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Cadmium (Cd), a harmful metal, exerts severe toxic effects on various tissues such as those in the kidney, liver, lung, and bone. In particular, renal toxicity with damage to proximal tubule cells is caused by chronic exposure to Cd. However, the molecular mechanism underlying chronic Cd renal toxicity remains to be understood. In this review, we present our recent findings since we examined to search for the target molecules involved in the renal toxicity of Cd using toxicogenomics. In NRK-52E rat renal tubular epithelial cells, we found using DNA microarrays that Cd suppressed the expression of the gene encoding Ube2d4, a member of the Ube2d family. The Ube2d family consists of selective ubiquitin-conjugating enzymes associated with p53 degradation. Moreover, Cd suppressed the expressions of genes encoding all Ube2d family members (Ube2d1/2/3/4) prior to the appearance of cytotoxicity in NRK-52E cells. Cd markedly increased p53 protein level and induced p53 phosphorylation and apoptosis in the cells. In vivo studies showed that chronic Cd exposure also suppressed Ube2d family gene expression and induced p53 accumulation and apoptosis in the renal tubules of the mouse kidney. These findings suggest that Cd causes p53-dependent apoptosis due to the inhibition of p53 degradation through the down-regulation of Ube2d family genes in NRK-52E cells and mouse kidney. Thus, the Ube2d family genes may be one of the key targets of renal toxicity caused by Cd.

Metallothionein (MT) is a cysteine-rich low molecular weight protein and thought to function in the detoxification of heavy metals and reactive oxygen species. We examined the induction of MT synthesis by cilostazol, an antiplatelet drug, in several vascular component cells to find a new pharmacological effect of cilostazol in vascular system. In human coronary artery endothelial cells, cilostazol significantly increased MT-IX and MT-IIA mRNA levels after the treatment for 6 h and endogenous MT-I/II protein levels after the treatment for 24 and 48 h. In addition, cadmium cytotoxicity was prevented by cilostazol in this endothelial cells. Moreover, cilostazol increased MT-IX and MT-IIA mRNA levels in human coronary artery smooth muscle cells, human brain microvascular endothelial cells, human brain microvascular pericytes and human colon carcinoma Caco-2 cells after the treatment for 6 h. Interestingly, cilostazol also increased MT-III mRNA level in brain microvascular endothelial cells more effectively than in other vascular cells. The present results suggest that cilostazol can protect the vascular system from toxic substances such as heavy metals via MT induction in vascular cells.

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.

In many research groups including our laboratory, metallothionein (MT)-I/II null mice have been used to clarify the biological function and physiological role of MT. Recent studies with MT-I/II null mice concerning the role of MT in the toxicity and distribution of metal, oxidative stress and chemical carcinogenesis were reviewed. Some reports, including our findings, showed that MT-I/II null mice have an increased sensitivity to harmful metals such as cadmium, mercury, zinc and arsenic. Moreover, it was clarified using MT-I/II null mice that MT plays a major role in the retention of cadmium, mercury and zinc in target tissues. MT-I/II null mice were found to be much more sensitive than wild-type mice to the toxicity caused by free radical-inducing factors, which include paraquat, acetaminophen, ethanol, X-ray, ultraviolet B, carbon tetrachloride, cisplatin, doxorubicin, cerulein and streptozotocin. In addition, MT-I/II null mice were highly susceptible to skin carcinogenesis induced by 7,12-dimethylbenz[a]anthracene and bladder carcinogenesis caused by N-butyl-N-(4-hydroxybutyl)nitrosamine. These results suggest that MT is an important protective factor against metal toxicity, oxidative stress and chemical carcinogenesis.

We have reviewed studies on cadmium (Cd) toxicity in laboratory animals and cultured cells with special attention to the disruption in cellular signal transduction, involvement in apoptosis of Cd, the cellular transport system for Cd and roles of metallothionein as a protective factor against Cd. Cd affects cellular functions by perturbing signal transductions, such as protein kinase C, mitogen-activated protein kinase and cyclic AMP pathways, but how the disruption of these pathways by Cd leads to the manifestation of toxicity in vivo is largely unknown. Exposure to cadmium at relatively high and low levels causes necrosis and apoptosis, respectively, which suggests that the mode of cell death by cadmium is dependent upon its exposure level. On the other hand, utilization of Ca2+ channels, DMT1 (divalent metal transporter 1) and a novel transport system having high-affinity for Mn2+ and Cd2+ were found to act as Cd transport systems via the cellular membrane. Metallothionein-I/II-null mice are highly susceptible to renal toxicity, hepatotoxicity, bone injury, hematotoxicity and immunotoxicity caused by chronic Cd exposure. Thus, metallothionein plays an important role in detoxification of Cd toxicity.

As the last manuscript in our series of review articles on cadmium (Cd) and health effects, we reviewed research articles on epidemiologic and experimental studies on exposure levels of Cd in occupational and environmental settings in various countries, disposition and body burden of Cd, critical concentrations of Cd in the kidney of humans and animals with a focus on biomarkers for renal dysfunction, and life expectancy in Cd-polluted areas and reference areas. After this manuscript was compiled, cadmium levels in rice crops received significant attention, since the risk assessment of cadmium is now under review and discussion by the Joint Expert Committee of Food Additives and Contaminants organized by the Food Agricultural Organization and World Health Organization in 2003. We hope that the information compiled in this review may provide directions for future studies on the health risk assessment of Cd.

We reviewed studies on genotoxicity and carcinogenicity of cadmium (Cd). Salmonella typhimurium and Escherichia coli exposed to Cd did not show mutagenicity, whereas cultured mammalian cells exposed to Cd showed mutation, DNA strand breaks, and chromosomal aberrations. Carcinogenicity tests showed that exposure to Cd increased the occurrence of tumors in testis, lung, prostate, hematopoietic tissues, and injection sites. On the other hand, recent epidemiologic studies are not supportive of earlier observations on the association between Cd and prostate cancer. The US NIOSH data on a possible association between Cd and lung cancer may need reevaluation. No studies which show a positive relationship between oral Cd exposure and carcinogenesis have been reported. All available data suggest that Cd should be reassigned to IARC Group 2A (probably carcinogenic to humans) from the current Group 1.

We have reviewed earlier studies on the possible involvement of cadmium in life-cycle related diseases and the reproductive toxicity of Cd including environmental disrupting actions. Experimental studies have suggested that Cd may be involved in the aggravation of life-cycle related diseases and the occurrence of reproductive toxicity. On the other hand, epidemiological studies did not necessarily support the experimental observations. Thus, we conclude that it is necessary to investigate further to determine whether Cd is responsible for the aggravation of life-cycle related diseases or has the capability to act as an environmental endocrine disrupter in humans.