Differences in the uptake of cadmium and mercury by rat hepatocyte primary cultures. Role of a sulfhydryl carrier

The Role of Glutathione and Sulfhydryl Groups in Cadmium Uptake by Cultures of the Rainbow Trout RTG-2 Cell Line

The aim of this study is to investigate the role of cellular sulfhydryl and glutathione (GSH) status in cellular cadmium (Cd) accumulation using cultures of the rainbow trout cell line RTG-2. In a first set of experiments, the time course of Cd accumulation in RTG-2 cells exposed to a non-cytotoxic CdCl2 concentration (25 μM) was determined, as were the associated changes in the cellular sulfhydryl status. The cellular levels of total GSH, oxidized glutathione (GSSG), and cysteine were determined with fluorometric high-performance liquid chromatography (HPLC), and the intracellular Cd concentrations were determined with inductively coupled plasma mass spectrometry (ICP-MS). The Cd uptake during the first 24 h of exposure was linear before it approached a plateau at 48 h. The metal accumulation did not cause an alteration in cellular GSH, GSSG, or cysteine levels. In a second set of experiments, we examined whether the cellular sulfhydryl status modulates Cd accumulation. To this end, the following approaches were used: (a) untreated RTG-2 cells as controls, and (b) RTG-2 cells that were either depleted of GSH through pre-exposure to 1 mM L-buthionine-SR-sulfoximine (BSO), an inhibitor of glutathione synthesis, or the cellular sulfhydryl groups were blocked through treatment with 2.5 μM N-ethylmaleimide (NEM). Compared to the control cells, the cells depleted of intracellular GSH showed a 25% reduction in Cd accumulation. Likewise, the Cd accumulation was reduced by 25% in the RTG-2 cells with blocked sulfhydryl groups. However, the 25% decrease in cellular Cd accumulation in the sulfhydryl-manipulated cells was statistically not significantly different from the Cd accumulation in the control cells. The findings of this study suggest that the intracellular sulfhydryl and GSH status, in contrast to their importance for Cd toxicodynamics, is of limited importance for the toxicokinetics of Cd in fish cells.

Structural Characterization of Toxicologically Relevant Cd2+-L-Cysteine Complexes

The exposure of humans to Cd exerts adverse human health effects at low chronic exposure doses, but the underlying biomolecular mechanisms are incompletely understood. To gain insight into the toxicologically relevant chemistry of Cd²⁺ in the bloodstream, we employed an anion-exchange HPLC coupled to a flame atomic absorption spectrometer (FAAS) using a mobile phase of 100 mM NaCl with 5 mM Tris-buffer (pH 7.4) to resemble protein-free blood plasma. The injection of Cd²⁺ onto this HPLC-FAAS system was associated with the elution of a Cd peak that corresponded to [CdCl₃]^-/[CdCl₄]^2- complexes. The addition of 0.1-10 mM L-cysteine (Cys) to the mobile phase significantly affected the retention behavior of Cd²⁺, which was rationalized by the on-column formation of mixed CdCys_xCl_y complexes. From a toxicological point of view, the results obtained with 0.1 and 0.2 mM Cys were the most relevant because they resembled plasma concentrations. The corresponding Cd-containing (~30 μM) fractions were analyzed by X-ray absorption spectroscopy and revealed an increased sulfur coordination to Cd²⁺ when the Cys concentration was increased from 0.1 to 0.2 mM. The putative formation of these toxicologically relevant Cd species in blood plasma was implicated in the Cd uptake into target organs and underscores the notion that a better understanding of the metabolism of Cd in the bloodstream is critical to causally link human exposure with organ-based toxicological effects.

Linking molecular targets of Cd in the bloodstream to organ-based adverse health effects

The chronic exposure of human populations to toxic metals remains a global public health concern. Although chronic Cd exposure is linked to kidney damage, osteoporosis and cancer, the underlying biomolecular mechanisms remain incompletely understood. Since other diseases could also be causally linked to chronic Cd exposure, a systems toxicology-based approach is needed to gain new insight into the underlying exposure-disease relationship. This approach requires one to integrate the cascade of dynamic bioinorganic chemistry events that unfold in the bloodstream after Cd enters with toxicological events that unfold in target organs over time. To this end, we have conducted a systematic literature search to identify all molecular targets of Cd in plasma and in red blood cells (RBCs). Based on this information it is impossible to describe the metabolism of Cd and the toxicological relevance of it binding to molecular targets in/on RBCs is elusive. Perhaps most importantly, the role that peptides, amino acids and inorganic ions, including HCO₃^-, Cl^- and HSeO₃^- play in terms of mediating the translocation of Cd to target organs and its detoxification is poorly understood. Causally linking human exposure to this metal with diseases requires a much better integration of the bioinorganic chemistry of Cd that unfolds in the bloodstream with target organs. This from a public health point of view important goal will require collaborations between scientists from different disciplines to untangle the complex mechanisms which causally link Cd exposure to disease.

Oleic acid protects against cadmium induced cardiac and hepatic tissue injury in male Wistar rats: A mechanistic study

AIMS

The aim of the present study was to evaluate the possible antioxidant role of oleic acid (OA) against Cd-induced injuries in the heart and liver tissues of male Wistar rats.

MAIN METHODS

Rats were treated with either vehicle (control), or OA (10 mg/kg b.w., fed orally), or Cd (0.44 mg/kg b.w., s.c.), or both (OA + Cd) for 15 days. Following completion of the treatment period, biomarkers of organ damage and oxidative stress including ROS, activities of antioxidant enzymes and their level, activities of Krebs cycle enzymes and respiratory chain enzymes were measured. Levels of interleukins (IL-1β, IL-6, IL-10), tumor necrosis factor (TNF-α) and nuclear factor kappa B (NFκB) were estimated to evaluate the state of inflammation. In addition, changes in mitochondrial membrane potential and status of cytochrome c (Cyt c) were also studied.

KEY FINDINGS

Pre-treatment of rats with OA significantly protected against Cd-induced detrimental changes possibly by decreasing endogenous ROS through regulation of antioxidant defense system, inflammatory responses and activities of metabolic enzymes. Moreover, OA was also found to restore mitochondrial membrane potential possibly by regulating Cyt c leakage thereby increasing mitochondrial viability.

SIGNIFICANCE

Our results for the first time demonstrated systematically that OA provided protection against Cd-induced oxidative stress mediated injuries in rat heart and liver tissues through its antioxidant mechanism. The results raise the possibility of using OA singly or in combination with other antioxidants or diet in the treatment of situations arising due to oxidative stress and may have future therapeutic relevance.

