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Abstract
Nuclear receptors are a family of transcription factors that can be activated by lipophilic ligands. They are fundamental regulators of development, reproduction, and energy metabolism. In bone, nuclear receptors enable bone cells, including osteoblasts, osteoclasts, and osteocytes, to sense their dynamic microenvironment and maintain normal bone development and remodeling. Our views of the molecular mechanisms in this process have advanced greatly in the past decade. Drugs targeting nuclear receptors are widely used in the clinic for treating patients with bone disorders such as osteoporosis by modulating bone formation and resorption rates. Deficiency in the natural ligands of certain nuclear receptors can cause bone loss; for example, estrogen loss in postmenopausal women leads to osteoporosis and increases bone fracture risk. In contrast, excessive ligands of other nuclear receptors, such as glucocorticoids, can also be detrimental to bone health. Nonetheless, the ligand-induced osteoprotective effects of many other nuclear receptors, e.g., vitamin D receptor, are still in debate and require further characterizations. This review summarizes previous studies on the roles of nuclear receptors in bone homeostasis and incorporates the most recent findings. The advancement of our understanding in this field will help researchers improve the applications of agonists, antagonists, and selective modulators of nuclear receptors for therapeutic purposes; in particular, determining optimal pharmacological drug doses, preventing side effects, and designing new drugs that are more potent and specific.
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Amso Z, Cornish J, Brimble MA. Short Anabolic Peptides for Bone Growth. Med Res Rev 2016; 36:579-640. [DOI: 10.1002/med.21388] [Citation(s) in RCA: 13] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/15/2015] [Revised: 01/24/2016] [Accepted: 02/15/2016] [Indexed: 12/11/2022]
Affiliation(s)
- Zaid Amso
- School of Chemical Sciences; The University of Auckland, 23 Symonds St; Auckland 1142 New Zealand
| | - Jillian Cornish
- Department of Medicine; The University of Auckland; Auckland 1010 New Zealand
| | - Margaret A. Brimble
- School of Chemical Sciences; The University of Auckland, 23 Symonds St; Auckland 1142 New Zealand
- Maurice Wilkins Centre for Molecular Biodiscovery, School of Biological Sciences; The University of Auckland; Auckland 1142 New Zealand
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Green AC, Martin TJ, Purton LE. The role of vitamin A and retinoic acid receptor signaling in post-natal maintenance of bone. J Steroid Biochem Mol Biol 2016; 155:135-46. [PMID: 26435449 DOI: 10.1016/j.jsbmb.2015.09.036] [Citation(s) in RCA: 47] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 08/19/2015] [Revised: 09/24/2015] [Accepted: 09/26/2015] [Indexed: 12/11/2022]
Abstract
Vitamin A and retinoid derivatives are recognized as morphogens that govern body patterning and skeletogenesis, producing profound defects when in excess. In post-natal bone, both high and low levels of vitamin A are associated with poor bone heath and elevated risk of fractures. Despite this, the precise mechanism of how retinoids induce post-natal bone changes remains elusive. Numerous studies have been performed to discover how retinoids induce these changes, revealing a complex morphogenic regulation of bone through interplay of different cell types. This review will discuss the direct and indirect effects of retinoids on mediators of bone turnover focusing on differentiation and activity of osteoblasts and osteoclasts and explains why some discrepancies in this field have arisen. Importantly, the overall effect of retinoids on the skeleton is highly site-specific, likely due to differential regulation of osteoblasts and osteoclasts at trabecular vs. cortical periosteal and endosteal bone surfaces. Further investigation is required to discover the direct gene targets of retinoic acid receptors (RARs) and molecular mechanisms through which these changes occur. A clear role for RARs in regulating bone is now accepted and the therapeutic potential of retinoids in treating bone diseases has been established.
