alpha 1-microglobulin in cadmium poisoning

Alpha 1-microglobulin: clinical laboratory aspects and applications

BACKGROUND

Urinary microproteins are becoming increasingly important in clinical diagnostics. They can contribute in the non-invasive early detection of renal abnormalities and the differentiation of various nephrological and urological pathologies. Alpha 1-microglobulin (A1M) is an immunomodulatory protein with a broad spectrum of possible clinical applications and seems a promising marker for evaluation of tubular function.

METHOD

We performed a systematic review of the peer-reviewed literature (until end of November 2003) on A1M with emphasis on clinical diagnostic utility and laboratory aspects.

CONCLUSIONS

A1M is a 27-kDa glycoprotein, present in various body fluids, with unknown exact biological function. The protein acts as a mediator of bacterial adhesion to polymer surfaces and is involved in inhibiting renal lithogenesis. Because A1M is not an acute phase protein, is stable in a broad range of physiological conditions and sensitive immunoassays have been developed, its measurement can be used for clinical purposes. Unfortunately, international standardisation is still lacking. Altered plasma/serum levels are usually due to impaired liver or kidney functions but are also observed in clinical conditions such as HIV and mood disorders. Urinary A1M provides a non-invasive, inexpensive diagnostic alternative for the diagnosis and monitoring of urinary tract disorders (early detection of tubular disorders such as heavy metal intoxications, diabetic nephropathy, urinary outflow disorders and pyelonephritis).

alpha(1)-Microglobulin: a yellow-brown lipocalin

alpha(1)-Microglobulin, also called protein HC, is a lipocalin with immunosuppressive properties. The protein has been found in a number of vertebrate species including frogs and fish. This review summarizes the present knowledge of its structure, biosynthesis, tissue distribution and immunoregulatory properties. alpha(1)-Microglobulin has a yellow-brown color and is size and charge heterogeneous. This is caused by an array of small chromophore prosthetic groups, attached to amino acid residues at the entrance of the lipocalin pocket. A gene in the lipocalin cluster encodes alpha(1)-microglobulin together with a Kunitz-type proteinase inhibitor, bikunin. The gene is translated into the alpha(1)-microglobulin-bikunin precursor, which is subsequently cleaved and the two proteins secreted to the blood separately. alpha(1)-Microglobulin is found in blood and in connective tissue in most organs. It is most abundant at interfaces between the cells of the body and the environment, such as in lungs, intestine, kidneys and placenta. alpha(1)-Microglobulin inhibits immunological functions of white blood cells in vitro, and its distribution is consistent with an anti-inflammatory and protective role in vivo.

alpha-1-Microglobulin: epidemiological indicator for tubular dysfunction induced by cadmium?

OBJECTIVES

To evaluate the suitability of alpha-1-microglobulin as a marker for cadmium induced renal dysfunction.

METHODS

alpha-1-Microglobulin was studied in a cross sectional survey in relation to the body burden of cadmium. Concentrations of alpha-1-microglobulin in 24 h urine of 831 people aged 2-87 years were analysed in association with urinary cadmium excretion, cadmium blood concentration, age, sex, occupational and smoking history, and estimated creatinine clearance. Participants came from a population residentially exposed to cadmium and from two control populations matched for socioeconomic status.

RESULTS

The excretion of alpha-1-microglobulin/24 h ranged from 0.1 mg to 176.3 mg and 44.4% of samples showed concentrations near the detection limit. Ordinal logistic regression analysis of people of all ages identified a high risk only for males compared with females (odds ratio (OR) 2.14; 95% confidence interval (95% CI) 1.56 to 2.94), age group, and duration of living on contaminated soil (OR 1.03/year; 95% CI 1.02 to 1.04), but not urinary cadmium excretion (OR 1.30; 95% CI 0.96 to 1.77) as significant predictors. For people < or = 50 years of age a weaker effect of sex (OR 1.76; 95% CI 1.13 to 2.73) and age group and an effect of similar magnitude for the duration of soil exposure (OR 1.03; 95% CI 1.01 to 1.04) were found. Also, the urinary cadmium excretion (OR 2.26; 95% CI 1.38 to 3.70) and occupational exposure (OR 1.71; 95% CI 1.03 to 2.83) were found to be significant in this younger age group. The estimated creatinine clearance had no significant impact on the alpha-1-microglobulin excretion.

CONCLUSION

alpha-1-Microglobulin is a suitable marker for early tubular changes only for people < or = 50 years. It may not be sufficiently specific for cadmium, and therefore not a suitable surrogate for cadmium exposure in epidemiological studies.

