Skip navigation. The Hayflick Limit is a concept that helps to explain the mechanisms behind cellular aging. The concept states that a normal human cell can only replicate and divide forty to sixty times before it cannot divide anymore, and will break down by programmed cell death or apoptosis. The concept of the Hayflick Limit revised Alexis Carrel's earlier theory, which stated that cells can replicate themselves infinitely. Leonard Hayflick developed the concept while at the Wistar Institute in Philadelphia, Pennsylvania, in The concept of the Hayflick Limit helped scientists study the effects of cellular aging on human populations from embryonic development to death, including the discovery of the effects of shortening repetitive sequences of DNA, called telomeres, on the ends of chromosomes.
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Cellular senescence was historically discovered as a form of cellular ageing of in vitro cultured cells. It has been under the spotlight following the evidence of oncogene-induced senescence in vivo and its role as a potent tumour suppressor mechanism.
In this short review, we summarise the many cellular pathways leading to cellular senescence and we discuss the latest experimental evidence and the questions emerging in the field. Since then, for the last five decades, the original definition of cellular senescence has withstood the test of time and the subject has become more and more relevant in the contexts of ageing and cancer. The loss of cell division potential and change in morphology of human lung and skin fibroblasts were primarily proposed as a model for ageing at the cellular level.
This structure is crucial in terms of end-protection of telomeres, since G-overhangs can serve as a primer for telomerase and contribute to the formation of a structure called T-loop for telomere loop which results from the invasion of G-overhang into duplex region of telomere. It also coordinates telomere maintenance by telomerase. Telomere uncapping that results from deregulation of components of the shelterin complex also initiates a DDR.
Indeed there is already some evidence of their alteration being detected in some tumours. Cellular senescence can be induced by a number of exogenous or endogenous stresses and historically telomere shortening was proposed to be the main mechanism leading to replicative cellular senescence establishment.
However, the exact critical length of such dysfunctional telomeres is still not fully clear, while it is clear instead that it varies widely in different species ultimately confounding our understanding of what ultimately triggers DDR activation and senescence establishment. A causative link between DDR signalling at individual telomeres and senescence was analysed using a single-cell detection method to detect upstream DDR events and cell cycle checkpoint in HDFs at a near-senescent stage.
These data suggest that even a single telomere-associated DNA damage event is sufficient to induce a long-term growth arrest. As the persistence of these foci seems to be the inherent feature of senescence-inducing DNA damage, this begs the question of why this DNA damage is not repaired. Answering this question will provide the essential molecular mechanism that establishes DNA damage-induced cellular senescence and ultimately why they senesce. The realisation that telomeres are fragile sites, 17 hard to replicate chromosomal regions, unveils an additional layer of complexity and suggests that telomere shortening and DDR activation may occur also during standard telomeric DNA replication.
Ataxia Telangiectasia and Rad3-related protein ATR involved in resolving replicational stress has an important role in the suppression of telomere fragility and recombination. Therefore, it appears that additional events, and not only progressive telomere shortening caused by the inherent limits of the DNA replication machinery, may contribute to telomere shortening or DNA damage generation at telomeres, with consequent replicative senescence establishment.
Upon exposure to various oncogenic stimuli normal mammalian cells can respond by activating DNA damage response DDR pathway. This response may commit cells to programmed cell death apoptosis in some cases 25 or may induce them to enter cellular senescence. Oncogene-induced cellular senescence OIS , also called premature senescence, was first observed in normal fibroblasts by the ectopic overexpression of oncogene H-RAS G12V expression. Further studies into these key observations lead to the proposal of DDR activation as a critical barrier to tumourigenesis, by forcing cells to stop their aberrant proliferation.
However, the requirements and dependency of these key players of both pathways, and their relative contributions seem to be dependent on the type of stress and cellular context. These data indicate that multiple pathways may be involved in senescence. While involvement of the p53 axis in human fibroblasts through the activation of DDR pathway has been well-demonstrated, in murine cells both ARF and p53 appear to be important in OIS establishment as inactivation of either p53 or ARF bypasses senescence in murine systems.
