The Role of MicroRNAs in the Fibroblasts of Triple Negative and Basal-like Breast Cancer

Breast cancer is the second most common cancer worldwide and the fifth biggest cause of cancer-related deaths. Breast cancer subtype has a huge influence on tumour behaviour and prognosis. The basal-like subtype, which is typically – but not always – triple negative for receptor expression status limiting treatment options, represents one of the main subtypes with relatively poor prognosis. Breast cancer is a disease that comprises not only transformed luminal epithelial cells but also modified stroma, as a consequence of interactions between the cancer cells and the stroma. One of the most numerous cellular components of cancer stroma are fibroblasts. MicroRNA (miRNA) are short, single-stranded RNA molecules that modify protein expression through regulation of mRNA translation and/or mRNA stability. I have explored the expression and roles of miRNAs in the stromal fibroblast compartment of triple negative breast cancer, in particular of miR-21. I have used a variety of clinical samples and cell culture models to assess the potential clinical relevance of these levels and their potential functional effects. I have found that miR-21 expression is significantly increased in stromal fibroblasts of triple negative breast cancer as compared to matched normal breast fibroblasts. However, miR-21 levels in the cancer associated fibroblasts did not significantly correlate with clinical outcomes. In tissue culture models, miR-21 was significantly up-regulated in breast fibroblasts by contact co-culture with epithelial cancer cells, suggesting that this interaction may be the cause of this increase in tumours. Using immortalised breast fibroblasts, I showed alteration of miR-21 levels did not significantly influence fibroblast migration or invasion. Nor did levels of miR-21 in these fibroblasts impact on the behaviour of breast epithelial cancer cells in a variety of co-culture settings. However, over-expression of miR-21 in primary breast cancer associated fibroblasts was associated with a small, but significant, increase in the migration of fibroblasts, and a small but significant decrease in the invasion of co- cultured tumour cells. Overall, I conclude that miR-21 does not have a striking and consistent cancer- related role in the fibroblast compartment of triple negative breast cancers, however further work is required to assess potential roles in primary settings.