Methotrexate-mediated inhibition of RAD51 expression and homologous recombination in cancer cells.

BACKGROUND: Methotrexate is an inhibitor of folic acid metabolism. Homologous recombination is one of the most important ways to repair double-stranded breaks in DNA and influence the radio- and chemosensitivity of tumor cells. But the relationship between methotrexate and homologous recombination repair has not been elucidated.
METHODS: Induction of double-strand breaks by methotrexate in HOS cells is assessed by the neutral comet assay. Inhibition of subnuclear repair foci by methotrexate is measured by immunofluorescence. Western blot and quantitative real-time PCR are conducted to detect whether methotrexate affects the expression level of genes involved in homologous recombination. In addition, we used a pCMV3xnls-I-SceI construct to determine whether methotrexate directly inhibits the process of homologous recombinational repair in cells, and the sensitivity to methotrexate in the Ku80-deficient cells is detected using clonogenic survival assays.
RESULTS: The result showed that methotrexate can regulate the repair of DNA double-strand breaks after radiation exposure, and methotrexate inhibition caused the complete inhibition of subnuclear repair foci in response to ionizing radiation. Mechanistic investigation revealed that methotrexate led to a significant reduction in the transcription of RAD51 genes. Treatment with methotrexate resulted in a decreased ability to perform homology-directed repair of I-SceI-induced chromosome breaks. In addition, enhancement of cell death was observed in Ku mutant cells compared to wild-type cells.
CONCLUSIONS: These results demonstrate that methotrexate can affect homologous recombination repair of DNA double-strand breaks by controlling the expression of homologous recombination-related genes and suppressing the proper assembly of homologous recombination-directed subnuclear foci.