Molecular Characterization of a Novel, Cadmium-inducible Gene from the Nematode Caenorhabditis elegans

Abstract

Cadmium is an environmental contaminant that is both a human toxicant and carcinogen. To inhibit cadmium-induced damage, cells respond by increasing the expression of genes that encode stress-response proteins. We previously reported the identification of 48 cadmium-inducible mRNAs in the nematode Caenorhabditis elegans. Here we describe a new cadmium-responsive gene, designated cdr-1, whose rate and level of inducible expression parallel those of the C. elegans metallothioneins. The CDR-1 mRNA contains an open reading frame of 831 bp and encodes a predicted 32-kDa, integral membrane protein. Following cadmium exposure, cdr-1 is transcribed exclusively in intestinal cells of post-embryonic C. elegans. In vivo, the CDR-1 protein is targeted specifically to the intestinal cell lysosomes. cdr-1 transcription is significantly induced by cadmium but not by other tested stressors. These results indicate that cdr-1 expression is regulated by cadmium and in a cell-specific fashion. Inhibition of CDR-1 expression renders C. elegans susceptible to cadmium toxicity. In conclusion, cdr-1 defines a new class of cadmium-inducible genes and encodes an integral membrane, lysosomal protein. This protein functions to protect against cadmium toxicity.The transition metal cadmium is considered a serious occupational and environmental health threat. It is continuously introduced into the atmosphere through the smelting of ores and the burning of fossil fuels and is commonly found in "Superfund" clean-up sites (1-3). Humans are exposed to cadmium primarily via inhalation and the ingestion of cadmium-containing foods (4). Toxicological responses of cadmium exposure include kidney damage, respiratory diseases, neurological disorders, and lung, kidney, prostate, and testicular cancers (4).Cadmium induces intracellular damage via the (a) nonspecific inactivation/denaturation of proteins, by binding to free sulfhydryl residues; (b) displacement of zinc co-factors from a variety of proteins, including transcription factors; and (c) generation of reactive oxygen species, which ultimately oxidize DNA, proteins, and lipids. Although cadmium itself is not redox active in vivo and cannot directly catalyze the reduction of molecular oxygen, it has been suggested that the production of reactive oxygen species is a consequence of a cadmium-induced depletion of cellular glutathione or the inactivation of copper/ zinc-superoxide dismutase (5, 6).To attenuate the toxic effects of cadmium, cells respond by increasing the steady-state levels of a variety of proteins. The functions of these proteins can be broadly defined, because those that repair intracellular damage or those that remove the toxicants (e.g. cadmium, reactive oxygen species). To remove toxicants, cells activate the transcription of genes that encode proteins that are involved in scavenging reactive oxygen species, including heme oxygenase, ␥-glutamylcysteine synthetase, superoxide dismutase, catalase, glutathione peroxidase, and glucose-6-phosphate dehydrogenase (7-11). Cadmium is esse...

Molecular characterization ofnumr-1andnumr-2: genes that increase both resistance to metal-induced stress and lifespan inCaenorhabditis elegans

“…Several of the genes identified in this analysis were previously associated with the C. elegans cadmium response, including mtl-1, mtl-2 and cdr-1 (Freedman et al, 1993;Liao et al, 2002). Many of the highly inducible genes, however, had no known biological or molecular function.…”

“…Many of the genes associated with metal detoxification are actively transcribed in the nematode intestine including mtl-1, mtl-2, cdr-1, gsto-1 and hmt-1 (Burmeister et al, 2008;Freedman et al, 1993;Liao et al, 2002;Vatamaniuk et al, 2005). Exposure to cadmium and other transition metals produce significant increases in numr-1/-2 transcription in the nematode intestine.…”

“…Intestinal numr-1/-2 expression is consistent with the finding of a highly conserved GATA motif within the numr-1/-2 promoter (Table 1). Like numr-1/-2, three other wellcharacterized cadmium-responsive C. elegans genes, mtl-1, mtl-2 and cdr-1, contain GATA elements (Freedman et al, 1993;Liao et al, 2002). The GATA motif is highly over-represented in the promoters of intestinally expressed genes and the ELT-2 GATAtype transcription factor regulates the expression of the majority of 2131 Novel C. elegans metal-responsive genes these genes (McGhee et al, 2009).…”

“…The maximal level of numr-1/-2 mRNA was detected following a 24-hour exposure. As a control, changes in the steady-state levels of mRNA for the cadmium-inducible gene cdr-1 were also measured (Liao et al, 2002). Both cdr-1 and numr-1/-2 showed the same pattern of cadmium-responsive mRNA accumulation.…”

Heme Utilization in the Caenorhabditis elegans Hypodermal Cells Is Facilitated by Heme-responsive Gene-2

“…Besides CDR-1, HRG-2 is also homologous to CE22631, CE02505, CE22138, and five other putative CDRs in C. elegans, all of which contain GST-N and GST-C-terminal like metaxin domains. Although CDR-1, CDR-4, and CDR-6 have been proposed to be involved in either cadmium detoxification or longevity determination (46,47), the specific molecular functions of these proteins are unknown. We have analyzed an hrg-2 (tm3798) deletion strain in which a 502-bp region, including part of the promoter and first two exons of hrg-2, was deleted.…”