Cellular senescence entails a state of an essentially irreversible proliferative arrest in which cells remain metabolically active and secrete a range of pro-inflammatory and proteolytic factors as part of the senescence-associated secretory phenotype. There are different types of senescent cells, and senescence can be induced in response to many DNA damage signals. Senescent cells accumulate in different tissues and organs where they have distinct physiological and pathological functions. Despite this diversity, all senescent cells must be able to survive in a nondividing state while protecting themselves from positive feedback loops linked to the constant activation of the DNA damage response. This capacity requires changes in core cellular programs. Understanding how different cell types can undergo extensive changes in their transcriptional programs, metabolism, heterochromatin patterns, and cellular structures to induce a common cellular state is crucial to preventing cancer development/progression and to improving health during aging. In this review, we discuss how senescent cells continuously evolve after their initial proliferative arrest and highlight the unifying features that define the senescent state.

Senescence is an irreversible withdrawal from cell proliferation that can be initiated after DNA damage-induced cell cycle arrest in G2 phase to prevent genomic instability. Senescence onset in G2 requires p53 (also known as TP53) and retinoblastoma protein (RB, also known as RB1) family tumour suppressors, but how they are regulated to convert a temporary cell cycle arrest into a permanent one remains unknown. Here, we show that a previously unrecognised balance between the cyclin-dependent kinase (CDK) inhibitor p21 and the checkpoint kinase Chk1 controls cyclin D–CDK activity during G2 arrest. In non-transformed cells, p21 activates RB in G2 by inhibiting cyclin D1 complexed with CDK2 or CDK4. The resulting G2 exit, which precedes the appearance of senescence markers, is associated with a mitotic bypass, Chk1 downregulation and reduction in the number of DNA damage foci. In p53/RB-proficient cancer cells, a compromised G2 exit correlates with sustained Chk1 activity, delayed p21 induction, untimely cyclin E1 re-expression and genome reduplication. Conversely, Chk1 depletion promotes senescence by inducing p21 binding to cyclin D1– and cyclin E1–CDK complexes and downregulating CDK6, whereas knockdown of the checkpoint kinase Chk2 enables RB phosphorylation and delays G2 exit. In conclusion, p21 and Chk2 oppose Chk1 to maintain RB activity, thus promoting the onset of senescence induced by DNA damage in G2.

Cells entering a state of senescence undergo a irreversible cell cycle arrest, associated by a set of functional and morphological changes. Senescence occurs following telomeres shortening (replicative senescence) or exposure to other acute or chronic physiologic stress signals (a phenomenon termed stasis: stress or aberrant signaling-induced senescence). In this review, I discuss the pathways of cellular senescence, the mechanisms involved and the role that these pathways have in regulating the initiation and progression of cancer. Telomere-initiated senescence or loss of telomere function trigger focal recruitement of protein sensors of the DNA double-strand breaks leading to the activation of the DNA damage checkpoint responses and the tumour suppressor gene product, p53, which in turn induces the cell-cycle inhibitor, p21(WAF1). Loss of p53 and pRb function allows continued cell division despite increasing telomere dysfunction and eventually entry into telomere crisis. Immortalisation is an essential prerequisite for the formation of a tumour cell. Therefore, a developing tumour cell must circumvent at least two proliferative barriers--cellular senescence and crisis--to achieve neoplastic transformation. These barriers are regulated by telomere shortening and by the p16(INK4a)/Rb and p53 tumour suppressor pathways. Elucidation of the genes and emerging knowledge about the regulatory mechanisms that lead to senescence and determine the pattern of gene expression in senescent cells may lead to more effective treatments for cancer.

Telomeres cap the ends of chromosomes and regulate the replicative life span of human somatic cells. Telomere function is lost upon critical shortening and a p53-dependent checkpoint that detects altered telomere states at the G1/S transition was proposed to act as a regulator of the telomere damage response. We show that telomerase-negative human fibroblasts spend more time in G2 phase as they approach senescence and this delay is associated with manifestations of telomere dysfunction and the triggering of an ATM/ATR-dependent DNA damage signal. This correlates with a partial release of telomeric proteins TRF1 and TRF2. Analysis of the consequences of TRF1 and TRF2 depletion or over-expression of mutated versions revealed that telomere uncapping or telomere replication stress also led to DNA damage signalling in G2. Progression through mitosis of these cells was associated with signs of incomplete telomere terminal processing. We also observed an increase in sister chromatid-type telomere aberrations in senescing fibroblasts indicating that defects of telomere post-replicative events increased as cells age. Our results link a post-replicative damage response at eroded telomeres to G2 arrest signalling and challenge the current paradigm that the checkpoint response to short telomeres occurs primarily at the G1/S transition in human cells.

Loss of telomere integrity can have two outcomes with opposite predicted effects on tumorigenesis. On the one hand, shortened telomeres in normal cells may trigger cell cycle arrest, leading to tum...

The TP53 gene encodes p53, a transcription factor involved in tumor suppression. However, TP53 also encodes other protein isoforms, some of which can disrupt the tumor suppressor functions of p53 even in the absence of TP53 mutations. In particular, elevated levels of the Δ133TP53 mRNA are detected in many cancer types and can be associated with poorer disease-free survival. We investigated the mechanisms of action of the two proteins translated from the Δ133TP53 mRNA: the Δ133p53α and Δ160p53α isoforms, both of which retain the oligomerization domain of p53. We discovered that the Δ133p53α and Δ160p53α isoforms adopt an altered conformation compared to full-length p53, exposing the PAb240 epitope (RHSVVV), which is inaccessible to the PAb240 antibody in the functional conformation of p53 (reactive to PAb1620). The Δ133p53α and/or Δ160p53α isoforms form hetero-oligomers with p53, regulating the stability, the conformation and the transcriptional activity of the p53 hetero-oligomers. Under basal conditions, Δ133p53α and Δ160p53α, in complex with p53, prevent proteasome-dependent degradation leading to the accumulation of PAb240 reactive Δ133p53α/Δ160p53α/p53 hetero-oligomers without increasing p53 transcriptional activity. Conversely, depletion of endogenous Δ133p53α isoforms in human fibroblasts is sufficient to restore p53 transcriptional activity, towards p53-target genes involved in cell cycle arrest. In the DNA damage response (DDR), PAb240 reactive Δ133p53α/Δ160p53α/p53 hetero-oligomers are highly phosphorylated at Ser15 compared to PAb1620-reactive p53 complexes devoid of Δ133p53α and Δ160p53α. This suggests that PAb240-reactive p53 hetero-oligomers integrate DNA damage signals. Δ133p53α accumulation is a late event in the DDR that depends on p53, but not on its transcriptional activation. The formation of Δ133p53α and p53 complexes increases at later DDR stages. We propose that Δ133p53α isoforms regulate p53 conformation as part of the normal p53 biology, modulating p53 activity and thereby adapting the cellular response to the cell signals.