Deregulation of cyclins D1 and E and suppression of cdk2 and cdk4 in senescent human fibroblasts
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F C Lucibello1, A Sewing, S Brüsselbach
1Institut für Molekularbiologie und Tumorforschung (IMT), Philipps-Universität Marburg, Germany.
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.
Insights
Cellular senescence involves irreversible G1 arrest. Senescent cells show reduced cdk2/cdk4 expression but elevated cyclin D1 and E, indicating gene deregulation not sufficient for DNA replication.
Area of Science:
- Cell Biology
- Molecular Biology
- Gerontology
Background:
- Cellular senescence is a state of irreversible cell cycle arrest, primarily in the G1 phase.
- Previous studies identified the absence of cyclin A, cyclin B, and cdc2 in senescent human fibroblasts.
- These known cell cycle genes do not fully explain the persistent G1 arrest.
Purpose of the Study:
- To investigate the expression patterns of G1 phase cell cycle regulators in senescent human fibroblasts.
- To identify novel genes that are deregulated during cellular senescence.
- To understand the molecular mechanisms underlying the irreversible G1 arrest.
Main Methods:
- Analysis of transcript abundance for cell cycle genes (cdk2, cdk4, cyclin D1, cyclin E) in senescent WI-38 fibroblasts.
- Quantification of cyclin D1 protein synthesis and steady-state levels.
- Assay of cyclin E-associated kinase activity.
Main Results:
- Senescent WI-38 cells exhibited significantly diminished transcripts for cdk2 and cdk4.
- Expression of cyclin D1 and cyclin E was unexpectedly elevated in senescent cells, particularly under serum starvation.
- Cyclin D1 protein levels increased 3- to 6-fold, and cyclin E-associated kinase activity decreased approximately 14-fold.
- The observed deregulation of cyclin D1 and E did not induce DNA replication in senescent cells.
Conclusions:
- Cyclins D1 and E are the first identified genes to be overexpressed or deregulated in senescent cells.
- The deregulation may stem from a missing regulatory loop controlling their expression in senescent cells.
- Altered expression of cyclin D1 and E alone is insufficient to overcome the G1 arrest characteristic of senescence.