressure. Mechanistically, PDE4D activated fatty acid translocase CD36 signaling which facilitates hepatic lipid deposition, resulting in TGF-β1 production by hepatocytes and excessive TGF-β1 signaling in vessels and consequent hypertension. Specific silencing of TGF-β1 in hepatocytes by siRNA using poly (β-amino ester) nanoparticles significantly normalized hepatic PDE4D overexpression-activated TGF-β1 signaling in vessels and hypertension. Together, the conclusions indicated that PDE4D plays an important role in the pathogenesis of NAFLD and associated hypertension via activation of CD36-TGF-β1 signaling in the liver. PDE4 inhibitor such as roflumilast, which is clinically approved for chronic obstructive pulmonary disease (COPD) treatment, has the potential to be used as a preventive or therapeutic drug against NAFLD and associated hypertension in the future.

Alterations in Lysosome Homeostasis in Lipid-Related Disorders: Impact on Metabolic Tissues and Immune Cells

Lipid-related disorders, which primarily affect metabolic tissues, including adipose tissue and the liver are associated with alterations in lysosome homeostasis. Obesity is one of the more prevalent diseases, which results in energy imbalance within metabolic tissues and lysosome dysfunction. Less frequent diseases include Niemann-Pick type C (NPC) and Gaucher diseases, both of which are known as Lysosomal Storage Diseases (LSDs), where lysosomal dysfunction within metabolic tissues remains to be fully characterized. Adipocytes and hepatocytes share common pathways involved in the lysosome-autophagic axis, which are regulated by the function of cathepsins and CD36, an immuno-metabolic receptor and display alterations in lipid diseases, and thereby impacting metabolic functions. In addition to intrinsic defects observed in metabolic tissues, cells of the immune system, such as B cells can infiltrate adipose and liver tissues, during metabolic imbalance favoring inflammation. Moreover, B cells rely on lysosomes to promote the processing and presentation of extracellular antigens and thus could also present lysosome dysfunction, consequently affecting such functions. On the other hand, growing evidence suggests that cells accumulating lipids display defective inter-organelle membrane contact sites (MCSs) established by lysosomes and other compartments, which contribute to metabolic dysfunctions at the cellular level. Overall, in this review we will discuss recent findings addressing common mechanisms that are involved in lysosome dysregulation in adipocytes and hepatocytes during obesity, NPC, and Gaucher diseases. We will discuss whether these mechanisms may modulate the function of B cells and how inter-organelle contacts, emerging as relevant cellular mechanisms in the control of lipid homeostasis, have an impact on these diseases.

Contribution of rs3211938 polymorphism at CD36 to glucose levels, oxidized low-density lipoproteins, insulin resistance, and body mass index in Mexican mestizos with type-2 diabetes from western Mexico

Introduction

Background: type-2 diabetes mellitus (T2DM) is a chronic metabolic disorder characterized by hyperglycemia, insulin resistance (IR), and abnormal fatty acid metabolism in which the CD36 receptor has been implicated in glucose and lipid dysregulation. Objective: to evaluate the contribution of polymorphism CD36 rs3211938 to metabolic profile in T2DM Mexican mestizos from western Mexico. Methods: we included 115 individuals classified as non-T2DM (NT2DM) adults and T2DM patients. Polymorphism CD36 rs3211938 was assessed by PCR-RFLP. Anthropometric and metabolic markers were measured by routine methods, and insulin and oxidized LDL (ox-LDL) were measured by ELISA. Results: the distribution of genotypes between NT2DM and T2DM patients was different (p < 0.001), as was the allele frequency (p = 0.002). NT2DM TG carriers showed the lowest levels of basal insulin and HOMA-IR index in comparison with TT carriers (p < 0.05 and p < 0.05, respectively). In the T2DM group TG carriers showed high BMI, WHR, and weight values (p = 0.001; p ≤ 0.05 and p < 0.05, respectively), and the highest levels of basal glucose, HDL-cholesterol, ox-LDL, and HOMA-IR (p < 0.001; p < 0.001; p < 0.001, and p = 0.001, respectively) in comparison with diabetic TT carriers. Conclusion: the CD36 rs3211938 TG genotype is associated with high levels of glucose, ox-LDL, HDL-cholesterol, and IR, and with increased BMI in Mexican mestizo T2DM patients from western Mexico.

Hypoxia-inducible factor 2α drives hepatosteatosis through the fatty acid translocase CD36

BACKGROUND & AIMS

Molecular mechanisms by which hypoxia might contribute to hepatosteatosis, the earliest stage in non-alcoholic fatty liver disease (NAFLD) pathogenesis, remain still to be elucidated. We aimed to assess the impact of hypoxia-inducible factor 2α (HIF2α) on the fatty acid translocase CD36 expression and function in vivo and in vitro.

