Telomerase Does Not Improve DNA Repair in Mitochondria upon Stress but Increases MnSOD Protein under Serum-Free Conditions

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.

Cellular aging beyond cellular senescence: Markers of senescence prior to cell cycle arrest in vitro and in vivo

“…This could be due to telomere-independent functions of telomerase (Ale-Agha et al, 2014;Chiodi & Mondello, 2012). For example, in conditions of oxidative stress, telomerase is translocated from the nucleus to the mitochondria, where it protects mitochondrial DNA and improves mitochondrial function (Ahmed et al, 2008;Martens et al, 2019). Similarly, in conditions of proteotoxic stress, telomerase prevents apoptosis (Zhou et al, 2014).…”

Increased telomerase improves motor function and alpha-synuclein pathology in a transgenic mouse model of Parkinson’s disease associated with enhanced autophagy

“…TA-65 as a highly purified extract contains at least 5% non-telomerase activator compounds ( Harley et al, 2011 ) which could also have antioxidant capacities. Another possibility is that more TERT is associated with higher MnSOD levels ( Martens et al, 2019 ) or other antioxidants such as glutathione and peroxiredoxin as described previously for cultured cells ( Indran et al, 2011 ). While we found a slight increase in the expression of MnSOD in TA-65 treated brains, this could not be confirmed on the protein level (data not shown).…”

Telomerase in Brain: The New Kid on the Block and Its Role in Neurodegenerative Diseases

“…It has been speculated that under conditions of oxidative stress, nuclear TERT could possibly interfere with DNA repair mechanisms [ 16 , 26 , 27 ]. Mitochondrial TERT is also able to upregulate antioxidant enzymes [ 10 , 28 , 29 ] thereby contributing to changes in cellular redox balance. Moreover, an improvement of cellular respiration by mitochondrial TERT localisation has been suggested as a possible mechanism of decreased ROS generation within mitochondria [ 13 ].…”