Ganglioside Inhibition of (¹²⁵ I)-Plasmin Binding to Colorectal Carcinoma Cells

Abstract

The pre-incubation of human colorectal carcinoma cells SW 1116 with 25 to 100 uM purified gangliosides resulted in 35–60% inhibition of specific (¹²⁵ I)-plasmin binding to the cell surface. After 5 to 6 days in culture, tumor cells were pre-incubated at 4° for 1 to 4 h followed by post-incubation with (¹²⁵ I)-plasmin by techniques previously described. At 25 uM the capacity for inhibition of plasmin binding was GTlb > GQ lb ≥ GD la > GM 1 ≤ GgOse Cer. Thus a terminal sialyl moiety appears to be necessary (p < 0.05) although exogenous N-acetyl neuraminic acid was ineffective (p > 0.05), indicating a role for the lipid portion of the ganglioside. Other (glyco)lipids such as sphingosine, fucolipid H-1 and sulfatide were without significant effect. The inhibition could not be reversed by the presence of 10 mM Ca EDTA, pre-treatment of the cell with carboxypeptidase or pretreatment of plasmin with neuraminidases. The inhibition was however reversed by post-incubation in control medium without exogenous ganglioside. Cell counts determined prior to, and after ganglioside incubation showed that the effect was not due to cell death or detachment from the culture surface. The dissociation constant for (¹²⁵l)-plasmin binding was 5.6 × 10⁻⁸ M (700,000 sites/cell), but in the presence of trisialoganglioside (GT1b), Scatchard plots suggested diversification of binding sites with 280,000 site/cell at Kd 2.6 × 10⁻⁸ M and 820,000 sites/cell at Kd 2.1 × 10⁻⁷ M. Another interpretation of the Scatchard plot in the presence of ganglioside was that the glycolipid imposed negative cooperativity on plasmin binding to the cell surface. These results suggest that certain gangliosides can affect tumor cell invasiveness by altering protease binding to the cell surface.