Chronic uremia stimulates LDL carbamylation and atherosclerosis

The association between monocyte to high-density lipoprotein cholesterol ratio and chronic kidney disease in a Chinese adult population: a cross-sectional study

BACKGROUND

Monocyte to high-density lipoprotein cholesterol ratio (MHR) was confirmed as a novel inflammatory marker and strongly associated with the risk of several diseases. This study aimed to investigate the relationship between MHR and chronic kidney disease (CKD) in a Chinese adult population.

METHODS

In this cross-sectional study, 232,775 community-dwelling adults in Binhai who completed health checkups in 2021 were enrolled. Participants were categorized based on the MHR quartiles. Clinical characteristics of participants across different groups were compared using one-way ANOVA, Kruskal-Wallis h-test, and Chi-squared test as appropriate. Univariate and multivariable logistic regression analyses were taken to assess the relationship between MHR and the presence of CKD, as well as its association with low estimated glomerular filtration rate (eGFR) and proteinuria. Subgroup analyses were further executed to confirm the reliability of this relationship.

RESULTS

A total of 21,014 (9.0%) individuals were diagnosed with CKD. Characteristic indicators including waist circumference, body mass index (BMI), blood pressure (BP), serum uric acid (SUA), triglyceride, and fasting blood glucose (FBG) showed a gradual increase with higher MHR quartiles, whereas parameters such as age, total cholesterol, high-density lipoprotein cholesterol (HDL-C), and eGFR decreased (p < .001). In the multivariable logistic regression analysis, we observed independent associations between MHR (per 1 SD increase) and CKD, as well as low eGFR and proteinuria, with odds ratio (ORs) and 95% confidence intervals (95%CIs) of 1.206 (1.186-1.225), 1.289 (1.260-1.319), and 1.150 (1.129-1.171), respectively (p < .001). Similar conclusions were confirmed in subgroup analysis stratified by gender, age, BMI, central obesity, hypertension, and diabetes mellitus, after justification for confounding factors.

CONCLUSION

Elevated MHR level was independently associated with the presence of CKD, suggesting that it might serve as a useful clinical tool for risk stratification, offering valuable insights to inform preventive and therapeutic approaches for clinicians in their routine medical practice.

Leukocyte-endothelial interaction in CKD

Chronic kidney disease (CKD) represents an independent risk factor for cardiovascular diseases (CVD). Accordingly, CKD patients show a substantial increased risk of cardiovascular mortality. Inflammation represents an important link between CKD and CVD. The interaction between endothelial cells and effector cells of the innate immune system plays a central role in the development and progression of inflammation. Vascular injury causes endothelial dysfunction, leading to augmented oxidative stress, increased expression of leukocyte adhesion molecules and chronic inflammation. CKD induces numerous metabolic changes, creating a uremic milieu resulting in the accumulation of various uremic toxins. These toxins lead to vascular injury, endothelial dysfunction and activation of the innate immune system. Recent studies describe CKD-dependent changes in monocytes that promote endothelial dysfunction and thus CKD progression and CKD-associated CVD. The NLR family pyrin domain containing 3-interleukin-1β-interleukin-6 (NLRP3-IL-1β-IL-6) signaling pathway plays a pivotal role in the development and progression of CVD and CKD alike. Several clinical trials are investigating targeted inhibition of this pathway indicating that anti-inflammatory therapeutic strategies may emerge as novel approaches in patients at high cardiovascular risk and nonresolving inflammation. CKD patients in particular would benefit from targeted anti-inflammatory therapy, since conventional therapeutic regimens have limited efficacy in this population.

B- and T-lymphocyte attenuator could be a new player in accelerated atherosclerosis associated with chronic kidney disease

BACKGROUND

In chronic kidney disease (CKD), cardiovascular morbi-mortality is higher than in general population. Atherosclerotic cardiovascular disease is accelerated in CKD, but specific CKD-related risk factors for atherosclerosis are unknown.

METHODS

CKD patients from the NEFRONA study were used. We performed mRNA array from blood of patients free from atheroma plaque at baseline, with (n=10) and without (n=10) de novo atherosclerotic plaque development 2 years later. Selected mRNA candidates were validated in a bigger sample (n=148). Validated candidates were investigated in vivo in an experimental model of CKD-accelerated atherosclerosis, and in vitro in murine macrophages.

RESULTS

mRNA array analysis showed 92 up-regulated and 67 down-regulated mRNAs in samples from CKD patients with de novo plaque development. The functional analysis pointed to a paramount role of the immune response. The validation in a bigger sample confirmed that B- and T-lymphocyte co-inhibitory molecule (BTLA) down-regulation was associated with de novo plaque presence after 2 years. However, BTLA down-regulation was not found to be associated with atherosclerotic progression in patients with plaque already present at baseline. In a model of CKD-accelerated atherosclerosis, mRNA and protein expression levels of BTLA were significantly decreased in blood samples and atheroma plaques. Plaques from animals with CKD were bigger, had more infiltration of inflammatory cells, higher expression of IL6 and IL17 and less presence of collagen than plaques from control animals. Incubation of macrophages with rat uremic serum decreased BTLA expression.

CONCLUSIONS

BTLA could be a potential biomarker or therapeutic target for atherosclerosis incidence in CKD patients.

Protein posttranslational modifications in health and diseases: Functions, regulatory mechanisms, and therapeutic implications

Protein posttranslational modifications (PTMs) refer to the breaking or generation of covalent bonds on the backbones or amino acid side chains of proteins and expand the diversity of proteins, which provides the basis for the emergence of organismal complexity. To date, more than 650 types of protein modifications, such as the most well-known phosphorylation, ubiquitination, glycosylation, methylation, SUMOylation, short-chain and long-chain acylation modifications, redox modifications, and irreversible modifications, have been described, and the inventory is still increasing. By changing the protein conformation, localization, activity, stability, charges, and interactions with other biomolecules, PTMs ultimately alter the phenotypes and biological processes of cells. The homeostasis of protein modifications is important to human health. Abnormal PTMs may cause changes in protein properties and loss of protein functions, which are closely related to the occurrence and development of various diseases. In this review, we systematically introduce the characteristics, regulatory mechanisms, and functions of various PTMs in health and diseases. In addition, the therapeutic prospects in various diseases by targeting PTMs and associated regulatory enzymes are also summarized. This work will deepen the understanding of protein modifications in health and diseases and promote the discovery of diagnostic and prognostic markers and drug targets for diseases.

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.

