Structure-antioxidant activity relationships of flavonoids and phenolic acids

Flavonoids are nearly ubiquitous in plants and are recognized as the pigments responsible for the colors of leaves, especially in autumn. They are rich in seeds, citrus fruits, olive oil, tea, and red wine. They are low molecular weight compounds composed of a three-ring structure with various substitutions. This basic structure is shared by tocopherols (vitamin E). Flavonoids can be subdivided according to the presence of an oxy group at position 4, a double bond between carbon atoms 2 and 3, or a hydroxyl group in position 3 of the C (middle) ring. These characteristics appear to also be required for best activity, especially antioxidant and antiproliferative, in the systems studied. The particular hydroxylation pattern of the B ring of the flavonoles increases their activities, especially in inhibition of mast cell secretion. Certain plants and spices containing flavonoids have been used for thousands of years in traditional Eastern medicine. In spite of the voluminous literature available, however, Western medicine has not yet used flavonoids therapeutically, even though their safety record is exceptional. Suggestions are made where such possibilities may be worth pursuing.

The Effects of Plant Flavonoids on Mammalian Cells:Implications for Inflammation, Heart Disease, and Cancer

Dietary antioxidant flavonoids and risk of coronary heart disease: the Zutphen Elderly Study.

Increased generation of reactive oxygen species (ROS) and an altered redox status have long been observed in cancer cells, and recent studies suggest that this biochemical property of cancer cells can be exploited for therapeutic benefits. Cancer cells in advanced stage tumours frequently exhibit multiple genetic alterations and high oxidative stress, suggesting that it might be possible to preferentially eliminate these cells by pharmacological ROS insults. However, the upregulation of antioxidant capacity in adaptation to intrinsic oxidative stress in cancer cells can confer drug resistance. Abrogation of such drug-resistant mechanisms by redox modulation could have significant therapeutic implications. We argue that modulating the unique redox regulatory mechanisms of cancer cells might be an effective strategy to eliminate these cells.

Targeting cancer cells by ROS-mediated mechanisms: a radical therapeutic approach?

The antioxidant activities and total phenolics of 28 plant products, including sunflower seeds, flaxseeds, wheat germ, buckwheat, and several fruits, vegetables, and medicinal plants were determined. The total phenolic content, determined according to the Folin−Ciocalteu method, varied from 169 to 10548 mg/100 g of dry product. Antioxidant activity of methanolic extract evaluated according to the β-carotene bleaching method expressed as AOX (Δ log A470/min), AA (percent inhibition relative to control), ORR (oxidation rate ratio), and AAC (antioxidant activity coefficient) ranged from 0.05, 53.7, 0.009, and 51.7 to 0.26, 99.1, 0.46, and 969.3, respectively. The correlation coefficient between total phenolics and antioxidative activities was statistically significant. Keywords: Antioxidant activity; phenolics; medicinal plants; oilseeds; buckwheat; vegetables; fruits; wheat products

Antioxidant Activity and Total Phenolics in Selected Fruits, Vegetables, and Grain Products

Plants consumed by humans contain thousands of phenolic compounds. The effects of dietary polyphenols are of great current interest due to their antioxidative and possible anticarcinogenic activities. A popular belief is that dietary polyphenols are anticarcinogens because they are antioxidants, but direct evidence for this supposition is lacking. This chapter reviews the inhibition of tumorigenesis by phenolic acids and derivatives, tea and catechins, isoflavones and soy preparations, quercetin and other flavonoids, resveratrol, and lignans as well as the mechanisms involved based on studies in vivo and in vitro. Polyphenols may inhibit carcinogenesis by affecting the molecular events in the initiation, promotion, and progression stages. Isoflavones and lignans may influence tumor formation by affecting estrogen-related activities. The bioavailability of the dietary polyphenols is discussed extensively, because the tissue levels of the effective compounds determine the biological activity. Understanding the bioavailability and blood and tissue levels of polyphenols is also important in extrapolating results from studies in cell lines to animal models and humans. Epidemiological studies concerning polyphenol consumption and human cancer risk suggest the protective effects of certain food items and polyphenols, but more studies are needed for clear-cut conclusions. Perspectives on the application of dietary polyphenols for the prevention of human cancer and possible concerns on the consumption of excessive amounts of polyphenols are discussed.

