Introduction

The metal-regulatory transcription factor 1 (MTF-1) is a highly conserved transcriptional activator (Radtke et al., 1993, 1995; Heuchel et al., 1994). We previously cloned mouse and human MTF-1, which share a 93% identity in amino acid sequence (Radtke et al., 1993; Brugnera et al., 1994). MTF-1 contains six zinc fingers of the C₂H₂-type as a DNA-binding domain and at least three distinct domains responsible for transcriptional activation. It binds to a number of metal responsive elements (MREs) present in the promoter regions of metallothionein genes I and II (MT-I and MT-II) and regulates their expression. The MREs present in metallothionein gene promoters are short DNA sequence motifs of 9–12 conserved base pairs, whereby a central core of 7 base pairs is strongly conserved (Stuart et al., 1985; Searle et al., 1987; Radtke et al., 1993; Ç.Günes, unpublished results). Binding of MTF-1 to MRE sequences is dependent on zinc cations, as has been shown by in vivo and in vitro experiments (Mueller et al., 1988; Westin and Schaffner, 1988; Searle, 1990; Radtke et al., 1993). However, MTF-1 activates the expression of metallothionein genes not only by zinc but also by other heavy metal cations such as cadmium or copper (Heuchel et al., 1994).

There are four types of metallothioneins in mammals, namely MT-I, MT-II, MT-III and MT-IV (Uchida et al., 1991; Palmiter et al., 1992; reviewed in Heuchel et al., 1995). The expression of MT-I and MT-II is stress inducible and ubiquitous, while MT-III and MT-IV are tissue-specific and only weakly stress-inducible. The expression of MT-III is restricted to neuronal tissue after embryonic day 17, whereas MT-IV expression is restricted to certain squamous epithelia and no transcripts can be detected before day 7 post partum (Palmiter et al., 1992; Quaife et al., 1994; Palmiter and Findley, 1995). The expression of MT-I/MT-II genes is enhanced by a great number of stimuli, most notably by adverse conditions such as an excess of heavy metal cations, reactive oxygen intermediates (ROI), UV- and X-irradiation (Searle et al., 1984; Kagi, 1987, 1991; Heuchel et al., 1995). Consequently, these proteins have been implicated in heavy metal detoxification, metal homeostasis and radical scavenging (Thornally and Vasak, 1985; Angel et al., 1986; Thomas et al., 1986; Kagi, 1991). Since constitutive MT-I and MT-II expression is very high in fetal liver (Kern et al., 1981; Ouellette, 1982; Quaife et al., 1986), it has been suggested that these MTs might play a pivotal role in fetal liver development. However, two groups have shown independently that mouse strains with combined deletion of the MT-I and MT-II genes, as a result of targeted gene disruption, show no altered phenotype under normal conditions. However, such null mutant mice were more sensitive to cadmium exposure, an observation which supports the proposed role of metallothioneins in metal detoxification (Michalska and Choo, 1993; Masters et al., 1994).

We have shown previously that MTF-1 is essential for basal and heavy metal-induced transcriptional activation of MT-I and MT-II genes in cultured cells (Heuchel et al., 1994). Here, using the power of knockout technology, we show that MTF-1 is essential for embryonic liver function in mice. At the molecular level, expression of the known MTF-1 target genes encoding MT-I and MT-II is undetectable. Furthermore, the mRNA for -glutamylcysteine synthetase (-GCS), an essential enzyme in glutathione (GSH) biosynthesis, is reduced in MTF-1^-/- embryos. Since both metallothionein and GSH share similar functions in that they are involved in cellular metal homeostasis and in detoxification processes, we propose that MTF-1 is involved in regulating cellular stress response.

