CD36 and lipid metabolism in the evolution of atherosclerosis

BACKGROUND

CD36 is a multi-functional class B scavenger receptor, which acts as an important modulator of lipid homeostasis and immune responses.

SOURCES OF DATA

This review uses academic articles.

AREAS OF AGREEMENT

CD36 is closely related to the development and progression of atherosclerosis.

AREAS OF CONTROVERSY

Both persistent up-regulation of CD36 and deficiency of CD36 increase the risk for atherosclerosis. Abnormally up-regulated CD36 promotes inflammation, foam cell formation, endothelial apoptosis, macrophage trapping and thrombosis. However, CD36 deficiency also causes dyslipidemia, subclinical inflammation and metabolic disorders, which are established risk factors for atherosclerosis.

GROWING POINTS

There may be an 'optimal protective window' of CD36 expression.

AREAS TIMELY FOR DEVELOPING RESEARCH

In addition to traditionally modulating protein functions using gene overexpression or deficiency, the modulation of CD36 function at post-translational levels has recently been suggested to be a potential therapeutic strategy.

Determination of Plasma Calcium Fractions

A simple, precise, and sensitive method is described for the determination of ionised calcium in plasma at normal body temperature and pH, using an ultrafiltrate of the plasma. This technique is based on a two-wave-length (480 nm, 550 nm) spectrophotometric method, using pH independent dye ammonium tetramethylmurexide (ATM). By this method, the normal (mean + 2 S.D.) plasma ionised calcium in 19 normal subjects is 1.04 + 0.14 mmol/l which is similar to some of those reported for ion-selective electrode measurements.

The PPARα ligand fenofibrate: meeting multiple targets in diabetic nephropathy

Fibrate peroxisome proliferator-activated receptor (PPAR)-alpha ligands are mainly used as hypolipidemic drugs. But this commentary highlights their potential in treating insulin resistance, dyslipidemia, and hypertension and in preventing diabetic nephropathy, inflammation, and cardiovascular disease. Because diabetes is a major contributor to chronic kidney disease and cardiovascular disease, PPAR-alpha agonists may provide greater opportunities for hitting multiple targets in this complex metabolic disease.

Comparison of biochemical with radiological findings in renal osteodystrophy

In a group of 121 adult patients with end-stage chronic renal failure who had been undergoing maintenance haemodialysis for up to 10 years a number of biochemical variables have been measured and related to a set of objective radiological changes in the same patients. The changes in plasma calcium, magnesium, phosphate, total protein, and albumin concentration did not distinguish the patients who were grouped on a radiological basis. Plasma alkaline phosphatase activity increased with the severity of the radiological findings but did not provide a sensitive discriminatory index between the different radiological groups. Plasma hydroxyproline concentration was found to be more sensitive than plasma alkaline phosphatase activity in detecting a radiological abnormality in some of the patients.

Nitric oxide production by human peritoneal mesothelial cells

Nitric oxide (NO) has multiple actions, ranging from immunomodulation to regulation of vascular tone and capillary flow. Thus NO generation within the peritoneum could potentially affect peritoneal transport by increasing capillary vasodilatation, and regulate the response to bacterial invasion. Peritoneal mesothelial cells have a common embryological derivation with endothelial cells. As mesothelial cells are the predominant cell type lining the peritoneal cavity, they could potentially be a major source of locally produced nitric oxide. Nitric oxide was measured using the Griess reaction, as total nitrite and nitrate, in fresh unused and spent dialysate effluent (SPDE) from both healthy peritoneal dialysis patients, and during episodes of bacterial peritonitis. Whereas fresh CAPD dialysate was nitrite free (5 +/- 0.1 microM), SPDE from a standard 4 h day time exchange contained 10.2 +/- 0.6 microM/L/h, and that from the overnight dwell 9.1 +/- 0.7 microM/L/h. During an episode of peritonitis, dialysate nitrite and nitrate increased significantly from 9.0 +/- 1.0 microM/L/h, when not infected to 17.5 +/- 2.4, from the first CAPD bag drained at presentation, and 15.2 +/- 1.8 for the second and 16.0 +/- 2.5 for the third exchange (p<0.01). By the following day nitrite levels had returned to baseline, 7.0 +/- 1.0 microM/L/h. Human peritoneal mesothelial cells (HPMC) were cultured and found to produce nitric oxide (261 nmol/mg cell protein), which increased in a dose dependent manner with the addition of spent uninfected CAPD dialysate. The addition of L-arginine, a NO substrate resulted in a 10% increase in nitric oxide production, whereas the addition of the blocker L-NMMA produced a 10% reduction. RNA for inducible nitric oxide synthase (iNOS) was sought using northern blotting technique following combination stimulation with lipopolysaccharide and cytokines (IL-1beta, TNFalpha and gamma-INF, and/or spent dialysate from patients with bacterial peritonitis). However, we could not demonstrate RNA production for iNOS. Peritoneal mesothelial cells may be an important source of locally generated nitric oxide within the peritoneal cavity under basal conditions, but as they do not contain iNOS, the markedly increased NO production observed with episodes of acute bacterial peritonitis is more likely due to a combination of increased NO production by peritoneal macrophages and endothelial cells.

