Abstract 2399: Loss of ubiquitin-specific peptidase 18 (USP18) destabilizes the regulator of thermogenesis uncoupling protein-1 (UCP-1) and represses lung cancer growth

Abstract

Abstract USP18 is the ISG15 (Interferon-Stimulated Gene 15) deconjugase that removes ISG15 from substrate proteins. We found USP18 null mice are markedly cold sensitive versus their wild-type littermates. USP18 null mice had significantly (P < 0.05) greater temperature declines than wild-type mice. These mice also had lower lipolysis rates and altered fat to body weight ratios. This implicated USP18 as a regulator of lipid and fatty acid metabolism. Protein expression profiles of thermogenic regulators were examined in brown fat of USP18 null versus wild-type mice. Strikingly, the thermoregulatory protein UCP-1 was substantially repressed in brown fat of USP18 null mice. To establish that UCP-1 repression was caused by loss of USP18, stable USP18 knock-down was independently achieved in a panel of murine lung cancer cell lines using transfected small hairpin RNAs (shRNAs). USP18 down-regulation by different shRNAs reduced UCP-1 levels as compared to controls. Engineered gain of USP18 expression stabilized UCP-1 expression in these lung cancer cell lines. UCP-1 destabilization followed complex formation with the ubiquitin-like protein ISG15. Immunoprecipitation assays established that complexes formed between ISG15 and UCP-1. Prior work found that USP18 regulates lung tumorigenesis. We therefore examined UCP-1 levels in human lung adenocarcinomas using The Cancer Genome Atlas (TCGA). Lung adenocarcinomas with undetected UCP-1 expression had significantly improved survival versus cases that expressed UCP-1. Notably, gain of UCP-1 expression in both human and murine lung cancer cell lines promoted their growth. In marked contrast, UCP-1 knockdown reduced proliferation. We explored whether UCP-1 expression affected fatty acid metabolism. The beta-hydroxybutyrate (ketone body) colorimetric assay confirmed that UCP-1 overexpression promoted fatty acid beta-oxidation. Gain of UCP-1 expression also increased fatty acid beta-oxidation using seahorse assays. Thus, a direct link exists between USP18 and expression of the thermoregulator UCP-1. This affects UCP-1 stability and lung cancer growth by altering fatty acid metabolism. 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. Citation Format: Xi Liu, Yun Lu, Weiguo Hu, Zibo Chen, Lisa M. Mustachio, Jason Roszik, Lin Zheng, Masanori Kawakami, Yulong Chen, Sarah J. Freemantle, Ethan Dmitrovsky. Loss of ubiquitin-specific peptidase 18 (USP18) destabilizes the regulator of thermogenesis uncoupling protein-1 (UCP-1) and represses lung cancer growth [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 2399.