Biochemical characterization of 1,25(OH)2D3 receptors in chick embryonal duodenal cytosol

Synthesis and biological evaluations of A-ring isomers of 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3

The activated vitamin D3 derivative 26,27-F6-1alpha,25(OH)2D3 (2a), its three A-ring diastereomers (2b, 2c, 2d), and 5,6-trans isomer (2e) were prepared. Two analogues (2b, 2c) of these isomers were synthesized by a palladium catalyzed coupling reaction using vinyl bromide 5 and enynes (6a, 6b), which were derived from readily commercially available 2S-(+)-glycidyl p-toluenesulfonate 7, as a common starting material. Competitive vitamin D receptor (VDR) binding affinities of these diastereomers of 2a were evaluated. Interestingly, the stereochemical effects at C-1,3 of 2a were considerably more moderate than those of 1alpha,25(OH)2D3 (1). In particular, isomerization at the 5,6-double bond of 2a only slightly reduced VDR affinity, whereas 5,6-trans-1alpha,25(OH)2D3 had a significantly lower binding affinity than 1.

Synthesis and biological evaluations of C-23-modified 26,26,26,27,27,27-F6-vitamin D3 analogues

A convenient synthetic method which could allow flexible modification at C-23 of 26,26,26,27,27,27-hexafluoro-1alpha,25-dihydroxyvitamin D3 (3) has been developed. An effective construction of hexafluoroacetone (HFA) aldol part on the side chain of 10 was achieved by aldol reaction with HFA gas. This route is also attractive as an approach to diverse 26,27-modified vitamin D3 analogues. The preliminary biological activities of 23-modifed 26,27-F6 vitamin D3 analogues are evaluated. The potency of VDR affinities of the C-23-substituted analogues (keto group (4); OH group (5a,5b); fluorine atom (6a,6b); and oxetane ring (7a,7b)) was found to vary depending upon both the nature and stereochemistry of the substituents. In contrast, the HL-60 cell differentiation property was less varied than VDR affinity, and depended upon the nature rather than the stereochemistry of the substituents.

Positive and negative regulations of the renal 25-hydroxyvitamin D3 1alpha-hydroxylase gene by parathyroid hormone, calcitonin, and 1alpha,25(OH)2D3 in intact animals

Reflecting the prime role of 1alpha,25(OH)2D3 in calcium homeostasis, the activity of 25-hydroxyvitamin D3 1alpha-hydroxylase, a key enzyme for 1alpha,25(OH)2D3 biosynthesis, is tightly regulated by 1alpha,25(OH)2D3, PTH and calcitonin. Its significant activity is found in kidney, though the enzymatic activity is also reported in extra-renal tissues. In the present study, we found that the 1alpha-hydroxylase gene abundantly expresses in kidney, and at low levels in other tissues and in some cell lines. Positive and negative regulations of 1alpha-hydroxylase gene by PTH, calcitonin, or 1alpha,25(OH)2D3 were observed at transcriptional levels in kidneys of animals and in a mouse proximal tubule cell line. Moreover, the protein kinase A inhibitor abrogated the PTH-mediated positive regulation. In mice lacking the vitamin D receptor, the 1alpha-hydroxylase gene expression was overinduced, and the inducible effect of either PTH or calcitonin, but not the repression by 1alpha,25(OH)2D3, was evident. Thus, vitamin D receptor is essential for the negative regulation by 1alpha,25(OH)2D3. Moreover, we demonstrate that renal 1alpha-hydroxylase gene expression in chronic renal failure model rats was decreased and the positive effect by PTH and calcitonin was diminished. The present study demonstrates that PTH and calcitonin positively regulate renal 1alpha-hydroxylase gene expression via PKA-dependent and independent pathway, respectively, and that 1alpha,25(OH)2D3 negatively regulates it mediated by vitamin D receptor. Furthermore, in a moderate state of chronic renal failure, renal cells expressing the 1alpha-hydroxylase gene appear to have diminished potential in response to PTH and calcitonin.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.

25-hydroxycholecalciferol in poultry nutrition

Vitamin D is a complex of secosteroids that must undergo metabolic alterations to reach optimal biological activity. The parent compounds 1) ergocalciferol (D2) and 2) cholecalciferol (D3) can be synthesized in the leaves of many plants or in the skin of most animals, respectively. Transport of vitamin D steroids after absorption is associated with vitamin D binding proteins (DBP). In general, the relative binding affinities of the vitamin D steroids are: 25-hydroxy vitamin D3 [25-(OH)D3] = 24,25-dihydroxy vitamin D3 [24,25-(OH)2D3] = 25,26-dihydroxy vitamin D3 [25,26-(OH)2D3] > 25-hydroxy vitamin D2 (25-(OH)D2) > 1,25-dihydroxy vitamin D3 [1,25-(OH)2D3] > vitamin D3. The DBP in poultry does not bind D2 forms effectively, and therefore poultry can not use this form of vitamin D adequately. The concentration of 25-(OH)D3 in blood seems to be well correlated with dietary vitamin D intake or exposure to ultraviolet light. The 1 alpha hydroxylase enzyme in the kidney is subject to negative feedback regulation and is critical for formation of the active metabolite 1,25-(OH)2D3. The intracellular vitamin D receptor (VDR) specifically binds 1,25-(OH)2D3 and is necessary for cellular action. Increased levels of two to three orders of magnitude are required for 25-(OH)D3 to compete with 1,25-(OH)2D3 for binding on VDR. Feeding studies with 25-(OH)D3 suggest it has nearly twice the activity of vitamin D3. Hatchability studies have shown that 25-(OH)D3 supports good fertility and hatchability, whereas hens fed only 1,25-(OH)2D3 did not have normal hatchability. Likewise, 1,25-(OH)2D3 seems to reach toxic levels at dietary concentrations only two to three times optimal dietary levels whereas feeding 25-(OH)D3 for extended periods at levels 8 to 10 times requirement seems to have no adverse effects. It seems that 25-(OH)D3 is the most active metabolite of vitamin D3, ultimately capable of supporting both cellular functions and embryonic development in chickens and turkeys when fed as the sole source of vitamin D3.

