Hemoglobin carbamylation in uremia

HbA1c overestimates the glucose management indicator: a pilot study in patients with diabetes, chronic kidney disease not on dialysis, and anemia using isCGM

Introduction

There are multiple mechanisms by which HbA1c values can be altered in chronic kidney disease (CKD), which limits its usefulness as a strategy to assess glycemic control in this population.

Methods

Concordance and agreement study between two diagnostic tests: HbA1c and glucose management indicator (GMI) measured by intermittently scanned continuous glucose monitoring (isCGM), based in a prospective cohort of patients with diabetes, CKD (glomerular filtration rate between 15 and 60 ml/min/1.73 m²), and anemia. The isCGM was performed for 3 months, and the GMI was compared with the HbA1c levels taken at the end of isCGM. Agreement was evaluated using Bland-Altman graph analysis and Lin's concordance correlation coefficient (CCC). The concordance of the measures with good glycemic control (<7%) was also evaluated.

Results

A total of 74 patients were enrolled (median age 68.5 years, 51.3% female, 64.9% with CKD stage 3, hemoglobin 11.1 ± 1.2 g/l). The Bland-Altman analysis shows a mean difference between GMI and HbA1c of 0.757 ± 0.687% (95% limits of agreement: -0.590 and 2.105). Difference was greater as the values of GMI and HbA1c increased. The agreement was poor [CCC 0.477; 95% confidence interval (CI): 0.360-0.594], as well as the concordance of values with good glycemic control according to GMI versus HbA1c (67.5% versus 29.7%, p < 0.001) (Kappa 0.2430; 95% CI: 0.16-0.32).

Conclusion

The HbA1c overestimates the GMI values with highly variable ranges of difference, which prevents a precise correction factor. isCGM probably is a safer option for monitoring and decision-making in this population, especially in patients treated with insulin where the risk of hypoglycemia is greater.

Long-term Visit-to-Visit Variability in Hemoglobin A1c and Kidney-Related Outcomes in Persons With Diabetes

RATIONALE & OBJECTIVE

To characterize associations between long-term visit-to-visit variability of hemoglobin A1c (HbA1c) and risk of adverse kidney outcomes in patients with diabetes.

STUDY DESIGN

Observational study.

SETTING & PARTICIPANTS

93,598 adults with diabetes undergoing routine care in Stockholm, Sweden.

EXPOSURES AND PREDICTORS

Categories of baseline and time-varying HbA1c variability score (HVS, the percentage of total HbA1c measures that vary by>0.5% [5.5mmol/mol] during a 3-year window): 0-20%, 21%-40%, 41%-60%, 61%-80%, and 81%-100%, with 0-20% as the reference group.

OUTCOME

Chronic kidney disease (CKD) progression (composite of>50% estimated glomerular filtration rate [eGFR] decline and kidney failure), acute kidney disease (AKI by clinical diagnosis or transient creatinine elevations according to KDIGO criteria), and worsening of albuminuria.

ANALYTICAL APPROACH

Multivariable Cox proportional hazards regression.

RESULTS

Compared with persons showing low HbA1c variability (HVS 0-20%), any increase in variability was associated with a higher risk of adverse kidney outcomes beyond mean HbA1c. For example, for patients with a baseline HbA1c variability of 81%-100%, the adjusted HR was 1.6 (95% CI, 1.47-1.74) for CKD progression, 1.23 [1.16-1.3] for AKI, and 1.28 [1.21-1.36] for worsening of albuminuria. The results were consistent across subgroups (diabetes subtypes, baseline eGFR, or albuminuria categories), in time-varying analyses and in sensitivity analyses including time-weighted average HbA1c or alternative metrics of variability.

LIMITATIONS

Observational study, limitations of claims data, lack of information on diet, body mass index, medication changes, and diabetes duration.

CONCLUSIONS

Higher long-term visit-to-visit HbA1c variability is consistently associated with the risks of CKD progression, AKI, and worsening of albuminuria.

PLAIN-LANGUAGE SUMMARY

The evidence for current guideline recommendations derives from clinical trials that focus on a single HbA1c as the definitive measure of efficacy of an intervention. However, long-term visit-to-visit fluctuations of HbA1c may provide additional value in the prediction of future kidney complications. We evaluated the long-term fluctuations in glycemic control in almost 100,000 persons with diabetes undergoing routine care in Stockholm, Sweden. We observed that higher long-term HbA1c fluctuation is consistently associated with the risks of chronic kidney disease progression, worsening of albuminuria and acute kidney injury. This finding supports a role for long-term glycemic variability in the development of kidney complications and illustrates the potential usefulness of this metric for risk stratification at the bedside beyond a single HbA1c test.

The Impact of Carbamylation and Anemia on HbA1c's Association With Renal Outcomes in Patients With Diabetes and Chronic Kidney Disease

OBJECTIVE

Glycated hemoglobin (HbA1c) can predict risk for microvascular complications in patients with diabetes. However, HbA1c's reliability in chronic kidney disease (CKD) has been questioned, with concerns including competition from another posttranslational protein modification, carbamylation, acting on the same amino groups as glycation, and anemia with reduced erythrocyte lifespans leading to altered glycation accumulation. We investigated whether carbamylation and anemia modify the impact of HbA1c on renal outcomes in patients with diabetes and CKD.

RESEARCH DESIGN AND METHODS

In 1,516 participants from the Chronic Renal Insufficiency Cohort study with diabetes and CKD, Cox regression models were applied to evaluate the association between HbA1c and CKD progression (composite of end-stage kidney disease or 50% decline in estimated glomerular filtration rate [eGFR]), stratified by carbamylated albumin (C-Alb) quartiles and anemia.

