title: RARγ is a negative regulator of osteoclastogenesis.
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.
authors: St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia.
authors: Alanna C
authors: AC
authors: Green
authors: St. Vincent's Institute, Fitzroy, Victoria 3065, Australia.
authors: Ingrid J
authors: IJ
authors: Poulton
authors: St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia.
authors: Christina
authors: C
authors: Vrahnas
authors: St. Vincent's Institute, Fitzroy, Victoria 3065, Australia.
authors: Karl D
authors: KD
authors: Häusler
authors: St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia.
authors: Carl R
authors: CR
authors: Walkley
authors: Division of Endocrinology, Stanford University School of Medicine, CA 94305, USA.
authors: Joy Y
authors: JY
authors: Wu
authors: St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia.
authors: T John
authors: TJ
authors: Martin
authors: MIMR-PHI Institute, Monash Medical Centre, Clayton, Victoria 3168, Australia.
authors: Matthew T
authors: MT
authors: Gillespie
authors: IO Therapeutics Inc., Santa Ana, CA 92705, USA.
authors: Roshantha A S
authors: RA
authors: Chandraratna
authors: MIMR-PHI Institute, Monash Medical Centre, Clayton, Victoria 3168, Australia; The Garvan Institute of Medical Research, Darlinghurst, New South Wales 2010 Australia.
authors: Julian M W
authors: JM
authors: Quinn
authors: St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia.
authors: Natalie A
authors: NA
authors: Sims
authors: St. Vincent's Institute, Fitzroy, Victoria 3065, Australia; Department of Medicine at St. Vincent's Hospital, The University of Melbourne, Victoria 3065, Australia. Electronic address: lpurton@svi.edu.au.
authors: Louise E
authors: LE
authors: Purton
mesh_terms: D000818:Animals; D015519:Bone Density / Q000187:drug effects; D001862:Bone Resorption / Q000235:genetics* / Q000378:metabolism / Q000473:pathology / Q000517:prevention & control; D001842:Bone and Bones / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D002454:Cell Differentiation; D015972:Gene Expression Regulation, Neoplastic; D016179:Granulocyte Colony-Stimulating Factor / Q000494:pharmacology; D006412:Hematopoietic Stem Cells / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018345:Mice, Knockout; D050778:NFATC Transcription Factors / Q000235:genetics / Q000378:metabolism; D010010:Osteoclasts / Q000187:drug effects / Q000378:metabolism* / Q000473:pathology; D061251:Primary Cell Culture; D053245:RANK Ligand / Q000235:genetics / Q000378:metabolism; D018168:Receptors, Retinoic Acid / Q000819:agonists / Q000037:antagonists & inhibitors / Q000235:genetics* / Q000378:metabolism; D000072482:Retinoic Acid Receptor alpha; D015398:Signal Transduction; D014212:Tretinoin / Q000031:analogs & derivatives / Q000494:pharmacology*; D000097768:Retinoic Acid Receptor gamma
chemical_list: D016179:Granulocyte Colony-Stimulating Factor; D050778:NFATC Transcription Factors; D053245:RANK Ligand; D018168:Receptors, Retinoic Acid; D000072482:Retinoic Acid Receptor alpha; D014212:Tretinoin; D000097768:Retinoic Acid Receptor gamma; C000607072:Rara protein, mouse; C505689:Tnfsf11 protein, mouse
keywords: Bone; Osteoclast; Osteoclastogenesis; Retinoic acid receptor; Vitamin A