IRG1 induced by heme oxygenase-1/carbon monoxide inhibits LPS-mediated sepsis and pro-inflammatory cytokine production

Abstract

The immunoresponsive gene 1 (IRG1) protein has crucial functions in embryonic implantation and neurodegeneration. IRG1 promotes endotoxin tolerance by increasing A20 expression in macrophages through reactive oxygen species (ROS). The cytoprotective protein heme oxygenase-1 (HO-1), which generates endogenous carbon monoxide (CO), is expressed in the lung during Lipopolysaccharide (LPS) tolerance and cross tolerance. However, the detailed molecular mechanisms and functional links between IRG1 and HO-1 in the innate immune system remain unknown. In the present study, we found that the CO releasing molecule-2 (CORM-2) and chemical inducers of HO-1 increased IRG1 expression in a time- and dose-dependent fashion in RAW264.7 cells. Furthermore, inhibition of HO-1 activity by zinc protoporphyrin IX (ZnPP) and HO-1 siRNA significantly reduced expression of IRG1 under these conditions. In addition, treatment with CO and HO-1 induction significantly increased A20 expression, which was reversed by ZnPP and HO-1 siRNA. LPS-stimulated TNF-α was significantly decreased, whereas IRG1 and A20 were increased by CORM-2 application and HO-1 induction, which in turn were abrogated by ZnPP. Interestingly, siRNA against IRG1 and A20 reversed the effects of CO and HO-1 on LPS-stimulated TNF-α production. Additionally, CO and HO-1 inducers significantly increased IRG1 and A20 expression and downregulated TNF-α production in a LPS-stimulated sepsis mice model. Furthermore, the effects of CO and HO-1 on TNF-α production were significantly reversed when ZnPP was administered. In conclusion, CO and HO-1 induction regulates IRG1 and A20 expression, leading to inhibition of inflammation in vitro and in an in vivo mice model.

Retracted: MicroRNA‐132 attenuates LPS‐induced inflammatory injury by targeting TRAF6 in neuronal cell line HT‐22

“…Lipopolysaccharide (LPS), as a major glycolipid in the outer membrane of Gram‐negative bacteria, is widely used to establish the experimental model of inflammation both in vivo and in vitro . Multiple investigations regarding to the mechanisms that drive epilepsy as well as the potential therapies for epilepsy are preceded by using murine hippocampal HT‐22 cells .…”

Food Fight: Role of Itaconate and Other Metabolites in Antimicrobial Defense

“…Such an immunoregulatory model is supported by studies showing that Irg1 / Acod1 is necessary for the immunosuppressive effects of HO-1 (Jamal Uddin et al, 2015) and that Irg1 / Acod1 plays a critical role in embryo implantation, a process thought to require immunosuppression (Chen et al, 2003; Cheon et al, 2003; Sherwin et al, 2004). However, such an immunosuppressive role is inconsistent with data from zebrafish macrophage-lineage cells showing that Irg1 / Acod1 is necessary to generate ROS from fatty acid oxidation (Hall et al, 2013).…”

“…Whether this mechanism extends to dendritic cells is unclear, but data exists to show that BMDCs also upregulate Irg1 / Acod1 in response to LPS (Hoshino et al, 2002). It is also possible that NO and itaconate cooperatively necessitate the switch to aerobic glycolysis, as treatment of macrophages with the NO donor S-Nitroso-N-Acetyl-D, L-Penicillamine (SNAP) is sufficient to induce Irg1/Acod1 expression (Jamal Uddin et al, 2015). This hypothesis is further supported by the fact that itaconate can suppress NO production, thereby establishing an immunoregulatory loop (Lampropoulou et al, 2016).…”

Targeting heme oxygenase-1 and carbon monoxide for therapeutic modulation of inflammation

“…54 Additional studies have associated the anti-inflammatory effect of CO with activation of the heat shock factor-1 (HSF-1), and the upregulation of the immunoresponsive gene-1 protein in macrophages. 181,186 …”