Definition
Hepatitis C virus (HCV) is a single-strand RNA virus. The genome encodes a large precursor polyprotein of approximately 3,000 aa which is processed into 10 viral proteins by host and viral proteases. Highly immunodominant HCV peptides are recognized by both cellular and humoral immunities.
Discovery
HCV was discovered by Choo et al., 1989. 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 hepatitis 1. Several HCV related peptides have been used to influence HCV viral replication, HCV Core Protein (1 - 20), HCV - 1 e2 Protein (484 - 499), 4A/4B peptide, 5A/5B peptide.
Structural Characteristics
HCV genome encodes a large precursor polyprotein which 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 proteases 2. HCV core protein forms the nucleocapsid, which is surrounded by the envelope containing E1 and E2 glycoproteins 3. HCV core protein has two major p23 and p21 forms. 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 processed by 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.
Sequence of HCV Core Protein (1 - 20) is H - Met - Ser - Thr - Asn - Pro - Lys - Pro - Gln - Arg - Lys - Thr - Lys - Arg - Asn - Thr - Asn - Arg - Arg - Pro - Gln – OH 4 and that of HCV - 1 e2 Protein (484 - 499) is H - Pro - Tyr - Cys - Trp - His - Tyr - Pro - Pro - Lys - Pro - Cys - Gly - Ile - Val - Pro - Ala – OH 5.
Mode of Action
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. 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 6. An in vivo transgenic murine model 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 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 suggests that core protein of HCV targets microsomal triglyceride transfer protein activity and modifies hepatic VLDL assembly and secretion 7.
Functions
HCV Core Protein (1 - 20), This N-terminal truncated (residues 1 to 20) HCV core protein fragment is able to bind to the HCV internal ribosome entry site (IRES), and as a result influences the level of HCV replication 4.
HCV-1 envelope 2 (e2) protein fragment 484 to 499 is one of the two major antigenic regions of the envelope 2 protein of the hepatitis C virus (HCV) 5.
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 resistance 6.
Microsomal triglyceride transfer, 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 HCV infection 7.
References
1. Choo QL, Kuo G, Weiner AJ, Overby LR, Bradley DW, Houghton M (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):6048-6055.
4. Li D, Takyar ST, Lott WB, Gowans EJ (2003). 313. Amino acids 1-20 of the hepatitis C virus (HCV) core protein specifically inhibit HCV IRES-dependent translation in HepG2 cells, and inhibit both HCV IRES- and cap-dependent translation in HuH7 and CV-1 cells. J. Gen. Virol., 84(4):815-825.
5. Zhang ZX, Sönnerborg A, Sällberg M (1994). Antigenic structure of the hepatitis C virus envelope 2 protein. Clin. Exp Immunol., 98(3):382-387.
6. 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.
7. 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., 16(2):185-194.