The inhibitory effect of ghrelin on sepsis-induced inflammation is mediated by the MAPK phosphatase-1
Hepatocellular dysfunction occurs early in sepsis and this appears to be caused by Kupffer cell-derived TNF-alpha production from the liver as a result of the increased release of the sympathetic neurotransmitter, norepinephrine, from the gut. Ghrelin, a novel stomach-derived peptide, is down-regulated in sepsis and administration of ghrelin into rodents decrease pro-inflammatory cytokines, attenuates hepatic and other organ injuries and improves survival. Ghrelin's beneficial effect in sepsis is mediated by the inhibition of the sympathetic nervous system (SNS), as evidenced by the reduced gut-derived norepineprine (NE) release in sepsis after ghrelin treatment. Recent data suggest that MKP-1, the MAPK phosphatase-1, is involved in the innate immune responses. To determine that the beneficial effect of ghrelin in sepsis is mediated by MKP-1, rats were subjected to sepsis by cecal ligation and puncture (CLP) alone, or treated with ghrelin, beginning at 5-h post-CLP and liver tissues were harvested and examined for MKP-1 mRNA and protein expression. CLP alone produced a significant decrease in MKP-1 gene expression in liver tissues at 20 h after CLP (P<0.05). MKP-1 mRNA was decreased by 30-40% at 2 and 5 h after CLP, but not statistically significant. MKP-1 protein expression was significantly decreased as early as 2 h after CLP and remained low at 5-20 h after CLP. While septic rats treated with vehicle produced significant decreases from sham rats, ghrelin treatment improved both mRNA and protein from vehicle group (0.58+/-0.069 vs. 0.91+/-0.16, P<0.05; 0.14+/-0.027 vs. 0.22+/-0.017, P=0.013), respectively. Since ghrelin's inhibitory effect is mediated by the SNS, we hypothesized that NE treatment in Kupffer cells may downregulate MKP-1. Kupffer cells were treated with NE and examined for MKP-1. Treatment with NE for 60 min showed an average of 46.9% decrease in MKP-1 mRNA expression compared to untreated cells (P<0.001). Likewise, NE treatment in RAW264.7 cells produced significantly lower MKP-1 mRNA than that of control cells. To further confirm the effect of NE on MKP-1, normal rats were infused with NE for 2 h through the portal vein and MKP-1 mRNA from the liver was examined. Infusion with NE produced a significant 73.7% decrease in MKP-1 mRNA. Therefore, ghrelin's inhibitory effect on gut-derived NE release in sepsis leading to the downregulation of pro-inflammatory cytokines is mediated by MKP-1.
Mediation by indole analogues of electron transfer during oxygen activation in variants of Escherichia coli ribonucleotide reductase R2 lacking the electron-shuttling tryptophan 48
Activation of dioxygen by the carboxylate-bridged diiron(II) cluster in the R2 subunit of class I ribonucleotide reductase from Escherichia coli results in the one-electron oxidation of tyrosine 122 (Y122) to a stable radical (Y122*). A key step in this reaction is the rapid transfer of a single electron from a near-surface residue, tryptophan 48 (W48), to an adduct between O(2) and diiron(II) cluster to generate a readily reducible cation radical (W48(+)(*)) and the formally Fe(IV)Fe(III) intermediate known as cluster X. Previous work showed that this electron injection step is blocked in the R2 variant with W48 replaced by phenylalanine [Krebs, C., Chen, S., Baldwin, J., Ley, B. A., Patel, U., Edmondson, D. E., Huynh, B. H., and Bollinger, J. M., Jr. (2000) J. Am. Chem. Soc. 122, 12207-12219]. In this study, we show that substitution of W48 with alanine similarly disables the electron transfer (ET) but also permits its chemical mediation by indole compounds. In the presence of an indole mediator, O(2) activation in the R2-W48A variant produces approximately 1 equiv of stable Y122* and more than 1 equiv of the normal (micro-oxo)diiron(III) product. In the absence of a mediator, the variant protein generates primarily altered Fe(III) products and only one-fourth as much stable Y122* because, as previously reported for R2-W48F, most of the Y122* that is produced decays as a consequence of the inability of the protein to mediate reductive quenching of one of the two oxidizing equivalents of the initial diiron(II)-O(2) complex. Mediation of ET is effective in W48A variants containing additional substitutions that also impact the reaction mechanism or outcome. In the reaction of R2-W48A/F208Y, the presence of mediator suppresses formation of the Y208-derived diiron(III)-catecholate product (which is predominant in R2-F208Y in the absence of reductants) in favor of Y122*. In the reaction of R2-W48A/D84E, the presence of mediator affects the outcome of decay of the peroxodiiron(III) intermediate known to accumulate in D84E variants, increasing the yield of Y122* by as much as 2.2-fold to a final value of 0.75 equiv and suppressing formation of a 490 nm absorbing product that results from decay of the two-electron oxidized intermediate in the absence of a functional ET apparatus.