Galectin-3 as a novel regulator of osteoblast-osteoclast interaction and bone homeostasis

Bone tissue undergoes permanent and lifelong remodeling with a concerted action of bone-building osteoblasts and bone-resorbing osteoclasts. A precise cooperation between those two cell types is critical in the complex process of bone renewal. Galectin-3 is a member of the β-galactoside-binding lectin family playing multiple roles in cell growth, differentiation and aggregation. As it has been described to be expressed in bone, galectin-3 might influence bone homeostasis by regulating the function and/or interplay of osteoblasts and osteoclasts. Here, we investigated the role of galectin-3 in osteoclastogenesis and osteoblast-osteoclast interactions. Bone histomorphometric analysis and μCT measurements revealed a decreased trabecular bone volume and an increased osteoclast number in 12 weeks old male galectin-3 knockout mice compared to wildtype littermates. Galectin-3 deficient bone marrow cells displayed a higher osteoclastogenic capacity in ex vivo differentiation assays, associated with elevated TRAF6 mRNA levels, suggesting an intrinsic inhibition of osteoclastogenesis by galectin-3 interfering with RANKL-mediated signaling. Furthermore, the addition of extracellular galectin-3 to murine or human osteoclastogenesis assays inhibited osteoclast formation and osteoclast numbers were higher in co-culture assays with galectin-3 deficient osteoblasts. In conclusion, our data suggest the secretion of galectin-3 as a novel mechanism for osteoblasts to control osteoclastogenesis and to maintain trabecular bone homeostasis independently of the RANKL/OPG-axis.