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

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-derived lysophosphatidic acid mediated LPAR1 activation as a therapeutic target for osteosarcoma metastasis

Osteosarcoma is the most common primary malignant bone cancer, with high rates of pulmonary metastasis. Osteosarcoma patients with pulmonary metastasis have worse prognosis than those with localized disease, leading to dramatically reduced survival rates. Therefore, understanding the biological characteristics of metastatic osteosarcoma and the molecular mechanisms of invasion and metastasis of osteosarcoma cells will lead to the development of innovative therapeutic intervention for advanced osteosarcoma. Here, we identified that osteosarcoma cells commonly exhibit high platelet activation-inducing characteristics, and molecules released from activated platelets promote the invasiveness of osteosarcoma cells. Given that heat-denatured platelet releasate maintained the ability to promote osteosarcoma invasion, we focused on heat-tolerant molecules, such as lipid mediators in the platelet releasate. Osteosarcoma-induced platelet activation leads to abundant lysophosphatidic acid (LPA) release. Exposure to LPA or platelet releasate induced morphological changes and increased invasiveness of osteosarcoma cells. By analyzing publicly available transcriptome datasets and our in-house osteosarcoma patient-derived xenograft tumors, we found that LPA receptor 1 (LPAR1) is notably upregulated in osteosarcoma. LPAR1 gene KO in osteosarcoma cells abolished the platelet-mediated osteosarcoma invasion in vitro and the formation of early pulmonary metastatic foci in experimental pulmonary metastasis models. Of note, the pharmacological inhibition of LPAR1 by the orally available LPAR1 antagonist, ONO-7300243, prevented pulmonary metastasis of osteosarcoma in the mouse models. These results indicate that the LPA-LPAR1 axis is essential for the osteosarcoma invasion and metastasis, and targeting LPAR1 would be a promising therapeutic intervention for advanced osteosarcoma.

Ferret Podoplanin Is Detected by PMab-241 in Immunohistochemistry

Podoplanin (PDPN) plays an important role in the development of many normal tissues and is expressed in various cancers. We have previously developed multiple monoclonal antibodies (mAbs) against PDPNs from a variety of animal species and characterized each of these PDPNs using the anti-PDPN mAbs. In this study, we evaluated whether these anti-PDPN mAbs possess cross-reactivity with ferret PDPN (ferPDPN) using flow cytometry. Comprehensive analysis using 17 differing anti-PDPN mAbs available for immunohistochemistry use, demonstrated that the anti-bear PDPN mAb (clone PMab-241) strongly cross-reacts with ferPDPN-overexpressed Chinese hamster ovary-K1 (CHO/ferPDPN) cells. Immunohistochemistry analysis demonstrated intense PMab-241 staining within Bowman's capsules and glomeruli of the ferret kidney, and lymphatic endothelial cells of the ferret lung. These results demonstrate that PMab-241 is suitable for the detection of PDPN in ferret tissues.

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.

Novel knock-in mouse model for the evaluation of the therapeutic efficacy and toxicity of human podoplanin-targeting agents

Podoplanin is a key molecule for enhancing tumor‐induced platelet aggregation. Podoplanin interacts with CLEC‐2 on platelets via PLatelet Aggregation–inducing domains (PLAGs). Among our generated antibodies, those targeting the fourth PLAG domain (PLAG4) strongly suppress podoplanin–CLEC‐2 binding and podoplanin‐expressing tumor growth and metastasis. We previously performed a single‐dose toxicity study of PLAG4‐targeting anti‐podoplanin–neutralizing antibodies and found no acute toxicity in cynomolgus monkeys. To confirm the therapeutic efficacy and toxicity of podoplanin‐targeting antibodies, a syngeneic mouse model that enables repeated dose toxicity tests is needed. Replacement of mouse PLAG1‐PLAG4 domains with human homologous domains drastically decreased the platelet‐aggregating activity. Therefore, we searched the critical domain of the platelet‐aggregating activity in mouse podoplanin and found that the mouse PLAG4 domain played a critical role in platelet aggregation, similar to the human PLAG4 domain. Human/mouse chimeric podoplanin, in which a limited region containing mouse PLAG4 was replaced with human homologous region, exhibited a similar platelet‐aggregating activity to wild‐type mouse podoplanin. Thus, we generated knock‐in mice with human/mouse chimeric podoplanin expression (Pdpn^KI/KI mice). Our previously established PLAG4‐targeting antibodies could suppress human/mouse chimeric podoplanin–mediated platelet aggregation and tumor growth in Pdpn^KI/KI mice. Repeated treatment of Pdpn^KI/KI mice with antibody‐dependent cell‐mediated cytotoxicity activity–possessing PG4D2 antibody did not result in toxicity or changes in hematological and biochemical parameters. Our results suggest that anti‐podoplanin–neutralizing antibodies could be used safely as novel anti‐tumor agents. Our generated Pdpn^KI/KI mice are useful for investigating the efficacy and toxicity of human podoplanin–targeting drugs.

