Person:

Zou, Lin

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-β-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 sGCα1−/−, 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 sGCα1−/− 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.

Background: Toll‐like receptor 3 (TLR3) was originally identified as the receptor for viral RNA and represents a major host antiviral defense mechanism. TLR3 may also recognize extracellular RNA (exRNA) released from injured tissues under certain stress conditions. However, a role for exRNA and TLR3 in the pathogenesis of myocardial ischemic injury has not been tested. This study examined the role of exRNA and TLR3 signaling in myocardial infarction (MI), apoptosis, inflammation, and cardiac dysfunction during ischemia‐reperfusion (I/R) injury. Methods and Results: Wild‐type (WT), TLR3−/−, Trif−/−, and interferon (IFN) α/β receptor‐1 deficient (IFNAR1−/−) mice were subjected to 45 minutes of coronary artery occlusion and 24 hours of reperfusion. Compared with WT, TLR3−/− or Trif−/− mice had smaller MI and better preserved cardiac function. Surprisingly, unlike TLR(2/4)‐MyD88 signaling, lack of TLR3‐Trif signaling had no impact on myocardial cytokines or neutrophil recruitment after I/R, but myocardial apoptosis was significantly attenuated in Trif−/− mice. Deletion of the downstream IFNAR1 had no effect on infarct size. Importantly, hypoxia and I/R led to release of RNA including microRNA from injured cardiomyocytes and ischemic heart, respectively. Necrotic cardiomyocytes induced a robust and dose‐dependent cytokine response in cultured cardiomyocytes, which was markedly reduced by RNase but not DNase, and partially blocked in TLR3‐deficient cardiomyocytes. In vivo, RNase administration reduced serum RNA level, attenuated myocardial cytokine production, leukocytes infiltration and apoptosis, and conferred cardiac protection against I/R injury. Conclusion: TLR3‐Trif signaling represents an injurious pathway during I/R. Extracellular RNA released during I/R may contribute to myocardial inflammation and infarction.