Host-targeting agents in the treatment of hepatitis C: a beginning and an end?

The development of two distinct classes of hepatitis C antiviral agents, direct-acting antivirals (DAAs) and host-targeting antivirals (HTAs), have distinctly impacted the hepatitis C virus (HCV) field by generating higher sustained virological response (SVR) rates within infected patients, via reductions in both adverse side effects and duration of treatment when compared to the old standard of care. Today DAAs are actively incorporated into the standard of care and continue to receive the most advanced clinical trial analysis. With a multitude of innovative and potent second-generation DAA compounds currently being tested in clinical trials, it is clear that the future of DAAs looks very bright. In comparison to the other class of compounds, HTAs have been slightly less impactful, despite the fact that primary treatment regimens for HCV began with the use of an HTA - interferon alpha (IFNα). The compound was advantageous in that it provided a broad-reaching antiviral response; however deleterious side effects and viral/patient resistance has since made the compound outdated. HTA research has since moved onward to target a number of cellular host factors that are required for HCV viral entry and replication such as scavenger receptor-BI (SR-BI), 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMGCoA reductase), cyclophilin A (CypA), fatty acid synthase (FASN) and miRNA-122. The rationale behind pursuing these HTAs is based upon the extremely low mutational rate that occurs within eukaryotic cells, thereby creating a high genetic barrier to drug resistance for anti-HCV compounds, as well as pan-genotypic coverage to all HCV genotypes and serotypes. As the end appears near for HCV, it becomes important to ask if the development of novel HTAs should also be analyzed in combination with other DAAs, in order to address potential hard-to-treat HCV patient populations. Since the treatment regimens for HCV began with the use of a global HTA, could one end the field as well?

Proteasomes can degrade a significant proportion of cellular proteins independent of ubiquitination.

The critical role of the ubiquitin-26S proteasome system in regulation of protein homeostasis in eukaryotes is well established. In contrast, the impact of the ubiquitin-independent proteolytic activity of proteasomes is poorly understood. Through biochemical analysis of mammalian lysates, we find that the 20S proteasome, latent in peptide hydrolysis, specifically cleaves more than 20% of all cellular proteins. Thirty intrinsic proteasome substrates (IPSs) were identified and in vitro studies of their processing revealed that cleavage occurs at disordered regions, generating stable products encompassing structured domains. The mechanism of IPS recognition is remarkably well conserved in the eukaryotic kingdom, as mammalian and yeast 20S proteasomes exhibit the same target specificity. Further, 26S proteasomes specifically recognize and cleave IPSs at similar sites, independent of ubiquitination, suggesting that disordered regions likely constitute the universal structural signal for IPS proteolysis by proteasomes. Finally, we show that proteasomes contribute to physiological regulation of IPS levels in living cells and the inactivation of ubiquitin-activating enzyme E1 does not prevent IPS degradation. Collectively, these findings suggest a significant contribution of the ubiquitin-independent proteasome degradation pathway to the regulation of protein homeostasis in eukaryotes.

20S proteasome differentially alters translation of different mRNAs via the cleavage of eIF4F and eIF3.

The molecular basis for coordinated regulation of protein synthesis and degradation is not understood. Here we report that the 20S proteasome endoproteolytically cleaves the translation initiation factors eIF4G, a subunit of eIF4F, and eIF3a, a subunit of eIF3. The cleavage of eIF4G or eIF3a differentially affects the assembly of ribosomal preinitiation complexes on different cellular and viral mRNAs in an in vitro system containing pure components. Inhibition of proteolytic activity of the 20S proteasome with specific inhibitors prevents cleavage of both factors in vitro and in vivo, restores assembly of ribosomal complexes in vitro, and differentially affects translation of different mRNAs in vivo. These studies demonstrate the importance of the endoproteolytic activity of proteasomes in regulation of cellular processes and suggest a link between protein synthesis and degradation.