Mechanism of cadmium-metallothionein-induced nephrotoxicity: relationship to altered renal calcium metabolism

Nephrotoxicity of cadmium-metallothionein: protection by zinc and role of glutathione

Chronic cadmium (Cd) exposure can cause renal proximal tubular dysfunction resulting from the release of Cd metallothionein (CdMT) from the liver and its accumulation and degradation in the renal tubular epithelial cells. Pretreatment with zinc (Zn) can protect against acute CdMT nephrotoxicity. While induction of MT by Zn plays a part in Zn protection, other factors, such as glutathione (GSH), may also be involved because protection is offered even in MT-null mice. The present study was designed to investigate the involvement of GSH in Zn protection against acute CdMT nephrotoxicity. The study was carried out in MT-null mice to remove the induction of MT by Zn as a confounding variable. Three approaches were used to modulate renal cortex GSH levels: buthionine sulfoximine (BSO) was administered to inhibit GSH synthesis, and GSH and Zn were administered to increase the GSH levels. Both GSH and Zn were effective in protecting against CdMT nephrotoxicity. Elevation in renal cortex GSH levels, however, was not essential for Zn protection, as a low dose of Zn that caused no significant increase in renal GSH also protected against CdMT. On the other hand, maintenance of normal GSH status was essential for Zn protection, as inhibition of GSH synthesis abolished this protection. Both GSH and Zn reduced the accumulation of Cd as well as MT in the renal cortex, with Zn causing greater reduction in Cd accumulation than that of MT. The relative intracellular distribution of Cd was unaltered. These results suggest that in MT-null mice Zn protects against CdMT nephrotoxicity by possibly displacing some of the Cd from CdMT as well as reducing the uptake of CdMT, and that this protection requires the maintenance of normal GSH status.

Induction of c-fos proto-oncogene in mesangial cells by cadmium

Cadmium is mitogenic under some circumstances and has been shown to cause accumulation of transcripts for several proto-oncogenes in a variety of cells, but the mechanism(s) remain to be delineated. Here we show that CdCl2 causes an increase in c-fos mRNA within 30 min of exposure of mesangial cells. At 10 microM Cd2+, this increase persists for at least 8 h in both rat and human cells. The half-life of c-fos mRNA is the same whether it accumulates following 4 h of treatment with Cd2+ or is induced transiently by phorbol ester. Cycloheximide, which stabilizes the transcript, causes a synergistic increase when administered with CdCl2. Nuclear run-on analysis confirms that Cd2+ causes transcriptional activation of the c-fos gene. Calmodulin and Ca2+/calmodulin-dependent kinase, and classical protein kinase C (PKC) isoforms represent two Ca2+-dependent signaling pathways that can lead to induction of c-fos, and Cd2+ has been shown to activate both calmodulin and PKC in vitro, possibly by virtue of the similar ionic radii of Cd2+ and Ca2+. Therefore, we investigated the effect of Cd2+ on these pathways in vivo. 10 microM CdCl2 did not increase total PKC activity or Ca2+/calmodulin-dependent kinase II activity and inhibited the latter at higher concentrations, ruling out either pathway in the Cd2+-dependent induction of c-fos. However, Cd2+ did lead to a sustained activation of the Erk family mitogen-activated protein kinases (MAPK) that correlated with induction of c-fos. A specific inhibitor of the MAPK kinases, PD98059, partially inhibited the induction of c-fos by Cd2+. We conclude that Cd2+ induces c-fos at least in part by causing a sustained activation of MAPK independent of its ability to activate PKC and calmodulin in vitro.

A multivariate study of protective effects of Zn and Cu against nephrotoxicity induced by cadmium metallothionein in rats

