PRG4/CD44/PP2A Signaling Axis: A Significant Regulator of Gout Arthritis

Abstract

Introduction: Gout is a common chronic arthritis caused by monosodium urate (MSU) crystal deposition in joints leading to activation of synovial resident macrophages and downstream production of interleukin-1b (IL-1b). Xanthine oxidase (XO) catalyzes purines into uric acid (UA) and reactive oxygen species (ROS) that activate NLRP3 inflammasome. Protein phosphatase-2A (PP2A) may be involved in regulating inflammatory pathways in macrophages. Proteoglycan 4 (PRG4) and its receptor CD44 and its transducer PP2A signaling axis play a biologically significant role in regulating urate crystal inflammation. Methods: Monocytes and macrophages were primed with TLR2 agonist and stimulated with MSU crystals. Fingolimod (PP2A activating drug) and febuxostat (XO inhibitor) were assessed for their ability to regulate PP2A and XO activity, UA production, ROS generation and IL-1b secretion. Anti-inflammatory effect of fingolimod was assessed in vivo. Phagocytic activation of gout and normal monocytes was compared along with intracellular ROS generation and IL-1b production. rhPRG4 and IL-1RA (IL-1b receptor antagonist) were examined for their ability to regulate phagocytosis, PP2A activity, ROS generation and IL-1b secretion. Peritoneal influx of classical monocytes (CMs) and non-classical monocytes (NCMs) in Prg4 deficient mice was assessed. Results: Fingolimod and febuxostat enhanced PP2A activity and suppressed XO activity, UA and ROS generation and IL-1b secretion. Fingolimod reduced influx of CMs and neutrophils and increased influx of NCMs in an in vivo model of acute gout. Phagocytic activation, ROS generation and IL-1b were higher in gout monocytes. rhPRG4 showed superior efficacy in reduction of phagocytosis, ROS generation and increasing PP2A activity. rhPRG4 showed faster resolution of inflammation in Prg4 deficient mice. Conclusion: Macrophage PP2A is inactivated in acute gout by ROS and a PP2A activator exhibited a broad anti-inflammatory effect in acute gout in vitro and in vivo. The anti-inflammatory effect of rhPRG4 in monocytes is mediated by PP2A and PRG4 plays a role in regulation of influx of immune cells in site of gout flare.