ReviewMultiple roles of the cell cycle inhibitor p21CDKN1A in the DNA damage response
Introduction
Exposure to different environmental stress conditions, including radiation (like ionizing and UV radiation), may induce the formation of a variety of DNA genotoxic lesions that cells must remove in order to avoid genomic instability, and to prevent cancer formation. To this end, virtually every organism has developed highly conserved genome surveillance and signaling mechanisms, collectively known as the DNA damage response (DDR). This pathway consists of DNA damage signaling cascade (cell cycle checkpoints), and of DNA repair processes able to recognize and remove a great number of DNA lesions [1].
Among the factors involved in this pathway, p21CDKN1A protein contributes to the cell response to DNA damage, by regulating fundamental processes, like cell cycle progression, apoptosis, and transcription [2], [3]. However, participation of p21 in all these pathways may occur not only after DNA damage, but also in physiological conditions (e.g. differentiation) [4], [5]. p21CDKN1A (also known as p21Cip1/Waf11/Sdi1), the first identified inhibitor of cyclin/cyclin-dependent kinase (CDK) complexes, was independently isolated as a CDK-binding protein [6], as a growth-inhibitory gene upregulated by p53 [7], or overexpressed in senescent fibroblasts [8]. The main role of p21 in cell-cycle regulation is performed by inhibiting the activity of cyclin-CDK complexes, thanks to direct interaction through specific sequences (termed CDK and Cy motifs) in the N-terminal domain of the protein [9], [10], [11], [12]. Cell cycle progression may be also regulated by p21 independently of cyclins and CDKs, thanks to the strong affinity binding to proliferating cell nuclear antigen (PCNA) [13], [14], [15], [16], a protein playing a central role in DNA replication and repair, as well as in other processes of DNA metabolism [17], [18]. This association may interfere with PCNA-dependent enzyme activities involved in DNA synthesis [18].
In contrast with the negative cell-cycle regulation, p21 may also serve as an assembly factor for cyclin D-CDK4/6, thus promoting (only at low concentrations) cyclin D-dependent events, and downstream activation of cyclin E-CDK2 [4], [5].
Being intrinsically unstructured in solution, p21 protein may adopt an extended conformation [19], which may facilitate and explain its ability to interact with a number of different proteins involved in the above processes (Fig. 1). Intriguingly, several reports have also suggested a direct participation of p21 in DNA repair, although this aspect has remained for long time controversial, not only because of different model systems investigated, but also because multiple mechanisms involved in the post-translational regulation of p21 protein, influence its functions [20].
This review will focus on the main functions of p21 in DDR, i.e. the regulation of cell cycle progression, transcription, and apoptosis, and will emphasize on the more recent findings supporting a direct participation of p21 in the DNA repair process, also in light of the relevance of post-translational regulation of p21 protein in the overall DDR.
Section snippets
Cell-cycle regulation
p21 is the principal mediator of cell cycle arrest in response to DNA damage, not only by inactivating G1-phase cyclins/CDKs complexes, but also through other processes, which possibly include direct interaction with PCNA to inhibit DNA replication, and indirect effects mediated by interaction with other cell cycle regulators. In addition, p21 has been shown to play a role in the maintenance of G2-phase arrest, through multiple mechanisms.
The demonstration that p21 is involved in cell response
Post-translational regulation of p21 in the DNA damage response
Most of the mechanisms regulating p21 expression at the transcriptional level, have been extensively described elsewhere [2], [5], [53], and they will be not further discussed. Instead, the relevance of post-translational modifications of p21 (i.e. phosphorylation and ubiquitination) in the regulation of protein stability will be approached here, in light of the outcome of DDR. Several studies have been devoted to explaining mechanisms, both at basal level and after DNA damage, regulating p21
Conclusions and perspectives
The protective effects of p21 against DNA damage and genome instability are supported not only by the role played by p21 in multiple processes evoked by DNA damage, but also by the evidence that lack of p21 may induce tumorigenesis [160]. Direct inhibition of cell cycle progression, together with indirect effects on cell-cycle related genes, are important but not unique functions of p21 helping cells to deal with DNA damage, as well as other types of stress (e.g. oncogene activation). The
Conflict of interest statement
The authors declare that they do not have any conflict of interest in this manuscript and in the work described therein.
Acknowledgements
We apologize to those colleagues whose work could not be cited owing to space limitation. Research in the author laboratories is supported in part by MIUR grants, and by the AIRC IG grant no. 5126.
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