Unoccupied and in vitro and in vivo occupied 1,25-dihydroxyvitamin D3 intestinal receptors. Multiple biochemical forms and evidence for transformation

Mitotic genome bookmarking by nuclear receptor VDR advocates transmission of cellular transcriptional memory to progeny cells

Mitosis is an essential process for the self-renewal of cells that is accompanied by dynamic changes in nuclear architecture and chromatin organization. Despite all the changes, the cell manages to re-establish all the parental epigenetic marks, post-mitotically. Recent reports suggest that some sequence-specific transcription factors remain attached to mitotic chromatin during cell division to ensure timely reactivation of a subset of transcription factors necessary to maintain cell identity. These mitotically associated factors are suggested to act as 'genome bookmarking factors' and the phenomenon is termed 'genome bookmarking'. Here, we studied this phenomenon with Vitamin D Receptor (VDR), a key regulator of calcium and phosphate homeostasis and a member of the nuclear receptor superfamily. This study, for the first time, has confirmed VDR as a mitotic bookmarking factor that may be playing a crucial role in the maintenance of cell identity and genome bookmarking. Full 'DNA binding domain (DBD)' present in VDR was identified as essential for enrichment of VDR on mitotic chromatin. Furthermore, the study also demonstrates that VDR evokes mitotic chromatin binding behaviour in its heterodimeric partner Retinoid X receptor (RXR). Interestingly, for promoting bookmarking behaviour in RXR, both DBD and/or ligand-binding domain (LBD) in conjunction with hinge region of VDR were required. Additionally, ChIP analysis showed that VDR remains associated with DR3 (direct repeat 3) region of its specific target gene promoter CYP24A1(Cytochrome P450 family 24 subfamily A member1), during mitosis. Altogether, our study illustrates a novel function of VDR in the epigenetic transmission and control of expression of target proteome for maintenance of cell identity and traits in progeny cells.

Association of vitamin D receptors with the nuclear matrix of human and rat genitourinary tissues

Calcitrol, 1,25 dihydroxyvitamin D3 (1,25-D3) has an important role in the antiproliferative and growth regulatory effects on normal and neoplastic cells (e.g. prostate cancer cells). 1,25-D3 binds to the vitamin D receptor (VDR), a member of the steroid receptor superfamily. Steroids, via intranuclear receptors, have been demonstrated to have high affinity binding to the nuclear matrix, the tissue specific scaffolding of the nucleus that is involved in the organization of DNA, replication and transcription. We hypothesized that the VDR interacts closely with the nuclear matrix in both human and rat tissues. In the studies described here, nuclear matrix proteins (NMP) were extracted from a number of rat and human tissues and immunoblot analysis performed using a rat anti-VDR antibody. The results from these studies reveal that the anti-VDR antibody detects six forms of the VDR in the NMP preparations: human testis demonstrated a protein of 57 and 52 kDa molecular weight compared with 57 and 37 kDa in the rat testis. Human prostate demonstrated proteins of 52 kDa compared to rat ventral (57 and 37 kDa) and dorsal prostate (52 and 26 kDa). Human and rat bladder NMP demonstrated a protein binding at 55 kDa and rat seminal vesicle NMP binding at 48 kDa. This is the first report of VDRs associated with the nuclear matrix. The varying molecular weight proteins reactive with the anti-VDR antibody within these tissues may represent different isoforms, proteolytic cleavage of a larger VDR or post-translational modification. The VDR-NMP interaction may be involved in the tissue specific actions of 1,25-D3 especially growth regulatory and antiproliferative effects.

The involvement of polyamines in the activation of vitamin D receptor from porcine intestinal mucosa

In the intracellular process of the action of calcitriol, vitamin D receptor is thought to undergo some kind of physiochemical change, called activation, before the receptor binds to the vitamin D response element of the gene. In this paper, the effects of polyamines and their analogues on the sedimentation properties of vitamin D receptor prepared from porcine intestinal mucosa, and on DNA binding activity of the receptor, were studied. In sucrose density gradient analysis, polyamines decreased the sedimentation coefficient of vitamin D receptor in a dose-dependent fashion. Polyamines increased DNA binding activity of vitamin D receptor dose-dependently. These findings show that polyamines can activate vitamin D receptor in vitro. Among naturally existing polyamines, spermidine and spermine, but not putrescine, were effective within their physiological intracellular concentrations, suggesting that both spermidine and spermine can activate vitamin D receptor in vivo as well. Sucrose density gradient analysis using various kinds of polyamine analogues having various numbers of cations showed that the number of cation of polyamines is important for the efficiency to change the sedimentation coefficient of vitamin D receptor, and that the distance between two cationic charges does not play an important role.