Renovascular disease is associated with low producer genotypes of the anti-inflammatory cytokine interleukin-10

Cytokines are important mediators of inflammatory and proliferative responses in disease states including atherosclerosis. Genetic variations in cytokine production could potentially influence the outcome of these responses. The aim of this study was to determine whether cytokine gene polymorphism might influence the development of atherosclerotic renal artery stenosis. Sixty-six patients with atherosclerotic renal artery stenosis and 100 normal healthy individuals were genotyped for interleukin-10 (IL-10), tumor necrosis factor-alpha (TNF-alpha), IL-6, and IL-2 promoter region polymorphism. TNF-a, TNF-d, and IL-10 microsatellite polymorphisms were also analyzed. The frequency of the anti-inflammatory cytokine IL-10 promoter (-1082 A positive) GA and AA genotypes which are associated with low production were higher in the patient group when compared to the control group. The AA-TT-AA homozygous genotype combination of three single-nucleotide polymorphisms at -1082, -819, and -592 in the IL-10 gene was also observed at a higher frequency in the patient group compared to the controls. The frequency of TNF-alpha, IL-6, and IL-2 polymorphisms did not show any significant difference between the patient and control groups. To correlate IL-10 genotypes with differences in IL-10 protein expression, in vitro mRNA and protein levels were analyzed in lipopolysaccharide-stimulated peripheral blood mononuclear cells from 22 patients with renal artery stenosis and 33 controls. Individuals genotyped as A positive at position -1082 produced lower levels of IL-10 protein and had lower copy numbers of mRNA when compared to individuals genotyped as A negative in both patient and control groups. The increased frequency of the low producer IL-10 promoter, -1082 A-positive genotype in patients with renal artery stenosis, suggests that IL-10 may protect against the development of atherosclerotic renovascular disease.

Regulation of lipoprotein trafficking in the kidney: role of inflammatory mediators and transcription factors

Inflammation and dyslipidaemia both play important roles in the development of glomerular atherosclerosis in renal diseases. We have demonstrated that inflammatory mediators induced Scr (scavenger receptor) expression and the formation of foam cells, and that AP-1 (activator protein 1)/ets were necessary transcriptional factors for Scr induction in HMCs (human kidney mesangial cells). Most cells are protected from excessive native LDL (low-density lipoprotein) accumulation by tight feedback regulation of the LDLr (LDL receptor). However, we observed that HMCs formed foam cells via the LDLr pathway when incubated with IL-1β (interleukin-1β; 5 ng/ml) and unmodified LDL (200 μg/ml), suggesting that inflammatory mediators may disrupt the cholesterol-mediated feedback regulation. This feedback involves cholesterol-mediated down-regulation of LDLr controlled by SCAP [SREBP (sterol responsive element-binding protein) cleavage-activating protein]. We have also demonstrated that both tumour necrosis factor α and IL-1β increased nuclear SREBP-1 levels by increasing SCAP mRNA expression, even in the presence of a high concentration of LDL. Since intracellular lipid content is governed by both influx and efflux mechanisms, we set out to examine the impact of inflammatory cytokines on cholesterol efflux, a process mediated by the protein ABCA1 (ATP binding cassette A1). IL-1β inhibited [3H]cholesterol efflux from HMCs by inhibition of the peroxisome-proliferator-activated receptor/LXR (liver X receptor)/ABCA1 pathway. Taken together, our results suggest that inflammatory mediators increase lipid accumulation in HMCs not only by promoting increased lipoprotein uptake by Scr and LDLr, but also by inhibiting ABCA1-mediated cholesterol efflux to high-density lipoprotein.

Does oral folic acid lower total homocysteine levels and improve endothelial function in children with chronic renal failure?

BACKGROUND

Accelerated vascular disease is common in chronic renal failure (CRF) and accounts for significant mortality and morbidity. Elevated homocysteine levels may contribute by an effect on endothelial function.

METHODS AND RESULTS

We performed a double-blind placebo-controlled randomized crossover trial of folic acid at 5 mg/m2 in 25 normotensive children 12+/-3 (7 to 17) years of age with CRF (glomerular filtration rate 26.8+/-13.2 mL/min per 1.73 m2) of noninflammatory etiology. Each subject underwent two 8-week periods of folic acid and placebo separated by an 8-week washout period. The effect of folic acid on homocysteine levels, LDL oxidation, and both endothelial-dependent and -independent vascular function were measured. After oral folic acid, serum folate levels rose from 11.7+/-4.25 to 635+/-519 microg/L (P=0.001), red cell folate levels rose from 364+/-195 to 2891+/-2623 microg/L (P<0.001), and total homocysteine levels fell from 10.28+/-4.16 to 8.62+/-2.32 micromol/L (P=0.03). In addition, there was a significant improvement in flow-mediated dilatation (FMD) (endothelial-dependent dilatation) from 7.21+/-2.8% to 8.47+/-3.01% (P=0.036) with no change in response to glyceryl trinitrate (endothelial-independent dilatation). There was no significant change in FMD or glyceryl trinitrate during the placebo phase. There was, however, no significant difference in final FMD after placebo or folic acid. Lag times for LDL oxidation were prolonged during the treatment phase (58.4+/-18.7 to 68.1+/-25.9 minutes, P=0.01).

CONCLUSION

Folic acid supplementation in children with CRF may improve endothelial function with an increased resistance of LDL to oxidation.

Dysregulation of LDL receptor under the influence of inflammatory cytokines: a new pathway for foam cell formation

BACKGROUND

Lipid-mediated renal injury is an important component of glomerulosclerosis and its similarity to atherosclerosis is well described. This study focused on the relationship between lipid-mediated injury and inflammation by examining the role of inflammatory cytokines in the regulation of human mesangial cell low-density lipoprotein (LDL) receptors.

METHODS

A human mesangial cell line (HMCL) was used to study the effects of tumor necrosis factor-alpha (TNF-alpha) and interleukin-1beta (IL-1beta) on the regulation of LDL receptor mRNA and protein in the presence of a high concentration of native LDL (250 microg/mL).

RESULTS

Native LDL caused foam cell formation in HMCL in the presence of antioxidants, TNF-alpha and IL-1beta. Both cytokines overrode LDL receptor suppression induced by a high concentration of LDL and increased LDL uptake by enhancing receptor expression. These cytokines also caused increased expression of SCAP [sterol responsive element binding protein (SREBP) cleavage activation protein], and an increase in the nuclear translocation of SREBP, which induces LDL receptor expression.

