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      Physiological and pathological consequences of cellular senescence

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          Abstract

          Cellular senescence, a permanent state of cell cycle arrest accompanied by a complex phenotype, is an essential mechanism that limits tumorigenesis and tissue damage. In physiological conditions, senescent cells can be removed by the immune system, facilitating tumor suppression and wound healing. However, as we age, senescent cells accumulate in tissues, either because an aging immune system fails to remove them, the rate of senescent cell formation is elevated, or both. If senescent cells persist in tissues, they have the potential to paradoxically promote pathological conditions. Cellular senescence is associated with an enhanced pro-survival phenotype, which most likely promotes persistence of senescent cells in vivo. This phenotype may have evolved to favor facilitation of a short-term wound healing, followed by the elimination of senescent cells by the immune system. In this review, we provide a perspective on the triggers, mechanisms and physiological as well as pathological consequences of senescent cells.

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          Most cited references80

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          Senescence and tumour clearance is triggered by p53 restoration in murine liver carcinomas.

          Although cancer arises from a combination of mutations in oncogenes and tumour suppressor genes, the extent to which tumour suppressor gene loss is required for maintaining established tumours is poorly understood. p53 is an important tumour suppressor that acts to restrict proliferation in response to DNA damage or deregulation of mitogenic oncogenes, by leading to the induction of various cell cycle checkpoints, apoptosis or cellular senescence. Consequently, p53 mutations increase cell proliferation and survival, and in some settings promote genomic instability and resistance to certain chemotherapies. To determine the consequences of reactivating the p53 pathway in tumours, we used RNA interference (RNAi) to conditionally regulate endogenous p53 expression in a mosaic mouse model of liver carcinoma. We show that even brief reactivation of endogenous p53 in p53-deficient tumours can produce complete tumour regressions. The primary response to p53 was not apoptosis, but instead involved the induction of a cellular senescence program that was associated with differentiation and the upregulation of inflammatory cytokines. This program, although producing only cell cycle arrest in vitro, also triggered an innate immune response that targeted the tumour cells in vivo, thereby contributing to tumour clearance. Our study indicates that p53 loss can be required for the maintenance of aggressive carcinomas, and illustrates how the cellular senescence program can act together with the innate immune system to potently limit tumour growth.
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            Persistent DNA damage signaling triggers senescence-associated inflammatory cytokine secretion

            Cellular senescence suppresses cancer by stably arresting the proliferation of damaged cells1. Paradoxically, senescent cells also secrete factors that alter tissue microenvironments2. The pathways regulating this secretion are unknown. We show that damaged human cells develop persistent chromatin lesions bearing hallmarks of DNA double-strand breaks (DSBs), which initiate increased secretion of inflammatory cytokines such as interleukin-6 (IL-6). Cytokine secretion occurred only after establishment of persistent DNA damage signaling, usually associated with senescence, not after transient DNA damage responses (DDR). Initiation and maintenance of this cytokine response required the DDR proteins ATM, NBS1 and CHK2, but not the cell cycle arrest enforcers p53 and pRb. ATM was also essential for IL-6 secretion during oncogene-induced senescence and by damaged cells that bypass senescence. Further, DDR activity and IL-6 were elevated in human cancers, and ATM-depletion suppressed the ability of senescent cells to stimulate IL-6-dependent cancer cell invasiveness. Thus, in addition to orchestrating cell cycle checkpoints and DNA repair, a novel and important role of the DDR is to allow damaged cells to communicate their compromised state to the surrounding tissue.
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              Extension of life-span by introduction of telomerase into normal human cells.

              Normal human cells undergo a finite number of cell divisions and ultimately enter a nondividing state called replicative senescence. It has been proposed that telomere shortening is the molecular clock that triggers senescence. To test this hypothesis, two telomerase-negative normal human cell types, retinal pigment epithelial cells and foreskin fibroblasts, were transfected with vectors encoding the human telomerase catalytic subunit. In contrast to telomerase-negative control clones, which exhibited telomere shortening and senescence, telomerase-expressing clones had elongated telomeres, divided vigorously, and showed reduced straining for beta-galactosidase, a biomarker for senescence. Notably, the telomerase-expressing clones have a normal karyotype and have already exceeded their normal life-span by at least 20 doublings, thus establishing a causal relationship between telomere shortening and in vitro cellular senescence. The ability to maintain normal human cells in a phenotypically youthful state could have important applications in research and medicine.
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                Author and article information

                Contributors
                +972-8-934-6575 , valery.krizhanovsky@weizmann.ac.il
                Journal
                Cell Mol Life Sci
                Cell. Mol. Life Sci
                Cellular and Molecular Life Sciences
                Springer Basel (Basel )
                1420-682X
                1420-9071
                31 July 2014
                31 July 2014
                2014
                : 71
                : 22
                : 4373-4386
                Affiliations
                Department of Molecular Cell Biology, The Weizmann Institute of Science, 76100 Rehovot, Israel
                Article
                1691
                10.1007/s00018-014-1691-3
                4207941
                25080110
                919436f5-621d-498d-ba8f-4bec6d3c878b
                © The Author(s) 2014

                Open AccessThis article is distributed under the terms of the Creative Commons Attribution License which permits any use, distribution, and reproduction in any medium, provided the original author(s) and the source are credited.

                History
                : 29 May 2014
                : 21 July 2014
                : 23 July 2014
                Categories
                Review
                Custom metadata
                © Springer Basel 2014

                Molecular biology
                aging,age-related disease,immune surveillance,dna damage response
                Molecular biology
                aging, age-related disease, immune surveillance, dna damage response

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