Quercetin greatly improved therapeutic index of doxorubicin against 4T1 breast cancer by its opposing effects on HIF-1 alpha in tumor and normal cells

The anthracycline antibiotic doxorubicin (DOX) has been used successfully for treating various types of cancers. However, the therapeutic efficacy of DOX was greatly restricted by its cumulative dose-related cardiotoxicity and common side effects such as bone marrow and immune suppression. Quercetin had better cardioprotective and hepatoprotective activities. The present study was to observe whether quercetin could improve therapeutic index of DOX and explore its mechanisms. Effects of quercetin on doxorubicin (DOX)-induced cytotoxicity were investigated in 4T1 cells and murine spleen cells by methylthiazoletetrazolium assay, flow cytometry and single cell gel electrophoresis. Influences of quercetin on therapeutic efficacy and systemic toxicity of DOX were evaluated in BALB/c mice with 4T1 breast cancer. Hypoxia-inducible factor-1 alpha (HIF-1 alpha) in tumor and normal cells was examined to explore mechanisms of quercetin by Western blot and enzyme-linked immunosorbent assay. In vitro, quercetin at dose less than 100 mu M had only slight effects on cell viability and DOX-induced cytotoxicity in 4T1 cells under normoxia, but it could reverse 4T1 cell resistance to DOX under hypoxia and protect spleen cells against DOX-induced cytotoxicity. In vivo, quercetin suppressed tumor growth and prolonged survival in BALB/c mice bearing 4T1 breast cancer. Importantly, quercetin enhanced therapeutic efficacy of DOX and simultaneously reduced DOX-induced toxic side effects. Further study showed that quercetin suppressed intratumoral HIF-1 alpha in a hypoxia-dependent way but increased its accumulation in normal cells. HIF-1 alpha siRNA abolished effects of quercetin on both tumor and normal cells. These results suggested that quercetin could improve therapeutic index of DOX by its opposing effects on HIF-1 alpha in tumor and normal cells, and was a promising candidate as anticancer agents.