CONCLUSION

These observations demonstrate that inflammatory cytokines can modify cholesterol-mediated LDL receptor regulation in mesangial cells, permitting unregulated intracellular accumulation of unmodified LDL and causing foam cell formation. These findings suggest that inflammatory cytokines contribute to lipid-mediated renal damage, and also may have wider implications for the study of inflammation in the atherosclerotic process.

Paradoxical increase in nitric oxide synthase activity in hypercholesterolaemic rats with impaired renal function and decreased activity of nitric oxide

BACKGROUND

We have shown that acute exposure of oxidized low-density lipoprotein (OX-LDL) induces vasoconstriction in renal vessels and reduces glomerular filtration rate (GFR) in an isolated perfused rat kidney model by decreasing the activity of nitric oxide (NO). L-arginine has a protective role against OX-LDl-induced vasoconstriction. Micropuncture studies have demonstrated that short-term diet-induced hypercholesterolaemia is associated with decreased GFR and renal blood flow and increased glomerular capillary pressure. This may be mediated by decreased activity of NO.

METHODS

Rats were made hypercholesterolaemic by supplementing the standard chow with 4% cholesterol and 1% sodium cholate. A group of rats on hypercholesterolaemic diet also received L-arginine in the drinking water. After 4 and 6 weeks, blood samples and 24-h urine samples were collected for the measurement of biochemical parameters. After 6 weeks, all rats were subjected to isolated perfusion of kidneys at a constant pressure of 100 mmHg. During isolated perfusion, the unused contralateral kidney was taken for morphological studies and for assessing the activity of nitric oxide synthase enzyme by beta-NADPH diaphorase histochemistry.

RESULTS

Rats fed a high-cholesterol diet had LDL levels 3-6 times greater than the rats fed standard chow. Rats that received L-arginine in the drinking water had serum L-arginine levels 5-6 times greater than control rats. At 6 weeks, creatinine clearance was significantly lower in the rats on the high-cholesterol diet compared to the rats on standard chow and rats on high-cholesterol diet plus L-arginine. Twenty-four-hour urinary total nitrate and nitrite excretion in the hypercholesterolaemic rats was 1.5-2 times greater than that of control rats. Twenty-four-hour urinary cGMP excretion was significantly lower in the rats on a high-cholesterol diet, but in the rats on high-cholesterol diet and L-arginine, 24-h urinary cGMP excretion was not significantly different from that of control rats. During isolated perfusion of kidneys, renal perfusate flow was found to be significantly reduced in the kidneys taken from the rats on a high-fat diet compared to controls. L-arginine supplementation in the drinking water almost completely reversed the effect of a high-fat diet. Inulin clearance was also significantly reduced in kidneys on a high-fat diet in contrast to controls but not in kidneys on high fat-diet and L-arginine. Basal cGMP excretion in urine was significantly lower in the kidneys taken from the rats on a high-fat diet compared to controls. L-arginine supplementation restored the basal cGMP excretion in these kidneys. NO synthase (NOS) enzyme activity as assessed by NADPH diaphorase activity showed that kidney sections taken from the rats on a high-fat diet showed more intense staining, indicating increased activity compared to the kidney sections taken from the rats on a normal diet.

CONCLUSION

Though activity of NO is diminished in hypercholesterolaemic rats with impaired renal function, there is a paradoxical increase in NO production and NOS activity. L-arginine reverses the effects of a high-fat diet.

Human mesangial cells express inducible macrophage scavenger receptor

BACKGROUND

Type A scavenger receptors (Scr) mediate the uptake of modified low-density lipoproteins by macrophages. The accumulation of lipids via this process is thought to lead to foam cell formation in atherosclerotic plaques. Human mesangial cells (HMCs) have not been previously shown to express Scr in normal culture. We therefore investigated whether there is an inducible form of Scr in a human mesangial cell line (HMCL).

METHODS

Scr activity was analyzed by cellular uptake of fluorescently labeled acetylated low-density lipoprotein using a flow cytometer. Scr mRNA expression was examined using reverse transcription-polymerase chain reaction, followed by Southern blotting. To investigate the molecular mechanism of Scr expression, several reporter gene constructs were designed. The first contained a full Scr promoter, the second a part of the Scr promoter that has both AP-1 and ets transcription factor binding sites. Other constructs were identical to the second, except that they contained either AP-1 or ets motif mutations.

RESULTS

Phorbol 12-Myristate 13-acetate (PMA) and angiotensin II (Ang II) increased both the percentage of Scr-positive cells and the Scr mean fluorescence intensity. PMA and Ang II also increased Scr mRNA and promoter activity in a time- and dose-responsive manner. Protein kinase C and calmodulin transduction pathways were involved in Scr up-regulation induced by PMA and Ang II. Additionally, a serine/threonine kinase was involved in PMA stimulation. Functional analysis showed that both AP-1 and ets motifs were specific response elements to PMA stimulation in HMCLs.

CONCLUSIONS

This study suggests that HMCs may express an inducible Scr, by which cells can acquire lipids and convert to foam cells in developing glomerulosclerosis.

Human mesangial cells express inducible macrophage scavenger receptor: an Ap-1 and ets mediated response

BACKGROUND

Type A scavenger (SR) mediate the uptake of modified low-density lipoproteins by macrophages. The accumulation of lipid via this process is thought to lead to foam cell formation in atherosclerotic plaques. Human mesangial cells (HMC), which can be converted to foam cells in vivo, have not previously been shown to express SR in normal culture. We investigated whether or not there was an inducible form of SR in a human mesangial cell line (HMCL).

METHODS

SR activity was analyzed by cellular uptake of fluorescently labeled acetylated low-density lipoprotein using a flow cytometer. SR mRNA expression was examined using RT-PCR followed by Southern blotting. To investigate the molecular mechanism of SR expression, several reporter gene constructs were designed. The first contained a full SR promoter, the second a part of the SR promoter that has both activated protein-1 (AP-1) and ets transcriptional factor binding sites. Other constructs were identical to the second except they contained either AP-1 or ets motif mutations.

