Abstract
Telomerase is best known for its function in maintaining telomeres but has also multiple additional, non-canonical functions. One of these functions is the decrease of oxidative stress and DNA damage due to localisation of the telomerase protein TERT into mitochondria under oxidative stress. However, the exact molecular mechanisms behind these protective effects are still not well understood. We had shown previously that overexpression of human telomerase reverse transcriptase (hTERT) in human fibroblasts results in a decrease of mitochondrial DNA (mtDNA) damage after oxidative stress. MtDNA damage caused by oxidative stress is removed via the base excision repair (BER) pathway. Therefore we aimed to analyse whether telomerase is able to improve this pathway. We applied different types of DNA damaging agents such as irradiation, arsenite treatment (NaAsO2) and treatment with hydrogen peroxide (H2O2). Using a PCR-based assay to evaluate mtDNA damage, we demonstrate that overexpression of hTERT in MRC-5 fibroblasts protects mtDNA from H2O2 and NaAsO2 induced damage, compared with their isogenic telomerase-negative counterparts. However, overexpression of hTERT did not seem to increase repair of mtDNA after oxidative stress, but promoted increased levels of manganese superoxide dismutase (MnSOD) and forkhead-box-protein O3 (FoxO3a) proteins during incubation in serum free medium as well as under oxidative stress, while no differences were found in protein levels of catalase. Together, our results suggest that rather than interfering with mitochondrial DNA repair mechanisms, such as BER, telomerase seems to increase antioxidant defence mechanisms to prevent mtDNA damage and to increase cellular resistance to oxidative stress. However, the result has to be reproduced in additional cellular systems in order to generalise our findings.
Highlights
Telomerase is best known for its role in telomere maintenance
Several studies demonstrated a protective function of telomerase/TERT on mitochondrial DNA after various stress types [3,4], as well as after transfection of a TERT mutant with a defective nuclear exclusion site (NES) resulting in an inability to leave the nucleus [6,7]
MtDNA damage levels were expressed as the relative amplification of the 11 kb long fragment, normalized against the 83 bp fragment
Summary
Various non-telomeric functions have been described for the catalytic subunit TERT (Telomerase Reverse Transcriptase) (for review see [1]). One of these non-canonical functions of TERT is the shuttling of the TERT protein from the nucleus to mitochondria under oxidative stress [2,3,4,5]. Several studies demonstrated a protective function of telomerase/TERT on mitochondrial DNA after various stress types [3,4], as well as after transfection of a TERT mutant with a defective nuclear exclusion site (NES) resulting in an inability to leave the nucleus [6,7]. While some authors reported an improved respiration, complex I and cytochrome c oxidase activity [3,8], others found an increased antioxidant defence in hTERT overexpressing cells [8,9]
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