Localization of apolipoprotein(a) and B-100 in various renal diseases

Preprocedural Lp(a) level and ApoB/ApoA-Ι ratio and the risk for contrast-induced acute kidney injury in patients undergoing emergency PCI

BACKGROUND

High serum Lipoprotein(a) (Lp(a)) level and Apolipoprotein B/Apolipoprotein AΙ (ApoB/ApoA-Ι) ratio are risk factors for cardiovascular disease and kidney disease and have been found to be correlated with the prevalence and prognosis of various kidney diseases. However, it is not clear whether the serum Lp(a) level and ApoB/ApoA-Ι ratio pre-PCI are correlated with the prevalence of contrast-induced acute kidney injury (CI-AKI).

METHODS

A total of 931 participants undergoing emergency PCI from July 2018 to July 2020 were included. According to whether the serum creatinine concentration was higher than the baseline concentration (by ≥25% or ≥ 0.5 mg/dL) 48-72 h after contrast exposure, these participants were divided into a CI-AKI group (n = 174) and a non-CI-AKI group (n = 757). Serum Lp(a), ApoA-Ι and ApoB concentration were detected in the patients when they were admitted to hospital, and the ApoB/ApoA-Ι ratio was calculated. Logistic regression and restricted cubic spline analyses were used to explore the correlation between the Lp(a) concentration or the ApoB/ApoA-Ι ratio and the risk of CI-AKI.

RESULTS

Among the 931 participants undergoing emergency PCI, 174 (18.69%) participants developed CI-AKI. Compared with the non-CI-AKI group, the Lp(a) level and ApoB/ApoA-Ι ratio pre-PCI in the CI-AKI group were significantly higher (P < 0.05). The incidence of CI-AKI was positively associated with the serum Lp(a) level and ApoB/ApoA-Ι ratio pre-PCI in each logistic regression model (P < 0.05). After adjusting for all the risk factors included in this study, restricted cubic spline analyses found that the Lp(a) level and the ApoB/ApoA-Ι ratio before PCI, within certain ranges, were positively associated with the prevalence of CI-AKI.

CONCLUSION

High Lp(a) levels and high ApoB/ApoA-Ι ratios before PCI are potential risk factors for CI-AKI.

Apolipoprotein(a), an enigmatic anti-angiogenic glycoprotein in human plasma: A curse or cure?

Angiogenesis is a finely co-ordinated, multi-step developmental process of the new vascular structure. Even though angiogenesis is regularly occurring in physiological events such as embryogenesis, in adults, it is restricted to specific tissue sites where rapid cell-turnover and membrane synthesis occurs. Both excessive and insufficient angiogenesis lead to vascular disorders such as cancer, ocular diseases, diabetic retinopathy, atherosclerosis, intra-uterine growth restriction, ischemic heart disease, stroke etc. Occurrence of altered lipid profile and vascular lipid deposition along with vascular disorders is a hallmark of impaired angiogenesis. Among lipoproteins, lipoprotein(a) needs special attention due to the presence of a multi-kringle protein subunit, apolipoprotein(a) [apo(a)], which is structurally homologous to many naturally occurring anti-angiogenic proteins such as plasminogen and angiostatin. Researchers have constructed different recombinant forms of apo(a) (rhLK68, rhLK8, RHACK2, KV-11, and AU-6) and successfully exploited its potential to inhibit unwanted angiogenesis during tumor metastasis and retinal neovascularization. Similar to naturally occurring anti-angiogenic proteins, apo(a) can directly interfere with angiogenic signaling pathways. Besides this, apo(a) can also exert its anti-angiogenic effect indirectly by inducing endothelial cell apoptosis, by inhibiting endothelial progenitor cell functions or by upregulating nuclear factors in endothelial cells via apo(a)-bound oxPLs. However, the impact of the anti-angiogenic potential of native apo(a) during physiological angiogenesis in embryos and wounded tissues is not yet explored. In this context, we review the studies so far done to demonstrate the anti-angiogenic activity of apo(a) and the recent developments in using apo(a) as a therapeutic agent to treat impaired angiogenesis during vascular disorders, with emphasis on the gaps in the literature.