RESULTS

Phorbol 12-myristate 13-acetate (PMA) increased both the percentage of SR positive cells and SR mean fluorescence intensity. PMA also increased SR mRNA and promoter activity in a time and dose responsive manner. Function analysis showed that both AP-1 and ets motifs were specific response elements to PMA stimulation in HMCL.

CONCLUSIONS

The present study suggests that the combination of interaction between AP-1 and ets transcriptional factors may mediate the inducible expression of the SR gene in HMCL, which may contribute to foam cell formation.

Lipopheresis in the nephrotic syndrome

BACKGROUND

Experimental models have established a role for lipoproteins in the pathogenesis of progressive renal failure. However, conventional treatment rarely normalizes the high serum cholesterol of the nephrotic syndrome. The removal of low-density lipoprotein by lipopheresis is discussed.

METHODS

Lipopheresis may be beneficial in nephrotic patients with focal segmental glomerulosclerosis. The authors studied the long-term effects of low-density lipoprotein cholesterol (LDL-C) removal using the Kaneka Liposorber system, which binds LDL-C to dextran sulfate in a controlled trial in 20 nephrotic patients with different renal diseases.

RESULTS

A 21-month clamp of plasma total cholesterol at 6.0 mmol/liter or below was significantly lower than controls (chi 2 = 84.3, P < 0.001), followed 12 aphereses over 6 to 12 weeks in all but three apheresed patients. 1/Cr slopes were unchanged when the 50-day average period of lipopheresis treatments was excluded from analysis. Proteinuria was not reduced, but serum albumin tended to rise (NS). Fibrinogen fell by 29.8%; high-density lipoprotein, apoA1, and Lp(a) were unchanged. Two apheresed patients had a prolonged remission with a reduction of proteinuria to less than 250 mg/24 hr. The reasons for prolonged reduction of total cholesterol include depletion of tissue cholesterol, an improved fractional catabolic rate of very low density lipoprotein (VLDL), increased hepatocyte LDL turnover, and the maintenance of statin therapy.

CONCLUSION

Lipopheresis is a safe and effective method for the control of LDL in nephrotic syndrome. Early clamping of total cholesterol in the normal range resulted in a prolonged and significant reduction of LDL compared with controls.

Calcineurin inhibitors enhance low-density lipoprotein oxidation in transplant patients

BACKGROUND

Our objective was to assess the pro-oxidant status of neoral and tacrolimus in renal transplant patients and monitor the protection provided by vitamin C and vitamin E in normalizing low density lipoprotein (LDL) oxidation lag time of tacrolimus-treated patients.

METHODS

Plasma LDL was isolated by density gradient ultracentrifugation from renal transplant patients receiving neoral, tacrolimus and tacrolimus with vitamin C and vitamin E. Oxidation was initiated by the addition of CuCl2 at 37 degrees C and monitored at 234 nm over 480 minutes and oxidation lag time was computed. Total antioxidant capacity of serum was measured using the enhanced chemiluminescent method.

RESULTS

LDL from tacrolimus-treated patients had significantly lower oxidation lag time and serum antioxidant activity in comparison with neoral-treated patients, and this was particularly significant during the first four months after transplantation. Vitamin C and E supplementation in tacrolimus treated patients provided protection against oxidation and normalized their oxidation lag time.

CONCLUSION

Calcineurin-inhibiting drugs, CsA and tacrolimus, have pro-oxidant activity and they increase the susceptibility of LDL to oxidation. Neoral formulation is fortified with DL-alpha tocopherol and therefore provides protection against oxidation. The present study clearly demonstrates the benefit of giving vitamin C and E supplements to patients taking tacrolimus and this seems to be particularly important during the early period after transplantation.

Lysophosphatidylcholine induces platelet-derived growth factor gene expression in a human mesangial cell line

BACKGROUND

Oxidized low-density lipoprotein (oxLDL) has been considered important in the pathogenesis of progressive renal injury. Lysophosphatidylcholine (lysoPC) is a major phospholipid component of oxLDL. On the other hand, platelet-derived growth factor (PDGF) has also been implicated in proliferative disease of the kidney. This study investigated the difference in the potential of PC and lysoPC to induce DNA synthesis and PDGF gene expression in a human glomerular mesangial cell line (HMCL).

METHODS

DNA synthesis in HMCL was measured by [3H] thymidine incorporation. The mRNA expression levels of the PDGF A chain and B chain genes were measured using reverse transcription-polymerase chain reaction.

RESULTS

LysoPC treatment up-regulated the [3H] thymidine incorporation level in a dose-dependent fashion. The [3H] thymidine incorporation level in HMCL coincubated with lysoPC started to increase after 4 hours of treatment, peaked at 24 hours, and decreased thereafter. The level in HMCL incubated with 100 microM of lysoPC (palmitoyl or stearoyl) increased to 7- or 10-fold of the control at peak time, respectively. However, PC treatment did not increase [3H] thymidine incorporation in HMCL. PC treatment did not induce mRNA expression of either PDGF A or B chain genes. LysoPC did not induce PDGF A chain mRNA expression either. The only B chain mRNA expression was induced by lysoPC. The mRNA expression level in HMCL treated with 50 microM lysoPC for two hours increased to 1.6-fold that of the control.

CONCLUSION

LysoPC may induce DNA synthesis in a mesangial cell through the induction of PDGF BB as an autocrine and paracrine growth factor.

LDL receptor gene expression in human mesangial cells under the influence of calcium channel blockers

BACKGROUND

Intracellular transport of lipid through the regulation of LDL receptor (LDLr) may be important in the progression of renal dysfunction.

