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Dyslipidemia and nephrotic syndrome: Recent advances
REVIEW
Dyslipidemia and Nephrotic Syndrome: Recent Advances Florian Kronenberg, MD Patients with nephrotic syndrome (NS) have one of the most pronounced secondary changes in lipoprotein metabolism known, and the magnitude of the changes correlates with the severity of the disease. These changes are of a quantitative as well as a qualitative nature. All apolipoprotein B (apo B)– containing lipoproteins, such as very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and lipoprotein(a) [Lp(a)], are elevated in nephrotic syndrome. High-density lipoproteins (HDL) are reported to be unchanged or reduced. In addition to these quantitative changes, the lipoprotein composition is markedly changed, with a higher ratio of cholesterol to triglycerides in the apo B– containing lipoproteins and an increase in the proportion of cholesterol, cholesterol ester, and phospholipids compared with proteins. Also apolipoproteins show major changes, with an increase in apolipoprotein A-I, A-IV, B, C, and E. Particularly the changes in apo C-II, which is an activator of the enzyme lipoprotein lipase (LPL), and apo C-III, an inhibitor of LPL, with an increase of the C-III to C-II ratio, might contribute to the impaired lipoprotein catabolism in NS. The mechanisms for these changes in lipoprotein metabolism are discussed in this review as far as they are known. Furthermore, the tremendous elevations of Lp(a) in nephrotic syndrome and its primary and secondary causes are reviewed. Primary causes became recently apparent by a significantly higher frequency of low-molecular-weight apo(a) phenotypes in patients compared with controls. The secondary causes were shown by an increase of Lp(a) in all apo(a) isoform groups. Because Lp(a) is an LDL-like particle that is usually included in the measured or calculated LDL cholesterol fraction, the influence of the extremely high Lp(a) levels in NS on the measurement of LDL cholesterol is discussed. © 2005 by the National Kidney Foundation, Inc.
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S AN INTRODUCTION, Table 1 describes the typical lipoprotein profile of a group of 207 patients with nephrotic syndrome (NS) and 274 controls that our group reported on recently.1,2 The patients had marked lipid abnorDivision of Genetic Epidemiology, Department of Medical Genetics, Molecular and Clinical Pharmacology, Innsbruck Medical University, Austria. The author’s work presented in this review was supported by the Austrian Science Fund, the Austrian National Bank, the Austrian Academy of Science, the Austrian Heart Fund, the Austrian Genome Research Programme GEN-AU (Project GOLD), and the Else Kröner-Fresenius Stiftung (Germany), which is highly appreciated. Address reprint requests to Florian Kronenberg, MD, Division of Genetic Epidemiology, Department of Medical Genetics, Molecular and Clinical Pharmacology, Innsbruck Medical University, Schöpfstr. 41, A-6020 Innsbruck, Austria. Email: [email protected] © 2005 by the National Kidney Foundation, Inc. 1051-2276/05/1502-0001$30.00/0 doi:10.1053/j.jrn.2004.10.003
Journal of Renal Nutrition, Vol 15, No 2 (April), 2005: pp 195-203
malities: they had strikingly elevated total and low-density lipoprotein (LDL) cholesterol levels and triglyceride concentrations, whereas the high-density lipoprotein (HDL) cholesterol levels were unchanged compared with controls. The total–HDL cholesterol ratio was markedly elevated to 8.0, compared with 4.9 in controls. Patients with NS showed a tremendous elevation in Lp(a) serum concentrations. The mean and median concentrations were 3 and 5 times higher, respectively, compared with controls. About 30% of the patients had Lp(a) levels above 70 mg/dL. This could only be observed in 8.4% of the controls. Most of the knowledge we have about the mechanisms of the disturbed lipoprotein metabolism in NS comes from studies in nephrotic animals mainly done by the groups of Vaziri and Kaysen. Nevertheless, turnover studies in nephrotic patients on those questions became avail195
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Table 1. Mean (⫾SD) Plasma Lipids and Lipoprotein(a) [Lp(a)] in Patients With Nephrotic Syndrome and Healthy Controls
Total cholesterol (mg/dL) Triglycerides (mg/dL) HDL cholesterol (mg/dL) Total/HDL cholesterol ratio LDL cholesterol (mg/dL) Lp(a) (mg/dL); mean ⫾ SD 25th percentile, median, 75th percentile Lp(a)-derived LDL cholesterol (mg/dL) Lp(a)-corrected LDL cholesterol (mg/dL)
Controls n ⫽ 274
Nephrotic Syndrome n ⫽ 207
203 ⫾ 42 136 ⫾ 92 44.0 ⫾ 12.6 4.9 ⫾ 1.6 132 ⫾ 37 20.0 ⫾ 32.8 2.0, 6.4, 18.5 9.0 ⫾ 14.7 123 ⫾ 39
302 ⫾ 92* 251 ⫾ 174† 43.7 ⫾ 16.7 8.0 ⫾ 4.5* 208 ⫾ 82* 60.4 ⫾ 85.4† 9.6, 29.8, 81.1 27.2 ⫾ 38.4† 181 ⫾ 82*
Abbreviations: Lp(a), lipoprotein(a); HDL, high-density lipoprotein; LDL, low-density lipoprotein. *P ⬍ .0001 by t-test for comparison between patients and controls. †P ⬍ .0001 by Wilcoxon rank sum test for comparison between patients and controls. Reprinted with permission from Blackwell Publishing.2
able during recent years. This article discusses the findings with a major focus on recent observations (Table 2).
Metabolism of LDL Cholesterol Particles Earlier turnover studies on LDL using 125Ilabeled lipoproteins reported conflicting results with an increased LDL synthesis3 or a decreased catabolism.4 Recent turnover studies that used stable isotopes, a technique coming closer to physiologic conditions, however, clearly showed an increased synthesis of LDL, whereas the fractional catabolic rate was not decreased.5 Vaziri et al contributed several pieces to the puzzle of understanding the disturbed metabolic mechanisms in these patients. They observed that the rate-limiting enzyme in the synthesis of cholesterol, the 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase, is markedly increased especially during the induction of NS in rats.6 This is the opposite of what is expected from a hypercholesterolemic condition, in which this enzyme is downregulated. In an hereditary analbuminemia the HMG-CoA reductase is also increased without an accompanying proteinuria, which suggests that the low albumin serum levels mediate the increase of HMG-CoA reductase.7 At the other end of the pathway, the rate-limiting enzyme of cholesterol catabolism to bile acids, the cholesterol 7␣-hydroxylase, was studied. Although this enzyme is markedly upregulated in diet-induced hypercholesterolemia, it remains unchanged in nephrotic rats, which contributes
only to a relative and not to an absolute decrease in cholesterol catabolism.8 Interestingly, the intracellular cholesterol concentration was found to be normal in nephrotic rats compared with diet-induced hypercholesterolemic rats, which show a marked increase in the total intracellular cholesterol content. In addition, a severe reduction in the amount of hepatic LDL receptor protein was reported in nephrotic rats, although the mRNA and the transcription rate were normal.8 A more prominent role in the intracellular cholesterol homeostasis, however, is Table 2. Characteristic Changes in Lipoprotein Metabolism in Patients With Nephrotic Syndrome Lipoproteins VLDL 1 IDL 1 LDL 1 Lp(a) 1 HDL ↔ or 1 Apolipoproteins apo B 1 apo E 1 apo A-I 1 apo A-IV 1 Enzymes ACAT 1 DGTA 1 HMG-CoA reductase 1 cholesterol 7␣-hydroxylase ↔ LCAT 22 LPL 22 HTGL 2 Receptors VLDL receptor 2 LDL receptor 2 HDL receptor 2
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played by the amount of free cholesterol compared with the cholesterol esters. Cholesterol esters constitute the majority of the cellular cholesterol content, but mainly the free cholesterol negatively influences the expression of the HMG-CoA reductase and the cholesterol 7␣hydroxylase. In line, free cholesterol concentrations are significantly reduced in NS. In this context another enzyme seems to play a key role: studies by Vaziri et al showed an upregulation of Acyl-coenzyme A:cholesterol acyltransferase (ACAT) in nephrotic rats.9 ACAT catalyzes the esterification of cholesterol, which is then incorporated in apo B– containing lipoproteins. ACAT-2 is expressed mainly in the liver and white fat. A very recent study by this group treated rats with puromycin-induced NS with an ACAT inhibitor.10 Before treatment, rats had an increased hepatic ACAT activity, ACAT-2 mRNA, and ACAT-2 protein level accompanied by reduced LDL and HDL receptor and plasma LCAT concentrations. The ACAT inhibitor reduced the plasma cholesterol and triglyceride concentrations and lowered the hepatic ACAT activity. This inhibition was also associated with near-normalization of plasma LCAT and hepatic HDL and LDL receptor protein and a marked improvement of proteinuria and hypoalbuminemia.10 It remains to be investigated whether an ACAT inhibition in nephrotic people results in a similar improvement of lipoprotein metabolism and NS.
