Many of the membrane-anchored serine proteases have restricted distribution in normal cells, while their expression is frequently dysregulated during cancer progression.17,38–40
Recent in vitro
data support the role for hepsin in prostate and ovarian tumor cell progression.24
Additionally, hepsin overexpression is associated with disorganization of the basement membrane and promotion of metastasis in a mouse model of prostate cancer.25
However, the mechanism by which hepsin contributes to tumori-genesis is unclear. Studies of the functional importance of hepsin overexpression in cell culture systems have resulted in conflicting results. While antihepsin antibodies or antisense oligonucleotides targeting hepsin significantly blocked hepatoma cell growth19
and inhibited invasive characteristics of ovarian and prostate cancer cell lines,24
stable transfection of hepsin cDNA in prostate and ovarian cancer cell lines promoted apoptosis and inhibited growth in soft agar and in nude mice.41,42
To gain insight into its function, we studied subcellular localization and expression of endogenous hepsin in human ovarian tumors. We showed that the majority of ovarian tumors display cytoplasmic and/or membrane staining and that the membrane staining was predominantly associated with clear cell and serous carcinomas, which may be related to the more intrinsic aggressive nature of these tumor types. Additionally, in patients with serous ovarian carcinomas, membranous expression of hepsin was associated with decreased survival (HR 5 7.7, p = 0.003) and shorter mean time to recurrence (1.0 years vs. 2.8 years; p = 0.008) (Elisa Lopez-Varela and Esther Oliva, unpublished), supporting the hypothesis that hepsin is involved in ovarian cancer progression through its interaction with the cell membrane.
The functional role of hepsin overexpression in ovarian cancer progression was evaluated in a mouse model of ovarian cancer. We demonstrated by using human and mouse ovarian cancer cell lines that overexpression of hepsin can promote cancer progression in vivo
and that proteolytic activity of hepsin is necessary for this function. The mechanism by which the proteolytic activity of hepsin promotes cancer progression is currently unknown. The catalytic domain of hepsin has a high degree of similarity to the catalytic domain of the Drosophila
type II serine protease, stubble-stubbloid, which is known to be involved in signal transduction, cytoskeletal reorganization and changes in cell shape during mor-phogenesis.43
As a transmembrane protein, hepsin is ideally suited for interaction with cell membrane proteins and proteins on adjacent cells. Additionally, the presence of a short cytoplasmic N-ter-minal domain suggests a role in intracellular signal transduction. However, evidence that these domains support interactions with extracellular, membrane, or intracellular proteins is still lacking.
We demonstrated that hepsin is localized in the cytoplasm and at desmosomal junctions in ovarian cancer cell lines. Desmosomes represent intracellular adhesive junctions that are involved in cell adhesion, differentiation and signal transduction.33
They also anchor intermediate filaments, thus providing structural integrity. There is cumulative evidence that functional desmosomes are involved in preventing cancer invasion and metastasis.44
The functional significance of hepsins localization at the desmosomal plaque is currently unclear. The only known proteolytic substrate for hepsin is pro-HGF. Hepsin can cleave pro-HGF to generate active HGF, the ligand for the c-Met receptor.36,37
It has been shown that c-Met overexpression is a prognostic factor in ovarian cancer and that targeting c-Met inhibits ovarian cancer peritoneal dissemination and invasion in a mouse model.45
c-Met is also known to form a complex with the desmosomal protein desmoglein 1.46
In turn, desmosome dissociation is known to be one of the first steps during HGF-induced epithelial-mesenchymal transition.47,48
Taken together, we propose that, in addition to the demonstrated role of hepsin in extracellular matrix degradation, hepsin and its substrate HGF may play a role in ovarian cancer progression through their interaction with desmosomes. The mechanism and functional significance of this interaction warrants further investigation.