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[Aging and cellular senescence]
1Department of Radiation Biophysics and Genetics, Kobe University School of Medicine.
Abstract:
Mechanisms of aging involve genetic programs and error accumulation. Cellular aging is an aspect of organismal aging from a point of view of age-dependent declines of tissue cells during the postreproductive aging process and a parallelism between enhanced individual and cellular aging in some genetic progeroid syndromes. Cellular senescence involves the gene-directed inhibition of replicative potential of cells. Cell fusion analysis has indicated that senescent normal and presenescent Werner syndrome cells cause the dominant suppression of DNA synthesis in the partner of either actively growing cells or any cells of the four complementation groups of immortalized human cells. Membrane proteins produced in senescent cells showed the biphasic DNA synthesis-inhibiting activity when assayed for young cells. Senescent cells showed the strong transcriptional repressions of early serum responsive genes (c-fos, c-jun, c-myc), late responsive genes of transcription factor E2F1 and cyclin E. In addition, the protein levels of CDK2 and cyclin E are also extremely low, with an increased level of the p53-dependent p21 Cip 1 protein which inhibits the kinase activity of cyclins/CDKs by forming complexes. Such characteristic molecular factors and mechanisms feature irreversible G1-arrest in cellular senescence.
Insights
Cellular senescence, a key aspect of aging, involves gene-directed cell cycle arrest. Senescent cells suppress DNA synthesis and exhibit altered gene expression, leading to irreversible G1-arrest.
Area of Science:
- Gerontology
- Cell Biology
- Molecular Biology
Background:
- Cellular aging contributes to organismal aging, marked by age-dependent tissue cell decline.
- Genetic progeroid syndromes exhibit parallels between organismal and cellular aging.
- Cellular senescence is characterized by a gene-directed inhibition of cellular replicative potential.
Purpose of the Study:
- To investigate the molecular mechanisms underlying cellular senescence.
- To identify factors within senescent cells that influence DNA synthesis and cell cycle progression.
Main Methods:
- Cell fusion experiments were used to assess the impact of senescent cells on DNA synthesis.
- Analysis of gene expression (transcriptional repression) and protein levels in senescent cells.
- Investigated the roles of specific genes and proteins like c-fos, c-jun, c-myc, E2F1, cyclin E, CDK2, and p21 Cip 1.
Main Results:
- Senescent cells, including those from Werner syndrome, dominantly suppress DNA synthesis in fused partner cells.
- Membrane proteins from senescent cells exhibit biphasic DNA synthesis-inhibiting activity.
- Senescent cells show repressed transcription of early serum-responsive genes and late genes involving E2F1 and cyclin E.
- Reduced protein levels of CDK2 and cyclin E, alongside increased p21 Cip 1, contribute to G1-arrest.
Conclusions:
- Cellular senescence is characterized by irreversible G1-arrest due to specific molecular factors and mechanisms.
- Senescent cells actively inhibit DNA synthesis in neighboring cells through secreted factors or cell-cell contact.
- The findings elucidate key molecular events driving cellular senescence and its role in aging.