Resistance to cadmium as a function of Caco-2 cell differentiation: role of reactive oxygen species in cadmium- but not zinc-induced adaptation mechanisms

Cadmium (Cd) is a highly toxic metal that enters the food chain. Following oral ingestion, the intestinal epithelium is the first biological barrier crossed by Cd and is also an important target tissue. In the present study, the human intestinal Caco-2 cell line was used to evaluate the impact of a low level of exposure on both undifferentiated and differentiated intestinal cells. As revealed by the LC50 values estimated with the 3-[4,5-dimethyl-2-thiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay, mature Caco-2 cells were more resistant to Cd. However, following a 24-h exposure to non-cytotoxic levels of Cd (10 mu M) or zinc (Zn, 100 mu M), threefold increases were obtained in the LC50 values of 7-day-old cells, whereas increased resistance in 21-day-old cells was observed exclusively with Zn. Induction of MT-IIa and HSP70 mRNAs was higher in undifferentiated cells and an increase in cellular glutathione (GSH) content was observed exclusively in these cell cultures. However, the results obtained with cycloheximide used for inhibiting protein synthesis and with l-buthionine sulfoximine (BSO), which inhibits GSH synthesis, revealed that protein synthesis is not a prerequisite to the development of resistance. The presence of 100 mM 3-amino-1,2,4-triazole (3AT), a catalase inhibitor, prevented Cd-induced but not Zn-induced resistance, as well as sensitized cells to Cd toxicity. These results show for the first time differences in constitutive and acquired resistance to Cd as a function of enterocytic differentiation status and suggest the involvement of different mechanisms for Cd- and Zn-induced adaptation in the intestinal cells. Redox signals may trigger Cd-induced adaptation mechanisms but pro-oxidant conditions would eliminate proliferative intestinal cells capability to develop resistance. This would be critical for Cd- but not Zn-induced mechanisms of resistance since Cd but not Zn may cause oxidative stress.