I've come across the hayflick theory in articles about human longevity. They theorise that as we age, we reach our hayflick limit & the cells start to make faulty copies...
A. Golubev, S. Khrustalev, A. Butov,
An in silico investigation into the causes of telomere length heterogeneity and its implications for the Hayflick limit, Journal of Theoretical Biology, Volume 225, Issue 2, 21 November 2003, Pages 153-170, ISSN 0022-5193, 10.1016/S0022-5193(03)00229-7.
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http://www.sciencedirect.com/science/article/pii/S0022519303002297)
Abstract: In telomerase-negative cell populations the mean telomere length (TL) decreases with increasing population doubling number (PD). A critically small TL is believed to stop cell proliferation at a cell-, age- and species-specific PD thus defining the Hayflick limit. However, positively skewed TL distributions are broad compared to differences between initial and final mean TL and strongly overlap at middle and late PD, which is inconsistent with a limiting role of TL. We used computer-assisted modelling to define what set of premises may account for the above. Our model incorporates the following concepts. DNA end replication problem: telomeres loose 1 shortening unit (SU) upon each cell division. Free radical-caused TL decrease: telomeres experience random events resulting in the loss of a random SU number within a remaining TL. Stochasticity of gene expression and cell differentiation: cells experience random events inducing mitoses or committing cells to proliferation arrest, the latter option requiring a specified number of mitoses to be passed. Cells whose TL reaches 1 SU cannot divide. The proliferation kinetics of such virtual cells conforms to the transition probability model of cell cycle. When no committing events occur and at realistic SU estimates of the initial TL, maximal PD values far exceed the Hayflick limit observed in normal cells and are consistent with the crisis stage entered by transformed cells that have surpassed the Hayflick limit. At intermediate PD, symmetrical TL distributions are yielded. Upon introduction of committing events making the ratio of the rates of proliferating and committing events (P/C) range from 1.10 to 1.25, TL distributions at intermediate PD become positively skewed, and virtual cell clones show bimodal size distributions. At P/C as high as 1.25 the majority of virtual cells at maximal PD contain telomeres with TL>1 SU. A 10% increase in P/C within the 1.10–1.25 range produces a two-fold increase in the maximal PD, which can reach values of up to 25 observed in rodent and some human cells. Increasing the number of committed mitoses from 0 to 10 can increases PD to about 50 observed in human fibroblasts. Introduction of the random TL breakage makes the shapes of TL distributions quite dissimilar from those observed in real cells. Conclusions: Telomere length decrease is a correlate of cell proliferation that cannot alone account for the Hayflick limit, which primarily depends on parameters of cell population kinetics. Free radical damage influences the Hayflick limit not through TL but rather by affecting the ratio of the rates of events that commit cells to mitoses or to proliferation arrest.
Keywords: Telomere; Hayflick's limit; Cell cycle; Cell aging; Model; Stochastic processes
David A. Juckett,
Cellular aging (The hayflick limit) and species longevity: A unification model based on clonal succession, Mechanisms of Ageing and Development, Volume 38, Issue 1, March 1987, Pages 49-71, ISSN 0047-6374, 10.1016/0047-6374(87)90110-2.
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http://www.sciencedirect.com/science/article/pii/0047637487901102)
Abstract: A model is presented which proposes a specific cause-and-effect relationship between a limited cell division potential and the maximum lifespan of humans and other mammals. It is based on the clonal succession hypothesis of Kay [1] which states that continually replicating cell beds (e.g. bone marrow, intestinal crypts, epidermis) could be composed of cells with short, well-defined division potentials. In this model, the cells of these beds are proposed to exist in an ordered hierarchy which establishes a specific sequence for cell divisions throughout the organism's lifespan. The depletion of division potential at all hierarchical levels leads to a loss of bed function and sets an intrinsic limit to species longevity. A specific hierarchy for cell proliferation is defined which allows the calculation of time to bed depletion and, ultimately, to organism mortality. The model allows the existence of a small number (n) of critical cell beds within the organism and defines organism death as the inability of any one of these beds to produce cells. The model is consistent with all major observations related to cellular and organismic aging. In particular, it links the PDLs (population doubling limit) observed for various species to their mean lifespan; it explains the slow decline in PDL as a function of age of the donor; it establishes a thermodynamically stable maximum lifespan for a disease-free population; and it can explain why tissue transplants outlive donors or hosts.