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Affiliation(s)
- Alanna C Green
- St Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia.
| | - T John Martin
- St Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia
| | - Louise E Purton
- St Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia
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Green AC, Poulton IJ, Vrahnas C, Häusler KD, Walkley CR, Wu JY, Martin TJ, Gillespie MT, Chandraratna RAS, Quinn JMW, Sims NA, Purton LE. RARγ is a negative regulator of osteoclastogenesis. J Steroid Biochem Mol Biol 2015; 150:46-53. [PMID: 25800721 DOI: 10.1016/j.jsbmb.2015.03.005] [Citation(s) in RCA: 23] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 12/01/2014] [Revised: 03/03/2015] [Accepted: 03/17/2015] [Indexed: 01/14/2023]
Abstract
Vitamin A is known to influence post-natal bone content, with excess intake being associated with reduced bone mineral density and increased fracture risk. Despite this, the roles retinoids play in regulating osteoclastogenesis, particularly in vivo, remain unresolved. This study therefore aimed to determine the effect of loss of retinoic acid receptors (RAR)α or RARγ on bone mass (analyzed by histomorphometry and dual-energy X-ray absorptiometry) and osteoclastogenesis in mice in vivo. RARγ null mice had significantly less trabecular bone at 8 weeks of age compared to wildtype littermates. In contrast, no change in trabecular bone mass was detected in RARα null mice at this age. Further histomorphometric analysis revealed a significantly greater osteoclast surface in bones from 8-week-old RARγ null male mice. This in vivo effect was cell lineage autonomous, and was associated with increased osteoclastogenesis in vitro from hematopoietic cells obtained from 8-week-old RARγ null male mice. The use of highly selective agonists in RANKL-induced osteoclast differentiation of wild type mouse whole bone marrow cells and RAW264.7 cells in vitro showed a stronger inhibitory effect of RARγ than RARα agonists, suggesting that RARγ is a more potent inhibitor of osteoclastogenesis. Furthermore, NFAT activation was also more strongly inhibited by RARγ than RARα agonists. While RARα and RARγ antagonists did not significantly affect osteoclast numbers in vitro, larger osteoclasts were observed in cultures stimulated with the antagonists, suggesting increased osteoclast fusion. Further investigation into the effect of retinoids in vivo revealed that oral administration of 5mg/kg/day ATRA for 10 days protected against bone loss induced by granulocyte colony-stimulating factor (G-CSF) by inhibiting the pro-osteoclastogenic action of G-CSF. Collectively, our data indicates a physiological role for RARγ as a negative regulator of osteoclastogenesis in vivo and in vitro, and reveals distinct influences of RARα and RARγ in bone structure regulation.
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Affiliation(s)
- Alanna C Green
- St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia
| | | | - Christina Vrahnas
- St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia
| | - Karl D Häusler
- St. Vincent's Institute, Fitzroy, Victoria 3065, Australia
| | - Carl R Walkley
- St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia
| | - Joy Y Wu
- Division of Endocrinology, Stanford University School of Medicine, CA 94305, USA
| | - T John Martin
- St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia
| | - Matthew T Gillespie
- MIMR-PHI Institute, Monash Medical Centre, Clayton, Victoria 3168, Australia
| | | | - Julian M W Quinn
- MIMR-PHI Institute, Monash Medical Centre, Clayton, Victoria 3168, Australia; The Garvan Institute of Medical Research, Darlinghurst, New South Wales 2010 Australia
| | - Natalie A Sims
- St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia
| | - Louise E Purton
- St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia.
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Lind T, Sundqvist A, Hu L, Pejler G, Andersson G, Jacobson A, Melhus H. Vitamin a is a negative regulator of osteoblast mineralization. PLoS One 2013; 8:e82388. [PMID: 24340023 PMCID: PMC3858291 DOI: 10.1371/journal.pone.0082388] [Citation(s) in RCA: 57] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/11/2013] [Accepted: 10/26/2013] [Indexed: 12/30/2022] Open
Abstract
An excessive intake of vitamin A has been associated with an increased risk of fractures in humans. In animals, a high vitamin A intake leads to a reduction of long bone diameter and spontaneous fractures. Studies in rodents indicate that the bone thinning is due to increased periosteal bone resorption and reduced radial growth. Whether the latter is a consequence of direct effects on bone or indirect effects on appetite and general growth is unknown. In this study we therefore used pair-feeding and dynamic histomorphometry to investigate the direct effect of a high intake of vitamin A on bone formation in rats. Although there were no differences in body weight or femur length compared to controls, there was an approximately halved bone formation and mineral apposition rate at the femur diaphysis of rats fed vitamin A. To try to clarify the mechanism(s) behind this reduction, we treated primary human osteoblasts and a murine preosteoblastic cell line (MC3T3-E1) with the active metabolite of vitamin A; retinoic acid (RA), a retinoic acid receptor (RAR) antagonist (AGN194310), and a Cyp26 inhibitor (R115866) which blocks endogenous RA catabolism. We found that RA, via RARs, suppressed in vitro mineralization. This was independent of a negative effect on osteoblast proliferation. Alkaline phosphatase and bone gamma carboxyglutamate protein (Bglap, Osteocalcin) were drastically reduced in RA treated cells and RA also reduced the protein levels of Runx2 and Osterix, key transcription factors for progression to a mature osteoblast. Normal osteoblast differentiation involved up regulation of Cyp26b1, the major enzyme responsible for RA degradation, suggesting that a drop in RA signaling is required for osteogenesis analogous to what has been found for chondrogenesis. In addition, RA decreased Phex, an osteoblast/osteocyte protein necessary for mineralization. Taken together, our data indicate that vitamin A is a negative regulator of osteoblast mineralization.
