Thionein gene expression in Cd++-variants of the CHO cell: correlation of thionein synthesis rates with translatable mRNA levels during induction, deinduction, and superinduction

Characterization of a stable eukaryotic cell line expressing the Rous sarcoma virus integrase

The Rous sarcoma virus integration protein (IN) is required for efficient integration of viral DNA into the host genome. IN was expressed in mouse C127 cells using a bovine papillomavirus vector. This system utilizes the mouse metallothionein promoter and the SV40 late polyadenylation signal for efficient expression of IN. A stable cell line derived from a single hygromycin-resistant colony was characterized. The expression of IN increased significantly upon Zn2+ induction of the metallothionein promoter, but did not respond to "superinduction" protocols. Full-length nonphosphorylated IN was the major product of expression. A minor product resulting from initiation of translation at an internal Met codon was also produced. The expressed IN did not exhibit the polypeptide heterogeneity at its COOH-terminus nor phosphorylation as is seen when IN is immunoprecipitated from virions. Using subcellular fractionation and indirect immunofluorescence, IN was primarily localized to nuclei and in some cells appeared to concentrate at discrete loci within the nuclei.

Differential regulation of metallothionein genes in rainbow trout fibroblasts, RTG-2

Exposure of a trout gonadal fibroblast (RTG-2) cell line to ZnCl2, CdCl2 and CuCl2 resulted in differential levels of accumulation of metallothionein (MT) mRNA. ZnCl2 being the most effective agent induced MT mRNA in 3 h, with 172-fold induction after 48 h and continued accumulation up to 144 h. Following CdCl2 treatment, mRNA could be detected after 24 h, reaching peak levels at 72 h. Furthermore, trout MT mRNA could be detected up to 8 days after withdrawal of extraneous ZnCl2. Using a novel technique of primer extension and DNA sequencing with total RNA as template, specificity of the trout MTa and MTb gene-specific primers was established. Primer extension studies revealed a higher response of MTa to ZnCl2 and CdCl2 compared to MTb. Insensitivity of MT mRNA induction to cycloheximide suggested that induction by the metals was independent of de novo protein synthesis. However, simultaneous exposure of cells to actinomycin D and metals completely inhibited MT mRNA synthesis implying control at the transcriptional level.

Enhanced cadmium cytotoxicity in A549 cells with reduced glutathione levels is due to neither enhanced cadmium accumulation nor reduced metallothionein synthesis

Glutathione (GSH) depletion sensitizes human lung carcinoma (A549-T27) cells to the cytotoxic effects of Cd++. The effects of GSH depletion on Cd++ accumulation and Cd++-induced metallothionein (MT) content were investigated to determine the possible role of these Cd++ responses in the sensitization process. Cellular GSH was depleted to 20% to 25% of control levels with buthionine sulfoximine (BSO), or diethyl maleate (DEM), respectively. Neither treatment significantly affected Cd++-induced accumulation of exogenous 35s-cysteine into intracellular MT in a dose-dependent fashion. The results indicate that neither enhanced Cd++ accumulation nor reduced MT synthesis plays a primary role in affecting enhanced Cd++ cytotoxicity in A549 cells with reduced GSH levels. Although BSO inhibition of GSH synthesis enhanced MT synthesis, it sensitized the cells to Cd++, which suggests an additive effect of GSH and MT in cadmium cytoprotection. This observation also raises the possibility that intracellular cysteine levels limit Cd++-induced MT accumulation rates.

Characterization and use of the Drosophila metallothionein promoter in cultured Drosophila melanogaster cells

The promoter from the metallothionein gene may be a useful conditional promoter for the construction of chimeric genes to be expressed in Drosophila cells in culture. To explore this possibility the responses of the endogenous metallothionein gene and an in vitro constructed chimeric gene containing the metallothionein promoter were examined. Copper and cadmium, when added to the growth medium of Drosophila Schneider's line 2 cells, can produce a 30-100 fold induction of metallothionein mRNA levels. The level of induction depends on the amount of copper or cadmium added to the medium and these mRNA levels remain high for at least four days. Copper is less toxic than cadmium and does not induce a typical heat-shock response in the cells. Finally, a chimeric gene containing the metallothionein promoter shows a similar induction when transformed into the cells.