Keywords: Aging; Stem cells; Clonal succession; Hayflick limit; Lifespan; Mathematical model
Rita B. Effros,
Replicative Senescence in the Immune System: Impact of the Hayflick Limit on T-Cell Function in the Elderly, The American Journal of Human Genetics, Volume 62, Issue 5, May 1998, Pages 1003-1007, ISSN 0002-9297, 10.1086/301845.
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http://www.sciencedirect.com/science/article/pii/S0002929707615183)
Keywords: Senescence; Immune system; Hayflick limit; T cells; Aging; Replicative senescence
Hans J. Bremermann,
Reliability of proliferation controls. The Hayflick limit and its breakdown in cancer, Journal of Theoretical Biology, Volume 97, Issue 4, 21 August 1982, Pages 641-662, ISSN 0022-5193, 10.1016/0022-5193(82)90364-2.
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http://www.sciencedirect.com/science/article/pii/0022519382903642)
Abstract: This paper presents a new theory of the Hayflick limit and its role in cancer. The Hayflick limit is identified as a fail-safe mechanism that limits to harmless size descendent clones of cells in which normal proliferation controls have broken down. Malignancy arises when the Hayflick limit is inactivated. It is argued that the Hayflick limit is due to differentiation towards a non-proliferating state. Redundant developmental clocks are envisioned as the mechanism. Chemical carcinogens and promoters can interfere with these clocks. Also, viral gene products and integration of viral DNA can stop the developmental clock and lead to malignant transformation in cells that have already suffered mutations in their normal regulatory mechanisms that control proliferation. Viral transformation can be understood as a viral strategy of survival and transmission to a new host. Malignant clones may constitute a niche for many slow viruses. Normal functioning of the Hayflick limit implies senescence of tissues due to differentiation towards a non-proliferating state. Hence, the limit may be the cause of senescence even though it is not due to an accumulation of somatic mutations.
Rita B. Effros, Roy L. Walford,
T cell cultures and the Hayflick limit, Human Immunology, Volume 9, Issue 1, January 1984, Pages 49-65, ISSN 0198-8859, 10.1016/0198-8859(84)90006-5.
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http://www.sciencedirect.com/science/article/pii/0198885984900065)
P. Naveilhan, C. Baudet, W. Jabbour, D. Wion,
A theory that may explain the Hayflick limit — a means to delete one copy of a repeating sequence during each cell cycle in certain human cells such as fibroblasts, Mechanisms of Ageing and Development, Volume 75, Issue 3, September 1994, Pages 205-213, ISSN 0047-6374, 10.1016/0047-6374(94)90010-8.
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http://www.sciencedirect.com/science/article/pii/0047637494900108)
Abstract: A model that may explain the limited division potential of certain cells such as human fibroblasts in culture is presented. The central postulate of this theory is that there exists, prior to certain key exons that code for materials needed for cell division, a unique sequence of specific repeating segments of DNA. One copy of such repeating segments is deleted during each cell cycle in cells that are not protected from such deletion through methylation of their cytosine residues. According to this theory, the means through which such repeated sequences are removed, one per cycle, is through the sequential action of enzymes that act much as bacterial restriction enzymes do — namely to produce scissions in both strands of DNA in areas that correspond to the DNA base sequence recognition specificities of such enzymes. After the first scission early in a replicative cycle, that enzyme becomes inhibited, but the cleavage of the first site exposes the closest site in the repetitive element to the action of a second restriction enzyme after which that enzyme also becomes inhibited. Then repair occurs, regenerating the original first site. Through this sequential activation and inhibition of two different restriction enzymes, only one copy of the repeating sequence is deleted during each cell cycle. In effect, the repeating sequence operates as a precise counter of the numbers of cell doubling that have occured since the cells involved differentiated during development.
