Carbamylated low-density lipoprotein induces proliferation and increases adhesion molecule expression of human coronary artery smooth muscle cells

Protein Carbamylation and the Risk of ESKD in Patients with CKD

SIGNIFICANCE STATEMENT

Protein carbamylation, a nonenzymatic post-translational protein modification partially driven by elevated blood urea levels, associates with mortality and adverse outcomes in patients with ESKD on dialysis. However, little is known about carbamylation's relationship to clinical outcomes in the much larger population of patients with earlier stages of CKD. In this prospective observational cohort study of 3111 individuals with CKD stages 2-4, higher levels of carbamylated albumin (a marker of protein carbamylation burden) were associated with a greater risk of developing ESKD and other significant adverse clinical outcomes. These findings indicate that protein carbamylation is an independent risk factor for CKD progression. They suggest that further study of therapeutic interventions to prevent or reduce carbamylation is warranted.

BACKGROUND

Protein carbamylation, a post-translational protein modification partially driven by elevated blood urea levels, associates with adverse outcomes in ESKD. However, little is known about protein carbamylation's relationship to clinical outcomes in the much larger population of patients with earlier stages of CKD.

METHODS

To test associations between protein carbamylation and the primary outcome of progression to ESKD, we measured baseline serum carbamylated albumin (C-Alb) in 3111 patients with CKD stages 2-4 enrolled in the prospective observational Chronic Renal Insufficiency Cohort study.

RESULTS

The mean age of study participants was 59 years (SD 10.8); 1358 (43.7%) were female, and 1334 (42.9%) were White. The mean eGFR at the time of C-Alb assessment was 41.8 (16.4) ml/minute per 1.73 m 2 , and the median C-Alb value was 7.8 mmol/mol (interquartile range, 5.8-10.7). During an average of 7.9 (4.1) years of follow-up, 981 (31.5%) individuals developed ESKD. In multivariable adjusted Cox models, higher C-Alb (continuous or quartiles) independently associated with an increased risk of ESKD. For example, compared with quartile 1 (C-Alb ≤5.80 mmol/mol), those in quartile 4 (C-Alb >10.71 mmol/mol) had a greater risk for ESKD (adjusted hazard ratio, 2.29; 95% confidence interval, 1.75 to 2.99), and the ESKD incidence rate per 1000 patient-years increased from 15.7 to 88.5 from quartile 1 to quartile 4. The results remained significant across numerous subgroup analyses, when treating death as a competing event, and using different assessments of eGFR.

CONCLUSIONS

Having a higher level of protein carbamylation as measured by circulating C-Alb is an independent risk factor for ESKD in individuals with CKD stages 2-4.

PODCAST

This article contains a podcast at https://dts.podtrac.com/redirect.mp3/www.asn-online.org/media/podcast/JASN/2023_04_24_JSN_URE_EP22_042423.mp3.

Carbamylated lipoproteins in diabetes

Diabetic dyslipidemia is characterized by quantitative and qualitative abnor-malities in lipoproteins. In addition to glycation and oxidation, carbamylation is also a post-translational modification affecting lipoproteins in diabetes. Patients with type 2 diabetes (T2D) exhibit higher levels of carbamylated low-density lipoproteins (cLDL) and high-density lipoproteins (cHDL). Accumulating evidence suggests that cLDL plays a role in atherosclerosis in diabetes. cLDL levels have been shown to predict cardiovascular events and all-cause mortality. cLDL facilitates immune cell recruitment in the vascular wall, promotes accumulation of lipids in macrophages, and contributes to endothelial dysf-unction, endothelial nitric oxide-synthase (eNOS) inactivation and endothelial repair defects. Lastly, cLDL induces thrombus formation and platelet aggregation. On the other hand, recent data have demonstrated that cHDL serum level is independently associated with all-cause and cardiovascular-related mortality in T2D patients. This relationship may be causative since the atheroprotective properties of HDL are altered after carbamylation. Thus, cHDL loses the ability to remove cholesterol from macrophages, to inhibit monocyte adhesion and recruitment, to induce eNOS activation and to inhibit apoptosis. Taken together, it seems very likely that the abnormalities in the biological functions of LDL and HDL after carbamylation contribute to atherosclerosis and to the elevated cardiovascular risk in diabetes.

Avenues for post-translational protein modification prevention and therapy

Non-enzymatic post-translational modifications (nPTMs) of proteins have emerged as novel risk factors for the genesis and progression of various diseases. We now have a variety of experimental and established therapeutic strategies to target harmful nPTMs and potentially improve clinical outcomes. Protein carbamylation and glycation are two common and representative nPTMs that have gained considerable attention lately as favorable therapeutic targets with emerging clinical evidence. Protein carbamylation is associated with the occurrence of cardiovascular disease (CVD) and mortality in patients with chronic kidney disease (CKD); and advanced glycation end products (AGEs), a heterogeneous group of molecules produced in a series of glycation reactions, have been linked to various diabetic complications. Therefore, reducing the burden of protein carbamylation and AGEs is an appealing and promising therapeutic approach. This review chapter summarizes potential anti-nPTM therapy options in CKD, CVD, and diabetes along with clinical implications. Using two prime examples-protein carbamylation and AGEs-we discuss the varied preventative and therapeutic options to mitigate these pathologic nPTMs in detail. We provide in-depth case studies on carbamylation in the setting of kidney disease and AGEs in metabolic disorders, with an emphasis on the relevance to reducing adverse clinical outcomes such as CKD progression, cardiovascular events, and mortality. Overall, whether specific efforts to lower carbamylation and AGE burden will yield definitive clinical improvement in humans remains largely to be seen. However, the scientific rationale for such pursuits is demonstrated herein.

Carbamylated Proteins in Renal Disease: Aggravating Factors or Just Biomarkers?

Carbamylation is a nonenzymatic post-translational modification resulting from the reaction between cyanate, a urea by-product, and proteins. In vivo and in vitro studies have demonstrated that carbamylation modifies protein structures and functions, triggering unfavourable molecular and cellular responses. An enhanced formation of carbamylation-derived products (CDPs) is observed in pathological contexts, especially during chronic kidney disease (CKD), because of increased blood urea. Significantly, studies have reported a positive correlation between serum CDPs and the evolutive state of renal failure. Further, serum concentrations of carbamylated proteins are characterized as strong predictors of mortality in end-stage renal disease patients. Over time, it is likely that these modified compounds become aggravating factors and promote long-term complications, including cardiovascular disorders and inflammation or immune system dysfunctions. These poor clinical outcomes have led researchers to consider strategies to prevent or slow down CDP formation. Even if growing evidence suggests the involvement of carbamylation in the pathophysiology of CKD, the real relevance of carbamylation is still unclear: is it a causal phenomenon, a metabolic consequence or just a biological feature? In this review, we discuss how carbamylation, a consequence of renal function decline, may become a causal phenomenon of kidney disease progression and how CDPs may be used as biomarkers.

