Bacterial lipopolysaccharides, phorbol myristate acetate, and zymosan induce the myristoylation of specific macrophage proteins

Monocytes treated with ciprofloxacin and oxyLDL express myristate, priming atherosclerosis

Antibiotics are essential in many life-threatening diseases. On the other hand, improper use of antibiotics can be disastrous. Cell morphological changes were observed in the ciprofloxacin-treated cells starting at 48 hours. Changes in cell morphology were continuously observed up to 14 days, which showed gradual morphological changes from monocyte to plaque-like cells at day 12, and foam cell, which is an intermediate stage in atherosclerosis was observed at day 8, which was confirmed with Oil Red O staining. Flow cytometry data revealed that oxidized LDL (oxyLDL)-induced cells showed 60.16% of CD64 (proinflammatory macrophage markers) and no expression of CD23 (anti-inflammatory macrophage markers), whereas ciprofloxacin-treated cells expressed 67.97% of CD64 and 13.78% of CD23. Chemokine antibody array analysis revealed that ciprofloxacin exposed cells showed a proinflammatory role (ENA78, Eotaxin1, Eotaxin2, IP-10, MIG, MIP-3β, SDF-1β, TECK, CXCL16, and Fractalkine). Liquid chromatography with tandem mass spectrometry (LC-MS/MS) revealed that myristic acid was incorporated into a protein with 68 kDa molecular mass in exposing oxyLDL-induced monocytes with ciprofloxacin, which could be a reason for the observed foam cells and in vitro plaque formation. As myristic acid primes atherosclerosis, it is better to limit the intake of antibiotics like ciprofloxacin for common illness, specifically the high-risk patients, which may contribute to atherosclerosis.

Myristoylation: An Important Protein Modification in the Immune Response

Protein N-myristoylation is a cotranslational lipidic modification specific to the alpha-amino group of an N-terminal glycine residue of many eukaryotic and viral proteins. The ubiquitous eukaryotic enzyme, N-myristoyltransferase, catalyzes the myristoylation process. Precisely, attachment of a myristoyl group increases specific protein–protein interactions leading to subcellular localization of myristoylated proteins with its signaling partners. The birth of the field of myristoylation, a little over three decades ago, has led to the understanding of the significance of protein myristoylation in regulating cellular signaling pathways in several biological processes especially in carcinogenesis and more recently immune function. This review discusses myristoylation as a prerequisite step in initiating many immune cell signaling cascades. In particular, we discuss the hitherto unappreciated implication of myristoylation during myelopoiesis, innate immune response, lymphopoiesis for T cells, and the formation of the immunological synapse. Furthermore, we discuss the role of myristoylation in inducing the virological synapse during human immunodeficiency virus infection as well as its clinical implication. This review aims to summarize existing knowledge in the field and to highlight gaps in our understanding of the role of myristoylation in immune function so as to further investigate into the dynamics of myristoylation-dependent immune regulation.

Lateral sequestration of phosphatidylinositol 4,5-bisphosphate by the basic effector domain of myristoylated alanine-rich C kinase substrate is due to nonspecific electrostatic interactions

A peptide corresponding to the basic (+13), unstructured effector domain of myristoylated alanine-rich C kinase substrate (MARCKS) binds strongly to membranes containing phosphatidylinositol 4,5-bisphosphate (PIP(2)). Although aromatic residues contribute to the binding, three experiments suggest the binding is driven mainly by nonspecific local electrostatic interactions. First, peptides with 13 basic residues, Lys-13 and Arg-13, bind to PIP(2)-containing vesicles with the same high affinity as the effector domain peptide. Second, removing basic residues from the effector domain peptide reduces the binding energy by an amount that correlates with the number of charges removed. Third, peptides corresponding to a basic region in GAP43 and MARCKS effector domain-like regions in other proteins (e.g. MacMARCKS, adducin, Drosophila A kinase anchor protein 200, and N-methyl-d-aspartate receptor) also bind with an energy that correlates with the number of basic residues. Kinetic measurements suggest the effector domain binds to several PIP(2). Theoretical calculations show the effector domain produces a local positive potential, even when bound to a bilayer with 33% monovalent acidic lipids, and should thus sequester PIP(2) laterally. This electrostatic sequestration was observed experimentally using a phospholipase C assay. Our results are consistent with the hypothesis that MARCKS could reversibly sequester much of the PIP(2) in the plasma membrane.

Possible role of Marcks in the cellular modulation of monocytic tissue factor-initiated hypercoagulation

The enhanced extrinsic tissue factor (TF)-initiated coagulation, often resulting from sepsis, could lead to disseminated intravascular coagulation presenting cardiovascular complications. Using model human leukaemia THP-1 monocytes, we studied monocytic TF (mTF) hypercoagulation and its regulation. After an 8 h exposure to bacterial endotoxin [lipopolysaccharide (LPS); 100 ng/ml], mTF activity was significantly upregulated as the result of the enhanced mTF synthesis. Thereafter, LPS induction declined, exhibiting a "quiescent-desensitizing' phenomenon. Such diminished LPS induction was,however,associated with sustained LPS-enhanced mTF synthesis, revealing the possible occurrence of a post-translational downregulation. It was noted that LPS desensitization was accompanied by the increased expression of myristoylated alanine-rich C kinase substrate (Marcks). In contrast, A23187 (20 micromol/l) or Quin-2AM (20 micromol/l) drastically activated mTF activity without detectable effect on mTF synthesis; both of which showed that sustained functional upregulation during 24 h culture did not enhance Marcks expression. These inverse correlations between mTF activity upregulation and Marcks expression suggested that Marcks could be inhibitory. Marcks phosphorylation site domain (151-175) (Marcks PSD) readily inhibited mTF-dependent FVII activation and diminished FVIIa formation in LPS-challenged cells. As a result, Marcks PSD offset LPS-induced mTF hypercoagulation upon inclusion in the single-stage clotting assays. The anticoagulant activity was confirmed by showing that Marcks PSD significantly blocked rabbit brain thromboplastin (rbTF) procoagulation and inhibited rbTF-dependent FVII activation as well as FVIIa formation. Our study suggests that Marcks expression plays a role in a novel cellular modulation to downregulate mTF hypercoagulation.

