Inducible gold-binding proteins in rat kidneys

Mechanisms of gold resistance

The efficacy of gold treatment in rheumatoid arthritis is hampered by toxicity, most prevalent in early phases, and unresponsiveness which can only be assessed after more than 6 months on therapy. There is some evidence that more severe disease increases the likelihood of drug resistance. No data indicate an altered pharmacokinetics in resistant individuals, although increased dosage of parenteral gold has been claimed effective in uncontrolled studies. Exposure to metals induces cell adaption and resistance in both prokaryotic and eukaryotic cell lines. In particular this has been shown for gold chloride, sodium aurothiomalate and auranofin. Auranofin induces increased synthesis of metallothionein, a low molecular weight cystein-rich peptide with metal-binding and -homeostatic properties. Thus there is experimental evidence suggesting cellular adaptation as a potential mechanism for gold resistance. However, with the possible exception of auranofin, the nature of the processes remains unknown.

Molecular aspects, physiological function, and clinical significance of metallothioneins

Metallothioneins (MTs) are well-characterized low molecular weight, heat-stable cytosolic proteins with exceptional high content of cysteinyl sulfur and are known to bind heavy metals like cadmium (Cd), zinc (Zn), and copper (Cu). Since these proteins are induced on exposure to heavy metals, it is now accepted that they have a detoxifying role during heavy metal toxicity. It has also been suggested that the primary function of Mt is in the homeostasis of the essential metals Zn and Cu. Recently, a role MT in selenium metabolism in primates has been established. Further, MT has gained considerable importance in the clinical disorders related to trace metal metabolism.

The effect of sodium aurothiomalate (myochrysin) on the distribution of calcium, magnesium, copper, zinc and iron in the rat

Sodium aurothiomalate was given to male Wistar rats (initial body weights: 150 g) by subcutaneous (s.c.) injection at doses of up to 7.5 mg/kg (corresponding to 4.27 mg gold/kg), twice a week, for 4-5 weeks. The concentrations of Ca, Mg, Fe, Cu and Zn were measured in serum, urine, faeces and in the liver, kidney, spleen, heart, lung, testis, bone and muscle. Kidney cytosol was separated by gel chromatography and the fractions analysed for protein, copper, zinc, iron and gold concentrations. The concentration of copper was increased 5-fold in kidney while smaller increases of zinc in kidney, copper in muscle, iron in muscle and testis and calcium in spleen were found. There was a significant reduction in the concentration of copper in serum. Kidney cytosol from gold-treated but not from control animals contained a low molecular weight protein which was associated with copper, zinc and gold. The rats developed proteinuria and microscopic changes to renal tubular cell structure were also observed. It is suggested that the gold-induced accumulation of copper may follow from an increased rate of synthesis of metallothionein and could be responsible for the renal dysfunction which develops in a proportion of rheumatoid arthritis patients who are treated with gold.

Subchronic treatment of rats with aurothioglucose; effects on plasma, hepatic, renal and urinary zinc, copper and metallothionein

Administration of sodium aurothioglucose (10 mg/kg per day) to female rats for up to 8 weeks resulted in no apparent effects on the kidney. Gold accumulated in kidney, liver, spleen, pancreas, skin and blood. Although plasma and hepatic gold levels increased with time, no remarkable change in either copper, zinc or metallothionein (MT) levels was observed. Gel filtration chromatography of plasma showed binding of gold to albumin, whereas copper was associated with albumin, ceruloplasmin and a protein eluting in the void volume of the Sephadex G-150 column. Almost all of the hepatic gold was bound to proteins other than MT. In the kidney, not only gold but also copper and MT increased rapidly, reached a maximum between 2 and 4 weeks and exhibited insignificant change thereafter. Gold-treated animals showed an increase in binding of copper to the very high molecular weight plasma protein, which may be involved in transport of copper to the kidneys. Urinary gold and MT followed a pattern similar to that in the kidney. Renal zinc also increased but returned to normal by week 8. In renal cytosol 57% and 54% of the gold and copper, respectively, were associated with MT. It appears that the elevated levels of copper and zinc, rather than gold, are responsible for the induction of MT synthesis. This then provides a mechanism by which gold and the inducing metals are retained by the kidney.

