Mechanisms of intestinal triacylglycerol synthesis

The intracellular chylomicron highway: novel insights into chylomicron biosynthesis, trafficking, and secretion

PURPOSE OF REVIEW

Chylomicron biosynthesis plays a vital role in supplying essential lipids and lipid soluble vitamins to peripheral tissues for various functions. Despite this, the intracellular synthesis, trafficking, and secretion of chylomicrons remains only partly understood. The purpose of this review is to summarize the role of established proteins in this process and bring attention to recently identified proteins to provide an up-to-date model of chylomicron biosynthesis.

RECENT FINDINGS

Recently, several proteins have been shown to play a role in the initial formation and lipidation of chylomicrons at the endoplasmic reticulum (ER), which include: TM6SF2, PLA2G12B, PRAP1, and SURF4. In addition, mitochondria have been implicated in chylomicron metabolism, but mechanistic insight is missing. The trafficking of chylomicrons from the ER to the Golgi, and the subsequent trafficking from the Golgi to the basolateral side of enterocytes, however, remains a mystery.

SUMMARY

Progress in the chylomicron biosynthesis field is largely associated with findings in VLDL biosynthesis. In addition, increased insight in events after prechylomicrons leave the ER is needed. Given the important role of chylomicron biosynthesis in whole-body lipid metabolism, further research into the molecular mechanisms is warranted.

Positional distribution of DHA in triacylglycerols: natural sources, synthetic routes, and nutritional properties

Docosahexaenoic acid (DHA, 22:6 n-3) is a long-chain polyunsaturated fatty acid (PUFA) present in high quantities in the mammalian brain and is a precursor of several metabolites. Clinical trials have demonstrated the benefits of dietary DHA in infants and adults. Triacylglycerols (TAGs) are the most abundant components of many natural oils, and in specific oils (e.g., fish, algal oils, etc.), they represent the main molecular form of dietary DHA. The positional distribution of DHA in the TAG glycerol backbone (sn-2 vs. sn-1/3) varied among different sources. Recent studies have shown that in human breast milk, DHA is mainly esterified at the sn-2 position (∼50% DHA of the total DHA), thus attracting research interest regarding the nutritional properties of sn-2 DHA. In this review, we summarize the different sources of TAG in natural oils with high amounts of DHA, including fish, algae, and marine mammal oils, with a focus on their positional distribution. Methods for analyzing the distribution of fatty acids in TAG of high-PUFA oils are discussed, and the lipase-catalyzed synthetic routes of specific triacylglycerols with sn-2 DHA are summarized. Furthermore, we discuss the recent research progress on the nutritional properties of DHA associated with its positional distribution on TAGs.

A Comprehensive Investigation of Lipid Profile During the Solid-State Fermentation of Rice by Monascus purpureus

Red yeast rice is a nutraceutical fermented product used worldwide for the symptomatic relief of dyslipidemia and cardiovascular disease. However, the fermentation-induced lipid transformation from rice to red yeast rice remains unclear. Herein, an ultra-high performance liquid chromatography coupled with hybrid quadrupole-orbitrap mass spectrometry method was developed for the comprehensive lipid analysis during fermentation. A total of 246 lipids fall in 21 subclasses were annotated in rice and red yeast rice, including 37 lysophospholipids, 14 phospholipids, 29 diglycerides, 114 triglycerides and fatty acid (15 species), ceramide (12 species), hexosylceramide (3 species), sitosterol ester (2 species), monogalactosyldiacylglycerol (2 species), digalactosyldiacylglycerol (2 species), monogalactosylmonoacylglycerol (8 species), digalactosylmonoacylglycerol (5 species), coenzyme Q (1 species), acyl hexosyl campesterol ester (1 species), and acylcarnitine (1 species). Results showed that lipid profiles changed, and new lipid species emerged. Notably, 18 medium- and long-chain triacylglycerols and triacylglycerols with short-chains were tentatively identified. These triacylglycerols also show the effects of body fat accumulation reduction, and hypolipidemic and hypoglycemic activities. Furthermore, lipid species that were profoundly changed were quantified, and the dynamic changes were investigated. This study clarified the molecular species and compositional changes in fermented rice from lipid aspect.