METHODS

CD36 expression and intracellular lipid content were determined in hypoxic hepatocytes, and in hypoxic CD36- or HIF2α -silenced human liver cells. Histological analysis, and HIF2α and CD36 expression were evaluated in livers from animals in which von Hippel-Lindau (Vhl) gene is inactivated (Vhl^f/f -deficient mice), or both Vhl and Hif2a are simultaneously inactivated (Vhl^f/f Hif2α^/f -deficient mice), and from 33 biopsy-proven NAFLD patients and 18 subjects with histologically normal liver.

RESULTS

In hypoxic hepatocytes, CD36 expression and intracellular lipid content were augmented. Noteworthy, CD36 knockdown significantly reduced lipid accumulation, and HIF2A gene silencing markedly reverted both hypoxia-induced events in hypoxic liver cells. Moreover livers from Vhl^f/f -deficient mice showed histologic characteristics of non-alcoholic steatohepatitis (NASH) and increased CD36 mRNA and protein amounts, whereas both significantly decreased and NASH features markedly ameliorated in Vhl^f/f Hif2α^f/f -deficient mice. In addition, both HIF2α and CD36 were significantly overexpressed within the liver of NAFLD patients and, interestingly, a significant positive correlation between hepatic transcript levels of CD36 and erythropoietin (EPO), a HIF2α -dependent gene target, was observed in NAFLD patients.

CONCLUSIONS

This study provides evidence that HIF2α drives lipid accumulation in human hepatocytes by upregulating CD36 expression and function, and could contribute to hepatosteatosis setup.

A novel bile acid analog, A17, ameliorated non-alcoholic steatohepatitis in high-fat diet-fed hamsters

Being endocrine signaling molecules that regulate lipid metabolism and affect energy balance, bile acids are potential drug candidates for non-alcoholic steatohepatitis (NASH). Obeticholic acid (OCA) could improve NASH accompanied by significant side effects. Therefore, it is worthwhile to develop safer and more effective bile acid analogs. In this study, a new bile acid analog A17 was synthesized and its potential anti-NASH effects were assessed in vitro and in vivo. The impact of A17 on steatosis was investigated in the rat primary hepatocytes challenged with oleic acid. It was found that A17 alleviated lipid accumulation by reducing fatty acid (FA) uptake and promoting FA oxidation. The reduction of FA uptake came from inhibiting fatty acid translocase (Cd36) expression. The promotion of FA oxidation came from stimulating the phosphorylation of adenosine monophosphate (AMP)-activated protein kinase alpha (AMPKα). In addition, A17 reduced lipopolysaccharide-induced inflammation in Raw264.7 cells by activating Takeda G protein-coupled receptor 5 (TGR5). In in vivo study, male Golden Syrian hamsters were fed with high fat (HF) diet and then treated with 50 mg/kg/d A17 for 6 weeks. A17 lowered the lipid profiles and liver enzyme levels in serum and improved liver pathological conditions with less side effects compared with OCA. Further studies confirmed that the molecular mechanisms of A17 in vivo were similar to those in vitro. In conclusion, a novel bile acid analog A17 was identified to ameliorate NASH in HF-fed hamsters. The potential mechanisms could be contributed to reducing FA uptake, stimulating FA oxidation and relieving inflammation.

Understanding lipotoxicity in NAFLD pathogenesis: is CD36 a key driver?

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease worldwide. NAFLD stages range from simple steatosis (NAFL) to non-alcoholic steatohepatitis (NASH) which can progress to cirrhosis and hepatocellular carcinoma. One of the crucial events clearly involved in NAFLD progression is the lipotoxicity resulting from an excessive fatty acid (FFA) influx to hepatocytes. Hepatic lipotoxicity occurs when the capacity of the hepatocyte to manage and export FFAs as triglycerides (TGs) is overwhelmed. This review provides succinct insights into the molecular mechanisms responsible for lipotoxicity in NAFLD, including ER and oxidative stress, autophagy, lipoapotosis and inflammation. In addition, we highlight the role of CD36/FAT fatty acid translocase in NAFLD pathogenesis. Up-to-date, it is well known that CD36 increases FFA uptake and, in the liver, it drives hepatosteatosis onset and might contribute to its progression to NASH. Clinical studies have reinforced the significance of CD36 by showing increased content in the liver of NAFLD patients. Interestingly, circulating levels of a soluble form of CD36 (sCD36) are abnormally elevated in NAFLD patients and positively correlate with the histological grade of hepatic steatosis. In fact, the induction of CD36 translocation to the plasma membrane of the hepatocytes may be a determining factor in the physiopathology of hepatic steatosis in NAFLD patients. Given all these data, targeting the fatty acid translocase CD36 or some of its functional regulators may be a promising therapeutic approach for the prevention and treatment of NAFLD.

SIRT3 deficiency exacerbates fatty liver by attenuating the HIF1α-LIPIN 1 pathway and increasing CD36 through Nrf2

Background

Deficiency of mitochondrial sirtuin 3 (SIRT3), a NAD⁺-dependent protein deacetylase that maintains redox status and lipid homeostasis, contributes to hepatic steatosis. In this study, we investigated additional mechanisms that might play a role in aggravating hepatic steatosis in Sirt3-deficient mice fed a high-fat diet (HFD).

