Radiation Effects on Mesenchymal Stem Cells in a Model of Fibrosarcoma

Background: Radiotherapy is a mainstay of sarcoma treatment, but can cause fibrosis, characterized by production of extra-cellular matrix proteins such as collagen by cancer-associated fibroblasts (CAFs) in the cancer stroma and surrounding normal tissues, which makes tumours more aggressive and resistant to further treatment. Mesenchymal stem cells (MSCs) can be recruited to irradiated tumours and can differentiate into CAF-like cells but the mechanisms of these effects remain unclear.
Aim: Determine the mechanisms of radiation effects on the recruitment of MSCs to tumours and their differentiation into CAF-like cells.
Methods: Mouse MSCs were irradiated directly or exposed to irradiated mouse fibrosarcoma cells (FS120 or FS188) or their conditioned media (CM) and/or irradiated endothelial cells. Expression of CAF/fibrosis markers (collagen, fibronectin, PDGF receptor-β and α-SMA) by MSCs was assessed 3-4 days’ post radiation. Trans-well migration assays were also performed. Candidate proteins were investigated for their ability to stimulate migration and maturation of MSCs to CAF-like cells and for the ability of radiation to stimulate their production in fibrosarcoma cells. Irradiated FS120 and FS188 solid tumours were analysed for collagen, using Masson’s trichrome staining, and α-SMA using IHC and immunofluorescence.
Results: Direct irradiation of MSCs had limited effects on their expression of CAF markers and migration, but exposure to irradiated tumour cells or CM and/or endothelial cells increased these effects. Candidate proteins TGF-β1, MCP-1, and SDF-1α all significantly enhanced the migration of MSCs, and radiation increased their production in fibrosarcoma cells. FS188 cells produced more MCP-1 than FS120 cells and FS188 and FS120 cells and their CM increased MSC migration in a radiation-dependent manner. Migration could be at least partially blocked by an MCP-1 blocking antibody. MSC expression of the MCP-1 receptor, CCR2, was increased after exposure to irradiated FS188 cells or their CM. In vivo, irradiated fibrosarcomas showed significant increases in collagen content, with more collagen in FS188 than in FS120 tumours.
Conclusion: Results support the notion that MSCs play an important role in radiation-induced CAF activity and fibrosis in sarcoma. MCP-1 was identified as an important mediator of these effects. Moreover, endothelial cells were shown to play an important role in the recruitment of MSCs in response to radiation. In vitro results identifying FS188 cells as being more pro-fibrotic than FS120 cells were consistent with in vivo results. Further work to understand these processes should help to develop novel treatment strategies for combination with radiotherapy.