p21
Definition
p21, also known as p21CIP1/WAF1, is a cyclin-dependent kinase inhibitor encoded by the CDKN1A gene. It helps regulate cell-cycle arrest, especially after DNA damage or other cellular stress, and is often induced through p53-dependent signalling. In ageing biology, p21 is commonly studied as a marker and mediator of cellular senescence. [1] [2] [3]
Why It Matters in Ageing Research
p21 matters because it connects DNA damage responses, cell-cycle control, tumour suppression, tissue repair, and senescence. Increased p21 expression can help stop damaged cells from dividing, but persistent activation may also contribute to senescent-cell accumulation and reduced regenerative capacity. Like p16INK4a, p21 is most informative when interpreted with other senescence markers and tissue context. [4] [5] [6]
Common Confusion
- p21 is a senescence-associated marker, but p21 expression alone does not prove that a cell is senescent.
- p21 is not the same as p16INK4a; both regulate cell-cycle arrest, but they are encoded by different genes and respond to different signals.
- p21 can be protective in some contexts by limiting proliferation of damaged cells, while persistent activation can contribute to ageing-related dysfunction.
Related Reading
References
- Harper, J. W., Adami, G. R., Wei, N., Keyomarsi, K., & Elledge, S. J. (1993). The p21 Cdk-interacting protein Cip1 is a potent inhibitor of G1 cyclin-dependent kinases. https://doi.org/10.1016/0092-8674(93)90499-G
- El-Deiry, W. S., Tokino, T., Velculescu, V. E., et al. (1993). WAF1, a potential mediator of p53 tumor suppression. https://doi.org/10.1016/0092-8674(93)90500-P
- Sherr, C. J., & Roberts, J. M. (1999). CDK inhibitors: positive and negative regulators of G1-phase progression. https://doi.org/10.1101/gad.13.12.1501
- Campisi, J., & d'Adda di Fagagna, F. (2007). Cellular senescence: when bad things happen to good cells. https://doi.org/10.1038/nrm2233
- Hernandez-Segura, A., Nehme, J., & Demaria, M. (2018). Hallmarks of Cellular Senescence. https://doi.org/10.1016/j.tcb.2018.02.001
- Childs, B. G., Durik, M., Baker, D. J., & van Deursen, J. M. (2015). Cellular senescence in aging and age-related disease: from mechanisms to therapy. https://doi.org/10.1038/nm.4000
This glossary entry is provided for educational purposes only and does not constitute medical advice.