RESULTS

The mean eGFR was 38.1 mL/min/1.73 m2, mean HbA1c was 7.5% (58 mmol/mol), and median C-Alb was 8.4 mmol/mol. HbA1c was lower in the higher C-Alb quartiles. During a median follow-up of 6.9 years, 763 participants experienced CKD progression. Overall, higher HbA1c was associated with an increased risk of CKD progression (adjusted hazard ratio 1.07 [95% CI 1.02-1.13]). However, using stratified analyses, HbA1c was no longer associated with CKD progression in the highest C-Alb quartile, but did show a monotonic increase in CKD progression risk across each lower C-Alb quartile (P-interaction = 0.022). Anemia also modified the association between HbA1c and CKD progression (P-interaction = 0.025).

CONCLUSIONS

In patients with coexisting diabetes and CKD, the association between HbA1c and CKD progression is modified by carbamylation and anemia.

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.

Diabetes mellitus in dialysis and renal transplantation

Diabetes mellitus is the commonest cause of end-stage kidney failure worldwide and is a proven and significant risk factor for the development of cardiovascular disease. Renal impairment has a significant impact on the physiology of glucose homeostasis as it reduces tissue sensitivity to insulin and reduces insulin clearance. Renal replacement therapy itself affects glucose control: peritoneal dialysis may induce hyperglycaemia due to glucose-rich dialysate and haemodialysis often causes hypoglycaemia due to the relatively low concentration of glucose in the dialysate. Autonomic neuropathy which is common in chronic kidney disease (CKD) and diabetes increases the risk for asymptomatic hypoglycaemia. Pharmacological options for improving glycaemic control are limited due to alterations to drug metabolism. Impaired glucose tolerance and diabetes are also common in the post-kidney-transplant setting and increase the risk of graft failure and mortality. This review seeks to summarise the literature and tackle the intricacies of glycaemic management in patients with CKD who are either on maintenance haemodialysis or have received a kidney transplant. It outlines changes to glycaemic targets, monitoring of glycaemic control, the use of oral hypoglycaemic agents, the management of severe hyperglycaemia in dialysis and kidney transplantation patients.

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

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

Do anti-carbamylated protein antibodies in rheumatoid arthritis reflect local and systemic osteoporosis? A study of osteoprotegrin and receptor activator for nuclear factor kappa B ligand and radiological assessment

Background

One of the most important and dangerous complications of rheumatoid arthritis (RA) is bone loss, which manifested by erosions and juxta-articular or systemic bone loss. Anti-carbamylated protein (anti-CarP) antibodies which are also called anti-homocitrulline antibodies have recently been found in RA. Increase anti-CarP antibody titres may lead to severe disease and increase the progression of bone loss. Osteoprotegrin and receptor activator for nuclear factor kappa B and its ligand (RANKL) are the main players in the pathogenesis of osteoporosis. Thus, we aimed to investigate and detect the presence and prevalence of anti-CarP in rheumatoid arthritis and their association with disease severity and osteoporosis, as well as with OPG/RANKL in 80 Egyptian RA patients to highlight this relationship which could be useful in managing RA patients with osteoporosis.

Results

Serum anti-CarP levels were significantly increased in the RA group compared with the control group (P< 0.001). We found a negative association between anti-CarP and anti-CCP and disease activity (r=−0.878, −0.534, respectively, P<0.001). We also found a positive correlation between anti-CarP and the Larsen score, DEXA score, RF, HAQ, and RANKL (r=0.646, 0.287, 0.243, 0.892, 0.671, 0.869 [respectively], P<0.001) and there was negative correlation between anti-CarP and OPG (r=−0.553, P<0.001).

Conclusion

Anti-CarP antibodies are associated with disease severity and disability in RA patients. They could play an important and significant role in the pathogenesis of osteoporosis in these patients.

Glycemic Monitoring and Management in Advanced Chronic Kidney Disease

Glucose and insulin metabolism in patients with diabetes are profoundly altered by advanced chronic kidney disease (CKD). Risk of hypoglycemia is increased by failure of kidney gluconeogenesis, impaired insulin clearance by the kidney, defective insulin degradation due to uremia, increased erythrocyte glucose uptake during hemodialysis, impaired counterregulatory hormone responses (cortisol, growth hormone), nutritional deprivation, and variability of exposure to oral antihyperglycemic agents and exogenous insulin. Patients with end-stage kidney disease frequently experience wide glycemic excursions, with common occurrences of both hypoglycemia and hyperglycemia. Assessment of glycemia by glycated hemoglobin (HbA1c) is hampered by a variety of CKD-associated conditions that can bias the measure either to the low or high range. Alternative glycemic biomarkers, such as glycated albumin or fructosamine, are not fully validated. Therefore, HbA1c remains the preferred glycemic biomarker despite its limitations. Based on observational data for associations with mortality and risks of hypoglycemia with intensive glycemic control regimens in advanced CKD, an HbA1c range of 7% to 8% appears to be the most favorable. Emerging data on the use of continuous glucose monitoring in this population suggest promise for more precise monitoring and treatment adjustments to permit fine-tuning of glycemic management in patients with diabetes and advanced CKD.

