Disturbance of sialic acid metabolism by chronic cadmium exposure and its relation to proteinuria

Renal function after reduction in cadmium exposure: an 8-year follow-up of residents in cadmium-polluted areas

BACKGROUND AND OBJECTIVE

Long-term exposure to cadmium (Cd) causes renal dysfunction, but the change in renal function with exposure is unknown. We assessed the evolution of Cd-induced renal effects after a reduction in dietary exposure to Cd in rice.

METHODS

Four hundred twelve residents in previously Cd-polluted and nonpolluted areas were examined twice, in 1998 and in 2006. Changes in blood Cd, urinary Cd, and kidney function [N-acetyl-β-d-glucosaminidase (NAG), β2-microglobulin, and albumin in urine] were measured.

RESULTS

In the most polluted area, mean blood Cd was 8.9 μg/L and 3.3 μg/L in 1998 and in 2006, respectively, and urinary Cd was 11.6 and 9.0 μg/g creatinine. Urinary albumin in 1998 increased with urinary Cd, but no such exposure-response relation appeared for 2006 albumin versus urinary Cd 1998, indicating recovery. Other biomarkers of kidney function were also elevated in 1998. Partial recovery was observed for NAG among women and was suggested for β2-microglobulin among young individuals. The probability of having β2-microglobulin levels above the 95th percentile in 2006 was high in those with elevated β2-microglobulin in 1998 [odds ratio (OR) = 24.8; 95% confidence interval (CI): 11.2, 55.3] compared with albumin (OR = 3.0; 95% CI: 1.2, 7.5) and NAG (OR = 2.6; 95% CI: 1.6, 4.4).

CONCLUSIONS

Results suggest that a Cd-mediated increase in urinary albumin excretion is reversible upon substantial reduction of exposure. For markers of tubular effects, we observed a tendency toward improvement but not complete recovery. Data from repeated observations suggest that β2-microglobulin may be more informative than NAG as an indicator for an individual's future tubular function.

Binding ability of sialic acid towards biological and toxic metal ions. NMR, potentiometric and spectroscopic study

The binary complexes of 5-amino-3,5-dideoxy-D-glycero-D-galactononulosic acid (NANA), commonly called N-acetyl neuraminic acid, formed with biological metal ions such as Co(II) and Cu(II) and toxic metal ions such as Cd(II) and Pb(II) were investigated in aqueous solution by means of potentiometry, UV and NMR spectroscopy. The corresponding ternary systems with 2,2'-bipyridine were studied in aqueous solution by potentiometry and UV spectroscopy. NANA co-ordinates all metal ions, in both binary and ternary systems through the carboxylic group (protonated or deprotonated according to pH), pyranosidic ring oxygen and glycerol chain alcoholic hydroxy groups. The prevailing species in the pH range 2-7 are of [M(NANA)(2)] type, and their stability constants are greater than those of simple carboxylate complexes. Above pH 7, the species [M(NANA)(2)OH](-) are also formed, but they do not prevent the precipitation of metal hydroxides. This work provides information on the solution state chemistry of NANA in the presence of bivalent metal ions; its great affinity for the toxic metals Cd(II) and Pb(II), near physiological conditions, and the relatively high stability of the complex species found may also account for the mechanism of toxicity.

Occurrence of sialic acids in healthy humans and different disorders

Sialic acid (SA), N-acetylated derivatives of neuraminic acid, play a central role in the biomedical functioning of humans. The normal range of total sialic acid (TSA) level in serum/plasma is 1.58-2.22 mmol L-1, the free form of SA only constituting 0.5-3 mumol L-1 and the lipid-associated (LSA) forms 10-50 mumol L-1. Notably, considerably higher amounts of free SA are found in urine than in serum/plasma (approximately 50% of the total SA). In inherited SA storage diseases such as Salla's disease, SA levels are elevated many times over, and their determination during clinical investigation is well established. Furthermore, a number of reports describe elevated SA levels in various other diseases, tentatively suggesting broader clinical utility for SA markers. Increased SA concentrations have been reported during inflammatory processes, probably resulting from increased levels of richly sialylated acute-phase glycoproteins. A connection between increased SA levels and elevated stroke and cardiovascular mortality risk has also been reported. In addition, SA levels are slightly increased in cancer, positively correlating with the degree of metastasis, as well as in alcohol abuse, diabetes, chronic renal failure and chronic glomerulonephritis. Several different mechanisms are assumed to underlie the elevated SA concentrations in these disorders. The apparent non-specificity of SA to a given disease limits the potential clinical usefulness of SA determination. In addition, some non-pathological factors, such as aging, pregnancy and smoking, may cause changes in SA concentrations. The absolute increases in SA levels are also rather small (save those in inherited SA storage disorders); this further limits the clinical potential of SA as a marker. Tentatively, SA markers might serve as adjuncts, when combined with other markers, in disease screening, disease progression follow-up, and in the monitoring of treatment response. To become clinically useful, however, the existing SA determination assays need to be considerably refined to reduce interferences, to be specific for certain SA forms, and to be more easy to use.

