Novel role of PKR in inflammasome activation and HMGB1 release

Double-stranded RNA-dependent protein kinase (PKR) is shown to be a key regulator of the inflammasome; PKR is central for caspase-1 activation and the release of interleukin (IL)-1β, IL-18 and high-mobility group box 1 (HMGB1) in response to a diverse range of stimuli. The inflammasome, an intracellular oligomeric protein complex involved in innate immunity, has been linked to the pathogenesis of obesity, type 2 diabetes, atherosclerosis, gout, sepsis and colitis. This paper identifies the double-stranded RNA-dependent protein kinase (PKR) as an important regulator of inflammasome activation. PKR autophosphorylation is required for its interaction with the Nod-like receptor NLRP3, for caspase 1 activation and for the release of the cytokines interleukin (IL)-1β, IL-18 and HMGB1 in response to a diverse set of stimuli. This finding raises the possibility of designing therapeutics to inhibit inflammasome activity in inflammatory conditions without broadly impairing immunity. The inflammasome regulates the release of caspase activation-dependent cytokines, including interleukin (IL)-1β, IL-18 and high-mobility group box 1 (HMGB1)1,2,3,4,5. By studying HMGB1 release mechanisms, here we identify a role for double-stranded RNA-dependent protein kinase (PKR, also known as EIF2AK2) in inflammasome activation. Exposure of macrophages to inflammasome agonists induced PKR autophosphorylation. PKR inactivation by genetic deletion or pharmacological inhibition severely impaired inflammasome activation in response to double-stranded RNA, ATP, monosodium urate, adjuvant aluminium, rotenone, live Escherichia coli, anthrax lethal toxin, DNA transfection and Salmonella typhimurium infection. PKR deficiency significantly inhibited the secretion of IL-1β, IL-18 and HMGB1 in E. coli-induced peritonitis. PKR physically interacts with several inflammasome components, including NOD-like receptor (NLR) family pyrin domain-containing 3 (NLRP3), NLRP1, NLR family CARD domain-containing protein 4 (NLRC4), absent in melanoma 2 (AIM2), and broadly regulates inflammasome activation. PKR autophosphorylation in a cell-free system with recombinant NLRP3, apoptosis-associated speck-like protein containing a CARD (ASC, also known as PYCARD) and pro-caspase-1 reconstitutes inflammasome activity. These results show a crucial role for PKR in inflammasome activation, and indicate that it should be possible to pharmacologically target this molecule to treat inflammation.