The Multifarious Functions of Leukotrienes in Bone Metabolism

Leukotrienes (LTs) are derived from arachidonic acid metabolism by the 5-lipoxygenase (5-LO) enzyme. The production of LTs is stimulated in the pathogenesis of rheumatoid arthritis, osteoarthritis, and periodontitis, with a relevant contribution to bone resorption. However, its role in bone turnover particularly the suppression of bone formation by modulating the function of osteoclasts and osteoblasts remains unclear. We investigated the effects of LTs on bone metabolism and their impact on osteogenic differentiation and osteoclastogenesis using a 5-LO knockout mouse model. Results from μCT analysis of femur from eight-week-old 5-LO-deficient mice showed increased cortical bone and medullary region in females and males and decreased trabecular bone in females. In the vertebra, we observed increased marrow area in both females and males 5-LO KO and decreased trabecular bone only in females 5-LO KO. IHC analysis showed higher levels of osteogenic markers TNAP and OPN and lower expression of osteoclastogenic marker TRAP in the femurs of 5-LO KO mice vs. WT. Alkaline phosphatase activity and mineralization assay results showed that the 5-LO absence enhances osteoblasts differentiation and mineralization but decreases the proliferation. ALP, Bglap, and Sp7 gene expression were higher in 5-LO KO osteoblasts compared to WT cells. Eicosanoids production was higher in 5-LO KO osteoblasts except for thromboxane 2 which was lower in 5-LO deficient mice. Proteomic analysis identified the downregulation of proteins related to ATP metabolism in 5-LO KO osteoblasts, and the upregulation of transcription factors such as AP-1 complex in long bones from 5-LO KO mice leading to an increased bone formation pattern in 5-LO deficient mice. We observed enormous differences in the morphology and function of osteoclasts with reduced bone resorption markers and impaired osteoclasts in 5-LO KO compared to WT osteoclasts. Altogether these results demonstrate that the absence of 5-LO is related to the greater osteogenic profile.