The determination of alpha 1-microglobulin by means of an automated latex immunoassay

Polystyrene (latex) particles coated with human anti-alpha 1 microglobulin antibodies are used in an automated "kinetic" assay for alpha 1-microglobulin in urine. For values below 12 mg/l, there was no significant difference between two kinds of standard, but above 12 mg/l the results depend on the origin of the alpha 1-microglobulin standard. Correlation between values obtained with both standards was good (r2 = 0.968). The method has a between-run imprecision (CV) of 11-16%. Comparison with two commercial enzyme immunoassays gave a bias of -10% to +40%, while two nephelometric methods differed by 0% to 23%, possibly because the standards used in these methods were of different origin. These results indicate the necessity of standardization of the determination of alpha 1-microglobulin. The detection limit of our method was 0.8 mg/l, enabling the application of the method for epidemiological investigations.

Lead-induced nephropathy: relationship between various biological exposure indices and early markers of nephrotoxicity

Lead nephropathy in adults is silent and insidious, characterized by the absence of proteinuria in its early phase. Of the early markers of nephrotoxicity, urinary N-acetyl-beta-D-glucosaminidase (NAG) appears to be the only one that is elevated in early lead nephropathy. However, the elevation in urinary NAG activity may be a response to a sharp increase in renal burden of lead. Its usefulness as a marker of chronic lead nephropathy is thus in doubt. There is a need, then, to identify a reliable early biological indicator of lead-induced kidney damage. Furthermore, there is also a need to identify suitable markers of chronic exposure to describe meaningful dose-response and dose-effect relationships. Traditionally, blood lead (PbB) was used, but the current blood lead level (PbBrec) is more an indicator of recent exposure. Time-integrated blood lead indices (PbBint) derived from repeated serial PbB measurements can be used as indices of chronic exposure. In 128 lead-exposed workers, the PbBint was the most important exposure variable in describing the variability in urinary alpha 1-microglobulin (U alpha 1 m), urinary beta 2-microglobulin (U beta 2m), and urinary retinol binding protein (URBP). U alpha 1m was the only marker that was significantly higher in the exposed group, with a good dose-response and dose-effect relationship with PbBint. The lack of dose-response and dose-effect relationships in other studies may be due to inappropriate exposure markers as well as less sensitive response markers. PbBint has a better correlation than PbBrec. Furthermore, U alpha 1m may be the most sensitive of the markers because of its higher molecular weight.

Proteinuria: changes and mechanisms in toxic nephropathies

During the last few decades, considerable progress has been made in the understanding of the pathophysiological mechanisms of proteinuria. A great variety of hemodynamic or biochemical mechanisms acting at different sites of the nephron have been shown to alter the renal handling and the urinary excretion of proteins. The finding which perhaps has had most practical implications is that the pattern of protein excretion quantitatively and qualitatively varies with the site and severity of renal damage. This has led to the development of a large array of methods for the identification and quantitation of specific urinary proteins. These methods have been most extensively used by toxicologists in experimental, epidemiological, or clinical studies on potentially nephrotoxic chemicals (e.g., drugs, heavy metals, solvents, etc.). The present review summarizes the current state of knowledge on the mechanisms of proteinuria and the use of urinary proteins as indicators of nephrotoxicity.

Asymptomatic low molecular weight proteinuria in children

We reviewed the clinical features in 7 boys with asymptomatic low molecular weight proteinuria. The disease had first been reported by us in 1980. The distinctive features common to all patients were male dominance, asymptomatic proteinuria with a large amount of low molecular weight proteins, normal development and growth, normal renal function tests in childhood, and minimal histological changes in renal biopsy specimens. In 1990, 35 cases have been reported in Japan. Thus, asymptomatic low molecular weight proteinuria is not a rare disease and is one of the causes of isolated proteinuria. Furthermore, this condition is recognized as a new entity.

Reversible low-molecular-weight proteinuria in patients with distal renal tubular acidosis

Four patients with untreated renal tubular acidosis had a urinary excretion of low-molecular-weight (LMW) proteins which was restored to normal by alkali therapy. Hypokalaemic proximal tubular damage in untreated patients with distal renal tubular acidosis is believed to be the cause of LMW proteinuria. An examination of urinary excretion of LMW proteins is useful for determining hypokalaemic proximal tubular dysfunction, as well as the efficiency of alkali therapy.

Assessment of urinary protein 1 and transferrin as early markers of cadmium nephrotoxicity

Transferrin and protein 1, a sex linked alpha 2-microprotein, were assayed in urine from 58 workers exposed to cadmium (Cd) in a non-ferrous smelter and from 58 age matched referents. These two new markers of nephrotoxicity were compared with urinary beta 2-microglobulin (beta 2-m), retinol binding protein (RBP), albumin, and beta-N-acetyl-glucosaminidase (NAG). The response of protein 1 to Cd tubulotoxicity was similar to that of beta 2-m, RBP, and NAG. In Cd workers, protein 1 had a correlation with urinary Cd (r = 0.56) similar to beta 2-m (r = 0.48), RBP (r = 0.58), and NAG (r = 0.49). Values of these three low molecular weight proteins and of NAG were increased only in workers with urinary Cd higher than 10 micrograms/g creatinine. Urinary transferrin and albumin were similarly affected by exposure to Cd. Their response, however, was clearly more sensitive than that of low molecular weight proteins. Prevalences of positive values of these two high molecular weight proteins were not only higher but also tended to rise at lower concentrations of Cd in urine or blood. This finding suggests that in some subjects subtle defects in glomerular barrier function may precede the onset of proximal tubular impairment after chronic exposure to Cd. It remains to be assessed whether these subjects are more at risk of developing renal insufficiency.