Impact of Posttranslational Modification in Pathogenesis of Rheumatoid Arthritis: Focusing on Citrullination, Carbamylation, and Acetylation

Rheumatoid arthritis (RA) is caused by prolonged periodic interactions between genetic, environmental, and immunologic factors. Posttranslational modifications (PTMs) such as citrullination, carbamylation, and acetylation are correlated with the pathogenesis of RA. PTM and cell death mechanisms such as apoptosis, autophagy, NETosis, leukotoxic hypercitrullination (LTH), and necrosis are related to each other and induce autoantigenicity. Certain microbial infections, such as those caused by Porphyromonasgingivalis, Aggregatibacter actinomycetemcomitans, and Prevotella copri, can induce autoantigens in RA. Anti-modified protein antibodies (AMPA) containing anti-citrullinated protein/peptide antibodies (ACPAs), anti-carbamylated protein (anti-CarP) antibodies, and anti-acetylated protein antibodies (AAPAs) play a role in pathogenesis as well as in prediction, diagnosis, and prognosis. Interestingly, smoking is correlated with both PTMs and AMPAs in the development of RA. However, there is lack of evidence that smoking induces the generation of AMPAs.

Carbamylation of elastic fibers is a molecular substratum of aortic stiffness

Because of their long lifespan, matrix proteins of the vascular wall, such as elastin, are subjected to molecular aging characterized by non-enzymatic post-translational modifications, like carbamylation which results from the binding of cyanate (mainly derived from the dissociation of urea) to protein amino groups. While several studies have demonstrated a relationship between increased plasma concentrations of carbamylated proteins and the development of cardiovascular diseases, molecular mechanisms explaining the involvement of protein carbamylation in these pathological contexts remain to be fully elucidated. The aim of this work was to determine whether vascular elastic fibers could be carbamylated, and if so, what impact this phenomenon would have on the mechanical properties of the vascular wall. Our experiments showed that vascular elastin was carbamylated in vivo. Fiber morphology was unchanged after in vitro carbamylation, as well as its sensitivity to elastase degradation. In mice fed with cyanate-supplemented water in order to increase protein carbamylation within the aortic wall, an increased stiffness in elastic fibers was evidenced by atomic force microscopy, whereas no fragmentation of elastic fiber was observed. In addition, this increased stiffness was also associated with an increase in aortic pulse wave velocity in ApoE^-/- mice. These results provide evidence for the carbamylation of elastic fibers which results in an increase in their stiffness at the molecular level. These alterations of vessel wall mechanical properties may contribute to aortic stiffness, suggesting a new role for carbamylation in cardiovascular diseases.

Proatherogenic Importance of Carbamylation-induced Protein Damage and Type 2 Diabetes Mellitus: A Systematic Review

INTRODUCTION & BACKGROUND

Protein carbamylation is a non-enzymatic and irreversible posttranslational process. It affects functions of numerous enzymes, hormones and receptors playing several roles in diabetes pathogenesis by changing their native structures. Detrimental consequences of oxidative protein damage comprise, but are not limited to glyoxidation, lipoxidation and carbonylation reactions. Since the carbamylated plasma proteins are strongly related to the glycemic control parameters of diabetes, they may have an additive value and emerge as potential biomarkers for the follow up, prognosis and treatment of diabetes mellitus.

METHODS & RESULTS

To conduct our systematic review, we used PubMed and Semantic Scholar, and used 'Protein carbamylation and diabetes' and 'Protein carbamylation and atherosclerosis' as keywords and looked into about five hundred manuscripts. Manuscripts that are not in English were excluded as well as manuscripts that did not mention carbamylation to maintain the focus of the present article. Similar to glycation, carbamylation is able to alter functions of plasma proteins and their interactions with endothelial cells and has been shown to be involved in the development of atherosclerosis.

CONCLUSION

At this stage, it seems clear that protein carbamylation leads to worse clinical outcomes. To improve patient care, but maybe more importantly to improve healthcare-prevention, we believe the next stage involves understanding how exactly protein carbamylation leads to worse outcomes and when and in what group of people anti-carbamylation therapies must be employed.

Usefullnes of atherogenic indices and Ca-LDL level to predict subclinical atherosclerosis in patients with psoriatic arthritis?

BACKGROUND

To investigate the link between carbamylated low-density lipoprotein (ca-LDL), atherogenic index of plasma (AIP), atherogenic coefficient (AC), Castelli's risk indices I and II (CRI I and II) and subclinic atherosclerosis in psoriatic arthritis (PsA).

METHODS

Thirty-ninepatients and 19 age, sex, body mass index matched healthy controls were included. Insulin resistance (IR) was assessed with homeostasis of model assessment-IR (HOMA-IR). Carotid intima-media thickness (CIMT) was measured at both common carotid arteries and mean CIMT was calculated.

RESULTS

The mean age was 49.50 ± 11.86 years and 64.1% were females in PsA group. In the PsA group, CIMT and HOMA-IR were significantly higher (p = 0.003, p = 0.043, respectively). AIP, AC, TG/HDL, CRI-1, CRI-2 and ca-LDL levels were similar between groups. In PsA group, CIMT was positively correlated with HOMA-IR, TG/HDL and AIP. Although ca-LDL was positively correlated with serum amyloid A (r = 0.744, p < 0.001), no correlation was detected between ca-LDL and CIMT (r = 0.215, p = 0.195). PsA patients with IR tended to have higher ca-LDL levels than patients without IR, but this difference lacked statistical significance (33.65 ± 26.94, 28.63 ± 28.06, respectively, p = 0.237).

CONCLUSIONS

A significant increase in CIMT was seen in PsA patients without clinically evident cardiovascular disease or any traditional atherosclerosis risk factors. CIMT was correlated with HOMA-IR, TG/HDL and AIP.

The Role of Nonenzymatic Post-translational Protein Modifications in Uremic Vascular Calcification

Considerable technological advances have enabled the identification and linkage of nonenzymatic post-translationally modified proteins to the pathogenesis of cardiovascular disease (CVD) in patients with kidney failure. Through processes such as the nonenzymatic carbamylation reaction as well as the formation of advanced glycation end products, we now know that protein modifications are invariably associated with the development of CVD beyond a mere epiphenomenon and this has become an important focus of nephrology research in recent years. Although the specific mechanisms by which protein modifications occurring in kidney failure that may contribute to CVD are diverse and include pathways such as inflammation and fibrosis, vascular calcification has emerged as a distinct pathological sequelae of protein modifications. In this review, we consider the biological mechanisms and clinical relevance of protein carbamylation and advanced glycation end products in CVD development with a focus on vascular calcification.

New Insights into the Roles of Monocytes/Macrophages in Cardiovascular Calcification Associated with Chronic Kidney Disease

Cardiovascular disease (CVD) is an important cause of death in patients with chronic kidney disease (CKD), and cardiovascular calcification (CVC) is one of the strongest predictors of CVD in this population. Cardiovascular calcification results from complex cellular interactions involving the endothelium, vascular/valvular cells (i.e., vascular smooth muscle cells, valvular interstitial cells and resident fibroblasts), and monocyte-derived macrophages. Indeed, the production of pro-inflammatory cytokines and oxidative stress by monocyte-derived macrophages is responsible for the osteogenic transformation and mineralization of vascular/valvular cells. However, monocytes/macrophages show the ability to modify their phenotype, and consequently their functions, when facing environmental modifications. This plasticity complicates efforts to understand the pathogenesis of CVC-particularly in a CKD setting, where both uraemic toxins and CKD treatment may affect monocyte/macrophage functions and thereby influence CVC. Here, we review (i) the mechanisms by which each monocyte/macrophage subset either promotes or prevents CVC, and (ii) how both uraemic toxins and CKD therapies might affect these monocyte/macrophage functions.

