The Nuclear Receptor AhR Controls Bone Homeostasis by Regulating Osteoclast Differentiation via the RANK/c-Fos Signaling Axis

Abstract

The aryl hydrocarbon receptor (AhR) pathway plays a key role in receptor activator of NF-κB ligand (RANKL)–mediated osteoclastogenesis. However, the mechanism underlying the regulation of AhR expression in osteoclasts and the signaling pathway through which AhR controls osteoclastogenesis remain unclear. We found that the expression of AhR in bone marrow–derived osteoclasts was upregulated by RANKL at an earlier stage than was the expression of signature osteoclast genes such as those encoding cathepsin K and NFAT, cytoplasmic, calcineurin-dependent 1. In response to RANKL, bone marrow macrophages isolated from AhR−/− mice exhibited impaired phosphorylation of Akt and MAPK as well as NF-κB, whereas their response to M-CSF remained unchanged. Osteoclast differentiation mediated by the AhR signaling pathway was also regulated in an RANKL/c-Fos–dependent manner. Furthermore, ligand activation of AhR by the smoke toxin benzo[a]pyrene accelerated osteoclast differentiation in a receptor-dependent manner, and AhR-dependent regulation of mitochondrial biogenesis in osteoclasts was observed. Moreover, AhR−/− mice exhibited impaired bone healing with delayed endochondral ossification. Taken together, the present results suggest that the RANKL/AhR/c-Fos signaling axis plays a critical role in osteoclastogenesis, thereby identifying the potential of AhR in treating pathological, inflammatory, or metabolic disorders of the bone.

Crosstalk between Cytokine RANKL and AhR Signalling in Osteoclasts Controls Bone Homeostasis

“…Recently, we showed that AhR expression was upregulated in bone marrow macrophages (BMMs), similar to c-Fos, 24 h after RANKL treatment. Together with the aforementioned finding that AhR −/− osteoclastic cells have disrupted RANKL-stimulated osteoclastogenic signaling, but normal responsivity to M-CSF [23], these findings suggest that impaired osteoclastogenesis of AhR −/− BMMs may be consequent to RANKL signaling pathway disruption ( Figure 1A). …”

“…Examining whether AhR plays a role in RANKL/c-Fos/NFATc1 osteoclastogenesis, we found that, compared to control BMMs, BMMs induced to express c-Fos by retroviral gene transfer exhibited a dramatic increase in the number of multinucleated osteoclasts [23]. In the same study, we also showed that RANKL stimulation of BMMs failed to induce osteoclast differentiation in the absence of the AhR-c-Fos complex.…”

“…BaP exposure increases c-Fos levels in RANKL-stimulated WT BMMs, but fails to induce c-Fos expression in AhR −/− BMMs [23]. These findings provide hints regarding the relationship between smoking and bone metabolism, provide insight into c-Fos-AhR-axis-mediated osteoclastogenesis, and support the pursuit of AhR-targeted therapies for osteoporosis ( Figure 1B).…”

“…Osteoclast precursors express the macrophage colony-stimulating factor (M-CSF) receptor c-Fms, and stimulation by M-CSF activates MAPK pathways such as Akt and extracellular signal-regulated kinase (ERK) signaling [22]. In a recent study examining whether AhR activation may involve RANKL or M-CSF in AhR −/− cells, we found that the absence of AhR disrupted activation of downstream cell signaling pathways in response to RANKL, but not M-CSF, indicating that AhR promotion of osteoclast differentiation may be mediated by way of sensitization of osteoclast precursors to RANKL [23].…”

“…Mitochondrial biogenesis during osteoclast development requires peroxisome proliferator-activated receptorgamma coactivator 1β (PGC-1β), a key mitochondrial oxidative energy metabolism factor [42][43][44][45][46][47]. Recent experimental results have shown that RANKL induction of PGC-1β mRNA and protein expression is suppressed in AhR −/− cells [23]. Additionally, the AhR −/− mouse phenotype includes defective calcium signaling and mitochondrial function leading to mast cell deficiency as well as enhanced intracellular reactive oxygen species and apoptosis [34].…”