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-α-linolenoyl-3-O-β-galactopyranosyl-sn-glycerol (DLGG, 1) and a mixture of two compounds, 1-O-α-linolenoyl-2-O-palmitoyl-3-O-β-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 α-linolenic acid, β-carotene, or (−)-epigallocatechin gallate. In a two-stage carcinogenesis experiment on ICR mouse skin with dimethylbenz[α]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 α-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.