Retinol-binding protein 4 levels and susceptibility to ischaemic events in men

Plasma proteome association with coronary heart disease and carotid intima media thickness: results from the KORA F4 study

BACKGROUND AND AIMS

Atherosclerosis is the main cause of stroke and coronary heart disease (CHD), both leading mortality causes worldwide. Proteomics, as a high-throughput method, could provide helpful insights into the pathological mechanisms underlying atherosclerosis. In this study, we characterized the associations of plasma protein levels with CHD and with carotid intima-media thickness (CIMT), as a surrogate measure of atherosclerosis.

METHODS

The discovery phase included 1000 participants from the KORA F4 study, whose plasma protein levels were quantified using the aptamer-based SOMAscan proteomics platform. We evaluated the associations of plasma protein levels with CHD using logistic regression, and with CIMT using linear regression. For both outcomes we applied two models: an age-sex adjusted model, and a model additionally adjusted for body mass index, smoking status, physical activity, diabetes status, hypertension status, low density lipoprotein, high density lipoprotein, and triglyceride levels (fully-adjusted model). The replication phase included a matched case-control sample from the independent KORA F3 study, using ELISA-based measurements of galectin-4. Pathway analysis was performed with nominally associated proteins (p-value < 0.05) from the fully-adjusted model.

RESULTS

In the KORA F4 sample, after Bonferroni correction, we found CHD to be associated with five proteins using the age-sex adjusted model: galectin-4 (LGALS4), renin (REN), cathepsin H (CTSH), and coagulation factors X and Xa (F10). The fully-adjusted model yielded only the positive association of galectin-4 (OR = 1.58, 95% CI = 1.30-1.93), which was successfully replicated in the KORA F3 sample (OR = 1.40, 95% CI = 1.09-1.88). For CIMT, we found four proteins to be associated using the age-sex adjusted model namely: cytoplasmic protein NCK1 (NCK1), insulin-like growth factor-binding protein 2 (IGFBP2), growth hormone receptor (GHR), and GDNF family receptor alpha-1 (GFRA1). After assessing the fully-adjusted model, only NCK1 remained significant (β = 0.017, p-value = 1.39e-06). Upstream regulators of galectin-4 and NCK1 identified from pathway analysis were predicted to be involved in inflammation pathways.

CONCLUSIONS

Our proteome-wide association study identified galectin-4 to be associated with CHD and NCK1 to be associated with CIMT. Inflammatory pathways underlying the identified associations highlight the importance of inflammation in the development and progression of CHD.

Lipocalin family proteins and their diverse roles in cardiovascular disease

The lipocalin (LCN) family members, a group of small extracellular proteins with 160-180 amino acids in length, can be detected in all kingdoms of life from bacteria to human beings. They are characterized by low similarity of amino acid sequence but highly conserved tertiary structures with an eight-stranded antiparallel β-barrel which forms a cup-shaped ligand binding pocket. In addition to bind small hydrophobic ligands (i.e., fatty acids, odorants, retinoids, and steroids) and transport them to specific cells, lipocalins (LCNs) can interact with specific cell membrane receptors to activate their downstream signaling pathways, and with soluble macromolecules to form the complex. Consequently, LCNs exhibit great functional diversity. Accumulating evidence has demonstrated that LCN family proteins exert multiple layers of function in the regulation of many physiological processes and human diseases (i.e., cancers, immune disorders, metabolic disease, neurological/psychiatric disorders, and cardiovascular disease). In this review, we firstly introduce the structural and sequence properties of LCNs. Next, six LCNs including apolipoprotein D (ApoD), ApoM, lipocalin 2 (LCN2), LCN10, retinol-binding protein 4 (RBP4), and Lipocalin-type prostaglandin D synthase (L-PGDS) which have been characterized so far are highlighted for their diagnostic/prognostic values and their potential effects on coronary artery disease and myocardial infarction injury. The roles of these 6 LCNs in cardiac hypertrophy, heart failure, diabetes-induced cardiac disorder, and septic cardiomyopathy are also summarized. Finally, their therapeutic potential for cardiovascular disease is discussed in each section.

Retinoid Homeostasis and Beyond: How Retinol Binding Protein 4 Contributes to Health and Disease

Retinol binding protein 4 (RBP4) is the specific transport protein of the lipophilic vitamin A, retinol, in blood. Circulating RBP4 originates from the liver. It is secreted by hepatocytes after it has been loaded with retinol and binding to transthyretin (TTR). TTR association prevents renal filtration due to the formation of a higher molecular weight complex. In the circulation, RBP4 binds to specific membrane receptors, thereby delivering retinol to target cells, rendering liver-secreted RBP4 the major mechanism to distribute hepatic vitamin A stores to extrahepatic tissues. In particular, binding of RBP4 to 'stimulated by retinoic acid 6' (STRA6) is required to balance tissue retinoid responses in a highly homeostatic manner. Consequently, defects/mutations in RBP4 can cause a variety of conditions and diseases due to dysregulated retinoid homeostasis and cover embryonic development, vision, metabolism, and cardiovascular diseases. Aside from the effects related to retinol transport, non-canonical functions of RBP4 have also been reported. In this review, we summarize the current knowledge on the regulation and function of RBP4 in health and disease derived from murine models and human mutations.

