Carbamylation-induced alterations in low-density lipoprotein metabolism

Effect of Carbamylated Erythropoietin in a Chronic Model of TNBS-Induced Colitis

BACKGROUND

Inflammatory bowel disease (IBD) is a public health issue with a growing prevalence, which can be divided into two phenotypes, namely Crohn's disease (CD) and ulcerative colitis (UC). Currently, used therapy is based only on symptomatic and/or palliative pharmacological approaches. These treatments seek to induce and maintain remission of the disease and ameliorate its secondary effects; however, they do not modify or reverse the underlying pathogenic mechanism. Therefore, it is essential to investigate new potential treatments. Carbamylated erythropoietin (cEPO) results from the modification of the Erythropoietin (EPO) molecule, reducing cardiovascular-related side effects from the natural erythropoiesis stimulation. cEPO has been studied throughout several animal models, which demonstrated an anti-inflammatory effect by decreasing the production of several pro-inflammatory cytokines.

AIM

This study aimed to evaluate the efficacy and safety of cEPO in a chronic TNBS-induced colitis model in rodents.

METHODS

Experimental colitis was induced by weekly intrarectal (IR) administrations of 1% TNBS for 5 weeks in female CD-1 mice. Then, the mice were treated with 500 IU/kg/day or 1000 IU/kg/day of cEPO through intraperitoneal injections for 14 days.

RESULTS

cEPO significantly reduced the concentration of alkaline phosphatase (ALP), fecal hemoglobin, tumor necrosis factor (TNF)-α, and interleukin (IL)-10. Also, it demonstrated a beneficial influence on the extra-intestinal manifestations, with the absence of significant side effects of its use.

CONCLUSION

Considering the positive results from cEPO in this experiment, it may arise as a new possible pharmacological approach for the future management of IBD.

Distribution and clinical significance of anti-carbamylation protein antibodies in rheumatological diseases among the Chinese Han population

Objective

Several studies have demonstrated that anti-carbamylation protein antibodies (Anti-CarPA) are persistent in patients with rheumatoid arthritis (RA), systemic lupus erythematosus (SLE), systemic sclerosis (SSC), primary Sjögren's syndrome (pSS), and interstitial lung disease associated with RA (RA-ILD). However, the relationship between anti-CarPA and other rheumatic diseases (RDs) and non-RA-ILD is not known till now. This study sought to examine the presence of anti-CarPA in Chinese Han patients with RDs and its clinical significance.

Methods

The study included 90 healthy controls (HCs) and 300 patients with RDs, including RA, SLE, polymyositis/dermatomyositis (PM/DM), pSS, SSC, spondyloarthritis (SpA), anti-neutrophil cytoplasmic autoantibodies associated with vasculitis (AAV), undifferentiated connective tissue disease (UCTD), and Behcet's disease (BD). Antibodies against carbamylated human serum albumin were detected using commercial enzyme-linked immunosorbent assay kits. Correlations between clinical and laboratory parameters were analyzed.

Result

Serum levels of anti-CarPA in RA (34.43 ± 33.34 ng/ml), SLE (21.12 ± 22.23 ng/ml), pSS (16.32 ± 13.54 ng/ml), PM/DM (30.85 ± 17.34 ng/ml), SSC (23.53 ± 10.70 ng/ml), and UCTD (28.35 ± 21.91 ng/ml) were higher than those of anti-CarPA in the HCs (7.30 ± 5.05 ng/ml). The concentration of serum anti-CarPA was higher in patients with rheumatic disease-related interstitial lung disease (RD-ILD), especially RA-ILD, PM/DM-ILD, and pSS-ILD. Patients with RD-ILD who tested positive for anti-CarPA were more likely to have a more severe radiographic classification (grades II, p = 0.045; grades III, p = 0.003). Binary logistic regression analysis suggested that anti-CarPA had an association with ILD in RA (p = 0.033), PM/DM (p = 0.039), and pSS (p = 0.048). Based on receiver operating characteristics (ROC) analysis, anti-CarPA cutoffs best discriminated ILD in RA (>32.59 ng/ml, p = 0.050), PM/DM (>23.46 ng/ml, p = 0.038), and pSS (>37.08 ng/ml, p = 0.040). Moreover, serum levels of anti-CarPA were correlated with antibodies against transcription intermediary factor 1 complex (anti-TIF1) (R = -0.28, p = 0.044), antibodies against glycyl-transfer ribonucleic acid synthetase (anti-EJ) (R = 0.30, p = 0.031), and antibodies against melanoma differentiation-associated gene 5 (anti-MDA5) (R = 0.35, p = 0.011).

Conclusion

Serum anti-CarPA could be detected in patients with RA, PM/DM, pSS, SSC, and UCTD among the Chinese Han population. And it may also assist in identifying ILD in patients with RA, PM/DM, and pSS, which emphasized attention to the lung involvement in anti-CarPA-positive patients.

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.