Serum Levels of Carbamylated LDL and Soluble Lectin-Like Oxidized Low-Density Lipoprotein Receptor-1 Are Associated with Coronary Artery Disease in Patients with Metabolic Syndrome

Background and objectives: Lectin-like oxidized low density lipoprotein receptor-1 (LOX-1) has been recognized as the primary receptor for carbamylated low-density lipoproteins (cLDL) and is increasingly being viewed as a critical mediator of vascular inflammation and atherosclerosis. The aim of the current study was to evaluate the possible role of circulating cLDL and soluble LOX-1 (sLOX-1) as potential biomarkers of metabolic syndrome (MetS) as well as of coronary artery disease (CAD) among MetS patients. Materials and Methods: The serum levels of cLDL and sLOX-1 were measured by ELISA in 30 MetS patients without CAD, 30 MetS patients with CAD, and 30 healthy controls. Results: Patients with MetS had significantly higher serum levels of both cLDL and sLOX-1 than the healthy controls but lower in comparison to MetS + CAD subjects. Serum sLOX-1 concentration correlated significantly with fasting glucose (rs = 0.414, p = 0.001) and high-density lipoprotein (HDL)-cholesterol (rs = -0.273, p = 0.035) in the whole MetS cohort, whereas it correlated with cLDL only in the MetS + CAD subgroup (rs = 0.396, p = 0.030). The receiver-operating characteristic (ROC) curves of cLDL and sLOX-1 for MetS diagnosis had area under the curve (AUC) values of 0.761 and 0.692, respectively. AUC values of cLDL and sLOX-1 for CAD diagnosis among MetS patients were 0.811 and 0.739. Elevated serum levels of cLDL and sLOX-1 were associated with a higher risk of MetS development [odds ratio (OR) 24.28, 95% confidence interval (CI): 5.86-104.61, p < 0.001 and OR 4.75; 95% CI: 1.58-14.25, p = 0.009] as well as with presence of CAD among MetS subjects (OR 11.23; 95% CI: 3.10-40.71, p < 0.001 and OR 4.03; 95% CI: 1.73-11.84, p = 0.019, respectively). Conclusions: The present study underscores the potential of cLDL and sLOX-1 as promising biomarkers for diagnosis and risk assessment of MetS and CAD among the MetS population.

Degenerative protein modifications in the aging vasculature and central nervous system: A problem shared is not always halved

Aging influences the pathogenesis and progression of several major diseases affecting both the cardiovascular system (CVS) and central nervous system (CNS). Defining the common molecular features that underpin these disorders in these crucial body systems will likely lead to increased quality of life and improved 'health-span' in the global aging population. Degenerative protein modifications (DPMs) have been strongly implicated in the molecular pathogenesis of several age-related diseases affecting the CVS and CNS, including atherosclerosis, heart disease, dementia syndromes, and stroke. However, these isolated findings have yet to be integrated into a wider framework, which considers the possibility that, despite their distinct features, CVS and CNS disorders may in fact be closely related phenomena. In this work, we review the current literature describing molecular roles of the major age-associated DPMs thought to significantly impact on human health, including carbamylation, citrullination and deamidation. In particular, we focus on data indicating that specific DPMs are shared between multiple age-related diseases in both CVS and CNS settings. By contextualizing these data, we aim to assist future studies in defining the universal mechanisms that underpin both vascular and neurological manifestations of age-related protein degeneration.

Estimating the Level of Carbamoylated Plasma Non-High-Density Lipoproteins Using Infrared Spectroscopy

BACKGROUND

The increased cardiovascular morbidity and mortality observed in chronic kidney disease (CKD) patients can be partly explained by the presence of carbamoylated lipoproteins. Lipid profiles can be determined with infrared spectroscopy. In this paper, the effects of carbamoylation on spectral changes of non-high-density lipoproteins (non-HDL) were studied.

METHODS

In the present study, fasting serum samples were obtained from 84 CKD patients (CKD stage 3-5: n = 37 and CKD stage 5d (hemodialysis): n = 47) and from 45 healthy subjects. In vitro carbamoylation of serum lipoproteins from healthy subjects was performed using increasing concentrations of potassium cyanate. Lipoprotein-containing pellets were isolated by precipitation of non-HDL. The amount of carbamoylated serum non-HDL was estimated using attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, followed by soft independent modelling by class analogy analysis.

RESULTS

Carbamoylation resulted in a small increase of the amide I band (1714-1589 cm^-1) of the infrared spectroscopy (IR) spectrum. A significant difference in the amide II/amide I area under the curves (AUC) ratio was observed between healthy subjects and CKD patients, as well as between the two CKD groups (non-dialysis versus hemodialysis patients).

CONCLUSIONS

ATR-FTIR spectroscopy can be considered as a novel method to detect non-HDL carbamoylation.

Isocyanic acid (HNCO) and its fate in the atmosphere: a review

Isocyanic acid (HNCO) has recently been identified in ambient air at potentially concerning concentrations for human health. Since its first atmospheric detection, significant progress has been made in understanding its sources and sinks. The chemistry of HNCO is governed by its partitioning between the gas and liquid phases, its weak acidity, its high solubility at pH above 5, and its electrophilic chemical behaviour. The online measurement of HNCO in ambient air is possible due to recent advances in mass spectrometry techniques, including chemical ionization mass spectrometry for the detection of weak acids. To date, HNCO has been measured in North America, Europe and South Asia as well as outdoors and indoors, with mixing ratios up to 10s of ppbv. The sources of HNCO include: (1) fossil fuel combustion such as coal, gasoline and diesel, (2) biomass burning such as wildfires and crop residue burning, (3) secondary photochemical production from amines and amides, (4) cigarette smoke, and (5) combustion of materials in the built environment. Then, three losses processes can occur: (1) gas phase photochemistry, (2) heterogenous uptake and hydrolysis, and (3) dry deposition. HNCO lifetimes with respect to photolysis and OH radical oxidation are on the order of months to decades. Consequently, the removal of HNCO from the atmosphere is thought to occur predominantly by dry deposition and by heterogeneous uptake followed by hydrolysis to NH3 and CO2. A back of the envelope calculation reveals that HNCO is an insignificant global source of NH3, contributing only around 1%, but could be important for local environments. Furthermore, HNCO can react due to its electrophilic behaviour with various nucleophilic functionalities, including those present in the human body through a reaction called protein carbamoylation. This protein modification can lead to toxicity, and thus exposure to high concentrations of HNCO can lead to cardiovascular and respiratory diseases, as well as cataracts. In this critical review, we outline our current understanding of the atmospheric fate of HNCO and its potential impacts on outdoor and indoor air quality. We also call attention to the need for toxicology studies linking HNCO exposure to health effects.

Protein Carbamylation in Chronic Kidney Disease and Dialysis

Protein carbamylation is a nonenzymatic posttranslational protein modification that can be driven, in part, by exposure to urea's dissociation product, cyanate. In humans, when kidney function is impaired and urea accumulates, systemic protein carbamylation levels increase. Additional mediators of protein carbamylation have been identified including inflammation, diet, smoking, circulating free amino acid levels, and environmental exposures. Carbamylation reactions on proteins are capable of irreversibly changing protein charge, structure, and function, resulting in pathologic molecular and cellular responses. Carbamylation has been mechanistically linked to the biochemical pathways implicated in atherosclerosis, dysfunctional erythropoiesis, kidney fibrosis, autoimmunity, and other pathological domains highly relevant to patients with chronic kidney disease. In this review, we describe the biochemical impact of carbamylation on human proteins, the mechanistic role carbamylation can have on clinical outcomes in kidney disease, the clinical association studies of carbamylation in chronic kidney disease, including patients on dialysis, and the promise of therapies aimed at reducing carbamylation burden in this vulnerable patient population.

