James D. Clark

Crystal structure of human cytosolic phospholipase A2 reveals a novel topology and catalytic mechanism.

Cytosolic phospholipase A2 initiates the biosynthesis of prostaglandins, leukotrienes, and platelet-activating factor (PAF), mediators of the pathophysiology of asthma and arthritis. Here, we report the X-ray crystal structure of human cPLA2 at 2.5 A. cPLA2 consists of an N-terminal calcium-dependent lipid-binding/C2 domain and a catalytic unit whose topology is distinct from that of other lipases. An unusual Ser-Asp dyad located in a deep cleft at the center of a predominantly hydrophobic funnel selectively cleaves arachidonyl phospholipids. The structure reveals a flexible lid that must move to allow substrate access to the active site, thus explaining the interfacial activation of this important lipase.

Discovery and Development of Janus Kinase (JAK) Inhibitors for Inflammatory Diseases

The Janus kinases (JAKs) are a family of intracellular tyrosine kinases that play an essential role in the signaling of numerous cytokines that have been implicated in the pathogenesis of inflammatory diseases. As a consequence, the JAKs have received significant attention in recent years from the pharmaceutical and biotechnology industries as therapeutic targets. Here, we provide a review of the JAK pathways, the structure, function, and activation of the JAK enzymes followed by a detailed look at the JAK inhibitors currently in the clinic or approved for these indications. Finally, a perspective is provided on what the past decade of research with JAK inhibitors for inflammatory indications has taught along with thoughts on what the future may hold in terms of addressing the opportunities and challenges that remain.

Cytosolic phospholipase A2α-deficient mice are resistant to collagen-induced arthritis

Pathogenic mechanisms relevant to rheumatoid arthritis occur in the mouse model of collagen-induced arthritis (CIA). Cytosolic phospholipase A2α (cPLA2α) releases arachidonic acid from cell membranes to initiate the production of prostaglandins and leukotrienes. These inflammatory mediators have been implicated in the development of CIA. To test the hypothesis that cPLA2α plays a key role in the development of CIA, we backcrossed cPLA2α-deficient mice on the DBA/1LacJ background that is susceptible to CIA. The disease severity scores and the incidence of disease were markedly reduced in cPLA2α-deficient mice compared with wild-type littermates. At completion of the study, >90% of the wild-type mice had developed disease whereas none of the cPLA2α-deficient mice had more than one digit inflamed. Furthermore, visual disease scores correlated with severity of disease determined histologically. Pannus formation, articular fibrillation, and ankylosis were all dramatically reduced in the cPLA2α-deficient mice. Although the disease scores differed significantly between cPLA2α mutant and wild-type mice, anti-collagen antibody levels were similar in the wild-type mice and mutant littermates. These data demonstrate the critical role of cPLA2α in the pathogenesis of CIA.

Cytosolic phospholipase A2α–deficient mice are resistant to experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE), a Th1-mediated inflammatory disease of the central nervous system (CNS), is a model of human multiple sclerosis. Cytosolic phospholipase A2 α (cPLA2 α), which initiates production of prostaglandins, leukotrienes, and platelet-activating factor, is present in EAE lesions. Using myelin oligodendrocyte glycoprotein (MOG) immunization, as well as an adoptive transfer model, we showed that cPLA2 α −/− mice are resistant to EAE. Histologic examination of the CNS from MOG-immunized mice revealed extensive inflammatory lesions in the cPLA2 α +/− mice, whereas the lesions in cPLA2 α −/− mice were reduced greatly or completely absent. MOG-specific T cells generated from WT mice induced less severe EAE in cPLA2 α −/− mice compared with cPLA2 α +/− mice, which indicates that cPLA2 α plays a role in the effector phase of EAE. Additionally, MOG-specific T cells from cPLA2 α −/− mice, transferred into WT mice, induced EAE with delayed onset and lower severity compared with EAE that was induced by control cells; this indicates that cPLA2 α also plays a role in the induction phase of EAE. MOG-specific T cells from cPLA2 α −/− mice were deficient in production of Th1-type cytokines. Consistent with this deficiency, in vivo administration of IL-12 rendered cPLA2 α −/− mice susceptible to EAE. Our data indicate that cPLA2 α plays an important role in EAE development and facilitates differentiation of T cells toward the Th1 phenotype.