Inhibition of carcinogenesis by dietary polyphenolic compounds

The effects of topically applied curcumin, chlorogenic acid, caffeic acid, and ferulic acid on 12- O -tetradecanoylphorbol-13-acetate (TPA)-induced epidermal ornithine decarboxylase activity, epidermal DNA synthesis, and the promotion of skin tumors were evaluated in female CD-1 mice. Topical application of 0.5, 1, 3, or 10 µmol of curcumin inhibited by 31, 46, 84, or 98%, respectively, the induction of epidermal ornithine decarboxylase activity by 5 nmol of TPA. In an additional study, the topical application of 10 µmol of curcumin, chlorogenic acid, caffeic acid, or ferulic acid inhibited by 91, 25, 42, or 46%, respectively, the induction of ornithine decarboxylase activity by 5 nmol of TPA. The topical application of 10 µmol of curcumin together with 2 or 5 nmol of TPA inhibited the TPA-dependent stimulation of the incorporation of [ 3 H]-thymidine into epidermal DNA by 49 or 29%, respectively, whereas lower doses of curcumin had little or no effect. Chlorogenic acid, caffeic acid, and ferulic acid were less effective than curcumin as inhibitors of the TPA-dependent stimulation of DNA synthesis. Topical application of 1, 3, or 10 µmol of curcumin together with 5 nmol of TPA twice weekly for 20 weeks to mice previously initiated with 7,12-dimethylbenz[ a ]anthracene inhibited the number of TPA-induced tumors per mouse by 39, 77, or 98%, respectively. Similar treatment of mice with 10 µmol of chlorogenic acid, caffeic acid, or ferulic acid together with 5 nmol of TPA inhibited the number of TPA-induced tumors per mouse by 60, 28, or 35%, respectively, and higher doses of the phenolic acids caused a more pronounced inhibition of tumor promotion. The possibility that curcumin could inhibit the action of arachidonic acid was evaluated by studying the effect of curcumin on arachidonic acid-induced edema of mouse ears. The topical application of 3 or 10 µmol of curcumin 30 min before the application of 1 µmol of arachidonic acid inhibited arachidonic acid-induced edema by 33 or 80%, respectively.

https://cancerres.aacrjournals.org/content/canres/48/21/5941.full.pdf

Inhibitory Effect of Curcumin, Chlorogenic Acid, Caffeic Acid, and Ferulic Acid on Tumor Promotion in Mouse Skin by 12-O-Tetradecanoylphorbol-13-acetate

Topical application of curcumin, the yellow pigment in turmeric and curry, strongly inhibited 12- O -tetradecanoylphorbol-13-acetate (TPA)-induced ornithine decarboxylase activity, DNA synthesis, and tumor promotion in mouse skin (Huang et al. , Cancer Res., 48: 5941–5946, 1988). Chlorogenic acid, caffeic acid, and ferulic acid (structurally related dietary compounds) were considerably less active. In the present study, topical application of curcumin markedly inhibited TPA- and arachidonic acid-induced epidermal inflammation (ear edema) in mice, but chlorogenic acid, caffeic acid, and ferulic acid were only weakly active or inactive. The in vitro addition of 3, 10, 30, or 100 µm curcumin to cytosol from homogenates of mouse epidermis inhibited the metabolism of arachidonic acid to 5-hydroxyeicosatetraenoic acid (5-HETE) by 40, 60, 66, or 83%, respectively, and the metabolism of arachidonic acid to 8-HETE was inhibited by 40, 51, 77, or 85%, respectively [IC 50 (concentration needed for 50% inhibition) = 5–10 µm]. Chlorogenic acid, caffeic acid, or ferulic acid (100 µm) inhibited the metabolism of arachidonic acid to 5-HETE by 36, 10, or 16%, respectively, and these hydroxylated cinnamic acid derivatives inhibited the metabolism of arachidonic acid to 8-HETE by 37, 20, or 10%, respectively (IC 50 > 100 µm). The metabolism of arachidonic acid to prostaglandin E 2 , prostaglandin F 2α , and prostaglandin D 2 by epidermal microsomes was inhibited approximately 50% by the in vitro addition of 5–10 µm curcumin. Chlorogenic acid, caffeic acid, and ferulic acid (100 µm) were inactive. In vitro rat brain protein kinase C activity was not affected by 50–200 µm curcumin, chlorogenic acid, caffeic acid, or ferulic acid. The inhibitory effects of curcumin, chlorogenic acid, caffeic acid, and ferulic acid on TPA-induced tumor promotion in mouse epidermis parallel their inhibitory effects on TPA-induced epidermal inflammation and epidermal lipoxygenase and cyclooxygenase activities.

https://cancerres.aacrjournals.org/content/canres/51/3/813.full.pdf

Inhibitory effects of curcumin on in vitro lipoxygenase and cyclooxygenase activities in mouse epidermis.