D2-40/podoplanin expression in cancer stroma by immunohistochemical staining is associated with poor prognosis in bladder cancer patients after radical cystectomy

OBJECTIVES

We assessed whether D2-40/podoplanin (PDPN) could be used to identify bladder cancer patients with a higher probability of benefiting from cisplatin-based combination chemotherapy.

PATIENTS AND METHODS

We investigated PDPN expression by immunohistochemical analysis of cystectomy specimens from 96 bladder cancer patients who had undergone radical cystectomy without neoadjuvant or adjuvant cisplatin-based combination chemotherapy until recurrence. We classified the cases into 2 groups according to the achievement of 2-year recurrence-free survival (RFS) and evaluated whether PDPN expression was associated with patient prognosis. We also classified the 96 cases into 3 groups according to the possible need for perioperative chemotherapy based on the response to chemotherapy after recurrence as "unnecessary" (achieving 2-year RFS), "responder" (recurring within 2 years and responding to chemotherapy after recurrence), and "non-responder" (not responding chemotherapy following recurrence) and compared PDPN expression between these groups.

RESULTS

Among 13 cases diagnosed with clinically <T2 disease, all 13 cases achieved 2-year RFS. Only 6 (8.7%) and 9 cases (13.0%) were classified as "responders" and "non-responders," respectively, out of 69 cases with clinical T2 or higher disease who received cisplatin-based chemotherapy after recurrence. We identified PDPN expression in cancer stroma cells were morphologically fibroblasts, but not in cancer cells. Cases with <1% PDPN staining positivity ratios showed significantly better prognosis compared to that in cases with ≧1% (log-rank tests). Moreover, the positive predictive value and specificity of a <1% PDPN positive staining ratio were 89.5% and 86.7%, respectively, for "unnecessary" perioperative chemotherapy in the 69 cases.

CONCLUSION

Our results confirmed that cisplatin-based perioperative chemotherapy is unnecessary in many cases requiring radical cystectomy. To our knowledge, this is the first report of PDPN expression in bladder cancer stroma cells that are morphologically fibroblasts. Although PDPN expression might be associated with poor prognosis after surgery, we did not find that it helped to predict chemotherapy response. Whether PDPN expression by fibroblasts in high-risk bladder cancer cases is a marker for prognosis after radical cystectomy and response to perioperative cisplatin-based chemotherapy to avoid unnecessary chemotherapy warrants further investigation.

Urothelial Carcinoma of the Bladder Induces Endothelial Cell Activation and Hypercoagulation

Cancer-related venous thromboembolisms (VTE) are associated with metastasis and reduced survival in patients with urothelial cancer of the bladder. Although previous reports suggest the contribution of tissue factor and podoplanin, the mechanistic linkage between VTE and bladder cancer cell-derived molecules is unknown. Therefore, we compared distinct procoagulant pathways in four different cell lines. In vitro findings were further confirmed by microfluidic experiments mimicking the pathophysiology of tumor blood vessels and in tissue samples of patients with bladder cancer by transcriptome analysis and immunohistology. In vitro and microfluidic experiments identified bladder cancer-derived VEGF-A as highly procoagulant because it promoted the release of von Willebrand factor (VWF) from endothelial cells and thus platelet aggregation. In tissue sections from patients with bladder cancer, we found that VWF-mediated blood vessel occlusions were associated with a poor outcome. Transcriptome data further indicate that elevated expression levels of enzymes modulating VEGF-A availability were significantly connected to a decreased survival in patients with bladder cancer. In comparison with previously postulated molecular players, we identified tumor cell-derived VEGF-A and endothelial VWF as procoagulant mediators in bladder cancer. Therapeutic strategies that prevent the VEGF-A-mediated release of VWF may reduce tumor-associated hypercoagulation and metastasis in patients with bladder cancer. IMPLICATIONS: We identified the VEGF-A-mediated release of VWF from endothelial cells to be associated with bladder cancer progression.