Protein Carbamylation: Chemistry, Pathophysiological Involvement, and Biomarkers

Protein carbamylation refers to a nonenzymatic modification, which consists in the binding of isocyanic acid on protein functional groups. This reaction is responsible for the alteration in structural and functional properties of proteins, which participate in their molecular aging. Protein molecular aging is now considered a molecular substratum for the development of chronic and inflammatory diseases, including atherosclerosis, chronic kidney disease, or rheumatoid arthritis. As a consequence, carbamylation-derived products have been proposed as interesting biomarkers in various pathological contexts and appropriate analytical methods have been developed for their quantification in biological fluids. The purpose of this review is (i) to describe the biochemical bases of the carbamylation reaction, (ii) to explain how it contributes to protein molecular aging, (iii) to provide evidence of its involvement in aging and chronic diseases, and (iv) to list the available biomarkers of carbamylation process and the related analytical methods.

Known players, new interplay in atherogenesis: Chronic shear stress and carbamylated-LDL induce and modulate expression of atherogenic LR11 in human coronary artery endothelium

In this study we examined whether low-density lipoprotein (LDL) receptor family members represent a link between blood flow characteristics and modified low-density lipoproteins involved in endothelial injury, a pivotal factor in atherogenesis. We demonstrated the expression of pro-atherogenic LDL receptor relative (LR11) for the first time in human coronary artery endothelial cells (HCAEC) in vitro and in vivo. Next, LR11 expression and regulation were explored in HCAEC cultured conventionally or on the inner surface of hollow fiber capillaries under exposure to shear stress for 10 days in the presence or absence of LDL. There was no LR11 expression under static conditions. When exposed to chronic low shear stress (2.5 dynes/cm2) transmembrane and soluble endothelial-LR11 were detected in high levels irrespective of the type of LDL added (carbamylated or native). In contrast, chronic high shear stress (25 dynes/cm2) inhibited the LR11-inducing effect of LDL such that transmembrane and soluble LR11 expression became non-detectable with native LDL. Carbamylated LDL significantly counteracted this atheroprotective effect of high shear stress as shown by lower, yet sustained expression of soluble and transmembrane LR11. Oxidised LDL showed similar effects compared to carbamylated LDL but caused significantly lower LR11 expression under chronic high shear stress. Medium from HCAEC under LR11-inducing conditions enhanced vascular smooth muscle cell migration, which was abrogated by the anti-LR11 antibody. Expression of LR11 depended entirely on p38MAPK phosphorylation. We conclude that coronary endothelial LR11 expression modulated by LDL and chronic shear stress contributes to atherogenesis. LR11 and p38MAPK are potential targets for prevention of atherosclerosis.

Carbamylated Low-Density Lipoproteins Induce a Prothrombotic State Via LOX-1: Impact on Arterial Thrombus Formation In Vivo

BACKGROUND

Carbamylation alters low-density lipoprotein (LDL) structure and is thought to promote vascular inflammation and dysfunction in patients with chronic kidney disease (CKD).

OBJECTIVES

This study sought to determine whether carbamylated LDL (cLDL) exerts prothrombotic effects in vascular cells and platelets and whether cLDL enhances arterial thrombus formation in vivo.

METHODS

LDL was isolated from healthy subjects or patients with CKD by sequential ultracentrifugation. Ex vivo carbamylation of LDL from healthy subjects was induced with potassium cyanate. Arterial thrombus formation was analyzed in a murine carotid artery photochemical injury model. Protein expression and mRNA levels were analyzed by Western blotting, flow cytometry, and real-time PCR. Platelet aggregation was measured by impedance aggregometry.

RESULTS

Intravenous administration of cLDL in mice accelerated arterial thrombus formation compared to treatment with native LDL (nLDL) or vehicle. Tissue lysates of mouse carotid arteries revealed that cLDL induced the expression of TF, PAI-1, and LOX-1 mRNA in vascular cells. In human aortic smooth muscle and endothelial cells, cLDL induced TF and PAI-1 expression. In contrast, nLDL had no effect on either cell type. While nLDL and cLDL had no aggregatory effect on resting platelets, cLDL enhanced platelet aggregation in response to different agonists. This effect was mediated by mitogen-activated protein kinase p38 phosphorylation and LOX-1 translocation to the surface. LDL isolated from patients with CKD mimicked the prothrombotic effects of cLDL on vascular cells, platelets, and thrombus formation in vivo.

CONCLUSIONS

We found that cLDL induces prothrombotic effects in vascular cells and platelets by activation of the LOX-1 receptor and enhances thrombus formation in vivo. This observation reveals a new mechanism underlying the increased incidence of acute thrombotic events observed in patients with CKD and may lead to the development of new lipid-targeting therapies in this population.

Carbamylated Low-Density Lipoprotein (cLDL)-Mediated Induction of Autophagy and Its Role in Endothelial Cell Injury

Patients with chronic kidney disease (CKD) have high risk of cardiovascular complications. Plasma levels of carbamylated proteins produced by urea-derived isocyanate or thiocyanate are elevated in CKD patients and that they are significant predictors of cardiovascular events and all-cause mortality. Carbamylated LDL (cLDL) has pro-atherogenic properties and is known to affect major biological processes relevant to atherosclerosis including endothelial cell injury. The underlying mechanisms of cLDL-induced endothelial cell injury are not well understood. Although autophagy has been implicated in atherosclerosis, cLDL-mediated induction of autophagy and its role in endothelial cell injury is unknown. Our studies demonstrate that human coronary artery endothelial cells (HCAECs) respond to cLDL by specific induction of key autophagy proteins including LC3-I, beclin-1, Atg5, formation of lipid-conjugated LC3-II protein, and formation of punctate dots of autophagosome-associated LC3-II. We demonstrated that autophagy induction is an immediate response to cLDL and occurred in a dose and time-dependent manner. Inhibition of cLDL-induced autophagy by a specific siRNA to LC3 as well as by an autophagy inhibitor provided protection from cLDL-induced cell death and DNA fragmentation. Our studies demonstrate that autophagy plays an important role in cLDL-mediated endothelial cell injury and may provide one of the underlying mechanisms for the pathogenesis of cLDL-induced atherosclerosis in CKD patients.

Carbamylated LL-37 as a modulator of the immune response

Carbamylation of lysine residues and protein N-termini is an ubiquitous, non-enzymatic post-translational modification. Carbamylation at sites of inflammation is due to cyanate formation during the neutrophil oxidative burst and may target lysine residues within the antimicrobial peptide LL-37. The bactericidal and immunomodulatory properties of LL-37 depend on its secondary structure and cationic nature, which are conferred by arginine and lysine residues. Therefore, carbamylation may affect the biological functions of LL-37. The present study examined the kinetics and pattern of LL-37 carbamylation to investigate how this modification affects the bactericidal, cytotoxic and immunomodulatory function of the peptide. The results indicated that LL-37 undergoes rapid modification in the presence of physiological concentrations of cyanate, yielding a spectrum of diverse carbamylated peptides. Mass spectrometry analyses revealed that theN-terminal amino group of Leu-1 was highly reactive and was modified almost instantly by cyanate to generate the predominant form of the modified peptide, named LL-37(C1) This was followed by the sequential carbamylation of Lys-8, Lys-12, and Lys-15 to yield LL-37(C8), and Lys-15 to yield LL-37(C12,15) Carbamylation had profound and diverse effects on the structure and biological properties of LL-37. In some cases, anti-inflammatory LL-37 was rapidly converted to pro-inflammatory LL-37.