Effects of the chrysotherapeutic agents auranofin and gold sodium thiomalate on hepatic and renal drug metabolism and heme metabolism

These studies were designed to investigate the effects of the chrysotherapeutic agents auranofin and myochrysine (GST) on hepatic and renal drug-metabolizing enzymes and heme metabolism. Male Sprague-Dawley rats were either administered a single dose of auranofin (17, 34, or 68 mg/kg, p.o.) or administered daily doses of auranofin (0.2, 0.6, 2, 9, or 40 mg/kg/day, p.o.) or GST (1.2 or 5.8 mg/kg/day, i.p.) for 3 or 14 days. Rats were killed 24 h after the final treatment, and subcellular fractions of liver and kidney were prepared. Cytochrome P-450 (P-450) content and ethoxycoumarin-O-deethylase (ECOD), benzphetamine-N-demethylase (BPND), delta-aminolevulinic acid (ALA) synthetase, and heme oxygenase activities were determined. Twenty-four hours following single doses of auranofin, no effects on hepatic P-450, ECOD, or BPND were observed. Treatment with the positive control compounds, CoCl2 (60 mg/kg) and Co-protophorphyrin IX (33 mg/kg), produced decreases in all three variables at 24 hr. Auranofin, at 2 mg/kg, and GST treatment, at both doses, reduced hepatic P-450 and ECOD activity at 3 days. This effect was reversed with continued treatment for 14 days. BPND activity was unaffected at 3 days but was decreased at 14 days. Heme oxygenase activity was enhanced at 3 days and had returned to control activity at 14 days, while ALA synthetase was unaffected. With the exception of heme oxygenase, which was increased, renal variables were unaltered at 3 days. At 14 days, renal P-450 content was decreased in the high-dose auranofin group, heme oxygenase activity was increased in all groups, and ALA synthetase activity was elevated in high-dose auranofin animals. These data indicate that, at doses twenty times the human dose, auranofin and GST administration produced reversible decreases in hepatic and renal P-450 which may be the result of altered heme metabolism.

Metallothionein in cultured human epithelial cells and synovial rheumatoid fibroblasts after in vitro treatment with auranofin

Radioimmunoassay (RIA) and reversed-phase high-pressure liquid chromatography (HPLC) were used to investigate gold-binding proteins of possible metallothionein (MT) nature occurring upon auranofin exposure of cultured human cells. An epithelial cell line (HE) and two sub-strains were examined. The HEAF sub-strain had been made resistant to 2 mumole auranofin/l culture medium. The resistance was associated with the appearance of gold-binding substances with gel filtration characteristics like MT. The HE100 sub-strain had been made resistant to 100 mumole CdCl2/l and contained high amounts of cytosolic Cd-induced MT. In addition, cultured synovial fibroblasts, derived from normal (SN) and rheumatoid (SRA) synovial tissues, were investigated. Evidence was obtained by RIA that the low molecular weight (mol.wt. 6000-7000) gold-binding proteins occurring in the HEAF cells and SRA cells following auranofin exposure, were of MT nature. The relative amounts of MT in the epithelial cell lines were: HE:HEAF:HE100 = 1:18:100. The relative amounts in the synovial fibroblasts were: SN:SRA:SRA treated with auranofin = 1:3:10. The HPLC methods used were found suitable for isolation of Cd-MT in the HE100 cells, but not for the Au-MT in the HEAF cells. By HPLC, the Cd-MT in the HE100 cells was resolved into 3 MT-1 and 1 MT-2 iso-proteins exhibiting the amino acid composition typical of MT. Judged by HPLC, the MT in these cells constituted 0.4% of the cytosolic proteins.

