Deregulated Lipid Sensing by Intestinal CD36 in Diet-Induced Hyperinsulinemic Obese Mouse Model

Interplay of CD36, autophagy, and lipid metabolism: insights into cancer progression

CD36, a scavenger receptor B2 that is dynamically distributed between cell membranes and organelle membranes, plays a crucial role in regulating lipid metabolism. Abnormal CD36 activity has been linked to a range of metabolic disorders, such as obesity, nonalcoholic fatty liver disease, insulin resistance and cardiovascular disease. CD36 undergoes various modifications, including palmitoylation, glycosylation, and ubiquitination, which greatly affect its binding affinity to various ligands, thereby triggering and influencing various biological effects. In the context of tumors, CD36 interacts with autophagy to jointly regulate tumorigenesis, mainly by influencing the tumor microenvironment. The central role of CD36 in cellular lipid homeostasis and recent molecular insights into CD36 in tumor development indicate the applicability of CD36 as a therapeutic target for cancer treatment. Here, we discuss the diverse posttranslational modifications of CD36 and their respective roles in lipid metabolism. Additionally, we delve into recent research findings on CD36 in tumors, outlining ongoing drug development efforts targeting CD36 and potential strategies for future development and highlighting the interplay between CD36 and autophagy in the context of cancer. Our aim is to provide a comprehensive understanding of the function of CD36 in both physiological and pathological processes, facilitating a more in-depth analysis of cancer progression and a better development and application of CD36-targeting drugs for tumor therapy in the near future.

Desulfovibrio in the Gut: The Enemy within?

Desulfovibrio (DSV) are sulfate-reducing bacteria (SRB) that are ubiquitously present in the environment and as resident commensal bacteria within the human gastrointestinal tract. Though they are minor residents of the healthy gut, DSV are opportunistic pathobionts that may overgrow in the setting of various intestinal and extra-intestinal diseases. An increasing number of studies have demonstrated a positive correlation between DSV overgrowth (bloom) and various human diseases. While the relationship between DSV bloom and disease pathology has not been clearly established, mounting evidence suggests a causal role for these bacteria in disease development. As DSV are the most predominant genera of SRB in the gut, this review summarizes current knowledge regarding the relationship between DSV and a variety of diseases. In this study, we also discuss the mechanisms by which these bacteria may contribute to disease pathology.

Quartet of APOCs and the Different Roles They Play in Diabetes

APOA1 and APOB are the structural proteins of high-density lipoprotein and APOB-containing lipoproteins, such as low-density lipoprotein and very low-density lipoprotein, respectively. The 4 smaller APOCs (APOC1, APOC2, APOC3, and APOC4) are exchangeable apolipoproteins; they are readily transferred among high-density lipoproteins and APOB-containing lipoproteins. The APOCs regulate plasma triglyceride and cholesterol levels by modulating substrate availability and activities of enzymes interacting with lipoproteins and by interfering with APOB-containing lipoprotein uptake through hepatic receptors. Of the 4 APOCs, APOC3 has been best studied in relation to diabetes. Elevated serum APOC3 levels predict incident cardiovascular disease and progression of kidney disease in people with type 1 diabetes. Insulin suppresses APOC3 levels, and accordingly, elevated APOC3 levels associate with insulin deficiency and insulin resistance. Mechanistic studies in a mouse model of type 1 diabetes have demonstrated that APOC3 acts in the causal pathway of diabetes-accelerated atherosclerosis. The mechanism is likely due to the ability of APOC3 to slow the clearance of triglyceride-rich lipoproteins and their remnants, thereby causing an increased accumulation of atherogenic lipoprotein remnants in lesions of atherosclerosis. Less is known about the roles of APOC1, APOC2, and APOC4 in diabetes.

From worms to humans: Understanding intestinal lipid metabolism via model organisms

The intestine is responsible for efficient absorption and packaging of dietary lipids before they enter the circulatory system. This review provides a comprehensive overview of how intestinal enterocytes from diverse model organisms absorb dietary lipid and subsequently secrete the largest class of lipoproteins (chylomicrons) to meet the unique needs of each animal. We discuss the putative relationship between diet and metabolic disease progression, specifically Type 2 Diabetes Mellitus. Understanding the molecular response of intestinal cells to dietary lipid has the potential to undercover novel therapies to combat metabolic syndrome.