Management of the Diabetic Patients with End Stage Renal Disease

Twenty diabetics with end-stage renal disease who had never previously received dialysis treatment were treated with continuous ambulatory peritoneal dialysis for periods of two to 36 months (average, 14.5). Intraperitoneal administration of insulin achieved good control of blood sugar Even though creatinine clearance decreased significantly (P = 0.001), contro of blood urea nitrogen and serum creatinine was adequate. Hemoglobin and serum albumin levels increased significantly (P = 0.005 and 0.04 respectively). Similarly, there was a significant increase in serum triglycerides and alkaline phosphatase (P = 0.02 and 0.05). Blood pressure became normal without medications in all but one of the patients. Retinopathy, neuropathy, and osteodystrophy remained unchanged. Peritonitis developed once in every 20.6 patient-months a rate similar to that observed in nondiabetics. The calculated survival rate was 92 per cent at one year; the calculated rate of continuation on ambulatory peritoneal dialysis was 87 per cent.

An overview of autoantibodies in rheumatoid arthritis

Rheumatoid arthritis (RA) is a systemic auto-immune disease principally effecting synovial joints. RA is characterized by immune cell infiltration in the joint. The presence of autoantibodies is a hallmark for the disease, among these are rheumatoid factor and antibodies against post-translational modified proteins like citrullination (ACPA) and carbamylation (anti-CarP antibodies). These autoantibodies may form immune complexes in the joint, leading to the attraction of immune cells. Based on the presence of these autoantibodies, RA patients can be subdivided in autoantibody positive and negative disease. Both subsets can be associated with genetic and environmental risk factors for RA, like the human leukocyte antigen (HLA) allele and smoking. Autoantibodies can already be detected years before disease onset in a subgroup of patients and at symptom onset a broad isotype spectrum is observed. This suggests that various events occur prior to the development of RA in which the first autoantibodies develop in predisposed individuals. Therefore, the presence of these autoantibodies can be useful in predicting future RA patients. Research on the characteristics and effector function of these autoantibodies is ongoing and will give more knowledge in the inflammatory responses underlying RA. This will give insight in the pathogenic role of autoantibodies in RA. Recent data are suggestive of a role for mucosal surfaces in the development of auto-immune responses associated with (the development of) RA. In conclusion, investigating the potential pathogenic effector functions of autoantibody isotypes and their molecular- and physicochemical-compositions might improve understanding of the disease origin and its underlying immunological processes. This may lead to the development of new therapeutic targets and strategies.

Two cases with Hb Andrew-Minneapolis showing high or low-normal HbA1c levels depending on the measurement method

We experienced two cases of Hb Andrew-Minneapolis with high or low-normal HbA1c levels depending on the measurement method. Case 1 was a 25-year-old male, and case 2 was a 32-year-old pregnant woman. Both cases showed normal glucose tolerance levels and glycated albumin within the reference range. In both cases, the high-performance liquid chromatography (HPLC) method (standard mode) showed high HbA1c levels of 6.8% and 6.5%, respectively, while the HbA1c levels measured by immunoassay were low normal at 4.6% in both cases. Globin gene analysis detected heterozygous β-chain mutations (β144Lys → Asn) in both cases, which resulted in the diagnosis of Hb Andrew-Minneapolis. In case 1, a high-resolution HPLC chromatogram showed multiple abnormal peaks; two unknown peaks in addition to variant hemoglobin (HbX0) and glycation products of variant hemoglobin (HbX1c) were observed after in vitro glycation reaction. Although the details of unknown peaks were not identified, those might be modified hemoglobin associated with variant hemoglobin. The presence of unknown peaks could cause high HbA1c levels measured by HPLC (standard mode). Furthermore, the HbA1c level measured by immunoassay was increased to 4.9% within the reference range after adjustment for modified hemoglobin in case 1. Consequently, the high HbA1c levels measured by HPLC (standard mode) and the low-normal HbA1c level measured by immunoassay might be due to modified hemoglobin associated with variant hemoglobin.

Serologic Diagnosis of Rheumatoid Arthritis

Accurate diagnosis of inflammatory arthritides remains a challenge because of substantial clinical overlap. To achieve a granular classification for informing clinical decisions, numerous potential serologic biomarkers have been identified. Rheumatologists have settled on rheumatoid factor and anti-citrullinated protein antibodies for the diagnosis of rheumatoid arthritis (RA) based on specificity and sensitivity and their ability to be integrated into clinical algorithms. These biomarkers should be interpreted in their specific clinical context. This article discusses the serologic basis for the diagnosis of RA, how these biomarkers have framed conceptualization of the pathogenesis of RA, and the inherent limitations in their use.

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.

Influence of iron deficiency on Hb A1c levels in type 2 diabetic patients

AIMS

Hemoglobin A1c (HbA1c) is gold-standard for the assessment of glycemic control in diabetic patients. Previous studies have reported that iron deficiency may elevate A1c concentrations, independent of glycemia. This study aimed to analyze the effect of iron status on HbA1c levels in diabetic patients.

METHODS

661 patients 336 females (228 menopausal and 108 premenopausal) and 325 males (237 age> 50 years and 88 age < 50 years) were recruited. HbA1c, ferritin, fasting plasma glucose, hemogram and medical history were recorded. Analysis of variance ANOVA and Pearson's regression were applied.

RESULTS

patients were divided according gender, age, glycemia and iron status (normal, latent iron deficiency LID, iron deficiency anemia IDA).All groups presented increasing HbA1c values in parallel with iron deficiency, subclinical and anemia, but the level of significance was not homogeneous in the different groups. Controlled premenopausal women HbA1c in normal iron status and IDA groups P = 0.0048, between normal and LID, P = 0.033. Not controlled premenopausal women Normal group and IDAP < 0.001, normal iron status and LID P = 0.019. Controlled menopausal women normal group and IDAP < 0.0001, LID and IDA P = 0.01. Not controlled menopausal women normal group and IDA P = 0.04. Controlled men over 50 years normal and IDA groups P = 0.002, LID and IDA P = 0.02. Controlled young men normal group and LID P = 0.03.