Effect of 1,25-(OH)2-vitamin D3 on growth, homologous receptor and c-myc regulation in C3H/10T1/2 cells

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) receptor concentration, cell proliferation, and the steady-state level of c-myc mRNA were examined in the C3H/10T1/2 mouse embryo fibroblasts, before and after exposing the cells to 1,25-(OH)2D3. The non-transformed, logarithmically growing C3H/10T1/2 Cl 8 cells contained a high concentration of 1,25-(OH)2D3 receptor (164 fmol/mg of protein). An up-regulation of the 1,25-(OH)2D3 receptor and a potent inhibition of cell growth were observed by exposing the cells to 10 nM 1,25-(OH)2D3. The concentration of 1,25-(OH)2D3 receptor in the two chemically transformed, tumorigenic cell lines. C3H/10T1/2 Cl 16 and C3H/10T1/2 TPA 482, was 218 and 63 fmol/mg of protein, respectively. In the two transformed cell lines, 10 nM 1,25-(OH)2D3 had only negligible effect on cell growth. In the Cl 16 cells, an up-regulation of the 1,25-(OH)2D3 receptor was demonstrated, but only a weak up-regulation was found in the TPA 482 cells by the 1,25-(OH)2D3 treatment. No major changes were found in c-myc mRNA levels by the 1,25-(OH)2D3 treatment. Despite inhibition of cell growth, the steady-state level of c-myc mRNA was slightly induced (35%, mean) in the Cl 8 cells compared to control cells. In the transformed cells, no consistent change of the c-myc level was found. In contrast to earlier reports, we did not find any correlation between the 1,25-(OH)2D3 receptor and c-myc level, nor did we find any decrease of c-myc mRNA by 1,25-(OH)2D3 treatment in the C3H/10T1/2 fibroblasts.

Effect of age on duodenal 1,25-dihydroxyvitamin D-3 receptors in Wistar rats

We confirmed our previous observation that duodenal Ca2+ absorption and serum 1,25-dihydroxyvitamin D (1,25-(OH)2D) levels declined concurrently in old (24 months old) rats as compared to young (6 months old) rats. It is well known that 1,25-dihydroxyvitamin D-3 (1,25-(OH)2D3) expresses its action after binding to specific receptor molecules. In this paper, we compared certain properties of rat duodenal 1,25-(OH)2D3 receptors from old and young animals. Receptor preparations were incubated with [3H]1,25-(OH)2D3 to quantitate the number of unoccupied and total receptor sites and showed that total and unoccupied receptor sites decreased by 22 and 16%, respectively in old rats. Endogenously occupied sites were reduced by 43% in duodenum of the old rat and, consequently, the percentage of receptor occupancy also declined. Age did not affect the dissociation constant (KD) of 1,25-(OH)2D3 from the receptor; the sedimentation coefficient (3.3 S) of the tritiated 1,25-(OH)2D3-receptor complex in sucrose density centrifugation; or its affinity for DNA. The data are consistent with the hypothesis that the age-related decline in Ca2+ absorption in the intestine may be due, in part, to the decrement in the circulating level of 1,25-(OH)2D and a reduction of intestinal 1,25-(OH)2D3 receptor occupancy status.

Lymphocyte cell lines from vitamin D-dependent rickets type II show functional defects in the 1 alpha,25-dihydroxyvitamin D3 receptor