Urea levels and cardiovascular disease in patients with chronic kidney disease

BACKGROUND

Elevated serum urea levels are common in moderate-to-advanced CKD. Several studies have shown that urea is a direct and indirect uremic toxin, especially with regard to cardiovascular disease. We sought to determine whether serum urea levels are associated with adverse cardiovascular events and death before renal replacement therapy (RRT) in patients with CKD.

METHODS

CKD-REIN is a prospective cohort of CKD nephrology outpatients not receiving maintenance dialysis. The 2507 patients included in the analysis were divided into three groups according to the baseline serum urea level (T1 < 10.5, T2:10.5 to 15.1, and T3 ≥ 15.1 mmol/L). Cox proportional hazard models were used to estimate hazard ratios (HRs) for first atheromatous or nonatheromatous cardiovascular (CV) events, and all-cause mortality before RRT. The models were adjusted for baseline comorbidities, laboratory data, and medications.

FINDINGS

Of the 2507 included patients (median [interquartile range (IQR)] age: 69[61-77]; mean (standard deviation) eGFR 33.5(11.6) mL/min/1.73 m²), 54% had a history of cardiovascular disease. After multiple adjustments for cardiovascular risk factors (including eGFR), patients in T3 had a higher risk of atheromatous and nonatheromatous cardiovascular events than patient in T1 (n events = 451, HR[95%CI]: 1.93[1.39-2.69]). The adjusted HRs for death before RRT (n events = 407) were 1.31[0.97; 1.76] and 1.73[1.22; 2.45] for patients T2 and those in T3, respectively.

INTERPRETATION

Our data suggested that urea is a predictor of cardiovascular outcomes beyond CV risk factors including eGFR.

Losartan Attenuates Atherosclerosis in Uremic Mice by Regulating Treg/Th17 Balance via Mediating PTEN/PI3K/Akt Pathway

INTRODUCTION

Uremia could accelerate atherosclerosis (AS) formation involving Treg/Th17 imbalance. Losartan regulates the imbalance between regulatory T cells (Treg cells) and T helper 17 cells (Th17 cells). However, their interactions in uremia accelerated AS (UAAS) remained poorly understood.

METHODS

UAAS mice model was established, and after losartan and VO-OHpic (VO, phosphatase and tensin homolog [PTEN] inhibitor) injection, biological indexes, and inflammatory cytokines (transforming growth factor-β1, TGF-β1; interleukin-10 [IL-10]; IL-17 and IL-6) levels were determined using enzyme-linked immunosorbent assay. Pathological changes on aorta were observed using hematoxylin-eosin staining. Percentages of Treg cells (CD4+CD25+Foxp3+) and Th17 cells (CD4+IL-17+) in total CD4+ T cells were determined using flow cytometry. PTEN expressions were measured using Western blot, quantitative real-time polymerase chain reaction, and immunohistochemistry staining as needed.

RESULTS

After UAAS mice model construction, biological indexes (urea, cholesterol, and triglycerides) levels were increased, and aortic atherosclerotic plaque was formed. In UAAS mice, in total CD4+ T cells, Treg cells percentage was decreased yet Th17 cells percentage was increased, and TGF-β1 and IL-10 levels were downregulated yet IL-17 and IL-6 levels were upregulated. An opposite effect was found after losartan treatment. PTEN was downregulated in UAAS mice, and suppressing PTEN reversed the alleviating effects of losartan in UAAS mice.

CONCLUSION

Losartan attenuated UAAS in mice by regulating Treg/Th17 cells balance via mediating PTEN/PI3K/Akt pathway, providing possible therapeutic method for UAAS in clinical practice.

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.

The Protective Effects of Lipid-Lowering Agents on Cardiovascular Disease and Mortality in Maintenance Dialysis Patients: Propensity Score Analysis of a Population-Based Cohort Study

Lipid-lowering agents display limited benefits on cardiovascular diseases and mortality in patients undergoing dialysis. Therefore, they are not routinely recommended for dialysis patients. The aim of this study was to assess the effects of lipid-lowering agents on clinical outcomes in dialysis patients on the basis of real-world evidence. This research used Taiwan’s National Health Insurance Research Database to identify dialysis patients from January 2009 to December 2015; patients were then categorized into a case group treated with lipid-lowering agents (n = 3,933) and a control group without lipid-lowering agents (n = 24,267). Patients were matched by age, sex, and comorbidities in a 1:1 ratio. This study used the Cox regression model to estimate the hazard ratios (HRs) for mortality and major adverse cardiovascular events (MACEs) for events recorded until December 2017. During a mean follow-up period of approximately 3.1 years, 1726 [43.9%, incidence 0.123/person-year (PY)] deaths and 598 (15.2%, incidence 0.047/PY) MACEs occurred in the case group and 2031 (51.6%, incidence 0.153/PY) deaths and 649 (16.5% incidence 0.055/PY) MACEs occurred in the control group. In the multivariable analysis of the Cox regression model, lipid-lowering agent users showed a significantly lower risk of death [HR: 0.75; 95% confidence interval (CI): 0.70–0.80] and MACEs (HR: 0.88; 95% CI: 0.78–0.98) than lipid-lowering agent non-users. Moreover, the survival benefit of lipid-lowering agents was significant across most subgroups. Dialysis patients treated with lipid-lowering agents display a 25 and 12% reduction in their risk of mortality and MACEs, respectively. Therefore, lipid-lowering agents might be considered when treating dialysis patients with hyperlipidemia.

Impact of Uremic Toxins on Endothelial Dysfunction in Chronic Kidney Disease: A Systematic Review

Patients with chronic kidney disease (CKD) are at a highly increased risk of cardiovascular complications, with increased vascular inflammation, accelerated atherogenesis and enhanced thrombotic risk. Considering the central role of the endothelium in protecting from atherogenesis and thrombosis, as well as its cardioprotective role in regulating vasorelaxation, this study aimed to systematically integrate literature on CKD-associated endothelial dysfunction, including the underlying molecular mechanisms, into a comprehensive overview. Therefore, we conducted a systematic review of literature describing uremic serum or uremic toxin-induced vascular dysfunction with a special focus on the endothelium. This revealed 39 studies analyzing the effects of uremic serum or the uremic toxins indoxyl sulfate, cyanate, modified LDL, the advanced glycation end products N-carboxymethyl-lysine and N-carboxyethyl-lysine, p-cresol and p-cresyl sulfate, phosphate, uric acid and asymmetric dimethylarginine. Most studies described an increase in inflammation, oxidative stress, leukocyte migration and adhesion, cell death and a thrombotic phenotype upon uremic conditions or uremic toxin treatment of endothelial cells. Cellular signaling pathways that were frequently activated included the ROS, MAPK/NF-κB, the Aryl-Hydrocarbon-Receptor and RAGE pathways. Overall, this review provides detailed insights into pathophysiological and molecular mechanisms underlying endothelial dysfunction in CKD. Targeting these pathways may provide new therapeutic strategies reducing increased the cardiovascular risk in CKD.