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Affiliation(s)
- Thomas Lind
- Department of Medical Sciences, Section of Clinical Pharmacology, Uppsala University, Uppsala, Sweden
| | - Anders Sundqvist
- Department of Medical Sciences, Section of Clinical Pharmacology, Uppsala University, Uppsala, Sweden
| | - Lijuan Hu
- Department of Medical Sciences, Section of Clinical Pharmacology, Uppsala University, Uppsala, Sweden
| | - Gunnar Pejler
- Department of Anatomy, Physiology and Biochemistry, Swedish University of Agricultural Sciences, Uppsala, Sweden
| | - Göran Andersson
- Division of Pathology, Department of Laboratory Medicine, Karolinska Institute, Karolinska University Hospital, Huddinge, Sweden
| | - Annica Jacobson
- Department of Medical Sciences, Section of Clinical Pharmacology, Uppsala University, Uppsala, Sweden
| | - Håkan Melhus
- Department of Medical Sciences, Section of Clinical Pharmacology, Uppsala University, Uppsala, Sweden
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Yeh LCC, Zavala MC, Lee JC. C-type natriuretic peptide enhances osteogenic protein-1-induced osteoblastic cell differentiation via Smad5 phosphorylation. J Cell Biochem 2006; 97:494-500. [PMID: 16187313 DOI: 10.1002/jcb.20657] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022]
Abstract
In the present study, we examined the hypothesis that the C-type natriuretic peptide (CNP) enhances osteogenic protein-1 (OP-1) action in stimulating osteoblastic cell differentiation in primary cultures of fetal rat calvaria cell (FRC). CNP enhanced synergistically the OP-1-induced Alkaline Phosphatase (AP) activity and mineralized bone nodule formation in a dose- and time-dependent manner. To examine possible mechanism of the synergy between OP-1 and CNP, the expression levels of key BMP receptors and signaling molecules were examined. Western blot analysis showed that BMPR-IB and -II receptor protein expression was not affected by CNP alone, but was stimulated by OP-1 alone. The combination of OP-1 and CNP did not further increase their protein levels. The Runx2 protein expression level was not altered by CNP alone, but was elevated by OP-1 alone, and was slightly reduced by the combination. The Smad5 protein expression level was slightly decreased by CNP alone, but was stimulated by OP-1 alone, and was not further stimulated by the combination. Smad5 phosphorylation was not stimulated by CNP alone, but was stimulated significantly by OP-1 alone. The combination of OP-1 and CNP further stimulated the OP-1-induced Smad5 phosphorylation. Thus, one mechanism of the observed synergy between OP-1 and CNP involves enhancement of the Smad5 phosphorylation.
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Affiliation(s)
- Lee-Chuan C Yeh
- Department of Biochemistry, The University of Texas Health Science Center, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900, USA.
