Amplification of the metallothionein‐1 and metallothionein‐2 genes in copper‐resistant hepatoma cells

The molecular basis for increased metallothionein concentrations in copperresistant hepatoma cells was examined. The copper‐resistant cell line HAC₆₀₀, which is maintained in 600 μm copper, had increased steady‐state mRNA levels for both the metallothionein‐1 (MT‐1) and the metallothionein‐2 (MT‐2) genes. Levels of mRNA were increased 11‐fold for MT‐1 and 15‐fold for MT‐2, with no significant change in α‐tubulin mRNA content. HAC₆₀₀NM cells, which are copper‐resistant cells kept in a normal copper concentration for over 1 year, also had eight‐ and tenfold increases in MT‐1 and MT‐2 mRNA levels. Nuclear run‐on assays showed that MT‐1 and MT‐2 gene transcription was increased nine‐ and eightfold in HAC₆₀₀ cells and seven‐ and tenfold in HAC₆₀₀NM cells, respectively. Southern blot analysis showed amplification of both metallothionein genes in HAC₆₀₀ and HAC₆₀₀NM cells. Thus the molecular basis of increased metallothionein in these hepatoma cells involved a stable gene amplification of both MT genes. The greater increase in metallothionein mRNA levels in HAC₆₀₀ cells relative to the changes in transcription suggests that posttranscriptional mechanisms of gene regulation may also be acting in these cells.