Hypoxia Inactivates Inducible Nitric Oxide Synthase in Mouse Macrophages by Disrupting Its Interaction with α-Actinin 4

Abstract

Nitric oxide, produced in macrophages by the high output isoform inducible NO synthase (iNOS), is associated with cytotoxic effects and modulation of Th1 inflammatory/immune responses. Ischemia and reperfusion lead to generation of high NO levels that contribute to irreversible tissue damage. Ischemia and reperfusion, as well as their in vitro simulation by hypoxia and reoxygenation, induce the expression of iNOS in macrophages. However, the molecular regulation of iNOS expression and activity in hypoxia and reoxygenation has hardly been studied. We show in this study that IFN-gamma induced iNOS protein expression (by 50-fold from control, p < 0.01) and nitrite accumulation (71.6 +/- 14 micro M, p < 0.01 relative to control), and that hypoxia inhibited NO production (7.6 +/- 1.7 micro M, p < 0.01) without altering iNOS protein expression. Only prolonged reoxygenation restored NO production, thus ruling out the possibility that lack of oxygen, as a substrate, was the cause of hypoxia-induced iNOS inactivation. Hypoxia did not change the ratio between iNOS monomers and dimers, which are essential for iNOS activity, but the dimers were unable to produce NO, despite the exogenous addition of all cofactors and oxygen. Using immunoprecipitation, mass spectroscopy, and confocal microscopy, we demonstrated in normoxia, but not in hypoxia, an interaction between iNOS and alpha-actinin 4, an adapter protein that anchors enzymes to the actin cytoskeleton. Furthermore, hypoxia caused displacement of iNOS from the submembranal zones. We suggest that the intracellular localization and interactions of iNOS with the cytoskeleton are crucial for its activity, and that hypoxia inactivates iNOS by disrupting these interactions.