Toll-like Receptor 4 Signaling Confers Cardiac Protection against Ischemic Injury via Inducible Nitric Oxide Synthase- and Soluble Guanylate Cyclase-dependent Mechanisms

Background:Prior administration of a small dose of lipopolysaccharide confers a cardiac protection against ischemia-reperfusion injury. However, the signaling mechanisms that control the protection are incompletely understood. We tested the hypothesis that Toll-like receptor 4 (TLR4) mediates the ability of lipopolysaccharide to protect against cardiac ischemia-reperfusion injury through distinct intracellular pathways involving myeloid differentiation factor 88 (MyD88), TIR-domain-containing adaptor protein-inducing interferon-&bgr;-mediated transcription factor (Trif), inducible nitric oxide synthase (iNOS), and soluble guanylate cyclase (sGC). Methods:Wild-type mice and genetically modified mice, that is TLR4-deficient (TLR4−def), TLR2 knockout (TLR2−/−), MyD88−/−, Trif−/−, iNOS−/−, and sGC&agr;1−/−, were treated with normal saline or 0.1 mg/kg lipopolysaccharide intraperitoneally. Twenty-four hours later, isolated hearts were perfused in a Langendorff apparatus and subsequently subjected to 30 min global ischemia and reperfusion for as long as 60 min. Left ventricular function and myocardial infarction sizes were examined. Results:Compared with saline-treated mice, lipopolysaccharide-treated mice had markedly improved left ventricular developed pressure and dP/dtmax (P < 0.01) and reduced myocardial infarction sizes (37.2 ± 3.4% vs. 19.8 ± 4.9%, P < 0.01) after ischemia-reperfusion. The cardiac protective effect of lipopolysaccharide was abolished in the TLR4−def and MyD88−/− mice but remained intact in TLR2−/− or Trif−/− mice. iNOS−/− mice or wild-type mice treated with the iNOS inhibitor 1400W failed to respond to the TLR4-induced nitric oxide production and were not protected by the lipopolysaccharide preconditioning. Although sGC&agr;1−/− mice had robust nitric oxide production in response to lipopolysaccharide, they were not protected by the TLR4-elicited cardiac protection. Conclusions:TLR4 activation confers a potent cardiac protection against ischemia-reperfusion injury via a MyD88-dependent, but Trif-independent, mechanism. iNOS/sGC are essential for the TLR4-induced cardiac protection.