The ubiquitin-specific protease 8 antagonizes melatonin-induced endocytic degradation of MT1 receptor to promote lung adenocarcinoma growth

Introduction: The human genome encodes two melatonin receptors (MT1 and MT2) 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 recep-tors in lung adenocarcinoma have so far remained incompletely understood.Objectives: This study aims to investigate the response of the MT1 receptor to melatonin treatment and its dynamic regulation by ubiquitin-specific protease 8 (USP8) in lung adenocarcinoma.Methods: The mRNA levels of MT1 and MT2 receptors were analyzed with sequencing data. The expres-sion and localization of the MT1 receptor with melatonin treatment were investigated by immunoblot-ting, immunofluorescence and confocal microscopy assays. Endocytic deubiquitylases were screened to identify MT1 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 MT1 receptor in lung adenocarci-noma tissues. Melatonin treatment leads to the downregulation of the MT1 receptor in lung adenocarci-noma cells, which is attributed to receptor endocytosis and lysosomal degradation via the canonical endo-lysosomal route. USP8 negatively regulates the endocytic degradation of the MT1 receptor incurred by melatonin exposure and thus protects lung adenocarcinoma cell growth. USP8 suppression by knock-down or pharmacological inhibition effectively deters cancer cell proliferation and sensitizes lung adeno-carcinoma cells to melatonin in vitro. Furthermore, USP8 silencing significantly potentiates the anticancer effects of melatonin in xenograft tumor models.Conclusion: The MT1 receptor responds to melatonin treatment and is endocytosed for lysosomal degra-dation 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.(c) 2022 The Authors. Published by Elsevier B.V. on behalf of Cairo University. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).