METHODS

We explored LDLr gene expression in human mesangial cell line (HMCL) under influence of calcium channel blockers using cell proliferation, LDL binding, Northern blot and LDLr promoter activity assay.

RESULTS

Diltiazem and verapamil increased the expression of LDLr mRNA in a dose-dependent manner. Increased LDLr mRNA paralleled LDL binding. Nifedipine did not increase the expression of LDLr mRNA and LDL binding to HMCL at 1 - 100 micromol/l. The LDLr promoter activity assay showed that treatment with 100 micromol/ of diltiazem and verapamil increased LDLr promoter activity by 126.72 +/- 10.68%, and 166.41 +/- 11.41%, respectively, at 24 hours (control as 100%), while treatment with 100 micromol/l of nifedipine had an inhibitory effect on LDLr promoter activity. High concentration of LDL (250 microg/ml) inhibited promoter activity. Diltiazem or verapamil coincubated with LDL (250 microg/ml) could not override transcriptional inhibition by LDL. CCBs inhibited the proliferation of HMCL, therefore, CCBs-induced LDLr expression did not depend on a proliferative response. Signal transduction pathway experiments showed that the calmodulin transduction pathway was involved in LDLr upregulation induced by diltiazem or verapamil. Additionally, tyrosine kinase and PKC pathways were involved in the induction of LDLr induced by verapamil.

CONCLUSION

These studies show that diltiazem and verapamil increase LDLr gene transcription and expression which is independent of cell proliferation in HMCL.

Renal vasoconstriction induced by oxidized LDL is inhibited by scavengers of reactive oxygen species and L-arginine

BACKGROUND

Low density lipoprotein (LDL) may be involved in the pathogenesis of glomerulosclerosis and progressive renal dysfunction associated with atherosclerotic renal artery stenosis (RAS). This study was undertaken to investigate the effects of native (n-LDL) and oxidized LDL (ox-LDL) on renal vascular response and function in an isolated perfused rat kidney (IPRK) model.

MATERIAL AND METHOD

IPRK model was used for the study at a constant pressure of 100 mm of Hg in the renal artery with continuous monitoring of pressure and renal perfusate flow. Urine and perfusate samples were collected to determine [14C] Inulin clearance and fractional reabsorption of sodium. To elucidate the role of nitric oxide (NO) urinary c-GMP, nitrate and nitrite excretion were measured and the responses to the NO synthase inhibitor N-monomethyl-L-arginine (LNMMA) and the NO donor Nitroso-glutathione (GSNO) were assessed. The effect of L-arginine supplementation and the role of reactive oxygen species were also studied by adding superoxide dismutase (SOD) and catalase.

RESULTS

Ox-LDL but not n-LDL caused vasoconstriction in IPRK, as evidenced by a significant dose dependent reduction in renal perfusate flow. [14C] Inulin clearance and fractional reabsorption of sodium were reduced during ox-LDL infusion whereas no significant change occured with n-LDL. There was a significant decrease in urinary excretion of c-GMP during ox-LDL infusion. 10 microM LNMMA significantly increased and GSNO (10 microM) significantly diminished the vasoconstrictory effect of ox-LDL. The presence of L-arginine (100 & 500 microM) significantly decreased ox-LDL induced vasoconstriction. SOD (150 U/ml) and catalase (1200 U/ml) both had a significant inhibitory effect and the combination of SOD and catalase almost completely abolished the vasoconstriction due to ox-LDL.

CONCLUSION

These results suggest that ox-LDL induced vasoconstriction in IPRK is mediated by decreased activity of NO probably due to inactivation of NO by reactive oxygen species. The free radical scavengers SOD, catalase and L-arginine provided protection against ox-LDL induced vasoconstriction in this model.

Cytokine regulation of low-density lipoprotein receptor gene transcription in human mesangial cells

BACKGROUND

The intracellular transport of lipids through regulation of the LDL receptor (LDLr) may be important in the progression of renal dysfunction. The present study was undertaken to investigate whether cytokines have any major effects on LDLr regulation and lipid-mediated glomerular injury in human mesangial cells (HMC).

METHODS

We explored the effects of 50 ng/ml of tumour necrosis factor alpha (TNF alpha), 5 ng/ml of transforming growth factor beta (TGF beta), platelet-derived growth factor (PDGF), and interleukin-1beta (IL-1beta) on the regulation of LDLr gene transcription in a human mesangial cell line (HMCL) using cell proliferation, LDL binding, northern blot and LDLr promoter activity assays.

RESULTS

TNF alpha, TGF beta, PDGF or IL-1beta did not significantly stimulate HMCL proliferation at the concentrations given above, but maximally stimulated LDLr mRNA expression and increased LDLr promoter activity by 167.48+/-23.56%, 150.47+/-24.41%, 127.71+/-24.65% and 163.01+/-31.91% respectively, at 24 h. An increased LDL binding was observed in parallel with increased LDLr mRNA. The tyrosine kinase transduction pathway was involved in LDLr upregulation induced by all four cytokines. Additionally, TGF beta involved serine/threonine kinase and G-protein pathways, and IL-1beta involved calmodulin, serine/threonine kinase and PKC pathways in upregulating LDLr. A high concentration of LDL (250 microg/ml) inhibited promoter activity, but TNF alpha, TGF beta, PDGF and IL-1beta co-incubated with LDL could override transcriptional inhibition by LDL.

CONCLUSION

TNF alpha, TGF beta, PDGF and IL-1beta increased LDLr gene expression by increasing sterol-independent and mitogenesis-independent gene transcription. This process may contribute to lipid deposition and foam cell formation in HMC.

