Prevention of hematogenous metastasis by neutralizing mice and its chimeric anti-Aggrus/podoplanin antibodies

Maackia amurensis seed lectin (MASL) and soluble human podoplanin (shPDPN) sequence analysis and effects on human oral squamous cell carcinoma (OSCC) cell migration and viability

Maackia amurensis lectins serve as research and botanical agents that bind to sialic residues on proteins. For example, M. amurensis seed lectin (MASL) targets the sialic acid modified podoplanin (PDPN) receptor to suppress arthritic chondrocyte inflammation, and inhibit tumor cell growth and motility. However, M. amurensis lectin nomenclature and composition are not clearly defined. Here, we sought to definitively characterize MASL and its effects on tumor cell behavior. We utilized SDS-PAGE and LC-MS/MS to find that M. amurensis lectins can be divided into two groups. MASL is a member of one group which is composed of subunits that form dimers, evidently mediated by a cysteine residue in the carboxy region of the protein. In contrast to MASL, members of the other group do not dimerize under nonreducing conditions. These data also indicate that MASL is composed of 4 isoforms with an identical amino acid sequence, but unique glycosylation sites. We also produced a novel recombinant soluble human PDPN receptor (shPDPN) with 17 threonine residues glycosylated with sialic acid moieties with potential to act as a ligand trap that inhibits OSCC cell growth and motility. In addition, we report here that MASL targets PDPN with very strong binding kinetics in the nanomolar range. Moreover, we confirm that MASL can inhibit the growth and motility of human oral squamous cell carcinoma (OSCC) cells that express the PDPN receptor. Taken together, these data characterize M. amurensis lectins into two major groups based on their intrinsic properties, clarify the composition of MASL and its subunit isoform sequence and glycosylation sites, define sialic acid modifications on the PDPN receptor and its ability to act as a ligand trap, quantitate MASL binding to PDPN with KD in the nanomolar range, and verify the ability of MASL to serve as a potential anticancer agent.

Targeting Podoplanin for the Treatment of Osteosarcoma

PURPOSE

Osteosarcoma, the most common bone malignancy in children, has a poor prognosis, especially when the tumor metastasizes to the lungs. Therefore, novel therapeutic strategies targeting both proliferation and metastasis of osteosarcoma are required. Podoplanin (PDPN) is expressed by various tumors and is associated with tumor-induced platelet activation via its interaction with C-type lectin-like receptor 2 (CLEC-2) on platelets. We previously found that PDPN contributed to osteosarcoma growth and metastasis through platelet activation; thus, in this study, we developed an anti-PDPN humanized antibody and evaluated its effect on osteosarcoma growth and metastasis.

EXPERIMENTAL DESIGN

Nine osteosarcoma cell lines and two osteosarcoma patient-derived cells were collected, and we evaluated the efficacy of the anti-DPN-neutralizing antibody PG4D2 and the humanized anti-PDPN antibody AP201, which had IgG4 framework region. The antitumor and antimetastasis effect of PG4D2 and AP201 were examined in vitro and in vivo. In addition, growth signaling by the interaction between PDPN and CLEC-2 was analyzed using phospho-RTK (receptor tyrosine kinase) array, growth assay, or immunoblot analysis under the supression of RTKs by knockout and inhibitor treatment.

RESULTS

We observed that PG4D2 treatment significantly suppressed tumor growth and pulmonary metastasis in osteosarcoma xenograft models highly expressing PDPN. The contribution of PDGFR activation by activated platelet releasates to osteosarcoma cell proliferation was confirmed, and the humanized antibody, AP201, suppressed in vivo osteosarcoma growth and metastasis without significant adverse events.

CONCLUSIONS

Targeting PDPN with a neutralizing antibody against PDPN-CLEC-2 without antibody-dependent cell-mediated cytotoxicity and complement-dependent cytotoxicity is a novel therapeutic strategy for PDPN-positive osteosarcoma.

Roles of Podoplanin in Malignant Progression of Tumor

Podoplanin (PDPN) is a cell-surface mucin-like glycoprotein that plays a critical role in tumor development and normal development of the lung, kidney, and lymphatic vascular systems. PDPN is overexpressed in several tumors and is involved in their malignancy. PDPN induces platelet aggregation through binding to platelet receptor C-type lectin-like receptor 2. Furthermore, PDPN modulates signal transductions that regulate cell proliferation, differentiation, migration, invasion, epithelial-to-mesenchymal transition, and stemness, all of which are crucial for the malignant progression of tumor. In the tumor microenvironment (TME), PDPN expression is upregulated in the tumor stroma, including cancer-associated fibroblasts (CAFs) and immune cells. CAFs play significant roles in the extracellular matrix remodeling and the development of immunosuppressive TME. Additionally, PDPN functions as a co-inhibitory molecule on T cells, indicating its involvement with immune evasion. In this review, we describe the mechanistic basis and diverse roles of PDPN in the malignant progression of tumors and discuss the possibility of the clinical application of PDPN-targeted cancer therapy, including cancer-specific monoclonal antibodies, and chimeric antigen receptor T technologies.

Platelet-mediated tumor metastasis mechanism and the role of cell adhesion molecules

Mounting evidence suggests that platelets play an essential role in cancer metastasis. The interactions between platelets and circulating tumor cells (CTCs) promote cancer metastasis. CTCs induce platelet activation and aggregation, and activated platelets gather and protect CTCs from shear stress and natural killer cells. Finally, platelets stimulate CTC anoikis resistance, epithelial-to-mesenchymal transition, angiogenesis, extravasation, and eventually, metastasis. Cell adhesion molecules (CAMs) have been identified as active players during the interaction of CTCs with platelets, but the specific mechanism underlying the contribution of platelet-associated CAMs to CTC metastasis remains unclear. In this review, we introduce the mechanism of platelet-related tumor metastasis and particularly focus on the role of CAMs in it.