Association between gut microbiota and short-chain fatty acids in children with obesity

The gut microbiome and its metabolites may be important role in regulating the pathogenesis of obesity. This study aimed to characterize the gut microbiome and short-chain fatty acid (SCFA) metabolome in obese children. This case-control study recruited children aged 7‒14 years and divided them into a normal group (NG) and an obese group (OG) based on their body mass index. Whole-genome shotgun metagenomic analysis was performed on fecal samples from the OG and NG groups to characterize the signatures and functional potential of the gut microbiota. Serum metabolite profiles were analyzed using high-performance liquid chromatography/mass spectrometry (LC/MS). The Statistical Package for the Social Sciences (SPSS, version 26) and R software were used for data analysis. A total of 99 children were recruited, with 49 in the OG and 50 in the NG. At the phylum level, Proteobacteria were significantly more abundant in children in the OG than those in the NG. At the genus level, Oscillibacter and Alistipes were significantly lower in children in the OG than those in the NG. Caproate levels significantly increased, whereas butyrate and isobutyrate levels decreased in children in the OG than those in the NG. Kyoto encyclopedia of genes and genomes (KEGG) functional analysis revealed 28 enriched KEGG pathways, of which/with the phosphotransferase system (PTS) and enhanced biofilm formation by Escherichia coli were particularly significant in the OG. Spearman's correlation analysis indicated that the genus Oscillibacter and species Clostridium_sp._CAG:302 connect serum metabolites and the gut microbiota in childhood obesity. Childhood obesity is correlated with the symbiotic status of the gut microbiota. The microbiota influences human metabolism via specific pathways, particularly butyrate, caproate, and the genus Oscillibacter, all closely associated with obesity.

Neuron-derived neurotrophic factor promotes the differentiation of intramuscular and subcutaneous adipocytes in goat

Adipocyte play an important role in human health and meat quality by influencing the tenderness, flavor, and juiciness of mutton It has been shown that neuron-derived neurotrophic factor (NENF) is closely related to energy metabolism and adipocyte differentiation in bovine. However, the role of NENF in the goats remains unclear. The aim of this study was to detect the expression of NENF in goat subcutaneous and intramuscular adipocytes, temporal expression profiles of the NENF, and overexpressed NENF on the differentiation of different adipocytes. In this study, PCR amplification successfully cloned the goat NENF gene with a fragment length of 521 bp. In addition, the time point of highest expression of NENF differed between these two adipocytes differentiation processes. Overexpression of NENF in intramuscular and subcutaneous adipocytes promoted the expression levels of differentiation markers CEBPβ and SREBP, which in turn promoted the differentiation of intramuscular and subcutaneous adipocytes. This study will provide basic data for further study of the role of goats in goat adipocyte differentiation and for the final elucidation of its molecular mechanisms in regulating goat adipocyte deposition.

Mice Lacking Mrs2 Magnesium Transporter are Hypophagic and Thin When Maintained on a High-Fat Diet

Genes regulating body fat are shared with high fidelity by mice and humans, indicating that mouse knockout (KO) phenotyping might identify valuable antiobesity drug targets. Male Mrs2 magnesium transporter (Mrs2) KO mice were recently reported as thin when fed a high-fat diet (HFD). They also exhibited increased energy expenditure (EE)/body weight and had beiged adipocytes that, along with isolated hepatocytes, demonstrated increased oxygen consumption, suggesting that increased EE drove the thin phenotype. Here we provide our data on these and additional assays in Mrs2 KO mice. We generated Mrs2 KO mice by homologous recombination. HFD-fed male and female Mrs2 KO mice had significantly less body fat, measured by quantitative magnetic resonance, than wild-type (WT) littermates. HFD-fed Mrs2 KO mice did not demonstrate increased EE by indirect calorimetry and could not maintain body temperature at 4 °C, consistent with their decreased brown adipose tissue stores but despite increased beige white adipose tissue. Instead, when provided a choice between HFD and low-fat diet (LFD), Mrs2 KO mice showed a significant 15% decrease in total energy intake resulting from significantly lower HFD intake that offset numerically increased LFD intake. Food restriction studies performed using WT mice suggested that this decrease in energy intake could explain the loss of body fat. Oral glucose tolerance test studies revealed significantly improved insulin sensitivity in Mrs2 KO mice. We conclude that HFD-fed Mrs2 KO mice are thin with improved insulin sensitivity, and that this favorable metabolic phenotype is driven by hypophagia. Further evaluation is warranted to determine the suitability of MRS2 as a drug target for antiobesity therapeutics.