Adipokine Retinol Binding Protein 4 and Cardiovascular Diseases

The morbidity and mortality of cardiovascular diseases (CVDs) have been increasing year by year all over the world and expanding greatly to the younger population, which becomes the leading causes of death globally that threatens human life safety. Prediction of the occurrence of diseases by using risk related adverse events is crucial for screening and early detection of CVDs. Thus, the discovery of new biomarkers that related to risks of CVDs are of urgent in the field. Retinol-binding protein 4 (RBP4) is a 21-kDa adipokine, mainly secreted by adipocytes. Besides its well-established function in the induction of insulin resistance, it has also been found in recent years to be closely associated with CVDs and other risk factors, such as hypertension, coronary heart disease, heart failure, obesity, and hyperlipidemia. In this review, we mainly focus on the progress of research that establishes the correlation between RBP4 and CVDs and the corresponding major risk factors in recent years.

Retinol-Binding Protein 4 Promotes Cardiac Injury After Myocardial Infarction Via Inducing Cardiomyocyte Pyroptosis Through an Interaction With NLRP3

Background

Acute myocardial infarction (AMI) is one of the leading causes of cardiovascular morbidity and mortality worldwide. Pyroptosis is a form of inflammatory cell death that plays a major role in the development and progression of cardiac injury in AMI. However, the underlying mechanisms for the activation of pyroptosis during AMI are not fully elucidated.

Methods and Results

Here we show that RBP4 (retinol‐binding protein 4), a previous identified proinflammatory adipokine, was increased both in the myocardium of left anterior descending artery ligation‐induced AMI mouse model and in ischemia‐hypoxia‒induced cardiomyocyte injury model. The upregulated RBP4 may contribute to the activation of cardiomyocyte pyroptosis in AMI because overexpression of RBP4 activated NLRP3 (nucleotide‐binding oligomerization domain‐like receptor family pyrin domain‐containing 3) inflammasome, promoted the precursor cleavage of Caspase‐1, and subsequently induced GSDMD (gasdermin‐D)‐dependent pyroptosis. In contrast, knockdown of RBP4 alleviated ischemia‐hypoxia‒induced activation of NLRP3 inflammasome signaling and pyroptosis in cardiomyocytes. Mechanistically, coimmunoprecipitation assay showed that RBP4 interacted directly with NLRP3 in cardiomyocyte, while genetic knockdown or pharmacological inhibition of NLRP3 attenuated RBP4‐induced pyroptosis in cardiomyocytes. Finally, knockdown of RBP4 in heart decreased infarct size and protected against AMI‐induced pyroptosis and cardiac dysfunction in mice.

Conclusions

Taken together, these findings reveal RBP4 as a novel modulator promoting cardiomyocyte pyroptosis via interaction with NLRP3 in AMI. Therefore, targeting cardiac RBP4 might represent a viable strategy for the prevention of cardiac injury in patients with AMI.

Glycosylated apolipoprotein J in cardiac ischaemia: molecular processing and circulating levels in patients with acute ischaemic events

AIM

Using proteomics, we previously found that serum levels of glycosylated (Glyc) forms of apolipoprotein J (ApoJ), a cytoprotective and anti-oxidant protein, decrease in the early phase of acute myocardial infarction (AMI). We aimed to investigate: (i) ApoJ-Glyc intracellular distribution and secretion during ischaemia; (ii) the early changes in circulating ApoJ-Glyc during AMI; and (iii) associations between ApoJ-Glyc and residual ischaemic risk post-AMI.

METHODS AND RESULTS

Glycosylated apolipoprotein J was investigated in: (i) cells from different organ/tissue origin; (ii) a pig model of AMI; (iii) de novo AMI patients (n = 38) at admission within the first 6 h of chest pain onset and without troponin T elevation at presentation (early AMI); (iv) ST-elevation myocardial infarction patients (n = 212) who were followed up for 6 months; and (v) a control group without any overt cardiovascular disease (n = 144). Inducing simulated ischaemia in isolated cardiac cells resulted in an increased intracellular accumulation of non-glycosylated ApoJ forms. A significant decrease in ApoJ-Glyc circulating levels was seen 15 min after ischaemia onset in pigs. Glycosylated apolipoprotein J levels showed a 45% decrease in early AMI patients compared with non-ischaemic patients (P < 0.0001), discriminating the presence of the ischaemic event (area under the curve: 0.934; P < 0.0001). ST-elevation myocardial infarction patients with lower ApoJ-Glyc levels at admission showed a higher rate of recurrent ischaemic events and mortality after 6-month follow-up (P = 0.008).

