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Introduction
There is immense current interest in the effects of thyroid hormones on bone. This is largely due to concern that patients on thyroxine replacement therapy are at increased risk of developing osteoporosis; this concern follows a number of reports describing reduced bone mineral density in this group of patients. The issue is, however, uncertain and the purpose of this review is (i) to summarize what is known about the effects of thyroid hormones on bone at both an experimental and clinical level and (ii) to try to reach a greater understanding of the problem and its implications for patient management.
Bone biology
Bone remodelling requires the tightly coupled actions of osteoclasts and osteoblasts. A normal bone remodelling cycle takes approximately 200 days. Each cycle begins with activation of cells which become osteoclasts and start resorbing bone. This phase lasts for about 50 days and is terminated
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ABSTRACT
The effect of excessive tri-iodothyronine (T3) in vivo was assessed using normal human lymphocytes. Cells from normal subjects were frozen in liquid nitrogen before and after oral administration of T3 for 1 week to permit a direct comparison under identical culture conditions. Within the group of individuals studied, some subjects did show changes in B or T cell function but hypertri-iodothyroninaemia produced no consistent effect for the whole group on circulating T cell subsets or T and B cell activation measured by short-term culture or stimulation of lymphocyte cultures with phytohaemagglutinin or pokeweed mitogen. Tri-iodothyronine supplementation of cultures in vitro did not affect pokeweed mitogen stimulation. These findings suggest that the immunological abnormalities in Graves' disease are not the result of increased circulating thyroid hormone levels and that remission following medical treatment is due to an immuno-suppressive effect of the drug rather than the restoration of euthyroidism.
J. Endocr. (1984) 101,81–86
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ABSTRACT
A robust, rapid, sensitive and specific enzyme-linked immunosorbent assay (ELISA), using an in-house immunoglobulin-A-sub-class mouse monoclonal human-thyroglobulin antibody (WNSM2), with a sensitivity of 1·51 pmol/l has been established for the measurement of thyroglobulin in serum. Standard curves in varying dilutions of human serum were similar to standard curves obtained in serum-free medium, thus demonstrating no significant cross-reactivity with any serum proteins other than thyroglobulin. Levels of serum thyroglobulin detected by ELISA correlated significantly (r = 0·93, P < 0·001) with those from a standardized and well-characterized radioimmunoassay. The coefficients of variation within and between ELISA assays were 3·9 and 7·1% respectively. Thyroglobulin was detectable in 87% of 54 normal subjects who had no history of thyroid or autoimmune disease, the mean (±s.d.) for this group being 15 ± 6·6 pmol/l with a range of 1·51–53 pmol/l. Using this assay, levels of thyroglobulin were shown to be significantly (P < 0·002) increased in patients with untreated hyperthyroid Graves' disease compared with normal subjects.
J. Endocr. (1985) 107, 383–387
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ABSTRACT
By suitable immunization of mice and fusion of their spleen cells with a non-secretor mouse myeloma line, monoclonal antibodies have been produced which react with the human thyroid microsomal (M) antigen. These monoclonal antibodies showed no reactivity by enzyme-linked immunoassay with liver microsomes or thyroglobulin and their specificity was confirmed by immunolocalization studies, in which they showed the staining characteristics of human M antibodies. All four monoclonal antibodies tested were immunoglobulin M; three were cytotoxic to thyroid cell monolayers. The lack of cytotoxicity with the fourth monoclonal supports the concept that certain epitopes of the M antigen may be partially or completely absent at the thyroid cell surface. These monoclonal antibodies should permit further characterization of the thyroid M antigen in view of their absence of cross-reactivity with thyroglobulin.
J. Endocr. (1985) 105, 47–52
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ABSTRACT
Tri-iodothyronine (T3) increases bone resorption in vivo and in vitro. In order to understand further the mechanisms by which this occurs we studied the effects of T3 at concentrations in the range of 1 pmol/l–1 μmol/l on bone resorption by osteoclasts isolated from neonatal rat long bones. Osteoclasts were disaggregated and incubated either with or without UMR 106 cells or with mixed bone cells. We found that there was no effect of T3 on bone resorption by osteoclasts incubated alone or co-cultured with UMR 106 cells. However, in culture with mixed bone cells there was a significant relationship between the concentration of T3 and bone resorption (r = 0·54, P= 0·01) The greatest effect was observed at a T3 concentration of 1 μmol/l at which a 1·8-fold increase in resorption was seen compared with control (P <0·005; paired t-test). We conclude that the ability of T3 to increase osteoclastic bone resorption is not due to a direct action of T3 on osteoclasts but is mediated by another cell present in bone. The observation that UMR 106 cells are unable to mediate this effect suggests that either the mediating cell is not osteoblastic or the phenotype of UMR 106 does not conform to the phenotype of osteoblastic cells that mediate the T3 responsiveness of bone.
