Cellular uptake of cadmium and zinc

Transport of toxic heavy metals across cell membranes

Membrane transport of nonessential toxic heavy metals (type D heavy metals) not only controls their access to intracellular target sites but also helps determine their uptake, distribution, and excretion from the body. The critical role of membranes in the toxicology of class D metals has attracted the attention of many investigators, and extensive information has been collected on the mechanism(s) of metal transfer across membranes. Characteristics of metal transport in different cells, or even on opposite sides of the same cell, or under different physiological conditions, are not identical, and no unitary hypothesis has been formulated to explain this process in all cells. However, it seems possible that the mechanisms proposed for different cells represent variations on a few common themes.

Cytotoxicity and accumulation of Hg, Ag, Cd, Cu, Pb and Zn in human peripheral T and B lymphocytes and monocytes in vitro

1. The cytotoxic effects of various heavy metals were assayed by trypan blue exclusion in vitro in human peripheral immune cells separated to high purity. T and B lymphocytes and monocytes were equally sensitive to metals. The individual metals could be ranked in the following decreasing order of cytotoxic potency, Hg approximately Ag > Cd approximately Cu > Pb approximately Zn, based on exposure time and concentration needed to give a particular percentage of dead cells. 2. The cytotoxic effects became irreversible after about 13 hr of metal exposure. 3. Examination by scanning electron microscopy showed that the heavy metals caused serious destruction of the cell membranes. 4. Solubility and uptake of metals into the cells were studied and discussed in relation to the cytotoxic effects. It was concluded that metal binding to cell surfaces or precipitate formation could inhibit ordinary uptake, thereby affecting cytotoxicity. For Pb in monocytes this appeared to lead to uptake of non-toxic complexes, probably by phagocytosis.

Cadmium-resistant Chinese hamster V79 cells with decreased accumulation of cadmium

Cells resistant to 3 x 10(-5) M CdCl2 (Cdr cells) were isolated from cultures of Chinese hamster V79 cells by a procedure that involved stepwise increase in the concentration of Cd2+ and subsequent mass selection. Cdr cells grew as fast as wild-type cells (Cds) in medium without cadmium. Cdr cells were not cross-resistant to other divalent metal ions, such as Hg2+, Ni2+, Pb2+, and Zn2+. Both Cds and Cdr cells induced similar levels of metallothioneins (MT) in response to zinc. Depletion of glutathione (GSH) did not significantly influence the sensitivity of Cdr cells to Cd2+ but markedly enhanced the sensitivity to Cd2+ of Cds cells. Furthermore, the rate of synthesis of GSH after depletion did not differ greatly between sensitive and resistant cells. The rate of uptake of 109Cd2+ by Cdr cells was only 10-15% that by Cds cells. The difference in rates of uptake between Cds and Cdr cells was observed irrespective of the presence or absence of serum in the culture medium. These results indicate that, in this system, resistance to Cd2+ is attributable neither to increased inducibility of MT nor to increases in intracellular levels of GSH, and that only a decrease in the rate of uptake of Cd2+ contributes to the acquisition of resistance to Cd2+. Uptake of Cd2+ by cells was dependent on temperature and the rate of uptake of Cd2+ by Cdr cells was lower at all temperatures examined than the rate of uptake by Cds cells. Cycloheximide did not suppress the uptake of Cd2+, suggesting that uptake does not require synthesis of cell proteins de novo. Preincubation of cells with N-ethylmaleimide suppressed the uptake of Cd2+ to some extent, a result that suggests the involvement of surface SH groups in the uptake of Cd2+ by these cells.