Cadmium-associated renal disease

Cadmium is widely used in industry, causing exposure of workers and environmental pollution because of its persistence in the biosystems. Its very long half-life in the human organism causes its accumulation over the lifetime in liver and kidneys. Cadmium ions have a high affinity for tissue thiols, induce the synthesis of a carrier cysteine-rich polypeptide called metallothionein, and impair proteoglycan metabolism. Significant renal effects include tubular nephropathy manifested by proteinuria, amino aciduria, glucosuria, phosphaturia, and calcium wastage. Chronic sequels include decrease in the glomerular filtration rate and increased risk of kidney stone disease. Biological monitoring of cadmium absorption includes determination of urinary cadmium and of low molecular weight marker proteins, such as beta2-microglobulin or retinol binding protein, the tubular reabsorption of which is impaired before a frank proteinuria.

Association between NAG-B and cadmium in urine with no evidence of a threshold

OBJECTIVES

To explore the significance of the increase in urinary excretion of the lysosomal enzyme beta-N-acetylglucosaminidase (NAG) at low exposures to cadmium (Cd) that is frequently found in the absence of any other sign of renal dysfunction.

METHODS

The activity was measured of the two main isoenzymes of NAG (NAG-A secreted by exocytosis and NAG-B released with cell membranes) in the urine of 49 male workers employed in a Cd smelter and of 20 age matched controls.

RESULTS

An increased urinary excretion of low molecular weight proteins was noted only in subjects who excreted > 10 micrograms Cd/g creatinine. The urinary activity of NAG-B showed a dose related increase that was already significant in the group excreting 0.5-2 micrograms Cd/g creatinine. In multiple regression analysis the NAG-B activity correlated with the excretion of Cd but not with that of lead or mercury. The NAG-A activity was by contrast unaffected by exposure to Cd but correlated with the urinary excretion of lead and copper.

CONCLUSIONS

As NAG-B is considered to be the lesional form of NAG, the existence of a specific association between this enzyme and urinary Cd excretion with no detectable threshold suggests that this metal produces cellular alterations at exposures commonly found in the general population.

Occupational hydrocarbon exposure and chronic nephropathy

This review aims at discussing the questions raised by the hydrocarbon-related chronic nephropathy and its possible consequence, the hydrocarbon-related chronic renal failure. It has been attempted to adopt the point of view of the clinician. Therefore, the most important part of the review is devoted to a presentation and an analysis of the available data on humans. The main features of the available studies on human subjects are presented, their conclusions discussed in the light of the possible methodological flaws, and practical conclusions drawn. After a discussion of the main difficulties encountered for selecting the suitable exposure indicator, the studies are discussed in order of decreasing quality of the study design (cohort, case-control, cross-sectional studies, and the case reports). It is concluded that a great deal of controversies about chronic hydrocarbon-related nephropathy is explained by differences in the study design and that hydrocarbon-induced nephropathy is probably more than a mere hypothesis, although a causal relationship has not yet been proven. Finally, some practical consequences for dealing with a hydrocarbon-exposed patient diagnosed with a kidney disease and the need for further research are discussed.

