The mechanism of regulation of hepatitis C viral propagation and pathogenesis

 

Signal-peptide peptidase (SPP) is a nine transmembrane protein that belongs to the GxGD-type intramembrane cleaving proteases. SPP is required for the generation of peptide ligands for a histocompatibility antigen, α chain E (HLA-E), and the maturation of core proteins of hepatitis C virus (HCV) and equine hepacivirus (EHcV).

SPP was also reported to recognize haem oxygenase-1 (HO-1) and the unspliced variant of X-box binding protein 1 (XBP1 μ) as substrates. Although it has been suggested that SPP is involved in the endoplasmic reticulum (ER)-associated degradation (ERAD) process through interaction with UBAC2, PDI (TRC8 or Derlin1), the physiological functions of SPP in ERAD remain largely unknown.

HCV belongs to the Flaviviridae family and possesses a single positive-strand RNA that encodes a single polyprotein of ~3,000 amino acids that is processed into 10 viral proteins by viral and host proteases. The core protein is the first viral protein to be translated and cleaved from the precursor polyprotein by a host signal peptidase at amino-acid position 191/192.

The immature core protein is further processed by SPP at the C-terminal transmembrane region to form the mature core protein³. The maturation of the core protein by SPP is crucial for the production of infectious HCV particles.

Although the mature core protein participates in particle formation, transgenic mice expressing the HCV core protein in the liver (CoreTg) developed insulin resistance, steatosis and hepatocellular carcinoma. The levels of core protein in CoreTg livers were equivalent to those of HCV patients, suggesting that the HCV core protein also plays crucial roles in HCV pathogenesis.

The C terminus of the mature HCV core protein was shown to be Phe177 in insect cells and mammalian cells. Mutation of the HCV core at Phe177 abolished cleavage by SPP and impaired infectious viral particle production¹⁵. However, the biological significance of cleavage of the HCV core protein by SPP on virus production and pathogenesis remains unknown.

In this study, authors generated SPP gene-knockout (SPPKO) cell lines and mice to investigate the roles of SPP on HCV propagation and pathogenesis. They found that the immature HCV core protein produced in SPPKO cells or cells treated with an SPP inhibitor was quickly degraded by the ubiquitin–proteasome pathway.

Authors demonstrated that the administration of an SPP inhibitor to CoreTg and single-allele deletion of SPP genes in CoreTg reduced the expression of the core protein and ameliorated insulin resistance and liver steatosis. Moreover, the production of infectious HCV was severely impaired in SPPKO cells.

siRNA-mediated screening revealed that the TRC8 gene, which encodes an ER-resident E3 ubiquitin-ligase, was responsible for the degradation of the immature HCV core protein. Finally, authors found that expression of the HCV core protein induced an alteration of the ER structure and ER stress in cells in which both the SPP and TRC8 genes have been knocked out (SPP/TRC8DKO).

The recovery of either SPP or TRC8 expression abrogated the induction of ER stress in SPP/TRC8DKO cells, suggesting that the immature HCV core protein retained in the ER membrane induces ER stress. Taken together, this data indicate that the inhibition of SPP activity induces the production of the immature HCV core protein, and TRC8 is involved in the degradation of the immature core protein by the proteasome to circumvent the induction of ER stress.

http://www.nature.com/ncomms/2016/160504/ncomms11379/full/ncomms11379.html

Edited May 9th 2016