Acquired resistance to auranofin in cultured human cells

A substrain (HEAF) of cultured human epithelial cells, grown as monolayers, was selected for resistance to auranofin (AF), a gold-containing anti-arthritic drug, by growing the parental HE cells with stepwise increased concentrations of AF in the medium. HEAF cells acquired resistance to 2 mumol AF/l, twice the concentration tolerated by the sensitive HE cells. Resistance to AF was also demonstrated in another substrain (HE100) originally selected for by its cadmium resistance, and characterized by a high cytosolic metallothionein (MT) content. Following continuous exposure to 2 mumol AF/l for 4 days, 58% of the HEAF cells, 67% of the HE100 cells, and 16% of the HE cells remained adherent to the flasks, compared with non-treated controls. Following 24 h AF exposure to living cells, HEAF cells had one-half and HE100 cells twice the cellular and cytosolic gold concentration per mg protein, as compared with HE cells. Gel filtration of cell cytosols revealed gold-binding proteins with a mol. wt. of about 10 000 apparently occurring on AF exposure in HEAF and HE cells. They bound 10-15% of cytosolic gold. MT in HE100 cells bound AF-gold to about the same extent. We suggest that the ability of cells to maintain the gold concentration at a low level (HEAF) and trapping of gold by MT (HE100) or low molecular weight proteins occurring on AF treatment (HEAF) may be mechanisms contributing to the observed cellular resistance to AF.

Acquired resistance against gold(III)-chloride in cultured human cells

Acquired resistance against gold(III)-chloride was developed in cultures of human epithelial cells derived from normal skin, by growing them with stepwise increased concentrations of the compound in the medium. Resistance to 350 mumol gold-chloride/l, about twice the commonly tolerated concentration and an otherwise lethal concentration to the cells, was attained after an adaptation period of 2-3 months. These cells had preserved their normal appearance except from small inclusion bodies in the cytoplasm. The effect of 350 mumol gold-chloride/l culture medium on cell growth of the sensitive (HE) and resistant (HEAu350) cell strain, was tested over a period of 9 days. On day 9, 2% of the HE cells and 80% of the HEAu350 cells remained adherent to the flasks compared with their nontreated control cells. The doubling time of the HE cells grown without gold was 24 h, that of the HEAu350 cells grown continuously on 350 mumol/l was 36 h. The HE cells died on prolonged exposure to this concentration. Measurement of the cellular content of Au, Cu and Zn by atomic absorption spectrophotometry revealed that the HEAu350 cells contained 1.8 times as much cellular and cytosolic Au per mg cell protein as the HE cells. Sixty per cent of the cytosolic gold in the HEAu350 cells was bound to high molecular weight proteins; no metallothionein was detected. Both strains contained minor amounts of zinc and copper compared with gold. The HEAu350 cells contained twice as much cellular Cu/mg cell protein as the HE cells, whereas the zinc content remained unchanged. The mechanism of acquired resistance against gold-chloride in these cells remains unknown.(ABSTRACT TRUNCATED AT 250 WORDS)

Variation of Se, Zn, Co, Fe and Rb distribution in rats upon sequence of injection with SeO2 and glutathione

The contents of Se, Zn, Co, Fe and Rb in several organs of Wistar rats were determined by instrumental neutron activation analysis (INAA) after injections of SeO2 and glutathione (GSH). Se was incorporated in all the examined organs, and the efficiency of incorporation does not depend upon the sequence of injection with SeO2 and GSH. The sequence of these injections affects the contents of the other elements in all the examined organs.

Triethylphosphine gold: cellular uptake and disposition after single and repeated oral doses in rats

The tissue and subcellular pharmacokinetics of gold following single and repeated oral doses of triethylphosphine gold (auranofin) has been studied in rats. After a single dose, the tissue and subcellular gold levels were 5-10 times lower than those reached with injectable gold compounds. In the liver tissues, gold concentrations peaked within 24 h followed by a biphasic clearance, with an initial rapid phase (t1/2 32 h) and a slow terminal phase (t 1/2 11 days). Renal gold concentrations continued to increase for 3 to 5 days and then decreased exponentially with a first order t 1/2 of about 7 days. Intracellularly, between 60-80% of hepatic and 50-70% of renal gold was present in the cytosol. In rats given repeated doses of auranofin, the hepatic and renal gold concentrations were 3-5 times higher than those measured after a single dose. The proportion of cellular gold present in the cytosol was markedly lower, with 43% in the liver and 30% in kidney tissues. In both the liver and kidney, gold concentrations were dose-dependent, whereas in the gastrointestinal tissues the increases as a function of dose were minimal.