Metallothionein gene expression in fish cell lines: its activation in embryonic cells by 5-azacytidine

We have investigated the regulation of metallothionein gene expression in two fish cell lines. Rainbow trout hepatoma (RTH) cells synthesized metallothionein in response to heavy metal exposure. The maximum level of metallothionein synthesis detected during zinc exposure was much greater than during cadmium exposure. The time-courses of metallothionein synthesis were different for the different metal inducers, suggesting that metallothionein may be differentially regulated by cadmium and zinc in these cells. The metal-induced synthesis of metallothionein was correlated with increased translational activity and accumulation of metallothionein-mRNA, suggesting that metallothionein may be regulated at the transcriptional and post-transcriptional levels in RTH cells. Chinook salmon embryo (CHSE) cells, unlike RTH cells, did not synthesize metallothionein or metallothionein-mRNA in response to heavy metal exposure. However, when these cells were treated with 5-azacytidine prior to heavy metal exposure, the synthesis of metallothionein was induced, suggesting that DNA methylation may play a role in metallothionein gene expression in fish.

Expression of cloned monkey metallothionein in Escherichia coli

Expression vectors were constructed in which a cDNA specifying the monkey kidney metallothionein-II (MT-II) was linked directly to the lambda PR promoter. Enhanced expression of MT-II in Escherichia coli was observed when two initiation signals were tandemly linked to the MT-II gene and the lambda cI+ host cells were induced by nalidixic acid.

Procedures for enhancing the utility of the metallothionein promoter for the regulated expression of downstream open reading frames

Procedures which enhance the inducibility of the mouse metallothionein I (mMT-I) transcriptional promoter in mouse C127 cells transformed by bovine papilloma virus have been investigated. These include: (i) induction with Zn2+ at low serum concentration, and (ii) use of a 'superinduction' protocol (presence of 1 microgram/ml of cycloheximide during induction with Zn2+, followed by 2 micrograms/ml of actinomycin D). Use of procedure (i) alone gave a 15- to 20-fold induction of expression of a downstream open reading frame (ORF), which is comparable to the maximum inducibility achieved with mMT-I in other systems. Use of procedures (i) and (ii) in combination allowed a 50-fold induction. Three different reporter ORFs (rabbit ferritin L subunit, human chorionic gonadotropin alpha subunit, and human lutropin beta subunit), in three different chromosomal contexts, responded to these procedures. The maximum rate of expression achieved was estimated at over 10(9) molecules per cell per day, which is 20% of the transformed cell's protein synthetic capacity. At these extremely high levels some of the induced products were cytotoxic.

Chemistry and biochemistry of metallothionein

A wealth of chemical, spectroscopic and structural data attest to the uniqueness of the metallothioneins as a group of novel bioinorganic structures. Their earmarking feature is the arrangement of "soft" metal ions in complexes with cysteine side chains to form discrete metal-thiolate clusters. In this review an account is given of the chemical characteristics of the 52 metallothioneins whose primary structures are now known completely or in part. Also included is an up-to-date summary of the spectroscopic properties and of the spatial structure models derived from X-ray diffraction crystallographic analysis and from two-dimensional nuclear magnetic resonance spectroscopy.

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.

Cd2+-induced synthesis of metallothionein in HeLa cells

HeLa cells synthesize metallothioneins in response to Cd2+. The kinetics of thionein (apoprotein of metallothionein) synthesis was studied by pulse-labelling the cells with [35S]cysteine and measuring relative amounts of the labelled thioneins separated by electrophoresis. Thionein synthesis rapidly increases in the first 6-8 h after exposure to 0.5 micrograms of Cd2+/ml, and begins to decrease in a few hours after reaching the maximum. However, the rate of synthesis never returns to the basal value at least in 30 h after Cd2+ exposure; instead, the second increase in thionein synthesis occurs at 16-18 h. A possible regulatory mechanism of thionein synthesis is discussed from these results, together with the data on intracellular accumulation and subcellular distribution of 109Cd2+. The initial increase of thionein synthesis is accompanied by an increase of mRNA coding for thioneins (MT-mRNA). The induction of MT-mRNA is sensitive to Actinomycin D, but not to cycloheximide, suggesting transcriptional regulation without any mediating protein synthesis. Two thionein isomers are coded for by mRNA molecules of almost the same size, which is similar to that of hepatic MT-mRNA of mouse and rat.

Coordinate amplification of metallothionein I and II genes in cadmium-resistant Chinese hamster cells: implications for mechanisms regulating metallothionein gene expression

We describe here the derivation, characterization, and use of clonal cadmium-resistant (Cdr) strains of the Chinese hamster cell line CHO which differ in their metallothionein (MT) induction capacity. By nondenaturing polyacrylamide gel electrophoresis, we showed that the stable Cdr phenotype is correlated with the augmented expression of both isometallothioneins (MTI and MTII). In cells resistant to concentrations of CdCl2 exceeding 20 microM, coordinate amplification of genes encoding both isometallothioneins was demonstrated by using cDNA MT-coding sequence probes and probes specific for 3'-noncoding regions of Chinese hamster MTI and MTII genes. Molecular and in situ hybridization analyses supported close linkage of Chinese hamster MTI and MTII genes, which we have mapped previously to Chinese hamster chromosome 3. This suggests the existence of a functionally related MT gene cluster in this species. Amplified Cdr variants expressing abundant MT and their corresponding Cds parental CHO cells should be useful for future studies directed toward elucidating the mechanisms that regulate expression of the isometallothioneins.

