Influenza A virus (IAV) is an important zoonotic pathogen that can cause disease in animals such as poultry and pigs, and it can cause infection and even death in humans, posing a serious threat to public health. IAV is an enveloped virus that relies on host cell metabolic systems, especially lipid metabolism systems, to complete its life cycle in host cells. On the other side, host cells regulate their metabolic processes to prevent IAV replication and maintain their normal physiological functions.
1. Positive and Negative Regulation of FA Metabolism in IAV Infection
FA is closely related to the inflammatory and allergic responses of organisms and widely involved in the regulation of IAV infection. Studies have shown that compared with non-obese mice, obese mice will have more severe inflammatory reaction after IAV infection due to abnormal FA metabolism
[1]. By analyzing the lipid metabolites of IAV-infected mice, Morita et al. also found that polyunsaturated fatty acids (PUFA) 12-hydroxyeicosatetraenoic acid (12-HETE), 15-hydroxyeicosatetraenoic acid (15-HETE), 17-hydroxy-4,7,10,13,15,19-docosahexaenoic acid (17-HDoHE) and omega-3 PUFA-derived lipid mediator protectin D1 (PD1) inhibited IAV infection after exogenous addition and reduced the lethality of IAV infection
[2]. Further studies showed that PD1 specifically inhibited the nuclear export of IAV mRNA and suppressed viral replication
[3]. Another in vitro study showed that lipoxygenin B4 (LXB4), derived from omega-6 PUFA arachidonic acid (AA), promoted the expression of B lymphocyte-induced maturation protein 1 (Blimp1) and X-box binding protein 1 (XBP1), the key transcription factors involved in plasma cell differentiation, thereby increasing the number of effector B cells and production of IgG, which promoted host resistance to IAV
[4]. In addition, through the study on the basic metabolism of lung injury and multiple organ failure caused by human H7N9 virus infection, it was found that palmitic acid, a saturated fatty acid and erucic acid, omega-9 long chain monounsaturated fatty acid were negatively correlated with the severity of the disease, and could be used as key molecules for the prognosis of the disease
[5]. In conclusion, FA such as PD1, LXB4, palmitic acid and erucic acid have negative regulatory effects on the infection process of IAV, which provides a scientific reference for the prevention and control of influenza and disease prognosis.
FA also have a positive regulatory effect on IAV infection that can exacerbate the severity of IAV infection. After PGE2 binds to its receptors EP2 and EP4, it can activate intracellular phosphatidylinositol 3-kinase (PI3K)-Akt and protein kinase A (PKA) signaling pathways, thereby inhibiting IRF3-mediated type I IFN activation and antigen presentation as well as T cell activation, exacerbating the damage caused by IAV infection
[6][7][8][9]. In addition, phospholipase D (PLD), a key enzyme in FA metabolism, can promote IAV replication by inhibiting the expression of myxovirus resistance gene A (MxA)
[10]. In 2010, PLD was identified as a host factor required for IAV replication
[11], and PLD has potential use as an anti-influenza target drug
[12].
2. Chol Plays a Major Role in Fusion and Release during the IAV Life Cycle
Chol is an indispensable substance in cells, not only participating in the formation of cell membranes but also serving as a precursor for the synthesis of a variety of bioactive substances. Additionally, Chol is an important component of the IAV envelope, accounting for approximately 11–12% of its total mass
[13], and is very important for IAV fusion and release
[14]. After binding to the sialic acid receptor, the successful entry of IAV into the cell further depends on fusion of the viral envelope with the intracellular body membrane
[15], and viral envelope Chol plays a key role in this process. Pretreatment of IAV particles with different concentrations of methyl-β-cyclodextrin (soluble Chol) resulted in a dose-dependent reduction in the cellular infectivity of IAV via solubilization of Chol, but this inhibition was limited to the viral fusion process
[16][17]. In addition to the critical role in the entry stage as the viral envelope Chol, cell membrane Chol is essential to ensure effective budding of progeny virions; the infectivity and stability of progeny virions released from cells deficient in cell membrane Chol are severely impaired because progeny virions acquire their envelope from the host cell membrane during budding, and Chol plays a key role in maintaining the IAV envelope structure
[18][19][20]. In vivo, Chol homeostasis is regulated by Annexin A6 (ANXA6)
[21][22][23]. Overexpression of ANXA6 decreased Chol levels in the cell membrane, thereby inhibiting IAV replication and propagation, and when siRNA was used to reduce the expression of ANXA6, the titer of the progeny IAV virions was increased
[24]. In addition, ANXA6 inhibits IAV budding by directly interacting with influenza matrix protein 2 (M2)
[25]. The CRAC motif of the M2 protein is a potential binding motif for Chol, but whether ANXA6 competes with progeny virions for binding to the CRAC motif is unknown
[26]. ANXA6 may be a target for anti-influenza drugs by regulating Chol
[21].
It is worth mentioning that Chol distributed on the cell membrane forms lipid rafts together with sphingolipids
[27][28] Host lipid raft is selected by IAV as a host attachment factor for multivalent binding, and play a critical role in assembly and budding of progeny virions. The polyvalent binding of IAV HA protein with sialic acid receptor will cause the aggregation of lipid rafts, which leads to the concentration and aggregation of tyrosine protein kinase (RTK) and activation, and then causes the response of PI3K/Akt and other related signaling pathways, and promotes the endocytosis of IAV
[29][30]. IAV also uses the “lipid raft” region on the host’s cell membrane as the virus budding site.