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.

Carbamylated sortilin associates with cardiovascular calcification in patients with chronic kidney disease

Sortilin, an intracellular sorting receptor, has been identified as a cardiovascular risk factor in the general population. Patients with chronic kidney disease (CKD) are highly susceptible to develop cardiovascular complications such as calcification. However, specific CKD-induced posttranslational protein modifications of sortilin and their link to cardiovascular calcification remain unknown. To investigate this, we examined two independent CKD cohorts for carbamylation of circulating sortilin and detected increased carbamylated sortilin lysine residues in the extracellular domain of sortilin with kidney function decline using targeted mass spectrometry. Structure analysis predicted altered ligand binding by carbamylated sortilin, which was verified by binding studies using surface plasmon resonance measurement, showing an increased affinity of interleukin 6 to in vitro carbamylated sortilin. Further, carbamylated sortilin increased vascular calcification in vitro and ex vivo that was accelerated by interleukin 6. Imaging by mass spectrometry of human calcified arteries revealed in situ carbamylated sortilin. In patients with CKD, sortilin carbamylation was associated with coronary artery calcification, independent of age and kidney function. Moreover, patients with carbamylated sortilin displayed significantly faster progression of coronary artery calcification than patients without sortilin carbamylation. Thus, carbamylated sortilin may be a risk factor for cardiovascular calcification and may contribute to elevated cardiovascular complications in patients with CKD.

The effect of dihydroceramide desaturase 1 inhibition on endothelial impairment induced by indoxyl sulfate

Protein-bound uremic toxins (PBUTs) have adverse effects on vascular function, which is imperative in the progression of cardiovascular and renal diseases. The role of sphingolipids in PBUT-mediated vasculo-endothelial pathophysiology is unclear. This study assessed the therapeutic potential of dihydroceramide desaturase 1 (Des1) inhibition, the last enzyme involved in de novo ceramide synthesis, to mitigate the vascular effects of the PBUT indoxyl sulfate (IS). Rat aortic rings were isolated and vascular reactivity was assessed in organ bath experiments followed by immunohistochemical analyses. Furthermore, cultured human aortic endothelial cells were assessed for phenotypic and mechanistic changes. Inhibition of Des1 by a selective inhibitor CIN038 (0.1 to 0.3 μM) improved IS-induced impairment of vasorelaxation and modulated immunoreactivity of oxidative stress markers. Des1 inhibition also reversed IS-induced reduction in endothelial cell migration (1.0 μM) by promoting the expression of angiogenic cytokines and reducing inflammatory and oxidative stress markers. These effects were associated with a reduction of TIMP1 and the restoration of Akt phosphorylation. In conclusion, Des1 inhibition improved vascular relaxation and endothelial cell migration impaired by IS overload. Therefore, Des1 may be a suitable intracellular target to mitigate PBUT-induced adverse vascular effects.

Neuropathy - Exponent of Accelerated Involution in Uremia: The Role of Carbamylation

Premature loss of functional integrity of the nervous system in chronic renal failure (CRF) as a consequence of persistent biological activities of the general uremic milieu is almost identical to its structural and functional involution during the process of physiological ageing, but disproportionate and independent of chronological age. In the hyperuremic status of CRF (urea - carbamide), forced carbamylation, as a non-enzymatic post-translational modification (NEPTM) of proteins and amino acids, by changing their biological properties and decreasing proteolysis capacity, represents pathogenetic potential of intensified molecular ageing and accelerated, pathological involution. Physiological predisposition and the exposure of neuropathy before complications of other organs and organ systems in CRF, due to the simultaneous and mutually pathogenetically related uremic lesion and the tissue and vascular segment of the nervous system, direct interest towards proteomic analytical techniques of quantification of carbamylated products as biomarkers of uremic neurotoxicity. Hypothetically, identical to the already established applications of other NEPTM products in practice, they have the potential of clinical methodology in the evaluation of uremic neuropathy and its contribution to the general prediction, but also to the change of the conventional CRF classification. In addition, the identification and therapeutic control of the substrate of accelerated involution, responsible for the amplification of not only neurological but also general degenerative processes in CRF, is attractive in the context of the well-known attitude towards aging.

Effects of blood urea nitrogen independent of the estimated glomerular filtration rate on the development of anemia in non-dialysis chronic kidney disease: The results of the KNOW-CKD study

BACKGROUND

Anemia is a common complication of chronic kidney disease (CKD). Blood urea nitrogen (BUN) in CKD represents nitrogenous uremic toxin accumulation which could be involved in anemia of CKD. We investigated the effects of BUN independent of estimated glomerular filtration rate (eGFR) on anemia in non-dialysis CKD (NDCKD).

METHODS

This prospective study included 2,196 subjects enrolled in the KoreaN Cohort Study for Outcome in Patients With Chronic Kidney Disease (KNOW-CKD) cohort with BUN and hemoglobin level data. Initially, we investigated the association between BUN and hemoglobin level. To examine the impact of baseline BUN on the incident anemia, a longitudinal study was performed on 1,169 patients without anemia at study enrollment. BUN residuals were obtained from the fitted curve between BUN and eGFR. Anemia was defined as a hemoglobin level of <13.0 g/dL for men and <12.0 g/dL for women.

RESULTS

BUN residuals were not related to eGFR but to daily protein intake (DPI), while BUN was related to both eGFR and DPI. BUN was inversely associated with hemoglobin level (β -0.03; 95% confidence interval [CI] -0.04, -0.03; P <0.001) in the multivariable linear regression analysis adjusted for multiple confounders including eGFR, and BUN residual used instead of BUN was also inversely associated with hemoglobin level (β -0.03; 95% CI -0.04, -0.02; P <0.001). Among the 1,169 subjects without anemia at baseline, 414 (35.4%) subjects newly developed anemia during the follow-up period of 37.5 ± 22.1 months. In the multivariable Cox regression analysis with adjustment, both high BUN level (Hazard ratio [HR] 1.02; 95% CI 1.01, 1.04; P = 0.002) and BUN residual used instead of BUN (HR 1.02; 95% CI 1.00, 1.04; P = 0.031) increased the risk of anemia development. Moreover, BUN, rather than eGFR, increased the risk of anemia development in patients with CKD stage 3 in the multivariable Cox regression.

