Compositional Lipoprotein Changes and Low ... - Semantic Scholar

Original Paper Received: November 1, 2002 Accepted: August 18, 2003

Nephron Clin Pract 2003;95:c77–c83 DOI: 10.1159/000074320

Compositional Lipoprotein Changes and Low-Density Lipoprotein Susceptibility to Oxidation in Chronic Renal Failure Patients with Heavy Proteinuria Sonia Athena Karabina a Haralampos Pappas b George Miltiadous c Eleni Bairaktari d Dimitris Christides c Alexandros Tselepis a Moses Elisaf c Konstantinos Siamopoulos b Departments of a Chemistry, Laboratory of Biochemistry, b Nephrology and c Internal Medicine, and of Biological Chemistry, Medical School, University of Ioannina, Ioannina, Greece

d Laboratory

Key Words Proteinuria W Low-density lipoprotein subfraction profile W Small dense low-density lipoprotein W Uremia W Oxidative modification of low-density lipoprotein

Abstract Background: There are limited data regarding qualitative lipoprotein abnormalities in undialysed uremic patients without proteinuria. In this report, we focused on lipoprotein changes observed in uremic patients with proteinuria as well as on the susceptibility of low-density lipoprotein (LDL) of these patients to oxidative modification in vitro. Methods: 20 patients with chronic renal failure [serum creatinine 11.6 mg/dl (141.4 Ìmol/l)], but not yet on renal replacement therapy, and with heavy proteinuria (1 2 g/24 h), and 18 age- and sex-matched healthy individuals participated in the study. In both patients and controls, venous blood was collected for determination of serum lipid and lipoprotein levels, lipoprotein subfraction profile and chemical composition, as well as the susceptibility of LDL subfractions to oxidation. Results: Patients exhibited a more atherogenic lipid profile compared with the control population. Further-

more, the total very LDL + intermediate-density lipoprotein mass was increased in patients compared with controls, while this subfraction was triglyceride enriched in uremic patients. The total LDL concentration was significantly higher in patients compared with controls due mainly to an increase in the mass of all lipoprotein subfractions. It is noteworthy that the mass of small dense LDL was significantly elevated in patients compared with controls (135 B 12 vs. 115 B 11 mg/dl, p = 0.01), an increase which was more pronounced in hypertriglyceridemic patients. Furthermore, the subfraction high-density lipoprotein-2 mass was significantly lower in uremic patients compared with controls. Finally, no significant differences in the lag time, the rate of oxidation and the relative electrophoretic mobility values in each LDL subfraction between the two groups were observed. Conclusion: We conclude that uremic patients with heavy proteinuria exhibit compositional lipoprotein changes that are less marked than those observed in nonuremic patients with nephrotic syndrome. However, there is no evidence that circulating LDL isolated from these patients is more susceptible to oxidation in vitro than lipoprotein isolated from age- and gender-matched controls. Copyright © 2003 S. Karger AG, Basel

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Moses Elisaf, MD, FRSH, FACA Department of Internal Medicine University of Ioannina GR–451 10 Ioannina (Greece) Tel. +30 2651 0 97509, Fax +30 2651 0 97016, E-Mail [email protected]

