1,25-dihydroxyvitamin D3 regulates the synthesis of nerve growth factor in primary cultures of glial cells.

The effect of 1,25-dihydroxyvitamin D3 (1,25-(OH)2 D3) on nerve growth factor (NGF) synthesis was investigated in primary cultures of astrocytes prepared from brain of neonatal rats. 1,25-(OH)2 D3 elicited a dose-dependent increase of NGF mRNA with a maximal effect at 10(-7) M, which persisted for at least 48 h. Northern blot analysis revealed an expression of the vitamin D3 receptor (VDR) gene in primary glial cells. Treatment of cells with 1,25-(OH)2 D3 led to an increase in the VDR mRNA levels. Similar results were obtained in C6 glioma cells. Exposure of primary glial cells to 10(-8) M 1,25-(OH)2 D3 caused only a 2-fold increase of the levels of cell-secreted NGF after 3 days of treatment. However, a 5-fold increase was observed three days after a second addition of vitamin D3. Likewise, a pretreatment with lower doses of hormone such as 10(-10) M or 10(-9) M enhanced the responsiveness of the cells to a 24 h treatment with 10(-8) M hormone. It appears, therefore, that the duration of the treatment influences the level of synthesis of NGF, possibly as a consequence of the increase of the VDR gene expression. The specificity of 1,25-(OH)2 D3 is supported by the fact that a concentration of 10(-7) M of an another vitamin D3 metabolite, 24,25-(OH)2 D3, had no effect on NGF synthesis. Several lines of evidence indicate that astrocytes constitute the major cell type responsive to 1,25-(OH)2 D3 in primary cultures of glial cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical trial of 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3 in uremic patients on hemodialysis: preliminary report.

A clinical trial was done by the Group, Japan to evaluate the efficacy of 26,27-F6-1,25(OH)2D3 on the calcium and bone metabolism of 43 uremic patients on hemodialysis, 24 men and 19 women with a mean age of 50.9 +/- 2.1 years. The initial dose administered orally was 0.05 micrograms/day for 2 weeks. Then the dose was increased every 2 weeks by 0.05 micrograms each time until the dose of 0.3 micrograms/day was reached or until serum calcium increased. 26,27-F6(OH)2D3 increased serum calcium levels significantly at a mean dose of 0.08 +/- 0.03 micrograms/day and at 0.05 micrograms/day of dose comparison in hemodialyzed patients. It decreased the serum level of PTH significantly at a mean dose of 0.14 +/- 0.06 micrograms/day and at 0.3 micrograms/day by dose comparison. The serum level of bone Gla protein increased significantly at a mean dose of 0.18 +/- 0.07 micrograms/day and at 0.25 micrograms/day by dose comparison in the same patients. These results suggest that 26,27-F6-1,25(OH)2D3 has a higher potency in calcium mobilization than 1,25(OH)2D3 in uremic patients on hemodialysis.

Metabolism and binding properties of 24,24-difluoro-25-hydroxyvitamin D3.

To evaluate possible functional roles for 24,25-dihydroxyvitamin D3, 24,24-difluoro-25-hydroxyvitamin D3 has been synthesized and shown to be equally as active as 25-hydroxyvitamin D3 in all known functions of vitamin D. The use of the difluoro compound for this purpose is based on the assumption that the C-F bonds are stable in vivo and that the fluorine atom does not act as hydroxyl in biological systems. No 24,25-dihydroxyvitamin D3 was detected in the serum obtained from vitamin D-deficient rats that had been given 24,24-difluoro-25-hydroxyvitamin D3, while large amounts were found when 25-hydroxyvitamin D3 was given. Incubation of the 24,24-difluoro compound with kidney homogenate prepared from vitamin D-replete chickens failed to produce 24,25-dihydroxyvitamin D3, while the same preparations produced large amounts of 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. Kidney homogenate prepared from vitamin D-deficient chickens produced 24,24-difluoro-1,25-dihydroxyvitamin D3 from 24,24-difluoro-25-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3. In binding to the plasma transport protein for vitamin D compounds, 24,24-difluoro-25-hydroxyvitamin D3 is less active than 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D3. In binding to the chick intestinal cytosol receptor, 24,24-difluoro-25-hydroxyvitamin D3 is more active than 25-hydroxyvitamin D3 which is itself more active than 24R,25-dihydroxyvitamin D3. The 24,24-difluoro-1,25-dihydroxyvitamin D3 is equal to 1,25-dihydroxyvitamin D3, and both are 10 times more active than 1,24R,25-trihydroxyvitamin D3 in this system. These results provide strong evidence that the C-24 carbon of 24,24-difluoro-25-hydroxyvitamin D3 cannot be hydroxylated in vivo, and, further, the 24-F substitution acts similar to H and not to OH in discriminating binding systems for vitamin D compounds.

Temperature-dependent inactivation of nucleic acid binding and aggregation of the 1,25-dihydroxyvitamin D3 receptor.

The interaction of the 1 alpha,25-dihydroxyvitamin D3 receptor with immobilized calf thymus DNA has been compared with its sedimentation properties on hypotonic sucrose gradients. Forty to sixty percent of total hormone:receptor complexes formed at 4 degrees C were retained by DNA-cellulose and could be eluted by 0.18 to 0.2 M KCl. In contrast, heating preparations to 25 degrees C rapidly and irreversibly converted receptor to a form which bound hormone and DEAE-cellulose normally, but was unable to associate with DNA. Similarly, the ability of receptor to aggregate to a 6 S species was labile at 25 degrees C. Stabilization of receptor in the DNA binding aggregating form was accomplished using Ca2+, Mg2+, Mn2+, or Na2MoO4 while several protease and phosphatase inhibitors were ineffective. An examination of DNA binding properties of aggregating and nonaggregating receptor forms revealed that only receptor competent to enter into aggregates could bind DNA suggesting that a functional nucleic acid binding site, and, hence, a nucleic acid interaction is necessary for aggregate formation. Consistent with this view, an RNA:receptor interaction appears to be involved in formation of the 6 S complex since removal of RNA by ribonuclease treatment or purification of receptor reduced aggregation, an effect that could be reversed by addition of purified RNA.

