Discoidal HDL and apoA-I-derived peptides improve glucose uptake in skeletal muscle

Human HDL subclasses modulate energy metabolism in skeletal muscle cells

In addition to its antiatherogenic role, HDL reportedly modulates energy metabolism at the whole-body level. HDL functionality is associated with its structure and composition, and functional activities can differ between HDL subclasses. Therefore, we studied if HDL2 and HDL3, the two major HDL subclasses, are able to modulate energy metabolism of skeletal muscle cells. Differentiated mouse and primary human skeletal muscle myotubes were used to investigate the influences of human HDL2 and HDL3 on glucose and fatty uptake and oxidation. HDL-induced changes in lipid distribution and mRNA expression of genes related to energy substrate metabolism, mitochondrial function, and HDL receptors were studied with human myotubes. Additionally, we examined the effects of apoA-I and discoidal, reconstituted HDL particles on substrate metabolism. In mouse myotubes, HDL subclasses strongly enhanced glycolysis upon high and low glucose concentrations. HDL3 caused a minor increase in ATP-linked respiration upon glucose conditioning but HDL2 improved complex I-mediated mitochondrial respiration upon fatty acid treatment. In human myotubes, glucose metabolism was attenuated but fatty acid uptake and oxidation were markedly increased by both HDL subclasses, which also increased mRNA expression of genes related to fatty acid metabolism and HDL receptors. Finally, both HDL subclasses induced incorporation of oleic acid into different lipid classes. These results, demonstrating that HDL subclasses enhance fatty acid oxidation in human myotubes but improve anaerobic metabolism in mouse myotubes, support the role of HDL as a circulating modulator of energy metabolism. Exact mechanisms and components of HDL causing the change, require further investigation.

HDL Function and Atherosclerosis: Reactive Dicarbonyls as Promising Targets of Therapy

Epidemiologic studies detected an inverse relationship between HDL (high-density lipoprotein) cholesterol (HDL-C) levels and atherosclerotic cardiovascular disease (ASCVD), identifying HDL-C as a major risk factor for ASCVD and suggesting atheroprotective functions of HDL. However, the role of HDL-C as a mediator of risk for ASCVD has been called into question by the failure of HDL-C-raising drugs to reduce cardiovascular events in clinical trials. Progress in understanding the heterogeneous nature of HDL particles in terms of their protein, lipid, and small RNA composition has contributed to the realization that HDL-C levels do not necessarily reflect HDL function. The most examined atheroprotective function of HDL is reverse cholesterol transport, whereby HDL removes cholesterol from plaque macrophage foam cells and delivers it to the liver for processing and excretion into bile. Indeed, in several studies, HDL has shown inverse associations between HDL cholesterol efflux capacity and ASCVD in humans. Inflammation plays a key role in the pathogenesis of atherosclerosis and vulnerable plaque formation, and a fundamental function of HDL is suppression of inflammatory signaling in macrophages and other cells. Oxidation is also a critical process to ASCVD in promoting atherogenic oxidative modifications of LDL (low-density lipoprotein) and cellular inflammation. HDL and its proteins including apoAI (apolipoprotein AI) and PON1 (paraoxonase 1) prevent cellular oxidative stress and LDL modifications. Importantly, HDL in humans with ASCVD is oxidatively modified rendering HDL dysfunctional and proinflammatory. Modification of HDL with reactive carbonyl species, such as malondialdehyde and isolevuglandins, dramatically impairs the antiatherogenic functions of HDL. Importantly, treatment of murine models of atherosclerosis with scavengers of reactive dicarbonyls improves HDL function and reduces systemic inflammation, atherosclerosis development, and features of plaque instability. Here, we discuss the HDL antiatherogenic functions in relation to oxidative modifications and the potential of reactive dicarbonyl scavengers as a therapeutic approach for ASCVD.

Scavenging dicarbonyls with 5'-O-pentyl-pyridoxamine increases HDL net cholesterol efflux capacity and attenuates atherosclerosis and insulin resistance

OBJECTIVE

Oxidative stress contributes to the development of insulin resistance (IR) and atherosclerosis. Peroxidation of lipids produces reactive dicarbonyls such as Isolevuglandins (IsoLG) and malondialdehyde (MDA) that covalently bind plasma/cellular proteins, phospholipids, and DNA leading to altered function and toxicity. We examined whether scavenging reactive dicarbonyls with 5'-O-pentyl-pyridoxamine (PPM) protects against the development of IR and atherosclerosis in Ldlr^-/- mice.

METHODS

Male or female Ldlr^-/- mice were fed a western diet (WD) for 16 weeks and treated with PPM versus vehicle alone. Plaque extent, dicarbonyl-lysyl adducts, efferocytosis, apoptosis, macrophage inflammation, and necrotic area were measured. Plasma MDA-LDL adducts and the in vivo and in vitro effects of PPM on the ability of HDL to reduce macrophage cholesterol were measured. Blood Ly6C^hi monocytes and ex vivo 5-ethynyl-2'-deoxyuridine (EdU) incorporation into bone marrow CD11b+ monocytes and CD34+ hematopoietic stem and progenitor cells (HSPC) were also examined. IR was examined by measuring fasting glucose/insulin levels and tolerance to insulin/glucose challenge.

RESULTS

PPM reduced the proximal aortic atherosclerosis by 48% and by 46% in female and male Ldlr^-/- mice, respectively. PPM also decreased IR and hepatic fat and inflammation in male Ldlr^-/- mice. Importantly, PPM decreased plasma MDA-LDL adducts and prevented the accumulation of plaque MDA- and IsoLG-lysyl adducts in Ldlr^-/- mice. In addition, PPM increased the net cholesterol efflux capacity of HDL from Ldlr^-/- mice and prevented both the in vitro impairment of HDL net cholesterol efflux capacity and apoAI crosslinking by MPO generated hypochlorous acid. Moreover, PPM decreased features of plaque instability including decreased proinflammatory M1-like macrophages, IL-1β expression, myeloperoxidase, apoptosis, and necrotic core. In contrast, PPM increased M2-like macrophages, Tregs, fibrous cap thickness, and efferocytosis. Furthermore, PPM reduced inflammatory monocytosis as evidenced by decreased blood Ly6C^hi monocytes and proliferation of bone marrow monocytes and HSPC from Ldlr^-/- mice.

CONCLUSIONS

PPM has pleotropic atheroprotective effects in a murine model of familial hypercholesterolemia, supporting the therapeutic potential of reactive dicarbonyl scavenging in the treatment of IR and atherosclerotic cardiovascular disease.

Progressive, Qualitative, and Quantitative Alterations in HDL Lipidome from Healthy Subjects to Patients with Prediabetes and Type 2 Diabetes

Prediabetes is a clinically silent, insulin-resistant state with increased risk for the development of type 2 diabetes (T2D) and cardiovascular disease (CVD). Since glucose homeostasis and lipid metabolism are highly intersected and interrelated, an in-depth characterization of qualitative and quantitative abnormalities in lipoproteins could unravel the metabolic pathways underlying the progression of prediabetes to T2D and also the proneness of these patients to developing premature atherosclerosis. We investigated the HDL lipidome in 40 patients with prediabetes and compared it to that of 40 normoglycemic individuals and 40 patients with established T2D using Nuclear Magnetic Resonance (NMR) spectroscopy. Patients with prediabetes presented significant qualitative and quantitative alterations, potentially atherogenic, in HDL lipidome compared to normoglycemic characterized by higher percentages of free cholesterol and triglycerides, whereas phospholipids were lower. Glycerophospholipids and ether glycerolipids were significantly lower in prediabetic compared to normoglycemic individuals, whereas sphingolipids were significantly higher. In prediabetes, lipids were esterified with saturated rather than unsaturated fatty acids. These changes are qualitatively similar, but quantitatively milder, than those found in patients with T2D. We conclude that the detailed characterization of the HDL lipid profile bears a potential to identify patients with subtle (but still proatherogenic) abnormalities who are at high risk for development of T2D and CVD.