mediately esterified by the key enzyme lecithin: cholesterol acyltransferase (LCAT). The now cholesterol-rich HDL-2 particles exchange in the circulation cholesterol against triglycerides from very-low-density lipoprotein (VLDL) remnants facilitated by cholesteryl ester transfer protein. In the liver, the HDL-2 particles bind to HDL receptors13 and unload their content of cholesterol esters and fatty acids. After dissociation from the receptor, the lipid-depleted HDL-3 particle is available for another cycle. In addition to this major pathway, some HDL-2 particles can be taken up as whole particles by the low-density lipoprotein receptor–related protein (LRP) or LDL receptor. One of the major disturbances in this pathway in NS is the marked reduction of LCAT protein in NS, which is lost by urine and results in an LCAT deficiency.14 As a result, the maturation of cholesterol-poor HDL-3 to lipid-rich HDL-2 particles is disturbed and less apo C-II and apo E in the HDL particles for transport to VLDL is available, which results in disturbances of the metabolism of triglyceride-rich lipoproteins (see below).15 The other key element in reverse cholesterol transport, the HDL receptor, shows a significantly reduced amount of HDL receptor protein. Furthermore, the hepatic triglyceride lipase, which hydrolyzes triglycerides from the HDL-2 particles during the docking to the HDL receptor, is also severely depressed in NS.16 Apo A-I concentrations are elevated.17
Metabolism of HDL Cholesterol Particles and Reverse Cholesterol Transport
Metabolism of Triglyceride-Rich Particles
Plasma HDL concentrations are often reported to be normal in nephrotic patients.1-3,11 However, the normal quantities are accompanied by qualitative changes: because of disturbances in the maturation of lipid-poor HDL-3 particles to cholesterol-rich HDL-2 particles, the ratio of the 2 particles is shifted to a high concentration of HDL-3 and low concentrations of HDL-2 particles.12 As a consequence, major disturbances in the reverse cholesterol pathway can be observed. Usually lipid-poor HDL-3 particles are formed from nascent HDL particles that bind to the cell surface binding protein vigilin of peripheral cells. Cellular free cholesterol undergoes a diffusion to the surface of HDL-3 particles, where it is im-
Triglyceride-rich particles, including chylomicrons, VLDLs, and intermediate-density lipoproteins (IDLs), are significantly elevated in NS, and the catabolism of these particles seems to be decreased.3-5,18 Turnover studies showed that the elevated levels of VLDLs are not caused by an increased production but by a decreased fractional catabolic rate.5 Although it was believed for a long time that proteinuria is responsible for the delayed chylomicron and VLDL clearance, recent work by Shearer et al showed that both hypoalbuminemia and proteinuria contribute separately to the reduced lipoprotein catabolism in NS.19 The key enzyme involved in the lipolysis is lipoprotein lipase (LPL), which is responsible for about
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Lipoprotein(a) and Apolipoprotein(a) Polymorphism
apo(a).27 The molecular weight of apo(a) isoforms ranges from 300 to ⬎800 kDa. Analyses of genomic DNA have shown that this size polymorphism is caused by a varying number of K-IV repeats in the apo(a) gene (ranging from 11 to ⬎40 K-IV repeats). Isoforms with up to 22 K-IV repeats are called low-molecular-weight (LMW) or small apo(a) isoforms, those with more than 22 K-IV repeats are called high-molecularweight (HMW) or large apo(a) isoforms. An inverse correlation exists between the molecular weight of apo(a) and the Lp(a) plasma concentrations. This means that individuals with HMW or large apo(a) phenotypes have on average low Lp(a) concentrations, and those with LMW or small isoforms usually show high concentrations. Depending on the population under investigation, this association explains between 30% and 70% of the variability in Lp(a) levels. For a detailed introduction to this lipoprotein, see Utermann.26 Numerous retrospective and also a remarkable number of prospective case-control studies have reported significantly higher Lp(a) levels in patients with coronary artery disease, cerebrovascular disease, and peripheral atherosclerosis when compared with controls.28,29 Lp(a) levels above 30 mg/dL were proposed to be associated with an increased risk. Those studies that have studied apo(a) isoforms as well as Lp(a) levels observed that short apo(a) alleles/isoforms were significantly more frequent in patients with coronary artery disease or other forms of atherothrombosis than in controls.30-32
Short Introduction to Lp(a) Lp(a) is an LDL-like lipoprotein that consists of an LDL particle to which the glycoprotein apolipoprotein(a) [apo(a)] is attached. The mean and median concentrations of Lp(a) in white subjects are about 15 to 20 mg/dL and 8 mg/dL, respectively, with an extremely broad range from ⬍0.1 mg/dL to ⬎300 mg/dL. The distribution of Lp(a) plasma concentrations in the general white and East Asian populations is skewed, and most people have low concentrations. Individuals of African descent have 2- to 4-fold higher median plasma levels of Lp(a) with a more Gaussian distribution than do white people.26 The apo(a) gene located on chromosome 6 (6q26-q27) determines a size polymorphism of
Lp(a) in Kidney Disease Numerous studies reported elevated Lp(a) levels in patients with kidney diseases. This increase, however, depends markedly on the impairment of kidney function, the amount of proteinuria, and the treatment modality.33,34 In addition to these parameters, there is strong evidence that the relative increase of Lp(a) also depends on the apo(a) K-IV repeat polymorphism. Our group described this interesting but still unexplained phenomenon for the first time more than 10 years ago in hemodialysis patients.35 We have seen in these patients that an increase of Lp(a) on average can only be observed in patients with only long (HMW) apo(a) isoforms when compared with isoform-matched controls. Patients with at least 1 short (LMW) apo(a) isoform had nearly identical
70% of the reduction in triglycerides of chylomicrons and VLDL. In NS this key enzyme, however, is markedly downregulated in skeletal muscle, adipose tissue, and myocardium.18,20,21 The VLDL receptor that shows not only a similar tissue distribution to LPL but also a functional relationship with LPL is also markedly reduced in NS.22,23 Another enzyme called acyl-CoA:diacylglycerol aycltransferase (DGAT) catalyzes the final step in the biosynthesis of triglycerides. Hepatic DGAT-1 mRNA and activity was significantly increased in rats with NS. Functionally, this increase was associated with a reduction of microsomal free fatty acid concentrations in the livers of nephrotic animals.24 Finally, VLDL from nephrotic patients itself seems to influence the binding and internalization of VLDL: the investigation of LPL-mediated lipolysis was significantly lower with VLDL preparations from nephrotic rats, and this impairment was corrected by the addition of HDL from normal rats.25 Shearer et al found in this context a significant reduction in the apo E to apo A-I ratio in nephrotic HDL.19 Apo E is an important ligand for the VLDL receptor. Because nascent VLDL receives apo E from HDL, HDL particles poor in apo E, as seen in NS, might contribute to an impaired lipolysis of triglyceride-rich lipoproteins by LPL.
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Lp(a) levels when compared with matched controls. This phenomenon of differences in the relative increase of Lp(a) between these 2 apo(a) isoform groups was also observed in other stages of kidney impairment or treatment modalities, as reviewed recently.34 The Lp(a) level itself seems to be less discriminative for cardiovascular disease in kidney patients compared with the general population. Instead of the Lp(a) concentrations, the apo(a) phenotype is a very good and consistent predictor for cardiovascular disease in hemodialysis patients. Almost all studies that investigated the apo(a) polymorphism observed that LMW apo(a) isoforms predict the risk for atherosclerotic complications (recently reviewed by Kronenberg34). The analysis of apo(a) phenotypes is therefore a useful tool for risk stratification.
Lp(a) and Nephrotic Syndrome Some studies suggested that nonnephrotic proteinuria has an influence on Lp(a) levels. However, a recent study in patients with mild and moderate kidney failure observed that the correlation of Lp(a) with glomerular filtration rate (r ⫽ ⫺0.18) was more pronounced than that of Lp(a) with nonnephrotic range proteinuria (r ⫽ 0.14). In a multiple regression analysis model, proteinuria was not independently associated with Lp(a) but modified the association between glomerular filtration rate and Lp(a) with borderline statistical significance.36 Obviously a major protein loss by urine must occur to have a significant impact on Lp(a) levels. This is clearly the case in NS, which is the condition with the highest Lp(a) levels ever reported.37,38 Concentrations up to 600 mg/dL were described.1 The disease-related elevated levels have been elegantly shown in longitudinal studies that showed a decrease of Lp(a) after remission of NS.37,38 In a recent study in 207 patients and 274 controls, we showed that primary and secondary causes contribute to this elevation.1 The primary or genetic causes became apparent by a markedly elevated number of LMW apo(a) phenotypes, which are usually associated with high Lp(a) levels. This frequency was 35.7% in patients compared with only 24.8% in controls (P ⫽ .009). We hypothesized that particularly patients who have already high Lp(a)
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Figure 1. Median Lp(a) serum concentrations in patients with nephrotic syndrome and controls stratified by 6 apo(a) isoform groups. Lp(a) concentrations were calculated for each expressed apo(a) isoform separately from the measured Lp(a) concentration by estimating the relative proportion of the 2 isoforms in the sodium dodecyl sulfate agarose gel electrophoresis. Adapted and reprinted with permission from Blackwell Publishing.1
levels because of an inherited LMW apo(a) isoform tend more often toward a nephrotic course of kidney disease if the particular disease allows for a nonnephrotic or a nephrotic course of disease. This is supported by 3 observations: in 2 recent studies, high Lp(a) levels predicted future relapse in children with steroid-sensitive NS39,40; and in patients with nonnephrotic glomerulonephritis who do not show these tremendous elevations of Lp(a), an apo(a) phenotype frequency distribution was observed, which is not different from controls.36 In addition to the primary reasons for the increase in Lp(a), secondary causes by the pathogenetic changes of the NS itself resulted in a different increase of Lp(a) in the various apo(a) isoform groups. LMW isoforms were associated with 40% to 75% elevated Lp(a) concentrations when compared with matched isoforms from controls. HMW apo(a) isoforms showed 100% to 500% elevated Lp(a) levels compared with matched isoforms from controls (Fig 1). The severity of the NS as well as the degree of kidney impairment did not influence the Lp(a) concentrations. Taken together, it seems that the large amount of protein loss in nephrotic patients results in an increased production of Lp(a) by the liver. This was indeed observed in recent turnover studies in patients with NS, which showed a similar fractional catabolic rate of Lp(a) in 5 patients and 5 controls, which suggested that differences in Lp(a) levels are caused by differ-
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Figure 2. LDL-C and Lp(a)-corrected LDL-C levels in patients with LMW and HMW apo(a) phenotypes. Only patients are presented who showed at least a 30-mg/dL difference between the 2 LDL-C levels. This was significantly less the case in patients with HMW than in those with LMW apo(a) phenotypes (16% versus 58%; P ⬍ .00001). Adapted and reprinted with permission from Blackwell Publishing.2
ences in synthesis rate.41 The number of studied patients did not allow investigation of the turnover stratified for apo(a) phenotypes. The data from our recent study,1 however, suggest that even when Lp(a) is elevated in all apo(a) isoform groups, we can observe again that HMW apo(a) isoforms show a higher relative (but not absolute) increase in Lp(a) levels than LWM apo(a) isoforms (Fig 1).
