Synthesis and function of metallothioneins

Crambescin C1 Exerts a Cytoprotective Effect on HepG2 Cells through Metallothionein Induction

The Mediterranean marine sponge Crambe crambe is the source of two families of guanidine alkaloids known as crambescins and crambescidins. Some of the biological effects of crambescidins have been previously reported while crambescins have undergone little study. Taking this into account, we performed comparative transcriptome analysis to examine the effect of crambescin-C1 (CC1) on human tumor hepatocarcinoma cells HepG2 followed by validation experiments to confirm its predicted biological activities. We report herein that, while crambescin-A1 has a minor effect on these cells, CC1 protects them against oxidative injury by means of metallothionein induction even at low concentrations. Additionally, at high doses, CC1 arrests the HepG2 cell cycle in G0/G1 and thus inhibits tumor cell proliferation. The findings presented here provide the first detailed approach regarding the different effects of crambescins on tumor cells and provide a basis for future studies on other possible cellular mechanisms related to these bioactivities.

Difference in hepatic metallothionein content in Antarctic red-blooded and haemoglobinless fish: undetectable metallothionein levels in haemoglobinless fish is accompanied by accumulation of untranslated metallothionein mRNA

Icefish (family Channichthyidae, suborder Nothothenioidei) are a group of Antarctic fish that have evolved unique phenotypes in order to adapt to the environment in which they live. Besides the lack of haemoglobin and the drastic reduction in the number of erythrocyte-like cells, another striking feature of the icefish is that their liver is devoid of metallothionein. These cysteine-rich heavy-metal-binding proteins are usually present in large amounts in a large variety of organisms, from bacteria to mammals. Despite the failure to detect appreciable levels of metallothionein in icefish liver, a cDNA encoding metallothionein was produced from total RNA by reverse transcriptase PCR. The icefish metallothionein showed high percentage identity with metallothionein from Trematomus bernachii, a red-blooded Antarctic fish in which a normal content of hepatic metallothionein was found. Steady-state mRNA levels were assessed in fish liver by high-stringency hybridization of the metallothionein probe with total RNA. The results showed that icefish livers retain large amounts of untranslated metallothionein mRNA. The stability of the icefish transcript might be correlated with the lack of specific motifs in the untranslated 3' ends of mRNA.

How lead can easily enter the food chain--a study of plant roots

Using the root test, the relationship between the amount of lead in plant tissues and the level of the root growth inhibition was examined for twelve food plant species. 345 mg to 8152 mg Pb.kg-1 dry wt. inhibited root growth only by 5 to 36% as compared to the control. At least 96.6% of lead was bound in cells of root tips. Most of the lead was accumulated in cell walls, vacuoles and sometimes in dictyosomal vesicles. Lead accumulating in these cell compartments is separated from cell cytoplasm and therefore is no longer toxic for root cells. The results show that amounts of lead much bigger than those observed in the environment can now easily enter the food chain via plants. High tolerance to lead in plant roots is quite unfavourable for other members in the food chain, including man.

Inhibition of copper-associated erythrocyte ghost membrane lipid peroxidation by hepatic cytosolic low molecular weight proteins

Male weanling Fischer rats were injected ip once daily with either 12.5 mg/kg body weight cupric chloride or 2 ml/kg body weight saline for up to 70 days. As the hepatic cytosolic copper increased in copper-treated rats, copper bound to proteins of different molecular weights; this was determined by gel filtration chromatography. Hepatic cytosolic copper from rats treated with cupric chloride for 14 days eluted in 3 peaks. These included a 150,000 + dalton peak, a 29,000 dalton peak and an 11,000-12,800 dalton peak. In addition to these peaks, hepatic cytosolic copper from rats treated with cupric chloride for greater than or equal to 28 days also eluted in a 4th, but shorter, 6,000-7,000 dalton peak. Hepatic cytosolic copper from saline-treated rats eluted only in a single 29,000 dalton peak. Experiments using an erythrocyte ghost membrane model of copper-associated lipid peroxidation demonstrated that incubation of membranes with protein-bound copper eluted in the 11,000-12,000 dalton peak was associated with less lipid peroxidation than incubation of membranes with cupric chloride or protein-bound copper eluted in the 150,000+ dalton peak. Experimental results suggest that the ability of copper to catalyze lipid peroxidation is significantly reduced by binding with hepatic cytosolic low molecular weight proteins but not by binding with hepatic cytosolic high molecular weight proteins.

Metallothionein induction in human peripheral blood lymphocytes by heavy metals

Human peripheral blood lymphocytes have the capacity to produce metallothioneins (MTs) as a protective response to cadmium exposure. To define the range of metal species inducing lymphocyte MTs, cellular proteins synthesized after exposure to each of 11 heavy metals were analyzed by gel electrophoresis. Toxic metals such as cadmium, mercury and silver were found to induce thioneins (apoproteins of MTs) at relatively low concentrations (maximum at approximately 10 microM), whereas less toxic metals such as zinc, copper and nickel were inductive at relatively high concentrations (maximum at approximately 200 microM). Tin, lead, iron, cobalt, and manganese did not induce thioneins. The heavy metal specificity of MT induction in the lymphocyte resembles that in the liver, and the regulatory mechanism of MT production seems to be similar in both of these tissues. In the cells exposed to highly toxic metals such as cadmium and mercury, expression of cytotoxicity (represented by decline of cysteine uptake) was remarkable at the metal concentrations higher than those saturating thionein induction, supporting the protective role of MTs against heavy metals.

