Background/Purpose: Interleukin-1β (IL-1β) and thapsigargin (TG)-augmented endoplasmic reticulum (ER) stress modulate the receptor activator of nuclear factor kappa-B ligand (RANKL)-mediated osteoclastogenesis. However, the mechanism by which IL-1β and TG affect osteoclastogenesis remains elusive. Thus, we investigated the relationships between RANKL-mediated osteoclast specific pathways and ER stress relevant signals in osteoclast differentiation of bone marrow-derived cells.

Methods: Bone marrow cells (BMCs) were obtained from 5-week-old male ICR mice. The cells were cultured to be differentiated into osteoclasts with macrophage-colony stimulating factor (M-CSF) and RANKL in the presence or absence of IL-1β, thapsigargin (TG, ER stress inducer), or 4-phenylbutyric acid (PBA, ER stress inhibitor). The formation of osteoclasts was evaluated by tartrate-resistant acid phosphatase (TRAP) staining and resorption pit assay with dentine slice. The molecular mechanisms of IL-1β and ER stress in osteoclastogenesis of BMCs were investigated using reverse transcription-polymerase chain reaction (RT-PCR) and immunoblotting. Osteoclast specific and ER stress relevant signaling molecules were analyzed. Transfections of small interfering RNA (siRNA) for glucose-regulated protein 78 (GRP78), protein kinase RNA-like ER kinase (PERK) and inositol-requiring enzyme 1 (IRE1) were performed to knockdown ER stress initiating signals to verify the relationships between osteoclast specific pathways and ER stress signals.

Results: The formation of osteoclasts was increased by IL-1β and TG augmented ER stress. PBA significantly inhibited IL-1β and TG induced osteoclats formation. The expressions of osteoclast specific pathways such as c-Fos and NFATc1, and ER stress associated signals such as PERK, IRE1, GRP78, and eIF2α were significantly increased by IL-1β and TG which was inhibited by PBA. Inhibition of ER stress initiating signals by siRNA inhibited the expression of above mentioned osteoclast specific signals, thus reduced IL-1β and/or TG-induced osteoclastogenesis.

Conclusion: IL-1β and/or TG-augmented ER stress significantly increased osteoclast formation which was inhibited by PBA. The mechanisms were mainly associated with up-regulation of ER stress signals such as GRP78, PERK, p-eIF2α and IRE1. Thus the modulation of ER stress signals affecting osteoclast formation might be a new therapeutic strategy to prevent inflammatory and destructive arthritis diseases such as rheumatoid arthritis (RA) and diverse osteoporotic diseases.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/receptor-activator-of-nuclear-factor-eb-ligand-mediated-osteoclastogenesis-is-augmented-by-interleukin-1a-via-up-regulation-of-endoplasmic-reticulum-stress-signals/