Temporal Dynamics of High-Density Lipoprotein Proteome in Diet-Controlled Subjects with Type 2 Diabetes

Type 2 Diabetes and HDL Dysfunction: A Key Contributor to Glycemic Control

High-density lipoproteins (HDL) have been shown to exert multiple cardioprotective and antidiabetic functions, such as their ability to promote cellular cholesterol efflux and their antioxidant, anti-inflammatory, and antiapoptotic properties. Type 2 diabetes (T2D) is usually associated with low high-density lipoprotein cholesterol (HDL-C) levels as well as with significant alterations in the HDL composition, thereby impairing its main functions. HDL dysfunction also negatively impacts both pancreatic β-cell function and skeletal muscle insulin sensitivity, perpetuating this adverse self-feeding cycle. The impairment of these pathways is partly dependent on cellular ATP-binding cassette transporter (ABC) A1-mediated efflux to lipid-poor apolipoprotein (apo) A-I in the extracellular space. In line with these findings, experimental interventions aimed at improving HDL functions, such as infusions of synthetic HDL or lipid-poor apoA-I, significantly improved glycemic control in T2D patients and experimental models of the disease. Cholesteryl ester transfer protein (CETP) inhibitors are specific drugs designed to increase HDLC and HDL functions. Posthoc analyses of large clinical trials with CETP inhibitors have demonstrated their potential anti-diabetic properties. Research on HDL functionality and HDL-based therapies could be a crucial step toward improved glycemic control in T2D subjects.

Searching for Atherosclerosis Biomarkers by Proteomics: A Focus on Lesion Pathogenesis and Vulnerability

Plaque rupture and thrombosis are the most important clinical complications in the pathogenesis of stroke, coronary arteries, and peripheral vascular diseases. The identification of early biomarkers of plaque presence and susceptibility to ulceration could be of primary importance in preventing such life-threatening events. With the improvement of proteomic tools, large-scale technologies have been proven valuable in attempting to unravel pathways of atherosclerotic degeneration and identifying new circulating markers to be utilized either as early diagnostic traits or as targets for new drug therapies. To address these issues, different matrices of human origin, such as vascular cells, arterial tissues, plasma, and urine, have been investigated. Besides, proteomics was also applied to experimental atherosclerosis in order to unveil significant insights into the mechanisms influencing atherogenesis. This narrative review provides an overview of the last twenty years of omics applications to the study of atherogenesis and lesion vulnerability, with particular emphasis on lipoproteomics and vascular tissue proteomics. Major issues of tissue analyses, such as plaque complexity, sampling, availability, choice of proper controls, and lipoproteins purification, will be raised, and future directions will be addressed.

High-Density Lipoprotein Alterations in Type 2 Diabetes and Obesity

Alterations affecting high-density lipoproteins (HDLs) are one of the various abnormalities observed in dyslipidemia in type 2 diabetes mellitus (T2DM) and obesity. Kinetic studies have demonstrated that the catabolism of HDL particles is accelerated. Both the size and the lipidome and proteome of HDL particles are significantly modified, which likely contributes to some of the functional defects of HDLs. Studies on cholesterol efflux capacity have yielded heterogeneous results, ranging from a defect to an improvement. Several studies indicate that HDLs are less able to inhibit the nuclear factor kappa-B (NF-κB) proinflammatory pathway, and subsequently, the adhesion of monocytes on endothelium and their recruitment into the subendothelial space. In addition, the antioxidative function of HDL particles is diminished, thus facilitating the deleterious effects of oxidized low-density lipoproteins on vasculature. Lastly, the HDL-induced activation of endothelial nitric oxide synthase is less effective in T2DM and metabolic syndrome, contributing to several HDL functional defects, such as an impaired capacity to promote vasodilatation and endothelium repair, and difficulty counteracting the production of reactive oxygen species and inflammation.