Therapeutic Implications of a High Lp(a) Level on LDL Cholesterol Lowering Lp(a) is an LDL-like particle consisting of 45% cholesterol.42,43 The usual methods for determining LDL cholesterol (LDL-C) do not distinguish between cholesterol derived from LDL and Lp(a) and are thus the net result of cholesterol levels from both lipoproteins. High Lp(a) concentrations therefore significantly contribute to the measured or calculated LDL-C levels. In a recent study, we observed that the LDL-C fraction that derived from Lp(a) was on average 27 mg/dL and therefore the highest in patients with NS, compared with only 9 mg/dL in controls.2 However, up to more than 250 mg/dL of LDL-C derived from Lp(a) cholesterol in patients with Lp(a) levels
higher than 500 mg/dL. Hemodialysis and continuous ambulatory peritoneal dialysis patients showed an overestimation of the true LDL cholesterol levels of on average 11 and 16 mg/dL, respectively.44 Figure 2 shows all patients of the mentioned study who showed an Lp(a)-corrected LDL-C level that was at least 30 mg/dL lower than uncorrected levels. However, we observed patients who had up to more than 250 mg/dL differences between the 2 levels. Because statins have no influence on Lp(a) levels,45 this observation might have an influence on the investigation of the efficacy of a therapeutic intervention. It will be more accurate to consider and monitor only the amount of LDL-C that is accessible for intervention. This can have a considerable influence on the results: for example, a not-so-rarely-observed nephrotic patient with an Lp(a) concentration of 200 mg/dL and a total LDL-C of 200 mg/dL has an Lp(a)-corrected LDL-C of 110 mg/dL. If in this case a statin leads to only a 30-mg/dL reduction in LDL-C, the effectiveness will be either 15% or 27%, depending whether total or Lp(a)-corrected LDL-C is used for calculation. Therefore, cases of nonresponders or low responders to statin therapy could be hidden behind tremendously elevated Lp(a) levels. No study up to now has investigated this question in patients with kidney disease in a direct way. However, Scanu and Hinman investigated the influence of statins on hypercholesterolemic subjects with high Lp(a) levels and observed in some cases an Lp(a)-corrected LDL-C decrease as low as 10 mg/dL.46 The decrease of the Lp(a)-corrected LDL-C levels was proportional to the pretreatment ratio of the Lp(a)corrected LDL-C to Lp(a) concentrations. This means that the decrease in LDL-C was strongly dependent on the accessible LDL-C, showing that only cholesterol from LDL-C and not Lp(a) is affected by statin therapy.46 Ongoing large trials with kidney patients will provide the possibility to study such an effect easily in the setting of kidney disease.
Apo E Polymorphism Apo E plays an important role in lipoprotein metabolism because of its function as a ligand for receptors. More recently, cell culture stud-
DYSLIPIDEMIA AND NEPHROTIC SYNDROME
ies showed that apo E is renal protective by regulating mesangial cell proliferation and matrix expansion.47 Apo E knockout mice develop renal lesions in addition to atherosclerosis.48 Therefore several studies investigated an association between the apo E polymorphism and NS in humans, and had contrasting findings. Smaller studies found an increase in the ⑀4 allele in childhood NS,49which was not confirmed by 2 larger studies.50,51 A recent study found a tendency toward a higher frequency of ⑀4 allele in 190 nephrotic children, which became significant in those who had frequent disease relapses.52
Interventional Studies Affecting Dyslipidemia In addition to the use of statins, there is no doubt that one of the most important strategies for treating dyslipidemia in NS is to treat the proteinuria. In many patients, this will result in an improvement in the lipid profile. It is well known that angiotensin converting enzyme (ACE) inhibitors have a proteinuriareducing effect. A recent study in 28 patients with nondiabetic chronic nephropathies with a daily proteinuria of at least 2 g investigated the extent to which upward titration of the ACE inhibitor lisinopril to a maximum tolerated doses would influence proteinuria and dyslipidemia.53 The maximum dose reduced not only proteinuria but also total and LDL cholesterol as well as triglycerides without affecting HDL cholesterol levels. The reduction in total and LDL cholesterol correlated with increases of serum albumin concentrations and oncotic pressure and was most pronounced in patients with severe hypoalbuminemia. The changes in triglycerides were strongly correlated with lisinopril doses. A study in 11 children with steroid and cyclosporine A–resistant NS showed that a combined LDL apheresis and prednisolone therapy not only decreased cholesterol concentrations but resulted also in a remission of the disease in 7 patients; in 5 of them it was a complete remission.54 Studies in Imai rats, which are models for spontaneous focal glomerulosclerosis with NS, tested the effect of angiotensin II receptor type 1 (AT-1) blockade with and without a combination with HMG-CoA reductase inhibitors. The treatment with an HMG-CoA reductase
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inhibitor alone resulted in an improvement in hyperlipidemia and renal disease. The treatment with an AT-1 blocker with and without an HMG-CoA reductase inhibitor resulted in a normalization of blood pressure, urinary protein excretion, plasma cholesterol, triglyceride, LDL, VLDL, and albumin concentrations as well as renal function. Long-term treatment even resulted in a prevention of the usually observed glomerulosclerosis and in a reduced tubulointerstitial injury.55 Experimental studies with 1 mg adrenocorticotropic hormone twice per week in 14 patients with membranous nephropathy up to 12 months showed reductions of 30% to 60% in the serum concentrations of cholesterol, triglycerides, apo B, and Lp(a) as well as an increase of HDL cholesterol and apo A-I of 30% to 40%. The urinary albumin excretion decreased by 90%, and the glomerular filtration rate increased by 25%.56 The mechanism behind the lipid-lowering effect is not clear. It can be excluded that it is simply caused by a reduction of proteinuria because it was even observed in hemodialysis patients.57,58 In vitro studies suggested an increased hepatic uptake of LDL.
Acknowledgements The author thanks Mrs. Petra Jassmann for technical assistance during the preparation of the manuscript, and all collaborators who contributed to the studies as well as all patients who were participants in the research projects.
