The use of malaria glycosaminoglycan to block cancers—lessons from the human placenta

The placenta is a specialized organ that is required for the establishment of human pregnancy. During placentation, trophoblast cell proliferation, migration and invasion occur in a highly regulated manner (1). Dysregulated trophoblast cell proliferation and invasion lead to pregnancies with adverse outcomes (2-4). By comparison, malignant tumours arise when cell proliferation, migration and invasion occur in an uncontrolled manner (5,6). As such, trophoblast and cancer cell invasion may share similar mechanisms, and a recent study by Salanti et al. (7) has compared the role of a cell surface protein in human trophoblast and cancer cells.

The malaria parasite Plasmodium falciparum, which replicates within infected erythrocytes, expresses the cell surface membrane glucosaminoglycan, VAR2CSA which enables the parasite to attach to host vascular cells via binding to specific receptors on the cells (8). Salanti et al. had demonstrated a number of years ago that VAR2CSA mediates binding of Plasmodium falciparum infected erythrocytes to the placental syncytiotrophoblast which lines the placenta, and is in direct contact with maternal blood (9,10). In placental cells, VAR2CSA binds a distinct chondroitin sulphate (CS) glycosaminoglycan (GAG) chain A (CSA) (11). Importantly, VAR2CSA expressing malaria parasites bind only to a distinct placental specific CS subtype. The placental CS subtype is thought to play roles in trophoblast cell proliferation and invasion, features also shared by cancer cells. This led Salanti et al. (7) to investigate whether human cancer cells also express the placental CS subtype.

Similarly, P. falciparum-infected RBC cannot bind anywhere in vascularised tissue except the placenta.

Salanti et al. (7) investigated the form of CS present in cancer cells. The authors produced recombinant VAR2CSA (rVAR2) protein and showed that it detected a distinct form of placental-CS produced exclusively in the human placenta and not in other normal tissues or cells. VAR2CSA in Plasmodium falciparum infected erythrocytes bound to human cancer cell lines in vitro but not to normal non-malignant primary cells. rVAR2 bound to 95% of patient derived human cancer cell lines of haematologic and mesenchymal origin in a highly specific manner. Human cancer and placental trophoblast cells were demonstrated to express a common and distinct form of CS which can be specifically recognised by recombinant malarial VAR2CSA. Human epithelial derived tumours bound strongly to rVAR2 compared to very minimal binding in adjacent normal tissue from the same patient. The authors analysed 676 malignant tissues from patients with localised stages I–III invasive ductal breast cancer (n=124), stage IIb (n=20) and stage I–III soft tissue sarcomas (n=532). The majority of the cancer tissues demonstrated binding for placental specific CS (placental-CS). Of the sarcoma sub-types investigated, only Ewing sarcomas showed weak or absent binding of rVAR2 in the majority of cases. The authors analysed tissue microarrays containing 47 Ewing sarcoma specimens from adult and pediatric patients and noted weak binding of rVAR2 was due to a general lack of CS chains or a specific lack of placental-CS. Most malignant tumours were strongly positive for placental-CS.

The authors then analysed whether there was an association with the progression and outcome of melanoma and placental-CS (detected by rVAR) in melanoma. There was increased staining for placental-CS (indicating potential binding) in metastatic and recurrent disease, suggesting placental-CS is a marker for disease progression of melanoma. Similarly, with non-small cell lung cancer, high expression correlated with poor relapse free survival. Overall, diverse human malignant tumour types originating from distinct germ layers displayed distinct placental-CS signatures that the authors suggested may predict disease progression and outcome.

Placental, melanoma, some breast cancer and melanoma cells express CSA-modified proteoglycan, CSPG4 (12,13). CSPG4 is associated with placental trophoblast cell proliferation and invasion (12,14). The authors therefore investigated whether rVAR2 binds tumour cells that do or do not express CSPG4, and their data demonstrated that rVAR2 binds cells that do not express CSPG4 suggesting that placental-CS can be displayed by other proteoglycan protein cores. The authors identified at least 17 other proteins, including CD44, that can bind rVAR2. However, they did not investigate whether all of the proteoglycans can be modified with CS in vivo.

The authors investigated inter-tumour diversity in expression of proteoglycans able to display placental-CS, and concluded that rVAR2 may be utilized to broadly target placental-CS chains in human cancers that have different proteoglycan expression profiles. This led the investigators to examine whether VAR2CSA expression in human tumours can be used to deliver cytotoxic therapeutics directly to tumours.

The authors tested whether fluorescently tagged rVAR2 was internalised in cancer cell lines in vitro and in vivo in mice. They found that rVAR2 was located in the xenograft tumours in vivo and in the cell lines in vivo suggesting that rVAR2 can be internalised to deliver cytotoxic compounds to placental-CS expressing cells in vivo and proved this in vivo in cancer xenografts of castration resistant prostate cancer (CRPC) tumours. Importantly, there was little morphological evidence of adverse effects in normal cells. The authors then produced a drug conjugate consisting of toxin linked to rVAR2 and used this in vivo. They found that they could target human tumours in vivo with this approach, with no apparent adverse effects in mice without tumours. As non-pregnant mice injected with the ‘drug conjugate’ had no adverse effects of treatment, it suggests that placental-CS is expressed in low amounts in non-tumour cells and that the binding of the conjugate is specific.

In summary, the data presented in the study by Salanti et al. (7) strongly supports the use of placental-CS as a target generally for rVAR2-based cancer therapeutics. It is interesting to speculate whether it may also be useful in therapies that target the placenta, as abnormalities in placentation result in a wealth of severe pregnancy disorders. It remains, however, to be determined whether any potential therapeutic crosses the placenta which may result in embryotoxic effects. While the in vivo mouse studies were in xenografts, it will be interesting to determine the effects in non-immune deficient mouse models. It will be fascinating to determine whether there are patient sub-groups who may benefit the most. Overall, the use of rVAR2 to specifically target malignant cancers is an exciting possibility that has the potential to block the development of many tumours.

Acknowledgments

Funding: I acknowledge the support of the Victorian Government’s Operational Infrastructure Support Program. ED was supported by a National Health and Medical Research Council of Australia, Senior Research Fellowship (#550905).

Footnote

Provenance and Peer Review: This article was commissioned and reviewed by the Section Editor Chen Qian (Center for Inflammation & Epigenetics, Houston Methodist Hospital Research Institute, Houston, TX, USA).

Conflicts of Interest: The author has completed the ICMJE uniform disclosure form (available at http://dx.doi.org/10.21037/tcr.2016.11.38). The author has no conflicts of interest to declare.

Ethical Statement: The author is accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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