Vitamin D metabolites stimulate phosphatidylcholine transfer to renal brush-border membranes

The phosphatidylcholine content of both the intestinal and renal brush-border membranes and ion transport are affected by 1,25-dihydroxycholecalciferol (1,25(OH)2D3). To investigate the mechanism of this effect, liposomes were prepared containing self-quenching concentrations of fluorescent phospholipid derivatives. When these liposomes were incubated with rat renal brush-border membrane vesicles, an immediate increase in the relative fluorescence of N-4-nitrobenz-2-oxa-1,3-diazole phosphatidylcholine (NBD-PC) was detected, indicating transfer of NBD-PC into a non-quenched membrane. Addition of 1,25(OH)2D3 to the liposomes produced a dose-dependent stimulation of NBD-PC transfer to the acceptor brush-border membrane vesicles. Peripheral fluorescence was visible when the brush-border membrane vesicles were viewed with a fluorescent microscope. Using brush-border membrane vesicles from kidneys of vitamin D-deficient animals, quantitation of lipid transfer revealed a 1,25(OH)2D3 (10(-7) M) stimulation of NBD-PC transfer from 1.38 +/- 0.27 to 2.07 +/- 0.26 micrograms/h, and of PC transfer, assessed by vesicle phosphatidylcholine content, from 49.7 +/- 12 to 57.3 +/- 12 micrograms/mg protein per h (P less than 0.05). There was no significant transfer of N-(lissamine rhodamine B sulfonyl)dioleoylphosphatidylethanolamine (N-Rh-PE). In the absence of hormone, the amount of NBD-PC transferred to brush-border membrane vesicles prepared from normal rats was significantly greater than that transferred to brush-border membrane vesicles prepared from vitamin D-deficient animals (2.12 +/- 0.02 vs. 1.39 +/- 0.27 micrograms of NBD-PC/h, P less than 0.05). Both physiologic and pharmacologic concentrations of 1,25(OH)2D3 stimulated NBD-PC transfer with maximum response at 10(-14) M (2.98 +/- 0.15 micrograms/h). 24,25-Dihydroxycholecalciferol and 25-hydroxycholecalciferol (25(OH)D3) also stimulated transfer, although dose-response curves were less effective than for 1,25(OH)2D3. Cortisol and vitamin D-3 did not stimulate transfer. 1,25(OH)2D3 did not stimulate NBD-PC transfer between liposome populations.     

The effect of essential fatty acid deficiency on the stimulation of intestinal calcium transport by 1,25-dihydroxyvitamin D3

The effect of altering the lipid composition of the brush-border membrane on the ability of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) to stimulate calcium transport across the intestinal mucosa was examined by raising chicks on a vitamin D, essential fatty acid-deficient diet (-DEFAD) and measuring calcium absorption from duodenal sacs in situ and calcium uptake into brush-border membrane vesicles in vitro. Administration of 1,25-(OH)2D3 to -DEFAD and to -D control chicks led to the same increase in calcium transport in situ, whereas calcium transport in isolated brush-border membrane vesicles was not stimulated in the EFAD group, but responded normally in the control group. When the incubation temperature was increased to 34 degrees C, brush-border membrane vesicles from 1,25-(OH)2D3-treated essential fatty acid-deficient (+DE-FAD) chicks accumulated calcium at a faster rate than did vesicles from -DEFAD chicks. There was a marked decrease in the linoleic acid content and an increase in the oleic acid content of both the total lipid extract of the brush-border membrane as well as the phosphatidylcholine and phosphatidylethanolamine fractions, which could explain the temperature sensitivity of the in vitro system. When the diet of the EFAD chicks was supplemented with linoleic acid, the rate of calcium uptake into subsequently isolated vesicles from +DE-FAD chicks correlated with the amount of linoleic acid in the brush-border membranes. These results support the concept that the action of 1,25-(OH)2D3 on membrane lipid turnover and structure plays a critically important role in the 1,25-(OH)2D3-mediated cellular transport responses.     