CONCLUSION

This study found a positive correlation between iron deficiency and increased HA1c levels. In diabetic patients with IDA should be interpreted with caution, due to the possibility of spurious increment in HbA1c.

The clinical usefulness of glycated albumin in patients with diabetes and chronic kidney disease: Progress and challenges

Prolonged hyperglycemia leads to a non-enzymatic glycation of proteins, and produces Amadori products, such as glycated albumin (GA) and glycated hemoglobin (HbA1c). The utility of HbA1c in the setting of chronic kidney disease (CKD) may be problematic since altered lifespan of red blood cells, use of iron and/or erythropoietin therapy, uremia and so on. Therefore, as an alternative marker, GA has been suggested as a more reliable and sensitive glycemic index in patients with CKD. In addition to the mean plasma glucose concentration, GA also reflects postprandial plasma glucose and glycemic excursion. Besides, with a half-life of approximately 2-3 weeks, GA may reflect the status of blood glucose more rapidly than HbA1c. GA is also an early precursor of advanced glycation end products (AGEs), which cause alterations in various cellular proteins and organelles. Thus, high GA levels may correlate with adverse outcomes of patients with CKD. In this review, the clinical usefulness of GA was discussed, including a comparison of GA with HbA1c, the utility and limitations of GA as a glycemic index, its potential role in pathogenesis of diabetic nephropathy and the correlations between GA levels and outcomes, specifically in patients with diabetes and CKD.

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.

Inpatient Glycemic Management in the Setting of Renal Insufficiency/Failure/Dialysis

PURPOSE OF THIS REVIEW

Chronic diabetic nephropathy and renal dysfunction from other causes are common in hospitalized patients with diabetes. Available diabetes management guidelines aim to reduce hyperglycemia and hypoglycemia, both independent risk factors for hospital outcomes. Renal dysfunction, which increases the risk of hypoglycemia, adds a layer of complexity in diabetes management. Therefore, modified glucose goals and treatment regimens may be required.

RECENT FINDINGS

Recent prospective and retrospective studies provide direction on safe insulin therapy for diabetes inpatients with renal compromise. Studies of newer diabetes pharmacotherapy provide data on oral agent use in the inpatient setting. Diabetes therapy should be modified with changing renal function. Glucose management in patients on peritoneal or hemodialysis is challenging. Reducing weight-based doses of insulin and use of newer insulins can reduce hypoglycemia risk. Safety and efficacy of DPP-4 inhibitors has been evaluated in the hospital and nursing home setting. Metformin, SGLT-2 inhibitors, and GLP1 receptor agonists can be used in several stages of renal dysfunction prior to and at discharge.

Anemia modifies the prognostic value of glycated hemoglobin in patients with diabetic chronic kidney disease

A common complication of chronic kidney disease (CKD), anemia can influence glycated hemoglobin (HbA1c) levels. In diabetic patients, anemia occurs earlier and with higher severity over the course of CKD stages. To elucidate the effect of hemoglobin (Hb) on the predictive value of HbA1c, we enrolled 1558 diabetic patients with stages 3-4 CKD, categorized according to baseline Hb and HbA1c quartiles. Linear regression revealed that higher HbA1c correlated significantly with higher Hb in the Hb < 10 g/dL group (β = 0.146, P = 0.004). A fully-adjusted Cox regression model revealed worse clinical outcomes in patients with higher HbA1c quartiles in the Hb ≥ 10 g/dL group. Hazard ratios for end-stage renal disease (ESRD), all-cause mortality, and composite endpoint (cardiovascular events and all-cause mortality) in patients with Hb ≥ 10 g/dL and the highest HbA1c quartile were 1.92 (95% confidence interval [CI], 1.17-3.15), 1.76 (95% CI, 1.02-3.03), and 1.54 (95% CI, 1.03-2.31), respectively. By contrast, HbA1c was not associated with clinical outcomes in the Hb < 10 g/dL group. In conclusion, in stages 3-4 diabetic CKD, higher HbA1c is associated with a higher risk of poor clinical outcomes in patients with Hb ≥ 10 g/dL.

The Role of Autoantibodies in Bone Metabolism and Bone Loss

Many autoimmune diseases are associated with deranged bone metabolism. The resulting localized or systemic bone loss can compromise the quality of life of patients by causing local bone deformities or fragility fractures. There is emerging evidence that antibodies have a direct impact on key players of bone homeostasis, in particular osteoclasts. Clinical and pre-clinical studies provide insight into the function of autoantibodies related to Rheumatoid Arthritis (rheumatoid factor, anti-citrullinated protein antibodies, and anti-carbamylated protein antibodies) and their inflammation-independent interaction with bone cells. Furthermore, we summarize the current knowledge about neutralizing antibodies to the antiresorptive protein osteoprotegerin, which have been described in patients with Coeliac Disease, Rheumatoid Arthritis, and Spondyloarthritis.

Is the management of diabetes different in dialysis patients?

Diabetes is highly and increasingly prevalent in the dialysis population and negatively impacts both quality and quantity of life. Nevertheless, the best approach to these patients is still debatable. The question of whether the management of diabetes should be different in dialysis patients does not have a clear yes or no answer but is divided into too many sub-issues that should be carefully considered. In this review, lifestyle, cardiovascular risk, and hyperglycemia management are explored, emphasizing the possible pros and cons of a similar approach to diabetes in dialysis patients compared to the general population.