Results

Targeted disruption of MTF-1 locus

To investigate the function of MTF-1 in vivo, we generated mice lacking functional MTF-1 by targeted disruption of the MTF-1 genomic locus (Figure 1). 129/SV ES cell clones containing one disrupted MTF-1 allele were injected into blastocysts to generate mice heterozygous for the mutated MTF-1 allele (see Materials and methods). These mice (MTF-1^+/-) with a mixed 129/SVC57BL/6 genetic background appeared normal and were intercrossed. Among 41 born pups, none was homozygous for the MTF-1 mutation, implying that MTF-1^-/- embryos die in utero (Table I). Genotype analyses of embryos from MTF-1^+/- intercrosses revealed no viable homozygous mutant embryo older than E14.5 days (0 of 25). Before day E13.5, homozygous mutant embryos were obtained at about the expected Mendelian frequency (10 of 41), indicating that implantation and early post-implantation development were not impaired. At day E13.5, only one of 14 MTF-1 homozygous mutant embryos was dead. Conversely, at day E14.5, only one homozygous mutant embryo was alive (Table I).

RT–PCR analysis of RNA from 13.5-day-old mutant embryos revealed an MTF-1 transcript with an internal deletion giving rise to a stop codon 39 bases after the point of deletion (Figure 1). As expected, total RNA from 13.5-day-old embryos did not show an MTF-1 signal with a probe covering the deleted segment (Figure 2, top lane). We also analyzed expression of the MT-I and MT-II genes in 13.5-day-old MTF-1^-/- embryos by RNase protection assay and found no detectable MT-I and MT-II expression whereas expression of the unrelated transcription factor Sp1 was not affected (Figure 2). This result agrees with the previous analysis of MTF-1^-/- ES cells (Heuchel et al., 1994). Hence, we conclude that homozygous mutant embryos do not produce any functional MTF-1 protein.

Phenotype of mice lacking MTF-1

Viable MTF-1^-/- embryos at day E13.5 were macroscopically indistinguishable from their heterozygous and wild-type littermates. They were uniformly pink in color, well-vascularized and showed no internal bleeding. A histological analysis of whole embryos did not reveal any differences (data not shown) except pronounced liver damage (Figure 3). The liver structure of MTF-1^-/- embryos at this stage was disrupted to a variable degree with enlarged, congested sinusoids and dissociation of the epithelial compartment (Figure 3D). Furthermore, although individual epithelial cells of MTF-1^-/- liver at day E13.5 appeared morphologically normal in conventional histology, immunohistochemical analysis showed that these cells had a reduced cytokeratin expression (Figure 3F) in comparison with wild-type littermates (Figure 3E). This deficiency was liver-specific since cytokeratin expression in the gut from the same embryo was not affected (Figure 3F, inset). At day E14.5, the only null mutant embryo still alive showed severe liver necrosis (Figure 3H), diffuse bleeding and generalized edema as well as an almost complete lack of cytokeratin expression in the liver (data not shown). By contrast, -actin expression in the vessel walls was still indistinguishable from heterozygous or wild-type littermates at day E14.5 (data not shown). An obvious explanation for the absence of cytokeratin might have been a dependence of liver cytokeratin genes on MTF-1; however, we have not found any binding sites for MTF-1 in the known cytokeratin promoters, and it remains to be seen whether the absence of cytokeratin is a cause or a consequence of liver pathology.

At day E13.5, we could not detect any significant increase in the frequency of apoptotic cells in the liver of MTF-1^-/- embryos, neither morphologically nor with the TUNEL in situ staining for DNA fragmentation (data not shown). At day E14.5, there was extensive DNA fragmentation, most likely resulting from necrosis rather than apoptosis. We also found that the cell proliferation rate in the MTF-1 homozygous mutant was not affected since bromodeoxyuridine incorporation at day E13.5 (Figure 3E and F) revealed no decrease when compared with wild-type or heterozygous littermates. Since the liver takes over as a blood-forming organ at this stage, we considered the possibility of a defect in the hematopoetic system in null mutant embryos. However, living 13.5-day-old homozygous mutant embryos showed not only no macroscopic color change indicative of anemia, but also no change in the number or appearance of erythrocytes or hematopoietic precursor cells observable in histological sections (data not shown). At day E13.5, the contribution of liver-derived enucleated erythrocytes is small, and thus a deficiency would not be easily detectable. However, the only living null mutant embryo at day E14.5 also showed no reduced representation of enucleated erythrocytes, again arguing against a defect in hematopoiesis (data not shown).

