Increased intestinal chromatin template activity. Influence of 1alpha,25-dihydroxyvitamin D3 and hormone-receptor complexes

Hormone-nuclear receptor interactions in health and disease. Vitamin D resistance

Tissue resistance to vitamin D, or vitamin D-dependent rickets (VDDR), can be classified as two separate conditions--VDDR type I and VDDR type II--both of which present with the classical clinical, radiological and biochemical features of rickets despite adequate vitamin D intake. VDDR II can also be associated with alopecia, for reasons that are not clear. The two syndromes result from distinct disorders of vitamin D metabolism or action. Both are inherited in an autosomal recessive fashion. VDDR I is caused by decreased production of the active form of vitamin D, 1,25-dihydroxycholecalciferol, with the proposed defect being in the gene encoding the enzyme 1 alpha-hydroxylase. VDDR II results from mutations in the gene for the intracellular receptor for 1,25-dihydroxycholecalciferol (vitamin D receptor), resulting in changes in hormone or DNA binding, depending on the mutation. These mutations are analogous to those affecting receptors for other steroid-thyroid hormones, which have also been shown to cause resistance to hormone action.

Effect of cortisol on [3H] 1,25-dihydroxyvitamin D3 uptake and 1,25-dihydroxyvitamin D3-induced DNA-dependent RNA polymerase activity in chick intestinal cells

The influence of cortisol on intestinal DNA-dependent RNA polymerase activity was studied in purified nuclei of vitamin D-deficient or 1,25-dihydroxyvitamin D3-treated chicks. Six- to 7-week-old vitamin D-deficient cockerels were given 5 mg of cortisol or vehicle intraperitoneally 24 and 48 hours before sacrifice. Three hours before sacrifice, 200 ng of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) was administered intracardially. Cortisol did not alter the uptake or metabolism of 1,25(OH)2D3 in the intestinal mucosa. After a 200 ng dose of 1,25(OH)2D3 the in situ intestinal ligated loop technique revealed a 39% increase in calcium absorption compared to control birds (P less than 0.001). The administration of cortisol (5 mg) to chickens given 1,25(OH)2D3, however, resulted in a significant decrease in intestinal calcium transport in vivo (P less than 0.0025). When intestinal nuclei were prepared from birds treated in a manner identical with that described above, 1,25(OH)2D3-treated and 1,25(OH)2D3 plus cortisol-treated chicks had intestinal RNA polymerase II transcriptional activities that were significantly greater than those of vitamin D-deficient controls (P less than or equal to 0.02, P less than or equal to 0.005). There was no difference between RNA polymerase II and I + III activities of the 1,25(OH)2D3-treated birds and that of the cortisol plus 1,25(OH)2D3-treated birds. Vitamin D-deficient chicks treated with cortisol alone showed RNA polymerase I + III activity that was significantly higher (P less than or equal to 0.01) than that of birds treated with vehicle alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in responsiveness to vitamin D metabolites

We have demonstrated that epiphyseal chondroblasts contain specific receptors for 24R,25-dihydroxy vitamin D3(24,25(OH)2D3) while diaphyseal osteoblasts contain specific receptors for 1 alpha 25-dihydroxy vitamin D3(1,25(OH)2D3). Both metabolites induce DNA synthesis and creatine kinase (CKBB) activity. We have also found that the responsiveness of rat kidney to these metabolites changes during development. In embryonic and early postnatal stages, the kidney responds to 24,25(OH)2D3, later to both 24,25(OH)2D3 and 1,25(OH)2D3, and the mature kidney only to 1,25(OH)2D3. These responses correlate with changes in the specific receptors present in the kidney. Furthermore, we have compared developmental changes in skeletal (epiphysis, diaphysis and mandibular condyle) and non-skeletal (kidney, cerebellum, cerebrum, liver and pituitary) tissue in both rat (a postnatal developer) and rabbit (a perinatal developer). Epiphyseal or diaphyseal chondroblasts at any stage of development were predominantly responsive to 24,25(OH)2D3, whereas osteoblasts were responsive to 1,25(OH)2D3. In contrast, condylar chondroblasts, kidney, cerebellum and pituitary responded to 24,25(OH)2D3 during early development and subsequently developed responsiveness to 1,25(OH)2D3. Using primary cell cultures from kidneys at different stages of maturation, we showed the same developmental pattern as in vivo. Chronic treatment of the cells with 24,25(OH)2D3, but not 1,25(OH)2D3, caused precocious development of responsiveness to 1,25(OH)2D3 in culture. We suggest that 24,25(OH)2D3 acts as a maturation factor, during early development in kidney, and probably in other tissues, possibly by induction of receptor to 1,25(OH)2D3, accompanied by down-regulation of its own receptor.

