Induction of metallothionein by cadmium-metallothionein in rat liver: a proposed mechanism.

Distribution of Cd to various organs following iv administration of CdCl2 (3.5 mg Cd/kg) resulted in more than 43% of total tissue Cd accumulating in the liver. In contrast, after CdMT administration (0.5 mg Cd/kg), only 1% of the Cd was found in liver. Rats administered CdCl2 (1.0 mg Cd/kg) had hepatic MT values 30-fold greater than controls and a hepatic Cd concentration of 17 micrograms/g. In comparison, rats treated with CdMT (0.4 mg Cd/kg) had hepatic MT concentrations 7-fold greater than controls and a hepatic Cd concentration of 0.80 micrograms/g. However, when hepatic MT levels were normalized to tissue Cd concentrations, induction of MT by CdMT was 5-fold greater than by CdCl2. Northern and slot-blot analyses of mRNA showed that both CdCl2 and CdMT coordinately increased MT mRNA. These data suggest that both CdMT and CdCl2 increase hepatic MT by similar mechanisms. A dose-response increase in MT produced by CdCl2 indicated a biphasic response, with low doses producing relatively more hepatic MT than higher doses. In addition, the amount of MT produced per unit Cd after CdMT treatment was similar to those observed after low doses of CdCl2 in the dose-response experiment. These data provide strong evidence to support the conclusion that the apparent potency of CdMT observed here and in previous studies is most likely due to the small amount of Cd distributed to the liver, which is relatively more effective in inducing MT than are higher concentrations.     

Metallothionein protects against the nephrotoxicity produced by chronic CdMT exposure.

Metallothionein (MT) is a low-molecular-weight, cysteine-rich, metal-binding protein. Induction of MT has been proposed to be an important adaptive mechanism in decreasing Cd toxicity. MT has been shown to protect against CdCl2-induced lethality and hepatotoxicity; however, MT does not protect against acute CdMT-induced nephrotoxicity. This study was aimed at clarifying the role of metallothionein in chronic CdMT-induced renal injury. Wild type and MT-I/II knockout (MT-null) mice were therefore given sc injections of CdMT (25 and 100 microg Cd/kg) or saline daily, 6 times/week for 6 weeks, and renal injury was evaluated. Multiple injections of CdMT to wild-type mice resulted in renal Cd concentrations up to 120 microg/g kidney, along with a 100-fold increase in renal MT (450 microg/g kidney). In contrast, renal Cd concentration in MT-null mice administered multiple injections of CdMT reached a much lower level than in wild-type mice (<10 microg/g kidney). Although less Cd accumulated in their kidneys, MT-null mice were more susceptible than wild-type mice to CdMT-induced nephrotoxicity, as indicated by increased urinary excretion of protein and N-acetyl-beta-D-glucosaminidase, as well as by elevated blood urea nitrogen levels. At the higher daily dose of CdMT (100 microg Cd/kg), kidneys of MT-null mice were enlarged. Chronic CdMT administration eventually damaged the entire kidney, which included glomerular swelling, interstitial inflammation, edema, tubular cell degeneration, and atrophy. In contrast to a single injection of CdMT that produces proximal tubular necrosis, chronic injection of CdMT results in tubular cell apoptosis in both wild-type and MT-null mice. These data indicate that chronic CdMT administration produces similar renal injury to that observed after chronic CdCl2 administration, and that intracellular MT protects against nephrotoxicity produced by chronic CdMT administration.     

Kidney synthesizes less metallothionein than liver in response to cadmium chloride and cadmium-metallothionein

