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

Inhaled Cadmium Oxide Nanoparticles: Their in Vivo Fate and Effect on Target Organs

The increasing amount of heavy metals used in manufacturing equivalently increases hazards of environmental pollution by industrial products such as cadmium oxide (CdO) nanoparticles. Here, we aimed to unravel the CdO nanoparticle destiny upon their entry into lungs by inhalations, with the main focus on the ultrastructural changes that the nanoparticles may cause to tissues of the primary and secondary target organs. We indeed found the CdO nanoparticles to be transported from the lungs into secondary target organs by blood. In lungs, inhaled CdO nanoparticles caused significant alterations in parenchyma tissue including hyperemia, enlarged pulmonary septa, congested capillaries, alveolar emphysema and small areas of atelectasis. Nanoparticles were observed in the cytoplasm of cells lining bronchioles, in the alveolar spaces as well as inside the membranous pneumocytes and in phagosomes of lung macrophages. Nanoparticles even penetrated through the membrane into some organelles including mitochondria and they also accumulated in the cytoplasmic vesicles. In livers, inhalation caused periportal inflammation and local hepatic necrosis. Only minor changes such as diffusely thickened filtration membrane with intramembranous electron dense deposits were observed in kidney. Taken together, inhaled CdO nanoparticles not only accumulated in lungs but they were also transported to other organs causing serious damage at tissue as well as cellular level.

Gadolinium chloride pretreatment ameliorates acute cadmium-induced hepatotoxicity

Cadmium is a known industrial and environmental pollutant. It causes hepatotoxicity upon acute administration. Features of cadmium-induced acute hepatoxicity encompass necrosis, apoptosis, peliosis and inflammatory infiltration. Gadolinium chloride (GdCl3) may prevent cadmium-induced hepatotoxicity by suppressing Kupffer cells. The effect of GdCl3 pretreatment on a model of acute cadmium-induced liver injury was investigated. Male Wistar rats 4–5 months old were injected intraperitoneally with normal saline followed by cadmium chloride (CdCl2; 6.5 mg/kg) or GdCl3 (10 mg/kg) followed by CdCl2 (6.5 mg/kg; groups I and II, respectively). Rats of both the groups were killed at 9, 12, 16, 24, 48 and 60 h after cadmium intoxication. Liver sections were analyzed for necrosis, apoptosis, peliosis and mitoses. Liver regeneration was also evaluated by tritiated thymidine incorporation into hepatic DNA. Serum levels of aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were also determined. Hepatic necrosis, hepatocyte and nonparenchymal cell apoptosis and macroscopic and microscopic types of peliosis hepatis were minimized by gadolinium pretreatment. Serum levels of AST and ALT were also greatly diminished in rats of group II. Tritiated thymidine incorporation into hepatic DNA was increased in gadolinium pretreatment rats. Kupffer cell activation was minimal in both the groups of rats. Gadolinium pretreatment attenuates acute cadmium-induced liver injury in young Wistar rats, with mechanisms other than Kupffer cell elimination.

Proteomic characterization of the late and persistent effects of cadmium at low doses on the rat liver

To investigate the late and persistent effects of cadmium (Cd) at low doses on the liver and its potential mechanisms, male Wistar rats were given i.p. injection of Cd as CdCl2 at 20 nmol kg(-1) body weight every other day for 4 weeks. At weeks 20, 44 and 52, the livers from Cd-treated and age-matched control rats were examined pathologically and biochemically. Chronic exposure of rats to Cd at low doses induced mild pathological changes and persistent oxidative damage as well as cell proliferation. Hepatic proteins were analyzed with two-dimensional electrophoresis (2-DE) and mass spectrometry. More than 1000 protein spots were detected by 2-DE. Ten proteins were distinguishable between Cd-treated and age-matched control groups at week 52 week after Cd treatment. Two of them were significantly down-regulated: prohibitin (PHB) and d-dopachrome tautomerase (DDT). By western blotting the down-regulated expression of PHB and DDT in the livers of Cd-treated rats was confirmed in both early (week 20) and late (week 52) time points. To further examine the down-regulation of antioxidant status in the Cd-treated livers, other common antioxidants, including superoxide dismutase and glutathione and one metal detoxification specific protein metallothionein, were also detected and found to be decreased, particularly at the late stage. These results suggest that mild histopathological changes, persistent oxidative damage and cell proliferation remained at the late stages (weeks 44-52) after rats were exposed to low-dose Cd. These persistent changes may be associated with the persistent down-regulation of cellular antioxidant systems.