Role of Podoplanin-Positive Cells in Cardiac Fibrosis and Angiogenesis After Ischemia

New insights into the cellular and extra-cellular composition of scar tissue after myocardial infarction (MI) have been identified. Recently, a heterogeneous podoplanin-expressing cell population has been associated with fibrogenic and inflammatory responses and lymphatic vessel growth during scar formation. Podoplanin is a mucin-like transmembrane glycoprotein that plays an important role in heart development, cell motility, tumorigenesis, and metastasis. In the adult mouse heart, podoplanin is expressed only by cardiac lymphatic endothelial cells; after MI, it is acquired with an unexpected heterogeneity by PDGFRα-, PDGFRβ-, and CD34-positive cells. Podoplanin may therefore represent a sign of activation of a cohort of progenitor cells during different phases of post-ischemic myocardial wound repair. Podoplanin binds to C-type lectin-like receptor 2 (CLEC-2) which is exclusively expressed by platelets and a variety of immune cells. CLEC-2 is upregulated in CD11b^high cells, including monocytes and macrophages, following inflammatory stimuli. We recently published that inhibition of the interaction between podoplanin-expressing cells and podoplanin-binding cells using podoplanin-neutralizing antibodies reduces but does not fully suppress inflammation post-MI while improving heart function and scar composition after ischemic injury. These data support an emerging and alternative mechanism of interactome in the heart that, when neutralized, leads to altered inflammatory response and preservation of cardiac function and structure. The overarching objective of this review is to assimilate and discuss the available evidence on the functional role of podoplanin-positive cells on cardiac fibrosis and remodeling. A detailed characterization of cell-to-cell interactions and paracrine signals between podoplanin-expressing cells and the other type of cells that compose the heart tissue is needed to open a new line of investigation extending beyond the known function of these cells. This review attempts to discuss the role and biology of podoplanin-positive cells in the context of cardiac injury, repair, and remodeling.

Molecular Aspects of Pathophysiology of Platelet Receptors

Receptor is a dynamic instrumental surface protein that helps to interact with specific molecules to respond accordingly. Platelet is the smallest in size among the blood components, but it plays many pivotal roles to maintain hemostasis involving its surface receptors. It (platelet) has cell adhesion receptors (e.g., integrins and glycoproteins), leucine-rich repeats receptors (e.g., TLRs, glycoprotein complex, and MMPs), selectins (e.g., CLEC, P-selectin, and CD), tetraspanins (e.g., CD and LAMP), transmembrane receptors (e.g., purinergic—P2Y and P2X1), prostaglandin receptors (e.g., TxA2, PGH2, and PGI2), immunoglobulin superfamily receptors (e.g., FcRγ and FcεR), etc. on its surface. The platelet receptors (e.g., glycoproteins, protease-activated receptors, and GPCRs) during platelet activation are over expressed and their granule contents are secreted (including neurotransmitters, cytokines, and chemokines) into circulation, which are found to be correlated with different physiological conditions. Interestingly, platelets promote metastasis through circulation protecting from cytolysis and endogenous immune surveillance involving several platelets receptors. The updated knowledge about different types of platelet receptors in all probable aspects, including their inter- and intra-signaling mechanisms, are discussed with respect to not only its (platelets) receptor type but also under different pathophysiological conditions.

Neoadjuvant chemoradiation changes podoplanin expression in esophageal cancer patients

BACKGROUND

Locally advanced adenocarcinoma of the esophagus (EAC) and squamous cell carcinoma (ESCC) result in a worse prognosis. Neoadjuvant treatment improves survival, however, only for responders. The transmembrane glycoprotein podoplanin is overexpressed in squamous cell carcinomas, miRNA-363 is associated to its regulation in head and neck cancer.

AIM

To predict therapy response and prognosis markers, and targets for novel therapies would individualize treatments leading to more favourable outcomes.

METHODS

Expression of podoplanin protein has been visualized by immunohistochemistry in surgical specimens of 195 esophageal cancer patients who underwent transthoracic esophagectomy: 90 ESCC and 105 EAC with clinical T2-3, Nx, M0. One hundred and six patients received neoadjuvant chemoradiation. RNA was extracted from paraffin-embedded tissue, and miRNA-363 quantified by real-time TaqMan-real-time-PCR. D2-40 mab staining of > 5% was scored as high podoplanin expression (HPE). We related podoplanin and miRNA-363 expression to histopathologic response after neoadjuvant treatment and clinicopathological characteristics, such as histological tumor type, survival rate or clinical tumor category.

RESULTS

We confirmed expression of membrane-bound podoplanin in 90 ESCC patients. 26% showed HPE of > 5%. In addition, absence in EAC patients (only 2% with HPE) was shown. Lower podoplanin expression has been detected in resection-specimen of 58 ESCC patients after neoadjuvant (RTx/CTx) treatment, only 11% with HPE, compared to 50% HPE of 32 non-pretreated primary surgery patients, P = 0.0001. This difference of podoplanin expression was confirmed comparing pre-treatment biopsies with matching post-treatment surgical specimens, P < 0.001. Podoplanin has been identified as a prognostic marker in 32 patients that underwent primary surgery without neoadjuvant treatment. Low (0-5%) podoplanin expression was associated with better prognosis compared to patients with HPE, P = 0.013. Podoplanin expression has been associated with post-transcriptional regulation by miRNA-363. At a cut-off value of miR-363 < 7, lower miR-363 expression correlated with HPE in surgical tissue specimens of primary surgery patients, P = 0.013. Therefore, ESCC patients with miRNA-363 expression < 7 had a worse prognosis than patients expressing miRNA-363 ≥ 7, P = 0.049.

CONCLUSION

Analysis of the molecular process that leads to decrease in podoplanin expression during neoadjuvant treatment and its regulation may provide novel markers and targets to improve targeted therapy of ESCC.

Association of high PDPN expression with pulmonary metastasis of osteosarcoma and patient prognosis

Podoplanin (PDPN) is an important positive regulator of platelet aggregation and functions as a lymphatic endothelial marker. PDPN has been observed to be expressed in human tumor tissues and various cancer cell lines. In the present study, PDPN expression in patients with primary osteosarcoma was assessed at the mRNA and protein levels, and the associations between PDPN expression and pulmonary metastasis (PM) and prognosis were examined. Reverse transcription-quantitative PCR (RT-qPCR) analysis was used to detect the expression levels of PDPN in primary osteosarcoma tissues and paired normal bone tissues (n=20 pairs). In addition, immunohistochemical analysis of PDPN expression was performed in 168 paraffin-embedded osteosarcoma tissue specimens and 23 matched normal tissues. The RT-qPCR results revealed higher mRNA expression levels of PDPN in patients with PM compared with patients without PM. Further survival analyses identified Enneking stage and PM as two independent prognostic indicators. Finally, univariate analysis revealed that high PDPN protein expression was significantly associated with Enneking stage and PM in patients with osteosarcoma.

Multifaceted role of cancer educated platelets in survival of cancer cells

Platelets, the derivatives of megakaryocytes, pose dynamic biological functions such as homeostasis and wound healing. The mechanisms involved in these processes are utilized by cancerous cells for proliferation and metastasis. Platelets through their activation establish an aggregate termed as Tumor cell induced platelet aggregation (TCIPA) that aids in establishing a niche for the primary tumor at secondary site while recruiting granulocytes and monocytes. The study of these close interactions between the tumor and the platelets can be exploited as biomarkers in liquid biopsy for early cancer detection, thereby increasing the life expectancy of cancer patients.

