Viruses, lacking their self-contained metabolism, rely on the host cell machinery to facilitate their replication. This dependency drives viruses to manipulate the intracellular flow of carbon, modifying the uptake and breakdown of glucose and amino acids within the host cell, consequently creating an environment conducive to their replication [1]. In vitro, cells depend upon two primary carbon sources: glucose and glutamine. The cell prefers glucose oxidation through glycolysis, tricarboxylic acid (TCA) cycle, and oxidative phosphorylation, efficiently generating ATP and fulfilling biosynthetic requirements. Glycolysis intermediates branch into the pentose phosphate pathway (PPP), amino acid synthesis, and lipid biosynthesis. Nonetheless, increased anabolic activity or limited pyruvate levels, which might arise due to stress, pathology, or viral infections, can prompt cells to switch to glutamine utilization. Glutamine serves as a carbon source, replenishing intermediates within the TCA cycle through anaplerosis [2, 3]. Cells utilize glutamine as a carbon and nitrogen source for creating nucleotides, amino acids, and lipids. Further, glutamine controls gene expression, protein assembly, and intracellular signaling pathways [3].
Reports have extensively documented the distinct metabolic changes that occur during the replication of various viruses. Viruses such as HCMV and vaccinia exhibit an increased glutamine utilization by the infected cells, effectively manipulating their host's metabolic pathways [4, 5]. Conversely, dengue, rhinovirus, and hepatitis C stimulate heightened glucose consumption [6, 7, 8]. These metabolic alterations are a strategic means for viruses to dysregulate crucial enzymes and metabolites involved in glucose, amino acid, nucleotide, and fatty acid metabolism. The virus-driven activation of host factors, phosphoinositide 3-kinase, hypoxia-inducible factor 1, and heat shock proteins (HSPs) has been shown to modulate host cell metabolism [9, 10, 11]. HEV also has been shown to interact with these host factors, which in turn might modulate host metabolic processes [12, 13]. Collectively, these studies reveal a dual nature in viral metabolic requirements, showing shared attributes and distinctive alterations in the host cell's metabolome induced by different viruses [14].
Hepatitis E virus (HEV), a positive-sense single-stranded RNA [ssRNA(+)] virus belonging to the Hepeviridae family, is classified into eight genotypes (GT): GTs 1 to 8. HEV GT1 predominates in endemic regions across Asia and Africa, predominantly transmitting to susceptible human hosts through drinking water contaminated with fecal matter containing the virus. Approximately 20 million HEV infections contribute to an annual risk of nearly 70,000 fatalities globally [15]. HEV exhibits rapid replication kinetics upon infection, leading to peak viremia within days of infiltrating hepatocytes. This is succeeded by a gradual decline, reaching undetectable levels and eventual clearance within 2 to 3 weeks [16]. A recent longitudinal study has demonstrated an association between acute hepatitis E and altered serum metabolite and lipid profiles, suggesting a substantial metabolic impact of the virus [17]. The HEV-infected human liver tissue analysis faces limitations, while the absence of a cell culture system for HEV GT1 severely impedes insights into host-virus interactions in vitro; hence, its metabolic requirements are not very well understood.
In a previous study, viral variants from a created full-length mutant (MT) HEV genomes demonstrated replication ability [18]. The full-length genomes can produce genetically linked viral variants, a characteristic that is strongly desired for studying the biology of HEV, an error-prone ssRNA(+) virus. We took advantage of this system to investigate the changes in global cellular metabolism resulting from HEV replication. The replication of HEV induced distinct changes, particularly impacting glycolysis, the TCA cycle, and nitrogen metabolism. Multiple metabolites within the hexosamine biosynthesis pathway (HBP) exhibited upregulation, a response linked to elevating critical cellular protector, HSP70, known to alleviate cellular stress [19, 20]. Additionally, inhibition of glycolysis and HSP70 activity or knockdown of glutamine fructose-6-phosphate amidotransferase (GFAT), the rate-limiting enzyme of HBP, reduced viral replication and HSP70 levels. The preliminary findings of this study highlight the critical roles of glucose and glutamine in driving HEV replication, with the HBP appearing to play a supporting role in this process.