Role of metallothionein in cellular uptake and disposition of gold sodium thiomalate

Rat liver and kidney tissue uptake of gold and its localization in the cytosol was studied following various doses of gold sodium thiomalate (GST). The timecourse of gold incorporation into intracellular gold-binding ligands following repeated injections of GST was also investigated (11 injections, one dose/week). Results show that between 30 and 60% of the hepatic and renal gold was localized in the cytosol over a wide range of GST doses. This was also true following repeated doses. In the kidney, the binding of gold to high molecular weight (HMW) proteins was saturated after the third GST dose, while incorporation into the metallothioneins (MT) continued to increase, accounting for as much as 50% of cytosolic gold. On the other hand the binding to hepatic MT was about 10x lower, and the proportion of cytosolic gold incorporated into the MT, decreased from 30% (after first 3 GST injections) to about 15% (following the last 3 injections). The results show that the stimulation of MT biosynthesis in different tissues as a response to the injected GST is not the same and varies within each organ with the dose and/or the duration of repeated exposure. In the liver, the ability of gold to induce MT synthesis was limited and the importance of MT in the cellular uptake and disposition of gold may largely be confined to the kidneys. It is suggested that besides playing a possible role in the detoxification of cellular gold, particularly in the kidney, MT may also contribute towards the retention and localization of gold in the tissues.

Cellular adaptation in metal toxicology and metallothionein

The cellular adaptation to toxicity of metals is one of the important factors in the evaluation of health effects of increased exposure to metals. Two major types of cellular effects can be distinguished during divalent metal exposure. Some of the experimental evidences on the role of these processes in the cellular toxicity of metals are reviewed in this article. Both these cellular effects are somewhat specific to certain metals and involve two distinct types of protein binding. One of these processes can be considered as a nuclear process, involving binding of metals to nuclear proteins and also the formation of morphologically distinct inclusion bodies. A number of metals such as lead, bismuth, mercury, copper and aluminium are accumulated intranuclearly and bind with non-histone protein in the nuclei. In addition, morphologically distinct intranuclear inclusion bodies are formed in the kidneys of experimental animals and in humans on continuous exposure to lead or bismuth salts. Another cellular effect of divalent metals is a cytoplasmic process involving a specific metal binding protein, metallothionein. This is a unique metalloprotein containing 2 types of metal clusters and its synthesis is induced by both essential (Zn2+ and Cu2+) and non-essential (Cd2+ and Hg2+) metals. A hypothetical model for metal induced synthesis of metallothionein is postulated and is partly based on the recent immunohistochemical localization of metallothionein in the nucleus and cytoplasm of both hepatic and renal cells.

Time-dependent uptake and metallothionein-binding of gold, copper and zinc in the rat kidney

In rats, in which the whole body burden of Au decreases rapidly, but biphasically, maximum kidney concns are not attained until 10-15 days after a single intraperitoneal dose of either Au(I) or Au(III). The concn of metallothionein-bound Au and of total kidney Cu, which also increases after the administration of the Au compounds, however, reach maxima at 5 days. Between at least 6 and 24 hr after Au treatment, the increases in the concn of Au and Cu in the metallothionein fraction are highly correlated. Measurements on the kidneys of animals at early times (15 min-6 hr) after dosing with Au(III) indicate that the Zn content of the (endogenous) metallothionein is depressed during the first hour, shows a transient increase at 2 hr and then falls to a minimum at 6 hr. Subsequent (6-24 hr) changes in metallothionein-bound Zn parallel those of metallothionein-bound Au and Cu. It seems, therefore, that Au and Cu are incorporated simultaneously into rat kidney metallothionein and this incorporation may be mediated by an initial displacement of Zn. In rats exposed to five doses of Au(III) the half-times of total and metallothionein-bound Au in the kidneys are appreciably longer than those in animals given a single dose. In both groups, the concns of Cu and Zn in the renal metallothionein do not decrease in parallel with that of Au, but change roughly in proportion to their whole kidney concns. In consequence, the metal composition of the metallothionein fraction, which remains above the endogenous concn in the normal kidney throughout an experimental period of 90-140 days, alters considerably with time.