Markers of early renal changes induced by industrial pollutants. II. Application to workers exposed to lead

The present study has been carried out in the framework of a collaborative research project on the development of new markers of nephrotoxicity. A battery of more than 20 potential indicators of renal changes has been applied to 50 workers exposed to lead (Pb) and 50 control subjects. After application of selection criteria 41 exposed and 41 control workers were eventually retained for the final statistical analysis. The average blood Pb concentration of exposed workers was 480 micrograms/l and their mean duration of exposure was 14 years. The battery of tests included parameters capable of detecting functional deficits (for example, urinary proteins of low or high molecular weight), biochemical alterations (for example, urinary eicosanoids, glycosaminoglycans, sialic acid) or cell damage (for example, urinary tubular antigens or enzymes) at different sites of the nephron or the kidney. The most outstanding effect found in workers exposed to Pb was an interference with the renal synthesis of eicosanoids, resulting in lower urinary excretion of 6-keto-PGF1 alpha and an enhanced excretion of thromboxane (TXB2). The health significance of these biochemical alterations, detectable at low exposure to Pb is unknown. As they were not associated with any sign of renal dysfunction, they may represent reversible biochemical effects or only contribute to the degradation of the renal function from the onset of clinical Pb nephropathy. The urinary excretion of some tubular antigens was also positively associated with duration of exposure to Pb. Another effect of Pb that might deserve further study is a significant increase in urinary sialic acid concentration.

Markers of early renal changes induced by industrial pollutants. III. Application to workers exposed to cadmium

Cadmium (Cd) was the third heavy metal investigated in the European collaborative research project on the development and validation of new markers of nephrotoxicity. Fifty workers exposed to Cd and 50 control workers were examined. After application of selection criteria 37 workers (mean age 43) exposed to Cd for an average of 11.3 years; and 43 age matched referents were retained for final analysis. The average concentrations of Cd in blood (Cd-B) and urine (Cd-U) of exposed workers were 5.5 micrograms Cd/l and 5.4 micrograms Cd/g creatinine respectively. By contrast with lead and mercury, Cd had a broad spectrum of effects on the kidney, producing significant alterations in amounts of almost all potential indicators of nephrotoxicity that were measured in urine--namely, low and high molecular weight proteins, kidney derived antigens or enzymes, prostanoids, and various other biochemical indices such as glycosaminoglycans and sialic acid. An increase in beta 2-microglobulin and a decrease of sialic acid concentration were found in serum. Dose-effect/response relations could be established between most of these markers and Cd-U or Cd-B. The thresholds of Cd-U associated with a significantly higher probability of change in these indicators were estimated by logistic regression analysis. Three main groups of thresholds could be identified: one around 2 micrograms Cd/g creatinine mainly associated with biochemical alterations, a second around 4 micrograms Cd/g creatinine for high molecular weight proteins and some tubular antigens or enzymes, and a third one around 10 micrograms Cd/g creatinine for low molecular weight proteins and other indicators. The recent recommendation by the American Conference of Governmental Industrial Hygienists (ACGIH) of 5 micrograms Cd/g creatinine in urine as the biological exposure limit for occupational exposure to Cd appears thus justified, although for most of the effects occurring around this threshold the link with the subsequent development of overt Cd nephropathy is not established. In that respect, the very early interference with production of some prostanoids (threshold 2 micrograms Cd/g creatinine) deserves further investigation; although this effect might contribute to protect the filtration capacity of the kidneys, it might also play a part in the toxicity of Cd on bone.

Incorporation of [35S]sulfate into glomerular membranes of rats chronically exposed to cadmium and its relation with urinary glycosaminoglycans and proteinuria

The aim of the present work was to assess the effects of long-term exposure to Cd on the sulfatation of glomerular membranes and their relation with proteinuria and urinary glycosaminoglycans (GAG). For this purpose the in vitro incorporation of [35S]sulfate was investigated in female Sprague-Dawley rats given 100 ppm of Cd in drinking water for 7 months. When compared with their controls, glomeruli from Cd-treated rats showed a 12.8% decrease in the incorporation of the label into glomerular membranes. This effect, which was not explained by differences in viability or in sulfate uptake by the glomeruli, suggests that sulfatation of glomerular membranes is impaired in Cd-treated rats. In support of this, in another independent experiment, a decrease, 17.4% on average, of the sulfate content of glomerular membranes was observed in long-term Cd-treated rats (100 ppm in drinking water for 4 months). This effect was significantly correlated with albuminuria and transferrinuria but not with beta 2-microglobinuria, suggesting that a loss of heparan sulfate of the glomerular capillary wall could be involved in the Cd-induced glomerular proteinuria. On the other hand an enhanced urinary excretion of GAG, negatively correlated with the sulfate content of glomerular membranes, was also observed in Cd-treated rats. Moreover GAG excretion was associated with tubular and glomerular proteinuria, which suggests that GAG might be a useful marker of Cd-induced nephrotoxicity.

