Heparin-binding agglutinin on human teratocarcinoma cells

Characterization and biological implications of membrane lectins in tumor, lymphoid and myeloid cells

Complex carbohydrates and sugar receptors at the surface of eukaryotic cells are involved in recognition phenomena. Membrane lectins have been characterized, using biochemical, biological and cytological methods. Their biological activities have been assessed using labeled glycoproteins or neoglycoproteins. Specific glycoproteins or neoglycoproteins have been used to inhibit their binding capacity in both in vitro and in vivo experiments. In adults, lymphoid and myeloid cells as well as tumor cells grow in a given organ and eventually migrate and home in another organ; these phenomena are known as the homing process or metastasis, respectively. In specific cases, membrane lectins of endothelial cells recognize cell surface glycoconjugates of lymphocytes or tumor cells, while membrane lectins of lymphocytes and of tumor cells recognize glycoconjugates of extracellular matrices or of non-migrating cells. Therefore, membrane lectins are involved in cell-cell recognition phenomena. Membrane lectins are also involved in endocytosis and intracellular traffic of glycoconjugates. This property has been demonstrated not only in hepatocytes, fibroblasts, macrophages and histiocytes but also in tumor cells, monocytes, thyrocytes, etc. Upon endocytosis, membrane lectins are present in endosomes, whose luminal pH rapidly decreases. In cells such as tumor cells or macrophages, endosomes fuse with lysosomes; it is therefore possible to target cytotoxic drugs or activators, by binding them to specific glycoconjugates or neoglycoproteins through a linkage specifically hydrolyzed by lysosomal enzymes. In cells such as monocytes, the delivery of glycoconjugates to lysosomes is not active; in this case, it would be preferable to use an acid-labile linkage. Cell surface membrane lectins are developmentally regulated; they are present at given stages of differentiation and of malignant transformation. Cell surface membrane lectins usually bind glycoconjugates at neutral pH but not in acidic medium: their ligand is released in acidic specialized organelles; the internalized ligand may be then delivered into lysosomes, while the membrane lectin is recycled. Some membrane lectins, however, do bind their ligand in relatively acidic medium as in the case of thyrocytes. The presence of cell surface membrane lectins which recognize specific sugar moieties opens the way to interesting applications: for instance, isolation of cell subpopulations such as human suppressor T cells, targeting of anti-tumor or anti-viral drugs, targeting of immunomodulators or biological response modifiers.

Endogenous lectins as mediators of tumor cell adhesion

Endogenous carbohydrate-binding proteins have been found in various normal tissues and cells. Although lectins with different sugar-binding specificities have been described, the most prevalent ones are those that bind beta-galactosides. The ability of some normal and malignant cells to bind exogenous carbohydrate-containing ligands suggested that lectinlike activity is associated with the cell surface and that carbohydrate-binding proteins might mediate intercellular recognition and adhesion. We found that extracts of various cultured murine and human tumor cells exhibit a galactoside-inhibitable hemagglutinating activity. This activity was associated with two proteins of molecular weights of 34,000 and 14,500 daltons, which were purified by affinity chromatography by using immobilized asialofetuin. That these lectins are present on the cell surface was indicated by the binding of monoclonal antilectin antibodies to the surface of various tumor cells and by the immunoprecipitation of 125I-labeled lectins from solubilized cell-surface iodinated cells by polyclonal antilectin antibodies. That these cell surface lectins are functional was demonstrated by the ability of the galactose-terminating asialofetuin to enhance cell aggregation and of asialofetuin glycopeptides to block this homotypic aggregation as well as to suppress cell attachment to substratum, and by the inhibition of both asialofetuin-induced cell aggregation and cell attachment to substratum by the binding of monoclonal antilectin antibodies to the cell surface. These findings implicate cell surface lectins as mediators of cell-cell and cell-substratum adhesion. Some of these cellular interactions might be important determinants of tumor cell growth and metastasis.

