Decreased clearance of low-density lipoprotein in uremic patients under dialysis treatment

The Decrease in Serum Total Cholesterol and Low-Density Lipoprotein (LDL) Concentrations With the Initiation of Hemodialysis Despite a Concomitant Increase in Serum Albumin Concentrations

Background and objective Hemodialysis patients often have lower serum low-density lipoprotein (LDL) and total cholesterol concentrations compared to the general population. It is unclear if this is due to a persistent decline in the values due to kidney disease or if the hemodialysis itself is contributing to the lower values. It is often assumed that malnutrition and anorexia are the main causes of the low lipid concentration in hemodialysis patients. In this study, we aimed to determine the association between hemodialysis initiation and serum lipid and albumin concentrations. Methodology The medical records of all patients initiating hemodialysis over an 11-year period at a single center were retrospectively reviewed. The data of 145 patients who had all the required lab values available were ultimately included in the study. Serum lipid levels at the initiation of hemodialysis were compared to values obtained mostly 6-18 months later. In order to determine if poor nutritional status is the reason for the decline in serum lipid levels, the serum albumin concentration at the initiation of hemodialysis was compared to that obtained during follow-up labs. Results We observed that serum cholesterol concentration declined from an average of 147 mg/dL to 137 mg/dL, while LDL decreased from an average of 78 mg/dL to 68 mg/dL, and serum albumin concentration increased from 3.4 g/dL to 3.8 g/dL after an average follow-up period of 10.8 months. Conclusions Based on our findings, the decline in serum LDL and total cholesterol concentrations with the initiation of hemodialysis may not be attributed to poor nutritional intake.

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

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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.

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.

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.

Residual Renal Function Affects Lipid Profile in Patients Undergoing Continuous Ambulatory Peritoneal Dialysis

Objective

To determine whether lipoprotein abnormalities associated with continuous ambulatory peritoneal dialysis (CAPD) are influenced by residual renal function (RRF).

Design

Open, non randomized prospective and com -parative study.

Setting

Single university teaching hospital dialysis unit and outpatient clinic.

Patients

Twenty adult patients on standard CAPD (1 -38 months) were divided into two groups: group A (RRF ≤ 0.8 mL/min, n = 10) and group B (RRF ≥ 1.1 mL/ min, n = 10). Patients in the two groups were matched for age, time on dialysis, body weight, body mass index, serum urea and albumin levels, peritoneal and urinary albumin losses, and peritoneal transport characteristics such as overnight 8hour peritoneal creatinine and β2-microglobulin clearances and overnight 8-hour effluent glucose concentrations.

Results

The degree of uremia in patients with preserved RRF (group B) was obviously lower than in patients with negligible RRF (group A), that is, patients in group B had significantly lower serum creatinine and β2-microglobulin levels and significantly higher weekly KTN than group A patients. Despite the prevalence of allele 4 of apolipoprotein E genotype in group A patients, their levels of serum total cholesterol, low-density lipoprotein cholesterol, lipoprotein (a) [Lp(a)], apolipoprotein B (ApoB), and apolipoprotein A1 (ApoA1) were significantly lower than those of patients with preserved RRF (group B). The two groups did not differ significantly in the serum levels of triglyceride or high-density lipoprotein cholesterol. Serum concentrations of Lp(a) and ApoA1, as well as ratios of ApoA1 to ApoB, were correlated significantly with RRF (r = 0.63, r = 0.51, and r = 0.61, respectively).

Conclusions

The findings suggest that RRF affects the lipid profile of CAPD patients, especially serum levels of cholesterol-rich lipoproteins.

Dyslipidemia in Peritoneal Dialysis — Relation to Dialytic Variables

Objective

To investigate whether the specific lipoprotein (LP) abnormalities of peritoneal dialysis (PD) are associated with functional variables of this mode of dialysis.

Design

A survey of the LP profile in relation to peritoneal dialysis capacity (PDC) variables. The LP profile was compared to that of a group of age- and sex-matched controls.

Setting

The Peritoneal Dialysis Unit at Sahlgrenska University Hospital in Gothenburg, Sweden.

Patients

Twenty-two nondiabetic PD patients (5 women, 17 men) who had been on PD for at least 6 months.

Main Outcome Measures

The LP profile included plasma lipids, apolipoproteins (Apo), and individual ApoA- and ApoB-containing LP. The PDC measurement determined peritoneal glucose uptake, protein losses, effective peritoneal surface area, and total weekly creatinine clearance.

