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Oltipraz, a synthetic derivative of the cruciferous vegetable product 1,2-dithiole-3-thione, is considered as one of the most potent chemoprotectants. It modulates both cytochrome P-450 (CYP) and glutathione S-transferase expression and activities in rat tissues. Its effects, however, are variable according to the enzyme, tissue, and species. We show here that, as previously found in rat lung and kidney, CYP1A1 is inducible by oltipraz in both rat intestine and Caco-2 cells, a cell line originated from a human colon adenocarcinoma. In these cells, a 50 microm oltipraz treatment increased CYP1A1 mRNA ( approximately 30-fold), protein and activity. mRNA level was augmented as early as 2 h after the beginning of treatment, suggesting a transcriptional activation, and was maximal between 8 and 12 h. Transient transfection of Caco-2 cells with constructs containing different sizes of the 5'-flanking region of the CYP1A1 gene upstream of the luciferase reporter gene showed an increase in luciferase activity in oltipraz-treated cells, which correlates with the presence of the xenobiotic responsive element (XRE). Furthermore we demonstrated that resveratrol, an antagonist of the aryl hydrocarbon (Ah) receptor, inhibited the induction of both CYP1A1 promoter activity and mRNA by oltipraz, supporting the involvement of the Ah receptor in this induction. In an attempt to further characterize the mechanism of CYP1A1 induction, we showed a rapid increase in intracellular calcium concentration upon treatment of Caco-2 cells with oltipraz. Moreover, the effect of this compound on CYP1A1 was strongly abolished in the presence of BAPTA-AM, a well known chelator of intracellular calcium, and 2-aminoethyl diphenylborate, an inhibitor of store-operated calcium channels. These results bring the first demonstration that oltipraz activates transcription of the CYP1A1 gene through the Ah receptor-XRE pathway in Caco-2 cells and that CYP1A1 induction relies upon an increase of intracellular calcium concentration.

The human multidrug-resistance protein (MRP1), known to mediate cellular efflux of a wide range of xenobiotics, including anticancer drugs, has also been shown to transport antimony, thereby conferring resistance to this heavy metal. The aim of the present study was to investigate whether other cytotoxic metals could be handled by MRPI using MRP1-overexpressing lung tumor GLC4/Sb30 cells. Such cells were found to be 3.4-, 12.7- and 16.3-fold more resistant than parental GLC4 cells to mercuric ion, arsenite and arsenate, respectively, whereas they remained sensitive to other cytotoxic metals tested such as copper, chromium, cobalt or aluminium. MK571, a potent inhibitor of MRP1 activity, almost totally reversed resistance of GLC4/Sb30 cells to mercuric ions and arsenic while it did not significantly alter sensitivity of GLC4 cells to metals. Arsenate-treated GLC4/Sb30 cells were found to poorly accumulate arsenic through increased MK571-inhibitable efflux of the metal. Arsenate, however, failed to alter MRP1-mediated transport of known MRP1 substrates such as calcein and vincristine. In conclusion, these findings demonstrated that MRP1 likely handled some, but not all, cytotoxic metals such as arsenic and mercuric ions in addition to antimony, therefore resulting in reduced toxicity of these compounds towards MRP1-overexpressing cells.