3. PL Metabolism Is Involved in Multiple Stages of IAV Infection
PL includes mainly glycerophospholipids and sphingomyelin (SM), which are the main components of biological membranes. Glycerophospholipids can be further divided into many categories depending on the substitution groups, for example, phosphatidylcholine (PC), phosphatidylglycerol (PG), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI) and cardiolipin (CL). As an enveloped virus, IAV needs to acquire an envelope from the host cell membrane during budding of progeny virions, which inevitably leads to changes in the PL content of the host cell. Alveolar type II (ATII) cells are the most important target cells of IAV; among the pulmonary surfactants synthesized by these cells, the PL content is as high as 35–50 mg/mL
[31]. When infected by IAV, ATII cells undergo reprogramming of PL metabolism, and the cell surface levels of PC, PG and PE are reduced, while those of PS, PI and SM are increased
[32].
Changes in glycerophospholipid levels affect IAV replication. On the one hand, glycosylation in the vicinity of cellular sialic acid receptors leads to blockade of virion binding to cellular receptors, but elevated PS levels can abolish this blockade and increase cellular susceptibility to enveloped viruses by up to 20-fold
[33][34]. On the other hand, PI can exert anti-IAV effects by directly binding to IAV with high affinity, disrupting its adsorption to host cells and binding to sialic acid receptors
[35]. In addition, PI can antagonize the activation of homologous ligands of Toll-like receptors (TLRs), central components of innate immunity, thereby inhibiting the transcription of proinflammatory genes, suppressing the cytokine storm induced by IAV infection and reducing damage to the organism
[36][37]. This observation indicates that the reprogramming of PL metabolism is multifaceted and shows that during IAV infection, glycerophospholipids act not only as the body’s “guardian” against IAV but also as an “accomplice” of IAV.
SM is of interest due to its important intracellular signaling and regulatory roles
[37], and its three metabolites of ceramide (Cer), sphingosine (Sph), and sphingosine-1-phosphate (S1P) are biologically active signaling molecules
[38]. SM and Cer play roles in promoting and inhibiting IAV infection, respectively, and some related metabolic regulatory enzymes involved in SM metabolism have been reported to have related effects (
Figure 1).
Figure 1. The role of sphingomyelin metabolism in IAV infection. In the red area, when the levels of sphingolipids or related metabolic enzymes are up-regulated or down regulated as shown by the arrow, IAV infection can be promoted through a variety of ways; conversely, in the green area, when the levels of sphingolipids or related metabolic enzymes are up-regulated or down regulated as shown by the arrow, IAV infection can be inhibited.
S1P is a terminal active metabolite of SM metabolism, and requires binding to different S1P receptor (S1PR1-S1PR5) to function. When S1PR1 is activated with CYM-5442, it can inhibit intercellular cell adhesion molecule-1 (ICAM1) production in a β-arrestin2-dependent manner, thereby reducing lung injury mediated by the strong immune response triggered by IAV infection, but it cannot reduce the viral titer
[39][40][41], indicating that S1PR1 cannot reduce viral replication but can reduce pathological damage. Sphingosine kinase 1 (SPHK1) is the key enzyme that catalyzes the generation of S1P from Sph
[42][43]. In infected cells, SPHK1 was shown to promote viral RNA synthesis by activating NF-κB and to maintain CRM1/RanBP3-mediated nuclear export of the viral ribonucleoprotein (vRNP) complex by activating ERK and AKT
[44][45][46][47][48]. S1P lyase (SPL), which induces S1P degradation, has also been shown to participate in the host anti-IAV response. SPL induces rapid activation of ERK and STAT1 and enhances the activation of IKKε, thereby promoting the host immune response
[49]. The above studies indicate that metabolites and enzymes in the SM metabolic pathway are widely involved in the regulation of IAV infection.
4. An Appropriate GL Concentration Is Important for IAV Infection
GLs are a class of amphiphilic lipid compounds containing glycosyl ligands that are widespread in living organisms, and glycosphingolipids (GSLs) are a main component of the GL family with multiple cellular roles. Some previous studies concluded that IAV must bind to cell surface GSLs to successfully infect cells
[50][51], but other studies demonstrated that GSL is not required for IAV infection but acts as an adsorption stabilizer only during the entry stage of the IAV life cycle
[52][53][54]. Glucosylceramide (GlcCer) is the main component of GSLs and can be converted into glucose and Cer under the catalytic activity of glucocerebrosidase (GBA)
[55], while glucosylceramide synthase (GCS) catalyzes the synthesis of GlcCer from Cer. After using CRISPR-Cas9 to knock out GBA, the level of GlcCer was increased by approximately three- to four-fold, delaying the transport of IAV to late endosomes and inhibiting IAV infection
[56]. In addition, when CRISPR/Cas9 was used to knock out the GCS gene, the level of GlcCer was reduced, and the infectivity of IAV was weakened
[57]. The above findings suggest that either excessive or insufficient levels of GlcCer inhibit IAV replication, suggesting that an appropriate concentration of GlcCer is extremely important for maintaining optimal IAV infection ability without dose dependency.