The ubiquitin-specific protease 8 antagonizes melatonin-induced endocytic degradation of MT(1) receptor to promote lung adenocarcinoma growth

INTRODUCTION: The human genome encodes two melatonin receptors (MT(1) and MT(2)) that relay melatonin signals to cellular interior. Accumulating evidence has linked melatonin to multiple health benefits, among which its anticancer effects have become well-established. However, the implications of its receptors in lung adenocarcinoma have so far remained incompletely understood. OBJECTIVES: This study aims to investigate the response of the MT(1) receptor to melatonin treatment and its dynamic regulation by ubiquitin-specific protease 8 (USP8) in lung adenocarcinoma. METHODS: The mRNA levels of MT(1) and MT(2) receptors were analyzed with sequencing data. The expression and localization of the MT(1) receptor with melatonin treatment were investigated by immunoblotting, immunofluorescence and confocal microscopy assays. Endocytic deubiquitylases were screened to identify MT(1) association. The effects of USP8 were assessed with shRNA-mediated knockdown and small molecule inhibitor. The combined efficacy of melatonin and USP8 suppression was also evaluated using xenograft animal models. RESULTS: Bioinformatic analysis revealed increased expression of the MT(1) receptor in lung adenocarcinoma tissues. Melatonin treatment leads to the downregulation of the MT(1) receptor in lung adenocarcinoma cells, which is attributed to receptor endocytosis and lysosomal degradation via the canonical endo-lysosomal route. USP8 negatively regulates the endocytic degradation of the MT(1) receptor incurred by melatonin exposure and thus protects lung adenocarcinoma cell growth. USP8 suppression by knockdown or pharmacological inhibition effectively deters cancer cell proliferation and sensitizes lung adenocarcinoma cells to melatonin in vitro. Furthermore, USP8 silencing significantly potentiates the anticancer effects of melatonin in xenograft tumor models. CONCLUSION: The MT(1) receptor responds to melatonin treatment and is endocytosed for lysosomal degradation that is counteracted by USP8. The inhibition of USP8 demonstrates tumor-suppressive effects and thus can be exploited as potential therapeutic strategy either as monotherapy or combined therapy with melatonin.