TMPRSS2, a Serine Protease Expressed in the Prostate on the Apical Surface of Luminal Epithelial Cells and Released into Semen in Prostasomes,Is Misregulated in Prostate Cancer Cells

TMPRSS2, a type II transmembrane serine protease, is highly expressed by the epithelium of the human prostate gland. To explore the regulation and function of TMPRSS2 in the prostate, a panel of monoclonal antibodies with high sensitivity and specificity were generated. Immunodetection showed TMPRSS2 on the apical plasma membrane of the prostate luminal cells and demonstrated its release into semen as a component of prostasomes, organelle-like vesicles that may facilitate sperm function and enhance male reproduction. In prostate cancer cells, TMPRSS2 expression was increased and the protein mislocalized over the entire tumor cell membrane. In both LNCaP prostate cancer cells and human semen, TMPRSS2 protein was detected predominantly as inactive zymogen forms as part of an array of multiple noncovalent and disulfide-linked complexes, suggesting that TMPRSS2 activity may be regulated by unconventional mechanisms. Our data suggested that TMPRSS2, an apical surface serine protease, may have a normal role in male reproduction as a component of prostasomes. The aberrant cellular localization, and increased expression of the protease seen in cancer, may contribute to prostate tumorigenesis by providing access of the enzyme to nonphysiological substrates and binding-proteins.TMPRSS2 is an androgen responsive gene that encodes a type II transmembrane serine protease (TTSP).^1,2,3 The members of the TTSP family share common protein structures including a transmembrane domain at the N terminus, linker regions with a variety of protein–protein interaction domains, and a canonical serine protease domain at the C terminus.^4,5,6 TTSPs have been found to play important roles in the development and homeostasis of mammals, and the aberrant expression of TTSP genes are reported to contribute to the etiology of several human disorders, including cancer.⁷ The importance of TMPRSS2 in vivo remains unclear because homozygous TMPRSS2-null mice are essentially phenotypically normal.⁸ However, TMPRSS2 was reported to regulate epithelial sodium channel (ENaC) activity in vitro, implying a possible role in epithelial sodium homeostasis.⁹ TMPRSS2 may play a role in angiogenesis and tubulogenesis in microvesicular endothelial cells, potentially modulating several aspects of prostate tumor biology.¹⁰ In addition to its proteolytic activity, TMPRSS2 may also serve as a cell receptor, conducting external signaling or interacting with the extracellular matrix through its extracellular protein binding domains. Overexpression of TMPRSS2 has been demonstrated in poorly differentiated prostate cancer with significant increase in the mRNA level.^2,11,12,13 TMPRSS2 was also reported to be involved in the majority of prostate cancer because of the gene fusion of the 5′-untranslational region of TMPRSS2 with ETS family members, which is implicated in the overexpression of ETS genes in the majority of prostate cancer.^2,14,15 The coding sequence of TMPRSS2 is not involved in the gene fusion, and as a consequence there is no resultant recombinant protein for the TMPRSS2-ETS gene fusion and the promoter-less copy of TMPRSS2 is silenced, resulting in reduced expression of TMPRSS2 mRNA in those prostate cancer patients with the gene fusion.¹⁶Despite these potentially interesting and important roles for TMPRSS2, characterization of the protein itself remains incomplete and somewhat confusing. Significant discrepancies in the apparent molecular mass of TMPRSS2 protein, with a calculated size around 54 kDa, have been reported in previous studies, with the most commonly sited being around 70 kDa and 32 kDa^11,17,18 as the full-length and serine protease domain of TMPRSS2, respectively. In this study, we have generated a panel of mouse monoclonal antibodies (mAbs) directed against TMPRSS2 that are highly sensitive and specific. Using the newly generated TMPRSS2 mAb, we have investigated the pathophysiological role of TMPRSS2 in prostate tissues by comparing its expression and subcellular distribution in prostate tumors and adjacent normal prostate glands. Functional and regulatory aspects of TMPRSS2 biology were also investigated by examining the activation status of the enzyme and the nature of complexes formed between TMPRSS2 and other proteins.