The pattern of apolipoprotein A-I lysine carbamylation reflects its lipidation state and the chemical environment within human atherosclerotic aorta

Protein lysine carbamylation is an irreversible post-translational modification resulting in generation of homocitrulline (N-ε-carbamyllysine), which no longer possesses a charged ε-amino moiety. Two distinct pathways can promote protein carbamylation. One results from urea decomposition, forming an equilibrium mixture of cyanate (CNO⁻) and the reactive electrophile isocyanate. The second pathway involves myeloperoxidase (MPO)-catalyzed oxidation of thiocyanate (SCN⁻), yielding CNO⁻ and isocyanate. Apolipoprotein A-I (apoA-I), the major protein constituent of high-density lipoprotein (HDL), is a known target for MPO-catalyzed modification in vivo, converting the cardioprotective lipoprotein into a proatherogenic and proapoptotic one. We hypothesized that monitoring site-specific carbamylation patterns of apoA-I recovered from human atherosclerotic aorta could provide insights into the chemical environment within the artery wall. To test this, we first mapped carbamyllysine obtained from in vitro carbamylation of apoA-I by both the urea-driven (nonenzymatic) and inflammatory-driven (enzymatic) pathways in lipid-poor and lipidated apoA-I (reconstituted HDL). Our results suggest that lysine residues within proximity of the known MPO-binding sites on HDL are preferentially targeted by the enzymatic (MPO) carbamylation pathway, whereas the nonenzymatic pathway leads to nearly uniform distribution of carbamylated lysine residues along the apoA-I polypeptide chain. Quantitative proteomic analyses of apoA-I from human aortic atheroma identified 16 of the 21 lysine residues as carbamylated and suggested that the majority of apoA-I carbamylation in vivo occurs on “lipid-poor” apoA-I forms via the nonenzymatic CNO⁻ pathway. Monitoring patterns of apoA-I carbamylation recovered from arterial tissues can provide insights into both apoA-I structure and the chemical environment within human atheroma.

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.

Protein carbamylation and chronic kidney disease progression in the Chronic Renal Insufficiency Cohort Study

BACKGROUND

Protein carbamylation is a post-translational protein modification caused, in part, by exposure to urea's dissociation product cyanate. Carbamylation is linked to cardiovascular outcomes and mortality in dialysis-dependent end-stage kidney disease (ESKD), but its effects in earlier pre-dialysis stages of chronic kidney disease (CKD) are not established.

METHODS

We conducted two nested case-control studies within the Chronic Renal Insufficiency Cohort Study. First, we matched 75 cases demonstrating CKD progression [50% estimated glomerular filtration rate (eGFR) reduction or reaching ESKD] to 75 controls (matched on baseline eGFR, 24-h proteinuria, age, sex and race). In the second study, we similarly matched 75 subjects who died during follow-up (cases) to 75 surviving controls. Baseline carbamylated albumin levels (C-Alb, a validated carbamylation assay) were compared between cases and controls in each study.

RESULTS

At baseline, in the CKD progression study, other than blood urea nitrogen (BUN) and smoking status, there were no significant differences in any matched or other parameter. In the mortality group, the only baseline difference was smoking status. Adjusting for baseline differences, the top tertile of C-Alb was associated with an increased risk of CKD progression [odds ratio (OR) = 7.9; 95% confidence interval (CI) 1.9-32.8; P = 0.004] and mortality (OR = 3.4; 95% CI 1.0-11.4; P = 0.05) when compared with the bottom tertile. C-Alb correlated with eGFR but was more strongly correlated with BUN.

CONCLUSIONS

Our data suggest that protein carbamylation is a predictor of CKD progression, beyond traditional risks including eGFR and proteinuria. Carbamylation's association with mortality was smaller in this limited sample size.

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.

Anti-carbamylated protein antibodies and skin involvement in patients with systemic sclerosis: An intriguing association

Carbamylation is a post-translational modification that mostly affects proteins with low turnover, such as dermal proteins. Carbamylated proteins accumulate in skin in an age-dependent manner, contributing to tissue alterations. As dermis is affected by systemic sclerosis (SSc) and anti-carbamylated protein antibodies (anti-CarP Ab) are found in SSc patients, we sought to evaluate the specificity of anti-CarP Ab and their relationship with clinical parameters reflecting skin involvement in SSc. This study investigated serum samples and clinical data from 124 patients with SSc. Anti-CarP Ab were affinity purified from pooled SSc sera, and their specificity was assessed by western blotting and ELISA with carbamylated proteins from two species (human and bovine albumin; human fibrinogen). Anti-CarP Ab were measured in SSc serum samples and in 41 healthy aged-matched individuals. Affinity-purified anti-CarP Ab recognized carbamylated epitopes irrespective of the protein type or species origin. Anti-CarP Ab levels inversely correlated with the modified Rodnan skin score (mRss) (Spearman's R = -0.32, p<0.001), independently of patients' age. Receiver operating characteristics (ROC) analysis identified anti-CarP Ab cut-offs that best discriminated dichotomized clinical variables related to skin involvement: the only clinical variables that were significantly different between groups were mRss (p = 0.001) and scleredema (p<0.001). Low anti-CarP Ab levels were associated with worse skin involvement. Future prospective studies are needed to assess their usefulness in the clinical setting.

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.

Cyanate improves insulin sensitivity and hepatic steatosis in normal and high fat-fed mice: Anorexic and antioxidative effects

Obesity is an important contributing factor to progression of chronic kidney disease. Cyanate, known as uremic toxin, is an electrophile produced spontaneously from urea or by myeloperoxidase-catalyzed oxidation of thiocyanate. Herein, we explored metabolic effects of cyanate in normal chow diet (NCD)- and high fat diet (HFD)-fed mice. Mice were treated with cyanate (1 mg/mL in drinking water) and fed NCD or HFD. Peritoneal glucose tolerance test (PGTT) and insulin tolerance test (ITT) were performed. Blood urea nitrogen (BUN) and creatinine concentrations were determined. Kidney and liver tissues were analyzed for reactive oxygen species (ROS) and lipid accumulations. Human albumin was carbamylated and evaluated for ROS scavenging activities. Contrary to our expectations, we found that cyanate treatment improved increased insulin sensitivity and alleviated hepatic steatosis in NCD- and HFD-fed mice. PGTT and ITT revealed faster and immediate glucose clearance in cyanate-treated NCD- and HFD-fed mice. Histological analysis of kidney and serum levels of BUN and creatinine showed no significant differences between cyanate-treated and control mice groups. Cyanate treatment reduced appetite and body weight in both NCD- and HFD-fed mice groups. Cyanate also decreased lipid peroxidation levels in the sera and the kidney, attenuated ROS levels in the kidney, which lead us to the findings that cAlb significantly reduced ROS levels compared to Alb in Caki-1 kidney and human umbilical vein endothelial cells. The results in this study may indicate that cyanate improves insulin sensitivity and hepatic steatosis possibly via exerting anorexic and antioxidative effects.

