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Kalfalah F, Janke L, Schiavi A, Tigges J, Ix A, Ventura N, Boege F, Reinke H. Crosstalk of clock gene expression and autophagy in aging. Aging (Albany NY) 2017; 8:1876-1895. [PMID: 27574892 PMCID: PMC5076443 DOI: 10.18632/aging.101018] [Citation(s) in RCA: 31] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2016] [Accepted: 08/18/2016] [Indexed: 01/13/2023]
Abstract
Autophagy and the circadian clock counteract tissue degeneration and support longevity in many organisms. Accumulating evidence indicates that aging compromises both the circadian clock and autophagy but the mechanisms involved are unknown. Here we show that the expression levels of transcriptional repressor components of the circadian oscillator, most prominently the human Period homologue PER2, are strongly reduced in primary dermal fibroblasts from aged humans, while raising the expression of PER2 in the same cells partially restores diminished autophagy levels. The link between clock gene expression and autophagy is corroborated by the finding that the circadian clock drives cell-autonomous, rhythmic autophagy levels in immortalized murine fibroblasts, and that siRNA-mediated downregulation of PER2 decreases autophagy levels while leaving core clock oscillations intact. Moreover, the Period homologue lin-42 regulates autophagy and life span in the nematode Caenorhabditis elegans, suggesting an evolutionarily conserved role for Period proteins in autophagy control and aging. Taken together, this study identifies circadian clock proteins as set-point regulators of autophagy and puts forward a model, in which age-related changes of clock gene expression promote declining autophagy levels.
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Affiliation(s)
- Faiza Kalfalah
- University of Düsseldorf, Medical Faculty, Institute of Clinical Chemistry and Laboratory Diagnostics, 40225 Düsseldorf, Germany
| | - Linda Janke
- University of Düsseldorf, Medical Faculty, Institute of Clinical Chemistry and Laboratory Diagnostics, 40225 Düsseldorf, Germany.,IUF-Leibniz Research Institute for Environmental Medicine, 40225 Düsseldorf, Germany
| | - Alfonso Schiavi
- University of Düsseldorf, Medical Faculty, Institute of Clinical Chemistry and Laboratory Diagnostics, 40225 Düsseldorf, Germany.,IUF-Leibniz Research Institute for Environmental Medicine, 40225 Düsseldorf, Germany
| | - Julia Tigges
- IUF-Leibniz Research Institute for Environmental Medicine, 40225 Düsseldorf, Germany
| | - Alexander Ix
- University of Düsseldorf, Medical Faculty, Institute of Clinical Chemistry and Laboratory Diagnostics, 40225 Düsseldorf, Germany
| | - Natascia Ventura
- University of Düsseldorf, Medical Faculty, Institute of Clinical Chemistry and Laboratory Diagnostics, 40225 Düsseldorf, Germany.,IUF-Leibniz Research Institute for Environmental Medicine, 40225 Düsseldorf, Germany
| | - Fritz Boege
- University of Düsseldorf, Medical Faculty, Institute of Clinical Chemistry and Laboratory Diagnostics, 40225 Düsseldorf, Germany
| | - Hans Reinke
- University of Düsseldorf, Medical Faculty, Institute of Clinical Chemistry and Laboratory Diagnostics, 40225 Düsseldorf, Germany.,IUF-Leibniz Research Institute for Environmental Medicine, 40225 Düsseldorf, Germany
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Waldera-Lupa DM, Kalfalah F, Florea AM, Sass S, Kruse F, Rieder V, Tigges J, Fritsche E, Krutmann J, Busch H, Boerries M, Meyer HE, Boege F, Theis F, Reifenberger G, Stühler K. Proteome-wide analysis reveals an age-associated cellular phenotype of in situ aged human fibroblasts. Aging (Albany NY) 2015; 6:856-78. [PMID: 25411231 PMCID: PMC4247387 DOI: 10.18632/aging.100698] [Citation(s) in RCA: 48] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/14/2022]
Abstract
We analyzed an ex vivo model of in situ aged human dermal fibroblasts, obtained from 15 adult healthy donors from three different age groups using an unbiased quantitative proteome-wide approach applying label-free mass spectrometry. Thereby, we identified 2409 proteins, including 43 proteins with an age-associated abundance change. Most of the differentially abundant proteins have not been described in the context of fibroblasts’ aging before, but the deduced biological processes confirmed known hallmarks of aging and led to a consistent picture of eight biological categories involved in fibroblast aging, namely proteostasis, cell cycle and proliferation, development and differentiation, cell death, cell organization and cytoskeleton, response to stress, cell communication and signal transduction, as well as RNA metabolism and translation. The exhaustive analysis of protein and mRNA data revealed that 77% of the age-associated proteins were not linked to expression changes of the corresponding transcripts. This is in line with an associated miRNA study and led us to the conclusion that most of the age-associated alterations detected at the proteome level are likely caused post-transcriptionally rather than by differential gene expression. In summary, our findings led to the characterization of novel proteins potentially associated with fibroblast aging and revealed that primary cultures of in situ aged fibroblasts are characterized by moderate age-related proteomic changes comprising the multifactorial process of aging.