Acute cadmium uptake by rabbit kidneys: mechanism and effects

Trapping of an arterial bolus in the kidney by 40 sec aortic occlusion permitted demonstration of diffusion-limited and Zn-sensitive renal uptake of nonfiltered but diffusible Cd from plasma; mercaptoethanol (ME) or 2,3-dimercaptopropanol (BAL) had been added to the bolus to prevent sequestration of the metals by plasma protein. Cadmium taken up by the kidney under these conditions, whether from blood or glomerular filtrate, unlike Zn, did not return to blood over a period of 2-3 min. Nor was Cd thus accumulated removed by EDTA; it had presumably been transferred into cells. The diffusion dependence of this internalization shows it to be a relatively slow process, as it is in jejunum. In contrast, uptake of Cd tightly bound in a lipid-soluble complex with diethyldithiocarbamate (DDTC) is rapid and flow dependent. Slow uptake in the presence of ME and its inhibition by Zn are therefore not likely to involve movement of undissociated ME complex across the cell membranes. Instead, it is suggested that (1) Cd-binding sites on the membrane possess a relatively high affinity for Cd and can compete for it with ME, and (2) it is the resulting Cd-membrane interaction which, as in jejunum, is depressed by Zn. During transient occlusion kidneys could be loaded with up to 25 micrograms Cd/g cortex without evidence of immediate malfunction. Inhibition of amino acid transport, as previously described, is seen only after an initiation period of 1-2 days following CdME injection, although cortical Cd levels at that time have decreased. The finding of a slowly developing inhibition of amino acid carrier systems suggests an indirect action of Cd; by implication, unique threshold concentrations should not be defined for Cd in renal cortex following subchronic exposures without reference to the duration of exposure.

Factors influencing the uptake of cadmium into cells in vitro

The uptake of cadmium was studied in Chinese Hamster Ovary (CHO) cells. Cadmium accumulates rapidly against a concentration gradient and uptake is dependent on incubation temperature. The presence of serum or albumin results in a reduction in Cd uptake. Several substances known to influence certain metabolic steps of cell metabolism were used to influence Cd accumulation. Of these compounds, those that blocked the SH groups of the plasma membrane exerted the strongest influence. The effect of inhibitors of endocytosis was less pronounced.

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.

Cytotoxicity of metals, metal-metal and metal-chelator combinations assayed in vitro

A simple, rapid assay, based on the lysosomal incorporation of neutral red by cells, conveniently carried out in 96-well microtiter plates, was used to evaluate the cytotoxic effect of cationic and anionic metal salts on BALB/c mouse 3T3 fibroblasts. Ranking of the metals according to their decreasing potency was based on spectrophotometrically determined absorbance of the neutral red, extracted from surviving viable cells. The rank order was Cd greater than Hg greater than Ag greater than Zn greater than Mn greater than Cu greater than Co greater than Ni greater than Cr(III) for the cationic metals and Cr2O7 greater than CrO4 greater than AsO2 greater than AsO4 greater than SeO3 greater than SeO4 greater than MnO4 for the anionic metals tested. Cationic metals incubated with cultures in medium containing 1% fetal bovine serum (FBS) were 3-4 times more toxic than in medium with 10% FBS. Cadmium served as a representative metal for the use of this assay not only for concentration, but also for time dependent exposures. Thus a 10% cytotoxic effect after 1 h of incubation with 60 microM cadmium was increased to 90% after 6 h. Examination of the effect of metal-metal interaction on cytotoxicity showed a marked reduction of cadmium toxicity by zinc and to a lesser degree, by nickel. The neutral red assay was also effectively used to investigate the effect of the chelators ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA) and 2,3-dimercaptosuccinic acid (DMSA) on cadmium-induced injury. Cytotoxicity by cadmium was completely inhibited by EDTA, and partially by NTA, but DMSA was ineffective. Reduction of copper toxicity by chelation was less efficient than for cadmium. Use of a chelator as therapy against metal poisoning was only partially effective and limited to administration within 2 h after incubation of cells with cadmium. It is believed that the neutral red assay can be a valuable tool for the screening of cytotoxic and potentially therapeutic agents under controlled in vitro conditions.

