Tpl2 knockout mice produce low levels of TNF-alpha when exposed to lipopolysaccharide (LPS) and they are resistant to LPS/D-Galactosamine-induced pathology. LPS stimulation of peritoneal macrophages from these mice did not activate MEK1, ERK1, and ERK2 but did activate JNK, p38 MAPK, and NF-kappaB. The block in ERK1 and ERK2 activation was causally linked to the defect in TNF-alpha induction by experiments showing that normal murine macrophages treated with the MEK inhibitor PD98059 exhibit a similar defect. Deletion of the AU-rich motif in the TNF-alpha mRNA minimized the effect of Tpl2 inactivation on the induction of TNF-alpha. Subcellular fractionation of LPS-stimulated macrophages revealed that LPS signals transduced by Tpl2 specifically promote the transport of TNF-alpha mRNA from the nucleus to the cytoplasm.

SUMMARY MicroRNAs regulated by LPS target genes that contribute to the inflammatory phenotype. Here we show that Akt1, which is activated by LPS, differentially regulates miRNAs including let-7e, miR-155, miR-181c and miR125b. In silico analyses and transfection studies revealed that let-7e represses TLR4 while miR-155 represses SOCS1, two genes critical for LPS-driven TLR signalling, which regulate endotoxin sensitivity and tolerance. As a result, Akt1−/− macrophages exhibited increased responsiveness to LPS in culture and Akt1−/− mice did not develop endotoxin tolerance in vivo. Overexpression of let-7e and suppression of miR-155 in Akt1−/− macrophages restores sensitivity and tolerance to LPS in culture and in animals. These results indicate that Akt1 regulates the response of macrophages to LPS by controlling miRNA expression.

Macrophages become activated initiating innate immune responses. Depending on the signals, macrophages obtain an array of activation phenotypes, described by the broad terms of M1 or M2 phenotype. The PI3K/Akt/mTOR pathway mediates signals from multiple receptors including insulin receptors, pathogen-associated molecular pattern receptors, cytokine receptors, adipokine receptors, and hormones. As a result, the Akt pathway converges inflammatory and metabolic signals to regulate macrophage responses modulating their activation phenotype. Akt is a family of three serine-threonine kinases, Akt1, Akt2, and Akt3. Generation of mice lacking individual Akt, PI3K, or mTOR isoforms and utilization of RNA interference technology have revealed that Akt signaling pathway components have distinct and isoform-specific roles in macrophage biology and inflammatory disease regulation, by controlling inflammatory cytokines, miRNAs, and functions including phagocytosis, autophagy, and cell metabolism. Herein, we review the current knowledge on the role of the Akt signaling pathway in macrophages, focusing on M1/M2 polarization and highlighting Akt isoform-specific functions.

