Functional Evidence that Cell Surface Galectin-3 Mediates Homotypic Cell Adhesion1

Galectin-3 (Gal-3) ¡sa ß-galactoside-binding protein with Mr ~30,000. Cell surface Gal-3 is postulated to be involved in homotypic aggregation of tumor cells in the circulation during metastasis through attachment to a complementary serum glycoprotein(s), which serves as a cross-linking bridge between adjacent cells. To test this hypothesis a recombinant strain of baculovirus encoding Gal-3 was used to infect Sf9 insect cells, which lack endogenous Gal-3. Immunoblotting and indirect immunofluorescence studies revealed that the infection with recombinant virus conferred Gal-3 expression on Sf9 cells, and the Gal-3 was localized on the cell surface as well as in the cytoplasm. Sf9 cells infected with recombinant virus underwent homotypic aggregation in the presence of exogenous glycoprotein (i.e., asialofetuin), whereas control cells uninfected or in fected with wild-type virus did not. Lactose and Fab' fragments of antiGal-3 antibodies markedly inhibited the cell-cell aggregation. Moreover, cosuspension of SIV cells infected with the recombinant virus with unin fected cells in the presence of asialofetuin resulted in a preferential cell-cell adhesion of the Gal-3-expressing cells. These results directly demonstrate the ability of cell surface Gal-3 molecules to mediate homo typic cell adhesion by bridging through branched, soluble complementary glycoconj ugales.