Cadmium-induced synthesis of metallothioneins in human T and B cell purified by a fluorescence activated cell sorter

Human peripheral blood lymphocytes were reacted with fluorescein-conjugated antibodies specific to T or B cell surface antigen and fractionated with a fluorescence activated cell sorter. The isolated T and B cells were examined for their capacity to synthesize metallothioneins (MTs). Analysis by gel electrophoresis indicated that both T and B cells were able to produce MTs in a Cd2+-inducible manner, suggesting that both cells types have a mechanism of protection against Cd2+ toxicity.

Some medical applications of the oxford scanning proton microprobe

Results from several medical investigations carried out using the Oxford scanning proton microprobe are presented, including maps of the hepatic copper-distribution in primary biliary cirrhosis and maps of the iron distribution in primary hemochromatosis. Preliminary studies of human nervous tissue reveal that morphologically recognizable structures can be differentiated using precision elemental mapping, and this may lead to a powerful new way of distinguishing subtle perturbations of structure and function. In further prelimianry studies, localized aluminum structures have been observed in Alzheimer's disease tissue at bulk levels of less than parts per million.

Hepatic copper distribution in primary biliary cirrhosis shown by the scanning proton microprobe

A number of conditions are associated with abnormalities of trace metal handling by the liver. We report the application of the Oxford scanning proton microprobe to the analysis of hepatic copper in one such condition, primary biliary cirrhosis. The scanning proton microprobe analyses conventional tissue sections (5-10 micron thickness) and produces simultaneous elemental distribution maps of biologically relevant elements with a spatial resolution of 1 micron and a detection limit better than 1 ppm. We have confirmed the localisation of excess copper to periportal areas and suggest that such accumulation is confined to a proportion of periportal hepatocytes. We have also shown a close spatial correlation between regions of copper accumulation and areas of high sulphur concentration. The copper to sulphur ratio in these areas is consistent with their identity as aggregates of copper loaded metallothionein, and the scanning proton microprobe was further able to show that the aggregates contain less than 30 ppm zinc.

Characterization and measurement of metallothionein messenger RNA of C57BL mouse liver

Liver poly(A)+RNA of Cd2+-treated C57BL mouse was characterized by cell-free translation, particularly intending to establish a procedure to measure the levels of messenger RNA coding for metallothioneins (MT-mRNA). Intact polysomes were obtained by Mg2+ precipitation from the liver cytoplasm of mice injected with 1 mg Cd2+/kg body wt. Poly(A)+RNA isolated from the polysomes was translated by a wheat germ cell-free system and the [35S]cysteine-labeled translation products were analyzed by sodium dodecyl sulfate (SDS)-15% polyacrylamide gel electrophoresis and fluorography. MTs were identified in the translation products directed by the RNA from the Cd2+-treated mice, but not in the translation products directed by the RNA from untreated mice. Relative incorporation of [35S]cysteine into MTs was determined by densitometrical quantification of the MT bands, and was found to be linear up to a RNA concentration of 150 micrograms/ml in the translation reaction mixture, showing that this system is suitable for the measurement of translatable MT-mRNA levels. Cd2+ stimulated the total levels of cell-free translation (1.4-fold at 20-60 micrograms/ml), not specifically to MT-mRNA. MT-mRNA sedimented at 9S in a sucrose gradient, and its size was comparable with rat and human MT-mRNAs.

Protein mapping of two metallothionein-rich cell strains and their parent lines, using high-resolution two-dimensional electrophoresis

A high-resolution two-dimensional gel electrophoresis (2-D) technique was used to characterize one human and one murine cadmium-resistant substrain and their parental wild-type lines. The substrains are cultured on 100 microM cadmium and contain high levels of the cysteine-rich protein metallothionein (MT). All four cell lines were labeled with [35S]methionine during growth. A remarkable consistency was found in the protein maps of the resistant strains compared to those obtained from their corresponding wild-type lines. Thus, in the maps from the human substrain only two spots were detected which were not found in the parent cells. In the murine substrain, two spots were more abundant and two diminished compared to the parent cells. No distinct spots corresponding to authentic MT were detected in any of the autoradiographs from the cadmium-resistant cells. The reason for this was found to be failure of the protein to focus in the first dimension. Purified [35S]cystine-labeled MT appeared as a diffuse labeling over the entire gel, and subsequently as wide horizontal bands in the second dimension. These bands were also clearly visible in the protein maps when MT-rich cells had been labeled with [35S]cysteine. This study shows that the standardized 2-D gel system used in many laboratories cannot be used to screen cell populations for MT.

The possible role of metallothioneins in copper tolerance of Silene cucubalus

Growth and copper-binding of a copper-tolerant and a copper-sensitive population of Silene cucubalus (L.) Wib. have been studied. The copper-tolerant plants showed a much lower uptake and a proportionally higher transport of copper from root to shoot. A copper-binding protein with an apparent Mr of 8500 resembling metallothionein has been isolated from the roots of copper-treated plants of the tolerant population. After 20 d, the protein was observed to be inducible upon copper supply in the copper-tolerant plants, but not yet in the sensitive ones. This could be an indication of a difference in metalregulated synthesis of the protein. Ion-exchange chromatography of the 8500 protein yielded a major copper-containing fraction eluting at high ionic strength. Other characteristics such as UV absorption and amino-acid composition resembled strongly those of metallothioneins. The involvement of metallothioneins in the detoxification of copper within Cu-tolerant plants is discussed in relation to other mechanisms.