Temperature-dependent Mobility of Concanavalin a Sites on Tumour Cell Surfaces

Abstract

AUB and his colleagues found that a contaminant in wheat germ lipase preparations agglutinated certain tumour cells, but not normal cells^1,2. The agglutinating molecule was later purified by Burger and Goldberg³ and characterized as a plant lectin (wheat germ agglutinin) that binds to specific terminal oligosaccharide residues containing N-acetyl-D-glucosamine. The ability to agglutinate tumour cells at concentrations which do not affect normal cells has been found for several other lectins with different saccharide binding specificities: concanavalin A^4,5 (Con A), soy bean agglutinin⁶, Ricinus communis agglutinin⁷, Pisum sativum agglutinin⁸ and others⁹. Burger proposed that the difference in agglutinability of tumour cells was a consequence of the exposure of “cryptic” lectin binding sites during transformation^3,10. Several investigators have found, however, that there is essentially no difference in the number of lectin sites on normal and transformed cells^11–15. Another explanation for the differential agglutinability of tumour cells was that the distribution of lectin sites at the cell surface could change to a topographic (“clustered”) state which is more favourable for cell agglutination^16–18. Experimental evidence for a topographic difference in the distribution of Con A sites on transformed, compared with normal, fibroblast cell surfaces has been obtained using ferritin-conjugated Con A¹⁶ and peroxidase-bound Con A^19,20. In these studies mounted cell membranes¹⁶ or intact cells^19,20 were labelled at room temperature with the Con A reagents. The mounted cell membranes were observed directly to obtain the topographic distribution of ferritin-Con A sites¹⁶; or the intact cells^19,20 were treated with Con A and peroxidase, fixed, dehydrated and embedded in plastic for thin section observation of an electron-dense peroxidase product at the cell surface^19,20. By either method the Con A membrane sites on tumour cells were distributed in a more clustered state than in normal cells^16,19,20. The clustered Con A sites seem to be involved in cell agglutination, because trypsinized normal cells (which are agglutinable) also show a similar reorganization of Con A sites¹⁸. When trypsinized 3T3 cells were agglutinated with ferritin-Con A and the cell aggregates removed, fixed, embedded in ‘Epson’ 812, sectioned and their cell membranes examined, the ferritin-Con A was found in clusters at the sites of contact between the agglutinated cells¹⁸.