The metabolism and mechanism of action of 1,25-dihydroxyvitamin D3

Much has been learned about the formation of the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3. Information concerning its formation and catabolism has allowed a clear understanding of factors involved in the maintenance of plasma concentrations of the hormone. The effects of 1,25-dihydroxyvitamin D3 on calcium transporting cells in the intestine are marked and well defined. The tissue (intestinal tissue) is easily isolated and manipulated and hence, this is an ideal tissue in which to examine the mechanism of divalent cation transport. The mechanism by which 1,25-dihydroxyvitamin D3 brings about this effect should help in understanding sterol hormone action.

Solubilization of 1,25-dihydroxyvitamin D3 receptor with pyridoxal 5'-phosphate from hen intestinal mucosa

1,25-Dihydroxyvitamin D3(1,25-(OH)2D3) receptor was solubilized in cytosol fractions upon homogenization of hen intestinal mucosa with pyridoxal 5'-phosphate contained in a low ionic strength buffer. Pyridoxal 5'-phosphate did not inhibit the binding of 1,25-(OH)2D3 to its receptor. The receptor solubilized with pyridoxal 5'-phosphate was similar to the KCl-solubilized receptor in its binding affinity to the hormone and sedimentation coefficient. A majority (greater than 90%) of the mucosal 1,25-(OH)2D3 receptors were obtained as associating with crude chromatin which was prepared with a low ionic strength buffer, and this fraction of the receptor was solubilized with pyridoxal 5'-phosphate. Ten millimolar pyridoxal 5'-phosphate was as effective as approx 0.2 M KCl in solubilizing the receptor from the crude chromatin. Pyridoxal 5'-phosphate also showed a potency to dissociate the 1,25-(OH)2D3-receptor complex previously bound to DNA-cellulose. Pyridoxal 5'-phosphate-related compounds such as pyridoxamine 5'-phosphate and pyridoxal did not show this potency. These results suggest that pyridoxal 5'-phosphate reduced the interaction of 1,25-(OH)2D3 receptor with its nuclear binding components without inhibiting the binding of the receptor to the hormone.

1,25-Dihydroxyvitamin D3 inhibits the clonogenic growth of transformed cells via its receptor

Anchorage-independent growth in soft agar is a unique property of transformed cells which is known to be correlated with tumorigenicity. We report here that 1,25-dihydroxyvitamin D3 suppresses colony formation by a number of cultured cancer cell lines in soft agar in a dose dependent manner with an ID50 of 5-7 X 10(-10) M. This effect is also achieved with analogues of 1,25-dihydroxyvitamin D3 in accordance with their binding affinity for the hormone's receptor. Only cells with 1,25-dihydroxyvitamin D3 receptor protein are inhibited in their colony formation by vitamin D analogs indicating that the hormone receptor complex may be integrally involved in the in vitro suppression of the anchorage-independent phenotype.