Tandem immunoaffinity chromatography for plasma 1 alpha,25-dihydroxyvitamin D3 utilizing two antibodies having different specificities: a novel and powerful pretreatment tool for 1 alpha,25-dihydroxyvitamin D3 radioreceptor assays

We report here a novel and powerful pretreatment method for radioreceptor assays (RRAs) for human plasma 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) based on "Tandem" immunoaffinity chromatography (Tandem IAC). Two antibodies having different specificities were each immobilized on agarose gel with cyanogen bromide to produce immunosorbents which were stable and repeatedly usable. An ethyl ether extract of plasma was applied to the first affinity column, from which 1,25(OH)2D3 could be preferentially eluted and separated from 1 alpha-deoxy type metabolites. The effluent was then submitted to the second column, and the 1,25(OH)2D3 retained was eluted after non- or weakly-absorbed interfering substances were washed out. This procedure allowed efficient purification without careful handling or strict time-management in the entire operation and enabled avoiding preparative high-performance liquid chromatography (HPLC) from RRA even with a conventional chick intestinal vitamin D receptor. Mean (+/- SD) plasma 1,25(OH)2D3 values of 56 normal subjects and 10 patients with chronic renal failure, obtained with this Tandem IAC/RRA system, were 36.4 (8.7) and 11.2 (4.0) pg/ml, respectively. The Tandem IAC will also be useful for developing immunoassays or gas chromatography-mass spectrometry of 1,25(OH)2D3.

Calcium regulating activity of 26,27-dimethyl analog of 24R,25-dihydroxyvitamin D3

To determine the possibility that methyl substitution in 26- and 27-positions of 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3] alters activities of the original compound, the effects of 24,25(OH)2D3 on calcium (Ca) regulating activity were compared with those of its methyl analog [24,25(OH)2(CH3)2D3] in addition to 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3]. 24,25(OH)2D3 at 10(-6) M and 24,25(OH)2(CH3)2D3 at 10(-7) M and above significantly stimulated both bone resorption in neonatal mouse calvaria cultures and formation of osteoclast-like multinucleated cells (MNC) in mouse bone marrow cultures. A stimulative effect of 1,25(OH)2D3 on bone resorption and MNC formation was recognized in very low concentrations (10(-11) M and above). Although a potency of 24,25(OH)2(CH3)2D3 in stimulating bone calcium (Ca) mobilization and intestinal Ca transport was higher than that of 24,25(OH)2D3, the potencies of both compounds were similar to that of 1,25(OH)2D3 unlike in vitro experiments. As 1,24R,25-trihydroxy-26,27-dimethylvitamin D3 showed almost the same effect as 24,25(OH)2(CH3)2D3, the dihydroxy form is suggested to be hydroxylated at 1 alpha position and converted to trihydroxy form in vitamin D-deficient rats. From these results, methyl substitution in 26- and 27-position of 24,25(OH)2D3 was found to elevate Ca regulating activity of the original compound. In addition, it is suggested that the basis for a similarity in potency between 1,25(OH)2D3 and 24,25(OH)2D3 or its dimethyl analog in vitamin D-deficient rats is likely the result of 1 alpha-hydroxylation.

Can renal handling of phosphate predict response to growth hormone therapy in normal variant short children by short-term treatment?

We investigated if renal handling of phosphate could predict height velocity in 28 normal variant short children (16 boys and 12 girls). Before and after human growth hormone was given for four consecutive days, the ratio of maximum tubular reabsorption rate for phosphorus to glomerular filtration rate (TmP/GFR) was calculated. Based on increments in TmP/GFR (delta TmP/GFR) with growth hormone administration, the patients were divided into two groups; children in whom the levels of delta TmP/GFR were 0.8 mg/dl GF or more (group A, n = 7) and those with levels less than 0.8 mg/dl GF (group B, n = 21). All children in group A and some in group B (n = 9) were injected with 0.5 IU/kg/week of recombinant human growth hormone for over one year. Height velocity during therapy was significantly greater in treated children in group A than in group B and was similar among treated (n = 9) and untreated (n = 12) children in group B. The present study suggests that change in renal handling of phosphate during short-term growth hormone administration can serve to select normal variant short children who will respond well to growth hormone therapy.

Calcium regulating activity of 24a-homo-24,24-difluoro-1 alpha,25-dihydroxyvitamin D3 and 26,27-dimethyl-24,24-difluoro-1 alpha,25-dihydroxyvitamin D3

Two fluoro analogs of 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3], 24a-homo-24,24-difluoro-1 alpha,25-dihydroxyvitamin D3 [24aF2-homo-1,25(OH)2D3], and 26,27-dimethyl-24,24-difluoro-1 alpha,25-dihydroxyvitamin D3 [24F2-1,25(OH)2(Me)2D3] were examined for calcium (Ca)-regulating activity. The objective of the present study was to determine whether or not fluoro substitution at 24-position would alter activities of the original compounds, that is, 26,27-dimethyl 1 alpha,25-dihydroxyvitamin. D3[1,25(OH)2(Me)2D3] and 24-homo-1 alpha,25-dihydroxyvitamin D3 [24homo-1,25(OH)2D3], respectively. The relative activities of 24aF2-homo-1,25(OH)2D3, 24F2-1,25(OH)2(Me)2D3, and 1,25(OH)2D3 in competing with 1,25(OH)2D3 for binding to chick intestinal cytosol receptor were 0.28:0.5:1.0. The relative potencies of the same series of compounds in competition for the vitamin D-deficient rat serum binding sites were 0.04:0.15:1. Bone-resorbing activities of two fluoro analogs in cultures of neonatal mouse parietal bones were more potent than that of 1,25(OH)2D3. Similar results were recognized in stimulating activities of osteoclast-like cell formation. Responses of two fluoro analogs to intestinal Ca absorption were similar to that of 1,25(OH)2D3. The potencies of 1,25(OH)2D3 and its fluoro analogs in bone Ca mobilization were the highest with 1,25(OH)2D3, followed by 24F2-1,25(OH)2(Me)2D3 and 24aF2-homo-1,25(OH)2D3, in that order. From these results and the data of Paulson et al., fluoro substitution in 24-position of 1,25(OH)2D3 apparently does not alter their activities,hence, the fluoro substitution at 24-position of 1,25(OH)2D3 and the elongation of side chain of 1,25(OH)2D3 may not intensify Ca-regulating activity.