Aqueous bark extract of Terminalia arjuna protects against cadmium-induced hepatic and cardiac injuries in male Wistar rats through antioxidative mechanisms

Cadmium (Cd) is one of the most ubiquitous toxic heavy metal in the environment. The present study was conducted to evaluate the protective role of aqueous bark extract of Terminalia arjuna (TA) against Cd induced oxidative damage in hepatic and cardiac tissues as the TA bark extract has folkloric medicinal use in the treatment of various hepatic and cardiac disorders. Male Wistar rats were divided into 4 groups. The control group was treated with normal saline as the vehicle; the second group orally administered with TA (20 mg/kg bw) daily for 15 days; the third group injected with Cd-acetate (0.44 mg/kg bw, s.c.) every alternate day for a period of 15 days; and the fourth group was administered with TA, 60 min prior to Cd treatment. The biomarkers of organ damage were significantly increased in the Cd treated groups. Besides, a significant alteration in the tissue levels of biomarkers of oxidative stress, the activities and the levels of antioxidant enzymes was observed following treatment with Cd. Additionally, some of the enzymes were found to be inhibited uncompetitively by Cd when tested in an in vitro system. Furthermore, evidence gathered from studies on the histological parameters and mitochondrial membrane potential in both the tissues argue in favour of the possible protective role of TA against Cd induced damage. Finally, gas chromatography-mass spectrometry revealed the presence of eight major bioactive phytochemicals in aqueous bark extract of TA having potent free radical scavenging property. The results indicate that the extract could protect hepatic and cardiac tissues against Cd-induced oxidative stress mediated damages through antioxidant mechanism(s).

Physiologically relevant plasma d,l-homocysteine concentrations mobilize Cd from human serum albumin

Although low-level chronic exposure of humans to cadmium (Cd(2+)) can result in a variety of adverse health effects, little is known about the role that its interactions with plasma proteins and small molecular weight (SMW) ligands in the bloodstream may play in delivering this metal to its target organs. To gain insight, a Cd-human serum albumin (HSA) 1:1 (molar ratio) complex was analyzed by size exclusion chromatography (SEC) coupled on-line to a flame atomic absorption spectrometer (FAAS). Using a phosphate buffered saline (PBS)-buffer mobile phase, the stability of the Cd-HSA complex was investigated in the presence of 2.0mM of SMW ligands, including taurine, acetaminophen, l-methionine, l-cysteine (Cys), d,l-homocysteine (hCys) or l-cysteine methyl-ester (Cys-Me). While taurine, acetaminophen and l-methionine did not affect its integrity, Cys, hCys and Cys-Me completely abstracted Cd from HSA. Subsequent investigations into the effect of 1.5, 1.0 and 0.5mM Cys and hCys on the integrity of the Cd-HSA complex revealed clear differences with regard to the nature of the eluting SMW-Cd species between these structurally related endogenous thiols. Interestingly, the Cd-specific chromatograms that were obtained for 0.5mM hCys revealed the elution of an apparent mixture of the parent Cd-HSA complex with a significant contribution of a structurally uncharacterized CdxhCysy species. Since this hCys concentration is encountered in blood plasma of hyperhomocysteinemia patients and since previous studies by others have revealed that a SH-containing carrier mediates the uptake of Cd into hepatocytes, our results suggest that plasma hCys may play a role in the toxicologically relevant translocation of Cd from the bloodstream to mammalian target organs.

Prenatal and perinatal acrylamide disrupts the development of cerebellum in rat: Biochemical and morphological studies

Acrylamide is known to cause neurotoxicity in the experimental animals and humans. The literature on its neurotoxic effect in the adult animals is huge, but the effect of acrylamide on the embryonic and postnatal development is relatively less understood. The present study examined its effects on the development of external features and cerebellum in albino rats. Acrylamide was orally administered to non-anesthetized pregnant females by gastric intubation 10 mg/kg/day. The animals were divided into three groups as follows. (1) Group A, newborn from control animals; (2) Group B; newborns from mothers treated with acrylamide from day 7 (D7) of gestation till birth (prenatal intoxicated group); (3) Group C; newborns from mothers treated with acrylamide from D7 of gestation till D28 after birth (perinatally intoxicated group). Acrylamide administered either prenatally or perinatally has been shown to induce significant retardation in the newborns’ body weights development, increase of thiobarbituric acid-reactive substances (TBARS) and oxidative stress (significant reductions in glutathione reduced [GSH], total thiols, superoxide dismutase [SOD] and peroxidase activities) in the developing cerebellum. Acrylamide treatment delayed the proliferation in the granular layer and delayed both cell migration and differentiation. Purkinje cell loss was also seen in acrylamide-treated animals. Ultrastructural studies of Purkinje cells in the perinatal group showed microvacuolations and cell loss. The results of this study show that prenatal and perinatal acrylamide or its metabolites disrupts the biochemical machinery, cause oxidative stress and induce structural changes in the developing rat cerebellum.

Chios mastic, a natural supplement for zinc to enhance male sexuality and prostate function

Mastic is a natural resin extracted from the stem of the evergreen tree Pistacia lentiscus var. Chia (Duham) (Anacardiaceae). For a long time, mastic has been esteemed for its aphrodisiac properties. To test this hypothesis, the trace element zinc was determined while the quantity released after a certain time of chewing was studied. For comparison, three commercial chewing-gums were analyzed as well. A portion of natural mastic or commercial gum was uniformly chewed for 1, 2, 3, and 4 h and the zinc content measured. The zinc content of mastic from P. lentiscus var. Chia was compared to that of other natural resins from the same genus (Pistacia terebinthus L.) or conifer [Pinus halepensis Mill. (Pinaceae)], having a different secretion mechanism and also used as an additive in human nutrition. Secreted resin and plant tissues from the above trees were sampled and the zinc content was determined. Zinc concentrations in the resin were lower than in the plant tissues. The Chios mastic showed a slightly greater zinc content compared to the other analyzed specimens. Among all gums studied, only the Chios mastic released a small amount of about 0.7 mg kg(-1) zinc in the mouth and gastrointestinal system after 4 h chewing time. With commercial gums, the zinc content increased to a large degree (up to 2 mg kg(-1)) after the same treatment, a fact which was attributed to the zinc uptake from salivary secretions, indicating zinc deprivation for the human organism.

Effect of prenatal and perinatal acrylamide on the biochemical and morphological changes in liver of developing albino rat

Acrylamide has been employed as an experimental probe to investigate biochemical and morphological changes in developing rat liver following toxin administration in pregnant rats. Non-anesthetized pregnant rats were given acrylamide by gastric intubation at a dose of 10 mg/kg/day. The pups were divided into three groups: Group A, mothers were treated with saline (control group); Group B, mothers were treated with acrylamide from day D7 of gestation till birth (prenatal intoxication); Group C, mothers were treated with acrylamide from D7 of gestation to D28 after birth (perinatal intoxication). Acrylamide-induced biochemical changes (in liver and serum) and morphological changes (in liver) were studied in control and acrylamide-treated developing pups. Prenatally and perinatally administered acrylamide significantly increased lipid peroxidation and reduced glutathione and total thiol levels in liver. Significant inhibition of peroxidase and superoxide dismutase activities was observed in liver tissue. Total lipids including cholesterol and triglycerides were significantly increased in the serum. Acrylamide treatment increased serum alanine aminotransferase and aspartate aminotransferase activities and inhibited alkaline phosphatase activity. Sodium and potassium concentrations were increased, but calcium, phosphorus and iron levels were significantly reduced in the serum. Acrylamide produced significant electrophoretic changes in serum proteins. The most noticeable change was splitting of beta-globulin into beta1- and beta2-globulins. Light microscopy showed acrylamide-induced fatty deposits, congested central vein, vacuolization and chromatolysis in hepatocytes. Ultrastructural studies revealed vacuolated cytoplasm, lipid droplets of variable size and mitochondria with damaged cristae and vacuolization. The nuclei in acrylamide-treated groups showed marked decrease in the staining of nuclear DNA.

