Urinary metallothionein as an indicator of cadmium body burden and of cadmium-induced nephrotoxicity.

There is a need to identify specific biological indicator(s) of cadmium exposure so that the renal damage can be prevented. Towards this end, we have examined the usefulness of urinary metallothionein as an indicator of cadmium body burden. It is found that, in both animals and humans, urinary metallothionein level is related to the hepatic and renal cadmium burdens. Significant correlations are also found between the urinary metallothionein and urinary cadmium and beta 2-microglobulin. Furthermore, it is noted that cadmium-exposed individuals with renal dysfunction excrete significantly more metallothionein than those with normal renal function. Thus it appears that there is merit to include metallothionein among the clinical parameters monitored in cadmium-exposed individuals. More tests are needed to define a critical concentration of metallothionein in urine which is related to the onset of renal dysfunction.

Urinary Metallothionein as an Indicator of Cadmium Body Burden and of Cadmium-Induced Nephrotoxicity by Zahir A. Shaikh* and Chiharu Tohyamat There is a need to identify specific biological indicator(s) of cadmium exposure so that the renal damage can be prevented. Towards this end, we have examined the usefulness of urinary metallothionein as an indicator of cadmium body burden. It is found that, in both animals and humans, urinary metallothionein level is related to the hepatic and renal cadmium burdens. Significant correlations are also found between the urinary metallothionein and urinary cadmium and 02-microglobulin. Furthermore, it is noted that cadmium-exposed individuals with renal dysfunction excrete significantly more metallothionein than those with normal renal function. Thus it appears that there is merit to include metallothionein among the clinical parameters monitored in cadmium-exposed individuals. More tests are needed to define a critical concentration of metaliothionein in urine which is related to the onset of renal dysfunction.
The biological function of metallothionein (MT) remains unclear. However, it appears that in chronic cadmium (Cd) exposure the protein is induced in liver, kidney and other tissues and is responsible for sequestration of the metal. The metal-protein complex is synthesized on free polysomes and, therefore, remains largely intracellular. Nevertheless, small amounts of Cd bound to MT-like protein have been detected in plasma of animals injected with Cd (1,2). Studies by Tohyama and Shaikh (3) and by others (4) have confirmed the existence of MT in plasma by immunological methods. The circulating Cd-MT is efficiently filtered and taken up by the kidneys. If the concentration of Cd-MT is increased by experimental injection, the proximal renal tubular epithelium is damaged, resembling very much the pathology seen after chronic Cd administration (5)(6)(7). The filtered Cd-MT is, however, not completely reabsorbed and low concentrations of the protein are detectable in urine (2,8,9). With the progression of renal damage, the excretion of Cd-MT, like other low molecular weight plasma pro- teins (i.e., 32-microglobulin, retinol-binding protein, lysozyme, etc.), increases in urine (2). Recent studies by our group (10-12) have examined the practical importance of quantitating extracellular MT for estimation of Cd body burden and also for determining the Cd-induced renal dysfunction. This paper summarizes some of the main points.

Studies in Animals
Using a radioimmunassay developed in our laboratory, MT was quantitated in the plasma and urine of rats injected with Cd. As shown in Figure  1, the concentration of the protein increases in both plasma and urine with the duration of treatment, up to about 8-10 weeks. The renal dysfunction which is evident at this time and presents itself as proteinuria and glucosuria (2) causes the plasma MT concentration to plateau, while elevating the MT concentration in urine. Further analysis showed that, in the absence of renal dysfunction, the urinary MT is related to both hepatic and renal Cd burdens (11), thus strengthening the argument that it is an indicator of body burden.

Studies in Humans
The association between Cd exposure and urinary MT level was further explored in workers at a Cd smelter (11). In general, the workers with longer employment history had higher MT levels in their urine than the workers with shorter employment at the smelter (11,13). When the urinary MT values were compared with the hepatic or renal values, determined by in vivo neutron activation analysis (14), there was a significant correlation between the MT and tissue Cd (Fig. 2). This observation confirmed the studies in animals where a similar correlation was noted (11). There are two other interesting points regarding the data shown in Figure 2. Whereas the hepatic Cd concentration of all workers (with and without renal dysfunction) was related to the urinary MT, the renal Cd was related to MT only in case of subjects with normal renal function. This may be indirect evidence that the main source of urinary MT is probably the liver.
The assocation between the urinary MT and Cd exposure was further evaluated in a study in environmentally exposed Japanese women (12). There have been reports stating that urinary Cd is related to the body burden under chronic exposure situations (15). We compared the Cd and MT concentrations in urine to evaluate whether there existed any relationship between the two. The data depicted in Figure 3 show that there is indeed a significant relationship between Cd and MT levels in urine. It may be argued that the determination of MT in urine, for the purpose of estimating the body burden, is no better a parameter than Cd in urine, except that the latter is prone to contamination errors. Further work is needed to evaluate the potential importance of urinary MT as a superior marker of Cd body burden, especially under occupational exposure situations, where blood and urinary Cd may fluctuate on a daily basis, depending on the extent of exposure. An important point to remember, though, is that urinary Cd-MT is an indicator of tissue Cd burden whereas the non-MT-bound Cd in urine, like blood Cd, may be more related to recent exposure.

Metallothionein and Renal Function Studies in Animals
The filtered MT is efficiently reabsorbed by the kidney under normal circumstances. Any impairment in the tubular function, however, results in marked excretion of the protein in urine. As shown in Figure 1 and previous studies (2,11), upon development of renal dysfunction, the Cd- treated rats do excrete high concentrations of MT in urine. Thus, the excretion of MT in urine appears to be dependent on renal function.

Studies in Humans
,32-microblogulin, a low molecular weight protein, is a non-specific but widely used indicator of renal tubular function. Since the molecular weight of MT is even smaller than P2-microglobulin, we examined the correlation between MT and 02microglobulin in urine to see if the excretion of the two proteins follows the same pattern (12). This indeed was the case, i.e., the concentration of MT in urine increased with the increase in 12microglobulin in urine (Fig. 4). However, whereas the 032-microglobulinuria levels varied four orders of magnitude, those of MT varied only about one and a half orders of magnitude. Thus, one may conclude that ,32-microglobulin in urine is a more sensitive indicator of renal function than MT. Nevertheless, MT is more specific for an indicator of Cd-induced renal dysfunction than 12-microglobulin. Additional data from Itai-Itai disease patients and other Cd-exposed Japanese women revealed that there was significantly greater excretion of MT associated with proteinuria, glucosuria and ,32-microglobulin (Fig. 5) than with normal urine chemistry.

Conclusions
From the data presented in this paper, it appears that the urinary MT level is related to the tissue Cd under normal renal function and that the elevated MT levels are an indication of impaired renal function due to chronic Cd exposure.