Cadmium is a persistent environmental toxicant that is associated with a variety of human diseases. Target organs of cadmium toxicity include kidney, testis, liver, prostate, lung and tissues, including muscle, skin and bone. Cadmium has also been classified as a category 1 human carcinogen by the International Agency for Research on Cancer [1
]. In addition, cadmium exposure is associated with teratogenic responses, including fetal limb malformations, hydrocephalus, and cleft palate [2
At low levels of exposure, the toxicological effects of cadmium are prevented by the activation of intracellular defense and repair systems, namely the stress response. Cadmium-induced expression of stress-responsive genes has been reported in a variety of species [6
]. Cadmium can activate transcription of many stress-responsive genes, including those that encode metallothioneins, glutathione-S-transferases (GSTs) and heat shock proteins, all of which play important roles in the resistance to metal toxicity or cellular repair. The emergence of microarray technology has enabled genome-wide investigations of gene regulation, and the subsequent identification of genes that were not previously associated with responses to cadmium exposure. For example, treatment of HeLa cells with cadmium affected the expression of more than 50 genes, out of 7,075 genes that were examined [11
]. Exposure of the human T-cell line CCRF-CEM to cadmium altered the mRNA levels of more than 100 genes in a dose- and time-dependent manner [11
]. The results obtained from these and other studies provide valuable knowledge on the ability of cadmium to alter gene expression [13
In most cases, the relationship between cadmium-induced changes in mRNA levels and the biological consequence of the alteration has not been established. Only a few cadmium-responsive genes have been tested for a role in the resistance to cadmium toxicity. Mammalian metallothioneins and the Caenorhabditis elegans cdr-1
genes are highly cadmium-inducible. Inactivation of both MT-1 and MT-2, in MT-1/2 double knockout mice, or inhibition of cdr-1
by RNA interference (RNAi) in C. elegans
results in hypersensitivity to cadmium [15
]. These results confirmed the important roles of these proteins in the defense against cadmium toxicity.
In the present study, we utilized whole genome C. elegans DNA microarrays to monitor global changes in the nematode transcription profile following cadmium exposure. Bioinformatic analysis of Gene Ontology (GO) and protein interaction networks were used to identify potentially novel pathways involved in the cadmium defense response. The biological role of the cadmium-responsive genes and the cognate pathways in the defense against cadmium toxicity were studied by inhibition of gene expression using RNAi. Genes and pathways previously associated with cadmium exposure were identified, confirming the efficacy of the study. In addition, genes and pathways not previously associated with cadmium exposure were discovered.