Acute exposure to Cd produces liver injury, whereas chronic exposure results in kidney injury. Tolerance to the hepatotoxicity is observed during chronic exposure to Cd due to the induction of metallothionein (MT). The nephrotoxicity produced by chronic Cd exposure purportedly results from renal uptake of Cd-metallothionein (CdMT) synthesized in liver. The change in target organ from liver to kidney might be due to a lower amount of MT synthesized in the kidney in response to CdMT. Therefore, the purpose of the present study was to quantitate hepatic and renal MT induced by CdCl2 and CdMT. MT levels in mice were quantitated using the Cd-heme assay 24 hr after administration of CdCl2 (0.5-3.0 mg Cd/kg) and CdMT (0.1-0.5 mg Cd/kg). In both liver and kidney, MT reached higher levels following administration of CdCl2 (220 and 60 micrograms/g, respectively) than of CdMT (25 and 35 micrograms/g, respectively), probably because higher dosages of CdCl2 than CdMT are tolerated. CdMT produced 19 and 3 micrograms MT/micrograms Cd in liver and kidney, respectively, while CdCl2 produced 11 and 6 micrograms MT/micrograms Cd, respectively. In conclusion, induction of MT occurs in both the liver and kidney after administration of CdCl2 and CdMT. However, the kidney is less responsive than the liver to the induction of MT by both forms of Cd, which may contribute to making the kidney the target organ of toxicity during chronic Cd exposure.     

Resistance to acute nephrotoxicity induced by cadmium-metallothionein dependence on pretreatment with cadmium chloride

Three groups of rats (B-D) were given various daily doses of CdCl2 (0.5-2 mg Cd/kg) continuously or in intervals during time periods of 1-8 weeks. Another group of animals (A) were kept untreated. At the end of the period, selected subgroups of groups A-D were given a single subcutaneous injection of 109Cd-metallothionein (109CdMT) 0.05 or 0.4 mg Cd/kg ("challenge dose"). Subsequently, urinary creatinine, protein, Cd, 109Cd and MT and kidney cortex Cd, 109Cd and MT were determined. In group A (no long term pretreatment), an increased proteinuria was observed after the rats had received the lower of the challenge doses of 109CdMT, and an even greater increase after the higher challenge dose of 109CdMT. No such increase appeared in group B, C and D (repeatedly pretreated with CdCl2) at either of the challenge doses. Higher metallothionein concentrations in kidney cortex observed in the pretreated groups constitute a plausible explanation of the protective effects of pretreatment against the development of increased proteinuria after challenge dosing. It is likely that increasing Cd concentrations, gradually accumulating in the renal cortex (22-226 micrograms/g wet wt.) as a result of the pretreatment, served to induce the synthesis of metallothionein in the renal cortical cells, thus making them resistant to the challenge from 109CdMT.     

Modulation of calciuria by cadmium pretreatment in rats with cadmium-metallothionein-induced nephrotoxicity.

One group of male Wistar rats (Group B) was pretreated by a daily subcutaneous injection with CdCl2 during 5 days with increasing doses (0.5, 1, 1, 2 and 2 mg Cd/kg). Another group of rats (Group A) was daily given normal saline subcutaneously for 5 days. On the second day after the last injection, a single s.c. injection of 109Cd-metallothionein (CdMT, 0.4 mg Cd/kg) was given to each animal in both groups. Urinary calcium, protein, metallothionein (MT), N-acetyl-beta-D-glucosaminidase (NAG) and gamma glutamyltransferase (gamma-GT) were measured. In Group A, calciuria, proteinuria, metallothioneinuria and enzymuria was induced by CdMT. Calciuria reached a peak during 0-6 h after the administration of CdMT, thus appearing earlier than other effects. Enzymuria was displayed at 6-12 h for gamma-GT and 12-24 h for NAG. A prominent increase of proteinuria appeared at 24-48 h after the challenge of CdMT. In Group B, no significant increase of urinary calcium, protein, or NAG was observed after the CdMT injection and urinary gamma-GT was only slightly elevated, thus demonstrating the protective action of pretreatment. This study demonstrates for the first time that calciuria, one of the signs of cadmium nephrotoxicity, can be prevented by cadmium pretreatment. Urinary MT increased slightly during the 4-5 days of CdCl2 pretreatment. This is in accordance with previous observations that cadmium pretreatment induces new synthesis of MT which is likely to constitute the background for the resistance to the CdMT challenge to the kidney.     