Cadmium-induced hepatotoxicity in rats and the protective effect of naringenin

This experiment pertains to the protective role of naringenin against cadmium (Cd)-induced oxidative stress in the liver of rats. Cadmium is a major environmental pollutant and is known for its wide toxic manifestations. Naringenin is a naturally occurring citrus flavonone which has been reported to have a wide range of pharmacological properties. In the present investigation cadmium (5mg/kg) was administered orally for 4 weeks to induce hepatotoxicity. Liver damage induced by cadmium was clearly shown by the increased activities of serum hepatic marker enzymes namely aspartate transaminase (AST), alanine transaminase (ALT), alkaline phosphatase (ALP), lactate dehydrogenase (LDH), gamma glutamyl transferase (GGT) and serum total bilirubin (TB) along with the increased level of lipid peroxidation indices (thiobarbituric acid reactive substances (TBARS) and lipid hydroperoxides) and protein carbonyl contents in liver. The toxic effect of cadmium was also indicated by significantly decreased levels of enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione S-transferase (GST)) and non-enzymatic antioxidants (reduced glutathione (GSH), vitamin C and vitamin E). Administration of naringenin at a dose of (50mg/kg) significantly reversed the activities of serum hepatic marker enzymes to their near-normal levels when compared to Cd-treated rats. In addition, naringenin significantly reduced lipid peroxidation and restored the levels of antioxidant defense in the liver. The histopathological studies in the liver of rats also showed that naringenin (50mg/kg) markedly reduced the toxicity of cadmium and preserved the normal histological architecture of the tissue. The present study suggested that naringenin may be beneficial in ameliorating the cadmium-induced oxidative damage in the liver of rats.

Manganese and its role in Parkinson's disease: from transport to neuropathology

The purpose of this review is to highlight recent advances in the neuropathology associated with Mn exposures. We commence with a discussion on occupational manganism and clinical aspects of the disorder. This is followed by novel considerations on Mn transport (see also chapter by Yokel, this volume), advancing new hypotheses on the involvement of several transporters in Mn entry into the brain. This is followed by a brief description of the effects of Mn on neurotransmitter systems that are putative modulators of dopamine (DA) biology (the primary target of Mn neurotoxicity), as well as its effects on mitochondrial dysfunction and disruption of cellular energy metabolism. Next, we discuss inflammatory activation of glia in neuronal injury and how disruption of synaptic transmission and glial-neuronal communication may serve as underlying mechanisms of Mn-induced neurodegeneration commensurate with the cross-talk between glia and neurons. We conclude with a discussion on therapeutic aspects of Mn exposure. Emphasis is directed at treatment modalities and the utility of chelators in attenuating the neurodegenerative sequelae of exposure to Mn. For additional reading on several topics inherent to this review as well as others, the reader may wish to consult Aschner and Dorman (Toxicological Review 25:147-154, 2007) and Bowman et al. (Metals and neurodegeneration, 2009).

Naringenin protects against cadmium-induced oxidative renal dysfunction in rats

Cadmium (Cd) is an environmental and industrial pollutant that affects various organs in human and experimental animals. Naringenin is a naturally occurring plant bioflavonoid found in citrus fruits, which has been reported to have a wide range of pharmacological properties. A body of evidence has accumulated implicating the free radical generation with subsequent oxidative stress in the biochemical and molecular mechanisms of cadmium toxicity. Since kidney is the critical target organ of chronic Cd toxicity, we carried out this study to investigate the effects of naringenin on Cd-induced toxicity in the kidney of rats. In experimental rats, oral administration of cadmium chloride (5mg/(kgday)) for 4 weeks significantly induced the renal damage which was evident from the increased levels of serum urea, uric acid, creatinine with a significant (p<0.05) decrease in creatinine clearance. Cadmium also significantly decreased the levels of urea, uric acid and creatinine in urine. A markedly increased levels of lipid peroxidation markers (thiobarbituric acid reactive substances and lipid hydroperoxides) and protein carbonyl contents with significant (p<0.05) decrease in non-enzymatic antioxidants (total sulfhydryl groups, reduced glutathione, vitamin C and vitamin E) and enzymatic antioxidants (superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx) and glutathione S-transferase (GST)) as well as glutathione metabolizing enzymes (glutathione reductase (GR) and glutathione-6-phosphate dehydrogenase (G6PD)) were also observed in cadmium-treated rats. Co-administration of naringenin (25 and 50mg/(kgday)) along with Cd resulted in a reversal of Cd-induced biochemical changes in kidney accompanied by a significant decrease in lipid peroxidation and an increase in the level of renal antioxidant defense system. The histopathological studies in the kidney of rats also showed that naringenin (50mg/(kgday)) markedly reduced the toxicity of Cd and preserved the normal histological architecture of the renal tissue. The present study suggest that the nephroprotective potential of naringenin in Cd toxicity might be due to its antioxidant and metal chelating properties, which could be useful for achieving optimum effects in Cd-induced renal damage.

Cytoprotective and antioxidant role of diallyl tetrasulfide on cadmium induced renal injury: An in vivo and in vitro study

Cadmium (Cd) is an environmental and industrial pollutant that affects various organs in humans and animals. A body of evidence has accumulated implicating the free radical generation with subsequent oxidative stress in the biochemical and molecular mechanisms of Cd toxicity. Since kidney is the critical target of Cd toxicity, we carried out this study to investigate the effects of diallyl tetrasulfide (DTS), an organosulfur compound derived from garlic on Cd induced toxicity in the kidney of rats and also in the kidney cell line (vero cells). In experimental rats, subcutaneous administration of Cd (3 mg/kg bw/day) for 3 weeks induced renal damage, which was evident from significantly increased levels of serum urea and creatinine with significant decrease in creatinine clearance. A markedly increased levels of lipid peroxidation markers (thiobarbituric acid reactive substances and lipid hydroperoxides) and protein carbonyl contents with significant decrease in nonenzymic antioxidants (total sulphydryl groups, reduced glutathione, vitamin C and vitamin E) and enzymic antioxidants (superoxide dismutase, catalase, glutathione peroxidase and glutathione-S-transferase) as well as glutathione metabolizing enzymes (glutathione reductase, and glucose-6-phosphate dehydrogenase) were also observed in Cd intoxicated rats. Coadministration of DTS (40 mg/kg bw/day) and Cd resulted in the reversal of the kidney function accompanied by a significant decrease in lipid peroxidation and increase in the antioxidant defense system. In vitro studies with vero cells showed that incubation of DTS (5-50 microg/ml) with Cd (10 microM) significantly reduced the cell death induced by Cd. DTS at 40 microg/ml effectively blocked the cell death and lipid peroxidation induced by Cd (10 microM) indicating its cytoprotective property. Further, the flow cytometric assessment on the level of intracellular reactive oxygen species using a fluorescent probe 2', 7'-dichlorofluorescein diacetate (DCF-DA) confirmed the Cd induced intracellular oxidative stress in vero cells, which was significantly suppressed by DTS (40 microg/ml). The histopathological studies in the kidney of rats also showed that DTS (40 mg/kg bw/day) markedly reduced the toxicity of Cd and preserved the architecture of renal tissue. The present study suggests that the cytoprotective potential of DTS in Cd toxicity might be due to its antioxidant and metal chelating properties, which could be useful for achieving optimum effects in Cd induced renal damage.

