Cadmium-induced hepatic and renal injury in chronically exposed rats: likely role of hepatic cadmium-metallothionein in nephrotoxicity

Metallothionein: an intracellular protein to protect against cadmium toxicity

Metallothioneins (MT) are low-molecular-weight, cysteine-rich, metal-binding proteins. MT genes are readily induced by various physiologic and toxicologic stimuli. Because the cysteines in MT are absolutely conserved across species, it was suspected that the cysteines are necessary for function and MT is essential for life. In attempts to determine the function(s) of MT, studies have been performed using four different experimental paradigms: (a) animals injected with chemicals known to induce MT; (b) cells adapted to survive and grow in high concentrations of MT-inducing toxicants; (c) cells transfected with the MT gene; and (d) MT-transgenic and MT-null mice. Most often, results from studies using the first three approaches have indicated multiple functions of MT in cell biology: MT (a) is a "storehouse" for zinc, (b) is a free-radical scavenger, and (c) protects against cadmium (Cd) toxicity. However, studies using MT-transgenic and null mice have not strongly supported the first two proposed functions but strongly support its function in protecting against Cd toxicity. Repeated administration of Cd to MT-null mice results in nephrotoxicity at one tenth the dose that produces nephrotoxicity in control mice. Human studies indicate that 7% of the general population have renal dysfunction from Cd exposure. Therefore, if humans did not have MT, "normal" Cd exposure would be nephrotoxic to humans. Thus, it appears that during evolution, the ability of MT to protect against Cd toxicity might have taken a more pivotal role in the maintenance of life processes, as compared with its other proposed functions (i.e. storehouse for zinc and free radical scavenger).

Protection against chronic cadmium toxicity by glycine

A Japanese drug containing glycine, glycyrrhizin, and cysteine (Stronger Neo-Minophagen C) has been reported to protect against chronic cadmium (Cd) toxicity. The present study was conducted to evaluate which of the three constituents of this drug was the main antagonist for Cd toxicity and whether the mechanism of protection involved antioxidant action. Adult female Sprague-Dawley rats were injected sc with 5 micromol CdCl2/kg per day, five times per week, for 15 weeks. Four groups of Cd-injected animals received co-treatments with either 10 mg glycyrrhizin/kg, 100 mg glycine/kg, 5 mg cysteine/kg, or with a mixture of all three compounds, five times per week, starting from week 7. An additional Cd-injected group was co-treated with vitamin E (100 mg/kg, five times per week, starting from week 7) as a positive control. Only those animals that received vitamin E, Minophagen mixture, or glycine were protected against Cd-induced hepatotoxicity as well as nephrotoxicity. All three co-treatments suppressed Cd-induced hepatic and renal lipid peroxidation. We conclude that the reported beneficial effects of Stronger Neo-Minophagen C are due to glycine, which appears to protect against chronic Cd toxicity by reducing oxidative stress.

Cadmium binding and sodium-dependent solute transport in renal brush-border membrane vesicles

Exposure to cadmium (Cd) impairs renal transport systems for glucose, amino acids, phosphate, and dicarboxylates. To investigate if these changes are directly related to a Cd binding to the renal brush-border membrane, Cd binding and the Na+-dependent uptakes of d-glucose, l-alanine, phosphate, and succinate were determined in rat renal brush-border membrane vesicles (BBMV) exposed to CdCl2. Cd uptake by BBMV showed time and concentration dependence. Changes in medium osmolality had no effect on Cd uptake, indicating that the process primarily involves binding of Cd to the membrane. Scatchard analysis indicated the presence of two types of Cd binding sites, differing in affinity and number. Increasing the medium Cd concentration from 50 to 200 microM resulted in a progressive increase in Cd binding to the membrane and decrease in Na+-dependent transport of d-glucose, l-alanine, inorganic phosphate, and succinate. In all cases, the inhibition of transport was directly proportional to the total amount of Cd binding to the membrane. These results suggest that, during chronic exposure to Cd, free Cd ions liberated in renal tubular cells may directly interact with brush-border membranes and impair Na+-dependent solute transports.

Oxidative stress as a mechanism of chronic cadmium-induced hepatotoxicity and renal toxicity and protection by antioxidants

The role of oxidative stress in chronic cadmium (Cd) toxicity and its prevention by cotreatment with antioxidants was investigated. Adult female Sprague-Dawley rats were injected sc with 5 micromol CdCl2/kg/day, 5 times a week, for up to 22 weeks. Serum alanine amino transferase and lactate dehydrogenase activities were elevated after 9 weeks of Cd administration, indicating hepatic damage. Renal toxicity, indicated by elevation in urinary lactate dehydrogenase activity and protein, was also observed around this time. Chronic Cd administration resulted in a gradual rise in hepatic as well as renal cortex glutathione levels. In spite of this, lipid peroxidation increased in both tissues, particularly during the second half of the Cd exposure period. Depletion of glutathione following buthionine sulfoximine administration at the end of Week 5, or inhibition of catalase by aminotriazole at the end of Week 7, resulted in the development of acute nephrotoxicity within 6 h. Coadministration of antioxidants, N-acetylcysteine (50-100 mg/kg, sc), or vitamin E (100-150 mg/kg, sc) with Cd, starting from the early phases of Cd exposure, controlled Cd-induced lipid peroxidation and protected the animals against hepatic as well as renal toxicity. A Japanese hepatoprotective drug, Stronger Neo-Minophagen C, containing glycyrrhizin, glycine, and cysteine, was also effective in reducing the chronic Cd nephrotoxicity. In conclusion, oxidative stress appears to play a major role in chronic Cd-induced hepatic and renal toxicity since inhibition of components of the antioxidant defense system accelerated and administration of antioxidants protected against Cd toxicity.

Treatment of chronic cadmium nephrotoxicity by N-acetyl cysteine

Chronic cadmium (Cd)-induced nephrotoxicity is believed to be irreversible at advanced stages and no treatment is currently available. This study examined the beneficial effect of N-acetyl cysteine (NAC) on Cd-induced nephrotoxicity. Female Sprague-Dawley rats were injected s.c. with 5 micromol CdCl2/kg per day, five times/week for up to 26 weeks. Nephrotoxicity was detected after 10 weeks by elevation in urinary lactate dehydrogenase activity and protein. NAC co-administration from week 13 prevented the progression of nephrotoxicity. In these animals, with low-level nephrotoxicity, discontinuation of Cd exposure at the end of week 22 resulted in gradual recovery over the next several weeks, without the need for treatment with NAC. On the other hand, discontinuation of NAC co-treatment at the end of week 22 resulted in quick progression of nephrotoxicity, indicating that NAC protection was short-lived. Resumption of NAC treatment and cessation of Cd exposure after 26 weeks resulted in rapid recovery from advanced nephrotoxicity. It is concluded that protection from Cd-induced nephrotoxicity is possible by continued co-administration of NAC and that recovery from advanced nephrotoxicity can also be achieved with NAC, provided that Cd exposure is stopped.

Cadmium-induced time-dependent oxidative stress in liver of mice: a correlation with kidney

In the present investigation a time-responsive oxidative stress, both in liver and kidney were carried out following cadmium (Cd) exposure. Cadmium was administered subcutaneously on each other day in normal saline solution. Mice were sacrificed on the 15th, 30th and 45th days post-exposure. For oxidative stress study different biochemical markers, e.g. lipid peroxidation (LPO), reduced glutathione (GSH) and glutathione S-transferase (GST) activity were considered. Time-responsive exponential increase in the lipid peroxidation and decrease in glutathione level and glutathione S-transferase activity occurred both in hepatic and renal tissues following cadmium treatment. Though both the organs responded in a similar fashion, their magnitude of response was found to be different. Body weight did not differ but relative liver and kidney weight were found to be significantly increased at different time points. The possible mechanism of time-dependent cadmium-induced toxic effects and relation between hepatic and renal biochemical response have been discussed.