Results

The patients had been on PD for 6 to 48 months (mean 15.3 months) and had a total weekly creatinine clearance of 69.7 ± 13.3 L/1.73m2body surface area, an average peritoneal glucose uptake corresponding to 446 ± 162 kcal/24 hour, and a protein loss of 8.1 ± 2.5 g/24 hr. The patients had significantly higher total cholesterol (7.1 mmol/L), VLDL-cholesterol (1.0 mmol/L), LDL-cholesterol (4.7 mmol/L), and triglyceride levels (2.5 mmol/L); whereas the HDL-cholesterol level (1.2 mmol/L) was significantly lower than in controls. The PD patients had increased levels of ApoB-containing LPs, both of the cholesterol-rich LP-B and of the triglyceride-rich LP-B complex, reflected in higher plasma concentrations of ApoB, ApoC-III, and ApoE. Furthermore, they had significantly lower levels of LP-A-I:A-II, as well as of ApoA-I and ApoA-II. The LP-A-I:A-II and ApoA-II levels correlated inversely with the duration of PD treatment ( r = 0.54, p < 0.01 and r = 0.52, p < 0.05, respectively). The ApoA-II level was inversely correlated with the peritoneal surface area ( r = 0.53, p < 0.05). There were no other correlations between LP variables and PDC variables, nor did any of the LP variables correlate with peritoneal glucose uptake or protein losses.

Conclusion

The proatherogenic lipoprotein profile of patients on PD is characterized by increased concentrations of cholesterol-rich and triglyceride-rich ApoB-containing LPs. While the duration of treatment appears to have some influence on the development of this type of dyslipidemia, the pathophysiological links to the dialysis mode must be further explored.

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 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.

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

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

Kinetic studies of atherogenic lipoproteins in hemodialysis patients: do they tell us more about their pathology?

Patients with chronic kidney disease have one of the highest risks for atherosclerotic complications. Several large epidemiological studies described an opposite association of total and low density lipoprotein (LDL) cholesterol with cardiovascular complications and total mortality compared to the general population, a circumstance often called "reverse epidemiology." Many factors might contribute to this reversal such as interaction with malnutrition/inflammation, pronounced fluctuations of atherogenic lipoproteins during the course of renal disease, heterogeneity of lipoprotein particles with preponderance of remnant particles, and chemical modification of lipoproteins caused by the uremic environment. A vicious cycle has been suggested in uremia in which the decreased catabolism of atherogenic lipoproteins such as LDL, IDL and Lp(a) leads to their increased plasma residence time and further modification of these lipoproteins by oxidation, carbamylation, and glycation. Using stable isotope techniques, it has been shown recently that the plasma residence time of these particles is more than twice as long in hemodialysis patients as in nonuremic subjects. This reduced catabolism, however, is masked by the decreased production of LDL, resulting in near-normal plasma levels of LDL. The production rate of Lp(a) in hemodialysis patients is similar to that in controls which together with the doubled residence time results in elevated Lp(a) levels. An increased clearance of these altered lipoproteins via the scavenger receptors of macrophages leads to the transformation of macrophages into foam cells in the vascular wall and might contribute to the pronounced risk for cardiovascular complications of these patients. These observations suggest that the real danger of these particles is not reflected by the measured concentrations but by their metabolic qualities.

Lipoprotein metabolism in chronic renal insufficiency

Chronic renal insufficiency (CRI) is associated with a characteristic dyslipidemia. Findings in children with CRI largely parallel those in adults. Moderate hypertriglyceridemia, increased triglyceride-rich lipoproteins (TRL) and reduced high-density lipoproteins (HDL) are the most usual findings, whereas total and low-density lipoprotein cholesterol (LDL-C) remain normal or modestly increased. Qualitative abnormalities in lipoproteins are common, including small dense LDL, oxidized LDL, and cholesterol-enriched TRL. Measures of lipoprotein lipase and hepatic lipase activity are reduced, and concentrations of apolipoprotein C-III are markedly elevated. Still an active area of research, major pathophysiological mechanisms leading to the dyslipidemia of CRI include insulin resistance and nonnephrotic proteinuria. Sources of variability in the severity of this dyslipidemia include the degree of renal impairment and the modality of dialysis. The benefits of maintaining normal body weight and physical activity extend to those with CRI. In addition to multiple hypolipidemic pharmaceuticals, fish oils are also effective as a triglyceride-lowering agent, and the phosphorous binding agent sevelamer also lowers LDL-C. Emerging classes of hypolipidemic agents and drugs affecting sensitivity to insulin may impact future treatment. Unfortunately, cardiovascular benefit has not been convincingly demonstrated by any trial designed to study adults or children with renal disease. Therefore, it is not possible at this time to endorse general recommendations for the use of any agent to treat dyslipidemia in children with chronic kidney disease.

