Deubiquitination and stabilization of programmed cell death ligand 1 by ubiquitin-specific peptidase 9, X-linked in oral squamous cell carcinoma

Abstract

Background: The immune checkpoint protein programmed cell death ligand 1 (PD-L1) binds to PD1 to promote tumor cell escape from the killing effect of the immune system. However, there are few studies on the regulatory mechanisms of PD-L1 in tumors. Although PD-L1 has been reported to undergo ubiquitination in some cancers, its regulatory mechanisms in oral squamous cell carcinoma (OSCC) are unclear. Therefore, we aimed to investigate this phenomenon.

Methods: We examined the expression and function of USP9X and PD-L1 in human oral keratinocytes (HOK) and OSCC cell lines (HN4 and HN30) as the control and relevant cancer cells using western blotting, immunoprecipitation, immunohistochemistry (IHC), T-cell-mediated tumor cell killing assay, and liquid chromatography-mass spectrometry.

Results: Programmed cell death ligand 1 was highly expressed in OSCC by the regulation of the ubiquitin-proteasome pathway. Furthermore, we discovered that ubiquitin-specific peptidase 9, X-linked (USP9X) could be combined with PD-L1 to induce its deubiquitination and stabilize its protein expression in OSCC.

Conclusion: Our data indicate that USP9X deubiquitinates and stabilizes PD-L1. Suppressing the expression of USP9X blocks tumor cell growth. The results provide a theoretical basis for USP9X as a therapeutic target.

Keywords: oral squamous cell carcinoma; programmed cell death ligand 1; programmed cell death protein 1; ubiquitin-specific peptidase 9, X-linked.