Muscular carnosine is a marker for cardiorespiratory fitness and cardiometabolic risk factors in men with type 1 diabetes

PURPOSE

Muscle is an essential organ for glucose metabolism and can be influenced by metabolic disorders and physical activity. Elevated muscle carnosine levels have been associated with insulin resistance and cardiometabolic risk factors. Little is known about muscle carnosine in type 1 diabetes (T1D) and how it is influenced by physical activity. The aim of this study was to characterize muscle carnosine in vivo by proton magnetic resonance spectroscopy (¹H MRS) and evaluate the relationship with physical activity, clinical characteristics and lipoprotein subfractions.

METHODS

16 men with T1D (10 athletes/6 sedentary) and 14 controls without diabetes (9/5) were included. Body composition by DXA, cardiorespiratory capacity (VO₂peak) and serum lipoprotein profile by proton nuclear magnetic resonance (¹H NMR) were obtained. Muscle carnosine scaled to water (carnosine_W) and to creatine (carnosine_CR), creatine and intramyocellular lipids (IMCL) were quantified in vivo using ¹H MRS in a 3T MR scanner in soleus muscle.

RESULTS

Subjects with T1D presented higher carnosine CR levels compared to controls. T1D patients with a lower VO₂peak presented higher carnosine_CR levels compared to sedentary controls, but both T1D and control groups presented similar levels of carnosine_CR at high VO₂peak levels. Carnosine_W followed the same trend. Integrated correlation networks in T1D demonstrated that carnosine_W and carnosine_CR were associated with cardiometabolic risk factors including total and abdominal fat, pro-atherogenic lipoproteins (very low-density lipoprotein subfractions), low VO₂peak, and IMCL.

CONCLUSIONS

Elevated muscle carnosine levels in persons with T1D and their effect on atherogenic lipoproteins can be modulated by physical activity.

HDL and type 2 diabetes: the chicken or the egg?

HDL is a complex macromolecular cluster of various components, such as apolipoproteins, enzymes and lipids. Quality evidence from clinical and epidemiological studies led to the principle that HDL-cholesterol (HDL-C) levels are inversely correlated with the risk of CHD. Nevertheless, the failure of many cholesteryl ester transfer protein inhibitors to protect against CVD casts doubts on this principle and highlights the fact that HDL functionality, as dictated by its proteome and lipidome, also plays an important role in protecting against metabolic disorders. Recent data indicate that HDL-C levels and HDL particle functionality are correlated with the pathogenesis and prognosis of type 2 diabetes mellitus, a major risk factor for CVD. Hyperglycaemia leads to reduced HDL-C levels and deteriorated HDL functionality, via various alterations in HDL particles' proteome and lipidome. In turn, reduced HDL-C levels and impaired HDL functionality impact the performance of key organs related to glucose homeostasis, such as pancreas and skeletal muscles. Interestingly, different structural alterations in HDL correlate with distinct metabolic abnormalities, as indicated by recent data evaluating the role of apolipoprotein A1 and lecithin-cholesterol acyltransferase deficiency in glucose homeostasis. While it is becoming evident that not all HDL disturbances are causatively associated with the development and progression of type 2 diabetes, a bidirectional correlation between these two conditions exists, leading to a perpetual self-feeding cycle.

Uncovering the role of apolipoprotein C-III in insulin resistance

Apolipoprotein C-III (apoC-III) is a small protein that is predominantly synthesized in the liver and mainly resides at the surface of triglyceride-rich lipoproteins. Its expression is upregulated by glucose and reduced by insulin, with enhanced apoC-III promoting hypertriglyceridemia and inflammation in vascular cells. The protein is also elevated in patients with diabetes, suggesting that enhanced apoC-III levels might contribute to the development of type 2 diabetes mellitus. The present review focuses on the key mechanisms by which apoC-III could promote type 2 diabetes mellitus, including exacerbation of insulin resistance in skeletal muscle, activation of β-cell apoptosis, promotion of weight gain through its effects on white adipose tissue and hypothalamus, and attenuation of the beneficial effects of high-density lipoproteins on glucose metabolism. Therapeutic strategies aimed at reducing apoC-III levels may not only reduce hypertriglyceridemia but also might improve insulin resistance, thus delaying the development of type 2 diabetes mellitus.

A short peptide of the C-terminal class Y helices of apolipoprotein A-I has preserved functions in cholesterol efflux and in vivo metabolic control

Apolipoprotein A-I (ApoA-I) of high-density lipoprotein (HDL) induces glucose uptake by muscle tissues and stimulates pancreatic insulin secretion, and also facilitates cholesterol transport in circulation, and is explored for anti-diabetic and anti-atherosclerotic treatments. As the better alternative to complex protein-lipid formulations it was recently established that the C-terminal region of the ApoA-I protein singly improves the metabolic control and prevents formation of atherosclerotic plaques. Additional investigations of peptides based on the ApoA-I structure may lead to novel anti-diabetic drugs. We here investigate a short peptide (33mer, RG33) that corresponds to the two last helical segments (aa 209-241) of the ApoA-I structure (so-called class Y-helices which forms amphipathic helices) for stability and solubility in serum, for in vitro cholesterol efflux capability, and for providing in vivo glucose control in an insulin resistant mouse model. The RG33 peptide efficiently solubilizes lipid-vesicles, and promotes the efflux of cholesterol from cultured macrophages. The efflux capacity is significantly increased in the presence of lipids compared to non-lipidated RG33. Finally, acute treatment with the RG33 peptide significantly improves the glucose clearance capacity of insulin resistant mice. The impact of the RG33 peptide on glucose control and cholesterol transport, as well as the physicochemical properties, makes it a good candidate for translational exploration of its therapeutic potential in diabetes treatment.

Probing the Structure-Function relationship and amyloidogenic propensities in natural variants of apolipoprotein A-I

Background

Apolipoprotein A-I (apoA-I) protects against atherosclerosis and participates in the removal of excess cellular cholesterol from peripheral organs. Several naturally occurring apoA-I mutations are associated with familial systemic amyloidosis, with deposition of amyloid aggregates in peripheral organs, resulting in multiple organ failure. Systematic studies on naturally occurring variants are needed to delineate their roles and involvement in pathogenesis.

Methods

We performed a comparative structure–function analysis of five naturally occurring apoA-I variants and the wild-type protein. Circular dichroism, Fourier-transform infrared spectroscopy, thioflavin T and congo red fluorescence assays, thermal, chemical, and proteolytic stability assays, and 1,2-Dimyristoyl-sn-glycero-3-phosphocholine clearance analyses were used to assess the effects of mutations on the structure, function, stability, aggregation, and proteolytic susceptibility of the proteins to explore the mechanisms underlying amyloidosis and hypercholesterolemia.