Origin of Glycosylated Hemoglobin Al in Chronic Renal Failure

In chronic renal failure both HbAI and HbAlc levels have been reported to be elevated. In order to investigate the causes of such increase we measured HbAI (cation-exchange chromatography), blood urea nitrogen, arterial blood pH, plasma bicarbonate, phosphatemia, serum iron and serum ferritin before dialysis in 60 uremic patients receiving long term hemodialysis. The increased levels of HbAI do not correlate with glucose intolerance, phosphatemia, blood urea nitrogen, time averaged concentration of urea, serum iron and serum ferritin. On the contrary the presence of a highly significant correlation between HbAI and arterial blood pH (p < 0.001) and between HbAI and plasma bicarbonate (p < 0.001) seems to emphasize a major role for acidosis in increasing the HbAI levels in uremic patients on long term hemodialysis.

Post-translational regulation of macrophage migration inhibitory factor: Basis for functional fine-tuning

Macrophage migration inhibitory factor (MIF) is a chemokine-like protein and an important mediator in the inflammatory response. Unlike most other pro-inflammatory cytokines, a number of cell types constitutively express MIF and secretion occurs from preformed stores. MIF is an evolutionarily conserved protein that shows a remarkable functional diversity, including specific binding to surface CD74 and chemokine receptors and the presence of two intrinsic tautomerase and oxidoreductase activities. Several studies have shown that MIF is subject to post-translational modification, particularly redox-dependent modification of the catalytic proline and cysteine residues. In this review, we summarize and discuss MIF post-translational modifications and their effects on the biological properties of this protein. We propose that the redox-sensitive residues in MIF will be modified at sites of inflammation and that this will add further depth to the functional diversity of this intriguing cytokine.

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MIF is a pro-inflammatory cytokine with tautomerase and oxidoreductase activity.

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MIF is susceptible to post-translational modifications, including redox modification.

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Oxidants and electrophiles generated at inflammatory sites can modify MIF.

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The biological consequences of redox modification need detailed characterization.

Carbamylated albumin is one of the target antigens of anti-carbamylated protein antibodies

Objectives. Anti-carbamylated protein (anti-CarP) antibodies are detected in RA patients. Fetal calf serum is used as an antigen source in anti-CarP ELISA, and the precise target antigens have not been found. We aimed to identify the target antigens of anti-CarP antibodies.

Methods. Western blotting of anti-CarP antibodies was conducted. Anti-carbamylated human albumin (CarALB) antibody was detected by in-house ELISA for 493 RA patients and 144 healthy controls (HCs). An inhibition ELISA of anti-CarP antibodies by CarALB and citrullinated albumin (citALB) was performed using eight RA patients’ sera. Serum CarALB was detected by liquid chromatography–tandem mass spectroscopy (LC/MS/MS), and the serum MPO concentration was measured by ELISA.

Results. We focused on carbamylated albumin because it corresponded to the size of the thickest band detected by western blotting of anti-CarP antibodies. Anti-CarALB antibody was detected in 31.4% of RA patients, and the correlation of the titres between anti-CarALB and anti-CarP was much closer than that between anti-citALB and anti-CCP antibodies (ρ = 0.59 and ρ = 0.16, respectively). The inhibition ELISA showed that anti-CarP antibodies were inhibited by CarALB, but not by citALB. CarALB was detected in sera from RA patients by LC/MS/MS. The serum MPO concentration was correlated with disease activity and was higher in RA patients with anti-CarALB antibody than in those without.

Conclusion. We found that carbamylated albumin is a novel target antigen of anti-CarP antibodies, and it is the first reported target antigen that has not been reported as the target of ACPA.

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.

Beyond citrullination: other post-translational protein modifications in rheumatoid arthritis

The presence of autoantibodies is one of the hallmarks of rheumatoid arthritis (RA). In the past few decades, rheumatoid factors (autoantibodies that recognize the Fc-tail of immunoglobulins) as well as anti-citrullinated protein antibodies (ACPAs) have been studied intensively. ACPAs recognize post-translationally modified proteins in which the amino acid arginine has been converted into a citrulline. More recently, other autoantibody systems recognizing post-translationally modified proteins have also gained attention, including autoantibodies recognizing fragmented immunoglobulin (anti-hinge antibodies), autoantibodies recognizing acetylated proteins and autoantibodies recognizing proteins that are modified by adducts formed under oxidative stress. In particular, detailed insights have been obtained on the presence and properties of autoantibodies recognizing carbamylated proteins, commonly called anti-carbamylated protein (anti-CarP) antibodies. In this Review, we summarize the current knowledge relating to these emerging autoantibodies that recognize post-translationally modified proteins identified in RA, with an emphasis on anti-CarP antibodies.

Glycated Hemoglobin and Outcomes in Patients with Advanced Diabetic Chronic Kidney Disease

Diabetes is the major risk factor for end-stage renal disease (ESRD) worldwide. In advanced chronic kidney disease (CKD), less is known about the predictive value of HbA1c. We enrolled 2401 diabetic patients with stage 3–4 and stage 5 CKD, who were classified into 4 groups according to their baseline HbA1c values (<6%, 6%–7%, 7%–9%, and >9%). During the median follow-up of 3 years, 895 patients developed ESRD, and 530 died. In linear regression analysis, higher HbA1c correlated with higher eGFR in patients with stage 5 CKD but not in stage 3–4 CKD. In Cox regression analysis, a trend toward worse clinical outcomes existed when the HbA1c level exceeded 6% in stage 3–4 CKD, but the significance was only observed for >9%. The hazard ratios (HRs) for ESRD, all-cause mortality and combined CV events with mortality in the group of HbA1c >9% were 1.6 (95% CI, 1.07 to 2.38), 1.52 (95% CI, 0.97 to 2.38) and 1.46 (95% CI, 1.02 to 2.09), respectively. This study demonstrates that the higher HbA1c level is associated higher risks for clinical outcomes in diabetic patients with stage 3–4 CKD but not in stage 5 CKD.