Inhibitors of cytosolic phospholipase A2

BackgroundActivation of phospholipase A2 (PLA2) and the subsequent metabolism of arachidonic acid (AA) to prostaglandins have been shown to play an important role in neuronal death in neurodegenerative disease. Here we report the effects of the prion peptide fragment HuPrP106-126 on the PLA2 cascade in primary cortical neurons and translocation of cPLA2 to neurites.ResultsExposure of primary cortical neurons to HuPrP106-126 increased the levels of phosphorylated cPLA2 and caused phosphorylated cPLA2 to relocate from the cell body to the cellular neurite in a PrP-dependent manner, a previously unreported observation. HuPrP106-126 also induced significant AA release, an indicator of cPLA2 activation; this preceded synapse damage and subsequent cellular death. The novel translocation of p-cPLA2 postulated the potential for exposure to HuPrP106-126 to result in a re-arrangement of the cellular cytoskeleton. However p-cPLA2 did not colocalise significantly with F-actin, intermediate filaments, or microtubule-associated proteins. Conversely, p-cPLA2 did significantly colocalise with the cytoskeletal protein beta III tubulin. Pre-treatment with the PLA2 inhibitor, palmitoyl trifluoromethyl ketone (PACOCF3) reduced cPLA2 activation, AA release and damage to the neuronal synapse. Furthermore, PACOCF3 reduced expression of p-cPLA2 in neurites and inhibited colocalisation with beta III tubulin, resulting in protection against PrP-induced cell death.ConclusionsCollectively, these findings suggest that cPLA2 plays a vital role in the action of HuPrP106-126 and that the colocalisation of p-cPLA2 with beta III tubulin could be central to the progress of neurodegeneration caused by prion peptides. Further work is needed to define exactly how PLA2 inhibitors protect neurons from peptide-induced toxicity and how this relates to intracellular structural changes occurring in neurodegeneration.

Indole Cytosolic Phospholipase A2 α Inhibitors: Discovery and in Vitro and in Vivo Characterization of 4-{3-[5-Chloro-2-(2-{[(3,4-dichlorobenzyl)sulfonyl]amino}ethyl)-1-(diphenylmethyl)-1H-indol-3-yl]propyl}benzoic Acid, Efipladib

The optimization of a class of indole cPLA 2 alpha inhibitors is described herein. The importance of the substituent at C3 and the substitution pattern of the phenylmethane sulfonamide region are highlighted. Optimization of these regions led to the discovery of 111 (efipladib) and 121 (WAY-196025), which are shown to be potent, selective inhibitors of cPLA 2 alpha in a variety of isolated enzyme assays, cell based assays, and rat and human whole blood assays. The binding of these compounds has been further examined using isothermal titration calorimetry. Finally, these compounds have shown efficacy when dosed orally in multiple acute and chronic prostaglandin and leukotriene dependent in vivo models.

Selective functional inhibition of JAK‐3 is sufficient for efficacy in collagen‐induced arthritis in mice

OBJECTIVE All gamma-chain cytokines signal through JAK-3 and JAK-1 acting in tandem. We undertook this study to determine whether the JAK-3 selective inhibitor WYE-151650 would be sufficient to disrupt cytokine signaling and to ameliorate autoimmune disease pathology without inhibiting other pathways mediated by JAK-1, JAK-2, and Tyk-2. METHODS JAK-3 kinase selective compounds were characterized by kinase assay and JAK-3-dependent (interleukin-2 [IL-2]) and -independent (IL-6, granulocyte-macrophage colony-stimulating factor [GM-CSF]) cell-based assays measuring proliferation or STAT phosphorylation. In vivo, off-target signaling was measured by IL-22- and erythropoietin (EPO)-mediated models, while on-target signaling was measured by IL-2-mediated signaling. Efficacy of JAK-3 inhibitors was determined using delayed-type hypersensitivity (DTH) and collagen-induced arthritis (CIA) models in mice. RESULTS In vitro, WYE-151650 potently suppressed IL-2-induced STAT-5 phosphorylation and cell proliferation, while exhibiting 10-29-fold less activity against JAK-3-independent IL-6- or GM-CSF-induced STAT phosphorylation. Ex vivo, WYE-151650 suppressed IL-2-induced STAT phosphorylation, but not IL-6-induced STAT phosphorylation, as measured in whole blood. In vivo, WYE-151650 inhibited JAK-3-mediated IL-2-induced interferon-gamma production and decreased the natural killer cell population in mice, while not affecting IL-22-induced serum amyloid A production or EPO-induced reticulocytosis. WYE-151650 was efficacious in mouse DTH and CIA models. CONCLUSION In vitro, ex vivo, and in vivo assays demonstrate that WYE-151650 is efficacious in mouse CIA despite JAK-3 selectivity. These data question the need to broadly inhibit JAK-1-, JAK-2-, or Tyk-2-dependent cytokine pathways for efficacy.