References 1. Kronenberg F, Lingenhel A, Lhotta K, et al: The apolipoprotein(a) size polymorphism is associated with nephrotic syndrome. Kidney Int 65:606-612, 2004 2. Kronenberg F, Lingenhel A, Lhotta K, et al: Lipoprotein(a)- and low-density lipoprotein-derived cholesterol in nephrotic syndrome: Impact on lipid-lowering therapy? Kidney Int 66:348-354, 2004 3. Joven J, Villabona C, Vilella E, et al: Abnormalities of lipoprotein metabolism in patients with the nephrotic syndrome. N Engl J Med 323:579-584, 1990 4. Warwick GL, Packard CJ, Demant T, et al: Metabolism of apolipoprotein B-containing lipoproteins in subjects with nephrotic-range proteinuria. Kidney Int 40:129-138, 1991 5. De Sain-van der Velden M, Kaysen GA, Barrett HA, et al: Increased VLDL in nephrotic patients results from a decreased catabolism while increased LDL results from increased synthesis. Kidney Int 53:994-1001, 1998 6. Vaziri ND, Liang KH: Hepatic HMG-CoA reductase gene expression during the course of puromycin-induced nephrosis. Kidney Int 48:1979-1985, 1995
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7. Liang K, Vaziri ND: HMG-CoA reductase, cholesterol 7alpha-hydroxylase, LCAT, ACAT, LDL receptor, and SRB-1 in hereditary analbuminemia. Kidney Int 64:192-198, 2003 8. Vaziri ND, Sato T, Liang K: Molecular mechanisms of altered cholesterol metabolism in rats with spontaneous focal glomerulosclerosis. Kidney Int 63:1756-1763, 2003 9. Vaziri ND, Liang K: Up-regulation of acyl-coenzyme A: Cholesterol acyltransferase (ACAT) in nephrotic syndrome. Kidney Int 61:1769-1775, 2002 10. Vaziri ND, Liang KH: Acyl-coenzyme A: Cholesterol acyltransferase inhibition ameliorates proteinuria, hyperlipidemia, lecithin-cholesterol acyltransferase, SRB-1, and low-density lipoprotein receptor deficiencies in nephrotic syndrome. Circulation 110:419-425, 2004 11. Appel GB, Blum CB, Chien S, et al: The hyperlipidemia of nephrotic syndrome: Relation to plasma albumin concentration, oncotic pressure and viscosity. N Engl J Med 312:15441548, 1985 12. Gherardi E, Rota E, Calandra S, et al: Relationship among the concentrations of serum lipoproteins and changes in their chemical composition in patients with untreated nephrotic syndrome. Eur J Clin Invest 7:563-570, 1977 13. Acton S, Rigotti A, Landschulz KT, et al: Identification of scavenger receptor SR-BI as a high density lipoprotein receptor. Science 271:518-520, 1996 14. Vaziri ND, Liang K, Parks JS: Acquired lecithin-cholesterol acyltransferase deficiency in nephrotic syndrome. Am J Physiol Renal Physiol 280:F823-F828, 2001 15. Deighan CJ, Caslake MJ, McConnell M, et al: Patients with nephrotic-range proteinuria have apolipoprotein C and E deficient VLDL1. Kidney Int 58:1238-1246, 2000 16. Liang K, Vaziri ND: Down-regulation of hepatic lipase expression in experimental nephrotic syndrome. Kidney Int 51:1933-1937, 1997 17. Kaysen GA, Hoye E, Jones H Jr: Apolipoprotein AI levels are increased in part as a consequence of reduced catabolism in nephrotic rats. Am J Physiol 268:F532-F540, 1995 18. Kaysen GA, Mehendru L, Pan XM, et al: Both peripheral chylomicron catabolism and hepatic uptake of remnants are defective in nephrosis. Am J Physiol 263:F335-F341, 1992 19. Shearer GC, Stevenson FT, Atkinson DN, et al: Hypoalbuminemia and proteinuria contribute separately to reduced lipoprotein catabolism in the nephrotic syndrome. Kidney Int 59:179-189, 2001 20. Garber DW, Gottlieb BA, Marsh JB, et al: Catabolism of very low density lipoproteins in experimental nephrosis. J Clin Invest 74:1375-1383, 1984 21. Vaziri ND: Molecular mechanisms of lipid disorders in nephrotic syndrome. Kidney Int 63:1964-1976, 2003 22. Sato T, Liang K, Vaziri ND: Down-regulation of lipoprotein lipase and VLDL receptor in rats with focal glomerulosclerosis. Kidney Int 61:157-162, 2002 23. Vaziri ND, Liang K: Down-regulation of VLDL receptor expression in chronic experimental renal failure. Kidney Int 51:913-919, 1997 24. Vaziri ND, Kim CH, Phan D, et al: Up-regulation of hepatic Acyl CoA: Diacylglycerol acyltransferase-1 (DGAT-1) expression in nephrotic syndrome. Kidney Int 66:262-267, 2004 25. Furukawa S, Hirano T, Mamo JC, et al: Catabolic defect of triglyceride is associated with abnormal very-low-density lipoprotein in experimental nephrosis. Metabolism 39:101-107, 1990
26. Utermann G: Lipoprotein(a), in Scriver CR, Beaudet AL, Sly WS, Valle D (eds): The metabolic and molecular bases of inherited disease (ed 8). New York, McGraw-Hill, 2000, pp 2753-2787 27. Utermann G, Menzel HJ, Kraft HG, et al: Lp(a) glycoprotein phenotypes: Inheritance and relation to Lp(a)-lipoprotein concentrations in plasma. J Clin Invest 80:458-465, 1987 28. Danesh J, Collins R, Peto R: Lipoprotein(a) and coronary heart disease: Meta-analysis of prospective studies. Circulation 102:1082-1085, 2000 29. Craig WY, Neveux LM, Palomaki GE, et al: Lipoprotein(a) as a risk factor for ischemic heart disease: Metaanalysis of prospective studies. Clin Chem 44:2301-2306, 1998 30. Sandholzer C, Saha N, Kark JD, et al: Apo(a) isoforms predict risk for coronary heart disease: A study in six populations. Arterioscler Thromb 12:1214-1226, 1992 31. Wild SH, Fortmann SP, Marcovina SM: A prospective case-control study of lipoprotein(a) levels and apo(a) size and risk of coronary heart disease in Stanford Five-City Project participants. Arterioscler Thromb Vasc Biol 17:239-245, 1997 32. Kronenberg F, Kronenberg MF, Kiechl S, et al: Role of lipoprotein(a) and apolipoprotein(a) phenotype in atherogenesis: Prospective results from the Bruneck Study. Circulation 100: 1154-1160, 1999 33. Kronenberg F, Utermann G, Dieplinger H: Lipoprotein(a) in renal disease. Am J Kidney Dis 27:1-25, 1996 34. Kronenberg F: Epidemiology, pathophysiology and therapeutic implications of lipoprotein(a) in kidney disease. Expert Rev Cardiovasc Ther 2:729-743, 2004 35. Dieplinger H, Lackner C, Kronenberg F, et al: Elevated plasma concentrations of lipoprotein(a) in patients with end-stage renal disease are not related to the size polymorphism of apolipoprotein(a). J Clin Invest 91:397-401, 1993 36. Kronenberg F, Kuen E, Ritz E, et al: Lipoprotein(a) serum concentrations and apolipoprotein(a) phenotypes in mild and moderate renal failure. J Am Soc Nephrol 11:105-115, 2000 37. Wanner C, Rader D, Bartens W, et al: Elevated plasma lipoprotein(a) in patients with the nephrotic syndrome. Ann Intern Med 119:263-269, 1993 38. Stenvinkel P, Berglund L, Heimburger O, et al: Lipoprotein(a) in nephrotic syndrome. Kidney Int 44:1116-1123, 1993 39. Kawasaki Y, Suzuki J, Nozawa R, et al: Prediction of relapse by plasma lipoprotein(a) concentration in children with steroidsensitive nephrotic syndrome. Nephron 92:807-811, 2002 40. Merouani A, Levy E, Mongeau JG, et al: Hyperlipidemic profiles during remission in childhood idiopathic nephrotic syndrome. Clin Biochem 36:571-574, 2003 41. De Sain-van der Velden MGM, Reijngoud DJ, Kaysen GA, et al: Evidence for increased synthesis of lipoprotein(a) in the nephrotic syndrome. J Am Soc Nephrol 9:1474-1481, 1998 42. Kostner GM, Ibovnik A, Holzer H, et al: Preparation of a stable fresh frozen primary lipoprotein[a] (Lp[a]) standard. J Lipid Res 40:2255-2263, 1999 43. Marcovina SM, Albers JJ, Scanu AM, et al: Use of a reference material proposed by the international federation of clinical chemistry and laboratory medicine to evaluate analytical methods for the determination of plasma lipoprotein(a). Clin Chem 46:1956-1967, 2000 44. Kronenberg F, Lingenhel A, Neyer U, et al: Prevalence of dyslipidemic risk factors in hemodialysis and CAPD patients. Kidney Int 63:S113-S114, 2003. (suppl 84)
DYSLIPIDEMIA AND NEPHROTIC SYNDROME 45. Kostner GM, Gavish D, Leopold B, et al: HMG CoA reductase inhibitors lower LDL cholesterol without reducing Lp(a) levels. Circulation 80:1313-1319, 1989 46. Scanu AM, Hinman J: Issues concerning the monitoring of statin therapy in hypercholesterolemic subjects with high plasma lipoprotein(a) levels. Lipids 37:439-444, 2002 47. Chen G, Paka L, Kako Y, et al: A protective role for kidney apolipoprotein E. Regulation of mesangial cell proliferation and matrix expansion. J Biol Chem 276:49142-49147, 2001 48. Zhang SH, Reddick RL, Piedrahita JA, et al: Spontaneous hypercholesterolemia and arterial lesions in mice lacking apolipoprotein E. Science 258:468-471, 1992 49. Asami T, Ciomartan T, Hayakawa H, et al: Apolipoprotein E epsilon 4 allele and nephrotic glomerular diseases in children. Pediatr Nephrol 13:233-236, 1999 50. Attila G, Noyan A, Karabay BA, et al: Apolipoprotein E polymorphism in childhood nephrotic syndrome. Pediatr Nephrol 17:359-362, 2002 51. Bruschi M, Catarsi P, Candiano G, et al: Apolipoprotein E in idiopathic nephrotic syndrome and focal segmental glomerulosclerosis. Kidney Int 63:686-695, 2003 52. Kim SD, Kim IS, Lee BC, et al: Apolipoprotein E polymorphism and clinical course in childhood nephrotic syndrome. Pediatr Nephrol 18:230-233, 2003
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53. Ruggenenti P, Mise N, Pisoni R, et al: Diverse effects of increasing lisinopril doses on lipid abnormalities in chronic nephropathies. Circulation 107:586-592, 2003 54. Hattori M, Chikamoto H, Akioka Y, et al: A combined low-density lipoprotein apheresis and prednisone therapy for steroid-resistant primary focal segmental glomerulosclerosis in children. Am J Kidney Dis 42:1121-1130, 2003 55. Rodriguez-Iturbe B, Sato T, Quiroz Y, et al: AT-1 receptor blockade prevents proteinuria, renal failure, hyperlipidemia, and glomerulosclerosis in the Imai rat. Kidney Int 66:668-675, 2004 56. Berg AL, Nilsson-Ehle P, Arnadottir M: Beneficial effects of ACTH on the serum lipoprotein profile and glomerular function in patients with membranous nephropathy. Kidney Int 56:1534-1543, 1999 57. Arnadottir M, Berg AL, Dallongeville J, et al: Adrenocorticotropic hormone lowers serum Lp(a) and LDL cholesterol concentrations in hemodialysis patients. Kidney Int 52:16511655, 1997 58. Arnadottir M, Berg A-L, Kronenberg F, et al: Corticotropin-induced reduction of plasma lipoprotein(a) concentrations in healthy individuals and hemodialysis patients: Relation to apolipoprotein(a) size polymorphism. Metabolism 48:342-346, 1999
Dyslipidemia and Nephrotic Syndrome: Recent Advances Florian Kronenberg, MD Patients with nephrotic syndrome (NS) have one of the most pronounced secondary changes in lipoprotein metabolism known, and the magnitude of the changes correlates with the severity of the disease. These changes are of a quantitative as well as a qualitative nature. All apolipoprotein B (apo B)– containing lipoproteins, such as very-low-density lipoproteins (VLDL), intermediate-density lipoproteins (IDL), low-density lipoproteins (LDL), and lipoprotein(a) [Lp(a)], are elevated in nephrotic syndrome. High-density lipoproteins (HDL) are reported to be unchanged or reduced. In addition to these quantitative changes, the lipoprotein composition is markedly changed, with a higher ratio of cholesterol to triglycerides in the apo B– containing lipoproteins and an increase in the proportion of cholesterol, cholesterol ester, and phospholipids compared with proteins. Also apolipoproteins show major changes, with an increase in apolipoprotein A-I, A-IV, B, C, and E. Particularly the changes in apo C-II, which is an activator of the enzyme lipoprotein lipase (LPL), and apo C-III, an inhibitor of LPL, with an increase of the C-III to C-II ratio, might contribute to the impaired lipoprotein catabolism in NS. The mechanisms for these changes in lipoprotein metabolism are discussed in this review as far as they are known. Furthermore, the tremendous elevations of Lp(a) in nephrotic syndrome and its primary and secondary causes are reviewed. Primary causes became recently apparent by a significantly higher frequency of low-molecular-weight apo(a) phenotypes in patients compared with controls. The secondary causes were shown by an increase of Lp(a) in all apo(a) isoform groups. Because Lp(a) is an LDL-like particle that is usually included in the measured or calculated LDL cholesterol fraction, the influence of the extremely high Lp(a) levels in NS on the measurement of LDL cholesterol is discussed. © 2005 by the National Kidney Foundation, Inc.