CONCLUSION

Higher BUN levels derived from inappropriately high protein intake relative to renal function were associated with low hemoglobin levels and the increased risk of anemia independent of eGFR in NDCKD patients.

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.

Arterial Remodelling in Chronic Kidney Disease: Impact of Uraemic Toxins and New Pharmacological Approaches

Chronic kidney disease (CKD) is a major public health concern that affects around 10 percent of the world’s population. The severity of CKD is mainly due to the high prevalence of cardiovascular (CV) complications in this population. The aim of this review is to describe the arterial remodelling associated with CKD, to provide a quick overview of the mechanisms involved and to review the recent pharmacological approaches aimed at improving vascular health in CKD. CKD patients are exposed to metabolic and haemodynamic disorders that may affect the CV system. Large artery functional and geometric abnormalities have been well documented in CKD patients and are associated with an increase in arterial stiffness and a maladaptive remodelling. Uraemic toxins, such as indoxyl sulphate, p-cresyl sulphate, protein carbamylation and advanced glycation products, exert various effects on vascular smooth muscle cell functions. The low-grade inflammation associated with CKD may also affect arterial wall composition and remodelling. It is worth noting that the CV risk for CKD patients remains high despite the pharmacological control of traditional CV risk factors, suggesting the need for innovative therapeutic strategies. An interventional study targeting the NLRP3 inflammasome has provided some interesting preliminary results that need to be confirmed, especially in terms of safety.

Irreversible post-translational modifications - Emerging cardiovascular risk factors

Despite the introduction of lipid-lowering drugs, antihypertensives, antiplatelet and anticoagulation therapies for primary prevention of cardiovascular and heart diseases (CVD), it remains the number one cause of death globally, raising the question for novel/further essential factors besides traditional risk factors such as cholesterol, blood pressure and coagulation. With continuous identification and characterization of non-enzymatic post-translationally modified isoforms of proteins and lipoproteins, it is becoming increasingly clear that irreversible non-enzymatic post-translational modifications (nPTMs) alter the biological functions of native proteins and lipoproteins thereby transforming innate serum components into CVD mediators. In particular renal insufficiency and metabolic imbalance are major contributors to the systemically increased concentration of reactive metabolites and thus increased frequency of nPTMs, promoting multi-morbid disease development centering around cardiovascular disease. nPTMs are significantly involved in the onset and progression of cardiovascular disease and represent a significant and novel risk factor. These insights represent potentially new avenues for risk assessment, prevention and therapy. This review chapter summarizes all forms of nPTMs found in CKD and under metabolic imbalance and discusses the biochemical connections between molecular alterations and the pathological impact on increased cardiovascular risk, novel nPTM-associated non-traditional cardiovascular risk factors, and clinical implication of nPTM in cardiovascular disease.

Lipoproteins and fatty acids in chronic kidney disease: molecular and metabolic alterations

Chronic kidney disease (CKD) induces modifications in lipid and lipoprotein metabolism and homeostasis. These modifications can promote, modulate and/or accelerate CKD and secondary cardiovascular disease (CVD). Lipid and lipoprotein abnormalities - involving triglyceride-rich lipoproteins, LDL and/or HDL - not only involve changes in concentration but also changes in molecular structure, including protein composition, incorporation of small molecules and post-translational modifications. These alterations modify the function of lipoproteins and can trigger pro-inflammatory and pro-atherogenic processes, as well as oxidative stress. Serum fatty acid levels are also often altered in patients with CKD and lead to changes in fatty acid metabolism - a key process in intracellular energy production - that induce mitochondrial dysfunction and cellular damage. These fatty acid changes might not only have a negative impact on the heart, but also contribute to the progression of kidney damage. The presence of these lipoprotein alterations within a biological environment characterized by increased inflammation and oxidative stress, as well as the competing risk of non-atherosclerotic cardiovascular death as kidney function declines, has important therapeutic implications. Additional research is needed to clarify the pathophysiological link between lipid and lipoprotein modifications, and kidney dysfunction, as well as the genesis and/or progression of CVD in patients with kidney disease.

Lipoproteins in chronic kidney disease: from bench to bedside

Chronic kidney disease (CKD) is associated with high cardiovascular risk. CKD patients exhibit a specific lipoprotein pattern termed 'uraemic dyslipidaemia', which is characterized by rather normal low-density lipoprotein cholesterol, low high-density lipoprotein cholesterol, and high triglyceride plasma levels. All three lipoprotein classes are involved in the pathogenesis of CKD-associated cardiovascular diseases (CVDs). Uraemia leads to several modifications of the structure of lipoproteins such as changes of the proteome and the lipidome, post-translational protein modifications (e.g. carbamylation) and accumulation of small-molecular substances within the lipoprotein moieties, which affect their functionality. Lipoproteins from CKD patients interfere with lipid transport and promote inflammation, oxidative stress, endothelial dysfunction as well as other features of atherogenesis, thus contributing to the development of CKD-associated CVD. While, lipid-modifying therapies play an important role in the management of CKD patients, their efficacy is modulated by kidney function. Novel therapeutic agents to prevent the adverse remodelling of lipoproteins in CKD and to improve their functional properties are highly desirable and partially under development.

The value of a low-protein diet and ketoanalogues of essential amino acids in the сontrol of protein carbamylation and toxic effects of urea in chronic kidney disease

Chronic kidney disease (CKD) is characterized by high mortality from cardiovascular diseases, the development of which is facilitated by traditional risk factors (typical for the general population) and by nontraditional ones (specific to patients with CKD) as well. These factors include also uremic toxins, for which a causal relationship has been established with specific pathological processes in patients with CKD, comprising the development of vascular dysfunction and accelerated progression of atherosclerosis. Urea has long been considered not as a uremic toxin, but as a marker of metabolic imbalance or dialysis efficiency (Kt/V) in CKD patients. In recent years, more and more publications have appeared on the study of the toxic effects of urea with the development of toxic-uremic complications and the phenotype of premature aging, common in CKD. It was found that an increase in urea levels in uremic syndrome causes damage to the intestinal epithelial barrier with translocation of bacterial toxins into the bloodstream and the development of systemic inflammation, provokes apoptosis of vascular smooth muscle cells, as well as endothelial dysfunction, which directly contributes to the development of cardiovascular complications. The indirect effects of increased urea levels are associated with carbamylation reactions, when isocyanic acid (a product of urea catabolism) changes the structure and function of proteins in the body. Carbamylation of proteins in CKD patients is associated with the development of renal fibrosis, atherosclerosis and anemia. Thus, urea is now regarded as an important negative agent in the pathogenesis of complications in CKD. Studies on a low-protein diet with using ketoanalogues of essential amino acids to minimize the accumulation of urea and other uremic toxins demonstrate the clinical benefit of such an intervention in slowing the progression of CKD and the development of cardiovascular complications.

