The human fatty acid synthase: a new therapeutic target for coxsackievirus B3-induced diseases?

PPAR Ligands Induce Antiviral Effects Targeting Perturbed Lipid Metabolism during SARS-CoV-2, HCV, and HCMV Infection

Simple Summary

The current coronavirus disease 2019 pandemic turned the attention of researchers to developing novel strategies to counteract virus infections. Despite several antiviral drugs being commercially available, there is an urgent need to identify novel molecules efficacious against viral infections that act through different mechanisms of action. In this context, our attention is focused on novel compounds acting on nuclear receptors, whose activity could be beneficial in viral infections, including coronavirus, hepatitis C virus, and cytomegalovirus.

Abstract

The manipulation of host metabolisms by viral infections has been demonstrated by several studies, with a marked influence on the synthesis and utilization of glucose, nucleotides, fatty acids, and amino acids. The ability of virus to perturb the metabolic status of the infected organism is directly linked to the outcome of the viral infection. A great deal of research in recent years has been focusing on these metabolic aspects, pointing at modifications induced by virus, and suggesting novel strategies to counteract the perturbed host metabolism. In this review, our attention is turned on PPARs, nuclear receptors controlling multiple metabolic actions, and on the effects played by PPAR ligands during viral infections. The role of PPAR agonists and antagonists during SARS-CoV-2, HCV, and HCMV infections will be analyzed.

Enterovirus Replication Organelles and Inhibitors of Their Formation

Enteroviral replication reorganizes the cellular membrane. Upon infection, viral proteins and hijacked host factors generate unique structures called replication organelles (ROs) to replicate their viral genomes. ROs promote efficient viral genome replication, coordinate the steps of the viral replication cycle, and protect viral RNA from host immune responses. More recent researches have focused on the ultrastructure structures, formation mechanism, and functions in the virus life cycle of ROs. Dynamic model of enterovirus ROs structure is proposed, and the secretory pathway, the autophagy pathway, and lipid metabolism are found to be associated in the formation of ROs. With deeper understanding of ROs, some compounds have been found to show inhibitory effects on viral replication by targeting key proteins in the process of ROs formation. Here, we review the recent findings concerning the role, morphology, biogenesis, formation mechanism, and inhibitors of enterovirus ROs.

Phospholipid synthesis fueled by lipid droplets drives the structural development of poliovirus replication organelles

Rapid development of complex membranous replication structures is a hallmark of picornavirus infections. However, neither the mechanisms underlying such dramatic reorganization of the cellular membrane architecture, nor the specific role of these membranes in the viral life cycle are sufficiently understood. Here we demonstrate that the cellular enzyme CCTα, responsible for the rate-limiting step in phosphatidylcholine synthesis, translocates from the nuclei to the cytoplasm upon infection and associates with the replication membranes, resulting in the rerouting of lipid synthesis from predominantly neutral lipids to phospholipids. The bulk supply of long chain fatty acids necessary to support the activated phospholipid synthesis in infected cells is provided by the hydrolysis of neutral lipids stored in lipid droplets. Such activation of phospholipid synthesis drives the massive membrane remodeling in infected cells. We also show that complex membranous scaffold of replication organelles is not essential for viral RNA replication but is required for protection of virus propagation from the cellular anti-viral response, especially during multi-cycle replication conditions. Inhibition of infection-specific phospholipid synthesis provides a new paradigm for controlling infection not by suppressing viral replication but by making it more visible to the immune system.

Host lipidome analysis during rhinovirus replication in HBECs identifies potential therapeutic targets

In patients with asthma or chronic obstructive pulmonary disease, rhinovirus (RV) infections can provoke acute worsening of disease, and limited treatment options exist. Viral replication in the host cell induces significant remodeling of intracellular membranes, but few studies have explored this mechanistically or as a therapeutic opportunity. We performed unbiased lipidomic analysis on human bronchial epithelial cells infected over a 6 h period with the RV-A1b strain of RV to determine changes in 493 distinct lipid species. Through pathway and network analysis, we identified temporal changes in the apparent activities of a number of lipid metabolizing and signaling enzymes. In particular, analysis highlighted FA synthesis and ceramide metabolism as potential anti-rhinoviral targets. To validate the importance of these enzymes in viral replication, we explored the effects of commercially available enzyme inhibitors upon RV-A1b infection and replication. Ceranib-1, D609, and C75 were the most potent inhibitors, which confirmed that FAS and ceramidase are potential inhibitory targets in rhinoviral infections. More broadly, this study demonstrates the potential of lipidomics and pathway analysis to identify novel targets to treat human disorders.

