Regulation of phospholipase A2 in human leukemia cell lines: its implication for intracellular signaling

Vanadium Compounds as Pro-Inflammatory Agents: Effects on Cyclooxygenases

This paper discusses how the activity and expression of cyclooxygenases are influenced by vanadium compounds at anticancer concentrations and recorded in inorganic vanadium poisonings. We refer mainly to the effects of vanadate (orthovanadate), vanadyl and pervanadate ions; the main focus is placed on their impact on intracellular signaling. We describe the exact mechanism of the effect of vanadium compounds on protein tyrosine phosphatases (PTP), epidermal growth factor receptor (EGFR), PLCγ, Src, mitogen-activated protein kinase (MAPK) cascades, transcription factor NF-κB, the effect on the proteolysis of COX-2 and the activity of cPLA₂. For a better understanding of these processes, a lot of space is devoted to the transformation of vanadium compounds within the cell and the molecular influence on the direct targets of the discussed vanadium compounds.

Cyclooxygenase-1 as the main source of proinflammatory factors after sodium orthovanadate treatment

Vanadium is a metal present in air pollution. Its compounds may have both anticancer and carcinogenic properties. Vanadium compounds are tested in treatment of diabetes and cancer. An important research direction aimed at better understanding of the mechanisms of action of the vanadium compounds is a more detailed insight into their impact on inflammatory reactions. The aim of this study was to examine the effect of micromolar concentrations of sodium orthovanadate, Na3VO4, on the activity and expression of cyclooxygenases: COX-1 and COX-2. PMA-activated THP-1 macrophages were incubated in vitro for 48 h with micromolar concentrations of sodium orthovanadate. As shown by an ELISA assay, sodium orthovanadate increases the quantity of prostaglandin E2 being released into the medium in a dose-dependent manner as well as impacts the quantity of the stable metabolite of thromboxane A2: thromboxane B2. The use of a COX-2 inhibitor, NS-398, revealed that this effect was independent of changes in the activity of COX-2. Western blotting analysis showed that sodium orthovanadate increased the expression of COX-2 when used with NS-398. Quantitative real-time PCR measurements of mRNA levels of genes PTGS1 and PTGS2 revealed no effect of the tested vanadium compound on the levels of analyzed transcripts.

Role of the ubiquitin proteasome system in regulating skin pigmentation

Pigmentation of the skin, hair and eyes is regulated by tyrosinase, the critical rate-limiting enzyme in melanin synthesis by melanocytes. Tyrosinase is degraded endogenously, at least in part, by the ubiquitin proteasome system (UPS). Several types of inherited hypopigmentary diseases, such as oculocutaneous albinism and Hermansky-Pudlak syndrome, involve the aberrant processing and/or trafficking of tyrosinase and its subsequent degradation which can occur due to the quality-control machinery. Studies on carbohydrate modifications have revealed that tyrosinase in the endoplasmic reticulum (ER) is proteolyzed via ER-associated protein degradation and that tyrosinase degradation can also occur following its complete maturation in the Golgi. Among intrinsic factors that regulate the UPS, fatty acids have been shown to modulate tyrosinase degradation in contrasting manners through increased or decreased amounts of ubiquitinated tyrosinase that leads to its accelerated or decelerated degradation by proteasomes.

Intracellular composition of fatty acid affects the processing and function of tyrosinase through the ubiquitin-proteasome pathway

Proteasomes are multicatalytic proteinase complexes within cells that selectively degrade ubiquitinated proteins. We have recently demonstrated that fatty acids, major components of cell membranes, are able to regulate the proteasomal degradation of tyrosinase, a critical enzyme required for melanin biosynthesis, in contrasting manners by relative increases or decreases in the ubiquitinated tyrosinase. In the present study, we show that altering the intracellular composition of fatty acids affects the post-Golgi degradation of tyrosinase. Incubation with linoleic acid (C18:2) dramatically changed the fatty acid composition of cultured B16 melanoma cells, i.e. the remarkable increase in polyunsaturated fatty acids such as linoleic acid and arachidonic acid (C20:4) was compensated by the decrease in monounsaturated fatty acids such as oleic acid (C18:1) and palmitoleic acid (C16:1), with little effect on the proportion of saturated to unsaturated fatty acid. When the composition of intracellular fatty acids was altered, tyrosinase was rapidly processed to the Golgi apparatus from the ER (endoplasmic reticulum) and the degradation of tyrosinase was increased after its maturation in the Golgi. Retention of tyrosinase in the ER was observed when cells were treated with linoleic acid in the presence of proteasome inhibitors, explaining why melanin synthesis was decreased in cells treated with linoleic acid and a proteasome inhibitor despite the abrogation of tyrosinase degradation. These results suggest that the intracellular composition of fatty acid affects the processing and function of tyrosinase in connection with the ubiquitin-proteasome pathway and suggest that this might be a common physiological approach to regulate protein degradation.

