Interferon stimulated gene 15, a proviral factor in hepatitis C virus replication, is regulated by the Proteasome subunit alpha type-6

Aim and Background: The interferon stimulated gene 15 (ISG15) plays an important role in the pathogenesis of hepatitis C virus (HCV) infection. We recently identified seven ISG15-regulated proteins, which putatively affect this proviral interaction. Therefore, the aim of this study was to further elucidate the relation between ISG15 and these proteins during HCV infection.

Methods: Expression of selected ISGs (ISG15 and interferon-induced protein with tetratricopeptide repeats 1 (IFI-T1)) and ISG15-regulated genes (Medium-chain specific acyl-CoA dehydrogenase (ACADM), Heterogeneous nuclear ribonucleoprotein A3 (HnrnpA3), HnrnpK, Hydroxymethylglutaryl-CoA synthase (Hmgcs1), Isocitrate dehydrogenase cytoplasmic (Idh1), Thioredoxin domain-containing protein 5 (Txndc5) and Proteasome subunit alpha type-6 (PSMA6)) were analyzed in liver samples of treatment-naïve HCV-infected patients (n = 54), HBV-infected patients (n = 23), uninfected controls (n = 12) as well as in primary human hepatocytes (PHH). Gene expression of ISG15 and PSMA6 was suppressed in a subgenomic HCV replicon cell line using specific siRNAs.

Results: Analysis of the hepatic expression of ISG15 and the seven ISG15-regulated genes only revealed a correlation between the expression of ISG15 and PSMA6 (r = 0.332, p < 0.01). ISG15 and PSMA6 expression patterns even showed a stronger correlation in PHH (r = 0.5629, p < 0.001). Furthermore, an elevated hepatic expression of ISG15, IFI-T1 and PSMA6 could be shown for HCV infected patients, as well as in PHH isolated from HCV-infected individuals, compared to uninfected controls. In contrast, hepatic expression profile of HBV patients also revealed an elevated PSMA6 expression but no induction of ISGs. Whereas the elevated hepatic ISG expression was associated with the HCV load and the HCV genotype, PSMA6 expression occurred independently of these viral parameters. To further analyze the relation between ISG15 and PSMA6, both genes were suppressed separately and simultaneously in the HCV replicon system, using specific siRNAs. Interestingly, the suppression of PSMA6 led to significant induction of ISG15 expression (fold change 2.5 (p < 0.001). Thus combined knockdown of both genes abrogated the antiviral effect induced by the separate suppression of ISG15.

Conclusions: Our data indicate that the proteasome subunit alpha type-6 is up-regulated during viral hepatitis and implies a negative regulator function for the ISG15, a proviral factor in the pathogenesis of HCV infection. These findings led to hypothesize, that the proteasome affects the enigmatic interaction between ISG15 and HCV.