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Peroxiredoxin-ⅡI(PRDX3)is a mitochondrial peroxide reductase that reduces hydrogen peroxide to regulates cellular redox state.PRDX3 is overexpressed in many tumors including liver cancer.It is important to investigate functions and action mechanisms of PRDX3 in cellular processes and its involvement in hepatocellular carcinoma(HCC)tumorigenesis and progression.In the present study,we established multiple stable cell lines by overexpressing or knocking down PRDX3 in HepG2 cells.We found that PRDX3 silencing decreased the growth rate of HepG2 cells and increased mitochondrial DNA(mtDNA)oxidation.Quantitative proteomics identified 475 differentially expressed proteins between the PRDX3 knockdown and the control cells.These proteins were involved in antioxidant activity,angiogenesis,cell adhesion,cell growth,ATP synthesis,nucleic acid binding,redox,and chaperones.PRDX3 knockdown led to the down-regulation of ATP synthases and the decreased cellular ATP level as determined by mass spectrometry,contributing to the slow-down of cell growth.Furthermore,silencing PRDX3 enhanced invasive properties of HepG2 cells via TIMP-1 down-regulation and the increased ECM degradation.To understand the molecular mechanism underlying PRDX3-mediated mtDNA protection,we treated cells with hydrogen peroxide,and identified that PRDX3 bounded to isoform 1 of uracil-DNA glycosylase(UNG1)via a disulfide linkage under oxidative stress.UNG1 is the major protein for initiating base-excision repair in mitochondria and is in close proximity to the respiratory chain that generates reactive oxygen species(ROS).We further demonstrated that the UNG1-PRDX3 interaction protected UNG1 from ROS-mediated degradation and prevented mtDNA oxidation.Moreover,our results show that ROS-mediated UNG1 degradation was Lon protease1(LonP1)-dependent and mitochondrial UNG1 degradation was aggravated by knockdown of PRDX3 expression.Taken together,these results reveal a novel function of UNG1 in the recruitment of PRDX3 to mtDNA under oxidative stress,enabling protection of UNG1 and UNG1-bound DNA from ROS damage and enhancing cell resistance to oxidative stress,and indicate that PRDX3 promotes HCC growth and mediates cell migration and invasiveness.