Anticancer therapeutic potential of soy isoflavone, genistein

Genistein (4'5, 7-trihydroxyisoflavone) occurs as a glycoside (genistin) in the plant family Leguminosae, which includes the soybean (Glycine max). A significant correlation between the serum/plasma level of genistein and the incidence of gender-based cancers in Asian, European and American populations suggests that genistein may reduce the risk of tumor formation. Other evidence includes the mechanism of action of genistein in normal and cancer cells. Genistein inhibits protein tyrosine kinase (PTK), which is involved in phosphorylation of tyrosyl residues of membrane-bound receptors leading to signal transduction, and it inhibits topoisomerase II, which participates in DNA replication, transcription and repair. By blocking the activities of PTK, topoisomerase II and matrix metalloprotein (MMP9) and by down-regulating the expression of about 11 genes, including that of vascular endothelial growth factor (VEGF), genistein can arrest cell growth and proliferation, cell cycle at G2/M, invasion and angiogenesis. Furthermore, genistein can alter the expression of gangliosides and other carbohydrate antigens to facilitate their immune recognition. Genistein acts synergistically with drugs such as tamoxifen, cisplatin, 1,3-bis 2-chloroethyl-1-nitrosourea (BCNU), dexamethasone, daunorubicin and tiazofurin, and with bioflavonoid food supplements such as quercetin, green-tea catechins and black-tea thearubigins. Genistein can augment the efficacy of radiation for breast and prostate carcinomas. Because it increases melanin production and tyrosinase activity, genistein can protect melanocytes of the skin of Caucasians from UV-B radiation-induced melanoma. Genistein-induced antigenic alteration has the potential for improving active specific immunotherapy of melanoma and carcinomas. When conjugated to B43 monoclonal antibody, genistein becomes a tool for passive immunotherapy to target B-lineage leukemias that overexpress the target antigen CD19. Genistein is also conjugated to recombinant EGF to target cancers overexpressing the EGF receptor. Although genistein has many potentially therapeutic actions against cancer, its biphasic bioactivity (inhibitory at high concentrations and activating at low concentrations) requires caution in determining therapeutic doses of genistein alone or in combination with chemotherapy, radiation therapy, and/or immunotherapies. Of the more than 4500 genistein studies in peer-reviewed primary publications, almost one fifth pertain to its antitumor capabilities and more than 400 describe its mechanism of action in normal and malignant human and animal cells, animal models, in vitro experiments, or phase I/II clinical trials. Several biotechnological firms in Japan, Australia and in the United States (e.g., Nutrilite) manufacture genistein as a natural supplement under quality controlled and assured conditions.

Inhibitory effects of surfactant protein A on surfactant phospholipid hydrolysis by secreted phospholipases A2

Hydrolysis of surfactant phospholipids by secreted phospholipases A(2) (sPLA(2)) contributes to surfactant dysfunction in acute respiratory distress syndrome. The present study demonstrates that sPLA(2)-IIA, sPLA(2)-V, and sPLA(2)-X efficiently hydrolyze surfactant phospholipids in vitro. In contrast, sPLA(2)-IIC, -IID, -IIE, and -IIF have no effect. Since purified surfactant protein A (SP-A) has been shown to inhibit sPLA(2)-IIA activity, we investigated the in vitro effect of SP-A on the other active sPLA(2) and the consequences of sPLA(2)-IIA inhibition by SP-A on surfactant phospholipid hydrolysis. SP-A inhibits sPLA(2)-X activity, but fails to interfere with that of sPLA(2)-V. Moreover, in vitro inhibition of sPLA(2)-IIA-induces surfactant phospholipid hydrolysis correlates with the concentration of SP-A in surfactant. Intratracheal administration of sPLA(2)-IIA to mice causes hydrolysis of surfactant phosphatidylglycerol. Interestingly, such hydrolysis is significantly higher for SP-A gene-targeted mice, showing the in vivo inhibitory effect of SP-A on sPLA(2)-IIA activity. Administration of sPLA(2)-IIA also induces respiratory distress, which is more pronounced in SP-A gene-targeted mice than in wild-type mice. We conclude that SP-A inhibits sPLA(2) activity, which may play a protective role by maintaining surfactant integrity during lung injury.

