Inhibition of cytosolic phospholipase A(2) attenuates activation of mitogen-activated protein kinases in human monocytic cells

Diesel Particulate Extract Accelerates Premature Skin Aging in Human Fibroblasts via Ceramide-1-Phosphate-Mediated Signaling Pathway

Both intrinsic (i.e., an individual’s body clock) and extrinsic factors (i.e., air pollutants and ultraviolet irradiation) accelerate premature aging. Epidemiological studies have shown a correlation between pollutant levels and aging skin symptoms. Diesel particle matter in particular leads to some diseases, including in the skin. Our recent study demonstrates that diesel particulate extract (DPE) increases apoptosis via increases in an anti-mitogenic/pro-apoptotic lipid mediator, ceramide in epidermal keratinocytes. Here, we investigated whether and how DPE accelerates premature skin aging using cultured normal human dermal fibroblasts (HDF). We first demonstrated that DPE increases cell senescence marker β-galactosidase activity in HDF. We then found increases in mRNA and protein levels, along with activity of matrix metalloprotease (MMP)-1 and MMP-3, which are associated with skin aging following DPE exposure. We confirmed increases in collagen degradation in HDF treated with DPE. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) is activated by DPE and results in increased ceramide production by sphingomyelinase activation in HDF. We identified that ceramide-1-phosphate (C1P) (produced from ceramide by ceramide kinase activation) activates MMP-1 and MMP-3 through activation of arachidonate cascade, followed by STAT 1- and STAT 3-dependent transcriptional activation.

Inhibitory effect of a potent and selective cytosolic phospholipase A2alpha inhibitor RSC-3388 on skin inflammation in mice

Cytosolic phospholipase A2alpha (cPLA2alpha) preferentially hydrolyzes membrane phospholipids containing arachidonic acid, resulting in the biosynthesis of eicosanoids such as prostaglandins and leukotrienes. To examine the contribution of cPLA2alpha to skin inflammation, we evaluated the effect of (E)-N-[(2S,4R)-4-[N-(biphenyl-2-ylmethyl)-N-2-methylpropylamino]-1-[2-(2,4-difluorobenzoyl)benzoyl]pyrrolidin- 2-yl]methyl-3-[4-(2,4-dioxothiazolidin-5-ylidenemethyl) phenyl]acrylamide (RSC-3388), a potent and selective cPLA2alpha inhibitor, on 2,4,6-trinitro-1-chlorobenzene (TNCB)-induced ear inflammation and mite antigen-induced dermatitis in mice. Topical application of RSC-3388 showed a significant inhibitory activity against TNCB-induced ear swelling and eicosanoid production in mice. Comprehensive expression analysis using Gene-Chip technology and subsequent experiments concerning mRNA and protein expression demonstrated that RSC-3388 clearly reduced the levels of interleukin-1beta, macrophage inflammatory protein-1alpha (MIP-1alpha) and MIP-1beta in a TNCB-induced mouse model. In addition, RSC-3388 ointment significantly alleviated atopic dermatitis-like skin lesions induced by repeated application of mite antigen. Furthermore, increased expression of cPLA(2)alpha, assessed by anti-phospho-cPLA2alpha antibody, was observed in the skin lesions of mite-antigen-induced dermatitis. These results indicate that cPLA2alpha is involved in the development of skin inflammation in mice, and RSC-3388 is expected to be useful for the treatment of inflammatory skin disorders such as atopic dermatitis.

The phospholipase A2 superfamily and its group numbering system

The superfamily of phospholipase A(2) (PLA(2)) enzymes currently consists of 15 Groups and many subgroups and includes five distinct types of enzymes, namely the secreted PLA(2)s (sPLA(2)), the cytosolic PLA(2)s (cPLA(2)), the Ca(2+) independent PLA(2)s (iPLA(2)), the platelet-activating factor acetylhydrolases (PAF-AH), and the lysosomal PLA(2)s. In 1994, we established the systematic Group numbering system for these enzymes. Since then, the PLA(2) superfamily has grown continuously and over the intervening years has required several updates of this Group numbering system. Since our last update, a number of new PLA(2)s have been discovered and are now included. Additionally, tools for the investigation of PLA(2)s and approaches for distinguishing between the different Groups are described.

Activation of cytosolic phospholipase A2 in human T-lymphocytes involves inhibitor-kappaB and mitogen-activated protein kinases

The group IV 85 kDa cytosolic phospholipase A(2) regulates many aspects of innate immunity. However, the function of this enzyme in T-cells remains controversial. We show here that human peripheral blood lymphocytes and Jurkat cells express cytosolic phospholipase A(2) and produce prostaglandin A(2) and leukotriene B(4). Selective inhibitors of this enzyme suppressed Ca(2+)-ionophore-, mitogen- and T-cell receptor-mediated expression of interleukin-2 at the level of transcription from the promoter. Activation of mitogen-activated protein kinases (MAPK), degradation of inhibitor-kappaBalpha and transactivation by nuclear factor-kappaB (NFkappaB) were impaired as was the antigen-, lectin- and interleukin-2-driven proliferation of T-cells in vitro. Ligands of peroxisome proliferator-activated receptor-gamma (PPARgamma) induced rapid phosphorylation of MAPK in human monocytic but not in Jurkat cells. These data indicated that in T-cells, eicosanoids generated upon signal-activated cytosolic phospholipase A(2) promote NFkappaB-dependent interleukin-2 transcription via a PPARgamma-independent mechanism involving the MAPK-pathway.