Protein carbamylation is a hallmark of aging

Aging is a progressive process determined by genetic and acquired factors. Among the latter are the chemical reactions referred to as nonenzymatic posttranslational modifications (NEPTMs), such as glycoxidation, which are responsible for protein molecular aging. Carbamylation is a more recently described NEPTM that is caused by the nonenzymatic binding of isocyanate derived from urea dissociation or myeloperoxidase-mediated catabolism of thiocyanate to free amino groups of proteins. This modification is considered an adverse reaction, because it induces alterations of protein and cell properties. It has been shown that carbamylated proteins increase in plasma and tissues during chronic kidney disease and are associated with deleterious clinical outcomes, but nothing is known to date about tissue protein carbamylation during aging. To address this issue, we evaluated homocitrulline rate, the most characteristic carbamylation-derived product (CDP), over time in skin of mammalian species with different life expectancies. Our results show that carbamylation occurs throughout the whole lifespan and leads to tissue accumulation of carbamylated proteins. Because of their remarkably long half-life, matrix proteins, like type I collagen and elastin, are preferential targets. Interestingly, the accumulation rate of CDPs is inversely correlated with longevity, suggesting the occurrence of still unidentified protective mechanisms. In addition, homocitrulline accumulates more intensely than carboxymethyl-lysine, one of the major advanced glycation end products, suggesting the prominent role of carbamylation over glycoxidation reactions in age-related tissue alterations. Thus, protein carbamylation may be considered a hallmark of aging in mammalian species that may significantly contribute in the structural and functional tissue damages encountered during aging.

Cholesterol Metabolism in CKD

Patients with chronic kidney disease (CKD) have a substantial risk of developing coronary artery disease. Traditional cardiovascular disease (CVD) risk factors such as hypertension and hyperlipidemia do not adequately explain the high prevalence of CVD in CKD. Both CVD and CKD are inflammatory states and inflammation adversely affects lipid balance. Dyslipidemia in CKD is characterized by elevated triglyceride levels and high-density lipoprotein levels that are both decreased and dysfunctional. This dysfunctional high-density lipoprotein becomes proinflammatory and loses its atheroprotective ability to promote cholesterol efflux from cells, including lipid-overloaded macrophages in the arterial wall. Elevated triglyceride levels result primarily from defective clearance. The weak association between low-density lipoprotein cholesterol level and coronary risk in CKD has led to controversy over the usefulness of statin therapy. This review examines disrupted cholesterol transport in CKD, presenting both clinical and preclinical evidence of the effect of the uremic environment on vascular lipid accumulation. Preventative and treatment strategies are explored.

Protein carbamylation and cardiovascular disease

Carbamylation constitutes a posttranslational modification of proteins or amino acids and results from different pathways in vivo. First is the non-enzymatic reaction between isocyanic acid, a decomposition product of urea, and either the N-terminus or the ɛ-amino group of lysine residues. Isocyanic acid levels, while low in vivo, are in equilibrium with urea and are thus increased in chronic and end-stage renal diseases. An alternative pathway involves the leukocyte heme protein myeloperoxidase, which catalyzes the oxidation of thiocyanate in the presence of hydrogen peroxide, producing isocyanate at inflammation sites. Notably, plasma thiocyanate levels are increased in smokers, and leukocyte-driven protein carbamylation occurs both within human and animal atherosclerotic plaques, as well as on plasma proteins. Protein carbamylation is considered a hallmark of molecular aging and is implicated in many pathological conditions. Recently, it has been shown that carbamylated low-density lipoprotein (LDL) induces endothelial dysfunction via lectin-like-oxidized LDL receptor-1 activation and increased reactive oxygen species production, leading to endothelial nitric oxide synthase uncoupling. Moreover, carbamylated LDL harbors atherogenic activities, including both binding to macrophage scavenger receptors inducing cholesterol accumulation and foam-cell formation, as well as promoting vascular smooth muscle proliferation. In contrast, high-density lipoprotein loses its anti-apoptotic activity after carbamylation, contributing to endothelial cell death. In addition to involvement in atherogenesis, protein carbamylation levels have emerged as a particularly strong predictor of both prevalent and incident cardiovascular disease risk. Recent studies also suggest that protein carbamylation may serve as a potential therapeutic target for the prevention of atherosclerotic heart disease.

[Role of protein carbamylation in chronic kidney disease complications]

Carbamylation corresponds to the non-enzymatic binding of isocyanic acid, mainly derived from urea decomposition, on amino groups of proteins, and participates in their molecular aging. This process is increased during chronic kidney disease (CKD) because of hyperuremia, and in other pathologies like atherosclerosis, where isocyanic may be formed from thiocyanate by myeloperoxidase in atheroma plates. Carbamylation triggers structural and functional modifications of proteins, thus impairing their biological roles and their interactions with cells. Much experimental evidence in vitro has shown the potential deleterious effects of carbamylated proteins on cell and tissue functions. Carbamylation-derived products (CDPs), and especially their major component homocitrulline, accumulate in organism in long half-life proteins, and may participate in the development of different complications of CKD, especially cardiovascular diseases, renal fibrosis, or nutritional and metabolic troubles. Recent clinical studies have confirmed the link between serum protein carbamylation and morbi-mortality in patients suffering from CKD or undergoing hemodialysis. Some CDPs could be used as biomarkers in these pathologies.

Increased serum homocitrulline concentrations are associated with the severity of coronary artery disease

Carbamylation is a non-enzymatic post-translational modification of proteins that has been recently identified as a non-traditional risk factor for atherosclerosis. The aim of this study was to determine whether serum homocitrulline (HCit), a characteristic carbamylation-derived product, was related to the presence and the severity of coronary artery disease (CAD).

Forty-five control subjects and 109 patients were included in this cross-sectional study. After coronary angiography, the patients were classified as non-CAD patients (patients with normal arteries, n=33) and CAD patients (n=76). The severity of CAD was then evaluated using the Gensini scoring system. Serum total HCit concentrations were determined by LC-MS/MS.

Serum HCit concentrations were significantly (p<0.001) higher in CAD patients than in control or non-CAD subjects. The receiver operating characteristic curve analysis showed an area under the curve equal to 0.908 (95% confidence interval, 0.853–0.964, p<0.001) and a threshold HCit concentration of 0.16 mmol/mol Lys for predicting the presence of CAD (78.9% sensitivity and 78.8% specificity). HCit concentrations significantly (p<0.001) increased concomitantly with the severity of CAD and were positively correlated with Gensini scores (r=0.725, p<0.001) as well as with the number of stenotic coronary arteries (p<0.001). Furthermore, in a multiple stepwise regression analysis, HCit was significantly (p<0.001) and independently associated with the presence of CAD, the Gensini score, and the number of stenotic arteries (standardized β values of 0.525, 0.722, and 0.642, respectively).

Our results demonstrate that serum HCit concentrations are increased during CAD and are positively associated with the severity of the disease.