The effects of inflammation on lipid accumulation in the kidneys of children with primary nephrotic syndrome

This study aimed to characterize the relationship between inflammation and lipid accumulation in children with primary nephrotic syndrome (PNS). Local expression of interleukin-1β (IL-1β), transforming growth factor-β1 (TGF-β1), low-density lipoprotein receptor (LDLr), sterol regulatory element binding protein-2 (SREBP-2), SREBP cleavage-activating protein (SCAP), and apolipoprotein B100 (apoB100) was analyzed by immunohistochemistry in kidney tissues obtained from children with PNS. Renal histopathology was evaluated by hematoxylin and eosin and periodic acid-Schiff staining. Serum levels of IL-1β and TGF-β1 were measured by enzyme-linked immunosorbent assays. Expression of IL-1β, TGF-β1, LDLr, SREBP-2, SCAP, and apoB100 was higher in samples from patients with non-minimal change necrotic syndrome (NMCNS) compared to both controls and patients with minimal change necrotic syndrome. Deposition of apoB100 was significantly correlated with expression of IL-1β, TGF-β1, LDLr, SREBP-2, and SCAP and with the glomerulosclerosis index, but not with plasma lipid levels. Expression of IL-1β and TGF-β1 was significantly correlated with expression of LDLr, SREBP-2, and SCAP. These findings suggest that inflammation leads to lipid accumulation in the kidney through disruption of the expression of proteins in the SCAP/SREBP-2/LDLr signaling pathway, which may underlie glomerulosclerosis and tubulointerstitial fibrosis in NMCNS.

Antiproteinuric effect of niceritrol, a nicotinic acid derivative, in chronic renal disease with hyperlipidemia: a randomized trial

PURPOSE

Lipoprotein (a) [Lp(a)] levels increase in patients with renal disease. We administered niceritrol, a nicotinic acid derivative, to patients with chronic renal disease and a high serum Lp(a) level, and studied its effects on lipid metabolism, proteinuria, and renal function.

METHODS

Thirty-three patients with chronic renal disease whose serum Lp(a) levels were > or = 15 mg/dL were randomly (but not blindly) assigned to treatment with niceritrol (n = 16) or to an untreated control group (n = 17). Parameters of lipid metabolism, excretion of urinary protein, and renal function were examined for 12 months.

RESULTS

Changes in urinary protein excretion, as well as Lp(a) levels, differed significantly between the two groups. The mean (+/- SD) change from baseline in excretion of urinary protein was 0.77 +/- 1.23 g/d in the control group compared with -1.41 +/- 2.26 g/d in the niceritrol group at 12 months (P =0.003). Mean Lp(a) levels increased by 3 +/- 10 mg/dL in the control group compared with a decrease of 10 +/- 13 mg/dL in the niceritrol group at 12 months (P =0.004). The mean creatinine clearance declined by 10 +/- 12 mL/min in the control group, compared with 1 +/- 13 mL/min in the niceritrol group at 12 months (P =0.06).

CONCLUSION

Lipid levels improved with niceritrol treatment, whereas the excretion of urinary protein decreased, perhaps slowing the rate of loss of renal function in chronic renal disease.

Association of kidney function with serum lipoprotein(a) level: the third National Health and Nutrition Examination Survey (1991-1994)

BACKGROUND

Elevated lipoprotein(a) (Lp[a]) levels have been observed in patients on dialysis therapy. However, few studies explored the relationship between kidney function and Lp(a) levels in patients with mild to moderate chronic kidney disease.

METHODS

We examined the association of estimated glomerular filtration rate (GFR) with Lp(a) level in 7,675 participants in the second phase of the Third National Health and Nutrition Examination Survey.

RESULTS

There was no association between Lp(a) level and estimated GFR in the overall sample (geometric mean, 10.4 mg/dL [95% confidence interval (CI), 9.2 to 11.8] in the group with a GFR of 90 to 149 mL/min/1.73 m2 versus 9.3 mg/dL [95% CI, 7.9 to 11.0] in the group with a GFR of 60 to 89 mL/min/1.73 m2 versus 12.1 mg/dL [95% CI, 9.0 to 15.9] in the group with a GFR of 15 to 59 mL/min/1.73 m2; P = 0.77 for linear trend) or non-Hispanic whites (geometric mean, 8.9 mg/dL [95% CI, 7.8 to 10.2] versus 8.5 mg/dL [95% CI, 7.1 to 10.2] versus 10.9 mg/dL [95% CI, 8.1 to 14.7]; P = 0.54 for linear trend). However, non-Hispanic blacks (geometric mean, 30.4 mg/dL [95% CI, 28.0 to 33.0] versus 35.2 mg/dL [95% CI, 31.4 to 39.4] versus 40.2 mg/dL [95% CI, 27.7 to 58.2]; P = 0.01 for linear trend) and Mexican Americans (geometric mean, 6.2 mg/dL [95% CI, 5.3 to 7.2] versus 7.4 mg/dL [95% CI, 6.4 to 8.5] versus 11.0 mg/dL [95% CI, 5.7 to 20.3]; P = 0.04 for linear trend) showed modestly, but significantly, greater Lp(a) levels with lower GFRs. In a weighed quantile regression model adjusted for age, sex, and race, a lower GFR was associated with greater 95th percentile serum Lp(a) values in the overall sample and non-Hispanic whites and with greater median Lp(a) levels in Mexican Americans.

