Potent inhibition of NFAT activation and T cell cytokine production by novel low molecular weight pyrazole compounds.

NFAT (nuclear factor of activated T cell) proteins are expressed in most immune system cells and regulate the transcription of cytokine genes critical for the immune response. The activity of NFAT proteins is tightly regulated by the Ca(2+)/calmodulin-dependent protein phosphatase 2B/calcineurin (CaN). Dephosphorylation of NFAT by CaN is required for NFAT nuclear localization. Current immunosuppressive drugs such as cyclosporin A and FK506 block CaN activity thus inhibiting nuclear translocation of NFAT and consequent cytokine gene transcription. The inhibition of CaN in cells outside of the immune system may contribute to the toxicities associated with cyclosporin A therapy. In a search for safer immunosuppressive drugs, we identified a series of 3,5-bistrifluoromethyl pyrazole (BTP) derivatives that block Th1 and Th2 cytokine gene transcription. The BTP compounds block the activation-dependent nuclear localization of NFAT as determined by electrophoretic mobility shift assays. Confocal microscopy of cells expressing fluorescent-tagged NFAT confirmed that the BTP compounds block calcium-induced movement of NFAT from the cytosol to the nucleus. Inhibition of NFAT was selective because the BTP compounds did not affect the activation of NF-kappaB and AP-1 transcription factors. Treatment of intact T cells with the BTP compounds prior to calcium ionophore-induced activation of CaN caused NFAT to remain in a highly phosphorylated state. However, the BTP compounds did not directly inhibit the dephosphorylation of NFAT by CaN in vitro, nor did the drugs block the dephosphorylation of other CaN substrates including the type II regulatory subunit of protein kinase A and the transcription factor Elk-1. The data suggest that the BTP compounds cause NFAT to be maintained in the cytosol in a phosphorylated state and block the nuclear import of NFAT and, hence, NFAT-dependent cytokine gene transcription by a mechanism other than direct inhibition of CaN phosphatase activity. The novel inhibitors described herein will be useful in better defining the cellular regulation of NFAT activation and may lead to identification of new therapeutic targets for the treatment of autoimmune disease and transplant rejection.

3,5-Bis(trifluoromethyl)pyrazoles: a novel class of NFAT transcription factor regulator.

A series of bis(trifluoromethyl)pyrazoles (BTPs) has been found to be a novel inhibitor of cytokine production. Identified initially as inhibitors of IL-2 synthesis, the BTPs have been optimized in this regard and even inhibit IL-2 production with a 10-fold enhancement over cyclosporine in an ex vivo assay. Additionally, the BTPs show inhibition of IL-4, IL-5, IL-8, and eotaxin production. Unlike the IL-2 inhibitors, cyclosporine and FK506, the BTPs do not directly inhibit the dephosphorylation of NFAT by calcineurin.

Inhibition of p56(lck) tyrosine kinase by isothiazolones.

Lck encodes a 56-kDa protein-tyrosine kinase, predominantly expressed in T lymphocytes, crucial for initiating T cell antigen receptor (TCR) signal transduction pathways, culminating in T cell cytokine gene expression and effector functions. As a consequence of a high-throughput screen for selective, novel inhibitors of p56(lck), an isothiazolone compound was identified, methyl-3-(N-isothiazolone)-2-thiophenecarboxylate(A-125800), which inhibits p56(lck) kinase activity with IC50 = 1-7 microM. Under similar assay conditions, the isothiazolone compound was equipotent in blocking the ZAP-70 tyrosine kinase activity but was 50 to 100 times less potent against the catalytic activities of p38 MAP kinase and c-Jun N-terminal kinase 2alpha. A-125800 blocked activation-dependent TCR tyrosine phosphorylation and intracellular calcium mobilization in Jurkat T cells (IC50 = 35 microM) and blocked T cell proliferation in response to alloantigen (IC50 = 14 microM) and CD3/CD28-induced IL-2 secretion (IC50 = 2.2 microM) in primary T cell cultures. Inhibition of p56(lck )by A-125800 was dose- and time-dependent and was irreversible. A substitution of methylene for the sulfur atom in the isothiazolone ring of the compound completely abrogated the ability to inhibit p56(lck) kinase activity and TCR-dependent signal transduction. Incubation with thiols such as beta-ME or DTT also blocked the ability of the isothiazolone to inhibit p56(lck) kinase activity. LC/MS analysis established the covalent modification of p56(lck) at cysteine residues 378, 465, and 476. Together these data support an inhibitory mechanism, whereby cysteine -SH groups within the p56(lck) catalytic domain react with the isothiazolone ring, leading to ring opening and disulfide bond formation with the p56(lck) enzyme. Loss of p56(lck) activity due to -SH oxidation has been suggested to play a role in the pathology of AIDS. Consequently, a similar mechanism of sulfhydryl oxidation leading to p56(lck) inhibition, described in this report, may occur in the intact T cell and may underlie certain T cell pathologies.