Protein carbamylation in kidney disease: pathogenesis and clinical implications

Carbamylation describes a nonenzymatic posttranslational protein modification mediated by cyanate, a dissociation product of urea. When kidney function declines and urea accumulates, the burden of carbamylation naturally increases. Free amino acids may protect proteins from carbamylation, and protein carbamylation has been shown to increase in uremic patients with amino acid deficiencies. Carbamylation reactions are capable of altering the structure and functional properties of certain proteins and have been implicated directly in the underlying mechanisms of various disease conditions. A broad range of studies has demonstrated how the irreversible binding of urea-derived cyanate to proteins in the human body causes inappropriate cellular responses leading to adverse outcomes such as accelerated atherosclerosis and inflammation. Given carbamylation's relationship to urea and the evidence that it contributes to disease pathogenesis, measurements of carbamylated proteins may serve as useful quantitative biomarkers of time-averaged urea concentrations while also offering risk assessment in patients with kidney disease. Moreover, the link between carbamylated proteins and disease pathophysiology creates an enticing therapeutic target for reducing the rate of carbamylation. This article reviews the biochemistry of the carbamylation reaction, its role in specific diseases, and the potential diagnostic and therapeutic implications of these findings based on recent advances.

Post-translational modification derived products (PTMDPs): toxins in chronic diseases?

In living organisms, proteins are progressively modified by spontaneous non-enzymatic reactions generating many post-translational modification derived products (PTMDPs) which exert deleterious effects and may be considered endogenous toxins in diabetes mellitus and chronic renal failure. Non-enzymatic glycation, which refers to the spontaneous binding of reducing sugars to free amino groups, is increased in diabetes mellitus because of hyperglycemia and is amplified by oxidative processes ('glycoxidation'). Glycoxidation leads to the formation of 'advanced glycation end products' (AGEs), together with products of other oxidative pathways. AGEs alter tissue organization and cell-protein interactions, mainly in the case of long-lived extracellular matrix proteins, and interact with membrane receptors, among which RAGE (receptor of AGEs), a multiligand receptor which triggers intracellular signaling pathways stimulating inflammatory functions. Another major protein modification, carbamylation, is increased in chronic renal failure, which may occur during the course of diabetes mellitus. Carbamylation is due to the binding of isocyanic acid on the α-NH2 extremity of proteins or amino acids, or on ε-NH2 lysine groups, generating homocitrulline, a potential biomarker in atherosclerosis. Isocyanic acid is formed in vivo either by spontaneous dissociation of urea or by myeloperoxidase action on thiocyanate. Carbamylated proteins exhibit altered properties. For example, carbamylated collagen is unable to stimulate oxidative functions of polymorphonuclear neutrophils but increases matrix metalloproteinase-9 production by monocytes. Lipoprotein functions are altered by carbamylation and may contribute to atherogenesis. Thus, the numerous PTMDPs may be considered both hallmarks of protein damage in chronic diseases and endogenous toxins acting at the molecular and cellular levels.

Modified low density lipoprotein and lipoprotein-containing circulating immune complexes as diagnostic and prognostic biomarkers of atherosclerosis and type 1 diabetes macrovascular disease

In atherosclerosis; blood low-density lipoproteins (LDL) are subjected to multiple enzymatic and non-enzymatic modifications that increase their atherogenicity and induce immunogenicity. Modified LDL are capable of inducing vascular inflammation through activation of innate immunity; thus, contributing to the progression of atherogenesis. The immunogenicity of modified LDL results in induction of self-antibodies specific to a certain type of modified LDL. The antibodies react with modified LDL forming circulating immune complexes. Circulating immune complexes exhibit prominent immunomodulatory properties that influence atherosclerotic inflammation. Compared to freely circulating modified LDL; modified LDL associated with the immune complexes have a more robust atherogenic and proinflammatory potential. Various lipid components of the immune complexes may serve not only as diagnostic but also as essential predictive markers of cardiovascular events in atherosclerosis. Accumulating evidence indicates that LDL-containing immune complexes can also serve as biomarker for macrovascular disease in type 1 diabetes.

The effect of lipoic acid on cyanate toxicity in the rat heart

BACKGROUND

Cyanate is a uremic toxin formed principally via spontaneous urea biodegradation. Its active isoform, isocyanate, is capable of reaction with proteins by N and S carbamoylation, which influences their structure and function. Sulfurtransferases implicated in anaerobic cysteine transformation and cyanide detoxification belong to the enzymes possessing SH groups in their active centers. The present studies aimed to demonstrate the effect of cyanate and lipoic acid on the activity of these enzymes as well as on the level of antioxidants and prooxidants in the rat heart.

METHODS

Wistar rats, which received intraperitoneal injections of cyanate and lipoic acid alone and in combination were sacrificed 2.5 h after the first injection. The hearts were isolated and homogenized in phosphate buffer and next biochemical assays were performed comprising determination of the level of glutathione, malondialdehyde and sulfane sulfur and the activity of antioxidant enzymes as well as glutathione S-transferase and gamma glutamyl transferase.

RESULTS

Sulfurtransferases and glutathione S-transferase were deactivated by cyanate treatment. It was accompanied by the decreased level of glutathione and sulfane sulfur and the increased level of reactive oxygen species and malondialdehyde. In parallel, antioxidant enzymes: catalase, glutathione peroxidase and gamma glutamyl transferase were activated under such circumstances. Lipoic acid, administered in combination with cyanate prevented the decrease in the level of glutathione and reduction of a pool of sulfane sulfur-containing compounds, concomitantly preserving the activity of antioxidant enzymes.

CONCLUSIONS

Since uremia, characterized by the elevated cyanate/isocyanate level, is accompanied by frequent cases of cardiovascular diseases, the addition of lipoic acid to the therapy seems promising in prophylaxis of heart diseases in uremic patients.

Carbamylation of LDL and its relationship with myeloperoxidase in type 2 diabetes mellitus

LDL (low-density lipoprotein) is subjected to pro-atherogenic modifications in the circulation. A novel uraemia-independent mechanism of carbamylation of lipoproteins mediated by MPO (myeloperoxidase) has recently been reported. We have investigated whether carbamylation of LDL was increased in patients with Type 2 diabetes without renal impairment and the role of MPO. cLDL (carbamylated LDL) and MPO were measured by ELISA in a cross-sectional study of 198 patients and 174 non-diabetic controls. The impact of lowering MPO on plasma cLDL was determined by assaying cLDL and MPO in archived samples from a previous randomized open-label parallel group study comparing rosiglitazone (n=20) and sulfonylurea (n=24). Both plasma cLDL (P<0.05) and MPO levels (P<0.01) were higher in patients with Type 2 diabetes than controls in the cross-sectional study. Plasma cLDL correlated with MPO (r=0.42 and P<0.01) in subjects with diabetes, and plasma MPO was an independent determinant of plasma cLDL even after adjusting for age, gender, BMI (body mass index), apoB (apolipoprotein B), urea and HbA1c (glycated haemoglobin). In the randomized trial, rosiglitazone significantly lowered MPO (P<0.01) and cLDL (P<0.05), whereas no changes were observed in the sulfonylurea group despite a similar reduction in HbA1c. The magnitude of reduction in plasma cLDL correlated with changes in MPO, but not with HbA1c in the rosiglitazone group, suggesting that lowering MPO reduced plasma cLDL. Plasma cLDL is increased in patients with Type 2 diabetes even in the absence of renal impairment and carbamylation of LDL in these subjects is mainly mediated by MPO and not by urea.

