24,25-Dihydroxyvitamin D(3) and bone metabolism

The 1alpha-hydroxylated metabolite of 25-hydroxyvitamin D(3), 1,25-dihydroxyvitamin D(3), is the biologically most active metabolite of vitamin D. The 24-hydroxylated metabolites were generally considered as degradation products of a catabolic pathway finally leading to excretion of calcitroic acid. Studies with analogues fluorinated at the C-24 position did not indicate a physiological function for 24R,25(OH)(2)D(3). Nevertheless throughout the years various studies showed biologic effects of other metabolites than 1alpha,25(OH)(2)D(3). In particular the metabolite 24R,25(OH)(2)D(3) has been functionally analyzed, e.g. with respect to a role in normal chicken egg hatchability and effects on chondrocytes in the resting zone of cartilage. Numerous studies have shown the presence of the vitamin D receptor in bone cells and effects of 1alpha,25(OH)(2)D(3) on bone and bone cells. Also for 24R,25(OH)(2)D(3) studies have been performed focusing on effects on bone and bone cells. The purpose of this review is to summarize the data regarding 24R,25(OH)(2)D(3) and bone and to evaluate its role in bone biology.     

WASHING THE CITIZEN: WASHING, CLEANLINESS AND CITIZENSHIP IN MENTAL HEALTH CARE

Participation in the community and citizenship for patients are common ideals that inspire improvements in mental health care. But what is meant by citizenship? Here an analysis is made of washing practices in psychiatric nursing in long-term mental health institutions. Four repertoires of washing are described, each oriented towards a specific notion of citizenship. In the first repertoire, washing is part of individual privacy; the patient is “enacted” as an individual whose authenticity should be respected in order to equip him or her for participation in the community. In the second repertoire, washing is a basic skill; the patient must learn to take care of her body in order to become an independent citizen. In the third repertoire washing is a precondition to citizenship; patients are to be helped to develop their potentials so that they can find their way in the community. In the fourth repertoire, washing is one opportunity among others to develop social relations; the extent and quality of these relations define a citizen. This analysis opens up not the question if, but which type of citizenship should be promoted.     

Evidence that the self-induced metabolism of 1,25-dihydroxyvitamin D-3 limits the homologous up-regulation of its receptor in rat osteosarcoma cells

The osteoblast-like osteosarcoma cell line UMR-106 has been shown to possess high-affinity receptors for 1,25-dihydroxyvitamin D (1,25-(OH)2D3). Also, these cells metabolize 1,25-(OH)2D3 to more polar metabolites. As previously demonstrated (Pols, H.A.P., et al. (1987) Biochim. Biophys. Acta 931, 115-119) the time course of specific binding of 1,25-(OH)2D3 in intact UMR-106 cells was found to be characterised by (a) an ascending phase, representing association with receptor, (b) a maximum at 90-120 min and (c) a rapid descending phase, closely associated with a decrease of medium 1,25-(OH)2D3 due to the metabolism of the hormone. The purpose of the present study was to investigate further the self-induced metabolism of 1,25-(OH)2D3 in relation to the homologous up-regulation of its receptor in these cells. Inhibition of metabolism of 1,25-(OH)2D3 with ketoconazole resulted, after a lag-time of about 90 min, in a sharp increase of receptor accumulation. This increase in receptor level in the presence of ketoconazole was blocked by coincubation with cycloheximide and actinomycin D. Preincubation experiments with unlabeled 1,25-(OH)2D3 showed that the elevation of hormone binding was 1,25-(OH)2D3-concentration dependent (ED50 200-300 pM). Addition of ketoconazole during these preincubations resulted in an even more pronounced accumulation of receptors, whereby the ED50 (50-60 pM) was comparable with the dissociation constant of the 1,25-(OH)2D3 receptor (41.3 +/- 4.3 pM). In summary, these data support the concept that the self-induced metabolism of 1,25-(OH)2D3 has a dual effect: (1) directly, by the regulation of the cellular concentration of and, consequently, receptor occupancy by the active form of vitamin D and (2) indirectly by its ability to modulate the ligand-dependent regulation of the 1,25-(OH)2D3.     

