Abstract
Garlic has been widely used as a therapeutic agent as well as a spice for more than 2000 years. Besides its well-known cardioprotective effects, garlic is also considered to possess potential cancer preventive properties. Chemopreventive effects of garlic have been attributed to its oil-soluble sulfur ingredients, such as diallyl sulfide, diallyl disulfide, and diallyl trisulfide (DATS), but their underlying molecular mechanisms remain largely unresolved. In the present study, we have compared the effects of aforementioned allyl sulfides on the growth of cultured human breast carcinoma (MCF-7) cells. DATS inhibited the growth of MCF-7 cells to a greater extent than did the other allyl sulfides as determined by the MTT assay. DATS also induced apoptosis in MCF-7 cells, which was mediated through accumulation of reactive oxygen species with subsequent activation of JNK that catalyzes phosphorylation of Bcl-2 at Ser70. In another experiment, DATS prevented tumor formation in a mouse xenograft model. Aberrant upregulation of cycloocygenase-2 (COX-2) has been frequently observed in several types of cancer cells and is considered as a molecular target for cancer chemoprevention. Topically applied DATS inhibited the 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced COX-2 expression in dorsal skin of female ICR mice. DATS inhibited the DNA binding activity of AP-1 which is one of key transcription factors regulating the inflammation and expression of COX-2. DATS inhibited TPA-induced phosphorylation of Akt and JNK, which are major MAPKs regulating AP-1. A pharmacologic Akt inhibitor LY294002 and a JNK inhibitor SP600125 abrogated the TPA-induced COX-2 expression, suggesting that suppression of COX-2 expression by DATS in TPA stimulated mouse skin is mediated by blocking the PI3K-Akt and JNK signaling. Topical application of DATS protected against mouse skin carcinogenesis induced by 7,12-dimethylbenz[a]anthracene plus TPA. In addition, DATS strongly inibited DNA binding activity of NF-κB compared with other allyl sulfides in human mammary epithelial (MCF10A) cells treated with TPA. DATS inhibited the transcriptional activity of NF-κB, phosphorylation of IκBα, and activity of IKKβ. Inhibition of NF-κB DNA binding activity and IKKβ activity by DATS were blunted by addition of the antioxidant N-acetyl-L-cysteine (NAC) and the reducing agent dithiothreitol. Therefore, anti-inflammatory activity of DATS may be associated with oxidation or covalent modification of thiol groups contained in IKKβ.