Anti-carbamylated Protein Antibody Levels Correlate with Anti-Sa (Citrullinated Vimentin) Antibody Levels in Rheumatoid Arthritis

OBJECTIVE

The presence of anticitrullinated protein antibodies (ACPA) in rheumatoid arthritis (RA) indicates a breach in immune tolerance. Recent studies indicate that this breach extends to homocitrullination of lysines with the formation of anti-carbamylated protein (anti-CarP) antibodies. We analyzed the clinical and serologic relationships of anti-CarP in 2 RA cohorts.

METHODS

Circulating levels of immunoglobulin G anti-CarP antibodies were determined by ELISA in established (Dartmouth-Hitchcock Medical Center) and early (Sherbrooke University Hospital Center) cohorts and evaluated for anticyclic citrullinated peptide antibodies (anti-CCP), specific ACPA, and rheumatoid factor (RF) levels using the Student t test and correlation analysis.

RESULTS

We identified elevated anti-CarP antibodies titers in 47.0% of seropositive patients (Dartmouth, n = 164), with relationships to anti-CCP (p < 0.0001) and IgM-RF (p = 0.001). Similarly, 38.2% of seropositive patients from the Sherbrooke cohort (n = 171) had elevated anti-CarP antibodies; titers correlated to anti-CCP (p = 0.01) but not IgM-RF (p = 0.09). A strong correlation with anti-Sa was observed: 47.9% anti-Sa+ patients were anti-CarP antibodies+ versus only 25.4% anti-Sa- in the Sherbrooke cohort (p = 0.0002), and 62.6% anti-Sa+ patients versus 26.9% anti-Sa- were anti-CarP antibodies+ in Dartmouth (p < 0.0001). We found a more variable response for reactivity to citrullinated fibrinogen or to citrullinated peptides from fibrinogen and α enolase.

CONCLUSION

In 2 North American RA cohorts, we observed a high prevalence of anti-CarP antibody positivity. We also describe a surprising and unexpected association of anti-CarP with anti-Sa antibodies that could not be explained by cross-reactivity. Further, considerable heterogeneity exists between anti-CarP reactivity and other citrullinated peptide reactivity, raising the question of how the pathogenesis of antibody responses for carbamylated proteins and citrullinated proteins may be linked in vivo.

Macrophage migration inhibitory factor (MIF) is rendered enzymatically inactive by myeloperoxidase-derived oxidants but retains its immunomodulatory function

Macrophage migration inhibitory factor (MIF) is an important player in the regulation of the inflammatory response. Elevated plasma MIF is found in sepsis, arthritis, cystic fibrosis and atherosclerosis. Immunomodulatory activities of MIF include the ability to promote survival and recruitment of inflammatory cells and to amplify pro-inflammatory cytokine production. MIF has an unusual nucleophilic N-terminal proline with catalytic tautomerase activity. It remains unclear whether tautomerase activity is required for MIF function, but small molecules that inhibit tautomerase activity also inhibit the pro-inflammatory activities of MIF. A prominent feature of the acute inflammatory response is neutrophil activation and production of reactive oxygen species, including myeloperoxidase (MPO)-derived hypochlorous acid and hypothiocyanous acid. We hypothesized that MPO-derived oxidants would oxidize the N-terminal proline of MIF and alter its biological activity. MIF was exposed to hypochlorous acid and hypothiocyanous acid and the oxidative modifications on MIF were examined by LC-MS/MS. Imine formation and carbamylation was observed on the N-terminal proline in response to MPO-dependent generation of hypochlorous and hypothiocyanous acid, respectively. These modifications led to a complete loss of tautomerase activity. However, modified MIF still increased CXCL-8/IL-8 production by peripheral blood mononuclear cells (PBMCs) and blocked neutrophil apoptosis, indicating that tautomerase activity is not essential for these biological functions. Pre-treatment of MIF with hypochlorous acid protected the protein from covalent modification by the MIF inhibitor 4-iodo-6-phenylpyrimidine (4-IPP). Therefore, oxidant generation at inflammatory sites may protect MIF from inactivation by more disruptive electrophiles, including drugs designed to target the tautomerase activity of MIF.

What is the Role of HbA1c in Diabetic Hemodialysis Patients?

The definition of a good glycemic control in patients with diabetes mellitus on hemodialysis is far from settled. In the general population, hemoglobin A1c is highly correlated with the average glycemia of the last 8-12 weeks. However, in hemodialysis patients, the correlation of hbA1c with glycemia is weaker as it also reflects changes in hemoglobin characteristics and red blood cells half-life. As expected, studies show that the association between HbA1c and outcomes in these patients differ from the general population. Therefore, the value of HbA1c in the treatment of hemodialysis patients has been questioned. Guidelines are generally cautious in their recommendations about possible targets of HbA1c in this population. Indeed, the risk of not treating hyperglycemia should be weighed against the particularly high risk of precipitating hypoglycemia in dialysis patients. In this review, a critical analysis of the current role of HbA1c in the care of hemodialysis patients is presented.

