title: Apoptotic osteocytes regulate osteoclast precursor recruitment and differentiation in vitro.
abstract: Fatigue loading causes a spatial distribution of osteocyte apoptosis co-localized with bone resorption spaces peaking around microdamage sites. Since osteocytes have been shown to regulate osteoclast formation and activity, we hypothesize that osteocyte apoptosis regulates osteoclastogenesis. In this study, we used serum-starvation to mimic reduced nutrient transport in microdamaged bone and induce apoptosis in MLO-Y4 osteocyte-like cells; conditioned medium was used to apply soluble factors released by apoptotic osteocytes (aOCY) to healthy non-apoptotic MLO-Y4 cells. Osteoclast precursor (RAW264.7 monocyte) migration and differentiation were assessed in the presence of conditioned media (CM) from: (A) aOCY, (B) osteocytes treated with apoptosis conditioned medium (i.e., healthy osteocytes in the presence of apoptosis cues; apoptosis CM-treated osteocytes (atOCY)), and (C) osteocytes treated with non-apoptosis conditioned medium (i.e., healthy osteocytes in the absence of apoptosis cues; non-apoptosis CM-treated osteocytes (natOCY)). Receptor activator for nuclear factor-κB ligand (RANKL), macrophage colony stimulating factor (M-CSF), vascular endothelial growth factor (VEGF) and osteoprotegerin (OPG) mRNA, and protein expression were measured. Our findings indicate that soluble factors released by aOCY and atOCY promoted osteoclast precursor migration (up to 64% and 24% increase, respectively) and osteoclast formation (up to 450% and 265% increase, respectively). Osteoclast size increased up to 233% in the presence of aOCY and atOCY CM. Recruitment, formation and size were unaltered by natOCY. RANKL mRNA and protein expression were upregulated only in aOCY, while M-CSF and VEGF increased in atOCY. Addition of RANKL-blocking antibody abolished aOCY-induced osteoclast precursor migration and osteoclast formation. VEGF and M-CSF blocking antibodies abolished atOCY-induced osteoclastogenesis. These findings suggest that aOCY directly and indirectly (through atOCY) initiate targeted bone resorption by regulating osteoclast precursor recruitment and differentiation.
authors.affiliation: Institute of Biomaterials and Biomedical Engineering, University of Toronto, Toronto, ON, Canada.
authors.forename: Saja A
authors.initials: SA
authors.lastname: Al-Dujaili
authors.forename: Esther
authors.initials: E
authors.lastname: Lau
authors.forename: Hashem
authors.initials: H
authors.lastname: Al-Dujaili
authors.forename: Kyle
authors.initials: K
authors.lastname: Tsang
authors.forename: Axel
authors.initials: A
authors.lastname: Guenther
authors.forename: Lidan
authors.initials: L
authors.lastname: You
mesh_terms: D000818:Animals; D017209:Apoptosis; D002454:Cell Differentiation; D002460:Cell Line; D002465:Cell Movement; D048429:Cell Size; D017077:Culture Media, Conditioned / Q000378:metabolism; D016895:Culture Media, Serum-Free; D015870:Gene Expression; D016173:Macrophage Colony-Stimulating Factor / Q000235:genetics / Q000378:metabolism; D051379:Mice; D009000:Monocytes / Q000166:cytology / Q000378:metabolism; D010010:Osteoclasts / Q000166:cytology / Q000502:physiology*; D010011:Osteocytes / Q000502:physiology*; D053244:Osteoprotegerin / Q000235:genetics / Q000378:metabolism; D053245:RANK Ligand / Q000235:genetics / Q000378:metabolism*; D015854:Up-Regulation; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics / Q000378:metabolism
chemical_list: D017077:Culture Media, Conditioned; D016895:Culture Media, Serum-Free; D053244:Osteoprotegerin; D053245:RANK Ligand; C505689:Tnfsf11 protein, mouse; D042461:Vascular Endothelial Growth Factor A; D016173:Macrophage Colony-Stimulating Factor