Intestinal ABCA1 directly contributes to HDL biogenesis in vivo

The Role of Organelles in Intestinal Function, Physiology, and Disease

The intestine maintains homeostasis by coordinating internal biological processes to adjust to fluctuating external conditions. The intestinal epithelium is continuously renewed and comprises multiple cell types, including absorptive cells, secretory cells, and resident stem cells. An important feature of this organ is its ability to coordinate many processes including cell proliferation, differentiation, regeneration, damage/stress response, immune activity, feeding behavior, and age-related changes by using conserved signaling pathways. However, the subcellular spatial organization of these signaling events and the organelles involved has only recently been studied in detail. Here we discuss how organelles of intestinal cells serve to initiate, mediate, and terminate signals, that are vital for homeostasis.

Gut Microbiota Functional Dysbiosis Relates to Individual Diet in Subclinical Carotid Atherosclerosis

Gut Microbiota (GM) dysbiosis associates with Atherosclerotic Cardiovascular Diseases (ACVD), but whether this also holds true in subjects without clinically manifest ACVD represents a challenge of personalized prevention. We connected exposure to diet (self-reported by food diaries) and markers of Subclinical Carotid Atherosclerosis (SCA) with individual taxonomic and functional GM profiles (from fecal metagenomic DNA) of 345 subjects without previous clinically manifest ACVD. Subjects without SCA reported consuming higher amounts of cereals, starchy vegetables, milky products, yoghurts and bakery products versus those with SCA (who reported to consume more mechanically separated meats). The variety of dietary sources significantly overlapped with the separations in GM composition between subjects without SCA and those with SCA (RV coefficient between nutrients quantities and microbial relative abundances at genus level = 0.65, p-value = 0.047). Additionally, specific bacterial species (Faecalibacterium prausnitzii in the absence of SCA and Escherichia coli in the presence of SCA) are directly related to over-representation of metagenomic pathways linked to different dietary sources (sulfur oxidation and starch degradation in absence of SCA, and metabolism of amino acids, syntheses of palmitate, choline, carnitines and Trimethylamine n-oxide in presence of SCA). These findings might contribute to hypothesize future strategies of personalized dietary intervention for primary CVD prevention setting.

Impaired cholesterol efflux capacity in patients with Helicobacter pylori infection and its relation with inflammation

BACKGROUND

Gut microorganisms are associated with atherosclerosis and related cardiovascular disease. Helicobacter pylori (H. pylori) infection is associated with dyslipidemia and inflammation contributing to the progression of atherosclerosis.

OBJECTIVE

Several studies have reported reduced HDL-C levels in H. pylori infected patients, but HDL cholesterol efflux capacity (CEC) as the most important function of HDL has not been evaluated yet.

METHODS

This cross-sectional study was conducted with 44 biopsy confirmed H. pylori patients and 43 controls. ABCA1-mediated, non-ABCA1 and total CEC were measured in ApoB-depleted serum and levels of ApoA-I, ApoB and hsCRP were estimated using ELISA technique.

RESULTS

Total and ABCA1 mediated-CEC were reduced in patients compared to controls, independent of age, sex, body mass index and HDL-C (p < 0.001), while non-ABCA1 CEC indicated no significant change between the groups. In addition, patients showed lower serum levels of ApoA-I but increased levels of hsCRP when compared to controls. Total CEC and ABCA1-mediated CEC positively correlated with ApoA-I and HDL-C, furthermore, ABCA1-mediated CEC as well as ApoA-I inversely correlated with hsCRP.

CONCLUSION

The results of the present study indicate reduced CECs in H. pylori infected patients, especially ABCA1-mediated CEC which is associated with decreased ApoA-I and increased inflammation.

HDL cholesterol protects from liver injury in mice with intestinal specific LXRα activation

BACKGROUND AND AIMS

Liver X receptors (LXRs) exert anti-inflammatory effects even though their hepatic activation is associated with hypertriglyceridemia and hepatic steatosis. Selective induction of LXRs in the gut might provide protective signal(s) in the aberrant wound healing response that induces fibrosis during chronic liver injury, without hypertriglyceridemic and steatogenic effects.

METHODS

Mice with intestinal constitutive LXRα activation (iVP16-LXRα) were exposed to intraperitoneal injection of carbon tetrachloride (CCl₄ ) for 8 weeks, and in vitro cell models were used to evaluate the beneficial effect of high-density lipoproteins (HDL).

RESULTS

After CCl₄ treatment, the iVP16-LXRα phenotype showed reduced M1 macrophage infiltration, increased expression M2 macrophage markers, and lower expression of hepatic pro-inflammatory genes. This anti-inflammatory effect in the liver was also associated with decreased expression of hepatic oxidative stress genes and reduced expression of fibrosis markers. iVP16-LXRα exhibited increased reverse cholesterol transport in the gut by ABCA1 expression and consequent enhancement of the levels of circulating HDL and their receptor SRB1 in the liver. No hepatic steatosis development was observed in iVP16-LXRα. In vitro, HDL induced a shift from M1 to M2 phenotype of LPS-stimulated Kupffer cells, decreased TNFα-induced oxidative stress in hepatocytes and reduced NF-kB activity in both cells. SRB1 silencing reduced TNFα gene expression in LPS-stimulated KCs, and NOX-1 and IL-6 in HepG2.

CONCLUSIONS

Intestinal activation of LXRα modulates hepatic response to injury by increasing circulating HDL levels and SRB1 expression in the liver, thus suggesting this circuit as potential actionable pathway for therapy.

Alisol B 23-acetate activates ABCG5/G8 in the jejunum via the LXRα/ACAT2 pathway to relieve atherosclerosis in ovariectomized ApoE-/- mice

Phytosterols have been shown to improve blood lipid levels and treat atherosclerosis. This research investigated the effects of phytosterol Alisol B 23-acetate (AB23A) on jejunum lipid metabolism and atherosclerosis. The results show that intragastric administration of AB23A can significantly reduce atherosclerotic plaque area and lipid accumulation in the jejunum of ovariectomized ApoE-/- mice fed a high-fat diet and can also improve the lipid mass spectra of the plasma and jejunum. In vitro studies have shown that AB23A can increase cholesterol outflow in Caco-2 cells exposed to high fat concentrations and increase the expression of ATP-binding cassette transfer proteins G5/G8 (ABCG5/G8), the liver X receptor α (LXRα). Furthermore, inhibition of LXRα can significantly eliminate the active effect of AB23A on decreasing intracellular lipid accumulation. We also confirmed that AB23A has a negative effect on Acyl-CoA cholesterol acyltransferase 2 (ACAT2) in Caco-2 cells cultured in the high concentrations of fat, and we found that AB23A further reduces ACAT2 expression in cells treated with the ACAT2 inhibitor pyripyropene or transfected with ACAT2 siRNA. In conclusion, we confirmed that AB23A can reduce the absorption of dietary lipids in the jejunum by affecting the LXRα-ACAT2-ABCG5/G8 pathway and ultimately exert an anti-atherosclerotic effect.

Lipid metabolism in colon cancer: Role of Liver X Receptor (LXR) and Stearoyl-CoA Desaturase 1 (SCD1)

Colorectal cancer (CRC) is one of the most commonly occurring cancers worldwide. Although several genetic alterations have been associated with CRC onset and progression, nowadays the reprogramming of cellular metabolism has been recognized as a fundamental step of the carcinogenic process. Intestinal tumor cells frequently display an aberrant activation of lipid metabolism. Indeed, to satisfy the growing needs of a continuous proliferation, cancer cells can either increase the uptake of exogenous lipids or upregulate the endogenous lipogenesis and cholesterol synthesis. Therefore, strategies aimed at limiting lipid accumulation are now under development in order to counteract malignancies. Two major players of lipids metabolism have been so far identified for their contribution to CRC development: the nuclear receptor Liver X Receptor (LXRs) and the enzyme Stearoyl-CoA Desaturase 1 (SCD1). Whereas LXR is mainly recognized for its role as a cholesterol sensor, finally promoting the loss of cellular cholesterol and whole-body homeostasis, SCD1 acts as the major regulator of new fatty acids, finely tuning the monounsaturated fatty acids (MUFA) to saturated fatty acids (SFA) ratio. Intriguingly, SCD1 is directly regulated by LXRs. Despite LXRs agonists have elicited great interest as a promising therapeutic target for cancer, LXR's ability to induce SCD1 and new fatty acids synthesis represent a major obstacle in the development of new effective treatments. Thus, further investigations are required to fully dissect the concomitant modulation of both players, to develop specific therapies aimed at blocking intestinal cancer cells proliferation, eventually counteracting CRC progression.

