ISG15 enhances glioma cell stemness by promoting Oct4 protein stability

The effects of ISG15 or ISGylation on tumor progression have been widely revealed; however, its roles in glioma progression are largely unknown. This study aims to explore the roles and underlying mechanisms of ISG15 in glioma progression. Here, ISG15 level was found to be upregulated in glioma tissues compared to the paired/unpaired normal tissues, and positively correlated with the level of stemness markers in glioma tissues. Loss of functional experiments indicated that ISG15 positively regulated glioma cell stemness, as evident by the increase of sphere formation ability, ALDH activity, stemness marker expression, and tumor-initiating ability. Further mechanistic studies revealed that ISG15 directly interacted with Oct4 protein, a critical stemness promoter, induced the ISGylation of Oct4 protein, and thus enhanced Oct4 protein stability. Additionally, it was found that Oct4 was ISGylated at lysine 284 (K284), which has been confirmed to be the ubiquitination site of Oct4 protein, and ISG15 knockdown did not degrade K284R mutant Oct4. Furthermore, ISG15 knockdown-induced downregulation of glioma cell stemness was rescued by Oct4 overexpression, but not by K284R mutant Oct4. Altogether, we suggest that ISG15-induced ISGylation of Oct4 protein is essential for glioma cell stemness.