Ceramides increase the activity of the secretory phospholipase A2 and alter its fatty acid specificity

Modulation of human recombinant secretory type II phospholipase A(2) activity by ceramide and cholesterol was investigated using model glycerophospholipid substrates composed of phosphatidylethanolamine and phosphatidylserine dispersed in aqueous medium. Enzyme activity was monitored by measurement of released fatty acids using capillary GC-MS. Fatty acids from the sn-2 position of the phospholipids were hydrolysed by the enzyme in proportion to the relative abundance of the phospholipid in the substrate. Addition of increasing amounts of ceramide to the substrate progressively enhanced phospholipase activity. The increased activity was accomplished largely by preferential hydrolysis of polyunsaturated fatty acids, particularly arachidonic acid, derived from phosphatidylethanolamine. The addition of sphingomyelin to the substrate glycerophospholipids inhibited phospholipase activity but its progressive substitution by ceramide, so as to mimic sphingomyelinase activity, counteracted the inhibition. The presence of cholesterol in dispersions of glycerophospholipid-substrate-containing ceramides suppressed activation of the enzyme resulting from the presence of ceramide. The molecular basis of enzyme modulation was investigated by analysis of the phase structure of the dispersed lipid substrate during temperature scans from 46 to 20 degrees C using small-angle synchrotron X-ray diffraction. These studies indicated that intermediate structures created after ceramide-dependent phase separation of hexagonal and lamellar phases represent the most susceptible form of the substrate for enzyme hydrolysis.

Ceramides increase the activity of the secretory phospholipase A2 and alter its fatty acid specificity

Modulation of human recombinant secretory type II phospholipase A(2) activity by ceramide and cholesterol was investigated using model glycerophospholipid substrates composed of phosphatidylethanolamine and phosphatidylserine dispersed in aqueous medium. Enzyme activity was monitored by measurement of released fatty acids using capillary GC-MS. Fatty acids from the sn-2 position of the phospholipids were hydrolysed by the enzyme in proportion to the relative abundance of the phospholipid in the substrate. Addition of increasing amounts of ceramide to the substrate progressively enhanced phospholipase activity. The increased activity was accomplished largely by preferential hydrolysis of polyunsaturated fatty acids, particularly arachidonic acid, derived from phosphatidylethanolamine. The addition of sphingomyelin to the substrate glycerophospholipids inhibited phospholipase activity but its progressive substitution by ceramide, so as to mimic sphingomyelinase activity, counteracted the inhibition. The presence of cholesterol in dispersions of glycerophospholipid-substrate-containing ceramides suppressed activation of the enzyme resulting from the presence of ceramide. The molecular basis of enzyme modulation was investigated by analysis of the phase structure of the dispersed lipid substrate during temperature scans from 46 to 20 degrees C using small-angle synchrotron X-ray diffraction. These studies indicated that intermediate structures created after ceramide-dependent phase separation of hexagonal and lamellar phases represent the most susceptible form of the substrate for enzyme hydrolysis.

Mildly oxidized LDL induces expression of group IIa secretory phospholipase A(2) in human monocyte-derived macrophages

Phospholipase A(2)s (PLA(2)s) constitute a family of enzymes that hydrolyze fatty acids of membrane phospholipids, thus initiating the synthesis of proinflammatory mediators. Various PLA(2)s have been detected in human atherosclerotic arteries (advanced lesions); however, only the secretory group of PLA(2) has been shown to specifically hydrolyze low density lipoprotein (LDL)-associated phospholipids and, as such, may play a potential role in atherogenesis. In the present study, we investigated the expression pattern of group IIa, IV, and V PLA(2)s in human macrophages, which are the key cells involved in the onset and perpetuation of atherosclerosis. Immunohistochemical staining by double labeling showed that the secretory nonpancreatic PLA(2) (snpPLA(2)) is detectable in macrophages in the intima of early atherosclerotic lesions. Reverse transcription-polymerase chain reaction analysis of RNA extracted from human monocytes clearly showed that expression of group IV PLA(2) was enhanced during differentiation into macrophages, with an onset of induction at days 2 to 3 of differentiation. Group V snpPLA(2) was constitutively expressed on differentiation, whereas the detection of group IIa snpPLA(2) was dependent on both differentiation and subsequent stimulation of macrophages. Indeed, the transcription of group IIa snpPLA(2) in macrophages was induced by treatment with minimally modified or mildly oxidized LDL, whereas native, extensively oxidized, or acetylated LDL had no effect. To our knowledge, this is the first report describing induction of group IIa snpPLA(2) expression in human monocyte-derived macrophages. The mRNA levels of cytosolic PLA(2) group IV and snpPLA(2) group V remained unchanged on LDL treatment. Thus, our results show that the expression of distinct PLA(2) enzymes is regulated not only during differentiation of monocytes into macrophages but also on exposure of macrophages to distinct LDL species. Consequently, our results indicate a potential role for both cytosolic and secretory PLA(2) enzymes in inflammation and in macrophage functions related to atherosclerosis, with a specific role for group IIa snpPLA2 in LDL scavenging.