Methods

Studies were conducted in wild-type (WT) and Sirt3^−/− mice fed a standard diet or a HFD and in SIRT3-knockdown human Huh-7 hepatoma cells.

Results

Sirt3^−/− mice fed a HFD presented exacerbated hepatic steatosis that was accompanied by decreased expression and DNA-binding activity of peroxisome proliferator-activated receptor (PPAR) α and of several of its target genes involved in fatty acid oxidation, compared to WT mice fed the HFD. Interestingly, Sirt3 deficiency in liver and its knockdown in Huh-7 cells resulted in upregulation of the nuclear levels of LIPIN1, a PPARα co-activator, and of the protein that controls its levels and localization, hypoxia-inducible factor 1α (HIF-1α). These changes were prevented by lipid exposure through a mechanism that might involve a decrease in succinate levels. Finally, Sirt3^−/− mice fed the HFD showed increased levels of some proteins involved in lipid uptake, such as CD36 and the VLDL receptor. The upregulation in CD36 was confirmed in Huh-7 cells treated with a SIRT3 inhibitor or transfected with SIRT3 siRNA and incubated with palmitate, an effect that was prevented by the Nrf2 inhibitor ML385.

Conclusion

These findings demonstrate new mechanisms by which Sirt3 deficiency contributes to hepatic steatosis.

The Multifunctionality of CD36 in Diabetes Mellitus and Its Complications-Update in Pathogenesis, Treatment and Monitoring

CD36 is a multiligand receptor contributing to glucose and lipid metabolism, immune response, inflammation, thrombosis, and fibrosis. A wide range of tissue expression includes cells sensitive to metabolic abnormalities associated with metabolic syndrome and diabetes mellitus (DM), such as monocytes and macrophages, epithelial cells, adipocytes, hepatocytes, skeletal and cardiac myocytes, pancreatic β-cells, kidney glomeruli and tubules cells, pericytes and pigment epithelium cells of the retina, and Schwann cells. These features make CD36 an important component of the pathogenesis of DM and its complications, but also a promising target in the treatment of these disorders. The detrimental effects of CD36 signaling are mediated by the uptake of fatty acids and modified lipoproteins, deposition of lipids and their lipotoxicity, alterations in insulin response and the utilization of energy substrates, oxidative stress, inflammation, apoptosis, and fibrosis leading to the progressive, often irreversible organ dysfunction. This review summarizes the extensive knowledge of the contribution of CD36 to DM and its complications, including nephropathy, retinopathy, peripheral neuropathy, and cardiomyopathy.

High dietary n6/n3 ratio decreases eicosapentaenoic to arachidonic acid ratios and upregulates NFκB/p50 expression in short-term low-dose streptozotocin and high-fructose rat model of diabetes

We studied the influence of dietary n6/n3 ratio and docosahexaenoic (DHA) and eicosapentaenoic (EPA) acids supplementation on fatty acid profile, lipid peroxidation and NFκ/p50 expression in diabetes type 2. Treatments consisted of three dietary n6/n3 ratios: 6 (Control), 50 (high n6) and 1 (DHA and EPA supplemented). Half of the rats in each of the dietary treatments were made diabetic using the fructose/low-streptozotocin model. The Control and high n6 diets decreased EPA/ARA (arachidonic acid) ratios in the plasma and in the hepatic tissue suggesting proinflammatory fatty acid profile. The high n6 diet additionally increased the 4-HNE and NFκ/p50 expression in the hepatic tissue. These changes were the consequence of a decrease in the plasma content of DHA and EPA and an increase in the content of arachidonic acid in the liver neutral lipids. The supplementation with the DHA and EPA attenuated the change in EPA/ARA ratios, which imply the importance of the n6/n3 ratio in diabetes type 2.

[Conjugated linoleic acid improves glucose and lipid metabolism in diabetic mice]

OBJECTIVE

To analyze the effect of conjugated linoleic acid (CLA) on glucose and lipid metabolism in obese diabetic (db/db) mice.

METHODS

db/db mice were randomized for treatment with saline or CLA mixture administered intragastrically. The changes in body weight, dietary intake, water intake, oral glucose tolerance, triglyceride and total cholesterol were recorded after the treatments. HE staining and oil red O staining were used to assess liver pathologies and fatty acid content. The expression levels of PPARα, PPARγ, CD36, CHREBP and SREBP-1c were detected using real-time PCR and Western blotting. HepG2 cells were treated with CLA and linoleic acid and the expressions of PPARα, ACC, P-ACC, and CD36 were detected; the level of acetyl-CoA in the cell supernatant was detected using ELISA.