Curcumin (diferuloylmethane), a yellow pigment that is obtained from the rhizomes of Curcuma longa Linn., is a major component of turmeric and is commonly used as a spice and food-coloring agent. The inhibitory effects of feeding commercial grade curcumin (77% curcumin, 17% demethoxycurcumin, and 3% bisdemethoxycurcumin) in AIN 76A diet on carcinogen-induced tumorigenesis in the forestomach, duodenum, and colon of mice were evaluated. Administration p.o. of commercial grade curcumin in the diet inhibited benzo(a)pyrene-induced forestomach tumorigenesis in A/J mice, N-ethyl-N'-nitro-N-nitrosoguanidine-induced duodenal tumorigenesis in C57BL/6 mice, and azoxymethane (AOM)-induced colon tumorigenesis in CF-1 mice. Dietary commercial grade curcumin was given to mice at: (a) 2 weeks before, during, and for 1 week after carcinogen administration (during the initiation period); (b) 1 week after carcinogen treatment until the end of the experiment (during the postinitiation period); or (c) during both the initiation and postinitiation periods. Feeding 0.5-2.0% commercial grade curcumin in the diet decreased the number of benzo(a)pyrene-induced forestomach tumors per mouse by 51-53% when administered during the initiation period and 47-67% when administered during the postinitiation period. Feeding 0.5-2.0% commercial grade curcumin in the diet decreased the number of N-ethyl-N'-nitro-N-nitrosoguanidine-induced duodenal tumors per mouse by 47-77% when administered during the postinitiation period. Administration of 0.5-4.0% commercial grade curcumin in the diet both during the initiation and postinitation periods decreased the number of AOM-induced colon tumors per mouse by 51-62%. Administration of 2% commercial grade curcumin in the diet inhibited the number of AOM-induced colon tumors per mouse by 66% when fed during the initiation period and 25% when fed during the postinitiation period. The ability of commercial grade curcumin to inhibit AOM-induced colon tumorigenesis is comparable to that of pure curcumin (purity greater than 98%). Administration of pure or commercial grade curcumin in the diet to AOM-treated mice resulted in development of colon tumors which were generally smaller in number and size as compared to the control group of AOM-treated mice. These results indicate that not only did curcumin inhibit the number of tumors per mouse and the percentage of mice with tumors but it also reduced tumor size. Histopathological examination of the tumors showed that dietary curcumin inhibited the number of papillomas and squamous cell carcinomas of the forestomach as well as the number of adenomas and adenocarcinomas of the duodenum and colon.

https://cancerres.aacrjournals.org/content/canres/54/22/5841.full.pdf

Inhibitory Effects of Dietary Curcumin on Forestomach, Duodenal, and Colon Carcinogenesis in Mice

A methanol extract of the leaves of the plant Rosmarinus officinalis L. (rosemary) was evaluated for its effects on tumor initiation and promotion in mouse skin. Application of rosemary to mouse skin inhibited the covalent binding of benzo(a)pyrene [B(a)P] to epidermal DNA and inhibited tumor initiation by B(a)P and 7,12-dimethylbenz[a]anthracene (DMBA). Topical application of 20 nmol B(a)P to the backs of mice once weekly for 10 weeks, followed 1 week later by promotion with 15 nmol 12-O-tetradecanoylphorbol-13-acetate (TPA) twice weekly for 21 weeks, resulted in the formation of 7.1 tumors per mouse. In a parallel group of animals that were treated topically with 1.2 or 3.6 mg of rosemary 5 min prior to each application of B(a)P, the number of tumors per mouse was decreased by 54 or 64%, respectively. Application of rosemary to mouse skin also inhibited TPA-induced ornithine decarboxylase activity, TPA-induced inflammation, arachidonic acid-induced inflammation, TPA-induced hyperplasia, and TPA-induced tumor promotion. Mice initiated with 200 nmol DMBA and promoted with 5 nmol TPA twice weekly for 19 weeks developed an average of 17.2 skin tumors per mouse. Treatment of the DMBA-initiated mice with 0.4, 1.2, or 3.6 mg of rosemary together with 5 nmol TPA twice weekly for 19 weeks inhibited the number of TPA-induced skin tumors per mouse by 40, 68, or 99%, respectively. Topical application of carnosol or ursolic acid isolated from rosemary inhibited TPA-induced ear inflammation, ornithine decarboxylase activity, and tumor promotion. Topical application of 1, 3, or 10 mumol carnosol together with 5 nmol TPA twice weekly for 20 weeks to the backs of mice previously initiated with DMBA inhibited the number of skin tumors per mouse by 38, 63, or 78%, respectively. Topical application of 0.1, 0.3, 1, or 2 mumol ursolic acid together with 5 nmol TPA twice weekly for 20 weeks to DMBA-initiated mice inhibited the number of tumors per mouse by 45-61%.

https://cancerres.aacrjournals.org/content/canres/54/3/701.full.pdf

Mou-Tuan Huang

Papers

Inhibition of carcinogenesis by dietary polyphenolic compounds

Inhibitory Effect of Curcumin, Chlorogenic Acid, Caffeic Acid, and Ferulic Acid on Tumor Promotion in Mouse Skin by 12-O-Tetradecanoylphorbol-13-acetate

Inhibitory effects of curcumin on in vitro lipoxygenase and cyclooxygenase activities in mouse epidermis.

Inhibitory Effects of Dietary Curcumin on Forestomach, Duodenal, and Colon Carcinogenesis in Mice

Inhibition of skin tumorigenesis by rosemary and its constituents carnosol and ursolic acid.