Novel role of PKR in inflammasome activation and HMGB1 release

Abstract

The inflammasome regulates release of caspase activation-dependent cytokines, including IL-1β, IL-18, and high-mobility group box 1 (HMGB1)1-5. During the course of studying HMGB1 release mechanisms, we discovered an important role of double-stranded RNA dependent protein kinase (PKR) 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 aluminum, rotenone, live E. coli, anthrax lethal toxin, DNA transfection, and S. Typhimurium infection. PKR deficiency significantly inhibited the secretion of IL-1beta, IL-18 and HMGB1 in E. coli-induced peritonitis. PKR physically interacts with multiple inflammasome components, including NLR family pyrin domain-containing 3 (NLRP3), NLR family pyrin domain-containing 1 (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, ASC and pro-casapse-1 reconstitutes inflammasome activity. These results reveal a critical role of PKR in inflammasome activation, and indicate that it should be possible to pharmacologically target this molecule to treat inflammation.

Tissue damage negatively regulates LPS-induced macrophage necroptosis

“…7 During tissue injury, HMGB1 is released from cells and serves as a necessary and sufficient mediator of inflammation to induce a variety of cellular responses including cell chemotaxis and the release of pro-inflammatory cytokines. 8,9 Inflammatory functions of HMGB1 are mediated by binding to cell surface receptors, including the receptor for advanced glycation end products (RAGE), Toll-like receptor (TLR)2, TLR4, and TLR9.…”

Review of the Molecular Pathogenesis of Osteosarcoma

“…PKR deficiency significantly inhibited the secretion of IL-1β, IL-18 and HMGB1 in E. coli-induced peritonitis (Lu et al, 2012). Anticancer agents including doxorubicin, cisplatin, and methotrexate each induced HMGB1 upregulation in human OSA cells, and RNA interference-mediated knockdown of HMGB1 restored the chemosensitivity of OSA cells in vivo and in vitro .…”

PKR stirs up inflammasomes

“…PKR kinase domain deficient mice had diminished levels of IL-1β cytokines or HMGB1 and showed reduced neutrophil infiltration to the peritoneal cavity, indicating compromised inflammasome activation. Collectively, this evidence suggests that PKR regulates multiple inflammasomes and PKR could thus be central to the immune response towards microbes and during the development of sterile inflammation [4]. As the different inflammasomes are engaged by structurally diverse activators and are most likely activated by different upstream processes, this broad function of PKR raises the question, by which mechanisms PKR influences the activity of these inflammasomes.…”

“…Recent findings have uncovered another piece to the puzzle, showing that PKR is involved in the activation of the NLRP1, NLRP3, NLRC4 and the AIM2 inflammasomes [4]. This new link is, at first sight, quite surprising for a protein that primarily functions as an antiviral protein.…”

“…They discovered that peritoneal macrophages from mice lacking the kinase domain of PKR released lower levels of HMGB1 and IL-1β cytokines compared to wild-type macrophages in response to stimulation with doublestranded DNA or RNA, crystals, anthrax lethal toxin or certain bacteria [4]. Furthermore, consistent with a role of PKR in inflammasome activation, cells lacking the PKR kinase domain were partially protected from pyroptosis, a caspase-1-dependent form of cell death [4,11]. Strikingly, the in vivo response of PKR kinase domain deficient mice towards live E. coli was also blunted when compared to wild-type mice.…”