Cytosolic phospholipase A2 (cPLA2) distribution in murine brain and functional studies indicate that cPLA2 does not participate in muscarinic receptor-mediated signaling in neurons.

Cytosolic phospholipase A2 (cPLA2) catalyzes the selective release of arachidonic acid from the sn-2 position of membrane phospholipids and has been suggested as an effector in the receptor-mediated release of arachidonic acid in signal transduction. The potential role of cPLA2 as an effector in muscarinic acetylcholine receptor signaling was investigated through ectopic expression of either the m1 or m5 receptor in combination with cPLA2 in COS-1, CHO and U-373 MG cell lines. U-373 MG and COS-1 cells express undetectable or very low levels of cPLA2. CHO cell extracts are characterized by a significant endogenous PLA2 activity that was increased over 20-fold following transient expression with cPLA2 cDNA. However, in none of the cells lines did the co-expression of muscarinic receptor and cPLA2 result in a significant increase in muscarinic receptor-mediated arachidonic acid release over cells expressing muscarinic receptor alone. The distribution of cPLA2 mRNA and cPLA2 immunoreactivity in murine brain were determined in order to investigate a potential role for cPLA2 in neurotransmission. cPLA2 mRNA was expressed in white matter, including cells contained within linear arrays characteristic of interfascicular oligodendrocytes. cPLA2 immunoreactivity in white matter was evident throughout the processes of fibrous astrocytes. cPLA2 expression in gray matter was confined to astrocytes at the pial surface of the brain. cPLA2 mRNA was detected in pia mater, both at the brain surface and inner core of the choroid plexus. cPLA2 may not be directly linked to neurotransmission since enzyme expression, mRNA, and cPLA2 immunoreactivity were undetectable in neurons of murine brain. Support or regulation of neurotransmission may be provided through the activity of cPLA2 in glial cells.

Identification of essential residues for the catalytic function of 85-kDa cytosolic phospholipase A2. Probing the role of histidine, aspartic acid, cysteine, and arginine.

Cytosolic phospholipase A2 (cPLA2) hydrolyzes the sn-2-acyl ester bond of phospholipids and shows a preference for arachidonic acid-containing substrates. We found previously that Ser-228 is essential for enzyme activity and is likely to function as a nucleophile in the catalytic center of the enzyme (Sharp, J. D., White, D. L., Chiou, X. G., Goodson, T., Gamboa, G. C., McClure, D., Burgett, S., Hoskins, J., Skatrud, P. L., Sportsman, J. R., Becker, G. W., Kang, L. H., Roberts, E. F., and Kramer, R. M.(1991) J. Biol. Chem. 266, 14850-14853). cPLA2 contains a catalytic aspartic acid motif common to the subtilisin family of serine proteases. Substitution within this motif of Ala for Asp-549 completely inactivated the enzyme, and substitutions with either glutamic acid or asparagine reduced activity 2000- and 300-fold, respectively. Additionally, using mutants with cysteine replaced by alanine, we found that Cys-331 is responsible for the enzyme's sensitivity to N-ethylmaleimide. Surprisingly, substituting alanine for any of the 19 histidines did not produce inactive enzyme, demonstrating that a classical serine-histidine-aspartate mechanism does not operate in this hydrolase. We found that substituting alanine or histidine for Arg-200 did produce inactive enzyme, while substituting lysine reduced activity 200-fold. Results obtained with the lysine mutant (R200K) and a coumarin ester substrate suggest no specific interaction between Arg-200 and the phosphoryl group of the phospholipid substrate. Arg-200, Ser-228, and Asp-549 are conserved in cPLA2 from six species and also in four nonmammalian phospholipase B enzymes. Our results, supported by circular dichroism, provide evidence that Asp-549 and Arg-200 are critical to the enzyme's function and suggest that the cPLA2 catalytic center is novel.

