The Novel Platelet Activation Receptor CLEC-2 Regulates Blood/Lymphatic Vessel Separation by Inhibiting Proliferation and Migration of Lymphatic Endothelial Cells

Abstract 160

CLEC-2 has been described recently as playing crucial roles in thrombosis/hemostasis, tumor metastasis, and lymphangiogenesis. The snake venom rhodocytin is known as a strong platelet activator, and we have shown that this effect is mediated by CLEC-2. Podoplanin, which is expressed on the surface of tumor cells, is an endogenous ligand for CLEC-2 and facilitates tumor metastasis by inducing platelet aggregation. Mice deficient in podoplanin, which is also expressed on the surface of lymphatic endothelial cells, show abnormal patterns of lymphatic vessel formation. We have recently reported on the generation and phenotype of CLEC-2-deficient mice. These mice are lethal at the embryonic/neonatal stages associated with disorganized and blood-filled lymphatic vessels and severe edema, indicating that CLEC-2 is essential for blood/lymphatic vessel separation. However, CLEC-2 is expressed in both platelets and neutrophils in mice and whether CLEC-2 in platelets, but not that in other cells, plays a role in the separation has not been directly proved yet. Moreover, the mechanism how CLEC-2 regulates of blood/lymphatic vessel separation has not been elucidated to date. In the present study, we found that specific deletion of CLEC-2 from platelets mediated by PF4-Cre conferred the defect of blood/lymphatic vessel separation, identifying that CLEC-2 expressed in platelets is required to regulate lymphatic vascular development. Tube formation of lymphatic endothelial cells, but not that of vascular endothelial cells, was inhibited in the presence of platelets. Further, proliferation and migration of lymphatic endothelial cells, but not those of vascular endothelial cells, were inhibited by CLEC-2+/+ platelets, but not by CLEC-2−/− platelets. We propose that CLEC-2 regulates blood/lymphatic vessel separation by inhibiting proliferation and migration of lymphatic endothelial cells through the interaction between CLEC-2 in platelets and podoplanin in lymphatic endothelial cells. CLEC-2 may be a novel therapeutic target protein for lymphatic metastasis of tumor if the interaction of CLEC-2 and podoplanin also regulates lymphangiogenesis by tumor cells.