Human and experimental studies on renal eicosanoid response to long-term cadmium exposure

In order to assess the effects of long-term exposure to cadmium (Cd) on the renal metabolism of eicosanoids, the urinary excretion of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha), prostaglandin E2 (PGE2), prostaglandin F2 alpha (PGF2 alpha), and thromboxane B2 (TXB2) was determined in 37 workers exposed to Cd and in female Sprague-Dawley rats given 100 ppm Cd in drinking water for 10 months. Urinary output of sodium and calcium was also determined. The Cd-exposed workers showed an increased urinary concentration of 6-keto-PGF1 alpha, PGE2, sodium, and calcium. The rise of 6-keto-PGF1 alpha was related to Cd levels in blood and weakly correlated with urinary sodium. Calcium in urine was not related to the concentration of the metal in blood and urine. A slight elevation in urinary TXB2 was also observed in workers with blood Cd higher than 5 micrograms/liter. After 10 months of exposure to Cd, female Sprague-Dawley rats presented an enhanced urinary excretion of albumin, transferrin, beta 2-microglobulin, sodium, and PGE2 in urine. The latter was significantly correlated with albuminuria and transferrinuria. In conclusion the results show that chronic exposure to Cd induces changes in the urinary excretion of some eicosanoids. The possible relation of these changes to Cd-induced kidney dysfunction are discussed.

Effects of CdCl2 and Cd-metallothionein on cultured mesangial cells

Cadmium is a potent nephrotoxin known to cause damage to the proximal tubular epithelium in vivo. The renal glomerulus is less frequently a target and is sensitive to Cd in vitro. We have previously presented evidence that the mesangial cell is a major target for Cd2+ toxicity in the isolated glomerulus (D. M. Templeton and N. Chaitu, Toxicology 61, 119-133, 1990). The present study was undertaken to investigate the sensitivity to Cd of rat mesangial cells grown in homogeneous culture. At a concentration of 1 microM, Cd2+ was a potent inducer of its binding protein metallothionein (MT). Cd2+ inhibited DNA synthesis in these cells with an EC50 of 5.4 +/- 0.4 microM, while preinduction of MT with Zn2+ was protective, raising the EC50 to 17.6 +/- 0.7 microM Cd2+. DNA synthesis in these cells is especially sensitive to Cd2+; only at concentrations of 20 microM Cd2+ and higher were significant effects on cell viability, attachment, and protein synthesis observed. Renal function depends in part on synthesis of specialized matrices by glomerular cells. Synthesis of both matrix and secreted proteoglycans was specifically affected by Cd2+ with an EC50 of about 10 microM for proteoglycan sulfation. We also investigated the effects of Cd-MT on these parameters. Contrary to observations that extracellular Cd-MT is a potent nephrotoxin in vivo, we were unable to demonstrate any effects of Cd-MT on DNA and protein synthesis at Cd concentrations below 60 microM in the cultured cells. Nor did Cd-MT at these concentrations affect DNA or protein synthesis in LLC-PK1 cells, a proximal tubule cell line. This was not due to failure of the cells to take up Cd because they accumulated comparable amounts of Cd whether it was provided as CdCl2 or Cd-MT. We conclude that ionic Cd2+ is the most toxic form of this metal to cultured mesangial cells. While these cells respond to micromolar concentrations of Cd2+ by increasing their content of metallothionein, presumably a protective response, only slightly higher levels may impair the regenerative capacity of mesangial cells, in addition to interfering with the specialized function of matrix synthesis.