Human lung tissue macrophages, but not alveolar macrophages, express matrix metalloproteinases after direct contact with activated T lymphocytes

Human alveolar macrophages (AM) and lung tissue macrophages (LTM) have a distinct localization in the cellular environment. We studied their response to direct contact with activated T lymphocytes in terms of the production of interstitial collagenase (MMP-1), 92-kD gelatinase (MMP-9), and of TIMP-1, one of the counter-regulatory tissue inhibitors of metalloproteinases. Either AM obtained by bronchoalveolar lavage or LTM obtained by mincing and digestion of lung tissue were exposed for 48 h to plasma membranes of T lymphocytes previously activated with phorbol myristate acetate and phytohemagglutinin for 24 h. Membranes of activated T cells strongly induced the production of MMP-1, MMP-9, and TIMP-1 exclusively in LTM but not in AM, whereas membranes from unstimulated T cells failed to induce the release of MMPs. Both populations of mononuclear phagocytes spontaneously released only small amounts of MMPs and TIMP-1. Similar results were obtained when MMP and TIMP-1 expression was analyzed at pretranslational and biosynthetic levels, respectively. Blockade experiments with cytokine antagonists revealed the involvement of T-cell membrane-associated interleukin-1 and tumor necrosis factor-alpha in MMP production by LTM upon contact with T cells. These data suggest that the ability of lung macrophages to produce MMPs after direct contact with activated T cells is related to the difference in phenotype of mononuclear phagocytes and cell localization. In addition, these observations indicate that cell-cell contact represents an important biological mechanism in potentiating the inflammatory response of mononuclear phagocytes in the lungs.

N-myristoyltransferase

Myristoylation refers to the co-translational addition of a myristoyl group to an amino-terminal glycine residue of a protein by an ubiquitously distributed enzyme myristoyl-CoA:protein N-myristoyltransferase (NMT, EC 2.3.1.97). This review describes the basic enzymology, molecular cloning and regulation of NMT activity in various pathophysiological processes such as colon cancer and diabetes.

Down-regulation of MARCKS-related protein (MRP) in macrophages infected with Leishmania

Leishmania, a protozoan parasite of macrophages, has been shown to interfere with host cell signal transduction pathways including protein kinase C (PKC)-dependent signaling. Myristoylated alanine-rich C kinase substrate (MARCKS) and MARCKS-related protein (MRP, MacMARCKS) are PKC substrates in diverse cell types. MARCKS and MRP are thought to regulate the actin network and thereby participate in cellular responses involving cytoskeletal rearrangement. Because MRP is a major PKC substrate in macrophages, we examined its expression in response to infection by Leishmania. Activation of murine macrophages by cytokines increased MRP expression as determined by Western blot analysis. Infection with Leishmania promastigotes at the time of activation or up to 48 h postactivation strongly decreased MRP levels. Leishmania-dependent MRP depletion was confirmed by [3H]myristate labeling and by immunofluorescence microscopy. All species or strains of Leishmania parasites tested, including lipophosphoglycan-deficient Leishmania major L119, decreased MRP levels. MRP depletion was not obtained with other phagocytic stimuli including zymosan, latex beads, or heat-killed Streptococcus mitis, a Gram-positive bacterium. Experiments with [3H]myristate labeled proteins revealed the appearance of lower molecular weight fragments in Leishmania-infected cells suggesting that MRP depletion may be due to proteolytic degradation.

Lipopolysaccharide induction of MARCKS-related protein and cytokine secretion are differentially impaired in microglia from LPS-nonresponsive (C3H/HeJ) mice

Many events involved in activation of microglia and leukocytes by lipopolysaccharide (LPS) are mediated by protein kinase C (PKC), and we have recently demonstrated that a major PKC substrate, MARCKS-related protein (MRP), is selectively induced by LPS in murine microglia. In microglia from LPS-nonresponsive (C3H/HeJ) mice, induction of MRP and secretion of CSF-1 required much higher LPS concentrations (> or = 100 ng/ml) than in normal (C3H/OuJ) microglia (< or = 10 ng/ml). By contrast, TNF alpha production was not significantly increased in C3H/HeJ microglia even at 1 microgram LPS/ml. Microglia expressed PKC isoforms alpha, beta, delta, and zeta (but not gamma and epsilon); PKC isoform levels were similar in both normal and C3H/HeJ microglia and no significant change in response to LPS was noted. Our results indicate that LPS alters PKC substrate (rather than kinase) expression, and that the Lpsd mutation in C3H/HeJ mice differentially affects regulation of several gene products implicated in microglial function.

Binding of MARCKS (myristoylated alanine-rich C kinase substrate)-related protein (MRP) to vesicular phospholipid membranes

The myristoylated alanine-rich C kinase substrate (MARCKS) protein family has two known members, MARCKS itself and MARCKS-related protein (MRP, also called MacMARCKS or F52). They are essential for brain development and are believed to regulate the structure of the actin cytoskeleton at the plasma membrane. Hence membrane binding is central to their function. MARCKS has been quite extensively characterized; MRP much less so. Despite the fact that MRP is only two thirds the size of MARCKS, it has hitherto been assumed that the two proteins have similar properties. Here we make a detailed study, including the effects of myristoylation, lipid composition, calmodulin and phosphorylation of the binding of MRP to phospholipid vesicles. We show that both the N-terminal myristoyl moiety and the central effector domain mediate binding. MRP behaves like MARCKS in the presence of neutral phospholipids. In contrast to MARCKS, however, the incorporation of 20% of negatively-charged phospholipids only marginally increases the affinity of myristoylated MRP. Co-operativity between the myristoyl moiety and the effector domain of MRP is weak and the protein has a significantly lower affinity for these vesicles compared with MARCKS. Furthermore, calmodulin or phosphorylation of the effector domain by the catalytic subunit of protein kinase C do not significantly decrease the binding of myristoylated MRP to negatively-charged phospholipid vesicles. Our results show that the mechanisms regulating the interactions of MARCKS and MRP with phospholipid vesicles are, at least quantitatively, different. In agreement with cellular studies, we therefore propose that MARCKS and MRP have different subcellular localization and, consequently, different functions.