1,25-Dihydroxyvitamin D stimulates sodium-dependent phosphate transport by renal outer cortical brush-border membrane vesicles by directly affecting membrane fluidity

We examined the effects of several forms of vitamin D added to renal brush-border membrane suspensions on phosphate and glucose transport and on membrane fluidity. The 1,25-D stimulated and the other vitamin D decreased phosphate uptake. In contrast, glucose uptake was not affected by the treatment of vitamin D. The 1,25-D resulted in a significant shift of the lower transition temperature in Arrhenius plots for phosphate, but not for glucose uptakes, from 15 degrees C to 11.5 degrees C. These data indicate that the 1,25-D may alter membrane fluidity, limited to the phosphate transporter, thus affecting the phosphate uptake.     

Effect of dietary calcium, phosphate and vitamin D deprivation on the pharmacokinetics of 1,25-dihydroxyvitamin D3 in the rat

The in vivo regulation of circulating 1,25(OH)2D3 concentrations by vitamin D status and by dietary calcium and phosphate deficiency was studied. Adult rats were cannulated in the jugular vein and the clearance of physiological doses of 1,25(OH)2D3 monitored. In vitamin D-replete rats we investigated the effects of dietary calcium and phosphate deficiency on the elimination half life of 1,25(OH)2D3 The results showed no effect of dietary phosphate deficiency on the elimination half life of 1,25(OH)2D3. Dietary calcium deficiency resulted in a small increase of the 1,25(OH)2D3 elimination half life (P = 0.04) (normal diet: 16.3 +/- 1.8 hrs, n = 6; -Ca diet: 18.6 +/- 1.1 hrs, n = 5; -P diet: 16.0 +/- 1.4 hrs, n = 6; mean +/- SD). The experiments with the vitamin D deficient rats showed a marked increase in the elimination half life of 1,25(OH)2D3 (36.4 +/- 6.8 hrs, n = 7), when compared to the rats on the normal diet (P = 0.001). From the experiments in the vitamin D replete rats one can infer that regulation of circulating 1,25(OH)2D3 concentrations by dietary calcium or phosphate takes place at the production site and not by changes in elimination rate. However, vitamin D status appears to regulate circulating 1,25(OH)2D3 concentrations also through an effect on the elimination rate.     

1,25-Dihydroxyvitamin D3-mediated intestinal calcium transport. Biochemical identification of lysosomes containing calcium and calcium-binding protein (calbindin-D28K)

A variety of intestinal cell organelles and proteins have been proposed to mediate 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3)-stimulated calcium absorption. In the present study biochemical analyses were undertaken to determine the subcellular localization of 45Ca after calcium transport in vivo in ligated duodenal loops of vitamin D-deficient chicks injected with 1.3 nmol of 1,25-(OH)2D3 or vehicle 15 h prior to experimentation. Separation of Golgi, mitochondria, basal lateral membrane, and lysosome fractions in the epithelial homogenates was achieved by differential sedimentation followed by centrifugation in Percoll gradients and evaluation of appropriate marker enzyme activities. Both vitamin D-deficient and 1,25-(OH)2D3-treated chicks had the highest levels of 45Ca-specific activity in lysosomal fractions. The lysosomes were also the only organelles to exhibit a 1,25-(OH)2D3-mediated difference in calcium content, increasing to 138% of controls. Lysosomes prepared from 1,25-(OH)2D3-treated chicks also contained the greatest levels of immunoreactive calbindin-D28k (calcium-binding protein). Chloroquine, a drug known to interfere with lysosomal function, was tested and found to inhibit 1,25-(OH)2D3-stimulated intestinal calcium absorption. Neither 1,25-(OH)2D3 nor chloroquine affected [3H]2O transport. In additional experiments, microsomal membranes (105,000 X g pellets) were subjected to gradient centrifugation. The highest levels of 45Ca-specific activity and calcium-binding protein in material from 1,25-(OH)2D3-treated chicks were found in fractions denser than endoplasmic reticulum and may represent endocytic vesicles. In studies on intestinal mucosa of 1,25-(OH)2D3-treated birds fractionated after 30 min of exposure to lumenal Ca2+ or Ca2+ plus chloroquine, 45Ca was found to accumulate in lysosomes and putative endocytic vesicles, relative to controls. A mechanism involving vesicular flow is proposed for 1,25-(OH)2D3-mediated intestinal calcium transport. Endocytic internalization of Ca2+, fusion of the vesicles with lysosomes, and exocytosis at the basal lateral membrane complete the transport process.     