Fatty Acid Transport and Signaling: Mechanisms and Physiological Implications

Long-chain fatty acids (FAs) are components of plasma membranes and an efficient fuel source and also serve as metabolic regulators through FA signaling mediated by membrane FA receptors. Impaired tissue FA uptake has been linked to major complications of obesity, including insulin resistance, cardiovascular disease, and type 2 diabetes. Fatty acid interactions with a membrane receptor and the initiation of signaling can modify pathways related to nutrient uptake and processing, cell proliferation or differentiation, and secretion of bioactive factors. Here, we review the major membrane receptors involved in FA uptake and FA signaling. We focus on two types of membrane receptors for long-chain FAs: CD36 and the G protein-coupled FA receptors FFAR1 and FFAR4. We describe key signaling pathways and metabolic outcomes for CD36, FFAR1, and FFAR4 and highlight the parallels that provide insight into FA regulation of cell function.

Perillartine protects against metabolic associated fatty liver in high-fat diet-induced obese mice

Metabolic associated fatty liver disease is the main cause of chronic liver disease in the world, but there is still no effective treatment. In the search for drugs to treat liver steatosis, we screened 303 natural products using HepG2 cells and discovered that perillartine derived from Perilla frutescens (L.) improved fat deposition as well as glucose homeostasis in hepatocytes. In vitro, perillartine reduced the expression of genes involved in lipid synthesis, lipid transport, and gluconeogenesis in hepatocytes, increased the number of mitochondria, and upregulated the phosphorylation of Akt. In vivo, perillartine reduced body weight gain and the fat rate, improved glucose metabolism and energy balance, and altered the gut microbial composition in mice given a high-fat diet. In addition, RORγ was identified as a possible target of perillartine through pharmacophore screening. Functional studies revealed that the overexpression of RORγ blocked the effects of perillartine, suggesting that it reduced lipid accumulation and regulated glucose metabolism by inhibiting the transcriptional activity of RORγ. Our results provide new information on a natural product inhibitor for RORγ and reveal that perillartine is a new candidate for the treatment of obesity and metabolic associated fatty liver disease.

Microbiota imbalance induced by dietary sugar disrupts immune-mediated protection from metabolic syndrome

How intestinal microbes regulate metabolic syndrome is incompletely understood. We show that intestinal microbiota protects against development of obesity, metabolic syndrome, and pre-diabetic phenotypes by inducing commensal-specific Th17 cells. High-fat, high-sugar diet promoted metabolic disease by depleting Th17-inducing microbes, and recovery of commensal Th17 cells restored protection. Microbiota-induced Th17 cells afforded protection by regulating lipid absorption across intestinal epithelium in an IL-17-dependent manner. Diet-induced loss of protective Th17 cells was mediated by the presence of sugar. Eliminating sugar from high-fat diets protected mice from obesity and metabolic syndrome in a manner dependent on commensal-specific Th17 cells. Sugar and ILC3 promoted outgrowth of Faecalibaculum rodentium that displaced Th17-inducing microbiota. These results define dietary and microbiota factors posing risk for metabolic syndrome. They also define a microbiota-dependent mechanism for immuno-pathogenicity of dietary sugar and highlight an elaborate interaction between diet, microbiota, and intestinal immunity in regulation of metabolic disorders.

Molecular mechanisms of metabolic associated fatty liver disease (MAFLD): functional analysis of lipid metabolism pathways

The metabolic-associated fatty liver disease (MAFLD) is a condition of fat accumulation in the liver in combination with metabolic dysfunction in the form of overweight or obesity and insulin resistance. It is also associated with an increased cardiovascular disease risk, including hypertension and atherosclerosis. Hepatic lipid metabolism is regulated by a combination of the uptake and export of fatty acids, de novo lipogenesis, and fat utilization by β-oxidation. When the balance between these pathways is altered, hepatic lipid accumulation commences, and long-term activation of inflammatory and fibrotic pathways can progress to worsen the liver disease. This review discusses the details of the molecular mechanisms regulating hepatic lipids and the emerging therapies targeting these pathways as potential future treatments for MAFLD.

A nexus of lipid and O-Glcnac metabolism in physiology and disease

Although traditionally considered a glucose metabolism-associated modification, the O-linked β-N-Acetylglucosamine (O-GlcNAc) regulatory system interacts extensively with lipids and is required to maintain lipid homeostasis. The enzymes of O-GlcNAc cycling have molecular properties consistent with those expected of broad-spectrum environmental sensors. By direct protein-protein interactions and catalytic modification, O-GlcNAc cycling enzymes may provide both acute and long-term adaptation to stress and other environmental stimuli such as nutrient availability. Depending on the cell type, hyperlipidemia potentiates or depresses O-GlcNAc levels, sometimes biphasically, through a diversity of unique mechanisms that target UDP-GlcNAc synthesis and the availability, activity and substrate selectivity of the glycosylation enzymes, O-GlcNAc Transferase (OGT) and O-GlcNAcase (OGA). At the same time, OGT activity in multiple tissues has been implicated in the homeostatic regulation of systemic lipid uptake, storage and release. Hyperlipidemic patterns of O-GlcNAcylation in these cells are consistent with both transient physiological adaptation and feedback uninhibited obesogenic and metabolic dysregulation. In this review, we summarize the numerous interconnections between lipid and O-GlcNAc metabolism. These links provide insights into how the O-GlcNAc regulatory system may contribute to lipid-associated diseases including obesity and metabolic syndrome.

