Induction and synthesis of metallothionein in isolated perfused rat liver

Metallothionein-like proteins and cell resistance to cis-dichlorodiammineplatinum(II) in L1210 cells

Our studies on the mechanism of resistance of the murine leukemia L1210-PDD line to cis-dichlorodiammineplatinum(II) (cis-DDP) have not shown why it is 10-fold more resistant to the drug than the L1210 line. For this reason we investigated metallothionein-like proteins ('MTs') in these cells. Soluble protein extracts from cultures treated for 24 h with cis-DDP, zinc sulphate or saline were anaerobically eluted from columns of chemically reduced Sephadex G-75, and the profiles of zinc, copper and platinum were determined along with those for incorporated radioactive cyst(e)ine and tyrosine. Both saline-treated cell lines contained similar levels of 'MTs', which were induced by exposure to a minimally toxic level of zinc (100 microM). Zinc induction of 'MTs' was nearly 4-fold greater in L1210 than in L1210-PDD cells. The levels of mRNA for metallothionein I (MTI) and II (MTII) in uninduced cells were measured by dot-blotting with a cDNA probe. The L1210-PDD cells contained 80% of the MTI and 41% of the MTII compared with L1210 cells, confirming the similar levels in uninduced cells. L1210-PDD cells were 2-fold more sensitive than L1210 cells to cadmium and equally sensitive to zinc. Thus, the resistance of L1210-PDD cells to cis-DDP was not associated with cross-resistance to group IIb metals, whereas their sensitivity to cadmium did reflect the relative inability of the cells to synthesize 'MTs'. The L1210 cells produced 'MTs' when treated with 0.5 and 5.0 microM cis-DDP, but the L1210-PDD cells did not when treated with 5.0-40 microM cis-DDP. Small amounts of platinum (less than 21% of the total eluted) were bound to 'MTs' in both cell lines, but platinum provided a minor portion of the 'MT'-bound metals, with zinc and copper contributing the bulk. The basis for the resistance of L1210-PDD cell to cis-DDP is neither an increased level of 'MTs' in the resistant cells nor an enhanced ability to increase the synthesis of 'MTs' after drug exposure.

Displacement of zinc and copper from copper-induced metallothionein by cadmium and by mercury: in vivo and ex vivo studies

The in vitro affinity of metals for metallothionein (MT) is Zn less than Cd less than Cu less than Hg. In a previous study Cd(II) and Hg(II) displaced Zn(II) from rat hepatic Zn7-MT in vivo and ex vivo (Day et al., 1984, Chem. Biol. Interact. 50, 159-174). The ability of Cd(II) or Hg(II) to displace Zn(II) and/or Cu(II) from metallothionein in copper-preinduced rat liver (Zn, Cu-MT) was assessed. Cd(II) and Hg(II) can displace zinc from (Zn, Cu)-MT both in vivo and ex vivo. The in vitro displacement of copper from MT by Hg(II) was not confirmed in vivo and ex vivo. Cd(II) treatment did not alter copper levels in (Zn, Cu)-MT, as expected. Hg(II) treatment, however, did not decrease copper levels in MT, but rather increased them. The sum of the copper increase and mercury incorporation into MT matched the zinc decrease under in vivo conditions and actually exceeded the zinc decrease under ex vivo conditions. Short-term exposure of rat liver to exogenous metals can result in incorporation of these metals into MT by displacement of zinc from pre-existing MT. Displacement of copper from pre-existing MT by mercury, as predicted by in vitro experiments, was not confirmed under the conditions of our in vivo and ex vivo experiments. This result is explainable based on the differing affinities and/or preferences of the two metal clusters in MT.

Mathematical model for cadmium kinetics in the isolated perfused rat liver system

The isolated perfused rat liver (IPRL) preparation has previously been used to investigate cadmium kinetics. A mathematical model which has been developed to simulate cadmium kinetics in the IPRL is described. The model takes into consideration binding of cadmium to both intra- and extracellular proteins and the mechanisms of membrane transport. In addition, the competitive interaction of cadmium with endogenous zinc is incorporated into the model. Model simulations of the behavior of cadmium and zinc in the perfusion medium, liver, and bile are compared to results from IPRL experiments involving cadmium doses ranging from 0.29 to 15.6 mumol. A major contribution of this model is the identification, from a kinetic point of view, of two high-molecular-weight classes of intracellular cadmium-binding species which can be identified by two distinct peaks in Sephadex G-75 profiles of hepatic cytosol. This model can be utilized for the quantitation of kinetics based on specific mechanisms involved in cadmium hepatic kinetics.