Combined application of rapamycin and atorvastatin improves lipid metabolism in apolipoprotein E-deficient mice with chronic kidney disease

Atherosclerosis arising from the pro-inflammatory conditions associated with chronic kidney disease (CKD) increases major cardiovascular morbidity and mortality. Rapamycin (RAPA) is known to inhibit atherosclerosis under CKD and non-CKD conditions, but it can cause dyslipidemia; thus, the co-application of lipid-lowering agents is recommended. Atorvastatin (ATV) has been widely used to reduce serum lipids levels, but its synergistic effect with RAPA in CKD remains unclear. Here, we analyzed the effect of their combined treatment on atherosclerosis stimulated by CKD in apolipoprotein E-deficient (ApoE^−/−) mice. Oil Red O staining revealed that treatment with RAPA and RAPA＋ ATV, but not ATV alone, significantly decreased the atherosclerotic lesions in the aorta and aortic sinus, compared to those seen in the control (CKD) group. The co-administration of RAPA and ATV improved the serum lipid profile and raised the expression levels of proteins involved in reverse cholesterol transport (LXRα, CYP7A1, ABCG1, PPARγ, ApoA1) in the liver. The CKD group showed increased levels of various genes encoding atherosclerosis-promoting cytokines in the spleen (Tnf-α, Il-6 and Il-1β) and aorta (Tnf-α and Il-4), and these increases were attenuated by RAPA treatment. ATV and RAPA＋ATV decreased the levels of Tnf-α and Il-1β in the spleen, but not in the aorta. Together, these results indicate that, in CKD-induced ApoE^−/− mice, RAPA significantly reduces the development of atherosclerosis by regulating the expression of inflammatory cytokines and the co-application of ATV improves lipid metabolism.

The effects of mutation and modification on the structure and stability of human lysozyme: A molecular link between carbamylation and atherosclerosis

Amino acid mutations in some proteins such as lysozyme lead to genetically disorder variants and adverse pathogenic consequences. Recently, amino acid modifications were known as a risk factor in many related diseases such as uremia and atherosclerosis, showing the importance of these surface-structure changes. Although the structural consequences of the hereditary proteins have been examined extensively, such effects for the protein modifications are known to a lesser extent. One drawback in the examination of protein modifications is hardness in experimental detection of modifications by techniques such as NMR and crystallography. Molecular modeling and simulation can help to understand such phenomena at the molecular levels. It is more rational that the effects of both mutation and modification can be compared in a single protein model. Here, molecular dynamics simulation is used to compare the effects of a disease-related carbamylation modification and an amyloidogenic mutation (D67H) in human lysozyme as a model protein. The results show that the carbamylation adversely effects on the tertiary structure, leading to the similar unfolding pathway to the hereditary amyloidogenic form. The carbamylation leads to the instability of the overall protein conformation, especially on the β-domain, which is a characteristic of hereditary amyloidosis in human lysozymes. The aggregation behaviors of both modified and mutant lysozyme were examined by molecular docking calculations. The results showed that the partially unfolded lysozyme might form tight protein aggregates upon carbamylation similar to the amyloidogenic variant. Both single and all-residues carbamylations impose serious conformational changes to the tertiary structure of lysozyme. It was obtained that carbamylation of lysozyme strongly effects on the stability of N-terminal β-sheet, which can produce a highly unstable conformation. The results of this study not only show the adverse structural consequences of a disease-associated post-translational modification, but it also may be very helpful to understand the molecular basis for many carbamylation-related diseases such as atherosclerosis in ESRD patients. The results show that non-native post-translational modifications may be as structurally important as hereditary mutations.

CKD Increases Carbonylation of HDL and Is Associated with Impaired Antiaggregant Properties

BACKGROUND

CKD is associated with increased oxidative stress that correlates with occurrence of cardiovascular events. Modifications induced by increased oxidative stress particularly affect circulating lipoproteins such as HDL that exhibit antiatheromatous and antithrombotic properties in vitro.

METHODS

To explore the specific role of oxidative modifications of HDL in CKD and their effect on the platelet-targeting antiaggregant properties of HDL, we used a CKD (5/6 nephrectomy) rabbit model. For ex vivo assessment of the antiaggregant properties of HDL, we collected blood samples from 15 healthy volunteers, 25 patients on hemodialysis, and 20 on peritoneal dialysis. We analyzed malondialdehyde, 4-hydroxynonenal (HNE), and 4-hydroxy-2-hexenal protein adduct levels. Platelet aggregation and activation were assessed by aggregometry, thromboxane B2 assay, or FACS. We modified HDL from controls by incubating it overnight at 37°C with 100 µM of HNE.

RESULTS

HDL from CKD rabbits and patients on hemodialysis had HNE adducts. The percentage of platelet aggregation or activation induced by collagen was significantly higher when platelets were incubated with HDL from CKD rabbit and hemodialysis groups than with HDL from the control group. In both rabbits and humans, platelet aggregation and activation were significantly higher in the presence of HNE-modified HDL than with HDL from their respective controls. Incubation of platelets with a blocking antibody directed against CD36 or with a pharmacologic inhibitor of SRC kinases restored the antiaggregative phenotype in the presence of HDL from CKD rabbits, patients on hemodialysis and peritoneal dialysis, and HNE-modified HDL.

CONCLUSIONS

HDL from CKD rabbits and patients on hemodialysis exhibited an impaired ability to inhibit platelet aggregation, suggesting that altered HDL properties may contribute to the increased cardiovascular risk in this population.

