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UVB‐induced 1,25(OH)2D3 production and vitamin D activity in intestinal CaCo‐2 cells and in THP‐1 macrophages pretreated with a sterol Δ7‐reductase inhibitor

Epidermal keratinocytes are able to produce 1,25‐dihydroxyvitamin D3 [1,25(OH)2D3] and induce vitamin D activity upon UVB irradiation. To find out whether this property is keratinocyte specific, we investigated this characteristic in two other cell types, namely intestinal CaCo‐2 cells and the macrophage‐like differentiated THP‐1 cells. THP‐1 macrophages and preconfluent CaCo‐2 cells contain the vitamin D receptor (VDR), possess 25‐hydroxylase (CYP2R1 and CYP27A1) and 1α‐hydroxylase (CYP27B1) activity, and survive the low UVB doses essential for vitamin D3 photoproduction. Upon irradiation, 24‐hydroxylase (CYP24) mRNA is induced in both cell types pretreated with the sterol Δ7‐reductase inhibitor BM15766 whereby the 7‐dehydrocholesterol (7‐DHC) content was increased. Transfection studies in CaCo‐2 cells with a vitamin D response element‐containing construct revealed the involvement of the VDR in this UVB‐dependent CYP24 induction. The CYP24 inducing activity in BM15766‐pretreated UVB‐irradiated CaCo‐2 cells and THP‐1 macrophages was identified as 1,25(OH)2D3 by combined high‐performance liquid chromatography radioimmunoassay. Addition of vitamin D binding protein to the CaCo‐2 cells attenuated UVB‐induced CYP24 induction suggesting the possibility of a paracrine or autocrine role for the photoproduced 1,25(OH)2D3. In conclusion, preconfluent CaCo‐2 cells and THP‐1 macrophages are able to induce vitamin D activity upon UVB irradiation and hence combine all parts of the vitamin D photoendocrine system, a characteristic which is therefore not keratinocyte specific. J. Cell. Biochem. © 2006 Wiley‐Liss, Inc.