Metallothionein expression and stress responses in aging human diploid fibroblasts.

Metallothioneins (MTs) are low molecular weight proteins with a high cysteine content that are inducible by heavy metals and by other conditions of environmental stress. This laboratory was investigated in human diploid fibroblasts the induction of MTs by cadmium and by dexamethasone, and the induction of heat shock proteins, as models for age-related changes in gene expression that reflect the ability of old cells to respond to environmental stress. Old cells were more sensitive to the toxic effects of CdCl2 in the concentration range 100-175 microM. Analysis of 35S-cysteine-labelled cell extracts by polyacrylamide gel electrophoresis and fluorography showed that in the absence of any inducer, old cells have a 3.7-fold increase over young cells in the basal level of MT. The rate and extent of induction of MT by CdCl2 was reduced in old cells: Exposure of old cells to 100 microM CdCl2 for 18 h resulted in MT levels about 33% of the amount in young cells. Northern blot analysis showed that the changes in MT protein levels occurred in parallel with changes in mRNA levels, which implicates transcriptional control as the origin of these aging changes. These young/old differences in MT synthesis were maintained in density-arrested cultures, indicating that the aging changes were not due to differences in the cell cycle status of these cell populations. The rate and extent of induction of a 68-kDa heat shock protein were also reduced in old cells, which showed an increase in basal, uninduced level of this protein similar to MT. In contrast, old cells retained the ability to synthesize MTs in response to dexamethasone at a rate similar to that in young cells.