Influence of cadmium-metallothionein pretreatment on tolerance of rat kidney cortical cells to cadmium toxicity in vitro and in vivo

Kidney cells were isolated from rats pretreated by daily subcutaneous doses of cadmium metallothionein (CdMT: 0.05-0.2 mg Cd/kg X 5) and from non-pretreated rats. Upon exposure to CdCl2 in vitro (0-200 micrograms Cd/ml), a concentration dependent decrease in viability was observed in the non-pretreated cells, while no such decrease occurred in the pretreated cells indicating that these cells were more resistant to the toxic action of cadmium. There was a higher in vitro uptake of Cd+2 and an increased metallothionein (MT) concentration in the pretreated cells (compared to non-pretreated cells). Subcellular distribution studies revealed that Cd was mainly recovered in the "cytosol" fraction. The higher total cadmium uptake in pretreated cells corresponded to an increase of Cd in "cytosol" and "nuclear" fractions. This observation may be explained by MT-binding of Cd in the cells and is in accordance with a possible protective effect of induced MT in the pretreated cells. In order to assess whether pretreatment-induced tolerance to cadmium toxicity--indicated by the cellular studies--could also be observed in vivo, some whole animal experiments were also performed. A dose-related proteinuria was observed in non-pretreated rats after a single subcutaneous administration of 109Cd-MT at doses of 0.05 and 0.4 mg Cd/kg. Urinary total Cd, 109Cd and MT was also increased in a dose-related fashion. Cadmium concentrations in kidney were dose related and reached 19 micrograms/g wet weight. In contrast, in animals repeatedly pretreated with CdMT according to 1), no proteinuria was observed after administration of the same single doses of 109CdMT. Total Cd. 109Cd and particularly MT-concentrations in urine were lower in such pretreated animals than in in non-pretreated ones in spite of the accumulation of higher tissue concentrations of total Cd (up to 80 micrograms/g). The pretreatment was thus shown to prevent some of the acute nephrotoxicity of CdMT, possibly by means of induction of MT synthesis.     

Comparisons of the toxicity of CdCl2 and Cd-metallothionein in isolated rat hepatocytes

In the intact animal, inorganic Cd distributes mainly to the liver and produces hepatotoxicity, while Cd-metallothionein (CdMT) distributes primarily to the kidney and produces nephrotoxicity. CdMT has also been demonstrated to be more toxic than Cd in cultured kidney cells, but it is not known if CdMT is more toxic to all cultured cells or if there is a good correlation between in vitro and in vivo toxicity. Therefore, hepatocytes, which were isolated and grown in monolayer culture for 24 h, were incubated with CdCl2 (1-100 microM) or CdMT (3-100 microM Cd). The intracellular K+ content was quantitated 24 h later as an index of toxicity. The K+ concentration of the hepatocytes was decreased 50% by 4 microM CdCl2, whereas 25 microM CdMT was required to produce similar injury. In the intact animal, zinc induces the synthesis of MT and decreases the hepatotoxicity of Cd. ZnCl2 added to the media (100 microM) for 24 h before exposure to Cd or CdMT increased the intracellular MT concentration 700%. This elevation in MT reduced the toxicity of CdCl2 approximately 80% but did not alter the toxicity of CdMT. In summary, CdCl2 is more toxic to cultured hepatocytes than Cd-MT, and MT induction decreases the toxicity of CdCl2 in hepatocytes, as has been observed in the intact animal. This indicates that cultured hepatocytes appear to be an excellent model for examining the hepatotoxicity of Cd.     

Protection by Zinc-Metallothionein (ZnMT) against Cadmium-Metallothionein-Induced Nephrotoxicity