Transformation of vitamin D3 to 1 alpha,25-dihydroxyvitamin D3 via 25-hydroxyvitamin D3 using Amycolata sp. strains

To enzymatically synthesize active metabolites of vitamin D3, we screened about 500 bacterial strains and 450 fungal strains, of which 12 strains were able to convert vitamin D3 to 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] via 25-hydroxyvitamin D3 [25(OH)D3]. The conversion activity was only detected in strains belonging to the genus Amycolata among all the organisms tested. A preparative-scale conversion of vitamin D3 to 25(OH)D3 and 1 alpha,25(OH)2D3 in a 200-1 tank fermentor using A. autotrophica FERM BP-1573 was accomplished, yielding 8.3 mg 25(OH)D3/l culture and 0.17 mg 1 alpha,25(OH)2D3/l culture. A related compound, vitamin D2, could be also converted to 25-hydroxyvitamin D2 and 1 alpha,25-dihydroxyvitamin D2 using the same strain. The cytochrome P-450 of FERM BP-1573 was detected by reduced CO difference spectra in whole-cell suspensions. Vitamin D3 in the culture induced cytochrome P-450 and the conversion activity simultaneously, suggesting that the hydroxylation at C-25 of vitamin D3 and at C-1 of 25(OH)D3 originates from cytochrome P-450.

Calcium regulating activity of 26,27-dialkyl analogs of 1 alpha,25-dihydroxyvitamin D3

A series of analogs of 1 alpha,25-dihydroxyvitamin D3[1,25(OH)2D3] with alkyl substitutions in 26- and 27-positions were tested for calcium (Ca) regulating activity. The potencies of dialkyl analogs in stimulating bone resorption in neonatal mouse calvaria cultures were the highest in 1 alpha,25-dihydroxy-26,27-dimethylvitamin D3[1,25(OH)2-(Me)2D3], followed by 1,25(OH)2D3, 1 alpha,25-dihydroxy-26,27-diethylvitamin D3[1,25(OH)2(Et)2D3], and 1 alpha,25-dihydroxy-26,27-dipropylvitamin D3[1,25(OH)2(Pr)2D3] in that order. A similar order of potential regarding formation of osteoclast-like cells in mouse bone marrow cell cultures and on bone Ca mobilization with long-term vitamin D-deficient rats was observed in the same series. The relative potencies of 1,25(OH)2D3, 1,25(OH)2(Me)2D3, 1,25(OH)2(Et)2D3, and 1,25(OH)2(Pr)2D3 in competing with 1,25(OH)2D3 for binding to chick intestinal cytosol receptors were 1:1:0.16:0.036. A similar order of potential in case of intestinal Ca transport in situ was observed in the same series. The potencies of dialkyl analogs in competing with 25-hydroxy-vitamin D3 for binding to rat serum vitamin D binding protein were much lower than that of 1,25(OH)2D3. Effect of 1,25(OH)2(Me)2D3 on osteopenia in rats induced by ovariectomy and right sciatic neurotomy was higher than that of 1,25(OH)2D3. From these results, the lengthening by one carbon at 26- and 27-positions was shown to maintain the Ca regulatory activity of 1,25(OH)2D3.

Evaluation of vitamin D-binding protein and vitamin D metabolite loss in children on continuous ambulatory peritoneal dialysis

We measured the serum concentration of vitamin D-binding protein (DBP) in children with chronic renal failure (CRF). We also evaluated the relationships between the peritoneal loss of vitamin D metabolites, DBP and albumin in nine children on continuous ambulatory peritoneal dialysis (CAPD). The serum levels of DBP in children with CRF were significantly higher than in normal children. The mean serum DBP level in CRF children undergoing CAPD was slightly lower than in CRF patients who were not on dialysis. In patients on CAPD, the peritoneal loss of 25-hydroxyvitamin D (25OHD) showed a significant positive correlation with the DBP concentration in the dialysate (r = 0.855, P less than 0.005). In contrast, the peritoneal loss of 1,25-dihydroxyvitamin D (1,25(OH)2D) showed a significant correlation with the loss of albumin in the dialysate (r = 0.779, P less than 0.01). The synthesis of 1,25(OH)2D3 is reduced in advanced renal failure, and the peritoneal losses of the active vitamin D sterols in patients on CAPD may aggravate this deficiency. We recommend that supplementation of active form of vitamin D, such as 1 alpha-hydroxyvitamin D3 or 1,25(OH)2D3, is important in CAPD patients, particularly those with elevated peritoneal loss of DBP and/or albumin.

Variation during development in the response of chicken embryos to calcitriol administered via slow-release pellets

Embryonic chickens were exposed to 0, 30, or 300 pg of calcitriol per day via slow-release pellets implanted adjacent to the chorioallantoic membrane. Pellets were placed in eggs on Days 10 and 15, and eggs were sampled on Days 12 and 17, respectively. The hormone induced high mortality among embryos receiving pellets on Day 10, but not among those whose treatment was begun on Day 15. Embryos receiving hormone were hypercalcemic and hypophosphatemic on both Day 12 and Day 17, but the concentration of magnesium in plasma was not affected. Size of embryos sampled on Day 12 was not affected by hormone treatment, but embryos sampled on Day 17 showed a dose-related reduction in size. Yolk-free carcasses of the embryos sampled on Day 17 also showed dose-dependent reductions in phosphorus and magnesium, but calcium content of carcasses on Day 17 was unaffected by treatment. These results indicate that both younger and older embryos respond to very small quantities of calcitriol administered via slow-release pellets. The absence of sustained hypercalcemia in earlier studies that used this protocol to dispense hormone was not caused by exposing older embryos to subthreshold quantities of calcitriol. The fact that reduced body size in late embryos receiving calcitriol was not accompanied by a reduction also in calcium content may mean that embryos deposit calcium in the carcass in an effort to deal with the extreme hypercalcemia induced by calcitriol.

