NEW INSIGHTS INTO MOLECULAR MECHANISMS OF HOST DEFENCE AGAINST INFLUENZA VIRUS REPLICATION
Acest articol este o trecere in revista a celor mai recente date din literatura cu privire la stresul indus reticulului endoplasmic de catre infectia cu virusul gripal A. S-a descoperit faptul ca virusul gripal A induce calea inositolului – dependent de enzima 1 (IRE1) a raspunsului reticulului endoplasmic (ER), iar inhibarea activitatii caii metabolice (IRE1), rezultand intr-o replicare virala redusa.
Se poate concluziona ca IRE1 poate fi considerat drept o tinta terapeutica potentiala pentru virusul gripal A.
Cuvinte-cheie: virusul gripal A, reticul endoplasmic (ER), calea metabolica IRE1
This article is a review of most recent literature data on role of endoplasmic reticulum(ER) stress in influenza A viral infection.It has been found that influenza A virus induces IRE1 pathway of the ER response and the inhibition of IRE1 activity leads to a decreased viral replication.It can be concluded that IRE1 may stand as a potential therapeutic target for influenza A virus. Decreasing viral replication by modulating the host ER stress response is a novel strategy that has important therapeutic implications
Keywords: influenza virus A, endoplasmic reticulum, IRE1 pathway
It is well known that Influenza A virus has been causing recurrent pandemics for centuries and continues to be a global health threat with a major economic burden.
For example, in the United States, seasonal influenza is estimated to cause 36,000 deaths and 226,000 hospitalizations annually. The annual Influenza virus epidemics are estimated to cost 10.4 billion in direct medical expenses and 16.4 billion in lost potential earnings [2, 3]. Furthermore antigenic shift continues o cause recurrent pandemics with a devastating death toll. More than 40 million people died during the 1918 pandemic outnumbering the death toll of World War 1 [4, 5]. With emergence of strains resistant to pharmacologic therapies, yearly vaccination remains the only available strategy for influenza A virus.
Taking into account the limited ability to produce large amounts of vaccines in a short time period which represents a problem at times of pandemics, further understanding of host cellular mechanisms involved in the pathogenesis of influenza A viral infection and identification of therapeutic targets in the host are needed for the development of new treatments less susceptible to resistance.
The recent attention has been focused on endoplasmic reticulum (ER) stress response, also known as the unfolded protein response (UPR), is a primitive, evolutionary conserved molecular signaling cascade that has been implicated in multiple biological phenomena including innate immunity and the pathogenesis of certain viral infections.
The researchers have investigated the effect of influenza A viral infection on ER stress pathways in lung epithelial cells. Influenza A virus induced ER stress in a pathway-specific manner. They have been shown that the virus activates the IRE1 pathway with little or no concomitant activation of the PERK and the ATF6 pathways.
When they have examined the effects of modulating the ER stress response on the virus, they have found that the molecular chaperone tauroursodeoxycholic acid (TUDCA) significantly inhibits influenza A viral replication. In addition, a specific inhibitor of the IRE1 pathway also blocked viral replication.
Their findings constitute the first evidence that ER stress plays a role in the pathogenesis of influenza A viral infection. Decreasing viral replication by modulating the host ER stress response is a novel strategy that has important therapeutic implications.
Therefore further understanding of host cellular mechanisms involved in the pathogenesis of influenza A viral infection and identification of therapeutic targets in the host are needed for the development of new treatments less susceptible to resistance.
The endoplasmic reticulum stress response, also known as the unfolded protein response (UPR), was originally discovered as an evolutionary conserved molecular signaling cascade the main function of which is to restore ER homeostasis and protein folding capacity during ER stress. Over the last decade, the growing body of knowledge about the UPR revealed a broader range of effects implicating it in multiple other cellular and disease processes including apoptosis, inflammation, and metabolism [6–12]. While ER stress has been shown to be involved in the pathogenesis of other viruses , its role in influenza A viral infection is unknown.
The adaptive effects of the UPR can be classified into three categories: (1) increasing protein folding capacity of the ER by transcriptional up-regulation of chaperone proteins, (2) decreasing ER protein load by attenuation of global protein translation, (3) increasing proteasome-mediated degradation of misfolded proteins. In addition to its adaptive effects, if ER stress is severe or prolonged, the UPR is also known to mediate apoptotic signals to protect the organism from necrotic cell death and inflammation. The factors that determine the balance between the adaptive versus apoptotic effects are still not well understood .
The upstream mediators of the UPR are three ER resident transmembrane proteins, activating transcription factor 6 (ATF6), PKR-like ER kinase (PERK), and inositol-requiring enzyme 1 (IRE1), usually held inactive by binding immunoglobulin protein (BiP) at their luminal N-terminal side.