CONCLUSION

In a cross-section of the US population, a low GFR is associated with only moderately greater Lp(a) levels, and this association may differ by race-ethnicity.

Renal handling of human apolipoprotein(a) and its fragments in the rat

The sites and mechanisms of the catabolism of atherogenic lipoprotein(a) (Lp(a)) are not well understood. Lp(a) is increased in patients with end-stage renal disease, suggesting a renal catabolism of Lp(a). To gain a better insight into renal handling of Lp(a), we established a heterologous rat model to study the renal catabolism of human Lp(a). Pure human Lp(a) was injected into Wistar rats, and animals were sacrificed at different time points (30 minutes to 24 hours). Intact Lp(a) was cleared from the circulation of injected rats with a half-life time of 14.5 hours. Strong intracellular immunostaining for apolipoprotein(a) (apo(a)) was observed in the cytoplasm of proximal tubular cells after 4, 8, and 24 hours. Apolipoprotein B (apoB) was colocalized with glomerular apo(a) 1 to 8 hours after Lp(a) injection, but renal capillaries and tubules remained negative. No relevant amounts of apo(a) fragments were found in the plasma of rats after injection of Lp(a). During all urine collection periods, apo(a) fragments with molecular weights of 50 to 160 kd were detected in the urine, however. Our results show that human Lp(a) injected into rats accumulates intracellularly in the rat kidney, and apo(a) fragments are excreted in the urine. The kidney apparently plays a major role in fragmentation of Lp(a). Despite the fact that rodents lack endogenous Lp(a), rats injected with human Lp(a) may provide a useful heterologous animal model to study the renal metabolism of Lp(a) further.

Cardiovascular disease and chronic renal disease: a new paradigm

Cardiovascular disease (CVD) is a major cause of morbidity and mortality among patients with chronic renal disease (CRD). Despite Improvement in treatment for CVD over the past 30 years, CVD mortality is approximately 15 times higher in dialysis patients than in the general population. The high prevalence of CVD among Incident dialysis patients suggests that CVD begins in earlier stages of CRD, and that implementation of risk factor reduction strategies earlier in the course of CRD may provide an opportunity to prevent CVD in CRD. Based on parallels between CVD and renal disease progression, we have proposed a paradigm that CVD and CRD are outcomes of the same underlying disorders. We propose that risk factor reduction strategies used to prevent CVD in the general population also be applied to patients with CRD, with the hope of preventing progression of renal disease, as well as preventing CVD.

Lipoprotein(a) stimulates growth of human mesangial cells and induces activation of phospholipase C via pertussis toxin-sensitive G proteins

BACKGROUND

Renal disease is commonly associated with hyperlipidemia and correlates with glomerular accumulation of atherogenic lipoproteins, for example, lipoprotein(a) [Lp(a)], and mesangial hypercellularity. Specific binding of Lp(a) to mesangial cells and induction of c-myc and c-fos expression has been demonstrated. Therefore, in this study, we investigated a possible growth stimulatory effect and mode of action of Lp(a) in human mesangial cells.

METHODS

Lp(a) was purified from the regenerate fluid of a dextran sulfate column-based low-density lipoprotein apheresis system. Human mesangial cells were isolated by a sequential sieving technique from patients undergoing tumor nephrectomy. DNA synthesis was measured by [3H]-thymidine incorporation. The intracellular calcium concentration ([Ca2+]i) was determined by Fura 2-fluorescence, and inositol 1,4,5-trisphosphate (1,4,5-IP3) concentration was measured by a radioreceptor assay.

RESULTS

The data show that Lp(a) bound to the cells with a Kd of 17.0 micrograms/ml and increased DNA synthesis and cell proliferation. Lp(a) caused a rapid increase in 1,4,5-IP3 and [Ca2+]i via a pertussis toxin-sensitive mechanism. The phospholipase C (PLC) inhibitor U73122 abolished Lp(a)-induced cell proliferation. In contrast, vasopressin-induced increase in 1,4,5-IP3 and [Ca2+]i was pertussis toxin insensitive.

