Lectins in Cancer Cells

Studies carried out over the last few years have demonstrated that tumor cells and malignant tissues contain lectins that are similar in sugar-binding specificity, molecular size, and antigenicity to the lectins found in normal cells and tissues. Lectins from tumor cells also share marked sequence homology with lectins from normal tissues. Lectins were purified from various tumor cells by affinity chromatography and monoclonal and polyclonal antilectin antibodies were prepared against them. These enabled us to establish the following: (1) Lectins are present on the surface of all the tumor cells that were examined, albeit at varying levels. (2) The level of cell surface lectins increases after normal cells are transformed by transfection with certain oncogenes or by retroviruses, or when cells transformed with a temperature-sensitive viral mutant are switched from growth at the nonpermissive to the permissive temperature. (3) Among tumor cells differing in metastatic propensity, those exhibiting a higher potential express higher levels of surface lectins. (4) Tumor cell surface lectins might be involved in cell-cell adhesion, cell attachment to substratum, the expression of the transformed phenotype (anchorage-independent growth), and blood-borne metastasis. (5) The levels of the lectins in tumor cells are modulated by agents that suppress the transformed phenotype (as represented by anchorage-independence) or enhance differentiation. Numerous studies by others have shown that cell surface carbohydrate-containing molecules are modified after transformation, and our findings demonstrate that the expression of cell surface carbohydrate-binding proteins is also altered by transformation. Obviously, any of these changes may result in alterations in cellular interactions. All the above findings implicate tumor cell lectins in cellular interactions (adhesion, attachment, possible binding of exogenous soluble glycoconjugates), cell growth and anchorage-independent growth, malignant transformation, tumor cell differentiation, and metastasis. It is clear that even if these lectins are involved in only a few of these fundamental processes, it is important to elucidate their functions and the mechanisms by which their expression is regulated during neoplastic transformation and tumor progression and the suppression of the transformed phenotype.