The Lipid-Metabolism-Associated Anti-Obesity Properties of Rapeseed Diacylglycerol Oil

To investigate the effects of rapeseed diacylglycerol oil (RDG) intake on lipid accumulation and metabolism in C57BL/6J mice, obese mice were fed a high-fat diet in which 45% of the total energy content came from RDG (RDGM group) or rapeseed triacylglycerol oil (RTGM group). This diet intervention was conducted for 12 weeks following the establishment of the obese mouse model. By the end of the experiment, the serum glucose levels of the mice in the RTGM and RDGM groups were 13.0 ± 1.3 mmol/L and 9.7 ± 1.5 mmol/L, respectively. Meanwhile, the serum triglyceride level in the RDGM group was 26.3% lower than that in the RTGM group. The weight-loss effect in the RDGM group was accompanied by a significant decrease in the white adipose tissue (WAT) index. The RDG intervention did not significantly change the antioxidant and anti-inflammatory properties of the rapeseed oil in vivo. The RDG diet improved the liver lipid metabolism abnormalities induced by a high-fat diet, leading to decreased liver damage index values (AST and ALT). Additionally, compared to that in the RTGM group, the expression of the adipogenic genes PPAR-γ and DGAT decreased in both the liver and intestine by 21.7% and 16.7% and by 38.7% and 47.2%, respectively, in the RDGM group. Further, most lipolytic genes in BAT showed no significant change after the RDG intervention. This implies that RDG regulates lipid metabolism by altering the expression of adipogenic genes in the liver, intestine, and adipose tissue, thereby reducing the accumulation of WAT. Furthermore, the RDG diet enhanced gut flora diversity, increasing the relative levels of unclassified Muribaculaceae and decreasing the levels of Dubosiella and Faecalibaculum in the mouse gut, potentially accelerating lipid metabolism. Thus, a three-month RDG diet intervention in obese mice exhibited benefits in regulating the somatotype, serum obesity-related indices, gut flora structure, and lipid metabolism in the adipose tissue, liver, and intestine.

The chylomicron saga: time to focus on postprandial metabolism

Since statins have had such tremendous therapeutic success over the last three decades, the field of atherosclerosis has become somewhat LDL-centric, dismissing the relevance of triglycerides (TG), particularly chylomicrons, in atherogenesis. Nonetheless, 50% of patients who take statins are at risk of developing atherosclerotic cardiovascular disease (ASCVD) and are unable to achieve their goal LDL-C levels. This residual risk is mediated, in part by triglyceride rich lipoproteins (TRL) and their remnants. Following his seminal investigation on the subject, Zilversmit proposed that atherosclerosis is a postprandial event in 1979 (1-4). In essence, the concept suggests that remnant cholesterol-rich chylomicron (CM) and very-low density lipoprotein (VLDL) particles play a role in atherogenesis. Given the foregoing, this narrative review addresses the most recent improvements in our understanding of postprandial dyslipidemia. The primary metabolic pathways of chylomicrons are discussed, emphasizing the critical physiological role of lipoprotein lipase and apoCIII, the importance of these particles' fluxes in the postprandial period, their catabolic rate, the complexities of testing postprandial metabolism, and the role of angiopoietin-like proteins in the partition of CM during the fed cycle. The narrative is rounded out by the dysregulation of postprandial lipid metabolism in insulin resistance states and consequent CVD risk, the clinical evaluation of postprandial dyslipidemia, current research limits, and potential future study directions.