Carbamylation and antibodies against carbamylated proteins in autoimmunity and other pathologies

Carbamylation is a non-enzymatic post-translational modification in which cyanate binds to molecules containing primary amine or thiol groups and forms carbamyl groups. Cyanate is in equilibrium with urea in body fluid and increased carbamylation was first reported in patients with increased urea levels such as patients suffering renal diseases. Next, increased carbamylation related to inflammation has also been described in other conditions such as cardiovascular disease. Recently, a new consequence of carbamylation has been observed: induction of an autoantibody response. We identified anti-carbamylated protein (anti-CarP) antibodies in rheumatoid arthritis (RA) patients and in patients having 'pre-RA' symptoms, arthralgia. The presence of anti-CarP antibodies in arthralgia patients is associated with an increased risk of developing RA. The presence of anti-CarP antibodies in RA patients is associated with more severe joint damage in RA patients who do not have anti-citrullinated protein antibodies. It is currently unknown to what extent carbamylation and/or the formation of anti-CarP antibodies contributes to the disease processes of chronic diseases such as renal diseases, cardiovascular diseases and RA. This review summarizes the current knowledge on carbamylation and the formation of anti-CarP antibodies and discusses their possibly important implications.

Carbamyl adducts on low-density lipoprotein induce IgG response in LDLR-/- mice and bind plasma autoantibodies in humans under enhanced carbamylation

AIMS

Post-translational modification of proteins via carbamylation predicts increased risk for coronary artery disease. Uremia and smoke exposure are known to increase carbamylation. The aim was to investigate the role of carbamylated low-density lipoprotein (LDL) immunization on antibody formation and atherogenesis in LDL receptor-deficient (LDLR-/-) mice, and to study autoantibodies to carbamylated proteins in humans with carbamylative load.

RESULTS

LDLR-/- mice immunized with carbamylated mouse LDL (msLDL; n=10) without adjuvant showed specific immunoglobulin G (IgG) antibody levels to carbamyl-LDL and malondialdehyde-modified LDL (MDA-LDL) but not to oxidized LDL or native LDL. Immunization did not influence the atherosclerotic plaque area compared with control LDLR-/- mice immunized with native msLDL (n=10) or phosphate-buffered saline (n=11). Humans with high plasma urea levels, as well as smokers, had increased IgG autoantibody levels to carbamyl-modified proteins compared to the subjects with normal plasma urea levels, or to nonsmokers.

INNOVATION

Carbamyl-LDL induced specific IgG antibody response cross-reactive with MDA-LDL in mice. IgG antibodies to carbamyl-LDL were also found in human plasma and related to conditions known to have increased carbamylation, such as uremia and smoking. Plasma antibodies to carbamylated proteins may serve as new indicator of in vivo carbamylation.

CONCLUSION

These data give insight into mechanisms of in vivo humoral recognition of post-translationally modified structures. Humoral IgG immune response to carbamylated proteins is suggested to play a role in conditions leading to enhanced carbamylation, such as uremia and smoking.

Carbamylation of serum albumin as a risk factor for mortality in patients with kidney failure

Urea, the toxic end product of protein catabolism, is elevated in end-stage renal disease (ESRD), although it is unclear whether or how it contributes to disease. Urea can promote the carbamylation of proteins on multiple lysine side chains, including human albumin, which has a predominant carbamylation site on Lys(549). The proportion of serum albumin carbamylated on Lys(549) (%C-Alb) correlated with time-averaged blood urea concentrations and was twice as high in ESRD patients than in non-uremic subjects (0.90% versus 0.42%). Baseline %C-Alb was higher in ESRD subjects who died within 1 year than in those who survived longer than 1 year (1.01% versus 0.77%) and was associated with an increased risk of death within 1 year (hazard ratio, 3.76). These findings were validated in an independent cohort of diabetic ESRD subjects (hazard ratio, 3.73). Decreased concentrations of serum amino acids correlated with higher %C-Alb in ESRD patients, and mice with diet-induced amino acid deficiencies exhibited greater susceptibility to albumin carbamylation than did chow-fed mice. In vitro studies showed that amino acids such as cysteine, histidine, arginine, and lysine, as well as other nucleophiles such as taurine, inhibited cyanate-induced C-Alb formation at physiologic pH and temperature. Together, these results suggest that chronically elevated urea promotes carbamylation of proteins in ESRD and that serum amino acid concentrations may modulate this protein modification. In summary, we have identified serum %C-Alb as a risk factor for mortality in patients with ESRD and propose that this risk factor may be modifiable with supplemental amino acid therapy.

The effect of lipoic acid on cyanate toxicity in different structures of the rat brain

Cyanate is formed mostly during nonenzymatic urea biodegradation. Its active form isocyanate reacts with protein -NH2 and -SH groups, which changes their structure and function. The present studies aimed to investigate the effect of cyanate on activity of the enzymes, which possess -SH groups in the active centers and are implicated in anaerobic cysteine transformation and cyanide detoxification, as well as on glutathione level and peroxidative processes in different brain structures of the rat: cortex, striatum, hippocampus, and substantia nigra. In addition, we examined whether a concomitant treatment with lipoate, a dithiol that may act as a target of S-carbamoylation, can prevent these changes. Cyanate-inhibited sulfurtransferase activities and lowered sulfide level, which was accompanied by a decrease in glutathione concentration and elevation of reactive oxygen species level in almost all rat brain structures. Lipoate administered in combination with cyanate was able to prevent the above-mentioned negative cyanate-induced changes in a majority of the examined brain structures. These observations can be promising for chronic renal failure patients since lipoate can play a double role in these patients contributing to efficient antioxidant defense and protection against cyanate and cyanide toxicity.

Atherosclerosis in chronic kidney disease: lessons learned from glycation in diabetes

In diabetes, glycation is a nonenzymatic posttranslational modification resulting from the bonding of a sugar molecule with a protein or lipid followed by oxidation, resulting in the development of advanced glycation end products (AGE). Like glycation, carbamylation is a posttranslational protein modification that is associated with AGE formation. Glycation of extracellular matrix proteins and low-density lipoprotein with subsequent deposition in the vessel wall could contribute to inflammatory response and atheroma formation. It is logical to extrapolate that carbamylation may result in modification of vessel wall proteins similar to glycation, and predispose to atherosclerosis.

Carbamylation-derived products: bioactive compounds and potential biomarkers in chronic renal failure and atherosclerosis

BACKGROUND

Carbamylation is a posttranslational modification of proteins resulting from the nonenzymatic reaction between isocyanic acid and specific free functional groups. This reaction alters protein structural and functional properties and thus contributes to molecular ageing. Many studies have shown the involvement of carbamylated proteins in diseases, especially in chronic renal failure and atherosclerosis.

CONTENT

In this review we describe the biochemical basis of the carbamylation process and its role in protein molecular ageing. We summarize the current evidence of protein carbamylation involvement in disease, identify available biomarkers of the carbamylation process and their related analytical methods, and discuss the practical relevance of these biomarkers.

SUMMARY

Carbamylation-induced protein alterations are involved in the progression of various diseases, because carbamylation-derived products (CDPs) are bioactive compounds that trigger specific and inappropriate cellular responses. For instance, carbamylation may promote hormone and enzyme inactivation, and carbamylated proteins, as diverse as collagen or LDLs, induce characteristic biochemical events of atherosclerosis progression. CDPs are potential biomarkers to monitor diseases characterized by an increased rate of carbamylation (e.g., chronic renal failure and atherosclerosis). Different methods (e.g., liquid chromatography-tandem mass spectrometry and immunoassays) to measure specific carbamylated proteins or general markers of carbamylation, such as protein-bound homocitrulline, have been described. Their use in clinical practice must still be validated by appropriate clinical studies.