The physiology and pathophysiology of vitamin D

The vitamin D endocrine system plays an important role in the maintenance of normal calcium homeostasis. Abnormalities of this system occur in many conditions, such as rickets, osteomalacia, hypoparathyroidism, and hyperparathyroidism. The diagnosis and treatment of these disorders will be facilitated if the clinician understands the general mechanisms by which defects in vitamin D metabolism and action occur. We review this information and discuss the use and limitations of vitamin D metabolite assays for diagnosis of clinical disorders of mineral metabolism.

Isolation of a cDNA clone containing a sequence complementary to the intestinal calcium-binding protein of the chick

The present work describes the construction of a cDNA library in pBR322 plasmid from an mRNA population enriched for the intestinal calcium-binding protein (CaBP) mRNA of the chick. We report the isolation of one recombinant clone containing a vitamin D-regulated sequence, which is complementary to part of the CaBP mRNA. Northern blot hybridization experiments allowed us to identify a 1900 nucleotide RNA species as the CaBP mRNA.

Inhibitory action of aurintricarboxylic acid and rifamycin AF/013 at the polynucleotide domain of 1,25-dihydroxyvitamin D3-receptor complexes

The binding of 1,25-dihydroxyvitamin D3-receptor complexes from chicken intestine to DNA-cellulose and isolated intestinal nuclei is inhibited in a dose-dependent manner by aurintricarboxylic acid and rifamycin AF/013. Since both nuclear- and cytoplasmic-associated receptors have been identified, some experiments were carried out on both populations of receptors. Concentrations resulting in 50% displacement of cytoplasmic receptor complexes were 3.2 X 10(-6) M and 1.2 X 10(-4) M for aurintricarboxylic acid and rifamycin AF/013 respectively. Moreover, rifamycin AF/013 was approximately nine times more potent at inhibiting nuclear receptor binding to DNA-cellulose compared to cytoplasmic receptors. Contrary to these findings, rifampicin, which does not inhibit eukaryotic RNA or DNA polymerases, did not cause a loss of receptor complex binding to DNA-cellulose at the doses tested. Neither aurintricarboxylic acid, rifampicin, nor rifamycin AF/013 resulted in any significant loss of sterol binding. Inhibition of receptor binding to DNA-cellulose by these polymerase inhibitors was not due to alteration of the DNA and was reversed by dialysis. Incubation of receptor complexes with aurintricarboxylic acid or rifamycin AF/013 inhibited binding to Cibacron blue-agarose and phosphocellulose. Furthermore, these polymerase inhibitors were utilized specifically to desorb receptor complexes from Cibacron blue-agarose columns. Sucrose density gradient analysis of inhibitor treated and untreated receptor revealed that rifamycin AF/013 treatment resulted in the appearance of a broadened 3.7 S sedimenting receptor in addition to specific bound 1,25-dihydroxyvitamin D3 in the 6.0 S region and in the pellet of the gradient.

Stimulation of intestinal chromatin template activity by dietary carbohydrates in adult rats

Oral administration of a 70% solution of sucrose to starved adult rats resulted 1 h after feeding in a 3.5-fold stimulation of intestinal chromatin template activity assayed in vitro using E. coli RNA polymerase. A similar stimulatory effect was observed with fructose, whereas glucose exhibited a weaker effect, indicating that the nature of the ingested carbohydrate may have a direct effect on the extent of intestinal chromatin template activation.

Reversal of decreased phosphorylation of sarcoplasmic reticulum calcium transport ATPase by 1,25-dihydroxycholecalciferol in experimental uremia

When compared to that from sham-operated controls, sarcoplasmic reticulum isolated from skeletal muscle of uremic rabbits had a lower rate of calcium uptake and storing capacity. In vivo administration of 1,25-dihydroxycholecalciferol [1,25(OH)2D3] restored the values in uremic animals toward normal. To obtain information about the mechanisms responsible for these differences, phosphorylation of the calcium transport ATPase was studied. The steady-state levels of phosphoprotein in uremic membranes were lower and returned to normal when the secosteroid was administered. Electrophoresis of the membranes phosphorylated with 32P-inosine triphosphate (32P-ITP) showed that the differences were related to a 100,000 dalton protein. The rate of phosphoprotein formation, determined with 32P-ITP and at 0 degrees C, was considerably lower in uremic than in control animals. Pretreatment with 1,25(OH)2D3 prevented this change. The hypothesis is advanced that the vitamin D metabolite affects the steady-state concentration and rate constant of formation of active sites in the Ca-ATPase. These results may partly explain the altered Ca transport function of the sarcoplasmic reticulum in experimental uremia.