A
S AN INTRODUCTION, Table 1 describes the typical lipoprotein profile of a group of 207 patients with nephrotic syndrome (NS) and 274 controls that our group reported on recently.1,2 The patients had marked lipid abnorDivision of Genetic Epidemiology, Department of Medical Genetics, Molecular and Clinical Pharmacology, Innsbruck Medical University, Austria. The author’s work presented in this review was supported by the Austrian Science Fund, the Austrian National Bank, the Austrian Academy of Science, the Austrian Heart Fund, the Austrian Genome Research Programme GEN-AU (Project GOLD), and the Else Kröner-Fresenius Stiftung (Germany), which is highly appreciated. Address reprint requests to Florian Kronenberg, MD, Division of Genetic Epidemiology, Department of Medical Genetics, Molecular and Clinical Pharmacology, Innsbruck Medical University, Schöpfstr. 41, A-6020 Innsbruck, Austria. Email: [email protected] © 2005 by the National Kidney Foundation, Inc. 1051-2276/05/1502-0001$30.00/0 doi:10.1053/j.jrn.2004.10.003
Journal of Renal Nutrition, Vol 15, No 2 (April), 2005: pp 195-203
malities: they had strikingly elevated total and low-density lipoprotein (LDL) cholesterol levels and triglyceride concentrations, whereas the high-density lipoprotein (HDL) cholesterol levels were unchanged compared with controls. The total–HDL cholesterol ratio was markedly elevated to 8.0, compared with 4.9 in controls. Patients with NS showed a tremendous elevation in Lp(a) serum concentrations. The mean and median concentrations were 3 and 5 times higher, respectively, compared with controls. About 30% of the patients had Lp(a) levels above 70 mg/dL. This could only be observed in 8.4% of the controls. Most of the knowledge we have about the mechanisms of the disturbed lipoprotein metabolism in NS comes from studies in nephrotic animals mainly done by the groups of Vaziri and Kaysen. Nevertheless, turnover studies in nephrotic patients on those questions became avail195
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Table 1. Mean (⫾SD) Plasma Lipids and Lipoprotein(a) [Lp(a)] in Patients With Nephrotic Syndrome and Healthy Controls
Total cholesterol (mg/dL) Triglycerides (mg/dL) HDL cholesterol (mg/dL) Total/HDL cholesterol ratio LDL cholesterol (mg/dL) Lp(a) (mg/dL); mean ⫾ SD 25th percentile, median, 75th percentile Lp(a)-derived LDL cholesterol (mg/dL) Lp(a)-corrected LDL cholesterol (mg/dL)
Controls n ⫽ 274
Nephrotic Syndrome n ⫽ 207
203 ⫾ 42 136 ⫾ 92 44.0 ⫾ 12.6 4.9 ⫾ 1.6 132 ⫾ 37 20.0 ⫾ 32.8 2.0, 6.4, 18.5 9.0 ⫾ 14.7 123 ⫾ 39
302 ⫾ 92* 251 ⫾ 174† 43.7 ⫾ 16.7 8.0 ⫾ 4.5* 208 ⫾ 82* 60.4 ⫾ 85.4† 9.6, 29.8, 81.1 27.2 ⫾ 38.4† 181 ⫾ 82*
Abbreviations: Lp(a), lipoprotein(a); HDL, high-density lipoprotein; LDL, low-density lipoprotein. *P ⬍ .0001 by t-test for comparison between patients and controls. †P ⬍ .0001 by Wilcoxon rank sum test for comparison between patients and controls. Reprinted with permission from Blackwell Publishing.2
able during recent years. This article discusses the findings with a major focus on recent observations (Table 2).
Metabolism of LDL Cholesterol Particles Earlier turnover studies on LDL using 125Ilabeled lipoproteins reported conflicting results with an increased LDL synthesis3 or a decreased catabolism.4 Recent turnover studies that used stable isotopes, a technique coming closer to physiologic conditions, however, clearly showed an increased synthesis of LDL, whereas the fractional catabolic rate was not decreased.5 Vaziri et al contributed several pieces to the puzzle of understanding the disturbed metabolic mechanisms in these patients. They observed that the rate-limiting enzyme in the synthesis of cholesterol, the 3-hydroxy-3-methyl-glutaryl coenzyme A (HMG-CoA) reductase, is markedly increased especially during the induction of NS in rats.6 This is the opposite of what is expected from a hypercholesterolemic condition, in which this enzyme is downregulated. In an hereditary analbuminemia the HMG-CoA reductase is also increased without an accompanying proteinuria, which suggests that the low albumin serum levels mediate the increase of HMG-CoA reductase.7 At the other end of the pathway, the rate-limiting enzyme of cholesterol catabolism to bile acids, the cholesterol 7␣-hydroxylase, was studied. Although this enzyme is markedly upregulated in diet-induced hypercholesterolemia, it remains unchanged in nephrotic rats, which contributes
only to a relative and not to an absolute decrease in cholesterol catabolism.8 Interestingly, the intracellular cholesterol concentration was found to be normal in nephrotic rats compared with diet-induced hypercholesterolemic rats, which show a marked increase in the total intracellular cholesterol content. In addition, a severe reduction in the amount of hepatic LDL receptor protein was reported in nephrotic rats, although the mRNA and the transcription rate were normal.8 A more prominent role in the intracellular cholesterol homeostasis, however, is Table 2. Characteristic Changes in Lipoprotein Metabolism in Patients With Nephrotic Syndrome Lipoproteins VLDL 1 IDL 1 LDL 1 Lp(a) 1 HDL ↔ or 1 Apolipoproteins apo B 1 apo E 1 apo A-I 1 apo A-IV 1 Enzymes ACAT 1 DGTA 1 HMG-CoA reductase 1 cholesterol 7␣-hydroxylase ↔ LCAT 22 LPL 22 HTGL 2 Receptors VLDL receptor 2 LDL receptor 2 HDL receptor 2
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197
played by the amount of free cholesterol compared with the cholesterol esters. Cholesterol esters constitute the majority of the cellular cholesterol content, but mainly the free cholesterol negatively influences the expression of the HMG-CoA reductase and the cholesterol 7␣hydroxylase. In line, free cholesterol concentrations are significantly reduced in NS. In this context another enzyme seems to play a key role: studies by Vaziri et al showed an upregulation of Acyl-coenzyme A:cholesterol acyltransferase (ACAT) in nephrotic rats.9 ACAT catalyzes the esterification of cholesterol, which is then incorporated in apo B– containing lipoproteins. ACAT-2 is expressed mainly in the liver and white fat. A very recent study by this group treated rats with puromycin-induced NS with an ACAT inhibitor.10 Before treatment, rats had an increased hepatic ACAT activity, ACAT-2 mRNA, and ACAT-2 protein level accompanied by reduced LDL and HDL receptor and plasma LCAT concentrations. The ACAT inhibitor reduced the plasma cholesterol and triglyceride concentrations and lowered the hepatic ACAT activity. This inhibition was also associated with near-normalization of plasma LCAT and hepatic HDL and LDL receptor protein and a marked improvement of proteinuria and hypoalbuminemia.10 It remains to be investigated whether an ACAT inhibition in nephrotic people results in a similar improvement of lipoprotein metabolism and NS.