Role of platelets and platelet receptors in cancer metastasis

The interaction of tumor cells with platelets is a prerequisite for successful hematogenous metastatic dissemination. Upon tumor cell arrival in the blood, tumor cells immediately activate platelets to form a permissive microenvironment. Platelets protect tumor cells from shear forces and assault of NK cells, recruit myeloid cells by secretion of chemokines, and mediate an arrest of the tumor cell platelet embolus at the vascular wall. Subsequently, platelet-derived growth factors confer a mesenchymal-like phenotype to tumor cells and open the capillary endothelium to expedite extravasation in distant organs. Finally, platelet-secreted growth factors stimulate tumor cell proliferation to micrometastatic foci. This review provides a synopsis on the current literature on platelet-mediated effects in cancer metastasis and particularly focuses on platelet adhesion receptors and their role in metastasis. Immunoreceptor tyrosine-based activation motif (ITAM) and hemi ITAM (hemITAM) comprising receptors, especially, glycoprotein VI (GPVI), FcγRIIa, and C-type lectin-like-2 receptor (CLEC-2) are turned in the spotlight since several new mechanisms and contributions to metastasis have been attributed to this family of platelet receptors in the last years.

A safety study of newly generated anti-podoplanin-neutralizing antibody in cynomolgus monkey (Macaca fascicularis)

Hematogenous metastases are enhanced by platelet aggregation induced by tumor cell-platelet interaction. Podoplanin is a key molecule to enhance the platelet aggregation and interacts with C-type lectin-like receptor 2 (CLEC-2) on platelet via PLAG domains. Our previous reports have shown that blocking podoplanin binding to platelets by neutralizing antibody specific to PLAG4 domain strongly reduces hematogenous metastasis. However, podoplanin is expressed in a variety of normal tissues such as lymphatic vessels and the question remains whether treatment of tumors with anti-podoplanin neutralizing antibodies would be toxic. Monkeys are the most suitable species for that purpose. PLAG3 and PLAG4 domains had high homology among various monkey species and human. PLAG domain deleted mutants were indicated that monkey PLAG4 domain played a more crucial role in podoplanin-induced platelet aggregation than did the PLAG3 domain as in human. Moreover, newly established neutralizing antibodies (1F6, 2F7, and 3F4) targeting the monkey PLAG4 domain blocked interaction between monkey podoplanin and CLEC-2. Especially, the 2F7 neutralizing antibody strongly suppressed platelet aggregation and pulmonary metastasis. Furthermore, inhibiting podoplanin function with 2F7 neutralizing antibody exhibited no acute toxicity in cynomolgus monkeys. Our results suggested that targeting podoplanin with specific neutralizing antibodies may be an effective anticancer treatment.

Platelet-activating factor podoplanin: from discovery to drug development

Tumor cell-induced platelet aggregation facilitates hematogenous metastasis by promoting tumor embolization, preventing immunological assaults and shear stress, and the platelet-releasing growth factors support tumor growth and invasion. Podoplanin, also known as Aggrus, is a type I transmembrane mucin-like glycoprotein and is expressed on wide range of tumor cells. Podoplanin has a role in platelet aggregation and metastasis formation through the binding to its platelet receptor, C-type lectin-like receptor 2 (CLEC-2). The podoplanin research was originally started from the cloning of highly metastatic NL-17 subclone from mouse colon 26 cancer cell line and from the establishment of 8F11 monoclonal antibody (mAb) that could neutralize NL-17-induced platelet aggregation and hematogenous metastasis. Later on, podoplanin was identified as the antigen of 8F11 mAb, and its ectopic expression brought to cells the platelet-aggregating abilities and hematogenous metastasis phenotypes. From the 8F11 mAb recognition epitopes, podoplanin is found to contain tandemly repeated, highly conserved motifs, designated platelet aggregation-stimulating (PLAG) domains. Series of analyses using the cells expressing the mutants and the established neutralizing anti-podoplanin mAbs uncovered that both PLAG3 and PLAG4 domains are associated with the CLEC-2 binding. The neutralizing mAbs targeting PLAG3 or PLAG4 could suppress podoplanin-induced platelet aggregation and hematogenous metastasis through inhibiting the podoplanin–CLEC-2 binding. Therefore, these domains are certainly functional in podoplanin-mediated metastasis through its platelet-aggregating activity. This review summarizes the platelet functions in metastasis formation, the role of platelet aggregation-inducing factor podoplanin in pathological and physiological situations, and the possibility to develop podoplanin-targeting drugs in the future.

Podoplanin emerges as a functionally relevant oral cancer biomarker and therapeutic target

Oral cancer has become one of the most aggressive types of cancer, killing 140,000 people worldwide every year. Current treatments for oral cancer include surgery and radiation therapies. These procedures can be very effective; however, they can also drastically decrease the quality of life for survivors. New chemotherapeutic treatments are needed to more effectively combat oral cancer. The transmembrane receptor podoplanin (PDPN) has emerged as a functionally relevant oral cancer biomarker and chemotherapeutic target. PDPN expression promotes tumor cell migration leading to oral cancer invasion and metastasis. Here, we describe the role of PDPN in oral squamous cell carcinoma progression, and how it may be exploited to prevent and treat oral cancer.

Podoplanin is a component of extracellular vesicles that reprograms cell-derived exosomal proteins and modulates lymphatic vessel formation

Podoplanin (PDPN) is a transmembrane glycoprotein that plays crucial roles in embryonic development, the immune response, and malignant progression. Here, we report that cells ectopically or endogenously expressing PDPN release extracellular vesicles (EVs) that contain PDPN mRNA and protein. PDPN incorporates into membrane shed microvesicles (MVs) and endosomal-derived exosomes (EXOs), where it was found to colocalize with the canonical EV marker CD63 by immunoelectron microscopy. We have previously found that expression of PDPN in MDCK cells induces an epithelial-mesenchymal transition (EMT). Proteomic profiling of MDCK-PDPN cells compared to control cells shows that PDPN-induced EMT is associated with upregulation of oncogenic proteins and diminished expression of tumor suppressors. Proteomic analysis of exosomes reveals that MDCK-PDPN EXOs were enriched in protein cargos involved in cell adhesion, cytoskeletal remodeling, signal transduction and, importantly, intracellular trafficking and EV biogenesis. Indeed, expression of PDPN in MDCK cells stimulated both EXO and MV production, while knockdown of endogenous PDPN in human HN5 squamous carcinoma cells reduced EXO production and inhibited tumorigenesis. EXOs released from MDCK-PDPN and control cells both stimulated in vitro angiogenesis, but only EXOs containing PDPN were shown to promote lymphatic vessel formation. This effect was mediated by PDPN on the surface of EXOs, as demonstrated by a neutralizing specific monoclonal antibody. These results contribute to our understanding of PDPN-induced EMT in association to tumor progression, and suggest an important role for PDPN in EV biogenesis and/or release and for PDPN-EXOs in modulating lymphangiogenesis.

