Human Apolipoprotein E is necessary and sufficient for production of infectious HCV particles in human non-liver cell lines

Abstract

HCV has a narrow tissue tropism. Accordingly, efficient replication is only observed in the liver and in cultured human liver cell lines. The molecular determinants that are responsible for the restricted tropism of HCV are incompletely understood. Recent evidence indicates that HCV assembly is tightly coupled to the synthesis of human lipoproteins that occurs in liver cells. Specifically, Apolipoprotein B (ApoB), ApoE, and microsomal triglyceride transfer protein (MTP), which are all essential for production of very low density lipoproteins (VLDL) have been implicated in HCV assembly. To define the minimal liver cell-specific requirements for production of infectious HCV progeny, we attempted to reconstitute assembly of HCV particles in human non-liver cell lines. To this end, we created stable HeLa (human cervical cancer) and Huh-7.5 (human hepatocarcinoma) cells replicating selectable subgenomic HCV RNA replicons. Co-expression of viral structural proteins reconstituted particle production only in Huh-7.5 replicon cells, but not in HeLa replicon cells. Importantly, ectopic expression of ApoE was necessary and sufficient to allow production of infectious HCV trans-complemented particles (HCVtcp) in HeLa cells. Comparison between Huh-7.5 and HeLa derived particles revealed no gros differences between the density distribution of intra- and extracellular particle. Using a similar approach, we could also observe ApoE-dependent production of infectious HCVtcp in several other non-liver cell lines. Enhancement of HCV replication by introduction of miR-122 into ApoE-expressing 293T and HeLa cells was sufficient for production of full-length HCV particles. More detailed analyses of particle composition and receptor usage of these particle types are currently ongoing. Collectively, these data indicate that ApoE may be the sole liver cell-specific cofactor for assembly of infectious HCV particles. In turn, ApoE may be major determinant limiting HCV tissue tropism.