Metabolic labeling unveils alterations in the turnover of HDL-associated proteins during diabetes progression in mice

Several aspects of diabetes pathophysiology and complications result from hyperglycemia-induced alterations in the structure and function of plasma proteins. Furthermore, insulin has a significant influence on protein metabolism by affecting both the synthesis and degradation of proteins in various tissues. To understand the role of progressive hyperglycemia on plasma proteins, in this study, we measured the turnover rates of high-density lipoprotein (HDL)-associated proteins in control (chow diet), prediabetic [a high-fat diet (HFD) for 8 wk] or diabetic [HFD for 8 wk with low-dose streptozotocin (HFD + STZ) in weeks 5-8 of HFD] C57BL/6J mice using heavy water (2H2O)-based metabolic labeling approach. Compared with control mice, HFD and HFD + STZ mice showed elevations of fasting plasma glucose levels in the prediabetic and diabetic range, respectively. Furthermore, the HFD and HFD + STZ mice showed increased hepatic triglyceride (TG) levels, total plasma cholesterol, and plasma TGs. The kinetics of 40 proteins were quantified using the proteome dynamics method, which revealed an increase in the fractional synthesis rate (FSR) of HDL-associated proteins in the prediabetic mice compared with control mice, and a decrease in FSR in the diabetic mice. The pathway analysis revealed that proteins with altered turnover rates were involved in acute-phase response, lipid metabolism, and coagulation. In conclusion, prediabetes and diabetes have distinct effects on the turnover rates of HDL proteins. These findings suggest that an early dysregulation of the HDL proteome dynamics can provide mechanistic insights into the changes in protein levels in these conditions.NEW & NOTEWORTHY This study is the first to examine the role of gradual hyperglycemia during diabetes disease progression on HDL-associated protein dynamics in the prediabetes and diabetic mice. Our results show that the fractional synthesis rate of HDL-associated proteins increased in the prediabetic mice whereas it decreased in the diabetic mice compared with control mice. These kinetic changes can help to elucidate the mechanism of altered protein levels and HDL dysfunction during diabetes disease progression.

Apolipoprotein A-II, a Player in Multiple Processes and Diseases

Apolipoprotein A-II (apoA-II) is the second most abundant apolipoprotein in high-density lipoprotein (HDL) particles, playing an important role in lipid metabolism. Human and murine apoA-II proteins have dissimilar properties, partially because human apoA-II is dimeric whereas the murine homolog is a monomer, suggesting that the role of apoA-II may be quite different in humans and mice. As a component of HDL, apoA-II influences lipid metabolism, being directly or indirectly involved in vascular diseases. Clinical and epidemiological studies resulted in conflicting findings regarding the proatherogenic or atheroprotective role of apoA-II. Human apoA-II deficiency has little influence on lipoprotein levels with no obvious clinical consequences, while murine apoA-II deficiency causes HDL deficit in mice. In humans, an increased plasma apoA-II concentration causes hypertriglyceridemia and lowers HDL levels. This dyslipidemia leads to glucose intolerance, and the ensuing high blood glucose enhances apoA-II transcription, generating a vicious circle that may cause type 2 diabetes (T2D). ApoA-II is also used as a biomarker in various diseases, such as pancreatic cancer. Herein, we provide a review of the most recent findings regarding the roles of apoA-II and its functions in various physiological processes and disease states, such as cardiovascular disease, cancer, amyloidosis, hepatitis, insulin resistance, obesity, and T2D.

HDL functionality in type 1 and type 2 diabetes: new insights

PURPOSE OF REVIEW

To critically appraise new insights into HDL structure and function in type 1 diabetes (T1DM) and type 2 diabetes (T2DM).