Calcium uptake in isolated brush-border vesicles from rat small intestine.

Ca2+ uptake in brush-border vesicles isolated from rat duodena was studied by a rapid-filtration technique. Ca2+ uptake showed saturation kinetics, was dependent on the pH and ionic strength of the medium and was independent of metabolic energy. Uptake activity was readily inhibited by Ruthenium Red, La3+, tetracaine, EGTA, choline chloride and Na+ or K+. The effect of variations in medium osmolarity on Ca2+ uptake and the ionophore A23187-induced efflux of the cation from preloaded vesicles indicated that the Ca2+-uptake process involved binding to membrane components, as well as transport into an osmotically active space. Scatchard-plot analyses of the binding data suggested at least two classes of Ca2+-binding sites. The high-affinity sites, Ka = (2.7 +/- 1.1) x 10(4) M-1 (mean +/- S.D.) bound 3.2 +/- 0.8 nmol of Ca2+/mg of protein, whereas the low-affinity sites (Ka = 60 +/- 6 M-1) bound 110 +/- 17 nmol of Ca2+/mg of protein. In the presence of 100 mM-NaCl, 1.7 and 53 nmol of Ca2+/mg of protein were bound to the high- and low-affinity sites respectively. Decreased Ca2+-uptake activity was observed in vesicles isolated from vitamin D-deficient as compared with vitamin D-replete animals and intraperitoneal administration of 1,25-dihydroxycholecalciferol to vitamin D-deficient rats 16 h before membrane isolation stimulated the initial rate of Ca2+ uptake significantly. The data indicated that Ca2+ entry and/or binding was passive and may involve a carrier-mediated Ca2+-uptake component that is associated with the brush-border membrane. Altering the electrochemical potential difference across the membrane by using anions of various permeability and selected ionophores appeared to increase primarily binding to the membrane rather than transport into the intravesicular space. Since there is considerable binding of Ca2+ to the vesicle interior, a comprehensive analysis of the transport properties of the brush-border membrane remains difficult at present.     

Enzymatic removal of alkaline phosphatase from renal brush-border membranes. Effect on phosphate transport and on phosphate binding

Brush-border membrane vesicles prepared from rabbit kidney cortex were incubated at 37 degrees C for 30 min with phosphatidylinositol-specific phospholipase C. This maneuver resulted in a release of approx. 85% of the brush-border membrane-linked enzyme alkaline phosphatase as determined by its enzymatic activity. Transport of inorganic [32P]phosphate (100 microM) by the PI-specific phospholipase C-treated brush-border membrane vesicles was measured at 20-22 degrees C in the presence of an inwardly directed 100 mM Na+ gradient. Neither initial uptake rates, as estimated from 10-s uptake values (103.5 +/- 6.8%, n = 7 experiments), nor equilibrium uptake values, measured after 2 h (102 +/- 3.4%) were different from controls (100%). Control and PI-specific phospholipase C-treated brush-border membrane vesicles were extracted with chloroform/methanol to obtain a proteolipid fraction which has been shown to bind Pi with high affinity and specificity (Kessler, R.J., Vaughn, D.A. and Fanestil, D.D. (1982) J. Biol. Chem. 257, 14311-14317). Phosphate binding (at 10 microM Pi) by the extracted proteolipid was measured. No significant difference in binding was observed between the two types of preparations: 31.0 +/- 9.37 in controls and 29.8 +/- 8.3 nmol/mg protein in the proteolipid extracted from PI-specific phospholipase C-treated brush-border membrane vesicles. It appears therefore that alkaline phosphatase activity is essential neither for Pi transport by brush-border membrane vesicles nor for Pi binding by proteolipid extracted from brush-border membrane. These results dissociate alkaline phosphatase activity, but not brush-border membrane vesicle transport of phosphate, from phosphate binding by proteolipid.     