Implications of Kidney Disease in the Cardiac Patient

Cardiovascular and renal diseases share common pathophysiological grounds, risk factors, and therapies. The 2 entities are closely interlinked and often coexist. The prevalence of kidney disease among cardiac patients is increasing. Patients have an atypical clinical presentation and variable disease manifestation versus the general population. Renal impairment limits therapeutic options and worsens prognosis. Meticulous treatment and close monitoring are required to ensure safety and avoid deterioration of kidney and heart functions. This review highlights recent advances in the diagnosis and treatment of cardiac pathologies, including coronary artery disease, arrhythmia, and heart failure, in patients with decreased renal function.

Novel biomarkers for the evaluation of aging-induced proteinopathies

Proteinopathies are characterized by aging related accumulation of misfolded protein aggregates. Irreversible covalent modifications of aging proteins may significantly affect the native three dimentional conformation of proteins, alter their function and lead to accumulation of misfolded protein as dysfunctional aggregates. Protein misfolding and accumulation of aberrant proteins are known to be associated with aging-induced proteinopathies such as amyloid ß and tau proteins in Alzheimer's disease, α-synuclein in Parkinson's disease and islet amyloid polypeptides in Type 2 diabetes mellitus. Protein oxidation processes such as S-nitrosylation, dityrosine formation and some of the newly elucidated processes such as carbamylation and citrullination recently drew the attention of researchers in the field of Gerontology. Studying over these processes and illuminating their relations between proteinopathies may help to diagnose early and even to treat age related disorders. Therefore, we have chosen to concentrate on aging-induced proteinopathic nature of these novel protein modifications in this review.

Carbamylated Lipoproteins and Progression of Diabetic Kidney Disease

BACKGROUND AND OBJECTIVES

Protein carbamylation is a consequence of uremia and carbamylated lipoproteins contribute to atherogenesis in CKD. Proteins can also be carbamylated by a urea-independent mechanism, and whether carbamylated lipoproteins contribute to the progression of CKD has not been investigated.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

A case-control study was performed to determine whether there were changes in plasma levels of carbamylated lipoproteins in individuals with type 2 diabetes with eGFR >60 ml/min per 1.73 m² compared with a group of age- and sex-matched healthy controls. A cohort of 1320 patients with type 2 diabetes with baseline eGFR ≥30 ml/min per 1.73 m² was longitudinally followed up to evaluate the association between carbamylated lipoproteins and progression of CKD. The primary kidney outcome was defined as doubling of serum creatinine and/or initiation of KRT during follow-up. Plasma carbamylated LDLs and HDLs was measured by ELISA.

RESULTS

In individuals with diabetes with eGFR >60 ml/min per 1.73 m², both plasma carbamylated LDL and HDL levels were higher compared with healthy controls (P<0.001). After a mean follow-up of 9 years of the diabetic cohort, individuals in the top quartile of carbamylated LDL (hazard ratio, 2.21; 95% confidence interval, 1.42 to 3.46; P<0.001) and carbamylated HDL (hazard ratio, 4.53; 95% confidence interval, 2.87 to 7.13; P<0.001) had higher risk of deterioration of kidney function compared with those in the lowest quartile. On multivariable Cox regression analysis, plasma carbamylated LDL was no longer associated with kidney outcome after adjusting for baseline eGFR and potential confounding factors. However, the association between plasma carbamylated HDL and kidney outcome remained significant and was independent of HDL cholesterol.

CONCLUSIONS

Plasma carbamylated HDL but not carbamylated LDL was independently associated with progression of CKD in patients with type 2 diabetes.

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.

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.

High urea induces depression and LTP impairment through mTOR signalling suppression caused by carbamylation

BACKGROUND

Urea, the end product of protein metabolism, has been considered to have negligible toxicity for a long time. Our previous study showed a depression phenotype in urea transporter (UT) B knockout mice, which suggests that abnormal urea metabolism may cause depression. The purpose of this study was to determine if urea accumulation in brain is a key factor causing depression using clinical data and animal models.

METHODS

A meta-analysis was used to identify the relationship between depression and chronic diseases. Functional Magnetic Resonance Imaging (fMRI) brain scans and common biochemical indexes were compared between the patients and healthy controls. We used behavioural tests, electrophysiology, and molecular profiling techniques to investigate the functional role and molecular basis in mouse models.

FINDINGS

After performing a meta-analysis, we targeted the relevance between chronic kidney disease (CKD) and depression. In a CKD mouse model and a patient cohort, depression was induced by impairing the medial prefrontal cortex. The enlarged cohort suggested that urea was responsible for depression. In mice, urea was sufficient to induce depression, interrupt long-term potentiation (LTP) and cause loss of synapses in several models. The mTORC1-S6K pathway inhibition was necessary for the effect of urea. Lastly, we identified that the hydrolysate of urea, cyanate, was also involved in this pathophysiology.

INTERPRETATION

These data indicate that urea accumulation in brain is an independent factor causing depression, bypassing the psychosocial stress. Urea or cyanate carbamylates mTOR to inhibit the mTORC1-S6K dependent dendritic protein synthesis, inducing impairment of synaptic plasticity in mPFC and depression-like behaviour. CKD patients may be able to attenuate depression only by strict management of blood urea.

Atherosclerosis in Chronic Kidney Disease: More, Less, or Just Different?

Patients with chronic kidney disease (CKD) are at an increased risk of premature mortality, mainly from cardiovascular causes. The association between CKD on hemodialysis and accelerated atherosclerosis was described >40 years ago. However, more recently, it has been suggested that the increase in atherosclerosis risk is actually observed in early CKD stages, remaining stable thereafter. In this regard, interventions targeting the pathogenesis of atherosclerosis, such as statins, successful in the general population, have failed to benefit patients with very advanced CKD. This raises the issue of the relative contribution of atherosclerosis versus other forms of cardiovascular injury such as arteriosclerosis or myocardial injury to the increased cardiovascular risk in CKD. In this review, the pathophysiogical contributors to atherosclerosis in CKD that are shared with the general population, or specific to CKD, are discussed. The NEFRONA study (Observatorio Nacional de Atherosclerosis en NEFrologia) prospectively assessed the prevalence and progression of subclinical atherosclerosis (plaque in vascular ultrasound), confirming an increased prevalence of atherosclerosis in patients with moderate CKD. However, the adjusted odds ratio for subclinical atherosclerosis increased with CKD stage, suggesting a contribution of CKD itself to subclinical atherosclerosis. Progression of atherosclerosis was closely related to CKD progression as well as to the baseline presence of atheroma plaque, and to higher phosphate, uric acid, and ferritin and lower 25(OH) vitamin D levels. These insights may help design future clinical trials of stratified personalized medicine targeting atherosclerosis in patients with CKD. Future primary prevention trials should enroll patients with evidence of subclinical atherosclerosis and should provide a comprehensive control of all known risk factors in addition to testing any additional intervention or placebo.