CONCLUSIONS

These results indicate that ischaemia induces an intracellular accumulation of non-glycosylated ApoJ and a reduction in ApoJ-Glyc secretion. Glycosylated apolipoprotein J circulating levels are reduced very early after ischaemia onset. Its continuous decrease indicates a worsening in the evolution of the cardiac event, likely identifying patients with sustained ischaemia after AMI.

Circulating retinol binding protein 4 levels in coronary artery disease: a systematic review and meta-analysis

BACKGROUND

Retinol binding protein 4 (RBP4) has been proposed to play a role in the pathophysiology of coronary artery disease (CAD), but previous findings on the association of RBP4 levels with CAD are inconsistent.

METHODS

A meta-analysis based on observational studies was conducted to evaluate the association between circulating RBP4 levels and CAD. Databases including PubMed, Web of Science, Embase, Google Scholar and ClinicalTrials.gov database were searched for eligible studies published up to 12 July 2021. Standard mean differences (SMDs) with 95% confidence intervals (CIs) were calculated using the inverse variance heterogeneity (IVhet) and random-effects model for data with moderate and high heterogeneity (I² > 30%) and data with low heterogeneity were analysed using a fixed-effects model (I² ≤ 30%). Moreover, a bias-adjusted quality-effects model was generated, and the prediction interval was also calculated under the random-effects model.

RESULTS

Two nested case-control studies, one cohort study and twelve case-control studies with a total of 7111 participants were included. Circulating RBP4 levels in patients with CAD were comparable to those in the controls under the IVhet model (SMD: 0.25, 95% CI: - 0.29-0.79, I²: 96.00%). The quality-effects model produced consistent results. However, the association turned to be significant under the random-effect model (SMD: 0.46, 95% CI: 0.17-0.75, I²: 96.00%), whereas the 95% predictive interval (PI) included null values (95% PI: - 0.82-1.74). Subgroup analyses illustrated a positive relationship between CAD and RBP4 levels in patients with complications (SMD: 1.34, 95% CI: 0.38-2.29, I²: 96.00%). The meta-regression analysis revealed that the mean BMI of patients (P = 0.03) and complication status (P = 0.01) influenced the variation in SMD.

CONCLUSIONS

There was low-quality evidence that patients with CAD exhibited similar circulating RBP4 levels compared with controls, and high inter-study heterogeneity was also observed. Thus, RBP4 might not be a potential risk factor for CAD. Comparisons among different subtypes of RBP4 with larger sample size are needed in the future.

The Relationship between Retinol-Binding Protein 4 and Markers of Inflammation and Thrombogenesis in Children with Kawasaki Disease

Background

Kawasaki disease (KD) is a self-limited vasculitis with unknown etiologies, and coronary artery lesions (CALs) are the most common and serious complications. Retinol-binding protein 4 (RBP4) has been confirmed effects on vasodilation, platelet activation inhibition, and cardiovascular diseases by researches. Therefore, this study was aimed at investigating the relationship between RBP4 and inflammation as well as thrombogenesis in children with KD.

Methods

79 subjects were from 62 children with KD and 17 healthy controls (HCs). The KD group was divided into KD with CALs (KD-CALs) and KD without CALs (KD-NCALs), and the serum RBP4 levels were measured by enzyme-linked immunosorbent assay (ELISA).

Results

Compared with the HC group, serum RBP4 levels in the KD group were significantly decreased (p < 0.05). RBP4, hemoglobin (Hb), and mean platelet volume (MPV) levels were higher, while platelet counts (Plt) and thrombin time (TT) levels were lower in the KD-NCALs group than in the KD-CALs group (p < 0.05). RBP4 had positive correlation with time point of intravenous immunoglobulin (IVIG), Hb, and percentage of leukomonocytes (L%) and negative correlation with the percentage of neutrophils (N%), MPV, C-reactive protein (CRP), neutrophil-to-lymphocyte ratio (NLR), prothrombin time (PT), fibrinogen (Fbg), and D-dimer (DD) in the KD group; RBP4 had positive correlation with the time point of IVIG and L% and negative correlation with N%, MPV, and NLR in the KD-NCALs group; and RBP4 had positive correlation with Hb and L% and negative correlation with N%, CRP, NLR, and PT in the KD-CALs group (p < 0.05). Multiple linear regression analysis confirmed that Hb and CRP in the KD group, MPV and N% in the KD-NCALs group, and PT and CRP in the KD-CALs group were independent predictors of RBP4 (p < 0.05).

Conclusion

Lower RBP4 was observed in the KD group than in the HC group, and RBP4 had associations with markers of inflammation and thrombogenesis in children with KD.

