Protein ISGylation and free ISG15 levels are increased by interferon gamma in breast cancer cells

The induction of ISG15 by interferon (IFN)-α/β and subsequent protein ISGylation has been demonstrated in several cell types. However, regulation of free ISG15 levels and ISGylation by other IFNs and its implications in some carcinomas have not yet been completely evaluated. Here, we demonstrated that free ISG15 and ISGylation levels are enhanced by IFN-γ treatment in the estrogen receptor-α-positive and –negative breast cancer cells, MCF-7 and MDA-MB-231, respectively. Specifically, IFN-γ increases free ISG15 levels in the cytoplasm and ISGylation in the nucleus and cytoplasm, but in a manner distinct between MCF-7 and MDA-MB-231 cells. Therefore, free ISG15 and ISGylation may play central roles in mammary tumors by differentially modulating certain tumorigenic characteristics of estrogen receptor-α-positive and -negative breast cancer cells. •IFN-γ is a regulator of ISGylation in breast cancer cells.•ERα- and ERα+ breast cancer cells display different ISGylation pattern.•Subcellular compartmentalization of free ISG15 and ISGylation marks in breast cancer cells.