Galectin-3 Phosphorylation Is Required for Its Anti-apoptotic Function and Cell Cycle Arrest

Galectin-3, a β-galactoside-binding protein, is implicated in cell growth, adhesion, differentiation, and tumor progression by interactions with its ligands. Recent studies have revealed that galectin-3 suppresses apoptosis and anoikis that contribute to cell survival during metastatic cascades. Previously, it has been shown that human galectin-3 undergoes post-translational signaling modification of Ser6 phosphorylation that acts as an “on/off” switch for its sugar-binding capability. We questioned whether galectin-3 phosphorylation is required for its anti-apoptotic function. Serine to alanine (S6A) and serine to glutamic acid (S6E) mutations were produced at the casein kinase I phosphorylation site in galectin-3. The cDNAs were transfected into a breast carcinoma cell line BT-549 that innately expresses no galectin-3. Metabolic labeling revealed that only wild type galectin-3 undergoes phosphorylation in vivo. Expression of Ser6 mutants of galectin-3 failed to protect cells from cisplatin-induced cell death and poly(ADP-ribose) polymerase from degradation when compared with wild type galectin-3. The non-phosphorylated galectin-3 mutants failed to protect cells from anoikis with G1 arrest when cells were cultured in suspension. In response to a loss of cell-substrate interactions, only cells expressing wild type galectin-3 down-regulated cyclin A expression and up-regulated cyclin D1 and cyclin-dependent kinase inhibitors, i.e.p21WAF1/CIP1 and p27KIP1 expression levels. These results demonstrate that galectin-3 phosphorylation regulates its anti-apoptotic signaling activity.