Coordinate amplification of metallothionein I and II genes in cadmium-resistant Chinese hamster cells: implications for mechanisms regulating metallothionein gene expression

We describe here the derivation, characterization, and use of clonal cadmium-resistant (Cdr) strains of the Chinese hamster cell line CHO which differ in their metallothionein (MT) induction capacity. By nondenaturing polyacrylamide gel electrophoresis, we showed that the stable Cdr phenotype is correlated with the augmented expression of both isometallothioneins (MTI and MTII). In cells resistant to concentrations of CdCl2 exceeding 20 microM, coordinate amplification of genes encoding both isometallothioneins was demonstrated by using cDNA MT-coding sequence probes and probes specific for 3'-noncoding regions of Chinese hamster MTI and MTII genes. Molecular and in situ hybridization analyses supported close linkage of Chinese hamster MTI and MTII genes, which we have mapped previously to Chinese hamster chromosome 3. This suggests the existence of a functionally related MT gene cluster in this species. Amplified Cdr variants expressing abundant MT and their corresponding Cds parental CHO cells should be useful for future studies directed toward elucidating the mechanisms that regulate expression of the isometallothioneins.

Interaction of cadmium chloride and gamma irradiation on blood parameters of the young adult rat

Two hundred and sixteen male Sprague-Dawley (S-D) rats, 80 +/- 5 days old and weighing 220-250 g each, were assigned at random to nine groups of 24 rats each. Rats were injected with cadmium (Cd) intraperitoneally every 3 days for 29 days for a total of nine injections. Injections doses were 0, 1.0, or 2.5 mg Cd kg-1 body wt. Twenty-four hours after the last Cd injection (Day 30), each rat received an acute whole-body 60Co gamma radiation dose of 0, 3.62, or 5.43 Gray (Gy) at a dose rate of 33.04 Gy min-1. The irradiated groups exhibited significant decreases in the total number of white blood cells (WBCs) and the percentage of lymphocytes. Significant increases were seen in the percentage of polyneutrophils, serum triacylglycerols (TG), serum iron, and serum lactate dehydrogenase (LDH). Cd-treated groups had increased total WBCs, percentage of polyneutrophils, and serum glutamate oxaloacetate transaminase (SGOT). Significant decreases were observed in the percentage of lymphocytes, hemoglobin, total number of red blood cells (RBCs), and hematocrit. In the co-insult, significant decreases were seen in the total number of WBCs and RBCs, the percentage of lymphocytes, hemoglobin, and hematocrit. Significant increases were observed in the percentage of polyneutrophils and serum iron. In general, Cd acted as a debilitator which enhanced the overall effect of ionizing radiation when applied as the second insult. On the other hand, Cd also provided protection against radiation; that is, some parameters such as total WBCs, serum TG, serum iron, and serum LDH were not as adversely affected by the co-insult as when radiation only was used. The mechanism of this Cd anomaly is not known.

Effect of molybdenum on the acute toxicity of mercuric chloride. IV. Effect of molybdenum on mercury-mediated metallothionein mRNA induction

In order to elucidate the mechanism of the stimulative effect of molybdenum on mercury-mediated renal metallothionein induction, the levels of translatable metallothionein mRNA (MT mRNA) in the kidneys of rats treated with saline or Na2MoO4 or HgCl2 or Na2MoO4 and HgCl2 were measured by translation experiments in cell-free protein synthesizing systems. The time course of accumulation of mercury in renal nuclei of rats given HgCl2 with or without Na2MoO4-pretreatment was also investigated. Molybdenum, itself, did not elevate levels of MT mRNA compared to saline controls at all time points (0, 6 and 14 h after exposure to HgCl2) but rapidly elevated the levels of the mRNA more than Hg-dosed rats when HgCl2 was also administered. On the other hand, the time course study in renal nuclei showed that the mercury content of nuclei was consistently lower in Mo-Hg-dosed rats than in Hg-dosed rats at all time points (4, 8 and 24 h after exposure to HgCl2). These results suggest that the stimulative effect of molybdenum on mercury-mediated metallothionein induction is coupled with an increase of the mRNA coding for the low molecular weight protein and that such an increase in the levels of translatable MT mRNA is not due to the difference in uptake of mercury into renal nuclei.