The role of retinol-binding protein 4 and its relationship with sex hormones in coronary artery disease

The role of retinol-binding protein 4 (RBP4) in patients with coronary artery disease (CAD) with different sexes has not been clearly established. Sex hormones, especially testosterone (T) and estradiol (E2), have been considered to play an important role in CAD. This study aimed to investigate the role of RBP4 and the possible association between RBP4 and T and E2 in CAD. The study included 658 individuals who underwent coronary angiography (CAG); they were assigned to CAD group (n = 440) and controls (n = 218). CAD group was subdivided into three subgroups. Serum RBP4 and T were assayed by enzyme-linked immunosorbent assay. Serum E2 was measured using electrochemiluminescence immunoassay. For men, RBP4 levels were lower in CAD group, especially those with acute myocardial infarction, than in controls (P < 0.05, P < 0.01, respectively). For women, no significant difference was found in RBP4 levels between both groups. RBP4 was positively correlated with T in male patients with CAD (r = 0.124, P < 0.05). Logistic regression analysis showed that RBP4 was a protective factor for CAD (odds ratio 0.975, 95% confidence interval 0.958-0.993; P = 0.007). In conclusion, RBP4 levels were significantly decreased and positively related with T in men with CAD. Higher RBP4 levels were associated with lower risk of CAD. RBP4 may play a potential protective role for CAD among men.

Proteomic Analysis of Urothelium of Rats with Detrusor Overactivity Induced by Bladder Outlet Obstruction

Overactive bladder (OAB) syndrome is a condition that has four symptoms: urgency, urinary frequency, nocturia, and urge incontinence and negatively affects a patient's life. Recently, it is considered that the urinary bladder urothelium is closely linked to pathogenesis of OAB. However, the mechanisms of pathogenesis of OAB at the molecular level remain poorly understood, mainly because of lack of modern molecular analysis. The goal of this study is to identify a potential target protein that could act as a predictive factor for effective diagnosis and aid in the development of therapeutic strategies for the treatment of OAB syndrome. We produced OAB in a rat model and performed the first proteomic analysis on the mucosal layer (urothelium) of the bladders of sham control and OAB rats. The resulting data revealed the differential expression of 355 proteins in the bladder urothelium of OAB rats compared with sham subjects. Signaling pathway analysis revealed that the differentially expressed proteins were mainly involved in the inflammatory response and apoptosis. Our findings suggest a new target for accurate diagnosis of OAB that can provide essential information for the development of drug treatment strategies as well as establish criteria for screening patients in the clinical environment.

Protein changes in non-LDL-lipoproteins in familial hypercholesterolemia: implications in cardiovascular disease manifestation and outcome

PURPOSE OF REVIEW

Familial hypercholesterolemia, represents one of the most extreme clinical entities associated with premature coronary artery disease (CAD). However, clinical manifestation of CAD varies across cohorts and individual patients suggesting the existence of additional non-LDL factors potentially contributing to their cardiovascular burden.

RECENT FINDINGS

Changes in HDL-associated proteins appear as one of the potential additional factors contributing to the cardiovascular risk in familial hypercholesterolemia. Specifically, the content of Apo M-SP1 in HDL3 has been directly associated with cholesterol efflux capacity. In addition, a coordinated decrease in the content of Apo L1 and LCAT in HDL3 has been related to the presence of corneal arcus and to bad prognosis in familial hypercholesterolemia patients after an acute ischemic event. In fact, HDL3 particles of familial hypercholesterolemia patients have diminished antioxidant and anti-inflammatory function.

SUMMARY

The identification of the specific changes in HDL-associated proteins that contribute to the increased cardiovascular risk of familial hypercholesterolemia patients could be useful for the development of novel therapeutic targets. These novel strategies, in combination with current lipid-lowering therapies, may help to reduce the residual risk found in these patients.

Detrimental Effect of Hypercholesterolemia on High-Density Lipoprotein Particle Remodeling in Pigs

BACKGROUND

Beneficial effects of high-density lipoproteins (HDL) seem altered in patients with symptomatic cardiovascular disease. We recently demonstrated in a swine model of ischemia-reperfusion (IR) that hypercholesterolemia abolishes HDL-related cardioprotection.

OBJECTIVES

This study sought to investigate, using the same animal model, whether the reported impairment of HDL cardioprotective function was associated with alterations in HDL remodeling and functionality.

METHODS

Pigs were fed a normocholesterolemic (NC) or hypercholesterolemic (HL) diet for 10 days, reaching non-HDL cholesterol concentrations of 38.2 ± 3.5 mg/dl and 218.6 ± 27.6 mg/dl, respectively (p < 0.0001). HDLs were isolated, and lipidomics and differential proteomics tests were performed to determine HDL molecular changes. HDL functionality and particle size were determined.