In contrast to inorganic Cd, acute iv administration of Cd boundto metallothionein (CdMT) concentrates in renal tissue. Thisuptake of CdMT produces functional and morphological changesin kidneys, similar to those observed after chronic exposureto inorganic Cd. In order to examine the importance of the metalcomponent of MT in the renal uptake of MT, the renal concentrationof ³⁵S after administration of [³⁵S]ZnMT and [³⁵S]CdMT was compared.Renal uptake of ³⁵S from both CdMT and ZnMT was very rapid,with peak concentrations observed 15–30 min after administration.³⁵S in kidneys increased in a dose-dependent manner after administrationof various doses of [³⁵S]ZnMT, up to 1.3 µmole MT/kg;however, higher doses did not further increase renal ³⁵S concentrations.A similar saturation of ³⁵S reabsorption was observed for therenal uptake of [³⁵S]CdMT. CdMT produced renal injury with dosesas low as 0.26 µmol MT/kg (0.2 mg Cd/kg). In contrast,with a dose of ZnMT as high as 5.12 µmol MT/kg (2 mg Zn/kg),no histopathological changes were observed. Therefore, ZnMTappears to be nontoxic even though ZnMT delivers more MT tothe kidney than does CdMT. Because ZnMT and CdMT are apparentlyhandled by the same renal transport mechanism, the effects ofZnMT on ¹⁰⁹CdMT renal uptake and nephrotoxicity were determined.One group of mice was given a nephrotoxic dose of ¹⁰⁹CdMT (0.51µmol MT/kg containing 0.4 mg Cd/kg, iv), and the othergroup received an equimolar dose of unlabeled ZnMT 1 min before¹⁰⁹CdMT administration. Renal function was evaluated by measuringurinary glucose and protein excretion, as well as histopathology.Marked renal toxicity was observed 24 hr after ¹⁰⁹CdMT administration.In contrast, renal function appeared normal in mice receivingZnMT before ¹⁰⁹CdMT. However, a similar concentration of ¹⁰⁹Cdwas found in kidneys of both groups. The present results demonstratethat ZnMT is not only nontoxic to the kidney at a dose as highas 5 µmol MT/kg, but can also protect against the nephrotoxiceffect of CdMT without decreasing renal Cd concentration.     

半胱氨酸，谷胱甘肽和除去金属的金属硫蛋白对含镉金属硫蛋白肾毒性的保护作用

在大鼠不同部位ｓｃ巯基化合物与含镉金属硫蛋白（ＣｄＭＴ），观察除去金属的金属硫蛋白（ＡｐｏＭＴ），Ｌ－半胱氨酸（Ｃｙｓ），还原型谷胱甘肽（ＧＳＨ）对大鼠ＣｄＭＴ损伤肾的保护作用。测定不同时相尿蛋白和尿碱性磷酸酶活性以及肾组织形态学结果表明，与单独给予ＣｄＭＴ比较，肾损伤作用明显减轻。ＡｐｏＭＴ，ＧＳＨ促使大鼠尿Ｃｄ排泄量显著增加，肾组织细胞中金属硫蛋白（ＭＴ）结合Ｃｄ和游离Ｃｄ含量明显降低，以ＧＳＨ尤为明显Ｃｙｓ能增加Ｃｄ在肾细胞内蓄积，提示在Ｃｄ中毒时，除ＭＴ对肾损伤有保护作用外，也有其它巯基化合物的参与，且作用机理不尽相同     

Induction of metallothionein synthesis by zinc in cadmium pretreated rats

The ability of zinc (Zn) salts to induce the synthesis of metallothionein (MT) in liver, kidney and pancreas of rats pretreated with cadmium (Cd) salts was investigated. Twenty-four hours after either CdCl2 (2.0 mg Cd/kg, s.c.) or saline pretreatment, rats were injected with saline, CdCl2 (2.0 mg Cd/kg, s.c.) or ZnSO4 (20 mg Zn/kg, s.c.) and the concentrations of MT and MT-1 mRNA in tissues subsequently measured. After a single injection of Cd salts, concentrations of MT and MT-1 mRNA were significantly increased in liver as compared to control. With two injections of Cd, the accumulation of MT in liver was approximately twice the levels of MT following a single injection of Cd. In kidney, MT and MT-1 mRNA expression were significantly increased only after two injections of Cd and in the pancreas, Cd injections did not alter either MT content or MT-1 mRNA expression. Treatment with Zn salts increased MT concentrations in both liver and pancreas. However, the pancreas was the most responsive to injections of Zn salts as compared to the liver in terms of increases in both protein concentration and MT-1 mRNA expression. When Zn injection was preceded by a Cd injection, induction as measured by MT-1 mRNA and MT concentrations were approximately additive in liver. In kidney, although Cd or Zn treatment separately had no effect on MT or MT-1 mRNA content, injection of Cd followed by Zn resulted in significantly increased levels of renal MT and MT-1 mRNA. Fractionation of liver cytosols on a Sephadex G-75 column revealed that in animals receiving two injections of Cd, virtually all the Cd was associated with MT whereas Zn was distributed between both high molecular weight (HMW) proteins and MT. In animals receiving both Cd and Zn injections, cytosolic Cd was still bound predominantly to the MT fraction, while the proportion of cytosolic Zn associated with MT increased. The results of this study suggest that, treatment with Cd salts followed by Zn salt injection can induce further synthesis of MT in liver, kidney and pancreas with subsequent binding of both Zn and Cd to the intracellular MT.     

