Particulate Hexavalent Chromium Inhibits RAD51 Paralogs Necessary for RAD51 Filament Formation and Stabilization During Homologous Recombination Repair.

Hexavalent chromium [Cr(VI)] is a lung carcinogen. Central to its carcinogenic mechanism are Cr(VI)-induced DNA double strand breaks and chromosome instability. While breaks are usually repaired in healthy cells, Cr(VI) inhibits homologous recombination repair by targeting RAD51. RAD51 paralogs (RAD51B, RAD51C, RAD51D, XRCC2, and XRCC3) are responsible for RAD51 loading and the stabilization of nucleoprotein filaments necessary for DNA strand exchange and repair. This study aimed to investigate the effects of Cr(VI) exposure on RAD51 paralogs. WTHBF-6 cells, a human lung cell line, were exposed to various environmentally and occupationally relevant concentrations of zinc chromate for acute (24 h) and prolonged (120 h) exposure times. After exposure to Cr(VI), we collected RNA for sequencing and assessed the ability of DNA repair proteins to form foci using immunofluorescence. Protein levels were measured with western blotting, RNA-Seq was validated with RT-qPCR, and protein-protein interactions were assessed with the Proximity Ligation Assay (PLA) assay. Cr(VI) transcriptionally repressed all RAD51 paralogs. Further functional analyses showed that Cr(VI) inhibited the foci formation of RAD51D after acute and prolonged exposures and of XRCC2 and XRCC3 after prolonged exposure. Cr(VI) also inhibited overall RAD51D protein expression, as well as its interaction with RAD51. These findings suggest that Cr(VI) inhibits all RAD51 paralogs, but RAD51D might be an early target of Cr(VI), leading to the loss of RAD51 filament formation and function and the overall inhibition of homologous recombination repair.