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.

Protein carbamylation in end stage renal disease: is there a mortality effect?

PURPOSE OF REVIEW

Protein carbamylation is a posttranslational protein modification caused, in part, by exposure to urea's dissociation product cyanate. Additional modulators of protein carbamylation include circulating free amino acid levels, inflammation, diet, smoking, and environmental pollution exposures. Carbamylation reactions can modify protein charge, structure, and function, leading to adverse molecular and cellular responses. These changes have been linked to several pathologic biochemical pathways relevant to patients with end stage renal disease (ESRD) such as accelerated atherosclerosis and dysfunctional erythropoiesis, among others. This review examines the consequences of human protein carbamylation and the clinical impact this is thought to have in patients with ESRD.

RECENT FINDINGS

Recent well-conducted studies across diverse cohorts of patients have independently associated elevations in protein carbamylation to mortality and morbidity in patients with ESRD. Studies are now examining the best strategies to reduce carbamylation load, including interventions aimed at lowering urea levels and restoring amino acid balance. Whether such carbamylation lowering strategies yield clinical improvements remain to be determined.

SUMMARY

Numerous fundamental studies provide plausible mechanisms for the observed association between protein carbamylation burden and adverse clinical outcomes in ESRD. Studies employing nutritional and dialytic interventions to lower carbamylation may mitigate this risk but the net clinical benefit has not been established.

Could ornithine supplementation be beneficial to prevent the formation of pro-atherogenic carbamylated low-density lipoprotein (c-LDL) particles?

Carbamylation (or carbamoylation) is a non-enzymatic post-translational modification process of lysine residues and protein N-termini, which occurs throughout the lifespan of both various plasma proteins and low-density lipoprotein (LDL) particles. Carbamylation results from the binding of isocyanates spontaneously derived from high levels of blood urea, environmental pollutants, nutritional sources and leads to the formation of potentially atherogenic carbamylated-LDL (c-LDL) particles. The carbamylation of LDL apolipoproteins is associated unfavorable downstream effects. Ornithine is a non-proteinogenic amino acid, which plays a central role at the urea cycle function. The primary use of ornithine in supplements is to support athletic performance, liver function and wound recovery. Ornithine is structurally highly similar to lysine, and is only one carbon atom shorter in its side-chain. Therefore, we hypothesize that supplemented ornithine could compete with ε-amino groups of lysine residues found in apolipoproteins of native LDL particles in their binding to isocyanates and decrease c-LDL formation. This issue still remains unresolved in current literature and needs to be elucidated in experimental studies.

Urea and chronic kidney disease: the comeback of the century? (in uraemia research)

Urea, a marker of uraemic retention in chronic kidney disease (CKD) and of adequacy of intradialytic solute removal, has traditionally been considered to be biologically inert. However, a number of recent experimental data suggest that urea is toxic at concentrations representative for CKD. First of all, at least five studies indicate that urea itself induces molecular changes related to insulin resistance, free radical production, apoptosis and disruption of the protective intestinal barrier. Second, urea is at the origin of the generation of cyanate, ammonia and carbamylated compounds, which as such all have been linked to biological changes. Especially carbamylation has been held responsible for post-translational protein modifications that are involved in atherogenesis and other functional changes. In observational clinical studies, these carbamylated compounds were associated with cardiovascular and overall morbidity and mortality. These findings shed new light on the validity of Kt/Vurea as a marker of dialysis adequacy. Yet, also the views that the kinetics of urea are not representative of the kinetics of several other uraemic retention solutes, and that urea cannot be held responsible for all complex metabolic and clinical changes responsible for the uraemic syndrome, still remain valid. Future efforts to improve the outcome of patients with CKD might be directed at further improving removal of solutes implied in the uraemic syndrome, including but not restricted to urea, also taking into account the impact of the intestine and (residual) renal function on solute concentration.

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.

Vascular toxicity of urea, a new "old player" in the pathogenesis of chronic renal failure induced cardiovascular diseases

Chronic kidney disease in children is an irreversible process that may lead to end-stage renal disease. The mortality rate in children with end-stage renal disease who receive dialysis increased dramatically in the last decade, and it is significantly higher compared with the general pediatric population. Furthermore, dialysis and transplant patients, who have developed end-stage renal disease during childhood, live respectively far less as compared with age/race-matched populations. Different reports show that cardiovascular disease is the leading cause of death in children with end-stage renal disease and in adults with childhood-onset chronic kidney disease, and that children with chronic kidney disease are in the highest risk group for the development of cardiovascular disease. Urea, which is generated in the liver during catabolism of amino acids and other nitrogenous metabolites, is normally excreted into the urine by the kidneys as rapidly as it is produced. When renal function is impaired, increasing concentrations of blood urea will steadily accumulate. For a long time, urea has been considered to have negligible toxicity. However, the finding that plasma urea is the only significant predictor of aortic plaque area fraction in an animal model of chronic renal failure -accelerated atherosclerosis, suggests that the high levels of urea found in chronic dialysis patients might play an important role in accelerated atherosclerosis in this group of patients. The aim of this review was to provide novel insights into the role played by urea in the pathogenesis of accelerated cardiovascular disease in renal failure.

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.

Chronic Kidney Disease and Disproportionally Increased Cardiovascular Damage: Does Oxidative Stress Explain the Burden?

Chronic kidney disease (CKD) patients are among the groups at the highest risk for cardiovascular disease and significantly shortened remaining lifespan. CKD enhances oxidative stress in the organism with ensuing cardiovascular damage. Oxidative stress in uremia is the consequence of higher reactive oxygen species (ROS) production, whereas attenuated clearance of pro-oxidant substances and impaired antioxidant defenses play a complementary role. The pathophysiological mechanism underlying the increased ROS production in CKD is at least partly mediated by upregulation of the intrarenal angiotensin system. Enhanced oxidative stress in the setting of the uremic milieu promotes enzymatic modification of circulating lipids and lipoproteins, protein carbamylation, endothelial dysfunction via disruption of nitric oxide (NO) pathways, and activation of inflammation, thus accelerating atherosclerosis. Left ventricular hypertrophy (LVH) and heart failure are hallmarks of CKD. NADPH oxidase activation, xanthine oxidase, mitochondrial dysfunction, and NO-ROS are the main oxidative pathways leading to LVH and the cardiorenal syndrome. Finally, a subset of antioxidant enzymes, the paraoxonases (PON), deserves special attention due to abundant clinical evidence accumulated regarding reduced serum PON1 activity in CKD as a contributor to the increased burden of cardiovascular disease. Future, meticulously designed studies are needed to assess the effects of antioxidant therapy on patients with CKD.