Acute CdMT injection is not a good model to study chronic Cd nephropathy: comparison of chronic CdCl2 and CdMT exposure with acute CdMT injection in rats

Kidney is the main target organ of Cd toxicity in humans. Cd-induced nephrotoxicity is thought to be caused by the Cd-metallothionein complex (CdMT) that "leaks" out of the liver and is taken up by the kidney. A single injection of CdMT has therefore been used as a model to study Cd nephropathy for the last 20 years. However, our recent studies reveal discrepancies between renal Cd concentration and nephrotoxic potencies of CdCl2 and CdMT. This study was further designed to critically evaluate whether a single injection of CdMT is an appropriate model to study the mechanism of chronic CdCl2 nephropathy. Age-matched rats were given multiple sc injections of either CdCl2 (0.8 and 1.2 mg Cd/kg) or CdMT (0.05 mg Cd/kg) daily, 6 days/week for 6 weeks, or a single injection of CdMT (0.2-0.6 mg Cd/kg i.p. for 24 h), and the nephrotoxicity was compared. Histologically, chronic CdCl2 or CdMT administration produced damage to the whole kidney, including tubular cell degeneration, apoptosis, and atrophy; interstitial inflammation; glomerular swelling; and sclerosis. In contrast, acute CdMT injection produced severe proximal tubule necrosis as the major feature of its toxicity. Biochemically, chronic exposure to Cd produced polyuria and calciuria, while proteinuria, glucosuria, and enzymuria were mild (2-5x). In contrast, acute CdMT nephrotoxicity was characterized by marked increases in urinary protein (13x), glucose (25x), N-acetyl-beta-d-glucosaminidase (28x), lactate dehydrogenase (100x), and gamma-glutamyltranspeptidase (160x). Serum levels of creatinine and blood urea nitrogen were unchanged following chronic Cd exposure but were markedly elevated (5x) after acute injection of CdMT. Chronic exposure to either CdCl2 or CdMT produced nephrotoxicity at renal Cd concentration of 85 to 110 micrograms/g kidney, while acute CdMT injection produced nephrotoxicity at only 5 to 7 micrograms/g kidney. In conclusion, the present study indicates that the features and mechanisms of renal injury from chronic Cd exposure are quite different from those produced by a single injection of CdMT. Therefore, it is proposed that acute CdMT injection is not an appropriate model for the study of chronic Cd-induced nephrotoxicity.

Plasma cadmium-metallothionein, a biological exposure index for cadmium-induced renal dysfunction, based on the mechanism of its action

Thirteen rabbits were given subcutaneous cadmium (0.3 mg Cd/kg) daily. The plasma cadmium-metallothionein (CdMT) and the Cd-induced hepatic and renal functions were determined at 0, 5, 8, 11, 12, 13 and 14 weeks. Hepatic dysfunction, an elevated plasma CdMT and renal dysfunction were detected mostly between 12 and 14 weeks. The hepatic dysfunction parameters were closely related with the plasma CdMT, which was then found to correlate with the renal dysfunction parameters. All the above findings suggest the following mechanism for the Cd-induced renal dysfunction: hepatic CdMT is released into the plasma upon the Cd-induced hepatic dysfunction, and then excess plasma CdMT, whose concentration is proportional to the CdMT in the renal proximal tubular lumen, induces renal dysfunction. The critical concentration of plasma CdMT to induce renal dysfunction was estimated as 80 microg Cd/l. The plasma CdMT is proposed therefore as a biological exposure index for the Cd-induced renal dysfunction, based on the mechanism of its action.

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.

Efficacy of amphipathic dithiocarbamates in intracellular cadmium mobilization and in modulation of hepatic and renal metallothionein in cadmium pre-exposed rat

Forty-eight hours after an intraperitoneal injection of cadmium chloride (1.5 mg Cd/kg) to female albino rats, Cd was mainly localized in the hepatic and renal supernatant cytosolic fraction (SCF). Seventy-two hours later, the total hepatic burden remained unchanged but the total renal burden was enhanced, showing its tendency to accumulate in the kidney. A single dose (0.4 mmol/kg, i.p.) of sodium N-benzyl-D-glucamine dithiocarbamate (BG.DTC) or sodium N-(4-methoxybenzyl)-D-glucamine dithiocarbamate (MeO.BG.DTC), 24 h after Cd injection, efficiently mobilized Cd from hepatic SCF, apparently from cadmium-metallothionein (Cd-MT); MeO.BG.DTC also removed Cd from hepatic nuclear mitochondrial fraction. This treatment, however, increased the renal burden of Cd, indicating that the chelating agents, at least partly, transport Cd from the liver and possibly from other sites into the kidney. Three doses of the chelators further enhanced mobilization of Cd from hepatic as well as renal SCF, as corroborated by its enhanced urinary and, to a greater extent, fecal excretion. Hepatic and renal MT were induced several-fold above normal after a single dose of Cd as well as single or repeated doses of BG.DTC or MeO.BG.DTC. Seventy-two hours after a Cd injection, the hepatic MT declined to half of the induced level while the renal MT remained elevated. Administration of BG.DTC or MeO.BG.DTC in Cd pre-treated rats produced an additive response in hepatic MT, but the response in renal MT was less than additive at one dose and slightly declined after three doses. Hepatic Zn and Cu and renal Zn increased on treatment with Cd but were depleted after a single or repeated injection of BG.DTC or MeO.BG.DTC in normal as well as in Cd pre-exposed animals. The results indicate that intracellular access of amphipathic dithiocarbamates effectively mobilizes MT-bound Cd, which is preferentially excreted in the feces, and helps avoid further burden on the kidney and consequent nephrotoxicity. Additionally, MeO.BG.DTC was a better inducer of hepatic MT to help increased capture of toxic metal from the initial circulation and consequent toxicity.

Copper metabolism in the kidney of rats administered copper and copper-metallothionein

To gain a greater understanding of the mechanism of Cu metabolism in kidneys of rats, using autofluorescence of Cu-metallothioneins (Cu-MTs) we revealed the behavior of Cu-MT in the kidneys of rats administered Cu-MT. Yellow and orange fluorescent signals of Cu-MT were observed in the cortex. By microscopic studies, Cu-MT was dominant in the proximal convolute tubular cells of the cortex. A high concentration of Cu-MT presented in the lysosome-like organelles of the proximal convolute tubular adjacent to the glomeruli. During the time course after the injection, the orange signal in lysosome-like organelles gradually converted to a yellow signal, indicating that the Cu-MT was involved in a degradation process in lysosomes by oxidation, and the MT mRNA increased in the cortex, although the immunoreactivity of MT was almost constant in the same region. These results suggested that Cu bound to the injected MT was released in lysosomes and became a new inducer of MT biosynthesis in the cortex. In conclusion, the biosynthesis and degradation of Cu-MT occur repeatedly in the proximal convolute tubular cells.

Cadmium decreases SGLT1 messenger RNA in mouse kidney cells

Mouse renal cortical tubule cells in primary culture exposed to cadmium (Cd2+) develop decreased Na(+)-glucose cotransport activity as measured by uptake of the glucose analogue alpha-methyl-glucoside. RNA was isolated from kidney cell cultures, and after reversed transcription, the DNA was amplified with primers to rat SGLT1 (the high affinity isoform of the sodium glucose cotransporter) and mouse beta-actin. Only one product was identified after amplification with the rat SGLT1 primers, which on sequencing was 96% identical to rat SGLT1. Compared to beta-actin, the intensity of the SGLT1 message declined progressively as CdCl2 concentration in the medium increased from 0 to 10 microM. Similar decreases in SGLT1 mRNA were also observed as media zinc (Zn2+) concentrations rose from 0 to 75 microM or as copper (Cu) concentrations increased from 0 to 150 microM. Exposure to 8 microM Cd as Cd-metallothionein (Cd7-MT) also caused a fall in relative SGLT1 mRNA abundance, and at nearly identical internal Cd concentrations of 40-43 pmol/microgram DNA, both Cd7-MT and CdCl2 reduced SGLT1 mRNA to 33% of control. In general, the fall in SGLT1 mRNA was more rapid than the decline in Na(+)-dependent glucose uptake after cells were exposed to Cd2+. These findings suggest that the effects of Cd2+ and other metals on renal glucose transport are related to decreased expression of SGLT1 message.