Live homozygous MTF-1^-/- embryos at days E13.5 and E14.5 showed no reduced liver size which distinguishes them from the small liver phenotype seen in the knockout of HLX, a developmental control gene for liver formation (Hentsch et al., 1996). Also, the placenta of MTF-1^-/- embryos did not reveal any abnormalities (data not shown), in contrast with the combined liver and placental defects seen in hepatocyte growth factor/scatter factor (HGF/SF) knockouts (Schmidt et al., 1995; Uehera et al., 1995). Hence, our findings render unlikely a loss of HLX or HGF/SF function as a downstream effect of MTF-1 deletion.

MTF-1 regulates the expression of the -GCS gene

As mentioned before, MTF-1 is essential for basal and heavy metal-induced expression of MT-I and MT-II genes. Since mice lacking the MT-I and MT-II genes are viable (Michalska and Choo, 1993; Masters et al., 1994) and other metallothionein isoforms cannot account for redundancy in MT-I and MT-II knockout mice because of their temporal and spatial expression (Palmiter et al., 1992; Quaife et al., 1994), we considered it most likely that deregulated expression of at least one additional target gene, perhaps in combination with loss of MT-I and MT-II expression, is responsible for the liver damage phenotype of MTF-1^-/- mice.

Recently, MREs have been described in the promoter region of the heavy-chain subunit of -GCS (-GCS_hc), the key enzyme in the biosynthesis of GSH. We found additional putative MREs in the promoter of -GCS_hc and tested them for MTF-1 binding. By electrophoretic mobility shift assays (EMSA), we observed a strong, MTF-1-specific complex with one of these MREs (located at position -114) and a specific albeit weak interaction with the MRE located at position +296 (Figure 4A and B). No binding to other putative MREs could be observed in EMSA assays (data not shown).

We then inserted a 1.8 kb fragment of the -GCS_hc promoter upstream of a -globin reporter gene (Materials and methods) and tested the expression of the reporter gene in transient transfection assays in the human hepatoma cell line HepG2 (Figure 4C), which expresses MTF-1 endogenously. In addition to a basal level of expression (Figure 4C, lane 1), co-transfections with mouse or human MTF-1 expression vectors yielded an increased transcription of the reporter gene (Figure 4C, lanes 2 and 3). Moreover, induction with Zn or Cd further elevated reporter gene expression (Figure 4C, lanes 4–9). These results are in agreement with the recent observation that endogenous -GCS_hc expression can be induced by zinc or cadmium treatment (Ishikawa et al., 1996).

More importantly, by RT–PCR and Northern blot analysis we found reduced -GCS_hc gene expression in 13.5-day-old MTF-1 knockout embryos compared with wild-type embryos (Figure 4D and E). The expression of the 36B4 reference gene, which codes for the acidic ribosomal protein P0, was not altered in wild-type, heterozygous or knockout embryos whereas the expression of -GCS_hc was reduced in knockout embryos. In contrast, expression of the gene encoding the light chain subunit (regulatory subunit) of -GCS (-GCS_lc) or the gene for GSH synthetase (GSH-Syn), the second enzyme involved in GSH biosynthesis, was not altered (data not shown).