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Affiliation(s)
- Daniel M Waldera-Lupa
- Institute for Molecular Medicine, Heinrich-Heine-University, Düsseldorf, Germany. Molecular Proteomics Laboratory, Biomedical Research Centre (BMFZ), Heinrich-Heine-University, Düsseldorf, Germany
| | - Faiza Kalfalah
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Ana-Maria Florea
- Department of Neuropathology, Heinrich-Heine-University, Düsseldorf, and German Cancer Consortium (DKTK), German Cancer Research Center (DKFZ), Heidelberg, Germany
| | - Steffen Sass
- Institute of Computational Biology, Helmholtz Center Munich, German Research Center for Environmental Health, Neuherberg, Germany
| | - Fabian Kruse
- Institute for Molecular Medicine, Heinrich-Heine-University, Düsseldorf, Germany. Molecular Proteomics Laboratory, Biomedical Research Centre (BMFZ), Heinrich-Heine-University, Düsseldorf, Germany
| | - Vera Rieder
- Institute for Molecular Medicine, Heinrich-Heine-University, Düsseldorf, Germany. Molecular Proteomics Laboratory, Biomedical Research Centre (BMFZ), Heinrich-Heine-University, Düsseldorf, Germany
| | - Julia Tigges
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Ellen Fritsche
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Jean Krutmann
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Hauke Busch
- Institute of Molecular Medicine and Cell Research, Albert-Ludwigs-University Freiburg, Freiburg, Germany. German Cancer Consortium (DKTK), Freiburg, Germany. German Cancer Research Center (DKFZ), D-69120, Heidelberg, Germany
| | - Melanie Boerries
- Institute of Molecular Medicine and Cell Research, Albert-Ludwigs-University Freiburg, Freiburg, Germany. German Cancer Consortium (DKTK), Freiburg, Germany. German Cancer Research Center (DKFZ), D-69120, Heidelberg, Germany
| | - Helmut E Meyer
- Department of Biomedical Research, Leibniz-Institute for Analytical Science - ISAS, Dortmund, Germany
| | - Fritz Boege
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Fabian Theis
- Department of Mathematics, Technical University Munich, Garching, Germany
| | - Guido Reifenberger
- Department of Neuropathology, Heinrich-Heine-University, Düsseldorf, and German Cancer Consortium (DKTK), German Cancer Research Center (DKFZ), Heidelberg, Germany
| | - Kai Stühler
- Institute for Molecular Medicine, Heinrich-Heine-University, Düsseldorf, Germany. Molecular Proteomics Laboratory, Biomedical Research Centre (BMFZ), Heinrich-Heine-University, Düsseldorf, Germany
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3
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Kalfalah F, Seggewiß S, Walter R, Tigges J, Moreno-Villanueva M, Bürkle A, Ohse S, Busch H, Boerries M, Hildebrandt B, Royer-Pokora B, Boege F. Structural chromosome abnormalities, increased DNA strand breaks and DNA strand break repair deficiency in dermal fibroblasts from old female human donors. Aging (Albany NY) 2015; 7:110-122. [PMID: 25678531 PMCID: PMC4359693 DOI: 10.18632/aging.100723] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/22/2014] [Accepted: 01/01/2015] [Indexed: 06/04/2023]
Abstract
Dermal fibroblasts provide a paradigmatic model of cellular adaptation to long-term exogenous stress and ageing processes driven thereby. Here we addressed whether fibroblast ageing analysedex vivo entails genome instability. Dermal fibroblasts from human female donors aged 20-67 years were studied in primary culture at low population doubling. Under these conditions, the incidence of replicative senescence and rates of age-correlated telomere shortening were insignificant. Genome-wide gene expression analysis revealed age-related impairment of mitosis, telomere and chromosome maintenance and induction of genes associated with DNA repair and non-homologous end-joining, most notably XRCC4 and ligase 4. We observed an age-correlated drop in proliferative capacity and age-correlated increases in heterochromatin marks, structural chromosome abnormalities (deletions, translocations and chromatid breaks), DNA strand breaks and histone H2AX-phosphorylation. In a third of the cells from old and middle-aged donors repair of X-ray induced DNA strand breaks was impaired despite up-regulation of DNA repair genes. The distinct phenotype of genome instability, increased heterochromatinisation and (in 30% of the cases futile) up-regulation of DNA repair genes was stably maintained over several cell passages indicating that it represents a feature of geroconversion that is distinct from cellular senescence, as it does not encompass a block of proliferation.
