Genomic Instability and Invasion in Breast Cancer

Abstract

Breast cancer is the most common cancer diagnosed in women and the second leading cause of cancer-related death in women, with 1 in 8 women developing some form of the disease in their lifetime. It is estimated that in the United States, over 330,000 new cases of breast cancer will be diagnosed in 2021, with more than 80% of these cases being the invasive subtype. Currently available therapeutics typically target the highly proliferative nature of cancer and do not target or consider the phenotypic and genotypic changes such epithelial-mesenchymal transition (EMT), aneuploidy, and the dysregulated tumor microenvironment (TME), which although poorly understood, all play an important role in the progression of cancer. Therefore, this dissertation explores how mechanical and chemical signals in the cellular microenvironment regulate cellular processes such as EMT, cytokinesis, invasion, and intravasation.

We found that EMT signaling, which increases invasiveness, causes multinucleation by upregulating the midbody proteins septin-6 and kinesin-like protein (Kif23 or Mklp1) in mammary epithelial cells cultured on stiff microenvironments that have mechanical properties similar to those found in breast tumors, but not on soft microenvironments reminiscent of the normal mammary gland. Following these findings, we investigated the role of cell-matrix adhesion through β1-integrin and integrin-linked kinase (ILK) in regulating multinucleation downstream of EMT signaling. We found that expression of ILK and β1-integrin are required for EMT signaling to induce multinucleation in cells cultured on stiff substrata. These observations suggest signaling through focal adhesions causes failure of cytokinesis in cells actively undergoing EMT.

In addition to understanding the mechanisms regulating EMT-induced multinucleation, we also aimed to understand the cellular processes involved in invasion and intravasation. We found that the activity of matrix metalloproteinases (MMPs) is required for invasion and escape to occur, while proliferation is required for neither. In support of this, we found that the cells that initiate invasions are preferentially quiescent, while cell proliferation is associated with the extension of invasions. These data suggest that matrix degradation and proliferation are coupled during the invasion and escape of human breast cancer cells and highlight the critical role of matrix proteolysis in governing tumor phenotype. Altogether, our findings highlight the critical role that the TME plays in directing cancer progression.