Inhibition of human CD24 binding to platelet-bound P-selectin by monoclonal antibody.

P-selectin, an adhesive molecule present on the surface of activated platelets, participates in the interactions between platelets and a variety of cells, including cancer cells. Such interaction is an important step toward the establishment of thriving secondary tumors during blood-borne metastasis. CD24, one of the mucin-type ligands of P-selectin, is expressed in various adenocarcinomas and may mediate the interaction between platelets and cancer cells. To demonstrate the involvement and specificity of P-selectin and/or CD24 in the interaction, we adapted a fluorometric binding procedure to assay the binding of calcein AM-labeled platelets to DU145 cells grown on 96-well microplates in the presence or absence of antibodies against CD24 or P-selectin. The platelet binding on DU145 cells was inhibited by anti-P-selectin antibody (GA-6) by 83.6% (p<0.01). The similar inhibition seen with anti-CD24 (FL80) antibody, (68%, p<0.01), was preventable by preincubating with a cognate CD24 peptide. In conclusion, our results indicate that P-selectin ligand CD24 mediates the binding of platelets to prostate cancer line DU145 cells in vitro. The fluorometric assay for platelet binding may have a promising role in further defining parameters of the interaction.