Cadmium effects on bone metabolism: accelerated resorption in ovariectomized, aged beagles

The purpose of this study was to evaluate, in an animal whose skeleton is comparable to humans, the combined effects of estrogen depletion and Cd exposure on bone resorption by monitoring skeletal release of 45Ca and to determine whether Cd-induced bone resorption occurred independent of osteotropic hormone changes and renal dysfunction. Cd exposure following ovariectomy or sham surgery was for 7 months: 1 month by oral ingestion of capsules (1, 5, 15, 50 ppm) and 6 months via drinking water (15 ppm). Serum and fecal 45Ca were increased at 1 week following ovariectomy (OV) (54 +/- 9% and 122 +/- 40%, respectively), but this response was attenuated by 2 weeks. Five of seven exposed dogs had increased serum and fecal 45Ca during the 50-ppm Cd capsule period (15-40% and 15-190%, respectively). Serum 45Ca levels in OV/+Cd dogs showed a significant and consistent increase within 1 week of initiating each of three separate Cd.H2O exposure cycles. Blood Cd levels increased over time from 2 to 15 micrograms/l, coinciding with the elevated serum 45Ca concentrations. No correlation was observed between serum 45Ca increases and parathyroid hormone, 1,25-(OH)2-vitamin D, or calcitonin. No effects of ovariectomy and/or Cd were observed in total serum Ca, calciotropic hormone concentrations, serum or urinary phosphorus and creatinine, creatinine clearance, or urinary specific gravity. Urinary Cd concentrations ranged from 7 to 50 micrograms/l in exposed dogs but were not detectable in nonexposed dogs. Urinary protein concentrations showed no differences between groups. Cd increased bone resorption (skeletal 45Ca release) in ovariectomized and sham-operated dogs without renal dysfunction or calciotropic hormone interaction. Based on our results, Cd is an exogenous factor which exacerbates bone mineral loss in postmenopausal osteoporosis.

Depletion of sialic acid without changes in sialidase activity in glomeruli of uninephrectomized diabetic rats

The role of sialidase in the depletion of glomerular sialic acid induced by diabetes has been investigated in uninephrectomized rats. Four months after streptozotocin administration, diabetic rats showed an enhanced urinary excretion of albumin and transferrin, which was associated with a decrease of sialic acid concentration in isolated glomeruli. Despite the sialic acid depletion, the glomerular sialidase activity was unchanged. These results indicate that the decreased glomerular sialic acid concentration observed in diabetic nephropathy might be caused by a disturbance of the sialylation of glomerular structures.

Latex immunoassay of transferrin in urine

A fully automated assay based on latex particle agglutination has been developed for the determination of transferrin in urine. The assay consists of incubating urine samples for 30 min at 50 degrees C with latex particles on which an anti-transferrin antibody has been adsorbed and then quantifying the residual unagglutinated particles with an optical particle counter. The measurable concentration range is 0.5-10 micrograms/l. Intra- and interassay coefficients of variation varied from 2.1 to 11.8% and the recovery averaged 102.5% (SD 13.8). Correlation coefficients between transferrinuria and albuminuria were 0.79 in 100 healthy subjects and 0.97 in 90 subjects with established or suspected glomerular involvement. Transferrin and albumin were stable in urine over the pH range 5-9 for 24 h at 37 degrees C, or for 2 weeks at 4 degrees C or room temperature. Transferrin was, however, less stable than albumin during prolonged storage of urine at 4 degrees C. The present latex immunoassay of transferrin may also be adapted to give a turbidimetric reading in which agglutination is quantified by measuring the decrease of absorbance at 360 nm.

Potentiation of diabetic glomerulopathy in uninephrectomized rats subchronically exposed to cadmium

The renal effects of diabetes mellitus and cadmium (Cd), separately or in combination, were investigated in unilaterally nephrectomized female Sprague-Dawley rats. Diabetes was induced by injection of streptozotocin and Cd was administered in drinking water at a concentration of 100 p.p.m. for 2.5 months. Cd did not affect the reduction in glomerular filtration rate or the rise in beta 2-microglobulinuria caused by diabetes. By contrast, the effect of diabetes on the urinary excretion of albumin, transferrin or IgG was greatly enhanced by concomitant exposure to Cd. This interaction occurred at Cd levels in the renal cortex which are very similar to those found in the general population of industrialized countries. These observations, in agreement with the results of a recent epidemiological study, suggest that Cd polluting the environment might potentiate the development of diabetic nephropathy.