RESULTS

Using principal component analysis, we identified 255 molecular lipid species differentially clustered in NC-HDL and HL-HDL. Ninety lipid metabolites were differentially expressed, and 50 showed at least 1.5-fold variation (false discovery rate adjustment q value <0.05). HL-HDLs presented a core enriched in cholesteryl esters and a surface depleted of phosphatidylcholine species containing polyunsaturated and long-chain fatty acids, indicating the presence of mature HDL particles with low surface fluidity. Hypercholesterolemia induced an important change in HDL-transported proteins (576 spots in HL-HDL vs. 621 spots in NC-HDL). HL-HDLs showed a reduced content of lipocalin retinol binding protein 4 and apolipoprotein M and in the retinoic acid-transporter cellular retinoic acid binding protein 1 (p < 0.05 vs. NC-HDL). No changes were observed in apolipoprotein A-I content and profile. Functionally, HL-HDL showed lower antioxidant activity (-35%) and a reduced capacity to efflux cholesterol (-60%) compared to NC-HDL (p < 0.05). Hypercholesterolemia induced larger HDL particles.

CONCLUSIONS

We demonstrate that hypercholesterolemia induces HDL lipidomic changes, losing phosphatidylcholine-lipid species and gaining cholesteryl esters, and proteomic changes, with losses in cardioprotective proteins. These remodeling changes shifted HDL particles toward a dysfunctional state.

Erythrocyte-heme proteins and STEMI: implications in prognosis

The role of erythrocytes in thrombus formation has been often neglected, but some studies have highlighted their active role in thrombotic events. Free-haemoglobin (Hb) has shown to induce oxidative-stress damage. Herein we have investigated the coordinated changes in heme-related proteins in patients with acute-coronary-syndromes (ACS), their association to ongoing thrombosis and their impact on patients' prognosis. The serum proteome of STEMI-patients (N=27) within the first 6h after event-onset and 3d after were compared to controls (N=60). Changes in heme-metabolism were characterized in a second STEMI-group by a dual proteomic approach analyzing in-vivo aspirated coronary thrombi at PCI (N=24) and the associated peripheral-blood changes (N=10). A third STEMI-group (N=132) was studied to analyze the impact of the observed changes in prognosis at 6-months-follow-up. The haptoglobin/hemopexin(Hpg/Hpx)-scavenging-system revealed a time-dependent response after STEMI with an early increase in Hpg circulating levels in the acute phase (P=0.01) and a late increase in Hpx levels 3d after (P=0.045). Beta-Hb content in coronary thrombi was directly correlated with systemic beta-Hb and Hpg (R=0.804,P=0.0029; R=0.859,P=0.0007) levels. The presence of a fully-occlusive thrombus was associated to higher circulating levels of beta-Hb (P=0.03) and unbound-Hpg (P=0.03). ELISA validation demonstrated a decreased survival rate at 6-months follow-up in STEMI-patients with lower Hpg plasma levels at admission (P=0.027). Our results show active Hb-release form erythrocytes in ACS. This release is followed by a systemic early increase in Hpg levels and a late increase in Hpx levels that can co-ordinately help to prevent systemic pro-oxidative effects. The Hb-scavenging ability of haptoglobin is related to patients' prognosis.

Inflammation and hemostasis in older octogenarians: implication in 5-year survival

Social changes and medical advances have increased longevity, but the conditions governing healthy vs unhealthy cardiovascular (CV) aging are not fully known. Factors beyond classical CV risk factors may have an important unrecognized value. We sought to identify proteins differentially expressed in healthy octogenarians (HOs) without a history of cardiovascular disease (CVD) and preserved functional and cognitive state compared with octogenarians with a history of CVD and cognitive decline (UHOs) using a systems biology approach, and investigated how these proteins relate to CV mortality at 5-year follow-up. Plasmas obtained from older octogenarians (87 ± 0 years) were analyzed by 2-DE + MS and bioinformatic pathway analysis in HOs (N = 38) and UHOs with cognitive (MEC<25) and functional (Barthel<90) decline and a previous ischemic event (acute myocardial infarction and/or stroke; N = 27). Results were validated by ELISA in HOs and UHOs and in an additional group of older octogenarians without cognitive impairment but with a previous CVD manifestation (HO-CVD; N = 35). UHOs showed a coordinated change in several inflammation-related proteins (AMBP, RBP4, and ITIH4; P < 0.05), together with a significant increase in the major inducer of the acute-phase reaction, interleukin-6 (P = 0.03). UHOs also showed a coordinated increase in hemostatic proteins that was associated with an impairment of fibrinolysis and an increased 5-year CV mortality (P = 0.003). The combination of inflammation (ITIH4 and interleukin-6) and hemostatic markers (D-dimer, A2AP, and coagulation factor XIII) was able to discriminate the presence of an unhealthy phenotype in the elderly (AUC = 0.750; P = 0.001). Unhealthy older octogenarians show increased levels of several plasma proteins of inflammation and coagulation. In older octogenarians, the increase in hemostatic markers indicated an increase in 5-year CV mortality at follow-up.