Introduction

Cardiovascular disease remains the most common cause of mortality in uremic patients accounting for up to 50% of deaths in this patient population [1–3]. These patients exhibit a unique lipoprotein profile, called uremic dyslipidemia, characterized by hypertriglyceridemia, elevated very low-density lipoprotein (VLDL), accumulated remnant particles and decreased high-density lipoprotein (HDL) [4–6]. Proteinuria, on the other hand, is usually associated with lipid abnormalities consisting of increased total and LDL cholesterol levels as well as increased triglycerides [7–9]. It has been previously suggested that beyond quantitative changes, qualitative lipoprotein abnormalities are frequently encountered in both uremic and proteinuric patients, especially a predominance of atherogenic small dense LDL particles [10–14]. However, there are few studies examining compositional lipoprotein changes in undialysed uremic patients and little available information relating to individuals without proteinuria [11, 12]. In this report, we have focused on qualitative and quantitative lipoprotein changes observed in uremic patients with proteinuria, as well as on the susceptibility of LDL isolated from these patients to oxidative modification in vitro. Materials and Methods Patients Twenty patients (12 men, 8 women) and 18 controls (11 men, 7 women) were recruited into the study. The patients had chronic renal failure with serum creatinine levels 1 1.6 mg/dl (141.4 Ìmol/l), but were not yet on renal replacement therapy, and they had heavy proteinuria (1 2 g/24 h). Patients with other diseases or on treatment that might influence their lipid profile were excluded, and specifically patients with diabetes mellitus, hypothyroidism, neoplasia, or patients administered thiazide diuretics, corticosteroids, ß-blockers, hypolipidemic drugs, or immunosuppressants. These patients were compared with age- and sex-matched healthy controls who were members of the laboratory staff and had normal renal function [serum creatinine ! 1.4 mg/dl (123.8 Ìmol/l)]. The same exclusion criteria applied to both patients and controls. All study participants were free of acute illness at the time of the study, and had not experienced any acute vascular event in the preceding 6 months. All subjects were reviewed after an overnight fast. Venous blood was collected for the determination of serum creatinine, glucose, albumin, total protein, serum lipid and lipoprotein levels, lipoprotein subfractions and chemical composition and the susceptibility of LDL subfractions to oxidation. All patients collected a 24-hour urine specimen for the determination of protein excretion. Subfractionation of Plasma Lipoproteins Lipoproteins were fractionated by isopycnic density gradient ultracentrifugation as described [15]. After ultracentrifugation, 24

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Nephron Clin Pract 2003;95:c77–c83

fractions of 0.4 ml each were collected and analyzed for their protein content. Subsequently, equal volumes of certain gradient fractions were pooled to constitute the lipoprotein subfractions, as follows: fractions 1 and 2 [VLDL + IDL; density (d) ! 1.019 g/ml], 3 and 4 (LDL-1; d = 1.019–1.023 g/ml), 5 and 6 (LDL-2; d = 1.023– 1.029 g/ml), 7 and 8 (LDL-3; d = 1.029–1.039 g/ml), 9 and 10 (LDL-4; d = 1.039–1.050 g/ml), 11 and 12 (LDL-5; d = 1.050– 1.063 g/ml), 13–16 (HDL-2; d = 1.063–1.100 g/ml), 17–22 (HDL-3; d = 1.100–1.167 g/ml), and 23 and 24 (VHDL; d = 1.167–1.190 g/ ml). All subfractions were extensively dialyzed in 10 mM phosphatebuffered saline (PBS) containing 2 mM EDTA, pH 7.4 at 4 ° C, then filter-sterilized and stored at 4 ° C for up to 2 weeks. Oxidation of LDL Subfractions LDL subfractions were dialysed to remove EDTA against two changes of a 200-fold volume of 10 mM PBS, pH 7.4 for 24 h at 4 ° C in the dark. From the dialysed LDL subfractions, a volume of 1 ml containing 100 Ìg protein/ml PBS was oxidized in the presence of 5 ÌM CuSO4 [16]. The kinetics of oxidation were determined by monitoring the increase in the 234-nm absorbance band on a Perkin-Elmer L 15 spectrophotometer every 10 min for 3 h. The increase in the absorbance results from the formation of conjugated dienes during peroxidation of the polyunsaturated fatty acid content of LDL [17]. The sigmoid curve of oxidation plotted against time was divided into three consecutive phases, i.e. lag phase, propagation phase, and decomposition phase. The lag time, the maximal rate of conjugated dienes formation and the total amount of diene formed were calculated as described previously [18]. Analytical Methods Serum glucose, urea, total cholesterol and triglycerides were determined on an Olympus AU560 Clinical Chemistry analyzer (Hamburg, Germany). Serum HDL cholesterol levels were measured with the above method in the supernatant, after treatment of serum with dextran sulfate/magnesium chloride for the precipitation of apolipoprotein B (Apo B)-containing lipoproteins. Serum LDL cholesterol levels were calculated using the Friedewald formula. Serum Apo B and Apo AI were measured by immunonephelometry (Behring Diagnostics GmbH, Liederbach, Germany). Serum lipoprotein (a) levels were measured by an enzyme immunoassay method [Macra Lp(a), Terumo Medical Corporation Diagnostic Division, Elkton, Md., USA]. Serum and urine creatinine and protein levels were measured by the Jaffé method and the biuret method, respectively, whereas serum albumin was assessed using the bromocresol green method. The total cholesterol, triglyceride and phospholipid content in each lipoprotein subfraction was measured by enzymatic methods using the Bio-Mérieux kits. The free cholesterol content in each lipoprotein subfraction was measured by the Boehringer Mannheim kit.