Serine 228 is essential for catalytic activities of 85-kDa cytosolic phospholipase A2.

The Ca(2+)-sensitive cytosolic phospholipase A2 (cPLA2) displays both a phospholipase A2 and a lysophospholipase activity. Numerous hydrolases, including lipases, catalyze the hydrolysis of ester bonds by means of an active site triad of amino acids that includes a serine or a cysteine residue. We have examined whether human cPLA2 belongs to this class of enzymes by using site-directed mutagenesis. Although chemical inactivation of cPLA2 by the sulfhydryl reagent N-ethylmaleimide made it appear that cysteine(s) may be essential for catalysis, all 9 cysteine residues of cPLA2 proved dispensable, allowing near-normal enzyme activity when substituted by alanine. We noted that cPLA2 contains a 110-amino-acid region with sequence homology to phospholipase B (PLB) from Penicillium notatum. Interestingly, one of the conserved serines of cPLA2, Ser-228, within this domain aligns with the lipase consensus sequence Gly-X(Leu)-Ser(137)-X(Gly)-Gly of PLB. Replacement of Ser-228 by alanine (or threonine or cysteine) yielded catalytically inactive cPLA2, even though the native conformation was maintained as determined by CD spectroscopy. Likewise, the lysophospholipase activity was completely abolished by the Ser-228 mutations. In contrast, substitution by alanine of three different serines of cPLA2 (Ser-195, Ser-215, or Ser-577) that also aligned with the PLB sequence allowed for substantial enzymatic activity of cPLA2. Our findings provide evidence that 1) Ser-228 participates in the catalytic mechanism of cPLA2 and that 2) both the phospholipase A2 and the lysophospholipase activities of cPLA2 are catalyzed by the same active site residue(s).

Calcium-sensitive cytosolic phospholipase A2 (cPLA2) is expressed in human brain astrocytes.

Calcium-sensitive cytosolic phospholipase A2 (cPLA2) is responsible for receptor-mediated liberation of arachidonic acid, and thus plays an important role in the initiation of the inflammatory lipid-mediator cascade generating eicosanoids and platelet-activating factor. In this study we have investigated the cellular distribution of cPLA2 in brain using a monoclonal antibody raised against cPLA2 to immunostain tissue sections of human cerebral cortex. We have localized cPLA2 in astrocytes of the gray matter. Colocalization with glial fibrillary acidic protein (GFAP) confirmed that cPLA2 is associated predominantly with protoplasmic astrocytes. Astrocytes of the white matter, on the other hand, were not immunoreactive. In experiments using different human astrocytoma cell lines we found that cPLA2 can be immunochemically localized in UC-11 MG cells, but cannot be detected in U-373 MG cells. This finding is consistent with the observation that cPLA2 mRNA as well as cPLA2 enzymatic activity can be readily measured in UC-11 MG astrocytoma cells, yet cannot be detected in U-373 MG cells. Our data suggest that the astrocyte is a primary source of cPLA2 in the brain and provide further evidence for the importance of this cell type in inflammatory processes in the brain.

Molecular cloning and expression of human Ca(2+)-sensitive cytosolic phospholipase A2.

Phospholipases A2 (PLA2s) play a key role in inflammatory processes through production of precursors of eicosanoids and platelet-activating factor. Recently, we described the purification of a novel approximately 100-kDa cytosolic PLA2 (cPLA2) from human monoblast U937 cells that is activated by physiological (intracellular) concentrations of Ca2+ (Kramer, R. M., Roberts, E. F., Manetta, J., and Putnam, J. E. (1991) J. Biol. Chem. 266, 5268-5272). Here we report the isolation of the complementary DNA encoding human cPLA2 and confirm its identity by expression in bacteria and in hamster cells. The predicted 749-amino acid cPLA2 protein has no similarity to the well known secretory PLA2s, but contains a structural element homologous to the C2 region of protein kinase C. The molecular cloning of cPLA2 will allow further studies defining the structure, function, and regulation of this novel PLA2.