Comparative analysis of docking motifs in MAP-kinases and nuclear receptors

Nuclear receptor (NR) agonists induce activation of mitogen-activated protein kinases (MAPK) through an yet unknown rapid non-genomic mechanism. Vice versa, NR are targets for phosphorylation by MAPK. By multiple alignment of the amino acid sequences and comparative analysis of the secondary and tertiary structures we identified four peptides in MAPK with similarity to bona fide protein-protein-interaction motifs in NR. In both molecule species, these motifs mediate selective docking to dimerization partners, coregulators or phosphoacceptors. We therefore propose that similar motifs may direct the site-specific association of NR with MAPK. Based on mutual allosteric interactions within a kinase-receptor complex, we discuss a novel principle how NR-agonists may regulate kinase activity and thus expression of hormone-dependent genes.

Characterization of a novel inhibitor of cytosolic phospholipase A2alpha, pyrrophenone

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha), one of the three subtypes of cPLA(2) (alpha, beta and gamma), is thought to be a rate-limiting enzyme in eicosanoid biosynthesis. We developed a novel and potent cPLA(2)alpha inhibitor with an optically active pyrrolidine, termed pyrrophenone, and characterized this compound in detail using enzyme and cellular assay systems. Pyrrophenone, which shows strong inhibition of cPLA(2)alpha activity, is one of the most potent cPLA(2)alpha inhibitors reported to date. Similar inhibitory potencies for cPLA(2)alpha were obtained from three different assays. The inhibitory activity of pyrrophenone is two or three orders of magnitude more potent than arachidonyl trifluoromethyl ketone (AACOCF(3)) under the same assay conditions. Pyrrophenone shows reversible inhibition of cPLA(2)alpha and displays no characteristics of the slow-binding inhibition observed for AACOCF(3). Pyrrophenone also inhibited the esterase and lysophospholipase activities of cPLA(2)alpha. However, the inhibition by pyrrophenone of 14 kDa secretory PLA(2)s, types IB and IIA, was over two orders of magnitude less potent than that for cPLA(2)alpha. Pyrrophenone strongly inhibited arachidonic acid release in calcium ionophore (A23187)-stimulated human monocytic cells (THP-1 cells) in a dose-dependent manner with an IC(50) value of 0.024 microM, followed by suppression of eicosanoid synthesis, and also showed dose-dependent inhibition for interleukin-1-induced prostaglandin E(2) synthesis in human renal mesangial cells with an IC(50) value of 0.0081 microM. The mechanism of inhibition of eicosanoid synthesis in these cell-based assays was due to inhibition of only one step of arachidonic acid release without any effect on cyclo-oxygenase or lipoxygenase pathways. These results suggest that pyrrophenone could be a potential therapeutic agent for inflammatory diseases.

Activation of the Phosphatidylinositol 3-Kinase-Akt/Protein Kinase B Signaling Pathway in Arachidonic Acid-stimulated Human Myeloid and Endothelial Cells

Although arachidonic acid has been demonstrated to stimulate a wide variety of cellular functions, the responsible mechanisms remain poorly defined. We now report that arachidonic acid stimulated the activity of class Ia phosphatidylinositol 3-kinase (PI3K) in human umbilical vein endothelial cells, HL60 cells, and human neutrophils. Pretreatment of endothelial cells with AG-1478, an inhibitor of the ErbB receptor family, resulted in the suppression of PI3K activation by arachidonic acid. The fatty acid enhanced the tyrosine phosphorylation of ErbB4 but not of ErbB2 or ErbB3. The ability of arachidonic acid to stimulate PI3K activity in neutrophils was suppressed by indomethacin and nordihydroguaiaretic acid, inhibitors of the cyclooxygenases and lipoxygenases, respectively, but not by 17-octadecynoic acid, an inhibitor of omega-hydroxylation of arachidonic acid by cytochrome P450 monooxygenases. Consistent with this, the activity of PI3K in neutrophils was stimulated by 5-hydroxyeicosatetraenoic acid. Arachidonic acid also transiently stimulated the phosphorylation of Akt on Thr-308 and Ser-473. Although PI3K was not required for the activation of the mitogen-activated protein kinases, ERK1, ERK2, and p38, in arachidonic acid-stimulated neutrophils, the fatty acid acted via PI3K to stimulate the respiratory burst. These results not only define a novel mechanism through which some of the actions of arachidonic acid are mediated but also demonstrate that, in addition to ErbB1 (epidermal growth factor receptor), ErbB4 can also be transactivated by a non-epidermal growth factor-like ligand.

Could platelet activating factor play a role in developmental dyslexia?

Post-mortem studies by Galaburda and colleagues on the brains of developmental dyslexics found characteristic neuronal abnormalities: ectopias, microgyria, and fewer large-soma cells in sensory thalamus. An association between dyslexia and immune dysfunction has also been proposed. We describe a mechanism which may explain these observations. Platelet-activating factor (PAF) is a pro-inflammatory lipid implicated in neurological disorders. We propose that PAF may also be involved in dyslexia.