Dopamine enhances the response of sunitinib in the treatment of drug-resistant breast cancer: Involvement of eradicating cancer stem-like cells.

Growing evidence suggests that the efficacy of sunitinib in breast cancer may be limited by increasing the population of cancer stem-like cells (CSCs). Hence the concurrent use of CSCs-targeting agents is required. Previous results indicated that dopamine receptor (DR) may serve as a potential therapeutic target of anti-CSCs therapies. This study focused on evaluating the effect of dopamine (an agonist of DR) on the enhancement of sunitinib's efficacy in the treatment of drug-resistant breast cancer, investigating the involved activation type of DR pathway and exploring the underlying anti-CSCs mechanisms. MCF-7 cells, MCF-7/Adr cells and breast cancer stem-like cells (BCSCs) were used for in vitro study. Moreover, MCF-7/Adr cells and BCSCs were selected as drug-resistant cell lines and further used for in vivo development of the xenograft animal models. Our results showed that dopamine greatly synergized the inhibitory effect of sunitinib in the drug-resistant cells and strikingly enhanced the response of sunitinib in both xenograft models. It was found that dopamine significantly down-regulated the expression of BCSCs markers (CD44(+)/CD24(-)) in vitro and in vivo. In addition, dopamine remarkably induced the apoptosis of BCSCs, markedly inhibited the Wnt signaling pathway and activated the apoptotic associated signals. The activation of dopamine receptor D1 (D1DR) pathway may be involved in the underlying mechanism as D1DR's antagonist SCH23390 completely reversed the combined effects. In conclusion, dopamine may eradicate CSCs and it significantly enhances the response of sunitinib in the treatment of drug-resistant breast cancer.