Coumarin‐related compounds, auraptene and umbelliferone, have been isolated from the cold‐pressed oil of natsumikan (Citrus natsudaidai HAYATA), and tested as inhibitors of tumor promoter 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA)‐induced Epstein‐Barr virus activation in Raji cells. The 50% inhibitory concentration (IC50) of auraptene (18μM)was almost equal to that of genistein. Umbelliferone, which lacks a geranyloxyl group present in auraptene, was less active (IC50=450 μM). In a two‐stage carcinogenesis experiment with 7, 12‐dimethylbenz[α]anthracene (topical application at 0.19 μmol) and TPA (topical application at 1.6 nmol) in ICR mouse skin, topical application of auraptene (at 160 nmol) significantly reduced tumor incidence and the numbers of tumors per mouse by 27% (P < 0.01) and 23% (P < 0.05), respectively. Auraptene at a concentration of 50 μM markedly suppressed superoxide (O2−) generation induced by 100 nM TPA in differentiated human promyelocytic HL‐60 cells. Having no O2− ‐scavenging potential, auraptene may inhibit the multicomponent NADPH oxidase system. Inhibition of intracellular hydroperoxide formation in differentiated HL‐60 cells by auraptene was also confirmed by flow‐cytometric analysis using 2′,7′‐dichlorofluorescein diacetate as a fluorescence probe. Quantitative analyses using high‐performance liquid chromatography showed the occurrence of auraptene not only in both the peels and sarcocarps of natsumikan, but also in those of hassaku orange (C. hassaku) and grapefruit (C. paradisi,) and even in their bottled fresh juice form. These results indicate that auraptene is a chemopreventer of skin tumorigenesis, and implies that suppression of leukocyte activation might be the mechanism through which it inhibits tumor promotion.

The modifying effects of topical application of the phenolic antioxidant protocatechuic acid (PA) on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced mouse skin tumor promotion were investigated. Dimethylbenz[a]anthracene-initiated female ICR mice were treated with TPA (1.6 nmol) twice weekly for 20 weeks to promote papilloma formation. Pre-treatment with 16nmol PA 30 min prior to each TPA treatment significantly inhibited the number of papillomas per mouse by 52% (P < 0.05). On the other hand, PA pre-treatment at a high dose (1600 nmol) significantly enhanced tumor numbers by 38% (P < 0.05). Interestingly, in the group treated with a quite high dose (20000 nmol) of PA 5 min prior to each TPA application, the average number of tumors per mouse was reduced by 38%, whereas the same PA dose 3 h before TPA treatment significantly enhanced tumor numbers by 84% (P < 0.01). These results suggested that topically applied PA was converted to compound(s) lacking antioxidative properties and/or rather possessing the potential to enhance tumor development. A similar tendency was also observed in the short-term experiment of TPA-induced inflammation and oxidative stress; i.e. two groups pre-treated with PA at 20000 nmol, 30min and 3h before TPA treatment, did not show suppression or even significantly enhanced TPA-induced leukocyte infiltration, H(2)O(2) generation, thiobarbituric acid-reacting substances level and proliferating cell nuclear antigen index, while PA treatment together with TPA significantly suppressed these parameters. Treatment with a high dose (20000 nmol) of PA alone for 3h enhanced oxidative stress by reducing glutathione levels in mouse skin, which was counteracted by the tyrosinase inhibitor arbutin. Oxidative stress responses such as leukocyte infiltration and H(2)O(2) generation were also counteracted by arbutin. These results suggested that tyrosinase-dependent oxidative metabolism of PA was at least partially involved in the enhanced effects of PA on TPA-induced inflammatory responses and thus tumor promotion.

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Rosmarinic acid (RoA) has been isolated in high yield (0.1%) from the leaves of Perilla frutescens Britton var. acuta f. viridis, one of the most common garnishes in Japan, as a superoxide scavenger in a xanthine/xanthine oxidase system. The scavenging activity of RoA was significantly higher than that of ascorbic acid (AA). The structure−activity relationships using RoA, caffeic acid (CaA), and its related compounds indicated that the presence of an ortho dihydroxyphenyl group is essential for the scavenging effect. In addition, the conjugated double bond in the C3 carbon chain is important for enhancing the effect. The activity of RoA was then shown to be due to the additive effect of both CaA and (3,4-dihydroxyphenyl)lactic acid units, both of which are hydrolyzed products of RoA. RoA significantly inhibited both intracellular superoxide and peroxide formation in differentiated HL-60 cells, suggesting that RoA effectively exhibited antioxidative activity in the biological systems through the scavenging of superoxide. Keywords: Perilla frutescens; rosmarinic acid; superoxide; xanthine oxidase; HL-60