Protracted p53-independent stimulation of p21WAF1/Cip1 fuels genomic instability by deregulating the replication licensing machinery

The cyclin-dependent kinase inhibitor p21WAF1/Cip1 is the prototype downstream effector of the tumor suppressor protein p53. Yet, evidence from human cancer and mice models, imply that p21WAF1/Cip1, under certain conditions, can exercise oncogenic activity. The mechanism behind this behavior is still obscure. Within this context we unexpectedly noticed, predominantly in p53 mutant human cancers, that a subset of highly atypical cancerous cells expressing strongly p21WAF1/Cip1 demonstrated also signs of proliferation. This finding suggests either tolerance to high p21WAF1/Cip1 levels or that p21WAF1/Cip1 per se guided a selective process that led to more aggressive off-springs. To address the latter scenario we employed p21WAF1/Cip1-inducible p53-null cellular models and monitored them over a prolonged time period, using high-throughput screening means. After an initial phase characterized by stalled growth, mainly due to senescence, a subpopulation of p21WAF1/Cip1 cells emerged, demonstrating increased genomic instability, aggressiveness and chemo-resistance. At the mechanistic level unremitted p21WAF1/Cip1 production saturates the CRL4CDT2 and SCFSkp2 ubiquitin ligase complexes reducing the turn-over of the replication licensing machinery. Deregulation of replication licensing triggered replication stress fuelling genomic instability. Conceptually, the above notion should be considered when anti-tumor strategies are designed, since p21WAF1/Cip1 responds also to p53-independent signals, including various chemotherapeutic compounds.

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Galanos P, Vougas K, Walter D, Polyzos A, Maya-Mendoza A, Haagensen EJ, Kokkalis A, Roumelioti FM, Gagos S, Tzetis M, Canovas B, Igea A, Ahuja AK, Zellweger R, Havaki S, Kanavakis E, Kletsas D, Roninson IB, Garbis SD, Lopes M, Nebreda A, Thanos D, Blow JJ, Townsend P, Sørensen CS, Bartek J, Gorgoulis VG