Impacts of Cancer on Platelet Production, Activation and Education and Mechanisms of Cancer-Associated Thrombosis

Platelets are small anucleate cells that are traditionally described as the major effectors of hemostasis and thrombosis. However, increasing evidence indicates that platelets play several roles in the progression of malignancies and in cancer-associated thrombosis. A notable cross-communication exists between platelets and cancer cells. On one hand, cancer can “educate” platelets, influencing their RNA profiles, the numbers of circulating platelets and their activation states. On the other hand, tumor-educated platelets contain a plethora of active biomolecules, including platelet-specific and circulating ingested biomolecules, that are released upon platelet activation and participate in the progression of malignancy. The numerous mechanisms by which the primary tumor induces the production, activation and aggregation of platelets (also known as tumor cell induced platelet aggregation, or TCIPA) are directly related to the pro-thrombotic state of cancer patients. Moreover, the activation of platelets is critical for tumor growth and successful metastatic outbreak. The development or use of existing drugs targeting the activation of platelets, adhesive proteins responsible for cancer cell-platelet interactions and platelet agonists should be used to reduce cancer-associated thrombosis and tumor progression.

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.

Roles of the CLEC-2-podoplanin interaction in tumor progression

Podoplanin is a type-I transmembrane sialomucin-like glycoprotein expressed on the surface of several kinds of tumor cells. The podoplanin receptor is a platelet activation receptor known as C-type lectin-like receptor 2 (CLEC-2), which has been identified as a receptor for the platelet-activating snake venom protein rhodocytin. CLEC-2 is highly expressed in platelets and megakaryocytes and expressed at lower levels in liver Kupffer cells. Podoplanin is expressed in certain types of tumor cells, including squamous cell carcinomas, seminomas, and brain tumors. Podoplanin is also expressed in a wide range of normal cells, including fibroblastic reticular cells in lymph nodes, kidney podocytes, and lymphatic endothelial cells, but not vascular endothelial cells. Metastasis of podoplanin-positive lung tumors injected from the tail vein is greatly inhibited in CLEC-2-depleted mice or in anti-podoplanin antibody-treated mice. These findings suggest that the CLEC-2-podoplanin interaction facilitates hematogenous tumor metastasis. Platelets may increase the survival of tumor cells by covering tumor cells and physically protecting them from shear stress or immune cells in the bloodstream. Alternatively, platelets may stimulate the epithelial-mesenchymal transition of tumor cells to facilitate their extravasation from blood vessels. Cell proliferation is stimulated in podoplanin-expressing tumor cells by the coculture with platelets, but the effects of the CLEC-2-podoplanin interaction on tumor growth in vivo are not yet resolved. It is possible that the CLEC-2-podoplanin interaction facilitates tumor-related thrombosis, subsequent inflammation, inflammation-induced cachexia, and reduced survival. Considering these findings, anti-podoplanin and anti-CLEC-2 drugs are promising therapies for the prevention of tumor metastasis, progression, and tumor-related symptoms, which may result in longer survival in cancer patients. There are advantages and disadvantages of anti-podoplanin vs. anti-CLEC-2 therapy. Side effects in podoplanin-expressing normal tissues due to treatment with anti-podoplanin and temporal thrombocytopenia due to treatment with anti-CLEC2 are potential problems, although solutions to these problems have been reported.

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 enhances lung cancer cell growth in vivo by inducing platelet aggregation

Podoplanin/Aggrus, known as a platelet aggregation-inducing factor, is frequently overexpressed in lung squamous cell carcinomas (LSCC) and glioblastomas among other tumours, and its expression has been reported to be correlated with poor prognosis. However, the contribution of podoplanin to malignant progression has been elusive. Here we demonstrate that in podoplanin-positive LSCC cells, their growth was abrogated by podoplanin knockout in vivo but not in vitro. Conversely, ectopic expression of podoplanin promoted cell growth in vivo and facilitated intratumoral platelet activation. Consistently, LSCC cells evoked podoplanin-mediated platelet aggregation (PMPA), and the releasates from platelets during PMPA promoted the growth of LSCC cells in vitro. Phospho-receptor-tyrosine-kinase array analysis revealed that epidermal growth factor receptor (EGFR) phosphorylation of LSCC cells was responsible for the growth promotion induced by platelet releasates. Treatment with an antiplatelet agent or podoplanin-neutralizing antibody depressed the growth of an LSCC tumour xenograft via suppression of EGFR phosphorylation. These results suggested that podoplanin in LSCC enhanced cell growth by inducing PMPA in vivo and contributed to malignant progression.