RESULTS

CLA treatment obviously reduced the dietary and water intake of db/db mice, effectively reduced the body weight and decreased serum triglyceride and cholesterol levels (P &lt; 0.05). CLA significantly reduced fasting blood glucose, increased glucose tolerance, reduced the accumulation of lipid droplets in the liver and improved lipid metabolism in db/db mice. The mice showed significantly increased expression of PPARα (P &lt; 0.05) and lowered CD36 expression (P &lt; 0.001) in the liver after CLA treatment. Cellular experiments showed that CLA significantly up-regulated PPARα (P &lt; 0.001) and P-ACC and decreased the expression of CD36 (P &lt; 0.01). ELISA showed that acetyl-CoA was significantly up-regulated in the cells after CLA treatment (P &lt; 0.01).

CONCLUSIONS

The mixture of two conjugated linoleic acid isomers can reduce fasting blood glucose, increase glucose tolerance and improve glycolipid metabolism in db/db mice by enhancing the expression of PPARα, increasing P-ACC and inhibiting CD36 expression.

Inflammatory stress and altered angiogenesis evoked by very high-fat diets in mouse liver

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD), a condition that leads to fibrosis, is caused by intake of very high-fat diets (HFDs). However, while the negative impact on the liver of these diets has been an issue of interest, systematic research on the effect of HFDs are lacking.

OBJECTIVE

To characterize the overall impact of HFDs on both molecular and morphological signs of liver remodeling.

METHODS

A study was conducted on male C57BL/6J mice to assess the effect of 4- and 8-week HFDs (60% kcal from fat) on (i) liver steatosis and fibrosis, and (ii) expression of factors involved in inflammation and angiogenesis.

RESULTS

After an 8-week HFD, vascular endothelial growth factor type-2 receptor (VEGF-R2) and fatty acid translocase/trombospondin-1 receptor (CD36) were overexpressed in liver tissue of mice given HFDs. These changes suggest impaired liver angiogenesis and occurred together with (i) increased GPR78-BiP and EIF2α phosphorylation, suggesting endoplasmic reticulum stress, (ii) induction of Col1a1 gene expression, a marker of fibrosis, and (iii) increased CD31 immunolabeling, consistent with active angiogenesis and fibrosis.

CONCLUSION

Our data show that very HFDs promote a rapid inflammatory response, as well as deregulation of angiogenesis, both consistent with development of liver fibrosis.

Macrophage colony-stimulating factor increases hepatic macrophage content, liver growth, and lipid accumulation in neonatal rats

Signaling via the colony-stimulating factor 1 receptor (CSF1R) controls the survival, differentiation, and proliferation of macrophages. Mutations in CSF1 or CSF1R in mice and rats have pleiotropic effects on postnatal somatic growth. We tested the possible application of pig CSF1-Fc fusion protein as a therapy for low birth weight (LBW) at term, using a model based on maternal dexamethasone treatment in rats. Neonatal CSF1-Fc treatment did not alter somatic growth and did not increase the blood monocyte count. Instead, there was a substantial increase in the size of liver in both control and LBW rats, and the treatment greatly exacerbated lipid droplet accumulation seen in the dexamethasone LBW model. These effects were reversed upon cessation of treatment. Transcriptional profiling of the livers supported histochemical evidence of a large increase in macrophages with a resident Kupffer cell phenotype and revealed increased expression of many genes implicated in lipid droplet formation. There was no further increase in hepatocyte proliferation over the already high rates in neonatal liver. In conclusion, treatment of neonatal rats with CSF1-Fc caused an increase in liver size and hepatic lipid accumulation, due to Kupffer cell expansion and/or activation rather than hepatocyte proliferation. Increased liver macrophage numbers and expression of endocytic receptors could mitigate defective clearance functions in neonates. NEW & NOTEWORTHY This study is based on extensive studies in mice and pigs of the role of CSF1/CSF1R in macrophage development and postnatal growth. We extended the study to neonatal rats as a possible therapy for low birth weight. Unlike our previous studies in mice and pigs, there was no increase in hepatocyte proliferation and no increase in monocyte numbers. Instead, neonatal rats treated with CSF1 displayed reversible hepatic steatosis and Kupffer cell expansion.

Hepatic Overexpression of CD36 Improves Glycogen Homeostasis and Attenuates High-Fat Diet-Induced Hepatic Steatosis and Insulin Resistance

The common complications in obesity and type 2 diabetes include hepatic steatosis and disruption of glucose-glycogen homeostasis, leading to hyperglycemia. Fatty acid translocase (FAT/CD36), whose expression is inducible in obesity, is known for its function in fatty acid uptake. Previous work by us and others suggested that CD36 plays an important role in hepatic lipid homeostasis, but the results have been conflicting and the mechanisms were not well understood. In this study, by using CD36-overexpressing transgenic (CD36Tg) mice, we uncovered a surprising function of CD36 in regulating glycogen homeostasis. Overexpression of CD36 promoted glycogen synthesis, and as a result, CD36Tg mice were protected from fasting hypoglycemia. When challenged with a high-fat diet (HFD), CD36Tg mice showed unexpected attenuation of hepatic steatosis, increased very low-density lipoprotein (VLDL) secretion, and improved glucose tolerance and insulin sensitivity. The HFD-fed CD36Tg mice also showed decreased levels of proinflammatory hepatic prostaglandins and 20-hydroxyeicosatetraenoic acid (20-HETE), a potent vasoconstrictive and proinflammatory arachidonic acid metabolite. We propose that CD36 functions as a protective metabolic sensor in the liver under lipid overload and metabolic stress. CD36 may be explored as a valuable therapeutic target for the management of metabolic syndrome.

