Deregulation of cyclins D1 and E and suppression of cdk2 and cdk4 in senescent human fibroblasts

Abstract

The state of cellular senescence is characterised by an irreversible arrest in the G1 phase of the cell cycle. It has previously been shown that three cell cycle genes, cyclin A, cyclin B and cdc2, are not expressed in senescent human fibroblasts. All three gene products have functions after S-phase entry, so that their suppression cannot explain the irreversible G1 arrest. Here, we report that the abundance of transcripts from two other cell cycle genes, cdk2 and cdk4, thought to act during G1-->S progression, is significantly diminished in senescent cells of the diploid human fibroblast line WI-38. Surprisingly, two other cyclins, D1 and E, behave in a completely different way, in that their expression is elevated in senescent cells, especially under conditions of serum starvation. Both the synthesis and the steady-state level of cyclin D1 protein were also found to be markedly higher in senescent cells (3- to 6-fold). Cyclins D1 and E are thus the first genes shown to be overexpressed or deregulated in senescent cells. It is tempting to speculate that this deregulation may be due to the absence, in senescent cells, of a regulatory loop that would normally control their expression. This is supported by our finding that cyclin E-associated kinase activity in senescent cells is reduced approx. 14-fold. Our data also suggest that the deregulated expression of cyclin D1 and E is not sufficient to drive senescent cells into DNA replication.