Cadmium induces cell cycle arrest in rat kidney epithelial cells in G2/M phase

Cadmium (Cd) has been reported to cause cell cycle arrest in various cell types by p53-dependent and -independent mechanisms. This study was designed to investigate cell cycle progression in kidney cells that are the target of chronic Cd toxicity. Rat renal proximal tubular epithelial cells, NRK-52E, were treated with up to 20 microM CdCl2 in DMEM containing 10% calf serum for up to 24 h. Flow cytometric analysis revealed time- and concentration-dependent increases in cells in G2/M phase of the cell cycle. As compared to the control cells, the cells exposed to 20 microM Cd showed a doubling of the number of cells in this phase after 24 h. The cell cycle arrest was associated with a decrease in protein levels of both cyclins A and B. Further investigation into the mechanism revealed that Cd treatment led to down-modulation of cyclin-dependent kinases, Cdk1 and Cdk2, apparently by elevating the expression of cyclin kinase inhibitors, KIP1/p27 and WAF1/p21. Furthermore, the wild-type p53 DNA-binding activity was up-regulated. Based on these observations, it appears that Cd causes G2/M phase arrest in NRK-52E cells via elevation of p53 activity, increasing the expression of cyclin kinase inhibitors p27 and p21, and decreasing the expression of cyclin-dependent kinases Cdk1 and 2, and of cyclins A and B.

Reciprocal inhibition of Cd and Pb sulfocomplexes for uptake in Caco-2 cells

Cadmium-lead interactions for uptake were studied in the TC7 clone of human enterocytic-like Caco-2 cells as a function of inorganic metal speciation. We have previously shown that Cd uptake in these cells involves both the free cation Cd2+ and chlorocomplex (CdCln(2-n)) species. Here we show 1.9 times higher uptake levels for 109CdCln(2-n) compared to 210PbCln(2-n). Reciprocal inhibitions of chlorocomplexes were observed with a much higher inhibitory effect of Cd compared to Pb. Replacing Cl- by NO3- increased both the level of aquo ion 109Cd2+ and 109Cd accumulation. In contrast, higher levels of 210Pb2+ did not favor 210Pb uptake. For both metals, higher uptake data were recorded in the presence of SO4(2-), leading to sulfocomplex formation, compared with Cl-. Reciprocal inhibitions were minimal at high-cation levels but were significant and comparable in the presence of sulfo-complexes. We conclude that, in addition to Cd2+ (but not Pb2+), sulfocomplexes of both metals would preferentially be taken up compared to chlorocomplexes. NRAMP2 is not involved in Pb2+ uptake, and the NRAMP2-mediated Cd2+ uptake is insensitive to Pb. Uptake of Pb chlorocomplexes could involve specific mechanisms but of very low affinity, whereas uptake of Pb sulfocomplexes occurs with high affinity.

Protective role of vitamin E on the oxidative stress in Hansen's disease (Leprosy) patients

BACKGROUND

A constellation of reactive oxygen species (ROS) capable of damaging cellular constituents generated in excess during the chronic, inflammatory, neurodegenerative disease process of leprosy. The consequences of this leads to enhanced oxidative stress and lower antioxidant status. Enzymatic antioxidants provide first line defense against ROS. We have measured the levels of oxidative stress indices like lipid peroxidation (LPO), protein carbonyls together with enzymatic antioxidants in the blood samples of control and leprosy patients. Nutritional rehabilitation by way of exogenous supplementation of functionally efficient antioxidants like vitamin E reactivates the enzymatic antioxidant system and guards against the insult caused by ROS during the pathogenesis of the disease and antileprosy chemotherapy.

DESIGN

Untreated leprosy patients were selected on the basis of clinical examination and skin smear. All diagnosed untreated leprosy patients received multi drug therapy (MDT) consisting of rifampicin, dapsone and clofazimine as recommended by World Health Organization. A small number of untreated cases were selected for co-supplementation of vitamin E along with MDT. Oxidative stress indices, enzymatic and nonenzymatic antioxidant status were assayed in untreated, MDT treated and those supplemented vitamin E along with MDT.

STATISTICAL METHODS

We have compared the significance in the mean+/-s.d. values of the oxidative stress indices and the levels of antioxidants using one way analysis of variance (ANOVA) between control, untreated, MDT treated and those supplemented vitamin E with MDT and the results were significant at P < 0.05. Statistical analysis of the results suggests that oral administration of vitamin E lowers oxidative stress and augments antioxidant status in affected individuals.

RESULTS

Enhanced oxidative stress as evidenced by increased LPO and protein carbonyl in leprosy cases lowers the antioxidant status. Treatment with MDT has a limited impact on increased oxidative stress and decreased antioxidant status. Coadministration of vitamin E along with MDT decreases oxidative stress and activate the antioxidant status.

DISCUSSION

The excess production of ROS as seen in leprosy cases could lead to degeneration of tissues and derangement of internal organs. The possible reason for the decreased antioxidant status in leprosy cases may be increased production of ROS, deranged liver function, and the free radical producing ability of drugs used in MDT of leprosy. Intervention with antioxidant supplementation like vitamin E prevents oxidative stress mediated through ROS and activates the net antioxidant status during the chronic course of the disease and antileprosy chemotherapy.

Combined efficacies of lipoic acid and 2,3-dimercaptosuccinic acid against lead-induced lipid peroxidation in rat liver

Oxidative stress with subsequent lipid peroxidation has been postulated as one mechanism for lead toxicity. Hence in assessing the protective effects of lipoic acid (LA) and meso 2,3-dimercaptosuccinic acid (DMSA) on lead toxicity, they were tested either separately or in combination for their effects on selected indices of hepatic oxidative stress. Elevated levels of lipid peroxides were accompanied by altered antioxidant defense systems. Lead acetate (Pb - 0.2%) was administered in drinking water for five weeks to induce toxicity. LA (25 mg kg(-1) body wt. day(-1) i.p) and DMSA (20 mg kg(-1) body wt. day(-1) i.p) were administered individually and also in combination during the sixth week. Lead damage to the liver was evident in the decreases in hepatic enzymes alanine transaminase (-38%), aspartate transaminase (-42%) and alkaline phosphatase (-43%); increases in lipid peroxidation (+38%); decreases in the antioxidant enzymes catalase (-45%), superoxide dismutase (-40%), glutathione peroxidase (-46%) and decreases in glutathione (-43%) and decreases in glutathione metabolizing enzymes, glutathione reductase (-59%), glucose-6-phosphate dehydrogenase (-27%) and glutathione-S-transferase (-42%). In combination LA and DMSA completely ameliorated the lead induced oxidative damage. Either compound alone was however only partially protective against lead damage.

