Effect of cadmium on F-actin and microtubules of Madin-Darby canine kidney cells

Actin cytoskeleton redox proteome oxidation by cadmium

Epidemiological studies associate environmental cadmium (Cd) exposure with the risk of lung diseases. Although mechanisms are not fully elucidated, several studies demonstrate Cd effects on actin and actin-associated proteins. In a recent study of Cd at concentrations similar to environmental exposures, we found that redox-dependent inflammatory signaling by NF-κB was sensitive to the actin-disrupting agent, cytochalasin D. The goal of the present study was to use mass spectrometry-based redox proteomics to investigate Cd effects on the actin cytoskeleton proteome and related functional pathways in lung cells at low environmental concentrations. The results showed that Cd under conditions that did not alter total protein thiols or glutathione redox state caused significant oxidation of peptidyl Cys of proteins regulating actin cytoskeleton. Immunofluorescence microscopy of lung fibroblasts and pulmonary artery endothelial cells showed that low-dose Cd exposure stimulated filamentous actin formation and nuclear localization of destrin, an actin-depolymerizing factor. Taken together, the results show that redox states of peptidyl Cys in proteins associated with actin cytoskeleton pathways are selectively oxidized in lung by Cd at levels thought to occur from environmental exposure.

Enhanced HSP30 and HSP70 accumulation in Xenopus cells subjected to concurrent sodium arsenite and cadmium chloride stress

Heat shock proteins (HSPs) are molecular chaperones that aid in protein folding, translocation and in preventing stress-induced protein aggregation. The present study examined the effect of simultaneous sodium arsenite and cadmium chloride treatment on the pattern of HSP30 and HSP70 accumulation in A6 kidney epithelial cells of the frog, Xenopus laevis. Immunoblot analysis revealed that HSP30 and HSP70 accumulation in concurrent stressor treatments were significantly higher than the sum of HSP30 or HSP70 accumulation in individual treatments. This finding suggested a synergistic action between sodium arsenite and cadmium chloride. KNK437 inhibitor studies indicated that the combined stressor-induced accumulation of HSPs may be regulated, at least in part, at the level of transcription. Immunocytochemistry revealed that simultaneous treatment of cells with the two stressors induced HSP30 accumulation primarily in the cytoplasm in a punctate pattern with some dysregulation of F-actin structure. Increased ubiquitinated protein accumulation was observed with combined sodium arsenite and cadmium chloride treatment compared to individual stressors suggesting an impairment of the ubiquitin proteasome degradation system. The addition of a mild heat shock further enhanced the accumulation of HSP30 and HSP70 in response to relatively low concentrations of sodium arsenite plus cadmium chloride.

Disruption of blastomeric F-actin: a potential early biomarker of developmental toxicity in zebrafish

The expression of at least some biomarkers of toxicity is generally thought to precede the appearance of frank pathology. In the context of developmental toxicity, certain early indicators may be predictive of later drastic outcome. The search for predictive biomarkers of toxicity in the cells (blastomeres) of an early embryo can benefit from the fact that for normal development to proceed, the maintenance of blastomere cellular integrity during the process of transition from an embryo to a fully functional organism is paramount. Actin microfilaments are integral parts of blastomeres in the developing zebrafish embryo and contribute toward the proper progression of early development (cleavage and epiboly). In early embryos, the filamentous actin (F-actin) is present and helps to define the boundary of each blastomere as they remain adhered to each other. In our studies, we observed that when blastomeric F-actin is depolymerized by agents like gelsolin, the blastomeres lose cellular integrity, which results in abnormal larvae later in development. There are a variety of toxicants that depolymerize F-actin in early mammalian embryos, the later consequences of which are, at present, not known. We propose that very early zebrafish embryos (~5-h old) exposed to such toxicants will also respond in a like manner. In this review, we discuss the potential use of F-actin disruption as a predictive biomarker of developmental toxicity in zebrafish.