Identifying new Risk Markers and Potential Targets: The Value of the Proteome

Several protein biomarkers, including cardiac troponin T, cardiac troponin I, B-type natriuretic peptide, C-reactive protein and apolipoprotein A-I, are widely employed in the evaluation of cardiovascular disease. Several of such potential biomarkers, or their multiscores, have been assessed over the last years for the prediction of cardiovascular risk but only a few of them have been validated for clinical use. Substantial improvement in the cardiovascular risk prediction and reclassification relative to traditional models therefore remains a difficult task presently unresolved. Hence, a potential importance of alternative approaches which may rely on novel proteomic biomarkers among others. Plasma or serum concentrations of numerous proteins were measured using proteomic approaches to establish their relationships with cardiovascular disease; none of them was however evaluated for cardiovascular risk prediction and subject stratification in rigorous large-scale studies. Thus, further research is needed to identify novel candidates that can improve cardiovascular risk prediction, subject stratification and standard care. Proteomics will undoubtedly remain a key approach to address this major clinical and scientific challenge.

Retinol-Binding Protein 4 Induces Cardiomyocyte Hypertrophy by Activating TLR4/MyD88 Pathway

Insulin resistance plays a major role in the development and progression of cardiac hypertrophy and heart failure. Heart failure in turn promotes insulin resistance and increases the risk for diabetes. The vicious cycle determines significant mortality in patients with heart failure and diabetes. However, the underlying mechanisms for the vicious cycle are not fully elucidated. Here we show that circulating levels and adipose expression of retinol-binding protein 4 (RBP4), an adipokine that contributes to systemic insulin resistance, were elevated in cardiac hypertrophy induced by transverse aortic constriction and angiotensin-II (Ang-II) infusion. Ang-II increased RBP4 expression in adipocytes, which was abolished by losartan, an Ang-II receptor blocker. The elevated RBP4 in cardiac hypertrophy may have pathophysiological consequences because RBP4 increased cell size, enhanced protein synthesis, and elevated the expression of hypertrophic markers including Anp, Bnp, and Myh7 in primary cardiomyocytes. Mechanistically, RBP4 induced the expression and activity of toll-like receptor 4 (TLR4) and myeloid differentiation primary response gene 88 (MyD88) in cardiomyocytes, resulting in enhanced inflammation and reactive oxygen species production. Inhibition or knockdown of the TLR4/MyD88 pathway attenuated inflammatory and hypertrophic responses to RBP4 stimulation. Importantly, RBP4 also reduced the expression of glucose transporter-4 and impaired insulin-stimulated glucose uptake in cardiomyocytes. This impairment was ameliorated in cardiomyocytes from TLR4 knockout mice. Therefore, RBP4 may be a critical modulator promoting the vicious cycle of insulin resistance and heart failure by activating TLR4/MyD88-mediated inflammatory pathways. Potentially, lowering RBP4 might break the vicious cycle and improve both insulin resistance and cardiac hypertrophy.

Retinol binding protein 4 in relation to diet, inflammation, immunity, and cardiovascular diseases

Retinol binding protein 4 (RBP4), previously called retinol binding protein (RBP), is considered a specific carrier of retinol in the blood. It is also an adipokine that has been implicated in the pathophysiology of insulin resistance. RBP4 seems to be correlated with cardiometabolic markers in inflammatory chronic diseases, including obesity, type 2 diabetes, metabolic syndrome, and cardiovascular diseases (CVDs). It has recently been suggested that inflammation produced by RBP4 induces insulin resistance and CVD. The clinical relevance of this hypothesis is discussed in this review. Knowledge concerning the association of RBP4 with inflammation markers, oxidative stress, and CVDs as well as concerning the role of diet and antioxidants in decreasing RBP4 concentrations are discussed. Special attention is given to methodologies used in previously published studies and covariates that should be controlled when planning new studies on this adipokine.

7keto-stigmasterol and 7keto-cholesterol induce differential proteome changes to intestinal epitelial (Caco-2) cells

Recent studies have expanded the appreciation of the roles of oxysterols triggering inflammatory, immune cytotoxic and apoptotic processes, but have not been considered for proteome analysis. A comparative proteomic study in intestinal epithelial cell cultures incubated (60 μM/24 h) with 7keto-cholesterol or 7keto-stigmasterol was performed. The influence of both compounds was studied following the nLC-TripleTOF analysis. Findings were compared to results for control cultures. In the principal component analysis (PCA) of proteome patterns, two components were extracted accounting for 99.8% of the variance in the protein expression. PCA analysis clearly discriminated between the perturbations in the proteome of cell cultures incubated with 7keto-cholesterol and 7keto-stigmasterol. These proteins participate in mitochondrial function, lipid homeostasis, inflammation and immunity and cell proliferation. Remarkable differences between proteome patterns in cell cultures exposed to 7keto-cholesterol and 7keto-stigmasterol affect macrophage migration inhibitory factor, apolipoprotein E, Bcl-2-associated transcription factor and cellular retinoic acid-binding protein. Besides, exposure to 7keto-stigmasterol increased the concentration of ubiquitin-conjugating enzyme E2 and the mitochondrial superoxide dismutase protein. Such findings raise new questions about safety studies and the regulatory potential of oxysterols in the differentiation and function of intestinal and associated immune cells, their response to environmental stimuli and impairment of absorption processes.

