Pre-treatment of mice with lipopolysaccharides as well as toll-like receptor 4 (TLR4) deficiency reduces cardiac ischemia-reperfusion (I/R) injury (Stapel et al. 2006). However, it is not clear how TLR4-signalling affects cardiac I/R injury. Therefore we investigated whether TLR4 dependent regulation of nuclear factor (NF) kB, cytokines, and hypoxia inducible factor (HIF)-1a, as well as the HIF-1 target genes adrenomedullin (ADM) and iNOS influence myocardial I/R injury.

TLR4^-/- and C57BL/6 mice without (control) and with 16 h of LPS pre-treatment (1 mg/kg BW) were subjected to 60 min of left anterior descending artery occlusion and reperfusion. Cardiac and aortic tissues were collected at different time points after LPS application and after I/R for the evaluation of NFkB subunits p50, p52 and p65 (ELISA), HIF-1a (immuno blot), TNFa, IL-1ß, IL-6, IL-10, ADM, and iNOS (real-time RT PCR, ELISA).

After I/R C57BL/6 control mice exhibit a rise in the NFkB p50/p52 subunit ratio, and a strong elevation of relative cytokine values, accompanied by large infarct sizes. Mice with smaller infarct areas, C57BL/6 mice pre-treated with LPS and TLR4^-/- mice, did not develop such a p50/p52 increase. Pre-treatment of C57BL/6 mice with LPS induces TNFa, IL-1ß and IL-6 mRNA expression and HIF-1a protein accumulation after 6 h. 10 h later HIF-1a protein had returned to control level, but mRNA of all three cytokines was still increased, i.e. cytokines were elevated at the time point of ischemia. In addition, LPS pre-treatment of C57BL/6 resulted in a transient cardiac ADM rise prior to ischemia and persisting expression of ADM and iNOS in the aorta. LPS treatment did neither induce cytokine expression nor HIF-1a accumulation in TLR4^-/- animals. In animals with smaller infarcts, TLR4^-/- mice and C57BL/6 mice pre-treated with LPS, a HIF-1a protein accumulation was detected after I/R. Furthermore, TLR4^-/- mice expressed high cardiac ADM levels post I/R.

HIF-1a seems to be required for reduction of I/R injury and HIF-1a induced ADM appears to be important for cardioprotection in TLR4^-/- mice, whereas ADM and iNOS may act together in LPS pre-treated C57BL/6 animals.

Stapel H, Kim S-C, Osterkamp S, Knuefermann P, Hoeft A, Meyer R, Grohé C, Baumgarten G (2006) Toll-like Receptor 4 modulates myocardial ischemia-reperfusion injury: role of matrix metalloproteinases. Europ J Heart Fail 8, 665–672