Impact of carbamylation on three different methods, HPLC, capillary electrophoresis and TINIA of measuring HbA1c levels in patients with kidney disease

AIMS

Aim of this study was to investigate whether established IFCC traceable routine measurements of HbA1c levels in patients with kidney diseases result in comparable and valid results. Additionally, the influence of carbamylation as a marker of uremia on the measurement methods was assessed.

METHODS

We compared three different measurement methods (high-performance liquid chromatography (HPLC), capillary electrophoresis and turbidimetric inhibition immunoassay (TINIA)) based on 407 nephrology samples for HbA1c determination with specific analysis of the threshold areas where different HbA1c measurements could result in different diagnoses (diabetes mellitus vs prediabetes and prediabetes vs non-diabetes). Indirectly, a potential effect of carbamylated hemoglobin was assessed based on determination of BUN levels in serum.

RESULTS

In nephrological samples, we were able to show that three different measurement methods provide similar results regarding HbA1c in routine diagnostics. Our results show that with BUN concentrations <80 mg/dl and ≥80 mg/dl, similar HbA1c levels were determined, independent of measurement method.

CONCLUSION/DISCUSSION

While routine measurements follow IFCC standards, many interfering factors remain which can influence HbA1c determination, e.g. carbamylation of proteins. As kidney disease is the most common complication in patients with diabetes mellitus, interference in the measurement of HbA1c due to carbamylated proteins as markers of uremia must be recognized and if necessary corrected. We found not only a weak influence of carbamylation on all methods of HbA1c determination, but also that, in nephrological samples, the three different measurement methods provided similar results regarding HbA1c in routine diagnostics.

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.

Carbamylation of immunoglobulin abrogates activation of the classical complement pathway

Post-translational modifications of proteins significantly affect their structure and function. The carbamylation of positively charged lysine residues to form neutral homoitrulline occurs primarily under inflammatory conditions through myeloperoxidase-dependent cyanate (CNO-) formation. We analyzed the pattern of human IgG1 carbamylation under inflammatory conditions and the effects that this modification has on the ability of antibodies to trigger complement activation via the classical pathway. We found that the lysine residues of IgG1 are rapidly modified after brief exposure to CNO- . Interestingly, modifications were not random, but instead limited to only few lysines within the hinge area and the N-terminal fragment of the CH2 domain. A complement activation assay combined with mass spectrometry analysis revealed a highly significant inverse correlation between carbamylation of several key lysine residues within the hinge region and N-terminus of the CH2 domain and the proper binding of C1q to human IgG1 followed by subsequent complement activation. This severely hindered complement-dependent cytotoxicity of therapeutic IgG1 . The reaction can apparently occur in vivo, as we found carbamylated antibodies in synovial fluid from rheumatoid arthritis patients. Taken together, our data suggest that carbamylation has a profound impact on the complement-activating ability of IgG1 and reveals a pivotal role for previously uncharacterized lysine residues in this process.

Nonenzymatic protein acylation as a carbon stress regulated by sirtuin deacylases

Cellular proteins are decorated with a wide range of acetyl and other acyl modifications. Many studies have demonstrated regulation of site-specific acetylation by acetyltransferases and deacetylases. Acylation is emerging as a new type of lysine modification, but less is known about its overall regulatory role. Furthermore, the mechanisms of lysine acylation, its overlap with protein acetylation, and how it influences cellular function are major unanswered questions in the field. In this review, we discuss the known roles of acetyltransferases and deacetylases and the sirtuins as a conserved family of a nicotinamide adenine dinucleotide (NAD⁺)-dependent protein deacylases that are important for response to cellular stress and homeostasis. We also consider the evidence for an emerging idea of nonenzymatic protein acylation. Finally, we put forward the hypothesis that protein acylation is a form of protein "carbon stress" that the deacylases evolved to remove as a part of a global protein quality-control network.

Autoantibody-mediated bone loss

In rheumatoid arthritis (RA), the presence of autoantibodies such as the rheumatoid factor and antibodies against citrullinated proteins is highly correlated with the severity of disease and bone loss. For many years, the involvement of autoantibodies in bone resorption has merely been attributed to enhanced tissue infiltration and the production of inflammatory cytokines that promote osteoclastogenesis. However, recent research provides evidence for a direct activation of osteoclasts and their precursors by autoantibodies, which is independent of inflammation. The depletion of B-cells with rituximab that substantially reduces autoantibody levels seems to be as effective as the well-established treatment with tumor necrosis factor-antagonists in RA patients that do not respond to methotrexate, highlighting the significance of autoantibodies for RA and bone loss.

Updates on the management of diabetes in dialysis patients

Diabetes mellitus is the leading cause of end-stage renal disease (ESRD) in the U.S. and many countries globally. The role of improved glycemic control in ameliorating the exceedingly high mortality risk of diabetic dialysis patients is unclear. The treatment of diabetes in ESRD patients is challenging, given changes in glucose homeostasis, the unclear accuracy of glycemic control metrics, and the altered pharmacokinetics of glucose-lowering drugs by kidney dysfunction, the uremic milieu, and dialysis therapy. Up to one-third of diabetic dialysis patients may experience spontaneous resolution of hyperglycemia with hemoglobin A1c (HbA1c) levels <6%, a phenomenon known as "Burnt-Out Diabetes," which remains with unclear biologic plausibility and undetermined clinical implications. Conventional methods of glycemic control assessment are confounded by the laboratory abnormalities and comorbidities associated with ESRD. Similar to more recent approaches in the general population, there is concern that glucose normalization may be harmful in ESRD patients. There is uncertainty surrounding the optimal glycemic target in this population, although recent epidemiologic data suggest that HbA1c ranges of 6% to 8%, as well as 7% to 9%, are associated with increased survival rates among diabetic dialysis patients. Lastly, many glucose-lowering drugs and their active metabolites are renally metabolized and excreted, and hence, require dose adjustment or avoidance in dialysis patients.