Lymphocyte cell lines were established from five patients with vitamin D-dependent rickets, type II (VDDR-II). These lines were established by infection with human T-lymphotrophic virus type I (HTLV-I). Binding of [3H]1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) to its receptor in these cell lines was compared to binding studies using a T-lymphocyte cell line (S-LB1) from a normal individual. The 1,25(OH)2D3 receptor of S-LB1 was comparable to the well-characterized chick intestinal 1,25(OH)2D3 receptor in terms of its ligand binding affinity and capacity, its mobility on 5-20% sucrose gradients, and its adsorption to and elution properties from DNA-cellulose. Three cell lines established from patients with VDDR-II (Rh-VDR, Sh-VDR, and Ab-VDR) showed no specific binding of 1,25(OH)2D3 to a receptor and treatment of the cultured cells with 1,25(OH)2D3 did not stimulate production of 24,25-dihydroxy-vitamin D3 (24,25(OH)2D3), a response which is diagnostic of the presence of a functional 1,25(OH)2D3 receptor. In a fourth cell line, A1-VDR, the receptor for 1,25(OH)2D3 had a low binding capacity and 25(OH)D3-24-hydroxylase activity was not detectable. Induction of 24,25-(OH)2D3 synthesis by 1,25(OH)2D3 was observed in the fifth cell line, designated Ro-VDR, although the sensitivity to hormone treatment was lower than in the control cell line from a normal donor. The capacity of the receptor for 1,25(OH)2D3 was low in Ro-VDR. In all cell lines where 1,25(OH)2D3 binding to a receptor was detectable, the receptor had the typical sedimentation coefficient of 3.7 S on sucrose density gradient analysis. Binding and elution properties to DNA-cellulose, however, differed from normal in both Ro-VDR and A1-VDR cells where elution from DNA-cellulose occurred at a lower salt concentration than is typical of the 1,25(OH)2D3 receptor. While Ro-VDR cells showed typical nuclear localization of the unoccupied 1,25(OH)2D3 receptor, neither the unoccupied nor the occupied receptor from A1-VDR cells was completely localized in the nucleus. In a series of functional studies we found that modulation of the level of the mRNAs coding for both the c-myc oncogene and the growth factor known as granulocyte-monocyte colony stimulating activity by 1,25(OH)2D3 correlated with the 1,25(OH)2D3 receptor status of these cells. Use of these cell lines will facilitate further study of the molecular defect(s) in the receptor for 1,25(OH)2D3 in vitamin D-dependent rickets type II and will allow a correlation with impairment of cellular functions.

Agents affecting adenylate cyclase activity modulate the stimulatory action of 1,25-dihydroxyvitamin D3 on the production of osteocalcin by human bone cells

The stimulation of osteocalcin synthesis by human osteoblast-like cells in response to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is antagonised by several bone regulatory agents. We have shown that agents which activate adenylate cyclase inhibit this action of 1,25(OH)2D3 on human osteoblast-like cells. Activation of adenylate cyclase, either via the stimulatory GTP-binding protein using cholera toxin, or directly at the catalytic via the stimulatory GTP-binding protein using cholera toxin, or directly at the catalytic subunit using forskolin, results in a suppression of osteocalcin synthesis. Whilst the activation of adenylate cyclase induces this inhibitory response, neither exogenous dibutyryl cyclic AMP nor the phosphodiesterase inhibitor, IBMX, exerted any apparent effect on the production of osteocalcin. The tumour promoting phorbol ester, 4 beta-phorbol 12,13-dibutyrate, also inhibited 1,25(OH)2D3-stimulated osteocalcin production. This was not apparent in response to the non-tumour promoting phorbol ester 4 beta-phorbol suggesting the involvement of protein kinase C.

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.

Selective localization of calcium-binding protein in human brainstem, cerebellum and spinal cord

The 28,000-Da vitamin D-dependent calcium-binding protein, CaBP, which is induced by one hormonally active form of vitamin D3, 1,25-dihydroxyvitamin D3, was localized by immunocytochemistry in the human brainstem, cerebellum and cervical segment of the spinal cord. Positive structures (neurons and their processes) were restricted to some well-defined motor and sensory pathways. In motor regions, the highest density of immunoreactive sites was found in the Purkinje cell layer of the cerebellar cortex, and CaBP-positive neurons were also found in the reticular formation and the inferior olivary nucleus. In sensory pathways, positive neurons were mainly localized in structures associated with protopathic thermalgesia (pain and temperature), as well as in the solitary nucleus and parabrachial nucleus of the taste pathway.

Glucocorticoid mechanism of action: monoclonal antibodies as experimental tools

In order to provide a further insight into glucocorticoid receptor (GR)-mediated action of glucocorticoid hormones, we produced ten monoclonal antibodies against rat GR. In studies combining physicochemical separation methods with antibody methodology, we established that the molybdate-stabilised GR contains one steroid-binding monomer. Using a monoclonal anti-GR antibody-based immunoaffinity chromatographic procedure, we purified two non-ligand-binding proteins, with molecular weights of 80,000 and 90,000, present in the molybdate-stabilised GR complex. These proteins are not recognised by monoclonal antibodies directed against GR. The possible relation of these two proteins to heat shock proteins remains to be established. Immunohistochemical studies of GR in the central nervous system of the rat provided new information on the distribution of GR, particularly in the hypothalamus. Studies of intracellular receptor localisation in rat brain after endocrine manipulations gave results in support of the classical concept of translocation of GR from cytoplasm to cell nucleus. Studies with a cell culture system also supported the existence of GR in the cytoplasm as well as in the cell nucleus.