Postprandial control of fatty acid transport proteins' subcellular location is not dependent on insulin

Fatty acid transport proteins rapidly translocate to the plasma membrane in response to various stimuli, including insulin, influencing lipid uptake into muscle. However, our understanding of the mechanisms regulating postprandial fatty acid transporter subcellular location remains limited. We demonstrate that the response of fatty acid transporters to insulin stimulation is extremely brief and not temporally matched in the postprandial state. We further show that high-fat diet-induced accumulation of fatty acid transporters on the plasma membrane can occur in the absence of insulin. Altogether, these data suggest that insulin is not the primary signal regulating fatty acid transporter relocation in vivo.

Abnormalities in the Metabolism of Fatty Acids and Triacylglycerols in the Liver of the Goto-Kakizaki Rat: A Model for Non-Obese Type 2 Diabetes

The Goto-Kakizaki (GK) rat is widely used as an animal model for spontaneous-onset type 2 diabetes without obesity; nevertheless, little information is available on the metabolism of fatty acids and triacylglycerols (TAG) in their livers. We investigated the mechanisms underlying the alterations in the metabolism of fatty acids and TAG in their livers, in comparison with Zucker (fa/fa) rats, which are obese and insulin resistant. Lipid profiles, the expression of genes for enzymes and proteins related to the metabolism of fatty acid and TAG, de novo synthesis of fatty acids and TAG in vivo, fatty acid synthase activity in vitro, fatty acid oxidation in liver slices, and very-low-density-lipoprotein (VLDL)-TAG secretion in vivo were estimated. Our results revealed that (1) the TAG accumulation was moderate, (2) the de novo fatty acid synthesis was increased by upregulation of fatty acid synthase in a post-transcriptional manner, (3) fatty acid oxidation was also augmented through the induction of carnitine palmitoyltransferase 1a, and (4) the secretion rate of VLDL-TAG remained unchanged in the livers of GK rats. These results suggest that, despite the fact that GK rats exhibit non-obese type 2 diabetes, the upregulation of de novo lipogenesis is largely compensated by the upregulation of fatty acid oxidation, resulting in only moderate increase in TAG accumulation in the liver.

Targeting fatty acid metabolism to improve glucose metabolism

Disturbances in fatty acid metabolism in adipose tissue, liver, skeletal muscle, gut and pancreas play an important role in the development of insulin resistance, impaired glucose metabolism and type 2 diabetes mellitus. Alterations in diet composition may contribute to prevent and/or reverse these disturbances through modulation of fatty acid metabolism. Besides an increased fat mass, adipose tissue dysfunction, characterized by an altered capacity to store lipids and an altered secretion of adipokines, may result in lipid overflow, systemic inflammation and excessive lipid accumulation in non-adipose tissues like liver, skeletal muscle and the pancreas. These impairments together promote the development of impaired glucose metabolism, insulin resistance and type 2 diabetes mellitus. Furthermore, intrinsic functional impairments in either of these organs may contribute to lipotoxicity and insulin resistance. The present review provides an overview of fatty acid metabolism-related pathways in adipose tissue, liver, skeletal muscle, pancreas and gut, which can be targeted by diet or food components, thereby improving glucose metabolism.

Hyperinsulinemia Enhances Hepatic Expression of the Fatty Acid Transporter Cd36 and Provokes Hepatosteatosis and Hepatic Insulin Resistance

Hepatosteatosis is associated with the development of both hepatic insulin resistance and Type 2 diabetes. Hepatic expression of Cd36, a fatty acid transporter, is enhanced in obese and diabetic murine models and human nonalcoholic fatty liver disease, and thus it correlates with hyperinsulinemia, steatosis, and insulin resistance. Here, we have explored the effect of hyperinsulinemia on hepatic Cd36 expression, development of hepatosteatosis, insulin resistance, and dysglycemia. A 3-week sucrose-enriched diet was sufficient to provoke hyperinsulinemia, hepatosteatosis, hepatic insulin resistance, and dysglycemia in CBA/J mice. The development of hepatic steatosis and insulin resistance in CBA/J mice on a sucrose-enriched diet was paralleled by increased hepatic expression of the transcription factor Pparγ and its target gene Cd36 whereas that of genes implicated in lipogenesis, fatty acid oxidation, and VLDL secretion was unaltered. Additionally, we demonstrate that insulin, in a Pparγ-dependent manner, is sufficient to directly increase Cd36 expression in perfused livers and isolated hepatocytes. Mouse strains that display low insulin levels, i.e. C57BL6/J, and/or lack hepatic Pparγ, i.e. C3H/HeN, do not develop hepatic steatosis, insulin resistance, or dysglycemia on a sucrose-enriched diet, suggesting that elevated insulin levels, via enhanced CD36 expression, provoke fatty liver development that in turn leads to hepatic insulin resistance and dysglycemia. Thus, our data provide evidence for a direct role for hyperinsulinemia in stimulating hepatic Cd36 expression and thus the development of hepatosteatosis, hepatic insulin resistance, and dysglycemia.

