GLYCEROGLYCOLIPIDS FROM CITRUS HYSTRIX, A TRADITIONAL HERB IN THAILAND, POTENTLY INHIBIT THE TUMOR-PROMOTING ACTIVITY OF 12-O-TETRADECANOYLPHORBOL 13-ACETATE IN MOUSE SKIN

Two glyceroglycolipids were isolated from the leaves of Citrus hystrix (bitter orange), a traditional herb in Thailand. They were identified as 1,2-di-O-alpha-linolenoyl-3-O-beta-galactopyransyl-sn-glycerol (DLGG, I) and a mixture of two compounds, 1-O-alpha-linolenoyl-2-O-palmitoyl-3-O-beta-galactopyranosyl-sn-glycerol (2a) and its counterpart (2b) (LPGG, 2). Both lipids were potent inhibitors of tumor promoter-induced Epstein-Barr virus (EBV) activation. The IC50 values of 1 and 2 were strikingly lower than those of representative cancer preventive agents such as alpha-linolenic acid, beta-carotene, or (-)-epigallocatechin gallate. In a two-stage carcinogenesis experiment on ICR mouse skin with dimethylbenz[alpha]anthracene (DMBA) and 12-O-tetradecanoylphorbol 13-acetate (TPA), compound 1 exhibited anti-tumor-promoting activity even at a dose 10 times lower than that of alpha-linolenic acid. As some synthetic detergents or saponins were entirely inactive in the EBV activation inhibition test, detergency was suggested not to play a major role in the mode of inhibitory action in vivo. The inhibition of the arachidonic acid cascade may be involved in anti-tumor promotion since 1 inhibited TPA-induced edema formation in the anti-inflammation test using ICR mouse ears.