Mechanisms and consequences of carbamoylation

Protein carbamoylation is a non-enzymatic post-translational modification that binds isocyanic acid, which can be derived from the dissociation of urea or from the myeloperoxidase-mediated catabolism of thiocyanate, to the free amino groups of a multitude of proteins. Although the term 'carbamoylation' is usually replaced by the term "carbamylation" in the literature, carbamylation refers to a different chemical reaction (the reversible interaction of CO₂ with α and ε-amino groups of proteins). Depending on the altered molecule (for example, collagen, erythropoietin, haemoglobin, low-density lipoprotein or high-density lipoprotein), carbamoylation can have different pathophysiological effects. Carbamoylated proteins have been linked to atherosclerosis, lipid metabolism, immune system dysfunction (such as inhibition of the classical complement pathway, inhibition of complement-dependent rituximab cytotoxicity, reduced oxidative neutrophil burst, and the formation of anti-carbamoylated protein antibodies) and renal fibrosis. In this Review, we discuss the carbamoylation process and evaluate the available biomarkers of carbamoylation (for example, homocitrulline, the percentage of carbamoylated albumin, carbamoylated haemoglobin, and carbamoylated low-density lipoprotein). We also discuss the relationship between carbamoylation and the occurrence of cardiovascular events and mortality in patients with chronic kidney disease and assess the effects of strategies to lower the carbamoylation load.

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.

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.

Human monoclonal Fab and human plasma antibodies to carbamyl-epitopes cross-react with malondialdehyde-adducts

Oxidized low-density lipoprotein (OxLDL) plays a crucial role in the development of atherosclerosis. Carbamylated LDL has been suggested to promote atherogenesis in patients with chronic kidney disease. Here we observed that plasma IgG and IgM antibodies to carbamylated epitopes were associated with IgG and IgM antibodies to oxidation-specific epitopes (ρ = 0·65-0·86, P < 0·001) in healthy adults, suggesting a cross-reaction between antibodies recognizing carbamyl-epitopes and malondialdehyde (MDA)/malondialdehyde acetaldehyde (MAA) -adducts. We used a phage display technique to clone a human Fab antibody that bound to carbamylated LDL and other carbamylated proteins. Anti-carbamyl-Fab (Fab106) cross-reacted with oxidation-specific epitopes, especially with MDA-LDL and MAA-LDL. We showed that Fab106 bound to apoptotic Jurkat cells known to contain these oxidation-specific epitopes, and the binding was competed with soluble carbamylated and MDA-/MAA-modified LDL and BSA. In addition, Fab106 was able to block the uptake of carbamyl-LDL and MDA-LDL by macrophages and stained mouse atherosclerotic lesions. The observed cross-reaction between carbamylated and MDA-/MAA-modified LDL and its contribution to enhanced atherogenesis in uraemic patients require further investigation.

Carbamylated erythropoietin promotes neurite outgrowth and neuronal spine formation in association with CBP/p300

Both erythropoietin (EPO) and carbamylated EPO (cEPO) have been shown to increase the length of neurites and spine density in neurons. However, the molecular mechanism underlying the EPO- and cEPO-induced neuronal differentiation has yet to be investigated. To address this issue, we investigated epigenetic modifications that regulate gene expression in neurons. Neurons treated with EPO or cEPO display an upregulation of E1A-binding protein (p300) and p300-mediated p53 acetylation, possibly increasing the transactivation activity of p53 on growth-associated protein 43 (GAP43). Treatment of cells with cEPO markedly increases spine formation and potentiates p300-mediated transactivation of PSD95, Shank2 and 3 compared to EPO. These results demonstrate that cEPO controls neuronal differentiation via acetylation of transcription factors and subsequent transactivation of target genes. These findings have important medical implications because cEPO is of interest in the development of therapeutic agents against neuropsychiatric disorders.

Glutaraldehyde erythropoietin protects kidney in ischaemia/reperfusion injury without increasing red blood cell production

BACKGROUND AND PURPOSE

Recombinant human erythropoietin (rHuEPO) is currently the mainstay of renal anaemia treatment. Recently, rHuEPO has been shown to provide pleiotrophic tissue protection in various pathological conditions. However, the benefits of rHuEPO beyond anaemia treatment are limited because it increases red blood cell mass. Carbamylated erythropoietin (CEPO) is the first rHuEPO derivative that lacks erythropoietic activity but retains tissue protection properties. Since carbamylation targets lysine residues on rHuEPo, we hypothesized that targeted lysine modifications of rHuEPO may result in a novel non-erythropoietic erythropoietin.

EXPERIMENTAL APPROACH

rHuEPO was subjected to various targeted lysine modifications. In vitro cytoprotection and apoptosis were evaluated using P19 and HEK293 cells. In vivo erythropoiesis was performed by administering the derivatives to animals for 2 weeks. Renoprotection was tested on an ischaemia/reperfusion (I/R) model.