Kidney changes in multiparous, nulliparous and ovariectomized mice fed either a nutrient-sufficient or -deficient diet containing cadmium

As a simulation of the etiological factors known for Itai-Itai disease, a syndrome characterized by renal dysfunction and osteomalacia in its Japanese victims, female mice were subjected to the individual and combined stresses of dietary Cd, nutrient-deficient diet, multiparity and ovariectomy. Renal function as affected by the etiological factors was periodically evaluated by determination of protein, amino acid, glucose and Cd concentrations in urine; periodic changes in skeletal Ca status were assessed relative to current renal function. Renal metabolism of Cd, Zn and Cu was also examined. At age 68 days, female mice were given nutrient-sufficient (+) or -deficient (-), purified diets containing either 0.25 (environmental), 5, or 50 ppm Cd as CdCl(2); the nutritional composition of (-) diet simulated that of food consumed by Japanese victims of Itai-Itai disease. At age 70 days, half of the females began a breeding regimen of six consecutive, 42-day rounds of pregnancy/lactation (PL mice); the remainder were maintained as virgin, non-pregnant controls (NP mice). Limited numbers of PL and NP mice were sacrificed at the end of each reproductive round. PL( + ) mice taken in a given round had successively borne litters in that round and all preceding ones. PL(-) females taken at the end of round (R)-1, -2 and -3 had successively borne litters through those rounds; those taken at the end of R-5 or R-6 had nonsuccessively borne litters in four of five or three of six rounds, respectively. At the conclusion of the 252-day reproductive period, remaining females entered the 392-day, post-reproductive phase of the experiment. At age 546 days (mid-R-12), PL females having successfully borne at least three litters were ovariectomized (OV) to mimic human menopause; at the same time, NP females were either ovariectomized or sham-operated (SO). After surgery, all females were maintained to age 714 days (mid-R-16), then sacrificed. Spot urine samples were taken from individual mice at the end of most reproductive rounds (R-2-->6), prior to surgery (mid-R-10), and prior to final sacrifice (late-R-15); samples were also collected via metabolism cages at the end of R-10. Food consumption, monitored on a weekly basis over the first nine rounds, was generally not significantly affected by dietary Cd level or nutrient deficiencies in females of the same reproductive status; consumption was increased about 2.5-fold in PL versus NP groups during the reproductive period and about 1.4-fold during the post-reproductive period. At each of the three dietary Cd levels and after all reproductive rounds, mean renal Cd concentrations were 1.2- to 5.6-fold higher in PL than NP mice. After six reproductive rounds, renal Cd concentrations in PL(+) and (-) groups exposed to 50 ppm Cd had reached 155 and 179 microg Cd/g kidney, respectively. Although these levels fell within a concentration range (145-200 microg Cd/g) where cadmium-induced renal dysfunction could be anticipated, no significant, Cd-dependent changes in mean urinary amino acid or protein concentrations were found. Moreover, among the same population, a 12% incidence of elevated urinary Cd (> or = 250 ng/ml) was noted, however none of the affected individuals exhibited depressed total calcium content (TCa) or calcium:dry weight ratios (Ca:DW) for femur. Such results suggested that the Cd-induced, skeletal demineralization observed in mice during the reproductive period (Bhattacharyya et al., Toxicology 1988a; 50: 193-204; Whelton et al., Toxicology 1994: 91: 235-251) likely occurred in the general absence of cadmium-induced renal dysfunction. By the end of the post-reproductive period, the incidence of elevated urinary Cd increased to 26% among ovariectomized females: of these, 89% with urinary Cd > or = 345 ng/ml exhibited decreases in TCa and/or Ca:DW values for femur or lumbar vertebrae that exceeded one S.D. of their group mean. Such results suggested that skeletal demineralization observed at

Hepatic levels of cadmium, zinc and copper in multiparous, nulliparous and ovariectomized mice fed either a nutrient-sufficient or -deficient diet containing cadmium

As a simulation of etiological factors known for Itai-Itai disease, female mice were subjected to the individual and combined stresses of dietary cadmium, nutrient-deficient diet, multiparity and ovariectomy. From age 68 days, female mice were maintained on either nutrient-sufficient (+) or -deficient (-), purified diets containing either 0.25 (environmental), 5, or 50 ppm Cd as CdCl(2); the nutritional composition of (-) diet simulated that of food consumed by Japanese women who contracted Itai-Itai disease. At age 70 days, half of the mice began a breeding regimen of six consecutive, 42-day rounds of pregnancy/lactation (PL mice); the remainder were maintained as virgin, non-pregnant controls (NP mice). Limited numbers of PL and NP mice were sacrificed at the end of each reproductive round. PL(+) mice taken in a given round had successively borne litters in that round and all preceding ones. PL(-) females taken at the end of round (R)-1, -2 and -3 had successively borne litters through those rounds; those taken at the end of R-5 or -6 had nonsuccessively borne litters in four of five or three of six rounds, respectively. At the end of the 252-day reproductive period, remaining females entered the 392-day, post-reproductive phase of the experiment. At age 546 days (mid-R-12), PL females having successfully borne at least three litters were ovariectomized (OV) to mimic human menopause, while NP females were either ovariectomized or sham-operated (SO). After surgery, all females were maintained to age 714 days (mid-R-16), then sacrificed. Food consumption, monitored on a weekly basis over the first nine rounds, was in general not significantly affected by dietary Cd level or nutrient deficiencies for females of the same reproductive status; consumption was increased about 2.5-fold in PL versus NP groups during the reproductive period and about 1.4-fold during the post-reproductive period. Over the reproductive period, small increases in liver concentrations of Zn and Cu were observed (ca. 3.1- and 2.5-fold, respectively) with far larger increases for Cd (ca. 22200-fold). Threshold hepatic Cd concentrations below which the concentrations of Zn and Cu were relatively constant and independent of Cd concentration were identified; they were 2.7 microg Cd/g liver for Zn and 3.3 microg Cd/g liver for Cu for females consuming (+) diet, and 4.9 microg Cd/g liver for Zn and 4.5 microg Cd/g liver for Cu for females consuming (-) diet. Regardless of Cd exposure level, round-by-round hepatic concentrations of Cd were generally 2- to 6-fold higher in PL than NP mice, while Zn or Cu levels were generally only 1.1- to 2.5-fold higher. For each reproductive round, hepatic concentrations of Cd in NP females were consistently about 10-fold greater in mice exposed to 50 than 5 ppm dietary Cd: corresponding Zn levels were essentially equivalent. For PL females. Cd levels were about 7-fold greater in 50 than 5 ppm Cd-exposed groups, however Zn concentrations were about 45% decreased. The pattern of Cd, Zn and Cu sequestration established during the reproductive period clearly differed from that of the post-reproductive period. Between R-6 and -16, hepatic concentrations of Cd, Zn and Cu appreciably decreased (14-69%) in 5 ppm Cd-exposed NPOV and PLOV females regardless of diet-type consumed. At the 50 ppm Cd level, Cd and Zn concencentrations dramatically rose with increases in Cd (37-129%) exceeding those of Zn (12-21%).

Effects of zinc and copper on cadmium uptake by brush border membrane vesicles

The effects of essential metals, zinc (Zn) and copper (Cu), on cadmium (Cd) uptake were investigated in brush border membrane vesicles (BBMV) isolated from the rat renal cortex and LLC-PK1 cells. BBMV were incubated with Cd in the presence or absence of Zn or Cu, and then washed with a chelating agent, EGTA, to remove Cd bound to the outer surface of BBMV. Co-incubation with Zn or Cu decreased Cd accumulation in these BBMV in a concentration-dependent manner. Kinetic analysis of the initial accumulation of Cd suggested that Cd is taken up into rat BBMV via an unsaturable component and a saturable component (K(m) = 13.8 microM, V(max) = 1.44 nmol/mg protein/min), and co-incubation with Zn significantly increased the K(m) of the saturable component without affecting the V(max), whereas Cu significantly increased the K(m)-value and decreased the V(max)-value. Increasing the osmolarity of the incubation medium slightly decreased Cd accumulation in the absence of Zn or Cu, whereas it did not decrease Cd accumulation in the presence of these metals. These results suggest the possibility that, in addition to passive diffusion, Cd is also taken up from the renal brush border membrane via carrier-mediated mechanisms that are inhibited by Zn competitively and by Cu non-competitively. Furthermore, these results suggest that: (1) Cd binds externally and internally to BBMV, (2) little Cd is transported into the intravesicular space, and (3) both Zn and Cu decrease the binding and transport of Cd.

