Role of the interleukin-33 (IL-33)/suppressor of tumorigenicity 2 (ST2) signaling in superoxide anion-triggered inflammation and pain behavior in mice

Reactive oxygen species such as superoxide anion have varied roles in inflammation and pain, which can be mimicked by potassium superoxide (KO₂), the superoxide anion donor. Interleukin (IL)-33 has pleiotropic functions by activating its receptor suppression of tumorigenicity 2 (ST2). However, the role of IL-33/ST2 signaling in inflammatory pain initiated by reactive oxygen species (ROS) such as superoxide anion has not been investigated, which was the aim of the present study. IL-33 levels were assessed by enzyme-linked immunosorbent assay (ELISA). Mechanical and thermal hyperalgesia and overt pain were evaluated by electronic von Frey, hot plate, and abdominal writhing/paw flinching/licking, respectively. Edema and leukocyte recruitment (myeloperoxidase assay and total/differential cell count), antioxidant capacity, superoxide anion production and lipid peroxidation were assessed. Paw skin and spinal cord messenger ribonucleic acid (mRNA) expression of pro-inflammatory mediators and glial markers in the spinal cord were evaluated. Immunofluorescence was used to detect spinal glial and neuronal c-Fos activation. KO₂ injection triggered IL-33 production in the paw skin and spinal cord of mice, induced hyperalgesia, edema, neutrophil recruitment to the paw tissue, overt pain-like behavior, and leukocyte recruitment to the peritoneum that were reduced in ST2 deficient mice. In the paw skin and spinal cord, KO₂ triggered IL-33/ST2-dependent oxidative stress, and mRNA expression of inflammatory molecules, which were reduced by ST2 deficiency. KO₂ induced spinal cord glial (at mRNA/protein levels) and neuronal activation in IL-33/ST2-dependent manner. IL-33/ST2 signaling mediates, at least in part, superoxide anion-induced inflammatory pain by modulating local and spinal inflammatory events.