Targeting liver metabolism: a pathway to cure hepatitis B virus?

KEYWORDS:

• Antiviral
• functional cure
• host-targeting therapies
• subviral particles
• viral hepatitis

1. Introduction

Viruses are considered the ultimate exploiters of cellular processes. With a minimal set of instructions, they manipulate cells into producing and amplifying additional copies of themselves. Viruses cannot use the individual components of a cell to do this: infecting a dead cell is a dead end. Instead, viruses are obligate parasites of the metabolic processes of a cell, which present potential targets for antiviral therapies.

An ideal example of this approach is hepatitis C virus (HCV), a liver-tropic virus whose replication depends on (and stimulates production of) intracellular lipid droplets. HCV replication has been inhibited through therapeutic regulation of cellular metabolic processes, including cholesterol metabolism, fatty acid metabolism, and nucleotide metabolism [1].

For hepatitis B virus (HBV, an incurable and common blood-borne infection), treatments targeting metabolic pathways remain relatively under-explored. Current HBV antivirals have little effect on reducing persistent forms of the virus (covalently closed circular DNA and integrated DNA) or preventing the secretion of immunomodulatory subviral particles (SVP) [2]. The loss of circulating SVPs represents a ‘functional cure’ and has been linked to better clinical outcomes [3].

Several characteristics make HBV an ideal candidate for effective therapeutics targeting host metabolism pathways. HBV has co-evolved with humans for millennia and is highly dependent on cellular pathways for propagation. Moreover, the virus is evolutionarily constrained by its dense genomic structure (every nucleotide in its genome codes for a protein and one-third of the genome codes for two proteins) and cannot easily mutate around these restrictions. Indeed, multiple cellular metabolic pathways are used by HBV for replication and secretion, offering potential targets for antiviral therapies.

2. Host metabolic pathways and their interactions with HBV replication

2.1. Autophagy

Autophagy is the cellular process of degrading and recycling cellular components. Autophagy plays an essential role in maintaining cellular energy balance and can be used by HBV for its replication [4]. Supporting this, intracellular HBV nucleocapsids are physically associated with autophagosomes and phagophores in infected cells [5]. Moreover, increasing autophagosome activity with the autophagosome degradation inhibitor bafilomycin A1 increased HBV replication by threefold and secretion of virions by eightfold [5].

Disrupting the autophagy metabolism pathway may be a promising target for preventing HBV secretion and replication. In a HBV transfection model using Huh−7 cells, inhibiting autophagosome formation with 3-methyladenine reduced HBV DNA replication and virion secretion by 20% and 80% compared to controls, respectively [4]. In the same model, disrupting the fusion between autophagosomes and multivesicular bodies by silencing Rab11 reduced the secretion of HBV by 90% compared to controls [5]. It is unclear, however, if these autophagic pathways can be targeted with great enough specificity to lead to antiviral therapeutic effects.

2.2. Lipid metabolism

The liver is an essential site for the body’s breakdown, synthesis, transport, and storage of lipids. These processes can be usurped by virus infections: for example, HCV dysregulates lipid metabolism, leading to intracellular accumulation of lipid droplets that act as platforms for viral replication and assembly [6]. Furthermore, inhibiting host gene TM6SF2, which regulates lipid secretion through the ER/Golgi pathway (the HCV secretion pathway), decreases HCV secretion by 50% in multiple in vitro models [7].

HBV infection in vivo appears to induce less dramatic alterations in cellular phenotype, but instead induces changes in some lipid metabolic pathways [8]. Chemical inhibitors of key lipid metabolism enzymes, such as acetyl CoA carboxylase and fatty acid synthase, have been shown to reduce the secretion of HBV SVPs by 75% in HBV-infected HepG2-NTCP cells [8].

Bile acid synthesis is an additional specialized arm of the liver lipid metabolism pathways. A major controller of bile acid synthesis is the transcription factor farnesoid X receptor, which also affects HBV transcription. Agonists to the farnesoid X receptor have shown anti-HBV activity in both in vitro and in vivo models [9], but so far results in clinical trials have been limited [10].

