Induction of cell adhesion by galectin-8 and its target molecules in Jurkat T-cells.

We previously showed that tandem-repeat type galectin-8, which has two covalently linked carbohydrate recognition domains (CRDs), induces neutrophil-adhesion through binding to integrin alphaM. Here, we analysed the function of galectin-8 in Jurkat T-cells. Galectin-8, as well as tandem-repeat galectin-9, and several multivalent plant lectins, induced Jurkat T-cell adhesion to a culture plate, whereas single-CRD galectins-1 and -3 did not. Galectin-8 also induced the adhesion of peripheral blood leucocytes to human umbilical vein endothelial cells. These results suggest that the di- or multi-valent structure of galectin-8 is essential for the induction of cell adhesion and that this ability exhibits broad specificity for leucocytes. The galectin-8-induced cell adhesion was accompanied by stress fibre formation, which suggests that intracellular signalling is required. We have identified integrin alpha4 as one of the candidate target molecules associated with the induction of cell adhesion. Indeed, inhibition of the function of integrin alpha4 by treating cells with a blocking-antibody reduced the sensitivity to galectin-8. Also, the phosphorylation of Pyk and ERK1/2, indicators of integrin-mediated signalling, was up-regulated on treatment with galectin-8. Thus, a primary target of galectin-8 must be the sugar chains on members of the integrin family, which are abundantly expressed on the surface of leucocytic cells.

Characterization of galectin-9-induced death of Jurkat T cells.

Galectin-9, a mammalian lectin with affinity for beta-galactosides, is known as an apoptosis inducer of activated T lymphocytes. In the present study, we examined the properties of galectin-9-mediated cell death of Jurkat T cells. Galectin-9NC (wild-type), consisting of two CRDs (N-terminal and C-terminal carbohydrate recognition domains), and derivatives of it, galectins-9-NN and -9-CC, induced Jurkat T-cell apoptosis. However, a single CRD (galectin-9NT or -CT) had no effect, suggesting the stable dimeric structure of two CRDs is required for the activity. The apoptosis was inhibited by pretreatment with an N-glycan synthesis inhibitor, indicating that the expression of N-glycans in the cells is essential for galectin-9-induced apoptosis. We previously showed that the apoptosis of MOLT-4 cell is mediated by galectin-9 via a Ca(2+)-calpain-caspase-1-dependent pathway. In Jurkat cells, the cell death by galectin-9, was insufficiently suppressed by caspase inhibitors, Ca(2+)-chelator or calpain inhibitor. Furthermore, we observed the loss of mitochondrial membrane potential and significant AIF release in galectin-9-treated cells. These findings suggest that caspase-dependent and-independent death pathways exist in Jurkat cells, and the main pathway might vary with the T-cell type.