Upregulation of a myristylated 74-kDa protein by interferon treatment of Daudi cells

Labeling of unstimulated human Daudi B lymphoblastoid cells with exogenously added [3H]myristate resulted in acylation of a broad spectrum of different proteins, most of which are currently unknown. Among this array of labeled proteins, a unique 74-kDa acylated protein was induced in interferon (IFN)-treated cells. In the present study, we defined the myristylation kinetics of this protein and examined the subcellular distribution before and after activation with IFN-alpha/beta. This acylated protein was detected only at a very low level in the membrane fraction of untreated cells, and its level increased 3-4-fold by treatment with IFN. This induction occurred over a short period of time and was IFN-alpha/beta dose-dependent. No significant induction was observed with IFN-gamma. Incorporation of [3H]myristate was completely abolished by cycloheximide. The fatty acid associated with this protein was probably linked to a nascent chain through an amide linkage, as it was not released by alkaline hydroxylamine treatment and was identified as myristic acid by HPLC after its release from the polypeptide chain by acid methanolysis. In contrast to other IFN-induced proteins, whose synthesis started at 10 h and was maintained for 20 h, this protein was present in the plasma membrane for a short period of time, between 4 and 6 h after IFN-alpha/beta treatment, and was no longer present in this cellular compartment. This event appears to be transient and suggests that a degradation or a negative regulation of transcription starts from 6-7 h after continuous IFN treatment. As many other myristylated proteins are implicated in cellular regulation, it is possible that this 74-kDa protein may have a regulatory role in cell proliferation and the inhibition of viral replication.

Human Th1 cells preferentially induce interleukin (IL)-1beta while Th2 cells induce IL-1 receptor antagonist production upon cell/cell contact with monocytes

The role of human T cells in the induction and regulation, upon cell/cell contact, of inflammatory responses by monocytic cells was investigated. The production of interleukin (IL)-1beta and IL-1 receptor antagonist (IL-1Ra) by the monocytic THP-1 cell line was measured upon contact with either Th1 or Th2 cell clones. CD4+ T cell clones specific for purified protein derivative of Mycobacterium tuberculosis, predominantly Th1 [high interferon (IFN)-gamma and low IL-4 producers], or tetanus toxoid, predominantly Th2 (low IFN-gamma and high IL-4 producers), were generated. Cell membranes from antigen-stimulated, but not from resting T cell clones induced dose-dependent cytokine production by THP-1 cells. Th1 clones induced higher levels of IL-1beta production (484-806 pg/ml) than did Th2 clones (21-114 pg/ml). In contrast, Th1 clones induced lower levels of IL-IRa (0.9-7.8 ng/ml) than did Th2 clones (7.0-49.6 ng/ml). Similar results were obtained when T cell clones were activated by cross-linked CD3 and CD28. IL-1beta production by THP-1 cells correlated with IFN-gamma production by T cell clones but was unaffected by IFN-gamma neutralization. IL-1Ra production by THP-1 cells correlated with IL-4 production by T cells and was partially inhibited by IL-4 neutralization. These data indicate that activated Th1 and Th2 cells express different molecules on the cell surface able to induce distinct pro-inflammatory (IL-1beta) or anti-inflammatory (IL-1Ra) responses in monocytes. This differential induction of molecules with opposite effects on inflammation stresses the functional heterogeneity in CD4+ T cells.

Myristoylation does not modulate the properties of MARCKS-related protein (MRP) in solution

The members of the myristoylated alanine-rich C kinase substrate (MARCKS) family are proteins essential for brain development and phagocytosis. MARCKS proteins bind to actin filaments and calmodulin (CaM) and are phosphorylated by protein kinase C. In order to investigate how these interactions are regulated, we have characterized the properties of both the myristoylated (myr) and unmyristoylated (unmyr) forms of recombinant MARCKS-related protein (MRP), a 20-kDa member of the MARCKS family. Ultracentrifugation and circular dichroic spectroscopy reveal that MRP is an elongated protein, with an axis ratio estimated between 7 and 12 and with an apparent random coil conformation. MRP binds to CaM with high affinity (Kd,myr = 4 nM; Kd,unmyr = 7 nM) and with a second order rate constant, k+1,unmyr, of 1.6 x 10(8) M-1 s-1. In contrast to classical ligands such as the myosin light chain kinase, binding of MRP to CaM does not induce the formation of an alpha-helix in MRP. The catalytic subunit of protein kinase C (PKM) phosphorylates myr MRP with high affinity ([S]0.5 = 3.5 microM), positive cooperativity (nH = 2.5) and a turnover number of 130 min-1. CaM inhibits the phosphorylation of myr MRP with a half-maximum rate of phosphorylation at a [CaM]/[MRP] ratio of 0.7, indicating that CaM might efficiently regulate the phosphorylation of MRP in vivo. Interestingly, Ca2+ inhibits the binding of MRP to CaM as well as its phosphorylation by PKM in the millimolar concentration range, suggesting that MRP has a weak affinity for Ca2+. Finally, unmyr MRP can be stoichiometrically myristoylated by N-myristoyl transferase in vitro. Since neither binding of CaM nor phosphorylation by PKM inhibits myristoylation, the N terminus of unmyr MRP is exposed on the surface of the protein and is well separated from the effector domain. In view of the observations that unmyr and myr MRP do not exhibit significant differences in their properties in solution, the function of myristoylation is most probably to modulate the interactions of MRP with membranes.

