Expression and Localization of Cathepsins B, D, and G in Two Cancer Stem Cell Subpopulations in Moderately Differentiated Oral Tongue Squamous Cell Carcinoma

AIM: We have previously demonstrated the putative presence of two cancer stem cell (CSC) subpopulations within moderately differentiated oral tongue squamous cell carcinoma (MDOTSCC), which express components of the renin–angiotensin system (RAS). In this study, we investigated the expression and localization of cathepsins B, D, and G in relation to these CSC subpopulations within MDOTSCC. METHODS: 3,3-Diaminobenzidine (DAB) and immunofluorescent (IF) immunohistochemical (IHC) staining was performed on MDOTSCC samples to determine the expression and localization of cathepsins B, D, and G in relation to the CSC subpopulations. NanoString mRNA analysis and colorimetric in situ hybridization (CISH) were used to study their transcripts expression. Enzyme activity assays were performed to determine the activity of these cathepsins in MDOTSCC. RESULTS: IHC staining demonstrated expression of cathepsins B, D, and G in MDOTSCC. Cathepsins B and D were localized to CSCs within the tumor nests, while cathepsin B was localized to the CSCs within the peri-tumoral stroma, and cathepsin G was localized to the tryptase(+) phenotypic mast cells within the peri-tumoral stroma. NanoString and CISH mRNA analyses confirmed transcription activation of cathepsins B, D, and G. Enzyme activity assays confirmed active cathepsins B and D, but not cathepsin G. CONCLUSION: The presence of cathepsins B and D on the CSCs and cathspsin G on the phenotypic mast cells suggest the presence of bypass loops for the RAS which may be a potential novel therapeutic target for MDOTSCC.