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.

Characterization of the DNA-binding properties of the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin

The DNA-binding properties of the receptor for 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) were investigated using chromatography on DNA-cellulose columns. A maximal binding of about 40% of the total receptor complex to DNA-cellulose was observed. In order to interact with DNA, the receptor must first bind TCDD. A heat-activation step followed by gel permeation chromatography using Sephadex G-25 increased the binding of the cytosolic receptor to DNA. The DNA-binding ability of the receptor was almost lost following mild proteolysis using trypsin or alpha-chymotrypsin, although these treatments did not reduce its ligand binding capacity and had no apparent effect on its size. Furthermore, pre-treatment of the DNA-cellulose column with an intercalating drug, ethidium bromide, resulted in inhibition of the binding of the TCDD-receptor complex to DNA, indicating that not only electrostatic interactions but also the configuration of DNA are of importance in receptor-DNA interactions.

1,25-Dihydroxyvitamin D3 receptors: altered functional domains are associated with cellular resistance to vitamin D3

1,25-Dihydroxyvitamin D3 receptors are cytosoluble proteins detectable in a variety of tissues responsive to 1,25(OH)2D3. They are DNA binding-proteins analogous to other steroid receptors and it is this functional property which is likely involved in the activation of hormone-sensitive genes. Utilizing 1,25(OH)2D3 and DNA binding assays, as well as anti-receptor monoclonal antibodies, we have probed the relationship between the 1,25(OH)2D3 receptor binding domains after selective cleavage with trypsin. These studies reveal that the hormone and DNA binding regions are separable, and are consistent with the finding that tissue resistance to 1,25(OH)2D3 is a result of structural defects in these domains. Recently, a primate model, the LLC-MK2 monkey kidney line, has been uncovered which may exemplify a hormone-binding defect. Here, 25-hydroxyvitamin D3-24-hydroxylase induction, a 1,25(OH)2D3 bioresponse, requires 100-fold higher concentrations of the hormone for maximal response. Concomitantly, this cell contains a variant receptor form which displays a correspondingly lowered apparent affinity for the hormone despite its seemingly normal DNA binding characteristics. Taken together, these studies suggest that the 1,25(OH)2D3 receptor is a macromolecule with multiple domains each of which may produce modified cellular resistance to 1,25(OH)2D3 if structurally altered.

Hormone-dependent phosphorylation of the 1,25-dihydroxyvitamin D3 receptor in mouse fibroblasts

Experimental results, employing several immunologic techniques, suggest that the mouse receptor for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) undergoes hormone-dependent phosphorylation in intact cells. Treatment of monolayer cultures of mouse 3T6 fibroblasts with 1,25(OH)2D3 reveals that the occupied 1,25(OH)2D3 receptor displays a minor reduction in electrophoretic mobility as compared to its unoccupied 54,500 dalton counterpart, a change consistent with covalent modification. Similar results were obtained by immunoprecipitation of metabolically-labeled receptors after incubation of 3T6 cells with [35S]methionine. This technique also provided greater insight into the precursor-product relationship between the two receptor forms. [32P]Orthophosphate-labeling of 3T6 cells, followed by immunoprecipitation indicated that only the form exhibiting covalent modification was phosphorylated. The temporal correspondence between the binding of 1,25(OH)2D3 to its cellular receptor and its phosphorylation suggests that the biochemical role of 1,25(OH)2D3 may be to induce a conformational change susceptible to phosphorylation and possibly functional activation.

Activation of the 1,25-dihydroxyvitamin D3 receptor in cultured rat osteogenic sarcoma cells

Activation of the 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] receptor-hormone complex was studied in-vitro using cytosolic preparations of rat osteogenic sarcoma cell ROS 17/2-8 and a DNA-cellulose assay. We found that salt was required for extraction of the unoccupied receptor indicating its possible nuclear localization. The 1,25(OH)2D3 receptor underwent an activation process similar to other steroid hormones which could be stimulated by heat and salt. At physiological ionic strength 100% of the complexes were, however, activated at 2 degrees C, indicating that the activation process is not absolutely temperature dependent. In contrast to other steroid hormones, 30-50% of the complexes were in an activated state in the absence of heat and salt moreover, alkaline phosphatase and ammonium sulphate had no effect on activation. Activation was also stimulated by ATP and ATP plus 8BrcAMP indicating the possible role of phosphorylation in the activation process; however, further work is required to clarify this point.