Results

We observed structural changes in the mutants independent of fibril formation, suggesting the influence of the surrounding environment. The mutants were involved in aggregate formation to varying degree; L170P, R173P, and V156E showed an increased propensity to aggregate under different physiological conditions. β sheet formation indicates that L170P and R173P participate in amyloid formation. Compared to WT, V156E and L170P exhibited higher capacity for lipid clearance.

Conclusions

The selected point mutations, including those outside the hot spot regions of apoA-I structure, perturb the physiochemical and conformational behavior of the protein, influencing its function.

General significance

The study provides insights into the structure–function relationships of naturally occurring apoA-I variants outside the hot spot mutation sites.

•

Several apoA-I mutants are associated with systemic amyloidosis.

•

Structure–function analysis of five apoA-I variants and wild-type protein was done.

•

Point mutations alter the physicochemical behavior and conformation of the variants.

HDL cholesterol is an independent predictor of β-cell function decline and incident type 2 diabetes: A longitudinal study

BACKGROUND

Experimental evidence indicates that high-density lipoprotein (HDL) may stimulate glucose uptake and improve β-cell function. The aim of this study was to evaluate whether lower levels of HDL may affect the risk to develop type 2 diabetes.

METHODS

Incident rate of type 2 diabetes and changes in insulin sensitivity and β-cell function over 5.5-year follow-up were examined in 670 non-diabetic subjects stratified in tertiles according to basal HDL levels.

RESULTS

As compared to the highest tertile of HDL, individuals with lower levels of HDL have an increased risk to develop type 2 diabetes independently from several cardiometabolic risk factors (odds ratio: 2.88, 95% confidence interval: 1.05-7.91), and exhibited a greater deterioration of β-cell function, estimated by the disposition index, over 5.5-year follow-up. Conversely, changes in Matsuda index of insulin sensitivity over the follow-up were not significantly different amongst the three HDL groups. In a multivariable regression analysis model including age, sex, waist circumference, triglycerides, total cholesterol, C-reactive protein, fasting and 2-hour post-load glucose, family history of type 2 diabetes and smoking habit, HDL concentration at baseline was an independent predictor of β-cell function decline over the follow-up (β = .30, P = .0001). Mediation analysis showed that the association between lower HDL levels at baseline and increased risk of incident diabetes was mediated by β-cell function deterioration during the follow-up (t = -3.32, P = .001).

CONCLUSIONS

Subjects with lower levels of HDL have an increased risk to develop type 2 diabetes likely due to a greater β-cell function decline over time.

ApoA-1 improves glucose tolerance by increasing glucose uptake into heart and skeletal muscle independently of AMPKα2

OBJECTIVE

Acute administration of the main protein component of high-density lipoprotein, apolipoprotein A-I (ApoA-1), improves glucose uptake in skeletal muscle. The molecular mechanisms mediating this are not known, but in muscle cell cultures, ApoA-1 failed to increase glucose uptake when infected with a dominant-negative AMP-activated protein kinase (AMPK) virus. We therefore investigated whether AMPK is necessary for ApoA-1-stimulated glucose uptake in intact heart and skeletal muscle in vivo.

METHODS

The effect of injection with recombinant human ApoA-1 (rApoA-1) on glucose tolerance, glucose-stimulated insulin secretion, and glucose uptake into skeletal and heart muscle with and without block of insulin secretion by injection of epinephrine (0.1 mg/kg) and propranolol (5 mg/kg), were investigated in 8 weeks high-fat diet-fed (60E%) wild-type and AMPKα2 kinase-dead mice in the overnight-fasted state. In addition, the effect of rApoA-1 on glucose uptake in isolated skeletal muscle ex vivo was studied.

RESULTS

rApoA-1 lowered plasma glucose concentration by 1.7 mmol/l within 3 h (6.1 vs 4.4 mmol/l; p < 0.001). Three hours after rApoA-1 injection, glucose tolerance during a 40-min glucose tolerance test (GTT) was improved compared to control (area under the curve (AUC) reduced by 45%, p < 0.001). This was accompanied by an increased glucose clearance into skeletal (+110%; p < 0.001) and heart muscle (+100%; p < 0.001) and an increase in glucose-stimulated insulin secretion 20 min after glucose injection (+180%; p < 0.001). When insulin secretion was blocked during a GTT, rApoA-1 still enhanced glucose tolerance (AUC lowered by 20% compared to control; p < 0.001) and increased glucose clearance into skeletal (+50%; p < 0.05) and heart muscle (+270%; p < 0.001). These improvements occurred to a similar extent in both wild-type and AMPKα2 kinase-dead mice and thus independently of AMPKα2 activity in skeletal- and heart muscle. Interestingly, rApoA-1 failed to increase glucose uptake in isolated skeletal muscles ex vivo.

CONCLUSIONS

In conclusion, ApoA-1 stimulates in vivo glucose disposal into skeletal and heart muscle independently of AMPKα2. The observation that ApoA-1 fails to increase glucose uptake in isolated muscle ex vivo suggests that additional systemic effects are required.

Metabolomics Signatures in Type 2 Diabetes: A Systematic Review and Integrative Analysis

OBJECTIVE

Metabolic signatures have emerged as valuable signaling molecules in the biochemical process of type 2 diabetes (T2D). To summarize and identify metabolic biomarkers in T2D, we performed a systematic review and meta-analysis of the associations between metabolites and T2D using high-throughput metabolomics techniques.

METHODS

We searched relevant studies from MEDLINE (PubMed), Embase, Web of Science, and Cochrane Library as well as Chinese databases (Wanfang, Vip, and CNKI) inception through 31 December 2018. Meta-analysis was conducted using STATA 14.0 under random effect. Besides, bioinformatic analysis was performed to explore molecule mechanism by MetaboAnalyst and R 3.5.2.

RESULTS

Finally, 46 articles were included in this review on metabolites involved amino acids, acylcarnitines, lipids, carbohydrates, organic acids, and others. Results of meta-analysis in prospective studies indicated that isoleucine, leucine, valine, tyrosine, phenylalanine, glutamate, alanine, valerylcarnitine (C5), palmitoylcarnitine (C16), palmitic acid, and linoleic acid were associated with higher T2D risk. Conversely, serine, glutamine, and lysophosphatidylcholine C18:2 decreased risk of T2D. Arginine and glycine increased risk of T2D in the Western countries subgroup, and betaine was negatively correlated with T2D in nested case-control subgroup. In addition, slight improvements in T2D prediction beyond traditional risk factors were observed when adding these metabolites in predictive analysis. Pathway analysis identified 17 metabolic pathways may alter in the process of T2D and metabolite-related genes were also enriched in functions and pathways associated with T2D.

CONCLUSIONS

Several metabolites and metabolic pathways associated with T2D have been identified, which provide valuable biomarkers and novel targets for prevention and drug therapy.

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.

An unsupervised learning approach to identify novel signatures of health and disease from multimodal data

BACKGROUND

Modern medicine is rapidly moving towards a data-driven paradigm based on comprehensive multimodal health assessments. Integrated analysis of data from different modalities has the potential of uncovering novel biomarkers and disease signatures.

