Triple negative breast cancer (TNBC) is a BC subtype featured by lack of estrogen and progesterone receptor and HER2. It is an aggressive type linked with poor prognosis and chemotherapy-resistance. Cancer malignancy can be caused by epithelial-mesenchymal transition (EMT). It allows cells' phenotype change and is driven by proteins such as Twist, ZEB, SNAIL and SLUG. SWI/SNF chromatin remodeling complexes (CRCs) modify chromatin structure, hence changing expression patterns. Disruption of their stoichiometry can lead to development of many types of cancer including BC. This work is focused on the role of BRM – the catalytic subunit of SWI/SNF CRCs in the TNBC development.
102 paraffin embedded tissue samples were analysed by immunohistochemistry (IHC). In vitro analysis was performed on MDA-MB-231 and MCF7 cancer cell lines using Western Blot, coimmunoprecipitation (Co-IP) and bimolecular fluorescence complementation (BiFC).
Upon IHC analysis of data available for TNBC patients we identified subgroups based on BRM level in cancer samples (high >90% and low <30% expression). In patients with low BRM level the higher number of lymph node metastasis was observed. Moreover, these patients exhibited significantly higher number of brain and liver metastasis but no metastasis to lung. On the contrary patients with high BRM level had metastasis located mostly in lung. Additionally, the group of patients with low BRM level had higher amount of vimentin in cancer cells, providing a likely explanation for the higher number of lymph node metastasis. Moreover, using Co-IP and BiFC the direct interaction between SNAIL and SLUG proteins (EMT drivers) with SWI/SNF CRC subunits was found. The in vitro overexpression of SNAIL in MDA-MB-231 cell line caused BRM protein level downregulation. The inhibition of proteasome in this cell line resulted in the upregulation of BRM protein, suggesting the link between SNAIL, SLUG, BRM and proteasome in EMT.
Malignancy of TNBC might be dependent on BRM - SWI/SNF CRCs and its direct connection with EMT. SNAIL and SLUG, overexpressed during EMT, might influence the stoichiometry of SWI/SNF CRC in BC cells leading to development of aggressive TNBC. BRM protein level may have a predictive value for place of metastasis.
Clinical trial identification
Legal entity responsible for the study
The Maria Skłodowska Curie Memorial Cancer Centre and Institute of Oncology in Warsaw.
Ministry of Science and Higher Education of Poland, grant No. DI2014 022944 and National Science Center of Poland, grant No. UMO-2016/23/N/NZ1/01138 and No. UMO-2014/15/B/NZ5/03532.
All authors have declared no conflicts of interest.