FUNCTIONAL CHARACTERIZATION OF THE ENDOCYTIC PROTEIN EPSIN3 IN BREAST CANCER

Endocytosis plays a critical role in the maintenance of cellular homeostasis. Due to its ability to resolve extracellular signals, the endocytic pathway controls many cellular processes, including transcription, proliferation, cell migration and cell fate determination. One relevant, unanswered, question regarding the role of endocytosis in the cell is whether subversion of the endocytic route is involved in the development of pathological conditions, such as cancer. This possibility is supported by studies showing altered expression of several endocytic proteins in human tumors. In a previous study in our lab, the endocytic protein Epsin 3 (Epn3) was found in a gene signature prognostic for metastatic breast cancer. Epn3 belongs to the Epsin family of endocytic proteins. Unlike the other Epsin members, Epn1 and Epn2, which are ubiquitously expressed, Epn3 is exclusively expressed in gastric cells and in wounded or pathological tissues exhibiting altered cell-extracellular matrix interactions. While Epn1 and Epn2 have been well characterized as endocytic adaptors, the exact function of Epn3 protein in endocytosis or signaling is largely unknown. In the present study, we show that Epn3 is overexpressed in approximately 30% of breast tumors and that upregulation of this protein correlates with clinico-pathological parameters of aggressive disease. We also show that EPN3 gene is amplified in human breast tumors and that this genetic alteration can occur independently from HER2 amplification in these tumors. Through functional studies using in vitro and in vivo breast cancer models, we demonstrate that breast tumor cells harboring Epn3 amplification/overexpression are dependent on Epn3 deregulation for the maintenance of their tumorigenic potential. In agreement with a possible oncogenic role, overexpression of Epn3 in a breast tumor cell line increases the tumorigenic potential in vivo. Of note, Epn3 overexpression is also able to induce, both in normal and tumor breast cells, transcriptional and morphological changes typical of an epithelial-to-mesenchymal transition (EMT), in a TGFβ−dependent manner; indeed, Epn3 overexpression induces upregulation of TGFβ ligands and receptors and inhibition of TGFβ signaling is able to revert Epn3-induced EMT. In addition, we show that Epn3 increases the capacity of normal mammary epithelial cells to form mammospheres in vitro, suggesting that Epn3 may contribute to the expansion of the breast cancer stem cell compartment. Finally, we demonstrate that Epn3 is involved in E-cadherin internalization, by inducing its downregulation from the cell surface upon TGFβ-stimulation. In conclusion, our data suggest a novel oncogenic role for Epn3 in human breast cancer, through its action as an endocytic protein on E-cadherin trafficking. Overexpression of Epn3 might enhance E-cadherin internalization, and consequently induce EMT activating the expression of mesenchymal proteins that promote cell invasion and lead to tumor aggressiveness.