Adaptive anti-oxidative responses to chronic exposure to stress-signaling molecule, oxidized cell-free DNA, in rat neural cells

Stress is an adaptation reaction to harmful environment changes. Both oxidized and non-oxidized cell-free DNA (oxo-cfDNA and no-cfDNA) liberating from damaged cells possesses a characteristic of stress-molecules. CfDNA molecules modified by oxidation exhibit more oxidative (increasing 8-oxodG in DNA) and antioxidative (enhanced transcription of NRF2 and NRF2-dependent genes) activities than no-cfDNA in non-neural cell cultures. Whether the oxo-cfDNA molecules affect brain cells under different regimens of treatment that may mimic multiple, chronically applied stressful insults remained unclear. We studied if multiple stimulations of nervous system cells with oxidized cfDNA may further augment the DNA oxidative damage by inducing 8-oxodG modifications, and increase antioxidant Hmox1 gene expression and the NRF2 protein level in a primary cell culture of rat cerebellum (RCC). We found that daily treatment of RCC with oxo-cfDNA for 3 days increased accumulation of 8-oxodG in intracellular DNA, activated transcription of Hmox1 gene and enhanced the NRF2 protein content. However, after eleven-day daily treatment, the opposite effect was observed, in which 8-oxodG level in intracellular DNA and Hmox1 gene expression and NRF2 protein level in cerebellar cells were significantly decreased. Therefore, under subchronic, 3-day regimen of treatment, oxidized cfDNA further augmented accumulation of 8-oxodG in cellular DNA and promoted activation of the antioxidant system whereas its 11-day chronic action decreased both activities presumably due to inducing tolerance to stress-signaling via overactivation and/or exhaustion of molecular targets of oxo-cfDNA in cerebellar cells.