Induction of metallothionein and metallothionein mRNA in rainbow-trout liver following cadmium treatment

A low-molecular-weight cadmium-binding protein was induced in the livers of rainbow trout (Salmo gairdnerii) following a series of intraperitoneal injections of cadmium chloride. The subsequent purification and amino acid analysis of this protein showed it to be a true metallothionein. As in higher organisms, two major forms of metallothionein appeared to be present in the liver following cadmium treatment. Following a similar induction procedure total RNA was also isolated and shown to contain high levels of metallothionein-mRNA activity when assayed in a wheat-germ cell-free translation system. This activity was present in the Poly-A+-containing fraction of the total RNA. The bulk of this mRNA activity was shown to be in the 8-10S region of a sucrose gradient.

Copper metallothionein of yeast, structure of the gene, and regulation of expression

Addition of copper to yeast cells leads to the induction of a low molecular weight, cysteine-rich protein that binds copper. This protein, termed copper chelatin or thionein, is related to the metallothionein family of proteins that are induced in response to cadmium and zinc in vertebrate cells. We have determined the structure of the yeast copper-binding protein by DNA sequence analysis of the gene. Although the 6573-dalton yeast protein is substantially divergent from vertebrate metallothioneins, the arrangement of 12 cysteine residues, which is a hallmark of metal-binding proteins, is partially conserved. We analyzed the regulatory DNA sequence of the gene by fusing it with the Escherichia coli galactokinase gene and assaying the levels of enzyme activity in yeast in response to copper. The transcriptional activation has a specific requirement for copper. Zinc, cadmium, and gold were unable to regulate the galactokinase activity. The yeast copper metallothionein regulatory sequences represent a previously unreported class of yeast promoter that is regulated by copper.

Cloning and expression of a yeast copper metallothionein gene

The induction of a copper-binding metallothionein (Cu-MT) was studied in yeast, Saccharomyces cerevisiae, and a relationship between copper resistance and intracellular levels of Cu-MT in these eukaryotes was established. Poly(A)-containing RNA from a copper-resistant (Cur) yeast strain, which synthesized abundant quantities of Cu-MT and in which Cu-MT gene transcription was enhanced 50-fold upon exposure to CuSO4, was used to screen yeast genomic DNA clones. Restriction analysis revealed common XbaI and KpnI sites in five genomic clones isolated. The transcription of these clones was regulated by copper. Transformation of a copper-sensitive (Cus) yeast strain by one of these clones confers copper resistance in yeast. The results suggest that the expression of the Cu-MT gene is, in part, responsible for mediating copper resistance in yeast.

Identification of a cadmium-binding protein from a cadmium-resistant variant of human lymphoblastoid cells (WI-L2)

Cadmium-binding protein synthesis and induction by cadmium chloride were studied in the human lymphoblastoid cell line WI-L2. Lymphoblasts were adapted to growth in 5 microM cadmium chloride (Cdr) and these cells were 2.5-fold more resistant to cadmium than the parental line. There was no difference in the cellular protein profile between the parental line and lymphoblasts grown for a short period, less than 10 days, in cadmium chloride as measured by [35S]cysteine labelling and SDS-polyacrylamide gel electrophoresis. A basal level of cadmium binding protein was apparent, however, by gel filtration. The Cdr lymphoblasts were found to synthesize a substantial amount of cadmium-binding protein, approximately 25-fold more than the parental line. The cadmium-binding protein has the following properties which are consistent with its being a metallothionein: (1) [35S]Cysteine-labelled protein eluted at a Ve/Vo = 2.1 on a Sephadex G-75 column; (2) the molecular weight was estimated as 11 kDa on 7-17% SDS polyacrylamide gels; (3) the protein was heat-stable; (4) the unlabelled protein bound 109Cd2+.

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.

Cloning and sequence analysis of two monkey metallothionein cDNAs

We have isolated two metallothionein (MT) cDNA clones copied from the RNA of cadmium-resistant monkey kidney cells. The complete DNA sequences of these clones show that they encode two distinct MTs. One clone appears to represent monkey MT-II, as shown by its close homology to the human MT-II sequence, whereas the second may correspond to monkey MT-I or a related variant metallothionein. Conserved sequences were identified in both the 5' and 3' untranslated regions of these clones.

cDNA cloning and nucleotide sequence comparison of Chinese hamster metallothionein I and II mRNAs

Polyadenylated RNA was extracted from a cadmium resistant Chinese hamster (CHO) cell line, enriched for metal-induced, abundant RNA sequences and cloned as double-stranded cDNA in the plasmid pBR322. Two cDNA clones, pCHMT1 and pCHMT2, encoding two Chinese hamster isometallothioneins were identified, and the nucleotide sequence of each insert was determined. The two Chinese hamster metallothioneins show nucleotide sequence homologies of 80% in the protein coding region and approximately 35% in both the 5' and 3' untranslated regions. Interestingly, an 8 nucleotide sequence (TGTAAATA) has been conserved in sequence and position in the 3' untranslated regions of each metallothionein mRNA sequenced thus far. Estimated nucleotide substitution rates derived from interspecies comparisons were used to calculate a metallothionein gene duplication time of 45 to 120 million years ago.