Role of metallothionein in cadmium-induced hepatotoxicity and nephrotoxicity

MT, a cysteine-rich, metal-binding protein, exists in most tissues and is easily induced by many stimuli. There are four major MT isoforms in mammalian tissues, with MT-I and -II present in all tissues, MT-III only in brain, and MT-IV located in epithelium. Many factors regulate MT synthesis, such as age, species, hormones, inflammation, and various chemical treatments. Not only is MT synthesis important, but degradation of MT is also an important mechanism of MT regulation. The importance of MT in Cd toxicology has been extensively investigated. MT does not have a major effect on absorption and tissue distribution of Cd, but it does play a major role in binding Cd in the cell, thus decreasing its elimination from the body, especially into the bile. MT is at least partially responsible for the retention of Cd in tissues and the long biological half-life of the metal. MT plays an important role in Cd tolerance and Cd-induced hepatotoxicity. MT binds Cd in the hepatic cytosol and renders it "inert." Therefore, MT is beneficial to the liver. However, the Cd-MT complex is nephrotoxic and is proposed to be responsible for chronic Cd poisoning. MT appears to play less of a protective role in Cd-MT-induced acute nephrotoxicity, and Zn-induced protection against CdMT acute renal injury is not mediated by MT. The role of MT in chronic Cd nephrotoxicity needs to be further clarified.

Effects of dithiocarbamates on toxicity of cadmium in rat primary hepatocyte cultures

The protective effects of N-benzyl-D-glucamine dithiocarbamate (BGD) and N-p-hydroxymethylbenzyl-D-glucamine dithiocarbamate (HBGD) on the toxicity of Cd in the rat primary hepatocyte cultures were studied. Cytotoxicity was assessed by measuring cell viability, extra cellular lactic dehydrogenase (LDH) activity, and intracellular lipid peroxidation and active oxygen species. Primary hepatocyte cultures were treated with 109CdCl2 (5, 10 or 50 microM Cd and 1.7 KBq of 109Cd/well) for 30 min or 4 h. BGD or HBGD was added to the culture medium to make the final concentration of 100 microM and incubated for 4.5 h in 30 min Cd exposure or 1 h in 4 h Cd exposure. Decreases in the hepatocyte viability caused by all Cd exposure concentrations were significantly prevented by treatment with BGD or HBGD. The treatment with the chelating agents for 4.5 h after Cd exposure for 30 min significantly prevented increases in extracellular LDH activity. Increases in the lipid peroxidation in hepatocytes exposed to Cd for 30 min or 4 h were prevented significantly by treatment with BGD or HBGD for 4.5 h or 1 h, respectively. Moreover, the increases in the level of active oxygen species caused by Cd exposure for 30 min were significantly prevented by treatment with the chelating agents for 1.5 h. These findings suggest that BGD and HBGD protect against the cytotoxicity of Cd in rat primary hepatocyte cultures and that the protective effects of chelating agents presumably result from a decrease in the Cd level, the effective sequestration of the reactive Cd ion, and the direct preventive effect on the active oxygen species in the hepatocytes.

Effect of cadmium on purified hepatic flavokinase: involvement of reactive -SH group(s) in the inactivation of flavokinase by cadmium

The effect of cadmium (Cd2+) was studied in vitro on the flavokinase (ATP : riboflavin 5'-phosphotransferase, EC 2.7.1.26) activity purified from rat liver. Cadmium inhibited flavokinase activity in a concentration-dependent manner and the effect was completely reversed by increasing concentration of zinc (Zn2+), indicating a competition between Zn2+ and Cd2+ for binding with the enzyme. Further, a competition between riboflavin and Cd2+ hints at the possibility that Zn2+ and Cd2+ probably compete for the same site on the enzyme where riboflavin binds. Our studies further reveal that hepatic flavokinase contains essential, accessible and functional thiol group(s) as evidenced by a concentration-dependent inhibition by sulfhydryl reagents and protection by thiol protectors like glutathione or dithiothreitol. Furthermore, the enzyme could also be protected from the inhibitory effect of Cd2+ and Hg2+ by glutathione and dithiothreitol suggesting that Cd2+ probably interacts with reactive thiol group at or near the active site of the enzyme to cause inhibition.

Mechanism of nephrotoxicity induced by repeated administration of cadmium chloride in rats

To explore the mechanism of Cd nephrotoxicity, CdCl2 was subcutaneously injected to rats, at 3 mg Cd/kg body weight once a day, for 8 d. In the liver, Cd bound to metallothioneins (MTs-Cd) rose from d 1 after the initiation of CdCl2 administration, and reached a plateau after the administration ceased. In the plasma, MTs-Cd rose from d 4, peaked on d 8, and gradually fell thereafter. In the kidneys, leucine aminopeptidase (LAP) and N-acetyl beta-D-glucosaminidase (NAG) fell during d 6-20, and Cd bound to cellular membranes (Mem-Cd) rose from d 1 and reached a plateau during d 6-20. The Mem-Cd levels were significantly correlated with the reduction in the LAP and NAG activity; the values of MTs-Cd plus Mem-Cd were almost equivalent to those of total Cd. These findings showed that the hepatic synthesis of MTs-Cd occurred followed by its release into plasma; the extent of renal injury was aggravated as the plasma level of MTs-Cd rose; and a greater part of the renal Cd distributed intracellularly as the MTs-binding form, while the residual Cd distributed as the cellular membrane-binding form. Also, it was suggested that Cd that occurred as the cellular membrane- binding form in the kidneys was involved in manifestation of renal injury.

Immunomodulatory activities of extracellular metallothionein. II. Effects on macrophage functions

Metallothionein (MT) is a thiol-rich protein that is rapidly induced by exposure to heavy metal cations. We have previously demonstrated that exogenous MT stimulates murine splenocytes to proliferate, but inhibits humoral responses to antigen. These observations suggest that metallothionein released from cells has a complex role in heavy metal-mediated immune dysfunction. Here we examine one possible mechanism by which MT mediates suppression of humoral immunity. Exposure of macrophages to 20 microM MT did not affect their ability to engulf opsonized sheep erythrocytes, but in the presence of 20 microM MT, peritoneal macrophages were stimulated to produce increased levels of oxygen radicals. These results correlated with observations that while macrophage phagocytosis of opsonized Candida albicans was unaltered by the presence of exogenous MT, killing of the engulfed yeast cells was dramatically enhanced by 20 microM MT. Amounts of free cadmium and zinc equimolar to that added as Zn,Cd-MT had no effect on candidacidal activity. MT was also found to significantly decrease lymphocyte proliferation mediated by macrophage activity. Biotinylated MT (MT-b) bound specifically to the plasma membranes of these macrophages, suggesting that membrane-associated molecules of the macrophage may transduce a signal mediated by MT binding. These results demonstrate that macrophages are a sensitive target for MT-mediated immunomodulation and that some of the consequences of the MT interaction with macrophages may be alterations in the capacity to produce an effective immune response and increased extracellular exposure to damaging free radicals.

Kinetics of Cd2+ in plasma, liver and kidneys after single intravenous injection of Cd-metallothionein-II

To explore the kinetics of Cd2+ in the body, rats received a single intravenous injection of CdCl2 or Cd-saturated metallothionein-II at 0.3 mg Cd/kg body weight. Cd2+ in the two agents was biexponentially eliminated from plasma: rapidly in the first 5 min, and gradually later. Compared with CdCl2, Cd-saturated metallothionein-II showed lower Cd2+ concentrations in plasma during the first 30 min; larger values for parameters concerning distribution of Cd2+, its total body clearance and half-life time in the beta phase. Cd2+ uptake in the liver was higher with CdCl2, and, conversely, in the kidneys it was higher with Cd-saturated metallothionein-II. In Cd-saturated metallothionein-II, the renal content of Cd2+ reached a maximum (8 micrograms Cd2+/g tissue) on day 1, gradually decreasing thereafter; there was a higher area under the Cd2+ content-time curve, and a lower mean residence time of Cd2+; the kidneys showed severe necrosis and defluxion of proximal tubular cells at days 1 and 5, although there were regenerative and reversion signs on day 5. These findings suggested that, in the case of Cd-saturated metallothionein-II, Cd2+ being taken into the cells of proximal tubules was excluded predominantly due to cellular death and the resultant defluxion.

