Berberine (BBR) is a natural isoquinoline alkaloid, which is used in traditional medicine for its anti‐microbial, anti‐protozoal, anti‐diarrhoeal activities. Berberine interacts with DNA and displays anti‐cancer activities, yet its effects on cellular DNA repair and on synthetic treatments with chemotherapeutic drugs remain unclear. In this study, we investigated the effects of BBR on DNA repair and on sensitization of breast cancer cells to different types of DNA damage anti‐tumoural drugs. We found BBR arrested cells in the cell cycle S phase and induced DNA breaks. Cell growth analysis showed BBR sensitized MDA‐MB‐231 cells to cisplatin, camptothecin and methyl methanesulfonate; however, BBR had no synergistic effects with hydroxurea and olaparib. These results suggest BBR only affects specific DNA repair pathways. Western blot showed BBR down‐regulated XRCC1 expressions, and the rescued XRCC1 recovered the resistance of cancer cells to BBR. Therefore, we conclude that BBR interferes with XRCC1‐mediated base excision repair to sensitize cancer cells to chemotherapeutic drugs. These finding can contribute to understanding the effects of BBR on cellular DNA repair and the clinical employment of BBR in treatment of breast cancer.

Triptolide is a natural compound isolated from the Tripterygium wilfordii, which possesses anti-inflammatory and anti-tumor activities. Triptolide reportedly inhibits RNA polymerase II-mediated transcription and ATM activities to interfere with DNA repair. However, the roles of triptolide in DNA repair are still largely unknown. Triple negative breast cancer cells (TNBC) are insensitive to targeted anti-tumoral drugs, thus DNA damage chemotherapeutic drugs are the available treatments used in clinic, while the drug resistance of TNBC causes the challenge for successful cure. In this study, we investigated the efficiency of cisplatin in combination with triptolide in treatment of TNBC. 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay shows triptolide suppresses the growth of two triple-negative breast cancer cells, BT549 and MDA-MB-231. Triptolide induces DNA breaks and arrests TNBC in the cell cycle S phase, and sensitizes TNBC to cisplatin. Western blot analysis shows triptolide down-regulated the levels of PARP1 and XRCC1, and slightly decreases the levels of RAD51. The results demonstrate triptolide interferes with single strand-break and base excision repair. The over-expressed PARP1/XRCC1 help the TNBC to resist triptolide. Based on these results, we conclude triptolide confers sensitization of TNBC to cisplatin via interference with XRCC1/PARP1-mediated base excision repair.