Combined nephrotoxicity of methylmercury, lead, and cadmium in Pekin ducks: metallothionein, metal interactions, and histopathology

This report describes the metallothionein (MT) levels and accumulation of mercury, lead, and cadmium, as well as their interaction with tissue zinc, copper, and iron, and the histopathological changes in kidneys of ducks exposed to methylmercury chloride (MeHgCl), lead acetate (PbAc), and cadmium chloride (CdCl2), singly or in combination for 13 wk. Forty-eight female Pekin ducks, divided into 8 groups of 6 birds each, were fed diets containing no added metals (control), 8 mg MeHgCl/kg feed, 80 mg PbAc/kg feed, 80 mg CdCl2/kg feed, 8 mg MeHgCl + 80 mg PbAc/kg feed, 8 mg MeHgCl + 80 mg CdCL2/kg feed, 80 mg PbAc + 80 mg CdCl2/kg feed, and 8 mg MeHgCl + 80 mg PbAc + 80 mg CdCL2/kg feed. Cadmium (Cd) when administered alone or in combination caused a 60-fold increase in kidney MT levels, while methylmercury (MeHg) or lead (Pb) administration caused a threefold increase in kidney MT levels. No significant changes in kidney MT levels were observed when metals were administered concurrently when compared with single-treatment groups. Residue analysis revealed accumulation of administered metals in kidney tissue. However, lead administration resulted in accumulation of small amounts of this element in kidney tissue. Simultaneous administration of MeHgCl and PbAc significantly increased the accumulation of lead in kidney when compared with PbAc-treated group. Cadmium when administered alone or in combination caused an increase in the levels of zinc and copper in kidney. Administration of MeHgCl or PbAc either alone or in combination caused increased iron levels in kidney, while cadmium administration either alone or in combination caused decreased iron levels. Administration of cadmium either alone or in combination caused degenerative changes in kidney proximal tubules. The severity of degenerative lesions increased when cadmium was simultaneously administered with other metals. These results indicate that combined administration of MeHg, Pb, and Cd has no significant effect on kidney MT levels or on essential elements in kidney tissue when compared with single metal groups. However, there appears to be an increase in the severity of histopathologic changes.     

Accumulation and degradation of the protein moiety of cadmium-metallothionein (CdMT) in the mouse kidney.

Of major concern in Cd toxicity is its ability to produce renal damage after chronic exposure in humans and experimental animals. Renal injury affects predominantly the proximal tubules and more specifically the first segments of these tubules. Similar toxic effects to the kidneys are observed after administration of cadmium bound to metallothionein (CdMT). Therefore, CdMT was used in this study as a model to understand the mechanism(s) of Cd nephrotoxicity. It has been recently demonstrated that Cd from CdMT was preferentially taken up by the proximal convoluted tubules. Therefore, the purpose of these studies was to determine if the organic portion of the complex was also accumulated in these tubules. [35S]CdMT prepared from rat liver was administered intravenously to mice at a nonnephrotoxic dose (0.1 mg Cd/kg). The radioactivity in the kidney showed maximum level (80% of the dose) 15 min after the injection. This preferential renal uptake was also observed after administration of various doses of [35S]CdMT. In contrast to the earlier observed persistency of 109Cd in the kidney after 109CdMT administration, 35S disappeared rapidly (with a half-life of approximately 2 hr), and 24 hr after injection of [35S]CdMT, there was very little 35S left in the kidneys. These observations indicate that the protein portion of CdMT is rapidly degraded after renal uptake of CdMT and the released Cd is retained in the kidney. Within the kidney, 35S distributed mainly to the cortex. Light microscopic autoradiography showed that [35S]CdMT preferentially distributed to the proximal convoluted tubule (S1 and S2), which is the site of nephrotoxicity. Within the S1 and S2 segments, a greater distribution of 35S to the apical portion of the cells was observed after administration of both a nonnephrotoxic (0.1 mg Cd/kg) and a nephrotoxic (0.3 mg Cd/kg) dose. 109Cd administered as 109CdMT also distributed to the apical portion of the S1 and S2 cells. Therefore, both the organic (35S) and inorganic (109Cd) portions of CdMT are rapidly and efficiently taken up by the S1 and S2 cells of the proximal tubules, the site of nephrotoxicity. These observations support the concept that CdMT is readily taken up by the proximal tubular cells as a complex, and then its protein portion is rapidly degraded to release Cd that binds permanently to intracellular sites and produces nephrotoxicity.     

