Role Of RecQL4in Maintaining Mitochondrial Genome Stability And Regulating Multidrug Resistance

RecQL4, one of the five human RecQ helicases, is crucial in maintaining genomic stability and loss of its function leads to premature aging and tumor predisposition in humans. Recent study demonstrated that RecQL4upregulation due to amplification of its genomic locus is associated with tumorigenicity of human tumor cells. Unlike other family members of human RecQ helicases whose proteins are only located in nuclear, RecQL4was shown to be distributed in both nuclear and cytoplasm. However, the underlying mechanism for regulating RecQL4nuclear exporting and the functional role of RecQL4in cytoplasm are not well understood. In this study, two functional nuclear exporting signals (NESs) were identified at the C-terminus of RecQLA Deletion of pNES2drastically diminished its cytoplasmic localization. Further studies by imrnunofluorescence staining and western blotting revealed that the cytoplasmic RecQL4was localized in mitochondria and further regulated by NES signal. Consistent with its mitochondrial localization, ectopic expression of RecQL4in HEK293cells significantly increased the mtDNA copy number while RecQL4knock down in U2OS cells markedly decreased the mtDNA copy number. Furthermore, a substantially increased level of mitochondrial superoxide production and a markedly decreased repair capacity for oxidative DNA damage were observed in the mitochondria of both RecQL4deficient human fibroblasts and RecQL4-suppressed cancer cells. These data strongly suggest a regulatory role for RecQL4in mitochondrial DNA replication and oxidative damage repair. Additionally, RecQL4was found to physically interact with YB-1. MDR1, a downstream gene of YB1related to multidrug resistance, was substancially induced in cisplatin-treated U2OS cells with high level of endogenous RecQL4expression, whereas RecQL4suppression leads to a largely dimished induction of MDR1expression and a significantly increased sensitivity to cisplatin treatment. Our data provide the evidence that RecQL4is involved in maintaining mitochondrial genome stability, and may play an important role in regulating chemoresistance in human tumor cells.