KEY RESULTS

We synthesized a novel derivative, a glutaraldehyde erythropoietin (GEPO). This construct abolished in vivo erythropoiesis. Biochemical characterization showed that GEPO was more electrostatically negative than rHuEPO. Immunoprecipitation experiments revealed that GEPO bound to the IL3RB/EPOR heterotrimeric receptor and ameliorated cellular apoptosis via the activation of Bcl-2. Notably, Bcl-2 activation was suppressed by the JAK2 inhibitor, tyrphostin AG490. In vivo experiments showed that GEPO also ameliorated kidney damage due to I/R injury both functionally and histologically.

CONCLUSIONS AND IMPLICATIONS

Herein, we describe a novel lysine-modified rHuEPO, glutaradehyde-EPO (GEPO), obtained from a simple reaction. This derivative has no erythropoietic properties but retains cell-protective characteristics both in vitro and in vivo, with promise for future use as an adjunctive treatment of kidney disease.

Endogenous carbamylation of renal medullary proteins

Protein carbamylation is a post-translational modification that can occur in the presence of urea. In solution, urea is in equilibrium with ammonium cyanate, and carbamylation occurs when cyanate ions react with the amino groups of lysines, arginines, protein N-termini, as well as sulfhydryl groups of cysteines. The concentration of urea is elevated in the renal inner medulla compared with other tissues. Due to the high urea concentration, we hypothesized that carbamylation can occur endogenously within the rat inner medulla. Using immunoblotting of rat kidney cortical and medullary homogenates with a carbamyl-lysine specific antibody, we showed that carbamylation is present in a large number of inner medullary proteins. Using protein mass spectrometry (LC-MS/MS) of rat renal inner medulla, we identified 456 unique carbamylated sites in 403 proteins, including many that play important physiological roles in the renal medulla [Data can be accessed at https://helixweb.nih.gov/ESBL/Database/Carbamylation/Carbamylation_peptide_sorted.html]. We conclude that protein carbamylation occurs endogenously in the kidney, modifying many physiologically important proteins.

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.

Carbamylated-oxidized LDL: proatherosclerotic effects on endothelial cells and macrophages

AIM

Both oxidized LDL and carbamylated LDL are considered important for initiating atherosclerosis in patients with end-stage kidney disease through vascular endothelial cell dysfunction or injury. However their effects on each other and their relationship related to pro-atherosclerotic effects on endothelial cells and macrophages have not been investigated. In this study, we analyzed the competition between LDL carbamylation and oxidation, tested biological effects of carbamylated-oxidized LDL (coxLDL) toward the endothelial cells, assessed its ability to cause foam cell development, and determined the roles of scavenger receptors in this process.

METHODS

Cross-competition between carbamylation and oxidation of LDL particles was tested using cell-free fluorescent ligand-receptor assay. Pro-atherogenic properties (cell proliferation, cytotoxicity, and foam cell formation) of all LDL isoforms were tested in vitro and ex vivo using endothelial cells and peritoneal macrophages. In addition, coxLDL was assessed in human sera and in vivo atherosclerotic plaques which were developed in mouse model of uremia-induced atherosclerosis.

RESULTS

Our data suggest that there is potential competition between carbamylation and oxidation of LDL, and that oxidation is a much stronger inhibitor of carbamylation than vice versa. coxLDL is highly cytotoxic to endothelial cells and strongly induce their proliferation measured by DNA synthesis. All three tested LDL isoforms demonstrated strong ability for transformation of primary mouse peritoneal macrophages to foam cells using predominantly CD36 scavenger receptor. coxLDL was the most potent inducer of foam cell development and macrophages/foam cell injury assessed by cell count and TUNEL, respectively. Finally, LDL particles modified by oxidation and carbamylation were detected in blood and shown to co-localize in atherosclerotic plaques in mice.

CONCLUSION

Our study demonstrated that LDL particles can be simultaneously carbamylated and oxidized and modifications are likely coexisting in the same LDL particle. We also demonstrated proatherosclerotic properties of coxLDL and proposed its role in atherosclerosis.

Protein carbamylation in chronic systolic heart failure: relationship with renal impairment and adverse long-term outcomes

BACKGROUND

Protein carbamylation, a posttranslational modification promoted during uremia and catalyzed by myeloperoxidase (MPO) at sites of inflammation, is linked to altered protein structure, vascular dysfunction, and poor prognosis. We examine the relationship between plasma protein-bound homocitrulline (PBHCit) levels, a marker of protein lysine residue carbamylation, with cardiorenal function and long-term outcomes in chronic systolic heart failure (HF).

METHODS AND RESULTS

In 115 patients with chronic systolic HF (left ventricular ejection fraction ≤35%), we measured plasma PBHCit by quantitative mass spectrometry and performed comprehensive echocardiography with assessment of cardiac structure and performance. Adverse long-term events (death, cardiac transplantation) were tracked for 5 years. In our study cohort, the median PBHCit level was 87 (interquartile range 59-128) μmol/mol lysine. Higher plasma PBHcit levels were associated with poorer renal function (estimated glomerular filtration rate [eGFR]: Spearman r = -0.37; P < .001), cystatin C (r = 0.31; P = .001), and elevated plasma amino-terminal pro-B-type natriuretic peptide (NT-proBNP) levels (r = 0.26; P = .006), but not with markers of systemic inflammation or oxidant stress (high-sensitivity C-reactive protein and myeloperoxidase [MPO]: P > .10 for each). Furthermore, elevated plasma PBHCit levels were not related to indices of cardiac structure or function (P > .10 for all examined) except modestly with increased right atrial volume index (r = 0.31; P = .002). PBHCit levels predicted adverse long-term events (hazard ratio [HR] 1.8, 95% confidence interval [CI] 1.3-2.6; P < .001), including after adjustment for age, eGFR, MPO, and NT-proBNP (HR 1.9, 95% CI 1.2-3.1; P = .006).