Results

Table 1 lists the anthropometric and clinical characteristics of the study population. As expected, patients exhibited increased serum urea, creatinine, but also total and LDL cholesterol, triglyceride, Apo B, LDL cholesterol/ HDL cholesterol, and lipoprotein (a) levels and decreased albumin levels compared with the control population.

Karabina/Pappas/Miltiadous/Bairaktari/ Christides/Tselepis/Elisaf/Siamopoulos

Table 1. Morphometric and clinical characteristics of patients and controls

Characteristic

Patients

Controls

Number Age, years Mean B SD Range Body mass index Serum glucose, mg/dl (mmol/l) Serum urea, mg/dl (mmol/l) Serum creatinine, mg/dl (Ìmol/l) Serum total cholesterol, mg/dl (mmol/l) Serum triglycerides, mg/dl (mmol/l) Serum LDL cholesterol, mg/dl (mmol/l) Serum HDL cholesterol, mg/dl (mmol/l) Serum LDL cholesterol/HDL cholesterol Serum lipoprotein (a), mg/dl Median Range Serum Apo AI, mg/dl (g/l) Serum Apo B, mg/dl (g/l) Serum total proteins, g/dl (g/l) Serum albumin, g/dl (g/l) 24-hour urine protein, mg

20

18

33B7 40U62 23.9B2.3 108B6 (5.94B0.33) 71.1B19.9 (25.4B7.1) 2.2B0.3 (194.5B26.5) 285B52 (7.4B1.34) 147B63 (1.66B0.7) 205B49 (5.3B1.27) 49B12 (1.26B0.3) 4.4B1.4 33.7 0.8U82 171B20 (1.71B0.2) 190B46 (1.9B0.46) 7.2B0.9 (72B9) 4.0B0.6 (40B6) 4,874.6B3,973

p

35B6 45U63 22.7B1.3 98B6 (5.4B0.33) 36.2B7 (12.9B2.5) 0.76B0.2 (67.2B17.7) 193B28 (5B0.72) 87B25 (0.98B0.28) 120B15 (3.1B0.39) 52B7 (1.34B0.18) 2.3B0.5 13.2 0.8U46 177B14 (1.77B0.14) 98B14 (0.98B0.14) 7.0B0.35 (70B3.5) 4.4B0.6 (44B6) ! 150

NS NS NS 0.01 0.01 0.01 0.01 0.01 NS 0.01 0.01 NS 0.01 NS 0.05 –

Values are expressed as mean B SD, except for the lipoprotein (a) levels, which are expressed as medians.

