Usp18 Promotes Lipolysis, Fatty Acid Oxidation And Lung Cancer Growth

Ubiquitin specific peptidase 18 (USP18) is the Interferon-Stimulated Gene 15 (ISG15) deconjugase that stabilizes target proteins by removing ISG15 from substrate proteins. We found that USP18 null mice have lower lipolysis rates, altered fat to body weight ratios and marked cold sensitivity. These findings implicated USP18 as a regulator of lipid and fatty acid metabolism. Prior work established that USP18 can promote lung tumorigenesis. We examined whether this occurs in lung cancer cells through altered lipid and fatty acid metabolism. The siRNA knockdown of USP18 downregulated Adipose Triglyceride Lipase (ATGL) and this reduced lipolysis. In contrast, engineered gain of USP18 expression upregulated ATGL and increased lipolysis. Notably, Ubiquitin Activating Enzyme E1 Like Protein (UBE1L) promoted ISG15-conjugated destabilization of ATGL protein. Immunoprecipitation experiments confirmed that ISG15 can covalently conjugate to ATGL. Protein expression profiles of thermogenic regulators were examined in brown fat of USP18 null versus wild-type mice. Strikingly, the master thermoregulatory protein Uncoupling Protein 1 (UCP1) was markedly repressed in brown fat of USP18 null mice. In contrast, USP18 reconstitution augmented UCP1 levels. Engineered gain of USP18 expression stabilized UCP1 protein through reduced Ubiquitin conjugation to UCP1 in the examined lung cancer cell lines. Intriguingly, gain of UCP1 expression in both human and murine lung cancer cell lines promoted cellular proliferation and UCP1 knockdown reduced proliferation. The beta-hydroxybutyrate colorimetric assay determined that UCP1 overexpression promoted fatty acid beta-oxidation in lung cancer cells. Seahorse assays independently confirmed that gain of UCP1 expression increased fatty acid beta-oxidation. Expression profiles of USP18, ATGL and UCP1 species were interrogated using The Cancer Genome Atlas (TCGA). Lung adenocarcinoma and squamous cell carcinoma cases having higher USP18, ATGL and UCP1 levels had a statistically-significantly unfavorable survival. This association also occurred in renal chromophobe carcinoma and low grade gliomas. Together, these findings implicate the deubiquitinase USP18 as a novel molecular pharmacologic target that controls fatty acid metabolism as an energy source for lung cancer growth. These observations provide a strong rationale to uncover USP18 pharmacologic inhibitors that would combat lung and potentially other human cancers. Citation Format: Xi Liu, Yun Lu, Zibo Chen, Xiuxia Liu, Weiguo Hu, Lin Zheng, Yulong Chen, Mi Shi, Lisa Mustachio, Jason Roszik, Masanori Kawakami, Sarah Freemantle, Ethan Dmitrovsky. USP18 promotes lipolysis, fatty acid oxidation and lung cancer growth [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2547.