Transformation of Mammary Epithelial Cells by Regulators of Cell Polarity.

During the progression to breast cancer, organized growth-arrested epithelial cells become hyperproliferative and lose their polarized glandular organization. Many studies have shown that proliferation is a driving force behind cancer formation, however little is known about the mechanisms that disrupt polarized acinar structures. To determine how breast cancer progresses we need to understand not only the mechanisms of cell proliferation, but also the mechanisms that regulate epithelial cell polarity and how these pathways may become altered during cell transformation. We found that the polarity regulator, Par6, is overexpressed in precancerous breast lesions and in advanced carcinoma. When Par6 was ectopically expressed in the non-transformed human breast epithelial cell line, MCF-10A, we found that Par6 promoted hyperproliferation. We determined that Par6 promoted proliferation through activation of the MAPK pathway and that this proliferative effect depended on Par6/aPKC/Cdc42 interaction. MCF-10A cells, when plated on basement membrane, form acinar-like structures that recapitulate the resting acinar structures within human breast tissue. Acinar structures derived from Par6 overexpressing cells are hyperproliferative and resemble the precancerous breast lesions observed in vivo. The fact that overexpression of Par6 was found in both early and late stage lesions, suggests that Par6 may impart an advantage that promotes cell transformation. Activation of the oncogene ErbB2 in acinar structures also resembles early breast lesions, termed ductal carcinoma in situ (DCIS). Both DCIS and ErbB2 transformed acini are characterized by disruption of acinar organization and hyperproliferation. We found that activated ErbB2 recruits Par6/aPKC and that this interaction is required for ErbB2-induced disruption of cell polarity, but not re-initiation of cell proliferation. This finding suggests that ErbB2 uses at least two distinct pathways to promote transformation; one to initiate proliferation and the other to disrupt epithelial cell polarity. We also found that downregulation of the cytoarchitectural regulator RhoA promoted invasion of ErbB2 transformed acinar structures. This suggests that further deregulation of the cell organization by disrupting cellular cytoarchitecture can promote invasion of ErbB2 transformed acini. Collectively, the studies of this dissertation found that inference with the normal expression and function of the regulators of cell polarity can promote proliferation and transformation of mammary epithelial cells.