Localization of metallothionein (MT) and expression of MT isoforms induced by cadmium in rat dental pulp

We investigated the induction of metallothionein (MT) by cadmium (Cd) in the dental pulp of rat incisors. Time-course studies of MT mRNA expression after single Cd injection were observed by Northern-blot analysis. The isoform-specific expressions of MT mRNAs (MT-I, MT-II and MT-III) were observed using the reverse transcriptase-polymerase chain reaction (RT-PCR) method. Both MT-I and MT-II mRNA levels increased within 3 h, peaked at 3 h and then decreased. These findings demonstrated that MT-I and MT-II mRNA were rapidly induced by Cd in dental pulp. MT-III mRNA was constitutively expressed in rat dental pulp, but the expression level did not change by Cd treatment. The localization of MT protein in Cd-treated rat dental pulp was determined by immunohistochemical staining using anti-MT antibody against MT-I and MT-II. MT protein was localized in the specific cell type of odontoblasts (secretory odontoblasts and resting odontoblasts). In conclusion, it is likely that stained MT in the immunohistochemical study should be MT-I and/or MT-II. Furthermore, MT-I and/or MT-II in Cd-treated rat dental pulp was localized in odontoblasts, in which accumulation of Cd were reported. The cell-specific synthesis of MT may be associated with its metal storage and detoxification role in dental tissues.

Mechanisms of cadmium-mediated acute hepatotoxicity

The mechanism of cadmium-mediated acute hepatotoxicity has been the subject of numerous investigations and although some uncertainties persist, sufficient evidence has emerged to provide a reasonable account of the toxic process. Acute hepatotoxicity involves two pathways, one for the initial injury produced by direct effects of cadmium and the other for the subsequent injury produced by inflammation. Primary injury appears to be caused by the binding of Cd2+ to sulfhydryl groups on critical molecules in mitochondria. Thiol group inactivation causes oxidative stress, the mitochondrial permeability transition, and mitochondrial dysfunction. Although cadmium may injure hepatocytes directly, there are compelling reasons to believe that hepatocellular injury is produced in vivo as the result of ischemia caused by damage to endothelial cells. Secondary injury from acute cadmium exposure is thought to occur from the activation of Kupffer cells and a cascade of events involving several types of liver cells and a large number of inflammatory and cytotoxic mediators. In this regard, it is clear that Kupffer cell activation and neutrophil infiltration are important events in the toxic process, and the involvement of proinflammatory cytokines and chemokines has also been implicated. The precise roles of the soluble mediators of inflammation warrant further investigation.

The inhibitory effect of zinc on cadmium-induced cell apoptosis and reactive oxygen species (ROS) production in cell cultures

The prevention of apoptosis by Zn(2+) is a well-known phenomenon. Both in in vitro and in vivo Zn(2+) supplementation prevents apoptosis induced by a variety of agents, among them by cadmium ions. The target for protective action of Zn ions on cell apoptosis is still unknown. In this paper we have evaluated the effect of in vitro ZnCl(2) supplementation at a concentration corresponding to the physiological level (10 microM) and higher (50 microM), on apoptosis induced with different Cd concentrations in two cell types: HeLa human tumor cell line and bovine aorta endothelial cells (BAECs). We demonstrated that Zn supplementation, especially at 10 microM concentration, significantly inhibited apoptosis in both types of cells. To assess the mechanism involved in the Zn effect we examined the influence of Zn supplementation on Cd accumulation in cells, Cd-induced superoxide anion (O(2)(-)) and hydrogen peroxide (H(2)O(2)) production. Zn caused 1.2-2.0-fold inhibition of Cd accumulation, 1.2-2.0-fold inhibition of Cd-induced apoptotic cell death, 1.1-2.0-fold decrease in reactive oxygen species (ROS) production in HeLa cells and in BAECs. These results indicate that inhibition of Cd-induced apoptosis in cells by Zn might be due, not only by inhibition of Cd accumulation in cells but, at least in part, to inhibition of Cd-induced production of ROS, which in turn are known as strong inducers of apoptosis.

