Replicative senescence and differentiated gene expression in cultured adrenocortical cells

We have used a differentiated endocrine cell type, the adrenocortical cell, to investigate the interrelationship of senescence and differentiation, the effects of the environment on differentiated gene expression, and the interrelationship of differentiated gene expression and proliferation. In bovine adrenocortical cells, expression of some differentiated functions is maintained to very late points in the replicative life span, whereas expression of others is lost at various times prior to senescence. There is clonal variation in the rate and extent of loss of functions. For steroid 17 alpha-hydroxylase, in situ hybridization shows that the observed decline in induction of 17 alpha-hydroxylase mRNA during senescence results from a decline in the fraction of cells expression the gene. Descendants of expressing cells in the primary cell population randomly become nonexpressing. Among clones there is a correlation between the fraction of cells expressing the gene and remaining replicative potential, although several experiments show no direct mechanism linking replicative senescence and 17 alpha-hydroxylase expression. Transfection with SV40 early region genes also dissociates the decline in growth and the change in 17 alpha-hydroxylase expression. SV40 T antigen selectively affects growth; expression of 17 alpha-hydroxylase is stabilized either in the on state, when cells are transfected early in the culture life span, or in the off state, when senescent cells are transfected. Thus, although the switching off of 17 alpha-hydroxylase expression and the loss of replicative potential are independent events, the switching process requires DNA replication. Because the switch is irreversible, changes in replicative potential occurring after the switch-off event do not affect the state of expression of the switched-off gene. Changes in differentiated cell properties and changes in replicative potential may be two facets of a general phenomenon of stochastic changes in gene expression in normal cells during senescence.