Background MieroRNAs (miRNAs) are very important regulators in biological processes such as development,cellular differentiation,and carcinogenesis.Given the important role of miRNAs in tumorigenesis and development,it is worth investigating whether some miRNAs play roles in the anticancer mechanism of flavonoids.However,such role has not been reported yet.We have previously selected a promising anti-cancer agent 3,6-dihydroxyflavone (3,6-DHF) in pharmacodynamic experiments,which may serve as a leading compound for developing more potent anticancer drags or chemopreventive supplements.The present study aims to investigate the chemopreventive activities of 3,6-DFF against mammary carcinogenesis.Methods The experimental model of breast carcinogenesis was developed by intraperitoneal injection of 1-methyl-1-nitrosourea (MNU).The bioavailability of 3,6-DHF in rats was detected by high performance liquid chromatography (HPLC).The expression of microRNA-34a (miR-34a) and microRNA-21 (miR-21) was evaluated by Real time quantitative reverse transcription PCR (qRT-PCR).Cell apoptosis was analyzed by flow cytometry or Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay.Mitochondrial membrane potential (ΔΨm)was assayed using 5,5,6,6-Tetrachloro-1,1,3,3-tetraethyl-imidacarbocyanine iodide (JC-1) dye by confocal laser scanning microscopy.The level ofcytochrome C in cytosol was evaluated by Western blotting.Results Our study showed that oral administration of 3,6-DHF effectively suppressed MNU-induced breast carcinogenesis in rats,decreasing the cancer incidence by 35.7%.The detection of bioavailability indicated that the concentration of 3,6-DHF was 2.5+0.4 μg/ml in the plasma of rats within 2 hours after administration,and was 21.7±3.8 μg/ml in the urine within 24 hours.Oral administration of 3,6-DHF to BALB/c nude mice beating breast cancer cell xenografts also significantly suppressed tumor growth in v/vo.Furthermore,our study revealed that the global up-regulation of miR-21 and down-regulation ofmiR-34a in breast carcinogenesis could be reversed by 3,6-DHF,which significantly up-regulated miR-34a expression and decreased miR-21 expression;inducing apoptosis of breast cancer cells in vitro and in vivo.Overexpression of miR-34a induced by plasmid transfection or inhibition of miR-21 by oligonucleotides markedly promoted the pro-apoptotic effect of 3,6-DHK Inactivation of miR-34a or over-production of miR-21 compromised the anticancer effects of 3,6-DHF.Conclusions These findings indicate that 3,6-DHF is a potent natural chemopreventive agent,and that miR-34a and miR-21 play roles in MNU-induced breast carcinogenesis and the anticancer mechanism of flavonoids.