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)

Potent Antiproliferative Effects of 25‐Hydroxy‐16‐ene‐23‐yne‐vitamin D3 That Resists the Catalytic Activity of Both CYP27B1 and CYP24A1

“…6A) shows no evidence of metabolism. We highlighted this important point in our previous study in which we investigated the metabolism of 25(OH)D 3 into 1a,25(OH) 2 D 3 in human bone cells [Siu-Caldera et al, 1995]. The HPLC profile of 25(OH)D 3 (Fig.…”

Vitamin D and calcium deficits predispose for multiple chronic diseases

“…Importantly, 1,25‐(OH) 2 D 3 is also produced at a number of extra‐renal sites, as apart from renal tubular cells, many other cell types express the 25‐(OH)D‐1α‐hydroxylase (CYP27B1). For example, CYP27B1 activity has been localized in osteoblasts [11], colonocytes [12,13], macrophages [14,15], synovial cells [16], keratinocytes [17], hair follicles, adrenal medulla, pancreatic islets [18], and vascular endothelial cells [19]; for details see Table 1.…”

“…In other words, in proliferating osteoblasts, 1,25‐(OH) 2 D 3 reduces expression of type I collagen, osteopontin and osteocalcin, whereas in mature postproliferative osteoblasts, 1,25‐(OH) 2 D 3 up‐regulates expression of genes encoding not only for matrix proteins but also for osteocalcin. It is conceivable therefore that to the extent to which osteoblast differentiation is under autocrine/paracrine control by 1,25‐(OH) 2 D 3 , low CYP27B1 activity in osteoblasts [11] will become a limiting factor for mineralized matrix maturation and bone formation. A compromised vitamin D status therefore not only causes rickets and osteomalacia but is also an important risk factor for osteoporosis, which is often caused by an insufficient rate of bone formation.…”

1α,25‐Dihydroxy‐3‐epi‐vitamin D3: In vivo metabolite of 1α,25‐dihydroxyvitamin D3 in rats

“…It is now well established that the secosteroid hormone, 1α,25‐dihydroxyvitamin D 3 (1α,25(OH) 2 D 3 ) is metabolized in its various target tissues, such as the kidney [1, 2], intestine [3–5], keratinocytes [6, 7], bone [8–14]and cartilage [8, 16], through both C‐24 and C‐23 oxidation pathways. The C‐24 oxidation pathway, initiated by C‐24 hydroxylation, leads to the conversion of 1α,25(OH) 2 D 3 into calcitroic acid [2, 17], while the C‐23 oxidation pathway, initiated by C‐23 hydroxylation, leads to the conversion of 1α,25(OH) 2 D 3 into calcitriol lactone [1, 4, 18, 19].…”