USP45 Represses Melanoma Development by Deubiquitinating and Stabilizing Tumor Suppressor MRGPRF

Melanoma, a highly malignant skin cancer, has seen a rising incidence and death toll. MRGPRF is a novel melanoma suppressor that inhibits the PI3K/AKT pathway. However, the regulation of MRGPRF in melanoma remains unclear. Here, 40 ubiquitin-specific proteases (USPs) are screened and USP45 is identified as a significant stabilizer of MRGPRF. Immunohistochemistry on melanoma patient biopsies demonstrates that USP45 expression is markedly reduced in melanoma tissues compared to adjacent noncancerous epidermis. Bioinformatic analyses corroborate that USP45 mRNA levels are downregulated in melanoma, and low USP45 expression is associated with poor patient prognosis. Functional assays demonstrate that USP45 overexpression inhibits melanoma cell malignancy, whereas USP45 knockdown promotes it. Mechanistically, USP45's catalytic domain directly binds to the N-terminal of MRGPRF and stabilizes MRGPRF, likely by removing its K63-linked ubiquitination in melanoma cells. The antimelanoma effects of USP45 are mitigated by MRGPRF depletion, while MRGPRF overexpression rescues the enhanced malignant phenotype induced by USP45 deficiency. In vivo, a melanoma xenograft mouse model shows that USP45 overexpression significantly impairs melanoma progression. These findings establish USP45 as a melanoma suppressor, at least partially through its stabilization of MRGPRF, highlighting a novel mechanism in melanoma pathogenesis and suggesting that USP45 agonists may serve as potential therapeutic agents.