Natural product toosendanin reverses the resistance of human breast cancer cells to adriamycin as a novel PI3K inhibitor

Adriamycin (ADM) is a commonly used drug in clinical breast cancer treatment. However, some breast cancer types or breast cancers subjected to repeated ADM exposure develop strong resistance to ADM thus limiting its clinical efficacy. In this study, we found for the first time that toosendanin (TSN), a triterpenoid extracted from the traditional Chinese medicine Melia toosendan Sieb et Zucc, could successfully reverse adriamycin resistance in human breast cancer cells. Immunofluorescence and HPLC analysis demonstrated that TSN promoted adriamycin accumulation in breast cancer cells, especially in the nucleus. Furthermore, TSN could significantly reduce ABCB1 expression. We then found that TSN was capable of suppressing adriamycin-induced Akt phosphorylation, probably due to downregulation of the PI3K catalytic subunits P110α and P110β, and inhibition of DNA-PKcs. Importantly, the inhibitory effect of TSN on PI3K P110α and P110β expression was specifically observed in breast cancer cells but not in normal human cells. Moreover, TSN significantly potentiated the anti-cancer effect of ADM in the 4T1 breast cancer model and its inhibition rate was nearly 90%. Thus, TSN could be used as a novel PI3K inhibitor to reverse breast cancer resistance. The combination of ADM and TSN may represent a useful strategy for human breast cancer therapy.