Cadmium uptake in rat hepatocytes in relation to speciation and to complexation with metallothionein and albumin

Cadmium (Cd) uptake has been studied in primary cultures of rat hepatocytes focusing on the impact of inorganic and organic speciation. Uptake time-course studies over a 60-min exposure to 0.3 microM (109)Cd revealed a zero-time uptake and a slower process of accumulation which proceeds within minutes. (109)Cd uptake showed saturation kinetics (K(m) = 3.5 +/- 0.8 microM), and was highly sensitive to inhibition by Zn and Hg. There was no evidence for sensitivity to the external pH nor for any preferential transport of the free cation Cd(2+) over CdCl(n) (2-n) chloro-complexes. According to the assumption that only inorganic metal species are available, metal uptake decreased upon albumin (BSA) addition to the exposure media. In contrast, higher levels of (109)Cd accumulation were obtained under optimal conditions for Cd complexation by MT. Comparison among uptake data obtained under inorganic and organic conditions revealed that Cd-MT would be taken up 0.4 times as rapidly as Cd(inorg). We conclude that uptake of Cd in rat hepatocytes involves specific transport mechanism(s) subjected to Zn or Hg interactions. Uptake of inorganic Cd is not proportional to the levels of free Cd(2+) and does not involve the divalent cation transporter DCT1 nor the co-transporter Fe(2+)-H(+) NRAMP2. We found Cd-MT but not Cd-BSA to be available for the liver cells, and have estimated a binding affinity four orders of magnitude higher for Cd complexation with MT compared to BSA; MT may have a significant role in Cd delivery to the liver.

Different transport mechanisms for cadmium and mercury in Caco-2 cells: inhibition of Cd uptake by Hg without evidence for reciprocal effects

Cadmium/Hg interactions have been studied in the TC7 clone of the enterocytic-like Caco-2 cells to test the hypothesis that these metals may compete for intestinal transport. Comparison of the kinetic parameter values for 203Hg(II) and 109Cd(II) uptake in a serum-free medium revealed that Hg is accumulated much more rapidly and to higher concentrations. The very rapid uptake/binding step and the initial uptake rate of 109Cd were both significantly inhibited by an excess of unlabeled Cd or Hg (apparent K(i) for Hg of 9.3 +/- 1.2 microM) without reciprocal effects. 109cadmium uptake was highly sensitive to temperature and a significant fraction of accumulation (12%) was EDTA extractable. 203Hg uptake remained insensitive to temperature or the EDTA washing procedure. However, the uptake of both tracers was half-decreased when an excess of the respective unlabeled metal was added in the stop solution, suggesting an exchange mechanism for adsorption. Cell pretreatment with N-ethylmaleimide (NEM) led to a 30% decrease or a 73% increase in the 3-min specific transport of 109Cd when NEM was still present in or removed from the uptake medium, respectively. NEM had no effect on 203Hg uptake. Overall our results suggest the involvement of a saturable specific mechanism for Cd, which is highly sensitive to inhibition by Hg and NEM under some conditions, and a nonspecific passive diffusion for Hg. The Hg- or NEM-induced inhibition of Cd uptake likely involves a thiol-mediated reaction, but our results suggest that NEM pretreatment may activate other cellular mechanisms leading to a stimulatory effect.

Involvement of calcium channels in the sexual dimorphism of cadmium-induced hepatotoxicity

Cadmium toxicity has been evaluated in a number of in vivo and in vitro toxicological studies. In vivo Cd toxicity exhibits sexual dimorphism with females being more susceptible to Cd uptake, accumulation, and toxicity in the liver. Research to date does not explain why females are more sensitive to Cd-induced hepatotoxicity. Recent studies demonstrate that progesterone sensitizes female F(344) rats and TRL-1215 cells to Cd toxicity, however the mode of action is still unclear. Approximately one half of the Cd entering the cytoplasm does so through receptor operated Ca(2+) channels. Progesterone treatment of human spermatozoa and Xenopus laevis oocytes causes a rapid influx of Ca(2+) suggesting a possible mechanism. Since hepatocytes have progesterone receptors on their cellular membrane and Ca(2+) influx into the cytoplasm occurs following progesterone treatment we evaluated the hypothesis that progesterone facilitates the uptake and accumulation of Cd via Ca(2+) channels, leading to enhanced toxicity. Primary isolated rat hepatocytes were treated with Cd, progesterone, and/or verapamil for 4 h and cytolethality was measured. Pretreatment with the Ca(2+) channel blocker verapamil increased the Cd concentration producing 50% lethality (LC(50)) by 2-fold, thus decreasing Cd cytolethality. In contrast, pretreatment with progesterone decreased the Cd LC(50) by 2-fold resulting in enhanced Cd cytolethality. Verapamil treatment reversed the progesterone enhanced Cd cytolethality. Verapamil and/or progesterone in the absence of Cd did not affect hepatocyte viability. Overall, the results of this study demonstrate that inhibition of progesterone-induced Ca(2+) influx with the Ca(2+) channel blocker verapamil, decreases Cd cytolethality in primary isolated rat hepatocytes. These findings indicate that progesterone activation of receptor-mediated Ca(2+) channels is involved in the sexually dimorphic hepatotoxicity seen following acute Cd exposure.

The effects of sulfhydryl blockers and metal ions on nickel accumulation by rat primary hepatocyte cultures

Previously, we found that nickel (Ni) accumulation by rat hepatocytes involves Ca channel transport processes. However, other mechanisms responsible for Ni accumulation are still unclear. Therefore, in the present study we examined the effects of sulfhydryl (SH) blockers on Ni accumulation by hepatocytes. Hepatocytes were exposed to various concentrations of N-ethylmaleimide (NEM) (0.5, 1 or 2 mM) or monoiodoacetic acid (MIA) (0.1, 0.25 or 0.5 mM), potent blockers of SH ligands, for 30 min and subsequently exposed to 10 microM NiCl(2) for an additional 60 min. Pretreatment with NEM and MIA enhanced the Ni accumulation by hepatocytes to maximum of 156 and 73%, respectively. The effects of essential and nonessential metal ions on Ni accumulation were also investigated. Pretreatment with 10 microM of Cu, Zn, Co, Cd and Mn, decreased Ni accumulation by 46, 30, 20, 18 and 11%, respectively. In contrast, pretreatment with Hg (10 and 20 microM) enhanced the Ni accumulation by almost 81 and 140%, respectively. Furthermore, significant decrease in SH concentration in the hepatocyte membrane was observed by the treatment with NEM, MIA and Hg, but not with Cu, Zn and Cd. These results suggest that Ni accumulation by hepatocytes does not appear to be dependent on the SH carrier-mediated transport processes, and that to block the SH ligands in the plasma membrane may facilitate the Ni crossing of the cell membrane.

Ethanol potentiates lead-induced inhibition of rat brain antioxidant defense systems

We investigated the effect of alcohol (3 g/kg body weight intragastrically) on lead-induced (50 mg/kg body weight intragastrically) oxidative stress in adult rat brain. Ethanol was found to potentiate the accumulation of lead in the rat brain by 100%. Lead and ethanol in combination also enhanced lipid peroxidation, a deteriorative process of biomembranes, and markedly decreased the antioxidant capacity of neuronal cells in terms of reduced activities of antioxidant enzymes i.e., superoxide dismutase, catalase and glutathione peroxidase. Further, the activity of glutathione reductase was also significantly decreased in lead and ethanol co-exposed animals as compared to only lead-treated animals, which had altered glutathione status. The results of the present study show that ethanol makes the adult rat brain more susceptible to the neurotoxic effects of lead by accentuating the oxidative stress induced by lead.

