Relationship: 1711



p21 (CDKN1A) expression, increase leads to cell cycle, disrupted

Upstream event


p21 (CDKN1A) expression, increase

Downstream event


cell cycle, disrupted

Key Event Relationship Overview


AOPs Referencing Relationship


AOP Name Adjacency Weight of Evidence Quantitative Understanding
Histone deacetylase inhibition leading to testicular toxicity adjacent High Moderate

Taxonomic Applicability


Term Scientific Term Evidence Link
Homo sapiens Homo sapiens High NCBI
Rattus norvegicus Rattus norvegicus High NCBI
Mus musculus Mus musculus High NCBI

Sex Applicability


Sex Evidence
Unspecific High

Life Stage Applicability


Term Evidence
Not Otherwise Specified High

Key Event Relationship Description


Cell cycle regulation through p21 (cyclin dependent kinase inhibitor 1A; CDKN1A) activation is demonstrated by the interactions of p21 with cyclins [Dotto, 2000]. p21 interacts directly with cyclins through a conserved region in close to its N-terminus (amino acids 17-24; Cy1) [Dotto, 2000]. The cyclin dependent kinase inhibitor, p21 has the secondary weak cyclin binding domain near its C-terminus region (amino acids 153-159), which overlaps with its proliferating cell nuclear antigen (PCNA) binding domain [Dotto, 2000]. Kinase activity of cyclin-dependent kinase (Cdk) was inhibited by Cy1 site of p21 that is important for the interaction of p21 with cyclin-Cdk complexes [Chen, 1996]. The p21 inhibits Cdk complexes such as cyclin A/E-Cdk2 or cyclin D-Cdk4 complexes, leading to the cell cycle disruption as G1/S arrest [Chen, 1996].

Evidence Supporting this KER


Biological Plausibility


p21 has a separate cyclin-dependent kinase 2 (CDK2) binding site in its N-terminus region (amino acids 53-58) and optimal cyclin/CDK inhibition requires binding by this site as well as one of the cyclin binding sites [Dotto, 2000]. The peptide containing Cy1 site inhibited the kinase activity of cyclin E-Cdk2 and cyclin A-Cdk2 [Chen, 1996]. The p21WAF1/CIP1/sdi1 gene product inhibits the cyclin D/cdk4/6 and the cyclin E/cdk2 complexes in response to DNA-damage, resulting in G1/S arrest [Moussa, 2015, Ogryzko, 1997]. p21 inhibits cyclin-dependent kinases and regulates cell cycle to promote cell cycle arrest. Deletion of either cyclin binding site in N-terminus or C-terminus of p21, or CDK binding domain was sufficient for the kinase activity inhibition [Chen, 1996].

Empirical Evidence


  • TSA induces p21 expression leading to cell cycle arrest [Gartel, 2002].
  • The up-regulation of p21 signaling and in testicular germ cells was observed in diabetes [Kilarkaje, 2015].
  • A study investing the effects of miR-6734 that has a sequence homology with a specific region of p21WAF1/CIP1 promoter on HCT-116 colon cancer cell growth indicated that miR-6734 up-regulated p21 gene expression and induced cell cycle arrest [Kang, 2016]. This result suggests that the direct enhancement of p21 gene expression is related to the alteration of the cell cycle distribution [Kang, 2016].
  • The study of postnatal telomere indicated that dysfunction of premature telomere induces cell-cycle arrest through p21 activation in mammalian cardiomyocytes [Aix, 2016].
  • The p21WAF1/CIP1/sdi1 gene product inhibits the cyclin D/cdk4/6 and the cyclin E/cdk2 complexes in response to DNA-damage, resulting in G1/S arrest [Moussa, 2015, Ogryzko, 1997].

Uncertainties and Inconsistencies


TSA promotes apoptosis via HDAC inhibition and p53 signaling pathway activation [Deng, 2016a]. It is suggested that furazolidone induces reactive oxygen species leading to suppression of p-AKT and p21, and induction of apoptosis [Deng, 2016b]. The dual roles of p21 in cell cycle arrest and anti-apoptotic effect in the testicular germ cells of diabetic rats are suggested [Kilarkaje, 2015]. The anti-apoptotic effect of p21 is mediated by caspase-3 inhibition, which demonstrates the possibility of cell-cycle independent effect on apoptosis [Deng, 2016b]. It has been demonstrated that p21 induces apoptosis in human cervical cancer cell lines [Tsao, 1999], whereas p21 is implicated in apoptosis inhibition by blocking activation of caspase-3 or interacting with ASK1 [Gartel, 2002, Zhan, 2007]. Up-regulation of p21 is implicated in the activation of DNA damage pathways, and deletion of p21 improved stem cell function and lifespan without accelerating chromosomal instability, which indicates that p21-dependent checkpoint induction affects the longevity limit [Choudhury, 2007].