Understanding different facets of cardiovascular diseases based on model systems to human studies: a proteomic and metabolomic perspective

Cardiovascular disease has remained as the largest cause of morbidity and mortality worldwide. From dissecting the disease aetiology to identifying prognostic markers for better management of the disease is still a challenge for researchers. In the post human genome sequencing era much of the thrust has been focussed towards application of advanced genomic tools along with evaluation of traditional risk factors. With the advancement of next generation proteomics and metabolomics approaches it has now become possible to understand the protein interaction network & metabolic rewiring which lead to the perturbations of the disease phenotype. Further, elucidating different post translational modifications using advanced mass spectrometry based methods have provided an impetus towards in depth understanding of the proteome. The past decade has observed a plethora of studies where proteomics has been applied successfully to identify potential prognostic and diagnostic markers as well as to understand the disease mechanisms for various types of cardiovascular diseases. In this review, we attempted to document relevant proteomics based studies that have been undertaken either to identify potential biomarkers or have elucidated newer mechanistic insights into understanding the patho-physiology of cardiovascular disease, primarily coronary artery disease, cardiomyopathy, and myocardial ischemia. We have also provided a perspective on the potential of proteomics in combating this deadly disease.

BIOLOGICAL SIGNIFICANCE

This review has catalogued recent studies on proteomics and metabolomics involved in understanding several cardiovascular diseases (CVDs). A holistic systems biology based approach, of which proteomics and metabolomics are two very important components, would help in delineating various pathways associated with complex disorders like CVD. This would ultimately provide better mechanistic understanding of the disease biology leading to development of prognostic biomarkers. This article is part of a Special Issue entitled: Proteomics in India.

Retinol-binding protein 4 and its potential roles in hypercholesterolemia revealed by proteomics

Effects of hypercholesterolemia on alterations of serum proteins have not been fully elucidated. Herein, using two-dimensional gel electrophoresis (2-DE) in conjunction with LC-MS searching has successfully been carried out to investigate the change of protein expression profiles as consequences of raised blood cholesterol at different levels (normal group: total cholesterol 200 mg/dL; borderline high group: total cholesterol 200-239 mg/dL; and high group: total cholesterol ≥ 240 mg/dL) (n = 45). Results revealed that down-regulation of retinol-binding protein 4 (RBP4) (-2.26 fold), transthyretin (-1.25 fold) and gelsolin (-1.47 fold) was observed in the high group. Meanwhile, the other proteins such as haptoglobin, complement factor B and CD5 antigen-like protein were up-regulated upto +3.24, +1.96 and +2.04 fold, respectively. Confirmation by Western blotting revealed a significant reduction of RBP4 (approximately 50 %) in individual samples derived from the high group. Presumptive conclusion can be drawn that down-regulation of RBP4 might be attributable to the inflammation of adipocytes caused by the release of proinflammatory cytokines (e.g. tumor necrosis factor α and interleukin-1β) from adipose tissues. Moreover, the decrease of transthyretin might also be taken into accounts since it is known that the transthyretin usually forms complex with RBP4 to prevent glomerular filtration and excretion through the kidney. The suppressing effect on RBP4 should be potentiated by the increase of complement factor B and CD5 antigen-like protein, which rendered the adipose tissues to overwhelm the liberation of RBP4 to blood circulation by metabolic and inflammatory processes. Such inflammation could further modulate the induction of cytokine release (e.g. IL-6 and IL-1β), resulting in the synthesis of acute phase protein, in particular, haptoglobin and C-reactive proteins from hepatocytes. However, the mechanism of gelsolin reduction remains unclear. Among these differentially expressed proteins, the RBP4 has been proposed as a major linkage between hypercholesterolemia, adipose tissues, liver and kidney, which is believed to be a potential biomarker for metabolic and cardiovascular disorders associated with dyslipidemia in the future.

Glycoproteome of human apolipoprotein A-I: N- and O-glycosylated forms are increased in patients with acute myocardial infarction

