Does Direct and Indirect Exposure to Ionising Radiation Influence the Metastatic Potential of Breast Cancer Cells

Abstract

Ionising radiation (IR) is commonly used for cancer therapy; however, its potential influence on the metastatic ability of surviving cancer cells exposed directly or indirectly to IR remains controversial. Metastasis is a multistep process by which the cancer cells dissociate from the initial site, invade, travel through the blood stream or lymphatic system, and colonise distant sites. This complex process has been reported to require cancer cells to undergo epithelial-mesenchymal transition (EMT) by which the cancer cells convert from an adhesive, epithelial to motile, mesenchymal form and is also associated with changes in glycosylation of cell surface proteins, which may be functionally involved in metastasis. In this paper, we give an overview of metastatic mechanisms and of the fundamentals of cancer-associated glycosylation changes. While not attempting a comprehensive review of this wide and fast moving field, we highlight some of the accumulating evidence from in vitro and in vivo models for increased metastatic potential in cancer cells that survive IR, focusing on angiogenesis, cancer cell motility, invasion, and EMT and glycosylation. We also explore the indirect effects in cells exposed to exosomes released from irradiated cells. The results of such studies need to be interpreted with caution and there remains limited evidence that radiotherapy enhances the metastatic capacity of cancers in a clinical setting and undoubtedly has a very positive clinical benefit. However, there is potential that this therapeutic benefit may ultimately be enhanced through a better understanding of the direct and indirect effects of IR on cancer cell behaviour.

Keywords: EMT; epithelial mesenchymal transition; exosomes; glycosylation; invasion; ionising radiation; metastasis.

Figure 1

The metastatic cascade: (1) hypoxia in the growing tumour stimulates new blood vessel formation (angiogenesis) to supply the tumour with oxygen and nutrients and remove waste products. (2) Hypoxia is also one of the signals that induces epithelial-mesenchymal transition (EMT) whereby rounded, adherent epithelial cancer cells break cell–cell and cell–basement membrane contacts and change to a motile, mesenchymal phenotype. (3) Cancer cells produce matrix degrading enzymes and migrate through the stroma towards blood vessels or lymphatics (4a) cancer cells enter blood vessels or lymphatics. (4b) They are transported through blood/lymph in blood vessels often forming protective aggregates with leukocytes and platelets (5a) at a distant site, cancer cells may become mechanically trapped in small vessels and/or may actively adhere to endothelium, (5b) they then extravasate and invade through basement membrane surrounding vessels and through local stroma using similar molecular mechanisms to those employed in step 3. (6) For successful establishment of a tumour at the new site, cancer cells must once again stimulate angiogenesis, as in step 1, and adapt to and flourish in a new environment.

Figure 2

Examples of cancer-associated glycosylation changes (a), (i) N-linked glycans are based on a trimannosyl core linked to an asparagine (Asn) residue on the polypeptide. This can be extended by sequential addition of monosaccharides to form a huge variety of complex bi-, tri- tetra-, and penta-antennary structures. (ii) In cancer, N-glycans tend to be larger and more branched than in normal cells. One commonly reported finding is an increase in beta 1,6 branched glycans that can be detected by the binding of a lectin, PHA-L. The region required for PHA-L binding lies within the boxed area. (b) The commonest form of O-linked glycosylation begins with the attachment of a single N-acetylgalactosamine residue to a serine (Ser) or threonine (Thr) residue of the polypeptide. This yields the Tn antigen. Tn can be extended in several ways. The addition of a beta1,3 linked galactose, catalysed by beta 1,3 galactosyltransferase (β1,3 GalT), yields the Thomsen-Friedenreich antigen. Alternatively, the addition of an alpha 2,6 linked sialic acid (NeuAc), catalysed by alpha 2,6 sialyltransferase (α2,6 ST), results in sialyl Tn. Key to symbols: shaded square = N-acetylglucosamine, GlcNAc; open circle = mannose, Man; shaded circle = galactose, Gal; open square = N-acetylgalactosamine, GalNAc; shaded diamond = sialic acid, NeuAc.

Figure 3

Exosome biogenesis, secretion, and cell communication via exosomes.