Primary mouse embryo fibroblasts from MTF-1 null mutant embryos are more sensitive to cadmium and hydrogen peroxide

To obtain more insights into the physiological role of MTF-1, we cultured primary mouse embryo fibroblasts (MEFs) from 12.5-day-old mouse embryos that were either wild-type, heterozygous or null mutant for the MTF-1 locus (Materials and methods). Since we observe a downregulation of genes responsible for metallothionein and GSH biosynthesis in the MTF-1 null mutant embryos, we decided to test the response of MTF-1-deficient cells against cadmium or H₂O₂-induced cytotoxicity. For this purpose, we treated primary MEFs with cadmium or H₂O₂ and determined the survival of MEFs by the neutral red assay (Figure 5A–D; see Materials and methods for details). As shown in Figure 5, MTF-1-deficient cells (MTF-1^-/-) are more susceptible to cadmium or H₂O₂ treatment when compared with wild-type cells (MTF-1^+/+). After 8 h of exposure to 40 M cadmium, viability of the MTF-1-deficient cells was 65% compared with that of wild-type cells (Figure 5A). This effect was even more pronounced when the cells were treated with 60 M cadmium; the viability of MTF-1 deficient cells was 40% compared with that of wild-type cells (Figure 5B). Similarly, deficient cells were more susceptible to exposure to H₂O₂; after an 8 h induction with 20 M and 40 M H₂O₂, 75 and 60% of the knockout cells, respectively, were viable when compared with wild-type cells (Figure 5C and D). Primary MEFs heterozygous for the MTF-1 locus (MTF-1^+/-) were also more susceptible to the cytotoxic effects of cadmium and H₂O₂ when compared with wild-type cells. However, these MTF-1^+/- cells showed more variation than wild-type or knockout cells from experiment to experiment (see Discussion).

Similar experiments were performed with permanent cell lines established from primary embryo fibroblasts. All MTF-1 knockout cell lines were more susceptible to cadmium and H₂O₂ than their wild-type counterparts (data not shown).

Discussion

The metal-regulatory transcription factor MTF-1 is expressed in embryonic stem cells, throughout mouse gestation and in adult mice where it is ubiquitously expressed (Ç.Günes, unpublished results), and thus is likely to influence gene expression in many cell types. Nevertheless, the observed phenotype of liver degeneration in MTF-1 null mutant embryos implies that MTF-1 is particularly important for proper liver development/function at the embryonic stage.

As expected, no expression of the known target genes, MT-I and MT-II, could be detected in MTF-1 null mutant embryos. Metallothioneins are induced via MRE promoter sequences not only by heavy metal load but also by ROI and thus may help to protect the cell against oxidative stress (Thornally and Vasak, 1985; Girotti, 1986; Thomas et al., 1986; Kagi, 1991; Dalton et al., 1994, 1997). Null mutant mice for both of the metal-inducible metallothioneins, MT-I and MT-II, are viable but show greater sensitivity to cadmium toxicity (Michalska and Choo, 1993; Masters et al., 1994). Also, cultured hepatocytes from such mice are highly sensitive to both Cd-induced cytotoxicity (Zheng et al., 1996) or tert-butylhydroperoxide-induced oxidative stress (Lazo et al., 1995).

In addition to these known MTF-1 target genes, we observed a reduced transcript level of another gene, namely the gene coding for -GCS_hc. -GCS is the first and the rate-limiting enzyme in the biosynthesis of the GSH de novo pathway. -GCS is an essential enzyme (Maellaro et al., 1990; Meister, 1995). Similarly, malfunction of other enzymes involved in GSH metabolism in mice and human results in severe phenotypes (Liebermann et al., 1996; Shi et al., 1996). In spite of its ubiquitous presence, GSH seems of particular importance in the liver, perhaps due to high levels of mitochondrial respiration and lipid peroxidation within this organ. In fact, GSH homeostasis is mainly regulated by the liver.