Influence of copper-(II) on colloidal carbon-induced Kupffer cell-dependent oxygen uptake in rat liver: relation to hepatotoxicity

Formation of reactive O2 species in biological systems can be accomplished by copper-(II) (Cu2+) catalysis, with the consequent cytotoxic response. We have evaluated the influence of Cu2+ on the respiratory activity of Kupffer cells in the perfused liver after colloidal carbon infusion. Studies were carried out in untreated rats and in animals pretreated with the Kupffer cell inactivator gadolinium chloride (GdCl3) or with the metallothionein (MT) inducing agent zinc sulphate, and results were correlated with changes in liver sinusoidal efflux of lactate dehydrogenase (LDH) as an index of hepatotoxicity. In the concentration range of 0.1-1 microM, Cu2+ did not modify carbon phagocytosis by Kupffer cells, whereas the carbon-induced liver O2 uptake showed a sigmoidal-type kinetics with a half-maximal concentration of 0.23 microM. Carbon-induced O2 uptake occurred concomitantly with an increased LDH efflux, effects that were significantly correlated and abolished by GdCl3 pretreatment or by MT induction. It is hypothesized that Cu2+ increases Kupffer cell-dependent O2 utilization by promotion of the free radical processes related to the respiratory burst of activated liver macrophages, which may contribute to the concomitant development of hepatocellular injury.

The effect of combined administration of cadmium and furosemide on auditory function in the rat

A number of heavy metals have been associated with toxic effects to the peripheral or central auditory system. These include lead, arsenic, mercury, platinum and organic tin compounds. In addition, the ototoxic effects of some metals may be potentiated by other factors. However, the auditory effects of cadmium have not previously been reported. The purpose of the present study was to investigate the potential ototoxic effects of cadmium from an acute dosage, and its potentiation by furosemide. Auditory brainstem response (ABR) thresholds were measured in adult Sprague-Dawley rats. Rats received either cadmium chloride (5 mg/kg, i.p.) followed by saline (4 ml/kg, i.p.). cadmium chloride followed by furosemide (200 mg/kg, i.p.), or furosemide alone. Follow-up ABRs were carried out 7 days post-treatment and threshold changes were compared between each treatment group. No significant threshold change was seen for the cadmium chloride plus saline treated or the furosemide treated animals. However, significant threshold elevations were observed in animals receiving cadmium chloride plus furosemide. In addition, scanning electron microscopy revealed extensive hair cell loss in animals treated with cadmium chloride and furosemide. Although functional auditory changes were not seen after the administration of cadmium alone, the potentiation of threshold changes by furosemide suggests that cadmium may be ototoxic under certain conditions.

Toxic metals stimulate inflammatory cytokines in hepatocytes through oxidative stress mechanisms

Hepatocytes, as well as nonparenchymal cells, secrete proinflammatory cytokines and chemokines that are involved in the pathology of many liver diseases. In particular, tumor necrosis factor-alpha (TNFalpha), as well as members of the CXC family of chemokines, including interleukin (IL)-8 in humans and macrophage inflammatory protein (MIP)-2 in rodents, have been implicated in both damage and repair processes associated with various hepatotoxins. In the liver, cytokine secretion is usually associated with nonparenchymal cells, particularly Kupffer cells. In the present studies, cytokine gene expression and secretion were investigated in hepatocytes treated with cadmium chloride (CdCl2) or vanadium pentoxide (V2O5). Using human Hep G2 cells and freshly isolated rodent hepatocytes, it was demonstrated that metals increase gene expression and secretion of CXC chemokines and TNFalpha. IL-8 and MIP-2 secretion induced either by the metals or H2O2 were inhibited by antioxidants such as tetramethyl-thiourea and N-acetyl-cysteine. In vitro neutralization experiments with TNFalpha and in vivo studies with TNFalpha receptor knockout mice indicated that the metals directly stimulate CXC chemokine secretion without the need for TNFalpha. Taken together these studies indicate that, in addition to other inflammatory mediators and acute phase proteins, cytokines and chemokines are produced by hepatocytes, which may participate in hepatotoxic responses. The events responsible for their expression involve cellular redox changes.

Induction of metallothionein in astrocytes and microglia in the spinal cord from the myelin-deficient jimpy mouse

Jimpy is a shortened life-span murine mutant whose genetic disorder results in severe pathological alterations in the CNS, including hypomyelination, oligodendrocyte death and strong astroglial and microglial reaction. The knowledge of metallothionein (MT) regulation in the CNS and especially of MT presence in specific glial cell types under pathological conditions is scarce. In the present study, immunocytochemical detection of MT-I + II has been performed in spinal cord sections from 10-12- and 20-22-day-old jimpy and normal animals. The identification of MT-positive glial cells was achieved through double labeling combining MT immunocytochemistry and selective markers for oligodendrocytes, astrocytes and microglia. MT was found in glial cells and was present in the spinal cord of jimpy and normal mice at both ages, but there were remarkable differences in MT expression and in the nature of MT-positive glial cells depending on the type of mouse. The number of MT-positive cells was higher in jimpy than in normal spinal cords. This was apparent in all spinal cord areas, although it was more pronounced in white than in the gray matter and at 20-22 days than at 10-12 days. The mean number of MT-positive glia in the jimpy white matter was 1.9-fold (10-12 days) and 2.4-fold (20-22 days) higher than in the normal one. Astrocytes were the only parenchymal glial cells that were positively identified as MT-producing cells in normal animals. Interestingly, MT in the jimpy spinal cord was localized not only in astrocytes but also in microglial cells. The occurrence of MT induction in relation to reactive astrocytes and microglia, and its role in neuropathological conditions is discussed.