Applying Mode-of-Action and Pharmacokinetic Considerations in Contemporary Cancer Risk Assessments: An Example with Trichloroethylene

The guidelines for carcinogen risk assessment recently proposed by the U.S. Environmental Protection Agency (U.S. EPA) provide an increased opportunity for the consideration of pharmacokinetic and mechanistic data in the risk assessment process. However, the greater flexibility of the new guidelines can also make their actual implementation for a particular chemical highly problematic. To illuminate the process of performing a cancer risk assessment under the new guidelines, the rationale for a state-of-the-science risk assessment for trichloroethylene (TCE) is presented. For TCE, there is evidence of increased cell proliferation due to receptor interaction or cytotoxicity in every instance in which tumors are observed, and most tumors represent an increase in the incidence of a commonly observed, species-specific lesion. A physiologically based pharmacokinetic (PBPK) model was applied to estimate target tissue doses for the three principal animal tumors associated with TCE exposure: liver, lung, and kidney. The lowest points of departure (lower bound estimates of the exposure associated with 10% tumor incidence) for lifetime human exposure to TCE were obtained for mouse liver tumors, assuming a mode of action primarily involving the mitogenicity of the metabolite trichloroacetic acid (TCA). The associated linear unit risk estimates for mouse liver tumors are 1.5 x 10(-6) for lifetime exposure to 1 microg TCE per cubic meter in air and 0.4 x 10(-6) for lifetime exposure to 1 microg TCE per liter in drinking water. However, these risk estimates ignore the evidence that the human is likely to be much less responsive than the mouse to the carcinogenic effects of TCA in the liver and that the carcinogenic effects of TCE are unlikely to occur at low environmental exposures. Based on consideration of the most plausible carcinogenic modes of action of TCE, a margin-of-exposure (MOE) approach would appear to be more appropriate. Applying an MOE of 1000, environmental exposures below 66 microg TCE per cubic meter in air and 265 microg TCE per liter in drinking water are considered unlikely to present a carcinogenic hazard to human health.

ClC-2 chloride secretion mediates prostaglandin-induced recovery of barrier function in ischemia-injured porcine ileum

BACKGROUND & AIMS

Ischemia results in the breakdown of the intestinal barrier, predisposing patients to sepsis and multiple organ failure. Prostaglandins play a critical role in mediating recovery of barrier function in ischemia-injured intestine through a mechanism involving stimulation of Cl - secretion. In the present study, we investigated the contributory role of individual Cl - channels in the recovery of barrier function in ischemia-injured porcine ileum.

METHODS

Ischemia-injured porcine ileal mucosa was mounted in Ussing chambers. Short-circuit current (Isc) and transepithelial resistance (TER) were measured in response to prostaglandin E 2 (PGE 2 ) and pharmacologic inhibitors of epithelial Cl - channels. Immunoassays were used to assess the expression and localization of ion channels.

RESULTS

Application of PGE 2 to ischemia-injured ileal mucosa stimulated increases in Isc, an indicator of Cl - secretion, that was followed by marked increases in TER, an indicator of barrier function recovery. In vitro studies revealed that although PGE 2 induced Cl - secretion via at least 3 distinct secretory pathways, recovery of barrier function was initiated by Cl - secretion via ClC-2 Cl - channels co-expressed with occludin and localized to tight junctions within restituting epithelium. Intravenous administration of furosemide to pigs subjected to 1 hour of ileal ischemia impaired recovery of barrier function, as evidenced by decreased TER and increased mucosal-to-serosal 3 H-mannitol flux after a 2-hour reperfusion/recovery period, confirming an important role for Cl - secretory pathways in vivo.

CONCLUSIONS

ClC-2-mediated intestinal Cl - secretion restores TER in ischemia-injured intestine. These data may provide the basis for targeted pharmacologic therapy for diseases associated with impaired barrier function.