Cholesterol Absorption and Synthesis in Pediatric Kidney, Liver, and Heart Transplant Recipients

BACKGROUND

Hypercholesterolemia after organ transplantation is common. Previously, we observed higher serum total cholesterol (TC) concentrations in our pediatric kidney than liver or heart transplant recipients. To find an explanation to the observed difference, our kidney recipients' cholesterol synthesis and absorption efficiency was compared to those of liver and heart recipients.

METHODS

Serum noncholesterol sterol ratios (10 x mmol to the mol of TC, surrogate estimates of hepatic cholesterol synthesis and intestinal absorption) were studied in 50 pediatric kidney, 25 liver and 12 heart transplant recipients without diabetes or cholestasis, and in 29 controls.

RESULTS

The kidney recipients had lower Delta-cholesterol (P=0.031), similar lathosterol and higher desmosterol ratios (markers of cholesterol synthesis) (P=0.020), and similar campesterol and sitosterol ratios (markers of cholesterol absorption) when compared to the controls. The liver recipients had lower campesterol ratios than the kidney recipients and controls (P=0.002). Glomerular filtration rates were not associated with the ratios of noncholesterol sterols. Multivariate analysis showed markers of cholesterol synthesis to be lower and absorption to be higher in the kidney than the liver or the heart transplant recipients. Weight-adjusted dosages of immunosuppressive agents were associated with some ratios of noncholesterol sterols and cholestanol though these varied between the transplant recipient groups.

CONCLUSIONS

Serum TC concentration in kidney recipients was not significantly associated with absorption efficiency or synthesis of cholesterol, though kidney transplantation was associated with low synthesis and high absorption efficiency of cholesterol. Immunosuppressive therapy with cyclosporine and methylprednisolone may modulate absorption efficiency and synthesis of cholesterol.

Delayed In Vivo Catabolism of Intermediate-Density Lipoprotein and Low-Density Lipoprotein in Hemodialysis Patients as Potential Cause of Premature Atherosclerosis

Objective— Premature cardiovascular disease is the leading cause of death in patients with end-stage renal disease treated by hemodialysis (HD). Low-density lipoprotein (LDL) levels are not generally increased in HD patients, but their LDL metabolism is still poorly understood. We therefore investigated the in vivo metabolism of apoB-containing lipoproteins in two different ethnic populations of HD patients and controls.

Methods and Results— We performed stable isotope kinetic studies using a primed constant infusion of deuterated leucine in 12 HD patients and 13 healthy controls. Tracer/tracee ratio of apoB was determined by means of gas chromatography/mass spectrometry, and the modeling program SAAMII was used to estimate the fractional catabolic rate (FCR) of apoB. Mean LDL-apoB plasma concentrations were almost identical in both groups (HD: 95±30 mg/dL, controls: 91±40 mg/dL), whereas LDL-apoB FCR was 50% lower in HD patients as compared with controls (0.22±0.12 days −1 versus 0.46±0.20 days −1 , P =0.001) with concomitantly decreased production rates of LDL. Compared with controls, intermediate-density lipoprotein (IDL)-apoB FCR was 65% lower (2.87±1.02 days −1 versus 8.89±4.94 days −1 , P =0.014), accompanied by 1.5-fold higher IDL-apoB levels in HD. Very low-density lipoprotein metabolism was similar in both study groups.

Conclusions— In vivo catabolism of LDL and IDL is severely impaired in HD patients but misleadingly masked by normal plasma cholesterol levels. The resulting markedly prolonged residence times of both IDL and LDL particles might thus significantly contribute to the well-documented high risk for premature cardiovascular disease in HD patients.

A broad-based metabolic approach to study VLDL apoB100 metabolism in patients with ESRD and patients treated with peritoneal dialysis

BACKGROUND

Dyslipidemia is often observed in patients with end-stage renal disease (ESRD) and is associated with cardiovascular diseases. Peritoneal dialysis treatment may further deteriorate the lipoprotein abnormalities, suggesting that peritoneal dialysis alters lipid metabolism.

