Role Of Tumor Microenvironment in Breast Cancer Metastasis

2019-06-10T17:28:44Z (GMT) by Aparna B. Shinde

Metastasis of primary mammary tumors to vital secondary organs is the primary cause of breast cancer-associated death, with no effective treatment. Metastasis is a highly selective process that requires cancer cells to overcome multiple barriers to escape the primary tumor, survive in circulation, and eventually colonize distant secondary organs. One of the important aspects of metastatic cancers is the ability to undergo epithelial-mesenchymal transition (EMT) and the reverse process mesenchymal-epithelial transition (MET) process. Constant interconversion of tumor cells between these phenotypes creates epithelial-mesenchymal heterogeneity (EMH) and interaction between these tumor cell types and the stromal cell compartment is clearly important to metastasis. In healthy tissues, stromal cells maintain the composition and structure of the tissue through the production of extracellular matrix (ECM) proteins and paracrine signaling with epithelial cells. However, little is known about how EMH promotes changes in the ECM to promote breast cancer progression and metastasis. Cancer cells also secret exosomes, nano-size extracellular vesicles, to establish intercellular communication with distant organs in order to induce metastasis. These exosomes contain a plethora of different proteins including extracellular matrix proteins and matrix crosslinking enzymes. Fibronectin, an important ECM protein, plays an active role in tumor progression and is often crosslinked by tissue transglutaminase 2 (TGM2) to promote fibrosis in cancer. Both FN and TGM2 exist in exosomes and are expressed by heterogenous breast tumors. Although FN and TGM2 have been reported to play essential roles in cancer, their involvement in metastasis remains unclear. This work utilizes a variety of approaches to investigate the role of tumor heterogeneity and ECM proteins in promoting breast cancer metastasis. In this dissertation, we establish that mesenchymal cells expressing intracellular FN are held in a stable non-metastatic mesenchymal phenotype and produce cellular fibrils containing functionalized FN capable of supporting the growth of metastatic competent epithelial cells. We introduce a novel 3D culture system consisting of a tessellated scaffold which is capable of recapitulating cellular and matrix phenotypes in vivo. Further, we also demonstrate breast tumor cells secrete exosomes containing TGM2 crosslinked FN fibrils to promote premetastatic niche formation and induction of metastasis. Using genetic approaches, we establish TGM2 is essential and sufficient to drive metastasis. Finally, we demonstrate pharmacological inhibition of TGM2 offers a potential therapeutic strategy to treat metastatic breast cancer. Altogether, our research provides insights into the mechanism through which TGM2 promotes metastatic breast cancer. This work will help in developing new drugs to target TGM2 aimed at reducing breast cancer metastasis.