Stability of cadmium resistance and metallothionein levels in vitro and in vivo

Cultured cells can be adapted to large concentrations of the toxic cadmium ion, apparently by induction of synthesis of the Cd-binding protein metallothionein (MT). One human epithelial line derived from normal skin (HE100) and one murine fibroblast line, derived from L cells (C1 1D100), were used to study the stability of Cd resistance, the MT levels following omission of Cd, and the inducibility of MT synthesis in cells on reexposure to Cd. In the murine cells there was no significant loss of resistance during a 4-week period either after cultivation in vitro or after growing the cells in nude mice. In the human cells a decrease (50%) in resistance was noted the first week after Cd omission. After removing Cd from the cells, a rapid decrease in MT content was demonstrated. After 3 weeks of cultivation only trace amounts were left in both cell lines. However, approximately 60% (HE100) and 80% (C1 1D100) of the previous levels were demonstrated after reexposure to maximum tolerable doses for 24 hr. The data indicate that the degree of stability of Cd resistance is dependent on the capacity in cells for an immediate de novo synthesis of large amounts of MT on reexposure to Cd. The animal experiments demonstrate that Cd resistance is maintained even after growing the cells in vivo.

Differential expression of cloned mouse metallothionein sequences in Escherichia coli

A cDNA for mouse hepatic metallothionein I has been cloned into pBR322 (Mbikay et al., 1981). Although this recombinant plasmid (M135) possesses the metallothionein sequence in the same reading frame as that of beta-lactamase, it fails to direct the synthesis of a fused beta-lactamase-metallothionein protein in Escherichia coli. Another plasmid (M244) was derived from M135 by deleting an internal 390-bp segment made of the 5' noncoding region of metallothionein, the dG-dC tail, and some beta-lactamase sequences. Bacteria harboring the new plasmid now contain in their periplasmic space a cysteine-rich, cadmium-binding protein of 12,000 daltons, as expected. These observations demonstrate that expression of cloned DNA in bacteria could depend as much on its primary structure as on proper insertion in relation to a promoter.

Inducible protective proteins: a potentially novel approach to chemotherapy

A number of toxic chemical and physical agents elicit the induction of a series of protein species, some of which react with the agents and render them nontoxic. A few of the induced species (such as metallothionein) are rich in thiol groups that might be expected to react with alkylating agents and render them nontoxic. If a safe means could be found for selectively enhancing the synthesis of alkylating-agent-reactive species in normal but not tumor cells, such a procedure would have ramifications in the area of cancer chemotherapy. In this report, we have utilized a variety of trace elements (Zn, Se, Cu, As) as inducers of synthesis of protective species in line CHO Chinese hamster cells and in a number of derived variants to determine whether this type of approach can be utilized to increase resistance to alkylating-agent toxicity. Our results indicate that Zn, Se and Cu elicit a protective response (increased survival, monitored by colony-forming ability) against the toxic effects of iodoacetate or melphalan, and, at least in the case of zinc, at levels that are physiologically reasonable. Arsenite appears to be a marginally effective inducer in the CHO cell and an ineffective inducer in the Cdr20F4 variant cell. The increased survival is not attributable to metallothionein inducibility, decreased availability of the alkylating agent in the medium, or decreased uptake of the drug into the trace-element-pretreated cells. The protective responses induced by zinc or selenite alone are additive in cells receiving both trace elements prior to exposure to alkylating agent, which suggests that different domains of response are elicited by the two metals. In view of reported differences in inducibility of protective proteins between normal and tumor cells, a possibility is raised for a novel approach to alkylating-agent chemotherapy that is somewhat analogous to the protocol utilized in high-dose methotrexate therapy.

Ultraviolet light inactivation of zinc-mediated metallothionein induction in normal and repair-deficient human cells

Synthesis of the low molecular weight, thiol-rich, metal-binding metallothioneins (MTS) is undetectable in normal human (NF) or xeroderma pigmentosum (XP) fibroblasts grown in the absence of excess ZnCl2. Addition of 200 microM ZnCl2 to the growth medium produces an increased MT synthesis rising from the basal rate to a rate at least 50-fold greater than basal rate within 7 h. MT induction kinetics in confluent and in exponentially growing subconfluent monolayers were indistinguishable. Zn2+-mediated MT induction is sensitive to actinomycin D suggesting that the induction process is under transcriptional control. Ultraviolet light irradiation causes a dose-dependent inactivation of Zn2+-mediated MT induction in both NF and XP cells. Post-irradiation incubation of UV-irradiated cells using liquid holding techniques leads to reactivation of Zn2+-mediated MT induction in NF cells but not in XP cells. These findings suggest the utility of MT induction produce transcription-terminating lesions, and (b) in evaluating cellular repair capacity for this class of DNA lesions.

