Human keratinocyte line HaCaT metabolizes 1α-hydroxyvitamin D3 and vitamin D3 to 1α,25-dihydroxyvitamin D3 (calcitriol)

Cultured human keratinocytes have the property to hydroxylate exogenous 25-hydroxyvitamin D-3 (25OHD(3)) at the C-1 alpha position thus producing 1 alpha,25-dihydroxyvitamin D-3 (1 alpha,25(OH)(2)D-3). In this study we investigated whether keratinocytes can also hydroxylate vitamin D-3 and one of its metabolites at the C-25 position. We could demonstrate that HaCaT keratinocytes can metabolize 1 alpha-hydroxyvitamin D-3 (1 alpha-OHD3) and vitamin D-3 to 1 alpha,25(OH)(2)D-3. Identification of the generated product as 1 alpha,25(OH)(2)D-3 was based on its elution pattern in two different high performance liquid chromatography systems, on its specific binding in a calf thymus receptor assay and on its gas chromatography-mass spectrometry characteristics. The hydroxylation of vitamin D-3 to 1 alpha,25(OH)(2)D-3 was dose- and time-dependent. Bovine serum albumin added up to 1.5% (w/v) to the culture medium greatly increased the hydroxylation rates. These results show that HaCaT cells have the capacity to hydroxylate vitamin D-3 at the C-1/25 positions. The generation of endogenous 1 alpha,25(OH)(2)D-3 from vitamin D-3 within the skin may indicate a novel pathway which is of importance for the regulation of epidermal cell growth and differentiation. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.