Hepatic fatty acid uptake is regulated by the sphingolipid acyl chain length

Ceramide synthase 2 (CerS2) null mice cannot synthesize very-long acyl chain (C22-C24) ceramides resulting in significant alterations in the acyl chain composition of sphingolipids. We now demonstrate that hepatic triacylglycerol (TG) levels are reduced in the liver but not in the adipose tissue or skeletal muscle of the CerS2 null mouse, both before and after feeding with a high fat diet (HFD), where no weight gain was observed and large hepatic nodules appeared. Uptake of both BODIPY-palmitate and [VH]-palmitate was also abrogated in the hepa- tocytes and liver. The role of a number of key proteins involved in fatty acid uptake was examined, including FATP5, CD36/FAT, FABPpm and cytoplasmic FABP1. Levels of FATP5 and FABP1 were decreased in the CerS2 null mouse liver, whereas CD36/FAT levels were significantly elevated and CD36/FAT was also mislocalized upon insulin treatment. Moreover, treatment of hepatocytes with C22-C24-ceramides down-regulated CD36/FAT levels. Infection of CerS2 null mice with recombinant adeno-associated virus (rAAV)-CerS2 restored normal TG levels and corrected the mislocalization of CD36/FAT, but had no effect on the intracellular localization or levels of FATP5 or FABP1. Together, these results demonstrate that hepatic fatty acid uptake via CD36/FAT can be regulated by altering the acyl chain composition of sphingolipids.

S-Adenosylmethionine increases circulating very-low density lipoprotein clearance in non-alcoholic fatty liver disease

BACKGROUND & AIMS

Very-low-density lipoproteins (VLDLs) export lipids from the liver to peripheral tissues and are the precursors of low-density-lipoproteins. Low levels of hepatic S-adenosylmethionine (SAMe) decrease triglyceride (TG) secretion in VLDLs, contributing to hepatosteatosis in methionine adenosyltransferase 1A knockout mice but nothing is known about the effect of SAMe on the circulating VLDL metabolism. We wanted to investigate whether excess SAMe could disrupt VLDL plasma metabolism and unravel the mechanisms involved.

METHODS

Glycine N-methyltransferase (GNMT) knockout (KO) mice, GNMT and perilipin-2 (PLIN2) double KO (GNMT-PLIN2-KO) and their respective wild type (WT) controls were used. A high fat diet (HFD) or a methionine deficient diet (MDD) was administrated to exacerbate or recover VLDL metabolism, respectively. Finally, 33 patients with non-alcoholic fatty-liver disease (NAFLD); 11 with hypertriglyceridemia and 22 with normal lipidemia were used in this study.

RESULTS

We found that excess SAMe increases the turnover of hepatic TG stores for secretion in VLDL in GNMT-KO mice, a model of NAFLD with high SAMe levels. The disrupted VLDL assembly resulted in the secretion of enlarged, phosphatidylethanolamine-poor, TG- and apoE-enriched VLDL-particles; special features that lead to increased VLDL clearance and decreased serum TG levels. Re-establishing normal SAMe levels restored VLDL secretion, features and metabolism. In NAFLD patients, serum TG levels were lower when hepatic GNMT-protein expression was decreased.

CONCLUSIONS

Excess hepatic SAMe levels disrupt VLDL assembly and features and increase circulating VLDL clearance, which will cause increased VLDL-lipid supply to tissues and might contribute to the extrahepatic complications of NAFLD.

Integrin α1-null mice exhibit improved fatty liver when fed a high fat diet despite severe hepatic insulin resistance

Hepatic insulin resistance is associated with increased collagen. Integrin α1β1 is a collagen-binding receptor expressed on hepatocytes. Here, we show that expression of the α1 subunit is increased in hepatocytes isolated from high fat (HF)-fed mice. To determine whether the integrin α1 subunit protects against impairments in hepatic glucose metabolism, we analyzed glucose tolerance and insulin sensitivity in HF-fed integrin α1-null (itga1(-/-)) and wild-type (itga1(+/+)) littermates. Using the insulin clamp, we found that insulin-stimulated hepatic glucose production was suppressed by ∼50% in HF-fed itga1(+/+) mice. In contrast, it was not suppressed in HF-fed itga1(-/-) mice, indicating severe hepatic insulin resistance. This was associated with decreased hepatic insulin signaling in HF-fed itga1(-/-) mice. Interestingly, hepatic triglyceride and diglyceride contents were normalized to chow-fed levels in HF-fed itga1(-/-) mice. This indicates that hepatic steatosis is dissociated from insulin resistance in HF-fed itga1(-/-) mice. The decrease in hepatic lipid accumulation in HF-fed itga1(-/-) mice was associated with altered free fatty acid metabolism. These studies establish a role for integrin signaling in facilitating hepatic insulin action while promoting lipid accumulation in mice challenged with a HF diet.