CONCLUSION

This study revealed that Lp(a) stimulates growth of human mesangial cells. Lp(a)-induced signaling involves binding to a receptor and stimulation of PLC via Gi proteins. Stimulation of PLC appears to be essential for the growth stimulatory effect of Lp(a). Whether these effects of Lp(a) contribute to the pathophysiology of renal disease needs to be determined.

Lipoprotein (a) stimulates mitogen activated protein kinase in human mesangial cells

Evidence suggests an important role of elevated serum lipoproteins in the progression of renal glomerulosclerosis. We report here that lipoprotein (a) (Lp(a)) increased phosphorylation and activity of mitogen activated protein kinase (MAPK) in human mesangial cells. When protein kinase C (PKC) was depleted by long-term incubation with the phorbol 12-O-myristate 13-acetate the effect of Lp(a) on MAPK activation was completely inhibited. Forskolin, a stimulator of the adenylyl cyclase, and dibutyryl-cAMP reduced the effect of Lp(a) on MAPK phosphorylation and activation. We conclude that Lp(a) stimulates the MAPK cascade via activation of PKC and that activation of protein kinase A counteracts Lp(a) induced MAPK activation in human mesangial cells.

Stimulatory effect of lipoprotein (a) on proliferation of human mesangial cells: role of lipoprotein (a) in renal disease

As lipoprotein (a) [Lp(a)] abnormalities would accelerate glomerular injury, we studied the effect of Lp (a) on proliferation of cultured human mesangial cells (MC). Transfection of human apo (a) gene into human hepatoma cells, HepG2 cells, producing human apo B, resulted in the formation of Lp (a), while no Lp (a) was detected in control cells. In contrast, free apo (a) was detected in the medium of apo (a)-transfected MC. Incubation of cultured medium of HepG2 cells transfected with apo (a) gene with MC resulted in a significant increase in cell number compared to control (P<0.01). In contrast, little effect of transfection of apo (a) gene directly into MC on growth of MC was observed. Of importance, addition of LDL into the medium of MC transfected with apo (a) vector resulted in a significant increase in number of MC compared to control, whereas LDL did not show any effects on MC growth. As active TGF-beta was not detected in the medium of MC, and addition of neutralizing anti-TGF-beta antibody did not alter growth of MC, Lp (a) stimulated growth of MC via the independent mechanisms from the inhibition of TGF-beta activation.

Role of dyslipidemia in the progression of chronic renal disease

The connection between lipids and the rate of progression of chronic renal disease was retrospectively examined in 70 patients who were divided into 2 groups according to their baseline creatinine clearance (CCr): Group 1 (Gp1) contained 30 patients with CCr 60-40 mL/min followed for 40.0 +/- 13.3 months; Group 2 (G2) contained 40 patients with CCr 39-15 mL/min followed for 39.0 +/- 18.2 months. The following parameters were considered: basal and final CCr proteinuria per unit of CCr (UProt/CCr); the difference between final and basal UProt/CCr (delta UProt/CCr); the change in CCr/month (delta CCr); baseline triglycerides (TG), total (TC), HDL (HDLC) and LDL (LDLC) cholesterol, Apo AI, Apo B, Lp(a). Besides in basal CCr the 2 groups significantly differed in the final CCr, final UProt/CCr, delta UProt/CCr, delta CCr. No differences were observed concerning lipid parameters except for Lp(a) (G1 14.8 +/- 13.6, G2 28.7 +/- 27.4 mg/dL; p < 0.05). Baseline TG (G1 184.1 +/- 61.3, G2 187.5 +/- 72.1 mg/dL) and Apo B (only G2 1.05 +/- 0.32 g/L) were significantly higher than normal subjects and the Apo AI/Apo B ratio (G1 1.42 +/- 0.43, G2 1.33 +/- 0.45) were significantly lower than in normal subjects. delta CCr, while inversely correlated in both groups with delta UProt/CCr (p < 0.01), only in G2 did it correlate directly with the Apo AI/Apo B ratio (p < 0.05) and inversely with Apo B and LDLC (p < 0.05). Although a correlation between Lp(a) and delta CCr was not found, 20/22 patients (3/5 G1, 17/17 G2) with a level > 30 mg% ran a progressive course. A natural progression of CRI, heralded by an increasing UProt, is highly frequent when baseline CCr is < 40 mL/min; only then lipids seem to add a burden to the renal damage.