Suppression of Kupffer cell function prevents cadmium induced hepatocellular necrosis in the male Sprague-Dawley rat

Exposure of humans to toxic metals and metalloids is a major environmental problem. Many metals, such as cadmium, can be hepatotoxic. However, the mechanisms by which metals cause acute hepatic injury are in many cases unknown. Previous reports suggest a major role for inflammation in acute cadmium induced hepatotoxicity. In initial experiments we found that a non-hepatotoxic dose of cadmium chloride (CdCl2; 2.0 mg/kg, i.v.) markedly increased the clearance rate of colloidal carbon from the blood, which is indicative of enhanced phagocytic activity by Kupffer cells (resident hepatic macrophages). Thus. the objective these studies was to determine the involvement of Kupffer cells in cadmium induced liver injury by inhibiting their function with gadolinium chloride (GdCl3). Male Sprague-Dawley rats were administered GdCl3 (10 mg/kg, i.v.) followed 24 h later by a single dose of CdCl2 (3.0 and 4.0 mg/kg, i.v.). Twenty four hours after CdCl2 administration animals were killed and the degree of liver toxicity was assessed using plasma alanine aminotransferase (ALT), as well as light microscopy. Cadmium chloride administration produced multifocal hepatocellular necrosis and increased plasma ALT activity. Pretreatment with GdCl3 significantly reduced both the morphological changes and hepatic ALT release caused by CdCl2. However, the protection was specific to the liver, and did not alter CdCl2 induced testicular injury, as determined by histopathological damage. In many cases, the inducible cadmium-binding protein, metallothionein (MT) is often an essential aspect of the acquisition of cadmium tolerance in the liver. Although cadmium caused a dramatic induction of hepatic MT (32-fold), GdCl3 caused only a minor increase (2-fold). Combined CdCl2 and GdCl3 treatment did not induce levels to an extent greater than CdCl2 alone. As expected, GdCl3 also caused a slight increase in the amount of cadmium associated with the liver. In cultured hepatocytes isolated from GdCl3 pretreated rats, CdCl2 induced cytotoxicity was not significantly altered compared to control hepatocytes, indicating that the mechanism of tolerance required the presence of other cell types. Thus, GdCl3 attenuation of CdCl2 induced hepatotoxicity does not appear to be caused by increased tissue MT content or a decreased susceptibility of hepatocytes to cadmium. From these data, we concluded that tolerance to cadmium induced hepatotoxicity involves the inhibition of Kupffer cell function which results in a decreased inflammatory response and an altered progression of hepatic injury. These data further indicate that Kupffer cell function is critical to cadmium induced hepatocellular necrosis.

Metallothionein in physiological and physiopathological processes

The multipurpose nature of MT that we have presented in this review has drawn attention from many different fields of research: biochemistry, molecular biology, toxicology, pharmacology, etc. In recent years, considerable advances have been made concerning the regulation of MT genes by metals. Little, however, is known at the molecular level about the mechanisms of MT induction by nonmetallic inducers such as growth factors. This is of particular interest since MT is highly expressed during liver regeneration, an event orchestrated by a series of growth stimulators and inhibitors. The significance of the nuclear distribution of MT in growing cells and what controls its translocation are questions that remain unanswered at the present time. The possibility that MT could participate in a DNA synthesis-related process through donation or abstraction of Zn to and from transcription factors has been inferred from in vitro studies. Such transfer mechanisms, however, have yet to be confirmed in vivo. Overexpression of MT is often accompanied by increased resistance towards a variety of alkylating agents and chemotherapeutic drugs. The mechanisms by which MT protects cells against these agents may depend on their distinct mode of toxic action. For some, MT cysteines can be the target of the direct attack from the parent compound. For others such as N-methyl-N-nitroso compounds, MT cysteines may serve as a sink for the reactive oxygen species now known to be derived from their metabolism. In either case, a primary consequence of such interactions is the release of the metals initially bound to MT. Therefore, the metal composition of MT appears to be an important factor to consider in determining the overall effect of MT in the resistance process.

