Metformin enhances doxorubicin sensitivity via inhibition of doxorubicin efflux in P-gp-overexpressing MCF-7 cells.

Resistance against chemotherapy is still a major problem in successful cancer treatment in the clinic. Therefore, identifying new compounds with lower side-effects and higher efficacy is an important approach to overcome multidrug resistance (MDR). Here, we investigated the activity and possible mechanism of the antidiabetic drug, metformin, in human doxorubicin (DOX)-resistant breast cancer (MCF-7/DOX) cells. The effect of metformin on the cytotoxicity of DOX was evaluated by MTT assay. The P-gp mRNA/protein expression levels following treatment with metformin were determined using real-time polymerase chain reaction and Western blot analysis, respectively. Intracellular rhodamine 123 accumulation assay was performed to evaluate the P-gp function. Cellular ATP content was determined using ATP assay kit. The effect of metformin on DOX-induced apoptosis was evaluated by annexin V/FITC assay. Exposure to metformin considerably enhanced the cytotoxicity of DOX. Metformin had no substantial effect on P-gp expression, while the activity of P-gp and intracellular ATP content decreased with metformin treatment in a dose-dependent manner. Furthermore, metformin significantly increased the DOX-induced apoptosis. These results indicate that metformin could reverse MDR in breast cancer cells by reducing P-gp activity. Therefore, metformin can be suggested as a potent adjuvant in breast cancer chemotherapy.