Altered expression of the twist gene in young versus senescent human diploid fibroblasts.

We have recently reported the cloning and characterization of the human twist gene (H-twist), which encodes an evolutionarily conserved helix-loop-helix transcription factor. In several species, twist has been shown to play a role in mesoderm differentiation. We now report on the differential expression of H-twist as a function of in vitro life span in human diploid fibroblasts. H-twist was found to be expressed at a relatively high level in quiescent young cells. Addition of fresh serum to young G0 cultures results in a rapid repression of H-twist mRNA expression. This repression depends on the presence of growth factors, specifically EGF, and requires protein synthesis. mRNA stability studies using either actinomycin D or alpha-amanitin indicate a similar decay rate of H-twist mRNA in both young and senescent cells, suggesting that the differential expression of H-twist in young versus old is not primarily regulated at the level of mRNA turnover.