Alterations in chromatin functions during aging in vitro

Expression of transcriptional repressor proteins mSin3A and 3B during aging and replicative senescence

Sin3 proteins have a key role in transcriptional repression mediated by histone deacetylation. Mammalian Sin3 proteins, mSin3A and 3B, act as adapter molecules which bind both to repressive transcription factors and to the methyl-CpG-binding proteins (MeCPs) and recruit histone deacetylases to assemble a multiprotein repressor complex. We have recently observed (Biochem. Biophys. Res. Commun. 252, 274-277, 1998) that the expression of mSin3A but not mSin3B protein is induced during neuronal apoptosis. The purpose of this study was to find out whether aging and replicative senescence affect the expression levels of mSin3A and 3B repressor proteins. We studied the expression levels of mSin3A and 3B mRNAs and proteins both in replicative senescence model of WI-38 fibroblasts and in liver and brain tissues of young (4-6 months) and old (26-30 months) male Wistar rats. Replicative senescence of human WI-38 fibroblasts did not affect the expression levels of mSin3A and 3B mRNAs. However, the late passage WI-38 fibroblasts showed a significant decline in the expression level of mSin3A protein. Immortalization of WI-38 fibroblasts with SV-40 transformation increased the expression level of 6.0 kb mSin3A mRNA. Aging of Wistar rats did not affect the expression levels of either mSin3A or mSin3B mRNAs in the liver and frontal cortex. Similarly, the protein levels of mSin3A and 3B were unaffected in the hippocampus, cerebellum and liver tissues during aging. These results show that aging in vivo, in contrast to replicative senescence, does not affect the expression levels of mSin3A and 3B repressor proteins. However, this does not exclude the possible age-related functional changes mediated by mSin3-histone deacetylase complexes.

Evidence that transcription changes in ageing cultures are terminal events occurring after the expression of a reduced replicative potential

There is a progressive decline in replicative capacity with increasing age as expressed in terms of percentage labelled nuclei with 3H-thymidine and altered saturation density at confluency. This expression of ageing in vitro is seen in three different lines of human embryo diploid fibroblasts, although the pattern and rate of decline is different in each case. Generalization about in vitro ageing from studies with one cell line should therefore be made with care or avoided. There was an increase in total cellular RNA content as cultures aged which was more pronounced as cells entered the senescent or terminal phase of their lifespan. This increase appeared to be accompanied by a slightly elevated uptake and incorporation of 3H-uridine per cell. Template activity of isolated nuclei was markedly reduced in very late passage or phase III cells, but did not show a progressive decline with increasing age. These studies show that there is a reduced replicative potential which is not accompanied by a detectable decline in transcription, and suggest that the altered template activity should be regarded as an effect of ageing in vitro.