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.

Vitamin D receptors in isolated rat parotid gland acinar cells

Rat parotid gland was examined for the presence of 1 alpha, 25-dihydroxycholecalciferol receptors using sucrose density gradient ultracentrifugation techniques. [3H] DHCC bound specifically and with high affinity to a 3.2 S protein present in nuclear and cytosolic fractions of isolated parotid acinar cells. Values for the equilibrium dissociation constant and for the receptor concentration were determined to be approx. 0.1 nM, and 12 fmol/mg protein, respectively. In competitive inhibition experiments, the 3.2 S protein displayed 100-fold lower affinity for 25-hydroxycholecalciferol than for DHCC, and did not bind estradiol or methylprednisolone. These results suggest that rat parotid gland acinar cells contain classical DHCC receptors. A similar approach failed to provide evidence of DHCC receptors in isolated pancreas acinar cells, lacrimal gland or submandibular gland. It has been previously reported that vitamin D is essential for normal exocrine secretion from the rat parotid gland (Tenenhouse, A. and Afari, G. (1978) Biochim. Biophys. Acta 538, 631-634). The present findings suggest that this effect is the result of a direct action of DHCC on the parotid gland acinar cell. The absence of DHCC receptors in other exocrine cells suggests that tissue sensitivity to DHCC is not a general property of exocrine systems.

Receptors for 1,25-dihydroxyvitamin D3 in chick pancreas: a partial physical and functional characterization

1,25-Dihydroxyvitamin D3(1,25-(OH)2D3) receptors from the rachitic chick pancreas have been partially characterized. Analyses of these receptors by isokinetic gradient centrifugation and analytical gel filtration reveal a sedimentation coefficient (S) of 3.3-3.7, a molecular weight (Mr) of 58,500-68,000, and a calculated Stokes molecular radius (Rs) of 34-36 A. Polyethylenimine-ammonium sulfate precipitation of pancreatic cytosol partially purifies aporeceptor and reduces nonspecific binding (in part, 5.8S DBP), thus providing material more amenable to kinetic analyses, Binding studies incorporating this fractionated cytosol reveal an equilibrium dissociation constant (K4) of approximately 0.112 nM at 2 degrees C for the 1,25-(OH)2D3-receptor interaction. Competition studies further demonstrate a particular preference for 1,25-(OH)2D3 over 1,24(R),25-trihydroxyvitamin D3, 24(R),25-dihydroxyvitamin C3, and 25-hydroxyvitamin D3. The pancreatic receptor also binds to immobilized group-selective affinity ligands such as DNA, cibacron blue, and heparin, and can be eluted as a single macromolecular species during standard linear KCl gradients. Its interaction with these ligands supports the premise that the 1,25-(OH)2D3 receptors' fundamental mode of action is at the level of the cellular genome. Salt-dependent nuclear uptake and chromatin localization studies with this receptor in vitro also support this potential site of action. Significantly, a physiologic dose of 1,25-(OH)2[3H]D3 to rachitic chicks leads to the in vivo formation of a receptor-hormone complex as identified by DNA-cellulose chromatography. These observations provide further evidence that the pancreatic protein is a biologically relevant component of the chick pancreas which functions to accumulate hormone intracellularly under physiologic situations.

