Accelerated fracture healing in mice lacking the 5-lipoxygenase gene.
OBJECTIVE
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.


CONCLUSIONS
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.
Department of Biochemistry and Molecular Biology, UMDNJ-New Jersey Medical School and Graduate School of Biological Sciences, Newark, USA.
Michaele B
MB
Manigrasso
J Patrick
JP
O'Connor
D000818:Animals; D001094:Arachidonate 5-Lipoxygenase / Q000235:genetics* / Q000378:metabolism; D002146:Bony Callus / Q000201:enzymology / Q000276:immunology / Q000378:metabolism; D051546:Cyclooxygenase 2 / Q000378:metabolism*; D005264:Femoral Fractures / Q000000981:diagnostic imaging / Q000201:enzymology / Q000503:physiopathology; D017102:Fracture Healing / Q000235:genetics* / Q000502:physiology; D006801:Humans; D015289:Leukotrienes / Q000096:biosynthesis; D051379:Mice; D008810:Mice, Inbred C57BL; D009154:Mutation; D011859:Radiography; D013718:Tensile Strength
D015289:Leukotrienes; D001094:Arachidonate 5-Lipoxygenase; D051546:Cyclooxygenase 2