14-3-3 zeta overexpression in early stage breast disease and tumor progression

Recent progress in diagnostic tools allows many breast cancers to be detected at an early pre-invasive stage. Thus, a better understanding of the molecular basis of early breast cancer progression is essential. 14-3-3 is a family of highly conserved and ubiquitously expressed proteins that are expressed in all eukaryotic organisms. In mammals there are seven isoforms, which bind to phosphor-serine/threonine residues regulating essential cellular processes such as signal transduction, cell cycle progression, and apoptosis. Our laboratory has discovered that a particular 14-3-3 family member, Zeta, is overexpressed in over 40% of breast tumor tissues. Furthermore, I examined the stage of breast disease in which 14-3-3zeta overexpression occurs and found that increased expression of 14-3-3zeta begins at the stage of atypical ductal hyperplasia, a very early stage of breast disease that confers increased risk for progress toward breast cancer. To determine whether 14-3-3zeta overexpression is a decisive early event in breast cancer, I overexpressed 14-3-3zeta in MCF10A cells, a non-transformed mammary epithelial cell (MEC) line and examined its impact on acini formation in a three dimensional (3D) culture model which simulates a basic unit of structure in the mammary gland. I discovered that 14-3-3zeta overexpression severely disrupted the acini architecture resulting in the disruption of polarity and luminal filling. Both are critical morphological events in the pre-neoplastic breast disease. This thesis focuses on the molecular mechanism of luminal filling. Proper lumen formation is a result of anoikis, a specific type apoptosis of cells not attached to the basement membrane. I found that 14-3-3zeta overexpression conferred a resistance to anoikis. Additionally, 14-3-3zeta overexpression in MCF10A cells and in MECs from 14-3-3zeta transgenic mice reduced expression of p53, which is known to mediate anoikis. Mechanistically, 14-3-3zeta induced hyperactivation of the PI3K/Akt pathway which led to phosphorylation and translocation of the MDM2 to the nucleus resulting in increased p53 degradation. Ectopic expression of p53 restored luminal apoptosis in 14-3-3zeta overexpressing MCF10A acini in 3D cultures. These data suggest that 14-3-3zeta overexpression is a critical event in early breast disease and down-regulation of p53 is one of the mechanisms by which 14-3-3zeta alters MEC acini structure and may increase the risk of progression to breast cancer.