Action of vitamin D on intestinal calcium transport

The means by which 1,25(OH)2D3 regulates calcium transport across the intestine still remains an enigma. The movement of calcium across the brush border membrane into the cell occurs down a steep concentration gradient. Stimulation of this process by 1,25(OH)2D3 occurs within 2h and does not require new protein synthesis. Although the mitochondria may be involved in moving the calcium across the cell under rather high concentrations of cytosol calcium (10(-5) M Ca), a different organelle (lysosome-like vesicle) may assume primary responsibility for intracellular transport at lower calcium concentrations. This change from mitochondrial to vesicular calcium movement requires time (within 18 h) and may involve new protein synthesis. In the absence of this changeover, calcium transport can still occur but only at the cost of higher intracellular calcium concentrations. Movement of calcium out of the cell across the basolateral membrane occurrs against a steep concentration gradient. The difficulty in preparing pure basolateral membranes with homogeneous orientation of the inner and outer surfaces has limited progress in our understanding of calcium movement across this membrane. However, the need for an energy-driven calcium pump at this membrane that is responsive to micromolar calcium concentrations is apparent. Whether 1,25(OH)2D3 induces the synthesis of such a pump or leads to its activation by factors such as CaBP, cAMP, and calmodulin remains to be determined.

Studies on the mechanism of action of 1 alpha, 24-dihydroxyvitamin D3. II. Specific binding of alpha, 24-dihydroxyvitamin D3 to chick intestinal receptor

The binding of vitamin D3 analogues to the chick intestinal cytosol receptor was studied. In intestinal cytosol fraction, receptor proteins having the sedimentation constant of 2.5 S and 3.7 S to which 1 alpha,25-dihydroxyvitamin D3 binds were present, and the latter was specific for the compound. The binding of 1 alpha,24(R)-dihydroxyvitamin D3 and 1 alpha,24(S)-dihydroxyvitamin D3 to the receptor was also observed, while very weak binding was seen in the case of 24(R)25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3. The binding affinity of 1 alpha,24(R)-dihydroxyvitamin D3 to the 3.7 S receptor was 1.3 times as high as that of 1 alpha,25-dihydroxyvitamin D3, whereas those of 1 alpha,24(S)-dihydroxyvitamin D3, 1 alpha-hydroxyvitamin D3 and 25-hydroxyvitamin D3 were 10, 304 and 652 times lower than 1 alpha,25-dihydroxyvitamin D3, respectively. The dissociation constant of the receptor-1 alpha,25-dihydroxyvitamin D3 complex at 0 degrees C was 3.0 x 10(-11) M, and the dissociation constants were calculated to be 2.4 x 10(-11) M and 2.7 x 10(-10) M for the complexes with 1 alpha,24(R)-dihydroxyvitamin D3 and 1 alpha,24(S)-dihydroxyvitamin D3, respectively.

Vitamin D metabolism and calcium absorption

Vitamin D along with parathyroid hormone (PTH) and calcitonin (CT) are the three principal effectors of calcium and phosphorus homeostasis. The secosteroid, vitamin D3, is subject to metabolic conversion to its biologically active form(s) 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] and 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] prior to initiation of its physiologic responses in the intestine and skeletal system. The production of 1,25(OH)2D3 is stringently regulated by a variety of endocrine signals including PTH as well as the "calcium needs" of the organism. At the target intestine, 1,25-(OH)2D3 stimulates the intestinal absorption of calcium via a mechanism analogous to that of other steroid hormones. Definitive biochemical evidence exists supporting the existence in the intestine of a highly specific protein receptor for 1,25(OH)2D3. After formation of the steroid-receptor complex, it migrates to the nucleus of the cell and stimulates messenger-RNA synthesis for proteins (including a calcium-binding protein) which are necessary for the generation of the biologic response. Current efforts to biochemically characterize vitamin D-mediated intestinal calcium transport include efforts to understand the role of calcium-binding protein in this process, as well as to identify other protein components present either in the brush border or basal lateral membranes.