Metallothionein is overexpressed by hamster fibroblasts selected for growth in 15 pm folinic acid and provides a growth advantage in low folate.

DC-3F/FA3 (FA3) cells, selected for growth in folic acid-free medium containing dialyzed serum and 15 pM [6S]-folinic acid, and parental DC-3F cells were compared by mRNA differential display to identify genetic changes occurring during selection. One of the genes found to be overexpressed in FA3 cells was metallothionein II (MT-II). Northern blots using a full-length hamster MT-II cDNA probe that recognizes both MT-I and MT-II RNA showed that the steady-state level of MT mRNA was elevated at least 10-fold in FA3 cells and in two other selected clones, FA7 and FA14, as well. Southern blot analysis of HindIII-digested genomic DNA indicated that amplification of neither the MT-I nor MT-II gene had occurred, and measurements of MT mRNA decay rates in the presence of actinomycin D suggested that no changes in its half-life had taken place. Hence, overexpression was due to an increase in transcription from the normal gene complement. In FA3 cells, the MT mRNA expression level was found to be directly sensitive and inversely proportional to media folate concentrations, whereas in DC-3F cells it was not, suggesting that MT gene expression is differentially regulated in these two cell lines. Overexpression of MT-II in transfected DC-3F cells was unable to support growth in 15 pM folinic acid. However, when plated in 15 nM folinic acid, a growth rate similar to FA3 cells was observed, whereas sham-transfected controls and double transfectants expressing antisense MT-II RNA and control levels of MT-II protein ceased to grow. Hence, overexpression of MT-II provides a growth advantage in low folate.