Double Discordance in the ROS-induction and Anti-tumor activity of Luteolin, a Classical Free Radical Scavenger

Luteolin is a traditional free radical scavenger and has been considered to be an antioxidant. But a recent study showed that luteolin induced ROS, and ROS was the driving cause of cell death in a luteolin-treated lung cancer cell line. The anti-melanoma effect of luteolin have not been well studied and the results were controversial in the limited publications. In our investigation we examined luteolin efficacy on growth inhibition of melanoma cells both in vitro and in vivo (xenografted melanoma cells), and investigated the molecular mechanism of luteolin in detail. We found that luteolin significantly inhibited the short-term and long-term proliferation of all 5 melanoma cell lines including Wm3211(BRAF wild type) and A375(BRAF mutant) in a dose-dependent manner. Luteolin inhibited protein accumulation of PDE4B, AKT2 and AKT3 in both cell lines, but over-expression of AKT2 or AKT3 showed mixed impacts on luteolin efficacy. Luteolin induced cellular ROS in a dose- and time-dependent manner, accompanied by ROS-metabolizing SOD1, SOD2 and catalase accumulation. When antioxidant NAC or mito-TEMPO was co-administered with luteolin, ROS levels were reversed but cell killing was not rescued. Depletion of mitochondria also showed no effect on luteolin efficacy. We further explored possible targets of luteolin, and identified the MAPK pathway was a direct target for luteolin; over-expression of MAPK pathway components significantly increased IC50s for luteolin in melanoma cell lines examined. A375 or Wm3211 melanoma cells were xenografted into a mouse model and treated with luteolin for 3 weeks. Luteolin showed significant tumor growth inhibition in both models. To the best of our knowledge this is the first report showing in vivo efficacy of luteolin for melanoma, with a molecular mechanism via the MAPK pathway, not through ROS induction per se.