The cyclin-dependent kinase inhibitor p21(WAF1/CIP1) (p21) is a cell-cycle checkpoint effector and inducer of senescence, regulated by p53. Yet, evidence suggests that p21 could also be oncogenic, through a mechanism that has so far remained obscure. We report that a subset of atypical cancerous cells strongly expressing p21 showed proliferation features. This occurred predominantly in p53-mutant human cancers, suggesting p53-independent upregulation of p21 selectively in more aggressive tumour cells. Multifaceted phenotypic and genomic analyses of p21-inducible, p53-null, cancerous and near-normal cellular models showed that after an initial senescence-like phase, a subpopulation of p21-expressing proliferating cells emerged, featuring increased genomic instability, aggressiveness and chemoresistance. Mechanistically, sustained p21 accumulation inhibited mainly the CRL4-CDT2 ubiquitin ligase, leading to deregulated origin licensing and replication stress. Collectively, our data reveal the tumour-promoting ability of p21 through deregulation of DNA replication licensing machinery-an unorthodox role to be considered in cancer treatment, since p21 responds to various stimuli including some chemotherapy drugs.

Biomedical Research Foundation of the Academy of Athens 4 Soranou Ephessiou St Athens GR 11527 Greece

Biotech Research and Innovation Centre University of Copenhagen Ole Maaloes Vej 5 Copenhagen DK 2200 Denmark

Cancer and Clinical Experimental Science Units Faculty of Medicine Institute for Life Sciences Center for Proteome Research University of Southampton University Road Southampton Southampton SO17 1BJ UK

Center for Targeted Therapeutics Department of Drug Discovery and Biomedical Sciences South Carolina College of Pharmacy University of South Carolina Coker Life Science Building 715 Sumter Street Columbia South Carolina 29208 USA

Centre for Gene Regulation and Expression College of Life Sciences University of Dundee Dow Street Dundee DD1 5EH UK

Department of Medical Genetics Medical School University of Athens Thivon and Levadias Str Athens GR 11527 Greece

Faculty Institute of Cancer Sciences University of Manchester Manchester Academic Health Science Centre Wilmslow Road Manchester M20 4QL UK

Genome Integrity Unit Danish Cancer Society Research Centre Strandboulevarden 49 Copenhagen DK 2100 Denmark

Institució Catalana de Recerca i Estudis Avançats 08010 Barcelona Spain

Institute for Research in Biomedicine Barcelona Institute of Science and Technology 08028 Barcelona Spain

Institute of Molecular and Translational Medicine Faculty of Medicine and Dentistry Palacky University Hněvotínská Olomouc 1333 5 779 00 Czech Republic

Institute of Molecular Cancer Research University of Zurich Winterthurerstrasse 190 Zurich CH 8057 Switzerland

Laboratory of Cell Proliferation and Ageing Institute of Biosciences and Applications National Centre for Scientific Research 'Demokritos' Agia Paraskevi Attikis PO Box 60228 Athens GR 153 10 Greece

Molecular Carcinogenesis Group Department of Histology and Embryology School of Medicine University of Athens 75 Mikras Asias Str Athens GR 11527 Greece

Research Institute for the Study of Genetic and Malignant Disorders in Childhood Aghia Sophia Children's Hospital Thivon Str Athens GR 11527 Greece

Science for Life Laboratory Division of Translational Medicine and Chemical Biology Department of Medical Biochemistry and Biophysics Karolinska Institute Stockholm SE 171 77 Sweden

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Cellular Senescence: Molecular Targets, Biomarkers, and Senolytic Drugs

Induction, regulation and roles of neural adhesion molecule L1CAM in cellular senescence

A prototypical non-malignant epithelial model to study genome dynamics and concurrently monitor micro-RNAs and proteins in situ during oncogene-induced senescence