GSH is essential to maintain the cellular redox-balance and is a scavenger of ROIs, which are mainly generated by respiration (Meister, 1995) or by exogenous cellular stress, such as that caused by heavy metal load (Bagchi et al., 1996). In addition, GSH has been shown to be the first line of defense against cadmium toxicity (Singhal et al., 1987; Shimizu and Morita, 1990; Chan and Cherian, 1992) and its biosynthesis can be induced by heavy metals (Iszard et al., 1995; Liu et al., 1995). A functional link between metallothionein and GSH in regulating the distribution of cellular zinc was proposed recently (Maret, 1995). This heavy metal-complexing ability of ubiquitous GSH could explain the viable phenotype of the MT-I and MT-II null mutant mice (Michalska and Choo, 1993; Masters et al., 1994). Conversely, it has been shown that Zn-MT, at least in part, can substitute for GSH as a cellular defense against ROI (Srivastava et al., 1993). Thus, GSH and metallothionein apparently have overlapping functions.

Using primary mouse embryo fibroblasts, we have shown that MTF-1-deficient cells are more susceptible to cadmium or H₂O₂ treatment when compared with their wild-type counterparts. Even though mice heterozygous for the MTF-1 locus develop normally and do not show any abnormalities under laboratory conditions, heterozygous (MTF-1^+/-) cells in culture were also more susceptible to cadmium or H₂O₂ treatment when compared with wild-type cells. MTF-1 is ubiquitously expressed, but its known target genes MT-I, MT-II and -GCS, although also expressed ubiquitously, play a particularly important role in the liver. It remains to be seen whether the knockout phenotype can be explained by the existence of a threshold for the accumulation of harmful agents during liver development, or whether MTF-1 has yet another role(s) in liver morphogenesis. Experiments with conditional MTF-1 knockout mice should help to resolve these questions.

The null mutant phenotype may be a compound effect of the aberrant expression of a number of genes. In fact, besides MT-I, MT-II and -GCS, we also found other candidate target genes involved in GSH synthesis and/or heavy metal homeostasis. Preliminary experiments suggest that MTF-1 may regulate the expression of the zinc transporter, ZnT-1 which is a ubiquitously expressed zinc export pump. As already described by Palmiter and Findley (1995), the promoter of ZnT-1 contains MTF-1 binding sites (MREs). We find that MTF-1 binds to these MREs and that the expression of the endogenous ZnT-1 transcript can be induced by zinc treatment in primary rat hepatocytes (Ç.Günes, unpublished results). In addition, we have identified MREs in the promoter of the -glutamyl transpeptidase gene which is involved in GSH metabolism and thus is another candidate target gene for MTF-1. Taken together, we consider it probable that MTF-1 plays a general role in cellular stress response.

Cellular stress response also relies on at least two further ubiquitous transcription factors, AP-1 (Jun/Fos) and NF-B, which are induced by a number of common stimuli, notably stress signals, and are involved in antioxidant redox regulation (Herrlich et al., 1994; Pahl and Baeuerle, 1994; Angel, 1995; Wilhelm et al., 1995; Sen and Packer, 1996; Karin, 1997). It has already been shown that the expression of some genes involved in GSH metabolism is regulated by AP-1 or NF-B (Friling et al., 1992; Rahman et al., 1996; Sekhar et al., 1997). Interestingly, the promoter of the -GCS_hc gene contains two AP-1 binding sites and one NF-B binding site (Mulchay and Gipp, 1995; Yao et al., 1995; Tomonari et al., 1997).

We note that c-Jun homozygous mutant embryos showed an embryonic liver phenotype similar, though not identical, to the one of MTF-1 null mutants (Hilberg et al., 1993), raising the possibility that one of these genes is epistatic over the other. However, preliminary experiments have not shown impaired c-Jun expression in MTF-1 knockouts (data not shown). Conversely, MTF-1 expression was not changed in fibroblasts derived from c-Jun knockout mice (A.Kolbus and P.Angel, personal communication). We also note that null mutants of the p65/relA subunit of NF-B exhibit a similar failure in fetal liver development but at a slightly later stage and concomitant with a strong increase of apoptosis (Beg et al., 1995; Bladt et al., 1995). Again, no altered expression of p65/relA was found in MTF-1 null mutants (data not shown).

