Metallothionein modulates the carcinogenicity of N-butyl-N-(4-hydroxybutyl)nitrosamine in mice

The two faces of metallothionein in carcinogenesis: photoprotection against UVR-induced cancer and promotion of tumour survival

Metallothionein is a multi-functional protein that protects the host against toxic heavy metals. Under stressful situations it can protect against oxidative damage, contribute to tissue repair, modulate immune responses and limit inflammatory processes. Recently, metallothionein's role in ultraviolet radiation (UVR)-induced injury has been investigated. These studies have shown that when metallothionein is upregulated following exposure to UVR, it can protect against UVR-induced damage and the subsequent development of skin cancer. We propose that this initial protection is achieved through its anti-oxidant role resulting in reduced oxidative stress, reduced apoptosis, reduced NFkappaB activation and enhanced repair of DNA damage. However, once UVR-induced neoplasia has occurred, the cancer cells can hijack metallothionein's protective functions, resulting in increased tumour progression and malignancy. These two discordant sets of attributes are context-dependent, and represent the two faces of metallothionein.

Protective role of metallothionein in benzo[a]pyrene-induced DNA damage

Metallothionein (MT) is known to reduce chemical carcinogenesis. Carcinogenesis induced by benzo[a]pyrene (B[a]P) which is an environmental chemical carcinogen is related to DNA adduct formation and oxidative damage through metabolic activation. Ten-week-old male MT-I/II null mice and wild-type mice were given a single injection of B[a]P (250 mg/kg, p.o.), and B[a]P-induced DNA damage was evaluated at 6-48 hr later. The frequencies of micronucleated reticulocytes (MNRET) in MT-I/II null mice were significantly increased compared with that of wild-type mice at 48 hr after B[a]P administration. At 48 hr after B[a]P administration, comet scores were significantly increased in MT-I/II null mice but not in wild-type mice. 8-Hydroxy-2'-deoxyguanosine (8-OHdG), a biomarker of oxidative DNA damage, was significantly increased in liver of MT-I/II null mice at 6 and 12 hr after B[a]P administration, although that of wild-type mice was only slightly changed. Because cytochrome P450 (CYP) plays a major role in the process of B[a]P metabolic activation, we attempted to reveal the effect of MT on metabolic activation of B[a]P. Although CYP1A activities were elevated in the livers of MT-I/II null mice and wild-type mice treated with B[a]P, it was not different between both strains of mice. In addition, MT levels in the livers of wild-type mice were significantly increased by the B[a]P treatment, whereas MT was not detected in livers of MT-I/II null mice with or without B[a]P treatment. These results demonstrate that MT acts as an endogenous defensive factor against B[a]P-induced DNA damage.

Low susceptibility to N-ethyl-N-nitrosourea-induced transplacental carcinogenesis in Long-Evans Cinnamon (LEC) rats

The Long-Evans Cinnamon (LEC) rat, an animal model of Wilson's disease, is resistant to a variety of chemical carcinogenesis except liver and colon. In the present study, N-ethyl-N-nitrosourea (ENU)-induced transplacental carcinogenesis was examined in male and female LEC, Long-Evans Agouti (LEA), a sibling line of the LEC rat, and F344 rats (n=21). ENU was administered to pregnant rats as a single s.c. injection at a dose of 60 mg/kg body weight on the 17th day after conception. Cerebral/spinal gliomas and trigeminal/spinal nerve schwannomas developed in both LEA and F344 rats at 30 weeks of age, but no nervous system tumors developed in LEC rats, the difference being statistically significant. Lung adenomas also developed in LEA and F344 rats, but not in LEC rats. Semiquantitative RT-PCR demonstrated that metallothionein (MT)1a, MT2 and O(6)-methylguanine-DNA methyltransferase (MGMT) mRNA levels in the liver of LEC rats were higher than those in F344 and LEA rats. In addition, Western blot analysis showed that MT (MT1 plus MT2) in the liver of LEC rats was also higher than that in other strains. Present results suggest that high levels of MT and/or MGMT contribute to the resistance to nitrosamine-induced carcinogenesis in LEC rats.