METHODS

To study the mechanisms involved in these abnormalities in peritoneal dialysis, we measured insulin sensitivity, free fatty acids release, de novo lipogenesis (DNL), very low-density lipoprotein (VLDL) apoB100 kinetics and cholesterol synthesis in vivo in ESRD (N= 6), peritoneal dialysis patients (N= 5), and controls (N= 7) using stable isotopes.

RESULTS

Insulin sensitivity, as assessed by an euglycemic hyperinsulinemic clamp, tended to be lower in ESRD and peritoneal dialysis compared to controls [P= 0.08 by analysis of variance (ANOVA)]. Free fatty acid release during the euglycemic hyperinsulinemic clamp tended to be higher in ESRD and peritoneal dialysis compared to controls (P= 0.08 by ANOVA), while DNL and fractional cholesterol synthesis were normal. VLDL-1 apoB100 (P < 0.05) and VLDL-2 apoB100 pool sizes (P < 0.05) were significantly higher in peritoneal dialysis patients compared to controls. The increased VLDL-1 apoB100 pool size was explained by increased VLDL-1 apoB100 synthesis (P < 0.05) in combination with reduced VLDL-1 apoB100 catabolism (P < 0.01), while the increased VLDL-2 apoB100 pool was explained by reduced catabolism (P < 0.01).

CONCLUSION

Both VLDL-1 apoB100 and VLDL-2 apoB100 pool sizes are increased in peritoneal dialysis patients, due to disturbances both in synthesis and catabolism. VLDL-1 apoB100 production is, at least partially, explained by increased free fatty acid availability secondary to peripheral insulin resistance, thus identifying insulin resistance as potential therapeutic target in peritoneal dialysis patients.

Oxidative stress during ex vivo hemodialysis of blood is decreased by a novel hemolipodialysis procedure utilizing antioxidants

The high cardiovascular mortality in patients receiving hemodialysis (HD) has been attributed, in part, to oxidative stress. Here we examined the effectiveness of antioxidants introduced by means of a novel hemolipodialysis (HLD) procedure in terms of reducing oxidative stress during ex vivo blood circulation. Oxidative stress was studied in a model HD system resembling the extracorporeal circulation of blood during clinical HD. Blood circulation produced an increase of up to 280% in free hemoglobin levels and an increase of 320% in electronegative LDL (LDL(-)) subfraction. A significant correlation between LDL(-) and free hemoglobin levels confirmed previous findings that LDL(-) formation during ex vivo circulation of blood can be mediated by the oxidative activity of free hemoglobin. These effects were significantly attenuated during HLD using a dialysis circuit containing vitamin E with or without vitamin C. By contrast, HLD with vitamin C alone had a marked pro-oxidant effect. TBARS, lipid hydroperoxides, vitamin E and beta-carotene content in LDL were not significantly altered by the HD procedure. These findings demonstrate the occurrence of oxidative stress in human plasma where lipoproteins are a target and indicate antioxidant-HLD treatment as a specific new approach to decreasing the adverse oxidative stress frequently associated with cardiovascular complications in high-risk populations of uremic patients.

Dyslipidemia in pediatric renal disease: epidemiology, pathophysiology, and management

Dyslipidemia increases the risk of cardiovascular events among individuals with renal disease, and there is a growing body of evidence that it hastens the progression of renal disease itself. Children with nephrotic syndrome or renal transplants have easily recognized hyperlipidemia. Among those with chronic renal insufficiency or end-stage renal disease, detection of dyslipidemia requires more careful analysis and knowledge of normal pediatric ranges. Disordered lipoprotein metabolism results from complex interactions among many factors, including the primary disease process, use of medications such as corticosteroids, the presence of malnutrition or obesity, and diet. The systematic treatment of dyslipidemia in children with chronic renal disease is controversial because conclusive data regarding the risks and benefits are lacking. Hepatic 3-methylglutaryl coenzyme A reductase inhibitors (statins), fibrates, plant stanols, bile acid-binding resins, and dietary manipulation are options for individualized treatment. Prospective investigations are required to guide clinical management.