Table 2. Chemical composition of the VLDL +IDL subfraction (d ! 1.019 g/ml) isolated from controls and patients by isopycnic density gradient ultracentrifugation

Table 3. LDL subfractions in patients and controls

Component, %

Patients

Controls

p

LDL

Patients

Controls

p

Cholesterol free Cholesterol esters Phospholipids Protein Triglycerides Lipoprotein mass mg/ml of plasma

6.3B1.6 19.1B2.0 18.4B2.4 14.2B3.6 42.1B3.6

7.4B3.2 17.2B4.0 19.7B2.3 16.8B2.6 38.6B2.1

NS NS NS 0.05 0.05

0.477B0.186

0.382B0.245

0.06

Values represent the mean B SD of duplicate analyses of each component and are in weight percent. Lipoprotein mass corresponds to the sum of all lipid and protein components.

Total LDL, mg/dl LDL-1, mg/dl LDL-2, mg/dl LDL-3, mg/dl LDL-4, mg/dl LDL-5, mg/dl LDL-1, % LDL-2, % LDL-3, % LDL-4, % LDL-5, %

448B36 72B13 79B18 153B34 92B20 42B8 17B2 17.5B2 35B2.5 18.5B2 11B3

320B42 26B1 52B18 109B44 74B36 43B18 8B4 18B6 33B9 21B6 14B8

0.01 0.05 0.05 0.05 0.05 NS 0.05 NS NS NS NS

Furthermore, serum total cholesterol, LDL cholesterol and Apo B levels correlated with proteinuria (p ! 0.01 for all correlations), while no lipid parameter was related to serum creatinine levels. Finally, low serum albumin was associated with increasing total cholesterol (p ! 0.05), LDL cholesterol (p ! 0.05) and Apo B (p ! 0.01) levels.

Results comparing the chemical composition of the VLDL + IDL subfraction isolated from patients and controls are shown in table 2. Even though the total VLDL + IDL mass was increased in patients compared with controls, this difference was of marginal significance (p = 0.06). However, the VLDL + IDL subfraction was triglyc-

Lipoprotein Abnormalities in Uremic Patients


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Table 4. Mean weight percent chemical composition of native LDL subfractions from patients and controls Component %

Cholesterol free Cholesterol esters Phospholipids Protein Triglycerides

LDL subfraction LDL-1 (d = 1.019–1.023 g/ml)

LDL-2 (d = 1.023–1.029 g/ml)

LDL-3 (d = 1.029–1.039 g/ml)

LDL-4 (d = 1.039–1.050 g/ml)

LDL-5 (d = 1.050–1.063 g/ml)

patients

controls

patients

controls

patients

patients

patients

9.3B0.9 38.1B0.72 19.1B1.4* 14.1B2.7 19.3B1.9*

9.8B3.6 36.1B3.2 23.4B2.7 16.7B1.7 13.8B2.4

9.5B1.1* 41.3B3.6 19.7B1.4* 16.2B1.9 13.4B1.9

6.5B2.7 41.1B5.5 25.2B4.8 15.4B3.6 11.7B7.3

7.5B0.8 7.8B3.3 42.1B3.9 46.0B7.6 19.9B1.4* 23.4B2.7 21.3B2.8* 14.2B4.4 9.2B2.2 8.7B4.2

controls

controls

7.9B2.1 8.5B3.3 42.1B4.9 44.3B7.1 19.9B1.0* 23.7B2.7 21.8B2.3* 16.6B4.0 8.4B1.4 6.9B1.8

controls

8.1B0.7 7.6B3.8 40.4B2.4 37.5B5.9 19.4B0.9* 26.0B3.8 23.4B1.1* 20.2B2.5 8.6B0.7 7.8B3.9

Values represent the mean B SD of duplicate analyses of each component in each LDL subfraction and are in weight percent. * p ! 0.05 compared with the LDL subfraction of controls.