METHODS

We collected 1385 data features from diverse modalities, including metabolome, microbiome, genetics, and advanced imaging, from 1253 individuals and from a longitudinal validation cohort of 1083 individuals. We utilized a combination of unsupervised machine learning methods to identify multimodal biomarker signatures of health and disease risk.

RESULTS

Our method identified a set of cardiometabolic biomarkers that goes beyond standard clinical biomarkers. Stratification of individuals based on the signatures of these biomarkers identified distinct subsets of individuals with similar health statuses. Subset membership was a better predictor for diabetes than established clinical biomarkers such as glucose, insulin resistance, and body mass index. The novel biomarkers in the diabetes signature included 1-stearoyl-2-dihomo-linolenoyl-GPC and 1-(1-enyl-palmitoyl)-2-oleoyl-GPC. Another metabolite, cinnamoylglycine, was identified as a potential biomarker for both gut microbiome health and lean mass percentage. We identified potential early signatures for hypertension and a poor metabolic health outcome. Additionally, we found novel associations between a uremic toxin, p-cresol sulfate, and the abundance of the microbiome genera Intestinimonas and an unclassified genus in the Erysipelotrichaceae family.

CONCLUSIONS

Our methodology and results demonstrate the potential of multimodal data integration, from the identification of novel biomarker signatures to a data-driven stratification of individuals into disease subtypes and stages-an essential step towards personalized, preventative health risk assessment.

Apolipoprotein-AI mimetic peptides D-4F and L-5F decrease hepatic inflammation and increase insulin sensitivity in C57BL/6 mice

BACKGROUND

Apolipoprotein-AI (apo-AI) is the major apolipoprotein found in high density lipoprotein particles (HDLs). We previously demonstrated that apo-AI injected directly into high-fat diet fed mice improved insulin sensitivity associated with decreased hepatic inflammation. While our data provides compelling proof of concept, apoA-I mimetic peptides are more clinically feasible. The aim of this study was to test whether apo-AI mimetic peptide (D-4F and L-5F) treatment will emulate the effects of full-length apo-AI to improve insulin sensitivity.

METHODS

Male C57BL/6 mice were fed a high-fat diet for 16 weeks before receiving D4F mimetic peptide administered via drinking water or L5F mimetic peptide administered by intraperitoneal injection bi-weekly for a total of five weeks. Glucose tolerance and insulin tolerance tests were conducted to assess the effects of the peptides on insulin resistance. Effects of the peptides on inflammation, gluconeogenic enzymes and lipid synthesis were assessed by real-time PCR of key markers involved in the respective pathways.

RESULTS

Treatment with apo-AI mimetic peptides D-4F and L-5F showed: (i) improved blood glucose clearance (D-4F 1.40-fold AUC decrease compared to HFD, P<0.05; L-4F 1.17-fold AUC decrease compared to HFD, ns) in the glucose tolerance test; (ii) improved insulin tolerance (D-4F 1.63-fold AUC decrease compared to HFD, P<0.05; L-5F 1.39-fold AUC compared to HFD, P<0.05) in the insulin tolerance test. The metabolic test results were associated with (i) decreased hepatic inflammation of SAA1, IL-1β IFN-γ and TNFα (2.61-5.97-fold decrease compared to HFD, P<0.05) for both mimetics; (ii) suppression of hepatic mRNA expression of gluconeogenesis-associated genes (PEPCK and G6Pase; 1.66-3.01-fold decrease compared to HFD, P<0.001) for both mimetics; (iii) lipogenic-associated genes, (SREBP1c and ChREBP; 2.15-3.31-fold decrease compared to HFD, P<0.001) for both mimetics and; (iv) reduced hepatic macrophage infiltration (F4/80 and CD68; 1.77-2.15-fold compared to HFD, P<0.001) for both mimetics.

CONCLUSION

Apo-AI mimetic peptides treatment led to improved glucose homeostasis. This effect is associated with reduced expression of inflammatory markers in the liver and reduced infiltration of macrophages, suggesting an overall suppression of hepatic inflammation. We also showed altered expression of genes associated with gluconeogenesis and lipid synthesis, suggesting that glucose and lipid synthesis is suppressed. These findings suggest that apoA-I mimetic peptides could be a new therapeutic option to reduce hepatic inflammation that contributes to the development of overnutrition-induced insulin resistance.

Search for Natural Compounds That Increase Apolipoprotein A-I Transcription in HepG2 Cells: Specific Attention for BRD4 Inhibitors

Although increasing apolipoprotein A‐I (apoA‐I) might lower the cardiovascular disease risk, knowledge on natural compounds that elevate apoA‐I transcription is limited. Therefore, the aim of this study was to discover natural compounds that increase apoA‐I transcription in HepG2 cells. Since BRD4 inhibition is known to elevate apoA‐I transcription, we focused on natural BRD4 inhibitors. For this, the literature was screened for compounds that might increase apoA‐I and or inhibit BRD4. This resulted in list A, (apoA‐I increasers with unknown BRD4 inhibitor capacity), list B (known BRD4 inhibitors that increase apoA‐I), and list C (BRD4 inhibitors with unknown effect on apoA‐I). These compounds were compared with the compounds in two natural compound databases. This resulted in (1) a common substructure (ethyl‐benzene) in 60% of selected BRD4‐inhibitors, and (2) four compounds that increased ApoA‐I: hesperetin, equilenin, 9(S)‐HOTrE, and cymarin. Whether these increases are regulated via BRD4 inhibition and the ethyl‐benzene structure inhibits BRD4 requires further study.

ApoAI-derived peptide increases glucose tolerance and prevents formation of atherosclerosis in mice

AIMS/HYPOTHESIS

Finding new treatment alternatives for individuals with diabetes with severe insulin resistance is highly desired. To identify novel mechanisms that improve glucose uptake in skeletal muscle, independently from insulin levels and signalling, we have explored the therapeutic potential of a short peptide sequence, RG54, derived from apolipoprotein A-I (ApoA-I).

METHODS

INS-1E rat clonal beta cells, C2C12 rat muscle myotubes and J774 mouse macrophages were used to study the impact of RG54 peptide on glucose-stimulated insulin secretion, glucose uptake and cholesterol efflux, respectively. GTTs were carried out on diet-induced insulin-resistant and Lepr^db diabetic mouse models treated with RG54 peptide, and the impact of RG54 peptide on atherosclerosis was evaluated in Apoe^-/- mice. Control mice received ApoA-I protein, liraglutide or NaCl.

RESULTS

The synthetic RG54 peptide induced glucose uptake in cultured muscle myotubes by a similar amount as insulin, and also primed pancreatic beta cells for improved glucose-stimulated insulin secretion. The findings were verified in diet-induced insulin-resistant and Lepr^db diabetic mice, jointly confirming the physiological effect. The RG54 peptide also efficiently catalysed cholesterol efflux from macrophages and prevented the formation of atherosclerotic plaques in Apoe^-/- mice.

CONCLUSIONS/INTERPRETATION

The RG54 peptide exhibits good prospects for providing glucose control and reducing the risk of cardiovascular disease in individuals with severe insulin resistance.