Interaction between high-density lipoproteins and inflammation: Function matters more than concentration!

High-density lipoprotein (HDL) plays an important role in lipid metabolism and especially contributes to the reverse cholesterol transport pathway. Over recent years it has become clear that the effect of HDL on immune-modulation is not only dependent on HDL concentration but also and perhaps even more so on HDL function. This review will provide a concise general introduction to HDL followed by an overview of post-translational modifications of HDL and a detailed overview of the role of HDL in inflammatory diseases. The clinical potential of HDL and its main apolipoprotein constituent, apoA-I, is also addressed in this context. Finally, some conclusions and remarks that are important for future HDL-based research and further development of HDL-focused therapies are discussed.

Functional pasta consumption in healthy volunteers modulates ABCG1-mediated cholesterol efflux capacity of HDL

BACKGROUNDS AND AIMS

Prevention of cardiovascular (CV) disease is considered a central issue in public health and great attention is payed to nutritional approaches, including consumption of functional foods to reduce CV risk in individuals without indications for anti-atherosclerotic drugs. Cholesterol efflux capacity (CEC) is an important anti-atherogenic property of HDL and a marker of CV risk. We evaluated the effect of a daily consumption of an innovative whole-wheat synbiotic pasta, compared to a control whole-wheat pasta, on serum ATP binding cassette G1 (ABCG1)-mediated CEC in healthy overweight or obese individuals.

METHODS AND RESULTS

Study participants (n = 41) were randomly allocated to either innovative or control pasta, consumed daily for twelve weeks. Serum CEC was measured before and after the dietary intervention, by a well-established radioisotopic technique on Chinese Hamster Ovary Cells transfected with human ABCG1. The innovative synbiotic pasta consumption was associated to a significantly higher post treatment/baseline ratio of ABCG1-mediated CEC values with respect to control pasta (mean ratio 1.05 ± 0.037 and 0.95 ± 0.042 respectively, p < 0.05). Analysis of the relationship between ABCG1-mediated CEC and glycemia, homocysteine, total folates and interleukin-6 showed specific changes in the correlations between HDL function and glycemia, oxidative and inflammatory markers only after synbiotic pasta consumption.

CONCLUSION

This is the first report on serum CEC improvement obtained by a new synbiotic functional pasta consumption, in absence of lipid profile modifications, in overweight/obese participants. This pilot study suggests that a simple dietary intervention can be a promising approach to CV preservation through improving of athero-protective HDL function.

Effect of fasting and feeding on apolipoprotein A-I kinetics in preβ1-HDL, α-HDL, and triglyceride-rich lipoproteins

The aim of this study was to compare the kinetics of apolipoprotein (apo)A-I during fed and fasted states in humans, and to determine to what extent the intestine contributes to apoA-I production. A stable isotope study was conducted to determine the kinetics of apoA-I in preβ₁ high-density lipoprotein (HDL) and α-HDL. Six healthy male subjects received a constant intravenous infusion of ²H₃-leucine for 14 h. Subjects in the fed group also received small hourly meals. Blood samples were collected hourly during tracer infusion and then daily for 4 days. Tracer enrichments were measured by mass spectrometry and then fitted to a compartmental model using asymptotic plateau of very-low-density lipoprotein (VLDL) apoB100 and triglyceride-rich lipoprotein (TRL) apoB48 as estimates of hepatic and intestinal precursor pools, respectively. The clearance rate of preβ₁-HDL-apoA-I was lower in fed individuals compared with fasted subjects (p < 0.05). No other differences in apoA-I production or clearance rates were observed between the groups. No significant correlation was observed between plasma apoC-III concentrations and apoA-I kinetic data. In contrast, HDL-apoC-III was inversely correlated with the conversion of α-HDL to preβ₁-HDL. Total apoA-I synthesis was not significantly increased in fed subjects. Hepatic production was not significantly different between the fed group (17.17 ± 2.75 mg/kg/day) and the fasted group (18.67 ± 1.69 mg/kg/day). Increase in intestinal apoA-I secretion in fed subjects was 2.20 ± 0.61 mg/kg/day. The HDL-apoA-I kinetics were similar in the fasted and fed groups, with 13% of the total apoA-I originating from the intestine with feeding.

Defects in High Density Lipoprotein metabolism and hepatic steatosis in mice with liver-specific ablation of Hepatocyte Nuclear Factor 4A

BACKGROUND

Aberrant concentration, structure and functionality of High Density Lipoprotein (HDL) are associated with many prevalent diseases, including cardiovascular disease and non-alcoholic fatty liver disease (NAFLD). Mice with liver-specific ablation of Hnf4α (H4LivKO) present steatosis and dyslipidemia by mechanisms that are not completely understood. The aim of this study was to explore the role of liver HNF4A in HDL metabolism and the development of steatosis.

METHODS AND RESULTS

Serum and tissue samples were obtained from 6-weeks old H4LivKO mice and their littermate controls. Liver and serum lipids were measured and HDL structure and functionality were assessed. Global gene expression changes in the liver were analyzed by expression arrays, validations were performed by RT-qPCR and DNA-protein interactions were studied by chromatin immunoprecipitation (ChIP). H4LivKO mice presented liver steatosis, increased liver triglyceride content and decreased concentration of serum total cholesterol, HDL cholesterol, triglycerides, phospholipids and cholesteryl esters. Most classes of phospholipids showed significant changes in species ratio and sphingosine-1-phosphate (S1P) levels were reduced. H4LivKO serum was enriched in the smaller, denser HDL particles, devoid of APOA2 and APOM apolipoproteins, exhibiting decreased activity of paraoxonase-1 but retaining macrophage cholesterol efflux capacity and phospho-AKT activation in endothelial cells. Global gene expression analysis revealed the association of liver HNF4A with known and novel regulators of HDL metabolism as well as NAFLD-susceptibility genes.

CONCLUSIONS

HNF4A ablation in mouse liver causes hepatic steatosis, perturbations in HDL structure and function and significant global changes in gene expression. This study reveals new targets of HNF4A involved in HDL metabolism and the development of steatosis and enriches our knowledge on HDL functionality in NAFLD.

Choline in cystic fibrosis: relations to pancreas insufficiency, enterohepatic cycle, PEMT and intestinal microbiota

BACKGROUND

Cystic Fibrosis (CF) is an autosomal recessive disorder with life-threatening organ manifestations. 87% of CF patients develop exocrine pancreas insufficiency, frequently starting in utero and requiring lifelong pancreatic enzyme substitution. 99% develop progressive lung disease, and 20-60% CF-related liver disease, from mild steatosis to cirrhosis. Characteristically, pancreas, liver and lung are linked by choline metabolism, a critical nutrient in CF. Choline is a tightly regulated tissue component in the form of phosphatidylcholine (Ptd'Cho) and sphingomyelin (SPH) in all membranes and many secretions, particularly of liver (bile, lipoproteins) and lung (surfactant, lipoproteins). Via its downstream metabolites, betaine, dimethylglycine and sarcosine, choline is the major one-carbon donor for methionine regeneration from homocysteine. Methionine is primarily used for essential methylation processes via S-adenosyl-methionine.