Effects of 1 alpha,25-dihydroxycholecalciferol on sodium-ion translocation across chick intestinal brush-border membrane.

By utilizing isolated brush-border vesicles, Na+ transport across the luminal membrane of chick small intestine was found to be a composite of (i) a saturable (Km 10mM-Na+) amiloride-sensitive Na+/H+ antiport and (ii) a potential-sensitive conductive pathway. No evidence was obtained for the existence of a Na+/Cl- symport system. With the exception of the duodenum, luminal Na+ transfer in the entire small intestine was subject to regulation by vitamin D. Repletion of vitamin D-deficient chicks with 1 alpha,25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] significantly decreased net Na+ uptake by isolated membrane vesicles (by approximately 30%). The sterol suppresses the conductive pathway (25-45% inhibition) as well as the Na+/H+ antiport system. Kinetic analysis of the latter revealed that 1 alpha,25(OH)2D3 altered Vmax (from 12.9 to 4.8 nmol of Na+/20s per mg of protein), but did not change Km. Diminution of Na+ transfer, entailing an increase in the electrochemical transmembrane Na+ gradient, provides an explanation of the simultaneously observed stimulatory action of 1 alpha,25(OH)2D3 on Na+-gradient-driven solute transport in chick small intestine. Indirect evidence was obtained that the luminal plasma membrane of chick small intestine displays a definite H+ permeability that is positively affected by 1 alpha,25(OH)2D3.     

Calcium and glucose uptake in rat small intestinal brush-border membrane vesicles. Modulation by exogenous hypercortisolism and 1,25-dihydroxyvitamin D-3

The effect of exogenous hypercortisolism and 1,25-dihydroxyvitamin D-3 on small-intestinal calcium and glucose transport in the rat was studied at the level of brush-border membrane vesicles generated from isolated villous cells by a freeze-thaw procedure. At 5 X 10(-5) M extravesicular calcium, initial uptake rates in vesicles prepared from triamcinolone-treated adult rats were decreased by 30% after 5 days. Since calcium ionophore A23187 virtually abolished the difference in calcium uptake, triamcinolone appeared to affect calcium channel density or activity rather than intravesicular binding capacity. Kinetic analysis showed that a decrease in Vmax of a saturable calcium transport system could entirely account for the diminished rate of vesicular calcium uptake. Calcium transport rates could be partially restored by in vivo administration of 1,25-dihydroxyvitamin D-3 at a dosage which did not affect vesicular calcium uptake in control animals. Conversely, sodium-driven glucose accumulation in brush-border vesicles from triamcinolone-treated rats was stimulated by 50-70% after 36 h and appeared insensitive to vitamin D. A specific triamcinolone action on the glucose carrier itself rather than on the driving force of the sodium gradient was indicated by (i) a similar stimulation of glucose transport under equilibrium exchange conditions and (ii) an opposite effect of triamcinolone on sodium-driven alanine transport. The triamcinolone-induced changes in calcium and glucose uptake were not accompanied by a gross alteration of membrane integrity in vitro or by major alterations in vesicular protein composition, intravesicular glucose space and sucrase or alkaline phosphatase activity. The modification of vesicular transport properties is discussed in relation to the vitamin D-antagonized inhibition of intestinal calcium uptake and the stimulation of glucose absorption in response to supraphysiologic amounts of glucocorticoids observed in intact epithelium.     