CONCLUSIONS

In chronic systolic HF, protein carbamylation is associated with poorer renal but not cardiac function, and portends poorer long-term adverse clinical outcomes even when adjusted for cardiorenal indices of adverse prognosis.

Effects of amino acids and albumin on erythropoietin carbamoylation

BACKGROUND

Anemia in chronic renal failure results from inadequate production of erythropoietin and decrease in its biological activity, and the reduced activity of erythropoietin is caused by the presence of plasma inhibitors of erythropoietin. It is reported that one of the inhibitors of erythropoietin is cyanate, a potential uremic toxin formed spontaneously from increased urea due to decreased renal function, and erythropoietin loses its biological activity due to negatively charged cyanate. The purpose of this study is to investigate the protective effects of amino acids, positively charged amino groups, and albumin binding of several toxins on erythropoietin carbamoylation.

METHODS

The degree of change in erythropoietin structure by cyanate was measured by the trinitrobenzenesulphonate reaction and Western blotting. The loss of biological activity of erythropoietin caused by cyanate was measured by injecting erythropoietin into rats with chronic renal failure.

RESULTS

The free amino groups in erythropoietin decreased under cyanate treatment in a time- and concentration-dependent manner. In the cyanate treatment group, of the twenty amino acids, phenylalanine, valine, tryptophan, threonine, and lysine prevented the structural modification of erythropoietin, according to Western blot analysis. In addition, of the three proteins, albumin prevented the structural modification of erythropoietin. As for the cyanate with erythropoietin treatment group, only lysine and albumin prevented the loss of biological activity of erythropoietin in the rats.

CONCLUSION

The results of this study suggest that lysine and albumin may play a protective role against renal anemia by erythropoietin carbamoylation in chronic renal failure.

Comparison of Neurite Outgrowth Induced by Erythropoietin (EPO) and Carbamylated Erythropoietin (CEPO) in Hippocampal Neural Progenitor Cells

A previous animal study has shown the effects of erythropoietin (EPO) and its non-erythropoietic carbamylated derivative (CEPO) on neurogenesis in the dentate gyrus. In the present study, we sought to investigate the effect of EPO on adult hippocampal neurogenesis, and to compare the ability of EPO and CEPO promoting dendrite elongation in cultured hippocampal neural progenitor cells. Two-month-old male BALB/c mice were given daily injections of EPO (5 U/g) for seven days and were sacrificed 12 hours after the final injection. Proliferation assays demonstrated that EPO treatment increased the density of bromodeoxyuridine (BrdU)-labeled cells in the subgranular zone (SGZ) compared to that in vehicle-treated controls. Functional differentiation studies using dissociated hippocampal cultures revealed that EPO treatment also increased the number of double-labeled BrdU/microtubule-associated protein 2 (MAP2) neurons compared to those in vehicle-treated controls. Both EPO and CEPO treatment significantly increased the length of neurites and spine density in MAP2(+) cells. In summary, these results provide evidences that EPO and CEPO promote adult hippocampal neurogenesis and neuronal differentiation. These suggest that EPO and CEPO could be a good candidate for treating neuropsychiatric disorders such as depression and anxiety associated with neuronal atrophy and reduced hippocampal neurogenesis.

High plasma thiocyanate levels modulate protein damage induced by myeloperoxidase and perturb measurement of 3-chlorotyrosine

Smokers have an elevated risk of atherosclerosis but the origin of this elevated risk is incompletely defined, though increasing evidence supports a role for the oxidant-generating enzyme myeloperoxidase (MPO). In previous studies we have demonstrated that smokers have elevated levels of thiocyanate ions (SCN(-)), relative to nonsmokers, and increased thiol oxidation, as SCN(-) is a favored substrate for MPO, and the resulting hypothiocyanous acid (HOSCN) targets thiol groups rapidly and selectively. In this study we show that increased HOSCN formation by MPO diminishes damage to nonthiol targets on both model proteins and human plasma proteins. Thus high SCN(-) levels protect against HOCl- and MPO-mediated damage to methionine, tryptophan, lysine, histidine, and tyrosine residues on proteins. Furthermore, levels of the HOCl-mediated marker compound 3-chlorotyrosine and the cross-linked product dityrosine are decreased. Plasma protein 3-chlorotyrosine levels induced by HOCl exposure in nonsmokers are elevated over the levels detected in smokers when exposed to identical oxidative insult (P<0.05), and a strong inverse correlation exists between plasma SCN(-) levels and 3-chlorotyrosine concentrations (r=0.6182; P<0.0001). These correlations were also significant for smokers (r=0.2724; P<0.05) and nonsmokers (r=0.4141; P<0.01) when analyzed as individual groups. These data indicate that plasma SCN(-) levels are a key determinant of the extent and type of protein oxidation induced by MPO on isolated and plasma proteins and that smoking status and resulting high SCN(-) levels can markedly modulate the levels of the widely used biomarker compound 3-chlorotyrosine.

Hypothiocyanous acid: benign or deadly?

Hypothiocyanous acid (HOSCN) is produced in biological systems by the peroxidase-catalyzed reaction of thiocyanate (SCN(-)) with H(2)O(2). This oxidant plays an important role in the human immune system, owing to its potent bacteriostatic properties. Significant amounts of HOSCN are also formed by immune cells under inflammatory conditions, yet the reactivity of this oxidant with host tissue is poorly characterized. Traditionally, HOSCN has been viewed as a mild oxidant, which is innocuous to mammalian cells. Indeed, recent studies show that the presence of SCN(-) in airways has a protective function, by preventing the formation of other, more damaging, inflammatory oxidants. However, there is an increasing body of evidence that challenges this dogma, showing that the selectivity of HOSCN for specific thiol-containing cellular targets results in the initiation of significant cellular damage. This propensity to induce cellular dysfunction is gaining considerable interest, particularly in the cardiovascular field, as smokers have elevated plasma SCN(-), the precursor for HOSCN. This review will outline the beneficial and detrimental aspects of HOSCN formation in biological systems.

