Conversion Of 25-hydroxyvitamin D3 Research Articles

Vitamin D upregulates the macrophage complement receptor immunoglobulin in innate immunity to microbial pathogens

Vitamin D deficiency remains a global concern. This ‘sunshine’ vitamin is converted through a multistep process to active 1,25-dihydroxyvitamin D3 (1,25D), the final step of which can occur in macrophages. Here we demonstrate a role for vitamin D in innate immunity. The expression of the complement receptor immunoglobulin (CRIg), which plays an important role in innate immunity, is upregulated by 1,25D in human macrophages. Monocytes cultured in 1,25D differentiated into macrophages displaying increased CRIg mRNA, protein and cell surface expression but not in classical complement receptors, CR3 and CR4. This was associated with increases in phagocytosis of complement opsonised Staphylococcus aureus and Candida albicans. Treating macrophages with 1,25D for 24 h also increases CRIg expression. While treating macrophages with 25-hydroxyvitamin D3 does not increase CRIg expression, added together with the toll like receptor 2 agonist, triacylated lipopeptide, Pam3CSK4, which promotes the conversion of 25-hydroxyvitamin D3 to 1,25D, leads to an increase in CRIg expression and increases in CYP27B1 mRNA. These findings suggest that macrophages harbour a vitamin D-primed innate defence mechanism, involving CRIg.

Side-chain oxidation of vitamin D3 in mouse kidney mitochondria: effect of the Hyp mutation and 1,25-dihydroxyvitamin D3 treatment.

Side-chain oxidation of vitamin D is an important degradative pathway. In the present study we compared the enzymes involved in side-chain oxidation in normal and Hyp mouse kidney. Homogenates of normal mouse kidney catalyze the conversion of 25-hydroxyvitamin D3 to 24,25-dihydroxyvitamin D3, 24-oxo-25-hydroxyvitamin D3 and 24-oxo-23,25-dihydroxyvitamin D3. After subcellular fractionation, total side-chain oxidative activity, estimated by the sum of the three products synthesized per milligram protein under initial rate conditions, coincided with the mitochondrial enzyme marker succinate-cytochrome-c reductase. Treatment of normal mice with 1,25-dihydroxyvitamin D3 (1.5 ng/g) resulted in an eightfold increase in mitochondrial enzyme activity, with no change in apparent Km but a significant rise in Vmax. With 24,25-dihydroxyvitamin D3 as the substrate, normal renal mitochondria produced 24-oxo-25-hydroxyvitamin D3 and 24-oxo-23,25-dihydroxyvitamin D3, and the synthesis of these metabolites could be increased sixfold by pretreatment with 1,25-dihydroxyvitamin D3. In the Hyp mouse, the side-chain oxidation pathway showed similar subcellular distribution of enzyme activity. However, product formation from 25-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3 was twofold greater in mutant than in normal mitochondria. Furthermore, 1,25-dihydroxyvitamin D3 pretreatment of Hyp mice resulted in a 3.4-fold increase over basal metabolism of both 25-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3. These results demonstrate that (i) kidneys from normal and Hyp mice possess basal and 1,25-dihydroxyvitamin D3 inducible enzyme system(s) in the mitochondrial fraction, which catalyze the side-chain oxidation of 25-hydroxyvitamin D3 and 24,25-dihydroxyvitamin D3, and (ii) the Hyp mutation appears to perturb the renal metabolism of both substrates only in the basal state.

Opposing effects of cyclic adenosine-3',5'-monophosphate and cyclic guanosine-3',5'-monophosphate on the metabolism of 24-hydroxyvitamin D3 in isolated chick renal tubules.

The conversion of 25-hydroxyvitamin D3 (25 OH D3) to 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), 24,25-dihydroxyvitamin D3 (24,25-(OH)2D3) and 1,24,25-trihydroxyvitamin D3 (1.24,25-(OH)3D3) was studied in renal tubules prepared from chicks raised on a vitamin D deficient diet with or without vitamin D supplementation. As described previously, in tubules from vitamin D deficient chicks, cyclic AMP caused an increase in the net accumulation of 1,25-(OH)2D3, the major metabolite formed under these circumstances. This stimulation was shown to be due to an increased maximum velocity of the hydroxylation reaction. There was also a significant inhibition of the net accumulation of 24,25-(OH)2D3. Cyclic GMP caused a significant inhibition of 1,25-(OH)2D3 formation and stimulation of the net accumulation of 24,25-(OH)2D3. In chicks supplemented with high doses of vitamin D, 24,25-(OH)2D3 was the major metabolite of 25 OH D3 detected and 1-hydroxylase activity was negligible. Under these circumstances, neither cyclic AMP nor cyclic GMP affected net accumulation of 24,25(OH)2D3. This suggested that the apparent effect of the nucleotides on formation of 24,25-(OH)2D3 may have been due to further metabolism of 24,25-(OH)2D3 when 1-hydroxylase activity was high. It is concluded that cyclic AMp and cyclic GMP have reciprocal effects on renal 25 OH D3-1-hydroxylase activity, and both should be considered potential intracellular regulators of 25 OH D3 metabolism.

Maleic acid-induced impaired conversion of 25(OH)D3 to 1,25(OH)2D3: Implications for Fanconi's syndrome

Conversion of 25-hydroxyvitamin D3 [25(OH)D3] to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] occurs exclusively in the renal cortex. To determine whether a disorder of the renal cortical tubule capable of causing Fanconi's syndrome can also impair the synthesis of 1,25(OH)2D3, we investigated whether conversion of 25(OH)D3 to 1,25(OH)2D3 was reduced by maleic acid. In vitamin D-deficient rats, maleic acid was administered i.v. over two hours. Thirty minutes after its initiation, when the complex renal tubule dysfunction had occurred, 3H-25(OH)D3 was administered i.v. as a bolus. Five hours afterwards, the amount of 3H-1,25(OH)2D3 recovered in the kidney, small intestine mucosa, and blood was one-third to one-half that in tissues of control rats that received acetazolamide or only saline or were subjected only to the surgical procedure. The glomerular filtration rate, as measured by inulin clearance, did not decrease significantly with maleic acid. In intact vitamin D-deficient chicks, 24 and 22 hr after i.p. administration of maleic acid and 14C-vitamin D3, respectively, the amount of 14C-1,25(OH)2D3 recovered in small intestine mucosa was reduced by one-half when compared to saline-treated controls. In kidney homogenates and isolated renal tubules of vitamin D-deficient chicks, activity of 25-hydroxyvitamin D3-1-hydroxylase was diminished immediately after maleic acid was administered in vivo or added in vitro to the incubation medium, respectively. These data provide the first demonstration that the renal capacity to convert 25(OH)D3 to 1,25(OH)2D3 can be substantially impaired in vivo by a renal disorder in which the glomerular filtration rate is not reduced.

Influence of dietary calcium, phosphorus, and vitamin D on the conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 by kidney tubules of diphosphonate-treated quails

Influence of dietary calcium, phosphorus, and vitamin D on the conversion of 25-hydroxyvitamin D3 to 1,25-dihydroxyvitamin D3 by kidney tubules of diphosphonate-treated quails