Hsp27, Hsp70, and metallothionein in MDCK and LLC-PK1 renal epithelial cells: effects of prolonged exposure to cadmium

Cadmium is a widely distributed industrial and environmental toxin. The principal target organ of chronic sublethal cadmium exposure is the kidney. In renal epithelial cells, acute high-dose cadmium exposure induces differential expression of proteins, including heat shock proteins. However, few studies have examined heat shock protein expression in cells after prolonged exposure to cadmium at sublethal concentrations. Here, we assayed total cell protein, neutral red uptake, cell death, and levels of metallothionein and heat shock proteins Hsp27 and inducible Hsp70 in cultures of MDCK and LLC-PK1 renal epithelial cells treated with cadmium for 3 days. Treatment with cadmium at concentrations equal to or greater than 10 microM (LLC-PK1) or 25 microM (MDCK) reduced measures of cell vitality and induced cell death. However, a concentration-dependent increase in Hsp27 was detected in both cell types treated with as little as 5 microM cadmium. Accumulation of Hsp70 was correlated only with cadmium treatment at concentrations also causing cell death. Metallothionein was maximally detected in cells treated with cadmium at concentrations that did not reduce cell vitality, and further increases were not detected at greater concentrations. These results reveal that heat shock proteins accumulate in renal epithelial cells during prolonged cadmium exposure, that cadmium induces differential expression of heat shock protein in epithelial cells, and that protein expression patterns in epithelial cells are specific to the cadmium concentration and degree of cellular injury. A potential role for Hsp27 in the cellular response to sublethal cadmium-induced injury is also implicated by our results.

Cd-MT causes endocytosis of brush-border transporters in rat renal proximal tubules

Nephrotoxicity in humans and experimental animals due to chronic exposure to cadmium (Cd) is manifested by defects in the reabsorptive and secretory functions of proximal tubules (PT). The main symptoms of Cd nephrotoxicity, including polyuria, phosphaturia, aminoaciduria, glucosuria, and proteinuria, suggest that various brush-border membrane (BBM) transporters are the main targets of Cd. Specific transporters may be either directly inhibited by Cd or lost from the BBM after Cd treatment, or both. We have recently proposed that Cd may impair the vesicle-dependent recycling of BBM transporters by inhibiting vacuolar H+-ATPase (V-ATPase) activity and endocytosis in PT cells (Herak-Kramberger CM, Sabolic I, and Brown D. Kidney Int 53: 1713-1726, 1998). The mechanism underlying the Cd effect was further explored in an in vivo model of experimental Cd nephrotoxicity induced by Cd-metallothionein (Cd-MT; 0.4 mg Cd/kg body mass; a single dose sc) in rats. The time-dependent redistribution of various BBM transporters was examined in this model by fluorescence and gold-labeling immunocytochemistry on tissue sections and by immunoblotting of isolated renal cortical BBM. In PT cells of Cd-MT-treated rats, we observed 1) shortening and loss of microvilli; 2) time-dependent loss of megalin, V-ATPase, aquaporin-1 (AQP1), and type 3 Na+/H+ exchanger (NHE3) from the BBM; 3) redistribution of these transporters into vesicles that were randomly scattered throughout the cell cytoplasm; and 4) redistribution of NHE3, but not megalin, into the basolateral plasma membrane. The internalization of BBM transporters was accompanied by fragmentation and loss of microtubules and by an increased abundance of alpha-tubulin monomers in PT cells. Transporter redistribution was detectable as early as 1 h after Cd-MT treatment and increased in magnitude over the next 12 h. We conclude that the early mechanism of Cd toxicity in PT cells may include a colchicine-like depolymerization of microtubules and impaired vesicle-dependent recycling of various BBM proteins. These processes may lead to a time-dependent loss of cell membrane components, resulting in reabsorptive and secretory defects that occur in Cd-induced nephrotoxicity.