Factorial experimental design was used to study the protective effects of Zn and Cu on cadmium-metallothionein(CdMT)-induced nephrotoxicity in male Wistar rats. In the factorial design two levels of Zn (0 and 25 mg/kg body weight), two levels of Cu (0 and 12.5 mg/kg), and two levels of CdMT (0.1 and 0.4 mg of Cd/kg) were used as varied factors. The factorial design was complemented with a center point with all three variables at an intermediate setting, i.e., Zn at 12.5 mg/kg, Cu at 6.25 mg/kg, and CdMT at 0.25 mg Cd/kg. Each of the nine combinations of settings was administered to one of nine groups with six rats in each. Zn and Cu were injected sc 24 hr prior to the injection of CdMT. The concentrations of protein and Ca in urine and Ca in renal cortex were used as effects. The relationship between the experimental design settings and the effects were modeled with multiple regression. The multiple regression analysis revealed that for the high dose of CdMT (i) the enhanced values of protein in urine caused by CdMT injection could be more efficiently reduced by Zn than by Cu, and (ii) excessive Ca in urine and renal cortex could be more efficiently reduced by Cu than by Zn. No significant synergism or antagonism between Cu and Zn was found. These models can be used to estimate the dose levels of Zn and Cu which will reduce the toxic effects of CdMT. The treatment of 20.4 mg/kg Zn, for example, will reduce the effects of 0.4 mg Cd/kg as CdMT on protein in urine, and 2.8 mg/kg Cu will reduce the Ca in urine to the levels of those caused by 0.25 mg Cd/kg (no Zn and Cu). Similarly, the effect of 0.4 mg Cd/kg on Ca level in renal cortex can be reduced to that of 0.28 mg Cd/kg as CdMT by 7.98 mg Cu/kg, which is three times as efficient as Zn. The obtained results might be of importance in understanding the mechanism of cadmium toxicity and the potential risk to the health of the population exposed to cadmium occupationally or environmentally.

Cadmium-metallothionein-induced kidney dysfunction increases magnesium excretion in the rat

Urinary excretion of the major minerals, calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K), as well as of protein and metallothionein, was studied following the injection of cadmium-metallothionein (CdMT) in rats. Animals were given vehicle (saline) and 0.4 and 0.8 mg Cd/kg body wt as CdMT. A marked, relatively early, and reversible increase in Mg excretion was seen. The increase was dose-related, indicating a close connection to the typical Cd-derived cellular damage in the renal tubular epithelium, including an early reversible Ca excretion and a late reversible protein excretion. The increase in Mg excretion was similar in magnitude to the one for Ca and much more prominent than that recorded for Na and K. The appearance of Mg and Ca excretion peaks at an early stage after CdMT injection makes it likely that this effect is an early event in the process of development of cellular damage and does not merely represent unspecific cellular damage giving rise to proteinuria.

Renal discrimination between strontium and calcium in rats. Effects of renal damage caused by cadmium-metallothionein

Renal functions were damaged at the reabsorptive site by the injection of cadmium-metallothionein (Cd-MT) and its effect on discrimination of chemically similar elements was examined for the two alkaline-earth elements, calcium (Ca) and strontium (Sr). Tubular damage was induced in female Wistar rats, 7 wk old, body wt 145.1 +/- 3.5 g (mean +/- S.D.), by an intraperitoneal injection of Cd-MT (400 micrograms Cd/kg body wt). The Cd-MT injection caused increases in urinary enzymes (lactate dehydrogenase; alkaline phosphatase), glucose, and total protein. Urinary Ca and Sr increased and urinary Sr/Ca ratio decreased rapidly after the Cd-MT injection. Both changes remained at significant levels throughout the experiment. On the other hand, renal Ca and Sr levels increased with time after 18 or 24 h and changed similarly. Although plasma levels of Ca and Sr slightly increased after 30 or 36 h post-injections, the plasma Sr/Ca level remained constant. A close exponential relationship formed between the relative clearances of Ca and Sr. There was no significant difference between the exponent of the following equation (K) for the Cd-MT-injected group and that for the control group. Sr clearance/creatinine clearance = (Ca clearance/creatinine clearance)K where K = 0.408 from experimental data for the two groups. This suggests that the discrimination mechanism between Sr and Ca during the reabsorptive step in the kidney is strictly regulated.

Modulation of metal toxicity by metallothionein

Toxic properties of several metals may be modified, since they are bound to metallothionein in vivo. Such modulation is particularly well known for cadmium (Cd), whose acute effects are prevented by metallothionein induction, whereas chronic effects on the kidney are partly explained on the basis of transport of cadmium-metallothionein (CdMt) into the kidney. Although intracellular Mt synthesis is induced by Cd, offering partial protection, nephrotoxicity may occur at times when such protection is insufficient. Perturbations in renal calcium metabolism may be an important basis for membrane dysfunction leading to proteinuria.