Transformation of prostatic epithelial cells and fibroblasts with cadmium chloride in vitro

Primary cultures of fibroblasts and epithelial cells were established from rat ventral prostate (RVP), canine (CP), baboon (BP), and human (HP) prostates, and were used in an assay system to evaluate cadmium chloride (CdCl2) cytotoxicity in vitro. Fibroblasts were always more susceptible to CdCl2 cytotoxicity than the epithelial cells of the same species. There was a distinct species variability to CdCl2 cytotoxicity, with RVP cells being greater than 200 times more susceptible than HP. Primary cultures treated with CdCl2 were subcultivated to establish cell lines. Only RVP fibroblast and epithelial cells resulted in permanent cell lines. Two fibroblast and two epithelial cell lines were derived from CdCl2-treated RVP cell cultures. The epithelial cell lines possessed tonofilaments, desmosomes and keratin. All four cell lines were resistant to CdCl2, had different karyotypes and an excess of chromosome 13. These results demonstrate the transforming potential of cadmium on prostate cells. The role of metallothionein and the significance of extra chromosomes 13 are discussed as possible factors of cadmium resistance.

Interactions between metals in rat jejunum: implications on the nature of cadmium uptake

Cadmium uptake from the lumen of the rat jejunum is characterized by apparent saturability and by sensitivity to inhibition by Zn and Ca. A variety of other heavy metals also inhibit intestinal Cd transport. The hypothesis was tested that Cd competes with other metals for their transport systems. The observed discrepancy between the apparent Ki and Km values for interaction between Cd and Zn argues against competition. Further, inhibition of Cd uptake by Zn, Ca and La is independent of Cd concentration over the range in which the system approaches partial apparent saturation (5-100 microM). Relatively high concentrations of other polyvalent cations, including Cd itself and Pb, Ni, Cr3+, Sr, Mg and polylysine, also depress 109Cd uptake. The possibility is suggested that these cations non-specifically neutralize membrane charges, and thus interfere with membrane binding and subsequent internalization of 109Cd. This explanation obviates the need for the unlikely assumption that specific membrane carriers exist to facilitate uptake of a toxic and non-essential metal like Cd.

Reversibility effects on renal and hepatic gluconeogenic enzymes in rats from chronic exposure of cadmium

The reversibility of cadmium chloride (CdCl2) effects on renal and hepatic gluconeogenic enzymes in rats was studied by removal of CdCl2 from the diet. Adult, male Sprague-Dawley rats were continuously treated with 0, 25, 50, and 75 ppm CdCl2 mixed in powdered diet for 180 d. After this period of chronic exposure, all treated rats were kept on CdCl2-free diet for another period of 180 d. The CdCl2 induced serum glucose and activities of glucose 6-phosphatase, fructose 1, 6-bisphosphatase, and phosphoenolpyruvate carboxykinase reversed to the levels of control values 120 d after the rats were fed with CdCl2-free diet. There were no significant differences in measured enzyme activities either 120 or 180 d after the cessation of CdCl2 treatment. The data indicate that CdCl2 effects on gluconeogenesis will gradually recover with time when the metal is removed from the diet.

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

Studies measuring the uptake of cadmium or mercury in isolated hepatocytes demonstrated that hepatocytes accumulated more cadmium than mercury in serum-containing medium, serum-free medium, or balanced salt solution. The preferential hepatocellular accumulation of cadmium, independent of medium composition, suggested that the uptake mechanism for cadmium and mercury might be different in hepatocytes. Pretreatment of hepatocytes with 50 microM N-ethylmaleimide decreased cadmium uptake by 23% while having no effect on the uptake of mercury was inhibited in a concentration-dependent manner. The uptake of cadmium was maximally inhibited (80%) with 75 microM parachloromercuribenzenesulfonate or 20 microM mercury respectively. Cadmium had no effect on mercury. Hepatocytes treated with parachloromercuribenzenesulfonate or mercury accumulated cadmium at a rate closely resembling the rate of mercury uptake in untreated hepatocytes. These results suggested that an SH-containing carrier may be operative in the uptake of cadmium by hepatocytes. Mercury can interact with this carrier to inhibit cadmium uptake; however, this carrier does not appear to facilitate mercury uptake.