Regulation of membrane and subunit interactions by N-myristoylation of a G protein alpha subunit in yeast

Initiation of the mating process in yeast Saccharomyces cerevisiae requires the action of secreted pheromones and G protein-coupled receptors. As in other eukaryotes, the yeast G protein alpha subunit undergoes N-myristoylation (GPA1 gene product, Gpa1p). This modification appears to be essential for function, since a myristoylation site mutation exhibits the null phenotype in vivo (gpa1(G2A)). Here we examine how myristoylation affects Gpa1p activity in vitro. We show that the G2A mutant of Gpa1p, when fused with glutathione S-transferase, can still form a complex with the G protein betagamma subunits. The complex is stabilized by GDP and is dissociated upon treatment with guanosine 5'-O-(thiotriphosphate). In addition, there is no apparent difference in the relative binding affinity of Gbetagamma for mutant and wild-type Gpa1p. Using sucrose density gradient fractionation of cell membranes, Gpa1p associates normally with the plasma membrane whereas Gpa1pG2A is mislocalized to a microsomal membrane fraction. A portion of Gbetagamma is also mislocalized in these cells, as it is in a gpa1Delta strain. In contrast, wild-type Gpa1p reaches the plasma membrane in cells that do not express Gbetagamma or cell surface receptors. These findings indicate that mislocalization of Gpa1pG2A is not caused by a redistribution of Gbetagamma, nor is it the result of any difference in Gbetagamma binding affinity. These data suggest that myristoylation is required for specific targeting of Gpa1p to the plasma membrane, where it is needed to interact with the receptor and to regulate the release of Gbetagamma.

Lipopolysaccharide stimulates differential expression of myristoylated protein kinase C substrates in murine microglia

Microglia rapidly respond to lipoplysaccharide (LPS) by transformation from resting to active states and secretion of several neuro- and immuno-regulators including tumour necrosis factor alpha (TNF-alpha), interleukin 1 beta (IL-1 beta), and interleukin 6 (IL-6). With longer LPS treatment, microglia are converted to reactive or phagocytic states with characteristics similar to macrophages in inflammation and injury processes. We have investigated LPS-mediated changes in two myristoylated substrates of protein kinase C (PKC): MARCKS (myristoylated alaninerich C kinase substrate) and MRP (MARCKS-related protein). Within 6 hours of addition, LPS induced a twofold increase in [3H]myristoylated and immunoreactive MARCKS protein and a sevenfold increase in MRP. The differential effect of LPS on expression of MRP vs. MARCKS was even more dramatic at the level of transcription: S1 nuclease protection assays revealed a 40-fold increase in MRP mRNA levels (maximum at 4-6 hours), whereas a threefold increase was observed for MARCKS. TNF alpha and colony-stimulating factor 1 (CSF-1), two cytokines which are induced by LPS, did not reproduce the observed effect of LPS on MARCKS and MRP gene transcription. CSF-1 also induced differential transcription of MRP, but of lower magnitude (threefold) and more sustained than by LPS. Accordingly, these two substrates for PKC are differentially up-regulated by LPS, apparently independent of TNF alpha or CSF-1.

Mammalian myristoyl CoA: protein N-myristoyltransferase

Myristoyl CoA:Protein N-myristoyltransferase (NMT) is the enzyme which catalyses the covalent transfer of myristate from myristoyl CoA to the amino-terminal glycine residue of protein substrates. Although NMT is ubiquitous in eukaryotic cells, the enzyme levels and cellular distribution vary among tissues. In this article, we describe the properties of mammalian NMT(s) with reference to subcellular distribution, molecular weights, substrate specificity and the possible involvement of NMT in pathological processes. The cytosolic fraction of bovine brain contains majority of NMT activity. In contrast, rabbit colon and rat liver NMT activity was predominantly particulate. Regional differences in NMT activity have been observed in both rabbit intestine and bovine brain. Results from our laboratory along with the existing knowledge, provide evidence for the existence of tissue specific isozymes of NMT.

MacMARCKS mutation blocks macrophage phagocytosis of zymosan

A major protein kinase C substrate, MacMARCKS (F52, MPR), was examined for its role in phagocytosis. In macrophage-phagocytosing zymosan particles, MacMARCKS was concentrated around nascent phagosomes as detected by immunofluorescent microscopy. The effector domain of MacMARCKS contains the phosphorylation sites, a calmodulin binding site, as well as a putative actin binding site. Stable J774 macrophage cell lines constitutively expressing effector domain deletion mutants of MacMARCKS were generated. When given zymosan particles, these transfectants showed approximately a 90% reduction in their phagocytic capacity. The receptor-mediated endocytosis of acetylated low density lipoproteins, however, was not affected by the mutant. These results strongly suggest the involvement of MacMARCKS in macrophage phagocytosis.

Lipid composition and fatty acid analysis of Helicobacter pylori

Lipids extracted from Helicobacter pylori were separated into lipid classes by thin-layer chromatography. Simple H. pylori lipids consisted of cholesterol esters, triglycerides, free fatty acids, cholesterol, diacylglycerols, and monoacylglycerols. Fatty acids were released from each lipid class by acid methanolysis, and analyzed by gas liquid chromatography and mass spectrometry. Unique methoxy fatty acids, including 11-methoxy heptadecanoic and 11-methoxy nonadecanoic acids, were the major components of the cholesterol esters and triglycerides. The predominance of methoxy fatty acids in the cholesterol esters of H. pylori may contribute to the acid-resistant characteristic of this bacillus.