Apolipoprotein A-I Protects Against Pregnancy-Induced Insulin Resistance in Rats

Objective- Insulin resistance and inflammation in pregnancy are risk factors for gestational diabetes mellitus. Increased plasma HDL (high-density lipoprotein) and apo (apolipoprotein) A-I levels have been reported to improve glucose metabolism and inhibit inflammation in animals and humans. This study asks whether increasing plasma apoA-I levels improves insulin sensitivity and reduces inflammation in insulin-resistant pregnant rats. Approach and Results- Insulin-resistant pregnant rats received intravenous infusions of lipid-free apoA-I (8 mg/kg) or saline on days 6, 9, 12, 15, and 18 of pregnancy. The rats were then subjected to a euglycemic-hyperinsulinemic clamp. Glucose uptake was increased in white and brown adipose tissue by 57±13% and 32±10%, respectively ( P<0.05 for both), and in quadriceps and gastrocnemius muscle by 35±9.7% and 47±14%, respectively ( P<0.05 for both), in the apoA-I-treated pregnant rats relative to saline-infused pregnant rats. The pregnant rats that were treated with apoA-I also had reduced plasma TNF-α (tumor necrosis factor-α) levels by 57±8.4%, plasma IL (interleukin)-6 levels by 67±9.5%, and adipose tissue macrophage content by 54±8.2% ( P<0.05 for all) relative to the saline-treated pregnant rats. Conclusions- These studies establish that apoA-I protects against pregnancy-induced insulin resistance in rats by increasing insulin sensitivity in adipose tissue and skeletal muscle and inhibiting inflammation. This identifies apoA-I as a potential target for preventing pregnancy-induced insulin resistance and reducing the incidence of gestational diabetes mellitus.

Apolipoprotein CIII and diabetes. Is there a link?

Apolipoprotein CIII (ApoCIII), a small protein that resides on the surface of lipoprotein particles, is a key regulator of triglyceride metabolism. The inhibition of lipoprotein lipase (LPL), the increased assembly and secretion of very low-density lipoproteins (VLDL) and the decreased reuptake of triglyceride-rich lipoproteins (TRLs) by the liver are mechanisms associating elevated serum ApoCIII levels and hypertriglyceridemia. ApoCIII concentration is high in individuals with diabetes mellitus, indicating a possible positive correlation with impairment of glucose metabolism. The aim of this review (based on a Pubmed search until August 2018) is to present the possible mechanisms linking ApoCIII and deterioration of carbohydrate homeostasis. ApoCIII enhances pancreatic β-cells apoptosis via an increase of the cytoplasmic Ca²⁺ levels in the insulin-producing cells. In addition, overexpression of ApoCIII enhances non-alcoholic fatty liver disease and exacerbates inflammatory pathways in skeletal muscles, affecting insulin signalling and thereby inducing insulin resistance. Moreover, recent studies reveal a possible mechanism of body weight increase and glucose production through a potential ApoCIII-induced LPL inhibition in the hypothalamus. Also, the presence of ApoCIII on the surface of high-density lipoprotein particles is associated with impairment of their antiglycemic and atheroprotective properties. Modulating ApoCIII may be a potent therapeutic approach to manage hypertriglyceridemia and improve carbohydrate metabolism.

Novel ABCA1 peptide agonists with antidiabetic action

Previously, apoE-derived ABCA1 agonist peptides have been shown to possess anti-atherosclerotic and possibly antidiabetic properties. Here we assessed the in vitro and in vivo actions of a second generation of ABCA1 peptide agonists, CS6253 and T6991-2, on glucose homeostasis. The results show that these two peptides improve glucose tolerance in a prediabetic diet-induced obesity mouse model by enhancing insulin secretion. It was further demonstrated that T6991-2 also improved glucose tolerance in leptin-deficient (ob/ob) mice. CS6253 increased insulin secretion both under basal conditions and in response to high glucose stimulation in pancreatic INS-1 β-cells rendered leptin receptor deficient with specific siRNA. Additional in vitro cell studies suggest that the CS6253 agonist attenuates hepatic gluconeogenesis and glucose transport. It also potentiates insulin-stimulated glucose uptake and utilization. These observed anti-diabetic actions suggest additional benefits of the CS6253 and T6991-2 ABCA1 peptide agonists for cardiovascular disease beyond their direct anti-atherosclerosis properties previously described.

CETP inhibition, statins and diabetes

Type 2 diabetes is a causal risk factor for the development of atherosclerotic cardiovascular disease (ASCVD). While treatment with a statin reduces the risk of having an ASCVD event in all people, including those with type-2 diabetes, statin treatment also increases the likelihood of new onset diabetes when given to those with risk factors for developing diabetes. Treatment with the cholesteryl ester transfer protein (CETP) inhibitor, anacetrapib, reduces the risk of having a coronary event over and above that achieved with a statin. However, unlike statins, anacetrapib decreases the risk of developing diabetes. If the reduced risk of new-onset diabetes is confirmed in another CETP inhibitor outcome trial, there will be a case for considering the use of the combination of a statin plus a CETP inhibitor in high ASCVD-risk people who are also at increased risk of developing diabetes.

Site-specific glycations of apolipoprotein A-I lead to differentiated functional effects on lipid-binding and on glucose metabolism

Prolonged hyperglycemia in poorly controlled diabetes leads to an increase in reactive glucose metabolites that covalently modify proteins by non-enzymatic glycation reactions. Apolipoprotein A-I (apoA-I) of high-density lipoprotein (HDL) is one of the proteins that becomes glycated in hyperglycemia. The impact of glycation on apoA-I protein structure and function in lipid and glucose metabolism were investigated. ApoA-I was chemically glycated by two different glucose metabolites (methylglyoxal and glycolaldehyde). Synchrotron radiation and conventional circular dichroism spectroscopy were used to study apoA-I structure and stability. The ability to bind lipids was measured by lipid-clearance assay and native gel analysis, and cholesterol efflux was measured by using lipid-laden J774 macrophages. Diet induced obese mice with established insulin resistance, L6 rat and C2C12 mouse myocytes, as well as INS-1E rat insulinoma cells, were used to determine in vivo and in vitro glucose uptake and insulin secretion. Site-specific, covalent modifications of apoA-I (lysines or arginines) led to altered protein structure, reduced lipid binding capability and a reduced ability to catalyze cholesterol efflux from macrophages, partly in a modification-specific manner. The stimulatory effects of apoA-I on the in vivo glucose clearance were negatively affected when apoA-I was modified with methylglyoxal, but not with glycolaldehyde. The in vitro data showed that both glucose uptake in muscle cells and insulin secretion from beta cells were affected. Taken together, glycation modifications impair the apoA-I protein functionality in lipid and glucose metabolism, which is expected to have implications for diabetes patients with poorly controlled blood glucose.

[Apolipoprotein A-I stimulates secretion of insulin and matrix metalloproteinases by islets of Langerhans]

The development of type 2 diabetes mellitus (DM2) is accompanied by disturbances in lipid metabolism. These include the increase in serum levels of atherogenic fractions of very low-density (VLDL) and low-density lipoproteins (LDL), total cholesterol, triglycerides and apo B. In contrast, the level of antiatherogenic high density lipoproteins (HDL) and the content of apolipoprotein A-I (apoA-I) decreased. To study the effect of the observed metabolic changes on insulin secretion in vitro, we used the islets of Langerhans isolated from the rat pancreas. It has been found that incubation of the islets in the presence of serum of the obese patients and patients with decompensated DM2 leads to a decrease in insulin secretion by 2.4 and 5.0 times, respectively. On the contrary, the addition of HDL to the incubation medium increased the insulin secretion by 3.4 times. A similar effect was observed in the presence of apoA-I, the main protein component of HDL. In the presence of apoA-I, the extracellular activity of matrix metalloproteinases (MMPs) demonstrated a 10-fold increase. The addition of LDL and VLDL to the islets did not change the secretion of insulin and activity of MMP. Our results testify to the important role of HDL and apoA-I in regulation of the insulin secretion by b-cells and the activity of MMPs in the islets of Langerhans.

