Lack of correlation between the inducibility of metallothionein mRNA and metallothionein protein in cadmium-exposed rodents

Abstract

Cadmium (Cd) is carcinogenic in humans and laboratory animals. Depending on the duration and route of exposure, Cd can also induce damage in the liver, kidneys and lungs. In certain tissues, metallothionein (MT) proteins are induced by Cd exposure and associated with native and acquired tolerance to the metal. Rats are generally more sensitive than mice to Cd carcinogenicity; however, sensitivity can vary markedly between different strains of the same rodent species. To further define the role of MT in Cd toxicity and Carcinogenesis, adult male Wistar rats and adult male C57 and DBA mice were treated with CdCl 2 and liver, kidney, and lung were analyzed for Cd, MT mRNA, and MT protein 24 h later. Dose-related increases in Cd were detected in the livers and kidneys of all animals tested; however, increases in pulmonary Cd were observed only in C57 mice, and only at the highest CdCl 2 dose. While hepatic Cd concentrations were similar in the rats and mice, renal Cd concentrations were similar in the rats and DBA mice but were nearly 2-fold higher in C57 mice at the highest CdCl 2 dose. Dose-related increases in MT mRNA occurred in the livers and lungs of all animals tested. Hepatic MT mRNA concentrations were highest in the rats, and C57 mice exhibited the greatest magnitude of hepatic MT mRNA induction. Dose-related increases in renal MT mRNA were also detected in both strains of mice, but between the two strains, C57 mice exhibited substantially higher levels of renal MT mRNA induction. Basal levels of renal MT mRNA were higher in the rats than in the mice, and transcription of the MT gene was not inducible in the rat kidney at any of the CdCl 2 doses used. In comparison, basal levels of pulmonary MT mRNA were similar in the rats and DBA mice, were substantially lower in C57 mice, and increases in pulmonary MT mRNA were most pronounced in the rats. Analysis of MT protein revealed dose-related increases in the livers and kidneys of all animals tested. C57 mice had the lowest basal and induced levels of hepatic MT, and basal levels of renal MT were much higher in the rats than in mice of either strain. Although dose-related increases in pulmonary MT were similar in both strains of mice, pulmonary MT levels were much lower and not inducible in the rats. Overall our experiments revealed complex profiles of Cd distribution and MT expression that varied between tissues, species and strains, and often did not directly correlate with sensitivity to damage. The results suggest that Cd distribution, inducibility of the MT gene, and levels of MT protein, must all be considered when predicting susceptibility to Cd toxicity and carcinogenicity at particular target sites.