ErbB2 overexpression is found in approximately 25% of invasive breast cancers (IBC) and is strongly associated with poor patient survival (Slamon et al., 1989
). Overexpression of ErbB2 has been demonstrated to promote breast cancer invasion and metastasis (Yu and Hung, 2000
). However, ErbB2 is overexpressed in 50-60% of ductal carcinomas in situ (DCIS) in general and 60-70% of high-grade DCIS (Nofech-Mozes et al., 2005
). DCIS, a precursor of IBC, consists of clonal proliferation of malignant cells within the lumen of mammary ducts, with no evidence of invasion through the basement membrane into the surrounding stroma (Burstein et al., 2004
). The apparent paradox that ErbB2, the well-known metastasis-promoting oncoprotein, is more frequently overexpressed in non-invasive DCIS than in IBC has been puzzling.
This stimulated debate about whether ErbB2 overexpression alone is sufficient to promote progression from non-invasive DCIS to IBC. The limited number of studies that have used patient follow-up data on invasive recurrence of primary DCIS have yielded ambiguous results. Some studies indicated that ErbB2-overexpressing DCIS had an increased risk of invasive recurrence (Provenzano et al., 2003
), while others suggested the opposite (Perin et al., 1996
; Ringberg et al., 2001
). Interestingly, studies using three-dimensional (3D) culture of mammary epithelial cells (MECs) showed that ErbB2 activation in preformed, growth-arrested, mammary acini led to disruption of the well-organized acinar structure that shared several properties with DCIS in vivo
, including uncontrolled cell proliferation, luminal filling, and no invasion (Muthuswamy et al., 2001
). Moreover, transgenic mice expressing neu
(rat homologue of human ErbB2) under its endogenous promoter developed DCIS-like mammary tumors after a long latency with rare metastasis (Andrechek et al., 2003
These indicate that ErbB2 activation/overexpression may be involved in DCIS formation and that ErbB2 overexpression alone is not sufficient to drive invasion/metastasis. It was suggested that greater ErbB2 activity or additional genetic/epigenetic events (“second hits”) are needed for MECs to gain invasive capability and for a subset of ErbB2-overexpressing DCIS to transition into IBC (Muthuswamy et al., 2001
). However, it remained unclear as to what the “second hits” are.
The transition from a normal cell to a malignant cell is a multistep process, and at least six hallmark alterations in cell physiology collectively drive the malignant progression (Hanahan and Weinberg, 2000
). 14-3-3 is a family of evolutionally conserved proteins that can bind to many target proteins involved in each of these cancer hallmark alterations (Tzivion et al., 2006
; Wilker and Yaffe, 2004
). It is conceivable that deregulation of 14-3-3 may contribute to cancer development. Generally, 14-3-3 proteins are divided into two subgroups: 14-3-3σ is a tumor suppressor, whereas the other 14-3-3 isoforms may have oncogenic functions. Increased 14-3-3ζ expression was observed in several tumor types and in the early stages of breast diseases such as DCIS (Danes et al., 2008
). This raised the interesting possibility that 14-3-3ζ overexpression might contribute to DCIS progression to IBC.
The epithelial-mesenchymal transition (EMT) is a process during which epithelial cells convert to a mesenchymal cell phenotype after losing cell polarity, disassembling cell-cell adhesion machinery, and subsequently acquiring cell motility (Guarino, 2007
). EMT promotes tumor invasion and metastasis by facilitating escape of tumor cells from the original rigid constraints of the surrounding tissue architecture (Guarino, 2007
). The EMT-mediated increase in invasion/metastasis is largely contributed by loss of E-cadherin function, because E-cadherin is essential for the maintenance of adherent junctions between neighboring cells, thus confers physical integrity on epithelial cells (Beavon, 2000
; Guarino, 2007
). E-cadherin loss has been shown to increase cell invasion in multiple in vitro
models, and has been correlated with increased metastasis in several epithelial tumor types (Strathdee, 2002
). Therefore, E-cadherin is considered a suppressor of tumor invasion.
Given that ErbB2 overexpression alone in DCIS is not sufficient for progression to IBC, we explored whether 14-3-3ζ overexpression in DCIS may serve as a “second hit” that cooperates with ErbB2 to drive a subset of ErbB2-overexpressing DCIS progression into IBC.