A Mendelian Randomization Study of Metabolite Profiles, Fasting Glucose, and Type 2 Diabetes

Mendelian randomization (MR) provides us the opportunity to investigate the causal paths of metabolites in type 2 diabetes and glucose homeostasis. We developed and tested an MR approach based on genetic risk scoring for plasma metabolite levels, utilizing a pathway-based sensitivity analysis to control for nonspecific effects. We focused on 124 circulating metabolites that correlate with fasting glucose in the Erasmus Rucphen Family (ERF) study (n = 2,564) and tested the possible causal effect of each metabolite with glucose and type 2 diabetes and vice versa. We detected 14 paths with potential causal effects by MR, following pathway-based sensitivity analysis. Our results suggest that elevated plasma triglycerides might be partially responsible for increased glucose levels and type 2 diabetes risk, which is consistent with previous reports. Additionally, elevated HDL components, i.e., small HDL triglycerides, might have a causal role of elevating glucose levels. In contrast, large (L) and extra large (XL) HDL lipid components, i.e., XL-HDL cholesterol, XL-HDL-free cholesterol, XL-HDL phospholipids, L-HDL cholesterol, and L-HDL-free cholesterol, as well as HDL cholesterol seem to be protective against increasing fasting glucose but not against type 2 diabetes. Finally, we demonstrate that genetic predisposition to type 2 diabetes associates with increased levels of alanine and decreased levels of phosphatidylcholine alkyl-acyl C42:5 and phosphatidylcholine alkyl-acyl C44:4. Our MR results provide novel insight into promising causal paths to and from glucose and type 2 diabetes and underline the value of additional information from high-resolution metabolomics over classic biochemistry.

Low apoA-I is associated with insulin resistance in patients with impaired glucose tolerance: a cross-sectional study

Background

Low apolipoprotein A-I (apoA-I) is an independent risk factor for atherosclerotic cardiovascular diseases. Insulin resistance predicts the progression of abnormal glucose metabolism, which is the main cause of atherosclerotic cardiovascular disease. In this study, we assessed the potential association between apoA-I levels and insulin resistance in patients with impaired glucose tolerance (IGT) and the possible link between apoA-I and IGT.

Methods

This study evaluated a cross-sectional study of 108 participants with impaired glucose tolerance (IGT group) and 84 controls (control group). ApoA-I and clinical characteristics were measured, and a homeostasis model assessment of insulin resistance (HOMA-IR) was calculated.

Results

The IGT group exhibited significantly lower apoA-I and higher HOMA-IR levels than the control group (apoA-I: 1.37 ± 0.36 vs 1.57 ± 0.39 g/L; HOMA-IR: 4.21 ± 1.56 vs 2.15 ± 0.99; P < 0.001 for both). ApoA-I was negatively correlated with HOMA-IR in both the IGT and control groups (IGT group: r = −0.269, P = 0.005; control group: r = −0.262, P = 0.016). Multiple stepwise regression analysis showed that low apoA-I levels (β = −1.470, P = 0.002) were independently correlated with high HOMA-IR levels in the IGT group. Moreover, logistic regression analysis identified that low apoA-I was an independent influencing factor for IGT (β = −1.170, OR = 0.310, P = 0.007).

Conclusions

ApoA-I is inversely associated with insulin resistance in patients with impaired glucose tolerance, and low apoA-I is an independent risk factor for impaired glucose tolerance. These results indicate that apoA-I plays an important role in regulating insulin sensitivity and glucose metabolism in patients with IGT.

Low levels of ApoA1 improve risk prediction of type 2 diabetes mellitus

BACKGROUND

Type 2 diabetes mellitus (T2DM) has reported to be a major public health crisis in China.

OBJECTIVE

We examined the incidence of new T2DM over 4 years for association of clinical factors and lipids with development of T2DM in a community-based population.

METHODS

We included 923 Chinese subjects who participated in community-organized health checkout in both 2009 and 2013. Health history was collected; physical examination was performed; biochemistry, lipids, and glucose were measured. Of 923, 819 were confirmed without T2DM in 2009 and included in the analysis. Unadjusted and adjusted logistic regression models were used to estimate the effects of clinical factors and biomarkers on the risk of new T2DM.

RESULTS

Of 819 subjects without T2DM in 2009, 65 were identified as T2DM in 2013, 8% over 4 years. These 65 subjects, compared with those 754 without new T2DM, were older, more likely to be male and smokers. They had higher body mass index (BMI), fasting glucose, blood pressure and triglycerides, and lower levels of high-density lipoprotein-cholesterol and apolipoprotein A1 (ApoA1). Multivariate logistic regression identified larger BMI (odds ratio [OR] = 1.7; 95% confidence interval [CI], 1.22-2.39, P = .002), higher fasting glucose levels (OR = 4.2, 95% CI, 2.90-6.19, P < .001), and low levels of ApoA1 (OR = 0.51, 95% CI 0.33-0.76, P = .002) were independently associated with new T2DM. Furthermore, receiver operating characteristics curves for multivariate models for new T2DM showed that area under the curve improved from 0.87 to 0.89 when adding ApoA1 to the Framingham Diabetes Risk Scoring Model and from 0.85 to 0.89 when adding ApoA1 to a 4-variable (age, BMI, glucose, and triglycerides) Chinese model.

CONCLUSIONS

There is a high incidence of new T2DM at 8% over 4 years among Chinese. Larger BMI, higher glucose levels, and lower levels of ApoA1 are significantly and independently associated with new T2DM. Lower ApoA1 improves the risk prediction of new type 2 diabetes when it was added to the existing risk models.

In vivo PET imaging with [(18)F]FDG to explain improved glucose uptake in an apolipoprotein A-I treated mouse model of diabetes

AIMS/HYPOTHESIS

Type 2 diabetes is characterised by decreased HDL levels, as well as the level of apolipoprotein A-I (apoA-I), the main apolipoprotein of HDLs. Pharmacological elevation of HDL and apoA-I levels is associated with improved glycaemic control in patients with type 2 diabetes. This is partly due to improved glucose uptake in skeletal muscle.

METHODS

This study used kinetic modelling to investigate the impact of increasing plasma apoA-I levels on the metabolism of glucose in the db/db mouse model.

