Definition
Hepatitis C virus (HCV) belongs to the genus Hepacivirus in the family Flaviviridae and possesses a viral genome consisting of a single, positive-strand RNA. The genome encodes a large precursor polyprotein of approximately 3,000 amino acids which is processed into at least 10 viral proteins by host and viral proteases.
Discovery
HCV was discovered by Choo et al., 1989 by investigators at Chiron. Portions of the HCV genome were isolated by screening cDNA expression libraries made from RNA and DNA from chimpanzees infected with serum from a patient with post-transfusion non-A, non-B hepatitis. To identify portions of the genome that encoded viral proteins, the libraries were screened with antibodies from patients who had non-A, non-B hepatitis1.
Structural Characteristics
HCV belongs to the genus Hepacivirus in the family Flaviviridae and possesses a viral genome consisting of a single, positive-strand RNA with a nucleotide length of about 9.4 kb. The genome encodes a large precursor polyprotein of approximately 3,000 amino acids. The polyprotein is processed co- and posttranslationally into at least 10 viral proteins by host and viral proteases. The structural proteins of HCV are located in the N-terminal one-fourth of the polyprotein and are cleaved by host membrane proteases2. HCV core protein forms the nucleocapsid, which is surrounded by the envelope containing E1 and E2 glycoproteins3. It has been suggested that HCV core protein is a multifunctional molecule that acts as a structural protein but is also involved in the pathogenesis of hepatitis C. HCV core protein has two major p23 and p21 forms4. HCV core protein p23 represents a 191-amino-acid product in which the C-terminal hydrophobic region also acts as a signal sequence for E1. HCV polyprotein is cleaved between residues 191 and 192 by host signal peptidase to generate C-terminal and N-terminal polypeptides encompassing the core and E1 proteins, respectively. For the full maturation of HCV core protein, the C-terminal signal-anchor sequence was thought to be further processed by an unidentified microsomal protease, and the 21-kDa isoform of core protein is predominantly detected both in cultured cells by transfection with expression plasmid and in viral particles obtained from sera of patients with hepatitis C. These results suggest that p21 is the mature form of HCV core protein3.
Functions
HCV core protein upregulates serine phosphorylation of insulin receptor substrate-1 and impairs the downstream Akt/PKB signaling pathway for insulin resistance - Insulin resistance is a critical component of type 2 diabetes mellitus (T2DM) pathogenesis. Several mechanisms are likely to be involved in the pathogenesis of HCV-related insulin resistance. Since previously it was observed that HCV core protein activates c-Jun N-terminal kinase (JNK) and mitogen-activated protein kinase, a study was conducted to examine the contribution of these pathways to insulin resistance in hepatocytes4. It was found that HCV core protein alone or in the presence of other viral proteins increases Ser312 phosphorylation of the insulin receptor substrate-1 (IRS-1). Hepatocytes infected with cell culture-grown HCV genotype 1a or 2a displayed a significant increase in the Ser473 phosphorylation status of the Ser/Thr kinase protein kinase B (Akt/PKB), while Thr308 phosphorylation was not significantly altered. HCV core protein-mediated Ser312 phosphorylation of IRS-1 was inhibited by JNK (SP600125) and phosphatidylinositol-3 kinase (LY294002) inhibitors. Furthermore, a functional assay also suggested that hepatocytes expressing HCV core protein alone or infected with cell culture-grown HCV exhibited a suppression of 2-deoxy-d-[3H] glucose uptake. Inhibition of the JNK signaling pathway significantly restored glucose uptake despite HCV core expression in hepatocytes. Taken together, HCV core protein increases IRS-1 phosphorylation at Ser312 which may contribute in part to the mechanism of insulin resistance5.
Hepatitis C virus core protein inhibits microsomal triglyceride transfer protein activity and very low density lipoprotein secretion - Liver steatosis, which involves accumulation of intracytoplasmic lipid droplets, is characteristic of hepatitis C virus (HCV) infection. By use of an in vivo transgenic murine model, a study demonstrates that hepatic overexpression of HCV core protein interferes with the hepatic assembly and secretion of triglyceriderich very low density lipoproteins (VLDL). Core expression led to reduction in microsomal triglyceride transfer protein (MTP) activity and in the particle size of nascent hepatic VLDL without affecting accumulation of MTP and protein disulfide isomerase. Hepatic human apolipoprotein AII (apo AII) expression in doublecore/ apo AII transgenic mice diminished intrahepatic core protein accumulation and abrogated its effects on VLDL production. Apo AII and HCV core colocalized in human HCV-infected liver biopsies. This implied that core protein of HCV targets microsomal triglyceride transfer protein activity and modifies hepatic VLDL assembly and secretion6.
References
1. Choo QL, G Kuo, AJ Weiner, LR Overby, DW Bradley, and M Houghton (1989). Isolation of a cDNA clone derived from a blood-borne non-A, non-B viral hepatitis genome. Science, 244:359-362.
2. Grakoui A, McCourt DW, Wychowski C, Feinstone SM, Rice CM (1993). Characterization of the hepatitis C virus-encoded serine proteinase: determination of proteinase-dependent polyprotein cleavage sites. J. Virol, 67(5):2832-2843.
3. Yasui K, Wakita T, Tsukiyama-Kohara K, Funahashi SI, Ichikawa M, Kajita T, Moradpour D, Wands JR, Kohara M (1998).The native form and maturation process of hepatitis C virus core protein. J. Virol, 72(7):6048-55.
4. Liu Q, Tackney C, Bhat RA, Prince AM, Zhang P (1997). Regulated processing of hepatitis C virus core protein is linked to subcellular localization. J. Virol, 71:657-662.
5. Banerjee S, Saito K, Ait-Goughoulte M, Meyer K, Ray RB, Ray R (2008). Hepatitis C Virus Core Protein Upregulates Serine Phosphorylation of Insulin Receptor Substrate-1 and Impairs the Downstream Akt/Protein Kinase B Signaling Pathway for Insulin Resistance. J Virol, 82(6): 2606–2612.
6. Perlemuter G, Sabile A, Letteron, P, Vona, G, Topilco, A, Chre´tien, Y, Koike K, Pessayre D, Chapman J, Barba G, Bre´chot C (2002). Hepatitis C virus core protein inhibits microsomal triglyceride transfer protein activity and very low density lipoprotein secretion: a model of viral-related steatosis. FASEB J, 16(2):185-94.