Human alpha 1-microglobulin: its measurement and clinical significance

alpha 1-Microglobulin (alpha 1-M), also called protein HC, is a low-molecular-weight (LMW) glycoprotein (about 30 kDa) with unique physicochemical properties. Using purified urinary alpha 1-M a standard and specific antibody against alpha 1-M, an assay system for alpha 1-M was developed, and the clinical significance of this protein was investigated by measuring total levels of alpha 1-M under physiological and pathological conditions. alpha 1-M is distributed in various body fluids: in serum, it consists mainly of free LMW alpha 1-M and monomeric IgA-alpha 1-M complex. The total alpha 1-M level in serum and urine usually reflects LMW alpha 1-M variation sensitively, and its determination is quite useful as an indicator of renal glomerulotubular dysfunction and hepatic dysfunction. Serum levels can vary, depending on IgA-alpha 1-M complex level, in parallel with the IgA concentration. The heterogeneity of alpha 1-M purified from different sources of urine by different procedures and underestimation of IgA-alpha 1-M complex by solid-phase antibody assays can be important causes for the discrepancy of serum levels between assays.

Epidemiological application of early markers of nephrotoxicity

This paper is a review of epidemiological studies in which sensitive markers of nephrotoxicity have been used to detect the early effects of chemicals on the kidney. Most of the studies are cross-sectional, and their objective was either to identify potentially nephrotoxic chemicals (organic solvents, heavy metals) in the working or general environment or to establish dose-response/effect relationships from which safe exposure levels can be defined (e.g., for cadmium and mercury vapour). A few longitudinal studies were conducted to determine the persistence of renal disturbances and to get information on their predictive value (e.g., microproteinuria in cadmium workers). Nephrotoxicity tests, which have proved to be the most useful in these epidemiological studies, rely on the determination of specific urinary proteins which, according to their size, reflect the functional integrity of the proximal tubule (e.g., retinol-binding protein or beta 2-microglobulin) or the glomerulus (e.g., albumin, immunoglobulin G). An increased urinary excretion of the lysosomal enzyme N-acetyl-beta-D-glucosaminidase has been reported in several studies (e.g., in lead-exposed workers), but the pathological significance of this finding remains to be clarified, particularly when it is not associated with changes in the urinary excretion of specific proteins. Further work is needed to assess the usefulness of tests introduced more recently such as the assay of renal antigens in urine and the use of red-blood-cell membrane negative charges as an index of the glomerular polyanion. With the exception of microproteinuria observed in chronic cadmium poisoning, no epidemiological data are available on the prognostic value of subclinical renal effects caused by nephrotoxic chemicals.

Urinary alpha 1-microglobulin as an indicator protein of renal tubular dysfunction caused by environmental cadmium exposure

An epidemiologic investigation was carried out to clarify the significance of the urinary excretion of alpha 1-microglobulin (alpha 1-MG) in people aged 50 years and over living in a Cd-polluted area in Japan. Approximately 80% of the population participated in the health examination. The urinary and serum levels and the relative clearance of alpha 1-MG to creatinine clearance were compared with various parameters (age, urinary beta 2-microglobulin (beta 2-MG), total protein, Cd, Cu and Zn, serum beta 2-MG, creatinine and blood urea nitrogen and relative clearances of alpha 1-MG, beta 2-MG, inorganic phosphate and uric acid). It was found that the urinary excretion of alpha 1-MG is closely associated with the urinary Cd and Cu and with the indices of renal dysfunction listed above. These results suggest that the urinary alpha 1-MG level markedly reflects a degree of proximal tubular dysfunction and that it may be useful as one of the screening measures for proximal tubular dysfunction caused by environmental Cd exposure.

A double antibody radioimmunoassay for human alpha 1-microglobulin

Using purified alpha 1-microglobulin from urine and specific rabbit antiserum, a double antibody radioimmunoassay has been developed. The assay is sensitive to 3 ng/ml and covers a working range from 25-800 ng/ml. The recovery rates on the average were 91.8% in serum and 97.7% in urine. The coefficients of intra-assay variation ranged from 3.6-4.5% and those of inter-assay variation from 4.0-8.8%. Correlations in serum and urine between the present assay and single radial immunodiffusion (r = 0.972; r = 0.978) and enzyme-linked immunosorbent assay (r = 0.935; r = 0.971) were satisfactory.