Investigating the effects of mesenchymal stem cells and exercise on breast cancer progression in vitro

Breast cancer is the most prevalent cancer in women worldwide. Whilst overall breast cancer (BC) survival rates have improved, metastatic breast cancers remain fatal.  Metastatic breast cancers are extremely complex diseases, generated by a plethora of multi-faceted processes and interactions that results in the metastatic dissemination of cancer cells. Critical to the pathophysiology of cancer, the tumour microenvironment is made up of tumour cells as well as various non-malignant stromal and immune cells. Critically, mesenchymal stem cells (MSCs) are a population of stromal cells that are found in most tumours. They have been widely implicated in breast cancer development and progression, but their role in the development of this disease remains discordant; research demonstrates that MSCs display both tumour-promoting and tumour-suppressive effects. Exercise oncology is a rapidly emerging field and physical activity has long been shown to reduce the risk of many diseases, including breast cancer. However, evidence supports the notion that physical activity not only reduces the risk of developing breast cancer but can also positively affect the progression of breast cancer. The studies described in this thesis investigated the role MSCs play in BC progression in vitro, as well as the role exercise plays in BC progression in vitro; two factors which are combined in the final chapter of this thesis. 

Chapter II describes the investigation of the effects of altering glucose concentration and the source of foetal bovine serum (FBS) on the growth of two breast cancer cell lines (T47D and MDA-MB-231) and human bone marrow-derived mesenchymal stem cells (hBM-MSCs) to determine successful conditions to enable their co-culture in 3D tumour spheroid models. The results herein demonstrated that FBS source had a significant effect on the growth parameters measured in this study. Three-dimensional tumour spheroids were then generated to create physiologically relevant models of BC that could be used to study the influence of MSCs on BC in vitro.  Results demonstrated that co-culture of hBM-MSCs and MDA-MB-231 cells dramatically reduced the invasiveness of both cell lines (P = 0.001) in vitro.

Chapter III contains a systematic review and meta-analysis that identified and summarised existing literature on the effects of human mesenchymal stem cells (hMSCs) from different sources on the migration of breast cancer cells (BCCs) in vitro. This study determined the direction of this relationship according to existing research and identified directions for future research. It revealed that combining studies using hMSCs from different sources resulted in an overall increase in migration of MDA-MB-231 and MCF-7 cell lines in vitro (P < 0.00001), demonstrating a pro-migratory effect of hMSCs on breast cancer cells. Importantly, the individual effects of hMSCs from different sources were investigated, and this study showed that hMSCs derived from adipose tissue, umbilical cord and bone-marrow each increased the migration of both MDA-MB-231 and MCF-7 cells to different levels (SMD = 3.93, P < .00001, SMD = 3.01, P < .00001, and SMD = 3.92, P = 0.06, respectively). 

Chapter IV presents a second systematic review and meta-analysis that summarised the overall effect of exercise on BC progression in vitro using different models, according to existing literature. It demonstrated that the identified exercise interventions on breast cancer cells (BCCs) in vitro significantly reduced breast cancer cell viability, proliferation, and tumourigenic potential (SMD = 1.76, P = 0.004, SMD = 2.85, P = 0.003, and SMD = 3.15, P = 0.0008, respectively). A clear direction of effect was found with exercise on breast cancer cell migration in vitro, however this effect was not significant (SMD = 0.62, P = 0.317). To our knowledge, this was the first meta-analysis and systematic review to investigate and summarise the existing literature on exercise and breast cancer progression in vitro, highlighting the different models used and identifying priority areas for future research focus. 

Chapter V investigated the effects of serum extracted from long-term endurance runners and sprinters of different ages on the invasion of breast cancer and mesenchymal stem cells in vitro. Using 3D models, the findings presented in this chapter suggested a possible protective effect of endurance running against the pro-invasive effects of ageing observed in this study. Critically, we demonstrated no significant difference in the invasion of MDA-MB-231 cells cultured in serum from older, long-term endurance runners (OE) compared to young, long-term endurance runners (YE; YE vs OE; F _(1,3) = 5.178, P=0.107). Invasion was significantly greater in cells cultured with serum from older, inactive participants (OC) compared to young, inactive participants (YC; YC vs OC; F _(1,3) = 37.135, P=0.009). 

The results of this thesis collectively demonstrate a role for MSCs in BC progression in vitro, whilst supporting the need for further research to fully understand the potential of these cells in BC progression. It also indicates an important role of exercise in the development of breast cancer progression in vitro. Interestingly, we established serum from trained human participants (Chapters IV and V), as well as exercise models (Chapter IV) can reduce breast cancer cell invasion in vitro, as well as hMSC invasion in vitro. Overall, the findings presented in this thesis demonstrate the promising potential of using exercise training as a therapeutic target for abating breast cancer progression, not only via its direct role on breast cancer cell invasion, but by the role it plays on reducing hMSCs invasion too.