Endonuclease G mediates endothelial cell death induced by carbamylated LDL

End-stage kidney disease is a terminal stage of chronic kidney disease, which is associated with a high incidence of cardiovascular disease. Cardiovascular disease frequently results from endothelial injury caused by carbamylated LDL (cLDL), the product of LDL modification by urea-derived cyanate. Our previous data suggested that cLDL induces mitogen-activated protein kinase-dependent mitotic DNA fragmentation and cell death. However, the mechanism of this pathway is unknown. The current study demonstrated that cLDL-induced endothelial mitotic cell death is independent of caspase-3. The expression of endonuclease G (EndoG), the nuclease implicated in caspase-independent DNA fragmentation, was significantly increased in response to cLDL exposure to the cells. The inhibition of EndoG by RNAi protected cLDL-induced DNA fragmentation, whereas the overexpression of EndoG induced more DNA fragmentation in endothelial cells. Ex vivo experiments with primary endothelial cells isolated from wild-type (WT) and EndoG knockout (KO) mice demonstrated that EndoG KO cells are partially protected against cLDL toxicity compared with WT cells. To determine cLDL toxicity in vivo, we administered cLDL or native LDL (nLDL) intravenously to the WT and EndoG KO mice and then measured floating endothelial cells in blood using flow cytometry. The results showed an increased number of floating endothelial cells after cLDL versus nLDL injection in WT mice but not in EndoG KO mice. Finally, the inhibitors of MEK-ERK1/2 and JNK-c-jun pathways decreased cLDL-induced EndoG overexpression and DNA fragmentation. In summary, our data suggest that cLDL-induced endothelial toxicity is caspase independent and results from EndoG-dependent DNA fragmentation.

The effect of uremic toxin cyanate (OCN–) on anaerobic sulfur metabolism and prooxidative processes in the rat kidney: a protective role of lipoate

Cyanate and its active form isocyanate are formed mainly in the process of nonenzymatic urea biodegradation. Cyanate is capable of protein S- and N-carbamoylation, which can affect their activity. The present studies aimed to demonstrate the effect of cyanate on activity of the enzymes implicated in anaerobic cysteine metabolism and cyanide detoxification and on glutathione (GSH) level and peroxidative processes in the kidney. In addition, we examined whether a concomitant treatment with lipoate, a dithiol that may act as a target of S-carbamoylation, can prevent these changes. The studies were conducted in Wistar rats. The animals were assigned to four groups, which received injections of physiological saline, cyanate (200 mg/kg), cyanate (200 mg/kg) + lipoate (100 mg/kg) and lipoate alone (100 mg/kg). The animals were killed 2 h after the first injection, the kidneys were isolated and kept at -80°C until biochemical assays were performed. Cyanate inhibited rhodanese (TST) and mercaptopyruvate sulfotransferase (MPST) activity, decreased GSH level and enhanced peroxidative processes in the kidney. All these changes were abolished by cyanate treatment in combination with lipoate.

Chronic uremia stimulates LDL carbamylation and atherosclerosis

Carbamylated LDL (cLDL) is a potential atherogenic factor in chronic kidney disease (CKD). However, whether elevated plasma cLDL associates with atherosclerosis in vivo is unknown. Here, we induced CKD surgically in apolipoprotein E-deficient (ApoE(-/-)) mice fed a high-fat diet to promote the development of atherosclerosis. These mice had two- to threefold higher plasma levels of both oxidized LDL (oxLDL) and cLDL compared with control mice. Oral administration of urea increased cLDL approximately eightfold in ApoE(-/-) mice subjected to unilateral nephrectomy and a high-fat diet, but oxLDL did not rise. Regardless of the model, the uremic mice with high plasma cLDL had more severe atherosclerosis as measured by intravital ultrasound echography and en face aortic staining of lipid deposits. Furthermore, cLDL accumulated in the aortic wall and colocalized with ICAM-1 and macrophage infiltration. In summary, these data demonstrate that elevated plasma cLDL may represent an independent risk factor for uremia-induced atherosclerosis.

Carbamylated LDL

Nonenzymatic modification of protein by cyanate, that is, carbamylation, has received new attention due to its apparent relevance in atherosclerosis. For example, carbamylation of low-density lipoprotein (LDL) is an important mechanism that potentially impacts high-risk atherosclerotic individuals with increased urea (renal insufficiency) or thiocyanate (tobacco smoking). Carbamylated LDL (cLDL) is increased in patients with end-stage kidney disease, especially those with atherosclerosis. In addition, cLDL exhibits distinct cytotoxic effects when tested in vitro on endothelial cells, induces the expression of adhesion molecules, and aggravates the monocyte adhesion to endothelial cells. It also facilitates the proliferation of vascular smooth-muscle cell (VSMC). Studies of potential pharmacological interruption of these processes in vivo may lead to discoveries of novel therapies for atherosclerosis.

Smoking and atherosclerotic cardiovascular disease: Part I: atherosclerotic disease process

The normal endothelium inhibits platelet and leukocyte adhesion to the vascular surface maintaining a balance of profibrinolytic and prothrombotic activity. Endothelial function is assessed largely as endothelium-dependent vasomotion, partly based on the assumption that impaired endothelium-dependent vasodilation reflects the alteration of important endothelial functions. Atherosclerotic risk factors, such as hypercholesterolemia, hypertension, diabetes and smoking, are associated with endothelial dysfunction. In the diseased endothelium, the balance between pro- and antithrombotic, pro- and anti-inflammatory, pro- and antiadhesive or pro- and antioxidant effects shifts towards a proinflammatory, prothrombotic, pro-oxidative and proadhesive phenotype of the endothelium. A common mechanism underlying endothelial dysfunction is related to the increased vascular production of reactive oxygen species. Recent studies suggest that inflammation per se, and C-reactive protein in particular, may contribute directly to endothelial dysfunction. The loss of endothelial integrity is a hallmark of atherosclerosis and the causal possible link between each individual risk factor, the development of atherosclerosis and the subsequent clinical events, such as myocardial infarction or stroke.

Carbamylation-dependent activation of T cells: a novel mechanism in the pathogenesis of autoimmune arthritis

The posttranslational modification of proteins has the potential to generate neoepitopes that may subsequently trigger immune responses. The carbamylation of lysine residues to form homocitrulline may be a key mechanism triggering inflammatory responses. We evaluated the role of carbamylation in triggering immune responses and report a new role for this process in the induction of arthritis. Immunization of mice with homocitrulline-containing peptides induced chemotaxis, T cell activation, and Ab production. The mice also developed erosive arthritis following intra-articular injection of peptides derived from homocitrulline and citrulline. Adoptive transfer of T and B cells from homocitrulline-immunized mice into normal recipients induced arthritis, whereas systemic injection of homocitrulline-specific Abs or intra-articular injection of homocitrulline-Ab/citrulline-peptide mixture did not. Thus, the T cell response to homocitrulline-derived peptides, as well as the subsequent production of anti-homocitrulline Abs, is critical for the induction of autoimmune reactions against citrulline-derived peptides and provides a novel mechanism for the pathogenesis of arthritis.