Transformation of 25- and 1 alpha-hydroxyvitamin D3 to 1 alpha, 25-dihydroxyvitamin D3 by using Streptomyces sp. strains

To enzymatically synthesize vitamin D derivatives, we screened about 300 Streptomyces sp. strains. Streptomyces sclerotialus FERM BP-1370 and Streptomyces roseoporus FERM BP-1574 were found to have the ability to convert 25-hydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3, respectively, to 1 alpha, 25-dihydroxyvitamin D3. The average rates of 1 alpha hydroxylation of 25-hydroxyvitamin D3 were 6.9 micrograms liter-1 min-1 with FERM BP-1370 and 7.0 micrograms liter-1 min-1 with FERM BP-1574. The specific cytochrome P-450 inhibitors carbon monoxide, SKF-525-A, and metyrapone inhibited the hydroxylation of 1 alpha- and 25-hydroxyvitamin D3 to 1 alpha, 25-dihydroxyvitamin D3 by FERM BP-1370 and FERM BP-1574. The cytochromes P-450 of these strains were detected by reduced CO difference spectra in the whole-cell suspensions. The appearance of cytochrome P-450 suggests that the cytochromes P-450 of FERM BP-1370 and FERM BP-1574 carry out the hydroxylation of 25- and 1 alpha-hydroxyvitamin D3 to 1 alpha, 25-dihydroxyvitamin D3.

Vitamin D metabolism in pre-operative extrahepatic biliary atresia

In order to clarify the pathogenesis of rickets in preoperative patients with extrahepatic biliary atresia, we evaluated baseline serum 25-OHD and 1,25(OH)2D levels and correlated serum 25-OHD levels with increase in age and season of birth in 16 preoperative patients. Further, parenteral vitamin D2 tolerance tests were performed in 5 cases. Serum 25-OHD and 1,25(OH)2D levels were significantly lower than those in 15 normal controls. There was a negative correlation between the serum 25-OHD levels and increase in age. The patients born during the winter had lower serum 25-OHD concentrations than those born in summer. The mean value of increased 25-OHD levels after the parenteral vitamin D2 tolerance tests did not differ from that of 6 controls. Since there was no impairment of vitamin D 25-hydroxylation, the reduction in serum 25-OHD may therefore be mainly due to disturbed intestinal vitamin D absorption. It was also concluded that season of birth and increase in age are pathogenic factors in the etiology of rickets in preoperative patients with extrahepatic biliary atresia.

Regulation of parathyroid hormone synthesis in chronic renal failure in rats

To clarify the mechanism of secondary hyperparathyroidism in chronic renal failure at the parathyroid hormone (PTH) synthesis level, we measured PTH messenger RNA (mRNA) levels in parathyroid glands in a rat model of chronic renal failure. Four weeks after 7/8 nephrectomy, hyperplasia of parathyroid glands was evident and serum PTH levels were elevated. Serum concentration of calcium, inorganic phosphate, and 1,25-dihydroxyvitamin D (1,25(OH)2D) of rats with chronic renal failure were not detectably different from those of sham-operated rats. In chronic renal failure rats, PTH mRNA levels were elevated both per RNA and per DNA of parathyroid cells, suggesting increased PTH mRNA levels per cell. The elevated levels of PTH mRNA were returned to normal levels by achieving supraphysiological concentrations of 1,25(OH)2D3 given i.p. twice at 24 and 48 hours before sacrifice, although this was attended by slight hypercalcemia. A synthetic analogue of vitamin D, 22-oxa-1,25(OH)2D3, also suppressed PTH mRNA to normal levels, but without hypercalcemia. These data suggest that secondary hyperparathyroidism in early chronic renal failure may be due in part to the resistance of parathyroid cells to the physiological concentration of 1,25(OH)2D in circulation on PTH synthesis and that 22-oxa-1,25(OH)2D3 may be useful in the management of secondary hyperparathyroidism of chronic renal failure.

Clinical and biochemical findings in parents of children with vitamin D-dependent rickets Type II

Vitamin D-dependent rickets type II is a rare disease caused by a disorder of the receptor for 1, 25-dihydroxyvitamin D (1, 25(OH)2D). Several parameters of this receptor-effector system were investigated to obtain biochemical information on the presumed heterozygotes of vitamin D-dependent rickets type II in parents of five patients and in their age-matched controls. It was found that the serum concentrations of 1, 25-(OH)2D and 24,25-dihydroxy-vitamin D (24,25(OH)2D), and the ratio of 1,25-(OH)2D/24,25-(OH)2D differed significantly in the parents from those of the patients and the respective control groups. In the parents' cultured skin fibroblasts, the activity of 25-hydroxyvitamin D-24-hydroxylase induced by 10(-8) mol/L 1, 25-(OH)2D3 ranged from 50 to 82% of that of their controls (versus 1-13% of controls for the patients). The binding capacity of the parents' [3H]1, 25-(OH)2D3 to the nucleus was 38-54% of that of their control subjects (versus 7-27% of controls for the patients). The parents' values were thus in a range between those of the patients and the control groups. These findings suggest that, in the parents, a partial impairment of the receptor system for 1, 25-(OH)2D led to an imbalance of vitamin D metabolism, thus confirming that vitamin D-dependent rickets type II is an autosomal recessive inherited disease. Serum concentrations of 1, 25-(OH)2D and 24, 25-(OH)2D may provide useful parameters for detecting heterozygotes of this disease.

