Characterizing ErbB2-induced mammary tumourigenesis

Abstract

Approximately 30% of human breast cancers demonstrate overexpression of the receptor tyrosine kinase ErbB2/HER2/Neu, with these cases correlating with recurrence and poor prognosis. While therapy targeting ErbB2 has met with some success, particularly in early-stage breast cancers, transformation and progression towards a later-stage metastatic phenotype is likely sustained by aberrant signaling from additional players, for instance that downstream of integrins. In fact, treatment with integrin-blocking antibodies and ß1-integrin ablation leads to reversion of the malignant phenotype of human breast cells in three-dimensional culture and in vivo. Moreover, ErbB2 has recently been found to interact with the ß4-integrin subunit to promote tumour formation and progression, as deletion of the ß4-integrin signaling domain led to suppression of mammary tumour onset and invasive growth, coupled with decreases in ErbB2-dependent signaling. ErbB2 may interact with integrin subunits by direct binding or via the intracellular kinases Src and focal adhesion kinase (FAK), both known to be activated downstream of ErbB2 and integrins and having well-established roles in cell adhesion, migration, and invasion. Using an inducible model of ErbB2 activation, we have demonstrated that controlled ErbB2 activation in human mammary epithelial cells leads to phosphorylation of Src Tyr215 and FAK Tyr861, consistent with a recently published clinical study examining phosphorylated forms of Src and FAK in ErbB2-positive human breast tumour samples. We have also confirmed that ErbB2 activation increases the capacity of cells for survival: Normally, MCF10A human mammary epithelial cells cultured in three-dimensional, laminin-rich extracellular matrix gel form mammary acini-like spheroids with hollow lumen surrounded by a single layer of polarized epithelial cells. However, ErbB2 activation prevents luminal clearance and induces luminal filling in acini formed in three-dimensional culture, and leads to activation of Akt, a known survival signal. Taken together these data indicate a potential role for ErbB2 at the apex of cell survival signaling via Src, FAK, and Akt, contributing to luminal cell survival in three-dimensional culture. We have thus confirmed Src, FAK, and Akt as potential players in early onset of breast cancer, and targeting these signaling players concurrently with ErbB2 may prove effective, especially in early stage breast cancers.