Receptors and channels regulating acrosome reactions

Prospective clinical studies informed by cloning and sequencing of sperm surface receptors and metal ion channels have elucidated critical early steps in the acrosome reaction that explain aspects of metal ion-related male infertility. Induction of the acrosome reaction is proposed to include non-nuclear progesterone receptor activation of Shaker-related sperm head voltage-gated potassium ion channels (VGKC). Men express VGKC isoforms with differing sensitivities to lead (Pb(2+)) inhibition, thus explaining interindividual variabilities in Pb(2+)-related male infertility. VGKC opening induces calcium (Ca(2+)) transients, and a signalling cascade induced by zona receptor aggregation requires an actin cytoskeleton created by the VGKC-induced Ca(2+) transients. Actin polymerization and stabilization, favoured by zinc (Zn(2+)) and depolymerized by cadmium (Cd(2+)), may mediate low Zn(2+) and high Cd(2+) infertile states. Zona receptor aggregation induces phosphotyrosine signals at sites, including sperm voltage-dependent Ca(2+) channels (VDCC), intermediate in electrophysiology between T- and L-type channels. Sperm surface VDCC localize at the sperm equatorial segment, the terminus of zona receptor translocation. Opening of VDCC admits a second Ca(2+) wave that activates phospholipase C phosphorylated in the zona receptor cascade. Phospholipase C induces fusogenic lipids and activates actin-severing proteins, depolymerizing the actin cytoskeleton and permitting apposition and fusion of acrosomal and plasma membranes.

Subchronic cadmium treatment affects the abundance and arrangement of cytoskeletal proteins in rat renal proximal tubule cells

Disfunction of proximal tubules (PT) in cadmium (Cd) nephrotoxicity in mammals results from the diminished functional capacity of brush-border membrane (BBM) caused by (a) direct inhibition of BBM transporters by Cd, (b) shortening and loss of microvilli, and (c) loss of specific BBM transporters. The loss of transporters may partially result from impaired intracellular vesicle recycling due to loss or/and inhibition of vacuolar H+-ATPase in the PT cell organelles. Cytoskeleton plays an important role in vesicle-mediated recycling and processing of BBM transporters in PT cells. Experiments in vitro have indicated that Cd may affect the state of polymerization of some cytoskeletal proteins. In this work we studied the in vivo effect of CdCl2-treatment in rats (2 mg Cd/kg b. m., s.c., daily for 14 days) upon abundance and arrangement of actin filaments, actin-bundling protein villin, and microtubules (MT) in PT cells. Cd-treatment elicited a dramatic accumulation of Cd in the kidney cortex (200 microg/g tissue wet mass after 14 days) and a strongly increased abundance of metallothionein in PT cells. As revealed by immunocytochemistry in tissue cryosections, the staining intensity of actin and villin in PT cells of Cd-treated rats was generally decreased, without a marked change in their intracellular distribution, whereas MT became largely irregular, diminished in most cells, and lost in many cells. However, the immunoblots revealed an increased content of villin and alpha-tubulin in cortical tissue homogenates from Cd-treated rats, thus indicating an impaired bundling of actin and greatly depolymerized MT in cells intoxicated with Cd. The partial loss of apical actin and villin in PT cells of Cd-treated rats may reflect (or cause) shortening and loss of microvilli, whereas derangement and depolymerization of MT may contribute to the impairment of intracellular recycling of BBM proteins, and lead to the loss of BBM transporters.

Evidence that E-cadherin may be a target for cadmium toxicity in epithelial cells

E-cadherin is a Ca(2+)-dependent cell adhesion molecule that plays an important role in the development and maintenance of epithelial polarity and barrier function. This commentary describes the results of recent studies showing that the environmental pollutant Cd(2+) can damage the E-cadherin-dependent junctions between many types of epithelial cells and reviews the evidence indicating that this effect results from the direct interaction of Cd(2+) with the E-cadherin molecule. In addition, the implications of these findings with respect to the mechanisms of Cd(2+) toxicity in specific target organs such as lung, kidney, bone, and the vascular endothelium are discussed.