CLINICAL IMPACT

CF patients with exocrine pancreas insufficiency frequently develop choline deficiency, due to loss of bile Ptd'Cho via feces. ~ 50% (11-12 g) of hepatic Ptd'Cho is daily secreted into the duodenum. Its re-uptake requires cleavage to lyso-Ptd'Cho by pancreatic and small intestinal phospholipases requiring alkaline environment. Impaired CFTR-dependent bicarbonate secretion, however, results in low duodenal pH, impaired phospholipase activity, fecal Ptd'Cho loss and choline deficiency. Low plasma choline causes decreased availability for parenchymal Ptd'Cho metabolism, impacting on organ functions. Choline deficiency results in hepatic choline/Ptd'Cho accretion from lung tissue via high density lipoproteins, explaining the link between choline deficiency and lung function. Hepatic Ptd'Cho synthesis from phosphatidylethanolamine by phosphatidylethanolamine-N-methyltransferase (PEMT) partly compensates for choline deficiency, but frequent single nucleotide polymorphisms enhance choline requirement. Additionally, small intestinal bacterial overgrowth (SIBO) frequently causes intraluminal choline degradation in CF patients prior to its absorption. As adequate choline supplementation was clinically effective and adult as well as pediatric CF patients suffer from choline deficiency, choline supplementation in CF patients of all ages should be evaluated.

Oral 15-Hydroxyeicosatetraenoic Acid Induces Pulmonary Hypertension in Mice by Triggering T Cell-Dependent Endothelial Cell Apoptosis

Pulmonary arterial hypertension (PAH) is a fatal disease characterized by increased mean pulmonary arterial pressure. Elevated plasma and lung concentrations of oxidized lipids, including 15-hydroxyeicosatetraenoic acid (15-HETE), have been demonstrated in patients with PAH and animal models. We previously demonstrated that feeding mice with 15-HETE is sufficient to induce pulmonary hypertension, but the mechanisms remain unknown. RNA sequencing data from the mouse lungs on 15-HETE diet revealed significant activation of pathways involved in both antigen processing and presentation and T cell-mediated cytotoxicity. Analysis of human microarray from patients with PAH also identified activation of identical pathways compared with controls. We show that in both 15-HETE-fed mice and patients with PAH, expression of the immunoproteasome subunit 5 is significantly increased, which was concomitant with an increase in the number of CD8/CD69 (cluster of differentiation 8 / cluster of differentiation 69) double-positive cells, as well as pulmonary arterial endothelial cell apoptosis in mice. Human pulmonary arterial endothelial cells cultured with 15-HETE were more prone to apoptosis when exposed to CD8 cells. Cultured intestinal epithelial cells secreted more oxidized lipids in response to 15-HETE, which is consistent with accumulation of circulating oxidized lipids in 15-HETE-fed mice. Administration of an apoA-I (apolipoprotein A-I) mimetic peptide, Tg6F (transgenic 6F), which is known to prevent accumulation of circulating oxidized lipids, not only inhibited pulmonary arterial endothelial cell apoptosis but also prevented and rescued 15-HETE-induced pulmonary hypertension in mice. In conclusion, our results suggest that (1) 15-HETE diet induces pulmonary hypertension by a mechanism that involves oxidized lipid-mediated T cell-dependent pulmonary arterial endothelial cell apoptosis and (2) Tg6F administration may be a novel therapy for treating PAH.

Effects of octylphenol exposure on the lipid metabolism and microbiome of the intestinal tract of Rana chensinensis tadpole by RNAseq and 16s amplicon sequencing

Octylphenol (OP) is a widely distributed endocrine disrupting chemical (EDC), and can be commonly found in various and diverse environmental media. Previous studies have reported that OP exposure could cause many adverse effects on aquatic animals. However, knowledge concerning the impact of OP on lipid metabolism in amphibians was still limited. In our study, Rana chensinensis tadpoles were exposed to different OP concentrations (0, 10^-8, 10^-7 and 10^-6 mol/L) from the Gosner stage (Gs) 25-38. The RNA-seq analysis of tadpole intestines was explored by RNA-seq, and six differentially expressed genes (DEGs) related to the fat digestion and absorption were validated by RT-qPCR. Moreover, we used 16s amplicon sequencing to evaluate effects of OP on intestinal microbiome in tadpoles, further determining the variations of lipid metabolism. Our results revealed that OP exposure influenced gene expression levels related to fat digestion and absorption and led to alteration of structure and composition of intestinal microbiome. At the phylum level, the Firmicutes/Bacteroidetes ratio was gradually decreased in OP exposure groups, which disrupted lipid metabolism. According to the results of intestinal microbial functional prediction, OP exposure interfered with metabolic function and increased risk of disease. These data provide us with powerful resources to assess the effects of OP on lipid metabolism by integrating RNAseq and 16s amplicon sequencing analysis of intestinal tract and intestinal microbiome.

Lipid efflux mechanisms, relation to disease and potential therapeutic aspects

Lipids are hydrophobic and amphiphilic molecules involved in diverse functions such as membrane structure, energy metabolism, immunity, and signaling. However, altered intra-cellular lipid levels or composition can lead to metabolic and inflammatory dysfunction, as well as lipotoxicity. Thus, intra-cellular lipid homeostasis is tightly regulated by multiple mechanisms. Since most peripheral cells do not catabolize cholesterol, efflux (extra-cellular transport) of cholesterol is vital for lipid homeostasis. Defective efflux contributes to atherosclerotic plaque development, impaired β-cell insulin secretion, and neuropathology. Of these, defective lipid efflux in macrophages in the arterial walls leading to foam cell and atherosclerotic plaque formation has been the most well studied, likely because a leading global cause of death is cardiovascular disease. Circulating high density lipoprotein particles play critical roles as acceptors of effluxed cellular lipids, suggesting their importance in disease etiology. We review here mechanisms and pathways that modulate lipid efflux, the role of lipid efflux in disease etiology, and therapeutic options aimed at modulating this critical process.

Impact of air pollution on intestinal redox lipidome and microbiome

Air pollution is a rising public health issue worldwide. Cumulative epidemiological and experimental studies have shown that exposure to air pollution such as particulate matter (PM) is linked with increased hospital admissions and all-cause mortality. While previous studies on air pollution mostly focused on the respiratory and cardiovascular effects, emerging evidence supports a significant impact of air pollution on the gastrointestinal (GI) system. The gut is exposed to PM as most of the inhaled particles are removed from the lungs to the GI tract via mucociliary clearance. Ingestion of contaminated food and water is another common source of GI tract exposure to pollutants. Recent studies have associated air pollution with intestinal diseases, including appendicitis, colorectal cancer, and inflammatory bowel disease. In addition to the liver and adipose tissue, intestine is an important organ system for lipid metabolism, and the intestinal redox lipids might be tightly associated with the intestinal and systematic inflammation. The gut microbiota modulates lipid metabolism and contributes to the initiation and development of intestinal disease including inflammatory bowel disease. Recent data support microbiome implication in air pollution-mediated intestinal and systematic effects. In this review, the associations between air pollution and intestinal diseases, and the alterations of intestinal lipidome and gut microbiome by air pollution are highlighted. The potential mechanistic aspects underlying air pollution-mediated intestinal pathology will also be discussed.

Organ-specific lymphatics play distinct roles in regulating HDL trafficking and composition

Recently, peripheral lymphatic vessels were found to transport high-density lipoprotein (HDL) from interstitial tissues to the blood circulation during reverse cholesterol transport. This function is thought to be critical to the clearance of cholesterol from atherosclerotic plaques. The role of organ-specific lymphatics in modulating HDL transport and composition is, however, incompletely understood. This study aimed to 1) determine the contribution of the lymphatics draining the intestine and liver (which are major sites of HDL synthesis) to total (thoracic) lymph HDL transport and 2) verify whether the HDLs in lymph are derived from specific organs and are modified during trafficking in lymph. The mesenteric, hepatic, or thoracic lymph duct was cannulated in nonfasted Sprague-Dawley rats, and lymph was collected over 5 h under anesthesia. Whole lymph and specific lymph lipoproteins (isolated by ultracentrifugation) were analyzed for protein and lipid composition. The majority of thoracic lymph fluid, protein, and lipid mass was sourced from the mesenteric, and to a lesser extent, hepatic lymph. Mesenteric and thoracic lymph were both rich in chylomicrons and very low-density lipoprotein, whereas hepatic lymph and plasma were HDL-rich. The protein and lipid mass in thoracic lymph HDL was mostly sourced from mesenteric lymph, whereas the cholesterol mass was equally sourced from mesenteric and hepatic lymph. HDLs were compositionally distinct across the lymph sources and plasma. The composition of HDL also appeared to be modified during passage from the mesenteric and hepatic to the thoracic lymph duct. Overall, this study demonstrates that the lipoproteins in lymph are organ specific in composition, and the intestine and liver appear to be the main source of HDL in the lymph.NEW & NOTEWORTHY High-density lipoprotein in lymph are organ-specific in composition and derive mostly from the intestine and liver. High-density lipoprotein also appears to be remodeled during transport through the lymphatics. These findings have implications to cardiometabolic diseases that involve perturbations in lipoprotein distribution and metabolism.