Translation of rat kidney mRNA after cadmium administration

1. Twenty-four hours after the administration of Cd2+ (11 mumol/kg body weight) to rats, the kidneys were removed and the RNA was extracted from the polysomes and used to prepare poly(A) RNA. 2. The poly(A)+ RNA was translated in rabbit reticulocyte lysates containing different labelled amino acids as precursors and the resultant proteins were separated by polyacrylamide gel electrophoresis. 3. The labelling of the proteins was similar using poly(A)+ RNA obtained from control and Cd2+ treated rats except for two proteins. 4. Regardless of labelled precursor used, proteins of mobility in sodium dodecylsulphate electrophoresis of mol. wt 50,000 contained approx twice as much radioactivity using the RNA from the kidney of treated rats. 5. Using labelled leucine, lysine, and cysteine, but not labelled phenylalanine or histidine, proteins of mobility in sodium dodecylsulphate electrophoresis of mol. wt 10,000 contained approx twice as much radioactivity using the RNA from the kidney of the Cd2+ treated rats. These results and the results following carboxymethylation of the proteins prior to electrophoresis, together with the results from co-electrophoresis of the products [125-I]-labelled liver metallothionein support the view that the poly(A)+ RNA contains kidney mRNA for metallothionein.

Differential reactivation of zinc-mediated metallothionein induction in ultraviolet-irradiated normal and repair-deficient human cells

The ubiquitous, low-molecular-weight, thiol-rich, metal-binding protein, metallothionein (MT), can be induced in cultured normal human fibroblasts (NF) and xeroderma pigmentosum (XP) cells by exposure to ZnCl2. Both NF and XP cells tolerate up to 200 microM ZnCl2 in the growth medium, upon addition of ZnCl2 (200 microM) to monolayer cultures, both NF and XP cells showed similar kinetics for the induction of MT synthesis: Within 7 hours the MT synthesis rate rose from a low, marginally detectable rate to a maximal rate at least 50-fold greater than the basal rate. The induction of MT synthesis in both cell types was inhibited by actinomycin D (5 microgram/ml), indicating that the induction process is controlled at the level of transcription. Exposure of NF and XP cells to far ultraviolet light (UV) followed by induction with ZnCl2 resulted in a UV dose-dependent decrease in the he maximal rate of MT synthesis measured 8.5 hours postirradiation. The UV sensitivity of the MT induction was greater in XP cells than in NF cells. However, considerations of the differential repair capacities of NF and XP cells superimposed upon the kinetics of MT induction were invoked to explain the apparent differential UV sensitivity of MT induction. Liquid holding recovery experiments showed that NF cells possess the capacity to reactivate this inducible gene function rapidly while XP cells are deficient in the reactivation capacity. These results are discussed in the context of both UV transcriptional mapping of this inducible gene function and development of techniques for measuring repair of transcription-blocking lesions.

Cellular uptake of cadmium and zinc

CHO mutants, resistant to over 100-fold of a normally toxic level of extracellular cadmium have been used to examine the mutually antagonistic effect of Cd and Zn on their uptake. Cadmium uptake in these mutants is only 7-10% that of the parental cells. Zinc uptake in these mutants is equal to or greater than that in the wild-type cells. Results of kinetic studies on uptake indicated that the two metals interact by competitive inhibition. TheK m andK i values for Cd and/or Zn were different in some of the mutants and indicate multiple carriers may be involved in the transport of these metals. The reduction in Cd uptake and concomitant increase in Zn uptake contribute to the increased Cd resistance in these mutants.

Induction of metallothionein mRNA in HeLa cells by dexamethasone and by heavy metals

Synthesis of metallothionein, a cysteine-rich heavy metal binding protein, is induced in cultured HeLa cells both by the heavy metals Cd2+, Zn2+ and Cu2+, and by the glucocorticoid hormone dexamethasone. The accumulation of [35S]cysteine-labeled metallothionein and the amount of translatable metallothionein mRNA show identical concentration dependences in response to dexamethasone treatment and in response to zinc exposure. Induction of translatable metallothionein mRNA is rapid in response to both the metal and glucocorticoid inducers. Increased synthesis and accumulation of metallothionein in response to either metal or glucocorticoid exposure is regulated by the level of translatable metallothionein mRNA in HeLa cells.