Pharmacologic Inhibition of Tpl2 Blocks Inflammatory Responses in Primary Human Monocytes, Synoviocytes, and Blood

Tumor necrosis factor α (TNFα) is a pro-inflammatory cytokine that controls the initiation and progression of inflammatory diseases such as rheumatoid arthritis. Tpl2 is a MAPKKK in the MAPK (i.e. ERK) pathway, and the Tpl2-MEK-ERK signaling pathway is activated by the pro-inflammatory mediators TNFα, interleukin (IL)-1β, and bacterial endotoxin (lipopolysaccharide (LPS)). Moreover, Tpl2 is required for TNFα expression. Thus, pharmacologic inhibition of Tpl2 should be a valid approach to therapeutic intervention in the pathogenesis of rheumatoid arthritis and other inflammatory diseases in humans. We have developed a series of highly selective and potent Tpl2 inhibitors, and in the present study we have used these inhibitors to demonstrate that the catalytic activity of Tpl2 is required for the LPS-induced activation of MEK and ERK in primary human monocytes. These inhibitors selectively target Tpl2 in these cells, and they block LPS- and IL-1β-induced TNFα production in both primary human monocytes and human blood. In rheumatoid arthritis fibroblast-like synoviocytes these inhibitors block ERK activation, cyclooxygenase-2 expression, and the production of IL-6, IL-8, and prostaglandin E2, and the matrix metalloproteinases MMP-1 and MMP-3. Taken together, our results show that inhibition of Tpl2 in primary human cell types can decrease the production of TNFα and other pro-inflammatory mediators during inflammatory events, and they further support the notion that Tpl2 is an appropriate therapeutic target for rheumatoid arthritis and other human inflammatory diseases.

Blockade of cytosolic phospholipase A2α prevents experimental autoimmune encephalomyelitis and diminishes development of Th1 and Th17 responses

Cytosolic phospholipase A2 alpha (cPLA2 alpha) is the rate-limiting enzyme for release of arachidonic acid, which is converted primarily to prostaglandins via the cyclooxygenase (COX) 1/2 pathways, and leukotrienes via the 5-lipoxygenase (LO) pathway. We utilized inhibitors of cPLA2 alpha, COX-1/2 and 5-LO to determine the potential roles of these enzymes in development of experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS). Blocking cPLA2 alpha prevented EAE development and greatly reduced antigen-induced production of Th1-type cytokines and IL-17. Blocking COX-1/2 delayed onset and reduced severity of EAE, and reduced production of Th1-type cytokines, but not IL-17. Blocking 5-LO delayed onset and reduced cumulative severity of EAE, but did not reduce production of Th1-type cytokines or IL-17. Finally, blockade of cPLA2 alpha from the onset of clinical EAE reduced duration of EAE relapses. Therefore, cPLA2 alpha represents a potential therapeutic target for treatment of MS.

Potential therapeutic uses of phospholipase A2 inhibitors

The cleavage of the sn-2 ester of membrane phospholipids by phospholipase A2 has long been ascribed as the important initiation step in the production of pro-inflammatory mediators, including prostaglandins, leukotrienes and platelet-activating factor. Consequently, this enzymatic action has been implicated in the pathogenesis of a multitude of diseases, ranging from lung inflammation and arthritis to neural injuries and cardiovascular disease. The three types of phospholipase A2 that have received the most research attention are: secretory phospholipase A2 (sPLA2), cytosolic phospholipase A2 (cPLA2) and lipoprotein-associated phospholipase A2 (LpPLA2). This review will discuss the likely roles of these PLA2 in the development of the disease conditions and recent advances in the development of specific inhibitors, based on the patent literature from December 2000 to December 2003.