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Modulation of ultrviolet light induced skin carcinogenesis by extracts of Rooibos and Honeybush using a mouse model:elucidating possible protective mechanisms
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This thesis provides the first scientific evidence of the photoprotective properties of rooibos and honeybush herbal tea extracts and to some extent, two major honeybush polyphenols, hesperidin and mangiferin. These properties were demonstrated using in vivo models by: Providing evidence for the inhibition of tumour promotion by ultraviolet B (UVB) radiation in a two-stage skin carcinogenesis mouse model. Topical application of polyphenol-rich extracts of rooibos and honeybush prior to UVB tumour promotion of 7,12-dimethylbenz[a]anthracene (DMBA)-initiated mouse skin, inhibited the formation of tumours. The rooibos and honeybush extracts decreased the incidence and volume of the tumours. Topical application of hesperidin and mangiferin were less effective than the honeybush extracts as only the tumour volume was decreased, but not the incidence. Providing evidence for the inhibition of photodamage of the skin by UVB exposure in a mouse model. Topical application of polyphenolic rich extracts of honeybush prior to UVB irradiation of mouse skin reduced erythema, peeling, oedema and hyperplasia. The depletion of antioxidant enzymes catalase and superoxide dismutase (SOD) was prevented. The extracts protected the skin from oxidative and direct DNA damage, and reduced lipid peroxidation. The induction of cyclooxygenase-2 (COX-2) and ornithine decarboxylase (ODC) was also reduced. Topical application of the polyphenols hesperidin and mangiferin showed reduced protective effects compared to the extracts. Suggesting the possible mechanisms by which honeybush and the polyphenols protect against photocarcinogenesis such as reducing tumour promotion, inflammation and oxidative stress. Suggesting the benefits of including honeybush and rooibos as cosmeceuticals in skin care products and sunscreens as part of the strategy for preventing skin cancer. Discussing the recommendations for further study such as investigating more specific chemopreventive activities of these two South African herbal teas and their polyphenols, dose response studies and clinical evaluations.