Protein kinase C regulates MARCKS cycling between the plasma membrane and lysosomes in fibroblasts

MARCKS is a protein kinase C (PKC) substrate that is phosphorylated during neurosecretion, phagocyte activation and growth factor-dependent mitogenesis. MARCKS binds calcium/calmodulin and crosslinks F-actin, and both these activities are regulated by PKC-dependent phosphorylation. We present evidence here that PKC-dependent phosphorylation also regulates the cycling of MARCKS between the plasma membrane and Lamp-1-positive lysosomes. Immuno-fluorescence and immunoelectron microscopy, and subcellular fractionation, demonstrated that MARCKS was predominantly associated with the plasma membrane of resting fibroblasts. Activation of PKC resulted in MARCKS phosphorylation and its displacement from the plasma membrane to Lamp-1-positive lysosomes. MARCKS phosphorylation is required for its translocation to lysosomes since mutating either the serine residues phosphorylated by PKC (phos-) or the PKC inhibitor staurosporine, prevented MARCKS phosphorylation, its release from the plasma membrane, and its subsequent association with lysosomes. In the presence of lysosomotropic agents or nocodazole, MARCKS accumulated on lysosomes and returned to the plasma membrane upon drug removal, further suggesting that the protein cycles between the plasma membrane and lysosomes. In contrast to wild-type MARCKS, the phos- mutant did not accumulate on lysosomes in cells treated with NH4Cl, suggesting that basal phosphorylation of MARCKS promotes its constitutive cycling between these two compartments.

Bacterial stimulators of macrophages

Our current understanding of the interaction between bacteria and macrophages, cells of the immune system that play a major role in the defense against infection, is summarized. Cell-surface structures of Gram-negative and Gram-positive bacteria that account for these interactions are described in detail. Besides surface structures, soluble bacterial molecules, toxins that are derived from pathogenic bacteria, are also shown to modulate macrophage functions. In order to affect macrophage functions, bacterial surface structures have to be recognized by the macrophage and toxins have to be taken up. Subsequently, signal transduction mechanisms are initiated that enable the macrophage to respond to the invading bacteria. To destroy bacteria, macrophages employ many strategies, among which antigen processing and presentation to T cells, phagocytosis, chemotaxis, and different bactericidal mechanisms are considered to be the main weapons.

Zanvil Alexander Cohn 1926-1993

Zanvil Alexander Cohn, an editor of this Journal since 1973, died suddenly on June 28, 1993. Cohn is best known as the father of the current era of macrophage biology. Many of his scientific accomplishments are recounted here, beginning with seminal studies on the granules of phagocytes that were performed with his close colleague and former editor of this Journal, James Hirsch. Cohn and Hirsch identified the granules as lysosomes that discharged their contents of digestive enzymes into vacuoles containing phagocytosed microbes. These findings were part of the formative era of cell biology and initiated the modern study of endocytosis and cell-mediated resistance to infection. Cohn further explored the endocytic apparatus in pioneering studies of the mouse peritoneal macrophage in culture. He described vesicular inputs from the cell surface and Golgi apparatus and documented the thoroughness of substrate digestion within lysosomal vacuoles that would only permit the egress of monosaccharides and amino acids. These discoveries created a vigorous environment for graduate students, postdoctoral fellows, and junior and visiting faculty. Some of the major findings that emerged from Cohn's collaborations included the radioiodination of the plasma membrane for studies of composition and turnover; membrane recycling during endocytosis; the origin of the mononuclear phagocyte system in situ; the discovery of the dendritic cell system of antigen-presenting cells; the macrophage as a secretory cell, including the release of proteases and large amounts of prostaglandins and leukotrienes; several defined parameters of macrophage activation, especially the ability of T cell-derived lymphokines to enhance killing of tumor cells and intracellular protozoa; the granule discharge mechanism whereby cytotoxic lymphocytes release the pore-forming protein perforin; the signaling of macrophages via myristoylated substrates of protein kinase C; and a tissue culture model in which monocytes emigrate across tight endothelial junctions. In 1983, Cohn turned to a long-standing goal of exploring host resistance directly in humans. He studied leprosy, focusing on the disease site, the parasitized macrophages of the skin. He injected recombinant lymphokines into the skin and found that these molecules elicited several cell-mediated responses. Seeing this potential to enhance host defense in patients, Cohn was extending his clinical studies to AIDS and tuberculosis. Zanvil Cohn was a consummate physician-scientist who nurtured the relationship between cell biology and infectious disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Pheromone action regulates G-protein alpha-subunit myristoylation in the yeast Saccharomyces cerevisiae

Myristic acid (C14:0) is added to the N-terminal glycine residue of the alpha subunits of certain receptor-coupled guanine nucleotide-binding regulatory proteins (G proteins). The G alpha subunit (GPA1 gene product) coupled to yeast pheromone receptors exists as a pool of both myristoylated and unmyristolyated species. After treatment of MATa cells with alpha factor, the myristoylated form of Gpa1p increases dramatically, and the unmyristoylated form decreases concomitantly. This pheromone-stimulated shift depends on the function of STE2 (alpha-factor receptor), STE11 (a protein kinase in the response pathway), and NMT1 (myristoyl-CoA:protein N-myristoyltransferase) genes and uses the existing pool of fatty acids (is not blocked by cerulenin). Myristoylated Gpa1p persists long after pheromone is removed. Because myristoylation is essential for proper G alpha-G beta gamma association and receptor coupling, pheromone-dependent stimulation of Gpa1p myristoylation may be an important contributing factor in adaptation after signal transmission.