Stress protein synthesis induced by cadmium-cysteine in rat kidney

Biomarkers are important tools which enable toxicologists to reliably predict and detect exposures to xenobiotics and resultant cell injury, ultimately improving risk assessments. Since the de novo synthesis of stress proteins can be detected early after exposure to some agents, analysis of toxicant-induced changes in gene expression, i.e. alterations in patterns of protein synthesis, may be useful to develop as biomarkers of exposure and toxicity. We are utilizing various xenobiotics as tools to study stress protein synthesis in target organs in order to evaluate the target tissue-specificity of this response. Previous data from this laboratory have demonstrated that induction of stress proteins in rat liver, but not kidney, after acute exposure to CdCl2 precedes hepatoxicity. Since kidney is a target tissue after chronic Cd exposure, it was of interest to examine stress protein synthesis in this tissue. However, dose-limiting hepatotoxicity precluded this evaluation. Cd complexed with molecules such as cysteine (cys) or metallothionein has been used in acute dosing regimens as a tool in order to study the nephrotoxicity of Cd. Therefore, this study was undertaken in order to evaluate Cd-induced stress protein synthesis in an important tissue known to be injured after chronic exposure, i.e. kidney. Specific objectives included comparing stress protein synthesis in rat kidney and liver after acute exposure to Cd-cys and CdCl2, determining the Cd threshold concentration for renal stress protein synthesis and assessing the relationship between stress protein synthesis and nephropathy. Male rats were exposed to equivalent doses of Cd as CdCl2 or Cd-cysteine (molar ratio Cd:cys = 1:15). Kidney Cd concentrations increased 5-fold after i.v. injection of Cd-cys compared to CdCl2, mimicking Cd distribution following chronic exposure. After exposure to Cd, tissue slices were incubated with 35S-methionine. Slices were subsequently homogenized and centrifuged, and the 16,000 g supernatants were subjected to SDS-polyacrylamide gel electrophoresis. Proteins which had incorporated 35S-methionine were detected by autoradiography. De novo synthesis of 70, 90 and 110 kDa proteins was enhanced in liver, but not in kidney, 4 h after injection of 2 mg Cd/kg as CdCl2. In contrast, dose-related increases in synthesis of these proteins were observed in kidney 4 h after injection of 1 and 2mg Cd/kg as Cd-cys, but not at lower dosages. In addition, synthesis of a 68 kDa kidney protein was inhibited at 2 mg Cd/kg as Cd-cys. The threshold for Cd-induced stress protein synthesis was shown to be between 4 and 8 micrograms Cd/g tissue.(ABSTRACT TRUNCATED AT 400 WORDS)

Protective effects of selenium on cadmium toxicity in rats: role of altered toxicokinetics and metallothionein

Selenium prevents the toxicity of the carcinogenic metal cadmium through undefined mechanisms. In this study, we determined the effects of selenium on cadmium toxicokinetics and on the ability of cadmium to induce metallothionein, a metal-binding protein that is thought to confer tolerance to cadmium toxicity. To assess the acute protective effects of selenium, male Wistar (WF/NCr) rats were given selenium (as SeO2; 10 mumol/kg, sc) at -24, 0, and +24 h relative to cadmium (as CdCl2; 45 mumol/kg, sc). Over a 14-d period this dose of cadmium killed 6 out of 10 rats, while 100% of the cadmium-treated rats given concurrent selenium treatments survived. The acute increases in testicular weight that were seen with cadmium, indicative of edematous damage, were also prevented by concurrent selenium treatments. Further studies assessed the distribution and excretion of cadmium and its ability to induce metallothionein in rats given 40 mumol Cd/kg, sc, at time 0 and selenium (10 mumol/kg, sc) at -24 and 0 h. Selenium treatments enhanced cadmium accumulation at 24 h in the liver (23%), testes (145%), and epididymis (35%) but reduced renal accumulation by more than half. Urine samples, collected at 0-3, 3-6, and 6-24 h following cadmium administration, indicated a markedly reduced excretion of cadmium in selenium treated rats during all time periods. The synthesis of metallothionein was stimulated to a much lesser extent by cadmium in selenium-treated rat kidney (41% decrease) but was unaffected in liver. The levels of cadmium-binding proteins within the testes were markedly reduced by cadmium treatment, an effect unmodified by selenium treatments. These results suggest selenium prevents acute cadmium toxicity through a mechanism that does not involve induction of metallothionein and in spite of a markedly enhanced retention of cadmium.

Comparison of the cytotoxic effects of cadmium chloride and cadmium-metallothionein in LLC-PK1 cells

Recent studies have shown that ionic cadmium (Cd2+) can selectively damage the tight junctions between LLC-PK1 cells. The objective of the present studies was to determine if cadmium that is bound to metallothionein (Cd-Mt) can also damage the junctions between these cells. Cells on Falcon Cell Culture Inserts were exposed to Cd2+ or Cd-Mt from the apical and basolateral compartments. The integrity of cell junctions was assessed by monitoring the transepithelial electrical resistance, and cell viability was evaluated by monitoring the release of lactate dehydrogenase into the medium. Exposure to Cd2+ for 1-4 hours caused a pronounced decrease in the transepithelial resistance without affecting cell viability. By contrast, exposure to Cd-Mt had little effect on the electrical resistance until the cells began to die, which did not occur until 24-48 hours of exposure. Additional results showed that the cells accumulated Cd2+ more rapidly than Cd-Mt. These results indicate that Cd-Mt does not damage the junctions between LLC-PK1 cells, but that it can kill the cells after prolonged exposure.

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.

Exogenous metallothionein and renal toxicity of cadmium and mercury in rats

The relative tissue distribution and toxicity of cadmium (Cd) and mercury (Hg) in the liver and kidneys of rats when the metals are administered as either inorganic salts or complexed with MT were studied. Male Sprague-Dawley rats were injected (i.v.) with Cd or Hg inorganic salt of chloride or in a complex of MT at a dose of 0.3 mg/kg body weight. The concentration of MT and metals in plasma and urine was monitored for 7 days, at the end of which the rats were killed. Injection of both HgCl2 and Hg-MT induced the synthesis of MT only in the kidney but not in the liver, whereas CdCl2 and Cd-MT injections induced MT synthesis in both liver and kidney, respectively. Plasma MT levels increased 3 days after CdCl2 but not after HgCl2 injection, suggesting that hepatic MT may be an important source of plasma MT under our experimental conditions. Renal toxicity was observed morphologically and by an increase in blood urea nitrogen, plasma creatinine, proteinuria in rats injected with Cd-MT and both forms of Hg. Urinary MT excretion was significantly elevated in Cd-MT injected rats compared with those injected with CdCl2. However, HgCl2 and Hg-MT injected rats showed no significant difference in urinary MT excretion. The magnitude in the renal accumulation of Hg is similar after the administration of Hg-MT or HgCl2, but our findings suggest that the site of epithelial injury may be different. Injury effects of Hg-MT localized mainly in the terminal portions of the proximal convoluted tubule and the initial portions of the proximal straight tubule whereas inorganic Hg caused necrosis in pars recta segments of the proximal tubule.

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.

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.

Immunohistochemical evidence of high concentrations of metallothionein in pancreatic hepatocytes induced by cadmium in rats

A recent study from our laboratory has shown that cadmium, a toxic heavy metal, is one of the most effective agents known for inducing hepatocytic transdifferentiation of the rat pancreas. With repeated injections of cadmium, the incidence of rats with pancreatic hepatocytic foci can be as high as 93%. Cadmium is also well known as a very potent inducer of metallothionein, a metal-binding protein that appears to be important in the biologic response to several toxic heavy metals in most tissues, including the pancreas. Therefore, the present study sought to determine if metallothionein was associated with cadmium-induced transdifferentiation of pancreatic cells. Expression of metallothionein was studied immunohistochemically by the peroxidase-antiperoxidase method in tissue sections of the pancreas of rats with pancreatic hepatocytes. High levels of metallothionein were localized primarily within the pancreatic hepatocytes. Surrounding normal pancreatic islet and acinar cells were not immunoreactive. Thus, metallothionein is expressed actively in cells transdifferentiated to hepatocytes by cadmium within the pancreas.