Lp(a) lipoprotein is a major risk factor for cardiovascular disease: pathogenic mechanisms and clinical significance

The results of two previous and two recent studies of middle-aged males and females are presented to exemplify the clinical importance of lipoprotein(a) (Lp(a)) as a risk factor for atherosclerosis and coronary heart disease. In these studies various conventional and recently suggested risk factors were included and different methods for Lp(a) quantification were used. Lp(a) was a significant risk factor in all four studies. In the recent prospective case-control study, Lp(a) and cholesterol were found to act synergistically and predict primary acute myocardial infarction in Swedish males. A cholesterol level above 6.5 mmol/l increased the risk of acute myocardial infarction if the Lp(a) level was above 200 mg/l. The plasma apo A-I level was a protective factor. In the other recent case-control study, an Lp(a) level above 500 mg/l was a highly significant risk factor in Black and White US women with myocardial infarction or advanced coronary artery disease in addition to low density lipoprotein cholesterol levels above 130 mg/dl. A high apo A-I level was a protective factor. In these studies no other factors tested reached significance in multivariate logistic regression analysis. A hypothetical association between high Lp(a) levels and intracellular infection with Chlamydia pneumoniae is discussed. The results suggest that the Lp(a) level is useful in identifying high-risk individuals. Lowering low density lipoprotein cholesterol below 100 mg/dl (<2.6 mmol/l) seems to be most important in both males and females with high-risk Lp(a) levels.

Apolipoprotein B independently predicts progression of very-low-level albuminuria in insulin-dependent diabetes mellitus

The purpose of the study was to examine the contribution of alterations in lipoprotein metabolism to the progression of very-low-level albuminuria in insulin-dependent diabetes mellitus (IDDM). We measured serum concentrations of lipids, lipoproteins, and apolipoproteins in 53 normoalbuminuric diabetic patients without overt hypertension, whom we restudied after 10 years. Albuminuria was measured as the urinary albumin to creatinine ratio (UA/UC) in repeated early-morning samples. Over 10 years, UA/UC increased significantly (P < .001), and five patients (9.4%) progressed to microalbuminuria. The increase in albuminuria was significantly and positively related to the baseline serum concentrations of total cholesterol (P < .05), low-density lipoprotein (LDL) cholesterol (P = .05), non-high-density lipoprotein (HDL) cholesterol (P < .05), and apolipoprotein (apo) B (P < .001), but no significant associations were found with triglycerides, HDL cholesterol, apo A-1, or lipoprotein(a) [Lp(a)]. The relative risk of developing microalbuminuria for a serum apo B concentration more than 1.1 g/L was 3.8 (95% confidence interval [CI], 1.9 to 7.7). In multiple linear regression analysis, serum apo B (P < .05) and glycated hemoglobin ([HbA] P < .05) at baseline were significant independent predictors of the increase in albuminuria, with no significant associations found for sex, smoking, duration of diabetes, mean arterial blood pressure (BP), or family history of cardiovascular disease and hypertension; the regression model predicted 42% of the variation in UA/UC at 10 years. The findings suggest that an abnormality in the metabolism of apo B may be independently associated with progression of very-low-level albuminuria and possibly with the development of early nephropathy in IDDM patients.

The role of proteinuria in the progression of chronic renal failure

The cause of the relentless progression of chronic renal failure of diverse origins remains unknown and is likely to be multifactorial. Numerous studies have now demonstrated a correlation between the degree of proteinuria and the rate progression of renal failure, which has led to the hypothesis that proteinuria may be an independent mediator of progression rather than simply being a marker of glomerular dysfunction. This article reviews the evidence underlying this hypothesis and the mechanisms by which particular proteins may cause renal pathology. The abnormal filtration of proteins across the glomerular basement membrane will bring them into contact with the mesangium and with the tubular cells. There is evidence to support a role of lipoproteins on mesangial cell function, which ultimately could contribute to glomerular sclerosis. The proximal tubular cells reabsorb proteins from the tubular fluid, which leaves them particularly vulnerable to any adverse effects proteins may have. It has been postulated that the sheer amount of protein to be metabolized by these cells may overwhelm the lysosomes and result in leakage of cytotoxic enzymes into the cells. In addition, the increased metabolism of proteins may result in production of ammonia, which can mediate inflammation through activation of complement. Specific proteins that have been shown to be cytotoxic are transferrin/iron, low-density lipoprotein, and complement components, all of which appear in the urine in proteinuric states. Other specific proteins have been shown to stimulate production of cytokines, chemoattractants, and matrix proteins by tubular cells and thus may stimulate interstitial inflammation and scarring. The mechanisms by which the presence of proteins in the tubular fluid alters tubular cell biology is yet to be determined.

