USP44 regulates HEXIM1 stability to inhibit tumorigenesis and metastasis of oral squamous cell carcinoma.

Oral squamous cell carcinoma (OSCC) is the most frequent type of oral malignancy with high metastasis and poor prognosis. The deubiquitinating enzyme Ubiquitin Specific Peptidase 44 (USP44) regulates the mitotic checkpoint, and its deficiency leads to aneuploidy and increases tumor incidence. However, the role of USP44 in OSCC is not well understood. Herein, we analyzed mRNA sequencing data of OSCC samples downloaded from the TCGA and GEO databases and found that USP44 was decreased in human OSCC tissues and was positively correlated to the survival of OSCC patients. To investigate the biological impact of USP44, we used recombinant lentiviruses to overexpress or knockdown USP44 expression in OSCC cell lines, which were also injected subcutaneously or into the lateral tail vein of Male BALB/c nude mice to model tumorigenesis or lung metastasis in vivo, respectively. The results showed that overexpression of USP44 inhibited malignant cell phenotypes in vitro and suppressed tumor growth and lung metastasis in vivo, while its downregulation had the opposite effects. Comprehensive proteomic analyses through Co-IP mass spectrometry and label-free quantitative LC-MS/MS methods identified 112 differentially expressed proteins positively regulated by USP44, among which 13 were involved in cancer-related pathways including apoptotic signaling and cell cycle regulation. PPI analysis identified Hexamethylene Bis-Acetamide-Inducible Protein 1 (HEXIM1) as the hub protein. Upregulation of USP44 enhanced HEXIM1 protein stability, leading to its higher expression in OSCC cells. Silencing of HEXIM1 further enhanced the malignant phenotype of OSCC cells. At the same time, HEXIM1 knockdown reversed the antitumor effects of USP44. These findings demonstrated that USP44 acted as a critical tumor suppressor in OSCC by inhibiting cell proliferation and metastasis through the stabilization of HEXIM1 protein, suggesting that USP44-HEXIM1 axis is a promising target for OSCC therapy.