## 4 Discussion  
Type II transmembrane serine proteases (TTSPs) are a large family, containing 17 proteases categorized into four subfamilies in humans. All subfamilies have a serine protease domain at the C-terminus where histidine, aspartate, and serine residues form a catalytic triad [20; 25; 26]. The substrates of TTSPs are cytokines, growth factors, and extracellular matrix components. Soluble forms of TTSPs are often detected in culture media, suggesting that their extracellular domains are shed from the cell surface [27; 28; 29; 30]. Since this shedding is dependent on their protease catalytic activity [30], it is assumed to occur as a result of self-cleavage. Among TTSPs, the hepsin/transmembrane protease serine (TMPRSS) subfamily characteristically contains a group A scavenger receptor domain in the stem region. Mosaic transmembrane serine protease (MSPL) and TMPRSS13, which belong to the TMPRSS subfamily, are splicing variants of a single gene cloned from a human lung cDNA library [19; 25]. MSPL/TMPRSS13 is highly expressed in the skin, lung, and bladder, but is not detected in colon tissue [19]. The physiological functions of MSPL and TMPRSS13 have been defined in epidermal barrier development [23] and virus infections [31; 32; 33]. By contrast, a number of reports describe other TTSP proteins as extensively associated with tumor growth and metastasis [26; 34]. For example, TMPRSS1 is upregulated in several types of cancer, including prostate [35] and ovarian cancers [36], and is involved in cancer cell migration and invasion [37]. TMPRSS1 expression is linked to poor prognosis in prostate cancer patients, suggesting that TMPRSS1 serves as a biomarker for prostate cancer [38]. Figure 5: Degradation of DC-SIGN expressed in the monocytic cell membrane. (**a**) Expression of full-length DC-SIGN in the human monocytic U937 cell line. U937 and U937-DC-SIGN cells were incubated with primary anti-DC-SIGN monoclonal antibody (R&D Systems) followed by Alexa Fluor 488 secondary antibody, and analyzed through flow cytometry (BD FACSCalibur). (**b**) The membrane protein fraction was prepared from U937-DC-SIGN cells (\(2.5\times 10^{6}\) cells). The fraction was incubated with and without MSPL-ECD (\(20\)\(\upmu\)g) at \(37\)\({}^{\circ}\)C for \(6\) h. Western blotting was performed using anti-DC-SIGN monoclonal antibody or anti-\(\beta\)-actin antibody. The arrows in lane 2 indicate digested DC-SIGN fragments. Here, we provide the first report that MSPL/TMPRSS13, which is in the same protein subfamily as TMPRSS1, is overexpressed in colorectal cancer cells. We demonstrated that MSPL/TMPRSS13 is recognized by DC-SIGN based on N-glycans and that this molecular interaction partially blocks MSPL/TMPRSS13 protease activity. Moreover, a previous report indicated that substrates containing Arg at the P1 position and Arg or Lys at position P2 are preferably cleaved by MSPL [25]. In contrast, the N-terminal amino acid sequence analysis in this study showed Leu-Lys and Ser-Arg at positions P1-P2 of the AAVGE and SNRFW sequences, respectively. These results indicate a novel substrate specificity of MSPL. Moreover, the physiological significance of the molecular interaction between DC-SIGN and MSPL/TMPRSS13 should be considered in light of its bidirectionality. In previous studies, we revealed that recognition of CEACAM-1 and Mac-2 BP by DC-SIGN attenuates DC maturation, resulting in promotion of immune escape by cancer cells [17; 18]. In this context, further investigation could reveal whether MSPL/TMPRSS13 recognition by DC-SIGN affects DC maturation. Meanwhile, MSPL/TMPRSS13 may be involved in cancer cell invasion and metastasis, as observed in other members of its subfamily, by degrading the basement membrane through protease activity. The physiological relevance of the direct involvement of MSPL/TMPRSS13 in tumor metastasis, including the influence of its interaction with DC-SIGN, requires further study.