Viral rewiring of cellular lipid metabolism to create membranous replication compartments

Positive-strand RNA (+RNA) viruses (e.g. poliovirus, hepatitis C virus, dengue virus, SARS-coronavirus) remodel cellular membranes to form so-called viral replication compartments (VRCs), which are the sites where viral RNA genome replication takes place. To induce VRC formation, these viruses extensively rewire lipid metabolism. Disparate viruses have many commonalities as well as disparities in their interactions with the host lipidome and accumulate specific sets of lipids (sterols, glycerophospholipids, sphingolipids) at their VRCs. Recent years have seen an upsurge in studies investigating the role of lipids in +RNA virus replication, in particular of sterols, and uncovered that membrane contact sites and lipid transfer proteins are hijacked by viruses and play pivotal roles in VRC formation.

Virus Factories and Mini-Organelles Generated for Virus Replication

Many viruses replicate and assemble in subcellular microenvironments called virus factories or ‘viroplasm.’ Virus factories increase the efficiency of replication and at the same time protect viruses from antiviral defenses. We describe how viruses reorganize cellular membrane compartments and cytoskeleton to generate these ‘mini-organelles’ and how these rearrangements parallel cellular responses to stress such as protein aggregation and DNA damage.

Activation of AMPK restricts coxsackievirus B3 replication by inhibiting lipid accumulation

Coxsackievirus B3 (CVB3) is the major pathogen of human viral myocarditis. CVB3 has been found to manipulate and modify the cellular lipid metabolism for viral replication. The cellular AMP-activated protein kinase (AMPK) is a key regulator of multiple metabolic pathways, including lipid metabolism. Here we explore the potential roles AMPK plays in CVB3 infection. We found that AMPK is activated by the viral replication during CVB3 infection in Hela cells and primary myocardial cells. RNA interference mediated inhibition of AMPK could increase the CVB3 replication in cells, indicating that AMPK contributed to restricting the viral replication. Next, we showed that CVB3 replication could be inhibited by several different pharmacological AMPK activators including metformin, A769662 and AICAR. And the constitutively active AMPK mutant (CA-AMPK) could also inhibit the CVB3 replication. Furthermore, we found that CVB3 infection increased the cellular lipid levels and showed that the AMPK agonist AICAR both restricted CVB3 replication and reduced lipid accumulation through inhibiting the lipid synthesis associated gene expression. We further found that CVB3 infection would also induce AMPK activated in vivo. The AMPK agonist metformin, which has been widely used in diabetes therapy, could decrease the viral replication and further protect the mice from myocardial histological and functional changes in CVB3 induced myocarditis, and improve the survival rate of infected mice. Lastly, it was demonstrated that the AICAR-mediated restriction of viral replication could be rescued partially by exogenous palmitate, the first product of fatty acid biosynthesis, demonstrating that AMPK activation restricted CVB3 infection through its inhibition of lipid synthesis. Taken together, these data in the present study suggest a model in which AMPK is activated by CVB3 infection and restricts viral replication by inhibiting the cellular lipid accumulation, and inform a potential novel therapeutic strategy for CVB3-associated diseases.

Liver X receptor activation enhances CVB3 viral replication during myocarditis by stimulating lipogenesis

AIMS

Viral myocarditis (VM) is severe cardiac inflammation that can result in sudden death or congestive heart failure in previously healthy adults, with no effective therapy. Liver X receptor (LXR) agonists have both anti-inflammatory and lipid-lowering properties. This study investigates whether LXR agonist T0901317 may modulate viral replication and cardiac inflammation during VM.