Carbamylated low-density lipoprotein induces oxidative stress and accelerated senescence in human endothelial progenitor cells

Carbamylated low-density lipoprotein (cLDL) plays a role in atherosclerosis. In this study we evaluate the effect of uremia on LDL carbamylation and the effect of cLDL and oxidized LDL (oxLDL; 200 μg/ml) on number, function, and genomic stability of endothelial progenitor cells (EPCs) obtained from healthy volunteers. cLDL was generated after incubation of native LDL (nLDL) with uremic serum from patients with chronic kidney disease (CKD) stages 2-4. Oxidative stress was measured by flow cytometry and fluorescent microscopy, mitochondrial depolarization by flow cytometry, senescence by β-galactosidase activity and telomere length, and DNA damage by phosphorylated histone H2AX (γH2AX). The percentage of cLDL by uremic serum was related to the severity of CKD. Compared with nLDL, cLDL induced an increase in oxidative stress (62±5 vs. 8±3%, P<0.001) and cells with mitochondrial depolarization (73±7 vs. 9±5%, P<0.001), and a decrease in EPC proliferation and angiogenesis. cLDL also induced accelerated senescence (73±16 vs. 12±9%, P<0.001), which was associated with a decrease in the expression of γH2AX (62±9 vs. 5±3%, P<0.001). The degree of injury induced by cLDL was comparable to that observed with oxLDL. This study supports the hypothesis that cLDL triggers genomic damage in EPCs, resulting in premature senescence. We can, therefore, hypothesize that EPCs injury by cLDL contributes to an increase in atherosclerotic disease in CKD.

Lipoprotein Metabolism Abnormalities in Patients with Chronic Renal Insufficiency

Patients with chronic renal insufficiency (CRI) on hemodialysis develop lipoprotein abnormalities that may contribute to increased risk for atherosclerosis. The objective of this study was to assess the atherogenic risk of chronic renal insufficiency patients and dialysis treated patients (DTP) by measuring total cholesterol (TC), triglycerides (TG), HDL-cholesterol (HDL-C), LDL-cholesterol (LDL-C) and calculating the risk factor ratio: TC/HDL-C and LDL-C/HDL-C. The examined group consisted of 18 chronic renal insufficiency patients and 60 patients on hemodialysis. The results were compared to a control group of 85 voluntary blood donors. Serum lipid parameters were examined by standard methods. All lipid parameters in hemodialysis patients were statistically different as compared to the control group (p<0.05) while chronic renal insufficiency patients showed significant difference only in triglycerides and HDL-cholesterol. Hypertriglyceridemia was present in both examined groups of patients and HDL-cholesterol was lower within both groups. All calculated atherogenic ratios were higher for patients than the control group. Lipid parameters were compared between chronic renal insufficiency and hemodialysis patients, but statistically significant difference was obtained only for HDL-cholesterol (p<0.05). The increased values of triglycerides and lower HDL-cholesterol in chronic renal insufficiency patients contribute to high incidence of cardiovascular disease. Chronic renal insufficiency patients have impaired reverse cholesterol transport from peripheral cells to lipoproteins, decreased levels of HDL-cholesterol, hypertriglyceridemia prevalence of small, dense LDL and increased levels of potentially atherogenic remnant particles.

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.

The role of hypothiocyanous acid (HOSCN) in biological systems

Hypohalous acids (HOX), produced by peroxidase-catalysed reactions of halide and pseudohalide ions with H(2)O(2), play an important role in the human immune system. However, there is compelling evidence that these oxidants also mediate host tissue damage and contribute to the progression of a number of inflammatory diseases. Although it is well established that significant amounts of hypothiocyanous acid (HOSCN) are formed under physiological conditions, the reactions of this oxidant with host biological systems are relatively poorly characterized. It is generally accepted that HOSCN is a mild oxidant that reacts selectively with thiols. However, it is becoming increasingly recognized that this selectivity can result in the induction of significant cellular damage, which may contribute to disease. This review will outline the formation and reactivity of HOSCN and the role of this oxidant in biological systems.

Scavenger receptors of endothelial cells mediate the uptake and cellular proatherogenic effects of carbamylated LDL

OBJECTIVE

Carbamylated LDL (cLDL) has been recently shown to have robust proatherogenic effects on human endothelial cells in vitro, suggesting cLDL may have a significant role in atherosclerosis in uremia. The current study was designed to determine which receptors are used by cLDL and thus cause the proatherogenic effects.

METHODS AND RESULTS

In ex vivo or in vitro models as well as in intact animals, administration of cLDL was associated with endothelial internalization of cLDL and subendothelial translocation (transcytosis). In vitro recombinant LOX-1 and SREC-1 receptors showed the greatest cLDL binding. However, pretreatment of the endothelial cells with specific inhibiting antibodies demonstrated that cLDL binds mainly to LOX-1 and CD36 receptors. The transcytosis was dependent on SR-A1, SREC-1, and CD36 receptors whereas LOX-1 receptor was not involved. The cytotoxicity was mediated by several studied scavenger receptors, but cLDL-induced monocyte adhesion depended only on LOX-1. The cLDL-induced synthesis of LOX-1 protein significantly contributed to both cytotoxicity and accelerated monocyte adhesion to endothelial cells.

