Topical β‐carotene is converted to retinyl esters in human skin ex vivo and mouse skin in vivo

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Abstract
Abstract:  Human epidermis contains endogenous retinoids (retinol and retinyl esters) and carotenoids (mostly β‐carotene). Previous studies have shown that the enzymes involved in retinoid metabolism are present in human epidermis. There is still a controversy about the presence in the skin of the enzymes able to convert β‐carotene into vitamin A (retinol), although a recent study demonstrated the conversion of β‐carotene into retinol in human cultured epidermal cells. In this study, we addressed the question of the possible bioconversion of topical β‐carotene into vitamin A or derivatives by human and mouse skin. Surgically excised human abdominal skin was mounted on Franz perfusion chambers to assess the cutaneous penetration of topical β‐carotene as well as its metabolism, after a 24‐h incubation period, whereas hairless mice received topical β‐carotene 24 h before assaying epidermal β‐carotene and retinoid concentrations. Epidermal retinoid and β‐carotene concentrations were determined by high‐pressure liquid chromatography. Topical β‐carotene penetrated well into human and mouse epidermis and induced a 10‐fold (human) and a threefold (mouse) increase of epidermal retinyl esters, which demonstrates that topical β‐carotene is converted into retinyl esters by human and mouse epidermis and thus appears as a precursor of epidermal vitamin A.