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Affiliation(s)
- Faiza Kalfalah
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Sabine Seggewiß
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
- Institute of Human Genetics and Anthropology, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Regina Walter
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Julia Tigges
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | | | - Alexander Bürkle
- Molecular Toxicology Group, Dept. of Biology, University of Konstanz, Konstanz, Germany
| | - Sebastian Ohse
- Systems Biology of the Cellular Microenvironment Group, Institute of Molecular Medicine and Cell Research, University of Freiburg, Freiburg, Germany
| | - Hauke Busch
- Systems Biology of the Cellular Microenvironment Group, Institute of Molecular Medicine and Cell Research, University of Freiburg, Freiburg, Germany
- German Cancer Research Centre (DKFZ), Heidelberg, Germany
- German Cancer Consortium (DKTK), Freiburg, Germany
| | - Melanie Boerries
- Systems Biology of the Cellular Microenvironment Group, Institute of Molecular Medicine and Cell Research, University of Freiburg, Freiburg, Germany
- German Cancer Research Centre (DKFZ), Heidelberg, Germany
- German Cancer Consortium (DKTK), Freiburg, Germany
| | - Barbara Hildebrandt
- Institute of Human Genetics and Anthropology, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Brigitte Royer-Pokora
- Institute of Human Genetics and Anthropology, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Fritz Boege
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
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Kalfalah F, Sobek S, Bornholz B, Götz-Rösch C, Tigges J, Fritsche E, Krutmann J, Köhrer K, Deenen R, Ohse S, Boerries M, Busch H, Boege F. Inadequate mito-biogenesis in primary dermal fibroblasts from old humans is associated with impairment of PGC1A-independent stimulation. Exp Gerontol 2014; 56:59-68. [PMID: 24699405 DOI: 10.1016/j.exger.2014.03.017] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/13/2013] [Revised: 03/15/2014] [Accepted: 03/22/2014] [Indexed: 01/17/2023]
Abstract
Extrinsic skin ageing converges on the dermis, a post-mitotic tissue compartment consisting of extracellular matrix and long-lived fibroblasts prone to damage accumulation and maladaptation. Aged human fibroblasts exhibit mitochondrial and nuclear dysfunctions, which may be a cause or consequence of ageing. We report on a systematic study of human dermal fibroblasts retrieved from female donors aged 20-67 years and analysed ex vivo at low population doubling precluding replicative senescence. According to gene set enrichment analysis of genome wide array data, the most prominent age-associated change of the transcriptome was decreased expression of mitochondrial genes. Consistent with that, mitochondrial content and cell proliferation declined with donor age. This was associated with upregulation of AMP-dependent protein kinase (AMPK), increased mRNA levels of PPARγ-coactivator 1α (PGC1A) and decreased levels of NAD(+)-dependent deacetylase sirtuin 1. In the old cells the PGC1A-mediated mito-biogenetic response to direct AMPK-stimulation by AICAR was undiminished, while the PGC1A-independent mito-biogenetic response to starvation was attenuated and accompanied by increased ROS-production. In summary, these observations suggest an age-associated decline in PGC1A-independent mito-biogenesis, which is insufficiently compensated by upregulation of the AMPK/PGC1A-axis leading under baseline conditions to decreased mitochondrial content and reductive overload of residual respiratory capacity.
