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...
