Abstract 4125: The retinoid-induced G0S2 protein undergoes ISG15ylation and destabilization.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC The interferon stimulated protein ISG15 was one of the first ubiquitin-like modifiers discovered. Similar to ubiquitin, ISG15 is conjugated to cellular proteins through a three-step enzymatic cascade involving an activating enzyme E1 (UBE1L), a conjugating enzyme E2 and a ligase E3. Conjugated ISG15 is removed from substrate proteins by the UBE1L-ISG15 deconjugase Usp18 (UBP43). To date, few direct target proteins of the ISG15 system were found and the biologic implications of their ISG15ylation are actively under study. We previously reported that the G0/G1 switch gene 2 (G0S2) was a direct target of all-trans-retinoic acid (RA) treatment in acute promyelocytic leukemia (APL). Prior work by others found that G0S2 was able to regulate lipolysis by inhibiting adipose triglyceride lipase (ATGL). The current study extends prior work by reporting that G0S2 was induced in APL cells by RA-treatment coordinately with UBE1L, ISG15 and the deconjugase Usp18. G0S2 was also shown to be a previously unrecognized target of ISG15ylation. G0S2 species were co-immunoprecipitated with ISG15. The functional consequence of this modification was confirmed by finding that G0S2 protein stability was directly affected by the ISG15 deconjugase Usp18. Loss of Usp18 expression by transfection of individual siRNAs into studied cells led to G0S2 protein destabilization. In contrast, engineered gain of Usp18 expression in the same cells stabilized G0S2 protein. Agents that can inhibit specific protein degradation pathways were also examined. Findings revealed that G0S2 protein was stabilized by treatment with proteasome or caspase inhibitors, but was not affected by lysosome or aggresome inhibitors. Thus, these findings indicate that G0S2 is a novel ISG15 substrate and that this modification substantially reduces G0S2 protein stability. Taken together, these results provide a mechanistic basis for cross-talk between interferon and retinoid signaling at the level of the G0S2 protein that undergoes ISG15ylation. This provides evidence for a role of lipolysis regulation in both interferon and retinoid signaling. The functional consequences of this are under study in mouse models. Citation Format: Yun Lu, Lisa Maria Mustachio, Fadzai Chinyengetere, Wei Yang, Tian Ma, Jessica Dong, David J. Sekula, Sarah J. Freemantle, Ethan Dmitrovsky. The retinoid-induced G0S2 protein undergoes ISG15ylation and destabilization. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4125. doi:10.1158/1538-7445.AM2013-4125