Effect of single oral phosphate loading on vitamin D metabolites in normal subjects and in X-linked hypophosphatemic rickets

There have been several reports that document abnormal vitamin D metabolism in X-linked hypophosphatemic rickets (XLH). Those reports indicate a blunted renal 25-hydroxyvitamin D-1 alpha-hydroxylase response to a potent stimulator, phosphorus restriction. We examined here its response to phosphate supplementation. Seven normal volunteers and 12 patients with XLH were submitted to single oral phosphate loading. This treatment produced a marked elevation of the serum phosphorus level, with a mild reduction in the serum calcium level. In normal subjects, although the concentrations of intact parathyroid hormone and mid-region parathyroid hormone were increased, with two peaks at 2 and 8 h after treatment, there were no significant changes in vitamin D metabolites including 25-hydroxyvitamin D (25(OH)D), 24,25-dihydroxyvitamin D (24,25(OH)2D) and 1,25-dihydroxyvitamin D (1,25(OH)2D). On the other hand, in the patients with XLH, the serum 1,25(OH)2D level increased from 23.4 +/- 12.0 (mean +/- SD) pg/ml to 44.3 +/- 33.6 pg/ml 6 h after ingestion without any significant change in 25(OH)D or 24,25(OH)2D.

Renal 25-hydroxyvitamin D3-1-hydroxylase in patients with renal disease

Renal 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) activity has been measured in 20 patients with renal disease using the remaining portion of needle renal biopsy specimens taken for diagnostic purposes and in five patients using kidney tissue removed during transplantation. The 1-hydroxylase activity of 12 patients with asymptomatic proteinuria and/or hematuria (group A) measured 83.2 +/- 37.7 pg/mg tissue/20 min. Since these 12 patients did not show impaired mineral metabolism or pathological changes in the renal tubules, we have presumed that these results indicate normal activity in man. We also measured the 1-hydroxylase activity in four patients treated with prednisolone (group B). The 1-hydroxylase activity (81.1 +/- 27.1 pg/mg tissue/20 min) of group B did not differ from that of group A. However, the urinary excretion of calcium (ratio of calcium/creatinine) was increased (0.18 +/- 0.07 vs. 0.07 +/- 0.03, P less than 0.01) by prednisolone therapy. These data suggest that glucocorticoid-induced changes in urinary calcium excretion are not the result of a direct effect of glucocorticoid on renal 1-hydroxylase. In the three patients with mild renal insufficiency (group C), the 1-hydroxylase activity (75.4 +/- 22.4 pg/mg tissue/20 min) did not differ from that of group A. However, in five patients with severe renal insufficiency (group D), the 1-hydroxylase activity (8.5 +/- 3.7 pg/mg tissue/20 min) was significantly decreased (P less than 0.01).

A hypercalcemic nude rat model that completely mimics human syndrome of humoral hypercalcemia of malignancy

Tumors causing humoral hypercalcemia of malignancy (HHM) were implanted to athymic nude rats. In one of these rat models transplanted with uterine cancer (UCC), a complete reproduction of human HHM syndrome was achieved: hypercalcemia, hypophosphatemia with increased urinary phosphate and cyclic AMP excretion, and suppressed serum 1,25-dihydroxy-vitamin D (1,25(OH)2D) level. In another hypercalcemic nude rat model implanted with oral cavity cancer (OCC), all the features were similar except for markedly elevated serum 1,25(OH)2D. Hypercalcemia disappeared by surgical removal of the tumors in both models, confirming the humoral mechanisms for causing these features. Furthermore, in UCC tumor-bearing rats, hypophosphatemia, increased renal phosphate excretion, and reduced serum 1,25(OH)2D concentration were already present when these rats were only marginally hypercalcemic. These results raise the possibility that the changes in renal tubular phosphate handling and vitamin D metabolism in HHM are not secondary to hypercalcemia but are due to direct effects of the humoral factor(s) that cause this syndrome. Extracts of both tumors exhibited stimulation of cyclic AMP production in osteoblastlike cells, UMR 106, which could be almost completely inhibited by parathyroid hormone (PTH) antagonist, human PTH(3-34). By comparing the nature and characteristics of humoral factor(s) from UCC and OCC models, mechanisms responsible for causing these abnormalities can be explored. Thus, these nude rat models can be useful for elucidating the underlying mechanism of the development of HHM.

Effect of 24,25-dihydroxyvitamin D3 on 1,25-dihydroxyvitamin D3 metabolism in calcium-deficient rats

The effect of 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] on 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] metabolism was examined in rats fed on a low-calcium diet. These rats exhibit hypocalcaemia, high urinary cyclic AMP excretion, a markedly elevated serum 1,25(OH)2D concentration and low serum concentrations of both 24,25(OH)2D and 25(OH)D. When the rats are treated orally with 1, 5 or 10 micrograms of 24,25(OH)2D3/100 g every day, there is a dramatic decrease in serum 1,25(OH)2D concentration in a dose-dependent manner concomitant with an increase in serum 24,25(OH)2D concentration. Serum calcium concentration and urinary cyclic AMP excretion are not significantly affected by the 24,25(OH)2D3 treatment, which suggests that parathyroid function is not affected by the 24,25(OH)2D3 treatment. The 25(OH)D3 1 alpha-hydroxylase activity measured in kidney homogenates is markedly elevated in rats on a low-calcium diet but is not affected by any doses of 24,25(OH)2D3. In contrast, recovery of intravenously injected [3H]1,25(OH)2D3 in the serum is decreased in 24,25(OH)2D3-treated rats. Furthermore, when [3H]1,25(OH)2D3 is incubated in vitro with kidney or intestinal homogenates of 24,25(OH)2D3-treated rats there is a decrease in the recovery of radioactivity in the total lipid extract as well as in the 1,25(OH)2D3 fraction along with an increase in the recovery of radioactivity in the water-soluble phase. These results are consistent with the possibility that 24,25(OH)2D3 has an effect on 1,25(OH)2D3 metabolism, namely that of enhancing the degradation of 1,25(OH)2D3. However, because a considerable proportion of the injected 24,25(OH)2D3 is expected to be converted into 1,24,25(OH)3D3 by renal 1 alpha-hydroxylase in 24,25(OH)2D3-treated rats, at least a part of the decrease in serum 1,25(OH)2D concentration may be due to a competitive inhibition by 24,25(OH)2D3 of the synthesis of 1,25(OH)2D3 from 25(OH)D3. Thus the physiological importance of the role of 24,25(OH)2D3 in regulating the serum 1,25(OH)2D concentration as well as the mechanism and metabolic pathway of degradation of 1,25(OH)2D3 remain to be clarified.