The Jinggangmycin-induced Mthl2 gene regulates the development and stress resistance in Nilaparvata lugens Stål (Hemiptera: Delphacidae)

Methuselah (Mth) belongs to the GPCR family B, which regulates various biological processes and stress responses. The previous transcriptome data showed jinggangmycin (JGM)-induced Mthl2 expression. However, its detailed functional role remained unclear in brown planthopper, Nilaparvata lugens Stål. In adult N. lugens, the Mthl2 gene showed dominant expressions, notably in ovaries and fat body tissues. The 3rd instar nymphs treated with JGM increased starvation, oxidative stress, and high temperature (34 °C) tolerance of the adults. On the contrary, under dsMthl2 treatment, completely opposite phenotypes were observed. The lipid synthesis genes (DGAT1and PNPLA3) of both females and males treated with JGM in the nymphal stage were observed with high expressions, while the lipolysis of the Lipase 3 gene was observed with low expressions. The JGM increased triglyceride (TG) content, fat body droplet size, and the number of fat body droplets. The same treatment also increased the Glutathione S-transferase (GST), catalase (CAT), and superoxide dismutase (SOD) activities. An increase in the heat shock protein (HSP70 and HSP90) expression levels was also observed under JGM treatment but not dsMthl2. The current study demonstrated the influential role of the Mthl genes, particularly the Mthl2 gene, in modulating the growth and development and stress-responsiveness in N. lugens. Thus, providing a platform for future applied research programs controlling N. lugens population in rice fields.

The Effect of the Feeding System on Fat Deposition in Yak Subcutaneous Fat

Fat deposition is very important to the growth and reproduction of yaks. In this study, the effect of the feeding system on fat deposition in yaks was explored by transcriptomics and lipidomics. The thickness of the subcutaneous fat in yaks under stall (SF) and graze feeding (GF) was evaluated. The transcriptomes and lipidomes of the subcutaneous fat in yaks under different feeding systems were detected by RNA-sequencing (RNA-Seq) and non-targeted lipidomics based on ultrahigh-phase liquid chromatography tandem mass spectrometry (UHPLC-MS), respectively. The differences in lipid metabolism were explored, and the function of differentially expressed genes (DEGs) was evaluated by gene ontology (GO) and Kyoto encyclopedia of genes and genome (KEGG) analysis. Compared with GF yaks, SF yaks possessed stronger fat deposition capacity. The abundance of 12 triglycerides (TGs), 3 phosphatidylethanolamines (PEs), 3 diglycerides (DGs), 2 sphingomyelins (SMs) and 1 phosphatidylcholine (PC) in the subcutaneous fat of SF and GF yaks was significantly different. Under the mediation of the cGMP-PKG signaling pathway, the blood volume of SF and GF yaks may be different, which resulted in the different concentrations of precursors for fat deposition, including non-esterified fatty acid (NEFA), glucose (GLU), TG and cholesterol (CH). The metabolism of C16:0, C16:1, C17:0, C18:0, C18:1, C18:2 and C18:3 in yak subcutaneous fat was mainly realized under the regulation of the INSIG1, ACACA, FASN, ELOVL6 and SCD genes, and TG synthesis was regulated by the AGPAT2 and DGAT2 genes. This study will provide a theoretical basis for yak genetic breeding and healthy feeding.

Transmembrane Protein 68 Functions as an MGAT and DGAT Enzyme for Triacylglycerol Biosynthesis

Triacylglycerol (TG) biosynthesis is an important metabolic process for intracellular storage of surplus energy, intestinal dietary fat absorption, attenuation of lipotoxicity, lipid transportation, lactation and signal transduction in mammals. Transmembrane protein 68 (TMEM68) is an endoplasmic reticulum (ER)-anchored acyltransferase family member of unknown function. In the current study we show that overexpression of TMEM68 promotes TG accumulation and lipid droplet (LD) formation in a conserved active sites-dependent manner. Quantitative targeted lipidomic analysis showed that diacylglycerol (DG), free fatty acid (FFA) and TG levels were increased by TMEM68 expression. In addition, TMEM68 overexpression affected the levels of several glycerophospholipids, such as phosphatidylcholine, phosphatidylethanolamine and phosphatidylinositol, as well as sterol ester contents. TMEM68 exhibited monoacylglycerol acyltransferase (MGAT) and diacylglycerol acyltransferase (DGAT) activities dependent on the conserved active sites in an in vitro assay. The expression of lipogenesis genes, including DGATs, fatty acid synthesis-related genes and peroxisome proliferator-activated receptor γ was upregulated in TMEM68-overexpressing cells. These results together demonstrate for the first time that TMEM68 functions as an acyltransferase and affects lipogenic gene expression, glycerolipid metabolism and TG storage in mammalian cells.