METHODS AND RESULTS

(i) Adult mice were administered T0901317 or vehicle with the onset of inflammation during CVB3 virus myocarditis or (ii) treated 2 days prior to CVB3 infection. Against what we expected, T0901317 treatment did not alter leucocyte infiltration after CVB3 infection; yet pre-administration with T0901317 resulted in increased mortality upon CVB3 infection, higher cardiac viral presence, and increased cardiomyocyte damage when compared with the vehicle. Furthermore, we show a correlation of fatty acid synthase (FAS) and sterol regulatory element-binding protein 1c (SREBP-1c) with CVB3 viral load in the heart and that T0901317 is able to enhance the cardiac expression of FAS and SREBP-1c. Finally, we show in vitro that T0901317 is able to exaggerate CVB3-mediated damage of Vero cells, whereas inhibitors of FAS and the SREBP-1c reduce the viral presence of CVB3 in neonatal cardiomyocytes.

CONCLUSION

LXR agonism does not modulate cardiac inflammation, but exacerbates virus-mediated myocardial damage during VM by stimulating lipid biosynthesis and enhancing CVB3 replication.

Antiviral activity of Bifidobacterium adolescentis SPM1605 against Coxsackievirus B3

Bifidobacteria are considered one of the most beneficial probiotics and have been widely studied for their effects against specific pathogens. The present study investigated the antiviral activity of probiotics isolated from Koreans against Coxsackievirus B3 (CVB3). The effect of probiotic isolates against CVB3 was measured by the plaque assay and cellular toxicity of bifidobacteria in HeLa cells was measured using 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Among 13 probiotic isolates, 3 Bifidobacterium adolescentis, 2 Bifidobacterium longum and 1 Bifidobacterium pseudocatenulatum had an antiviral effect against CVB3, while the others did not show such effect. B. adolescentis SPM1605 showed the greatest inhibitory properties against CVB3. When the threshold cycle (CT) values for the treated B. adolescentis SPM1605 samples were compared to the results for the non-treated samples, it was shown that the amplified viral sequences from the CVB3 had their copy number lowered by B. adolescentis SPM1605. Moreover, the gene expression in infected HeLa cells was also inhibited by 50%. The results suggest that B. adolescentis SPM1605 suppresses CVB3 and could be used as an alternative therapy against infectious diseases caused by coxsackieviruses.

Rewiring of cellular membrane homeostasis by picornaviruses

Viruses are obligatory intracellular parasites and utilize host elements to support key viral processes, including penetration of the plasma membrane, initiation of infection, replication, and suppression of the host's antiviral defenses. In this review, we focus on picornaviruses, a family of positive-strand RNA viruses, and discuss the mechanisms by which these viruses hijack the cellular machinery to form and operate membranous replication complexes. Studies aimed at revealing factors required for the establishment of viral replication structures identified several cellular-membrane-remodeling proteins and led to the development of models in which the virus used a preexisting cellular-membrane-shaping pathway "as is" for generating its replication organelles. However, as more data accumulate, this view is being increasingly questioned, and it is becoming clearer that viruses may utilize cellular factors in ways that are distinct from the normal functions of these proteins in uninfected cells. In addition, the proteincentric view is being supplemented by important new studies showing a previously unappreciated deep remodeling of lipid homeostasis, including extreme changes to phospholipid biosynthesis and cholesterol trafficking. The data on viral modifications of lipid biosynthetic pathways are still rudimentary, but it appears once again that the viruses may rewire existing pathways to generate novel functions. Despite remarkable progress, our understanding of how a handful of viral proteins can completely overrun the multilayered, complex mechanisms that control the membrane organization of a eukaryotic cell remains very limited.

Increased long chain acyl-Coa synthetase activity and fatty acid import is linked to membrane synthesis for development of picornavirus replication organelles

All positive strand (+RNA) viruses of eukaryotes replicate their genomes in association with membranes. The mechanisms of membrane remodeling in infected cells represent attractive targets for designing future therapeutics, but our understanding of this process is very limited. Elements of autophagy and/or the secretory pathway were proposed to be hijacked for building of picornavirus replication organelles. However, even closely related viruses differ significantly in their requirements for components of these pathways. We demonstrate here that infection with diverse picornaviruses rapidly activates import of long chain fatty acids. While in non-infected cells the imported fatty acids are channeled to lipid droplets, in infected cells the synthesis of neutral lipids is shut down and the fatty acids are utilized in highly up-regulated phosphatidylcholine synthesis. Thus the replication organelles are likely built from de novo synthesized membrane material, rather than from the remodeled pre-existing membranes. We show that activation of fatty acid import is linked to the up-regulation of cellular long chain acyl-CoA synthetase activity and identify the long chain acyl-CoA syntheatse3 (Acsl3) as a novel host factor required for polio replication. Poliovirus protein 2A is required to trigger the activation of import of fatty acids independent of its protease activity. Shift in fatty acid import preferences by infected cells results in synthesis of phosphatidylcholines different from those in uninfected cells, arguing that the viral replication organelles possess unique properties compared to the pre-existing membranes. Our data show how poliovirus can change the overall cellular membrane homeostasis by targeting one critical process. They explain earlier observations of increased phospholipid synthesis in infected cells and suggest a simple model of the structural development of the membranous scaffold of replication complexes of picorna-like viruses, that may be relevant for other (+)RNA viruses as well.

