Platelets and Cancer-associated Thrombosis: Focusing on the Platelet Activation Receptor CLEC-2 and Podoplanin

Patients with cancer have an increased risk of thromboembolism, which is the second leading cause of death in these patients. Several mechanisms of the prothrombotic state in these patients have been proposed. Among them are a platelet activation receptor, C-type lectin-like receptor 2 (CLEC-2), and its endogenous ligand podoplanin, which are the focus of this review. CLEC-2 is almost specifically expressed in platelets/megakaryocytes in humans. A membrane protein, podoplanin is expressed in certain types of cancer cells, including squamous cell carcinoma, brain tumor, and osteosarcoma, in addition to several normal tissues, including kidney podocytes and lymphatic endothelial cells but not vascular endothelial cells. In the bloodstream, podoplanin induces platelet activation by binding to CLEC-2 and facilitates hematogenous cancer metastasis and cancer-associated thrombosis. In an experimental lung metastasis model, the pharmacological depletion of CLEC-2 from platelets in mice resulted in a marked reduction of lung metastasis of podoplanin-expressing B16F10 cells. Control mice with B16F10 orthotopically inoculated in the back skin showed massive thrombus formation in the lungs, but the cancer-associated thrombus formation in CLEC-2-depleted mice was significantly inhibited, suggesting that CLEC-2-podoplanin interaction stimulates cancer-associated thrombosis. Thromboinflammation induced ectopic podoplanin expression in vascular endothelial cells or macrophages, which may also contribute to cancer-associated thrombosis. CLEC-2 depletion in cancer-bearing mice resulted in not only reduced cancer-associated thrombosis but also reduced levels of plasma inflammatory cytokines, anemia, and sarcopenia, suggesting that cancer-associated thrombosis may cause thromboinflammation and cancer cachexia. Blocking CLEC-2-podoplanin interaction may be a novel therapeutic strategy in patients with podoplanin-expressing cancer.