Heme Oxygenase 1 dampens the macrophage sterile inflammasome response and regulates its components in the hypoxic lung.

Exposure to hypoxia causes an inflammatory reaction in the mouse lung and this response can be modulated by overexpressing the hypoxia-inducible stress-response enzyme, heme oxygenase-1 (HO-1). We hypothesized that the inflammasome activity may be a central pathway by which HO-1 controls pulmonary inflammation following alveolar hypoxia. Therefore, we investigated whether HO-1 controls inflammasome activation by altering its expression in macrophages primed with classical NLRP3 inducers, and in murine lungs lacking HO-1 and exposed to acute hypoxia. We found that lack of HO-1 activated LPS and ATP-treated bone marrow derived-macrophages (BMDMs) causing an increase in secreted levels of cleaved IL-1B, IL-18, and caspase-1, markers of increased inflammasome activity, whereas HO-1 overexpression suppressed IL-1B, NLRP3, and IL-18. The production of cleaved IL-1B and the activation of caspase-1 in LPS and ATP-primed macrophages were inhibited by hemin, and two of the HO-1 enzymatic products (bilirubin and carbon monoxide (CO)). Exposure of mice to hypoxia induced the expression of several inflammasome mRNA components (IL-1B, Nlrp3 and Caspase-1) and this was further augmented by HO-1 deficiency. This pronounced inflammasome activation was detected as increased protein levels of ASC, IL-18, pro-caspase-1, and cleaved caspase-1 in the lungs of hypoxic mice. Systemically, Hmox1 deficient mice showed increased basal levels of IL-18 that were further increased after 48 hours of hypoxic exposure. Taken together, these finding points to a pivotal role for HO-1 in the control of baseline and hypoxic inflammasome signaling, perhaps through the antioxidant properties of bilirubin and CO's pleiotropic effects.