Cancer Stem Cells in Head and Neck Cutaneous Squamous Cell Carcinoma Express Cathepsins

Cancer stem cell (CSC) subpopulations within moderately differentiated head and neck cutaneous squamous cell carcinoma (MDHNcSCC) express the components of the renin-angiotensin system (RAS). This study investigated the expression of cathepsins B, D, and G, which constitute bypass loops of the RAS, by CSCs in MDHNcSCC.

Immunohistochemical staining was performed on MDHNcSCC tissue samples from 15 patients to determine the expression of cathepsins B, D, and G. Co-localization of these cathepsins with the embryonic stem cell markers Octamer-binding transcription factor 4 (OCT4) and c-MYC was investigated with immunofluorescence staining. Reverse transcription quantitative polymerase chain reaction was performed on 5 MDHNcSCC tissue samples to investigate transcript expression of cathepsins B, D and G. Western blotting and enzymatic activity assays were performed on 5 MDHNcSCC tissue samples and 6 MDHNcSCC-derived primary cell lines to confirm protein expression, transcript expression, and functional activity of these cathepsins, respectively.

Immunohistochemical staining demonstrated the expression of cathepsins B, D, and G in all MDHNcSCC tissue samples. Immunofluorescence staining showed localization of cathepsins B and D to the c-MYC⁺ CSC subpopulations and the OCT4⁺ CSC subpopulations within the tumor nests and the peritumoral stroma. Cathepsin G was expressed on the tryptase⁺/c-MYC⁺ cells within the peritumoral stroma. Reverse transcription quantitative polymerase chain reaction demonstrated transcript expression of cathepsins B, D and G in the MDHNcSCC tissue samples. Western blotting and enzymatic activity assays confirmed protein expression and functional activity of cathepsins B and D in the MDHNcSCC tissue samples and MDHNcSCC-derived primary cell lines, respectively.

Cathepsins B, D, and G are expressed in MDHNcSCC with functionally active cathepsins B and D localizing to the CSC subpopulations, and cathepsin G is expressed by mast cells, suggesting the potential use of cathepsin inhibitors in addition to RAS blockade to target CSCs in MDHNcSCC.