Activated macrophages are described as classically activated or M1 type and alternatively activated or M2 type, depending on their response to proinflammatory stimuli and the expression of genetic markers including iNOS, arginase1, Ym1, and Fizz1. Here we report that Akt kinases differentially contribute to macrophage polarization, with Akt1 ablation giving rise to an M1 and Akt2 ablation resulting in an M2 phenotype. Accordingly, Akt2 −/− mice were more resistant to LPS-induced endotoxin shock and to dextran sulfate sodium (DSS)-induced colitis than wild-type mice, whereas Akt1 −/− mice were more sensitive. Cell depletion and reconstitution experiments in a DSS-induced colitis model confirmed that the effect was macrophage-dependent. Gene-silencing studies showed that the M2 phenotype of Akt2 −/− macrophages was cell autonomous. The microRNA miR-155, whose expression was repressed in naive and in LPS-stimulated Akt2 −/− macrophages, and its target C/EBPβ appear to play a key role in this process. C/EBPβ, a hallmark of M2 macrophages that regulates Arg1, was up-regulated upon Akt2 ablation or silencing. Overexpression or silencing of miR-155 confirmed its central role in Akt isoform-dependent M1/M2 polarization of macrophages.inflammation | peritonitis | sepsis | inflammatory bowel disease A ctivated macrophages express proinflammatory factors and are known as classically activated or M1-type macrophages. Toll-like receptor (TLR) stimulation may induce the M1 phenotype through the activation of several signaling cascades, which regulate the induction of proinflammatory mediators such as TNF-α, IL-6, and iNOS. However, macrophages can also undergo alternative activation to become alternatively activated or M2-type macrophages. M2 macrophages are characterized by reduced responsiveness to TLR ligands, which results in the induction of low levels of proinflammatory cytokines and in the upregulation of arginase 1 (Arg1), IL-10, found in inflammatory zone 1 (Fizz1), and chitinase 3-like-3 (YM1/CHI3l3) (1). Although the molecular mechanisms that regulate M2 macrophage polarization are not well understood, it appears that STAT6 activation and the induction of C/EBPβ play a central role in this process (2-4). C/EBPβ regulates the expression of Arg1 (3), the gene that encodes the inducible arginase, and selective inhibition of C/EBPβ in macrophages blocks M2 polarization (4).Akt (also known as PKB) is a family of three serine/threonine protein kinases (Akt1, Akt2, and Akt3) that regulate a host of cellular functions, including cell survival, proliferation, differentiation, and intermediary metabolism (5-7). Even though the majority of the literature does not make a distinction between different Akt isoforms, there is a growing list of differences between them. Akt1 appears not to be dispensable for eNOS induction and endothelial cell function (8, 9), whereas Akt2 is not dispensable for insulin signaling (10). Deletion of Akt1 resulted in enhanced atherosclerosis in the APOE −/− mouse model (5), and Akt1 −/− mice do not d...

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The authors determined the association between metabolic syndrome in early pregnancy (mean, 11.96 weeks) and the risk of preterm birth in the mother-child cohort study ("Rhea" Study) in Crete, Greece, 2007-2009. Maternal fasting serum samples were collected, and blood pressure was measured at the time of the first major ultrasound examination (n = 625). Multivariable log-binomial regression models were used. Women with metabolic syndrome were at high risk for preterm birth (relative risk (RR) = 2.93, 95% confidence interval (CI): 1.53, 5.58), with the highest risk observed for medically indicated preterm births (RR = 5.13, 95% CI: 1.97, 13.38). Among the components of metabolic syndrome, the most significant risk factor was hypertension (RR = 2.32, 95% CI: 1.28, 4.20). An elevation of 10 mm Hg in diastolic blood pressure increased the relative risk for preterm birth by 29% (RR = 1.29, 95% CI: 1.08, 1.53), while a per unit increase in the low density lipoprotein/high density lipoprotein cholesterol ratio increased this risk by 19% (RR = 1.19, 95% CI: 1.02, 1.39). Fetal weight growth restriction was associated with elevated levels of insulin (RR = 1.14, 95% CI: 1.08, 1.20) and diastolic blood pressure (RR = 1.27, 95% CI: 1.00, 1.61) in early pregnancy. These findings suggest that women with metabolic syndrome in early pregnancy had higher risk for preterm birth.

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Provirus insertion in the last intron of the Tpl-2 gene in retrovirus-induced rat T-cell lymphomas results in the enhanced expression of a carboxy-terminally truncated Tpl-2 kinase. Here we show that the truncated protein exhibits an approximately sevenfold higher catalytic activity and is two-to threefold more efficient in activating the MAPK and SAPK pathways relative to the wild-type protein. The truncated Tpl-2 protein and a GST fusion of the Tpl-2 carboxy-terminal tail interact when coexpressed in Sf9 cells. Their interaction down-regulates the kinase activity of the truncated protein suggesting that tail-directed intramolecular interactions regulate the Tpl-2 kinase. Tpl-2 transgenic mice expressing the wild-type protein from the proximal Lck promoter fail to show a biological phenotype, whereas mice expressing the truncated protein develop large-cell lymphoblastic lymphomas of T-cell origin. These results show that Tpl-2 is an oncogenic kinase that is activated by carboxy-terminal truncation.