Binding of bromocresol green and bromocresol purple to albumin in hemodialysis patients

Background:

Colorimetric albumin assays based on binding to bromocresol purple (BCP) and bromocresol green (BCG) yield different results in chronic kidney disease. Altered dye binding of carbamylated albumin has been suggested as a cause. In the present study, a detailed analysis was carried out in which uremic toxins, acute phase proteins and Kt/V, a parameter describing hemodialysis efficiency, were compared with colorimetrically assayed (BCP and BCG) serum albumin.

Methods:

Albumin was assayed using immunonephelometry on a BN II nephelometer and colorimetrically based on, respectively, BCP and BCG on a Modular P analyzer. Uremic toxins were assessed using high-performance liquid chromatography. Acute phase proteins (C-reactive protein and α1-acid glycoprotein) and plasma protein α2-macroglobulin were assayed nephelometrically. In parallel, Kt/V was calculated.

Results:

Sixty-two serum specimens originating from hemodialysis patients were analyzed. Among the uremic toxins investigated, total para-cresyl sulfate (PCS) showed a significant positive correlation with the BCP/BCG ratio. The serum α1-acid glycoprotein concentration correlated negatively with the BCP/BCG ratio. The BCP/BCG ratio showed also a negative correlation with Kt/V.

Conclusions:

In renal insufficiency, the BCP/BCG ratio of serum albumin is affected by multiple factors: next to carbamylation, uremic toxins (total PCS) and α1-acid glycoprotein also play a role.

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.

Urea, a true uremic toxin: the empire strikes back

Blood levels of urea rise with progressive decline in kidney function. Older studies examining acute urea infusion suggested that urea was well-tolerated at levels 8-10× above normal values. More recent in vitro and in vivo work argue the opposite and demonstrate both direct and indirect toxicities of urea, which probably promote the premature aging phenotype that is pervasive in chronic kidney disease (CKD). Elevated urea at concentrations typically encountered in uremic patients induces disintegration of the gut epithelial barrier, leading to translocation of bacterial toxins into the bloodstream and systemic inflammation. Urea induces apoptosis of vascular smooth muscle cells as well as endothelial dysfunction, thus directly promoting cardiovascular disease. Further, urea stimulates oxidative stress and dysfunction in adipocytes, leading to insulin resistance. Finally, there are widespread indirect effects of elevated urea as a result of the carbamylation reaction, where isocyanic acid (a product of urea catabolism) alters the structure and function of proteins in the body. Carbamylation has been linked with renal fibrosis, atherosclerosis and anaemia. In summary, urea is a re-emerging Dark Force in CKD pathophysiology. Trials examining low protein diet to minimize accumulation of urea and other toxins suggest a clinical benefit in terms of slowing progression of CKD.

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.

ApoB and apoM - New aspects of lipoprotein biology in uremia-induced atherosclerosis

Chronic kidney disease affects as much as 13% of the population, and is associated with a markedly increased risk of developing cardiovascular disease. One of the underlying reasons is accelerated development of atherosclerosis. This can be ascribed both to increased occurrence of traditional cardiovascular risk factors, and to risk factors that may be unique to patients with chronic kidney disease. The latter is reflected in the observation that the current treatment modalities, mainly directed against traditional risk factors, are insufficient to prevent cardiovascular disease in the patient with chronic kidney disease. This review discusses mechanisms accelerating uremic atherosclerosis with a specific focus on the putative roles of apolipoprotein(apo)s B and M that may be particularly important in patients with chronic kidney disease.

Modified Lipids and Lipoproteins in Chronic Kidney Disease: A New Class of Uremic Toxins

Chronic kidney disease (CKD) is associated with an enhanced oxidative stress and deep modifications in lipid and lipoprotein metabolism. First, many oxidized lipids accumulate in CKD and were shown to exert toxic effects on cells and tissues. These lipids are known to interfere with many cell functions and to be pro-apoptotic and pro-inflammatory, especially in the cardiovascular system. Some, like F2-isoprostanes, are directly correlated with CKD progression. Their accumulation, added to their noxious effects, rendered their nomination as uremic toxins credible. Similarly, lipoproteins are deeply altered by CKD modifications, either in their metabolism or composition. These impairments lead to impaired effects of HDL on their normal effectors and may strongly participate in accelerated atherosclerosis and failure of statins in end-stage renal disease patients. This review describes the impact of oxidized lipids and other modifications in the natural history of CKD and its complications. Moreover, this review focuses on the modifications of lipoproteins and their impact on the emergence of cardiovascular diseases in CKD as well as the appropriateness of considering them as actual mediators of uremic toxicity.

Cyanate-Impaired Angiogenesis: Association With Poor Coronary Collateral Growth in Patients With Stable Angina and Chronic Total Occlusion

Background

Cyanate has recently gained attention for its role in the pathogenesis of vascular injury. Nonetheless, the effect of cyanate on angiogenesis remains unclear.

Methods and Results

In this study, we demonstrated that oral administration of cyanate impaired blood perfusion recovery in a mouse hind‐limb ischemia model. A reduction in blood perfusion recovery at day 21 was observed in the ischemic tissue of cyanate‐treated mice. Likewise, there were fewer capillaries in the ischemic hind‐limb tissue of cyanate‐exposed mice. Our in vitro study showed that cyanate, together with its carbamylated products, inhibited the migration, proliferation, and tube‐formation abilities of endothelial cells. Further research revealed that cyanate regulated angiogenesis partly by interrupting the vascular endothelial growth factor receptor 2/phosphatidylinositol 3‐kinase/Akt pathway. The serum concentrations of homocitrulline, a marker of cyanate exposure, were determined in 117 patients with stable angina and chronic total occlusion. Consistent with the antiangiogenic role of cyanate, homocitrulline levels were increased in patients with poor coronary collateralization (n=58) compared with those with high collateralization (n=59; 21.09±13.08 versus 15.54±9.02 ng/mL, P=0.009). In addition, elevated homocitrulline concentration was a strong predictor of poor coronary collateral growth.

Conclusions

Impaired angiogenesis induced by cyanate might contribute to poor coronary collateral growth.