Role of metallothionein as a protective factor against radiation carcinogenesis

In order to elucidate the involvement of metallothionein (MT) in radiation carcinogenesis, we examined the susceptibility of MT-I/II null mice to carcinogenesis and oxidative DNA damage resulting from X-irradiation. Eight-week-old female MT-I/II null mice and wild-type mice were exposed to whole-body X-irradiation at doses of 1.0, 1.5 or 2.0 Gy once a week for 6 weeks. Incidence of thymic lymphoma was determined at 24 weeks after the first exposure to X-irradiation. The frequency of thymic lymphomas induced by X-irradiation (at 1.5 and 2.0 Gy) was significantly higher in MT-I/II null mice than in wild-type mice. In addition, although the levels of 8-hydroxy-2'-deoxyguanosine (8-OHdG) were increased in the serum and urine of both strains of mice 24 hr after exposure to a single bout of whole body X-irradiation, these increases were significantly greater in the MT-I/II null mice than in the wild-type mice. Thus, the present results suggest that MT plays a protective role against carcinogenesis and oxidative DNA damage caused by X-irradiation.

Metallothionein Toxicology: Metal Ion Trafficking and Cellular Protection

The literature is replete with reports about the involvement of metallothionein in host defense against injurious chemical, biological, and physical agents. Yet, metallothionein's functional roles are still being debated. This review addresses the issues that have left the physiological significance of metallothionein in doubt and moves on to assess the MT's importance in cell toxicology. It is evident that the protein is broadly involved in protecting cells from injury due to toxic metal ions, oxidants, and electrophiles. Attention is focused on MT's structural and chemical properties that confer this widespread role in cell protection. Particular emphasis is placed on the implications of finding that metal ion unsaturated metallothionein is commonly present in many cells and tissues and the question, how does selectivity of reaction with metallothionein take place in the cellular environment that includes large numbers of competing metal binding sites and high concentrations of protein and glutathione sulfhydryl groups?

Protective role of metallothionein in bone marrow injury caused by X-irradiation

In order to elucidate the role of metallothionein (MT) in preventing the adverse effects of X-ray irradiation, we examined the susceptibility of MT-I/II null mice to bone marrow injury caused by X-irradiation and effects of pretreatment with MT-inducing metals on X-ray injury. Eight-week-old male mice were exposed to a single bout of whole-body X-irradiation at a dose between 0.1 and 6.0 Gy. The numbers of leukocytes, reticulocytes with micronuclei (MNRET) in the blood, and polychromatic erythrocytes with micronuclei (MNPCE) in the bone marrow were determined 24 hr after X-irradiation. X-irradiation significantly decreased the total number of leukocytes in MT-I/II null mice and wild-type mice in a dose-dependent manner, but the total number of leukocytes was significantly lower in MT-I/II null mice than in wild-type mice at a low dose of irradiation, between 0.1 and 1.0 Gy. X-irradiation (0.1 and 0.5 Gy) significantly increased the appearance of MNRET and MNPCE in both strains, but the increase was greater in the MT-I/II null mice than in the wild-type mice. Additional groups of mice were pre-administered bismuth nitrate or zinc sulfate to induce MT in the bone marrow cells prior to X-irradiation; the X-ray injury was prevented by such treatments in wild-type mice only. Thus, the present results suggest that MT plays a protective role against a low dose of X-ray injury.

[Analysis of toxicity using metallothionein knockout mice]

Two research groups produced metallothionein (MT)-I/II knockout mice with null mutation of MT-I and MT-II genes. In 1993, Choo et al. produced MT-I/II knockout mice with a mixed genetic background of 129 Ola and C57BL/6 strains. Palmiter et al. also produced MT-I/II knockout mice with a genetic background of 129/Sv strain in 1994. Subsequently, MT-I/II knockout mice have been used to clarify the biological function and physiological role of MT by many research groups. We were also provided MT-I/II knockout mice from Dr. Choo (Australia). F1 hybrid mice were mated with C57BL/6, and their offspring were back-crossed to C57BL/6 for ten generations. MT-I/II knockout (MT(-/-)) mice and wild-type (MT(+/+)) mice were obtained by mating of those heterozygous (MT(+/-)) mice. We have been investigating the susceptibility of MT-I/II knockout mice to toxicity of harmful factors and some diseases. Our present studies found that MT-I/II knockout mice have an increased sensitivity to harmful metals such as cadmium, mercury, and arsenic, oxidative stress, chemical carcinogenesis and neurodegenerative diseases. These results clearly indicate that MT plays an important role in defense of these toxicities. In this review, we present our findings and summarize recent reports with MT-I/II knockout mice concerning the role of MT as a biological protective factor.