Lack of association between five polymorphisms in the human glucocorticoid receptor gene and glucocorticoid resistance

Glucocorticoid resistance due to mutations in the gene for the glucocorticoid receptor has been suggested to be more common than is thought at present, owing to the relative mildness of its symptoms and the difficulty of its diagnosis. To investigate the prevalence of mutations in the glucocorticoid receptor gene responsible for relative insensitivity to glucocorticoids, we carried out polymerase chain reaction/single-strand conformation analysis of the glucocorticoid receptor gene in a group of 20, otherwise healthy, persons with a reduced response in a dexamethasone suppression test and in 20 controls. We did not find mutations or polymorphisms associated with a reduced sensitivity to glucocorticoids. However, we identified five novel polymorphisms in the gene for the human glucocorticoid receptor, which may be useful in analyzing whether loss of (part of) the glucocorticoid receptor gene plays a role in glucocorticoid-resistant malignancies. Although relative resistance to glucocorticoids seems to be rather frequent in otherwise healthy persons, it is not usually associated with mutations or polymorphisms in the glucocorticoid receptor gene. Received: 17 July 1996 / Revised: 26 November 1996     

Promoter and 3'-untranslated-region haplotypes in the vitamin d receptor gene predispose to osteoporotic fracture: the rotterdam study

Polymorphisms of the vitamin D receptor gene (VDR) have been shown to be associated with several complex diseases, including osteoporosis, but the mechanisms are unknown and study results have been inconsistent. We therefore determined sequence variation across the major relevant parts of VDR, including construction of linkage disequilibrium blocks and identification of haplotype alleles. We analyzed 15 haplotype-tagging SNPs in relation to 937 clinical fractures recorded in 6,148 elderly whites over a follow-up period of 7.4 years. Haplotype alleles of the 5' 1a/1e, 1b promoter region and of the 3' untranslated region (UTR) were strongly associated with increased fracture risk. For the 16% of subjects who had risk genotypes at both regions, their risk increased 48% for clinical fractures (P = .0002), independent of age, sex, height, weight, and bone mineral density. The population-attributable risk varied between 1% and 12% for each block and was 4% for the combined VDR risk genotypes. Functional analysis of the variants demonstrated 53% lower expression of a reporter construct with the 1e/1a promoter risk haplotype (P = 5 x 10(-7)) in two cell lines and 15% lower mRNA level of VDR expression constructs carrying 3'-UTR risk haplotype 1 in five cell lines (P = 2 x 10(-6)). In a further analysis, we showed 30% increased mRNA decay in an osteoblast cell line for the construct carrying the 3'-UTR risk haplotype (P = .02). This comprehensive candidate-gene analysis demonstrates that the risk allele of multiple VDR polymorphisms results in lower VDR mRNA levels. This could impact the vitamin D signaling efficiency and might contribute to the increased fracture risk we observed for these risk haplotype alleles.     

Genetics and biology of vitamin D receptor polymorphisms

The vitamin D endocrine system is involved in a wide variety of biological processes including bone metabolism, modulation of the immune response, and regulation of cell proliferation and differentiation. Variations in this endocrine system have, thus, been linked to several common diseases, including osteoarthritis (OA), diabetes, cancer, cardiovascular disease, and tuberculosis. Evidence to support this pleiotropic character of vitamin D has included epidemiological studies on circulating vitamin D hormone levels, but also genetic epidemiological studies. Genetic studies provide excellent opportunities to link molecular insights with epidemiological data and have therefore gained much interest. DNA sequence variations, which occur frequently in the population, are referred to as "polymorphisms" and can have modest and subtle but true biological effects. Their abundance in the human genome as well as their high frequencies in the human population have made them targets to explain variation in risk of common diseases. Recent studies have indicated many polymorphisms to exist in the vitamin D receptor (VDR) gene, but the influence of VDR gene polymorphisms on VDR protein function and signaling is largely unknown. So far, three adjacent restriction fragment length polymorphisms for BsmI, ApaI, and TaqI, respectively, at the 3' end of the VDR gene have been the most frequently studied. Because these polymorphisms are probably nonfunctional, linkage disequilibrium with one or more truly functional polymorphisms elsewhere in the VDR gene is assumed to explain the associations observed. Research is therefore focussed on documenting additional polymorphisms across the VDR gene to verify this hypothesis and on trying to understand the functional consequences of the variations. Substantial progress has been made that will deepen our understanding of variability in the vitamin D endocrine system and might find applications in risk assessment of disease and in predicting response-to-treatment.