mediately esterified by the key enzyme lecithin: cholesterol acyltransferase (LCAT). The now cholesterol-rich HDL-2 particles exchange in the circulation cholesterol against triglycerides from very-low-density lipoprotein (VLDL) remnants facilitated by cholesteryl ester transfer protein. In the liver, the HDL-2 particles bind to HDL receptors13 and unload their content of cholesterol esters and fatty acids. After dissociation from the receptor, the lipid-depleted HDL-3 particle is available for another cycle. In addition to this major pathway, some HDL-2 particles can be taken up as whole particles by the low-density lipoprotein receptor–related protein (LRP) or LDL receptor. One of the major disturbances in this pathway in NS is the marked reduction of LCAT protein in NS, which is lost by urine and results in an LCAT deficiency.14 As a result, the maturation of cholesterol-poor HDL-3 to lipid-rich HDL-2 particles is disturbed and less apo C-II and apo E in the HDL particles for transport to VLDL is available, which results in disturbances of the metabolism of triglyceride-rich lipoproteins (see below).15 The other key element in reverse cholesterol transport, the HDL receptor, shows a significantly reduced amount of HDL receptor protein. Furthermore, the hepatic triglyceride lipase, which hydrolyzes triglycerides from the HDL-2 particles during the docking to the HDL receptor, is also severely depressed in NS.16 Apo A-I concentrations are elevated.17
Metabolism of HDL Cholesterol Particles and Reverse Cholesterol Transport
Metabolism of Triglyceride-Rich Particles
Plasma HDL concentrations are often reported to be normal in nephrotic patients.1-3,11 However, the normal quantities are accompanied by qualitative changes: because of disturbances in the maturation of lipid-poor HDL-3 particles to cholesterol-rich HDL-2 particles, the ratio of the 2 particles is shifted to a high concentration of HDL-3 and low concentrations of HDL-2 particles.12 As a consequence, major disturbances in the reverse cholesterol pathway can be observed. Usually lipid-poor HDL-3 particles are formed from nascent HDL particles that bind to the cell surface binding protein vigilin of peripheral cells. Cellular free cholesterol undergoes a diffusion to the surface of HDL-3 particles, where it is im-
Triglyceride-rich particles, including chylomicrons, VLDLs, and intermediate-density lipoproteins (IDLs), are significantly elevated in NS, and the catabolism of these particles seems to be decreased.3-5,18 Turnover studies showed that the elevated levels of VLDLs are not caused by an increased production but by a decreased fractional catabolic rate.5 Although it was believed for a long time that proteinuria is responsible for the delayed chylomicron and VLDL clearance, recent work by Shearer et al showed that both hypoalbuminemia and proteinuria contribute separately to the reduced lipoprotein catabolism in NS.19 The key enzyme involved in the lipolysis is lipoprotein lipase (LPL), which is responsible for about
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Lipoprotein(a) and Apolipoprotein(a) Polymorphism
apo(a).27 The molecular weight of apo(a) isoforms ranges from 300 to ⬎800 kDa. Analyses of genomic DNA have shown that this size polymorphism is caused by a varying number of K-IV repeats in the apo(a) gene (ranging from 11 to ⬎40 K-IV repeats). Isoforms with up to 22 K-IV repeats are called low-molecular-weight (LMW) or small apo(a) isoforms, those with more than 22 K-IV repeats are called high-molecularweight (HMW) or large apo(a) isoforms. An inverse correlation exists between the molecular weight of apo(a) and the Lp(a) plasma concentrations. This means that individuals with HMW or large apo(a) phenotypes have on average low Lp(a) concentrations, and those with LMW or small isoforms usually show high concentrations. Depending on the population under investigation, this association explains between 30% and 70% of the variability in Lp(a) levels. For a detailed introduction to this lipoprotein, see Utermann.26 Numerous retrospective and also a remarkable number of prospective case-control studies have reported significantly higher Lp(a) levels in patients with coronary artery disease, cerebrovascular disease, and peripheral atherosclerosis when compared with controls.28,29 Lp(a) levels above 30 mg/dL were proposed to be associated with an increased risk. Those studies that have studied apo(a) isoforms as well as Lp(a) levels observed that short apo(a) alleles/isoforms were significantly more frequent in patients with coronary artery disease or other forms of atherothrombosis than in controls.30-32
Short Introduction to Lp(a) Lp(a) is an LDL-like lipoprotein that consists of an LDL particle to which the glycoprotein apolipoprotein(a) [apo(a)] is attached. The mean and median concentrations of Lp(a) in white subjects are about 15 to 20 mg/dL and 8 mg/dL, respectively, with an extremely broad range from ⬍0.1 mg/dL to ⬎300 mg/dL. The distribution of Lp(a) plasma concentrations in the general white and East Asian populations is skewed, and most people have low concentrations. Individuals of African descent have 2- to 4-fold higher median plasma levels of Lp(a) with a more Gaussian distribution than do white people.26 The apo(a) gene located on chromosome 6 (6q26-q27) determines a size polymorphism of
Lp(a) in Kidney Disease Numerous studies reported elevated Lp(a) levels in patients with kidney diseases. This increase, however, depends markedly on the impairment of kidney function, the amount of proteinuria, and the treatment modality.33,34 In addition to these parameters, there is strong evidence that the relative increase of Lp(a) also depends on the apo(a) K-IV repeat polymorphism. Our group described this interesting but still unexplained phenomenon for the first time more than 10 years ago in hemodialysis patients.35 We have seen in these patients that an increase of Lp(a) on average can only be observed in patients with only long (HMW) apo(a) isoforms when compared with isoform-matched controls. Patients with at least 1 short (LMW) apo(a) isoform had nearly identical
70% of the reduction in triglycerides of chylomicrons and VLDL. In NS this key enzyme, however, is markedly downregulated in skeletal muscle, adipose tissue, and myocardium.18,20,21 The VLDL receptor that shows not only a similar tissue distribution to LPL but also a functional relationship with LPL is also markedly reduced in NS.22,23 Another enzyme called acyl-CoA:diacylglycerol aycltransferase (DGAT) catalyzes the final step in the biosynthesis of triglycerides. Hepatic DGAT-1 mRNA and activity was significantly increased in rats with NS. Functionally, this increase was associated with a reduction of microsomal free fatty acid concentrations in the livers of nephrotic animals.24 Finally, VLDL from nephrotic patients itself seems to influence the binding and internalization of VLDL: the investigation of LPL-mediated lipolysis was significantly lower with VLDL preparations from nephrotic rats, and this impairment was corrected by the addition of HDL from normal rats.25 Shearer et al found in this context a significant reduction in the apo E to apo A-I ratio in nephrotic HDL.19 Apo E is an important ligand for the VLDL receptor. Because nascent VLDL receives apo E from HDL, HDL particles poor in apo E, as seen in NS, might contribute to an impaired lipolysis of triglyceride-rich lipoproteins by LPL.
DYSLIPIDEMIA AND NEPHROTIC SYNDROME
Lp(a) levels when compared with matched controls. This phenomenon of differences in the relative increase of Lp(a) between these 2 apo(a) isoform groups was also observed in other stages of kidney impairment or treatment modalities, as reviewed recently.34 The Lp(a) level itself seems to be less discriminative for cardiovascular disease in kidney patients compared with the general population. Instead of the Lp(a) concentrations, the apo(a) phenotype is a very good and consistent predictor for cardiovascular disease in hemodialysis patients. Almost all studies that investigated the apo(a) polymorphism observed that LMW apo(a) isoforms predict the risk for atherosclerotic complications (recently reviewed by Kronenberg34). The analysis of apo(a) phenotypes is therefore a useful tool for risk stratification.
Lp(a) and Nephrotic Syndrome Some studies suggested that nonnephrotic proteinuria has an influence on Lp(a) levels. However, a recent study in patients with mild and moderate kidney failure observed that the correlation of Lp(a) with glomerular filtration rate (r ⫽ ⫺0.18) was more pronounced than that of Lp(a) with nonnephrotic range proteinuria (r ⫽ 0.14). In a multiple regression analysis model, proteinuria was not independently associated with Lp(a) but modified the association between glomerular filtration rate and Lp(a) with borderline statistical significance.36 Obviously a major protein loss by urine must occur to have a significant impact on Lp(a) levels. This is clearly the case in NS, which is the condition with the highest Lp(a) levels ever reported.37,38 Concentrations up to 600 mg/dL were described.1 The disease-related elevated levels have been elegantly shown in longitudinal studies that showed a decrease of Lp(a) after remission of NS.37,38 In a recent study in 207 patients and 274 controls, we showed that primary and secondary causes contribute to this elevation.1 The primary or genetic causes became apparent by a markedly elevated number of LMW apo(a) phenotypes, which are usually associated with high Lp(a) levels. This frequency was 35.7% in patients compared with only 24.8% in controls (P ⫽ .009). We hypothesized that particularly patients who have already high Lp(a)
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Figure 1. Median Lp(a) serum concentrations in patients with nephrotic syndrome and controls stratified by 6 apo(a) isoform groups. Lp(a) concentrations were calculated for each expressed apo(a) isoform separately from the measured Lp(a) concentration by estimating the relative proportion of the 2 isoforms in the sodium dodecyl sulfate agarose gel electrophoresis. Adapted and reprinted with permission from Blackwell Publishing.1
levels because of an inherited LMW apo(a) isoform tend more often toward a nephrotic course of kidney disease if the particular disease allows for a nonnephrotic or a nephrotic course of disease. This is supported by 3 observations: in 2 recent studies, high Lp(a) levels predicted future relapse in children with steroid-sensitive NS39,40; and in patients with nonnephrotic glomerulonephritis who do not show these tremendous elevations of Lp(a), an apo(a) phenotype frequency distribution was observed, which is not different from controls.36 In addition to the primary reasons for the increase in Lp(a), secondary causes by the pathogenetic changes of the NS itself resulted in a different increase of Lp(a) in the various apo(a) isoform groups. LMW isoforms were associated with 40% to 75% elevated Lp(a) concentrations when compared with matched isoforms from controls. HMW apo(a) isoforms showed 100% to 500% elevated Lp(a) levels compared with matched isoforms from controls (Fig 1). The severity of the NS as well as the degree of kidney impairment did not influence the Lp(a) concentrations. Taken together, it seems that the large amount of protein loss in nephrotic patients results in an increased production of Lp(a) by the liver. This was indeed observed in recent turnover studies in patients with NS, which showed a similar fractional catabolic rate of Lp(a) in 5 patients and 5 controls, which suggested that differences in Lp(a) levels are caused by differ-
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Figure 2. LDL-C and Lp(a)-corrected LDL-C levels in patients with LMW and HMW apo(a) phenotypes. Only patients are presented who showed at least a 30-mg/dL difference between the 2 LDL-C levels. This was significantly less the case in patients with HMW than in those with LMW apo(a) phenotypes (16% versus 58%; P ⬍ .00001). Adapted and reprinted with permission from Blackwell Publishing.2
ences in synthesis rate.41 The number of studied patients did not allow investigation of the turnover stratified for apo(a) phenotypes. The data from our recent study,1 however, suggest that even when Lp(a) is elevated in all apo(a) isoform groups, we can observe again that HMW apo(a) isoforms show a higher relative (but not absolute) increase in Lp(a) levels than LWM apo(a) isoforms (Fig 1).