Uptake, cellular distribution and DNA damage produced by mercuric chloride in a human fetal hepatic cell line

A human hepatic cell line (WRL-68 cells) was employed to investigate the uptake of the toxic heavy metal mercury. Hg accumulation in WRL-68 cells is a time and concentration dependent process. A rapid initial phase of uptake was followed by a second slower phase. The transport does not require energy and at low HgCl2 concentrations (<50 microM) Hg transport occurs by temperature-insensitive processes. Subcellular distribution of Hg was: 48% in mitochondria, 38% in nucleus and only 8% in cytosolic fraction and 7% in microsomes. Little is known at the molecular level concerning the genotoxic effects following the acute exposure of eucaryotic cells to low concentrations of Hg. Our results showed that Hg induced DNA single-strand breaks or alkali labile sites using the single-cell gel electrophoresis assay (Comet assay). The percentage of damaged nucleus and the average length of DNA migration increased as metal concentration and time exposure increased. Lipid peroxidation, determined as malondialdehyde production in the presence of thiobarbituric acid, followed the same tendency, increased as HgCl2 concentration and time of exposure increased. DNA damage recovery took 8 h after partial metal removed with PBS-EGTA.

Cytotoxicity of cadmium and characteristics of its transport in cardiomyocytes

Cadmium (Cd) is reported to produce cardiotoxicity at doses and exposure conditions that cause no effect in kidney or liver. The purpose of the present investigation was to examine the cytotoxicity of Cd to neonatal rat cardiomyocytes in primary culture and to elucidate the transport characteristics of Cd in these cells at a nontoxic concentration. Cd concentrations of 0.1 microM and higher that are well tolerated by hepatocytes and renal cortical epithelial cells were toxic to the cardiomyocyte. The plot of initial uptake rate of Cd at various concentrations was nonlinear suggesting that, in addition to simple diffusion, other processes may also be involved. These processes required metabolic energy as pretreatment with dinitrophenol or sodium fluoride inhibited 58 and 59% of the Cd uptake, respectively. The uptake of Cd was also affected by the incubation temperature and lowering the temperature from 37 to 4 degreesC reduced Cd uptake over 30 min by 61%. Cd uptake required interaction with membrane sulfhydryl groups; pretreatment with p-chloromercuribenzenesulfonic acid or mercuric chloride reduced Cd uptake by 46 and 58%, respectively. Cd utilized the transport pathways for calcium (Ca), zinc (Zn), and copper (Cu). Coincubation with 1.26 mM Ca competitively inhibited Cd uptake by 77%. In the presence of Ca, 30 microM Zn or Cu further inhibited Cd accumulation competitively by as much as 63 and 32%, respectively. Cd could enter the cardiomyocytes through Ca channels and Ca channel blocker, verapamil, inhibited up to 76% of Cd uptake. From the above results it can be concluded that Cd is highly toxic to the cardiomyocytes. A majority of Cd enters these cells through transport processes that exist for Ca, Zn, and Cu. The transport processes utilized by Cd are temperature sensitive and dependent on metabolic energy. Furthermore, these involve membrane sulfhydryl groups and include Ca channels.

Cadmium suppresses delta 9 desaturase activity in rat hepatocytes

The effects of cadmium (Cd) on the fatty acid composition and delta 9 desaturation activity were studied in hepatocytes which had been cultured in serum-free medium. The aim of this study was to determine whether the inhibition of delta 9 desaturase seen in Cd-treated rat liver (Kudo et al. (1991) Toxicology, 68, 133-142) is the direct effect of Cd or not. When hepatocytes were cultured in serum-free medium, increases in the proportions of monounsaturated fatty acids such as oleic acid (18:1) were observed in the phospholipid (PL) and triacylglycerol (TG) fractions of hepatocytes, suggesting that delta 9 desaturase was induced. By contrast, essential fatty acids such as linoleic acid (18:2) and arachidonic acid (20:4) decreased. These changes were similar to those seen in the livers of essential fatty acid (EFA)-deficient rats. When hepatocytes were cultured in the presence of CdCl2, the changes in the fatty acid composition were suppressed. The effect of CdCl2 was concentration-dependent. The rate of the conversion of 18:0 to 18:1, which is catalyzed by delta 9 desaturase, was reduced by treatment with Cd, whereas the rate of conversion of 18:2 to 20:4, which is catalyzed by delta 6 desaturase, was slightly increased. Cd did not inhibit liver microsomal delta 9 desaturase activity in vitro. We concluded that Cd suppressed the induction of delta 9 desaturase in cultured hepatocytes. These phenomena can explain the selective inhibition of delta 9 desaturase in the livers of Cd-treated rats.

Lipoperoxidative damage on lead exposure in rat brain and its implications on membrane bound enzymes

We have investigated the effect of lead exposure on lipid peroxidation, a deteriorative process of the membranes, in the different regions of the brain. Lead treatment (50 mg/kg b.wt. intragastrically) for a period of eight weeks to rats resulted in a significant accumulation of lead in all the regions of brain, at maximum in hippocampus. The lipid peroxidation was accentuated following lead exposure and there was a linear correlation between the increase in lipid peroxidation and increase in lead levels (r = 0.75). The antioxidant capacity of the neuronal cells in terms of the activity of antioxidant enzymes superoxide dismutase, catalase and glutathione peroxidase was diminished. Lead treatment also altered the glutathione status i.e. levels of reduced glutathione were lowered, accompanied with the accumulation of oxidized glutathione. Furthermore, the activity of glutathione reductase was significantly lowered in lead-treated animals. The activity of membrane bound enzyme acetylcholinesterase was significantly inhibited following lead exposure and there was a linear correlation between the increase in lipid peroxidation and decrease in acetylcholinesterase activity (r = -0.83). It appears from the results that lead may exert its neurotoxic effects via peroxidative damage to the membranes.

Glutathione mercaptides as transport forms of metals

Among the many cellular functions of GSH, the roles of this tripeptide in metal transport, storage, and metabolism have recently received considerable attention. Although these roles had often been overlooked, they are critical for normal cellular metabolism and for protection from xenobiotics. Indeed, a number of the protective and regulatory functions of GSH are related to its ability to chelate reactive metals. GSH functions in the mobilization and delivery of metals between ligands, in the transport of metals across cell membranes, as a source of cysteine for metal binding, and as a reductant or cofactor in redox reactions involving metals. However, the interaction between GSH and metals can also produce or exacerbate cell injury. For example, GSH appears to be involved in the renal accumulation and toxicity of a number of metals, and in the carcinogenicity of chromium. Additional work is clearly needed to identify the mechanisms involved, and to better define the roles of GSH in metal homeostasis.

