Inhibitory effects of polypeptides derived from a snake venom C‐type lectin, aggretin, on tumor cell‐induced platelet aggregation

Summary Background and objectives Podoplanin, a transmembrane sialoglycoprotein, is expressed by lymphatic endothelial cells and many tumor cells, and is involved in tumor cell‐induced platelet aggregation and tumor metastasis. A recent study found that C‐type lectin‐like receptor 2 (CLEC‐2) is a physiologic receptor for podoplanin. Previous studies showed that aggretin, a snake venom‐derived protein, activates platelets by targeting platelet CLEC‐2. We hypothesized that the C‐terminal fragment of aggretin may bind to platelet CLEC‐2 and displace podoplanin, in turn exerting antitumor metastatic effects. Methods and results Aggretin α‐chain C‐terminus (residues 106–136; AACT) prolonged the lag phase of platelet aggregation induced by aggretin in human washed platelets, indicating that AACT may target the binding site of CLEC‐2. HepG2 cells, which are podoplanin‐expressing hepatoma cells, induced platelet aggregation with a lag phase. Pretreatment with AACT inhibited platelet aggregation and prolonged the lag phase induced by HepG2 cells. This inhibitory effect was also found with another hepatocarcinoma cell line, HuH‐7. AACT inhibited the interaction between HuH‐7 cells and platelets, and a specific binding assay demonstrated that CLEC‐2 was the binding site for AACT on platelets. In addition, the invasive ability of HepG2 cells was abolished by AACT in a chick embryo chorioallantoic membrane model. Furthermore, formation of lung metastases after intravenous administration of HuH‐7 cells was significantly reduced when mice were treated with AACT. Conclusions AACT interacts with CLEC‐2 of platelets, leading to interference with platelet aggregation and the subsequent metastatic potential of tumor cells. These results suggest that aggretin AACT is a potential candidate for the treatment of tumor metastasis through CLEC‐2 blockade.