Abstract
Fracture healing is a complicated, long-term, and multistage repair process. Intermittent administration of parathyroid hormone (PTH) has been proven effective on intramembranous and endochondral bone formation during the fracture healing process, however, the mechanism is unclear. In this study, we investigated the role of exogenous PTH and endogenous PTH deficiency in bone fracture healing and explored the mechanism by using PTH knockout (PTH-/-) mice and ATDC5 cells. In a mouse femur fracture model, endogenous PTH deficiency could delay endochondral ossification whereas exogenous PTH promotes accumulation of endochondral bone, accelerates cartilaginous callus conversion to bony callus, enhances maturity of bony callus, and attenuates impaired fracture healing resulting from endogenous PTH deficiency. In fracture callus tissue, endogenous PTH deficiency could inhibit chondrocyte proliferation and differentiation whereas exogenous PTH could activate the IHH signaling pathway to accelerate endochondral ossification and rescue impaired fracture healing resulting from endogenous PTH deficiency. In vitro, exogenous PTH promotes cell proliferation by activating IHH signaling pathway on ATDC5 cells. In mechanistic studies, by using ChIP and luciferase reporter assays, we showed that PTH could phosphorylate CREB, and subsequently bind to the promoter of IHH, causing the activation of IHH gene expression. Therefore, results from this study support the concept that exogenous PTH 1-34 attenuates impaired fracture healing in endogenous PTH deficiency mice via activating the IHH pathway and accelerating endochondral ossification. Hence, the investigation of the mechanism underlying the effects of PTH treatment on fracture repair might guide the exploration of effective therapeutic targets for fracture.
Highlights
Fracture healing is a complicated, long-term, and multistage repair process that is initiated in response to injury, resulting in the generation of new bone and connective tissue with similar anatomy and functionality to the pre-injury site (Meesters et al, 2018)
The results indicated that cAMP-response element-binding protein (CREB) could bind to the promoter of Indian HH (IHH) on -1642∼-1649 bp, and showed that parathyroid hormone (PTH) phosphorylates CREB, which directly resulted in an increase of IHH promoter activity (Figures 5B–D). These data strongly testified that PTH could phosphorylate CREB, and subsequently bind to the promoter of IHH, causing the activation of IHH gene expression. We demonstrated that both endogenous PTH deficiency and exogenous PTH cause a dramatic change of molecular and cellular events in the endochondral ossification process associated with bone fracture healing
Exogenous PTH1-34 resulted in increased new bone formation, improvements in bone mechanical strength, bone mineral content (BMC), external callus volume, and callus strength as well as accelerated remodeling and earlier replacement of woven bone by lamellar bone in animal models (Andreassen et al, 1999; Alkhiary et al, 2005; Mognetti et al, 2011).In addition, similar biological effects were observed in complicated fracture animal models, including osteoporotic fractures, diabetes, stress fractures, open fractures, and nonunion (Jahng and Kim, 2000; Tagil et al, 2010; Lin et al, 2012; Hamann et al, 2014)
Summary
Fracture healing is a complicated, long-term, and multistage repair process that is initiated in response to injury, resulting in the generation of new bone and connective tissue with similar anatomy and functionality to the pre-injury site (Meesters et al, 2018). It contains a variety of molecular and cellular events, controlled by numerous, complex cellular signaling pathways, and characterized as partially overlapping sequential phases, which is including inflammation, proliferation, callus formation, and bone remodeling. According to the characteristics of each stage of fracture healing, various methods and drugs are adopted to accelerate the process of fracture healing (Chen and Luan, 2019; Li et al, 2019; Mu et al, 2019; Nakata et al, 2020)
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