Co-expression of podoplanin and fibroblast growth factor 1 predicts poor prognosis in patients with lung squamous cell carcinoma

Podoplanin and fibroblast growth factor (FGF) 1 have been detected more frequently in lung squamous cell carcinoma (SQCC) compared with lung adenocarcinoma. Furthermore, it has been previous demonstrated that FGF1 is located on the edge of tumor nests in certain lung SQCC sections, which resembles the characteristic expression pattern of podoplanin. Podoplanin and FGF1 have roles in lymphangiogenesis and angiogenesis. Based on their consistently specific expression in lung SQCC and similar localization patterns, the present study aimed to investigate whether the expression of podoplanin in tumor cells is correlated with FGF1 expression in lung SQCC and whether their co-expression has clinicopathological significance, particularly for lymphangiogenesis/angiogenesis. The correlation between podoplanin and FGF1 expression in tumor cells of 82 lung SQCC cases was investigated by immunohistochemical staining and the association between the co-expression of podoplanin and FGF1, and clinicopathological factors such as microvessel density (MVD), was examined in these samples. In addition, the prognostic value of co-expression of podoplanin and FGF1 in tumor cells was determined, and the regulation of FGF1 expression and angiogenesis by podoplanin was examined in vitro in a human lung SQCC cell line. Immunohistochemical analysis demonstrated that there was a significant correlation between podoplanin and FGF1 expression in lung SQCC tumor cells (R=0.591; P<0.0001). Co-expression of podoplanin and FGF1 was significantly associated with larger primary tumor size, advanced TNM stage and higher intratumoral MVD. Survival analysis demonstrated that cases with podoplanin and FGF1 double-positive staining had a significantly lower survival rate compared with cases with double-negative staining. In vitro experiments revealed that podoplanin regulated FGF1 expression and affected tube formation of human umbilical vein endothelial cells. Combined, the results demonstrated that podoplanin was co-expressed with FGF1 in lung SQCC and this co-expression was correlated with poor prognosis.

Antiglycopeptide Mouse Monoclonal Antibody LpMab-21 Exerts Antitumor Activity Against Human Podoplanin Through Antibody-Dependent Cellular Cytotoxicity and Complement-Dependent Cytotoxicity

The interaction between podoplanin (PDPN) and C-type lectin-like receptor 2 (CLEC-2) is involved in tumor malignancy. We have established many monoclonal antibodies (mAbs) against human podoplanin using the cancer-specific mAb (CasMab) technology. LpMab-21, one of the mouse antipodoplanin mAbs, is of the IgG_2a subclass, and its minimum epitope was determined to be Thr76-Arg79 of the human podoplanin. Importantly, sialic acid is linked to Thr76; therefore, LpMab-21 is an antiglycopeptide mAb (GpMab). In this study, we investigated whether LpMab-21 shows antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) against human podoplanin-expressing cancer cell lines in vitro and also studied its antitumor activities using a xenograft model. LpMab-21 showed high ADCC and CDC activities against not only podoplanin-expressing Chinese hamster ovary cells but also LN319 glioblastoma cells and PC-10 lung cancer cells, both of which endogenously express podoplanin. Furthermore, LpMab-21 decreased tumor growth in vivo, indicating that LpMab-21 could be useful for antibody therapy against human podoplanin-expressing cancers.

Podoplanin promotes progression of malignant pleural mesothelioma by regulating motility and focus formation

Malignant pleural mesothelioma (MPM) is characterized by dissemination and aggressive growth in the thoracic cavity. Podoplanin (PDPN) is an established diagnostic marker for MPM, but the function of PDPN in MPM is not fully understood. The purpose of this study was to determine the pathogenetic function of PDPN in MPM. Forty‐seven of 52 tumors (90%) from Japanese patients with MPM and 3/6 (50%) MPM cell lines tested positive for PDPN. Knocking down PDPN in PDPN‐high expressing MPM cells resulted in decreased cell motility. In contrast, overexpression of PDPN in PDPN‐low expressing MPM cells enhanced cell motility. PDPN stimulated motility was mediated by activation of the RhoA/ROCK pathway. Moreover, knocking down PDPN with short hairpin (sh) RNA in PDPN‐high expressing MPM cells resulted in decreased development of a thoracic tumor in mice with severe combined immune deficiency (SCID). In sharp contrast, transfection of PDPN in PDPN‐low expressing MPM cells resulted in an increase in the number of Ki‐67‐positive proliferating tumor cells and it promoted progression of a thoracic tumor in SCID mice. Interestingly, PDPN promoted focus formation in vitro, and a low level of E‐cadherin expression and YAP1 activation was observed in PDPN‐high MPM tumors. These findings indicate that PDPN is a diagnostic marker as well as a pathogenetic regulator that promotes MPM progression by increasing cell motility and inducing focus formation. Therefore, PDPN might be a pathogenetic determinant of MPM dissemination and aggressive growth and may thus be an ideal therapeutic target.

LpMab-23: A Cancer-Specific Monoclonal Antibody Against Human Podoplanin

Human podoplanin (hPDPN), the ligand of C-type lectin-like receptor-2, is involved in cancer metastasis. Until now, many monoclonal antibodies (mAbs) have been established against hPDPN. However, it is still difficult to develop a cancer-specific mAb (CasMab) against hPDPN because the protein sequence of hPDPN expressed in cancer cells is the same as that in normal cells. Herein, we report LpMab-23 of the mouse IgG₁ subclass, a novel CasMab against hPDPN. In an immunohistochemical analysis, LpMab-23 reacted with tumor cells of human oral cancer, but did not react with normal cells such as lymphatic endothelial cells (LECs). In contrast, LpMab-17, another anti-hPDPN mAb, reacted with both tumor cells and LECs. Furthermore, flow cytometric analysis revealed that LpMab-23 reacted with hPDPN-expressing cancer cell lines (LN319, RERF-LC-AI/hPDPN, Y-MESO-14/hPDPN, and HSC3/hPDPN) but showed little reaction with normal cells (LECs and HEK-293T), although another anti-hPDPN mAb, LpMab-7, reacted with both hPDPN-expressing cancer cells and normal cells, indicating that LpMab-23 is a CasMab against hPDPN.