Altered intestinal epithelial nutrient transport: an underappreciated factor in obesity modulated by diet and microbiota

Dietary nutrients absorbed in the proximal small intestine and assimilated in different tissues have a profound effect on overall energy homeostasis, determined by a balance between body's energy intake and expenditure. In obesity, altered intestinal absorption and consequently tissue assimilation of nutrients may disturb the energy balance leading to metabolic abnormalities at the cellular level. The absorption of nutrients such as sugars, amino acids and fatty acids released from food digestion require high-capacity transporter proteins expressed in the intestinal epithelial absorptive cells. Furthermore, nutrient sensing by specific transporters/receptors expressed in the epithelial enteroendocrine cells triggers release of gut hormones involved in regulating energy homeostasis via their effects on appetite and food intake. Therefore, the intestinal epithelial cells play a pivotal role in the pathophysiology of obesity and associated complications. Over the past decade, gut microbiota has emerged as a key factor contributing to obesity via its effects on digestion and absorption of nutrients in the small intestine, and energy harvest from dietary fiber, undigested component of food, in the large intestine. Various mechanisms of microbiota effects on obesity have been implicated. However, the impact of obesity-associated microbiota on the intestinal nutrient transporters needs extensive investigation. This review marshals the limited studies addressing the altered structure and function of the gut epithelium in obesity with special emphasis on nutrient transporters and role of diet and microbiota. The review also discusses the thoughts and controversies and research gaps in this field.

Spontaneous liver disease in wild-type C57BL/6JOlaHsd mice fed semisynthetic diet

Mouse models are frequently used to study mechanisms of human diseases. Recently, we observed a spontaneous bimodal variation in liver weight in C57BL/6JOlaHsd mice fed a semisynthetic diet. We now characterized the spontaneous variation in liver weight and its relationship with parameters of hepatic lipid and bile acid (BA) metabolism. In male C57BL/6JOlaHsd mice fed AIN-93G from birth to postnatal day (PN)70, we measured plasma BA, lipids, Very low-density lipoprotein (VLDL)-triglyceride (TG) secretion, and hepatic mRNA expression patterns. Mice were sacrificed at PN21, PN42, PN63 and PN70. Liver weight distribution was bimodal at PN70. Mice could be subdivided into two nonoverlapping groups based on liver weight: 0.6 SD 0.1 g (approximately one-third of mice, small liver; SL), and 1.0 SD 0.1 g (normal liver; NL; p<0.05). Liver histology showed a higher steatosis grade, inflammation score, more mitotic figures and more fibrosis in the SL versus the NL group. Plasma BA concentration was 14-fold higher in SL (p<0.001). VLDL-TG secretion rate was lower in SL mice, both absolutely (-66%, p<0.001) and upon correction for liver weight (-44%, p<0.001). Mice that would later have the SL-phenotype showed lower food efficiency ratios during PN21-28, suggesting the cause of the SL phenotype is present at weaning (PN21). Our data show that approximately one-third of C57BL/6JOlaHsd mice fed semisynthetic diet develop spontaneous liver disease with aberrant histology and parameters of hepatic lipid, bile acid and lipoprotein metabolism. Study designs involving this mouse strain on semisynthetic diets need to take the SL phenotype into account. Plasma lipids may serve as markers for the identification of the SL phenotype.