Differential ability of cells to promote oxidation of low density lipoproteins in vitro

Atherosclerosis and focal segmental glomerulosclerosis share some common histological features and it is speculated that they result from similar pathobiological mechanisms. There is strong evidence that oxidation of low density lipoprotein (LDL) may be an initiating event in atherogenesis and that oxidised LDL may also be involved in the glomerulosclerotic process. In vitro studies have demonstrated that cells present in the arterial intima and kidney-derived cells promote LDL oxidation. The aim of this study was to compare LDL oxidation by kidney-derived human mesangial cells and proximal tubular cells, with human umbilical vein endothelial cells and the human monocyte cell line THP-1. We used the thiobarbituric acid assay and agarose gel electrophoresis to measure the extent of LDL oxidation. Our results demonstrate that all of the cell types used had the ability to oxidise LDL significantly more than cell-free controls and that endothelial cells induced the highest degree of oxidative modification of LDL under our experimental conditions.

Glomerular atherosclerosis

This paper suggests a two-hit model for lipoprotein-mediated progressive renal disease, in which postsecretory modification of low-density lipoprotein may favour the transformation of mesangial cells, monocytes and macrophages to glomerular foam cells. Proteinuria and lipiduria would mediate tubulointerstitial damage. Based on this, careful treatment with lipid-lowering agents, lipopheresis and antioxidants may ameliorate the progression of glomerular and tubulointerstitial pathology.

Do glomerular atherosclerosis and lipid-mediated tubulo-interstitial disease cause progressive renal failure in man?

The nephrotic syndrome presents the kidney with a new environment in which blood vessels, glomerular structures and tubules are exposed over substantial periods of time to lipoproteins. LDL has charge affinity with glomerular basement membrane glycosaminoglycans, so potentially increases or maintains albumin loss. This in turn stimulates LDL synthesis. HDL is small enough to be passed by the glomerular filter in substantial amounts and has been found to stimulate endothelin-1 production by human proximal tubular cells in culture. LDL also inhibits nitric oxide vasodilatory responses, an action which when added to that of endothelin-1 may result in decreased renal tissue oxygenation. Taken together, these aspects of the nephrotic syndrome broaden conventional definitions of atherosclerosis and offer a number of targets for therapy in progressive renal disease.

Simvastatin therapy for hypercholesterolemic patients with nephrotic syndrome or significant proteinuria

Experimental evidence suggests that lipid lowering therapy could slow the progression of renal disease in humans. We have conducted a double-blind, placebo controlled trial of the HMG CoA reductase inhibitor simvastatin in patients with the nephrotic syndrome or significant proteinuria (> 1 g/day) and hypercholesterolemia (> or = 6.5 mmol/liter). Patients were placed on a lipid lowering diet for at least 10 weeks before randomization. After a four-week placebo run-in, 30 adults were randomized to simvastatin or placebo therapy (10 mg/day, increasing to 20 to 40 mg/day as required) for 24 weeks. There were seven dropouts, none of whom were "definitely" related to drug therapy. Total and LDL cholesterol levels fell by a mean of 33 and 31%, respectively, in simvastatin treated patients, compared with only 5 and 1% in patients on placebo (P < 0.001, P = 0.002, respectively). Apolipoprotein B100 levels fell by a mean of 31% in the simvastatin group but rose 0.3% in the placebo group (P = 0.014). There were no significant changes in HDL levels. There were no significant differences between the groups in their urine protein levels, their rise in plasma creatinine, or decline in plasma inulin clearance. Simvastatin is a safe, effective therapy for hypercholesterolemia in proteinuric states. A much larger trial is needed to show if potent lipid-lowering therapy slows progression of hypercholesterolemic proteinuric diseases.

Lipiduria in renal disease

This article reviews the published data on lipiduria in both health and disease. Small amounts of lipid appear in the urine under normal circumstances but, in the nephrotic syndrome in humans, there is also a considerable amount of high-density lipoprotein in the urine as well as smaller amounts of other lipoproteins. Potential tubular re-uptake mechanisms for lipoproteins have been demonstrated in both animal and cell-culture models. In humans, there is no direct evidence for these specific re-uptake mechanisms--it is only through specific staining of renal biopsies for apolipoproteins that the presence of such mechanisms in intracellular vesicular structures is suggested. It is possible that lipoprotein filtration and re-uptake by the tubule are important mechanisms in tubular injury.

Calcium set point progressively worsens in hemodialysis patients despite conventional oral 1-alpha hydroxycholecalciferol supplementation

Calcium set point was measured in 12 patients on chronic hemodialysis. Dialysate calcium concentration was 1.65 mmol/l. Calcium carbonate (CaCO3) was used as the phosphate binder and oral 1-alpha hydroxycholecalciferol (alfacalcidol) was administered in a dose of 0.25-1.0 micrograms/day for 12 months. Comparing base line and post study values, there were no significant changes in ionized calcium (ICa++), intact immunoreactive parathyroid hormone (iPTH), plasma total calcium (TCa++), plasma phosphate (P), alkaline phosphatase (ALP), or aluminum (Al). However, the relative calcium set point significantly worsened (shifted to the right). Three patients developed hypercalcemia (25%) with a total calcium > 2.65 mmol/l. Total bone mineral content (BMC) fell suggesting demineralization. We conclude that this dose of oral alfacalcidol, CaCO3, and a dialysate calcium concentration of 1.65 mmol/l are not sufficient to halt the progression of secondary hyperparathyroidism in chronic hemodialysis patients. Measurement of calcium set point may be the best early measure of failure to prevent worsening of hyperparathyroidism.

Oxidation of low-density lipoproteins by rat mesangial cells and the interaction of oxidized low-density lipoproteins with rat mesangial cells in vitro

Glomerulosclerosis and atherosclerosis share common pathobiological mechanisms. Experiments carried out in vitro over the past decade using cells thought to be involved in the atherosclerotic process such as endothelial cells, smooth muscle cells, and monocyte/macrophages have shown that postsecretory modifications such as oxidation increase the atherogenicity of LDL. Animal experiments employing antioxidant therapy have also been shown to slow the progression of atherosclerotic lesions. We set out to investigate the interactions between oxidized LDL (oxLDL) and rat mesangial cells (RMC) that might be of importance in the glomerulosclerotic process. Our results show that RMC have the ability to oxidize LDL, that oxLDL binding was 2-3-fold greater than native LDL (nLDL), and that oxLDL was more cytotoxic to these cells than nLDL. We speculate that cell-mediated oxidation of LDL in vivo may play a role in the progression of the glomerulosclerotic process.

