Tumour Extracellular Vesicles in Cancer Metastasis & Thrombosis

Abstract

Tumour cells release lipid particles known as extracellular vesicles (EV) that contribute to cancer metastasis, to the immune response, and to thrombosis. There are many types of identified EV, and most live tumour cells spontaneously shed EV. For example, microvesicles (MV; 0.1-1 μm), are released by the direct budding of the membrane whereas a smaller type (50-200 nm), called exosomes, are shed via the endosomal pathway. Intriguingly, if tumour cells are exposed to chemotherapy or radiotherapy, one of the consequences of cell death is the abundant cell membrane release of another type of EV; the apoptotic vesicles (ApoV).

This research has first compared the three types of EV using a multitude of structural and molecular parameters. This led to their characterisation and creation of a distinguishable identity for each of them. This research then aimed to investigate and compare their functionality. We have addressed this by comparing the three EV types as prophylactic tumour vaccines and by their ability to clot the blood. The latter was investigated because the second leading cause of death in cancer patients undergoing chemotherapy is thrombosis (blood clot). Strikingly among the three EV, ApoV demonstrated superiority in both systems; superior protection against cancer and significantly rapid blood coagulation was induced by ApoV, as compared to the other EV types. Further investigation showed that ApoV possessed dramatically higher levels of procoagulant activity compared to exosomes, MV, or even equivalent protein fractions from intact living or dying tumour cells.

In addition, we have identified that the superior procoagulant activity of ApoV was dependent on phosphatidylserine and the extrinsic coagulation factor tissue factor. Surprisingly, the intrinsic coagulation pathway (Factors VIII and IX) was dispensable for the procoagulant activity of ApoV. We have also identified the endogenous expression of coagulation Factor V on ApoV and determined it to be functional but non-essential for the optimal procoagulant activity of ApoV.

To investigate the contribution of melanoma EV to lymph node metastasis, we examined primary tumour growth and lymph node metastasis in mice lacking the EV receptor (CD169; sialoadhesin). Although CD169-/- mice displayed a lower level of lymph node metastasis, as compared to C57BL/6 mice, this failed to reach statistical significance. Similarly, primary tumour progression and the response to immunisation with ApoV in CD169-/- mice were lower and higher, respectively, compared to wild type mice. However, the results suggest that the CD169 receptor has no major role in tumour growth and metastasis.

Together, these results emphasise the complexities of interaction between the host and tumour vesicles, and suggest an important role for ApoV released during chemotherapy in immunity, as well as in cancer / chemotherapy-related VTE. These findings illustrate how ApoV have the ability to “aid or aggravate” outcomes for cancer patients.