Renal cortical mitochondrial dysfunction upon cadmium metallothionein administration to Sprague-Dawley rats

A bolus dose of cadmium metallothionein (CdMT) produces renal proximal tubular dysfunction because it accumulates in the tubular epithelial cells and undergoes rapid degradation, releasing Cd. Morphologically, mitochondria appear to be the target organelle. The present study examined changes in renal cortical mitochondrial function following CdMT administration and investigated whether some of these effects could be ascribed to Cd2+ accumulation in the mitochondria. Sprague-Dawley rats were injected ip with 0.3 mg Cd as CdMT/kg and the animals were sacrificed after 6, 8, or 12 h. Two- to threefold increases in urinary protein excretion and LDH activity were evident at 8 h, with marked elevations (11- and 29-fold) thereafter. Renal cortical mitochondria were swollen and rounded at 12 h. The mitochondrial Cd level was 399 pmol/mg protein at 6 h and did not change significantly during the next 6 h; however, mitochondrial respiratory function declined with time. At 12 h, state 3 oxygen consumption, respiratory control ratio (RCR), and ADP:O (P/O) ratio were 48, 49, and 76% of control values, respectively, indicating inhibition of electron transfer and oxidative phosphorylation. The direct effect of Cd on mitochondrial function was examined by incubating mitochondria from untreated rats with 0.1-2 microM CdCl2. Rapid uptake of Cd resulted in concentration-dependent effects on respiration. After 1 min of incubation with 2 microM Cd, the mitochondria contained 262 microgCd/mg protein and state 3 respiration and RCR values were 75 and 33% of control levels, respectively. Thus, renal proximal tubular cell damage following a bolus dose of CdMT involves perturbations in mitochondrial respiration, brought on by the accumulation of Cd.     

The modulation by metallothionein of cadmium-induced cytotoxicity in primary hepatocyte cultures

The cytotoxicity of CdCl2 and 2 isoforms of hepatic cadmium-metallothionein (CdMT I and II), was investigated using primary cultures of rat hepatocytes. The cell cultures were exposed to cadmium as CdCl2 or as either isoform of CdMT for a 20-h period at concentrations ranging from 50 to 500 ng Cd X ml-1. Cytotoxicity was assessed by determining the amount of lactic dehydrogenase released from the cells into the incubation medium and the incorporation of [3H] arginine into cell protein. The uptake of Cd by the cells was also measured. Cadmium chloride and both isoforms of CdMT were found to be toxic to hepatocytes although partial protection was afforded by the binding of cadmium to metallothionein (MT). At the higher exposure concentrations and in accordance with the toxicity data, the cells exposed to CdCl2 were found to accumulate more cadmium than those exposed to CdMT. The distribution of cadmium in the culture medium was examined using Sephadex G-75 chromatography. The speciation of cadmium is discussed in relation to its cytotoxicity.     

Cadmium-metallothionein nephrotoxicity in the rat: transient calcuria and proteinuria