High Density Lipoproteins: Metabolism, Function, and Therapeutic Potential

High Density Lipoproteins (HDLs) have long been considered as “good cholesterol,” beneficial to the whole body and, in particular, to cardio-vascular health. However, HDLs are complex particles that undergoes dynamic remodeling through interactions with various enzymes and tissues throughout their life cycle, making the complete understanding of its functions and roles more complicated than initially expected. In this review, we explore the novel understanding of HDLs' behavior in health and disease as a multifaceted class of lipoprotein, with different size subclasses, molecular composition, receptor interactions, and functionality. Further, we report on emergent HDL-based therapeutics tested in small and larger scale clinical trials and their mixed successes.

The PPAR pan-agonist tetradecylthioacetic acid promotes redistribution of plasma cholesterol towards large HDL

Tetradecylthioacetic acid (TTA) is a synthetic fatty acid with a sulfur substitution in the β-position. This modification renders TTA unable to undergo complete β-oxidation and increases its biological activity, including activation of peroxisome proliferator activated receptors (PPARs) with preference for PPARα. This study investigated the effects of TTA on lipid and lipoprotein metabolism in the intestine and liver of mice fed a high fat diet (HFD). Mice receiving HFD supplemented with 0.75% (w/w) TTA had significantly lower body weights compared to mice fed the diet without TTA. Plasma triacylglycerol (TAG) was reduced 3-fold with TTA treatment, concurrent with increase in liver TAG. Total cholesterol was unchanged in plasma and liver. However, TTA promoted a shift in the plasma lipoprotein fractions with an increase in larger HDL particles. Histological analysis of the small intestine revealed a reduced size of lipid droplets in enterocytes of TTA treated mice, accompanied by increased mRNA expression of fatty acid transporter genes. Expression of the cholesterol efflux pump Abca1 was induced in the small intestine, but not in the liver. Scd1 displayed markedly increased mRNA and protein expression in the intestine of the TTA group. It is concluded that TTA treatment of HFD fed mice leads to increased expression of genes involved in uptake and transport of fatty acids and HDL cholesterol in the small intestine with concomitant changes in the plasma profile of smaller lipoproteins.

Regulation of intestinal lipid metabolism: current concepts and relevance to disease

Lipids entering the gastrointestinal tract include dietary lipids (triacylglycerols, cholesteryl esters and phospholipids) and endogenous lipids from bile (phospholipids and cholesterol) and from shed intestinal epithelial cells (enterocytes). Here, we comprehensively review the digestion, uptake and intracellular re-synthesis of intestinal lipids as well as their packaging into pre-chylomicrons in the endoplasmic reticulum, their modification in the Golgi apparatus and the exocytosis of the chylomicrons into the lamina propria and subsequently to lymph. We also discuss other fates of intestinal lipids, including intestinal HDL and VLDL secretion, cytosolic lipid droplets and fatty acid oxidation. In addition, we highlight the applicability of these findings to human disease and the development of therapeutics targeting lipid metabolism. Finally, we explore the emerging role of the gut microbiota in modulating intestinal lipid metabolism and outline key questions for future research.

Impact of natural products on the cholesterol transporter ABCA1

ETHNOPHARMACOLOGICAL RELEVANCE

In different countries and areas of the world, traditional medicine has been and is still used for the treatment of various disorders, including chest pain or liver complaints, of which we now know that they can be linked with altered lipid and cholesterol homeostasis. As ATP-binding cassette transporter A1 (ABCA1) plays an essential role in cholesterol metabolism, its modulation may be one of the molecular mechanisms responsible for the experienced benefit of traditional recipes. Intense research activity has been dedicated to the identification of natural products from traditional medicine that regulate ABCA1 expression.

AIMS OF THE REVIEW

This review surveys natural products, originating from ethnopharmacologically used plants, fungi or marine sources, which influence ABCA1 expression, providing a reference for future study.

MATERIALS AND METHODS

Information on regulation of ABCA1 expression by natural compounds from traditional medicine was extracted from ancient and modern books, materia medica, and electronic databases (PubMed, Google Scholar, Science Direct, and ResearchGate).

RESULTS

More than 60 natural compounds from traditional medicine, especially traditional Chinese medicine (TCM), are reported to regulate ABCA1 expression in different in vitro and in vivo models (such as cholesterol efflux and atherosclerotic animal models). These active compounds belong to the classes of polyketides, terpenoids, phenylpropanoids, tannins, alkaloids, steroids, amino acids and others. Several compounds appear very promising in vivo, which need to be further investigated in animal models of diseases related to ABCA1 or in clinical studies.

CONCLUSION

Natural products from traditional medicine constitute a large promising pool for compounds that regulate ABCA1 expression, and thus may prevent/treat diseases related to cholesterol metabolism, like atherosclerosis or Alzheimer's disease. In many cases, the molecular mechanisms of these natural products remain to be investigated.

"Dysfunctions" induced by Roux-en-Y gastric bypass surgery are concomitant with metabolic improvement independent of weight loss

Metabolic surgery has been increasingly recommended for obese diabetic patients, but questions remain as to its molecular mechanism that leads to improved metabolic parameters independently of weight loss from a network viewpoint. We evaluated the role of the Roux limb (RL) in Roux-en-Y gastric bypass (RYGB) surgery in nonobese diabetic rat models. Improvements in metabolic parameters were greater in the long-RL RYGB group. Transcriptome profiles reveal that amelioration of diabetes state following RYGB differs remarkably from both normal and diabetic states. According to functional analysis, RYGB surgery significantly affected a major gene group, i.e., the newly changed group, which represented diabetes-irrelevant genes abnormally expressed after RYGB. We hypothesize that novel "dysfunctions" carried by this newly changed gene group induced by RYGB rebalance diabetic states and contribute to amelioration of metabolic parameters. An unusual increase in cholesterol (CHOL) biosynthesis in RL enriched by the newly changed group was concomitant with ameliorated metabolic parameters, as demonstrated by measurements of physiological parameters and biodistribution analysis using [14C]-labeled glucose. Our findings demonstrate RYGB-induced "dysfunctions" in the newly changed group as a compensatory role contributes to amelioration of diabetes. Rather than attempting to normalize "abnormal" molecules, we suggest a new disease treatment strategy of turning "normal" molecules "abnormal" in order to achieve a new "normal" physiological balance. It further implies a novel strategy for drug discovery, i.e. targeting also on "normal" molecules, which are traditionally ignored in pharmaceutical development.

Rare dyslipidaemias, from phenotype to genotype to management: a European Atherosclerosis Society task force consensus statement

Genome sequencing and gene-based therapies appear poised to advance the management of rare lipoprotein disorders and associated dyslipidaemias. However, in practice, underdiagnosis and undertreatment of these disorders are common, in large part due to interindividual variability in the genetic causes and phenotypic presentation of these conditions. To address these challenges, the European Atherosclerosis Society formed a task force to provide practical clinical guidance focusing on patients with extreme concentrations (either low or high) of plasma low-density lipoprotein cholesterol, triglycerides, or high-density lipoprotein cholesterol. The task force also recognises the scarcity of quality information regarding the prevalence and outcomes of these conditions. Collaborative registries are needed to improve health policy for the care of patients with rare dyslipidaemias.