Effects of lipid-lowering drugs on intermediate-density lipoprotein in uremic patients

BACKGROUND

Patients with chronic renal failure often have alterations in lipoprotein profile including elevated very-low density lipoprotein (VLDL) and intermediate density lipoprotein (IDL), and reduced high density lipoprotein (HDL) levels. Among these changes, raised IDL has been shown as an independent risk factor for atherosclerosis in hemodialysis patients. There are a limited number of studies reporting pharmacological approaches to IDL reduction in a uremic population.

METHODS

We therefore summarize the effects of lipid-lowering drugs on IDL levels in patients with chronic renal failure treated by hemodialysis (HD) or continuous ambulatory peritoneal dialysis (CAPD).

RESULTS

First, a nicotinic acid analog niceritrol was given to hemodialysis patients. The drug increased HDL-cholesterol by 11%, but the reductions in VLDL-, IDL- and LDL-cholesterol were not significant. Second, CAPD patients were treated with a fibric acid derivative clinofibrate, which was excreted mainly into bile unlike other drugs in this class. The fibrate resulted in a remarkable reduction in VLDL-triglycerides, although it did not reduce IDL-cholesterol. Finally, an HMG-CoA reductase inhibitor (statin) pravastatin was used in HD and CAPD patients. Pravastatin reduced IDL- and LDL-cholesterol to the same extent (by 31%). None of these treatments caused serious adverse effects.

CONCLUSIONS

We propose that IDL is an important target in the management of uremic dyslipidemia. To date, statins have been shown to be suitable for this purpose, although it remains to be clarified whether such an intervention reduces the risk for atherosclerotic vascular events in the uremic population.

Oxidative stress in chronic renal failure

Cardiovascular disease is the major cause of morbidity and mortality in chronic renal failure. The aim of this review is to summarise current evidence suggesting that there is increased free radical production, antioxidant depletion and changes in lipoprotein composition in renal failure which will lead to oxidation of LDL and hence to accelerated development of atherosclerosis.

Carbamylated hemoglobin and carbamylated plasma protein in hemodialyzed patients

The carbamylation reaction in vivo involves the nonenzymatic, covalent attachment of isocyanic acid, the spontaneous dissociation product of urea, to proteins. Carbamylated proteins have been proposed as markers of uremia and indicators of uremic control. However, the utility of measuring carbamylated proteins has not been investigated adequately. Therefore, this study was done to determine the relationship between the carbamylation of long-lived protein (hemoglobin) with that of short-lived proteins (plasma proteins) in hemodialyzed patients. Significantly higher carbamylated hemoglobin (CHb; 157 +/- 40 microg valine hydantoin/g Hb) and carbamylated protein (CTP; 0.117 +/- 0.011 absorbance/mg protein) concentrations were found in hemodialyzed patients (N = 13) as compared to normal individuals (N = 9, 53 +/- 20 microg valine hydantoin/g Hb and 0.08 +/- 0.01 absorbance/mg protein, respectively). A high correlation was found between CHb and CTP concentrations (r = 0.87, P < 0.0001), demonstrating a strong relationship between these two different half-lived proteins. A six-month longitudinal study of seven hemodialyzed patients showed that the between subject correlations were significant for CHb versus CTP as well as CHb versus pre-dialysis urea. Correlations were not significant for CTP versus pre-dialysis urea or Kt/V, nor CHb versus Kt/V. Carbamylated hemoglobin fluctuated the most over this time period (30.1% +/- 20.2%), pre-dialysis urea and CTP varied less (18.3% +/- 13.4% and 14.9% +/- 7.5%, respectively), and Kt/V varied the least (6.3% +/- 3.3%). Within subject correlations were not significant between any two tests. It is unclear whether the lack of correlations found is real or a function of the small sample size. However, these data do show that CHb and CTP are positively associated and reflect the degree of urea exposure in the blood, but their usefulness for patients on maintenance hemodialysis is not clear.

Increased plasma concentrations of LDL-unbound apo(a) in patients with end-stage renal disease