Interaction of native and oxidized lipoprotein(a) with human mesangial cells and matrix

The trapping of apolipoprotein(a) and apolipoprotein B-100 in glomeruli of patients with the nephrotic syndrome seems to be linked to a less favorable course of renal disease. To evaluate the potential role of lipoprotein(a) as a mediator of glomerular injury, we measured uptake of native lipoprotein(a) [Lp(a)] and oxidatively modified Lp(a) by cultured human mesangial cells and matrix and studied the effects of Lp(a) on mesangial cell DNA-synthesis and cellular proliferation. Uptake of Lp(a) by mesangial cells occurred at a significantly lower affinity (KD 16 micrograms/ml vs. 39 micrograms/ml) and a lower maximum degradative capacity (6.7-fold) than for LDL. Specificity of receptor mediated uptake was 50% for Lp(a) compared to 84% for LDL. Oxidative modification of both Lp(a) and LDL was accompanied by a significant decrease in uptake and degradative capacities. Due to the limited uptake, native and oxidatively modified Lp(a) had only marginal effects on intracellular cholesterol metabolism, which was measured as inhibition of sterol synthesis and stimulation of cholesterol esterification. However, binding of Lp(a), oxidized Lp(a) and oxidized LDL to extracellular mesangial matrix was enhanced compared to LDL. Furthermore, incubation of mesangial cells with Lp(a) and oxLp(a) in concentrations of 2.5 micrograms/ml and higher resulted in a decrease of DNA synthesis. Regardless of the oxidative status, a maximal suppression of DNA synthesis was observed at 20 micrograms/ml Lp(a). Native Lp(a) also blunted the stimulatory effects of PDGF on mesangial cell DNA-synthesis. Lp(a) did not alter basal TGF-beta transcription in human mesangial cells. The avid interaction of Lp(a) and modified lipoproteins with mesangial matrix provides a concept for the enhanced entrapment of these lipoproteins in the diseased glomerulum. Native Lp(a) is a poor ligand for the LDL receptor; oxidation of Lp(a) even lowers the affinity towards this receptor. Further studies must be carried out to clarify the pathophysiological significance of Lp(a) trapping in the mesangial matrix.

Lipoprotein(a) in renal disease

Lipoprotein(a) [Lp(a)] is a genetically determined risk factor for atherosclerotic vascular disease. Several studies have described a correlation between high Lp(a) plasma levels and coronary heart disease, stroke, and peripheral atherosclerosis. In healthy individuals Lp(a) plasma concentrations are almost exclusively controlled by the apolipoprotein(a) [apo(a)] gene locus on chromosome 6q2.6-q2.7. More than 30 alleles at this highly polymorphic gene locus determine a size polymorphism of apo(a). There exists an inverse correlation between the size (molecular weight) of apo(a) isoforms and Lp(a) plasma concentrations. Average Lp(a) levels are high in individuals with low molecular weight isoforms and low in those with high molecular weight isoforms. Mean Lp(a) plasma levels are elevated over controls in patients with renal disease. Patients with nephrotic syndrome exhibit excessively high Lp(a) plasma concentrations, which can be reduced with antiproteinuric treatment. The mechanism underlying this elevation is unclear, but the general increase in protein synthesis caused by the liver due to high urinary protein loss is a likely explanation. Patients with end-stage renal disease (ESRD) also have elevated Lp(a) levels. These are even higher in patients treated by continuous ambulatory peritoneal dialysis than in those receiving hemodialysis. Lipoprotein(a) concentrations decrease to values observed in controls matched for apo(a) type following renal transplantation. This clearly demonstrates the nongenetic origin of Lp(a) elevation in ESRD. Both the increase in ESRD and the decrease following renal transplantation are apo(a) phenotype dependent. Only patients with high molecular weight phenotypes show the described changes in Lp(a) levels. In patients with low molecular weight types the Lp(a) concentrations remain unchanged during both phases of renal disease. As in the general population, Lp(a) is a risk factor for cardiovascular events in ESRD patients. In this patient group the apo(a) phenotype seems to be equally or better predictive of the degree of atherosclerosis than is Lp(a) concentration. Further prospective studies will be necessary to confirm these observations. Whether Lp(a) also plays a key role in the pathogenesis and progression of renal diseases needs further study. Controversial data on the role of the kidney in Lp(a) metabolism result from insufficient sample sizes of several studies. Due to the broad range and skewed distribution of Lp(a) plasma concentrations, large study groups must be investigated to obtain reliable results.

Impairment of endothelium-dependent dilation in rabbit renal arteries by oxidized lipoprotein(a). Role of oxygen-derived radicals

BACKGROUND

Hyperlipoproteinemia is associated with impairment of nitric oxide (NO)-mediated, endothelium-dependent dilation in renal arteries. In the present study, we assessed and compared the effects of human lipoprotein(a) and LDL on endothelium-dependent and -independent dilation in vitro.