RESULTS

Treatment of db/db mice with apoA-I for 2 h significantly improved both glucose tolerance (AUC 2574 ± 70 mmol/l × min vs 2927 ± 137 mmol/l × min, for apoA-I and PBS, respectively; p < 0.05) and insulin sensitivity (AUC 388.8 ± 23.8 mmol/l × min vs 194.1 ± 19.6 mmol/l × min, for apoA-I and PBS, respectively; p < 0.001). ApoA-I treatment also increased glucose uptake by skeletal muscle in both an insulin-dependent and insulin-independent manner as evidenced by increased uptake of fludeoxyglucose ([(18)F]FDG) from plasma into gastrocnemius muscle in apoA-I treated mice, both in the absence and presence of insulin. Kinetic modelling revealed an enhanced rate of insulin-mediated glucose phosphorylation (k 3) in apoA-I treated mice (3.5 ± 1.1 × 10(-2) min(-1) vs 2.3 ± 0.7 × 10(-2) min(-1), for apoA-I and PBS, respectively; p < 0.05) and an increased influx constant (3.7 ± 0.6 × 10(-3) ml min(-1) g(-1) vs 2.0 ± 0.3 × 10(-3) ml min(-1) g(-1), for apoA-I and PBS, respectively; p < 0.05). Treatment of L6 rat skeletal muscle cells with apoA-I for 2 h indicated that increased hexokinase activity mediated the increased rate of glucose phosphorylation.

CONCLUSIONS/INTERPRETATION

These findings indicate that apoA-I improves glucose disposal in db/db mice by improving insulin sensitivity and enhancing glucose phosphorylation.

Dual Actions of Apolipoprotein A-I on Glucose-Stimulated Insulin Secretion and Insulin-Independent Peripheral Tissue Glucose Uptake Lead to Increased Heart and Skeletal Muscle Glucose Disposal

Apolipoprotein A-I (apoA-I) of HDL is central to the transport of cholesterol in circulation. ApoA-I also provides glucose control with described in vitro effects of apoA-I on β-cell insulin secretion and muscle glucose uptake. In addition, apoA-I injections in insulin-resistant diet-induced obese (DIO) mice lead to increased glucose-stimulated insulin secretion (GSIS) and peripheral tissue glucose uptake. However, the relative contribution of apoA-I as an enhancer of GSIS in vivo and as a direct stimulator of insulin-independent glucose uptake is not known. Here, DIO mice with instant and transient blockade of insulin secretion were used in glucose tolerance tests and in positron emission tomography analyses. Data demonstrate that apoA-I to an equal extent enhances GSIS and acts as peripheral tissue activator of insulin-independent glucose uptake and verify skeletal muscle as an apoA-I target tissue. Intriguingly, our analyses also identify the heart as an important target tissue for the apoA-I-stimulated glucose uptake, with potential implications in diabetic cardiomyopathy. Explorations of apoA-I as a novel antidiabetic drug should extend to treatments of diabetic cardiomyopathy and other cardiovascular diseases in patients with diabetes.

High density lipoprotein and metabolic disease: Potential benefits of restoring its functional properties

Background

High density lipoproteins (HDLs) are thought to be atheroprotective and to reduce the risk of cardiovascular disease (CVD). Besides their antioxidant, antithrombotic, anti-inflammatory, anti-apoptotic properties in the vasculature, HDLs also improve glucose metabolism in skeletal muscle.

Scope of the review

Herein, we review the functional role of HDLs to improve metabolic disorders, especially those involving insulin resistance and to induce regression of CVD with a particular focus on current pharmacological treatment options as well as lifestyle interventions, particularly exercise.

Major conclusions

Functional properties of HDLs continue to be considered important mediators to reverse metabolic dysfunction and to regress atherosclerotic cardiovascular disease. Lifestyle changes are often recommended to reduce the risk of CVD, with exercise being one of the most important of these. Understanding how exercise improves HDL function will likely lead to new approaches to battle the expanding burden of obesity and the metabolic syndrome.

ABCA1 agonist peptides for the treatment of disease

PURPOSE OF REVIEW

The review summarizes information pertaining to the preclinical development of new apolipoprotein (apo) E mimetic peptides that stimulate cellular cholesterol efflux.

RECENT FINDINGS

Small α-helical peptides based on the C-terminal domain of apoE have been developed for therapeutic applications. These peptides stimulate cellular cholesterol efflux via the ATP-binding cassette transporter A1 (ABCA1) with high potency, like native apolipoproteins on a molar basis. This potent activity has been related to the unique ability of these peptides to maintain α-helix structure upon dilution. Recent structure-activity studies improving the safety features of these mimetic peptides have greatly improved their potential for clinical use. These studies have identified structural features of the class A α-helix motif that induce muscle toxicity and hypertriglyceridemia, which may have implications for the design of other HDL mimetic peptides.

SUMMARY

ABCA1 is an integral membrane protein that plays a central role in biology. Its principal function is to mediate the efflux of cholesterol and phospholipid from cells to extracellular apo, preventing a build-up of excess cholesterol in membranes. This process generates HDL particles that perform a variety of functions to protect against disease. A number of these functions can be viewed as directly or indirectly supporting ABCA1 activity, thus constituting a positive feedback system to optimize cellular lipid efflux responses and disease prevention. Consequently, therapeutic approaches that mimic the activities of apos may prove highly effective to combat disease. One such approach involves the use of peptides. The broad biological relevance of ABCA1 suggests these apo mimetic peptides may be useful for the treatment of a number of diseases, such as atherosclerosis, diabetes, and Alzheimer's disease.

Advances in high-density lipoprotein physiology: surprises, overturns, and promises

Emerging evidence strongly supports that changes in the HDL metabolic pathway, which result in changes in HDL proteome and function, appear to have a causative impact on a number of metabolic disorders. Here, we provide a critical review of the most recent and novel findings correlating HDL properties and functionality with various pathophysiological processes and disease states, such as obesity, type 2 diabetes mellitus, nonalcoholic fatty liver disease, inflammation and sepsis, bone and obstructive pulmonary diseases, and brain disorders.

Structural and Functional Analysis of the ApolipoproteinA-I A164S Variant

Apolipoprotein A-I (apoA-I) is the main protein involved in the formation of high-density lipoprotein (HDL), it is the principal mediator of the reverse cholesterol transfer (RCT) pathway and provides cardio-protection. In addition to functional wild-type apoA-I, several variants have been shown to associate with hereditary amyloidosis. In this study we have performed biophysical and biochemical analyses of the structure and functional properties of the A164S variant of apoA-I (1:500 in the Danish general population), which is the first known mutation of apoA-I that leads to an increased risk of ischaemic heart disease (IHD), myocardial infarction and mortality without associated low HDL cholesterol levels. Despite the fact that epidemiologically IHD is associated with low plasma levels of HDL, the A164S mutation is linked to normal plasma levels of lipids, HDL and apoA-I, suggesting impaired functionality of this variant. Using biophysical techniques (e.g., circular dichroism spectroscopy and electron microscopy) to determine secondary structure, stability and pro-amyloidogenic property of the lipid free A164S apoA-I variant, our observations suggest similarity in structural properties between apoA-I WT and apoA-I A164S. However, the A164S apoA-I variant exhibits lower binding affinity to lipids but forms similar sized HDL particles to those produced by WT.