High-density lipoprotein (HDL) functionality, which is closely associated with its composition and transport capabilities, determines its role in atheroprotection. During acute phase processes, HDL seems to lose its anti-inflammatory and cytoprotective properties. In this study, we hypothesized that after an acute myocardial infarction apolipoprotein (Apo) A-I, the main protein component of HDL, might undergo changes in its molecular processing. Therefore, we have characterized the Apo A-I proteome during the evolution of new-onset acute myocardial infarction (AMI). To this end, serum Apo A-I was characterized by 2-dimensional electrophoresis/mass-spectrometry in controls and AMI patients at admission (within the first 6 hours after pain onset) and 8 hours, 16 hours, 24 hours, and 3 days afterward. The Apo A-I glycoproteome was analyzed by lectin-based glycoprotein isolation methods and deglycosylation assays, and Apo A-I serum levels were evaluated by enzyme-linked immunosorbent assay (ELISA). The Apo A-I proteomic signature (5 spots: 28 kDa/pI:5-5.75) was significantly altered in AMI patients 3 days after the event with respect to controls. Increased levels of N- and O-glycosylated Apo A-I forms were found post-AMI. Apo A-I serum levels measured by ELISA were significantly changed and related to left ventricular ejection fraction, troponin-T, and C-reactive protein. The Apo A-I molecule measured by ELISA corresponded to the main glycosylated spots and was specifically O-GlcNAcylated in AMI patients. Therefore, our results demonstrate that Apo A-I is both N- and O-glycosylated and that there is an increase in Apo A-I glycosylation after AMI. Furthermore, the specific increase in the O-GlcNAcylated forms could have a relevant prognostic value and a protective role in the evolution of AMI.

Serum levels of retinol-binding protein-4 are associated with the presence and severity of coronary artery disease

Background

The interplay between the novel adipokine retinol-binding protein-4 (RBP4) and coronary artery disease (CAD) is still obscure. We investigated the relationship between RBP4 levels and the presence and severity of angiographically proven CAD and determined its possible role in acute myocardial infarction (AMI).

Methods

305 individuals with angiographically proven CAD (CAD-patients), were classified into 2 subgroups: 1) acute myocardial infarction (AMI, n = 141), and 2) stable angina (SA, n = 164). Ninety-one age- and sex-matched individuals without CAD, but with at least 2 classical cardiovascular risk factors, served as controls (non-CAD group). RBP4 serum levels were measured at hospital admission and were analyzed in relation to the coronary severity stenosis, assessed by the Gensini-score and the number of coronary narrowed vessels. Other clinical parameters, including insulin levels, HOMA-IR, hsCRP, glycaemic and lipid profile, and left-ventricular ejection fraction were also assessed.

Results

Serum RBP4 levels were significantly elevated in patients with CAD compared to non-CAD patients (39.29 ± 11.72 mg/L vs. 24.83 ± 11.27 mg/L, p < 0.001). We did not observe a significant difference in RBP4 levels between AMI and SA subgroups (p = 0.734). Logistic regression analysis revealed an independent association of CAD presence with serum RBP4 (β = 0.163, p = 0.006), and hsCRP (β = 0.122, p = 0.022) levels, in the whole study group. Among variables, hsCRP (β = 0.220), HDL (β = −0.150), and RBP4 (β = 0.297), correlated in both univariate and multivariate analysis with CAD severity (R² = 0.422, p < 0.001). Similarly, RBP4 concentrations increased with the number of coronary narrowed vessels (p < 0.05).

Conclusion

Patients with CAD, both SA and AMI, showed elevated RBP4 serum levels. Notably, increased RBP4 concentration seemed to independently correlate with CAD severity, but no with AMI.

Trial registration

The ClinicalTrials.gov Identifier is: NCT00636766

Increased RBP4 in a human model of activated anti-atherosclerotic and antiremodelling defences

BACKGROUND

Both increased and decreased levels of the adipokine retinol-binding protein 4 (RBP4) have been reported in cardiovascular disease, and levels of RBP4 have been related to diabetes, metabolic syndrome and cardiovascular risk. Recently, clear in vitro and ex vivo vasodilatory and inhibitory of platelet activation effects of RBP4 has been shown and a reduced RBP4 level was found in high cardiovascular risk patients, suggesting a potential cardiovascular protective role for increased levels of RBP4.

PATIENTS AND METHODS

Plasma level of RBP4 (ELISA) was determined in a cohort of Bartter's and Gitelman's syndrome (BS/GS) patients, a human model of endogenous Ang II signalling antagonism and activation of anti-atherosclerotic and antiremodelling defenses, the opposite of cardiovascular risk patients, and in healthy normotensive subjects. Haem Oxygenase (OH)-1 protein level (sandwich immunoassay) as a potential mediator of RBP4 stimulation of PI3K/Akt pathway and flow-mediated dilation (FMD) as a measure of endothelium (NO)-dependent response have also been measured.

RESULTS

RBP4 in BS/GS patients (40·59 ± 15·32 μg/mL vs. 25·05 ± 5·56, P = 0·011) along with HO-1 protein levels (9·44 ± 3·09 ng/mL vs. 5·49 ± 1·04, P = 0·003) and FMD (10·52% ± 2·22 vs. 7·99 ± 1·13 P = 0·006) were significantly increased compared with healthy normotensive subjects.

CONCLUSIONS

The increase of RBP4 in BS/GS, a human model of endogenous Ang II signalling antagonism and activation of anti-atherosclerotic and antiremodelling defenses, the opposite of cardiovascular risk patient, found in concert with an increased NO-mediated vasodilation and HO-1 levels supports a protective role for this adipokine in vascular protection/cardiovascular risk.