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Affiliation(s)
- Faiza Kalfalah
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich Heine University, Med. Faculty, Düsseldorf, Germany
| | - Stefan Sobek
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich Heine University, Med. Faculty, Düsseldorf, Germany
| | - Beatrice Bornholz
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich Heine University, Med. Faculty, Düsseldorf, Germany
| | - Christine Götz-Rösch
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Julia Tigges
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Ellen Fritsche
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Jean Krutmann
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Karl Köhrer
- Genomics & Transcriptomics Laboratory, Biomedical Research Centre (BMFZ), Heinrich Heine University, Düsseldorf, Germany
| | - René Deenen
- Genomics & Transcriptomics Laboratory, Biomedical Research Centre (BMFZ), Heinrich Heine University, Düsseldorf, Germany
| | - Sebastian Ohse
- Institute of Molecular Medicine and Cell Research, Albert-Ludwigs-University, Freiburg, Germany; Freiburg Institute for Advanced Studies - FRIAS, Albert-Ludwigs-University, Freiburg, Germany
| | - Melanie Boerries
- Institute of Molecular Medicine and Cell Research, Albert-Ludwigs-University, Freiburg, Germany; German Cancer Consortium (DKTK), Freiburg, Germany; German Cancer Research Center (DKFZ), Heidelberg, Germany; Freiburg Institute for Advanced Studies - FRIAS, Albert-Ludwigs-University, Freiburg, Germany
| | - Hauke Busch
- Institute of Molecular Medicine and Cell Research, Albert-Ludwigs-University, Freiburg, Germany; German Cancer Consortium (DKTK), Freiburg, Germany; German Cancer Research Center (DKFZ), Heidelberg, Germany; Freiburg Institute for Advanced Studies - FRIAS, Albert-Ludwigs-University, Freiburg, Germany
| | - Fritz Boege
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich Heine University, Med. Faculty, Düsseldorf, Germany.
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Tigges J, Krutmann J, Fritsche E, Haendeler J, Schaal H, Fischer JW, Kalfalah F, Reinke H, Reifenberger G, Stühler K, Ventura N, Gundermann S, Boukamp P, Boege F. The hallmarks of fibroblast ageing. Mech Ageing Dev 2014; 138:26-44. [PMID: 24686308 DOI: 10.1016/j.mad.2014.03.004] [Citation(s) in RCA: 150] [Impact Index Per Article: 15.0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/02/2013] [Revised: 03/11/2014] [Accepted: 03/18/2014] [Indexed: 12/26/2022]
Abstract
Ageing is influenced by the intrinsic disposition delineating what is maximally possible and extrinsic factors determining how that frame is individually exploited. Intrinsic and extrinsic ageing processes act on the dermis, a post-mitotic skin compartment mainly consisting of extracellular matrix and fibroblasts. Dermal fibroblasts are long-lived cells constantly undergoing damage accumulation and (mal-)adaptation, thus constituting a powerful indicator system for human ageing. Here, we use the systematic of ubiquitous hallmarks of ageing (Lopez-Otin et al., 2013, Cell 153) to categorise the available knowledge regarding dermal fibroblast ageing. We discriminate processes inducible in culture from phenomena apparent in skin biopsies or primary cells from old donors, coming to the following conclusions: (i) Fibroblasts aged in culture exhibit most of the established, ubiquitous hallmarks of ageing. (ii) Not all of these hallmarks have been detected or investigated in fibroblasts aged in situ (in the skin). (iii) Dermal fibroblasts aged in vitro and in vivo exhibit additional features currently not considered ubiquitous hallmarks of ageing. (iv) The ageing process of dermal fibroblasts in their physiological tissue environment has only been partially elucidated, although these cells have been a preferred model of cell ageing in vitro for decades.
