Interaction of Galectin-9 With Lipid Rafts Induces Osteoblast Proliferation Through the c-Src/ERK Signaling Pathway

Abstract

Galectin-9 is a beta-galactoside-binding lectin expressed in various tissues, including bone. The role of galectin-9 in human osteoblasts, however, remains unclear. This study showed that galectin-9 interacts with lipid rafts and induces osteoblast proliferation through the c-Src/ERK signaling pathway. Galectin-9 is a beta-galactoside-binding lectin that modulates many biological functions by interacting with particular carbohydrates attached to proteins and lipids. However, the role of galectin-9 in bone metabolism and osteoblast proliferation remains unclear. This study investigated the effects of galectin-9 on osteoblast proliferation and its signaling mechanisms. The effect of galectin-9 on osteoblast proliferation was tested by measuring the conversion of tetrazolium salt WST-8 to formazan. Protein phosphorylation was assayed by western blotting and confocal microscopy was used to localize lipid rafts. Galectin-9-induced proliferation of the obtained osteoblasts in a dose-dependent manner, whereas galectin-1, -3, and -4 did not. Galectin-9-induced phosphorylation of c-Src and subsequent ERK1/ERK2 in the osteoblasts. The galectin-9-induced phosphorylation and proliferation were inhibited by PP2, a selective inhibitor of c-Src. Galectin-9-induced clustering of lipid rafts detected by cholera toxin B (CTB; binding the raft-resident ganglioside GM1) using confocal microscopy. Cross-linking of the GM1 ganglioside with CTB by anti-CTB antibody-induced phosphorylation of c-Src, whereas disruption of galectin-9-induced lipid rafts by beta-methylcyclodextrin reduced c-Src phosphorylation and proliferation of the cells. These results suggest that galectin-9, but not other galectins, induced proliferation of human osteoblasts through clustering lipid rafts on membrane and subsequent phosphorylation of the c-Src/ERK signaling pathway.