After a s.c. injection of 0.4 mg Cd/kg as cadmium-metallothionein (CdMT) in rats, a marked increase in urinary protein concentration appeared at 16-40 h. There was a peak of urinary Cd content during the first 4 h after the treatment. Urinary Ca was increased at 8 h after the CdMT injection and returned to normal level at 32 h. Luminal and basolateral renal membrane vesicles were isolated from both control group and CdMT (0.4 mg Cd/kg) group at 24 h after the injection. Calcium uptake and binding of both fractions were decreased in the group treated with CdMT. Cd, Zn and MT concentrations in the kidney cortex were increased, but Ca concentration was not significantly changed. Since injected CdMT is probably only partly reabsorbed by tubular cells at the dose level of 0.4 mg Cd/kg as CdMT, excessive plasma CdMT is rapidly excreted in urine, explaining the increased Cd excretion during the first few hours observed in the present experiment. Decreased Ca binding in the luminal membranes as observed in vitro could be one of the mechanisms of production of calcuria if occurring in vivo. Another possible explanation of calcuria is that Cd ions released from CdMT into the cytoplasm of the tubular cell, may exert ionic interference with Ca transport across the luminal membranes and produce decreased Ca reabsorption. It is known that a disturbance of Ca metabolism could influence the membrane stability and such a change may contribute to explaining the proteinuria characteristic of CdMT nephrotoxicity. The reversibility of the proteinuria observed after a single dose of CdMT may be related to the induction of metallothionein synthesis in the renal cells.     

Renal cadmium deposition and injury as a result of accumulation of cadmium-metallothionein (CdMT) by the proximal convoluted tubules--A light microscopic autoradiography study with 109CdMT.

Chronic, but not acute, exposure to inorganic Cd produces renal damage. However, a single injection of cadmium bound to metallothionein (CdMT) produces renal injury. It is hypothesized that an interorgan redistribution of Cd as CdMT is responsible for the chronic nephrotoxic effect of Cd. To better understand the mechanism(s) of CdMT-induced nephrotoxicity, the intrarenal distribution of 109CdMT was examined. 109CdMT isolated from rat liver was injected into mice at a nonnephrotoxic dose (0.1 mg Cd/kg, iv). The radioactivity in the kidney reached a maximum level (85% of the dose) as early as 30 min following administration and remained essentially constant for up to 7 days after injection. Within the kidney, 109Cd distributed almost entirely to the cortex. Light microscopic autoradiography of the kidney showed that, within the cortex, 109Cd distributed preferentially to the S1 and S2 segments of the proximal convoluted tubules. Within the S1 and S2 segments, the concentration of 109Cd in the basal and apical parts of the cells was similar to that after the nonnephrotoxic dose of CdMT, but after a nephrotoxic dose (0.3 mg Cd/kg) the radioactivity distributed preferentially to the apical portion of the cells. In contrast, light microscopic autoradiography studies with 109CdCl2 revealed that 109Cd was more evenly distributed throughout the proximal tubules. Moreover, after administration of a large dose of inorganic Cd (3 mg Cd/kg), a similar concentration of Cd was found in the convoluted and straight proximal tubules. These data support the hypothesis that CdMT-induced nephrotoxicity might be due, at least in part, to its preferential uptake of CdMT into the S1 and S2 segments of the proximal tubules, the site of Cd-induced nephrotoxicity.     

肝脏损害对染镉大鼠镉分布的影响

大鼠腹腔内注射ＣｄＣｌ２０．５ｍｇＣｄ＾２＋／Ｋｇ体重，每周三次，共１０周。注射ＣｄＣｌ２第４周末，其中一组动物灌胃ＣＣｌ４９００ｍｇ／ｋｇ体重。结果表明ＣｄＣｌ２＋ＣＣｌ４组动物肝脏损害后肝镉浓度明显低于单纯ＣｄＣｌ２组，同时伴随血镉，肾镉水平显著升高。肝、肾中金属硫蛋白浓度也与相应组织中隔浓度呈类似的变化形式。ＣｄＣｌ２＋ＣＣｌ４组动物尿镉和尿金属硫蛋白浓度均明显高于ＣｄＣｌ２组。这些实验     

Differential hepatotoxicity induced by cadmium in Fischer 344 and Sprague-Dawley rats.

