Interaction of galectin-9 with lipid rafts induces osteoblast proliferation through the c-Src/ERK signaling pathway.
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.


BACKGROUND
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.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Tanikawa
Rena
R
First Department of Internal Medicine, University of Occupational and Environmental Health, School of Medicine, Kitakyushu, Japan.
Tanikawa
Takahisa
T
Okada
Yosuke
Y
Nakano
Kazuhisa
K
Hirashima
Mitsuomi
M
Yamauchi
Akira
A
Hosokawa
Ryuji
R
Tanaka
Yoshiya
Y
D049109:Cell Proliferation / Q000187:drug effects; D002478:Cells, Cultured; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D005260:Female; D037161:Galectins / Q000378:metabolism* / Q000494:pharmacology; D006801:Humans; D008297:Male; D021962:Membrane Microdomains / Q000187:drug effects / Q000378:metabolism*; D010006:Osteoblasts / Q000166:cytology* / Q000187:drug effects / Q000378:metabolism; D010766:Phosphorylation / Q000187:drug effects; D016392:Proto-Oncogene Proteins pp60(c-src) / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism; D034741:RNA, Small Interfering / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology
D037161:Galectins; C105032:LGALS9 protein, human; D034741:RNA, Small Interfering; D016392:Proto-Oncogene Proteins pp60(c-src); D048049:Extracellular Signal-Regulated MAP Kinases