Effect of vitamin D deficiency on D-glucose and citrate transport in rat renal basolateral membrane vesicles.

We had previously reported that feeding rats on Steenbock and Black's rickets-inducing diet markedly influences the metabolic picture of the kidney and the transmembrane transport systems of D-glucose and citrate in renal brush-border membrane vesicles. We have now studied D-glucose and citrate transport into basolateral membrane vesicles prepared from kidney cortex of control and rachitic rats and the effect of 1,25-dihydroxyvitamin D3 on these transport systems was also investigated. D-glucose and citrate uptake, determined in the presence of a Na(+)-gradient, was lowered in rachitic animals and 1,25-dihydroxyvitamin D3 administration proved to be ineffective in restoring normal values. Citrate transport, determined in the presence of a K(+)-gradient, was not influenced by both rickets and 1,25-dihydroxyvitamin D3 supply. The in vitro addition to vesicle preparations of calcium or phosphate or citrate or 1,25-dihydroxyvitamin D3 did not show a selective influence on D-glucose and citrate uptake.     

A specific binding protein for 1 alpha,25-dihydroxyvitamin D in the chick embryo chorioallantoic membrane

A 3.7 S binding protein for the steroid hormone and vitamin D metabolite 1 alpha-25-dihydroxyvitamin D (1,25-(OH)2-D) was observed in high salt cytosol extracts of chick embryo chorioallantoic membrane. The binding protein was characterized after partial purification of cytosol extracts by ammonium sulfate fractionation. The binding of 1,25-(OH)2-D was saturable, had a high affinity (Kd = 0.16 nM), and was specific for hormonally active vitamin D metabolites. Analysis of the displacement of [3H]1,25-(OH)2-D by unlabeled analogues showed the affinities of vitamin D metabolites to be in the order of 1,25-(OH)2-D = 1,24R,25-(OH)3-D much greater than 25-OH-D = 1-OH-D greater than 24R,25-(OH)2-D. Hormone binding was sensitive to pretreatment with sulfhydryl-blocking reagents. The chorioallantoic membrane 1,25-(OH)2-D-binding protein associated with the chromatin fraction after homogenization of membranes in low salt buffer, and bound to DNA-cellulose columns, eluting as a single peak at 0.215 M KCl. These findings support identification of this 1,25-(OH)2-D-binding protein as a steroid hormone receptor, with properties indistinguishable from 1,25-(OH)2-D receptors in other chick tissues. The chorioallantoic membrane functions in the last third of embryonic development to reabsorb calcium from the eff shell for deposition in embryonic bone. 1,25-(OH)2-D binding activity in the chorioallantoic membrane increased 4- to 5-fold from day 12 to day 16 of incubation, immediately preceding the onset of shell reabsorption. This finding suggests that 1,25-(OH)2-D may act to regulate shell mobilization and transepithelial calcium transport by the chorioallantoic membrane. Finally, the similarity of shell mobilization to bone resorption, which is also stimulated by 1,25-(OH)2-D, suggests that the chorioallantoic membrane is a useful alternate model for the study of 1,25-(OH)2-D action on bone mineral metabolism.     

Alkaline phosphatase activity does not mediate phosphate transport in the renal-cortical brush-border membrane.

We studied (1) the effect of primary modulators of phosphate transport, namely the hypophosphataemic mouse mutant (Hyp) and low-phosphorus diet, on alkaline phosphatase activity in mouse renal-cortex brush-border membrane vesicles and (2) the effect of several primary inhibitors of alkaline phosphatase on phosphate transport. Brush-border membrane vesicles from Hyp-mouse kidney had 50% loss of Na+-dependent phosphate transport, but only 18% decrease in alkaline phosphatase activity. The low-phosphorus diet effectively stimulated Na+/phosphate co-transport in brush-border membrane vesicles (+ 118%), but increased alkaline phosphatase activity only slightly (+13%). Levamisole (0.1 mM) and EDTA (1.0 mM) inhibited brush-border membrane-vesicle alkaline phosphatase activity of 82% and 93% respectively, but had no significant effect on Na+/phosphate co-transport. We conclude that alkaline phosphatase does not play a direct role in phosphate transport across the brush-border membrane of mouse kidney.     