Dietary fatty acids and CD36-mediated cholesterol homeostasis: potential mechanisms

Currently, the prevention and treatment of CVD have been a global focus since CVD is the number one cause of mortality and morbidity. In the pathogenesis of CVD, it was generally thought that impaired cholesterol homeostasis might be a risk factor. Cholesterol homeostasis is affected by exogenous factors (i.e. diet) and endogenous factors (i.e. certain receptors, enzymes and transcription factors). In this context, the number of studies investigating the potential mechanisms of dietary fatty acids on cholesterol homeostasis have increased in recent years. As well, the cluster of differentiation 36 (CD36) receptor is a multifunctional membrane receptor involved in fatty acid uptake, lipid metabolism, atherothrombosis and inflammation. CD36 is proposed to be a crucial molecule for cholesterol homeostasis in various mechanisms including absorption/reabsorption, synthesis, and transport of cholesterol and bile acids. Moreover, it has been reported that the amount of fatty acids and fatty acid pattern of the diet influence the CD36 level and CD36-mediated cholesterol metabolism principally in the liver, intestine and macrophages. In these processes, CD36-mediated cholesterol and lipoprotein homeostasis might be impaired by dietary SFA and trans-fatty acids, whereas ameliorated by MUFA in the diet. The effects of PUFA on CD36-mediated cholesterol homeostasis are controversial depending on the amount of n-3 PUFA and n-6 PUFA, and the n-3:n-6 PUFA ratio. Thus, since the CD36 receptor is suggested to be a novel nutrient-sensitive biomarker, the role of CD36 and dietary fatty acids in cholesterol metabolism might be considered in medical nutrition therapy in the near future. Therefore, the novel nutritional target of CD36 and interventions that focus on dietary fatty acids and potential mechanisms underlying cholesterol homeostasis are discussed in this review.

T cell-mediated regulation of the microbiota protects against obesity

The microbiota influences obesity, yet organisms that protect from disease remain unknown. During studies interrogating host-microbiota interactions, we observed the development of age-associated metabolic syndrome (MetS). Expansion of Desulfovibrio and loss of Clostridia were key features associated with obesity in this model and are present in humans with MetS. T cell-dependent events were required to prevent disease, and replacement of Clostridia rescued obesity. Inappropriate immunoglobulin A targeting of Clostridia and increased Desulfovibrio antagonized the colonization of beneficial Clostridia. Transcriptional and metabolic analysis revealed enhanced lipid absorption in the obese host. Colonization of germ-free mice with Clostridia, but not Desulfovibrio, down-regulated genes that control lipid absorption and reduced adiposity. Thus, immune control of the microbiota maintains beneficial microbial populations that constrain lipid metabolism to prevent MetS.

Requirement for the intestinal epithelial insulin-like growth factor-1 receptor in the intestinal responses to glucagon-like peptide-2 and dietary fat

The intestinal hormone, glucagon-like peptide-2 (GLP-2), enhances the enterocyte chylomicron production. However, GLP-2 is known to require the intestinal-epithelial insulin-like growth factor-1 receptor (IE-IGF-1R) for its other actions to increase intestinal growth and barrier function. The role of the IE-IGF-1R in enterocyte lipid handling was thus tested in the GLP-2 signaling pathway, as well as in response to a Western diet (WD). IE-IGF-1R knockout (KO) and control mice were treated for 11 days with h(GLY² )GLP-2 or fed a WD for 18 weeks followed by a duodenal fat tolerance test with C¹⁴ -labeled triolein. Human Caco-2BBE cells were treated with an IGF-1R antagonist or signaling inhibitors to determine triglyceride-associated protein expression. The IE-IGF-1R was required for GLP-2-induced increases in CD36 and FATP-4 in chow-fed mice, and for expression in vitro; FATP-4 also required PI3K/Akt. Although WD-fed IE-IGF-1R KO mice demonstrated normal CD36 expression, the protein was incorrectly localized 2h post-duodenal fat administration. IE-IGF-1R KO also prevented the WD-induced increase in MTP and decrease in APOC3, increased jejunal mucosal C¹⁴ -fat accumulation, and elevated plasma triglyceride and C¹⁴ -fat levels. Collectively, these studies elucidate new roles for the IE-IGF-1R in enterocyte lipid handling, under basal conditions and in response to GLP-2 and WD-feeding.

Apolipoprotein C-II Mimetic Peptide Promotes the Plasma Clearance of Triglyceride-Rich Lipid Emulsion and the Incorporation of Fatty Acids into Peripheral Tissues of Mice

Aim

Plasma apolipoprotein C-II (apoC-II) activates lipoprotein lipase (LPL) and thus lowers plasma triglycerides (TG). We previously reported that a human apoC-II mimetic peptide (C-II-a) decreased plasma TG in apoC-II mutant mice, as well as in apoE-knockout mice. Because it is unknown what tissues take up free fatty acids (FFAs) released from TG after C-II-a peptide administration, we investigated in mice TG plasma clearance and tissue incorporation, using ³H-triolein as a tracer, with and without C-II-a treatment.

