Dietary flavonoids have been extensively researched in relation to health benefits in humans. The regular consumption of green tea catechins (GTC) has been associated with a reduction in the risk of developing diabetes, cardiovascular disease and cancer. Flavonoids are known to protect plants from the damage caused by exposure to UV radiation, and this effect has also been observed when flavonoids are applied topically to human skin cells. The effect of oral consumption of flavonoids on skin protection from UV exposure is not clear.
The work presented within this thesis aimed to investigate the effect of GTC on the response of skin cells to UV induced-stress. Keratinocyte cells from an immortalised human skin cell line (HaCaT) were assessed after exposure to various stress conditions in vitro (FBS starvation, hydrogen peroxide and UV), in combination with a pre-treatment of green tea extract or a purified mixture of GTC. GTC reduced cell death induced by stress (decrease in LDH release), and maintained viability (increase in MTT uptake) in HaCaT cells, relative to control treatments. The uptake of vitamin C, a photo-protective agent depleted after UV exposure, was enhanced by treatment with GTC during stress conditions, as monitored by uptake of 14C-dehydroascorbic acid and evaluation of vitamin C transporters with qRT-PCR. In relation to in vivo conditions, GTC may provide protection and also enhance vitamin C uptake into skin cells undergoing stress.
Bioavailability of GTC and metabolites in human skin cells after daily consumption of green tea and vitamin C supplements for 3 months was also investigated. Catechin metabolites in a range of tissues (plasma, interstitial blister fluid, skin biopsies and urine) were identified with LC-MS-MS in unconjugated and conjugated (sulphate, methyl and glucuronide) forms. For the first time, conjugated catechin metabolites were identified in skin tissue samples and extracellular fluid surrounding skin cells; including M6/M6'-O-sulphate, O-methyl-EC-O-sulphate,EC-O-sulphate and EGC-O-glucuronide, with metabolites identified in urine and plasma post-consumption similar to data reported in the literature.
The work presented in this thesis provides new knowledge on bioavailability of GTC and metabolites in human skin, which together with vitamin C, may exert UV protection and other health benefits. Further research is required in vitro using pure conjugated standards (methyl, glucuronide and sulphate moieties), and data corresponding to the inflammatory biomarkers post-UV exposure (analysis at the University of Manchester and University of Bradford) is also required before a conclusive relationship can be drawn between oral consumption of flavonoids and UV protection.
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