Effect of growth and maturation on membrane-initiated actions of 1,25-dihydroxyvitamin D3-II: calcium transport, receptor kinetics, and signal transduction in intestine of female chickens

We recently reported (Larsson and Nemere [2003]: Endocrinology 144:1726) the effects of growth and maturation on 1,25(OH)2D3-membrane initiated effects in the intestine of male chickens. Here we extend our observations to studies on females with two stages of high calcium demand: growth (7-14 weeks) and egg laying (28-58 weeks). The rapid stimulatory effect of 130 pM 1,25(OH)2D3 on calcium transport was assessed as a physiological response in perfused duodena of 7-, 14-, 28-, and 58-week-old chickens, and determined to be 308%, 184%, 170%, and 153%, respectively, of corresponding controls after 40 min. Saturation analyses of [3H]1,25(OH)2D3 binding to nuclear vitamin D receptor (VDR) indicated an absence of cooperative binding, no changes in dissociation constant (Kd) with age, and an increase in maximum binding capacity (Bmax) between 7-week birds and older age groups. Analyses of saturable binding of [3H]1,25(OH)2D3 to the membrane associated rapid response steroid binding protein (1,25D3-MARRS bp) in basal lateral membranes (BLM), indicated cooperative binding, and an increase in both Bmax and Kd with age. No changes in the age-related expression of 1,25D3-MARRS bp were found, as judged by Western analyses, suggesting that a shift in ligand binding to lower affinity membrane components accounted for the increase in calculated Bmax. Basal levels of protein kinase C (PKC) activity decreased with age, as did hormone enhancement of activity. Basal levels of protein kinase A (PKA) activity remained constant with age, while the magnitude of hormone stimulation increased. Comparison of dose-response curves for ion transport and kinase activities in 7-week chicks suggested that PKC mediates phosphate transport while PKA mediates calcium transport. Thus, the age-related loss of calcium transport is most likely related to loss of PKC-mediated phosphate transport.     

The defect in transcellular transport of phosphate in the nephron is located in brush-border membranes in X-linked hypophosphatemia (Hyp mouse model)

We purified renal cortex brush-border membranes from mutant hemizygous hypophosphatemic (Hyp/Y) mice and male control (+/Y) littermates. Tenfold purification of mutant and wild-type membranes was obtained. Phosphate enters +/Y brush-border membrane vesicles by a saturable Na+-dependent arsenate-inhibited component and also by a diffusional component observed in the presence of a potassium gradient. Phosphate is not bound or incorporated significantly by mouse brush-border membrane vesicles. Parallel studies with rat renal cortex brush-border membrane vesicles revealed that phosphate and D-glucose transport in rat and mouse vesicles are similar and have the characteristics reported by other workers. Brush-border membrane vesicles prepared from Hyp/Y renal cortex have significant (p less than 0.001) partial loss of phosphate transport on the Na+-dependent arsenate-inhibited component. D-Glucose transport is not affected. Our previous studies reveal that other components of transcellular phosphate flux in kidney are normal. Therefore, we conclude that the mutant gene product in the Hyp mouse is confined to the brush-border membrane. Stability of the X-chromosome in mammalian evolution implied that the same gene product is involved in the classic human disease, familial 'vitamin D 'resistant' X-linked hypophosphatemia.     

Effect of parathyrin on the transport properties of isolated renal brush-border vesicles.

