Thionein gene expression in Cd++-variants of the CHO cell: correlation of thionein synthesis rates with translatable mRNA levels during induction, deinduction, and superinduction

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Abstract
The relationship of thionein synthesis rates to translatable cytoplasmic thionein mRNA levels was investigated for the first time in a cultured cell system. Thionein synthesis was induced in Cd r , a cadmium-resistant variant of CHO, by exposure to 2 μ M CdCl 2 . Following a short (1.5 hr) lag, thionein synthesis increases to a rate that is at least 30 times the uninduced rate 7–8 hr after addition of Cd ++ . This increase is blocked by the coincident addition of actinomycin D. Cytoplasmic thionein mRNA levels, measured by translation in a modified wheat germ system, increase rapidly following induction to values approximately 25 times uninduced levels within 6–8 hr. The increases in thionein mRNA precede proportionate increases in thionein synthesis by 0.5–1.0 hr. Continued exposure to Cd ++ results in a decreased thionein synthesis rate after 8 hr. By 30 hr, the rate is one-half that seen 6–8 hr after induction. Removal of Cd ++ after 8 hr results in a rapid decrease in thionein synthesis (t 1/2 - 4 hr). Both decreases are inhibited by the addition of actinomycin. In all instances--induction, deinduction, and actinoraycin-mediated “super-induction”--translatable thionein mRNA levels and thionein synthesis rates increase, decrease, or are maintained coordinately. The results suggest that thionein synthesis in Cd r is controlled primarily by the level of translatable cytoplasmic thionein mRNA.