Cd-metallothionein nephrotoxicity in inbred strains of mice

Genetic differences in the acute hepatic and testicular toxicity of Cd occur among different strains of mice. However, it is not known whether genetic variation to the renal damage caused by Cd-metallothionein (CdMT) exists. Therefore, male mice of the C3H/HeJ, C57/Bl10, CBA/CA, and DBA/2J strains, previously shown to differ in hepatic and testicular injury due to Cd, were treated with CdMT at dosages of 0.2, 0.4, 0.8, and 1.6 mg/kg (sc). For all strains of mice, tissue accumulation of Cd occurred predominantly in kidney, which had two to three times as much Cd as liver, while testes had no measurable amounts of Cd. Hepatic and renal metallothionein (MT) concentrations were increased with increasing dosage of CdMT, and no differences between strains were demonstrated. Urinary glucose was increased significantly at the three highest dosages of CdMT, with no differences between strains. At each dose level, light microscopic manifestations of CdMT nephropathy did not differ between strains. In summary, all CdMT-treated strains of mice responded similarly with respect to all measured renal parameters (accumulation of Cd and MT and nephrotoxicity). Unlike the strain differences in hepatic and testicular injury from Cd in these strains of mice, CdMT nephrotoxicity shows no such genetic variation.

Distribution of cadmium chloride and cadmium-metallothionein to liver parenchymal, Kupffer, and endothelial cells: their relative ability to express metallothionein

Acute exposure to cadmium (Cd) salts results in liver toxicity, while administration of cadmium-metallothionein (CdMT) iv, causes renal damage. When CdMT is administered iv there is a rapid accumulation of Cd in the proximal tubule cells of the kidney. In comparison, only small amounts of Cd accumulate in the liver following administration of CdMT. Thus, in order to better understand the regulation of MT as well as the toxicity of Cd, the present study has examined the ability of each of the three primary liver cells, parenchymal (PC), Kupffer (KC), and endothelial (EC), to accrue Cd after administration of either inorganic or organic forms of Cd. In addition, the relative ability of each cell type to express metallothionein (MT) mRNA and protein was examined. Following CdCl2 (3.5 mg Cd/kg) treatment, Cd concentrations increased to about the same degree in PC and KC, but EC had about 2-fold more than PC. After administration of CdCl2 (1.0 mg Cd/kg) each cell responded to the presence of Cd by increasing intracellular MT mRNA and protein. However, PC showed the greatest response, with a 30-fold increase in mRNA and a 21-fold increase in protein. Interestingly, KC and EC possessed intracellular Cd concentrations equal to or greater than that of PC, but contained less MT than would have been expected on the basis of their intracellular Cd concentrations. Thus, KC had a 7-fold increase in MT mRNA and a 2-fold increase in protein, while EC increased mRNA 3-fold and protein 2-fold over control values. In contrast, following CdMT (0.5 mg Cd/kg) administration, only low levels of Cd were detected, with similar concentrations in each cell type. After administration of CdMT (0.4 mg Cd/kg), PC again showed the greatest response, with a 3-fold increase in mRNA and a 6-fold increase in MT protein. Only slight changes were observed in KC and EC. In conclusion, the present study has shown the following: (1) Endogenous levels of MT in KC and EC are higher than those in PC. (2) Cd is readily accumulated by all three cell types, when administered as CdCl2, but not when given as CdMT. (3) PC, KC, and EC are capable of responding to intracellular Cd by increasing MT.

Toxicological principles of metal carcinogenesis with special emphasis on cadmium

Metals are an important and emerging class of carcinogens. At least three metals, specifically nickel, chromium, and arsenic, are confirmed human carcinogens, and several more are suspected to have carcinogenic potential in man. Considering that the list of known human carcinogens of any type is very small, it becomes clear that metals make up a substantial portion of the list. Furthermore, many metals are very potent carcinogens in laboratory animals. Despite this, relatively little attention has been given to the topic of metal carcinogenesis. The reasons for this relative lack of attention are not clear but perhaps are fostered by a perception that, because metals are the simplest of molecules, their mechanism of action must also be simple. This could not be farther from the truth and, although no clear mechanisms have emerged in the area of metal carcinogenesis, it has become apparent that they are anything but simple. Metal carcinogens possess several unique characteristics including a remarkable target site specificity. Detection of the mechanism, or mechanisms, of metal carcinogenesis has, however, proven elusive, in part because of a wide diversity of metallic carcinogenic agents and the intricate nature of metal interactions in biologic systems. The following review explores this broad topic, with special emphasis on toxicological principles including dose-response relationships and potential mechanisms, using cadmium as an example.

Effect of chronic dietary zinc deficiency on cadmium toxicity and carcinogenesis in the male Wistar [Hsd: (WI)BR] rat

Though it is known that excess zinc will prevent cadmium carcinogenesis, the impact of zinc deficiency on cadmium carcinogenesis has not been defined. This study assessed the effect of dietary zinc deficiency on the carcinogenic potential of cadmium in rats. Groups (n = 28 each) of male Wistar [Hsd: (WI)BR] rats were fed diets adequate (60 ppm) or deficient (7 ppm) in zinc and received a single sc dose of cadmium (5, 10, or 30 mumol Cd/kg). Lesions were assessed over the next 92 weeks. All cadmium doses increased the incidence of testicular interstitial cell tumors. The incidence of cadmium-induced testicular tumors was unaffected by dietary zinc status. However, when multiplicity of testicular lesions was considered, zinc-deficient diets markedly increased the number of testicular interstitial cell adenomas generated by cadmium exposure while significantly reducing the number of preneoplastic lesions (interstitial cell hyperplasias). The combined total number of neoplastic and preneoplastic lesions of the testes was independent of zinc status clearly indicating a shift from hyperplasia to neoplasia within the testes of zinc-deficient rats. The highest cadmium dose (30 mumol/kg) increased injection site sarcomas in zinc-deficient rats (7 tumors/27 rats at risk) but not zinc-adequate rats (3/26) when compared to control (0/49). Chronic progressive renal nephropathy was accelerated by cadmium in zinc-deficient rats. Results indicate that dietary zinc deficiency enhances carcinogenic response at the injection site of cadmium, promotes the neoplastic progression of cadmium-induced testicular lesions, and enhances chronic progressive nephropathy. Thus, dietary zinc deficiency appears to cause a generalized increase in the chronic toxic effects of cadmium.

Biochemical response to cadmium. Dose-time effect

Dose- and time-related effects of Cd (II) (0.5 or 1.0 mg/kg, Cd as CdCl2.H2O, subcutaneously, daily for 48 h, 1, 3, or 6 wk) were investigated in rats. A dose-related increase in the activity of plasma alkaline phosphatase (ALP), lactate dehydrogenase (LDH), aspartate aminotransferase (GOT), and alanine aminotransferase (GPT) was evident only at 6 wk, whereas an early rise in ALP and LDH was seen at 3 wk in 1.0 mg Cd group only. The hepatic and renal metallothionein (MT) induction displayed a dose- as well as time-related increase with Cd accumulation. A significant increase in hepatic Zn and renal Cu, no change in hepatic Cu, and a slight increase in renal Zn was observed. Urinary ALP and leucine aminopeptidase (LAP) showed an initial increase at 48 h, thereafter returned to near normal. A second phase of enzymuria (ALP, LAP, GOT, GPT, gamma-glutamyl transpeptidase), proteinuria, and aminoaciduria occurred at 6 wk in a dose-related manner. The urinary excretion of specific renal enzymes appeared closely related to the MT induction and organ Cd levels.

Induction of heme oxygenase in the small intestinal epithelium: a response to oral cadmium exposure

The effects of oral cadmium administration on heme oxygenase activity and cytochrome P-450-dependent drug metabolism in intestinal epithelium were examined in male Sprague-Dawley rats. Cadmium chloride was administered via drinking water (0, 5 or 50 ppm cadmium) for 5 or 30 days, and heme oxygenase, 7-ethoxycoumarin O-deethylase (ECOD), 7-ethoxyresorufin O-deethylase (EROD) and cytochrome P-450 were measured in the liver and in the epithelium of the proximal region of the small intestine. Cadmium exposure produced a marked, dose-related induction of intestinal heme oxygenase (up to 300% of control levels) in the small intestine at both time points examined. Concomitant decreases in intestinal ECOD (70%) and EROD (65%) activities were also observed, with a 65% decline in cytochrome P-450 levels at 30 days as compared with controls. Oral cadmium exposure, however, did not affect heme catabolism or cytochrome P-450 function in the liver, even at the highest concentration (50 ppm) administered, although cadmium levels accumulated in a dose-related manner in the liver as well as in the small intestine. Systemic absorption of cadmium was limited, as reflected by the relatively low accumulation of cadmium in the liver at 5 days (approximately 20 micrograms/g), as compared with the levels present in small intestine at this time ponit (approximately 100 micrograms/g). These findings emphasize the sensitivity of cytochrome P-450-dependent drug metabolism in small intestinal epithelium to orally ingested cadmium, and highlight the vulnerability of this tissue to low-dose exposure to this metal.