Methods and Results

Intralipid® fat emulsion was labeled with ³H-triolein and then mixed with or without C-II-a. Addition of the peptide did not alter mean particle size of the lipid emulsion particles (298 nm) but accelerated their plasma clearance. After intravenous injection into C57BL/6N mice, the plasma half-life of the ³H-triolein for control and C-II-a treated emulsions was 18.3 ± 2.2 min and 14.8 ± 0.1 min, respectively. In apoC-II mutant mice, the plasma half-life of ³H-triolein for injected control and C-II-a treated emulsions was 30.1 ± 0.1 min and 14.8 ± 0.1 min, respectively. C57BL/6N and apoC-II mutant mice at 120 minutes after the injection showed increased tissue incorporation of radioactivity in white adipose tissue when C-II-a treated emulsion was used. Higher radiolabeled uptake of lipids from C-II-a treated emulsion was also observed in the skeletal muscle of C57BL/6N mice only. In case of apoC-II mutant mice, decreased uptake of radioactive lipids was observed in the liver and kidney after addition of C-II-a to the lipid emulsion.

Conclusions

C-II-a peptide promotes the plasma clearance of TG-rich lipid emulsions in wild type and apoC-II mutant mice and promotes the incorporation of fatty acids from TG in the lipid emulsions into specific peripheral tissues.

Regulation of Chylomicron Secretion: Focus on Post-Assembly Mechanisms

Rapid and efficient digestion and absorption of dietary triglycerides and other lipids by the intestine, the packaging of those lipids into lipoprotein chylomicron (CM) particles, and their secretion via the lymphatic duct into the blood circulation are essential in maintaining whole-body lipid and energy homeostasis. Biosynthesis and assembly of CMs in enterocytes is a complex multistep process that is subject to regulation by intracellular signaling pathways as well as by hormones, nutrients, and neural factors extrinsic to the enterocyte. Dysregulation of this process has implications for health and disease, contributing to dyslipidemia and a potentially increased risk of atherosclerotic cardiovascular disease. There is increasing recognition that, besides intracellular regulation of CM assembly and secretion, regulation of postassembly pathways also plays important roles in CM secretion. This review examines recent advances in our understanding of the regulation of CM secretion in relation to mobilization of intestinal lipid stores, drawing particular attention to post-assembly regulatory mechanisms, including intracellular trafficking of triglycerides in enterocytes, CM mobilization from the lamina propria, and regulated transport of CM by intestinal lymphatics.

Inulin Improves Postprandial Hypertriglyceridemia by Modulating Gene Expression in the Small Intestine

Postprandial hyperlipidemia is an important risk factor for cardiovascular diseases in the context of obesity. Inulin is a non-digestible carbohydrate, known for its beneficial properties in metabolic disorders. We investigated the impact of inulin on postprandial hypertriglyceridemia and on lipid metabolism in a mouse model of diet-induced obesity. Mice received a control or a western diet for 4 weeks and were further supplemented or not with inulin for 2 weeks (0.2 g/day per mouse). We performed a lipid tolerance test, measured mRNA expression of genes involved in postprandial lipid metabolism, assessed post-heparin plasma and muscle lipoprotein lipase activity and measured lipid accumulation in the enterocytes and fecal lipid excretion. Inulin supplementation in western diet-fed mice decreases postprandial serum triglycerides concentration, decreases the mRNA expression levels of Cd36 (fatty acid receptor involved in lipid uptake and sensing) and apolipoprotein C3 (Apoc3, inhibitor of lipoprotein lipase) in the jejunum and increases fecal lipid excretion. In conclusion, inulin improves postprandial hypertriglyceridemia by targeting intestinal lipid metabolism. This work confirms the interest of using inulin supplementation in the management of dyslipidemia linked to obesity and cardiometabolic risk.

Intestinal CD36 and Other Key Proteins of Lipid Utilization: Role in Absorption and Gut Homeostasis

Several proteins have been implicated in fatty acid (FA) transport by enterocytes including the scavenger receptor CD36 (SR-B2), the scavenger receptor B1 (SR-B1) a member of the CD36 family and the FA transport protein 4 (FATP4). Here, we review the regulation of enterocyte FA uptake and its function in lipid absorption including prechylomicron formation, assembly and transport. Emphasis is given to CD36, which is abundantly expressed along the digestive tract of rodents and humans and has been the most studied. We also address the pleiotropic functions of CD36 that go beyond lipid absorption and metabolism to include recent evidence of its impact on intestinal homeostasis and barrier maintenance. Areas of progress involving contribution of membrane phospholipid remodeling and of cytosolic FA-binding proteins, FABP1 and FABP2 to fat absorption will be covered. © 2018 American Physiological Society. Compr Physiol 8:493-507, 2018.

