Identification of USP18 as an Important Regulator of the Susceptibility to IFN-α and Drug-Induced Apoptosis

Abstract

Gene products that modify the apoptotic susceptibility of cancer cells may offer novel drug response markers or therapeutic targets. In this study, we probed the contribution of 53 different isopeptidases to apoptosis triggered by bortezomib and etoposide. USP18, a type I IFN-induced protein that deconjugates the ubiquitinlike modifier ISG15 from target proteins, was found to limit apoptotic susceptibility to IFN-α or bortezomib. Ablating USP18 in cells treated with IFN-α increased tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) production; upregulated expression of transcription factors IFN-regulatory factor (IRF)-1, IRF-7, and IRF-9; and promoted the extrinsic pathway of apoptosis. The proapoptotic effects of ablating USP18 were abrogated by FLIP overexpression or TRAIL silencing. However, in bortezomib-treated cells, weak spontaneous signaling from type I IFNs was implicated in the proapoptotic effect of USP18 ablation. Ectopic USP18 repressed apoptotic signaling by IFN-α, TRAIL, or bortezomib. Similar effects were produced by a catalytically inactive USP18 mutant, indicating that the antiapoptotic function of USP18 is independent of its catalytic activity. These findings suggest that USP18 may significantly limit operation of the extrinsic apoptotic pathway triggered by type I IFN and drugs. Cancer Res; 70(2); 655-65. ©2010 AACR.

The DeISGylase USP18 limits TRAIL-induced apoptosis through the regulation of TRAIL levels

“…23 We have confirmed this observation and also demonstrated that, through the downregulation of USP18, a negative regulator of the IFN signaling, it is possible to sustain the spontaneous interferon response 29 and to strengthen apoptosis induced by rhTRAIL. Analysis of gene expression profiles in several cancer cell lines corroborated that cells expressing high levels of USP18 exhibit resistance to rhTRAILinduced apoptosis.…”

“…29,38 Stealth RNA interference RNAi for USP18, TRAIL, and nontargeting shRNA were purchased from Invitrogen. Cells were transfected 24 h after plating by adding the medium OptiMem, containing Lipofectamine 2000 (Invitrogen) plus the stealth RNAi oligos.…”

“…The pro-survival effect of USP18 is correlated with its function as inhibitor of the spontaneous interferon signaling. 29 To evaluate whether USP18 can influence TRAIL induced apoptosis we generated T98G cells stably expressing a shRNA specific for USP18. Clones were isolated and treated with IFN-α to evaluate the robustness of silencing.…”

“…29 Hence, we decided to explore whether also the improved apoptotic response to rhTRAIL, observed in cells with reduced USP18 expression was dependent on cellular TRAIL. E1A-transformed fibroblasts were co-transfected with two different RNAi specific for USP18 and for TRAIL, or the relative controls (Fig.…”

“…28 We have recently identified in USP18 an important regulator of the apoptotic response to interferons and to other pro-death stimuli such as proteasome inhibitors and genotoxic stresses. 29 The pro-survival function of USP18 depends on its ability to buffer the spontaneous interferon signaling. In this manuscript we have investigated the ability of USP18 to modulate TRAIL-induced apoptosis in the glioblastoma cell line T98G.…”