The human relevance of the renal tumor-inducing potential of d-limonene in male rats: implications for risk assessment

The monoterpene d-limonene is a naturally occurring chemical which is the major component in oil of orange. Currently, d-limonene is widely used as a flavor and fragrance and is listed to be generally recognized as safe (GRAS) in food by the Food and Drug Administration (21 CFR 182.60 in the Code of Federal Regulations). Recently, however, d-limonene has been shown to cause a male rat-specific kidney toxicity referred to as hyaline droplet nephropathy. Furthermore, chronic exposure to d-limonene causes a significant incidence of renal tubular tumors exclusively in male rats. Although d-limonene is not carcinogenic in female rats or male and female mice given much higher dosages, the male rat-specific nephrocarcinogenicity of d-limonene may raise some concern regarding the safety of d-limonene for human consumption. A considerable body of scientific data has indicated that the renal toxicity of d-limonene results from the accumulation of a protein, alpha 2u-globulin, in male rat kidney proximal tuble lysosomes. This protein is synthesized exclusively by adult male rats. Other species, including humans, synthesize proteins that share significant homology with alpha 2u-globulin. However, none of these proteins, including the mouse equivalent of alpha 2u-globulin, can produce this toxicity, indicating a unique specificity for alpha 2u-globulin. With chronic exposure to d-limonene, the hyaline droplet nephropathy progresses and the kidney shows tubular cell necrosis, granular cast formation at the corticomedullary junction, and compensatory cell proliferation. Both d-limonene and cis-d-limonene-1,2-oxide (the major metabolite involved in this toxicity) are negative in in vitro mutagenicity screens. Therefore, the toxicity-related renal cell proliferation is believed to be integrally involved in the carcinogenicity of d-limonene as persistent elevations in renal cell proliferation may increase fixation of spontaneously altered DNA or serve to promote spontaneously initiated cells. The scientific data base demonstrates that the tumorigenic activity of d-limonene in male rats is not relevant to humans. The three major lines of evidence supporting the human safety of d-limonene are (1) the male rat specificity of the nephrotoxicity and carcinogenicity; (2) the pivotal role that alpha 2u-globulin plays in the toxicity, as evidenced by the complete lack of toxicity in other species despite the presence of structurally similar proteins; and (3) the lack of genotoxicity of both d-limonene and d-limonene-1,2-oxide, supporting the concept of a nongenotoxic mechanism, namely, sustained renal cell proliferation.(ABSTRACT TRUNCATED AT 400 WORDS)

The mobilization of intracellular cadmium by alkoxyethyl esters of meso-2,3-dimercaptosuccinic acid

The cadmium mobilizing properties of two newly synthesized esters of meso-2,3-dimercaptosuccinic acid in mice have been examined. They are: di(2'-methoxyethyl) meso-2,3-dimercaptosuccinate ([-CH(SH)COOCH2CH2OR]2, R = CH3; MEDMS), and di(2'-ethoxyethyl) meso-2,3-dimercaptosuccinate ([-CH(SH)COOCH2CH2OR]2, R = CH2CH3; EEDMS), conveniently prepared from dimercaptosuccinate acid with 2-methoxyethanol and 2-ethoxyethanol, respectively. Mobilization studies in mice of aged in vivo cadmium deposits using five ip injections of 0.40 mmol/kg of each chelator in peanut oil clearly indicate that both compounds, MEDMS and EEDMS, are significantly superior to 2,3-dimercaptopropan-1-ol (BAL) in depleting the whole body burden of cadmium. The reductions caused by MEDMS and EEDMS were approximately 20 and 26%, respectively, whereas under similar dosage regimens BAL effected about only a 12% reduction. The esters were neither equal nor superior to BAL for the reduction of renal cadmium levels, MEDMS being the least effective. For the mobilization of hepatic cadmium deposits, both were quite promising (MEDMS, 20%; EEDMS, 34% reduction) compared to BAL (only 2% reduction). There was no accumulation of cadmium with either MEDMS or EEDMS in any of the other organs examined--spleen, testes, pancreas, and particularly the brain. These compounds enhance the fecal excretion of cadmium by a factor of 25- to 40-fold but have very little effect on the urinary excretion of this element. The present study reveals that the order of overall efficacy is EEDMS greater than MEDMS greater than BAL, considering the liver and whole body burdens of cadmium, but BAL greater than EEDMS greater than MEDMS in terms of the efficacy in reducing cadmium levels in the kidneys.

Acute cadmium chloride-induced renal toxicity in the Syrian hamster

It has previously been reported that cadmium (Cd) induces renal lesions only after sequestration by endogenous metallothionein (MT), and not in the form of simple salts. However, in this report we detail findings of acute CdCl2-induced renal lesions in the Syrian hamster, which appear to be species specific as neither rats nor mice showed such lesions. Adult rats and mice of different strains and Syrian hamsters (Cr:RGH) were given Cd doses ranging from 30-50 mumol/kg, sc, and examined histologically for renal lesions between 2 hr and 7 days later. Hamsters developed necrosis of the proximal renal tubules 12-24 hr after CdCl2 treatment at an average incidence of 60% in both sexes. Tubular regeneration occurred within 1 week as shown by immunocytochemical localization of DNA synthesis with 5-bromo-2'-deoxyuridine. By electron microscopy, initial changes with Cd (16 hr) included cytoplasmic vesiculation and dilatation of endoplasmic reticulum, and swelling of mitochondria followed rapidly by enlargement of vacuoles, nuclear changes, and cellular disintegration. Rats and mice showed no such lesions even at lethal doses of Cd (40-50 mumol/kg). At maximum tolerated doses of Cd (approximately LD 10: for rats and mice, 35 mumol/kg; for hamsters, 50 mumol/kg) renal Cd content was not higher in hamsters than in the other species 24 hr after injection; hamsters, in fact, had the lowest Cd content. Likewise, basal or Cd-induced levels of renal MT were not remarkably different between these species. These results indicate the hamster is uniquely susceptible to acute effects of Cd on the kidney and that this effect is not related to an unusually high concentration of CdCl2 or unusually low basal or induced levels of MT in the kidney.

Comparison of the toxicity of inorganic and liver-incorporated cadmium: a 4-wk feeding study in rats

The toxicity of cadmium was examined in rats fed diets containing either tissue-incorporated cadmium or cadmium salt for 4 wk. The test diets contained 30 mg cadmium/kg either as cadmium chloride, or as cadmium incorporated in pigs' livers; the control group was fed a diet containing liver from a pig not treated with cadmium. Over 90% of the cadmium present in the pigs' livers was bound to metallothionein. Analysis of the diet and determination of the food consumption revealed that both cadmium-fed groups were exposed to similar dietary cadmium levels. There was no adverse effect on general health or survival. Feeding cadmium resulted in growth retardation and slightly decreased water intake. Moreover, both cadmium-treated groups showed clear signs of anaemia and increased plasma aspartate and alanine aminotransferase activities. For the group fed cadmium chloride, all of these effects were more pronounced than for the group fed cadmium incorporated in liver. Microscopic examination of the liver and kidneys, however, did not reveal any lesion that could be attributed to the cadmium treatment. After exposure to cadmium the spleen showed decreased extramedullary haematopoiesis, an effect that was also more pronounced after feeding of the cadmium chloride than after feeding liver-incorporated cadmium. The differences in the extent of the toxic effects between the inorganic and the tissue-incorporated cadmium were accompanied by differences in the cadmium concentrations in liver and kidneys: the feeding of cadmium incorporated in pigs' livers resulted in about half the accumulation of cadmium in the rats' livers that took place after intake of a diet containing cadmium chloride. In contrast a much less marked difference in cadmium accumulation was observed in the kidneys. Since humans are usually exposed to tissue-incorporated cadmium these findings deserve further investigation, with special attention to the observed difference in tissue accumulation.