BiP is a major chaperone protein and is considered the master regulator of the UPR . During ER stress BiP is released from ATF6, PERK, and IRE1 because of competitive binding to the increasing levels of mis-folded proteins and thus allowing the activation of the UPR. When released from BIP, ATF6 translocates to the Golgi where it gets cleaved by resident proteases.
Cleaved ATF6 functions as a transcription factor for chaperone genes. PERK and IRE1 homodimerize, when released from BIP, which induces their auto-phosphorylation
and activation. PERK is a serine/threonine kinase that phosphorylates and inactivates eIF2_ (eukaryotic translation initiation factor 2_). Phosphorylation of eIF2_ induces global shut down of protein translation. Certain mRNAs, for example activating transcription factor 4 (ATF4) and BiP, escape that inhibition and gain a translational advantage .
The third ER stress regulator, IRE1, has an endoribonuclease domain as well as a kinase domain. The endonuclease activity induces splicing of a 26-base intron in the XBP1 mRNA leading to a reading frameshift and translation into an active transcription factor for genes involved in ER-associated degradation (ERAD).
The downstream effects of the IRE1 kinase function include phosphorylation
of JNK and p38 MAP kinases, both of which are implicated in mediating some of the apoptotic effects of the UPR [14, 15] .
In light of the growing evidence of diverse interactions between viral infections and ER stress the authors have investigated the effects of influenza A viral infection on the different pathways of the UPR and the potential role of ER stress in viral replication.
Over the last decade, several groups of laboratories have reported the induction of ER stress in the setting of some viral infections.
However, very little is known about the mechanisms by which ER stress is induced or about the role of the UPR in the pathogenesis of these viruses.
Until recently, the only studies linking influenza infection to ER stress are ones showing that hemagglutinin A induces ER stress. 26 years ago, researchers showed that ER stress can be induced by overexpressing a mutated misfolded form of the influenza A hemagglutinin protein in simian cells. In a more recent study, overexpression of wild type influenza A hemagglutinin protein was shown to induce NF-_B activation. The mechanism proposed by the investigators was the induction of ER stress but it was not directly proven .
The most recent literature data is first evidence that infection with wild type influenza A virus induces ER stress. Furthermore the researchers showed that influenza A viral infection induces the ER stress response, in primary human airway epithelial cells, in a pathway- specific manner. The virus specifically induces the IRE1 branch of the UPR with little or no concomitant activation of the PERK and ATF6 pathways. They also showed that the virus modulates the stress response in the setting of a preexisting stress by decreasing the activation of the ATF6 pathway as indicated by its inhibitory effect on ATF6-driven genes.
In airway epithelial cells, the effects of a virus on ER stress are different.
Although the mechanism by which influenza A virus induces ER stress could be the accumulation of viral proteins in the ER lumen, the differential effects on the different pathways of the UPR are indicative of additional levels of interaction between the virus and the ER stress signaling cascade. To date the only known mechanism of activation of PERK, ATF6, and IRE1 is
their release from BiP. The mechanism behind differential activation of the different arms of the UPR is unknown .
They showed that by modulating the ER stress response with the chemical chaperone TUDCA, viral replication was inhibited.
This suggests that the UPR plays a facilitating role in the replication cycle of the virus. The combined effects of influenza A viral infection on the ER stress pathways seem to be aiming at optimal activation of IRE1 pathway. By inhibiting the ATF6 pathway, the activation of which is known to increase the ER folding capacity and to alleviate the ER stress response, the virus may be insuring optimal and sustained activation of IRE1.
The inhibition of influenza A viral replication with TUDCA, which alleviates the ER stress response, suggests that one or more of the downstream effects of IRE1, the only ER stress pathway activated by the virus, is required for viral replication. This was confirmed in by the most recent studies studies by the inhibition of viral replication with the use of the IRE1 inhibitor, 3,5-dibromosalicylaldehyde.
The new data obtained by scientists regarding implication of the UPR in influenza A viral replication is a novel finding that could lead to new therapies for influenza A virus respiratory infection. By targeting a host cellular mechanism, such therapies are less susceptible to viral resistance mechanisms. The scientific interest has been in studying the effects of chemical chaperones, in particular TUDCA, on in vivo models of influenza A viral infection.
They have anticipated the effects to be more complex than merely inhibiting viral replication. The disease manifestations of influenza A viral infection are in part due to the host immune response and inflammatory cytokines .
The UPR is known to intersect with cytokine release mechanisms through NF-_B activation. Therefore modulation of the ER stress response by chemical chaperones may cause a change in the cytokine milieu created by the infection and therefore a change in the disease phenotype. Furthermore, MHC class I expression on the cell surface, which is important in antigen presentation and in mediating T cell immunity, is dependent on the ER function [18-20].
Improving the ER folding capacity with chemical chaperones may render dendritic
cells more efficient in presenting viral antigens and therefore accelerating viral clearance.
The combined effect of all these potential mechanisms is difficult to predict and an area of future study is needed.
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