Thus different mechanisms of senescence establishment and maintenance may make different contributions in different species. Murine models remain one of the best systems to prove causality of the mechanisms that in human samples can often only be proposed. However, mice retain their own specificities that occasionally must be taken into account.
Indeed, ATM in some murine models seems to display a more limited role in Ras-induced senescence compared to human systems. This occurs in the absence of DNA replication. Crucially, ATM inactivation fails to rescue senescence. Such a compound has recently been demonstrated to counteract ATR signalling in yeast. A hallmark of senescent cells is the condensation of chromatin. OIS cells display senescence-associated heterochromatin foci SAHF , which have been proposed to repress proliferative E2F-target genes 38 involving lysine 9 methylation of histone H3 and complex formation with HP1 at their promoters.
Indeed both cultured cells and human tumours can proliferate and display proliferative markers despite SAHF formation. The senescence condition is also associated with the activation of the expression of a number of inflammation-associated genes.
The production of ROS is an inevitable biochemical consequence of oxygen metabolism, which is essential for the life of aerobic species. Antioxidant systems, via both enzymatic superoxide dismutases, catalases, peroxidases and non-enzymatic small molecules like vitamin C, glutathione defenses, maintain a controlled balance against oxidative stress within the cell by conversion of such oxidants into harmless more reduced molecular species.
It has been proposed that both the rates of telomere shortening and replicative senescence can be modulated by simply modifying the amount of oxidative stress, 48 which leads to DNA breaks accumulating at telomeres. The chemical conversions of highly reactive superoxide species to a less reactive but more soluble hydrogen peroxide make it easier to penetrate through membranes quickly working as a secondary messenger signalling molecule.
However, the precise mechanism and downstream paths will need further investigative efforts. Oxidation of nucleotide pools by ROS has also been recently proposed to control senescence induction. Apparently, ROS is a mediator of OIS and participates both in the initiation and further maintenance of the senescence status of the cells.
However, there are many unanswered questions. Does the source of ROS that is activated upon oncogene activation and the one that locks the cells in the senescent state remain the same? Most importantly, how do ROS impact on cell-cycle progression? Most of the techniques to visualise or quantify ROS are limited by several restrictive factors, one of which is their short half-life. Moreover, most of the probes used for detection or quantification also promote further free radical production increasing background noise to signal ratio.
From the discovery of cellular senescence in cultured cells to the observation of its in vivo accumulation in various human pre-malignant lesions, 69 there is mounting evidence suggesting that senescence is a powerful natural anti-tumour mechanism. However, the cautionary marks that need to be considered are that the proposed Mdm2 inhibitor and p53 activator Nutlin3 may, reportedly, trigger a DNA damage response upon treatment in cancer cells by slowing DNA repair.
The mechanisms underlying the bypass of senescence response in the progression of tumours as well as the identification of multiple biomarkers in tissues will pave the road for successful clinical decisions. Such bioinformatic analysis could improve our understanding in tumour prognosis or response to the treatment. On the other hand, recent studies reveal a dark side of cellular senescence, which is associated with the secreted inflammatory factors, and may alter the microenvironment in the favour of tumour progression.
DDR appears to be instrumental not only in the establishment, but also in the maintenance of several aspects of the senescence programme Fig. Since widely used traditional cancer therapy relies on destruction of tumours by cytotoxic treatment, understanding the details of OIS within the context of DDR signalling may provide us invaluable information for translating our basic research knowledge into successful clinical outcomes.
According to the three-stage carcinogenesis model proposed previously for Ras-induced tumours using a dose-dependent mouse model, low levels of Ras activation promote cellular proliferation and are neither sufficient for cellular transformation nor senescence. The second stage leads to increasing the levels of Ras, and turns hyperplasic lesions to the oncogenic threshold signal, which activates DDR providing senescence barrier 74 The third stage is the bypass of senescence as a result of inactivation of tumour suppressor pathways Fig.