Increased hepatic CD36 expression with age is associated with enhanced susceptibility to nonalcoholic fatty liver disease

CD36 has been associated with obesity and diabetes in human liver diseases, however, its role in age-associated nonalcoholic fatty liver disease (NAFLD) is unknown. Therefore, liver biopsies were collected from individuals with histologically normal livers (n=30), and from patients diagnosed with simple steatosis (NAS; n=26). Patients were divided into two groups according to age and liver biopsy samples were immunostained for CD36. NAFLD parameters were examined in young (12-week) and middle-aged (52-week) C57BL/6J mice, some fed with chow-diet and some fed with low-fat (LFD; 10% kcal fat) or high-fat diet (HFD; 60% kcal fat) for 12-weeks. CD36 expression was positively associated with age in individuals with normal livers but not in NAS patients. However, CD36 was predominantly located at the plasma membrane of hepatocytes in aged NAS patients as compared to young. In chow-fed mice, aging, despite an increase in hepatic CD36 expression, was not associated with the development of NAFLD. However, middle-aged mice did exhibit the development of HFD-induced NAFLD, mediated by an increase of CD36 on the membrane. Enhanced CD36-mediated hepatic fat uptake may contribute to an accelerated progression of NAFLD in mice and humans. Therapies to prevent the increase in CD36 expression and/or CD36 from anchoring at the membrane may prevent the development of NAFLD.

Selective regulation of hepatic lipid metabolism by the AMP-activated protein kinase pathway in late-pregnant rats

The liver plays an essential role in maternal metabolic adaptation during late pregnancy. With regard to lipid metabolism, increased secretion of very low-density lipoprotein (VLDL) is characteristic of late pregnancy. Despite this well-described metabolic plasticity, the molecular changes underlying the hepatic adaptation to pregnancy remain unclear. As AMPK is a key intracellular energy sensor, we investigated whether this protein assumes a causal role in the hepatic adaptation to pregnancy. Pregnant Wistar rats were treated with vehicle or AICAR (5-aminoimidazole-4-carboxamide ribonucleotide) for 5 days starting at gestational day 14. At the end of treatment, the rats were subjected to an intraperitoneal pyruvate tolerance test and in situ liver perfusion with pyruvate. The livers were processed for Western blot analysis, quantitative PCR, thin-layer chromatography, enzymatic activity, and glycogen content measurements. Blood biochemical profiles were also assessed. We found that AMPK and ACC phosphorylation were reduced in the livers of pregnant rats in parallel with a reduced level of hepatic gluconeogenesis of pyruvate. This effect was accompanied by both a reduction in the levels of hepatic triglycerides (TG) and an increase in circulating levels of TG. Treatment with AICAR restored hepatic levels of TG to those observed in nonpregnant rats. Additionally, AMPK activation reduced the upregulation of genes related to VLDL synthesis and secretion observed in the livers of pregnant rats. We conclude that the increased secretion of hepatic TG in late pregnancy is concurrent with a transcriptional profile that favors VLDL production. This transcriptional profile results from the reduction in hepatic AMPK activity.

Lipid modulatory activities of Cichorium glandulosum Boiss et Huet are mediated by multiple components within hepatocytes

To investigate a possible methodology of exploiting herbal medicine and design polytherapy for the treatment of non-alcoholic fatty liver disease (NAFLD), we have made use of Cichorium glandulosum Boiss et Huet (CG), a traditional Chinese herbal medicine that has been proven to be effective in treating hepatic diseases. Here, we report that the extract of CG effectively reduced lipid accumulation under conditions of lipid overloading in vivo and in vitro (in a rat high-fat diet model and a hepG2 cell model of free fatty acid treatment). CG extract also protected hepatocytes from injury and inflammation to aid its lipid-lowering properties (in a rat high-fat diet model and a L02 cell model of acetaminophen treatment). Serum chemistry analysis accompanied by in vitro drug screening confirmed that CG-4, CG-10 and CG-14 are the lipo-effective components of CG. Western blotting analysis revealed that these components can regulate key lipid targets at the molecular level, including CD36, FATP5 and PPAR-α, thus the lipid oxidation and lipid absorption pathways. Finally, we adopted the experimental design and statistical method to calculate the best combination proportion (CG-4: CG-10: CG-14 = 2.065: 1.782: 2.153) to optimize its therapeutic effect.