The transport properties of brush-border membrane vesicles isolated by a calcium-precipitation method from the renal cortex of normal and parathyrin (parathyroid hormone)-treated rats were studied by a rapid-filtration technique. Parathyrin elicited a dose-dependent decrease in the Na+-dependent phosphate uptake by the brush-border membrane vesicles, but the uptake of D-glucose, Na+ and mannitol was not affected. A maximum inhibition of 30% was observed after the application of 30 U.S.P. units intramuscularly 1 h before the animals were killed. Intravenous infusion of dibutyryl cyclic AMP (0.5-1.5 MG) also decreased the phosphate uptake by the brush-border vesicles. Both dibutyryl cyclic AMP and parathyrin were ineffective when added in vitro to brush-border membrane vesicles isolated from normal rats. These data suggest that parathyrin exerts its action on the phosphate reabsorption in the renal proximal tubule by affecting the Na+/phosphate co-transport system in the brush-border membrane. The effects of parathyrin on Na+ and glucose transport, however, seem to be due to alterations to the driving forces for transport and not to the brush-border transport systems.     

The effect of vitamin D analogs and of vitamin D-binding protein on lymphocyte proliferation

In the absence of vitamin D-binding protein (DBP), 1,25-(OH)2D3 at 10(-12) M significantly inhibited the [3H]thymidine incorporation in human lymphocytes during mixed lymphocyte cultures (MLC) or after phyto-hemaglutinin (PHA) stimulation. In the presence of a physiological concentration of DBP (5 x 10(-6) M), the concentration of 1,25-(OH)2D3 required for inhibition was 10(-10) M (for PHA-cultures) and 10(-9) M (for MLC). Several vitamin D analogs were compared for their inhibitory action on PHA stimulation. In the absence of DBP, the concentration necessary for 50% inhibition of [3H]thymidine incorporation ranged from 10(-12) M [1,25-(OH)2D3 and 24,24-F2-1,25-(OH)2D3], over 10(-10) M [1,24R, 25-(OH)3D3; 1,25S, 26-(OH)3D3 and 26,27-F6-1,25-(OH)2D3] and 10(-8) M [25 OHD3 and 24,25-(OH)2D3] to 10(-6) M [calcitriol-lactone]. This rank order correlates with the binding affinity of the various analogs to the cytoplasmic 1,25-(OH)2D3-receptor. DBP counteracted the inhibitory effect of all analogs and the degree of counteraction was directly proportional to the binding affinity between DBP and the vitamin D analog. DBP thus decreased the in vitro inhibitory action of 1,25-(OH)2D3 and its analogs on lymphocyte proliferation. Of all analogs tested, only 1,25-(OH)2D3 had a significant effect at a physiological concentration.     

Dietary phosphate deprivation increases 1,25-dihyroxyvitamin D3 synthesis in rat kidney in vitro

A sensitive radioreceptor assay has been used to measure in vitro 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) synthesis in vitamin D-replete rats. Incubation of kidney cortical slices with 25-hydroxyvitamin D3 produced a product which co-migrated on high performance liquid chromatography with authentic 1,25(OH)2D3 in two different solvent systems and displaced 1,25(OH)2D3 from its intestinal receptor. In addition, mass spectral analysis of the product produced a mass fragmentation consistent with that of authentic 1,25(OH)2D3. Endogenous renal cortical 1,25(OH)2D3 content in phosphate-deprived rats averaged 1.1 +/- 0.3 pmol/g (n = 11), which was significantly greater than the renal cortical 1,25(OH)2D3 content of age-matched rats eating a normal diet which averaged 0.44 +/- 0.21 pmol/g (n = 8, p less than 0.001). After incubation, net 1,25(OH)2D3 synthesis in renal slices from phosphate-deprived rats averaged 51 pmol/g/h, about 13-fold greater than the mean of 3.8 pmol/g/h observed in renal slices from rats eating the normal diet. These results indicate that the elevated plasma 1,25(OH)2D3 levels observed in rats during dietary phosphate deprivation are due to increased renal synthesis of the hormone.     