METHODS AND RESULTS

Dilator responses were detected in isolated, saline-perfused, preconstricted arterial segments by a photoelectric device. Acetylcholine-induced, endothelium-dependent dilator responses of rabbit renal arteries were not significantly attenuated after 150 minutes of incubation with native lipoprotein(a) (30 and 100 micrograms/mL). However, exposure to in vitro oxidized lipoprotein(a) (150 minutes, 30 and 100 micrograms/mL) suppressed acetylcholine-induced dilator responses in a dose-dependent manner. At similar concentrations, native and oxidized LDL had no effect. Endothelium-independent dilations induced by the NO-donor sodium nitroprusside were also impaired by oxidized lipoprotein(a), whereas forskolin-induced dilator responses were unaffected, indicating that smooth muscle dilator capacity was not impaired. Attenuation of dilator responses by oxidized lipoprotein(a) was potentiated in the presence of superoxide dismutase (SOD). The SOD effect was completely blunted by coincubation with catalase (100 U/mL) or deferoxamine. In the absence of SOD, catalase or deferoxamine had no effect on dilator responses. Using a chemiluminescence assay, we could detect increased O2- production by arteries pretreated with oxidized lipoprotein(a), which suggested that enhanced NO inactivation by O2- could be the underlying mechanism for impairment of endothelium-dependent dilations.

CONCLUSIONS

These data indicate that oxidized lipoprotein(a) impairs endothelium-dependent dilation and is more potent than oxidized LDL in this effect. The mechanism of the impairment may involve formation of O2- and inactivation of NO.

Selected aspects of ACE inhibitor therapy for patients with renal disease: impact on proteinuria, lipids and potassium

Overt proteinuria is often accompanied by hypercholesterolemia and is associated with increased lipoprotein(a) levels. These lipid abnormalities are probably involved in the high incidence of macrovascular complications associated with diabetic nephropathy and possibly other kinds of non-diabetic proteinuric renal disease. Over the last decade many studies have shown that ACE inhibitors can reduce urinary protein excretion but little attention was paid to the impact of this form of therapeutic intervention on the lipid profile. In this article we review our recent data showing that fosinopril administration was associated with significant decreases in both urinary protein excretion, serum total cholesterol levels, and plasma lp(a) levels. The use of ACE inhibitors in patients with renal impairment can result in the development of hyperkalemia as a result of suppression of angiotensin II-driven aldosterone secretion by the adrenal gland. Inhibition of aldosterone secretion may depend on the degree of inhibition of angiotensin II formation in the circulation and also locally in the adrenal gland. Because the various ACE inhibitors exhibit different degrees of ACE inhibition at the tissue level, we have postulated that angiotensin II-dependent aldosterone production will be inhibited to a lesser degree by agents that have low tissue affinity for the adrenal gland. The implication of this theoretical concept for the development of hyperkalemia in patients with impaired renal function treated with ACE inhibitors is discussed.

Oxidized LDL and lipoprotein(a) stimulate renin release of juxtaglomerular cells

Atherogenic lipoproteins accumulate in the arterial wall and may potentially stimulate neighboring cells. In the glomerulus the vascular pole resembles afferent arteries in close vicinity to the juxtaglomerular apparatus. We examined the effects of native and oxidized LDL and lipoprotein(a) [Lp(a)] on renin release of juxtaglomerular cells (JG cells) prepared in primary culture from mouse kidneys. Renin activity of JG cells was measured in culture supernatants and cells between the 20th and 40th hour of culturing. Spontaneous renin release into the cell supernatant was 26 +/- 1% of total activity. Control stimulation of JG cells by melittin or forskolin dose-dependently increased renin release up to 90 +/- 2%. Incubation of JG cells with native LDL (50 and 300 micrograms/ml) or native Lp(a) (30 micrograms/ml) did not alter renin release. Oxidized LDL increased renin release to 34 +/- 1% and 43 +/- 1% at 50 and 300 micrograms/ml, while oxidized Lp(a) stimulated renin release to 33 +/- 1%, 42 +/- 1%, and 71 +/- 2% at 1, 10, and 30 micrograms/ml, respectively. Coincubation with superoxide dismutase and catalase, enzymes removing O2- and H2O2, completely eliminated oxidized LDL and Lp(a)-stimulated renin release. In the absence of lipoproteins, renin release was significantly stimulated by activation of O2- formation by the xanthine/xanthine oxidase reaction. These data indicate that oxidized LDL and Lp(a) stimulate renin release in JG cells by a mechanism involving oxygen-derived radicals. Thus, oxidatively modified atherogenic lipoproteins may contribute to renin-dependent hypertension in renoparenchymatous kidney disease.