Identification of a novel cholesterol-lowering dipeptide, phenylalanine-proline (FP), and its down-regulation of intestinal ABCA1 in hypercholesterolemic rats and Caco-2 cells

There has been no report about in vivo active cholesterol-lowering dipeptide in any protein origin, despite their potential health benefits. Cattle heart protein hydrolysate ultra-filtrate (HPHU, molecular weight < ca. 1,000 Da peptide mixture) exhibits cholesterol-lowering activity in hypercholesterolemic rats, but the active peptide in HPHU that lowers serum cholesterol levels and its molecular mechanism are unknown. In this study, we separated and purified HPHU to identify a novel cholesterol-lowering dipeptide (phenylalanine-proline, FP) and characterized the mechanism underlying its effects in vivo and in vitro. We identified FP as an active peptide from HPHU by MALDI-TOF mass spectrometry. FP significantly decreased serum total and non-HDL cholesterol and hepatic cholesterol levels in rats. FP significantly increased serum HDL cholesterol, accompanied by a significant decrease in the atherogenic index. FP also significantly increased fecal cholesterol and acidic steroid excretion. Moreover, FP significantly decreased ATP-binding cassette transporter A1 (ABCA1) expression in the rat jejunum and reduced cholesterol absorption in Caco-2 cells. We found a novel cholesterol-lowering dipeptide FP that could improve cholesterol metabolism via the down-regulation of intestinal ABCA1. The cholesterol-lowering action induced by FP was disappeared in PepT1KO mice. FP-induced cholesterol-lowering action is mediated via PepT1 in mice.

HDL therapy today: from atherosclerosis, to stent compatibility to heart failure

Epidemiologically, high-density lipoprotein (HDL) cholesterol levels have been inversely associated to cardiovascular (CV) events, although a Mendelian Randomisation Study had failed to establish a clear causal role. Numerous atheroprotective mechanisms have been attributed to HDL, the main being the ability to promote cholesterol efflux from arterial walls; anti-inflammatory effects related to HDL ligands such as S1P (sphingosine-1-phosphate), resolvins and others have been recently identified. Experimental studies and early clinical investigations have indicated the potential of HDL to slow progression or induce regression of atherosclerosis. More recently, the availability of different HDL formulations, with different phospholipid moieties, has allowed to test other indications for HDL therapy. Positive reports have come from studies on coronary stent biocompatibility, where the use of HDL from different sources reduced arterial cell proliferation and thrombogenicity. The observation that low HDL-C levels may be associated with an enhanced risk of heart failure (HF) has also suggested that HDL therapy may be applied to this condition. HDL infusions or apoA-I gene transfer were able to reverse heart abnormalities, reduce diastolic resistance and improve cardiac metabolism. HDL therapy may be effective not only in atherosclerosis, but also in other conditions, of relevant impact on human health.Key messagesHigh-density lipoproteins have as a major activity that of removing excess cholesterol from tissues (particularly arteries).Knowledge on the activity of high-density lipoproteins on health have however significantly widened.HDL-therapy may help to improve stent biocompatibility and to reduce peripheral arterial resistance in heart failure.

Regulation of glucose and lipid metabolism in health and disease

Glucose and fatty acids are the major sources of energy for human body. Cholesterol, the most abundant sterol in mammals, is a key component of cell membranes although it does not generate ATP. The metabolisms of glucose, fatty acids and cholesterol are often intertwined and regulated. For example, glucose can be converted to fatty acids and cholesterol through de novo lipid biosynthesis pathways. Excessive lipids are secreted in lipoproteins or stored in lipid droplets. The metabolites of glucose and lipids are dynamically transported intercellularly and intracellularly, and then converted to other molecules in specific compartments. The disorders of glucose and lipid metabolism result in severe diseases including cardiovascular disease, diabetes and fatty liver. This review summarizes the major metabolic aspects of glucose and lipid, and their regulations in the context of physiology and diseases.

Targeting Foam Cell Formation in Atherosclerosis: Therapeutic Potential of Natural Products

Foam cell formation and further accumulation in the subendothelial space of the vascular wall is a hallmark of atherosclerotic lesions. Targeting foam cell formation in the atherosclerotic lesions can be a promising approach to treat and prevent atherosclerosis. The formation of foam cells is determined by the balanced effects of three major interrelated biologic processes, including lipid uptake, cholesterol esterification, and cholesterol efflux. Natural products are a promising source for new lead structures. Multiple natural products and pharmaceutical agents can inhibit foam cell formation and thus exhibit antiatherosclerotic capacity by suppressing lipid uptake, cholesterol esterification, and/or promoting cholesterol ester hydrolysis and cholesterol efflux. This review summarizes recent findings on these three biologic processes and natural products with demonstrated potential to target such processes. Discussed also are potential future directions for studying the mechanisms of foam cell formation and the development of foam cell-targeted therapeutic strategies.

Emerging roles for high-density lipoproteins in neurodegenerative disorders

Lipoproteins are the complexes of different lipids and proteins, which are devoted to the transport and clearance of lipids or lipid-related molecules in the circulation. Lipoproteins have been found to play a crucial role in brain function and may influence myelination process. Among lipoproteins, high-density lipoproteins (HDLs) and their major protein component, apoA-I, are directly involved in cholesterol efflux in the brain. It has been suggested that inadequate or dysfunctional brain HDLs may contribute to cerebrovascular dysfunctions, neurodegeneration, or neurovascular instability. HDL deficiency could also promote cognitive decline through impacting on atherosclerotic risk. The focus of this review is to discuss knowledge on HDL dysregulation in neurological disorders. A better understanding on how changes in cellular HDL and apolipoprotein homeostasis affect central nervous system function may provide promising novel avenues for the treatment of specific HDL-related neurological disorders.

Lycopene Modulates Pathophysiological Processes of Non-Alcoholic Fatty Liver Disease in Obese Rats

Background: The higher consumption of fat and sugar are associated with obesity development and its related diseases such as non-alcoholic fatty liver disease (NAFLD). Lycopene is an antioxidant whose protective potential on fatty liver degeneration has been investigated. The aim of this study was to present the therapeutic effects of lycopene on NAFLD related to the obesity induced by a hypercaloric diet. Methods: Wistar rats were distributed in two groups: Control (Co, n = 12) and hypercaloric (Ob, n = 12). After 20 weeks, the animals were redistributed into the control group (Co, n = 6), control group supplemented with lycopene (Co+Ly, n = 6), obese group (Ob, n = 6), and obese group supplemented with lycopene (Ob+Ly, n = 6). Ob groups also received water + sucrose (25%). Animals received lycopene solution (10 mg/kg/day) or vehicle (corn oil) via gavage for 10 weeks. Results: Animals which consumed the hypercaloric diet had higher adiposity index, increased fasting blood glucose, hepatic and blood triglycerides, and also presented in the liver macro and microvesicular steatosis, besides elevated levels of tumor necrosis factor-α (TNF-α). Lycopene has shown therapeutic effects on blood and hepatic lipids, increased high-density lipoprotein cholesterol (HDL), mitigated TNF-α, and malondialdehyde (MDA) and further improved the hepatic antioxidant capacity. Conclusion: Lycopene shows therapeutic potential to NAFLD.

Modulation of Hepatic MRP3/ABCC3 by Xenobiotics and Pathophysiological Conditions: Role in Drug Pharmacokinetics

Liver transporters play an important role in the pharmacokinetics and disposition of pharmaceuticals, environmental contaminants, and endogenous compounds. Among them, the family of ATP-Binding Cassette (ABC) transporters is the most important due to its role in the transport of endo- and xenobiotics. The ABCC sub-family is the largest one, consisting of 13 members that include the cystic fibrosis conductance regulator (CFTR/ABCC7); the sulfonylurea receptors (SUR1/ABCC8 and SUR2/ABCC9) and the multidrug resistanceassociated proteins (MRPs). The MRP-related proteins can collectively confer resistance to natural, synthetic drugs and their conjugated metabolites, including platinum-containing compounds, folate anti-metabolites, nucleoside and nucleotide analogs, among others. MRPs can be also catalogued into "long" (MRP1/ABCC1, -2/C2, -3/C3, -6/C6, and -7/C10) and "short" (MRP4/C4, -5/C5, -8/C11, -9/C12, and -10/C13) categories. While MRP2/ABCC2 is expressed in the canalicular pole of hepatocytes, all others are located in the basolateral membrane. In this review, we summarize information from studies examining the changes in expression and regulation of the basolateral hepatic transporter MPR3/ABCC3 by xenobiotics and during various pathophysiological conditions. We also focus, primarily, on the consequences of such changes in the pharmacokinetic, pharmacodynamic and/or toxicity of different drugs of clinical use transported by MRP3.