Synthesis of a metallothionein-like protein in cultured human skin fibroblasts: relation to abnormal copper distribution in Menkes' disease

A metallothionein-like protein (MTP) is synthesized in normal diploid human skin fibroblasts cultured in Zn- or Cu-supplemented medium. Synthesis of MTP is not detected in cells cultured without metal supplementation of complete tissue-culture medium. Cultured fibroblasts from patients with Menkes' disease accumulate excess Cu which chromatographs both with high-molecular-weight protein(s) and with a Cu-MTP. Under normal culture conditions, the Menkes' MTP incorporates [35S]-cystine, but not appreciable amounts of 65Zn. However, Menkes fibroblasts retain the ability to incorporate 65Zn into MTP in response to Zn supplementation of the medium. The results do not support the idea that Menkes' disease results from a failure of Cu to bind to MTP, but rather that an elevated intracellular Cu concentration in Menkes' disease fibroblasts leads to association of excess Cu with high-molecular-weight protein, stimulating synthesis of a Cu-binding MTP.

The effect of cadmium on kidney ribosomes

Rats received 2.6 mg/kg CdCl2 24 h prior to removing the kidneys and preparing the ribosomal fraction. The amount of ribosomal RNA was decreased 28% and there was a significant increase in bound ribosomes relative to free ribosomes following cadmium. The activity of these ribosomes from the kidney of cadmium-treated animals was nearly doubled either using endogenous mRNA or poly(U), together with elongation factors prepared from control rat liver. When ribosomal subunits were prepared and recombined in a cell-free assay system, the increased ribosomal activity was located in the 60S subunit. In chronic experiments for 2.5 or 11 weeks, cadmium chloride was administered in the drinking water at 100 or 250 ppm. The ribosomal RNA concentration was not decreased at 100 ppm for 2.5 weeks, yet the activity of the ribosomes was increased about 50%. AT 250 ppm for 2.5 weeks or 100 ppm for 11 weeks the ribosomal RNA concentration was decreased 15% and 7%, respectively, while the activity of the ribosomes was once more increased by about 50%. The increase in ribosomal activity after cadmium chloride are discussed in relation to the reported effects of cadmium chloride and of mercuric chloride on renal metallothionein and on renal tubules.

Regulation of metallothionein synthesis in HeLa cells by heavy metals and glucocorticoids

Metallothioneins (MTs) are low molecular weight, cysteine-rich proteins that bind heavy metals. MT induction occurs in liver in response to either heavy metal (Zn++ or Cd++) administration or stress. The synthesis of MT can also be induced by either heavy metals or glucocorticoid hormones in HeLa cells cultured in serum-free medium. Induction of MT by zinc is subject to "desensitization." In contrast, dexamethasone (dex) induction results in a continued elevation in the rate of MT synthesis. The stability of MT is dependent on the availability of metal; consequently, MT induced by dex is degraded much more rapidly (half-life of 11 to 12 hours) than MT induced by elevated zinc levels (half-life of 36 to 38 hours). Removal of either inducer results in biphasic degradation curves, as apothionein and zinc come into balance. In contrast, deinduction kinetics for MT synthesis following removal of the two inducers (zinc and dex) are the same, with a half-life of two and one-half hours. Inhibition of RNA synthesis blocks deinduction after removal of inducer. Induction of MT occurs in a wide variety of species, from blue-green algae to man. This system should provide an excellent model for the comparative biochemistry of regulation of gene expression.

Amino acid analysis and cell cycle dependent phosphorylation of an H1-like, butyrate-enhanced protein (BEP; H1(0); IP25) from Chinese hamster cells