Oxidized lipoprotein(a) inhibits endothelium-dependent dilation: prevention by high density lipoprotein

We assessed the effects of human native and oxidized lipoprotein(a) (150 min, 30 and 100 micrograms/ml) on endothelium-dependent vasodilation of isolated rabbit renal arteries. Vasodilation was not attenuated after incubation of arteries with native lipoprotein(a). However, when the arteries were exposed to oxidized lipoprotein(a), acetylcholine-induced vasodilation was dose dependently significantly impaired. Concomitant incubation of segments with high density lipoprotein (HDL, 0.5 mg/ml) prevented the attenuation of dilations induced by oxidized lipoprotein(a). Thus, we report for the first time that oxidized lipoprotein(a) impairs endothelium-dependent vasodilation, and that HDL prevents its inhibitory effect.

The role of lipids in nephrosclerosis and glomerulosclerosis

Hyperlipidemia and lipoprotein abnormalities are often encountered in patients with nephrotic syndrome or chronic renal disease and also in those undergoing haemodialysis and with renal transplant. Even though the significance of lipid deposition in renal tissue and the role of lipoproteins in the pathogenesis of renal disease in man is unclear, experimental and clinical data indicate a possible damaging effect of a disturbed lipid metabolism on the kidney. In humans, glomerular lipid deposition is observed in genetic diseases such as Fabry's disease, lecithin:cholesterol acyltransferase activity (LCAT) deficiency and arteriohepatic dysplasia, and in diseases with acquired disturbance of lipid metabolism such as nephrotic syndrome and cholestatic liver disease. Studies on animals with lupus nephritis, aminonucleoside nephrosis, reduced renal mass, diabetes mellitus or systemic hypertension have shown that cholesterol can increase the incidence of glomerulosclerosis. As most of these studies have been performed in the rat, which has a different lipoprotein profile to that of man, these results should be carefully interpreted with regard to their relevance for humans. In vitro cell culture studies on human glomerular cells have given some preliminary insights into the cellular mechanisms of lipid induced glomerular damage. Apo E-containing lipoproteins, which are pathologically elevated in many renal diseases, are avidly taken up by human mesangial cells. These cells seem to play a central role in the initiation of glomerulosclerosis by inducing proliferation and production of excess extracellular matrix. Lipoproteins are able to stimulate DNA synthesis in these cells, and increase the synthesis of mitogens and extracellular matrix protein. The pathogenic role of oxidized lipoproteins has not yet been defined. Human mesangial cells do not seem to take up these modified lipoproteins. However, macrophages infiltrate glomeruli and may constitute the stimulus for the generation of minimally modified lipoproteins and their cellular uptake. The data from animal experiments suggest that treatment that corrects hyperlipidemia may have an ameliorative effect on renal function. Thus, there are strong indications that lipoproteins may play a critical role in mediating the development of glomerulosclerosis.

Lipoprotein(a) in nephrotic syndrome

Lipoprotein(a) [LP(a)] is an independent risk factor for cardiovascular disease, and it has also been speculated that it promotes thrombosis. Recent studies have shown that patients with gross proteinuria have greatly increased plasma levels of Lp(a), but the genesis is obscure. In the present study, plasma Lp(a) levels were measured in 31 patients with nephrotic syndrome (NS), 24 patients with IgA nephropathy and 43 healthy control subjects. Lp(a) levels were significantly elevated in NS (median 49.0 mg/dl), in contrast to the control subjects and patients with IgA nephropathy (median 7.0 and 9.7 mg/dl, respectively). Plasma Lp(a) levels fell markedly in 10 of 10 NS patients after remission. In NS, Lp(a) levels correlated directly with serum cholesterol levels (P < 0.05) and indirectly with plasma orosomucoid levels (P < 0.05), but not with serum albumin, triglycerides, HDL cholesterol, urinary protein excretion or GFR. In addition, Lp(a) tended to be higher in NS patients with edema (median 54.3 mg/dl) than in patients without edema (19.0 mg/dl; P = 0.06). Nine NS patients were further evaluated with plasma ANP levels and urinary sodium excretion. Plasma Lp(a) correlated directly with ANP (P < 0.01) and indirectly with urinary sodium excretion (P < 0.05). Excellent correlations were found between Lp(a) and VLDL cholesterol and VLDL triglycerides, respectively, suggesting a close link between Lp(a) and triglyceride-rich lipoproteins in nephrosis.