Therapeutic Implications of a High Lp(a) Level on LDL Cholesterol Lowering Lp(a) is an LDL-like particle consisting of 45% cholesterol.42,43 The usual methods for determining LDL cholesterol (LDL-C) do not distinguish between cholesterol derived from LDL and Lp(a) and are thus the net result of cholesterol levels from both lipoproteins. High Lp(a) concentrations therefore significantly contribute to the measured or calculated LDL-C levels. In a recent study, we observed that the LDL-C fraction that derived from Lp(a) was on average 27 mg/dL and therefore the highest in patients with NS, compared with only 9 mg/dL in controls.2 However, up to more than 250 mg/dL of LDL-C derived from Lp(a) cholesterol in patients with Lp(a) levels
higher than 500 mg/dL. Hemodialysis and continuous ambulatory peritoneal dialysis patients showed an overestimation of the true LDL cholesterol levels of on average 11 and 16 mg/dL, respectively.44 Figure 2 shows all patients of the mentioned study who showed an Lp(a)-corrected LDL-C level that was at least 30 mg/dL lower than uncorrected levels. However, we observed patients who had up to more than 250 mg/dL differences between the 2 levels. Because statins have no influence on Lp(a) levels,45 this observation might have an influence on the investigation of the efficacy of a therapeutic intervention. It will be more accurate to consider and monitor only the amount of LDL-C that is accessible for intervention. This can have a considerable influence on the results: for example, a not-so-rarely-observed nephrotic patient with an Lp(a) concentration of 200 mg/dL and a total LDL-C of 200 mg/dL has an Lp(a)-corrected LDL-C of 110 mg/dL. If in this case a statin leads to only a 30-mg/dL reduction in LDL-C, the effectiveness will be either 15% or 27%, depending whether total or Lp(a)-corrected LDL-C is used for calculation. Therefore, cases of nonresponders or low responders to statin therapy could be hidden behind tremendously elevated Lp(a) levels. No study up to now has investigated this question in patients with kidney disease in a direct way. However, Scanu and Hinman investigated the influence of statins on hypercholesterolemic subjects with high Lp(a) levels and observed in some cases an Lp(a)-corrected LDL-C decrease as low as 10 mg/dL.46 The decrease of the Lp(a)-corrected LDL-C levels was proportional to the pretreatment ratio of the Lp(a)corrected LDL-C to Lp(a) concentrations. This means that the decrease in LDL-C was strongly dependent on the accessible LDL-C, showing that only cholesterol from LDL-C and not Lp(a) is affected by statin therapy.46 Ongoing large trials with kidney patients will provide the possibility to study such an effect easily in the setting of kidney disease.
Apo E Polymorphism Apo E plays an important role in lipoprotein metabolism because of its function as a ligand for receptors. More recently, cell culture stud-
DYSLIPIDEMIA AND NEPHROTIC SYNDROME
ies showed that apo E is renal protective by regulating mesangial cell proliferation and matrix expansion.47 Apo E knockout mice develop renal lesions in addition to atherosclerosis.48 Therefore several studies investigated an association between the apo E polymorphism and NS in humans, and had contrasting findings. Smaller studies found an increase in the ⑀4 allele in childhood NS,49which was not confirmed by 2 larger studies.50,51 A recent study found a tendency toward a higher frequency of ⑀4 allele in 190 nephrotic children, which became significant in those who had frequent disease relapses.52
Interventional Studies Affecting Dyslipidemia In addition to the use of statins, there is no doubt that one of the most important strategies for treating dyslipidemia in NS is to treat the proteinuria. In many patients, this will result in an improvement in the lipid profile. It is well known that angiotensin converting enzyme (ACE) inhibitors have a proteinuriareducing effect. A recent study in 28 patients with nondiabetic chronic nephropathies with a daily proteinuria of at least 2 g investigated the extent to which upward titration of the ACE inhibitor lisinopril to a maximum tolerated doses would influence proteinuria and dyslipidemia.53 The maximum dose reduced not only proteinuria but also total and LDL cholesterol as well as triglycerides without affecting HDL cholesterol levels. The reduction in total and LDL cholesterol correlated with increases of serum albumin concentrations and oncotic pressure and was most pronounced in patients with severe hypoalbuminemia. The changes in triglycerides were strongly correlated with lisinopril doses. A study in 11 children with steroid and cyclosporine A–resistant NS showed that a combined LDL apheresis and prednisolone therapy not only decreased cholesterol concentrations but resulted also in a remission of the disease in 7 patients; in 5 of them it was a complete remission.54 Studies in Imai rats, which are models for spontaneous focal glomerulosclerosis with NS, tested the effect of angiotensin II receptor type 1 (AT-1) blockade with and without a combination with HMG-CoA reductase inhibitors. The treatment with an HMG-CoA reductase
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inhibitor alone resulted in an improvement in hyperlipidemia and renal disease. The treatment with an AT-1 blocker with and without an HMG-CoA reductase inhibitor resulted in a normalization of blood pressure, urinary protein excretion, plasma cholesterol, triglyceride, LDL, VLDL, and albumin concentrations as well as renal function. Long-term treatment even resulted in a prevention of the usually observed glomerulosclerosis and in a reduced tubulointerstitial injury.55 Experimental studies with 1 mg adrenocorticotropic hormone twice per week in 14 patients with membranous nephropathy up to 12 months showed reductions of 30% to 60% in the serum concentrations of cholesterol, triglycerides, apo B, and Lp(a) as well as an increase of HDL cholesterol and apo A-I of 30% to 40%. The urinary albumin excretion decreased by 90%, and the glomerular filtration rate increased by 25%.56 The mechanism behind the lipid-lowering effect is not clear. It can be excluded that it is simply caused by a reduction of proteinuria because it was even observed in hemodialysis patients.57,58 In vitro studies suggested an increased hepatic uptake of LDL.
Acknowledgements The author thanks Mrs. Petra Jassmann for technical assistance during the preparation of the manuscript, and all collaborators who contributed to the studies as well as all patients who were participants in the research projects.