The similarity of the phenotypes of MTF-1, c-Jun and p65/relA null mutant mice, as well as the similar activation stimuli suggests that these transcription factors may have overlapping but nonredundant functions during embryogenesis, for example by synergistic regulation of common target genes involved in stress response.

Materials and methods

Targeted disruption of the MTF-1 gene

The construction of the targeting vector used in the targeted disruption of the MTF-1 locus in cultures of embryonic stem cells of mouse strain 129/5v, ES cell line ES-GS, was published in Heuchel et al. (1994). ES cells from one clone heterozygous for the mutated MTF-1 allele (26E7) were injected into blastocysts of C57BL/6 mice. Fourteen of the 31 male mice born were highly chimeric. Ten of those 14 were intercrossed with wild-type C57BL/6 females and showed germline transmission. Genotype identification was done by PCR with genomic DNA prepared from tail biopsies of 3- to 6-week-old born pups or of genomic DNA prepared from the yolk sac of embryos. The PCR was perfomed as described elsewhere (Heuchel et al., 1994).

Histological analysis

Embryos were isolated and fixed in 4% formaldehyde solution overnight. Their yolk sacs were used for genomic DNA preparation and PCR genotyping. After fixation, embryos were embedded in paraffin and processed for conventional histology. Sections were stained with hematoxylin-eosin or used for further treatments.

Two hours before isolation of the embryos, mice were injected intraperitoneally with 300 l of bromodeoxyuridine (50 mg/ml)-containing solution. Bromodeoxyuridine integration was detected by incubation with peroxidase-conjugated Fab fragments. Sites of binding were visualized using the actin–biotin diaminobenzidine method. Double-staining of these sections was done using a monoclonal antibody (Lu 5, BMA) which detects all types of cytokeratins (concentration of 1:200), followed by an incubation with an AP-conjugated goat anti-mouse antisera.

RNA preparations, cloning of promoter constructs, transfections and transcript mappings

Total RNA from embryos was isolated using the guanidium thiocyanate/acidic phenol method (Chomczinski and Sacchi, 1987). mRNA from embryos was prepared directly by using the Qiagen direct mRNA midi/maxi kit. Preparation of cytoplasmic RNA from tissue culture was according to Radtke et al. (1993). RNase protection assay from MTF-1, Sp1 and MT-I and MT-II genes was performed as described by Heuchel et al. (1994). Northern blot analysis was performed according to standard procedures (Sambrook et al., 1989). To detect the -GCS_hc mRNA, 50 ng of the 2.9 kb human -GCS_hc cDNA was labeled by random hexanucleotide primers. Expression of 36B4 mRNA was used for RNA integrity. An 800 bp fragment of 36B4 cDNA was labeled by random hexanucleotide primers.

Transfections, cloning of promoter–OVEC constructs and S1 nuclease mapping of transcripts

Transfections and S1 nuclease mapping of transcripts were performed as described previously (Radtke et al., 1993). The following primers were used to amplify the 1.8 kb fragment of the -GCS_hc from a human genomic DNA library. The upstream primer (position -1442) was 5'-GGTCGAGCTCGAGCACTATTTAGTGTGGAGC-3' and the downstream primer (position +398) was 5'-TGGCGACGTCTGTTCCTCCGGGCTGACGGCGGTCG-3'. The resulting 1.8 kb fragment was subsequently cloned upstream of the -globin reporter gene (Westin et al., 1987). The human -GCS_hc promoter-reporter construct (10 g) was transfected either with or without 2 g of an expression plasmid carrying mouse MTF-1 or human MTF-1 cDNA. Cells were treated for 4 h with either 100 M zinc sulfate or with 40 M cadmium sulfate before harvest (as described previously by Radtke et al., 1993). The intensity of the bands was quantified by phosphoimager.

