Rapid publication: Constitutive expression of a vitamin D 1‐hydroxylase in a myelomonocytic cell line: A model for studying 1,25‐dihydroxyvitamin D production in vitro

The capacity of the v‐myc‐transformed, chicken myelomonocytic cell line HD‐11 to metabolize 25‐hydroxyvitamin D₃ (25‐OHD₃) was examined. HD‐11 cells produced and secreted a metabolite of 25‐OHD, that was bound with high affinity by receptor for 1,25‐dihydroxyvitamin D₃ [1,25‐(OH)₂D₃]. On normal‐phase HPLC, this metabolite cochromatographed with authentic 1,25‐(OH)₂D₃ in both hexane‐ and methylene chloride‐based solvent systems. The 25‐OHD, 1‐hydroxylation reaction was substrate saturable with a K_m of 73 nM 25‐OHD₃ and a maximal velocity of 167 fmol per 10⁶ cells per h. This reaction was inhibited by keto‐conazole, a recognized inhibitor of cytochrome P₄₅₀ mixed‐function oxidases including the authentic, renal 25‐OHD3 1‐hydroxylase. On the other hand, HD‐11 cell 1,25‐(OH)₂D₃ production was not affected by the antioxidant DPPD, a known inhibitor of free radical‐generated 1,25‐(OH)₂D₃. In addition to synthesizing 1,25‐(OH)₂D₃, this monocyte‐macrophage cell line also has the potential to be a target for the hormone; HD‐11 cells express high‐affinity receptor for 1,25‐(OH)₂D₃ (K_in = 0.06 nM). Division of Endocrinology and Metabolism, Cedars‐Sinai Medical Center‐UCLA School of Medicine, Los Angeles, CA 90048.