Deficiency of PDK1 in osteoclasts delays fracture healing and repair

Bone fractures are common traumatic injuries of the musculoskeletal system. However, delayed union and non-union fractures are a major clinical problem that present significant socioeconomic burden to patients and the public health sector. The bone-resorbing osteoclasts and bone-forming osteoblasts serve important roles in the fracture repair/healing process. Osteoclast deficiency or decreased osteoblast activity negatively impacts fracture healing. We previously demonstrated that the specific deletion of the serine/threonine kinase 3-phosphoinositide-dependent protein kinase 1 (PDK1) in osteoclasts leads to abrogated osteoclast formation and bone resorption in response to receptor activator of nuclear factor-κB in vitro and protected mice against ovariectomized-induced bone loss and lipopolysaccharide-induced osteolysis in vivo. Given the importance of osteoclasts in fracture repair, we hypothesized that the specific loss of PDK1 in osteoclasts will alter the fracture healing process. Mice of tibial fracture were constructed, and tibial specimens were sampled at 7-, 14-, 21- and 28-days post-fracture to observe the effect of PDK1 gene regulated osteoclasts on fracture healing process by X-ray radiography, microcomputed tomography scanning, histomorphological staining and biomechanical testing. The present study revealed, using the tibial fracture model, that the specific deletion of the PDK1 gene in osteoclasts impeded the fracture healing process by delaying the resorption of the cartilaginous callus and subsequent remodeling of immature woven bone to structurally and mechanically ensure lamellar bone is stronger. No effect on osteoblast bone formation and osteogenesis was observed, thus indicating that delayed fracture healing is primarily due to defective osteoclast activity. These results provide important clinical implications for the use of anti-resorptive agents, such as bisphosphonates, for the treatment of osteolytic conditions. Such anti-resorptive therapies may detrimentally delay fracture healing and repair.