Blood Ca2+ modulates responsiveness of renal 25(OH)D3-1 alpha-hydroxylase to PTH in rats

To clarify whether extracellular Ca2+ modulates renal 25-hydroxyvitamin D3 [25(OH)D3]-1 alpha-hydroxylase, thyroparathyroidectomized rats were infused with 15 mM CaCl2, 20 mM EGTA, and/or 2.5 U/h parathyroid hormone (PTH), and blood Ca2+, serum 1,25-dihydroxyvitamin D [1,25(OH)2D], and renal 1 alpha-hydroxylase activity were determined. Rats with CaCl2, EGTA, or PTH infusion (group 1) exhibited low blood Ca2+, serum 1,25(OH)2D, and 1 alpha-hydroxylase activities. Infusion of CaCl2 alone (group 2) caused a significant increase in blood Ca2+ and a reduction in serum 1,25(OH)2D and 1 alpha-hydroxylase compared with group 1. Administration of PTH alone (group 3) markedly elevated blood Ca2+, serum 1,25(OH)2D, and 1 alpha-hydroxylase activity. When EGTA was infused along with PTH (group 4), blood Ca2+ was significantly reduced compared with group 3, and serum 1,25(OH)2D and renal 1 alpha-hydroxylase were further elevated. In contrast, when CaCl2 was infused with PTH (group 5), blood Ca2+ was higher than that in group 3, and serum 1,25(OH)2D and 1 alpha-hydroxylase activities were significantly reduced compared with group 3. No significant difference in serum inorganic phosphate or urinary cAMP excretion was observed by CaCl2 or EGTA infusion in both PTH-treated and nontreated rats. These results demonstrate that extracellular Ca2+ modulates the responsiveness of renal 1 alpha-hydroxylase to PTH as well as the base-line activity of the enzyme in the absence of PTH. These effects of extracellular Ca2+ on renal 1 alpha-hydroxylase may serve to offer an efficient way of regulating 1,25(OH)2D production and serum 1,25(OH)2D concentration by altering the responsiveness of 1 alpha-hydroxylase to PTH and possibly other stimulations depending on the demand for Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of insulin in the stimulation of renal 25-hydroxyvitamin D3-1 alpha-hydroxylase by phosphorus deprivation in rats

The increase in serum 1,25(OH)2D concentration in response to dietary phosphorus (P) depreviation is dependent on the presence of insulin in rats. The present study was undertaken to clarify whether insulin exerts its effects by affecting the renal production of 1,25(OH)2D. The 25(OH)D-1 alpha-hydroxylase activity in kidney homogenates was markedly stimulated by P deprivation in control rats (0.20 +/- 0.06 pmol/g tissue/min in the rats on a normal P diet v 1.3 +/- 0.15 pmol/g/min in the rats on a low P diet; 6.5-fold increase). In contrast, in streptozotocin-diabetic rats, the increase in the renal 1 alpha-hydroxylase activity in response to P deprivation (0.25 +/- 0.01 pmol/g/min; 3.6-fold increase) as well as the enzyme activity in the rats on a normal P diet (0.07 +/- 0.01 pmol/g/min) was markedly suppressed. Furthermore, all the changes in the renal 1 alpha-hydroxylase activity in insulin-deficient rats disappeared by insulin replacement (0.16 +/- 0.01 pmol/g/min in the rats on a normal P diet v 1.3 +/- 0.01 pmol/g/min in the rats on a low P diet; eightfold increase). These results demonstrate that the stimulation of 1 alpha-hydroxylase in response to dietary P deprivation is blunted by insulin deficiency and is fully restored by insulin replacement. It is suggested that insulin, in addition to its direct stimulatory effect on 1 alpha-hydroxylase, alters the responsiveness of renal 1 alpha-hydroxylase to P deprivation. These effects of insulin on 1 alpha-hydroxylase may be responsible for the change in serum 1,25(OH)2D concentration in response to dietary P deprivation, although the possibility cannot be ruled out that insulin also affects the metabolic clearance of 1,25(OH)2D.

Emerging concepts on the biologic role and mechanism of action of 1,25-dihydroxyvitamin D3

The biologic actions of 1,25-(OH)2D3 are diverse, ranging from a major role in the regulation of mineral homeostasis in intestine, kidney, and bone to the control of such fundamental processes as myeloid progenitor cell differentiation. The central character in this action is the 1,25-(OH)2D3 receptor, a protein whose activity is focused at the level of the genome. The function of this polypeptide, by analogy with other steroid receptors, is to interact in a sequence-specific manner with unique regulatory elements of DNA, which serve to modify the activity of their respective promoters. The exact manner in which receptor binding to these sequences precipitates promoter activity is unclear. It is, however, a direct result of the structural organization of the steroid receptors, which represent a class of transcriptional controlling proteins. The deduced primary sequences emanating from the molecular cloning of estrogen, progesterone, glucocorticoid, and 1,25-(OH)2D3 receptors has revealed several important structure-function relationships. These include the identification of a highly conserved cysteine-rich domain that may interact with DNA and a steroid-binding domain that is hydrophobic and is located at the carboxy terminus of the protein. The similarity of this domain among heterologous steroid receptor species implies that each of these proteins belongs to a common gene family whose functional activities are similar if not identical. It is this structure within the 1,25-(OH)2D3 receptor that provides conclusive evidence that 1,25-(OH)2D3 is a steroid hormone that via its receptor modifies the activity of hormone-sensitive genes.