Journal of Endocrinology (1992) 133, 327–331
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Abstract
Changes in thyroid status have a major effect on the GH/insulin-like growth factor (IGF) axis. The majority of IGF in the circulation is bound to specific IGF-binding proteins (IGFBPs) of which six have been cloned and sequenced. We have studied changes in hepatic gene expression of IGFBP-1, -2 and -3, in male Wistar rats rendered hyperthyroid (thyroxine, 200 μg/kg per day) or hypothyroid (propylthiouracil, 0·1% daily). Littermates of the same age were used as controls (n=6 in each group). Thyroxine was measured by radioimmunoassay, and hepatic IGFBP-1, -2 and -3 mRNA levels by Northern blot analysis using specific rat cDNA probes with a 28S ribosomal probe as a loading control. Mean± s.e.m. thyroxine levels were 247·0±44·5 (hyperthyroid group), <9·0 (hypothyroid group) and 76·0 ± 4·5 nmol/l (control group). IGFBP-1 and -2 mRNA levels in the hypothyroid animals compared with the controls were significantly increased, but similar levels of expression were found in thyrotoxic and control rats. IGFBP-3 mRNA levels in hypothyroid animals were decreased, and increased in thyrotoxic animals. Thus, in the adult rat, hypothyroidism is associated with increased hepatic IGFBP-1 and -2 gene expression, but decreased IGFBP-3 gene expression, while in thyrotoxicosis there are normal IGFBP-1 and -2 mRNA levels but increased IGFBP-3 gene expression. These results suggest that there is specific and different transcriptional regulation for IGFBP-1, -2 and -3 in hypoand hyperthyroid rats.
Journal of Endocrinology (1994) 140, 251–255
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In order to facilitate the understanding of gonadotrophin-releasing hormone (GnRH) agonist and antagonist action in the primate animal model, the marmoset GnRH receptor (GnRH-R) was cloned and characterised. It was shown to have 95% and 85% sequence identity with the human and rat GnRH-Rs, respectively, and, when transiently expressed in COS-7 cells, it exhibited high-affinity des-Gly(10), [d-Trp(6)]-GnRH binding, with a K(d) value similar to those of both the rat and human forms, but with a greatly reduced B(max) value. The ED(50) for production of GnRH-induced total inositol phosphate (IP) for the marmoset GnRH-R was also similar to those of the rat and the human, but the maximal response compared with the rat receptor was markedly reduced. In all mammalian forms of the GnRH-R cloned to date, the conserved DRY region of G-protein-coupled receptors is substituted with DRS. The most interesting feature of the marmoset GnRH-R was the substitution of this motif with DRF. In order to investigate the DRS to DRF substitution, a Ser(140)Phe rat GnRH-R mutant was generated. The mutant had a K(d) value similar to that of the wild-type rat receptor, although the B(max) value was slightly lower, indicating that expression of functional mutant receptor at the cell surface was reduced. The ED(50) value for IP production was also similar to that of the wild-type receptor, with a reduction in maximal response. The level of internalisation for the rat wild-type and mutant GnRH-R constructs was also assessed and the Ser(140)Phe mutant was shown to have an increased rate of receptor internalisation, suggesting a role for this residue in regulating internalisation. These results show that the marmoset GnRH-R exhibits a substitution in the DRS motif and that this substitution may play a part in desensitisation and internalisation events.
St.Vincent's Institute of Medical Research, Department of Medicine at St. Vincent's Hospital Melbourne, Department of Cancer Research and Molecular Medicine, 9 Princes St, Fitzroy, Victoria 3065, Australia
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St.Vincent's Institute of Medical Research, Department of Medicine at St. Vincent's Hospital Melbourne, Department of Cancer Research and Molecular Medicine, 9 Princes St, Fitzroy, Victoria 3065, Australia
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St.Vincent's Institute of Medical Research, Department of Medicine at St. Vincent's Hospital Melbourne, Department of Cancer Research and Molecular Medicine, 9 Princes St, Fitzroy, Victoria 3065, Australia
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Parathyroid hormone (PTH) treatment stimulates osteoblast differentiation and bone formation, and is the only currently approved anabolic therapy for osteoporosis. In cells of the osteoblast lineage, PTH also stimulates the expression of members of the interleukin 6 (IL-6) cytokine superfamily. Although the similarity of gene targets regulated by these cytokines and PTH suggest cooperative action, the dependence of PTH anabolic action on IL-6 cytokine signaling is unknown. To determine whether cytokine signaling in the osteocyte through glycoprotein 130 (gp130), the common IL-6 superfamily receptor subunit, is required for PTH anabolic action, male mice with conditional gp130 deletion in osteocytes (Dmp1Cre.gp130 f/f ) and littermate controls (Dmp1Cre.gp130 w/w ) were treated with hPTH(1–34) (30 μg/kg 5× per week for 5 weeks). PTH dramatically increased bone formation in Dmp1Cre.gp130 w/w mice, as indicated by elevated osteoblast number, osteoid surface, mineralizing surface, and increased serum N-terminal propeptide of type 1 collagen (P1NP). However, in mice with Dmp1Cre-directed deletion of gp130, PTH treatment changed none of these parameters. Impaired PTH anabolic action was associated with a 50% reduction in Pth1r mRNA levels in Dmp1Cre.gp130 f/f femora compared with Dmp1Cre.gp130 w/w . Furthermore, lentiviral-Cre infection of gp130 f/f primary osteoblasts also lowered Pth1r mRNA levels to 16% of that observed in infected C57/BL6 cells. In conclusion, osteocytic gp130 is required to maintain PTH1R expression in the osteoblast lineage, and for the stimulation of osteoblast differentiation that occurs in response to PTH.
