Osteocyte expression of caspase-3, COX-2, IL-6 and sclerostin are spatially and temporally associated following stress fracture initiation
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Stress fractures are debilitating injuries and exact mechanisms that initiate their repair incompletely understood. We hypothesized that osteocyte apoptosis and expression of cytokines and proteins such as sclerostin, VEGF, TGFb, COX-2 and IL-6 were early signalling events to facilitate the formation of periosteal woven bone and recruitment of osteoclast precursors to the site of remodelling. A stress fracture was created in the right ulna of mature female wistar rats using cyclic end-loading. Rats were euthanized 1, 4 and 7 days after loading (n=5/group). Standard histological staining was used to examine stress fracture morphology and immunohistochemistry to detect the localization of these proteins and in-situ hybridization to detect mRNA along the stress fracture line or gene expression to quantify the target genes. Unloaded ulnae served as controls. The labelling index of caspase-3, COX-2 and IL-6 was significantly elevated in the region of stress fractures at all time points compared with controls (P<0.001). Additionally, the labelling index of sclerostin protein was significantly reduced in osteocytes adjacent to the stress fracture region when compared to controls at all three time points (P<0.001). Both VEGF and TGFb expression were only be localized in the woven bone. These data reinforce the involvement of osteocyte apoptosis in the healing of fatigue damage in bone, and demonstrate that local regulation of sclerostin, COX-2 and IL-6 are important signalling events associated with new bone formation and stress fracture remodelling.
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