Effect of 1 alpha-hydroxycholecalciferol on psoriasis vulgaris: a pilot study

We carried out a clinical trial of 1 alpha-hydroxycholecalciferol [1 alpha(OH)D3] at a dose of 1.0 microgram a day on 7 patients with psoriasis vulgaris. These patients had been treated by topical applications of corticosteroids before this study without improvement, and during the clinical trial, treatment of topical corticosteroids was continued on 6 of the 7 patients. Four of 7 patients showed complete remission and marked improvement and 2 additional patients showed minimal improvement of their skin lesions during and after the treatment with 1 alpha(OH)D3. No adverse reactions were noted during the treatment period. The mechanism of the phenomenon we observed has yet to be elucidated. Controlled trials of large numbers of patients with psoriasis vulgaris treated with 1 alpha(OH)D3 are under way.

Studies on the role of 1,25-dihydroxyvitamin D in chick embryonic development

Vitamin D-deficient laying hens were repleted with 25-hydroxy[26,27-3H]vitamin D3 or 1,25-dihydroxy[26,27-3H]vitamin D3. Egg production returned to normal for both groups of hens by the third week. Eggs from hens fed either 25-hydroxy[26,27-3H]vitamin D3 or 1,25-dihydroxy[26,27-3H]vitamin D3 contained 1,25-dihydroxy[26,27-3H]vitamin D3. Eggs from hens fed 25-hydroxy[26,27-3H]vitamin D3 contained substantial amounts of 25-hydroxy[26,27-3H]vitamin D3, while those from hens fed 1,25-dihydroxy[26,27-3H]vitamin D3 contained none. Plasma from 18-day embryos from hens fed 1,25-dihydroxy[26,27-3H]vitamin D3 contained little or no 1,25-dihydroxy[26,27-3H]vitamin D3, while that from 18-day embryos from hens given 25-hydroxy[26,27-3H]vitamin D3 had normal levels of 1,25-dihydroxy[26,27-3H]vitamin D3. No eggs from hens fed 1,25-dihydroxy[26,27-3H]vitamin D3 hatched, while eggs from hens fed 25-hydroxy[26,27-3H]vitamin D3 achieved a hatchability of 90%. It appears that embryos from hens maintained on 1,25-dihydroxyvitamin D3 as their sole source of vitamin D are essentially vitamin D deficient.

Decreased bone density in severely handicapped children and adults, with reference to the influence of limited mobility and anticonvulsant medication

Bone density and related biochemical parameters were investigated in institutionalised children and adults with severe handicaps, who were classified according to the degree of limited mobility (group 1, bed-ridden; group 2, capable of crawling; group 3, capable of walking) and according to whether or not they were receiving anticonvulsants. As determined by microdensitometric analysis of radiograms of the second metacarpal bone, bone width (D), bone pattern area (sigma GS) and bone salt density (sigma GS/D) were decreased in the patients, the decreases being most prominent in group 1, followed by groups 2 and 3, in that order. Significant decreases of sigma GS and sigma GS/D, but not of D, were found in patients on anticonvulsant treatment in comparison to patients without therapy. Serum alkaline phosphatase (Al-p) and parathyroid hormone (iPTH) as well as urinary calcium and cyclic adenosine-3',5'-monophosphate (cAMP) excretion were significantly increased in group 1. In comparison to patients without therapy, anticonvulsant-treated children showed significantly decreased levels of serum calcium (Ca), ionised Ca (Ca2+), 25-hydroxy vitamin D3 and urinary phosphate (PO4) excretion, and elevated levels of Al-p, iPTH and calcitonin (iCT). It is suggested that limited physical activity results in a mild hyperparathyroid state, which is aggravated in patients on anticonvulsant treatment.

Mineral homeostasis in very premature infants: serial evaluation of serum 25-hydroxyvitamin D, serum minerals, and bone mineralization

This study was designed to evaluate the role of vitamin D sufficiency, as reflected in serum 25-hydroxyvitamin D (25-OHD) concentrations, on serum minerals and bone mineralization in very premature infants. Seventy-two infants (mean +/- SD gestation 30.1 +/- 2.5 weeks, mean +/- SD birth weight 1178 +/- 278 gm) were observed serially for the first 3 months of life. Mean serum calcium and phosphorus values, but not magnesium, remained low prior to 12 weeks. The percentage of infants with moderate to severe hypomineralization was 75% at 3 weeks, 55% at 6 weeks, 54% at 9 weeks, and 15% at twelve weeks. Low serum calcium and phosphorus values, high alkaline phosphatase activity, and moderate-severe hypomineralization were more frequent in infants weighing less than 1000 gm and in those with lower mineral intake. With a 400 IU vitamin D supplement, 45% of infants could maintain an initially normal serum 25-OHD concentration or increase low concentrations, whereas 55% had falling or persistently low (less than or equal to 15 ng/ml) 25-OHD concentrations. Birth weight and mineral intakes were comparable in these two groups, yet the group with the lower serum 25-OHD concentration had lower serum calcium and higher alkaline phosphatase values, and a higher percentage of moderate to severe hypomineralization. Regardless of birth weight, mineral intake, or 25-OHD concentration, increases in serum calcium and phosphorus values and in mineralization were seen at postconception term (12 weeks in most infants, nine weeks in those weighing 1250 to 1600 gm). At 12 weeks of age, but not before, serum 25-OHD concentration was directly correlated with serum calcium (r = 0.47, P less than 0.01) and serum phosphorus (r = 0.47, P less than 0.01) and inversely correlated with alkaline phosphatase values (r = -0.71, P less than 0.01). Mineral availability and 25-OHD sufficiency both appear to be important and to act synergistically, with neither totally compensating for the other.