RECENT FINDINGS

In young T1DM patients with early renal impairment and a high inflammatory score, both HDL antioxidative activity and endothelial vasodilatory function were impaired, revealing a critical link between HDL dysfunction, subclinical vascular damage, systemic inflammation and end organ damage. HDL may inhibit development of T2DM by attenuating endoplasmic reticulum (ER) stress and apoptotic loss of pancreatic β-cells, an effect due in part to ABC transporter-mediated efflux of specific oxysterols with downstream activation of the hedghehog signalling receptor, Smoothened. The apoM-sphingosine-1-phosphate complex is critical to HDL antidiabetic activity, encompassing protection against insulin resistance, promotion of insulin secretion, enhanced β-cell survival and inhibition of hepatic glucose production. Structure-function studies of HDL in hyperglycemic, dyslipidemic T2DM patients revealed both gain and loss of lipidomic and proteomic components. Such changes attenuated both the optimal protective effects of HDL on mitochondrial function and its capacity to inhibit endothelial cell apoptosis. Distinct structural components associated with individual HDL functions.

SUMMARY

Extensive evidence indicates that both the proteome and lipidome of HDL are altered in T1DM and T2DM, with impairment of multiple functions.

The HDL Proteome Watch: Compilation of studies leads to new insights on HDL function

PURPOSE OF REVIEW

High density lipoproteins (HDL) are a heterogeneous family of particles that contain distinct complements of proteins that define their function. Thus, it is important to accurately and sensitively identify proteins associated with HDL. Here we highlight the HDL Proteome Watch Database which tracks proteomics studies from different laboratories across the world.

RECENT FINDINGS

In 45 published reports, almost 1000 individual proteins have been detected in preparations of HDL. Of these, 251 have been identified in at least three different laboratories. The known functions of these consensus HDL proteins go well beyond traditionally recognized roles in lipid transport with many proteins pointing to HDL functions in innate immunity, inflammation, cell adhesion, hemostasis and protease regulation, and even vitamin and metal binding.

SUMMARY

The HDL proteome derived across multiple studies using various methodologies provides confidence in protein identifications that can offer interesting new insights into HDL function. We also point out significant issues that will require additional study going forward.

Apolipoprotein Signature of HDL and LDL from Atherosclerotic Patients in Relation with Carotid Plaque Typology: A Preliminary Report

In the past years, it has become increasingly clear that the protein cargo of the different lipoprotein classes is largely responsible for carrying out their various functions, also in relation to pathological conditions, including atherosclerosis. Accordingly, detailed information about their apolipoprotein composition and structure may contribute to the revelation of their role in atherogenesis and the understanding of the mechanisms that lead to atherosclerotic degeneration and toward vulnerable plaque formation. With this aim, shotgun proteomics was applied to identify the apolipoprotein signatures of both high-density and low-density lipoproteins (HDL and LDL) plasma fractions purified from healthy volunteers and atherosclerotic patients with different plaque typologies who underwent carotid endarterectomy. By this approach, two proteins with potential implications in inflammatory, immune, and hemostatic pathways, namely, integrin beta-2 (P05107) and secretoglobin family 3A member 2 (Q96PL1), have been confirmed to belong to the HDL proteome. Similarly, the list of LDL-associated proteins has been enriched with 21 proteins involved in complement and coagulation cascades and the acute-phase response, which potentially double the protein species of LDL cargo. Moreover, differential expression analysis has shown protein signatures specific for patients with “hard” or “soft” plaques.

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.

Isolation of HDL by sequential flotation ultracentrifugation followed by size exclusion chromatography reveals size-based enrichment of HDL-associated proteins

High-density lipoprotein (HDL) particles have multiple beneficial and cardioprotective roles, yet our understanding of their full structural and functional repertoire is limited due to challenges in separating HDL particles from contaminating plasma proteins and other lipid-carrying particles that overlap HDL in size and/or density. Here we describe a method for isolating HDL particles using a combination of sequential flotation density ultracentrifugation and fast protein liquid chromatography with a size exclusion column. Purity was visualized by polyacrylamide gel electrophoresis and verified by proteomics, while size and structural integrity were confirmed by transmission electron microscopy. This HDL isolation method can be used to isolate a high yield of purified HDL from a low starting plasma volume for functional analyses. This method also enables investigators to select their specific HDL fraction of interest: from the least inclusive but highest purity HDL fraction eluting in the middle of the HDL peak, to pooling all of the fractions to capture the breadth of HDL particles in the original plasma sample. We show that certain proteins such as lecithin cholesterol acyltransferase (LCAT), phospholipid transfer protein (PLTP), and clusterin (CLUS) are enriched in large HDL particles whereas proteins such as alpha-2HS-glycoprotein (A2HSG), alpha-1 antitrypsin (A1AT), and vitamin D binding protein (VDBP) are enriched or found exclusively in small HDL particles.