Electrophoretic mobility shift assays (EMSA)

EMSA was performed as described by Radtke et al. (1993). Binding reactions were performed by incubating 2–5 fmol end-labeled 31 bp long MRE-containing oligonucleotides with nuclear extracts obtained as described by Schreiber et al. (1989). Identification of the MTF-1 binding was performed by using wild-type control extracts in the presence or absence of an MRE-containing oligonucleotide. As an internal control, Sp1 binding to the oligonucleotides was performed.

RT–PCR analysis

Titan One-Tube-RT–PCR was performed according to the supplier's information (Boehringer Mannheim). Briefly, 25 or 100 ng mRNA was incubated with 25 pmol each of both primer pairs in the same tube for 1 min at 94°C and 2 min at 65°C for denaturation and was stored on ice immediately until the RT–PCR mix was added. The RT–PCR was performed as follows. A pre-cycle of 35 min at 58°C and 2 min at 94°C, followed by 35 cycles of 30 s at 94°C, 30 s at 63°C and 1 min at 68°C, and a final cycle of 5 min at 68°C. As a control, the expression of the acidic ribosomal protein P0, which is encoded by the gene 36B4, was detected in the same RT–PCR. Aliquots from the RT–PCR were fractionated by agarose gel electrophoresis and bands were visualized by ethidium bromide (EtBr)-staining and the intensity of the bands was quantified by Fluor-Imager. The following primers were used to amplify respective fragments of either -GCS_hc or of 36B4 genes from 13.5-day-old embryos by RT–PCR. The first primer for the reverse transcriptase reaction and the following PCR for -GCS_hc was: 5'-CTATCATCTACAGATTCAGAAATCACTCCCCAGCG-3' while the second primer for this gene in PCR was: 5'-GCGGGCATGGGGCTGCTGTCCCA-3'. The first primer for the reverse transcriptase reaction and the following PCR for 36B4 was: 5'-GCAAATGCAGATGGATCAGCCAGGAAGGCCTTGACC-3' while the second primer for this gene in PCR was: 5'-GTGGGAGCAGACAACGTGGGCTCC-3'.

Primary mouse embryo fibroblasts

Primary embryo fibroblasts were isolated from 12.5-day-old mouse embryos by previously published methods. MTF-1 genotypes of cultured cells were determined by PCR and RT–PCR. Cells were grown in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% fetal calf serum (FCS).

Neutral red cell viability assay

Cytotoxicity of cadmium and H₂O₂ was assessed by determining the uptake of the neutral red dye essentially as described by Sigma, Biosciences™ according to Borenfreund and Puerner (1985). In brief, equal numbers of the indicated MEF cells were grown on 6-well dishes. After 12–16 h, media were replaced by fresh media containing the indicated amounts of cadmium or H₂O₂ for 8 h before adding the neutral red dye. Then 1/50 of a 0.33% neutral red solution was added to the plates and incubation was continued for 2 h in a standard 37°C incubator. Medium containing neutral red was removed and plates were carefully rinsed with 1/100 volume of fixation solution (0.1% CaCl₂ in 0.5% formaldehyde). A 1:1 volume of the neutral red solubilization solution (1% acetic acid in 50% ethanol) was then added and plates were placed on a shaker for 2 h at room temperature. The viability of cells was determined spectrophotometrically by measuring the absorbance of the neutral red dye extracted from the cells after solubilization at a wavelength of 540 nm.

Acknowledgements

We thank Drs Joy Alcedo, Alcide Barberis and Richard Eckner for critical reading of the manuscript and Dr Ken Zaret (Brown University) for valuable discussions. We also thank Drs Andrea Kolbus and Peter Angel (DKFZ, Heidelberg) for determining MTF-1 expression in a c-Jun^-/- cell line. The human -GCS_hc cDNA was a gift of R.T.Mulchay (University of Wisconsin-Madison) and the 36B4 reference cDNA was provided by Dr R.Eckner (Zürich). This work was supported by the Swiss National Science Foundation and by the Kanton Zürich.

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