The intertwined fate: exploring the hepatitis C virus – Nrf1 crosstalk
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Despite advances in knowledge and medicine, hepatitis C virus (HCV) infection remains a global challenge. The viral life cycle heavily depends on lipid metabolism; therefore, HCV infection is associated with profound changes in host lipid homeostasis. The transcription factor nuclear factor erythroid 2 related factor-1 (Nrf1) is one of the regulators maintaining this homeostasis. Nrf1 exists in multiple proteoforms that differ in their capacity to serve as cholesterol sensor, activator or inhibitor of gene expression. We have previously identified that the amount of full-length Nrf1 protein in HCV-replicating cells is significantly reduced. Here, we investigate whether HCV affects the formation of the different proteoforms and their functionality using Western blot, qPCR, CLSM and FRET acceptor-photobleaching methods. We report that HCV infection does not alter the onset of Nrf1 proteoforms generated through proteasomal cleavage of the protein. However, the amount of different Nrf1 proteoforms is significantly reduced in HCV-positive cells due to enhanced Nrf1 turnover. Furthermore, the Nrf1 proteoforms with transcriptional activator functions are prevented from translocation into the nucleus. Reduced Nrf1 activity contributes to elevated cholesterol levels and favors lipid droplets formation, which serve as a central platform for viral morphogenesis. Conversely, rescue of Nrf1 activity in HCV-replicating cells is associated with decreased intracellular cholesterol levels, reduced number of lipid droplets and impaired viral release, which is reflected by intracellular accumulation of the core protein and intact viral particles. Taken together, our results characterize the so far not investigated complex interplay between HCV and Nrf1. HCV-mediated inhibition of Nrf1 functionality leads to intracellular cholesterol accumulation, resulting in enhanced lipid droplet formation that supports the HCV life cycle and contributes to HCV-associated pathogenesis.
Author Summary
The lack of a vaccine and limited access to effective drugs (pan-genotypic direct-acting antivirals) for curing hepatitis C virus (HCV) infection means that HCV remains an ongoing and urgent challenge worldwide. In light of this, a deeper understanding of the virus-host interaction is required. In this study, we investigate the interplay between HCV and lipid metabolism, focusing on the uncharacterized role of the cholesterol sensor and transcription factor Nrf1 in this interaction. We observe the inhibition of Nrf1 activity in HCV-replicating cells, which leads to enhanced intracellular cholesterol accumulation and lipid droplet formation, resulting in microenvironment favorable for viral morphogenesis. We reveal the underlying mechanisms and describe their relevance to the viral life cycle and virus-associated pathogenesis.