Table 5. Chemical composition of HDL subfractions from patients and controls

Component, %

Cholesterol free Cholesterol esters Phospholipids Protein Triglycerides Lipoprotein mass, mg/dl of plasma

HDL-2 (d = 1.063–1.100 g/ml)

HDL-3 (d = 1.100–1.167 g/ml)

VHDL (d = 1.167–1.190 g/ml)

patients

controls

patients

controls

patients

controls

4.6B0.7 26.5B1.5 24.1B1.8* 38.7B3.5 6.2B1.5

3.8B2.1 23.8B7.8 30.4B4.8 35.9B8.9 5.9B3.2

1.8B0.7 21.7B2.4 26.2B1.6 45.5B2.3 4.9B1.7

2.4B1.6 20.3B5.6 28.5B4.4 44.8B6.3 3.9B1.6

0.4B0.1 7.0B2.2 12.9B0.9 76.3B2.5 3.4B1.1

0.6B0.4 7.9B1.6 11.6B1.9 76.8B2.9 3.0B1.2

1.28B0.23

1.35B0.35

1.64B0.24

1.67B0.17

0.99B0.23* 1.17B0.41

Values represent the mean B SD of duplicate of each component in each HDL subfraction and are in weight percent. Lipoprotein mass corresponds to the sum of all lipid and protein components. * p ! 0.05 compared with controls.

eride enriched in uremic patients compared with controls. As shown in table 3, the total LDL concentration was significantly higher in patients compared with controls due mainly to an increase in the mass of all lipoprotein subfractions, except for the LDL-5. It is noteworthy that the mass of the small dense LDL (subfractions LDL-4 + LDL5) was significantly elevated in patients compared with controls (135 B 12 mg/dl vs. 115 B 11 mg/dl, p = 0.01). This difference was more pronounced in hypertriglyceridemic patients [triglycerides 1150 mg/dl (1.7 mmol/l); 145 B 16 mg/dl (1.64 B 0.18 mmol/l) vs. 110 B 18 mg/dl (1.24 B 0.20 mmol/l); p ! 0.01]. These figures refer to the mass of the small dense LDL. However, marked heterogeneity was seen in the plasma concentration of small dense LDL, which ranged from 36 to 190 mg/dl. The LDL-4 + LDL-5 concentration was greater than 100 mg/dl in half

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of the patients, but only in 2 controls. The LDL subfraction distribution was similar in both groups, with an increase in the percentage of LDL-3. It should be mentioned that only the mean percentage of LDL-1, the more buoyant LDL subfraction, was increased in uremic patients compared with controls. The chemical composition of LDL subfractions in uremic patients (table 4) revealed that most of the LDL subfractions in uremic patients were relatively triglyceride rich compared with controls, though only in LDL-1 was this difference of statistical significance. With regard to HDL, the HDL-2 mass was significantly lower in uremic patients compared with controls (table 5). However, there were no significant changes in the chemical composition of HDL subfractions between patients and controls, except for phospholipids, which were


Table 6. Oxidation parameters of LDL subfractions Patients LDL-1

Controls LDL-2

LDL-3

LDL-4

LDL-5

LDL-1

LDL-2

LDL-3

LDL-4

LDL-5

Lag time, min 82.1B13.2 69.5B12.8 59.0B13.1 51.9B11.3 48.7B8.9 87.3B11.0 74.6B13.7 60.6B9.9 52.7B7.9 50.0B7.7 Rate of oxidation, nmol/mg protein per min 7.1B1.8 7.7B1.6 8.3B1.8 8.4B1.8 8.7B2.1 6.7B0.9 7.7B1.5 9.1B0.9 9.2B0.5 8.1B1.5 Total amount of dienes, nmol/mg protein 400.7B78.4 425.7B91.4 459.3B73.1* 419.8B81.9* 355.6B83.2* 433.6B78.6 455.5B82.9 527.2B84 501.4B42 430.1B113 REM 2.27B0.49 2.48B0.45 2.67B0.42 2.92B0.52 2.94B0.47 2.37B0.54 2.55B0.55 2.66B0.47 2.85B0.30 2.91B0.42 Oxidation was performed by incubating 0.1 mg/ml LDL subfraction protein with Cu2+, 10 ÌM final concentration at 37 ° C. The kinetics of oxidation were determined by monitoring the increase in absorbance at 134 nm every 10 min for 3 h. Values represent the mean B SD. REM = Relative electrophoretic mobility. * p ! 0.01 compared with controls.

