Role of galectin-8 as a modulator of cell adhesion and cell growth.

Galectin-8 belongs to the family of tandem-repeat type galectins. It consists as several isoforms, each made of two domains of approximately 140 amino-acids, both having a carbohydrate recognition domain (CRD). These domains are joined by a 'link peptide' of variable length. The human galectin-8 gene covers 33 kbp of genomic DNA. It is localized on chromosome 1 (1q42.11) and contains 11 exons. The gene produces by alternative splicing 14 different transcripts, altogether encoding 6 proteins. Galectin-8, like other galectins, is a secreted protein. Upon secretion galectin-8 acts as a physiological modulator of cell adhesion. When immobilized, it functions as a matrix protein equipotent to fibronectin in promoting cell adhesion by ligation and clustering of a selective subset of cell surface integrin receptors. Complex formation between galectin-8 and integrins involves sugar-protein interactions and triggers integrin-mediated signaling cascades such as Tyr phosphorylation of FAK and paxillin. In contrast, when present in excess as a soluble ligand, galectin-8 (like fibronectin) forms a complex with integrins that negatively regulates cell adhesion. Such a mechanism allows local signals emitted by secreted galectin-8 to specify territories available for cell adhesion and migration. Due to its dual effects on the adhesive properties of cells and its association with fibronectin, galectin-8 might be considered as a novel type of a matricellular protein. Galectin-8 levels of expression positively correlate with certain human neoplasms, prostate cancer being the best example studied thus far. The overexpressed lectin might give these neoplasms some growth and metastasis related advantages due to its ability to modulate cell adhesion and cellular growth. Hence, galectin-8 may modulate cell-matrix interactions and regulate cellular functions in a variety of physiological and pathological conditions.