Cultured human cells with a high content of metallothionein show resistance against gold-chloride

Resistance to gold(III)chloride was tested in cultures of human epithelial cells derived from normal skin. Two cell strains were used, a 'wild type' (HE) and a substrain, previously made resistant to 100 mumol/l of CdCl2 (HE100), with a high content of the cysteine-rich cytoplasmic protein, metallothionein. Cell growth was studied for a period of 4 days during exposition to gold-chloride (100-400 mumol/l of HAuCl4 X 4H2O). Gold-chloride produced a dose-dependent inhibition of cell proliferation of both cell strains. Compared with untreated control cells, however, the percentage survival after 4 days' treatment with 400 mumol/l was only 11% of HE cells, vis-à-vis 44% of the HE100 cells. After 24 hours' exposition to 200 mumol gold-chloride/l, the subcellular distribution of gold was determined by gel-filtration of cytosols, and subsequent analyses for Au by flame atomic absorption spectrophotometry. In the HE100 cells 30% of the cytosolic gold co-eluted with the metallothionein. In the HE cells, which do not contain detectable amounts of metallothionein, only traces of gold were found in the corresponding eluate fractions. The total amount of intracellular gold was 15% higher in HE100 than in HE cells. This study renders it probable that the binding of gold to pre-existing metallothionein in HE100 cells affords protection against otherwise lethal gold concentrations. The significance of the results is briefly discussed in relation to therapeutic use of gold substances.

The effect of zinc and copper pretreatment on the binding of gold (I) to hepatic and renal metallothioneins

In the first part of this study, single s.c. injections of gold (I) sodium thiomalate were given to male Wistar rats at various dose levels and sacrificed either at 48 hr or 4 days later. The results indicate that though metallothioneins play a significant role in the sequestration of renal gold, the ability of gold itself to induce synthesis of the metalloprotein was limited in the kidneys and practically insignificant in the liver. Pre-injection with zinc however, (4 mg/kg body wt), significantly (P less than 0.01) enhanced the uptake of gold into the metallothioneins both, in the liver as well as the kidneys. The pretreatment increased the proportion of cytosolic gold incorporated into the thioneins from 29% to 48% in the kidneys and from 22% to 47% in the liver. Concurrent increases in the uptake of zinc and copper into the thioneins was also observed. Pre-injection with copper induced a similar effect in the kidneys only. There was no significant difference in the overall uptake of gold into the cytosol itself between the control animals (injected with gold only) and those pretreated with either zinc or copper. This indicated that the increased uptake of gold into the thioneins in the pretreated animals were due to enhanced synthesis of metallothioneins and subsequent increased binding of gold to thioneins rather than increased levels of intracellular gold itself. It is suggested that increased protection against the deposition and possible toxicity of gold in the cell organelles of the liver and kidney tissues may be provided by an increased binding of gold to metallothioneins through pretreatment with zinc, and to a lesser extent copper.

The effects of cadmium and copper on the renal uptake and metallothionein binding of gold in the rat and hamster

Rats and hamsters, (pre)-treated with copper and cadmium, were used to investigate whether species-differences in renal metallothionein synthesis in response to gold were determined by changes in the kidney concentrations of other metals. The effects of both dietary copper limitation and excess on the renal metabolism of gold also were studied in the rat. In this species, all of the pre-treatments affected the renal concentrations of total and metallothionein-bound copper, but none of them altered either the kidney uptake or thionein-binding of gold. Incorporation of zinc into the metallothionein, which accompanied the binding of gold in this fraction of the kidney, however, was influenced slightly by the pretreatments, In hamsters, pretreatment with cadmium, which increased the concentrations of total and thionein-bound zinc in the kidneys, also did not affect the renal uptake of gold, although it increased significantly the binding of gold to the metallothionein fraction of the renal cytosol. This increased binding of gold also was accompanied by further increases in the zinc and copper contents of the metallothionein; the contents of total and thionein-bound cadmium, however, remained essentially unchanged. Concentrations of copper and zinc in the hamster kidney were not affected significantly by subcutaneous administration of copper alone (five daily doses, each of 3.2 mg Cu/kg body wt.), but were increased when gold was given during the copper-treatment. The concentrations of gold, copper and zinc in the renal metallothionein fraction also were increased under these conditions. From these results it seems that kidney metallothionein synthesis in response to gold may be related to the changes in either the concentration or distribution of zinc, rather than copper.

