Long‐term exposure to chromium(VI) oxide leads to defects in sulfate transport system in chinese hamster ovary cells

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Abstract
Chromium(VI) resistant Chinese hamster ovary (CHO) cell lines were established in this study by exposing parental CHO‐K1 cells to sequential increases in CrO3 concentration. The final concentration of CrO3 used for selection was 7 μM for Cr7 and 16 μM for Cr16 cells. Cr16‐1 was a subclone derived from Cr16 cells. Next, these resistant cells were cultured in media without CrO3 for more than 6 months. The resistance of these cells to CrO3 was determined by colony‐forming ability following a 24‐h treatment. The LD50 of CrO3 for chromium(VI) resistant cells was at least 25‐fold higher than that of the parental cells. The cellular growth rate, chromosome number, and the hprt mutation frequency of these chromium(VI) resistant cells were quite similar to their parental cells. The glutathione level, glutathione S‐transferase, catalase activity, and metallothionine mRNA level in Cr7 and Cr16‐1 cells were not significantly different from their parental cells. Furthermore, Cr16‐1 cells were as sensitive as CHO‐K1 cells to free‐radical generating agents, including hydrogen peroxide, nickel chloride, and methanesulfonate methyl ester, and emetine, i.e., a protein synthesis inhibitor. The uptake of chromium(VI) and the remaining amount of this metal in these resistant and the parental cell lines were assayed by atomic absorption spectrophotometry. Experimental results indicated that a vastly smaller amount of CrO3 entered the resistant cell lines than their parental cells did. A comparison was made of the sulfate uptake abilities of CHO‐K1 and chromium(VI) resistant cell lines. These results revealed that the uptake of sulfate anion was substantially reduced in Cr7 and Cr16‐1 cells. Extracellular chloride reduced sulfate uptake in CHO‐K1 but not in Cr16‐1 cells. Therefore, the major causative for chromium(VI) resistance in these resistant cells could possibly be due to the defects in SO42‐/C1− transport system for uptake chromium(VI).