Effects of cadmium chloride on the paracellular barrier function of intestinal epithelial cell lines

In the present study we characterized the functional and structural disruption of the paracellular barrier of intestinal epithelium in vitro in relation to cytotoxicity after apical Cd2+ exposure. For that purpose filter-grown Caco-2 and IEC-18 cells were apically exposed to 5 to 100 microM CdCl2 for 4 or 14 h. It was found that the effects of Cd2+ on the epithelial barrier were concentration- and time-dependent. The first detected effects of Cd2+ in Caco-2 cells after 4 h exposure were a decrease in transepithelial electrical resistance, increased permeabilities of mannitol and PEG-4000, and changes in intercellular localization of ZO-1, occludin, and e-cadherin. The effects were far more pronounced after prolonged exposure. The disruption of the paracellular barrier by 5 to 30 microM Cd2+ was detected without a significant loss of viability of the Caco-2 cells. In the IEC-18 cells, Cd2+ concentrations affecting the barrier (50 and 100 microM) also affected cell viability. In both cell lines the effects on the cell layers continued to develop after removal of extracellular Cd2+. This correlated with the cellular retention of Cd2+, which was high for the 12 h following 4 h accumulation. This study showed that the decreased epithelial barrier function of intestinal epithelial cells is accompanied by tight junction disruption. It is concluded that Cd2+ causes increased paracellular permeability by disruption of junctional function and structure. The initial junctional effects of Cd2+ suggest that Cd2+ increases its own bioavailability by causing disruption of the intestinal paracellular barrier.

Effect of calmodulin-inhibitors and verapamil on the nephrotoxicity of cadmium in rat

Recent reports indicate that calmodulin inhibitors (CIs) can modify cadmium (Cd) toxicity in rodents. Pretreatment with CIs prevents Cd-induced testicular damage in mice and reduces the severity of such damage in rats. On the other hand it has been suggested that the cellular transport of Cd can be partly inhibited by the calcium-channel inhibitor, verapamil. The aim of this study was to determine whether these inhibitors can prevent the toxic effects of Cd on the kidney which is the critical organ. For that purpose, we have examined the effects of two CIs (trifluoperazine and chlorpromazine) and of verapamil on the development of tubular damage in female Sprague-Dawley rats. The animals were injected subcutaneously 5 days a week for 8 weeks with cadmium chloride (1 mg Cd/kg), alone or in association with trifluoperazine (20 mg/kg), chlorpromazine (15 mg/kg) or verapamil (2 x 5 mg/kg). The development of renal dysfunction was followed by measuring the urinary excretion of the low molecular weight protein Clara cell protein (CC16). In Cd-treated rats, the urinary excretion of CC16 started to increase from week 6 to reach at the end of experiment values more than 100-times above normal. CIs or verapamil did not influence the rise of urinary CC16 induced by Cd. The three inhibitors, by contrast, enhanced the accumulation of Cd in the liver and, at the exception of chlorpromazine, in the kidneys of Cd-treated rats. Although interfering with the metabolism of Cd, CIs and verapamil do not prevent renal damage in rats chronically exposed to this heavy metal.

Cadmium (Cd2+) disrupts E-cadherin-dependent cell-cell junctions in MDCK cells

Previous studies from our laboratory have shown that Cd2+ can selectively disrupt E-cadherin-dependent cell-cell junctions in the porcine renal epithelial cell line, LLC-PK1. The objective of the present studies was to determine whether or not Cd2+ could produce similar effects in Madin-Darby canine kidney (MDCK) cells, an immortal epithelial cell line derived from dog kidney. This is an important issue because MDCK cells have been used extensively as a model system to study the basic mechanisms of E-cadherin-dependent cell-cell adhesion. MDCK cells on permeable membrane supports were exposed to Cd2+ by adding CdCl2 to either the apical or the basolateral compartment. The integrity of cell-cell junctions was assessed by morphologic observation of the cells and by monitoring the transepithelial electrical resistance. The results showed that exposure to 10-40 microM Cd2+ for 15 min-4 h caused the cells to separate from each other without detaching from the growing surface. The separation of the cells was accompanied by a marked drop in the transepithelial electrical resistance, a loss of E-cadherin from the cell-cell contacts, and a reorganization of the actin cytoskeleton. These effects were much more pronounced when Cd2+ was added basolaterally than when it was added apically. Moreover, the effects of Cd2+ were qualitatively similar to those observed when the cells were incubated in Ca(2+)-free medium. These results show that Cd2+ can disrupt E-cadherin-dependent cell-cell junctions in MDCK cells, and they indicate that this cell line would be an appropriate model for further mechanistic studies in this area.

