Accelerated fracture healing in mice lacking the 5-lipoxygenase gene

Abstract

Background and purpose Cyclooxygenase-2 (COX-2) promotes inflammation by synthesizing pro-inflammatory prostaglandins from arachidonic acid. Inflammation is an early response to bone fracture, and ablation of COX-2 activity impairs fracture healing. Arachidonic acid is also converted into leukotrienes by 5-lipoxygenase (5-LO). We hypothesized that 5-LO is a negative regulator of fracture healing and that in the absence of COX-2, excess leukotrienes synthesized by 5-LO will impair fracture healing.

Methods Fracture healing was assessed in mice with a targeted 5-LO mutation (5-LO^KO mice) and control mice by radiographic and histological observations, and measured by histomorphometry and torsional mechanical testing. To assess effects on arachidonic acid metabolism, prostaglandin E2, F2α, and leukotriene B4 levels were measured in the fracture calluses of control, 5-LO^KO COX-1^KO, and COX-2^KO mice by enzyme linked immunoassays.

Results Femur fractures in 5-LO^KO mice rapidly developed a cartilaginous callus that was replaced with bone to heal fractures faster than in control mice. Femurs from 5-LO^KO mice had substantially better mechanical properties after 1 month of healing than did control mice. Callus leukotriene levels were 4-fold higher in mice homozygous for a targeted mutation in the COX-2 gene (COX-2^KO), which indicated that arachidonic acid was shunted into the 5-LO pathway in the absence of COX-2.

Interpretation These experiments show that 5-LO negatively regulates fracture healing and that shunting of arachidonic acid into the 5-LO pathway may account, at least in part, for the impaired fracture healing response observed in COX-2^KO mice.

MBM performed the experiments and also data analysis. JPOC developed the hypothesis and experimental design, performed data analysis, and wrote the manuscript.

Research concerning the COX-1^KO and COX-2^KO mice was funded in part by a grant to JPOC from the Arthritis Foundation. MBM has no competing interests. JPOC has filed patents to protect intellectual property disclosed in this manuscript, founded Accelalox Inc. to commercialize this technology, is an owner of Accelalox Inc., and serves as Chief Scientific Officer at Accelalox.