The galactoside-binding protein galectin-3 is increasingly recognized as an important player in cancer development, progression, and metastasis via its interactions with various galactoside-terminated glycans. We have shown previously that circulating galectin-3, which is increased up to 30-fold in cancer patients, promotes blood-borne metastasis in an animal cancer model. This effect is partly attributable to the interaction of galectin-3 with unknown receptor(s) on vascular endothelial cells and causes endothelial secretion of several metastasis-promoting cytokines. Here we sought to identify the galectin-3-binding molecule(s) on the endothelial cell surface responsible for the galectin-3-mediated cytokine secretion. Using two different galectin-3 affinity purification processes, we extracted four cell membrane glycoproteins, CD146/melanoma cell adhesion molecule (MCAM)/MUC18, CD31/platelet endothelial cell adhesion molecule-1 (PECAM-1), CD144/VE-cadherin, and CD106/Endoglin, from vascular endothelial cells. CD146 was the major galectin-3-binding ligand and strongly co-localized with galectin-3 on endothelial cell surfaces treated with exogenous galectin-3. Moreover, galectin-3 bound to N-linked glycans on CD146 and induced CD146 dimerization and subsequent activation of AKT signaling. siRNA-mediated suppression of CD146 expression completely abolished the galectin-3-induced secretion of IL-6 and G-CSF cytokines from the endothelial cells. Thus, CD146/MCAM is the functional galectin-3-binding ligand on endothelial cell surfaces responsible for galectin-3-induced secretion of metastasis-promoting cytokines. We conclude that CD146/MCAM interactions with circulating galectin-3 may have an important influence on cancer progression and metastasis.Galectin-3 is a galactoside-binding protein that is expressed by many types of human cells. Overexpression of galectin-3 commonly occurs in most types of cancer (1, 2) and is increasingly recognized to influence cancer cell-cell and cancer-microenvironment communications and contributes to cancer development, progression, and metastasis as a result of galectin-3 interaction with various galactose-terminated glycans carried on the cell surface and in the extracellular matrix (2).Earlier studies by us as well as by other groups have revealed that the concentration of circulating galectin-3 is markedly increased, up to 30-fold, in the bloodstream of cancer patients, including those with colorectal, breast, lung, bladder, pancreatic, and head and neck cancers and melanoma (2-8). Patients with metastatic disease tend to have higher concentrations of circulating galectin-3 than those with only localized tumors (3). Our recent study has revealed that increased circulating galectin-3 is an important promoter of blood-borne metastasis in a mouse model (9). This effect of galectin-3 is shown to be partly attributed to galectin-3 interaction with the oncofetal Thomsen-Friedenreich (galactose1,3-N-acetyl-galactosamine ␣-, TF) disaccharide expressed on the cancer-associated transmembr...