Metallothionein expression as prognostic factor for transitional cell carcinoma of bladder

OBJECTIVES

To determine whether metallothionein (MT) protein expression is associated with clinical outcomes in patients with transitional cell carcinoma (TCC) of the bladder.

METHODS

Archival pathologic radical cystectomy and transurethrally resected specimens and medical charts were reviewed for 123 patients with TCC. Patients were divided into groups based on the TNM stage, tumor grade, and MT protein expression in the primary tumor. Survival and disease progression were correlated with MT expression.

RESULTS

The mean patient age was 66 years (range 41 to 92). Of the 123 tumors, 21, 13, 18, 24, 17, and 30 were pathologically staged as pTa, pT1, pT2, pT3, pT4, and pTis, respectively; 28, 15, 14, and 66 tumors had a histologic grade of X, 1, 2, and 3, respectively. On univariate analysis, TNM stage and tumor grade predicted survival and progression outcomes. MT expression was detected in 69 (56.9%) of 123 bladder cancer specimens. Greater MT protein expression was associated with worse overall survival, disease-specific survival, disease-free survival, and disease-free progression (P = 0.0004, P = 0.05, P = 0.0008, and P = 0.0005, respectively).

CONCLUSIONS

MT protein expression in the primary tumor of TCC specimens appeared to be associated with overall survival, disease-specific survival, disease-free survival, and disease-free progression. This finding requires additional validation using other data sets.

Metallothionein-III prevents gamma-ray-induced 8-oxoguanine accumulation in normal and hOGG1-depleted cells

Metallothioneins (MT) play an important biological role in preventing oxidative damage to cells. We have previously demonstrated that the efficiency of the protective effect of MT-III against the DNA degradation from oxidative damage was much higher than that of MT-I/II. As an extension of the latter investigation, this study aimed to assess the ability of MT-III to suppress 8-oxoguanine (8-oxoG), which is one of the major base lesions formed after an oxidative attack to DNA and the mutant frequency of the HPRT gene in human fibroblast GM00637 cells upon exposure to gamma-rays. We found that human MT-III expression decreased the level of 8-oxoG and mutation frequency in the gamma-irradiated cells. Using an 8-oxoguanine DNA glycosylase (OGG1)-specific siRNAs, we also found that MT-III expression resulted in the suppression of the gamma-radiation-induced 8-oxoG accumulation and mutation in the OGG1-depleted cells. Moreover, the down-regulation of MT in human neuroblastoma SKNSH cells induced by MT-specific siRNA led to a significant increase in the 8-oxoG level, after exposure to gamma-irradiation. These results suggest that under the conditions of gamma-ray oxidative stress, MT-III prevents the gamma-radiation-induced 8-oxoG accumulation and mutation in normal and hOGG1-depleted cells, and this suppression might, at least in part, contribute to the anticarcinogenic and neuroprotective role of MT-III.

Suppressed proliferative response of spleen T cells from metallothionein null mice

To investigate the role of metal-binding protein, metallothionein (MT), in lymphocyte activation, the mitogen-induced proliferation of freshly isolated spleen cells was compared among MT-I, II null, and control 129/Sv mice. Spleen cells from MT null mice exhibited a markedly reduced proliferation compared with control cells when stimulated by concanavalin A or anti-CD3(epsilon) mAb, but not by lipopolysaccharide, indicating that only the response of T cells to mitogens was suppressed in MT null mice. Flow cytometric analysis of unstimulated spleen cells demonstrated no significant difference in the relative percentages of either B220+ and CD3+ cells or CD4+ and CD8+ cells between the two strains of mice. The production of interleukin (IL)-2 by MT null spleen cells after the stimulation by anti-CD3(epsilon) mAb was lower than that of control spleen cells, especially within 24 hr after the stimulation. The addition of IL-2 recovered the proliferation of MT null spleen cells to the control level. The reduced proliferative response to mitogenic stimulation of MT null T cells was confirmed by using purified splenic T cells. These results suggest that the MT expressed at basal level in the splenocytes plays an important role in T cell mitogen-induced proliferative response, probably by positively regulating the production of IL-2.