Lipid A stimulates phospholipase D activity in rat mesangial cells via a G-protein

Stimulation of mesangial cells (MC) with the bacterial endotoxin Lipid A activated two enzymes involved in lipid metabolism. First, a phospholipase D hydrolyses phosphatidylethanolamine (PE) to phosphatidic acid (PA), followed by dephosphorylation of PA to 1,2-diacylglycerol (DAG) by PA phosphohydrolase. MC or microsomes from these cells were pre-labelled with [3H]glycerol. A 30-60 s stimulation with 10-100 ng of Lipid A/ml caused a decrease in [3H]glycerol in PE and increased radioactive glycerol in PA. The enzyme responsible for this hydrolysis preferred PE containing unsaturated acyl side chains. DAG was formed from PA within the first 1 min after Lipid A stimulation. Microsomes incubated with 25 mM-NaF to inhibit phospholipase C and to stimulate GTP-binding proteins also caused PE to be converted into PA. The [3H]glycerol and acyl mass of phosphatidylcholine, phosphatidylserine and phosphatidylinositol did not change with either Lipid A or NaF. Addition of guanosine 5'-[gamma-thio]triphosphate to MC microsomes caused the rapid decrease in proportion of PE and increase in PA, followed by an increase in DAG unsaturated acyl mass. These data suggest the concurrent G-protein-dependent activation by Lipid A of a PE-directed phospholipase D and a PA phosphohydrolase.

Lipid A activation of glomerular mesangial cells: mimicry of the bioactive lipid, phosphatidic acid

Lipid A, the active component of bacterial endotoxin, stimulates multiple cell types, including glomerular mesangial cells (MC), and yet the molecular mechanisms of cell activation remain unclear. Lipid A, in its monosaccharyl form, structurally resembles the biologically active lipid phosphatidic acid (PA). Given this, it was postulated that lipid A activates cells by acting as a structural and functional mimetic of PA. Lipid A was found to specifically stimulate an MC lyso-PA acyl transferase activity, leading to enhanced synthesis of sn-2-unsaturated forms of PA. Sn-2-unsaturated PA itself, in contrast to sn-2-saturated PA, also stimulated the lyso-PA acyl transferase activity, a positive feedback feature previously noted with lyso-lecithin acyl transferase. Structure-function correlations demonstrated that the phosphate moieties in both PA and lipid A were necessary to feedback stimulation of lyso-PA acyl transferase (AT), as dephosphorylated lipid A and 2-unsaturated 1,2-sn-diacylglycerol had no stimulatory effect on lyso-PA AT. The biologic relevance of the lipid A and PA-mediated increases in lyso-PA acyl transferase activity was shown, whereby limited exposure to these lipids rapidly induced identical MC morphologic and functional alterations characteristic of cellular activation. By mimicking the stimulatory action of PA, per se, on lyso-PA acyl transferase activity, lipid A may initiate a positive feedback cycle of acylation, yielding increased amounts of PA enriched in unsaturated fatty acids. This newly synthesized PA may subsequently act as the proximal mediator of cellular activation.

Rapid activation of phosphatidate phosphohydrolase in mesangial cells by lipid A

Knowledge of rapid events in cell signaling initiated by lipid A, the core moiety of bacterial lipopolysaccharide, is limited. In the present study we have demonstrated that cis-parinaric acid (cis-PnA) rapidly labels 1,2-sn-diacylglycerol (DAG) subsequent to labeling of phosphatidic acid (PA). Stimulation of microsomal membranes with lipid A decreased the level of PA labeled with cis-PnA within 5 s and increased the proportion of fluorescent label in DAG. Lipid A stimulation of DAG synthesis at 5-15 s was inhibited by incubation of mesangial cells with pertussis toxin prior to isolation of microsomal membranes. Inhibition of DAG formation was accompanied by an accumulation of the mass and fluorescent label in the cis-PnA-labeled phosphatidic acid pool. GTP gamma S caused a decrease in labeled PA and an increase in labeled 1,2-DAG. We conclude that the PA pool was enlarged via the lipid A sensitive lyso-PA acyl transferase (lyso-PA-AT) and was decreased by a phosphatidate phosphohydrolase to form DAG. The phosphatidate phosphohydrolase was at least partly regulated by a pertussis-sensitive G-protein. Lipid A or 1,2-dilinoleyl-PA, a product of lyso-PA-AT, induced cell activation as monitored by actin reorganization and cellular shape changes. Pretreatment of cells with pertussis toxin prevented the morphological changes normally induced by lipid A or 1,2-dilinoleyl-PA. In contrast, 1-oleoyl-2-acetylglycerol induced rapid actin reorganization and shape change, presumably bypassing the pertussis blockade. We propose that specific pools of PA and PA-derived DAG are key elements in rapid signaling in mesangial cells and are independent of the PI cycle and phospholipase C.

Cloning and molecular characterization of the murine macrophage "68-kDa" protein kinase C substrate and its regulation by bacterial lipopolysaccharide

We have isolated and characterized a cDNA clone encoding the murine macrophage 68-kDa protein kinase C substrate, which is homologous to the 80- to 87-kDa protein identified by the acronym MARCKS (myristoylated alanine-rich C kinase substrate). The murine MARCKS cDNA clone encodes an acidic protein of 309 amino acids with a calculated molecular weight of 29,661. Transfection of the murine MARCKS gene into TK-L fibroblasts produced a myristoylated protein kinase C substrate that migrated on SDS/PAGE with an apparent molecular mass of 68 kDa. Peptide mapping studies indicated that MARCKS produced by the transfected gene was indistinguishable from the endogenous murine macrophage protein. Comparison of the murine macrophage sequence with the previously published chicken and bovine brain sequences revealed two conserved domains: an N-terminal membrane-binding domain and a phosphorylation domain that also contains calmodulin and actin binding sites. In murine peritoneal macrophages, bacterial lipopolysaccharide increased MARCKS mRNA levels by greater than 30-fold. Multiple MARCKS transcripts were observed and could be accounted for by differential polyadenylylation and incomplete processing. Genomic Southern blot analysis suggested a single MARCKS gene per haploid genome.