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We have previously demonstrated that neutrophil recruitment to the heart following myocardial infarction (MI) is enhanced in mice lacking 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) that regenerates active glucocorticoid within cells from intrinsically inert metabolites. The present study aimed to identify the mechanism of regulation. In a mouse model of MI, neutrophil mobilization to blood and recruitment to the heart were higher in 11β-HSD1-deficient (Hsd11b1 − / − ) relative to wild-type (WT) mice, despite similar initial injury and circulating glucocorticoid. In bone marrow chimeric mice, neutrophil mobilization was increased when 11β-HSD1 was absent from host cells, but not when absent from donor bone marrow-derived cells. Consistent with a role for 11β-HSD1 in ‘host’ myocardium, gene expression of a subset of neutrophil chemoattractants, including the chemokines Cxcl2 and Cxcl5, was selectively increased in the myocardium of Hsd11b1 − / − mice relative to WT. SM22α-Cre directed disruption of Hsd11b1 in smooth muscle and cardiomyocytes had no effect on neutrophil recruitment. Expression of Cxcl2 and Cxcl5 was elevated in fibroblast fractions isolated from hearts of Hsd11b1 − / − mice post MI and provision of either corticosterone or of the 11β-HSD1 substrate, 11-dehydrocorticosterone, to cultured murine cardiac fibroblasts suppressed IL-1α-induced expression of Cxcl2 and Cxcl5. These data identify suppression of CXCL2 and CXCL5 chemoattractant expression by 11β-HSD1 as a novel mechanism with potential for regulation of neutrophil recruitment to the injured myocardium, and cardiac fibroblasts as a key site for intracellular glucocorticoid regeneration during acute inflammation following myocardial injury.
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Department of Genetics and Molecular Biology, University of Campinas, São Paulo, Brazil
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Department of Medicine, The University of Melbourne, St. Vincent’s Hospital, Melbourne, Australia
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Department of Medicine, The University of Melbourne, St. Vincent’s Hospital, Melbourne, Australia
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Bone strength is partially determined during cortical bone consolidation, a process comprising coalescence of peripheral trabecular bone and its progressive mineralisation. Mice with genetic deletion of suppressor of cytokine signalling 3 (Socs3), an inhibitor of STAT3 signalling, exhibit delayed cortical bone consolidation, indicated by high cortical porosity, low mineral content, and low bone strength. Since leptin receptor (LepR) is expressed in the osteoblast lineage and is suppressed by SOCS3, we evaluated whether LepR deletion in osteocytes would rectify the Dmp1cre.Socs3fl/fl bone defect. First, we tested LepR deletion in osteocytes by generating Dmp1cre.LepRfl/fl mice and detected no significant bone phenotype. We then generated Dmp1cre.Socs3fl/fl.LepRfl/fl mice and compared them to Dmp1cre.Socs3fl/fl controls. Between 6 and 12 weeks of age, both Dmp1cre.Socs3fl/fl.LepRfl/fl and control (Dmp1cre.Socs3fl/fl ) mice showed an increasing proportion of more heavily mineralised bone, indicating some cortical consolidation with time. However, at 12 weeks of age, rather than resolving the phenotype, delayed consolidation was extended in female Dmp1cre.Socs3fl/fl.LepRfl/fl mice. This was indicated in both metaphysis and diaphysis by greater proportions of low-density bone, lower proportions of high-density bone, and greater cortical porosity than Dmp1cre.Socs3fl/fl controls. There was also no change in the proportion of osteocytes staining positive for phospho-STAT3, suggesting the effect of LepR deletion in Dmp1cre.Socs3fl/fl mice is STAT3-independent. This identifies a new role for leptin signalling in bone which opposes our original hypothesis. Although LepR in osteocytes has no irreplaceable physiological role in normal bone maturation, when STAT3 is hyperactive, LepR in Dmp1Cre-expressing cells supports cortical consolidation.