The concept of critical levels of toxic heavy metals in target tissues

The high reactivity of heavy metals with biological systems is well documented, although some disagreement remains on the precise dose-effect relationships involved. This represents a question of considerable importance, especially in attempts to assess the risks of exposure. The implicit assumption is usually made that a threshold concentration of specific metals exists in the most sensitive target organ, so that an increased frequency of functional lesions will be expected if this threshold is exceeded. The threshold for the metal defines its so-called critical level, and this review was written in order to examine the theoretical and practical difficulties in establishing such a level. Among these may be cited, for instance, the dependence of what constitutes the target tissue on the speciation of the metal, the changes in apparent critical level with rate and route of metal administration, the short half-life of some of the metals as well as their compartmentation in the tissues, and the considerable initiation delay frequently preceding the appearance of lesions. For these and other reasons a useful approximate value for a critical concentration has only been proposed so far for the total Cd concentration in the renal cortex of chronically exposed human adults.

Correlation of cadmium-induced nephropathy and the metabolism of endogenous copper and zinc in rats

Female Wistar rats were injected (sc) every second day for 8 weeks with Cd (0.25 mg/kg) as CdCl2. After only a 2-week exposure, when cadmium (Cd) concentration in liver was about 13 micrograms/g, ultrastructural examinations revealed some irregular ergastoplasm systems and significant proliferation of smooth endoplasmic reticulum in the hepatocyte ultrastructure. The increase in Zn content occurred simultaneously with the increase in Cd concentration in the liver (Zn to Cd ratio was 1:1). In the kidneys after a 3-week exposure, when Cd concentration was 7 micrograms/g, the concentration of endogenous Cu increased. At the same time the urinary excretion of that metal was considerably higher than that of the control group. In the kidneys after a 4-week exposure, when Cd concentration in this organ exceeded 10 micrograms/g tissue, injured brush border microvilli and swollen mitochondria in the proximal convoluted tubular cells were seen. In renal corpuscules, fusion between the podocyte pedicles was also found. The changes in renal cortex ultrastructure became more pronounced when Cd concentration in kidney was increasing. Necrotic changes in the examined organ were observed when Cd concentration increased to about 30 micrograms/g tissue. The critical concentration in renal cortex of about 200 micrograms/g tissue should be revised. The present margin of safety with regard to risk of renal effects is small.

Non-metallothionein-bound cadmium in the pathogenesis of cadmium nephrotoxicity in the rat

Male rats were injected SC with 0.6 mg Cd/kg/day for 5 days per week for 2, 4, 6, and 8 weeks. Liver and kidney were examined morphologically and analyzed for metallothionein, cadmium, zinc, and copper. Morphologic changes were found in kidney but not in liver. The earliest ultrastructural change consisted of myelin figures in vacuoles in cytoplasm of proximal tubular lining cells reflecting degeneration of membranes. This change occurred after 4 weeks with 801 +/- 25 nmol/g (89.9 micrograms/g) total kidney cadmium or 390 nmol/g (43.7 micrograms/g) of cadmium not bound to metallothionein. Similar changes were observed after 6 weeks but after 8 weeks pathological changes consisted of focal cellular necrosis and interstitial fibrosis. Other ultrastructural changes included altered mitochondria and increased numbers of microbodies. Renal cadmium after 8 weeks exposure was 1827 +/- 48 nmol/g (215.3 +/- 5.8 micrograms/g) or 628 nmol/g (70.2 micrograms/g) of cadmium not bound to metallothionein. Total cadmium was higher in liver than in kidney but partitioning between bound and nonbound cadmium differed in the two organs. The fraction not bound to metallothionein increased with time of exposure in both liver and kidney. However, total cadmium in the liver did not exceed potentially available binding sites of metallothionein, whereas total cadmium did exceed potentially available binding sites of metallothionein in the kidney where pathologic changes occurred. The results indicated that degeneration of cellular membranes is an early cellular effect of cadmium exposure followed later by toxicity to organelles, cellular necrosis, and interstitial fibrosis. Cadmium-induced cellular toxicity is more directly related to the fraction of cadmium in the kidney that is not bound to metallothionein than is total cadmium per se.

Amelioration of the teratogenicity of cadmium by the metallothionein induced by bismuth nitrate

The participation of maternal hepatic metallothionein (MT) in the amelioration of cadmium teratogenicity in mice was examined. Pretreatment with bismuth nitrate (subcutaneously) ameliorated the teratogenicity, including exencephaly and abnormalities of the axial skeleton, caused by a single intraperitoneal injection of cadmium sulfate. Pretreatment with bismuth nitrate for 3 days induced MT drastically in maternal liver and kidney. Six and 24 hr after the injection of cadmium sulfate, the accumulation of maternal hepatic cadmium increased and that in the decidua, including embryos, decreased after pretreatment with bismuth nitrate. Mouse embryos on day 7 of gestation were cultured for 48 hr. Exposure to cadmium sulfate in vitro induced unfused brain fold, which corresponds to exencephaly in vivo. From the in vitro experiment, it was suggested that the teratogenicity of cadmium on day 7 of gestation is a direct action against the mouse embryo. In the present experiment it was suggested that pretreatment with bismuth nitrate induced maternal hepatic and renal MT; cadmium was therefore trapped and detoxicated, and consequently embryos were exposed to a lower concentration of cadmium.

Protection against cadmium toxicity and enzyme inhibition by dithiothreitol

In the present in vivo studies the alterations in cation transporting enzymes of the brain, kidney and liver tissues were assessed at intervals between 0 to 48 h after a single, acute (10 mg kg-1, i.p.) dose of cadmium (Cd). The inhibition of Na+-K+-ATPase during the first 24 h does not parallel the changes in K+-PNPPase suggesting differential effects on phosphorylation and dephosphorylation steps of the overall ATPase reaction. Between 30 min to 2 h the inhibition in enzyme activity was steep (27 per cent in brain, 54 per cent in liver) followed by a rapid reversal between 2-6 h. This critical period may correspond to the time of induction of metallothionein. This enzyme reversal was followed by a significant decrease in Na+-K+ ATPase (40-68 per cent) and K+-PNPPase (44-60 per cent) between 24 to 48 h. A similar pattern was observed in Ca2+-ATPase in all the three tissues. A 33 per cent mortality was observed in rats after 48 h of cadmium challenge. Administration of dithiothreitol (DTT, 20 mg kg-1, i.p.) to CdCl2 pretreated rats at 24 h resulted in mortality reduced from 33 per cent to 0 and reversal in the inhibition of Na+-K+-ATPase in brain and kidney and Ca2+-ATPase in brain. Since protection of brain and kidney enzymes by DTT paralleled its protection against Cd toxicity, their inhibition by Cd may, in part, constitute the biochemical basis of Cd toxicity.

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.

Detection of kidney damage by malathion impurities using a microdissection technique

O,O,S-Trimethylphosphorothioate (OOS-Me) and O,S,S-trimethylphosphorodithioate (OSS-Me) are impurities in technical grade malathion and related insecticides which have been shown to cause delayed death in rats following a single oral dose of 40-60 mg/kg. In connection with studies on the mode of action of these compounds, work on the microdissection and examination of nephrons was carried out. Nephrons from impurities-treated rats showed swelling, distortion and distension of glomeruli, as well as narrowing of the first part of the proximal tubule (swan neck). These results were similar to those observed from kidney tissue obtained from cadmium-chloride-treated rats and are indicative of OOS-Me and OSS-Me-induced kidney tubule damage.

Effect of N-benzyl-D-glucamine dithiocarbamate on the renal toxicity produced by subacute exposure to cadmium in rats

The effect of N-benzyl-D-glucamine dithiocarbamate (BGD) on the renal toxicity produced by subacute exposure to cadmium in rats was studied. Rats were injected sc with CdCl2 (1.5 mg Cd/kg) daily for 26 days and thereafter they received 13 injections of BGD (400 mumol/kg) every other day. Urinary protein concentration and AST activity significantly increased after 20 days of cadmium treatment. The pattern of the increase in the urinary excretion of cadmium after cadmium treatment was consistent with that in the urinary excretion of protein and AST. Urinary excretion of amino acid increased gradually after the cessation of cadmium treatment. BGD treatment significantly decreased the urinary excretion of protein, aspartate aminotransferase (AST), and amino acid. Plasma AST activity was elevated 8 days after the beginning of cadmium treatment, indicating that the hepatic damage occurred prior to the renal damage. In addition, the microscopic examination of renal tissue from cadmium-treated rats revealed the necrosis of the proximal tubular cells. The cadmium concentrations in liver and kidney were significantly decreased by BGD treatment. The results of this study indicate that BGD treatment is effective in decreasing the cadmium concentrations in liver and kidney, resulting in the therapeutic effect on the cadmium-induced renal damage.