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Lind PM, Johansson S, Rönn M, Melhus H. Subclinical hypervitaminosis A in rat: measurements of bone mineral density (BMD) do not reveal adverse skeletal changes. Chem Biol Interact 2005; 159:73-80. [PMID: 16289060 DOI: 10.1016/j.cbi.2005.10.104] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/24/2005] [Revised: 10/07/2005] [Accepted: 10/10/2005] [Indexed: 11/23/2022]
Abstract
We have previously shown that subclinical hypervitaminosis A in rats causes fragile bones. To begin to investigate possible mechanisms for Vitamin A action we extended our previous study. Forty-five mature female Sprague-Dawley rats were divided into three groups, each with 15 animals. They were fed a standard diet containing 12IU Vitamin A per g pellet (control, C), or a standard diet supplemented with 120 IU ("10xC") or 600 IU ("50xC") Vitamin A/g pellet for 12 weeks. At the end of the study, serum retinyl esters were elevated 4- and 20-fold. Although neither average food intake nor final body weights were significantly different between groups, a dose-dependent reduction in serum levels of Vitamin D and E, but not Vitamin K, was found. In the 50xC-group the length of the humerus was the same as in controls, but the diameter was reduced (-4.1%, p<0.05). Peripheral quantitative computed tomography (pQCT) at the diaphysis showed that bone mineral density (BMD) was unchanged and that periosteal circumference had decreased significantly (-3.7%, p<0.05). Ash weight of the humerus was not affected, but since bone volume decreased, volumetric BMD, as measured by the bone ash method, even increased (+2.5%, p<0.05). In conclusion, interference with other fat-soluble Vitamins is a possible indirect mechanism of Vitamin A action. Moreover, BMD measurements do not reveal early adverse skeletal changes induced by moderate excesses of Vitamin A in rats. Since the WHO criterium for osteoporosis is based on BMD, further studies are warranted to examine whether this is also true in humans.
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Affiliation(s)
- P M Lind
- Division of Biochemical Toxicology and Cancerm, Research, Institute of Environmental Medicine, Karolinska Institutet, Box 210, S-17177 Stockholm, Sweden.
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Jacobson A, Johansson S, Branting M, Melhus H. Vitamin A differentially regulates RANKL and OPG expression in human osteoblasts. Biochem Biophys Res Commun 2004; 322:162-7. [PMID: 15313187 DOI: 10.1016/j.bbrc.2004.07.092] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2004] [Indexed: 01/06/2023]
Abstract
All-trans-retinoic acid (ATRA) induces bone resorption, but the molecular mechanisms are unknown. We have studied the effect of ATRA on osteoprotegerin (OPG) and receptor activator of NF-kappaB ligand (RANKL) expression in human MG-63 osteosarcoma cells and primary osteoblast-like cultures. ATRA dose-dependently down-regulated protein levels of OPG in MG-63 cells, with a maximum (-56%) observed at a dose of 10(-6)M. This effect was confirmed with quantitative real-time PCR, where OPG mRNA was decreased after 4h (-68%) in primary cultures and after 8h (-87%) in MG-63 cells. The reduction in OPG expression was inhibited by a retinoic acid receptor (RAR)-antagonist and was mimicked by a RARbeta,gamma-agonist, indicating that the ATRA effect is mediated by these receptors. In primary cultures we found a threefold induction of RANKL mRNA expression. Thus, the RANKL/OPG ratio was markedly increased, suggesting a potential mechanism of ATRA-induced bone resorption.
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Affiliation(s)
- A Jacobson
- Department of Medical Sciences, Uppsala University Hospital, Uppsala, Sweden.
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Raouf A, Li V, Kola I, Watson DK, Seth A. The Ets1 proto-oncogene is upregulated by retinoic acid: characterization of a functional retinoic acid response element in the Ets1 promoter. Oncogene 2000; 19:1969-74. [PMID: 10773887 DOI: 10.1038/sj.onc.1203505] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022]
Abstract
The v-ets oncoprotein and its progenitor Ets1 belong to a family of transcription factors that are related by an 85 amino acid conserved DNA binding domain, the ets domain. Ets1 plays important role(s) in control of cell proliferation, differentiation and apoptosis. Abnormal expression of Ets1 could lead to disruption of these processes and contribute to development of malignancy. Retinoic acid (RA) inhibits proliferation, induces differentiation and regulates apoptosis in many different cell types. Here, we demonstrate that RA treatment increases the expression of Ets1 mRNA, but not that of Ets2, Elk1 or Fli1 in MC3T3-E1 cells. Ets1 induction is detectable after 4 h, can be maintained for at least 14 days, and is inhibited by Actinomycin D, which suggests that RA regulation of Ets1 occurs at the transcriptional level. The promoter region of Ets1 contains four retinoic acid response element (RARE) half sites located at -94, -152, -1765 and -2252 from the translation start site. We show that RARbeta is expressed by MC3T3-E1 cells in the presence of RA and demonstrate that it binds to the -94 RARE half site. Furthermore, RA induces transcription of Ets1 promoter-reporter constructs containing this RARE half site.
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Affiliation(s)
- A Raouf
- Department of Laboratory Medicine and Pathobiology, MRC group in Periodontal Physiology, University of Toronto, and Sunnybrook and Women's College Health Sciences Centre, Toronto, Ontario, Canada
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