CONCLUSIONS

Our data suggest that cLDL uses a unique pattern of scavenger receptors. They show that LOX-1 receptor, and partially CD36, SREC-1, and SR-A1 receptors, are essential for the proatherogenic effects of cLDL on human endothelial cells.

Myeloperoxidase-mediated lipoprotein carbamylation as a mechanistic pathway for atherosclerotic vascular disease

There is an emerging and significant body of research that suggests that MPO (myeloperoxidase) may be a critical mediator in dysfunctional lipoprotein formation and, hence, atherogenic initiation and progression. MPO is a haem peroxidase found in leucocytes and is abundant in macrophages surrounding atherosclerotic lesions. Several lines of evidence support the role of MPO-mediated carbamylation of proteins in atherogenesis. The generic mechanism of MPO-mediated protein carbamylation has been elucidated recently and has been identified as a potentially crucial pathway that links smoking, inflammation and atherogenesis. HDL (high-density lipoprotein) exerts a physiologically beneficial effect of reducing arterial cholesterol deposition; however, there are considerable gaps in current understanding of the molecular basis of dysfunctional HDL formation. Especially deserving of attention is a contextual understanding of dysfunctional pro-atherogenic HDL formation in light of inflammatory changes in atheroma. The present review is especially timely in light of the solved structures of nascent and discoidal HDL and integrates the biochemical significance of MPO carbamylation in the context of these structures. Various avenues of experimental investigation are explored which will be crucial in understanding the vascular consequences of dysfunctional HDL formation and the identification of novel mechanistic pathways in vascular disease. It is anticipated that further knowledge on the intricacies of dysfunctional HDL formation, potentially by an MPO-driven pathway, will lead to considerable progress in identifying novel drug targets for atherosclerosis and characterization of the primary atherogenic process.

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

AIM

Presence of accelerated atherosclerosis in dialysis patients cannot be entirely explained by conventional risk factors. Exposure to urea, which is elevated in patients with kidney disease, leads to the carbamylation of proteins. We investigated the effects of carbamylated low-density lipoprotein (cLDL) on human coronary artery vascular smooth muscle cells (VSMC).

METHODS

Native LDL (nLDL) was carbamylated with potassium cyanate. Cells were incubated with different concentrations of cLDL carbamylated at different time points. Cytotoxicity, apoptosis, proliferation (bromodeoxyuridine incorporation), expression of adhesion molecules and extracellular matrix protein synthesis were studied.

RESULTS

Carbamylated low-density lipoprotein exposure leads to morphological alterations and presence of cellular debris. Neither nLDL nor cLDL caused apoptosis. Lactate dehydrogenase (LDH) release was not different between groups. Carbamylated low-density lipoprotein led to a striking proliferation in VSMC compared to nLDL. Carbamylated low-density lipoprotein significantly increased intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 expression compared to the control. The effects of cLDL on proliferation and adhesion molecule expression were dose-dependent and correlated with the degree of low-density lipoprotein carbamylation. cLDL had no effect on extracellular matrix protein synthesis.

CONCLUSION

The results support the hypothesis that cLDL may contribute to the pathogenesis of atherosclerosis in uraemic patients.

Lipid abnormalities and cardiovascular risk in renal disease

The recent 4D study failed to provide definitive evidence for benefit of statin use in type 2 diabetics on dialysis. This finding stands in stark contrast to a number of other observations in patients with early stages of chronic kidney disease where substantial benefit of statins had been documented. Here we discuss some potential explanations for the unexpected finding of the 4D study and for the negative association between below average total cholesterol and vascular mortality among dialysis patients. Admittedly, in the absence of definite evidence in dialysis patients, we still conclude that the administration of statins is appropriate in patients with manifest coronary disease.

Protein carbamylation links inflammation, smoking, uremia and atherogenesis

Post-translational modification and functional impairment of proteins through carbamylation is thought to promote vascular dysfunction during end-stage renal disease. Cyanate, a reactive species in equilibrium with urea, carbamylates protein lysine residues to form epsilon-carbamyllysine (homocitrulline), altering protein structure and function. We now report the discovery of an alternative and quantitatively dominant mechanism for cyanate formation and protein carbamylation at sites of inflammation and atherosclerotic plaque: myeloperoxidase-catalyzed oxidation of thiocyanate, an anion abundant in blood whose levels are elevated in smokers. We also show that myeloperoxidase-catalyzed lipoprotein carbamylation facilitates multiple pro-atherosclerotic activities, including conversion of low-density lipoprotein into a ligand for macrophage scavenger receptor A1 recognition, cholesterol accumulation and foam-cell formation. In two separate clinical studies (combined n = 1,000 subjects), plasma levels of protein-bound homocitrulline independently predicted increased risk of coronary artery disease, future myocardial infarction, stroke and death. We propose that protein carbamylation is a mechanism linking inflammation, smoking, uremia and coronary artery disease pathogenesis.

Endocrinology and Dialysis Jean L. HolleySeries Editor: Lipid Abnormalities Associated with End-Stage Renal Disease

Patients undergoing chronic renal replacement therapy have a high incidence of dyslipidemia. In general, there are increased concentrations of triglyceride-rich apolipoprotein B-containing particles. These elevations lead to increased levels of non-high-density lipoprotein (HDL) levels. This pattern is further modified by the method of dialysis (peritoneal versus hemodialysis) and comorbidities such as diabetes. End-stage renal disease patients also demonstrate increased levels of lipoprotein(a) (Lp(a)) and oxidized low-density lipoprotein (LDL)both of which are highly atherogenic. This review focuses on the pathogenesis of these lipid abnormalities and their role in the atherosclerotic process.

