Cell-cell interactions during mammary gland development and breast cancer metastasis

Abstract

Abstract

Breast cancer arises in mammary epithelial tissue and causes mortality by metastasizing to various organs. While we have come to understand the various pathways regulating mammary gland development, tumor initiation and distant metastasis, our knowledge of how distinct subpopulations communicate with each other to facilitate such processes is rather limited in comparison. Epithelial tissue integrity of the mammary gland is established and maintained by numerous homo- and hetero-typic interactions among epithelial cells and between their stromal microenvironment. Furthering our understanding of how epithelial cells, both normal and malignant, regulate and rely on such cell-cell interactions may provide potential therapeutic targets to disrupt many critical processes in tumor initiation and distant metastasis. In the present dissertation, I investigated how mammary gland stem cells interact with each other and with their stromal microenvironment, and how metastatic breast cancer cells interact with their target stroma to facilitate their colonization of distant organs.

In the first study, we studied Perp, a tetra-span protein expressed on the cell surface of mammary gland stem cells (MaSCs), to elucidate its role in MaSC activity. We found that Perp expression is highly enriched in MaSCs and could potentially be used as a marker for isolating MaSCs. The suppression of Perp, however, did not affect MaSC activity in an in vivo transplantation setting, suggesting that Perp is not critical for mammary gland development, though it has been shown to be in skin development through a desmosome-dependent mechanism. Interestingly, Perp expression was elevated in tumors of spontaneous breast cancer models (MMTV-Wnt and -PyMT) and cancer cells with high Perp expression had higher tumor initiation potential. These results suggest that although both the skin and mammary glands are composed of epithelial cells, the mechanisms by which these cells establish and maintain desmosomes may be tissue-specific.

In the second study, we investigated the same molecule in the context of metastatic colonization to investigate whether Perp itself facilitates intercellular interactions between disseminating tumor cells and the stromal microenvironment provided by distant organs. Perp expression was enriched in epithelial-like metastatic breast cancer cell lines and the suppression of Perp significantly inhibited lung metastasis through a desmosome-independent mechanism. We also found that Perp is dynamically regulated during epithelial-to-mesenchymal transition (EMT) and mesenchymal-to-epithelial transition (MET), but Perp suppression alone did not affect these programs. Our results indicate that Perp is critical in successful distant metastasis of epithelial-like cancer cells and may be an effective target in blocking metastatic colonization in breast cancer patients.

In the third study, we searched for a vascular component to the MaSC microenvironment to modulate MaSC and/or tumor-initiating cell (TIC) activity through a putative (peri)vascular niche. We developed novel imaging methods to visualize MaSCs in their native microenvironment during mammary gland reconstitution, which we could enhance by inducing robust angiogenesis in the mammary fat pad. In addition, we have begun to distinguish specific subpopulations of endothelial cells that are modulated during pregnancy and tumorigenesis. Overall, our results suggest the existence of a (peri)vascular microenvironment for MaSCs. Further characterization of this niche during tumorigenesis may lead to the development of novel therapeutic strategies in breast cancer.