Keywords: Replicative senescence; Restriction enzyme; Aging; Mitosis
Geoff Watts, Leonard
Hayflick and the limits of ageing, The Lancet, Volume 377, Issue 9783, 18-24 June 2011, Page 2075, ISSN 0140-6736, 10.1016/S0140-6736(11)60908-2.
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http://www.sciencedirect.com/science/article/pii/S0140673611609082)
Rita B. Effros, Graham Pawelec,
Replicative senescence of T cells: does the Hayflick Limit lead to immune exhaustion?, Immunology Today, Volume 18, Issue 9, September 1997, Pages 450-454, ISSN 0167-5699, 10.1016/S0167-5699(97)01079-7.
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http://www.sciencedirect.com/science/article/pii/S0167569997010797)
Abstract: Extensive in vitro research on fibroblasts has defined numerous genetic and phenotypic changes associated with replicative senescence. Identification of T-cell replicative senescence as a feature of human immunodeficiency virus (HIV) disease and ageing suggests this phenomenon merits more careful consideration by immunologists, especially with regard to chronic infection, memory and adoptive immunotherapy.
Rita B. Effros,
Impact of the Hayflick Limit on T cell responses to infection: lessons from aging and HIV disease, Mechanisms of Ageing and Development, Volume 125, Issue 2, February 2004, Pages 103-106, ISSN 0047-6374, 10.1016/j.mad.2003.11.003.
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http://www.sciencedirect.com/science/article/pii/S0047637403002227)
Abstract: Aging and HIV disease show certain immunological similarities. In both situations, control over viral infection is diminished, and there is an increase in certain types of cancer. The immune cell type responsible for controlling viral infections and cancer is the so-called CD8 or cytotoxic T cell. In elderly persons and individuals chronically infected with HIV, there are high proportions of CD8 T cells that resemble cells that reach the end stage of replicative senescence in cell culture after repeated rounds of antigen-driven proliferation. Senescent cultures are characterized by irreversible cell cycle arrest, shortened telomeres, inability to upregulate telomerase, loss of CD28 expression, and apoptosis resistance. Strategies that retard replicative senescence may, therefore, provide novel approaches to enhancing immune function during aging and HIV disease.
Keywords: T cells; Replicative senescence; Aging; HIV disease
Olivier Toussaint, Jose Remacle, Jean-François Dierick, Thierry Pascal, Christophe Frippiat, Stéphanie Zdanov, Joao Pedro Magalhaes, Véronique Royer, Florence Chainiaux,
From the Hayflick mosaic to the mosaics of ageing.: Role of stress-induced premature senescence in human ageing, The International Journal of Biochemistry & Cell Biology, Volume 34, Issue 11, November 2002, Pages 1415-1429, ISSN 1357-2725, 10.1016/S1357-2725(02)00034-1.
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http://www.sciencedirect.com/science/article/pii/S1357272502000341)
Abstract: The Hayflick limit—senescence of proliferative cell types—is a fundamental feature of proliferative cells in vitro. Various human proliferative cell types exposed in vitro to many types of subcytotoxic stresses undergo stress-induced premature senescence (SIPS) (also called stress-induced premature senescence-like phenotype, according to the definition of senescence). The known mechanisms of appearance the main features of SIPS are reviewed: senescent-like morphology, growth arrest, senescence-related changes in gene expression, telomere shortening. Long before telomere-shortening induces senescence, other factors such as culture conditions or lack of ‘feeder cells’ can trigger either SIPS or prolonged reversible G0 phase of the cell cycle. In vivo, ‘proliferative’ cell types of aged individuals are likely to compose a mosaic made of cells irreversibly growth arrested or not. The higher level of stress to which these cells have been exposed throughout their life span, the higher proportion of the cells of this mosaic will be in SIPS rather than in telomere-shortening dependent senescence. All cell types undergoing SIPS in vivo, most notably the ones in stressful conditions, are likely to participate in the tissular changes observed along ageing. For instance, human diploid fibroblasts (HDFs) exposed in vivo and in vitro to pro-inflammatory cytokines display biomarkers of senescence and might participate in the degradation of the extracellular matrix observed in ageing.
Keywords: Senescence; Oxidative stress; Telomeres; Apolipoprotein J; TGF-β1; Ageing