A change of rhodocytin's suprastructure turns the agonist into an antagonist of tumor cell induced platelet aggregation.

Cancer cells circulating in the blood attach to platelets by direct cell-cell interactions via several receptor-counterreceptor contacts and indirectly by fibrin bridges which connect the two cell types by distinct integrin receptors. In the microenvironment of these tumor cell platelet aggregates (TCPAs), the tumor cells are shielded from the shear stress of the blood flow and from attack by the immune system. This supports hematogenous metastasis and tumor cell induced thrombosis. Platelet activation is triggered by binding of podoplanin on cancer cells to the platelet receptor Clec-2. Therefore, we hypothesize that targeting this initial step will prevent the entire cascade leading to the formation of TCPAs. Rhodocytin, a heterodimeric (αß)2 C-type lectin from the Malayan pit viper Calloselasma rhodostoma, binds to Clec-2 and thereby induces TCPA formation. Remarkably, mutations in rhodocytin that disturbed formation of oligomers, blocked the podoplanin-Clec-2 axis and prevented platelet activation. Therefore, we used lysine reactive chemicals to modify rhodocytin isolated from the crude snake venom. Blue native gel electrophoresis and far western blotting showed a change of rhodocytin's suprastructure triggered by acetylation and PEGylation. Mass spectrometry analysis of altered lysines suggested that their modifications interfered with the formation of rhodocytin tetramers. When tested in assays for tumor cell induced platelet aggregation, we found that derivatization turned rhodocytin from an agonist into an antagonist. This observation indicates that Clec-2 is a valid target receptor molecule to curb TCPA formation and to prevent hematogenous metastasis and tumor cell induced thrombosis in cancer patients.

Stimulation of platelet aggregation by affinity captured rhodocytin from the Malayan pit viper Calloselasma rhodostoma.

The receptor protein CLEC-2 on platelet membranes is the target of the endogenous ligand podoplanin found on cancer cells and of rhodocytin, a snake venom component of the Malayan pit viper Calloselasma rhodostoma. Ligand binding results in platelet activation, increased blood coagulation and thrombosis. In an effort to isolate rhodocytin, we have purified CLEC-2 as bait from E. coli. Affinity captured rhodocytin interacted with mammalian CLEC-2 and stimulated platelet aggregation in a dose dependent manner.