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      Regulatory roles of tankyrase 1 at telomeres and in DNA repair: suppression of T-SCE and stabilization of DNA-PKcs

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          Abstract

          Intrigued by the dynamics of the seemingly contradictory yet integrated cellular responses to the requisites of preserving telomere integrity while also efficiently repairing damaged DNA, we investigated roles of the telomere associated poly(adenosine diphosphate [ADP]-ribose) polymerase (PARP) tankyrase 1 in both telomere function and the DNA damage response following exposure to ionizing radiation. Tankyrase 1 siRNA knockdown in human cells significantly elevated recombination specifically within telomeres, a phenotype with the potential of accelerating cellular senescence. Additionally, depletion of tankyrase 1 resulted in concomitant and rapid reduction of the nonhomologous end-joining protein DNA-PKcs, while Ku86 and ATM protein levels remained unchanged; DNA-PKcs mRNA levels were also unaffected. We found that the requirement of tankyrase 1 for DNA-PKcs protein stability reflects the necessity of its PARP enzymatic activity. We also demonstrated that depletion of tankyrase 1 resulted in proteasome-mediated DNA-PKcs degradation, explaining the associated defective damage response observed; i.e., increased sensitivity to ionizing radiation-induced cell killing, mutagenesis, chromosome aberration and telomere fusion. We provide the first evidence for regulation of DNA-PKcs by tankyrase 1 PARP activity and taken together, identify roles of tankyrase 1 with implications not only for DNA repair and telomere biology, but also for cancer and aging.

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          Most cited references52

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          Tankyrase inhibition stabilizes axin and antagonizes Wnt signalling.

          The stability of the Wnt pathway transcription factor beta-catenin is tightly regulated by the multi-subunit destruction complex. Deregulated Wnt pathway activity has been implicated in many cancers, making this pathway an attractive target for anticancer therapies. However, the development of targeted Wnt pathway inhibitors has been hampered by the limited number of pathway components that are amenable to small molecule inhibition. Here, we used a chemical genetic screen to identify a small molecule, XAV939, which selectively inhibits beta-catenin-mediated transcription. XAV939 stimulates beta-catenin degradation by stabilizing axin, the concentration-limiting component of the destruction complex. Using a quantitative chemical proteomic approach, we discovered that XAV939 stabilizes axin by inhibiting the poly-ADP-ribosylating enzymes tankyrase 1 and tankyrase 2. Both tankyrase isoforms interact with a highly conserved domain of axin and stimulate its degradation through the ubiquitin-proteasome pathway. Thus, our study provides new mechanistic insights into the regulation of axin protein homeostasis and presents new avenues for targeted Wnt pathway therapies.
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            Defective telomere lagging strand synthesis in cells lacking WRN helicase activity.

            Cells from Werner syndrome patients are characterized by slow growth rates, premature senescence, accelerated telomere shortening rates, and genome instability. The syndrome is caused by the loss of the RecQ helicase WRN, but the underlying molecular mechanism is unclear. Here we report that cells lacking WRN exhibit deletion of telomeres from single sister chromatids. Only telomeres replicated by lagging strand synthesis were affected, and prevention of loss of individual telomeres was dependent on the helicase activity of WRN. Telomere loss could be counteracted by telomerase activity. We propose that WRN is necessary for efficient replication of G-rich telomeric DNA, preventing telomere dysfunction and consequent genomic instability.
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              Tankyrase, a poly(ADP-ribose) polymerase at human telomeres.

              Tankyrase, a protein with homology to ankyrins and to the catalytic domain of poly(adenosine diphosphate-ribose) polymerase (PARP), was identified and localized to human telomeres. Tankyrase binds to the telomeric protein TRF1 (telomeric repeat binding factor-1), a negative regulator of telomere length maintenance. Like ankyrins, tankyrase contains 24 ankyrin repeats in a domain responsible for its interaction with TRF1. Recombinant tankyrase was found to have PARP activity in vitro, with both TRF1 and tankyrase functioning as acceptors for adenosine diphosphate (ADP)-ribosylation. ADP-ribosylation of TRF1 diminished its ability to bind to telomeric DNA in vitro, suggesting that telomere function in human cells is regulated by poly(ADP-ribosyl)ation.
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                Author and article information

                Journal
                Aging (Albany NY)
                ImpactJ
                Aging (Albany NY)
                Impact Journals LLC
                1945-4589
                October 2010
                20 October 2010
                : 2
                : 10
                : 691-708
                Affiliations
                Department of Environmental & Radiological Health Sciences, Colorado State University, Fort Collins, CO 80523-1618, USA
                Author notes
                Corresponding author: Susan M. Bailey, PhD; sbailey@ 123456colostate.edu
                Article
                10.18632/aging.100210
                2993799
                21037379
                3b066112-8da1-4ae3-802a-5f4179afa8b6
                Copyright: © 2010 Dregalla et al.

                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited

                History
                : 12 October 2010
                : 18 October 2010
                Categories
                Research Paper

                Cell biology
                dna-pkcs,radiosensitivity,parsylation,aging,tankyrase 1,t-sce
                Cell biology
                dna-pkcs, radiosensitivity, parsylation, aging, tankyrase 1, t-sce

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