Uncovering Endolysosomal Alterations in HER2+ Breast Cancer

Abstract

Breast cancer (BC) is the most prevalent and frequently diagnosed type of cancer worldwide. HER2 is an oncogene overexpressed in 20% of all breast cancers and is associated with the most aggressive types of cancer. A part of the patients with HER2 positive BC still does not respond well to HER2 targeting therapy. Therefore, there is an unmet need of novel therapeutic approaches. Cancer cells hijack intracellular pathways, like their endolysosomal system, to overcome metabolic restrictions. Alterations in the function and structure of endolysosomal organelles are linked to cancer cell proliferation and invasion via unresolved molecular mechanisms. It has been previously shown that lysosomal markers are elevated in HER2 positive BC cells. However, it is yet not known how these changes contribute to cancer cell metabolism and breast cancer progression. We have engineered cellular models that inducibly express HER2 protein in Luminal A (MCF-7) and Triple-Negative (MDA-MB-231) BC cells, that are HER2 negative by origin. We used advanced light and electron microscopy to characterize lysosomal alterations in HER2-positive BC models. Our results showed significant alterations in endolysosomal organelles in our HER2 positive BC models. Looking at important regulators of lysosomal genes, TFEB is significantly translocated to the nucleus in MCF-7 cells, whereas TFE3 is significantly translocated to the nucleus in MDA-MB-231 cells. Ultrastructurally, we observed that there are more peripheral lysosomes in MCF-7 cells upon HER2 overexpression. We saw that the average size of Rab7+ late endosomes is bigger, and the protein levels of Rab7, as well as FYCO1 is higher in MDA-MB-231 cells. Besides this, we observed that Cathepsin B is located on the plasma membrane of MCF-7-cells upon HER2 induction. Importantly, both cell lines showed a decreased maturation and acidification of overall endolysosomal organelles. Overall, our results show that HER2 overexpression underlies functional alterations in the endolysosomal system of BC cells and induces a general maturation defect. Resolving HER2-specific endolysosomal maturation defects can possibly identify novel therapeutic targets in the future.