Antitumor activity of chLpMab-2, a human-mouse chimeric cancer-specific antihuman podoplanin antibody, via antibody-dependent cellular cytotoxicity

Human podoplanin (hPDPN), a platelet aggregation‐inducing transmembrane glycoprotein, is expressed in different types of tumors, and it binds to C‐type lectin‐like receptor 2 (CLEC‐2). The overexpression of hPDPN is involved in invasion and metastasis. Anti‐hPDPN monoclonal antibodies (mAbs) such as NZ‐1 have shown antitumor and antimetastatic activities by binding to the platelet aggregation‐stimulating (PLAG) domain of hPDPN. Recently, we developed a novel mouse anti‐hPDPN mAb, LpMab‐2, using the cancer‐specific mAb (CasMab) technology. In this study we developed chLpMab‐2, a human–mouse chimeric anti‐hPDPN antibody, derived from LpMab‐2. chLpMab‐2 was produced using fucosyltransferase 8‐knockout (KO) Chinese hamster ovary (CHO)‐S cell lines. By flow cytometry, chLpMab‐2 reacted with hPDPN‐expressing cancer cell lines including glioblastomas, mesotheliomas, and lung cancers. However, it showed low reaction with normal cell lines such as lymphatic endothelial and renal epithelial cells. Moreover, chLpMab‐2 exhibited high antibody‐dependent cellular cytotoxicity (ADCC) against PDPN‐expressing cells, despite its low complement‐dependent cytotoxicity. Furthermore, treatment with chLpMab‐2 abolished tumor growth in xenograft models of CHO/hPDPN, indicating that chLpMab‐2 suppressed tumor development via ADCC. In conclusion, chLpMab‐2 could be useful as a novel antibody‐based therapy against hPDPN‐expressing tumors.

Development and characterization of anti-glycopeptide monoclonal antibodies against human podoplanin, using glycan-deficient cell lines generated by CRISPR/Cas9 and TALEN

Human podoplanin (hPDPN), which binds to C-type lectin-like receptor-2 (CLEC-2), is involved in platelet aggregation and cancer metastasis. The expression of hPDPN in cancer cells or cancer-associated fibroblasts indicates poor prognosis. Human lymphatic endothelial cells, lung-type I alveolar cells, and renal glomerular epithelial cells express hPDPN. Although numerous monoclonal antibodies (mAbs) against hPDPN are available, they recognize peptide epitopes of hPDPN. Here, we generated a novel anti-hPDPN mAb, LpMab-21. To characterize the hPDPN epitope recognized by the LpMab-21, we established glycan-deficient CHO-S and HEK-293T cell lines, using the CRISPR/Cas9 or TALEN. Flow cytometric analysis revealed that the minimum hPDPN epitope, in which sialic acid is linked to Thr76, recognized by LpMab-21 is Thr76-Arg79. LpMab-21 detected hPDPN expression in glioblastoma, oral squamous carcinoma, and seminoma cells as well as in normal lymphatic endothelial cells. However, LpMab-21 did not react with renal glomerular epithelial cells or lung type I alveolar cells, indicating that sialylation of hPDPN Thr76 is cell-type-specific. LpMab-21 combined with other anti-hPDPN antibodies that recognize different epitopes may therefore be useful for determining the physiological function of sialylated hPDPN.

Targeting a novel domain in podoplanin for inhibiting platelet-mediated tumor metastasis

Podoplanin/Aggrus is a sialoglycoprotein expressed in various cancers. We previously identified podoplanin as a key factor in tumor-induced platelet aggregation. Podoplanin-mediated platelet aggregation enhances tumor growth and metastasis by secreting growth factors and by forming tumor emboli in the microvasculature. Thus, precise analysis of the mechanisms of podoplanin-mediated platelet aggregation is critical for developing anti-tumor therapies. Here we report the discovery of a novel platelet aggregation-inducing domain, PLAG4 (81-EDLPT-85). PLAG4 has high homology to the previously reported PLAG3 and contributes to the binding of its platelet receptor CLEC-2. Mutant analyses indicated that PLAG4 exhibits a predominant platelet-aggregating function relative to PLAG3 and that conserved Glu⁸¹/Asp⁸²/Thr⁸⁵ residues in PLAG4 are indispensable for CLEC-2 binding. By establishing anti-PLAG4-neutralizing monoclonal antibodies, we confirmed its role in CLEC-2 binding, platelet aggregation, and tumor emboli formation. Our results suggest the requirement of simultaneous inhibition of PLAG3/4 for complete suppression of podoplanin-mediated tumor growth and metastasis.

The chimeric antibody chLpMab-7 targeting human podoplanin suppresses pulmonary metastasis via ADCC and CDC rather than via its neutralizing activity

Podoplanin (PDPN/Aggrus/T1α) binds to C-type lectin-like receptor-2 (CLEC-2) and induces platelet aggregation. PDPN is associated with malignant progression, tumor metastasis, and poor prognosis in several types of cancer. Although many anti-human PDPN (hPDPN) monoclonal antibodies (mAbs), such as D2-40 and NZ-1, have been established, these epitopes are limited to the platelet aggregation-stimulating (PLAG) domain (amino acids 29-54) of hPDPN. Recently, we developed a novel mouse anti-hPDPN mAb, LpMab-7, which is more sensitive than D2-40 and NZ-1, using the Cancer-specific mAb (CasMab) method. The epitope of LpMab-7 was shown to be entirely different from that of NZ-1, a neutralizing mAb against the PLAG domain according to an inhibition assay and lectin microarray analysis. In the present study, we produced a mouse-human chimeric anti-hPDPN mAb, chLpMab-7. ChLpMab-7 showed high antibody-dependent cellular cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). Furthermore, chLpMab-7 inhibited the growth of hPDPN-expressing tumors in vivo. Although chLpMab-7 recognizes a non-PLAG domain of hPDPN, it suppressed the hematogenous metastasis of hPDPN-expressing tumors. These results indicated that chLpMab-7 suppressed tumor development and hematogenous metastasis in a neutralization-independent manner. In conclusion, hPDPN shows promise as a target in the development of a novel antibody-based therapy.

