Abstract B42: Platelets promote tumor growth and metastasis via direct interaction between Aggrus/podoplanin and CLEC-2
Authors: Takagi Satoshi , Sato Shigeo , Oh-hara Tomoko , Takami Miho , Koike Sumie , Fujita Naoya 
Published: May 2015

Abstract
Platelet–tumor cell interaction contributes to the progression of tumor malignancy by the following three modes of action: (i) tumor cells coated by platelets in the vasculature become bulky and adhesive, and these properties increase the rate of tumor embolization in the microvasculature; (ii) platelets protect tumor cells from shear stress and immunological assault in the blood stream by coating their cell surfaces; and (iii) growth factors secreted from granules of activated platelets enhance the growth and motility of tumors and tumor vasculature. Although some clinical trials have clarified the efficacy of antiplatelet drugs for tumor therapy, the most important issue related to administration of antiplatelet drugs for cancer therapy is the increased bleeding risk. Therefore, a novel and low-bleeding approach is necessary for suppressing specific platelet–tumor cell interactions.
Aggrus, also known as podoplanin, is a type-I transmembrane glycoprotein and upregulated in several cancers, including squamous cell carcinomas, mesothelioma, Kaposi's sarcoma, brain tumor, and bladder cancer. We previously discovered Aggrus as a platelet aggregation-inducing factor and its association with tumor malignancy by promoting hematogenous metastasis. Because Aggrus mutants lacking platelet aggregation-inducing abilities were unable to form hematogenous metastasis, Aggrus-induced platelet aggregation is directly linked to the metastasis formation. The C-type lectin-like receptor 2 (CLEC-2) expressed on platelets was identified as one of the counter receptors of Aggrus. Aggrus binding to CLEC-2 transmits platelet activation signals through Syk and phospholipase Cγ2 in platelets. The fact that CLEC-2–deficient platelets respond normally to platelet agonists, indicates that inhibition of the Aggrus–CLEC-2 interaction may not affect physiological hemostasis. Thus, Aggrus–CLEC-2 binding could be a therapeutic molecular mechanism for cancer therapy with minimum bleeding risk.
In this study, we generated an anti-human Aggrus monoclonal antibody (mAb), MS-1, that suppressed Aggrus–CLEC-2 binding, Aggrus-induced platelet aggregation, and Aggrus-mediated tumor metastasis. Interestingly, MS-1 mAb attenuated the growth of Aggrus-positive tumors in vivo. Moreover, the humanized chimeric MS-1 antibody, ChMS-1, also exhibited strong antitumor activity against Aggrus-positive lung squamous cell carcinoma, PC-10 tumor, xenografted in NOD-SCID mice. Immunohistochemical staining revealed the infiltration of platelets around PC-10 tumors but not around Aggrus-negative A549 tumors. To confirm the platelet–tumor cell interaction in tumor growth, we generated Aggrus knockdown PC-10 (PC-10/shAgg) cells. Although Aggrus knockdown did not affect PC-10 cell growth in vitro, drastic retardation of PC-10/shAgg tumor growth in vivo was observed. We next cultured PC-10 transfectants with washed platelets in vitro and found that addition of platelets greatly enhanced the proliferation of control PC-10 cells but not PC-10/shAgg cells. These results suggest that platelets may promote PC-10 cell growth only when they were activated by interacting with Aggrus protein on tumor cell surfaces. Furthermore, Aggrus is not only associated with tumor metastasis but also with tumor growth by promoting interaction with platelets. In conclusion, these data suggest that Aggrus-targeting drugs can be developed as antitumor and antimetastatic drugs.
This study was supported by the Advanced Research for Medical Products Mining Programme of the National Institute of Biomedical Innovation (NIBIO).
Citation Format: Satoshi Takagi, Shigeo Sato, Tomoko Oh-hara, Miho Takami, Sumie Koike, Naoya Fujita. Platelets promote tumor growth and metastasis via direct interaction between Aggrus/podoplanin and CLEC-2. [abstract]. In: Abstracts: AACR Special Conference on Cellular Heterogeneity in the Tumor Microenvironment; 2014 Feb 26-Mar 1; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(1 Suppl):Abstract nr B42. doi:10.1158/1538-7445.CHTME14-B42