Serum cholesterol falls spontaneously in nephrotic patients with progressive renal disease

Total cholesterol (TC) levels were lower than expected in some patients with advanced renal disease and nephrotic-range proteinuria. Studies of 35 clinically stable nonuremic patients and of 12 nephrotic patients with advancing renal failure were therefore performed. Analysis of pooled biochemical data from 35 patients who were hypercholesterolemic on entry to the clinic revealed a positive correlation between TC and reciprocal creatinine (l/Cr) while serum albumin (ALB) was negatively correlated with l/Cr and TC. In the 12 nephrotic patients with negative reciprocal creatinine slopes there was a strong correlation between the slopes of l/Cr and TC. These data suggest that plasma cholesterol falls in the nephrotic hypercholesterolemic patients in whom renal disease progresses, and that the slopes of plasma cholesterol and reciprocal creatinine are closely related.

Effect of cyclosporin A on endothelin synthesis by cultured human renal cortical epithelial cells

We report here for the first time that human renal proximal tubular cells secrete endothelin, clear evidence of de-novo endothelin synthesis by these cells and the effect of cyclosporin A (CsA) on endothelin synthesis both in short-term (24 h) and medium-term (5-day) culture. Human renal cortical epithelial cells were cultured and shown to possess proximal tubular characteristics. These cells produced endothelin in culture in a time-dependent manner, as measured by radioimmunoassay (291.6 +/- 51.4 pg/well/24 h). Furthermore, endothelin production by these cells was significantly decreased by up to 80% by cycloheximide (1051.8 +/- 54.9 pg/mg cell protein/24 h versus 253.2 +/- 12.6 pg/mg cell protein/24 h), showing that these cells actively synthesize endothelin. In short-term culture (24 h), CsA significantly inhibited endothelin synthesis at a medium concentration of 10,000 micrograms/l. No change in endothelin synthesis was seen at lower CsA concentrations. In contrast, over a 5-day period, a non-significant increase in endothelin synthesis was observed at CsA concentrations of 2000 micrograms/l (152.5 +/- 20.4%); however, cell growth was significantly decreased at this concentration (71.33 +/- 6.39%). Using a newly developed two-site immunoradiometric assay specific for endothelin-1 (ET-1), we demonstrate that ET-1 is the major endothelin isoform produced by human renal proximal tubular cells.

Effect of 24,25-dihydroxycholecalciferol on intestinal absorption of calcium and phosphate and on parathyroid hormone secretion in chronic renal failure

Twelve patients with chronic renal failure (CRF) requiring dialysis, 7 of whom were hypercalcaemic, were treated with 24,25-dihydroxycholecalciferol [24,25(OH)2D3; 10 micrograms daily for 3 months] or placebo and the treatment then crossed over for a further 3 months. Treatment with 24,25(OH)2D3 was associated with small but significant increments in the fractional rates of absorption of calcium (p less than 0.01) and phosphate (p less than 0.05) measured by a combined radioisotope test, but even during treatment, the absorption of both calcium and phosphate remained subnormal. A significant fall in immunoreactive parathyroid hormone (i-PTH) levels occurred during treatment (p less than 0.05) in the absence of significant changes in plasma ionised calcium and plasma phosphate levels. In 7 patients, treatment with 24,25(OH)2D3 converted parathyroid glands which were non-suppressible by calcium infusion before treatment to suppressible ones (greater than 50% suppression of i-PTH levels) at similar levels of ionised calcium. 24,25(OH)2D3 may have a role in the treatment of renal osteodystrophy, especially in patients with hypercalcaemic hyperparathyroidism.

Lipoprotein(a) in patients with proteinuria

Lipoprotein(a) (Lp(a)) has recently been recognized to be a risk factor for coronary heart disease. Lp(a) median values in the absence of renal disease are around 10 mg/dl. Higher levels (greater than or equal to 30 mg/dl) correlate with the occurrence of coronary heart disease, particularly in the presence of elevated cholesterol. We have studied Lp(a) in 76 adults with proteinuria. Fifty had glomerular diseases and 26 non-glomerular diseases, with renal function varying from normal to advanced chronic renal failure. Lp(a) values were shifted to the right, with a median of 21.0 mg/dl, and 25% of patients had values of 30 mg/dl or more. Lp(a) did not correlate with cholesterol, age, lipoprotein subclasses, apoproteins A-I or B-100, albumin, creatinine, or creatinine clearance. Median Lp(a) values did not differ significantly comparing men versus women, or glomerular versus non-glomerular disease. Lp(a) may inhibit fibrinolysis, and is deposited in atherosclerotic lesions. Although the cause of these elevated Lp(a) levels is uncertain, we propose that they contribute to the increased risk of coronary heart disease in the nephrotic syndrome, and may play a role in progressive renal disease.

