Expression of Osteoprotegerin, Receptor Activator of NF-κB Ligand (Osteoprotegerin Ligand) and Related Proinflammatory Cytokines During Fracture Healing

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Abstract
Fracture healing is a unique biological process regulated by a complex array of signaling molecules and proinflammatory cytokines. Recent evidence for the role of tumor necrosis family members in the coupling of cellular functions during skeletal homeostasis suggests that they also may be involved in the regulation of skeletal repair. The expression of a number of cytokines and receptors that are of functional importance to bone remodeling (osteoprotegerin [OPG], macrophage colony‐stimulating factor [M‐CSF], and osteoprotegerin ligand [receptor activator of NF‐κB ligand (RANKL)]), as well as inflammation (tumor necrosis factor α [TNF‐α] and its receptors, and interleukin‐1α [IL‐1α] and ‐β and their receptors) were analyzed over a 28‐day period after the generation of simple transverse fractures in mouse tibias. OPG was expressed constitutively in unfractured bones and elevated levels of expression were detected throughout the repair process. It showed two distinct peaks of expression: the first occurring within 24 h after fracture and the second at the time of peak cartilage formation on day 7. In contrast, the expression of RANKL was nearly undetectable in unfractured bones but strongly induced throughout the period of fracture healing. The peak in expression of RANKL did not correlate with that of OPG, because maximal levels of expression were seen on day 3 and day 14, when OPG levels were decreasing. M‐CSF expression followed the temporal profile of RANKL but was expressed at relatively high basal levels in unfractured bones. TNF‐α, lymphotoxin‐β (LT‐β), IL‐1α, and IL‐1β showed peaks in expression within the first 24 h after fracture, depressed levels during the period of cartilage formation, and increased levels of expression on day 21 and day 28 when bone remodeling was initiated. Both TNF‐α receptors (p55 and p75) and the IL‐1RII receptor showed identical patterns of expression to their ligands, while the IL‐1R1 was expressed only during the initial period of inflammation on day 1 and day 3 postfracture. Both TNF‐α and IL‐1α expression were localized primarily in macrophages and inflammatory cells during the early periods of inflammation and seen in mesenchymal and osteoblastic cells later during healing. TNF‐α expression also was detected at very high levels in hypertrophic chondrocytes. These data imply that the expression profiles for OPG, RANKL, and M‐CSF are tightly coupled during fracture healing and involved in the regulation of both endochondral resorption and bone remodeling. TNF‐α and IL‐1 are expressed at both very early and late phases in the repair process, which suggests that these cytokines are important in the initiation of the repair process and play important functional roles in intramembraneous bone formation and trabecular bone remodeling.