Mechanotransduction in Bone Does Not Require a Functional Cyclooxygenase-2 (COX-2) Gene

Authors

  • Imranul Alam,

    1. Department of Orthopaedic Surgery, Indiana University School of Medicine, Indianapolis, Indiana, USA
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    • These authors contributed equally to this study.

  • Stuart J Warden,

    1. Department of Orthopaedic Surgery, Indiana University School of Medicine, Indianapolis, Indiana, USA
    2. Department of Physical Therapy, School of Health and Rehabilitation Sciences, Indiana University, Indianapolis, Indiana, USA
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    • These authors contributed equally to this study.

  • Alexander G Robling,

    1. Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, Indiana, USA
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  • Charles H Turner PhD

    Corresponding author
    1. Department of Orthopaedic Surgery, Indiana University School of Medicine, Indianapolis, Indiana, USA
    2. Biomechanics and Biomaterials Research Center, Indiana University School of Medicine, Indianapolis, Indiana, USA
    • Department of Orthopaedic Surgery, Indiana University School of Medicine, 1120 South Drive, FH-115, Indianapolis, IN 46202, USA
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  • The authors have no conflict of interest.

Abstract

COX-2 is a key enzyme involved in the response of bone to loading. However, using mice with a null mutation of the COX-2 gene, we found that a functional COX-2 gene is not required for mechanotransduction. This paradoxical finding may have resulted, in part, from mechanically induced COX-1 activity.

Introduction: Cyclooxygenase-2 (COX-2) is an important mediator in the response of bone to mechanical loading, with pharmacological inhibition of COX-2 effectively eliminating or reducing mechanically induced bone formation. In this study, we further investigated the role of COX-2 in skeletal mechanotransduction using a genetic approach. The aim was to compare the skeletal responsiveness of COX-2 homozygous mutant (COX-2−/−) and wildtype control (COX-2+/+) mice to investigate whether a functional COX-2 gene is necessary for mechanotransduction.

Materials and Methods: Adult female COX-2+/+ and COX-2−/− mice on a C57BL/6×129/ola background were studied using the ulna axial loading model. The response to 2 days of loading for 120 cycles/day at 2 Hz was measured histomorphometrically. Phenotypic characterization of the femurs in these mice was also performed. In a separate group of animals, the expression of the remaining COX isozyme, COX-1, was assessed using real-time RT-PCR 4 h after one bout of 120 loading cycles.

Results: Null mutation of the COX-2 gene resulted in a consistent femoral phenotype of reduced bone mass, altered architecture, and inferior mechanical properties. Many of these differences were nullified after adjustment for body weight. Nevertheless, body weight-corrected values showed a consistent trend of reduced mechanical properties in COX-2−/− mice. Genotype did not influence the response to mechanical loading, with no histomorphometric differences being found between COX-2+/+ and COX-2−/− mice. Real-time RT-PCR showed COX-2−/− mice to express significantly greater COX-1 expression in loaded ulnas than in loaded ulnas in COX-2+/+ mice. There were no differences in COX-1 expression in nonloaded ulnas.

Conclusions: A functional COX-2 gene was not found to be required for skeletal mechanotransduction. This is in contrast to previous pharmacological studies showing that COX-2 is critical to the response of bone to loading. Investigating a potential reason for the absence of a genotype difference in this study, we found that mice with a null mutation in the COX-2 gene possess inductive skeletal COX-1 expression.

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