Fam3C alters Golgi apparatus morphology and function in triple negative breast cancer

Fam3C, also known as Interleukin-like EMT inducer (ILEI), is an established regulator of the epithelial to mesenchymal transition and breast cancer stem cell phenotypes. Multiple cancer cell models and orthotopic animal model experiments have demonstrated a role for Fam3C in tumor progression and metastasis. Here, we establish Fam3C's impact on triple negative breast cancer patients and genetically engineered mouse models of spontaneous breast cancer tumor progression. Though Fam3C is a known secreted protein, we discovered its retention in the Golgi apparatus through anchoring of its signal peptide into the membrane before its signal peptide and pro-peptide are processed and removed. While retained in the Golgi apparatus, Fam3C affects the overall morphology of the organelle and its biological functions, including alterations in protein secretion and invasive potential. Expanding our knowledge of the biological mechanisms behind EMT will help develop therapies to specifically target cells with increased metastatic potential in triple negative breast cancer.