Abstract 1510: Cell-cell contacts regulate HER2 cellular localization and the transitions between migration and proliferation in breast cancer cells

Introduction: Epithelial-to-mesenchymal transition (EMT) is widely recognized as the primary mechanism regulating invasion and metastatic dissemination in carcinomas. In contrast, the mechanisms triggering single disseminated cancer cells (DCC) to reacquire their epithelial identity at ectopic sites, initiating metastasis formation, remain unclear. Since we had previously noted that HER2 expression levels and cellular density impact on migration and proliferation, we sought to determine whether they are involved in molecular mechanisms regulating transitions between proliferation and migration.
Methods: To control HER2 expression levels we generated inducible cell models. Cell density was assessed by the number of cell-cell contacts or by the number of cells per area. Pathway analyses were performed by gene expression analysis, protein interactions by western blotting and mass spectrometry. Migration and proliferation were analyzed by functional assays including in vitro live cell imaging.
Results: Migrating epithelial cells undergo reduced protein trafficking, resulting in a perinuclear deposition of HER2. This was accompanied by increased calcium ion regulation via phospholipase-C in the endoplasmic reticulum (ER). Upon the establishment of cell-cell contacts, particularly between cells expressing desmosome cadherin proteins, HER2 is redirected to the cell surface through direct physical binding with desmoplakin, facilitating proliferation signals. Then, PIP2 to PIP3 is converted through the activation of the PI3K pathway, concomitant with the upregulation of PDPK1. The PH domain of PDPK1 exhibits a high binding affinity to PIP2 molecules. Consequently, membrane localization of HER2 significantly diminishes PIP2 availability for the PLC pathway, redirecting cells towards proliferation. Patient sample analysis confirmed the loss of membrane-bound HER2 in solitary circulating tumor cells (CTCs), which was paralleled by increased plakoglobin, elevated ZEB1, and reduced KI67 protein levels.
Conclusion: Cell-cell contacts determine the cellular localization and function of growth factor receptors like HER2. Single disseminating cancer cells therefore need to establish such contacts at metastatic sites to activate proliferation programs. Our data are in line with findings that circulating tumor cell clusters are associated with higher metastatic rates, the impact of specific metastatic niches for outgrowth and have implementation for the targeting of single DCC.