References 1. Kronenberg F, Lingenhel A, Lhotta K, et al: The apolipoprotein(a) size polymorphism is associated with nephrotic syndrome. Kidney Int 65:606-612, 2004 2. Kronenberg F, Lingenhel A, Lhotta K, et al: Lipoprotein(a)- and low-density lipoprotein-derived cholesterol in nephrotic syndrome: Impact on lipid-lowering therapy? Kidney Int 66:348-354, 2004 3. Joven J, Villabona C, Vilella E, et al: Abnormalities of lipoprotein metabolism in patients with the nephrotic syndrome. N Engl J Med 323:579-584, 1990 4. Warwick GL, Packard CJ, Demant T, et al: Metabolism of apolipoprotein B-containing lipoproteins in subjects with nephrotic-range proteinuria. Kidney Int 40:129-138, 1991 5. De Sain-van der Velden M, Kaysen GA, Barrett HA, et al: Increased VLDL in nephrotic patients results from a decreased catabolism while increased LDL results from increased synthesis. Kidney Int 53:994-1001, 1998 6. Vaziri ND, Liang KH: Hepatic HMG-CoA reductase gene expression during the course of puromycin-induced nephrosis. Kidney Int 48:1979-1985, 1995
202
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7. Liang K, Vaziri ND: HMG-CoA reductase, cholesterol 7alpha-hydroxylase, LCAT, ACAT, LDL receptor, and SRB-1 in hereditary analbuminemia. Kidney Int 64:192-198, 2003 8. Vaziri ND, Sato T, Liang K: Molecular mechanisms of altered cholesterol metabolism in rats with spontaneous focal glomerulosclerosis. Kidney Int 63:1756-1763, 2003 9. Vaziri ND, Liang K: Up-regulation of acyl-coenzyme A: Cholesterol acyltransferase (ACAT) in nephrotic syndrome. Kidney Int 61:1769-1775, 2002 10. Vaziri ND, Liang KH: Acyl-coenzyme A: Cholesterol acyltransferase inhibition ameliorates proteinuria, hyperlipidemia, lecithin-cholesterol acyltransferase, SRB-1, and low-density lipoprotein receptor deficiencies in nephrotic syndrome. Circulation 110:419-425, 2004 11. Appel GB, Blum CB, Chien S, et al: The hyperlipidemia of nephrotic syndrome: Relation to plasma albumin concentration, oncotic pressure and viscosity. N Engl J Med 312:15441548, 1985 12. Gherardi E, Rota E, Calandra S, et al: Relationship among the concentrations of serum lipoproteins and changes in their chemical composition in patients with untreated nephrotic syndrome. Eur J Clin Invest 7:563-570, 1977 13. Acton S, Rigotti A, Landschulz KT, et al: Identification of scavenger receptor SR-BI as a high density lipoprotein receptor. Science 271:518-520, 1996 14. Vaziri ND, Liang K, Parks JS: Acquired lecithin-cholesterol acyltransferase deficiency in nephrotic syndrome. Am J Physiol Renal Physiol 280:F823-F828, 2001 15. Deighan CJ, Caslake MJ, McConnell M, et al: Patients with nephrotic-range proteinuria have apolipoprotein C and E deficient VLDL1. Kidney Int 58:1238-1246, 2000 16. Liang K, Vaziri ND: Down-regulation of hepatic lipase expression in experimental nephrotic syndrome. Kidney Int 51:1933-1937, 1997 17. Kaysen GA, Hoye E, Jones H Jr: Apolipoprotein AI levels are increased in part as a consequence of reduced catabolism in nephrotic rats. Am J Physiol 268:F532-F540, 1995 18. Kaysen GA, Mehendru L, Pan XM, et al: Both peripheral chylomicron catabolism and hepatic uptake of remnants are defective in nephrosis. Am J Physiol 263:F335-F341, 1992 19. Shearer GC, Stevenson FT, Atkinson DN, et al: Hypoalbuminemia and proteinuria contribute separately to reduced lipoprotein catabolism in the nephrotic syndrome. Kidney Int 59:179-189, 2001 20. Garber DW, Gottlieb BA, Marsh JB, et al: Catabolism of very low density lipoproteins in experimental nephrosis. J Clin Invest 74:1375-1383, 1984 21. Vaziri ND: Molecular mechanisms of lipid disorders in nephrotic syndrome. Kidney Int 63:1964-1976, 2003 22. Sato T, Liang K, Vaziri ND: Down-regulation of lipoprotein lipase and VLDL receptor in rats with focal glomerulosclerosis. Kidney Int 61:157-162, 2002 23. Vaziri ND, Liang K: Down-regulation of VLDL receptor expression in chronic experimental renal failure. Kidney Int 51:913-919, 1997 24. Vaziri ND, Kim CH, Phan D, et al: Up-regulation of hepatic Acyl CoA: Diacylglycerol acyltransferase-1 (DGAT-1) expression in nephrotic syndrome. Kidney Int 66:262-267, 2004 25. Furukawa S, Hirano T, Mamo JC, et al: Catabolic defect of triglyceride is associated with abnormal very-low-density lipoprotein in experimental nephrosis. Metabolism 39:101-107, 1990
26. Utermann G: Lipoprotein(a), in Scriver CR, Beaudet AL, Sly WS, Valle D (eds): The metabolic and molecular bases of inherited disease (ed 8). New York, McGraw-Hill, 2000, pp 2753-2787 27. Utermann G, Menzel HJ, Kraft HG, et al: Lp(a) glycoprotein phenotypes: Inheritance and relation to Lp(a)-lipoprotein concentrations in plasma. J Clin Invest 80:458-465, 1987 28. Danesh J, Collins R, Peto R: Lipoprotein(a) and coronary heart disease: Meta-analysis of prospective studies. Circulation 102:1082-1085, 2000 29. Craig WY, Neveux LM, Palomaki GE, et al: Lipoprotein(a) as a risk factor for ischemic heart disease: Metaanalysis of prospective studies. Clin Chem 44:2301-2306, 1998 30. Sandholzer C, Saha N, Kark JD, et al: Apo(a) isoforms predict risk for coronary heart disease: A study in six populations. Arterioscler Thromb 12:1214-1226, 1992 31. Wild SH, Fortmann SP, Marcovina SM: A prospective case-control study of lipoprotein(a) levels and apo(a) size and risk of coronary heart disease in Stanford Five-City Project participants. Arterioscler Thromb Vasc Biol 17:239-245, 1997 32. Kronenberg F, Kronenberg MF, Kiechl S, et al: Role of lipoprotein(a) and apolipoprotein(a) phenotype in atherogenesis: Prospective results from the Bruneck Study. Circulation 100: 1154-1160, 1999 33. Kronenberg F, Utermann G, Dieplinger H: Lipoprotein(a) in renal disease. Am J Kidney Dis 27:1-25, 1996 34. Kronenberg F: Epidemiology, pathophysiology and therapeutic implications of lipoprotein(a) in kidney disease. Expert Rev Cardiovasc Ther 2:729-743, 2004 35. Dieplinger H, Lackner C, Kronenberg F, et al: Elevated plasma concentrations of lipoprotein(a) in patients with end-stage renal disease are not related to the size polymorphism of apolipoprotein(a). J Clin Invest 91:397-401, 1993 36. Kronenberg F, Kuen E, Ritz E, et al: Lipoprotein(a) serum concentrations and apolipoprotein(a) phenotypes in mild and moderate renal failure. J Am Soc Nephrol 11:105-115, 2000 37. Wanner C, Rader D, Bartens W, et al: Elevated plasma lipoprotein(a) in patients with the nephrotic syndrome. Ann Intern Med 119:263-269, 1993 38. Stenvinkel P, Berglund L, Heimburger O, et al: Lipoprotein(a) in nephrotic syndrome. Kidney Int 44:1116-1123, 1993 39. Kawasaki Y, Suzuki J, Nozawa R, et al: Prediction of relapse by plasma lipoprotein(a) concentration in children with steroidsensitive nephrotic syndrome. Nephron 92:807-811, 2002 40. Merouani A, Levy E, Mongeau JG, et al: Hyperlipidemic profiles during remission in childhood idiopathic nephrotic syndrome. Clin Biochem 36:571-574, 2003 41. De Sain-van der Velden MGM, Reijngoud DJ, Kaysen GA, et al: Evidence for increased synthesis of lipoprotein(a) in the nephrotic syndrome. J Am Soc Nephrol 9:1474-1481, 1998 42. Kostner GM, Ibovnik A, Holzer H, et al: Preparation of a stable fresh frozen primary lipoprotein[a] (Lp[a]) standard. J Lipid Res 40:2255-2263, 1999 43. Marcovina SM, Albers JJ, Scanu AM, et al: Use of a reference material proposed by the international federation of clinical chemistry and laboratory medicine to evaluate analytical methods for the determination of plasma lipoprotein(a). Clin Chem 46:1956-1967, 2000 44. Kronenberg F, Lingenhel A, Neyer U, et al: Prevalence of dyslipidemic risk factors in hemodialysis and CAPD patients. Kidney Int 63:S113-S114, 2003. (suppl 84)
DYSLIPIDEMIA AND NEPHROTIC SYNDROME 45. Kostner GM, Gavish D, Leopold B, et al: HMG CoA reductase inhibitors lower LDL cholesterol without reducing Lp(a) levels. Circulation 80:1313-1319, 1989 46. Scanu AM, Hinman J: Issues concerning the monitoring of statin therapy in hypercholesterolemic subjects with high plasma lipoprotein(a) levels. Lipids 37:439-444, 2002 47. Chen G, Paka L, Kako Y, et al: A protective role for kidney apolipoprotein E. Regulation of mesangial cell proliferation and matrix expansion. J Biol Chem 276:49142-49147, 2001 48. Zhang SH, Reddick RL, Piedrahita JA, et al: Spontaneous hypercholesterolemia and arterial lesions in mice lacking apolipoprotein E. Science 258:468-471, 1992 49. Asami T, Ciomartan T, Hayakawa H, et al: Apolipoprotein E epsilon 4 allele and nephrotic glomerular diseases in children. Pediatr Nephrol 13:233-236, 1999 50. Attila G, Noyan A, Karabay BA, et al: Apolipoprotein E polymorphism in childhood nephrotic syndrome. Pediatr Nephrol 17:359-362, 2002 51. Bruschi M, Catarsi P, Candiano G, et al: Apolipoprotein E in idiopathic nephrotic syndrome and focal segmental glomerulosclerosis. Kidney Int 63:686-695, 2003 52. Kim SD, Kim IS, Lee BC, et al: Apolipoprotein E polymorphism and clinical course in childhood nephrotic syndrome. Pediatr Nephrol 18:230-233, 2003
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53. Ruggenenti P, Mise N, Pisoni R, et al: Diverse effects of increasing lisinopril doses on lipid abnormalities in chronic nephropathies. Circulation 107:586-592, 2003 54. Hattori M, Chikamoto H, Akioka Y, et al: A combined low-density lipoprotein apheresis and prednisone therapy for steroid-resistant primary focal segmental glomerulosclerosis in children. Am J Kidney Dis 42:1121-1130, 2003 55. Rodriguez-Iturbe B, Sato T, Quiroz Y, et al: AT-1 receptor blockade prevents proteinuria, renal failure, hyperlipidemia, and glomerulosclerosis in the Imai rat. Kidney Int 66:668-675, 2004 56. Berg AL, Nilsson-Ehle P, Arnadottir M: Beneficial effects of ACTH on the serum lipoprotein profile and glomerular function in patients with membranous nephropathy. Kidney Int 56:1534-1543, 1999 57. Arnadottir M, Berg AL, Dallongeville J, et al: Adrenocorticotropic hormone lowers serum Lp(a) and LDL cholesterol concentrations in hemodialysis patients. Kidney Int 52:16511655, 1997 58. Arnadottir M, Berg A-L, Kronenberg F, et al: Corticotropin-induced reduction of plasma lipoprotein(a) concentrations in healthy individuals and hemodialysis patients: Relation to apolipoprotein(a) size polymorphism. Metabolism 48:342-346, 1999