Tumor necrosis factor alpha modifies agonist-dependent responses in human neutrophils by inducing the synthesis and myristoylation of a specific protein kinase C substrate

Tumor necrosis factor alpha (TNF-alpha) and bacterial lipopolysaccharide (LPS) induce the synthesis and cotranslational myristoylation of an 82-kDa specific protein kinase C substrate in human neutrophils. The myristic acid is covalently bound via a hydroxylamine-resistant amide linkage to the N-terminal glycine of the protein. The isoelectric point of the protein is at pH 4.6. The protein is rapidly phosphorylated when neutrophils are stimulated with chemotactic agonists or with phorbol 12-myristate 13-acetate, an activator of protein kinase C, and displays two characteristic phosphopeptides in one- and two-dimensional separation systems. Identical phosphopeptides were detected when the 82-kDa protein was phosphorylated in vitro with purified kinase C. The 82-kDa protein was immunoprecipitated by a polyclonal antiserum raised against the 87-kDa specific protein kinase C substrate from bovine brain. From these biochemical and immunological criteria it is concluded that the 82-kDa protein is the human neutrophil homolog of MARCKS, the myristoylated, alanine-rich C kinase substrate previously described in bovine and rat brain and in murine fibroblasts and macrophages. TNF-alpha and LPS prime human neutrophils for potentiated protein kinase C-dependent responses such as the respiratory burst and exocytosis. Consistent with this, these mediators do not induce the phosphorylation of MARCKS but prime the neutrophils for enhanced phosphorylation of this protein when the cells subsequently encounter activators of protein kinase C. This increase in MARCKS phosphorylation can be explained by the elevated levels of the protein observed in TNF-alpha- or LPS-treated neutrophils. Indeed, MARCKS constitutes 90% of all proteins synthesized in response to TNF-alpha or LPS. These data strongly suggest that MARCKS acts as a critical effector molecule in the transduction pathway of these important inflammatory mediators.

Fatty acylated proteins as components of intracellular signaling pathways

From the studies presented above, it is obvious that fatty acylation is a common modification among proteins involved in cellular regulatory pathways, and in certain cases mutational analyses have demonstrated the importance of covalent fatty acids in the functioning of these proteins. Indeed, certain properties provided by fatty acylation make it an attractive modification for regulatory proteins that might interact with many different substrates, particularly those found at or near the plasma membrane/cytosol interface. In the case of intracellular fatty acylated proteins, the fatty acyl moiety allows tight binding to the plasma membrane without the need for cotranslational insertion through the bilayer. For example, consider the tight, salt-resistant interaction of myristoylated SRC with the membrane, whereas its nonmyristoylated counterpart is completely soluble. Likewise for the RAS proteins, which associate weakly with the membrane in the absence of fatty acylation, while palmitoylation increases their affinity for the plasma membrane and their biological activity. Fatty acylation also permits reversible membrane association in some cases, particularly for several myristoylated proteins, thus conferring plasticity on their interactions with various signaling pathway components. Finally, although this has not been demonstrated, it is conceivable that covalent fatty acid may allow for rapid mobility of proteins within the membrane. Several questions remain to be answered concerning requirements for fatty acylation by regulatory proteins. The identity of the putative SRC "receptor" will provide important clues as to the pathways in which normal SRC functions, as well as into the process of transformation by oncogenic tyrosine kinases. The possibility that other fatty acylated proteins associate with the plasma membrane in an analogous manner also needs to be investigated. An intriguing observation that can be made from the information presented here is that at least three different families of proteins involved in growth factor signaling pathways encode both acylated and nonacylated members, suggesting that selective fatty acylation may provide a means of determining the specificity of their interactions with other regulatory molecules. Further studies of fatty acylated proteins should yield important information concerning the regulation of intracellular signaling pathways utilized during growth and differentiation.

A glycerol ether induces mobilization and 12-lipoxygenation of arachidonic acid in macrophages. Synergistic effect on mobilization and induction of leukotriene C formation by activators of protein kinase C

A glycerol triether, 1,2-isopropylidene 3-0-decanyl-sn-glycerol, was found to induce mobilization of arachidonic acid from ethanolamine phosphoglycerides and phosphatidylinositol in mouse peritoneal macrophages. This effect showed structural specificity, occurred without activation of protein kinase C and resulted in formation and release of predominantly 12-hydroxy-eicosatetraenoic acid. Activators of kinase C (4-beta-phorbol 12-myristate 13-acetate and 1,2-dioctanoyl-sn-glycerol) instead specifically enhance prostaglandin E2 formation. When macrophages were exposed to both a kinase C activator and the glycerol triether, the mobilization of arachidonic acid was synergistically enhanced and formation of leukotriene C was induced.

Lipopolysaccharide/lipid A receptors on lymphocytes and macrophages

Significant advances have been realized during the past five years in the understanding of the mechanism(s) by which endotoxic LPS interactions with mammalian lymphoreticular cells leads to characteristic cellular responses. There is now strong experimental evidence to support the concept that specific receptors for the lipid A component of LPS do, in fact, exist and are functional on these cells. While the available data do not rule out a potential contribution of nonspecific hydrophobic interactions of lipid A with the membrane bilayer in the cellular activation process, it would appear that interaction with the LPS receptor alone is sufficient to initiate triggering. Whether there exist more than one molecular entity which might function on mammalian cell membranes as a specific receptor for LPS, or whether different cell types may manifest different LPS receptors remains as an interesting area for future research. Further, the concept that molecular complexes of LPS with mammalian host proteins, such as the acute phase LPS binding protein, might trigger additional novel pathways for cell activation is both exciting and of potential importance. The precise mechanism or mechanisms by which LPS-receptor ligand interactions translate into appropriate transmembrane signalling events is currently uncertain. Clearly there exists evidence for contribution of many of the traditional second signals, although at present, the data are incomplete and not always consistent between laboratories. Of potential concern in this respect are the sometimes rather striking differences noted between lipid A and intact polysaccharide containing S-LPS. While such differences may be significant and important, it should be remembered that S-LPS itself is a potent stimulus for many lymphoreticular cell subpopulations, and any postulated pathways must encompass S-LPS as well as lipid A. In any case, it is likely that the further molecular-biochemical characterization of LPS receptors will yield crucial information for the eventual elucidation of the precise pathways for LPS transmembrane signalling. Such information will be invaluable in the future harnessing of the immunostimulatory potential of LPS as well as the abrogation of its profound deleterious pathophysiological effects in endotoxin shock.