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.

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

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

Carbamylation of serum albumin and erythropoietin resistance in end stage kidney disease

BACKGROUND AND OBJECTIVES

The mechanisms underlying erythropoietin resistance are not fully understood. Carbamylation is a post-translational protein modification that can alter the function of proteins, such as erythropoietin. The hypothesis of this study is that carbamylation burden is independently associated with erythropoietin resistance.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

In a nonconcurrent prospective cohort study of incident hemodialysis patients in the United States, carbamylated albumin, a surrogate of overall carbamylation burden, in 158 individuals at day 90 of dialysis initiation and erythropoietin resistance index (defined as average weekly erythropoietin dose [U] per kg body weight per hemoglobin [g/dl]) over the subsequent 90 days were measured. Linear regression was used to describe the relationship between carbamylated albumin and erythropoietin resistance index. Logistic regression characterized the relationship between erythropoietin resistance index, 1-year mortality, and carbamylation.

RESULTS

The median percent carbamylated albumin was 0.77% (interquartile range=0.58%-0.93%). Median erythropoietin resistance index was 18.7 units/kg per gram per deciliter (interquartile range=8.1-35.6 units/kg per gram per deciliter). Multivariable adjusted analysis showed that the highest quartile of carbamylated albumin was associated with a 72% higher erythropoietin resistance index compared with the lowest carbamylation quartile (P=0.01). Increasing erythropoietin resistance index was associated with a higher risk of death (odds ratio per unit increase in log-erythropoietin resistance index, 1.69; 95% confidence interval, 1.06 to 2.70). However, the association between erythropoietin resistance index and mortality was no longer statistically significant when carbamylation was included in the analysis (odds ratio, 1.44; 95% confidence interval, 0.87 to 2.37), with carbamylation showing the dominant association with death (odds ratio for high versus low carbamylation quartile, 4.53; 95% confidence interval, 1.20 to 17.10).

CONCLUSION

Carbamylation was associated with higher erythropoietin resistance index in incident dialysis patients and a better predictor of mortality than erythropoietin resistance index.

Measurement of Hba(1C) in patients with chronic renal failure

BACKGROUND

Carbamylated hemoglobin (carbHb) is reported to interfere with measurement and interpretation of HbA(1c) in diabetic patients with chronic renal failure (CRF). There is also concern that HbA1c may give low results in these patients due to shortened erythrocyte survival.

METHODS

We evaluated the effect of carbHb on HbA(1c) measurements and compared HbA(1c) with glycated albumin (GA) in patients with and without renal disease to test if CRF causes clinically significant bias in HbA(1c) results by using 11 assay methods. Subjects included those with and without renal failure and diabetes. Each subject's estimated glomerular filtration rate (eGFR) was used to determine the presence and degree of the renal disease. A multiple regression model was used to determine if the relationship between HbA(1c) results obtained from each test method and the comparative method was significantly (p<0.05) affected by eGFR. These methods were further evaluated for clinical significance by using the difference between the eGRF quartiles of >7% at 6 or 9% HbA(1c). The relationship between HbA(1c) and glycated albumin (GA) in patients with and without renal failure was also compared.

RESULTS

Some methods showed small but statistically significant effects of eGFR; none of these differences were clinically significant. If GA is assumed to better reflect glycemic control, then HbA(1c) was approximately 1.5% HbA(1c) lower in patients with renal failure.

CONCLUSIONS

Although most methods can measure HbA(1c) accurately in patients with renal failure, healthcare providers must interpret these test results cautiously in these patients due to the propensity for shortened erythrocyte survival in renal failure.

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.

Glycemic control in diabetic dialysis patients and the burnt-out diabetes phenomenon

Diabetes mellitus (DM) is the most common cause of end-stage kidney disease and a major risk of morbidity and mortality. It is not clear whether medical management of DM has any significant beneficial effect on clinical outcomes at the end-stage of diabetic nephropathy with full-blown micro- and macro-angiopathic complications. Both loss of kidney function and dialysis treatment interfere with glucose homeostasis and confound glycemic control. Given the unique nature of uremic milieu and dialysis therapy related alterations, there have been some debates about reliance on the conventional measures of glycemic control, in particular the clinical relevance of hemoglobin A1c and its recommended target range of <7 % in diabetic dialysis patients. Moreover, a so-called burnt-out diabetes phenomenon has been described, in that many diabetic dialysis patients experience frequent hypoglycemic episodes prompting cessation of their anti-diabetic therapies transiently or even permanently. By reviewing the recent literature we argue that the use of A1c for management of diabetic dialysis patients should be encouraged if appropriate target ranges specific for these patients (e.g. 6 to 8 %) are used. We also argue that "burnt-out diabetes" is a true biologic phenomenon and highly prevalent in dialysis patients with established history and end-stage diabetic nephropathy and explore the role of protein-energy wasting to this end. Similarly, the J- or U-shaped associations between A1c or blood glucose concentrations and mortality are likely biologically plausible phenomena that should be taken into consideration in the management of diabetic dialysis patients to avoid hypoglycemia and its fatal consequences in diabetic dialysis patients.