A fraction enriched in the butyrate-enhanced protein (BEP) has been isolated from Chinese hamster (line CHO) cells by perchloric acid extraction and Bio-Rex 70 chromatography. Amino acid analyses indicate that the composition of BEP resembles that of CHO H1; however, BEP contains 11% less alanine than H1, and, in contrast to H1, BEP contains methionine. Treatment of BEP with cyanogen bromide results in the cleavage of a small fragment of approximately 20 amino acids so that the large fragment seen in sodium dodecyl sulfate--acrylamide gels has a molecular weight of approximately 20 000. Radiolabeling and electrophoresis indicate that BEP is phosphorylated in a cell cycle dependent fashion. In G1-arrested cells, little or no phosphate is incorporated into BEP. As cells progress through interphase, BEP becomes phosphorylated so that 12--35% of the BEP molecules are phosphorylated at one to two sites by late interphase. During mitosis, all BEP molecules become phosphorylated at approximately four sites per molecule (BEPM). Electrophoresis and the analysis of cell populations by electron microscopy indicate that the appearance of BEPM is temporally correlated with the mitotic phosphorylation of histone H1 (H1M) and with chromosomal condensation during prophase, metaphase, and anaphase. During exit from mitosis, BEPM undergoes dephosphorylation. The dephosphorylation of BEPM is temporally correlated with dephosphorylation of H1M and with the unraveling of fully condensed chromosomes near the anaphase--telophase transition. These data suggest that (1) BEP is a specialized histone of the H1 class and (2) BEP is the species equivalent of calf lung histone H1(0) [Panyim, S., & Chalkley, R. (1969) Biochem. Biophys. Res. Commun. 37, 1042], rat H1(0) [Medvedev, Zh. A., Medvedeva, M. N., & Huschtscha, L. I. (1977) Gerontology (Basel) 23, 334], and IP25, a protein enhanced in differentiated Friend erythroleukemia cells [Keppel, F., Allet, B., & Eisen, H. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 653]. The data also indicate that putative HMG1 and HMG2 proteins do not undergo the extensive cell cycle dependent phosphorylations measured for histone H1 and BEP.

Metallothionein mRNA induction in HeLa cells in response to zinc or dexamethasone is a primary induction response

Metallothioneins (MTs) are low molecular weight, heavy metal binding proteins unique in their high cysteine content and high affinity for Zn2+, Cd2+, Hg2+, Ag2+ and Cu2+ (refs 1--3). The synthesis of MTs is induced by zinc or cadmium in the liver and kidney and in cultured cells. More recently MT induction by the steroid hormone dexamethasone (Dex) has been demonstrated in HeLa cells and adrenalectomized rats. Because glucocorticoid hormones lead to an intracellular accumulation of zinc, the question arises of whether the induction of MT gene expression by steroids is a 'primary induction response' (ref. 18), or due to elevated intracellular Zn2+. The glucocorticoid-induced transport of Zn2+ is dependent on concurrent protein synthesis. We now show that, in contrast to glucocorticoid-stimulated Zn2+ transport, the Zn2+ and Dex induction of translatable MT-mRNA is independent of concomitant protein synthesis but not RNA synthesis; that is, MT induction by either agent is a primary induction response.

Regulation of the stability of poly(I)xpoly(C)-induced human fibroblast interferon mRNA: selective inactivation of interferon mRNA and lack of involvement of 2',5'-oligo(A) synthetase activation during the shutoff of interferon production

The inactivation of interferon mRNA during the shutoff phase of interferon production in poly(I)xpoly(C)-induced human fibroblast cultures is selective. We have determined that the shutoff of interferon production, which takes place from 3 to 8 hr after the beginning of induction, is not associated with an appreciable declined in the rate of bulk cellular protein synthesis or of cellular protein secretion. While the amount of translatable interferon mRNA declined markedly during the shutoff phase, the level of translatable bulk cellular mRNA and the stability of [3H]uridine-labeled mRNA were unaffected. Superinduction with actinomycin D selectively stabilized interferon mRNA with no apparent effect on the stability of bulk cellular mRNA. Furthermore, an activation of the 2',5'-oligo(A) synthetase/endonuclease system does not appear to be involved in the shutoff phenomenon. Uninduced FS-4 cells contained a low basal level of 2'5'-oligo(A) synthetase activity, which was unchanged in poly(I)xpoly(C)-induced cells during the shutoff phase. Treatment of FS-4 cells with interferon for 16-18 hr prior to induction increased the enzyme activity by approximately 200-fold. However, this did not inhibit interferon production after induction with poly(I)xpoly(C) alone or after superinduction with cycloheximide or actinomycin D or both. Furthermore, the rates of decay of interferon production were comparable in cells with either a basal or an increased level of 2',5'-oligo(A) synthetase. Thus a 200-fold increase in 2',5'-oligo(A) synthetase level did not affect either the stability of interferon mRNA or the efficacy of interferon superinduction by metabolic inhibitors.

The interaction of cadmium and certain other metal ions with proteins and nucleic acids

The toxic effects of cadmium and other selected divalent cations are presumed to be related to specific chemical and physical characteristics of the ion. The chemistry of cadmium and metal ions in general is reviewed from the viewpoint of such relevant properties as ion polarizability, electronic structure, and the hard-soft characteristics. The softness of metal ions is seen as a useful single parameter to correlate with the affinity for nucleic acids and proteins and with toxic effects. The effects of cadmium on nucleic acids and proteins are examined for a number of specific cases to illustrate the variety of interactions that are well recognized and to demonstrate the utility of soft metal ions as reagents and probes for examining the relationship of structure and function in these macromolecules.