Actin assembly by cadmium ions

Cadmium is a highly toxic metal entering cells by a variety of mechanisms. Its toxic action is far from being completely understood, although specific interaction with the cellular calcium metabolism has been indicated. Metal ions that influence intracellular Ca2+ concentrations or compete with Ca2+ for protein binding sites may exert an effect on actin filaments, whose assembly and disassembly are both regulated by a number of calcium-dependent factors. Cadmium is such a metal. Much evidence demonstrates that cadmium interferes with the dynamics of actin filaments in various types of cells. Here we show that, at high (0.8-1.0 mM) concentrations, CdCl2 causes actin denaturation. At such Cd2+ concentrations, actin precipitates (really actin, as shown by SDS-PAGE, see Fig. 1B) in the form of irregular, disordered clots, clearly appreciable by electron microscopy. Denaturation seems to be reversible since, after Cd2+ removal by dialysis, the polymerizability of sedimented actin is restored almost completely. On the other hand, at concentrations ranging from 0.25 to 0.6 mM, CdCl2 is more effective as an actin polymerizing agent than both MgCl2 and CaCl2. The Cd-related increase in the actin assembly rate is ascribable to an enhanced nucleation rather than to an increased monomer addition to filament growing ends. The latter, in contrast, appears quite slow. Critical concentration measurements revealed that the extent of polymerization of both Mg- and Cd-assembled actin are very close (C(c) ranges from 0.25 to 0.5 microM), while Ca-polymerized actin shows a polymerization extent markedly lower (C(c) = 4.0 microM). By both the fluorescent Ca2+ chelator Quin-2 assay and limited proteolysis of actin by trypsin and alpha-chymotrypsin, the real substitution of G-actin-bound Ca2+ by Cd2+ has been appreciated. The increase in Quin-2 fluorescence after addition of excess CdCl2 indicates that, in our experimental conditions, Ca2+ tightly-bound to actin is partially (60-70%) replaced by Cd2+, forming Cd-actin. Electrophoretic patterns after limited proteolysis reveal that the trypsin cleavage sites in the segment 61-69 of the actin polypeptide chain are less accessible in Cd-actin than in Ca-actin, although the cation-dependent effect is less pronounced in Cd-actin than in Mg-actin. Our results are consistent with some of the consequences on microfilament organization observed in Cd2(+)-treated cells; however, considering the positive effect of Cd2+ on actin polymerization in solution we have noticed that this was never observed in vivo. A different indirect effect of Cd2+ on some cellular event(s) influencing cytoplasmic actin polymerization appears to be reasonable.

Calcium-independent effects of cadmium on actin assembly in mesangial and vascular smooth muscle cells

