Butyrate selectively activates the metallothionein gene in teratocarcinoma cells and induces hypersensitivity to metal induction

The expression of metallothionein genes (MT-I and MT-II) was shown to be enhanced within 2 h of addition of 2.5-5 mM sodium butyrate to cultures of teratocarcinoma cells. Both undifferentiated stem cells (F9 and OC15) and differentiated cells (PSA5E and OC15 END) reacted similarly to butyrate by increased accumulation of MT mRNAs. As expected, all of the teratocarcinoma cells that were tested also responded to Zn2+ and Cd2+ by 5- to 10-fold increases in MT mRNA accumulation within 2-24 h of metal addition to the culture media. Surprisingly, MT genes in cells pretreated with butyrate were hypersensitive to metal induction, and this was demonstrated by accumulated transcript levels and by synthesis of MT protein. The maximal metal response was obtained by exposure of cells to butyrate for around 5-8 h together with 10 microM heavy metals. Metal additions to culture media over a range of concentrations and times only induced half the levels of MT mRNA that were achieved by butyrate plus metals. Butyrate enhanced the rate of accumulation of MT mRNA in response to metals, increased the sensitivity of the MT gene to metals, and protected cells from toxic effects of high concentrations of metals. The butyrate and metal ion responses were selective in that no accumulation of c-myc, c-fms, HSP-70, or AFP mRNA was detected. However, c-fos mRNA accumulated in cells exposed to toxic concentrations of metals (50 microM and higher) and this was also potentiated by butyrate treatment. These results suggest that butyrate alters the chromatin conformation of both the MT-I and MT-II genes leading to an accentuated transcriptional response to metals.