Conversion of vitamin D3 to hormonally active 1α,25-dihydroxyvitamin D3 in cultured keratinocytes: Relevance to cell growth and differentiation ☆

Abstract The regulatory potential of intracellularly generated calcitriol on growth and differentiation of cultured keratinocytes is determined by the degree of cell confluence and availability of the highly lipophilic substrate vitamin D 3 to these cells. The enzymatic conversion of vitamin D 3 to calcitriol is considerably elevated in the presence of the nontoxic surfactant copolymer pluronic ® F127 (120 μg/ml medium) compared to the control without this agent. We found a positive correlation between the formation rate of calcitriol and inhibition of the 3 H-thymidine incorporation rate into the DNA of keratinocytes. Intracellularly generated calcitriol causes a clear increase of the cell diameter, and thus has a prodifferentiating effect on keratinocytes in vitro . These findings corroborate the hypothesis that UVB-induced production of vitamin D 3 in human skin results in formation of substantial amounts of calcitriol in keratinocytes which suppress the growth and initiate differentiation of these cells.