Galectin-9 is a beta-galactoside-binding lectin expressed in various tissues. It binds various glycoconjugates and modulates a variety of biological functions in various cell types. Although galectin-9 is expressed in bone, its function in human osteoblasts remains unclear. We demonstrate that galectin-9 induces osteoblast differentiation through the CD44/Smad signaling pathway in the absence of bone morphogenetic proteins (BMPs). Galectin-9 increases alkaline phosphatase activities in human osteoblasts and induces the phosphorylation of Smad1/5/8 and translocation of Smad4 to the nucleus in the absence of BMPs. Galectin-9 also induces binding of Smad4 to the Id1 promoter and increases its activity. Anti-CD44 antibody inhibits Smad1/5/8 phosphorylation by galectin-9. Galectin-9 binds to CD44 and induces the formation of a CD44/BMP receptor complex. Because Smad1 is phosphorylated by BMP receptors, we propose that formation of the CD44/BMP receptor complex induced by galectin-9 may provide a trigger for the activation of Smads. (C) 2010 Elsevier Inc. All rights reserved.