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"Cyclooxygenase-2 in Bone Fracture Healing"

Hsuan-Ni Lin
Interdisciplinary Program
B.S., Chung Shan Medical University, 2005, Taichung, Taiwan
M.S., 2007, New York University, New York, NY

Thesis Advisor: J. Patrick OíConnor, Ph.D.
Associate Professor
Department of Microbiology, Biochemistry and Molecular Genetics

Thursday, June 9, 2016
1:00 P.M., Rutgers NJMS Cancer Center, room F1196


This study invested the role of cyclooxygenase-2 (COX-2) in bone fracture healing. COX-2 catalyzes the conversion of arachidonic acid (ArA) into prostaglandins (PGs), which are important in regulation of normal bone biology and fracture healing. Genetic ablation or pharmaceutical inhibition of COX-2 results in impaired fracture repair; however the mechanism is not understood. To better understand the function of COX-2 during fracture healing, we performed immunohistochemistry to localize the expression of COX-2 in time-staged fracture callus specimens. COX-2 expression was detected in osteoclasts, leukocytes and proliferative chondrocytes during the inflammatory response and endochondral ossification of fracture healing. To investigate how COX-2 regulates fracture healing, we quantified the temporal expression pattern of COX derived lipid mediators during fracture healing. Our analysis found that levels of COX-derived lipid mediators are elevated during the early inflammatory response and remained at high levels for at least 7 days after fracture. The results suggest that COX-2 may regulate the inflammatory response and endochondral ossification during fracture healing, possibly through altering the levels of PGs.

The finding of COX-2 expression in osteoclasts during fracture healing is novel which reveals a regulatory role of osteoclasts in bone regeneration. To study the role of osteoclast COX-2, we first determined the necessities of osteoclasts during fracture healing. We deleted osteoclasts using clodronate liposomes (CLD-lips) and determine the effects on fracture healing. CLD-lip treatment impaired fracture healing, led to significantly reduced callus osteoclast populations, delayed resolution of callus cartilage, and decreased callus material properties. We then investigated the effects of tissue specific COX-2 deletion in osteoclasts using Ptgs2tm1Hrh;Lyz2tm1(cre)Ifo (cKOLyz2) mice on fracture healing. Similar results of impaired fracture repair were observed in cKOLyz2 mice, evident by delayed resorption of cartilaginous matrix and less new bone formation in fracture callus. In addition, the number of osteoclasts decreased in the fracture callus. These observations indicate osteoclast COX-2 may mediate osteoclast formation and function and regulate endochondral ossification.

Finally, we studied the regulation of COX-2 expression in osteoclasts. We found osteopontin (OPN), a cell attachment protein that can signal through cell adhesion molecules such as Šv‚3 integrin, induced dose and time-dependent COX-2 expression in cultured osteoclasts. We further demonstrated that OPN and integrin ‚3 are expressed in chondrocytes and osteoclasts, respectively, in fracture callus and tibia growth plate. Genetic deletion of OPN or integrin ‚3 gene in mice resulted in reduction of COX-2 expression in osteoclasts in vivo. These data demonstrates that OPN regulates osteoclast COX-2 through an integrin mediated signaling pathway. Together, our results indicate that COX-2 is an important mediator for osteoclast biology and that osteoclasts regulate fracture healing.

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