ChLpMab-23: Cancer-Specific Human-Mouse Chimeric Anti-Podoplanin Antibody Exhibits Antitumor Activity via Antibody-Dependent Cellular Cytotoxicity

Podoplanin is expressed in many cancers, including oral cancers and brain tumors. The interaction between podoplanin and its receptor C-type lectin-like receptor 2 (CLEC-2) has been reported to be involved in cancer metastasis and tumor malignancy. We previously established many monoclonal antibodies (mAbs) against human podoplanin using the cancer-specific mAb (CasMab) technology. LpMab-23 (IgG₁, kappa), one of the mouse anti-podoplanin mAbs, was shown to be a CasMab. However, we have not shown the usefulness of LpMab-23 for antibody therapy against podoplanin-expressing cancers. In this study, we first determined the minimum epitope of LpMab-23 and revealed that Gly54-Leu64 peptide, especially Gly54, Thr55, Ser56, Glu57, Asp58, Arg59, Tyr60, and Leu64 of podoplanin, is a critical epitope of LpMab-23. We further produced human-mouse chimeric LpMab-23 (chLpMab-23) and investigated whether chLpMab-23 exerts antibody-dependent cellular cytotoxicity (ADCC) and antitumor activity. In flow cytometry, chLpMab-23 showed high sensitivity against a podoplanin-expressing glioblastoma cell line, LN319, and an oral cancer cell line, HSC-2. chLpMab-23 also showed ADCC activity against podoplanin-expressing CHO cells (CHO/podoplanin). In xenograft models with HSC-2 and CHO/podoplanin, chLpMab-23 exerts antitumor activity using human natural killer cells, indicating that chLpMab-23 could be useful for antibody therapy against podoplanin-expressing cancers.

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.

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.

The multifaceted role of podoplanin expression in hepatocellular carcinoma

The role of podoplanin in hepatocellular carcinoma (HCC) is not clear yet. The aim of our study was to evaluate the expression of podoplanin in HCC and to determine its role in hepatocarcinogenesis. We performed immunohistochemistry with monoclonal D2-40 antibody, on paraffin-embedded tissue sections of 72 patients diagnosed with HCC. Lymphatic vessels density (LVD) was increased in patients who had vascular invasion at the time of diagnosis (P=0.018) and in those with associated cirrhosis (P=0.006). Tumor cells showing podoplanin expression were correlated with histological grade (P=0.040). Podoplanin-expressing cancer associated fibroblasts (CAFs) were correlated with both LVD (P=0.019) and tumor cells (P=0.015). Our results sustain the dual role of podoplanin in HCC by its involvement in both HCC tumorigenesis, lymphatic neovascularization and tumor invasion invasiveness. A possible crosstalk between epithelial and stromal tumor cells in HCC tumor microenvironment may be mediated by podoplanin, but this hypothesis needs further studies to elucidate this interrelation.

A critical role of platelet TGF-β release in podoplanin-mediated tumour invasion and metastasis

The tumour microenvironment is critical for various characteristics of tumour malignancies. Platelets, as part of the tumour microenvironment, are associated with metastasis formation via increasing the rate of tumour embolus formation in microvasculature. However, the mechanisms underlying the ability of tumour cells to acquire invasiveness and extravasate into target organs at the site of embolization remain unclear. In this study, we reported that platelet aggregation-inducing factor podoplanin expressed on tumour cell surfaces were found to not only promote the formation of tumour-platelet aggregates via interaction with platelets, but also induced the epithelial-mesenchymal transition (EMT) of tumour cells by enhancing transforming growth factor-β (TGF-β) release from platelets. In vitro and in vivo analyses revealed that podoplanin-mediated EMT resulted in increased invasiveness and extravasation of tumour cells. Treatment of mice with a TGF-β-neutralizing antibody statistically suppressed podoplanin-mediated distant metastasis in vivo, suggesting that podoplanin promoted haematogenous metastasis in part by releasing TGF-β from platelets that was essential for EMT of tumour cells. Therefore, our findings suggested that blocking the TGF-β signalling pathway might be a promising strategy for suppressing podoplanin-mediated haematogenous metastasis in vivo.