lower in the patients’ HDL-2 subfraction (table 5). However, all HDL subfractions were somewhat triglyceride enriched. Both the decrease in HDL-2 mass and the triglyceride enrichment of HDL were more pronounced in hypertriglyceridemic patients [triglycerides 1150 mg/dl (11.7 mmol/l); 0.88 B 0.12 mg/dl, HDL-2 triglycerides 8.9%, HDL-3 triglycerides 9.4% and VHDL triglycerides 5.1%]. These figures refer to the HDC-2 mass. The oxidation parameters of LDL subfractions are depicted in table 6. Interestingly, the total amounts of dienes were significantly higher in the LDL-3, LDL-4, and LDL-5 subfractions in the control population compared with those of the patients. No significant differences in the lag time, the rate of oxidation and the relative electrophoretic mobility values in each LDL subfraction between the two groups were observed.

Discussion

This is the first study to demonstrate the compositional lipoprotein changes in patients with nephrotic-range proteinuria and impaired renal function. In fact, the only data available in the literature refer to patients with nephrotic syndrome but reasonably well-preserved renal function as well as uremic patients without heavy proteinuria (!2 g/24 h) [11–14]. Our study was able to delineate both the qualitative lipid and lipoprotein changes observed in this population (increased total and LDL cholesterol and Apo B levels as well as increased VLDL + IDL and LDL mass and reduced HDL mass), but also the qualitative lipoprotein abnormalities, namely the presence of triglyceride-rich VLDL + IDL as well as triglyceride-rich HDL, and the


increased concentrations of small dense LDL, but without significant changes in the LDL subfraction distribution. The increased triglycerides, along with the increased VLDL + IDL mass, are characteristic of uremic dyslipidemia, are evident even with mild deterioration of renal function and are mainly related to defective catabolism of triglyceride-rich lipoproteins by the enzymes lipoprotein lipase and hepatic lipase [4, 5]. However, in contrast to that recently observed in chronic renal failure patients [11] or proteinuric patients [14], the VLDL + IDL fraction was triglyceride and not cholesterol enriched. This difference could be partly due to the lower serum triglyceride levels observed in predialysis patients participating in the previous study [115 mg/dl (1.3 mmol/l) vs. 147 mg/ dl (1.66 mmol/l) in our study] [11], taking into account that the composition of this subfraction is clearly influenced by the total triglyceride concentration. Our patients also had increased concentrations of almost all LDL subfractions compared with the control population. Furthermore, even though the concentration of small dense LDL (subfractions LDL-4 + LDL-5) was increased in patients compared with the control population, there was no difference in the LDL distribution profile between the two groups. This finding contrasts with the increased small dense LDL concentrations associated with a shift of LDL subfraction distribution towards small and dense LDL particles in patients with heavy proteinuria [13, 14]. However, in these studies, the patients had noticeably higher triglycerides [283 mg/dl (3.2 mmol/l) and 203.5 mg/dl (2.3 mmol/l), respectively, vs. 147 mg/dl (1.66 mmol/l) in our study], a finding that could explain the observed differences, since it is well known that the LDL subfraction profile is directly related to triglyceride-rich lipoprotein metabolism [19]. In fact, it is well known that