Bacterial transformations of and resistances to heavy metals

Bacteria carry out chemical transformations of heavy metals. These transformations (including oxidation, reduction, methylation, and demethylation) are sometimes byproducts of normal metabolism and confer no known advantage upon the organism responsible. Sometimes, however, the transformations constitute a mechanism of resistance. Many species of bacteria have genes that control resistances to specific toxic heavy metals. These resistances often are determined by extrachromosomal DNA molecules (plasmids). The same mechanisms of resistance occur in bacteria from soil, water, industrial waste, and clinical sources. The mechanism of mercury and organomercurial resistance is the enzymatic detoxification of the mercurials into volatile species (methane, ethane, metallic HgO) which are rapidly lost from the environment. Cadmium and arsenate resistances are due to reduced net accumulation of these toxic materials. Efficient efflux pumps cause the rapid excretion of Cd2+ and AsO4(3-). The mechanisms of arsenite and of antimony resistance, usually found associated with arsenate resistance, are not known. Silver resistance is due to lowered affinity of the cells for Ag+, which can be complexed with extracellular halides, thiols, or organic compounds. Sensitivity is due to binding of Ag+ more effectively to cells than to Cl-.

Cadmium and thallous ion permeabilities through lipid bilayer membranes

Cadmium (Cd2+) and thallous ion (Tl+) permeabilities were measured in planar (Mueller-Rudin) lipid bilayer membranes made from diphytanoylphosphatidylcholine in decane. Permeabilities of the electroneutral Cl- complexes, measured with tracers (109Cd and 204Tl), were about 10(-8) cm X s-1 for CdCl2 and 10(-6) cm X s-1 for TlCl. Electrical conductance measurements showed that permeabilities to Cd2+ and Tl+ were approx. 10(-11) cm X s-1, similar to the Na+ permeability. The low permeabilities to both Cd2+ and CdCl2 are consistent with biological studies which suggest that Cd transport and toxicity are protein mediated and correlated with Cd2+, not CdCl2, concentration. However, the low bilayer permeability to Tl+ raises questions about recent reports that Tl+ is a lipid permeable cation in biological membranes and liposomes. An alternative explanation for the lipid permeable behavior of Tl+ is presented, based on the diffusion of TlCl and other complexes of Tl+ with inorganic and organic anions.

Prevention of cadmium-induced sterility by zinc in the male rat

The potential of zinc (Zn) to antagonize the adverse effects of cadmium (Cd) on the male reproductive processes was studied. A significant reduction in the weights of the testis and epididymis, the testicular sperm population and oligospermia to azoospermia in the epididymis was recorded in rats treated once s.c. with 2 mg/kg Cd and sacrificed 20 days later. This was accompanied by a significantly reduced serum T and a loss of fertility. Administration of a single s.c. injection of Zn (80 mg/kg) did not alter significantly any of the reproductive parameters studied. On the other hand, Zn given 1/2 to 2 h after Cd resulted in partial recovery. Organ weights and sperm population in these groups were maintained and the fertility rate was 67% and 50% at 5 and 20 days as compared to 67% and 0% at corresponding period in Cd-exposed males. The protective effect diminished when Zn was given more than 2 h after Cd exposure. The most effective regimen of Zn therapy was an administration of a total dose of 80 mg/kg Zn given in 3 injections (15 mg/kg before, 50 mg/kg at the time and 15 mg/kg 2 h after Cd). The results provide evidence that Zn, if given before or within 2 h of Cd exposure, is capable of at least partially reversing its deleterious effects on spermatogenesis, steroidogenesis and fertility of the male rat.

Cadmium and zinc flux in wild-type and cadmium-resistant CHO cells

Cellular flux of cadmium-109 and zinc-65 is characterized in cultured Chinese hamster ovary cells. The transport of cadmium is primarily unidirectional and, following uptake, cadmium is strongly retained. Zinc transport is bidirectional and intracellular zinc continuously leaches out into the medium. Nonradioactive cadmium or zinc enhances the efflux of(65)Zn from prelabeled cells. Transport of these metals into wild-type cells is not affected by azide, ouabain, cycloheximide, or actinomycin D. A cadmium-resistant mutant was isolated that exhibited altered sensitivities to certain inhibitors of macromolecular synthesis as well as quantitative differences in metal transport and accumulation. Although the mutant accumulates less cadmium than the wild-type cell, that which is retained is bound much more tightly. In addition, this lower rate of cadmium uptake is significantly decreased by either cycloheximide or actinomycin D. This suggests that thede novo synthesis of a protein or proteins is required for much of the net cadmium retention by the cadmium-resistant cells.