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Affiliation(s)
- Julia Tigges
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Jean Krutmann
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Ellen Fritsche
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany
| | - Judith Haendeler
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany; Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Heiner Schaal
- Center for Microbiology and Virology, Institute of Virology, Heinrich-Heine-University, Med. Faculty, D-40225 Düsseldorf, Germany
| | - Jens W Fischer
- Institute for Pharmacology and Clinical Pharmacology, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Faiza Kalfalah
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Hans Reinke
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany; Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Guido Reifenberger
- Department of Neuropathology, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | - Kai Stühler
- Institute for Molecular Medicine, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany; Molecular Proteomics Laboratory, Centre for Biological and Medical Research (BMFZ), Heinrich-Heine-University, Düsseldorf, Germany
| | - Natascia Ventura
- Leibniz Research Institute for Environmental Medicine (IUF), Düsseldorf, Germany; Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany
| | | | - Petra Boukamp
- German Cancer Research Centre (DKFZ), Heidelberg, Germany
| | - Fritz Boege
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, Düsseldorf, Germany.
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Sobek S, Dalla Rosa I, Pommier Y, Bornholz B, Kalfalah F, Zhang H, Wiesner RJ, von Kleist-Retzow JC, Hillebrand F, Schaal H, Mielke C, Christensen MO, Boege F. Negative regulation of mitochondrial transcription by mitochondrial topoisomerase I. Nucleic Acids Res 2013; 41:9848-57. [PMID: 23982517 PMCID: PMC3834834 DOI: 10.1093/nar/gkt768] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022] Open
Abstract
Mitochondrial topoisomerase I is a genetically distinct mitochondria-dedicated enzyme with a crucial but so far unknown role in the homeostasis of mitochondrial DNA metabolism. Here, we present data suggesting a negative regulatory function in mitochondrial transcription or transcript stability. Deficiency or depletion of mitochondrial topoisomerase I increased mitochondrial transcripts, whereas overexpression lowered mitochondrial transcripts, depleted respiratory complexes I, III and IV, decreased cell respiration and raised superoxide levels. Acute depletion of mitochondrial topoisomerase I triggered neither a nuclear mito-biogenic stress response nor compensatory topoisomerase IIβ upregulation, suggesting the concomitant increase in mitochondrial transcripts was due to release of a local inhibitory effect. Mitochondrial topoisomerase I was co-immunoprecipitated with mitochondrial RNA polymerase. It selectively accumulated and rapidly exchanged at a subset of nucleoids distinguished by the presence of newly synthesized RNA and/or mitochondrial RNA polymerase. The inactive Y559F-mutant behaved similarly without affecting mitochondrial transcripts. In conclusion, mitochondrial topoisomerase I dampens mitochondrial transcription and thereby alters respiratory capacity. The mechanism involves selective association of the active enzyme with transcriptionally active nucleoids and a direct interaction with mitochondrial RNA polymerase. The inhibitory role of topoisomerase I in mitochondrial transcription is strikingly different from the stimulatory role of topoisomerase I in nuclear transcription.
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Affiliation(s)
- Stefan Sobek
- Institute of Clinical Chemistry and Laboratory Diagnostics, Heinrich-Heine-University, Med. Faculty, D-40225 Düsseldorf, Germany, Laboratory of Molecular Pharmacology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892, USA, Center for Physiology and Pathophysiology, Institute of Vegetative Physiology, University of Köln, D-50931 Köln, Germany, Center for Molecular Medicine Cologne, University of Köln, D-50931 Köln, Germany, Cologne Excellence Cluster on Cellular Stress Responses in Aging-associated Diseases, University of Köln, D-50931 Köln, Germany, Department of Pediatrics, Med. Faculty, University of Köln, D-50931 Köln, Germany and Center for Microbiology and Virology, Institute of Virology, Heinrich-Heine-University, Med. Faculty, D-40225 Düsseldorf, Germany
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