Atherogenic postprandial remnant lipoproteins; VLDL remnants as a causal factor in atherosclerosis

Oxidized LDL (Ox-LDL) and chylomicron (CM) remnants have been suggested to be the most atherogenic lipoproteins that initiate and exacerbate coronary atherosclerosis. In this review, we propose a hypothesis of the causal lipoproteins in atherosclerosis based on our recent findings on postprandial remnant lipoproteins (RLP). Plasma RLP-C and RLP-TG increased significantly after food intake, especially a fat load. More than 80% of the TG increase after the fat load consisted of the TG in RLP, which contained significantly greater apoB100 than apoB48 particles as VLDL remnants. The majority of the LPL in non-heparin plasma was found in RLP as an RLP-LPL complex and released into the circulation after hydrolysis. Plasma LPL did not increase after food intake, which may have caused the partial hydrolysis of CM and VLDL as well as the significant increase of RLP-TG in the postprandial plasma. LPL was inversely correlated with the RLP particle size after food intake. We showed that VLDL remnants are the major atherogenic lipoproteins in the postprandial plasma associated with insufficient LPL activity and a causal factor in the initiation and progression of atherosclerosis. We also propose "LPL bound TG-rich lipoproteins" as a new definition of remnant lipoproteins based on the findings of the RLP-LPL complex in the non-heparin plasma.

Apolipoprotein C-II: New findings related to genetics, biochemistry, and role in triglyceride metabolism

Apolipoprotein C-II (apoC-II) is a small exchangeable apolipoprotein found on triglyceride-rich lipoproteins (TRL), such as chylomicrons (CM) and very low-density lipoproteins (VLDL), and on high-density lipoproteins (HDL), particularly during fasting. ApoC-II plays a critical role in TRL metabolism by acting as a cofactor of lipoprotein lipase (LPL), the main enzyme that hydrolyses plasma triglycerides (TG) on TRL. Here, we present an overview of the role of apoC-II in TG metabolism, emphasizing recent novel findings regarding its transcriptional regulation and biochemistry. We also review the 24 genetic mutations in the APOC2 gene reported to date that cause hypertriglyceridemia (HTG). Finally, we describe the clinical presentation of apoC-II deficiency and assess the current therapeutic approaches, as well as potential novel emerging therapies.

Hexarelin, a Growth Hormone Secretagogue, Improves Lipid Metabolic Aberrations in Nonobese Insulin-Resistant Male MKR Mice

Despite the occurrence of dyslipidemia and its contribution to the development of insulin resistance in obese subjects, a growing number of studies have described abnormal lipid profiles among leaner persons. For example, individuals with an abnormal paucity or distribution of fat (lipodystrophy) develop severe insulin resistance, dyslipidemia, and hepatic steatosis. Deranged adipocyte metabolism and differentiation contribute to ectopic fat deposition and consequent development of insulin resistance. Growth hormone (GH) therapy has been shown to correct body composition abnormalities in some lipodystrophy patients. However, little is known about the effects of GH-releasing peptides in this regard. Hexarelin, a GH secretagogue, has recently been shown to have beneficial effects on fat metabolism via the CD36 receptor. In this study, the effects of twice daily intraperitoneal injections of hexarelin (200 μg/kg body weight) were examined in nonobese insulin-resistant MKR mice and corresponding wild-type FVB mice for 12 days. Hexarelin treatment significantly improved glucose and insulin intolerance and decreased plasma and liver triglycerides in MKR mice. These beneficial metabolic effects could be due to the improved lipid metabolism and enhanced adipocyte differentiation of white adipose tissue with hexarelin treatment. Interestingly, although food intake of hexarelin-treated MKR mice was significantly increased, this did not change total body weight. Moreover, hexarelin treatment corrected the abnormal body composition of MKR mice, as demonstrated by a decrease in fat mass and an increase in lean mass. Our results suggest a possible application of hexarelin in treatment of lipid disorders associated with the metabolic syndrome.

Developments in intestinal cholesterol transport and triglyceride absorption

PURPOSE OF REVIEW

To discuss recent advances in research focused on intestinal lipid handling.