2.3. Glucose metabolism

The liver is also heavily involved in the synthesis, transport, and storage of glucose, pathways that can be hijacked by HBV. HBV activates anaerobic glycolysis to suppress RIG-I-like receptor (RLR) signaling, nominally to avoid the interferon-mediated immune response [11]. In primary human hepatocytes infected with HBV, treatment with the pyruvate transport inhibitor UK5099 or hypoxia (which increases anaerobic glycolysis) significantly enhanced HBV replication by 1.2-fold and SVP secretion by up to 3.5-fold [11]. This effect may be in part due to cellular hypoxia-inducible factors that promote viral transcription by directly activating the HBV promoters [12]. Conversely, treatment with dichloroacetate and galactose (both of which promote oxidative phosphorylation) reduced HBV replication by 17% and SVP secretion by 60-70% [11]. This antiviral activity was suggested to be due to increased levels of lactate (resultant from anaerobic respiration) antagonizing antiviral signaling proteins [13].

2.4. Endoplasmic reticulum glucosidases

Endoplasmic reticulum glucosidases are a group of enzymes that play a crucial role in protein glycosylation and HBV SVP secretion [14]. For example, the treatment of HepG2.2.15 cells that stably express HBV with deoxynojirimycin (a small-molecule inhibitor of α-glucosidase) led to significant degradation of medium HBV surface protein by 86% as well as large HBV surface protein by 56% [15].

3. Future perspectives and challenges

Targeting host metabolism as an antiviral strategy presents several advantages over those targeting viral proteins. Firstly, host-directed therapies can potentially have broader antiviral effects with reduced risk of antiviral resistance, as the same host metabolic pathways are likely used by multiple strains, genotypes, or even other related viruses. Moreover, there is less genetic variation in the human population, compared to the diversity in viral genomes by several orders of magnitude, leading to improved target engagement and specificity of potential therapeutics.

Nevertheless, several hurdles remain to be overcome for host-directed therapeutic strategies to enter the clinical setting. One of the main challenges is the potential for side-effects. Given that chronic hepatitis B is asymptomatic for much of its course, it is important to minimize the risk of adverse effects for optimal treatment. Another challenge is the variation in human populations. Although not as diverse as virus populations, these include genetic differences, epigenetic changes, nutritional status, and the presence of comorbidities which could lead to undesired outcomes or side effects. These factors can influence the efficacy and safety of antiviral therapies and need to be carefully considered in the design and development of new approaches. Additionally, an incomplete understanding of cellular metabolic pathways could hinder the development of these therapies. The interactions between the virus and host are complex and inhibiting one pathway may lead to other, as yet unknown pathways that affect the pathogenesis of HBV.

4. Expert opinion

The targeting of host metabolic pathways could be a promising approach in the fight against viral infections. This strategy has been explored to inhibit HCV replication and secretion but is still nascent in the HBV field. Exploiting the dependence of HBV on various host metabolic pathways offers a potential avenue to overcome the limitations of directly targeting viral proteins. While current antiviral therapies have been effective in suppressing viral replication, they are rarely curative and hence require long-term treatment. In contrast, targeting host metabolic pathways could be a complementary approach to achieve a cure.

Targeting metabolic pathways involved in either the secretion of SVPs or the inhibition of the innate immune response could be promising strategies. Given that SVPs support the persistence of HBV, targeting lipid metabolism pathways could potentially inhibit their secretion. Host glucose metabolism pathways, which seem to be involved in suppressing the interferon response, could similarly be manipulated therapeutically to induce de-repression of the immune system and promote viral clearance. Combining host-targeted treatments with existing antiviral therapies would likely synergize their therapeutic effects and potentially achieve a functional cure for chronic hepatitis B. While such agents are not yet ready for clinical use, with continued research the field could develop additional approaches to shape a better future for the 350 million people worldwide living with chronic hepatitis B.

Declaration of interest

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

Work on this manuscript was in part supported by Ideas grants 2002565 and 2021586 from the National Health and Medical Research Council (NHMRC) of Australia, The University of Sydney, and by the Robert W Storr bequest to the Sydney Medical Foundation, University of Sydney.