USP20 competitively binds to STUB1 to enhance CTSL expression and promote epithelial-mesenchymal transition in head and neck squamous cell carcinoma.

RATIONALE: Metastatic head and neck squamous cell carcinoma (mHNSCC) poses a significant threat to patient survival. Previous studies have identified cathepsin L (CTSL) as a key driver of tumourigenesis, metastasis and chemoresistance. However, the regulatory mechanisms underlying CTSL expression remain poorly understood. METHODS: A specific deubiquitinase responsible for CTSL expression was identified through treatment with broad-spectrum deubiquitinase inhibitors and mass spectrometry analysis. The colocalization of CTSL and USP20 in the cytoplasm was examined using confocal microscopy. The effects of CTSL or USP20 depletion on tumour biological behaviour were evaluated through various in vitro and in vivo assays. RESULTS: We identified USP20 as a specific deubiquitinase of CTSL. USP20 mediates the deubiquitination and stabilization of CTSL, thereby promoting epithelial-to-mesenchymal transition and cancer stem cell renewal, ultimately enhancing metastatic potential and chemoresistance. Notably, USP20 competes with STUB1 for CTSL binding, further driving the malignant phenotype of HNSCC. Analysis of clinical samples revealed that both CTSL and USP20 are highly expressed in metastatic HNSCC tissues, with a positive correlation between their expression levels. CONCLUSIONS: Our study reveals a novel mechanism in which USP20 competitively interacts with STUB1 to stabilize CTSL and promote tumour progression. These findings provide preclinical evidence supporting USP20 as a potential therapeutic target for overcoming metastasis and chemotherapy resistance in HNSCC. KEY POINTS: USP20 deubiquitinates and stabilizes CTSL. STUB1 promotes CTSL ubiquitination and degradation. USP20 competitively binds to CTSL in competition with STUB1. Targeting USP20 sensitizes cancer cells to cisplatin or paclitaxel.