Several metal ions are known to cause depolymerization of the actin cytoskeleton under some circumstances. We found that in renal mesangial and vascular smooth muscle cells, micromolar concentrations of Cd2+ result in loss of phalloidinstainable filamentous (F-) actin. The decrease in F-actin was not accompanied by a corresponding increase in G-actin. The decrease in total actin could be accounted for in part by an inhibition by Cd2+ of total protein (and actin) synthesis after 6 to 8 h without an effect on actin degradation, and the equilibrium between F- and G-actin was shifted to maintain near-constant levels of G-actin. However, Cd2+ caused significant decreases in F-actin at earlier times, indicating effects on the polymerization equilibrium independent of those on actin synthesis. Only picomolar concentrations of free intracellular Cd2+ occur in these experiments. However, it is this Cd2+ pool which is responsible for F-actin depolymerization because equal cellular concentrations of cadmium delivered as Cd-metallothionein have no effect. The effect is also very specific for Cd2+ and under the same conditions neither Mg2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, nor Hg2+ result in any loss of F-actin. Addition of Cd2+ to mesangial and vascular smooth muscle cells had no immediate effect on free intracellular calcium concentrations ([Ca2+]i) even though Ca(2+)-signalling pathways were intact as shown with vasopressin and endothelin. Exposure to 10 microM CdCl2 for 8 h nevertheless caused an increase in [Ca2+]i to > 250 nM and increases in [Ca2+]i achieved with ionophores alone were sufficient to decrease F-actin concentrations. However, a rise in [Ca2+]i is not necessary for actin depolymerization. Depletion of cellular Ca2+ by treatment with thapsigargin did not protect F-actin against Cd2+; the effect of Cd2+ was enhanced in cells unable to increase their [Ca2+]i. We conclude that depolymerization of F-actin by Cd2+ in smooth muscle and mesangial cells is metal-specific, Ca(2+)-independent, and accompanied by a depletion of total actin protein.

Cell death and regeneration of renal proximal tubular cells in rats with subchronic cadmium intoxication

Male Sprague-Dawley rats were injected subcutaneously with 0.6 mg cadmium (Cd)/kg/day for 8 wk. The subsequent changes in renal proximal tubules were studied histologically, histochemically, and ultrastructurally. The urinary and tissue Cd concentrations were determined by atomic absorption spectrophotometry. After 4 wk of exposure, apoptosis was observed predominantly in segment S3 along with epithelial regeneration in the affected tubules, and these changes gradually became more pronounced as the experimental period was prolonged. The apoptotic cells were shed into the lumen and were found to contain a large quantity of Cd. Apoptotic cells were counted in paraffin sections after various periods of exposure to Cd. Nuclear bromodeoxyuridine uptake, mitotic count, and nuclear density were used as indicators of tubular regeneration. A correlation was found between the numerical increase of apoptotic cells and the rate of urinary Cd excretion, and the rate of increase in the tissue Cd concentration had a tendency to reduce after 4 wk as the rate of urinary Cd increased. These observations suggest that apoptosis might be helpful for the efficient excretion of Cd into urine. Progressive increases in the preceding indicators of regeneration were observed. From our results, it appears that Cd-induced tubular damage, i.e., cell deletion due to apoptosis, is reversible as a result of marked epithelial regeneration. On the basis of these histological changes, the critical concentration of Cd required to produce renal tubular damage was estimated to be 600 micrograms/g dry tissue.

Effects of cadmium and copper on zinc transport kinetics by isolated renal proximal cells

Zinc, cadmium, and copper are known to interact in many transport processes, but the mechanism of inhibition is widely debated, being either competitive or noncompetitive according to the experimental model employed. We investigated the mechanisms of inhibition of zinc transport by cadmium and copper using renal proximal cells isolated from rabbit kidney. Initial rates of 65Zn uptake were assessed after 0.5 min of incubation. The kinetics parameters of zinc uptake obtained at 20 degrees C were a Jmax of 208.0 +/- 8.4 pmol.min-1.(mg protein)-1, a Km of 15.0 +/- 1.5 microM and an unsaturable constant of 0.259 +/- 0.104 (n = 8). Cadmium at 15 microM competitively inhibited zinc uptake. In the presence of 50 microM cadmium, or copper at both 15 and 50 microM, there was evidence of noncompetitive inhibition. These data suggest that zinc and cadmium enter renal proximal cells via a common, saturable, carrier-mediated process. The mechanisms of the noncompetitive inhibition observed at higher concentrations of cadmium or with copper require further investigation, but may involve a toxic effect on the cytoskeleton.