Partial lecithin:cholesterol acyltransferase (LCAT) deficiency in Balkan endemic nephropathy

The role of lipid abnormalities has been also implicated in the progression of renal diseases. The search for lipid abnormalities in Balkan endemic nephropathy (BEN) has roused sporadic interest and has not been fully elucidated. This study was performed in 54 healthy subjects from the families affected with BEN (group A), 18 members from non-affected families living in the same location (group B), and 25 control subjects (group C). Lipid profiles and lecithin:cholesterol acyltransferase (LCAT) were determined in each subject. The most striking distinction between the groups was that of the LCAT activity, which was abnormally low in group A (39 +/- 2), significantly different (P less than 0.0001) from that of the other groups. Thirty individuals from group A were those accounting for the low LCAT activity (A1). This group had a significantly lower total cholesterol and free cholesterol than all of the other subjects. The entire group A subjects had a significantly lower percentage of free cholesterol than the other two groups. There was no significant difference in HDL cholesterol between any of the groups, but group A1 had significantly higher HDL than group C (P less than 0.04). What emerges from our study is that a certain proportion of subjects from BEN families have a peculiar form of lipid abnormalities associated with an abnormal LCAT activity. At present we have no explanation for these findings. We believe that these changes may have an important role in the pathogenesis of BEN.

Intraplatelet serotonin, beta-thromboglobulin, and histamine concentrations and thromboxane A2 synthesis in renal disease

Intraplatelet serotonin (5-HT), beta-thromboglobulin (beta-TG), and histamine content as well as platelet total thromboxane A2 (TXA2) synthesizing capacity were measured in 53 patients with chronic renal disease: nephrotic syndrome (n = 18); end-stage renal failure (ESRF; n = 13); continuous ambulatory peritoneal dialysis (CAPD; n = 9); hemodialysis (HD; n = 13). These indices of platelet function were correlated with plasma total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), and triglyceride (TG) concentrations. When compared with controls, intraplatelet 5-HT was significantly reduced in all patient groups studied and beta-TG was diminished in all patient groups except CAPD. Total platelet TXA2 synthesizing capacity was increased in ESRF and HD groups. Intraplatelet histamine content was not altered in any of the patient groups studied. There was a significant inverse correlation between intraplatelet 5-HT content on the one hand and plasma TC, LDL-C, and TG on the other. The depletion of intraplatelet 5-HT and beta-TG and the increase in total TXA2 synthesizing capacity are consistent with platelet activation in chronic renal disease. The correlation between these indices of platelet activation and TC, LDL-C, HDL-C, and TG suggests that changes in the concentrations of these lipids may contribute to the activation of platelets in these conditions.

Short report: plasma aluminium concentration and 24-hour urinary aluminium excretion before, during and after treatment with sucralfate

Ten dyspeptic patients were treated with 1 g sucralfate q.d.s. for six weeks. The plasma aluminium concentration and 24-h urinary aluminium excretion were measured at 3-weekly intervals before, during and after treatment with sucralfate. Compared with before treatment, there were significant rises in the median plasma aluminium concentration at 3 and 6 weeks during treatment with sucralfate (6 micrograms/L to 13 and 12 micrograms/L). The median 24-h urinary aluminium excretion rose significantly from a pretreatment level of 20 micrograms to 71 and 78 micrograms after 3 and 6 weeks of treatment; the significant increase of urinary aluminium excretion persisted for three weeks after cessation of treatment (52 micrograms/24 hours), but thereafter urinary excretion was not significantly different from pretreatment. The results are consistent with significant absorption and tissue accumulation of aluminium during standard treatment with sucralfate in individuals with normal renal function.

Effects of dietary fatty acids in an animal model of focal glomerulosclerosis

The obese Zucker rat develops hyperlipidemia, proteinuria and focal glomerulosclerosis without prior changes in renal hemodynamics. To study the effects of oral fatty acid intake on the development of renal injury in this model, rats were fed standard chow or chow supplemented with either 14% fish oil or 14% beef tallow after unilateral nephrectomy at the age of 10 weeks. At 32 weeks post-nephrectomy animals were sacrificed and renal tissue saved to assess histology and glomerular eicosanoid production. Fish-oil treated rats had lower mean plasma cholesterol levels and developed less proteinuria than control or tallow-fed animals although there was no difference in plasma creatinine or blood pressure. Histological analysis showed significantly fewer sclerosed glomeruli in the fish oil group (4.0 +/- 0.8% vs. control 19.4 +/- 4.1%, P less than 0.0005 and vs. beef tallow 10.8 +/- 1.9%, P less than 0.005). Glomeruli derived from rats on fish oil supplements produced smaller amounts of prostaglandin (PG)E2 and of the stable metabolites of PGI2 (6-oxo-PGF1 alpha), PGF2 (PGF2 alpha) and thromboxane (TX)A2 (TXB2) than those from tallow-fed animals. This study demonstrates that oral fatty acid intake may influence the development of glomerulosclerosis. The apparent beneficial effects of fish oil have not been fully defined, but may relate to favorable changes in plasma lipid concentration and renal eicosanoid production.

Characterisation of the binding of low-density lipoproteins to cultured rat mesangial cells

Mesangial cell lipid accumulation is a recognised feature of glomerular disease and has been implicated as a factor in the pathogenesis of renal injury. To investigate possible mechanisms of such accumulation, binding of 125I-labelled human low-density lipoprotein (LDL) to rat mesangial cells was studied in vitro. Experiments were performed at 4 degrees C to prevent ligand internalisation. LDL remained associated with the cells after repeated washing. Binding was time-dependent, was inhibited by addition of an excess of unlabelled LDL, but to a much lesser extent by apoprotein-A-rich high-density lipoprotein particles devoid of apoprotein E (HDL-A). Specific binding reached saturation at an LDL concentration of 21 micrograms/ml, required the presence of calcium, and was inhibited by heparin and dextran sulphate. Scatchard analysis suggested a single class of binding site (Kd 22.7 micrograms protein/ml). Higher binding affinities were obtained when rat LDL was substituted for human LDL (Kd 1.3 micrograms/ml) and when human fibroblasts were exposed to human LDL under identical experimental conditions (Kd 3.0 micrograms/ml). Further experiments at 37 degrees C demonstrated degradation of LDL by cells. These results suggest that mesangial cells possess apoprotein B, E receptors. Mesangial cell lipid accumulation may therefore result from receptor-mediated endocytosis of LDL particles.