HER2 Cellular Localization, Cell-Cell Contacts, and Cell Density Regulate Cancer Cell Plasticity in HER2+ Breast Cancer

Phenotypic plasticity in HER2+ breast cancer presents a significant challenge in comprehending and treating early-stage metastasis. We conducted an extensive study employing in vitro models and patient samples to investigate the relationships among cell density, HER2 expression, HER2 cellular localization, and their impact on the plasticity program of HER2+ breast cancer cells. Our research reveals a previously unexplored facet of HER2+ breast cancer biology. In conditions of low cellular density, the endoplasmic reticulum (ER) undergoes a remarkable transformation, shifting its function from protein trafficking to serving as a regulator of calcium flux via the PLC pathway. This alteration in ER function leads to the accumulation of HER2 protein within the perinuclear ER compartment. Furthermore, we discovered that in situations of low cellular density or loss of cell-cell contact among epithelial cells, an epithelial-to-mesenchymal transition (EMT) program is activated, resulting in a significant upregulation of desmosome junctions. Desmosome junctions persist as the longest-lasting epithelial cell junctions on the cell surface and represent the primary means for solitary epithelial cells to establish initial contact. Notably, our research revealed a physical binding between HER2 and Desmoplakin (DSP), facilitating HER2 membrane localization upon early cell-cell contact that initiates the proliferation of cancer cells. The switching from migration to proliferation hinges on a competition between the HER2 membrane pathway and the PLC pathway for the shared target molecule, PIP2. Upon HER2 membrane localization, the PI3K pathway is activated, converting PIP2 to PIP3. Simultaneously, HER2 activation upregulates PDPK1, whose PH domain exhibits a high binding affinity to PIP2 molecules. Consequently, membrane localization of HER2 substantially diminishes PIP2 availability for the PLC pathway, redirecting cells toward proliferation. Analysis of patient samples confirms the loss of membrane bound HER2 in circulating tumor cells (CTCs) and highlights phenotypic transitions in solitary cancer cells. These cells are characterized by increased plakoglobin, elevated ZEB1, and reduced KI67 protein levels. In summary, our findings introduce a novel mechanism of phenotypic plasticity in HER2+ breast cancer that regulates phenotype switching. This mechanism holds promise for the interception of early-stage metastasis, potentially leading to improved outcomes for HER2+ breast cancer patients. ### Competing Interest Statement The authors have declared no competing interest.