RECENT FINDINGS

An important strategy in reducing atherosclerosis and risk of cardiovascular events is to increase the rate of reverse cholesterol transport, including its final step; cholesterol excretion from the body. The rate of removal is determined by a complex interplay between the factors involved in regulation of intestinal cholesterol absorption. One of these factors is a process known as transintestinal cholesterol excretion. This pathway comprises transport of cholesterol directly from the blood, through the enterocyte, into the intestinal lumen. In humans, this pathway accounts for 35% of cholesterol excretion in the feces. Mechanistic studies in mice revealed that, activation of the bile acid receptor farnesoid X receptor increases cholesterol removal via the transintestinal cholesterol excretion pathway as well as decreases plasma cholesterol and triglyceride providing an interesting target for treatment of dyslipidemia in humans. The physical chemical properties of bile acids are under control of farnesoid X receptor and determine intestinal cholesterol and triglyceride solubilization as well as absorption, providing a direct link between these two important factors in the pathogenesis of cardiovascular disease. Besides bile acids, intestinal phospholipids are important for luminal lipid solubilization. Interestingly, phospholipid remodeling through LPCAT3 was shown to be pivotal for uptake of fatty acids by enterocytes, which may provide a mechanistic handle for therapeutic intervention.

SUMMARY

The importance of the intestine in control of cholesterol and triglyceride homeostasis is increasingly recognized. Recently, novel factors involved in regulation of cholesterol excretion and intestinal triglyceride and fatty acid uptake have been reported and are discussed in this short review.

Valproate induced hepatic steatosis by enhanced fatty acid uptake and triglyceride synthesis

Steatosis is the characteristic type of VPA-induced hepatotoxicity and may result in life-threatening hepatic lesion. Approximately 61% of patients treated with VPA have been diagnosed with hepatic steatosis through ultrasound examination. However, the mechanisms underlying VPA-induced intracellular fat accumulation are not yet fully understood. Here we demonstrated the involvement of fatty acid uptake and lipogenesis in VPA-induced hepatic steatosis in vitro and in vivo by using quantitative real-time PCR (qRT-PCR) analysis, western blotting analysis, fatty acid uptake assays, Nile Red staining assays, and Oil Red O staining assays. Specifically, we found that the expression of cluster of differentiation 36 (CD36), an important fatty acid transport, and diacylglycerol acyltransferase 2 (DGAT2) were significantly up-regulated in HepG2 cells and livers of C57B/6J mice after treatment with VPA. Furthermore, VPA treatment remarkably enhanced the efficiency of fatty acid uptake mediated by CD36, while this effect was abolished by the interference with CD36-specific siRNA. Also, VPA treatment significantly increased DGAT2 expression as a result of the inhibition of mitogen-activated protein kinase kinase (MEK) - extracellular regulated kinase (ERK) pathway; however, DGAT2 knockdown significantly alleviated VPA-induced intracellular lipid accumulation. Additionally, we also found that sterol regulatory element binding protein-1c (SREBP-1c)-mediated fatty acid synthesis may be not involved in VPA-induced hepatic steatosis. Overall, VPA-triggered over-regulation of CD36 and DGAT2 could be helpful for a better understanding of the mechanisms underlying VPA-induced hepatic steatosis and may offer novel therapeutic strategies to combat VPA-induced hepatotoxicity.

4,4'-Diaponeurosporene-Producing Bacillus subtilis Increased Mouse Resistance against Salmonella typhimurium Infection in a CD36-Dependent Manner

Deficient mucosal innate immunity is a hallmark of infectious diarrhea, such as Salmonella typhimurium (S. typhimurium)-induced gastroenteritis. Here, we report that oral administration of a 4,4′-diaponeurosporene-producing Bacillus subtilis (B.s-Dia) could improve mice mucosal immunity, as showed by an increased resistance against S. typhimurium infection. Intragastric administration of B.s-Dia for 7 days could increase the secretion of CCL20 by intestinal epithelial cells (IECs) and then recruit more dendritic cells. Meanwhile, the number of CD8αα⁺ intraepithelial lymphocytes, which play a critical role in downregulating immune responses, was also reduced, probably as a consequence of the decrease of IEC-derived TGFβ. Further study showed that CD36 played a critical role in B.s-Dia-induced immune enhancement, as blocking CD36 signal with a specific antagonist, sulfo-N-succinimidyl oleate, led to the inability of B.s-Dia to enhance mucosal innate immunity.

Appetite control by the tongue-gut axis and evaluation of the role of CD36/SR-B2

Understanding the mechanisms governing food intake is a public health issue given the dramatic rise of obesity over the world. The overconsumption of tasty energy-dense foods rich in lipids is considered to be one of the nutritional causes of this epidemic. Over the last decade, the identification of fatty acid receptors in strategic places in the body (i.e. oro-intestinal tract and brain) has provided a major progress in the deciphering of regulatory networks involved in the control of dietary intake. Among these lipid sensors, CD36/SR-B2 appears to play a significant role since this membrane protein, known to bind long-chain fatty acid with a high affinity, was specifically found both in enterocytes and in a subset of taste bud cells and entero-endocrine cells. After a short overview on CD36/SR-B2 structure, function and regulation, this mini-review proposes to analyze the key findings about the role of CD36/SR-B2 along of the tongue-gut axis in relation to appetite control. In addition, we discuss whether obesogenic diets might impair lipid sensing mediated by CD36/SR-B2 along this axis.