Hepatocyte-specific Ptpn6 deletion promotes hepatic lipid accretion, but reduces NAFLD in diet-induced obesity: potential role of PPARγ

Hepatocyte-specific Shp1 knockout mice (Ptpn6(H-KO)) are protected from hepatic insulin resistance evoked by high-fat diet (HFD) feeding for 8 weeks. Unexpectedly, we report herein that Ptpn6(H-KO) mice fed an HFD for up to 16 weeks are still protected from insulin resistance, but are more prone to hepatic steatosis, as compared with their HFD-fed Ptpn6(f/f) counterparts. The livers from HFD-fed Ptpn6(H-KO) mice displayed 1) augmented lipogenesis, marked by increased expression of several hepatic genes involved in fatty acid biosynthesis, 2) elevated postprandial fatty acid uptake, and 3) significantly reduced lipid export with enhanced degradation of apolipoprotein B (ApoB). Despite more extensive hepatic steatosis, the inflammatory profile of the HFD-fed Ptpn6(H-KO) liver was similar (8 weeks) or even improved (16 weeks) as compared to their HFD-fed Ptpn6(f/f) littermates, along with reduced hepatocellular damage as revealed by serum levels of hepatic enzymes. Interestingly, comparative microarray analysis revealed a significant up-regulation of peroxisome proliferator-activated receptor gamma (PPARγ) gene expression, confirmed by quantitative polymerase chain reaction. Elevated PPARγ nuclear activity also was observed and found to be directly regulated by Shp1 in a cell-autonomous manner.

CONCLUSION

These findings highlight a novel role for hepatocyte Shp1 in the regulation of PPARγ and hepatic lipid metabolism. Shp1 deficiency prevents the development of severe hepatic inflammation and hepatocellular damage in steatotic livers, presenting hepatocyte Shp1 as a potential novel mediator of nonalcoholic fatty liver diseases in obesity.

Increased soluble CD36 is linked to advanced steatosis in nonalcoholic fatty liver disease

BACKGROUND

Soluble CD36 (sCD36) clusters with insulin resistance, but no evidence exists on its relationship with hepatic fat content. We determined sCD36 to assess its link to steatosis in nonalcoholic fatty liver disease (NAFLD) and chronic hepatitis C (CHC) patients.

MATERIALS AND METHODS

Two hundred and twenty-seven NAFLD, eighty-seven CHC, and eighty-five patients with histologically normal liver (NL) were studied. Steatosis was graded by Kleiner's histological scoring system. Serum sCD36 and hepatic CD36 expression was assessed by immunoassay and immunohistochemistry, respectively.

RESULTS

In NAFLD, serum sCD36 levels were significantly higher in simple steatosis than in NL (361.4 ± 286.4 vs. 173.9 ± 137.4 pg/mL, respectively; P < 0.001), but not in steatohepatitis (229.6 ± 202.5 pg/mL; P = 0.153). In CHC, serum sCD36 levels were similar regardless of the absence (428.7 ± 260.3 pg/mL) or presence of steatosis (387.2 ± 283.6 pg/mL; P = 0.173). A progressive increase in serum sCD36 values was found in NAFLD depending on the histological grade of steatosis (P < 0.001), but not in CHC (P = 0.151). Serum sCD36 concentrations were independently associated with advanced steatosis in NAFLD when adjusted by demographic and anthropometric features [odds ratio (OR), 1.001; 95% confidence interval (CI), 1.000 to 1.002; P = 0.021] and by metabolic variables (OR, 1.002; 95% CI, 1.000 to 1.003; P = 0.001). Interestingly, a significant correlation was observed between hepatic CD36 and serum sCD36 (ρ = 0.499, P < 0.001).

CONCLUSIONS

Increased serum sCD36 is an independent factor associated with advanced steatosis in NAFLD.

1,3-Dichloro-2-propanol induced hyperlipidemia in C57BL/6J mice via AMPK signaling pathway

1,3-Dichloro-2-propanol (1,3-DCP) is a well-known contaminant that has been detected in a wide range of foods. Dietary intake represents the greatest source of exposure to 1,3-DCP. In the study, we first found 1,3-DCP could induce hyperlipidemia in C57BL/6J mice below 1 mg/kg/day. We investigated serum lipid profile, liver total cholesterol (TC) and triglyceride (TG), histopathology of Liver and adipose tissue. The results showed 1,3-DCP dose dependently increased serum TG, TC and low-density lipoprotein cholesterol (LDL-C), decreased serum high-density lipoprotein cholesterol (HDL-C), increased relative liver weight, liver TG and TC, relative adipose tissue weight and enlarged the size of adipose cells. Because AMPK signal pathway is important in the process of lipid metabolism, we further investigated the effects of 1,3-DCP on AMPK signaling pathway in murine models. The results showed that 1,3-DCP (0.1-1 mg/kg/day) decreased p-AMPK/tAMPK ratio, p-ACC/tACC ratio, PPARα expression, but increased FAT, SREBP1, HMGCR and FAS expression. These observations indicated that 1,3-DCP induced hyperlipidemia in C57BL/6J mice at least partially through regulating AMPK signaling pathway.