Contribution Towards Understanding the Role of Myoferlin During Breast Cancer ProgressionMyoferlin is a member of the ferlin protein family that has key physiological functions in plasma membrane fusion, repair and endocytosis events. Several studies show that in endothelial cells myoferlin is a regulator of tyrosine kinase receptors VEGFR2 and TIE2, suggesting a putative role in angiogenesis. Recently, highthroughput genomic and proteomic studies, including one from our group, have identified myoferlin as overexpressed in cancer. However, only few reports thus far associate a particular function to myoferlin in cancer. Owing to a proteomic approach, we discovered myoferlin as an overexpressed cell membrane associated protein in human breast adenocarcinoma. We validated this observation in 150 breast cancer cases. Immunohistochemistry evaluation and in silico gene expression analysis of different breast cancer subtypes reveal that myoferlin expression is particularly heterogeneous in triple negative breast cancer (TNBC) and correlates with poor patient outcome in this subgroup. Functionally, we show for the first time that myoferlin is a key functional regulator of the EGF receptor. Myoferlin depletion blocks EGF-induced breast cancer cell migration and epithelial-to-mesenchymal transition. In vitro, lack of myoferlin leads to impaired degradation of phosphorylated EGFR and causes sustained activation and malfunction of EGFR downstream targets like AKT. Mechanistically, we show that myoferlin directly interacts with caveolin-1 and is critical for the assembly of functional caveolae. This process is in turn needed for the proper proteasomal degradation of the receptor through a caveolin-mediated pathway. Along its role in receptor trafficking, we also suggest that myoferlin can modulate cancer cell metabolism through its involvement in endocytosis. Using transmission electron microscopy we demonstrate that myoferlin is an essential component of the endosomal system of TNBC cells, whose depletion severely impairs vesicle trafficking. Loss of myoferlin in vitro causes impaired mitochondrial function resulting in lowered oxygen consumption, ROS and ATP production. Correspondingly, mitochondria are found frequently depolarized and the cells are prone to apoptosis. The resulting metabolic imbalance provokes in vitro a shift towards glycolysis resulting in increased lactate production. The metabolic sensor AMPK escorts this adaptation process and its activation is dependent on caveolin-1 expression. Myoferlin depletion in vivo significantly suppresses tumor development in both chicken chorioallantoic membrane and in mouse xenograft models of TNBC. In the latter, post-tumorectomy follow-up shows that animals bearing myoferlin-deficient tumors develop significantly fewer and smaller lung metastases. In line with this, histological analysis demonstrates that myoferlin-depleted tumors are less invasive, display no necrosis and contain fewer lipid droplets compared to their control counterparts, suggesting in vivo metabolic adaptation.