Cadmium chloride (CdCl2)-induced metallothionein (MT) expression in neonatal rat primary astrocyte cultures

Metallothionein (MT) protein and mRNA levels were studied following exposure of rat neonatal primary astrocyte cultures to cadmium chloride (CdCl2). MT mRNA was probed on Northern blots with a 32P labeled synthetic cDNA probe specific for rat MT mRNA. The probe hybridizes to a single mRNA with a size appropriate for MT, approximately 550 bases. Expression of MT-I mRNA in astrocyte monolayers exposed to 2 x 10(-6) M CdCl2 for 6 h was increased approximately 5-fold (9.7 fg/micrograms total RNA) over MT-I mRNA levels in controls (2 fg/micrograms total RNA). MT-I mRNA could also be detected in untreated cells, suggesting constitutive MT expression in these cells. Western-blot analysis revealed a marked increase in MT protein levels upon exposure to CdCl2 (1 x 10(-6) M; 96 h). Consistent with the constitutive expression of MTs both at the mRNA level and protein level, we have also demonstrated a time-dependent increase in MT-immunoreactivity in astrocytes exposed to CdCl2. The present study suggests that astrocytes constitutively express MTs, and that MT-induction by CdCl2 may be an example of a generalized increase in MTs in response to heavy metal exposure, thus protecting astrocytes, and perhaps also indirectly, juxtaposed neurons from the neurotoxic effects of heavy metals.

Targeted disruption of metallothionein I and II genes increases sensitivity to cadmium

We inactivated the mouse metallothionein (MT)-I and MT-II genes in embryonic stem cells and generated mice homozygous for these mutant alleles. These mice were viable and reproduced normally when reared under normal laboratory conditions. They were, however, more susceptible to hepatic poisoning by cadmium. This proves that these widely expressed MTs are not essential for development but that they do protect against cadmium toxicity. These mice provide a means for testing other proposed functions of MT in vivo.

Metallothionein induction by cadmium, cytokines, thrombin and endothelin-1 in cultured vascular endothelial cells

Metallothionein induction was investigated using vascular endothelial cells derived from bovine aorta in a culture system. The induction occurred by cadmium (2 and 5 microM) but not by zinc (10 and 300 microM) after a 24-h incubation of the confluent cultures. It was revealed that cytokines including interleukin-1 beta, interleukin-6, tumor necrosis factor alpha and transforming growth factor beta (1 ng/ml each) have a capacity of metallothionein induction. In these inducers, only cadmium and tumor necrosis factor alpha exhibited significant cytotoxicity, suggesting that metallothionein is not induced simply in response to cytotoxicity. It was found that either thrombin or endothelin-1 which are coagulation factor or anti-fibrinolytic factor, respectively, also induced metallothionein synthesis. It was therefore suggested that metallothionein in endothelial cells may be involved in the regulation of the functions of these cells as well as the protection against cytotoxic agents.

Cadmium-induced hepatic endothelial cell injury in inbred strains of mice

Susceptibility to cadmium (Cd) hepatotoxicity differs among inbred strains of mice. For example, C3H/HeJ mice are sensitive to Cd-induced hepatotoxicity, whereas DBA/2J mice are resistant. The mechanism of genetic predisposition to Cd hepatotoxicity is unknown. A contemporary theory for acute target organ intoxication maintains that Cd initially damages vascular endothelium and parenchymal cell injury is a secondary event that results from localized ischemia. In the present study, the hypothesis that hepatic endothelial cells (EC) of C3H mice are more susceptible to Cd toxicity than those of DBA mice was tested. Hepatic parenchymal and endothelial cells were grown separately on monolayer cultures for 22 h and subsequently treated with various concentrations of Cd. Hepatocellular toxicity was assessed by lactate dehydrogenase leakage and intracellular K+ loss, whereas endothelial cell injury was assessed by trypan blue exclusion and the inhibition of protein synthesis. The susceptibility of hepatocytes to the cytotoxic effects of Cd was identical between strains. In contrast, the vulnerability of EC to Cd intoxication was strain-dependent. When exposed to 2.5-10.0 microM Cd, EC of Cd-sensitive mice were more susceptible to the cytotoxic effects of Cd than those of Cd-resistant mice. Basal metallothionein (MT) levels as well as Cd uptake into EC were similar in the two strains. Following Cd exposure, EC of Cd-sensitive mice accumulated similar amounts of MT as EC of Cd-resistant mice. These observations suggest that the microvasculature in livers of inbred mice is the target tissue responsible for strain-dependent susceptibility to Cd-induced liver injury. The mechanisms that account for this genetic variation in endothelial cell response to Cd are unknown, but do not appear to be related to the cellular disposition of Cd nor to a defect in the metabolism of MT.