Endogenous galactoside-binding lectins: a new class of functional tumor cell surface molecules related to metastasis

The formation of secondary tumors by circulating cancer cells (blood-borne metastasis) correlates with an increased tendency of the cells to form emboli by aggregation with other tumor cells or with host cells. Although it is evident that cell-cell recognition and adhesion are mediated by cell surface components, the identity of these molecules is only now being unraveled. Over the last decade an increasing number of studies have demonstrated the presence of endogenous carbohydrate-binding proteins on the surface of various normal cells, and it has been proposed that such lectin-like molecules might be involved in intercellular adhesion. We have shown that various tumor cell lines contain endogenous galactose-specific lectins. Lectin activity was detected at the cell surface by the binding of asialofetuin. This glycoprotein also enhanced the aggregation of the tumor cells. After purification by affinity chromatography on immobilized asialofetuin the lectin activity was associated with two proteins of Mr 14,500 and 34,000. By using polyclonal and monoclonal antilectin antibodies in conjunction with various immunologic techniques we have demonstrated that the endogenous lectins are present on the surface of different tumor cells. Quantitation of cell surface lectins by flow cytometric analyses of antilectin antibody binding revealed that among related tumor cells those exhibiting a higher metastatic potential expressed more lectin on their surface. The binding of monoclonal antilectin antibodies to metastatic cells decreased asialofetuin-induced homotypic aggregation in vitro and suppressed the ability of the cells to form lung metastases after intravenous injection in the tail vein of syngeneic mice. These results strongly implicate the tumor cell surface lectins in cell adhesion and metastasis. We propose that such lectins can increase the ability of tumor cells that enter the blood stream to form aggregates with other tumor cells, or to adhere to host cells or the extracellular matrix and thereby increase their metastatic potential. Other contributing components to tumor cell-host cell interactions are cell surface carbohydrate-binding proteins that have been detected on lymphocytes, platelets, macrophages, hepatocytes, and endothelial cells. These lectin-like molecules might recognize and bind carbohydrates expressed on the surface of tumor cells and enhance emboli formation and organ colonization.

Inhibition of ganglioside sialyltransferase activity and stimulation of neutral glycolipid exocytosis by heparin

Heparin treatment of human teratocarcinoma cells in culture has several manifestations. Accumulation of gangliosides is greatly decreased while the content of cellular neutral glycolipids is relatively unaffected. However, synthesis of neutral glycolipids is increased and large amounts of these glycolipids are exported out of the cells into the medium. In addition, sialyltransferase activity of heparin-treated teratocarcinoma cells is significantly inhibited, accounting for the decreased cellular content of gangliosides. These studies are not intended to infer any physiological role of heparin in the regulation of glycolipid biosynthesis. However, results do show that beta-D-galactoside 2,3-sialyltransferase is a heparin-binding protein and that inhibition of the enzyme activity by heparin is linked to an alteration in the secretion of most neutral glycolipid precursors and sialyltransferase acceptors. These data suggest that in addition to its biosynthetic function, this ganglioside transferase may play a regulatory role in glycolipid secretion.

Heparin-induced aggregation of lymphoid cells

The effects of different carbohydrates on cell-to-cell adhesion were examined in an aggregation assay, which consisted of swirling a suspension of cells and monitoring the loss of single cells with a Coulter Counter. Of the carbohydrates tested, only heparin and dextran sulfate induced cell aggregation. This effect occurred in freshly isolated mouse splenocytes and in cultured cells of lymphoid origin (P388, YAA-CI) but not in cell lines of fibroblastic origins (3T3, SV-3T3, BHK, and PY-BHK). Using the YAA-CI cell line for further study, we found that aggregation could be induced by relatively small amounts of heparin (less than 10 micrograms/ml). Binding experiments with 3H-heparin showed that under normal physiological conditions each YAA-CI cell bound approximately 2 X 10(6) molecules of heparin at saturation with a Kd of 3.5 X 10(-7) M. This binding was blocked by both unlabelled heparin and dextran sulfate but not by other carbohydrates. When the pH of the medium was decreased, the heparin-induced aggregation was inhibited, and the Kd of the 3H-heparin binding was increased. In a similar fashion, when the ionic strength of the medium was increased, heparin-induced aggregation was inhibited and the Kd of the interaction was increased. These results suggest that the aggregation is inversely related to the Kd of the interaction and that the binding of heparin to the cell surface is primarily of an ionic nature.