MOLECULAR MECHANISMS OF ERBB2-DRIVEN THREE-DIMENSIONAL BREAST TUMOR GROWTH.

Abstract

The ability of breast cancer cells to survive anchorage-independently is thought to be a critical pre-requisite for breast carcinoma progression. Normal breast epithelial cells grow in vivo attached to the extracellular matrix (ECM). Detachment from the ECM causes their apoptosis, a phenomenon called as 'anoikis'. Unlike normal breast epithelial cells, breast cancer cells remain viable after detaching from the ECM. This viability is critical for the ability of breast cancer cells to form three-dimensional tumors, invade other tissues and spread through the body. ErbB2/Her2 receptor tyrosine kinase is overproduced by 10-30% of breast cancers and is thought to play a critical role in their progression. ErbB2 blocks death of breast cancer cells detached from the ECM by partially understood mechanisms. In an effort to understand these mechanisms better we found that detachment of non-malignant breast epithelial cells causes upregulation of a transcription factor Irf6 which results in a further upregulation of a pro-apoptotic protein Perp. We further observed that Irf6 and Perp upregulation contributes to anoikis of these cells. We also noticed that ErbB2 blocks anoikis of breast cancer cells by downregulating Perp and Irf6 in a manner dependent on the activity of the mitogen-activated protein kinases (MAPK), which are well known to mediate ErbB2 signalling. Moreover, we observed that treatment with ErbB2-targeted drugs, such as an anti-ErbB2 antibody trastuzumab or an ErbB2/EGFR small molecule inhibitor lapatinib upregulates Perp and Irf6 in ErbB2-positive human breast cancer cells. In addition, ectopic expression of either Perp or Irf6 triggered anoikis of ErbB2-overproducing breast epithelial cells and blocked their anchorage-independent growth. We further found that detached ErbB2-overproducing breast cancer cells require the activity of a protein kinase Mek a major activator of the MAPKs, to block lysosomal degradation of ErbB2. We demonstrated that this effect of Mek on ErbB2 is required for anoikis resistance of breast cancer cells. Thus, we have identified novel molecular mechanisms by which ErbB2 promotes three-dimensional growth of breast cancer cells. These mechanisms are driven by ErbB2-induced activation of MAPK signalling, stabilization of ErbB2 protein in detached breast cancer cells and further ErbB2-dependent downregulation of Perp and Irf6.