Heterogeneity among human mononuclear phagocytes in their secretion of lysozyme, interleukin 1 and type-beta transforming growth factor: a quantitative analysis at the single-cell level

Human mononuclear phagocytes have the remarkable capacity to secrete a wide range of products. The reverse hemolytic plaque assay was used to visualize and quantify the secretion of lysozyme, interleukin 1 or type-beta transforming growth factor by individual monocytes and macrophages. With this sensitive immunoassay, the release of these products by either freshly isolated monocytes, macrophages derived from monocytes in vitro, or activated peritoneal macrophages was detected in both the presence and absence of secretagogues. When coupled with immunocytochemistry for EBM/11, a monoclonal marker for human cells of the monocyte/macrophage cell lineage, functional heterogeneity was evident both in the amount of lysozyme, interleukin 1 or type-beta transforming growth factor released per cell, and in the number of EBM/11+ cells which secreted detectable levels of these products at any one time. In addition, there was a size-dependent heterogeneity among human monocytes and culture-derived macrophages in their ability to secrete interleukin 1 or lysozyme, respectively. We conclude that the secretory activity of individual mononuclear phagocytes is markedly heterogeneous.

Monokine production by microglial cell clones

Cytokines have been suggested to act as intermediates between the immune and the central nervous system, but little is known about the type of cells synthesizing them in the brain. We have immortalized with oncogenic retroviruses primary brain cell cultures from mouse embryos and have generated clones of microglial cells that have been characterized. Three of the clones studied produce interleukin 1 (IL 1); IL 6 and tumor necrosis factor-alpha as assessed by biological assays and by Northern blot analysis. Our data raise the question on the role of these cytokines in the brain and suggest that early resident microglial cells might play an important role in development processes and in the adult brain.

Acylation of viral and eukaryotic proteins

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Bacterial lipopolysaccharides prime human neutrophils for enhanced production of leukotriene B4

Neutrophils can be "primed" for an enhanced respiratory burst by lipopolysaccharide (LPS) in concentrations measurable in patients with septic shock. Leukotriene B4 (LTB4) is the primary eicosanoid product of neutrophils and is felt to be a mediator of host defense and inflammation. We investigated the in vitro effects of LPS on neutrophil production of LTB4 and the omega-oxidation metabolites of LTB4. Incubation of neutrophils with LPS in concentrations ranging from 0.01 to 100 ng/ml did not result in production of LTB4 or metabolites in the absence of a second stimulus. Priming neutrophils with LPS and then stimulating with opsonized zymosan, phorbol-myristate-acetate or a low concentration of the calcium ionophore A23187 resulted in enhanced production of LTB4. LPS priming of neutrophils occurred in a concentration dependent manner. LPS did not result in LTB4 production in response to the chemoattractant peptide FMLP. LPS priming of neutrophils had no effect on cytosolic calcium concentrations of resting or zymosan-stimulated cells. These results suggest that LPS might effect host defense and tissue injury by potentiating the effect of other stimulants on neutrophil production of LTB4. This LPS induced enhancement may represent an important pathogenetic pathway in patients with gram negative sepsis.

Acylation of monocyte and glomerular mesangial cell proteins. Myristyl acylation of the interleukin 1 precursors

Acylation of cellular proteins with the fatty acids myristate or palmitate represents an important mechanism for the co- or posttranslational modification of proteins. Lipid A, the biologically active component of bacterial endotoxin, exerts a number of biochemical effects on responsive cell types. Evidence is presented that lipid A stimulates the synthesis and subsequent myristyl acylation of intracellular monocyte and glomerular mesangial cell proteins. Two of the myristylated monocyte proteins were identified by specific immunoprecipitation as the 33-kD IL 1 alpha and beta precursors; a similar myristylated protein was found in mesangial cells. The 17-kD secretory form of monocyte IL 1 beta did not contain covalently linked myristate. Myristyl acylation of the IL 1 precursor proteins may facilitate the processing or membrane localization of these proteins, which lack characteristic hydrophobic signal sequences. The acylated 33-kD IL 1 alpha may remain preferentially associated with the membrane in an active form, whereas limited proteolysis may convert the biologically inactive IL 1 beta precursor into the extracellular, nonacylated, active 17-kD protein.

Association of a myristoylated protein with a biological membrane and its increased phosphorylation by protein kinase C

A hydrophilic enzyme, lysozyme, was myristoylated in vitro by the N-hydroxysuccinimide ester of myristic acid, and the monomyristoylated lysozyme was isolated by CM-cellulose cation-exchange column chromatography. The monomyristoylated lysozyme associated with phospholipid vesicles, whereas the association of native lysozyme was negligible. The membrane-associated monomyristoylated lysozyme was phosphorylated with partially purified rat brain Ca2+- and phospholipid-dependent protein kinase (protein kinase C) in the presence of Ca2+, phosphatidylserine and phorbolmyristate acetate. Thus, the myristoylated lysozyme became a substrate of protein kinase C through its hydrophobic association with the membrane. The present results suggest that the myristoylation of cytoplasmic proteins may have an important role in signal transduction.