Immunologic impact of the intestine in metabolic disease

Obesity and diabetes are associated with increased chronic low-grade inflammation and elevated plasma glucose levels. Although inflammation in the fat and liver are established features of obesity-associated insulin resistance, the intestine is emerging as a new site for immunologic changes that affect whole-body metabolism. Specifically, microbial and dietary factors incurred by diet-induced obesity influence underlying innate and adaptive responses of the intestinal immune system. These responses affect the maintenance of the intestinal barrier, systemic inflammation, and glucose metabolism. In this Review we propose that an understanding of the changes to the intestinal immune system, and how these changes influence systemic immunity and glucose metabolism in a whole-body integrative and a neuronal-dependent network, will unveil novel intestinal pathologic and therapeutic targets for diabetes and obesity.

Lipid stimulation of fatty acid sensors in the human duodenum: relationship with gastrointestinal hormones, BMI and diet

BACKGROUND AND AIMS

The small intestinal free fatty acid (FFA) sensors, FFA receptor 1 (FFAR1), FFAR4, G-protein receptor 119 (GPR119) and cluster of differentiation-36 (CD36), mediate the fat-induced release of gastrointestinal (GI) hormones. We investigated whether expression of duodenal FFA sensors in humans was (i) altered by intraduodenal (ID) lipid infusion, (ii) disordered in overweight or obese individuals, (iii) related to lipid-induced GI hormone secretion or (iv) affected by habitual dietary patterns.

METHODS

Endoscopic duodenal biopsies were collected from 20 lean (body mass index (BMI): 22±1 kg m^-2), 18 overweight (BMI: 27±1 kg m^-2) and 19 obese (BMI: 35±1 kg m^-2) participants at baseline, and following a 30 min ID Intralipid infusion (2 kcal min^-1); FFA sensor expression was quantified by reverse transcription-PCR. On a separate day, participants underwent ID Intralipid infusion (2 kcal min^-1) for 120 min, to assess GI hormone responses. Habitual diet was evaluated using food frequency questionnaires.

RESULTS

Baseline FFAR1 and FFAR4 expression were lower, and CD36 was higher, in obese participants compared with lean participants. ID lipid increased GPR119 and FFAR1 expression equally across study groups, but did not alter FFAR4 or CD36 expression. Increased FFAR1 expression correlated positively with glucose-dependent insulinotropic polypeptide (GIP) secretion (r=0.3, P<0.05), whereas there was no relationship between habitual diet with the expression of FFA sensors.

CONCLUSIONS

Obesity is associated with altered duodenal expression of FFAR1, FFAR4 and CD36, suggesting altered capacity for the sensing, absorption and metabolism, of dietary lipids. GPR119 and FFAR1 are early transcriptional responders to the presence of ID lipid, whereas FFAR1 may be an important trigger for lipid-induced GIP release in humans.

CD36 actions in the heart: Lipids, calcium, inflammation, repair and more?

CD36 is a multifunctional immuno-metabolic receptor with many ligands. One of its physiological functions in the heart is the high-affinity uptake of long-chain fatty acids (FAs) from albumin and triglyceride rich lipoproteins. CD36 deletion markedly reduces myocardial FA uptake in rodents and humans. The protein is expressed on endothelial cells and cardiomyocytes and at both sites is likely to contribute to FA uptake by the myocardium. CD36 also transduces intracellular signaling events that influence how the FA is utilized and mediate metabolic effects of FA in the heart. CD36 transduced signaling regulates AMPK activation in a way that adjusts oxidation to FA uptake. It also impacts remodeling of myocardial phospholipids and eicosanoid production, effects exerted via influencing intracellular calcium (iCa(2+)) and the activation of phospholipases. Under excessive FA supply CD36 contributes to lipid accumulation, inflammation and dysfunction. However, it is also important for myocardial repair after injury via its contribution to immune cell clearance of apoptotic cells. This review describes recent progress regarding the multiple actions of CD36 in the heart and highlights those areas requiring future investigation. This article is part of a Special Issue entitled: Heart Lipid Metabolism edited by G.D. Lopaschuk.