Early Pro-Inflammatory Remodeling of HDL Proteome in a Model of Diet-Induced Obesity: 2H2O-Metabolic Labeling-Based Kinetic Approach

Mice fed a high-fat diet for 12 weeks or longer develop hyperglycemia, insulin resistance, dyslipidemia, and fatty liver. Additionally, a high-fat diet induces inflammation that remodels and affects the anti-inflammatory and antiatherogenic property of the high-density lipoprotein (HDL). However, the precise time course of metabolic disease progression and HDL remodeling remains unclear. Short-term (four weeks) high-fat feeding (60% fat calories) was performed in wild-type male C57BL/6J mice to gain insights into the early metabolic disease processes in conjunction with a HDL proteome dynamics analysis using a heavy water metabolic labeling approach. The high-fat diet-fed mice developed hyperglycemia, impaired glucose tolerance, hypercholesterolemia without hypertriglyceridemia or hepatic steatosis. A plasma HDL proteome dynamics analysis revealed increased turnover rates (and reduced half-lives) of several acute-phase response proteins involved in innate immunity, including complement C3 (12.77 ± 0.81 vs. 9.98 ± 1.20 h, p < 0.005), complement factor B (12.71 ± 1.01 vs. 10.85 ± 1.04 h, p < 0.05), complement Factor H (19.60 ± 1.84 vs. 16.80 ± 1.58 h, p < 0.05), and complement factor I (25.25 ± 1.29 vs. 19.88 ± 1.50 h, p < 0.005). Our findings suggest that an early immune response-induced inflammatory remodeling of the plasma HDL proteome precedes the diet-induced steatosis and dyslipidemia.

A Review of the Current Evidence: Impact of Metabolic Surgery on Diabetes Outcomes and Obesity-Associated Macrovascular Complications

PURPOSE OF REVIEW

Type 2 diabetes (T2D) and obesity are comorbidities that generally progress with time even when non-invasive therapies are prescribed. Indeed, weight loss that is achieved with behavioral modification alone is generally inconsistent and often short-lived. In contrast, although patients do experience weight regain with metabolic surgery, they still benefit from a significant net decrease in weight. As a result, T2D remission can be achieved in up to 60% of patients within 2 years after surgery. However, it is unknown if the positive effects of metabolic surgery extend to macrovascular disease risk reduction.

RECENT FINDINGS

As noted in four randomized controlled trials (RCTs), Roux-en-Y gastric bypass (RYGB) facilitates partial remission of T2D in about 30% of volunteers 5 years after surgery. Of the four RCTs, only one investigated the effects of sleeve gastrectomy (SG) at 5 years; that study found that the rate of partial relapse was slightly lower with SG (23%). However, observational studies indicate that the gap between RYGB and SG may be larger than that observed in RCTs. In contrast, the rate of full remission is noted infrequently 5 years after SG or RYGB. Metabolic surgery also mitigates macrovascular disease risk as indicated by multiple observational studies. The effects of metabolic surgery on cardiometabolic parameters are clinically meaningful. The weight loss that is facilitated by metabolic surgery reduces the metabolic and inflammatory stress caused by T2D and obesity. In turn, metabolic surgery likely mitigates macrovascular disease risk. Additional evidence from RCTs is needed to substantiate the effects of metabolic surgery on macrovascular disease risk.