Dietary Toll-Like Receptor Stimulants Promote Hepatic Inflammation and Impair Reverse Cholesterol Transport in Mice via Macrophage-Dependent Interleukin-1 Production

Background: The mechanisms connecting dietary intake of processed foods with systemic inflammatory markers and cardiovascular risk remain poorly defined. We sought to compare the abundance of pro-inflammatory stimulants of innate immune receptors in processed foods with those produced by the murine ileal and caecal microbiota, and to explore the impact of their ingestion on systemic inflammation and lipid metabolism in vivo. Methods and results: Calibrated receptor-dependent reporter assays revealed that many processed foods, particularly those based on minced meats, contain pro-inflammatory stimulants of Toll-like receptor (TLR)-2 and TLR4 at concentrations which greatly exceed those produced by the endogenous murine ileal microbiota. Chronic dietary supplementation with these stimulants, at concentrations relevant to those measured in the Western diet, promoted hepatic inflammation and reduced several markers of reverse cholesterol transport (RCT) in mice. Hepatocytes were found to be insensitive to TLR2- and TLR4-stimulants directly, but their secretion of functional cholesterol acceptors was impaired by interleukin (IL)-1β released by TLR-responsive hepatic macrophages. Hepatic macrophage priming by high-fat diet enhanced the impairment of RCT by ingested endotoxin, and this was reversed by macrophage depletion via clodronate liposome treatment, or genetic deficiency in the IL-1 receptor. Conclusion: These findings reveal an unexpected mechanism connecting processed food consumption with cardiovascular risk factors, and introduce the food microbiota as a potential target for therapeutic regulation of lipid metabolism.

Τhe Antioxidant Function of HDL in Atherosclerosis

Atherosclerosis is a multistep process that progresses over a long period of time and displays a broad range of severity. In its final form, it manifests as a lesion of the intimal layer of the arterial wall. There is strong evidence supporting that oxidative stress contributes to coronary heart disease morbidity and mortality and antioxidant high-density lipoprotein (HDL) could have a beneficial role in the prevention and prognosis of the disease. Indeed, certain subspecies of HDL may act as natural antioxidants preventing oxidation of lipids on low-density lipoprotein (LDL) and biological membranes. The antioxidant function may be attributed to inhibition of synthesis or neutralization of free radicals and reactive oxygen species by HDL lipids and associated enzymes or transfer of oxidation prone lipids from LDL and biological membranes to HDL for catabolism. A limited number of clinical trials suggest that the increased antioxidant potential of HDL correlates with decreased risk for atherosclerosis. Some nutritional interventions to increase HDL antioxidant activity have been proposed with limited success so far. The limitations in measuring and understanding HDL antioxidant function in vivo are also discussed.

Targeted Deletion of Hepatocyte Abca1 Increases Plasma HDL (High-Density Lipoprotein) Reverse Cholesterol Transport via the LDL (Low-Density Lipoprotein) Receptor

OBJECTIVE

The role of hepatocyte Abca1 (ATP binding cassette transporter A1) in trafficking hepatic free cholesterol (FC) into plasma versus bile for reverse cholesterol transport (RCT) is poorly understood. We hypothesized that hepatocyte Abca1 recycles plasma HDL-C (high-density lipoprotein cholesterol) taken up by the liver back into plasma, maintaining the plasma HDL-C pool, and decreasing HDL-mediated RCT into feces. Approach and Results: Chow-fed hepatocyte-specific Abca1 knockout (HSKO) and control mice were injected with human HDL radiolabeled with ¹²⁵I-tyramine cellobiose (¹²⁵I-TC; protein) and ³H-cholesteryl oleate (³H-CO). ¹²⁵I-TC and ³H-CO plasma decay, plasma HDL ³H-CO selective clearance (ie, ³H-¹²⁵I fractional catabolic rate), liver radiolabel uptake, and fecal ³H-sterol were significantly greater in HSKO versus control mice, supporting increased plasma HDL RCT. Twenty-four hours after ³H-CO-HDL injection, HSKO mice had reduced total hepatic ³H-FC (ie, ³H-CO hydrolyzed to ³H-FC in liver) resecretion into plasma, demonstrating Abca1 recycled HDL-derived hepatic ³H-FC back into plasma. Despite similar liver LDLr (low-density lipoprotein receptor) expression between genotypes, HSKO mice treated with LDLr-targeting versus control antisense oligonucleotide had slower plasma ³H-CO-HDL decay, reduced selective ³H-CO clearance, and decreased fecal ³H-sterol excretion that was indistinguishable from control mice. Increased RCT in HSKO mice was selective for ³H-CO-HDL, since macrophage RCT was similar between genotypes.

CONCLUSIONS

Hepatocyte Abca1 deletion unmasks a novel and selective FC trafficking pathway that requires LDLr expression, accelerating plasma HDL-selective CE uptake by the liver and promoting HDL RCT into feces, consequently reducing HDL-derived hepatic FC recycling into plasma.

Irisin Is Controlled by Farnesoid X Receptor and Regulates Cholesterol Homeostasis

Objective

The aim of this study was to investigate whether the nuclear receptor farnesoid X receptor (FXR) could regulate FNDC5/Irisin expression and the role of Irisin in hyperlipidemia and atherosclerosis in ApoE-/- mice.

Methods and Results

We treated primary human hepatocytes, HepG2 cells, and Rhesus macaques with FXR agonist (CDCA, GW4064, and ivermectin). FNDC5 expression was highly induced by CDCA and GW4064 in hepatocytes, HepG2 cells, and the circulating level of Irisin increased in Rhesus macaques. Luciferase reporter and CHIP assays were used to determine whether FXR could regulate FNDC5 promoter activity. Irisin-ApoE-/- and ApoE-/- mice were used to study the metabolic function of Irisin in dyslipidemia and atherosclerosis. Irisin-ApoE-/- mice showed improved hyperlipidemia and alleviated atherosclerosis as compared with ApoE-/- mice. Irisin upregulated the expression of Abcg5/Abcg8 in liver and intestine, which increased the transport of biliary cholesterol and fecal cholesterol output.

Conclusion

Activation of FXR induces FNDC5 mRNA expression in human and increased the circulating level of Irisin in Rhesus macaques. FNDC5/Irisin is a direct transcriptional target of FXR. Irisin may be a novel therapeutic strategy for dyslipidemia and atherosclerosis.

Dietary cholesterol and apolipoprotein A-I are trafficked in endosomes and lysosomes in the live zebrafish intestine

Difficulty in imaging the vertebrate intestine in vivo has hindered our ability to model nutrient and protein trafficking from both the lumenal and basolateral aspects of enterocytes. Our goal was to use live confocal imaging to increase understanding of intestinal trafficking of dietary cholesterol and apolipoprotein A-I (APOA-I), the main structural component of high-density lipoproteins. We developed a novel assay to visualize live dietary cholesterol trafficking in the zebrafish intestine by feeding TopFluor-cholesterol (TF-cholesterol), a fluorescent cholesterol analog, in a lipid-rich, chicken egg yolk feed. Quantitative microscopy of transgenic zebrafish expressing fluorescently tagged protein markers of early, recycling, and late endosomes/lysosomes provided the first evidence, to our knowledge, of cholesterol transport in the intestinal endosomal-lysosomal trafficking system. To study APOA-I dynamics, transgenic zebrafish expressing an APOA-I fluorescent fusion protein (APOA-I-mCherry) from tissue-specific promoters were created. These zebrafish demonstrated that APOA-I-mCherry derived from the intestine accumulated in the liver and vice versa. Additionally, intracellular APOA-I-mCherry localized to endosomes and lysosomes in the intestine and liver. Moreover, live imaging demonstrated that APOA-I-mCherry colocalized with dietary TF-cholesterol in enterocytes, and this colocalization increased with feeding time. This study provides a new set of tools for the study of cellular lipid biology and elucidates a key role for endosomal-lysosomal trafficking of intestinal cholesterol and APOA-I. NEW & NOTEWORTHY A fluorescent cholesterol analog was fed to live, translucent larval zebrafish to visualize intracellular cholesterol and apolipoprotein A-I (APOA-I) trafficking. With this model intestinal endosomal-lysosomal cholesterol trafficking was observed for the first time. A new APOA-I fusion protein (APOA-I-mCherry) expressed from tissue-specific promoters was secreted into the circulation and revealed that liver-derived APOA-I-mCherry accumulates in the intestine and vice versa. Intestinal, intracellular APOA-I-mCherry was observed in endosomes and lysosomes and colocalized with dietary cholesterol.