Creating a new paradigm in nutrition research within the National Cancer Institute

Almost two decades after Doll and Peto (1981) provided evidence that one third of cancer deaths are related to diet, it remains unclear which dietary components may be key in cancer prevention. Although the complexity of the diet can become overwhelming, the National Cancer Institute (NCI) of the National Institutes of Health (NIH) has remained steadfast in its commitment to defining the roles that diet and nutrition have in the development of cancer and has provided increased research and training support to assist in unraveling this interrelationship. Evidence for this sustained commitment is highlighted by a fourfold increase in NCI expenditures for nutrition research and training from 1983 to 1998; this substantial increase reflects a trend that is occurring in some universities and the private sector. More than one third of the nutrition-related NCI research is funded by the Division of Cancer Prevention. Supported investigations cover the gamut from basic mechanisms of action of dietary constituents, methodology development, human metabolic studies, clinical trials of dietary modification and the chemopreventive potential of individual nutrients to population-based studies.

Cadmium-induced mRNA expression of Hsp32 is augmented in metallothionein-I and -II knock-out mice

Cadmium is toxic and carcinogenic to humans and animals. The testis and lung are the target organs for cadmium carcinogenesis. Heat shock proteins (HSPs) as well as metallothionein (MT) and glutathione (GSH) play an important role in protection against its toxicity. HSP32, also known as heme oxygenase-1, is a 32-kDa protein induced by heme, heavy metals, oxidative stresses, and heat. We investigated expression of the Hsp32 gene of various organs (the liver, lung, heart, stomach, kidney, and testis) in transgenic mice deficient in the MT-I and -II genes (MT-KO) and in control mice (MT-W) after an injection of cadmium chloride (CdCl2). Survival of MT-W mice after a subcutaneously injection of CdCl2 was higher than that of MT-KO mice, while no significant difference was observed in the level of GSH in each organ between MT-W and MT-KO mice. Northern blot analysis showed that the MT-I mRNA was more extensively induced in the liver, kidney, and heart than other organs 6 h after an injection of CdCl2 (30 micromol/kg body wt, sc). There was little increase of the MT-I mRNA in the testis when induced by CdCl2. Expression of the Hsp32 gene in the liver and kidney in response to CdCl2 was more extensively augmented in MT-KO mice than in MT-W mice. In the lung and testis, there was little induction and no augmentation in expression of the Hsp32 gene induced by CdCl2 in both MT-W and MT-KO mice. In the stomach, there was little induction of the Hsp32 mRNA in MT-W mice, but was increased in MT-KO mice. Immunohistochemical staining revealed that the HSP32 protein was strongly expressed in the kidney and liver of MT-W mice 24 h after an injection of CdCl2 (20 micromol/kg body wt, sc), while the expression of HSP32 protein was not increased in the testis. In metabolically active organs such as the liver and kidney, expression of the Hsp32 gene as well as the MT-I gene was extensively induced by cadmium in MT-W mice, and more eminently induced in MT-KO mice. We suggest that organs of low stress response to cadmium such as the testis and lung may be vulnerable target sites for cadmium toxicity and carcinogenesis.

Low susceptibility of Long-Evans Cinnamon rats to N-butyl-N-(4-hydroxybutyl)-nitrosamine-induced urinary bladder carcinogenesis and inhibitory effect of urinary copper

We studied the susceptibilities to N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN)-induced urinary bladder carcinogenesis of male Long-Evans Cinnamon (LEC), F344 and Long-Evans Agouti (LEA) rats. Male rats (n=21) were given 0.1% BBN in their drinking water from week 6, 8 and 10 for one week, and killed in week 56. The incidences of transitional cell tumors (papillomas plus carcinomas) in BBN-treated LEC and F344 rats were 12% and 76%, respectively (P<0.001, experiment 1), and those in LEC and LEA rats were 11% and 95%, respectively (P<0.001, experiment 2). When male LEC and F344 rats were given 0.1% BBN in their drinking water for 7 days, the intake of BBN and the urinary concentration of its active metabolite, N-butyl-N-(3-carboxypropyl)nitrosamine (BCPN), were higher in the LEC rats (P<0.01). The urinary pHs of untreated LEC and F344 rats were similar between week 6 and 30. The urinary copper concentration was lower in LEC rats before jaundice than in F344 rats, but its concentrations in 28- and 50-week-old LEC rats were 1.7 and 2.3 times those in F344 rats. In a two-stage carcinogenesis study using F344 rats, i.p. injections of cupric nitrilotriacetate increased urinary copper excretion, and inhibited BBN-induced bladder carcinogenesis. In a two-stage carcinogenesis study using LEC rats, oral administration of D-penicillamine decreased urinary copper excretion, and increased BBN-induced bladder cancer, although the difference was not significant. These data show that LEC rats are resistant to bladder carcinogenesis and suggest that urinary copper has a significant role in their resistance.