Stimulation of rat intestinal protein synthesis by 1,25-dihydroxyvitamin D3.

To study general stimulatory effects of 1,25-dihydroxyvitamin D3 on intestinal protein synthesis, slices of duodenal villi from 1,25-dihydroxyvitamin D3-treated and vitamin D-deficient rats were incubated in vitro for 90 min at the surface of medium containing [3H]leucine. Incorporation of the [3H]leucine into TCA-precipitated protein, which was shown to be linear for 12 h and 90% inhibited by cycloheximide, was increased by 50-60% at 26 h after a single injection of 125 ng of 1,25-dihydroxyvitamin D3 (three experiments, P less than 0.001). The increase, which was not due to circadian rhythm fluctuations of the intestine, was in synchrony with the second Ca2+ transport response observed by Halloran and DeLuca (Arch. Biochem. Biophys. 208, 477-486, 1981). However, no significant difference in [3H]leucine incorporation was observed before or during the initial Ca2+ transport response observed by Halloran and DeLuca, i.e., at 1.0, 3.0, and 6.5 h following an injection of 1,25-dihydroxyvitamin D3. The late onset of the 1,25-dihydroxyvitamin D3-induced increase in total protein synthesis implies that it is an indirect rather than a direct effect of the hormone.

The distal nephron in the chick embryo as a target tissue for 1-alpha-25-dihydroxycholecalciferol.

A histological and ultrastructural study as well as an autoradiographic analysis after injection of tritiated 1,25-dihydroxycholecalciferol (1,25-DHCC) were conducted on the distal convoluted tubules of chick embryos. Distal tubules in the embryo were shown to have the same spatial distribution as described for the adult kidney; they presented a convoluted portion located in the vicinity of the central intralobular veins and straight portions irradiating from this region towards the periphery. The epithelium in these tubules was well differentiated; its cells had numerous interdigitating folds in their lateral boundaries which were especially numerous at the basal ends. This device greatly increased the membrane surface available for interchange and was interpreted as an expression of active water and/or mineral transport. Nuclear concentration of radioactivity was found 2 h after injection of tritiated 1,25-DHCC in both the pars convoluta and the pars recta of the distal tubules. This concentration could be blocked by the previous administration of large amounts of nonradioactive 1,25-DHCC. These facts were interpreted as indicating that distal convoluted tubules in the chick embryo are functionally differentiated and contain target cells for 1,25-DHCC.

1 alpha hydroxycholecalciferol and 25 hydroxycholecalciferol in renal bone disease.

Four microgrammes of 1-alpha-hydroxycholecalciferol (1-alpha-OH D3) or 200 mug of 25-hydroxycholecalciferol (25-OH D3) were given orally every other day respectively to 10 uraemic patients (8 on chronic haemodialysis) for 1-12 weeks and to 3 patients on chronic haemodialysis for 4-8 weeks. A transilial bone biopsy and serial evaluation of serum immunoreactive PTH (iPTH) calcium phosphate and alkaline phosphatase were performed before and at the end of therapy. Both 1-alpha-OH D3 and 25-OH D3 (the latter at a 50 times higher dose) were able to depress hyperparathyroidism in two-thirds of the cases and to consistently improve the mineralisation defect. In no case did iPTH or the bone histomorphometric parameters return to normal, so that long term evaluation of these two drugs is warranted.

Recent advances in our understanding of the vitamin D endocrine system.

Vitamin D3 gives rise to at least one hormone in which the kidney is utilized as an endocrine system. This hormone arises from 25-OH-D3 which in turn is synthesized in the liver from vitamin D3. The production of this calcium and phosphorus mobilizing hormone, namely 1,25-(OH)2D3, is strongly regulated by the need for calcium and phosphorus. The regulation of its production can occur only after initial 1,25-(OH)2D3 is made and brings about the appearance of 25-OH-D3-24hydroxylase. The need for calcium brings about a stimulation of parathyroid hormone secretion. The parathyroid hormone suppresses the 24-hydroxylase and stimulates the 1-hydroxylase. Alternatively, the need for phosphorus directly stimulates the 1-hydroxylase and suppresses the 24-hydroxylase. The 24-hydroxylation appears to be the initial reaction leading to the inactivation and excretion of vitamin D whereas the 1-hydroxylation is the reaction bringing about the activation of the molecule to 1,25-(OH)2D3. The 1,25-(OH)2D3, the 25-OH-D3 and an analog of 1,25-(OH)2D3, namely 1alpha-OH-D3, are potentially extremely useful in the treatment of metabolic bone diseases such as renal osteodystrophy, hepatically related disorders of calcium and bone metabolism, hypoparathyroidism, and vitamin D dependency disease. The 1alpha-OH-D3 is effective by virtue of its conversion to 1,25-(OH)2D3. The 25-hydroxylation of both 1alpha-OH-D3 and vitamin D3 itself occurs predominantly in the liver. Finally, it is not entirely settled whether 1,25-(OH)2D3 is active directly in all of the functions of viramin D or whether it must be further converted metabolically. A new metabolic pathway for vitamin D has been discovered in which 1,25-(OH)2D3 loses its 26 and 27 carbons to carbon dioxide, producing an unknown metabolite. It is not certain whether this pathway represents degradation of the 1,25-(OH)2D3 or its further activation.