Eukaryotic cells feature astonishing complexity of regulatory networks, yet control over this fine-tuned machinery is easily overrun by viruses with expression of just a handful of proteins. One of the striking examples of such hostile take-over is the rewiring of normal cellular membrane metabolism by (+)RNA viruses towards development of new membranous organelles harboring viral replication machinery. (+)RNA viruses of eukaryotes infect organisms from unicellular algae to humans. Many of them induce diseases resulting in significant economic losses, public health burden, human suffering and sometimes fatal consequences. We show how picornaviruses reorganize cellular lipid metabolism by targeting long chain acyl-CoA synthetase activity. This induces increased import of fatty acids in infected cells and up-regulation of phospholipid synthesis, resulting in formation of replication organelles different from the pre-existing cellular membranes. This mechanism is utilized by diverse viruses and may represent an attractive target for anti-viral interventions.

Virus factories, double membrane vesicles and viroplasm generated in animal cells

Many viruses reorganise cellular membrane compartments and the cytoskeleton to generate subcellular microenvironments called virus factories or 'viroplasm'. These create a platform to concentrate replicase proteins, virus genomes and host proteins required for replication and also protect against antiviral defences. There is growing interest in understanding how viruses induce such large changes in cellular organisation, and recent studies are beginning to reveal the relationship between virus factories and viroplasm and the cellular structures that house them. In this review, we discuss how three supergroups of (+)RNA viruses generate replication sites from membrane-bound organelles and highlight research on perinuclear factories induced by the nucleocytoplasmic large DNA viruses.

West Nile virus replication requires fatty acid synthesis but is independent on phosphatidylinositol-4-phosphate lipids

West Nile virus (WNV) is a neurovirulent mosquito-borne flavivirus, which main natural hosts are birds but it also infects equines and humans, among other mammals. As in the case of other plus-stranded RNA viruses, WNV replication is associated to intracellular membrane rearrangements. Based on results obtained with a variety of viruses, different cellular processes have been shown to play important roles on these membrane rearrangements for efficient viral replication. As these processes are related to lipid metabolism, fatty acid synthesis, as well as generation of a specific lipid microenvironment enriched in phosphatidylinositol-4-phosphate (PI4P), has been associated to it in other viral models. In this study, intracellular membrane rearrangements following infection with a highly neurovirulent strain of WNV were addressed by means of electron and confocal microscopy. Infection of WNV, and specifically viral RNA replication, were dependent on fatty acid synthesis, as revealed by the inhibitory effect of cerulenin and C75, two pharmacological inhibitors of fatty acid synthase, a key enzyme of this process. However, WNV infection did not induce redistribution of PI4P lipids, and PI4P did not localize at viral replication complex. Even more, WNV multiplication was not inhibited by the use of the phosphatidylinositol-4-kinase inhibitor PIK93, while infection by the enterovirus Coxsackievirus B5 was reduced. Similar features were found when infection by other flavivirus, the Usutu virus (USUV), was analyzed. These features of WNV replication could help to design specific antiviral approaches against WNV and other related flaviviruses.

Role of cellular lipids in positive-sense RNA virus replication complex assembly and function

Positive-sense RNA viruses are responsible for frequent and often devastating diseases in humans, animals, and plants. However, the development of effective vaccines and anti-viral therapies targeted towards these pathogens has been hindered by an incomplete understanding of the molecular mechanisms involved in viral replication. One common feature of all positive-sense RNA viruses is the manipulation of host intracellular membranes for the assembly of functional viral RNA replication complexes. This review will discuss the interplay between cellular membranes and positive-sense RNA virus replication, and will focus specifically on the potential structural and functional roles for cellular lipids in this process.