Carbamylated low-density lipoprotein induces death of endothelial cells: a link to atherosclerosis in patients with kidney disease

BACKGROUND

The presence of accelerated atherosclerosis in patients with kidney disease cannot be entirely explained by traditional cardiovascular risk factors. Exposure to urea, which is normally present in human blood plasma and elevated in patients with kidney disease, leads to the carbamylation of proteins. We postulated that low-density lipoprotein (LDL) carbamylated by urea has biologic effects relevant to atherosclerosis.

METHODS

To produce carbamylated LDL (cLDL), human native LDL (nLDL) was chemically modified in vitro by exposure to potassium cyanate. Human coronary artery endothelial cells (HCAECs) and human coronary artery smooth muscle cells (CASMCs) were treated in vitro with cLDL or nLDL. Irreversible cell death was measured using the lactate dehydrogenase (LDH) assay, apoptosis was assessed by annexin V binding, and proliferation was determined using bromodeoxyuridine (BrdU) incorporation. Total plasma protein carbamylation and plasma cLDL were measured in hemodialysis patients using the homocitrulline assay and enzyme-linked immunosorbent assay (ELISA).

RESULTS

Our studies demonstrated that cLDL but not nLDL induced dose-dependent vascular cell injuries relevant to atherosclerosis, which included the proliferation of vascular smooth muscle cells and endothelial cell death. Under light microscopy, endothelial cells treated with cLDL showed signs of morphologic alterations. The injury to endothelial cells measured by LDH release was time-dependent and correlated with the degree of LDL carbamylation. At least a part of the endothelial cell population treated with cLDL died by apoptosis. In patients with advanced renal disease on hemodialysis, total plasma protein carbamylation and plasma cLDL were several times higher than in control healthy individuals.

CONCLUSION

Collectively these data suggest the potential role of carbamylated LDL in accelerated atherosclerosis in patients with chronic renal disease and, possibly, in healthy individuals.

CELL SURVIVAL IN THE HOSTILE ENVIRONMENT OF THE RENAL MEDULLA

The countercurrent system in the medulla of the mammalian kidney provides the basis for the production of urine of widely varying osmolalities, but necessarily entails extreme conditions for medullary cells, i.e., high concentrations of solutes (mainly NaCl and urea) in antidiuresis, massive changes in extracellular solute concentrations during the transitions from antidiuresis to diuresis and vice versa, and low oxygen tension. The strategies used by medullary cells to survive in this hostile milieu include accumulation of organic osmolytes and heat shock proteins, the extensive use of the glycolysis for energy production, and a well-orchestrated network of signaling pathways coordinating medullary circulation and tubular work.

Quantification of carbamylated LDL in human sera by a new sandwich ELISA

BACKGROUND

We previously suggested that increased carbamylated LDL (cLDL), a product of nonenzymatic modification of LDL in human serum by urea-derived cyanate, may cause cardiovascular complications in patients with chronic renal insufficiency. An assay for precise measurement of cLDL in serum was not previously available.

METHODS

Polyclonal antibodies against human cLDL and nonmodified, native LDL (nLDL) were raised in rabbits and extensively purified by affinity chromatography. New sandwich ELISAs to measure cLDL and nLDL with use of these antibodies were developed. Serum concentrations of cLDL and nLDL were measured by the sandwich ELISAs in 41 patients with end-stage renal disease (ESRD) and 40 healthy controls.

RESULTS

Both assays showed satisfactory reproducibility, linearity, and recovery. The assays could detect 2.7 mg/L cLDL with a linear detection range of 5-1000 mg/L and 5 mg/L nLDL with a linear detection range of 50-1000 mg/L. These measurements showed that patients with ESRD have significantly increased serum cLDL [281.5 (46.9) mg/L compared with 86.1 (29.7) mg/L in a control group; P <0.001]. There was no significant difference in nLDL concentrations between the groups.

CONCLUSIONS

These assays are a potentially valuable tool for cardiovascular research in renal patients and healthy individuals. The cLDL concentration appears to be the highest among all previously described modified LDL isoforms in both controls and ESRD patients.

Inhibition of erythropoietin activity by cyanate

OBJECTIVE

Increased urea concentration is a measure of advanced renal failure and the adequacy of renal replacement therapy in end-stage renal disease (ESRD). Altered biologic activity due to changes in protein structure occurs when cyanate, formed spontaneously from urea, reacts with proteins. Carbamylation results in impaired erythropoietin (EPO) activity when high concentrations of cyanate react with EPO. In this study, the activity of carbamylated EPO (C-EPO), formed at a cyanate concentration which may occur in vivo, was studied in Sprague-Dawley rats.

MATERIAL AND METHODS

The extent of carbamylation, causing loss of free amino groups, was monitored using trinitrobenzenesulfonic acid. Erythrocyte, hemoglobin, hematocrit and leukocyte levels were measured after either EPO, incubated EPO, C-EPO, physiologic saline or cyanate (1.5 microM; 0.2 ml) were injected subcutaneous twice weekly for 3 weeks in rats.

RESULTS

In vitro carbamylation of EPO was time- and concentration-dependent. C-EPO concentration increased as the duration of exposure to cyanate increased from 6 to 72 h, or as cyanate concentration increased from 15 nM to 1.5 microM. Injections of EPO caused significant increases in vivo in all erythropoietic measures. In contrast, injections of C-EPO, physiologic saline or 1.5 microM cyanate caused no change from baseline.

CONCLUSIONS

These results demonstrated diminished biologic activity in healthy rats by C-EPO formed in vitro at cyanate concentrations that may be found in vivo. C-EPO and high urea-derived cyanate levels may contribute to suboptimal erythropoietic responses to EPO therapy for chronic renal failure and ESRD, and may provide another measurement indicating inadequate dialysis.