β-Carotene in the human body: metabolic bioactivation pathways - from digestion to tissue distribution and excretion

β-Carotene intake and tissue/blood concentrations have been associated with reduced incidence of several chronic diseases. Further bioactive carotenoid-metabolites can modulate the expression of specific genes mainly via the nuclear hormone receptors: retinoic acid receptor- and retinoid X receptor-mediated signalling. To better understand the metabolic conversion of β-carotene, inter-individual differences regarding β-carotene bioavailability and bioactivity are key steps that determine its further metabolism and bioactivation and mediated signalling. Major carotenoid metabolites, the retinoids, can be stored as esters or further oxidised and excreted via phase 2 metabolism pathways. In this review, we aim to highlight the major critical control points that determine the fate of β-carotene in the human body, with a special emphasis on β-carotene oxygenase 1. The hypothesis that higher dietary β-carotene intake and serum level results in higher β-carotene-mediated signalling is partly questioned. Alternative autoregulatory mechanisms in β-carotene / retinoid-mediated signalling are highlighted to better predict and optimise nutritional strategies involving β-carotene-related health beneficial mediated effects.

[Metabolism and Function of High-Density Lipoproteins (HDL)]

Metabolism and Function of High-Density Lipoproteins (HDL) Abstract. HDL has long been considered as 'good cholesterol', beneficial to the whole body and in particular to cardio-vascular health. However, HDL is a complex particle that undergoes dynamic remodeling through interactions with various enzymes and tissue types throughout its life cycle. In this review, we explore the novel understanding of HDL as a multifaceted class of lipoprotein, with multiple subclasses of different size, molecular composition, receptor interactions, and functionality, in health and disease. Further, we report on emergent HDL based therapeutics tested in small and larger scale clinical trials and their mixed successes.

ATP-binding cassette transporter A1 (ABCA1) expression in adipose tissue and its modulation with insulin resistance in obesity

PURPOSE

Adipose tissue dysfunction is at the center of metabolic dysfunctions associated with obesity. Through studies in isolated adipocytes and mouse models, ATP-binding cassette transporter A1 (ABCA1) expression in the adipose tissue has been shown to regulate high-density lipoprotein (HDL) cholesterol levels in the circulation and insulin sensitivity at both adipose tissue and whole-body levels. We aimed to explore the possible link between ABCA1 expression in the adipose tissue and metabolic derangements associated with obesity in humans.

PATIENTS AND METHODS

This exploratory study among individuals who were lean (body mass index [BMI]: 22.3±0.34 kg/m², n=28) and obese (BMI: 44.48±5.3 kg/m², n=34) compared the expression of ABCA1, adiponectin and GLUT4 (SLC2A4) in visceral and subcutaneous adipose tissue using quantitative real-time PCR and immunohistochemistry. Homeostatic model assessment for insulin resistance (HOMA-IR) and adipose tissue insulin resistance (adipo-IR) were used as insulin resistance markers.

RESULTS

Visceral adipose tissue from individuals who were obese had significantly lower ABCA1 (P=0.04 for mRNA and protein) and adiponectin (P=0.001 for mRNA) expression compared to that from lean individuals. Subcutaneous adipose tissue did not show any significant difference in the expression. When individuals were divided into insulin-sensitive (IS) and insulin-resistant (IR) groups based on HOMA-IR, IR individuals had lower ABCA1 (P=0.0001 for mRNA and P=0.009 for protein) expression compared to IS individuals in visceral adipose tissue, but not in subcutaneous adipose tissue. The difference was significant after adjusting for age, gender and BMI. ABCA1 mRNA expression in visceral adipose tissue correlated negatively with both HOMA-IR (r=-0.44, P=0.0003) and adipo-IR (r=-0.35, P=0.005) after adjusting for age, gender and BMI. ABCA1 expression in either visceral or subcutaneous adipose tissue did not have any significant correlation with HDL cholesterol levels or mean adipocyte area.

CONCLUSION

Obesity and insulin resistance are associated with lower expression of ABCA1 in visceral adipose tissue in humans.

Phospholipid Remodeling in Physiology and Disease

Phospholipids are major constituents of biological membranes. The fatty acyl chain composition of phospholipids determines the biophysical properties of membranes and thereby affects their impact on biological processes. The composition of fatty acyl chains is also actively regulated through a deacylation and reacylation pathway called Lands' cycle. Recent studies of mouse genetic models have demonstrated that lysophosphatidylcholine acyltransferases (LPCATs), which catalyze the incorporation of fatty acyl chains into the sn-2 site of phosphatidylcholine, play important roles in pathophysiology. Two LPCAT family members, LPCAT1 and LPCAT3, have been particularly well studied. LPCAT1 is crucial for proper lung function due to its role in pulmonary surfactant biosynthesis. LPCAT3 maintains systemic lipid homeostasis by regulating lipid absorption in intestine, lipoprotein secretion, and de novo lipogenesis in liver. Mounting evidence also suggests that changes in LPCAT activity may be potentially involved in pathological conditions, including nonalcoholic fatty liver disease, atherosclerosis, viral infections, and cancer. Pharmacological manipulation of LPCAT activity and membrane phospholipid composition may provide new therapeutic options for these conditions.

ATP binding cassette family A protein 1 determines hexosylceramide and sphingomyelin levels in human and mouse plasma

Sphingolipids, including ceramide, SM, and hexosylceramide (HxCer), are carried in the plasma by lipoproteins. They are possible markers of metabolic diseases, but little is known about their control. We previously showed that microsomal triglyceride transfer protein (MTP) is critical to determine plasma ceramide and SM, but not HxCer, levels. In human plasma and mouse models, we examined possible HxCer-modulating pathways, including the role of ABCA1 in determining sphingolipid plasma concentrations. Compared with control samples, plasma from patients with Tangier disease (deficient in ABCA1) had significantly lower HxCer (-69%) and SM (-40%) levels. Similarly, mice deficient in hepatic and intestinal ABCA1 had significantly reduced HxCer (-79%) and SM (-85%) levels. Tissue-specific ablation studies revealed that hepatic ABCA1 determines plasma HxCer and SM levels; that ablation of MTP and ABCA1 in the liver and intestine reduces plasma HxCer, SM, and ceramide levels; and that hepatic and intestinal MTP contribute to plasma ceramide levels, whereas only hepatic MTP modulates plasma SM levels. These results identify the contribution of ABCA1 to plasma SM and HxCer levels and suggest that MTP and ABCA1 are critical determinants of plasma sphingolipid levels.

Liver X receptors in lipid signalling and membrane homeostasis

Liver X receptors α and β (LXRα and LXRβ) are nuclear receptors with pivotal roles in the transcriptional control of lipid metabolism. Transcriptional activity of LXRs is induced in response to elevated cellular levels of cholesterol. LXRs bind to and regulate the expression of genes that encode proteins involved in cholesterol absorption, transport, efflux, excretion and conversion to bile acids. The coordinated, tissue-specific actions of the LXR pathway maintain systemic cholesterol homeostasis and regulate immune and inflammatory responses. LXRs also regulate fatty acid metabolism by controlling the lipogenic transcription factor sterol regulatory element-binding protein 1c and regulate genes that encode proteins involved in fatty acid elongation and desaturation. LXRs exert important effects on the metabolism of phospholipids, which, along with cholesterol, are major constituents of cellular membranes. LXR activation preferentially drives the incorporation of polyunsaturated fatty acids into phospholipids by inducing transcription of the remodelling enzyme lysophosphatidylcholine acyltransferase 3. The ability of the LXR pathway to couple cellular sterol levels with the saturation of fatty acids in membrane phospholipids has implications for several physiological processes, including lipoprotein production, dietary lipid absorption and intestinal stem cell proliferation. Understanding how LXRs regulate membrane composition and function might provide new therapeutic insight into diseases associated with dysregulated lipid metabolism, including atherosclerosis, diabetes mellitus and cancer.