In vivo and ex vivo displacement of zinc from metallothionein by cadmium and by mercury

Divalent cadmium and mercury ions are capable in vitro of displacement of zinc from metallothionein. This process has now been studied in vivo and ex vivo, using the isolated perfused rat liver system, in order to determine if this process can occur in the intact cell. Rats with normal and elevated (via preinduction with zinc) levels of hepatic zinc thionein were studied. Cd(II) completely displaces zinc from normal levels of metallothionein and on a one-to-one basis from elevated levels of metallothionein, both in vivo and ex vivo. Hg(II) displaces zinc from metallothionein (normal or elevated) rather poorly, as compared with Cd(II), in vivo, probably due to the kidneys preference for absorbing this metal. Ex vivo Hg(II) displaces zinc from metallothionein (normal or elevated) on a one-to-one basis, with considerably more mercury being incorporated into the protein than in vivo. The results of double-label ex vivo experiments using metal and [35S]cysteine (+/- cycloheximide) were consistent with the above experiments, indicating that de novo thionein synthesis was not required for short term incorporation of cadmium and mercury into metallothionein. These data are supportive of the hypothesis that cadmium and mercury incorporation into rat hepatic metallothionein during the first few hours after exposure to these metals can occur primarily by displacement of zinc from preexisting zinc thionein by a process which does not require new protein synthesis.

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.

Effect of adrenalectomy on cadmium- and turpentine-induced hepatic synthesis of metallothionein and alpha 2-macrofetoprotein in the rat

Recent reports have strongly implicated glucocorticoids in the induction of hepatic metallothionein synthesis and hypozincemia which occurs in certain pathophysiologic conditions. Studies were performed in rats to determine the effect of adrenalectomy and glucocorticoid treatment on the hepatic accumulation of metallothionein subsequent to the administration of cadmium and turpentine, two diverse substances known to induce hypozincemia and hepatic synthesis of metallothionein as well as alpha 2-macrofetoprotein in intact rats. By 24 h, both substances induced significant hypozincemia, hepatic metallothionein accumulation, and a severe tissue inflammatory response in adrenalectomized rats. Adrenalectomy only prevented the increase in plasma alpha 2-macrofetoprotein concentration. Results indicate that hepatic synthesis of alpha 2-macrofetoprotein, but not metallothionein, is mediated by adrenal hormones. Thus, glucocorticoids do not play a "vital" role in hepatic metallothionein accumulation or hypozincemia induced by inflammatory stress, as previously postulated.

The synthesis of metallothionein and cellular adaptation to metal toxicity in primary rat kidney epithelial cell cultures

Single cells were isolated from rat (100-150 g) kidney cortex slices by mild trypsinization. They were plated on plastic culture dishes and were maintained in a selective Eagle's D-Valine Minimum Essential Media (MEM) to form an epithelial cell monolayer culture. The fibroblast growth was retarded by D-Valine. The cells in monolayer culture accumulated cadmium when they were incubated with a 109CdCl2 (10(-5) M) containing medium. The synthesis of metallothionein by these cells was demonstrated by incorporation of [35S]cysteine and 109CdCl2 in a heat stable protein (mol. wt 10,000) within 8 h of exposure and also by immunoprecipitation using a specific antibody to rat liver metallothionein. The cytotoxic effects of Zn2+, Cu2+, Cd2+ and Hg2+ were studied in culture after addition of various concentrations of metal salts (10(-5)-10(-2) M). Disrupted cellular colonies with severe cell damage were observed after addition of 10(-3) M Cd2+ or Hg2+ as CdCl2 or HgCl2 while similar toxicity was observed only after addition of 10(-2) M Zn2+ or Cu2+ as ZnSO4 or CuSO4 to confluent cell cultures. The cellular damage due to Cd2+ was protected when the cells were pretreated with Cd2+ (10(-5) M) for 24 h and these cells could tolerate much higher concentrations of cadmium (10(-2) M). These results indicate a direct protective role of intracellular metallothionein in the cellular toxicity of cadmium.

Multiple forms of inducible drug-metabolizing enzymes: a reasonable mechanism by which any organism can cope with adversity

All organisms possess a number of genetically regulated mechanisms in order to cope with rapid adverse changes in the environment. The two systems which appear to respond to a seemingly endless array of chemical specificities are the immune response and the induction of drug-metabolizing enzymes. Similarities and differences between the immunoglobulin and the cytochrome P-450-mediated monooxygenase systems are described. DNA insertion sequences, plasmid "transposons," maize "controlling elements," gene duplication, intervening sequences, and high-frequency intergenic recombination are all discussed as possible methods by which organisms can "adapt" quickly to a new selective pressure. If the regulation of P-450 induction resembles in any way the other methods by which pro- and eukaryotes cope genetically with numerous forms of environmental adversity, therefore, it is very likely that mammalian tissues contain hundreds, if not thousands, of inducible forms of P-450.