Zinc alters actin filaments in Madin-Darby canine kidney cells

Exposure of semiconfluent cultures of Madin-Darby canine kidney cells to 10 microM zinc leads to a change in the organization of the actin filament system. Most of the stress fibers at the basal end of the cell are lost and the actin associated with the lateral membrane and junctional regions appears to retract into the cytoplasm. In addition, at the base of the cell in regions of cell-substratum contact, dense, actin-rich plaques appear. These alterations in actin filaments are associated with a change in cell shape. Microtubules were unaffected by exposure to 10 microM zinc. At zinc concentrations greater than or equal to 50 microM the microtubules depolymerized. Exposure to cadmium alters the actin filaments as well but the effect is different from the change seen with zinc. When the cells are exposed simultaneously to zinc and cadmium the cells appear the same as they would if exposed to zinc alone. Exposure of MDCK cells to either metal, individually or in combination, results in a significant and similar increase in F-actin content as determined spectrofluorometrically. The changes in organization and amount of F-actin are associated with a reduction in the ability of the cells to remain attached to the substrate, a toxic effect of these metals with regard to epithelial function. The results indicate that zinc, an essential metal, and cadmium, a highly toxic metal, interact with the actin cytoskeleton in intact cells.

Differential effect of cysteine-to-serine substitutions in metallothionein on cadmium resistance

A set of mutant coding sequences for Chinese hamster metallothionein (MT) 2 in which codons for individual cysteines were replaced by serine codons was cloned into a yeast expression system. MT gene expression was placed under control of a constitutive promoter on a multicopy Escherichia coli-yeast shuttle vector. MTs were expressed in a metal-sensitive host that lacks the endogenous MT gene. The expressed MTs conferred increased metal resistance to the yeast host. A sensitive assay for cadmium resistance was developed in which population doubling times were monitored in rich liquid medium supplemented with a sublethal dose of CdCl2. Measurements on mutants with single cysteine replacements at 12 positions revealed two mutant classes. One class (Cys----Ser at position 5, 13, 19, or 33) did not affect the detoxification capacity of MT. A second class (Cys----Ser at position 7, 15, 26, 29, 44, 48, 50, or 60) conferred to the host markedly less resistance to cadmium. Bridging cysteines were more critical to cadmium resistance. All five bridging cysteine mutants studied (at positions 7, 15, 44, 50, and 60) conferred lower cadmium resistance. In contrast, mutation of four out of seven terminal cysteines (at position 5, 13, 19, or 33) was shown to be inconsequential. Mutations tend to be more detrimental in the alpha domain than in the beta domain in conveying cadmium resistance, suggesting that the contribution of individual cysteine to the detoxification function of MT is site specific.

Cadmium inhibition of basolateral solute fluxes in rabbit renal tubules and the nature of cycloleucine uptake

The objective of this investigation was to determine whether Cd inhibition of amino acid transport across basolateral (BL) cell membranes in renal tubules results from a direct toxic action at that site. The concentration ratio (R) of cycloleucine in cell water/arterial plasma at steady state in nonfiltering rabbit kidneys consistently exceeded 1.0, thus confirming the active nature of BL amino acid uptake. BL cycloleucine extrusion, though down the concentration gradient, has previously been shown to be greatly slowed in Cd-poisoned animals; nevertheless, R remained unchanged. Active uptake and passive extrusion of cycloleucine must therefore be equally sensitive to Cd, a fact strongly suggesting an indirect action of the metal on BL solute transfer. This hypothesis is strengthened by the observation that R for paraaminohippurate (PAH), another solute actively accumulated across BL membranes, also remained unaffected by Cd poisoning. The reduction in R(PAH) by the direct transport inhibitor probenecid served as positive control. The additional finding that R(cycloleucine) is also depressed by probenecid, as well as by excess PAH, indicates some overlap in substrate specificities of the two carrier systems. The systems are not identical, however, as can be deduced from the observation that L-leucine affected only transport of cycloleucine, not that of PAH.