Analysis of Desmosomal Protein Desmocollin 2 and Plakophilin 1 Expression in Primary Breast Cancer and Their Roll in Metastases Development

Abstract

This study investigated the role of the desmosomal protein DSC2 in the malignant progression of primary breast cancer cells and the formation of distant metastases. Firstly, the expression of DSC2 in primary breast cancer samples was quantified at a protein level using microarray and western blot analyses. These results, when analysed in conjunction with patient data, highlighted the prognostic value of DSC2, showing significantly higher DSC2 expression in the comparatively aggressive molecular subtypes HER2 and TNBC, when compared to luminal breast cancers. Furthermore, an increase in cerebral and lung metastases, as well as a reduction in disease-free and overall survival, was observed in cases with high DSC2 expression.

The direct influence of DSC2 expression on primary breast cancer cell behaviour and survival was subsequently investigated at a cellular level in both in vitro and in vivo experiments. Firstly, DSC2 overexpression and DSC2 knock down experiments were performed using the TNBC cell line MDA-MB-231 and the brain-seeking subline MDAMB-231-BR. In vitro experiments on cell lines with DSC2 down regulation showed a significant decrease in cellular aggregation. Conversely, it could be shown that increased ectopic DSC2 expression resulted in increased cellular aggregation and improved chemoresistance in 3D structures. This influence could not be reproduced in 2D monolayered structures, further highlighting the importance of 3D cell aggregation for tumour cell drug resistance. The ensuing in vivo experiments using a brain dissemination xenograft mouse model underlined these findings, showing that DSC2 knock down in brain-seeking TNBC cells caused a significant reduction in both circulating tumour cells and tumour cell clusters, and, correspondingly, the formation of fewer and smaller brain metastases.

Our results suggest that DSC2 overexpression improves the metastatic capabilities of breast cancer cells by increasing tumour cell aggregation, thereby (1) enabling the formation of circulating tumour cell clusters - which are more resistant and prone to metastasise than single CTCs - and (2) hindering drug diffusion and protecting centrally located cells within both the primary tumour and in CTC clusters against drug exposition. Furthermore, we hypothesise that the increased DSC2 expression observed in the TNBC and HER2 subtypes is, at least in part, responsible for their comparatively higher metastatic potential and overall poorer prognosis.