High Level of APOA1 in Blood and Maternal Fetal Interface Is Associated With Early Miscarriage

Early miscarriage (EM) is one of the most devastating obstetrical complications globally affecting the quality of women's life. In the present study, we aimed to identify proteins that correlate with and could act as biomarkers for EM. We performed 2-dimensional gel electrophoresis in chorionic villi samples followed by mass spectrometry for identification of differential protein expression with EM. Proteomic studies detected a total 124 protein spots, out of which 83 spots were differentially expressed between EM and controls in chorionic villi samples. Matrix assisted laser desorbtion/ionization-time of flight (MALDI-TOF) mass spectrometry analysis revealed Apolipoprotein A1 (APOA1) to be the most upregulated protein in the EM group that was validated by Western blotting and Enzyme-linked immunosorbent assay (ELISA) . We found low but not statistically significant level of APOA1 on 21st day of menstruation in comparison to the 7th day. APOA1 level was observed to be the lowest in the first trimester. Hence, this study suggests that low APOA1 expression is critical in establishing pregnancy and elevated APOA1 expression in chorionic villi correlates with EM. Similar observation in serum samples suggests its potential as a marker for the risk of EM.

Unravelling HDL-Looking beyond the Cholesterol Surface to the Quality Within

High-density lipoprotein (HDL) particles have experienced a turbulent decade of falling from grace with widespread demotion from the most-sought-after therapeutic target to reverse cardiovascular disease (CVD), to mere biomarker status. HDL is slowly emerging from these dark times due to the HDL flux hypothesis wherein measures of HDL cholesterol efflux capacity (CEC) are better predictors of reduced CVD risk than static HDL-cholesterol (HDL-C) levels. HDL particles are emulsions of metabolites, lipids, protein, and microRNA (miR) built on the backbone of Apolipoprotein A1 (ApoA1) that are growing in their complexity due to the higher sensitivity of the respective “omic” technologies. Our understanding of particle composition has increased dramatically within this era and has exposed how our understanding of these particles to date has been oversimplified. Elucidation of the HDL proteome coupled with the identification of specific miRs on HDL have highlighted the “hormonal” characteristics of HDL in that it carries and delivers messages systemically. HDL can dock to most peripheral cells via its receptors, including SR-B1, ABCA1, and ABCG1, which may be a critical step for facilitating HDL-to-cell communication. The composition of HDL particles is, in turn, altered in numerous disease states including diabetes, auto-immune disease, and CVD. The consequence of changes in composition, however, on subsequent biological activities of HDL is currently poorly understood and this is an important avenue for the field to explore in the future. Improving HDL particle quality as opposed to HDL quantity may, in turn, prove a more beneficial investment to reduce CVD risk.

The Effect of Proprotein Convertase Subtilisin/Kexin Type 9 Inhibition on Sterol Absorption Markers in a Cohort of Real-World Patients

Background:

Proprotein convertase subtilisin/kexin type 9 (PCSK9) is expressed in multiple tissues, including the small intestine. The effect of PCSK9 inhibition on cholesterol absorption is not known.

Objectives:

Measure serum cholesterol absorption markers before and after initiation of PCSK9 inhibitors.

Methods:

Single-center retrospective cohort of patients administered evolocumab and alirocumab between July 2015 and January 2017. Paired t tests were used to compare mean serum cholesterol marker concentrations, and ratios to total cholesterol, before and after PCSK9 inhibitor initiation. Analyses were repeated for those taking and not taking statins and taking or not taking ezetimibe at both initiation and follow-up, for each PCSK9 inhibitor, and based on follow-up time (<60, 60-120, and >120 days).

Results:

There were 62 possible participants, 34 were excluded for lack of data or unknown PCSK9 inhibitor initiation date. Average follow-up was 92.5 days. Mean campesterol (before 3.14 μg/mL, 95% CI: 2.79-4.38 μg/mL; after 2.09 μg/mL, 95% CI: 1.87-2.31 μg/mL; P < .0001), sitosterol (before 2.46 μg/mL, 95% CI: 2.23-2.70 μg/mL; after 1.62 μg/mL, 95% CI: 1.48-1.75 μg/mL; P < .0001), and cholestanol (before 3.25 μg/mL, 95% CI: 3.04-3.47 μg/mL; after 2.08 μg/mL, 95% CI: 1.96-2.21 μg/mL; P < .0001) all significantly decreased at follow-up. There was no significant change in absorption marker to total cholesterol ratios. Findings were not influenced by statin or ezetimibe use or nonuse, which PCSK9 inhibitor was prescribed, or time to follow-up.

Conclusion:

Proprotein convertase subtilisin/kexin type 9 inhibition was associated with decreased cholesterol absorption markers.

Endocytosis of lipoproteins

During their metabolism, all lipoproteins undergo endocytosis, either to be degraded intracellularly, for example in hepatocytes or macrophages, or to be re-secreted, for example in the course of transcytosis by endothelial cells. Moreover, there are several examples of internalized lipoproteins sequestered intracellularly, possibly to exert intracellular functions, for example the cytolysis of trypanosoma. Endocytosis and the subsequent intracellular itinerary of lipoproteins hence are key areas for understanding the regulation of plasma lipid levels as well as the biological functions of lipoproteins. Indeed, the identification of the low-density lipoprotein (LDL)-receptor and the unraveling of its transcriptional regulation led to the elucidation of familial hypercholesterolemia as well as to the development of statins, the most successful therapeutics for lowering of cholesterol levels and risk of atherosclerotic cardiovascular diseases. Novel limiting factors of intracellular trafficking of LDL and the LDL receptor continue to be discovered and to provide drug targets such as PCSK9. Surprisingly, the receptors mediating endocytosis of high-density lipoproteins or lipoprotein(a) are still a matter of controversy or even new discovery. Finally, the receptors and mechanisms, which mediate the uptake of lipoproteins into non-degrading intracellular itineraries for re-secretion (transcytosis, retroendocytosis), storage, or execution of intracellular functions, are largely unknown.

Investigation of the influence of high glucose on molecular and genetic responses: an in vitro study using a human intestine model

Background

Dietary glucose consumption has increased worldwide. Long-term high glucose intake contributes to the development of obesity and type 2 diabetes mellitus (T2DM). Obese people tend to eat glucose-containing foods, which can lead to an addiction to glucose, increased glucose levels in the blood and intestine lumen, and exposure of intestinal enterocytes to high dietary glucose. Recent studies have documented a role for enterocytes in glucose sensing. However, the molecular and genetic relationship between high glucose levels and intestinal enterocytes has not been determined. We aimed to identify relevant target genes and molecular pathways regulated by high glucose in a well-established in vitro epithelial cell culture model of the human intestinal system (Caco-2 cells).

Methods

Cells were grown in a medium containing 5.5 and 25 mM glucose in a bicameral culture system for 21 days to mimic the human intestine. Transepithelial electrical resistance was used to control monolayer formation and polarization of the cells. Total RNA was isolated, and genome-wide mRNA expression profiles were determined. Molecular pathways were analyzed using the DAVID bioinformatics program. Gene expression levels were confirmed by quantitative reverse transcription polymerase chain reaction (RT-qPCR).

Results

Microarray gene expression data demonstrated that 679 genes (297 upregulated, 382 downregulated) were affected by high glucose treatment. Bioinformatics analysis indicated that intracellular protein export (p = 0.0069) and ubiquitin-mediated proteolysis (p = 0.024) pathways were induced, whereas glycolysis/gluconeogenesis (p < 0.0001), pentose phosphate (p = 0.0043), and fructose-mannose metabolism (p = 0.013) pathways were downregulated, in response to high glucose. Microarray analysis of gene expression showed that high glucose significantly induced mRNA expression levels of thioredoxin-interacting protein (TXNIP, p = 0.0001) and lipocalin 15 (LCN15, p = 0.0016) and reduced those of ATP-binding cassette, sub-family A member 1 (ABCA1, p = 0.0004), and iroquois homeobox 3 (IRX3, p = 0.0001).

Conclusions

To our knowledge, this is the first investigation of high glucose-regulated molecular responses in an intestinal enterocyte model. Our findings identify new target genes that may be important in the intestinal glucose absorption and metabolism during high glucose consumption.