IRE1α mediates PKR activation in response to Chlamydia trachomatis infection

Abstract

Protein kinase RNA activated (PKR) is a crucial mediator of anti-viral responses but is reported to be activated by multiple non-viral stimuli. However, mechanisms underlying PKR activation, particularly in response to bacterial infection, remain poorly understood. We have investigated mechanisms of PKR activation in human primary monocyte-derived dendritic cells in response to infection by Chlamydia trachomatis. Infection resulted in potent activation of PKR that was dependent on TLR4 and MyD88 signalling. NADPH oxidase was dispensable for activation of PKR as cells from chronic granulomatous disease (CGD) patients, or mice that lack NADPH oxidase activity, had equivalent or elevated PKR activation. Significantly, stimulation of cells with endoplasmic reticulum (ER) stress-inducing agents resulted in potent activation of PKR that was blocked by an inhibitor of IRE1α RNAse activity. Crucially, infection resulted in robust IRE1α RNAse activity that was dependent on TLR4 signalling and inhibition of IRE1α RNAse activity prevented PKR activation. Finally, we demonstrate that TLR4/IRE1α mediated PKR activation is required for the enhancement of interferon-β production following C. trachomatis infection. Thus, we provide evidence of a novel mechanism of PKR activation requiring ER stress signalling that occurs as a consequence of TLR4 stimulation during bacterial infection and contributes to inflammatory responses.

Toll‐like receptor 2 (TLR2) engages endoplasmic reticulum stress sensor IRE1α to regulate retinal innate responses in Staphylococcus aureus endophthalmitis

“…This is supported by multiple pieces of evidence, including, splicing of IRE1α downstream target XBP1 and the attenuation of an inflammatory response and severe disease pathology by pharmacological inhibition of IRE1α. These observations corroborate with studies where activation of IRE1-XBP1-mediated ER stress has also been reported via various intracellular pathogens such as Chlamydia trachomatis, 55 Brucella abortus, 56 and Francisella tularensis. 27 However, some other bacterial species and their toxins such as subtilase toxin produced by Escherichia coli, 57 and listeriolysin O, produced by Listeria monocytogenes 58 have been shown to activate other arms of the UPR as well.…”

Insights Into the Role of Endoplasmic Reticulum Stress in Infectious Diseases

“…The obligate intracellular pathogen Chlamydia induces the UPR by upregulating BiP (44,45). Chlamydia infection also induces TLR4/IRE1-mediated activation of PKR, which enhances IFN-β production (46). Brucella abortus localizes to the ER by transforming its fine reticular pattern into a thicker tubular structure (47).…”

Regulation of Cytokine Production by the Unfolded Protein Response; Implications for Infection and Autoimmunity

“…One group reported that persistent (non-productive) Chlamydia infection induced transient BiP upregulation and eIF2α phosphorylation but not ATF6 cleavage or XBP1 splicing ( 179 ). However, in another study, Chlamydia stimulated “robust” IRE1 activation and XBP1 splicing, and induced CHOP in a GCN2-dependent manner ( 180 ). Legionella actively inhibited XBP1 splicing via bacterial translation elongation inhibitors ( 181 ).…”

“…Chlamydia also induced PKR-dependent IFN-β through a mechanism requiring TLR4 and IRE1 RNase activity. Interestingly, this TLR4 activity may limit CHOP induction, stressing the importance of the multiple innate immune and ER stress inputs that impact cytokine production during infection ( 180 ). XBP1 deficiency significantly decreased TLR2-dependent TNF-α and IL-6 responses to Francisella in vitro .…”