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

The immunoresponsive gene 1 （IRG 1） 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 I RG 1 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-a 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-a production. Additionally, CO and HO-1 inducers significantly increased IRG1 and A20 expression and downregulated TNF-a 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 vitroand in an in vivomice model.

Carbon monoxide decreases interleukin-1β levels in the lung through the induction of pyrin

Carbon monoxide （CO） can act as an anti-inflammatory effector in mouse models of lung injury and disease, through the downregulation of pro-inflammatory cytokines production, though the underlying mechanisms remain unclear. The nucleotide-binding oligomerization domain-, leucine-rich region-, and pyrin domain-containing-3 （NLRP3） inflammasome is a protein complex that regulates the maturation and secretion of pro-inflammatory cytokines, including interleukin-1β （IL-1β）. In this report, we show that the CO-releasing molecule （CORM-2） can stimulate the expression of pyrin, a negative regulator of the NLRP3 inflammasome. CORM-2 increased the transcription of pyrin in the human leukemic cell line （THP-1） in the absence and presence of lipopolysaccharide （LPS）. In THP-1 cells, CORM-2 treatment dose-dependently reduced the activation of caspase-1 and the secretion of IL-lp, and increased the levels of IL-IO, in response to LPS and adenosine 5＇-triphosphate （ATP）, an NLRP3 inflammasome activation model. Genetic interference of IL-10 by small interfering RNA （siRNA） reduced the effectiveness of CORM-2 in inhibiting IL-1β production and in inducing pyrin expression. Genetic interference of pyrin by siRNA increased IL-lp production in response to LPS and ATP, and reversed CORM-2-dependent inhibition of caspase-1 activation. CO inhalation （250 ppm） in vivo increased the expression of pyrin and IL-10 in lung and spleen, and decreased the levels of IL-1β induced by LPS. Consistent with the induction of pyrin and IL-10, and the downregulation of lung IL-1β production, CO provided protection in a model of acute lung injury induced by intranasal LPS administration. These results provide a novel mechanism underlying the anti-inflammatory effects of CO, involving the IL-10-dependent upregulation of pyrin expression.

Metabolic signaling functions of the heme oxygenase/CO system in metabolic diseases

Immunometabolism is the intersection of immunology and metabolism.The heme oxygenase(HO)-1 and carbon monoxide(CO)(HO-1/CO)system can be induced by various stresses,including infection,and its function is to relieve stresses such as oxidative and inflammatory reactions.Here we demonstrate that the HO-1/CO system is intimately involved in the beneficial modulation of metabolic pathways via the production of ATF4 through an integrative stress response.

RIP140, a Janus metabolic switch involved in defense functions

Since its discovery almost 15 years ago, the receptor-interacting protein RIP140has been determined to be a corepressor of various transcription factors and nuclear receptors. Mice lacking the RIP140 gene are lean, exhibit resistance to high-fat diet- induced obesity and have increased glucose tolerance and insulin sensitivity. In 2008, RIP140 was determined to be a coactivator of nuclear factor-~cB, a master transcriptional regulator of inflammation in immune cells, including macrophages. This year, it was reported that RIP 140 degradation is involved in the resolution of inflammation and in endotoxin tolerance. The general picture that had emerged indicates that RIP140 is a ]anus metabolic switch that convergently regulates metabolic pathways involved in defense func- tions through its pleotropic interactions with transcription factors.