Possible mechanism for zinc protection against cadmium cytotoxicity in cultured vascular endothelial cells

We investigated the effect of zinc on cadmium cytotoxicity in vascular endothelial cells in a culture system. The cytotoxicity was evaluated by [3H]adenine release assay. Cadmium at 2 microM concentration and above significantly increased the [3H]adenine release, but zinc at 80 microM and below did not induce such a response after a 24-h incubation. Metallothionein was induced by cadmium at 0.1 microM and above but not by zinc at 300 microM and below; however, zinc at 10 microM and above significantly decreased cadmium-(2 and 5 microM) induced cytotoxicity. Zinc protection against cadmium cytotoxicity was also observed in the presence of 1 microM cycloheximide. Zinc caused significantly less accumulation of cadmium in the cell layer accompanied with a significant accumulation of zinc. The distribution (%) of cadmium in the particulate fraction of the cells was significantly decreased by zinc. In contrast, cadmium in the cytosol fraction was increased in the cells treated with both cadmium and zinc. Gel filtration chromatography of the cytosol showed that cadmium was capable of being bound to high-molecular-weight proteins and metallothionein. The metallothionein-bound cadmium was not increased by zinc; however, the relative distribution of cadmium in the high-molecular-weight fraction in the cytosol was decreased in cadmium plus zinc-treated cells. From these results, it was suggested that the mechanism by which zinc protects endothelial cells from cadmium cytotoxicity was decreased accumulation of cadmium in the particulate fraction and in the high-molecular-weight fraction in the cytosol of the cells. This alteration is postulated to be caused by both zinc-induced decrease in the intracellular cadmium accumulation and the sequestration of cadmium by cadmium-induced metallothionein.

Comparison of cadmium, mercury and calcium accumulations by isolated hepatocytes of the small skate (Raja erinacea) and rat

1. Accumulation of calcium, cadmium and mercury by isolated hepatocytes of the small skate (Raja erinacea) and rat was examined at 14 and 37 degrees C, respectively. 2. Metal uptakes by both species were biphasic, with rat cells accumulating more metal than the skate cells. 3. Total accumulation after 30 min was in the order: mercury = cadmium much greater than calcium. 4. In both species calcium and cadmium accumulations were reduced at 4 degrees C, while mercury accumulation was not. 5. Cd accumulation was increased by Cu and Hg in both species. 6. Hg accumulation was inhibited by Cu in both species, and increased by Cd only in the rat cells.

Cadmium and mercury accumulation in rat hepatocytes: interactions with other metal ions

The uptake of essential metals, such as calcium, copper (Cu), iron (Fe), and zinc (Zn), occurs through processes that include energy-independent carrier mechanisms as well as ion channels. Since cadmium (Cd) and mercury (Hg) inhibit the uptake of these metals, this study examined whether the essential metals in turn affect Cd and Hg accumulation. The uptake of 3 microM Cd was inhibited by Cu, Fe, Zn, and Hg, with 100 microM Zn having the greatest effect. Kinetic analysis indicated that these metals inhibited Cd accumulation in a competitive manner. In comparison, neither the essential metals nor Cd had any significant effect on Hg accumulation. At 4 degrees C the accumulation of Cd was reduced to 20% of that at 37 degrees C, while Hg uptake remained unaffected. The efflux of Cd from the hepatocytes was biphasic, energy-independent, and not affected by Zn, Cu, or Fe. Thus the essential metals decreased Cd accumulation by inhibiting its uptake. On the other hand, Hg decreased Cd accumulation by both inhibiting its uptake and enhancing its efflux. As determined by the organic SH blockers, nearly two-thirds of the Cd entered the hepatocytes through processes involving the SH ligands. The uptake of Hg, however, was not affected by the SH blockers. Furthermore, the fraction of membrane-bound Hg at 3 microM concentration was 2.5 times greater than Cd, indicating that Hg is associated with additional binding sites not utilized by Cd. These results suggest that in hepatocytes Cd uptake occurs mainly through the SH-containing transport processes associated with the uptake of Zn and, to a smaller extent, Cu and Fe. Hg can inhibit Cd uptake by binding to these sites; however, its own uptake occurs via other processes that remain to be elucidated.

Differences in cadmium and mercury uptakes by hepatocytes: role of calcium channels

Calcium uptake in cells occurs through specific membrane channels. Since cadmium and mercury inhibit calcium uptake, this study examined whether the calcium channels may also be involved in the uptake of these metals. Primary cultures of rat hepatocytes were incubated with 3 microM CdCl2 or HgCl2 in the absence or presence of four different organic calcium channel blockers or a calcium agonist. The calcium channel blockers had no significant effect on mercury accumulation. In comparison, the uptake of cadmium was inhibited by diltiazem and verapmil (50-250 microM) as well as by nifedipine and nitrendipine (25-100 microM), with a maximum inhibition of 31% after 30 min incubation with 250 microM verapamil. The calcium agonist vasopressin (20 nM) increased cadmium accumulation by 15%. This effect was blocked by 250 microM verapamil. Kinetic analysis showed that verapamil decreased the Vmax of cadmium uptake, without altering the Km, indicating a noncompetitive inhibition. The calcium channel blockers were ineffective at 4 degrees C. These data suggest that about a third of the cadmium enters hepatocytes through the calcium channels. The mechanism of mercury uptake, on the other hand, is very different as it does not appear to involve the calcium channels.

Mechanisms of metal transport across liver cell plasma membranes

The liver's pivotal role in the homeostasis of essential trace metals and detoxification of exogenous metals is attributed to its ability to efficiently extract metals from plasma, metabolize, store, and redistribute them in various forms either into bile or back into the bloodstream. Bidirectional transport across the sinusoidal plasma membrane allows the liver to control plasma concentrations and therefore availability to other tissues. In contrast, transport across the canalicular membrane is largely, but not exclusively, unidirectional and is a major excretory pathway. Although each metal has relatively distinct hepatic transport characteristics, some generalizations can be made. First, movement of metals from plasma to bile follows primarily a transcellular route. The roles of the paracellular pathway and of ductular secretion appear minimal. Second, intracellular binding proteins and in particular metallothionein play only indirect roles in transmembrane flux. The amounts of metallothionein normally secreted into plasma and bile are quite small and cannot account for total metal efflux. Third, metals traverse liver cell plasma membranes largely by facilitated diffusion, and by fluid-phase, adsorptive, and receptor-mediated endocytosis/exocytosis. There is currently no evidence for primary active transport. Because of the high rate of hepatocellular membrane turnover, metal transport via endocytic vesicles probably makes a larger contribution than previously recognized. Finally, there is significant overlap in substrate specificity on the putative membrane carriers for the essential trace metals. For example, zinc and copper share many transport characteristics and apparently compete for at least one common transport pathway. Similarly, canalicular transport of five of the metals discussed in this overview (Cu, Zn, Cd, Hg, and Pb) is linked to biliary GSH excretion. These metals may be transported as GSH complexes by the canalicular glutathione transport system(s). Unfortunately, none of the putative membrane carrier proteins have been studied at the subcellular or molecular level. Our knowledge of their biochemical properties is rudimentary and rests almost entirely on indirect evidence obtained in vivo or in intact cell systems. The challenge for the future is to isolate and characterize these putative metal carriers, and to determine how they are functionally regulated.