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the most important factor determining the formation of small dense LDL in the general population is the concentration of triglycerides [19]. In addition, it is recognized that serum triglyceride 1130 mg/dl (1.47 mmol/l) is the threshold above which atherogenic levels of small dense LDL are formed [20]. A model for the formation of small dense LDL has been proposed, which suggested that in the presence of hypertriglyceridemia and subsequently the increased concentration of VLDL-1, a neutral lipid exchange with LDL occurs through cholesterol ester transfer protein (CETP) [19, 22]. Thus, triglycerides are moved from the VLDL to LDL in exchange for cholesterol, which is transferred to VLDL. This leads to a triglyceride-rich LDL particle that is hydrolyzed by hepatic lipase thereby shrinking the particle and forming small dense LDL [19, 20, 22]. Accordingly, in our study, the increased concentration of small dense LDL was more evident in hypertriglyceridemic patients. Moreover, and despite the absence of marked hypertriglyceridemia in our cohort, the LDL subfractions were somewhat enriched in triglycerides, though they were not cholesterol depleted. It should be mentioned in this setting that the composition of LDL subfractions are also related to the activities of other enzymes not measured in our study, such as hepatic lipase and CETP [20]. Our results seem to agree with those of a previous study in which uremic patients also exhibited significantly higher LDL scores, signifying an increase in the number of smaller particles compared with the control population [12]. In another study involving uremic patients with lower serum triglyceride and cholesterol levels, there was no difference in the concentration of small dense LDL between predialysis patients and controls, though the LDL subfraction distribution showed an increase in the percentage of small dense LDL [11]. The difference noted between our results and those reported in that study may be clearly related to the differences in lipid profile, namely the lower triglyceride and cholesterol levels observed in that study. Previous studies have established that increased concentrations of small dense LDL are associated with enhanced cardiovascular risk [19, 21– 23] and patients with concentrations of small dense LDL 1100 mg/dl have a 7-fold increase in the risk of myocardial infarction [22]. Using these criteria, half of our patients exhibit a considerable risk of developing premature vascular disease. Interestingly, Ambrosch et al. [24] have recently provided new clues on the atherogenic mechanisms of small dense LDL, which sensitize vascular cells to inflammatory signals more effectively than normal-sized LDL.

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In agreement with previously published data, a significant decrease in total HDL and HDL-2 subfraction was observed in our patients [25]. There are not adequate data with regard to the HDL subfraction distribution in uremic or in proteinuric patients [25]. Our results clearly showed that the HDL-2 mass was significantly reduced in patients compared with the control population, implying a defective reverse cholesterol transport in this population. As expected, this decrease was more profound in hypertriglyceridemic patients, since HDL changes in uremic patients might be related to abnormalities in the metabolism of triglyceride-rich lipoproteins [4, 5]. All HDL subfractions were relatively triglyceride rich, suggesting an increased CETP-mediated lipid exchange between VLDL and HDL, which is a characteristic finding of hypertriglyceridemia [26]. It is now generally accepted that the oxidative modification of LDL plays an important role in the pathogenesis of atherosclerosis. In our report and in accordance with most investigators, we did not demonstrate an increased susceptibility of uremic LDL to oxidation [27–29]. This is surprising, since patients had increased concentrations of small dense LDL known to be more prone to oxidation than large buoyant LDL. This might explain the increased LDL oxidation susceptibility reported by some previous investigators [30]. Moreover, Annuk et al. [31] have recently reported that renal patients are in a state of oxidative stress compared with healthy controls. Differences in the observed findings may reflect a variety of confounding factors, including methodological procedures [27]. Additionally, dietary factors determining the fatty acid composition of LDL may play a role. In fact, higher polyunsaturated fatty acid content enhances the susceptibility of LDL to lipid peroxidation, while monounsaturates are less susceptible to oxidation [32–34]. In this setting, it should be mentioned that our patients, but not the controls, were given appropriate dietary advice on fat intake and composition. It is concluded that uremic patients with heavy proteinuria exhibit compositional lipoprotein changes, but of a lesser degree compared with nonuremic patients with nephrotic syndrome. However, there is no evidence that circulating LDL isolated from these patients is more susceptible to oxidation in vitro than that from age- and gender-matched controls.


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