title: Human osteoblasts in culture metabolize both 1 alpha, 25-dihydroxyvitamin D3 and its precursor 25-hydroxyvitamin D3 into their respective lactones.
abstract: 1 alpha, 25-Dihydroxyvitamin D3 [1 alpha, 25-(OH)2D3], the hormonal form of vitamin D3, is further metabolized in the kidney and intestine through the carbon 24 (C-24) oxidation pathway initiated by C-24 hydroxylation, and the carbon 23 (C-23) oxidation pathway initiated by C-23 hydroxylation. The C-24 oxidation pathway leading to the formation of calcitroic acid has been previously reported to be present in bone cells, but the C-23 oxidation pathway leading to the formation of 1 alpha, 25-(OH)2D3-26,23-lactone has not been described in bone cells, even though 1 alpha, 25-(OH)2D3-26,23-lactone is noted to have a significant effect on bone formation. Therefore, in the present study, we investigated the production of 1 alpha, 25-(OH)2D3-26,23-lactone in normal human osteoblasts, and our studies revealed that human osteoblasts possess the activity of both 24- and 23-hydroxylases constitutively. Thus, 1 alpha, 24(R),25-(OH)3D3, 1 alpha, 25-(OH)2-24-oxo-D3, 1 alpha, 23(S), 25-(OH)3-24-oxo-D3, 1 alpha, 23-(OH)2-24,25,26,27-tetranor D3, and calcitroic acid formed through the C-24 oxidation pathway and 1 alpha, 23(S),25-(OH)3D3 and 1 alpha, 25-(OH)2D3-26,23-lactone formed through the C-23 oxidation pathway were detected under basal conditions. Also, the synthesis of these metabolites was increased significantly when the cells were treated with 1 alpha, 25-(OH)2D3 (50 nM) for 24 h before incubation with the tracer. As 25-hydroxyvitamin D3 (25OHD3) follows similar side-chain modifications as 1 alpha, 25-(OH)2D3, the metabolism of 25OHD3 in normal human osteoblasts was studied under basal conditions. We found that 25OHD3 was also metabolized through both C-24 and C-23 oxidation pathways, resulting in significant synthesis of 24(R),25-(OH)2D3 along with 25OH-24-oxo-D3, 23(S),25-(OH)2-24-oxo-D3, 23(S),25-(OH)2D3, and 25OHD3-26,23-lactone. Under the same experimental conditions, we looked for 1 alpha, 25-(OH)2D3 synthesis, as earlier studies have shown production of 1 alpha, 25-(OH)2D3 in human bone cells. During a time-course study ranging from 1-24 h, we found that by 2 h, the 24(R), 25-(OH)2D3 concentration rose and accumulated considerably during the following 24 h, but 1 alpha, 25-(OH)2D3 did not accumulate at any time. However, other 1-hydroxylated metabolites, 1 alpha, 23(S),25-(OH)3D3, 1 alpha, 23(S),25-(OH)3-24-oxo-D3, as well as 1 alpha, 25-(OH)2D3-26,23-lactone were detected.(ABSTRACT TRUNCATED AT 400 WORDS)
authors.affiliation: Department of Pediatrics, Women and Infants Hospital of Rhode Island, Providence 02905, USA.
authors.forename: M L
authors.initials: ML
authors.lastname: Siu-Caldera
authors.forename: L
authors.initials: L
authors.lastname: Zou
authors.forename: M G
authors.initials: MG
authors.lastname: Ehrlich
authors.forename: E R
authors.initials: ER
authors.lastname: Schwartz
authors.forename: S
authors.initials: S
authors.lastname: Ishizuka
authors.forename: G S
authors.initials: GS
authors.lastname: Reddy
mesh_terms: D002112:Calcifediol / Q000378:metabolism*; D002117:Calcitriol / Q000378:metabolism*; D002478:Cells, Cultured; D006801:Humans; D007783:Lactones / Q000378:metabolism*; D010006:Osteoblasts / Q000378:metabolism*
chemical_list: D007783:Lactones; D002117:Calcitriol; D002112:Calcifediol