The role of cancer-associated fibroblasts in triple-negative breast cancer

Breast cancer is the leading cause of cancer death in women. Triple-negative breast cancer (TNBC) is a highly aggressive subtype of breast cancer which is associated with a poor clinical outcome in comparison to other breast cancer subtypes. In TNBC a high stroma content is associated with a poor prognosis. Cancer-associated fibroblasts (CAFs) are key components of the tumour stroma and are responsible for secreting and remodelling the extracellular matrix (ECM). The role of soluble signalling by CAFs in cancer progression is well established. In contrast, the means by which they contribute to cancer progression by altering the mechanical properties of the tumour microenvironment remains to be clarified. Therefore, the aim of this work was to identify differences in TNBC-associated fibroblasts as compared to normal breast fibroblasts and determine their function in TNBC progression. In order to achieve this, we have characterised normal breast fibroblasts and TNBC patient-derived CAFs using immunofluorescence, live cell imaging and traction force microscopy. We have then established 2D and 3D in vitro co-culture models to study the interactions between fibroblasts and HCC1143 cells using live cell imaging and immunofluorescence. Finally, we have analysed the effect of CAF-derived ECM, particularly hyaluronan, on HCC1143 by generating fibroblast-derived ECM. We observed that in 2D and 3D, the TNBC CAFs showed an increased capacity to induce the nuclear localisation of the oncogenic transcriptional co-regulator YAP in HCC1143, as compared to normal breast fibroblasts. The CAFs also exhibited increased cellular contractile forces, and these forces were required for CAFs to compress clusters of TNBC cells, and to induce the nuclear localisation of YAP. Finally, we show that CAF-derived hyaluronan is an alternative mechanism by which CAFs can promote the nuclear localisation of YAP in HCC1143. Taken together, these observations support the hypothesis that the contractile forces and hyaluronan production of CAFs promote TNBC via the nuclear translocation of YAP. This highlights the importance of the physical properties of the tumour microenvironment in and progression of TNBC and suggests that CAFs can be targets for future drug-development against TNBC.