Background/Purpose: Osteoclasts are specialized myeloid lineage cells that mediate both pathologic and physiologic bone remodeling   Fully differentiated and functional osteoclasts are found exclusively associated with the bone surface, indicating that interaction with the bone substrate plays a pivotal role in the regulation of osteoclast biology. Most in vitro studies on the formation and activation of osteoclasts have been performed using cells adhered to tissue culture plastic, and there is little information regarding the specific molecular mechanisms by which adherence to the bone surface regulates the terminal stages of osteoclast differentiation. To address this discrepancy, we have compared the transcriptional profiles of osteoclasts generated on a variety of substrates, including authentic bone and have identified the unique molecular signatures that are dependent and independent of integrin beta 3, a prototypical osteoclast regulator of osteoclast function.

Methods: Bone marrow derived macrophages from wild-type and integrin beta 3 deficient mice were cultured in the presence or absence of RANKL on plastic, hydroxyapatite, or calvarial bone discs. RNA was isolated at different stages of osteoclast generation and microarray analysis and pathway mapping were utilized to identify osteoclast signaling pathways regulated by the cytokine RANKL, bone substrate, and integrin beta 3. Pathway analysis was performed using GSEA and Ingenuity pathway analysis.

Results: Microarray analysis revealed RANKL-induced molecular profiles that were uniquely and specifically regulated in osteoclasts differentiated on the authentic bone substrate compared to the other substrates. Pathway analysis revealed coordinated downregulation by bone of a cluster of genes associated with cell cycle progression. The expression level of integrin beta 3, which is induced by RANKL during osteoclast differentiation, is further modulated by culture on the bone substrate. Interestingly, bone regulated genes could be divided into those that were dependent and independent of integrin beta 3.

Conclusion: In addition to cytokines and growth factors, interaction of osteoclasts and their precursor with the bone substrate regulates pathways that are involved in osteoclast formation and activation. Although integrin beta 3 has been regarded as the essential mediator of osteoclast attachment and activation, molecular pathways independent of this integrin also modulate the genetic program during osteoclastogenesis.  Analysis of these genes and their pathways provides novel targets and approaches for therapeutic targeting of osteoclast-mediated bone loss in inflammatory and related bone disorders.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/novel-targets-for-blocking-osteoclast-mediated-resorption-in-inflammatory-disorders/