Comparative studies on the distribution of gold, copper and zinc in the livers and kidneys of rats and hamsters after treatment with sodium [195Au]-aurothiomalate

The distribution of gold, copper and zinc in the livers and kidneys of female rats and hamsters was determined after intraperitoneal injection of sodium [195Au]-aurothiomalate. After five doses of sodium [195Au]-aurothiomalate (1 mg Au(I) per kg body weight), the hepatic and renal concentrations of Au were greater in rats than in hamsters. In the former species, treatment with the Au(I)-compound led to an increase in the Cu-concentration of the kidney and to the synthesis of a (Cu, Au)-metallothionein. In either species binding of Au to the hepatic metallothionein was insignificant. The renal (Cu, Au)-metallothionein from the sodium [195Au]-aurothiomalate-treated rat appeared to be extremely heterogeneous and was resolved into at least four components on ion exchange chromatography.

Effects of gold sodium thiomalate on cytosolic copper and zinc in the rat kidney and liver tissues

1. Male Wistar rats were given a single subcutaneous injection of gold sodium thiomalate and killed 7 days later. The binding of Au, Zn and Cu to the kidney and liver cytosolic proteins of control and gold-treated rats and determined. 2. In the renal cytosol of Au(I)- exposed rats, the binding of Cu to the low-molecular-weight (LMW) proteins increased by 62%, and to the high-molecular-weight (HMW) proteins the binding decreased by 54%. The incorporation of Cu into the liver cytosolic proteins increased, in both, the HMW and the LMW proteins. The binding of Zn into the renal cytosolic proteins was increased by 39% (HMW proteins) and 100% (LMW proteins). Au(I) had little effect on the binding of Zn to the cytosolic proteins in the liver. 3. It is suggested that the therapeutic action of gold complexes may be mediated, to some extent, by its effects on the metabolism of Cu and/or Zn.

The tissue distribution of gold, copper and zinc in animals treated with Au (III); species differences in the binding of these metals in the kidney

Whole body retention of Au and the distribution of Au, Cu and Zn have been measured in female rats, rabbits, guinea-pigs, hamsters and mice after either a single injection or multiple doses of Au(III). At 24 h after a single intraperitoneal injection whole body retention of Au was about 65% of the dose in the rabbit and 50% of the dose in other species. After five doses, retention (as a percentage of the total dose) ranged from 36% in mice to 49% in rats. Concentrations of Au in the kidneys were lowest in mice and highest in rats but, in all species, were greater than in other organs. In rats and guinea-pigs, but not in hamsters, rabbits and mice, treatment with Au(III) increased the Cu content of the kidneys and of the soluble fraction isolated therefrom. The latter from the rat and guinea-pig kidney contained both Au and Cu in association with a low molecular weight metalloprotein (metallothionein), which also contained Zn and was separated by ion exchange chromatography into three subspecies. Binding of Au by these metalloproteins appeared to be related to the renal accumulation of Cu. Apart from the mouse, in which renal accumulation of Au was low, slight damage resulted in the kidneys of all species after treatment with Au. It appears, therefore, that nephrotoxicity cannot be explained simply by the renal concentration of Au and the form in which it is accumulated within the tubular cells.

The effect of penicillamine and 2,3-dimercaptosuccinic acid on urinary excretion and tissue distribution of gold

In order to study interactions in vivo between Au+ and SH-containing agents, groups of mice were given 35 mumol/kg of radiolabelled [195Au] thiomalate (Myocrisin) intramuscularly. The administration of Myocrisin is known to result in protein-bound gold and free thiomalate (mercaptosuccinate). High doses of dimercaptosuccinate (1 mmol/kg daily) increased the urinary excretion of radiolabelled gold [195Au] for several days. Treatment for 7 days with 1 mmol/kg of penicillamine or dimercaptosuccinate reduced the blood and kidney levels of gold to 30--50% of the controls. The oral administration of penicillamine in high doses, 1--10 mmol/kg, increased significantly the urinary excretion of [195Au] the first day after the Myocrisin injection, but on the subsequent days the radio-metal excretion was unaffected by the treatment. A lower dose level of penicillamine (0.3 mmol/kg daily) gave rise to only a small and insignificant increase in the urinary excretion of gold. The present results indicate that penicillamine at low clinical doses is an inefficient chelator of gold, while high doses (presumably comparable to about 1 200 mg daily in humans) may mobilize certain amounts of the metal deposits.