Expression of a Mr 41,000 Glycoprotein Associated with Thrombin-independent Platelet Aggregation in High Metastatic Variants of Murine B16 Melanoma

In the previous study, we generated a monoclonal antibody, 8F11, against NL-17, a high metastatic clone derived from a metastatic variant of murine colon adenocarcinoma 26. 8F11 inhibited platelet aggregation induced by NL-17 and recognized a Mr 44,000 membrane protein as antigen. In the present study, the reactivity of 8F11 to murine B16 melanoma and its metastatic variants was examined, and the antigen recognized by 8F11 on the cell surface was characterized. 8F11 was found to strongly react with 3 metastatic variants of B16 melanoma. In contrast, only slight reactivity was observed with parent B16 melanoma. The reactivity of the antibody to these cells was in the order B16F10 > B16BL-6 > B16F1 ≫ B16. Western blot analysis showed a Mr 41,000 protein as the antigen recognized by 8F11 on the cell surface of B16F10 cells. The Mr 41,000 antigen appeared to be a glycoprotein that bound to wheat germ agglutinin as has been observed for the Mr 44,000 antigen of NL-17. To elucidate the functional role of the Mr 41,000 antigen in B16 melanoma, platelet aggregation induced by B16 and B16F10 was compared. B16 was reported to stimulate platelet aggregation by the generation of thrombin, whereas B16F10 was found to activate platelet by at least 2 mechanisms: one dependent on thrombin and the other independent on thrombin. The activity of B16 and its metastatic variants to induce platelet aggregation in the presence of MD805, a synthetic antagonist of thrombin, well correlated with the reactivity of 8F11 to these cells. 8F11 blocked platelet activation by B16F10 under conditions preventing thrombin activity such as enzymatic formation of lysolecithin through the treatment of the cell surface with phospholipase A2 or in the presence of MD805. These data indicate that Mr 41,000 glycoprotein recognized by 8F11 on metastatic variants of B16 melanoma is involved in the thrombin-independent platelet aggregation. A positive correlation was observed between the levels of Mr 41,000 glycoprotein expression of B16 and its metastatic variants and their pulmonary metastasis after i.v. injection, suggesting Mr 41,000 glycoprotein, as well as other factors reported previously, may play an important role in the hematogenous spread of B16 melanoma.