Abscisic acid (ABA) has shown efficacy in the treatment of diabetes and inflammation; however, its molecular targets and the mechanisms of action underlying its immunomodulatory effects remain unclear. This study investigates the role of peroxisome proliferator-activated receptor ␥ (PPAR ␥) and lanthionine synthetase C-like 2 (LANCL2) as molecular targets for ABA. We demonstrate that ABA increases PPAR ␥ reporter activity in RAW 264.7 macrophages and increases ppar ␥ expression in vivo, although it does not bind to the ligand-binding domain of PPAR ␥. LANCL2 knockdown studies provide evidence that ABAmediated activation of macrophage PPAR ␥ is dependent on lancl2 expression. Consistent with the association of LANCL2 with G proteins, we provide evidence that ABA increases cAMP accumulation in immune cells. ABA suppresses LPS-induced prostaglandin E 2 and MCP-1 production via a PPAR ␥-dependent mechanism possibly involving activation of PPAR ␥ and suppression of NF-B and nuclear factor of activated T cells. LPS challenge studies in PPAR ␥-expressing and immune cell-specific PPAR ␥ null mice demonstrate that ABA down-regulates toll-like receptor 4 expression in macrophages and T cells in vivo through a PPAR ␥-dependent mechanism. Global transcriptomic profiling and confirmatory quantitative RT-PCR suggest novel candidate targets and demonstrate that ABA treatment mitigates the effect of LPS on the expression of genes involved in inflammation, metabolism, and cell signaling, in part, through PPAR ␥. In conclusion, ABA decreases LPS-mediated inflammation and regulates innate immune responses through a bifurcating pathway involving LANCL2 and an alternative, ligand-binding domain-independent mechanism of PPAR ␥ activation.
Macrophage infiltration into adipose tissue is a hallmark of obesity. We recently reported two phenotypically distinct subsets of adipose tissue macrophages (ATM) based on the surface expression of the glycoprotein F4/80 and responsiveness to treatment with a peroxisome proliferator-activated receptor (PPAR) γ agonist. Hence, we hypothesized that F4/80hi and F4/80lo ATM differentially express PPAR γ. This study phenotypically and functionally characterizes F4/80hi and F4/80lo ATM subsets during obesity. Changes in gene expression were also examined on sorted F4/80lo and F4/80hi ATM by quantitative real-time RT-PCR. We show that while F4/80lo macrophages predominate in adipose tissue of lean mice, obesity causes accumulation of both F4/80lo and F4/80hi ATM. Moreover, accumulation of F4/80hi ATM in adipose tissue is associated with impaired glucose tolerance. Phenotypically, F4/80hi ATM express greater amounts of CD11c, MHC II, CD49b, and CX3CR1 and produce more TNF-α, MCP-1, and IL-10 than F4/80lo ATM. Gene expression analyses of the sorted populations revealed that only the F4/80lo population produced IL-4, whereas the F4/80hi ATM expressed greater amounts of PPAR γ, δ, CD36 and toll-like receptor-4. In addition, the deficiency of PPAR γ in immune cells favors expression of M1 and impairs M2 macrophage marker expression in adipose tissue. Thus, PPAR γ is differentially expressed in F4/80hi versus F4/80low ATM subsets and its deficiency favors a predominance of M1 markers in WAT.
Conjugated linoleic acid (CLA) exerts a protective effect on experimental inflammatory bowel disease and shows promise as a chemopreventive agent against colorectal cancer (CRC) in mice, although the mechanisms by which it exerts its beneficial effects against malignancies in the gut are not completely understood. Mice lacking PPARgamma in immune and epithelial cells and PPARgamma-expressing littermates were fed either control or CLA-supplemented (1 g CLA/100 g) diets to determine the role of PPARgamma in inflammation-induced CRC. To induce tumor formation and colitis, mice were treated with azoxymethane and then challenged with 2% dextran sodium sulfate, respectively. Dietary CLA ameliorated disease activity, decreased colitis, and prevented adenocarcinoma formation in the PPARgamma-expressing floxed mice but not in the tissue-specific PPARgamma-null mice. Dietary CLA supplementation significantly decreased the percentages of macrophages in the mesenteric lymph nodes (MLN) regardless of the genotype and increased regulatory T cell numbers in MLN of PPARgamma-expressing, but not in the tissue-specific, PPARgamma-null mice. Colonic tumor necrosis factor-alpha mRNA expression was significantly suppressed in CLA-fed, PPARgamma-expressing mice. This study suggests CLA ameliorates colitis and prevents tumor formation in part through a PPARgamma-dependent mechanism.
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