Cellular Determinants of Hepatitis C Virus Assembly, Maturation, Degradation, and Secretion

Abstract

Intracellular infectious hepatitis C virus (HCV) particles display a distinctly higher buoyant density than do secreted virus particles, suggesting that the characteristic low density of extracellular HCV particles is acquired during viral egress. We took advantage of this difference to examine the determinants of assembly, maturation, degradation, and egress of infectious HCV particles. The results demonstrate that HCV assembly and maturation occur in the endoplasmic reticulum (ER) and post-ER compartments, respectively, and that both depend on microsomal transfer protein and apolipoprotein B, in a manner that parallels the formation of very-low-density lipoproteins (VLDL). In addition, they illustrate that only low-density particles are efficiently secreted and that immature particles are actively degraded, in a proteasome-independent manner, in a post-ER compartment of the cell. These results suggest that by coopting the VLDL assembly, maturation, degradation, and secretory machinery of the cell, HCV acquires its hepatocyte tropism and, by mimicry, its tendency to persist.Hepatitis C virus (HCV) establishes persistent infection in Ͼ70% of infected individuals (25), and over 170 million people are persistently infected worldwide. Persistent HCV infection is associated with a chronic inflammatory disease (hepatitis) that ultimately leads to hepatic fibrosis, cirrhosis, and hepatocellular carcinoma (25). Chronic infection is also associated with disorders of lipid metabolism (54), with abnormal accumulation of lipids in the liver parenchymal cells (steatosis) and reduced serum beta-lipoprotein levels (52). Currently, the only approved antiviral therapy for HCV is the administration of type I interferon combined with ribavirin. However, this therapy is toxic and is effective in only a fraction of cases (43).HCV is the sole member of the genus Hepacivirus, which belongs to the Flaviviridae family. The virus is enveloped, and the single-stranded positive-strand RNA genome contains a single open reading frame flanked by untranslated regions (5Ј UTR and 3Ј UTR) that contain RNA sequences essential for RNA translation and replication (17,18). Translation of the single open reading frame is driven by an internal ribosomal entry site (IRES) sequence present within the 5Ј UTR (24), and the resulting polyprotein, of approximately 3,000 amino acids in length, is processed by cellular and viral proteases into its individual components (44). The nonstructural proteins NS3, NS4A, NS4B, NS5A, and NS5B are sufficient to support efficient HCV RNA replication in membranous compartments in the cytosol (10,35,39). Overexpression of the core, E1, and E2 proteins is sufficient for the formation of virus-like structures in insect cells (6), and expression of the viral polyprotein leads to the formation of virus-like particles in HeLa (38) and Huh-7 (23) cells. It has been proposed that infectious particles are assembled when genomic RNA-containing core particles bud through the endoplasmic reticulum (ER) membrane (47), acquiring the v...

Entry and replication of recombinant hepatitis C viruses in cell culture

“…It seems that HCV can hijack this function for its own assembly as it circulates in patient serum in association with lipoproteins [22,23]. Moreover, host factors crucial for production of VLDL like apolipoprotein E, apolipoprotein B and microsomal triglyceride transfer protein have been found to be important for production and release of infectious HCV particles [24][25][26][27][28][29]. The association of HCV with lipoproteins influences cell entry with the involvement of several lipoprotein (LDL receptor) [30,31] and lipid receptors (Scavenger receptor class B type I, SR-BI; Niemann-Pick C1-like 1 cholesterol uptake receptor, NPC1L1) [32,33].…”

Hepatitis C virus selectively perturbs the distal cholesterol synthesis pathway in a genotype-specific manner

“…13 Direct viral perturbation of cholesterol biosynthetic pathways and their regulation is suggested from experimental in vitro and animal models. [14][15][16][17][18] HCV-mediated oxidative stress has been suggested as a possible causative mechanism leading to both HCV-associated hepatic steatosis and IR, and has also been linked to cholesterol regulation. 15,19 Finally, from the host perspective, it is not known why compensatory regulation to increase cholesterol synthesis appears inadequate to resolve HCV-associated hypolipidemia.…”

High plasma level of nucleocapsid-free envelope glycoprotein-positive lipoproteins in hepatitis C patients

“…First, alteration of the lipoprotein pathway by inhibition of the microsomal triglyceride transfer protein (MTP) or of the diacylglycerol acyltransferase-1 (DGAT-1) or silencing of apoB or apoE expression decreases the production of infectious HCVcc virions. [12][13][14] Second, the phospholipid compositions of HCVcc and TRL share similar characteristics, whereas they strikingly differ from those of cellular membranes or envelopes of virus that assemble at cellular membranes. [15][16][17] Furthermore, lipoprotein lipases that specifically hydrolyse lipoprotein triacylglycerol modify HCVcc biochemical and physical features and decrease their infectivity.…”