Identification of a novel platelet antagonist that binds to CLEC-2 and suppresses podoplanin-induced platelet aggregation and cancer metastasis

Podoplanin (PDPN) enhances tumor metastases by eliciting tumor cell-induced platelet aggregation (TCIPA) through activation of platelet C-type lectin-like receptor 2 (CLEC-2). A novel and non-cytotoxic 5-nitrobenzoate compound 2CP was synthesized that specifically inhibited the PDPN/CLEC-2 interaction and TCIPA with no effect on platelet aggregation stimulated by other platelet agonists. 2CP possessed anti-cancer metastatic activity in vivo and augmented the therapeutic efficacy of cisplatin in the experimental animal model without causing a bleeding risk. Analysis of the molecular action of 2CP further revealed that Akt1/PDK1 and PKCμ were two alternative CLEC-2 signaling pathways mediating PDPN-induced platelet activation. 2CP directly bound to CLEC-2 and, by competing with the same binding pocket of PDPN in CLEC-2, inhibited PDPN-mediated platelet activation. This study provides evidence that 2CP is the first defined platelet antagonist with CLEC-2 binding activity. The augmentation in the therapeutic efficacy of cisplatin by 2CP suggests that a combination of a chemotherapeutic agent and a drug with anti-TCIPA activity such as 2CP may prove clinically effective.

Podoplanin associates with adverse postoperative prognosis of patients with clear cell renal cell carcinoma

Podoplanin, a transmembrane sialomucin-like glycoprotein, was recently shown to be involved in tumor progression and metastasis, and its potential role in facilitating platelet-based tumor embolization and promigratory phenotype of cancer cells was also demonstrated. In this study, we assessed the clinical significance of tumoral podoplanin expression in 295 patients with clear cell renal cell carcinoma (ccRCC) through immunohistochemistry on tissue microarrays and analyzing the staining intensity. Univariate analysis suggested an adverse prognostic effect of high tumoral podoplanin expression on patients' overall survival (OS) and recurrence-free survival (RFS) (P < 0.001 for both). In the multivariate analysis, high tumoral podoplanin expression (using staining intensity as either a continuous or dichotomous variable) was still an independent adverse prognostic factor for patient survival (OS, P < 0.001, RFS, P < 0.001 for continuous; OS, P < 0.001, RFS, P = 0.002 for dichotomous). Moreover, stratified analysis identified a higher prognostic power in the intermediate/high risk patient groups. After utilizing those parameters in the validated multivariate analysis, two nomograms were constructed to predict ccRCC patients' OS and RFS (c-index 0.815 and 0.805, respectively), and performed better than existing integrated models (P < 0.001 for all comparisons). In conclusion, high tumoral podoplanin expression could independently predict an adverse clinical outcome for ccRCC patients, and it might be useful in future for clinical decision-making and therapeutic developments.

Anti-podoplanin Monoclonal Antibody LpMab-7 Detects Metastatic Lesions of Osteosarcoma

Osteosarcoma is the most common primary malignant bone tumor and is highly metastatic to the lungs. Therefore, the development of a novel molecular targeting therapy against metastasis of osteosarcoma is necessary. A platelet aggregation-inducing factor, podoplanin/aggrus, is involved in tumor metastasis. Furthermore, podoplanin expression was reported to be involved in the poor prognosis of osteosarcoma patients. However, the association between podoplanin expression and metastasis of osteosarcoma remains unclear because of the lack of highly sensitive anti-podoplanin monoclonal antibodies (MAbs). In this study, we used a novel anti-podoplanin MAb, LpMab-7, which is more sensitive than well-known anti-podoplanin MAbs in immunohistochemistry. Immunohistochemical analysis using LpMab-7 showed that podoplanin expression at primary lesions is observed in 15 out of 16 (93.8%) cases. Furthermore, podoplanin expression at metastatic lesions was higher compared with primary lesions in three out of four (75%) cases with lung metastasis. Because LpMab-7 has high sensitivity against podoplanin, it is expected to be useful for molecular targeting therapy for osteosarcomas.

Characterization of monoclonal antibody LpMab-3 recognizing sialylated glycopeptide of podoplanin

Podoplanin (PDPN/Aggrus/T1α/gp36/OTS-8), a type I transmembrane sialoglycoprotein, is involved in platelet aggregation, cell invasion, and cancer metastasis. Podoplanin expression in cancer cells or cancer-associated fibroblasts was reported to be involved in poor prognosis of several cancers. Furthermore, podoplanin is expressed in lymphatic endothelial cells or lung type I alveolar cells. Although many anti-podoplanin monoclonal antibodies (MAbs), such as NZ-1 and D2–40, have been established, almost all anti-podoplanin MAbs are produced against a platelet aggregation-inducing (PLAG) domain. In this study, we produced and characterized a novel anti-podoplanin monoclonal antibody, LpMab-3, the epitope of which is a sialylated glycopeptide of podoplanin. We identified the minimum epitope of LpMab-3 as Thr76–Glu81 of human podoplanin, which is different from PLAG domain, using Western blot analysis and flow cytometry. Immunohistochemical analysis showed that LpMab-3 is useful for detecting lung type I alveolar cells and lymphatic endothelial cells. Because LpMab-3 detects only sialylated podoplanin, it could be useful for uncovering the physiological function of sialylated human podoplanin.

Interaction between circulating cancer cells and platelets: clinical implication

Metastasis is the main cause of cancer-associated mortality. During this complicated process, some cancer cells, also called circulating tumor cells (CTCs), detach from primary sites, enter bloodstream and extravasate at metastatic site. Thrombocytosis is frequently observed in patients with metastatic cancers suggesting the important role of platelets in metastasis. Therefore this review focuses on how platelets facilitate the generation of CTCs, protect them from various host attacks, such as immune assaults, apoptosis and shear stress, and regulate CTCs intravasation/extravasation. Platelet-derived cytokines and receptors are involved in this cascade. Identification the mechanisms underlie platelet-CTCs interactions could lead to the development of new platelet-targeted therapeutic strategy to reduce metastasis.

Targeted downregulation of platelet CLEC-2 occurs through Syk-independent internalization

Platelet aggregation at sites of vascular injury is not only essential for hemostasis, but may also cause acute ischemic disease states such as myocardial infarction or stroke. The hemi-immunoreceptor tyrosine-based activation motif-containing C-type lectinlike receptor 2 (CLEC-2) mediates powerful platelet activation through a Src- and spleen tyrosine kinase (Syk)-dependent tyrosine phosphorylation cascade. Thereby, CLEC-2 not only contributes to thrombus formation and stabilization but also plays a central role in blood-lymphatic vessel development, tumor metastasis, and prevention of inflammatory bleeding, making it a potential pharmacologic target to modulate these processes. We have previously shown that injection of the anti-CLEC-2 antibody, INU1, results in virtually complete immunodepletion of platelet CLEC-2 in mice, which is, however, preceded by a severe transient thrombocytopenia thereby limiting its potential therapeutic use. The mechanisms underlying this targeted CLEC-2 downregulation have remained elusive. Here, we show that INU1-induced CLEC-2 immunodepletion occurs through Src-family kinase-dependent receptor internalization in vitro and in vivo, presumably followed by intracellular degradation. In mice with platelet-specific Syk deficiency, INU1-induced CLEC-2 internalization/degradation was fully preserved whereas the associated thrombocytopenia was largely prevented. These results show for the first time that CLEC-2 can be downregulated from the platelet surface through internalization in vitro and in vivo and that this can be mechanistically uncoupled from the associated antibody-induced thrombocytopenia.