Lipoprotein(a) [Lp(a)] and its characteristic glycoprotein apolipoprotein(a) [apo(a)] are risk factors for atherosclerosis in the general population. Patients with renal disease show an elevation of Lp(a). Recent studies have described an arteriovenous difference of Lp(a) in the renovascular bed as well as the plasma-derived fragmented LDL-unbound apo(a) in urine, suggesting that the kidney is involved in the metabolism of Lp(a). We therefore investigated whether patients with chronic renal failure have higher levels of LDL-unbound apo(a) and whether this could account for the increased Lp(a) concentrations in these patients. In addition, we studied the possible generation of apo(a) fragments in vitro by mimicking uremic plasma conditions and by investigating the assembly of Lp(a) in cell culture experiments. Patients treated by hemodialysis (N = 185) and by continuous ambulatory peritoneal dialysis (CAPD; N = 20) had markedly elevated absolute (1.22 +/- 1.55 mg/dl and 2.14 +/- 2.86 mg/dl) as well as relative (7.5% and 7.3%) amounts of LDL-unbound apo(a) in comparison to controls (0.46 +/- 0.48 mg/dl or 4.5%). Following renal transplantation the absolute amount decreased significantly. Lp(a) plasma concentration was the most important determining variable for the absolute amount of LDL-unbound apo(a) and showed a positive correlation in both hemodialysis patients (r = 0.85) and controls (r = 0.92). In vitro experiments demonstrated that "uremization" of plasma samples did not generate a higher amount of LDL-unbound apo(a). Although LDL of renal patients has different chemical and structural properties as compared to control LDL, the extracellular assembly of Lp(a) did not differ between patients and controls. Therefore, the higher amounts of LDL-unbound apo(a) found in renal disease are not caused by an impaired assembly of Lp(a), but rather indicate a catabolic role of the kidney for LDL-unbound apo(a) as was already shown for Lp(a). Despite a small contribution, these elevated levels cannot explain the higher Lp(a) values found in patients with end-stage renal disease.

Oxidation of low-density lipoprotein and atherosclerosis in chronic renal failure

The major cause of death in patients with end-stage renal failure receiving renal replacement therapy is cardiovascular disease. Oxidation of low-density lipoprotein (LDL) is recognized as a key early stage in the development of atherosclerosis, leading to uptake of LDL by the macrophage scavenger receptor and hence to foam cell formation. However, several studies have suggested that the susceptibility of LDL to oxidation is not increased in chronic renal failure. We propose a number of mechanisms which may lead to increased lipoprotein oxidation in vivo, and hence contribute to increased atherosclerosis in renal failure.

Lipids, lipoproteins and apolipoproteins abnormalities in patients undergoing dialysis

Twenty hemodialysis (HD) patients and 20 patients on continuous ambulatory peritoneal dialysis (CAPD) were investigated for lipids, lipoproteins and apolipoproteins abnormalities. HD patients had elevated serum triglyceride, decreased high-density lipoprotein cholesterol (HDL-C) and apolipoprotein A-I (Apo A-I), while CAPD patients had elevated total cholesterol, triglyceride, low-density lipoprotein cholesterol (LDL-C), Apolipoprotein B (Apo B), Apo B/Apo A-I ratio, and decreased HDL-C, Apo A-I. Because of the molecular sieving effects of peritoneum, CAPD have a negative effect on these abnormalities. CAPD patients might be at greater risk of developing coronary artery disease than HD patients who are also at increased risk as compared with normals.

Effect of serum subfractions from peritoneal dialysis patients on Hep-G2 cell apolipoprotein A-I and B metabolism

We previously showed that uremic serum subfractions isolated from hemodialysis (HD) patients inhibited the production of apolipoprotein (apo) A-I by human hepatoblastoma cells, Hep-G2. Because of the reported differences in atherogenic cardiovascular mortality between HD and peritoneal dialysis (PD) patients, we examined the effect of similar subfractions from PD patients on apo A-I and apo B synthesis. After obtaining informed consent, serum samples from five normal subjects and nine stable PD patients were applied to Sephadex G-25 columns to obtain the serum subfractions used in the various experiments. Sephadex G-25 chromatograms of PD sera showed a broad peak from fractions 30 through 60 (molecular wt 500 to 2000 Da). Control serum showed no peak in this region. PD serum subfractions decreased apo A-I synthesis, secretion, and apo A-I mRNA expression by Hep-G2 cells when compared to subfractions from control subjects. Cholesterol efflux studies showed that conditioned media obtained from Hep-G2 cells incubated with PD serum subfractions inhibited cholesterol efflux from fibroblasts, suggesting a biologically-significant decrease in apo A-I synthesis. PD serum subfractions increased protein synthesis and mRNA expressions of apo B by Hep-G2 cells. Therefore, serum subfractions obtained from PD patients decreased apo A-I and increased apo B synthesis, findings consistent with their serum lipoprotein profiles suggesting that a biologically-active component in these subfractions could contribute to the risk of atherogenic cardiovascular disease in PD.