Metastasis suppressing properties of the cell-surface anchored serine protease prostasin: new functional and mechanistic insights from breast cancer

Serine proteases play multifaceted roles in cancer, affecting tumor formation, progression, and metastasis. While most serine proteases studied act as tumor promoters by remodeling the extracellular matrix and activating signaling pathways, others can function as tumor suppressors. Prostasin is a glycosylphosphatidylinositol-anchored serine protease that is expressed in epithelial tissues, including the ductal epithelium of the breast. We found that prostasin protein expression is lost in high-grade, poorly differentiated, invasive ductal carcinoma in both mice and humans. To test whether prostasin impacts tumor progression and metastasis, prostasin-deficient mice were crossed into the oncogene-induced transgenic MMTV-PymT mammary tumor model. While prostasin deficiency did not affect primary tumor growth, it resulted in a significantly increased spontaneous dissemination of cancer cells to the lungs, suggesting a causal relationship between the loss of prostasin expression and progression to distant metastasis of breast cancer. At the cellular level, re-expression of prostasin in human breast cancer cells that have lost endogenous prostasin attenuated their invasive properties. Importantly, silencing prostasin expression in non-transformed human mammary epithelial cells (HMECs) resulted in the disruption of epithelial integrity and the loss of tight junctions (TJs), an early hallmark of cells acquiring an invasive phenotype. Discovery proteomics identified HMEC-expressed fibronectin (FN) as a regulatory target of prostasin and revealed increased levels of FN upon prostasin silencing. Mechanistically, cellular FN plays a causal role in TJ integrity in HMECs, and concomitant silencing of FN and prostasin rescues the defects caused by prostasin loss. Prostasin-mediated FN regulation represents a novel mechanism for regulating mammary epithelial cell TJ integrity and a potential candidate pathway for targeted therapy in breast cancer patients.