A distinctive role of the leukotriene B ₄ receptor BLT1 in osteoclastic activity during bone loss

Abstract

Although leukotriene B4 (LTB4) is produced in various inflammatory diseases, its functions in bone metabolism remain unknown. Using mice deficient in the high-affinity LTB4 receptor BLT1, we evaluated the roles of BLT1 in the development of two bone resorption models, namely bone loss induced by ovariectomy and lipopolysaccharide. Through observations of bone mineral contents and bone morphometric parameters, we found that bone resorption in both models was significantly attenuated in BLT1-deficient mice. Furthermore, osteoclasts from BLT1-deficient mice showed reduced calcium resorption activities compared with wild-type osteoclasts. Osteoclasts expressed BLT1, but not the low-affinity LTB4 receptor BLT2, and produced LTB4. LTB4 changed the cell morphology of osteoclasts through the BLT1-Gi protein-Rac1 signaling pathway. Given the causal relationship between osteoclast morphology and osteoclastic activity, these findings suggest that autocrine/paracrine LTB4 increases the osteoclastic activity through the BLT1-Gi protein-Rac1 signaling pathway. Inhibition of BLT1 functions may represent a strategy for preventing bone resorption diseases.bone remodeling ͉ G protein-coupled receptor ͉ knockout mice ͉ lipid mediator ͉ osteoporosis ͉ L eukotriene B 4 (LTB 4 ), a metabolite of arachidonic acid, is a potent lipid mediator with various biological activities toward neutrophils and differentiated T cells, including chemotaxis, degranulation, and production of superoxide anions (1, 2). These actions of LTB 4 are mediated by specific cell surface receptors (BLTs). We previously cloned two distinct BLTs, BLT1 and BLT2 (3,4). BLT1 is a high-affinity receptor that mediates the inhibition of adenylate cyclase and calcium entry by coupling with the Gi-and Gq-classes of G proteins (5). BLT2 transduces comparable intracellular signals but has a lower affinity to LTB 4 (5). Although several hydroxyeicosatetraenoic acids were found to activate BLT2 (6), we recently identified 12(S)-hydroxyheptadeca-5Z, 8E, 10E-trienoic acid (12-HHT) as a very potent endogenous ligand for BLT2 (7). LTB 4 is produced in inf lammatory diseases such as psoriasis (8), bronchial asthma (9), ulcerative colitis (10), postischemic tissue injuries (11), and rheumatoid arthritis (12-15).Bone remodeling consists of old bone resorption by osteoclasts and new bone deposition by osteoblasts. Osteoclasts and osteoblasts participate in bone remodeling under the control of many hormones, cytokines (16,17), and autacoids, including lipid mediators (18). The effects of LTB 4 on bone resorption were investigated using organ cultures of mouse calvariae (19,20). LTB 4 enhanced calcium efflux from the mouse calvariae, suggesting that LTB 4 stimulates bone resorption (19). LTB 4 increased the formation of resorption pits by osteoclasts in rat bone tissues (20), suggesting that LTB 4 modulates bone resorption by increasing the number and/or activity of osteoclasts. However, few reports have provided definitive biochemical information about the mRNA/protein expression and ...

International Union of Basic and Clinical Pharmacology. LXXXIV: Leukotriene Receptor Nomenclature, Distribution, and Pathophysiological Functions

“…Finally, BLT 1 R knockout mice are protected in models of both collagenand K/BxN serum-induced arthritis (Kim et al, 2006a;Shao et al, 2006;Mathis et al, 2010), discussed in detail below. In this context, BLT 1 R knockout mice displayed a decrease in bone resorption induced by either LPS or ovariectomy (Hikiji et al, 2009).…”

Interplay between CXCR2 and BLT1 Facilitates Neutrophil Infiltration and Resultant Keratinocyte Activation in a Murine Model of Imiquimod-Induced Psoriasis

“…The resultant cDNA was amplified by PCR. The protocol and sequences of primers used to measure Ltb4r1, arachidonate 5-lipoxygenase (Alox5), and Actb (also known as b-actin) levels were described previously (15).…”

5-Lipoxygenase Mediates RANKL-Induced Osteoclast Formation via the Cysteinyl Leukotriene Receptor 1

“…These observations suggest that increased bone formation or decreased bone resorption may occur in the absence of the 5-LO enzyme and suggest that 5-LO and its metabolites may act as regulators of bone metabolism. Although some studies indicated that LTB4 functions by inhibiting bone formation and stimulating osteoclastic bone resorption (15)(16)(17), little is known about the roles of 5-LO and its metabolites in osteoclast differentiation. In the current study, we clarified their critical roles and intracellular signaling pathways in osteoclast differentiation.…”