The role of type II serine protease hepsin in breast cancer signaling

Proteolytic enzymes play a significant role in normal physiology and pathophysiology by regulating protein stability, processing, and activation of growth factors. The central role of proteolysis and the potential for pharmacological interventions make the enzyme class an attractive target for therapy in cancer. However, the regulatory mechanisms acting downstream of many proteolytic enzymes are poorly understood, which caused significant failures in clinical trials targeting matrix metalloproteinases in the past. This thesis describes the up- and downstream pathways of the Type II Transmembrane Serine Protease Hepsin. Hepsin is overexpressed in most prostate, ovarian, and breast cancers. Studies in mouse models suggest a role in promoting metastatic spread and tumor growth. However, the signaling pathways around hepsin remain largely unknown. We determined the upstream oncogenic signaling that upregulates hepsin protein in breast cancer. The hepsin overexpression in breast cancer is regulated by posttranslational mechanisms and involves the stabilization of hepsin by Heat Shock Proteins following RAS transformation. Hepsin upregulation causes loss of epithelial junctions and the basement membrane. To study the hepsin regulated pathways, we generated an inducible hepsin overexpression model, which allows upregulation of hepsin in cell lines and in vivo models of breast cancer. Ectopic hepsin overexpression experiments demonstrated that hepsin overactivation can explain the disruptive effects of RAS on epithelial integrity. Hepsin overexpression also increases pro-HGF processing. To study the pathways downstream of hepsin, we generated a hepsin knockout mouse strain via gene editing. The knockout mouse model revealed a novel mammary branching phenotype and a connection between hepsin and the TGFβ pathway in mammary gland development and cancer. Hepsin activates the TGFβ pathway by releasing the latent TGFβ from the extracellular matrix via the degradation of fibronectin. By using in vitro and mouse models, we found that fibronectin is a direct substrate of hepsin. The hepsin driven TGFβ signaling induces cell proliferation in 3D culture models, which can be suppressed with existing hepsin antibody inhibitors. Genetic inhibition of hepsin reduces primary and metastatic mouse tumor growth, providing proof of principle that targeting hepsin can be a strategy for reducing the growth of breast tumors