Hepatitis C Virus Diversity and Hepatic Steatosis

P. Roingeard

Disclosures

J Viral Hepat. 2013;20(2):77-84. 

In This Article

HCV Sequences Involved in Steatosis

Studies in early experimental in vitro expression systems and transgenic mouse models showed that the HCV core protein was sufficient to induce triglyceride accumulation in hepatocytes.[20,41,42] All these early reports were based on studies using constructs derived from a genotype 1 genome. More recent investigations have been conducted with in vitro cellular models expressing the HCV genotype 3 core protein that has been found to result in higher levels of triglyceride accumulation than the genotype 1 core protein.[8] These functional differences between genotype 1 and 3 core proteins may result from differences in their amino acid sequences, although these sequences are very similar. The search for viral sequences responsible for genotype 3 core protein-specific effects on triglyceride accumulation led to the identification of a single amino acid change, at position 164.[9] A phenylalanine (F) residue in this position, as found in genotype 3 core sequences, has been shown to be specifically associated with higher levels of lipid droplet accumulation in cellular models in vitro. In almost all the core sequences of viruses of other genotypes, this residue is replaced by a tyrosine (Y).[9] Similarly, higher levels of lipid droplet accumulation were observed in a study in which an HCV core protein sequence from a genotype 3 virus was produced in a cellular model.[8] However, the authors were unable to identify the residues involved in this phenomenon and did not implicate F164. Nevertheless, this particular residue was directly implicated in the stronger FAS activation observed with the genotype 3 core protein.[27] Another study reporting the sequencing of the core gene from genotype 3 viruses infecting patients with or without steatosis led to the identification of two additional residues correlated with lipid accumulation: phenylalanine-valine (FV) or leucine-isoleucine (LI) at positions 182 and 186 (as opposed to FI).[10] The production in vitro of core proteins with these FV or LI residues resulted in significantly higher intracellular lipid levels than the production of core proteins with FI residues in these positions. However, in two other independent studies, analyses of the sequences of various clinical isolates failed to implicate the FV and LI residues in positions 182 and 186, or any other genetic differences between genotype 3 core proteins from patients with and without steatosis, suggesting a possible role for other factors in the development of steatosis in patients infected with genotype 3 viruses.[43,44] In all these bioclinical studies, it was difficult to evaluate the role of the F164 residue in liver steatosis, because this residue was present in all genotype 3 isolates, even those from patients without steatosis[10,43,44] (Fig. 3). Nevertheless, an F residue was never detected in position 164 in any of the genotype 1 strains associated with severe steatosis.[44]

Figure 3.

Overview of the main results obtained in studies investigating the impact of the HCV core protein sequence on hepatic steatosis development. LD, lipid droplets. FAS, fatty acid synthetase. HCVcc, HCV in cell culture.

Overall, studies using in vitro cellular models to compare the intracellular lipid accumulation induced by genotype 3 and genotype 1 core proteins have not generated consistent results[8,9,10,27,43] (Fig. 3). These differences may be due to the different cell models and lipid droplet quantification methods used in these studies. One might assume that human liver cell lines would constitute the best model, but Huh7 cells have been reported to have intracellular lipid levels in standard culture conditions too high for the detection of meaningful differences between the proteins studied.[10] This was confirmed with the HCV in cell culture (HCVcc) model, as viruses bearing structural proteins (the core and envelope E1 and E2 proteins) from a genotype 2 or a genotype 3 strain induced the accumulation of similar numbers of lipid droplets when propagated in Huh7 cells.[45] Furthermore, despite a polymorphism between the genotype 2 strains J6 and JFH1 at residue 164 (Y in JFH-1, F in J6), the propagation of JFH-1 and J6/JFH-1 HCVcc in Huh7 cells did not result in any significant differences in lipid droplet amount (unpublished personal observation). However, one study comparing these two viruses has shown that a virus bearing the J6 core sequence is more efficiently assembled and secreted from infected cells but has lower ability to induce the LD clustering.[46] This suggests that the viral assembly efficiency by itself might influence the lipid storage, through the core protein dependent use of the LD as viral assembly platform. Thus, even if the core protein is central in this process, the effect of the HCV infection on the cellular lipid load might also depend on the interaction between this protein and other viral partners involved in virion assembly. Interestingly, the binding of the core protein to the nonstructural NS5A protein is critical for virus particle assembly,[47] and this protein has been shown to also interact with lipid metabolism,[48] inducing steatosis in a transgenic mouse model.[49] As a result, one bioclinical study also analysed the NS5A sequences of the viral variants circulating in patients with and without steatosis.[44] No differences in the sequence of the NS5A gene were found between patients with and without steatosis, suggesting that other viral proteins and, probably, host factors are involved in the development of steatosis in genotype 3 infection. However, although recent data have suggested that the polymorphism of various host genes, including the PPARγ,IL-28B,adiponutrin and MTP genes, may influence the development of more severe steatosis in chronic carriers of HCV, this phenomenon seems to concern principally patients infected with nongenotype 3 viruses.[50,51,52,53,54,55,56]

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