The cytotoxic effects of cadmium chloride and mercuric chloride mixtures in rat primary hepatocyte cultures

The toxic effects of Cd and Hg mixtures were studied using primary monolayer cultures of rat hepatocytes. Cytotoxicity was assessed by measuring the release of lactic dehydrogenase from the cells. Cytotoxic and non-cytotoxic metal levels were used. At the higher exposure concentrations (0.2 micrograms Cd.ml-1 and 2.0 micrograms Hg.ml-1), Cd was very toxic to hepatocytes whereas Hg was only marginally toxic. The combination of Cd and Hg was more toxic than predicted by summation of the individual metal toxicities. The incorporation of [35S]cysteine into protein of the cytosol and insoluble cell fraction was increased in response to Cd or Hg exposure and was directly related to cell 35S accumulation. Combinations of Cd and Hg significantly increased the proportion of total 35S which was incorporated in cell protein, an effect that was attributed to the accumulation of protein in the insoluble cell fraction. Cd uptake by hepatocytes was related to exposure concentration but was lower when Hg was also present in the incubation medium. Gel chromatography of the cytosol from Cd-exposed cells showed 3 Cd containing fractions which corresponded to the elution positions of high Mr proteins, metallothionein (MT) and low Mr molecules. When hepatocytes were exposed to Hg in combination with Cd, the MT-like fraction was no longer evident and Cd in the low Mr fraction was greatly reduced. Regardless of the presence or absence of Cd in the exposure medium, 98% of cytosol Hg in Hg-exposed cells was found to elute after the low Mr fraction, at a position equivalent to inorganic salts. This indicates that the enhanced cytotoxicity of Cd and Hg may be related to a decrease in the MT-like protein in the cytosol and not due to a direct competitive binding interaction in relation to the protein.

Response of rat hepatocyte cultures to cadmium chloride and cadmium-diethyldithiocarbamate

Cellular effects of cadmium (Cd) were studied in primary cultures of rat hepatocytes incubated with cadmium chloride (CdCl2) or cadmium-diethyldithiocarbamate (Cd(DTC)2), labelled with 109Cd. The lipid-soluble complex Cd(DTC)2 was rapidly taken up into the cells and a maximal concentration was reached after 4 h incubation. On the other hand, incubation with CdCl2 resulted in a slow, continuous accumulation of Cd for up to 20 h. Cd was found to be associated with proteins to a higher extent when added to the incubation medium as CdCl2 than when added as Cd(DTC)2, which in addition to differences in lipophilicity of the Cd compounds partly explains the differences in Cd uptake. Subcellular distribution studies showed that a significantly higher proportion of Cd was associated with the total particulates fraction in cells after incubation with Cd(DTC)2 compared to CdCl2 (32 and 19%, respectively). The activities of glutathione reductase and succinic dehydrogenase were inhibited to a similar extent by the 2 Cd compounds. Alcohol dehydrogenase was more strongly affected by CdCl2 than by Cd(DTC)2, although the uptake of Cd was 3-4 times higher in cells incubated with Cd(DTC)2 than in those incubated with CdCl2. The results from the present study show that DTC can increase the transport of Cd into the cell by complex formation with Cd. Compared to CdCl2 the Cd(DTC)2 complex was less toxic as indicated by the biochemical parameters used.

Role of zinc in protection against cadmium-induced toxicity in formation of embryonic chick bone in tissue culture

To clarify a possible mechanism of zinc (Zn)-induced tolerance to cadmium (Cd) toxicity on bone formation, femurs from 9-day-old chick embryos were cultured for 6 days by the roller-tube method in the presence of Cd (2, 4 or 9 microM) and/or Zn (60 microM). Zn prevented a decrease in bone growth caused by Cd at 4 and 9 microM. An increase in calcium (Ca) content of diaphysis was inhibited by Zn in both the presence and absence of Cd. Histologically, Zn protected a Cd-induced degenerative change of mesenchymal cells in the periosteum and that of osteoblasts around the trabecula at each Cd level. At 60 microM Zn, Cd accumulated less in the bone at 2 microM but more at 9 microM. From these results, it was concluded that Zn prevented Cd-induced toxicity in the process of ossification except calcification in a culture system by two different mechanisms, i.e. a decreasing Cd accumulation at a low level of Cd and probably an induction of metallothionein (MT)-like protein at a high level of Cd.

Cadmium uptake by rat red blood cells

Rat red blood cells were employed to study the uptake of cadmium (109Cd). Suspensions of red blood cells were exposed to Cd concentrations of 0.005-500 microM, which were representative of whole blood concentrations (both bound and free) observed following in vivo Cd administration. Cd uptake was biphasic with an initial rapid phase (less than 1 min) followed by a slower second phase. The rate of Cd uptake at 25 degrees C was one-fourth of that at 37 degrees C. The metabolic inhibitors; sodium fluoride (1 mM), potassium cyanide (1 mM) and carbonyl cyanide-m-chlorophenyl hydrazone (2 microM) and the Na+-K+-ATPase inhibitor, ouabain (1 mM) did not reduce Cd (50 microM) uptake into red blood cells. This suggests that the uptake of Cd into red blood cells was not an active process. Incubation of Cd (10 microM) with an equimolar concentration of Zn did not alter uptake of Cd into red blood cells, but at 5 and 10 times higher concentrations of Zn, Cd uptake was enhanced 5-fold. Mercury at one-tenth and equimolar concentrations of Cd increased Cd uptake by red blood cells 2-fold. N-Ethylmaleimide (0.5-5 mM), which irreversibly inactivates membrane sulfhydryl groups, decreased Cd uptake. The data indicate that Cd uptake into rat red blood cells occurs by passive transport and that alterations of sulfhydryls of red blood cell membrane may modulate the process.

The amelioration of cadmium-induced injury in isolated hepatocytes by reduced glutathione

Although various theories have been proposed to account for the mechanism of cadmium-induced cellular injury, none have received strong, direct support from experimental data. An interference with sulfhydryl groups is one of the proposed mechanisms. The ability of reduced glutathione (GSH) to protect hepatocytes from the toxic effects of cadmium has been investigated. When added just prior to cadmium, GSH has a pronounced protective effect, while an additional 15 min after cadmium resulted in partial protection against the ensuing loss of cellular potassium ion. The protection against loss of aspartate aminotransferase into the medium is pronounced, regardless of whether addition of GSH is at zero time or 15 min after cadmium. Addition of the GSH at either time resulted in reduced levels of cadmium associated with the hepatocytes, and may well account for these protective properties of GSH. Protein-bound sulfhydryl groups showed a small decrease in response to cadmium but only after cell injury had been initiated. These data allow the conclusion that cadmium does not exert its cytotoxic effects by simple, single irreversible interaction with cellular thiol groups. Furthermore, as the added GSH remains extracellular, these data indicate that protection can be afforded at an extracellular level even after intracellular exposure to toxic concentrations of cadmium.

Protective effects of dithiothreitol on cadmium-induced injury in isolated rat hepatocytes

The biochemical mechanism involved in cadmium-induced cellular injury remains to be elucidated. Various theories have been proposed to account for this action, one of which is an interaction with reduced sulfhydryl (SH) groups of membranes. The ability of a known SH group reducing agent, dithiothreitol (DTT), to interact with cadmium-induced damage to isolated hepatocytes was investigated. Cadmium chloride produced a cytotoxic response similar to that previously demonstrated. When added just prior to cadmium chloride, DTT was found to protect against the metal-induced injury. If the cells were first exposed to cadmium for 15 min before DTT was added, it was found that the cells could be rescued from the ensuing expression of toxicity due to cadmium. This result was regardless of the fact that by 15 min the cells had accumulated a quantity of cadmium that was associated with cell injury. Furthermore, DTT did not exert its action by decreasing the amount of cadmium accumulation by the cells. Under some experimental conditions, an increased cadmium uptake in the presence of DTT was observed. While it is not clear as to whether the chelating or thiol reductive properties of DTT are of prime importance in the demonstrated protection, the data show that toxicity is not due to a simple, single irreversible interaction between cadmium and membrane SH groups.