Retinoid X receptors orchestrate osteoclast differentiation and postnatal bone remodeling
María P. Menéndez-Gutiérrez, … , Annabel F. Valledor, Mercedes Ricote
María P. Menéndez-Gutiérrez, … , Annabel F. Valledor, Mercedes Ricote
Published January 9, 2015
Citation Information: J Clin Invest. 2015;125(2):809-823. https://doi.org/10.1172/JCI77186.
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Research Article Bone biology

Retinoid X receptors orchestrate osteoclast differentiation and postnatal bone remodeling

Abstract

Osteoclasts are bone-resorbing cells that are important for maintenance of bone remodeling and mineral homeostasis. Regulation of osteoclast differentiation and activity is important for the pathogenesis and treatment of diseases associated with bone loss. Here, we demonstrate that retinoid X receptors (RXRs) are key elements of the transcriptional program of differentiating osteoclasts. Loss of RXR function in hematopoietic cells resulted in formation of giant, nonresorbing osteoclasts and increased bone mass in male mice and protected female mice from bone loss following ovariectomy, which induces osteoporosis in WT females. The increase in bone mass associated with RXR deficiency was due to lack of expression of the RXR-dependent transcription factor v-maf musculoaponeurotic fibrosarcoma oncogene family, protein B (MAFB) in osteoclast progenitors. Evaluation of osteoclast progenitor cells revealed that RXR homodimers directly target and bind to the Mafb promoter, and this interaction is required for proper osteoclast proliferation, differentiation, and activity. Pharmacological activation of RXRs inhibited osteoclast differentiation due to the formation of RXR/liver X receptor (LXR) heterodimers, which induced expression of sterol regulatory element binding protein-1c (SREBP-1c), resulting in indirect MAFB upregulation. Our study reveals that RXR signaling mediates bone homeostasis and suggests that RXRs have potential as targets for the treatment of bone pathologies such as osteoporosis.

Authors

María P. Menéndez-Gutiérrez, Tamás Rőszer, Lucía Fuentes, Vanessa Núñez, Amelia Escolano, Juan Miguel Redondo, Nora De Clerck, Daniel Metzger, Annabel F. Valledor, Mercedes Ricote

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Figure 5

Low MAFB expression in osteoclast progenitors underlies the abnormal RXR-KO osteoclast phenotype.

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Low MAFB expression in osteoclast progenitors underlies the abnormal RXR...
(A and B) Relative mRNA expression of Mafb in the course of osteoclast differentiation in vitro (A) and in osteoclast progenitors (B). **P < 0.01; ***P < 0.001, compared with RXR-KO cells on the same day of differentiation. (C) MAFB protein expression in osteoclast progenitors (representative of 3 mice per genotype). (D) Mafb mRNA expression in bone marrow myeloid populations; cells were isolated as shown in Supplemental Figure 1D. (E–G) Proliferation assays in lentiviral-mediated MAFB-overexpressing RXR-KO osteoclast progenitors: number of CFUs (E) and number of cells per CFU (F) in bone marrow cell cultures infected with control lentivirus (WT-GFP and RXR-KO-GFP) and with MAFB lentivirus (RXR-KO-MAFB). n = 6 per group. (G) Flow cytometry analysis of the proliferative responses of osteoclast progenitors infected with control lentivirus (WT-GFP and RXR-KO-GFP) and with MAFB lentivirus (RXR-KO-MAFB). n = 3 per group. *P < 0.05, compared with WT-GFP; #P < 0.01, compared with RXR-KO-GFP. (H–K) siRNA assay: representative TRAP-positive cells (H), number of nuclei (I), Ctsk expression (J), and resorption pit area (K) in osteoclast cultures after 5 days of differentiation from bone marrow cells transfected with control or Mafb siRNAs. *P < 0.05; **P < 0.01, compared with siControl. Resorption activity was measured after culturing day-5 osteoclasts on bone bovine cortical bone slices for 2 additional days; the experiment shown is representative of 3 independent experiments done in triplicate. Scale bars: 100 μm. Data are presented as mean ± SEM. Statistical comparisons were made by unpaired 2-tailed Student’s t test.