In line with this model, two recent studies showed that the p a well-known downstream target of DDR pathway- is activated only when oncogene activation signalling reaches a critical threshold in these activated-kras mice model settings. However, in the later stage malignant tumour cells, reactivated p53 eliminates cells. Previous analysis of physiological levels of expression of kRas G12D mouse models questioned the presence of OIS as an artefact of kRas overexpression. At low levels of oncogenic signalling, either the signal amplification of the oncogene or the loss of negative regulators of the oncogene is 77 necessary for the development of the senescence phenotype.
Combination targeted therapy, which keeps the senescence associated secretory phenotype under control and restores tumour suppressors, could be a promise for future. Main roads to cellular senescence. While telomere maintenance is a critical regulator of replicative senescence, multiple pathways can lead to DDR activation and oncogene-induced cellular senescence. Dose-dependent oncogenic signalling outcomes.
While acquired Ras mutations are pro-growth signals that promote proliferation, the level of activation is an important predictor in terms of the outcome in vivo. National Center for Biotechnology Information , U. Sponsored Document from. Eur J Cancer. Author information Article notes Copyright and License information Disclaimer.
This document may be redistributed and reused, subject to certain conditions. This article has been cited by other articles in PMC. Abstract Cellular senescence was historically discovered as a form of cellular ageing of in vitro cultured cells.
Introduction 1. Pathways triggering the senescence programme 2. Reactive oxygen species — a cause or a consequence? Conclusions and perspectives From the discovery of cellular senescence in cultured cells to the observation of its in vivo accumulation in various human pre-malignant lesions, 69 there is mounting evidence suggesting that senescence is a powerful natural anti-tumour mechanism.
Open in a separate window. Conflict of interest statement None declared. Acknowledgements F. References 1. Hayflick L. The serial cultivation of human diploid cell strains. Exp Cell Res. The limited in vitro lifetime of human diploid cell strains.
Living on a break: cellular senescence as a DNA-damage response. Nat Rev Cancer. Collado M. Senescence in tumours: evidence from mice and humans. Zakian V. Telomeres: beginning to understand the end. Griffith J. Mammalian telomeres end in a large duplex loop. Lange Td. Shelterin: the protein complex that shapes and safeguards human telomeres. Genes Dev. Celli G. Ku70 stimulates fusion of dysfunctional telomeres yet protects chromosome ends from homologous recombination.
Nat Cell Biol.
Hayflick, His Limit, and Cellular Ageing
Cellular senescence was historically discovered as a form of cellular ageing of in vitro cultured cells. It has been under the spotlight following the evidence of oncogene-induced senescence in vivo and its role as a potent tumour suppressor mechanism. In this short review, we summarise the many cellular pathways leading to cellular senescence and we discuss the latest experimental evidence and the questions emerging in the field. Since then, for the last five decades, the original definition of cellular senescence has withstood the test of time and the subject has become more and more relevant in the contexts of ageing and cancer.
File:Hayflick Limit (1).svg
The Hayflick limit , or Hayflick phenomenon , is the number of times a normal human cell population will divide before cell division stops. Hayflick demonstrated that a normal human fetal cell population will divide between 40 and 60 times in cell culture before entering a senescence phase. This finding refuted the contention by French Nobel laureate Alexis Carrel that normal cells are immortal. Each time a cell undergoes mitosis , the telomeres on the ends of each chromosome shorten slightly. Cell division will cease once telomeres shorten to a critical length.
Part 4: Towards a metrology of aging with telomeres
Avi Roy does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment. In all this while, though, only one person lived beyond the age of This has led experts to believe that there may be a limit to how long humans can live. Animals display an astounding variety of maximum lifespan ranging from mayflies and gastrotrichs, which live for 2 to 3 days, to giant tortoises and bowhead whales, which can live to years. The record for the longest living animal belongs to the quahog clam, which can live for more than years.