Mechanistic explanation for platelet contribution to cancer metastasis

Cancer-associated mortality is frequently caused by metastasis, however, our understanding of this process remains incomplete and therapeutic options are limited. Metastasis is a dynamic multi-step process involving intravasation of tumor cells into the host's blood and lymphatic vessels, their dissemination within the circulation, and finally arrest and extravasation in a distant organ where they establish secondary tumors. It is generally conceived that platelets contribute to all steps of hematogenous tumor dissemination. In this review, we provide an overview of the current knowledge of the platelet receptors involved in tumor cell-induced platelet aggregation, an essential immune surveillance escape mechanism of circulating tumor cells. We discuss how platelets prevent immunological attack, contribute to tumor cell extravasation and thereby facilitate colonization of distant organs.

Podoplanin: a novel regulator of tumor invasion and metastasis

Podoplanin, a small mucin-type sialoglycoprotein, was recently shown to be involved in tumor progression. Podoplanin is overexpressed in cancer cells of various human malignancies, and recently, it is also detected in intratumoral stromal cells. We now appreciate that podoplanin plays a dual role in cancer: it can not only suppress tumor growth but also promote tumor progression. Researchers have identified several potential pathways invoked by podoplanin, which participate in the epithelial-to-mesenchymal transition, collective-cell migration, platelet activation and aggregation, and lymphangiogenesis, and thus regulate the tumor invasion and metastasis. Here, we discuss the current experimental and human clinical data on podoplanin to validate the multiple context-dependent functions in different microenvironments and to delineate the diverse regulatory mechanisms.

Suppression of Aggrus/podoplanin-induced platelet aggregation and pulmonary metastasis by a single-chain antibody variable region fragment

Almost all highly metastatic tumor cells possess high platelet aggregating abilities, thereby form large tumor cell-platelet aggregates in the microvasculature. Embolization of tumor cells in the microvasculature is considered to be the first step in metastasis to distant organs. We previously identified the platelet aggregation-inducing factor expressed on the surfaces of highly metastatic tumor cells and named as Aggrus. Aggrus was observed to be identical to the marker protein podoplanin (alternative names, T1α, OTS-8, and others). Aggrus is frequently overexpressed in several types of tumors and enhances platelet aggregation by interacting with the platelet receptor C-type lectin-like receptor 2 (CLEC-2). Here, we generated a novel single-chain antibody variable region fragment (scFv) by linking the variable regions of heavy and light chains of the neutralizing anti-human Aggrus monoclonal antibody MS-1 with a flexible peptide linker. Unfortunately, the generated KM10 scFv failed to suppress Aggrus-induced platelet aggregation in vitro. Therefore, we performed phage display screening and finally obtained a high-affinity scFv, K-11. K-11 scFv was able to suppress Aggrus-induced platelet aggregation in vitro. Moreover, K-11 scFv prevented the formation of pulmonary metastasis in vivo. These results suggest that K-11 scFv may be useful as metastasis inhibitory scFv and is expected to aid in the development of preclinical and clinical examinations of Aggrus-targeted cancer therapies.

A cancer-specific monoclonal antibody recognizes the aberrantly glycosylated podoplanin

Podoplanin (PDPN/Aggrus/T1α), a platelet aggregation-inducing mucin-like sialoglycoprotein, is highly expressed in many cancers and normal tissues. A neutralizing monoclonal antibody (mAb; NZ-1) can block the association between podoplanin and C-type lectin-like receptor-2 (CLEC-2) and inhibit podoplanin-induced cancer metastasis, but NZ-1 reacts with podoplanin-expressing normal cells such as lymphatic endothelial cells. In this study, we established a cancer-specific mAb (CasMab) against human podoplanin. Aberrantly glycosylated podoplanin including keratan sulfate or aberrant sialylation, which was expressed in LN229 glioblastoma cells, was used as an immunogen. The newly established LpMab-2 mAb recognized both an aberrant O-glycosylation and a Thr55-Leu64 peptide from human podoplanin. Because LpMab-2 reacted with podoplanin-expressing cancer cells but not with normal cells, as shown by flow cytometry and immunohistochemistry, it is an anti-podoplanin CasMab that is expected to be useful for molecular targeting therapy against podoplanin-expressing cancers.

Platelets promote osteosarcoma cell growth through activation of the platelet-derived growth factor receptor-Akt signaling axis

The interactions of tumor cells with platelets contribute to the progression of tumor malignancy, and the expression levels of platelet aggregation-inducing factors positively correlate with the metastatic potential of osteosarcoma cells. However, it is unclear how tumor-platelet interaction contributes to the proliferation of osteosarcomas. We report here that osteosarcoma-platelet interactions induce the release of platelet-derived growth factor (PDGF) from platelets, which promotes the proliferation of osteosarcomas. Co-culture of platelets with MG63 or HOS osteosarcoma cells, which could induce platelet aggregation, enhanced the proliferation of each cell line in vitro. Analysis of phospho-antibody arrays revealed that co-culture of MG63 cells with platelets induced the phosphorylation of platelet derived growth factor receptor (PDGFR) and Akt. The addition of supernatants of osteosarcoma-platelet reactants also increased the growth of MG63 and HOS cells as well as the level of phosphorylated-PDGFR and -Akt. Sunitinib or LY294002, but not erlotinib, significantly inhibited the platelet-induced proliferation of osteosarcoma cells, indicating that PDGF released from platelets plays an important role in the proliferation of osteosarcomas by activating the PDGFR and then Akt. Our results suggest that inhibitors that specifically target osteosarcoma-platelet interactions may eradicate osteosarcomas.

Expression of Aggrus/podoplanin in bladder cancer and its role in pulmonary metastasis

Platelet aggregation-inducing factor Aggrus, also known as podoplanin, is associated with tumor malignancy by promoting hematogenous metastasis. Aggrus overexpression has been reported in some tumor tissues including lung, esophagus, head and neck and brain. We here found the frequent upregulation of aggrus mRNA in urinary bladder cancers using cancer tissue panels from various organs. Immunohistochemical analysis confirmed Aggrus protein expression in urinary bladder cancers and suggested a positive correlation between Aggrus expression and metastatic tendency in bladder cancers. Endogenous expression of Aggrus protein on the cell surface was found in the mouse bladder cancer MBT-2 cell line and human bladder cancer SCaBER cell lines. Knockdown of Aggrus expression in MBT-2 cells decreased their ability to induce platelet aggregation and form pulmonary metastasis in syngeneic mouse models. Knockdown of Aggrus expression in the human bladder cancer SCaBER cells also attenuated their ability to induce platelet aggregation and form pulmonary metastasis in mice. Moreover, pulmonary metastasis of SCaBER cells was prevented by prior administration of our generated anti-Aggrus neutralizing monoclonal antibodies by attenuating their retention in lung. These results indicate that Aggrus plays an important role in bladder cancer metastasis. Thus, anti-Aggrus neutralizing antibodies would be useful for the prevention of hematogenous metastasis of Aggrus-positive bladder cancer.

