Tanshinone IIA is a derivative of phenanthrene-quinone isolated from Danshen, a widely used Chinese herbal medicine. It has antioxidant properties and cytotoxic activity against multiple human cancer cell lines, inducing apoptosis and differentiation of some human cancer cell lines. Our purpose was to confirm its anticancer activity on human breast cancer in vitro and in vivo and to elucidate the mechanism of its activity. Human breast cancer cells were tested in vitro for cytotoxicity, colony formation inhibition, BrdU incorporation and gene expression profiling after treatment with tanshinone IIA. Seven nude mice bearing human breast infiltrating duct carcinoma orthotopically were tested for anticancer activity and expression of caspase-3 in vivo by s.c. injection of tanshinone IIA at a dose of 30 mg/kg 3 times/week for 10 weeks. Tanshinone IIA demonstrated a dose-and time-dependent inhibitory effect on cell growth (IC 50 = 0.25 mg/ml), and it significantly inhibited colony formation and BrdU incorporation of human breast cancer cells. Oligonucleotide microarray analysis identified 41 upregulated (1.22%) and 24 downregulated (0.71%) genes after tanshinone IIA treatment. Upregulated genes were involved predominantly in cycle regulation, cell proliferation, apoptosis, signal transduction and transcriptional regulation; and downregulated genes were associated mainly with apoptosis and extracellular matrix/adhesion molecules. A 44.91% tumor mass volume reduction and significant increase of casepase-3 protein expression were observed in vivo. Our findings suggest that tanshinone IIA might have potential anticancer activity on both ER-positive and -negative breast cancers, which could be attributed in part to its inhibition of proliferation and apoptosis induction of cancer cells through upregulation and downregulation of multiple genes involved in cell cycle regulation, cell proliferation, apoptosis, signal transduction, transcriptional regulation, angiogenesis, invasive potential and metastatic potential of cancer cells. ADPRTL1 might be the main target at which tanshinone IIA acted. ' 2005 Wiley-Liss, Inc.Key words: tanshinone IIA; anticancer activity; breast cancer; growth inhibition; apoptosis Breast cancer is a major public health problem in the United States and in most industrialized countries.1 It is the most common cancer in women and the leading cause of cancer death among women 35-54 years of age.2 The rising incidence and poor prognosis of breast cancer cases have prompted a search for additional preventive and therapeutic modalities.Danshen (Salvia miltiorrhiza Bunge) is a widely used Chinese herbal medicine; its extracts contain diterpene quinone and phenolic acid derivatives, including tanshinone (I, IIA and IIB), cryptotanshinone, isocryptotanshinone, miltirone, tanshinol (I and II) and salviol. These compounds have antioxidant properties and protect against lipid peroxidation in vitro and in vivo, making them potential antidotes for free radical-based disorders.3-5 Tanshinone IIA is a derivative of...