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.

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.

Platelets in cancer metastasis: To help the "villain" to do evil

Cancer progress is accompanied by platelet activation and thrombotic complications. Platelets are a dangerous alliance of cancer cells, and are a close engager in multiple processes of cancer metastasis. Platelet adhesion to cancer cells forms a protective cloak that helps cancer cells to escape immune surveillance and natural killer cell-mediated cytolysis. Platelets facilitate tethering and arrest of disseminated cancer cells in the vasculature, enhance invasive potentials and thus extravasation of cancer cells. Moreover, platelets recruit monocytes and granulocytes to the sites of cancer cell arrest, and collaborate with them to establish a pro-metastatic microenvironment and metastatic niches. Platelets also secret a number of growth factors to stimulate cancer cell proliferation, release various angiogenic regulators to regulate tumor angiogenesis and subsequently promote cancer growth and progress. Albeit platelets are helping the "villain" cancer to do evil, the close engagements of platelets in cancer metastasis and progress can be used as the intervention targets for new anti-cancer therapeutic developments. Platelet-targeted anti-cancer strategy may bring in novel anti-cancer treatments that can synergize the therapeutic effects of chemotherapies and surgical treatments of cancer.

miR-15b regulates cisplatin resistance and metastasis by targeting PEBP4 in human lung adenocarcinoma cells

MicroRNAs (miRNAs) have been identified as important posttranscriptional regulators involved in various biological and pathological processes of cells, but their association with tumor chemoresistance has not been fully understood. We detected miR-15b expression in two lung adenocarcinoma cell lines, A549 and A549/CDDP, and then investigated the effects of miR-15b on the metastasis and the chemosensitivity of cancer cells, using both gain- and loss-of-function studies. The correlation between miR-15b level and chemoresistance was further investigated in clinical lung adenocarcinoma specimens. miR-15b was significantly upregulated in cisplatin-resistant lung adenocarcinoma A549/CDDP cells compared with parental A549 cells. miR-15b regulates epithelial-mesenchymal transition (EMT) and cisplatin resistance in vitro and modulates response of lung adenocarcinoma cells to cisplatin in vivo. Further studies identified phosphatidylethanolamine-binding protein 4 (PEBP4) as a direct and functional target of miR-15b. Small-interfering RNA-mediated PEBP4 knockdown revealed similar effects as that of ectopic miR-15b expression, whereas overexpression of PEBP4 attenuated the function of miR-15b in lung adenocarcinoma cells. Increased miR-15b expression was also detected in tumor tissues sampled from lung adenocarcinoma patients treated with cisplatin-based chemotherapy and was proved to be correlated with low expression of PEBP4, decreased sensitivity to cisplatin and poor prognosis. Our results suggest that upregulation of miR-15b could suppress PEBP4 expression and in turn contribute to chemoresistance of lung adenocarcinoma cells to cisplatin.

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.

The role of podoplanin in the biology of differentiated thyroid cancers

Podoplanin (PDPN), a mucin-type transmembrane glycoprotein specific to the lymphatic system is expressed in a variety of human cancers, and is regarded as a factor promoting tumor progression. The purpose of this study was to elucidate the molecular role of PDPN in the biology of thyroid cancer cells. PDPN expression was evaluated in primary thyroid carcinomas and thyroid carcinoma cell lines by RT-qPCR, Western blotting, IF and IHC. To examine the role of podoplanin in determining a cell's malignant potential (cellular migration, invasion, proliferation, adhesion, motility, apoptosis), a thyroid cancer cell line with silenced PDPN expression was used. We observed that PDPN was solely expressed in the cancer cells of 40% of papillary thyroid carcinoma (PTC) tissues. Moreover, PDPN mRNA and protein were highly expressed in PTC-derived TPC1 and BcPAP cell lines but were not detected in follicular thyroid cancer derived cell lines. PDPN knock-down significantly decreased cellular invasion, and modestly reduced cell migration, while proliferation and adhesion were not affected. Our results demonstrate that PDPN mediates the invasive properties of cells derived from papillary thyroid carcinomas, suggesting that podoplanin might promote PTC progression.