Platelets promote tumor growth and metastasis via direct interaction between Aggrus/podoplanin and CLEC-2

The platelet aggregation-inducing factor Aggrus, also known as podoplanin, is frequently upregulated in several types of tumors and enhances hematogenous metastasis by interacting with and activating the platelet receptor CLEC-2. Thus, Aggrus-CLEC-2 binding could be a therapeutic molecular mechanism for cancer therapy. We generated a new anti-human Aggrus monoclonal antibody, MS-1, that suppressed Aggrus-CLEC-2 binding, Aggrus-induced platelet aggregation, and Aggrus-mediated tumor metastasis. Interestingly, the MS-1 monoclonal antibody 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 xenografted into NOD-SCID mice compromising antibody-dependent cellular cytotoxic and complement-dependent cytotoxic activities. Because Aggrus knockdown suppressed platelet-induced proliferation in vitro and tumor growth of the lung squamous cell carcinoma in vivo, Aggrus may be involved in not only tumor metastasis but also tumor growth by promoting platelet-tumor interaction, platelet activation, and secretion of platelet-derived factors in vivo. Our results indicate that molecular target drugs inhibiting specific platelet-tumor interactions can be developed as antitumor drugs that suppress both metastasis and proliferation of tumors such as lung squamous cell carcinoma.

The physiological and pathophysiological roles of platelet CLEC-2

CLEC-2 is a C-type lectin receptor which is highly expressed on platelets but also found at low levels on different immune cells. CLEC-2 elicits powerful platelet activation upon engagement by its endogenous ligand, the mucin-type glycoprotein podoplanin. Podoplanin is expressed in a variety of tissues, including lymphatic endothelial cells, kidney podocytes, type I lung epithelial cells, lymph node stromal cells and the choroid plexus epithelium. Animal models have shown that the correct separation of the lymphatic and blood vasculatures during embryonic development is dependent on CLEC-2-mediated platelet activation. Additionally, podoplanin-deficient mice show abnormalities in heart, lungs, and lymphoid tissues, whereas absence of CLEC-2 affects brain development. This review summarises the current understanding of the molecular pathways regulating CLEC-2 and podoplanin function and suggests other physiological and pathological processes where this molecular interaction might exert crucial roles.

Platelet CLEC-2 and podoplanin in cancer metastasis

It has long been recognised that the function of platelets in health and disease span far beyond their roles in haemostasis and thrombosis. The observation that tumour cells induce platelet aggregation was followed by extensive experimental evidence linking platelets to cancer progression. Aggregated platelets coat tumour cells during their transit through the bloodstream and mediate adherence to vascular endothelium, protection from shear stresses, evasion from immune molecules, and release of an array of bioactive molecules that facilitate tumour cell extravasation and growth at metastatic sites. The sialyated membrane glycoprotein podoplanin is found on the leading edge of tumour cells and is thought to influence their migratory and invasive properties. Podoplanin elicits powerful platelet aggregation and is the endogenous ligand for the platelet C-type lectin receptor, CLEC-2, which itself regulates podoplanin signalling. Here, the bidirectional relationship between CLEC-2 and podoplanin is described and considered in the context of tumour growth and metastasis.

The impact of Aggrus/podoplanin on platelet aggregation and tumour metastasis

Platelets are small blood components that play indispensable roles in the initial stages of coagulation. In addition to their role in haemostasis, platelets participate in inflammation and tissue regeneration under physiological conditions. Recent studies also revealed the role of platelets under pathological conditions, including the oncogenic process. Platelets enhance tumour growth and metastasis by secreting many growth factors and angiogenic factors or by forming a coat around tumour cells in the blood stream. We previously discovered Aggrus (also known as podoplanin, gp36, gp38P, T1alpha and OTS-8) expressed on tumour cell surfaces as a key molecule for tumour-induced platelet aggregation. Aggrus expression is increased in various malignant tumours such as squamous cell carcinomas, mesotheliomas, glioblastomas and osteosarcomas. Detailed analysis revealed that Aggrus contains three tandem repeats of platelet aggregation-stimulating (PLAG) domains that are associated with its platelet aggregation-inducing ability. PLAG domains of Aggrus are involved in binding to its platelet receptor, C-type lectin-like receptor 2 (CLEC-2). Neutralizing monoclonal antibodies that interfere with Aggrus-CLEC-2 binding attenuate Aggrus-induced platelet aggregation, tumour cell growth and metastasis formation. Aggrus is also expressed in advanced atherosclerotic lesions, suggesting that Aggrus is associated with thrombus formation on disrupted atherosclerotic lesions. These data suggest that Aggrus is a promising cell surface target for developing new therapies against cancer and thrombosis.

Podoplanin expression in advanced atherosclerotic lesions of human aortas

Thrombus formation on disrupted atherosclerotic lesion is a key mechanism of cardiovascular events. Podoplanin (Aggrus), expressed on the surface of several tumor cells, is an endogenous ligand for C-type lectin-like receptor 2 (CLEC-2), and is involved in tumor cell-induced platelet aggregation and its malignant potency. Podoplanin, which is also expressed in lymphatic endothelial cells, facilitates blood/lymphatic vessel separation. However, podoplanin expression in atherosclerotic lesion has not been investigated. To clarify podoplanin expression in atherosclerotic lesion and to assess its importance for the onset of cardiovascular events, we examined podoplanin expression in abdominal aortas obtained from 31 autopsy cases. Immunohistochemical analysis indicated that podoplanin was localized to smooth muscle cells and macrophages. Moreover, podoplanin immunoreactivity was increased in advanced atherosclerotic lesions containing necrotic core, many macrophages and smooth muscle cells, compared with early lesions composed of smooth muscle cells and small numbers of macrophages. Furthermore, Western-blot and real time-PCR analyses showed that podoplanin expression was significantly enhanced in advanced atherosclerotic lesions, compared with early lesions. These results suggest that podoplanin contributes to thrombotic property of advanced stages of atherosclerosis and that it might be a novel molecular target for an anti-thrombus drug.