Serum 1,25-dihydroxyvitamin D concentrations in premature infants: preliminary results

Serum 1,25(OH)2D concentrations were measured in serial serum samples from 19 premature infants of 29.6 +/- 1.3 weeks gestation and 1,129 +/- 159 g birthweight. 1,25(OH)2D was always normal or elevated and mean concentrations increased with age (adult, 55.2 +/- 13; infants, 1-2 weeks, 81.5 +/- 37.7 pg/mg; 3 weeks, 65 +/- 21; 6 weeks, 90.0 +/- 17.3; 9 weeks, 99.0 +/- 25.1; 12 weeks, 103.3 +/- 26.6 pg/ml). No correlation was seen with 25-OHD. Infants given 800 IU D2 supplements had lower 1,25(OH)2D levels than infants given 400 IU D2. Breast fed infants had initially higher 1,25(OH)2D levels; however, this was not sustained. These preliminary data suggest that premature infants regulate 1,25(OH)2D production similar to more mature infants and children. Whether the premature infant has a normal gastrointestinal and/or bone responsiveness to 1,25(OH)2D and whether these elevated 1,25(OH)2D concentrations are "adequately elevated" requires further study.

The mechanism of end-organ resistance to 1 alpha,25-dihydroxycholecalciferol in the common marmoset

The common marmoset, a New World monkey, requires a large amount of cholecalciferol (110 i.u./day per 100g body wt.) to maintain its normal growth. In a previous report, we demonstrated that the circulating levels of 1 alpha, 25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] in the marmosets are much higher than those in rhesus monkeys and humans, but the marmosets are not hypercalcaemic [Shinki, Shiina, Takahashi, Tanioka, Koizumi & Suda (1983) Biochem. Biophys. Res. Commun. 14, 452-457]. To compare the effect of the daily intake of cholecalciferol, two rhesus monkeys were given a large amount of cholecalciferol (900 i.u./day per 100g body wt). Their serum levels of calcium, 25-hydroxycholecalciferol and 24R,25-dihydroxycholecalciferol were markedly elevated, but the serum 1 alpha,25(OH)2D3 levels remained within a range similar to those in the rhesus monkeys fed the normal diet (intake of cholecalciferol 5 i.u./day per 100g body wt). Intestinal cytosols prepared from both monkeys contained similar 3.5 S macromolecules to which 1 alpha,25(OH)2D3 was bound specifically. However, the cytosols from the marmosets contained only one-sixth as many 1 alpha,25(OH)2D3 receptors as those from the rhesus monkeys. Furthermore, the activity of the 1 alpha,25(OH)2D3-receptor complex in binding to DNA-cellulose was very low in the marmosets. These results suggest that the marmoset possesses an end-organ resistance to 1 alpha,25(OH)2D3 and is a useful animal model for studying the mechanism of vitamin D-dependent rickets, type II.

The appearance of 1,25-dihydroxyvitamin D3 receptor during chick embryo development

The appearance of the 1,25-dihydroxyvitamin D3 receptor in intestine, kidney, and chorioallantoic membrane of chick embryo was followed by sucrose density gradient sedimentation analysis and Scatchard plot analysis. The receptor from each of these organs sediments as a single 3.7S component. At 19 days of embryonic life, intestine had the highest specific 1,25-dihydroxyvitamin D3 binding activity followed by kidney and chorioallantoic membrane. The 1,25-dihydroxyvitamin D3 binding activity increased gradually at 12-15 days and rapidly until 20 days in intestine. In kidney, this protein increased rapidly from 12 to 16 days and did not change subsequently. In chorioallantoic membrane, the receptor increased slowly from 8 through 15 days, rapidly until 19 days, and decreased at 20 days. The injection of hydrocortisone into the chick embryo at 10 days increased receptor number in intestine, kidney, and chorioallantoic membrane by a factor of 2 at 12 days. Injection of this hormone after this time had little or no effect.

Calcium metabolism and its control--a review

It is now established that avians can only utilize the cholecalciferol form of vitamin D, which must be converted to the hormone 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] to perform normal calcium metabolism. Although 1,25(OH)2D3 is the final active form of vitamin D, hens fed only this form of vitamin D do not have normal hatchability of eggs. The problem appears to be caused by abnormal calcification and development of the embryonic beak. This appears to be caused by inadequate transport of 1,25(OH)2D3 into the egg. Although 1,25(OH)2D3 is not incorporated into the egg adequately, its precursor, 25-hydroxyvitamin D3 (25-OH-D3), is. The developing embryo however, can utilize 1,25(OH)2D and does so at least as early as Day 10 of incubation. During periods of maximal shell calcification and high circulating estradiol levels, the hen produces high levels of 1,25(OH)2D3. The kidney hydroxylase responsible for the final hydroxylation of the vitamin D hormone can be further stimulated by in vivo or in vitro administration of estradiol and, to a lesser extent, prolactin and parathyroid hormone. When eggs are not produced, as in the senescent or prepubertal stages of life, plasma 1,25(OH)2D3 concentrations are less than half that occurring during periods of active lay. Hens selected for their ability to produce thin or thick shells have 1,25(OH)2D3 concentrations in plasma that are positively correlated to their ability to produce egg shell.

Activity of renal 25-hydroxyvitamin D3-1 alpha-hydroxylase in a case of X-linked hypophosphataemic rickets

In 1974, a 2-year-old boy was diagnosed as having X-linked hypophosphataemic rickets (XLH) because of severe rickets and hypophosphataemia. The vitamin D metabolite concentrations, blood and urine chemistry and renal 25-hydroxyvitamin D3 (25OHD3)-1 alpha-hydroxylase were measured in 1982 (about 2 weeks after withdrawal of medication). 1 alpha-hydroxylase was 392 pg/mg tissue/20 min in the patient, which was high compared with aged-matched controls (69.7 +/- 28.5 pg/mg tissue/20 min, mean +/- SD, n = 7). Our present studies showed that the 1 alpha-hydroxylase activity in the patient with XLH was elevated. Therefore, the normal or low 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) concentrations in XLH patients could be due to accelerated catabolism of 1,25-(OH)2D3 or abnormally regulated 25OHD3-1 alpha-hydroxylase in response to hypophosphataemia, although significantly elevated above that in normal controls.