A number of reports document that Fischer 344 (F344) rats are more susceptible to chemically induced liver injury than Sprague-Dawley (SD) rats. Cadmium (CdCl2), a hepatotoxicant that does not require bioactivation, was used to better define the biological events that are responsible for the differences in liver injury between F344 and SD rats. CdCl2 (3 mg/kg) produced hepatotoxicity in both rat strains, but the hepatic injury was 18-fold greater in F344 rats as assessed by plasma alanine aminotransferase (ALT) activity. This difference in toxicity was not observed when isolated hepatocytes were incubated with CdCl2 in vitro, indicating that other cell types contribute to Cd-induced hepatotoxicity in vivo. Indeed, the sieve plates of hepatic endothelial cells (EC) in F344 rats were damaged to a greater degree than EC in SD rats. Additionally, Kupffer cell (KC) inhibition reduced hepatotoxicity in both strains, suggesting that this cell type is involved in the progression of CdCl2-induced hepatotoxicity. Moreover, enhanced synthesis of heat shock protein 72 occurred earlier in the SD rat. Maximal levels of hepatic metallothionein (MT), a protein associated with cadmium tolerance, were greater in SD rats. These protective factors may limit CdCl2-induced hepatocellular injury in SD compared with F344 rats by reducing KC activation and the subsequent inflammatory response that allows for the progression of hepatic injury.     

Metallothionein-null mice are more sensitive than wild-type mice to liver injury induced by repeated exposure to cadmium.

Liver is a major target organ of cadmium (Cd) toxicity following acute and chronic exposure. Metallothionein (MT), a low-molecular-weight, cysteine-rich, metal-binding protein has been shown to play an important role in protection against acute Cd-induced liver injury. This study investigates the role of MT in liver injury induced by repeated exposure to Cd. Wild-type and MT-I/II knockout (MT I/II-null) mice were injected sc with a wide range of CdCl(2) doses, 6 times/week, for up to 10 weeks, and their hepatic Cd content, hepatic MT concentration, and liver injury were examined. Repeated administration of CdCl(2) produced acute and nonspecific chronic inflammation in the parenchyma and portal tracts and around central veins. Higher doses produced granulomatous inflammation and proliferating nodules in liver parenchyma. Apoptosis and mitosis occurred concomitantly in liver following repeated Cd exposure, whereas necrosis was mild. As a result, significant elevation of serum enzyme levels was not observed. In wild-type mice, hepatic Cd concentration increased in a dose- and time-dependent manner, reaching 400 microgram/g liver, along with 150-fold increases in hepatic MT concentrations, the latter reaching 1200 microgram/g liver. In contrast, in MT I/II-null mice, hepatic Cd concentrations were about 10 microgram/g liver. Despite the lower accumulation of Cd in livers of MT I/II-null mice, the maximum tolerated dose of Cd was one-eighth lower than that for wild-type mice at 10 weeks, and liver injury was more pronounced in the MT I/II-null mice, as evidenced by increases in liver/body weight ratios and histopathological analyses. In conclusion, these data indicate that (1) nonspecific chronic inflammation, granulomatous inflammation, apoptosis, liver cell regeneration, and presumably, preneoplastic proliferating nodules are major features of liver injury induced by repeated Cd exposure, and (2) intracellular MT is an important protein protecting against this Cd-induced liver injury.     

The origin of metallothionein in red blood cells

The origin of metallothionein (MT) in red blood cells (RBCs) from a mouse given cadmium was studied in connection with RBC kinetics. Plasma Cd concentration rapidly decreased 3 hr following 109CdCl2 (2 mg/kg, sc) administration, whereas RBC Cd increased from 2 to 4 days, followed by a gradual decrease. RBC Cd was found to be distributed more in the high-molecular-weight fraction than in the MT fraction 12 hr after administration. But, thereafter, Cd increased rapidly in the MT fraction to show changes with time similar to Cd level in RBCs. Hepatic damage induced in a mouse given 21 injections of Cd, with resultant marked elevation of plasma MT concentrations, did not cause any change in RBC Cd concentration. MT was hardly transferred to RBC when a mouse RBC suspension was incubated with mouse hepatic MT. To examine the relationship of Cd-MT and erythropoietic function, mice in the normal group, the phenylhydrazine-induced anemia group (PH), the transfusion-induced plethora group (TR), and the erythropoietin administered plethora group (TR + EP) were given 109CdCl2. Three days after administration, Cd concentration in its RBCs and its MT fraction remarkably increased in the PH group, and was greatly decreased in the TR group. A significant increase was noted in the TR + EP group as compared with the TR group. These results indicate that MT in the RBCs is formed in erythroblasts.