Modulation of Human Keratinocyte Responses to Solar UV by Plant Polyphenols As a Basis for Chemoprevention of Non-Melanoma Skin Cancers

Excessive exposure to solar UVA and UVB radiation is widely considered to cause skin cancers such as squamous cell carcinoma and basalioma. Direct UVB damage to skin cell DNA as well as UV-induced chronic skin inflammation, accelerated keratinocyte proliferation, inhibited apoptosis, and immunosuppression seem to underlie the UV-induced carcinogenesis. Also, UVB induces cytochrome P450 subfamilies (CYP1A1 and CYP1B1) involved in metabolic activation of organic pro-carcinogens and their conversion to ultimate carcinogens. Here, the effects of several glycosylated and non-glycosylated plant polyphenols (verbascoside, resveratrol, polydatin, rutin, and quercetin) on the inflammatory, apoptotic, metabolic, and proliferative responses of cultured human epidermal keratinocytes (HEK) to non-cytotoxic doses of solar-simulated UVA+UVB and chemical mediators of UV signalling in HEK, 6- formylindolo[3,2-b]carbazole and squalene isolated from photo-oxidized skin surface lipids (SSL), were evaluated. We showed that the stilbenes and quercetin being exposed to UV were photo-destroyed within a short period of time, while verbascoside and rutin were photo-stable. When SSL were exposed to UV, the stilbenes and quercetin remarkably accelerated photo-oxidation of alpha-tocopherol, squalene, and cholesterol fractions, whilst verbascoside protected them. Verbascoside invariably inhibited molecular pathways in HEK leading to inflammatory cytokine expression (NFkappaB and EGFR/ERK phosphorylation), and cell proliferation (EGFR nuclear translocation), and displayed a stimulus-specific effect on the metabolic axis aryl hydrocarbon receptor (AhR)-CYP1A1/CYP1B1. By contrast, the stilbenes inhibited UV-connected inflammatory cytokines excluding IL-8, but they prevalently stimulated NFkappaB, EGFR nuclear translocation and the AhR-CYP pathway. We conclude that, among the PPs investigated, verbascoside does interfere with multiple UV-sensitive signalling in HEK in a way that it could have a major impact on skin cancer chemoprevention.