Klotho, a gene related to a syndrome resembling human premature aging, functions in a negative regulatory circuit of vitamin D endocrine system

The klotho gene encodes a novel type I membrane protein of beta-glycosidase family and is expressed principally in distal tubule cells of the kidney and choroid plexus in the brain. These mutants displayed abnormal calcium and phosphorus homeostasis together with increased serum 1,25-(OH)2D. In kl-/- mice at the age of 3 wk, elevated levels of serum calcium (10.9 +/- 0.31 mg/dl vs. 10.0 +/- 0.048 mg/dl in wild-type mice), phosphorus (14.7 +/- 1.1 mg/dl vs. 9.7 +/- 1.5 mg/dl in wild type) and most notably, 1,25-(OH)2D (403 +/- 99.7 mg/dl vs. 88.0 +/- 34.0 mg/dl in wild type) were observed.Reduction of serum 1,25-(OH)2D concentrations by dietary restriction resulted in alleviation of most of the phenotypes, suggesting that they are downstream events resulting from elevated 1,25-(OH)2D. We searched for the signals that lead to up-regulation of vitamin D activating enzymes. We examined the response of 1alpha-hydroxylase gene expression to calcium regulating hormones, such as PTH, calcitonin, and 1,25-(OH)2D3. These pathways were intact in klotho null mutant mice, suggesting the existence of alternate regulatory circuits. We also found that the administration of 1,25-(OH)2D3 induced the expression of klotho in the kidney. These observations suggest that klotho may participate in a negative regulatory circuit of the vitamin D endocrine system, through the regulation of 1alpha-hydroxylase gene expression.     

Effects of 1,25-dihydroxyvitamin D3 on cell-cycle kinetics of T 47D human breast cancer cells

The replication of several human and animal cancer cell lines is regulated in vitro and in vivo by 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], the hormonally active form of vitamin D3. We have examined the effects of concentrations of 1,25-(OH)2D3, which inhibit cellular replication, on the cell-cycle kinetics of a 1,25-(OH)2D3-responsive human breast cancer cell line, T 47D. After 6 or 7 days of treatment, a time period representing approximately five cell population doublings of control cultures, concentrations of 1,25-(OH)2D3 in the range 10(-9) M to 10(-6) M caused a time- and concentration-dependent decrease in cell numbers. Treatment of cells growing in charcoal-treated fetal calf serum with 10(-8) M 1,25-(OH)2D3 for 6 days reduced cell numbers to 49% +/- 9% (n = 9) of control, and this was associated with a marked increase in the proportion of cells in the G2 + M phase of the cell cycle from 9.7% +/- 0.5% (n = 11) to 19.6% +/- 2.3% (n = 9), significant by paired analysis (P less than 0.002). At higher concentrations of 1,25-(OH)2D3 (10(-7)-10(-6) M), there was a concentration-dependent decline in S phase and increases in both G0/G1 and G2 + M phase cells. Detailed analysis of the temporal changes in cell-cycle phase distribution following treatment with 2.5 X 10(-8) and 10(-7) M 1,25-(OH)2D3 showed an initial accumulation of cells in G0/G1 and depletion of S phase cells during the first 24 hr of treatment. This decline in S phase cells was not accompanied by a decline in % G2 + M indicating a transition delay in G2 or mitosis. At the lower dose these changes returned to control values at 48 hr and at later times were associated with a slight but consistent decline in G0/G1 phase and an increase in G2 + M. In contrast cells treated with 10(-7) M 1,25-(OH)2D3 had significantly elevated % G0/G1 cells at days 2 and 3, consistent with a transition delay through G1 phase. This was confirmed in stathmokinetic experiments which demonstrated an approximate sevenfold decrease in the rate of exit of cells from G0/G1 following 4 days of exposure to 10(-7) M 1,25-(OH)2D3. This accumulation of cells in G0/G1 was accompanied by a fall in % S phase cells.(ABSTRACT TRUNCATED AT 400 WORDS)