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

HDL and glucose metabolism: current evidence and therapeutic potential

High-density lipoprotein (HDL) and its principal apolipoprotein A-I (ApoA-I) have now been convincingly shown to influence glucose metabolism through multiple mechanisms. The key clinically relevant observations are that both acute HDL elevation via short-term reconstituted HDL (rHDL) infusion and chronically raising HDL via a cholesteryl ester transfer protein (CETP) inhibitor reduce blood glucose in individuals with type 2 diabetes mellitus (T2DM). HDL may mediate effects on glucose metabolism through actions in multiple organs (e.g., pancreas, skeletal muscle, heart, adipose, liver, brain) by three distinct mechanisms: (i) Insulin secretion from pancreatic beta cells, (ii) Insulin-independent glucose uptake, (iii) Insulin sensitivity. The molecular mechanisms appear to involve both direct HDL signaling actions as well as effects secondary to lipid removal from cells. The implications of glucoregulatory mechanisms linked to HDL extend from glycemic control to potential anti-ischemic actions via increased tissue glucose uptake and utilization. Such effects not only have implications for the prevention and management of diabetes, but also for ischemic vascular diseases including angina pectoris, intermittent claudication, cerebral ischemia and even some forms of dementia. This review will discuss the growing evidence for a role of HDL in glucose metabolism and outline related potential for HDL therapies.

Proteomic analysis of the palmitate-induced myotube secretome reveals involvement of the annexin A1-formyl peptide receptor 2 (FPR2) pathway in insulin resistance

Elevated levels of the free fatty acid palmitate are found in the plasma of obese patients and induce insulin resistance. Skeletal muscle secretes myokines as extracellular signaling mediators in response to pathophysiological conditions. Here, we identified and characterized the skeletal muscle secretome in response to palmitate-induced insulin resistance. Using a quantitative proteomic approach, we identified 36 secretory proteins modulated by palmitate-induced insulin resistance. Bioinformatics analysis revealed that palmitate-induced insulin resistance induced cellular stress and modulated secretory events. We found that the decrease in the level of annexin A1, a secretory protein, depended on palmitate, and that annexin A1 and its receptor, formyl peptide receptor 2 agonist, played a protective role in the palmitate-induced insulin resistance of L6 myotubes through PKC-θ modulation. In mice fed with a high-fat diet, treatment with the formyl peptide receptor 2 agonist improved systemic insulin sensitivity. Thus, we identified myokine candidates modulated by palmitate-induced insulin resistance and found that the annexin A1- formyl peptide receptor 2 pathway mediated the insulin resistance of skeletal muscle, as well as systemic insulin sensitivity.

HDL-mediated mechanisms of protection in cardiovascular disease

Low plasma levels of HDL-cholesterol (HDL-C) represent a strong and independent risk factor for cardiovascular disease. HDL particles display a wide spectrum of atheroprotective activities, which include effluxing cellular cholesterol, diminishing cellular death, decreasing vascular constriction, reducing inflammatory response, protecting from pathological oxidation, combating bacterial infection, lessening platelet activation, regulating gene expression by virtue of microRNAs, and improving glucose metabolism. It remains presently indeterminate as to whether some biological activities of HDL are more relevant for the protection of the endothelium from atherogenesis when compared with others. The multitude of such activities raises the question of a proper assay to assess HDL functionality ex vivo. Together with clear understanding of molecular mechanisms underlying atheroprotective properties of HDL, such assay will provide a basis to resolve the ultimate question of the HDL field to allow the development of efficient HDL-targeting therapies.

Single injections of apoA-I acutely improve in vivo glucose tolerance in insulin-resistant mice

AIMS/HYPOTHESIS

Apolipoprotein A-I (apoA-I), the main protein constituent of HDL, has a central role in the reverse cholesterol-transport pathway, which together with the anti-inflammatory properties of apoA-I/HDL provide cardioprotection. Recent findings of direct stimulation of glucose uptake in muscle by apoA-I/HDL suggest that altered apoA-I and HDL functionality may be a contributing factor to the development of diabetes. We have studied the in vivo effects of short treatments with human apoA-I in a high-fat diet fed mouse model. In addition to native apoA-I, we investigated the effects of the cardioprotective Milano variant (Arg173Cys).

METHODS

Male C57Bl6 mice on a high-fat diet for 2 weeks that received a single injection of human apoA-I proteins (wild-type and Milano) were analysed for blood glucose and insulin levels during a 3 h incubation followed by glucose tolerance tests. Incorporation of injected human apoA-I protein into HDLs was analysed by native gel electrophoresis.

RESULTS

ApoA-I treatment significantly improved insulin secretion and blood glucose clearance in the glucose tolerance test, with an efficiency exceeding that of lean control animals, and led to decreased basal glucose during the 3 h incubation. Notably, the two apoA-I variants triggered insulin secretion and glucose clearance to the same extent.

CONCLUSIONS/INTERPRETATION

ApoA-I treatment leads to insulin- and non-insulin-dependent effects on glucose homeostasis. The experimental model of short-term (2 weeks) feeding of a high-fat diet to C57Bl6 mice provides a suitable and time-efficient system to unravel the resulting tissue-specific mechanisms of acute apoA-I treatment that lead to improved glucose homeostasis.

Cardioprotective functions of HDLs

Multiple human population studies have established the concentration of high density lipoprotein (HDL) cholesterol as an independent, inverse predictor of the risk of having a cardiovascular event. Furthermore, HDLs have several well-documented functions with the potential to protect against cardiovascular disease. These include an ability to promote the efflux of cholesterol from macrophages in the artery wall, inhibit the oxidative modification of low density lipoproteins (LDLs), inhibit vascular inflammation, inhibit thrombosis, promote endothelial repair, promote angiogenesis, enhance endothelial function, improve diabetic control, and inhibit hematopoietic stem cell proliferation. There are undoubtedly other beneficial functions of HDLs yet to be identified. The HDL fraction in human plasma is heterogeneous, consisting of several subpopulations of particles of varying size, density, and composition. The functions of the different HDL subpopulations remain largely unknown. Given that therapies that increase the concentration of HDL cholesterol have varying effects on the levels of specific HDL subpopulations, it is of great importance to understand how distribution of different HDL subpopulations contribute to the potentially cardioprotective functions of this lipoprotein fraction. This review summarizes current understanding of the relationship of HDL subpopulations to their cardioprotective properties and highlights the gaps in current knowledge regarding this important aspect of HDL biology.

Regulation of signal transduction by HDL

High density lipoprotein (HDL) cholesterol has direct effects on numerous cell types that influence cardiovascular and metabolic health. These include endothelial cells, vascular smooth-muscle cells, leukocytes, platelets, adipocytes, skeletal muscle myocytes, and pancreatic β cells. The effects of HDL or apoA-I, its major apolipoprotein, occur through the modulation of intracellular calcium, oxygen-derived free-radical production, numerous kinases, and enzymes, including endothelial nitric-oxide synthase (eNOS). ApoA-I and HDL also influence gene expression, particularly genes encoding mediators of inflammation in vascular cells. In many paradigms, the change in intracellular signaling occurs as a result of cholesterol efflux, with the cholesterol acceptor methyl-β-cyclodextrin often invoking responses identical to HDL or apoA-I. The ABC transporters ABCA1 and ABCG1 and scavenger receptor class B, type I (SR-BI) frequently participate in the cellular responses. Structure-function relationships are emerging for signal initiation by ABCA1 and SR-BI, with plasma membrane cholesterol binding by the C-terminal transmembrane domain of SR-BI uniquely enabling it to serve as a sensor of changes in membrane cholesterol. Further investigation of the processes underlying HDL and apoA-I modulation of intracellular signaling will potentially reveal new prophylactic and therapeutic strategies to optimize both cardiovascular and metabolic health.