Quantitative Understanding of the Linkage


Response-response Relationship


The peptide containing cyclin-binding domain of p21 in N-terminus inhibited the kinase activity of cyclin E-Cdk2 with 296 nM of the concentration in which kinase activity is inhibited in 50% (Ki) [Chen, 1996].

The peptide containing cyclin-binding domain of p21 in C-terminus showed 32,000, 800, or >300,000 nM of Ki for inhibition of the kinase activity of cyclin E-Cdk2, cyclin A-Cdk2 or cyclin D1-Cdk4, respectively [Chen, 1996].



Known modulating factors


Known Feedforward/Feedback loops influencing this KER


Domain of Applicability


DNA replication in Xenopus was suppressed by the GST fusion protein of p21 without amino acids 17-24 or the peptide containing cyclin binding site in N-terminus of p21 protein [Chen, 1996]. P21 regulates the E2F transcriptional activity to control cell cycle in human U2OS osteosarcoma cells (Homo sapiens) [Delavaine, 1999]. Cell cycle is regulated by p21 through cyclins and CDKs in mice (Mus musculus) [Sherr CJ, 2004].  



Dotto GP (2000) p21WAF1/Cip1: more than a break to the cell cycle? Biochim Biophys Acta 1471: M43-M56

Chen J et al (1996) Cyclin-binding motifs are essential for the function of p21CIP1. Mol Cell Biol 16: 4673-4682

Moussa RS et al. (2015) Differential targeting of the cyclin-dependent kinase inhibitor, p21CIP/WAF1, by chelators with anti-proliferative activity in a range of tumor cell-types. Oncotarget 6:29694-29711

Ogryzko VV et al. (1997) WAF1 retards S-phase progression primarily by inhibition of cyclin-dependent kinases. Mol Cell Biol 17:4877-4882

Gartel AL and Tyner AL (2002) The role of the cyclin-dependent kinase inhibitor p21 in apoptosis. Mol Cancer Ther 1: 639-649

Kilarkaje N and Al-Bader MM. (2015) Diabetes-Induced Oxidative DNA Damage Alters p53-p21CIP1/Waf1 Signaling in the Rat Testis. Reproductive Sciences 22: 102–112

Kang MR et al (2016) miR-6734 up-regulates p21 gene expression and induces cell cycle arrest and apoptosis in colon cancer cells. PLoS One 11: e0160961

Aix E et al (2016) Postnatal telomere dysfunction induces cardiomyocyte cell-cycle arrest through p21 activation. J Cell Biol 213: 571-583

Deng Z et al. (2016a) Histone deacetylase inhibitor trichostatin A promotes the apoptosis of osteosarcoma cells through p53 signaling pathway activation. Int J Biol Sci 12:1298-1308

Deng S et al (2016b) P21Waf1/Cip1 plays a critical role in furazolidone-induced apoptosis in HepG2 cells through influencing the caspase-3 activation and ROS generation. Food Chem Toxicol 88: 1-12

Tsao YP et al (1999) Adenovirus-mediated p21WAF1/SDII/CIP1 gene transfer induces apoptosis of human cervical cancer cell lines. J Virology 73: 4983-4990

Zhan J et al (2007) Negative regulation of ASK1 by p21Cip1 involves a small domain that includes serine 98 that is phosphorylated by ASK1 in vivo. Mol Cell Biol 27: 3530-3541

Choudhury AR et al (2007) Cdkn1a deletion improves stem cell function and lifespan of mice with dysfunctional telomeres without accelerating cancer formation. Nat Genet 39: 99-105

Delavaine L and La Thangue NB (1999) Control of E2F activity by p21Waf1/Cip1. Oncogene 18: 5381-5392

Sherr CJ and Roberts JM (2004) Living with or without cyclins and cyclin-dependent kinases. Gene Dev 18: 2699-2711