Arachidonic acid as a messenger for the expression of long-term potentiation

Arachidonic acid is suggested to play a role in the expression of long-term potentiation (LTP), a synaptic analog of memory and learning. However, it is unknown whether this free fatty acid is actually released during LTP or not. To address this question, we assayed arachidonic acid with an HPLC system using 9-anthryldiazomethane (ADAM) as a fluorescent probe. High frequency stimulation (tetanic stimulation) to a hippocampal slice from the guinea pig brain caused a huge increase in the release of glutamate from presynaptic terminals and in turn, a gradual increase in the release of arachidonic acid. A similar increase in the release of arachidonic acid was induced by application of glutamate and the increase was inhibited by either the selective AMPA/kainate receptor antagonist, DNQX, or to a lesser extent by the selective NMDA receptor antagonist, APV. These findings suggest that arachidonic acid is produced by activation of ionotropic glutamate receptors involving expression of LTP. Arachidonic acid exerted a long-lasting facilitatory action on synaptic transmission in the CA1 region of rat hippocampal slices and the facilitation occluded the tetanic LTP. Arachidonic acid, thus, appears to be a significant factor for the expression of LTP.

Study on the activation of phospholipases A2 by purinergic agonists in rat submandibular ductal cells

Extracellular ATP and benzoyl-ATP (Bz-ATP) increased the release of [3H]arachidonic acid ([3H]AA) from prelabeled rat submandibular gland (RSMG) ductal cells respectively two- and threefold. Both agonists also increased the release of [3H]AA from acini but at a lower level (+50% and +100% respectively). Carbachol had no significant effect on either cellular population. In ductal cells phorbol myristate acetate, an activator of protein kinase C, slightly increased the basal release of [3H]AA but did not affect the release of [3H]AA in response to ATP. Staurosporine, an inhibitor of protein kinases, inhibited the response to the purines. The removal of calcium from the extracellular medium decreased the response to ATP and Bz-ATP. Only barium could partly substitute for calcium to restore the purinergic response. Zinc inhibited the release of [3H]AA. Permeabilization of the cells with streptolysin O (SLO) activated the calcium-independent phospholipase A2 activity (iPLA2). The iPLA2, not the calcium-dependent PLA2 (cPLA2), released [3H]oleic acid ([3H]OA) from RSMG ductal cells. It is concluded that RSMG ducts have a higher PLA2 activity when compared to acini. This activity is accounted for by iPLA2 and cPLA2. Both enzymes are activated by P2X agonists by a staurosporine-sensitive mechanism. Cells permeabilized with SLO or membranes from Escherichia coli as a substrate are not good models to study the regulation of these enzymes. In intact RSMG ductal cells the two activities can be distinguished by rather specific inhibitors, by different ionic conditions and also by the fatty acid used to label the cells.

Cholesterol relieves the inhibitory effect of sphingomyelin on type II secretory phospholipase A2

Secretory type II phospholipase A2 (sPLA2) is inhibited by sphingomyelin (SPH); cholesterol either mixed with the model glycerophospholipid substrate or added to the assay medium as separated liposomes counteracts this inhibition efficiently. The inhibition of fatty acid release assayed by quantitative gas chromatography-MS is observed when SPH is added to erythrocyte membranes as the substrate instead of a readily hydrolysable phosphatidylethanolamine/phosphatidylserine model mixture. Hydrolysis of SPH by Staphylococcus aureus sphingomyelinase suppresses its inhibitory potency. The addition of cholesterol to SPH liposomes with a 1:1 stoichiometry relieves completely the inhibition of sPLA2 exerted by SPH. The mechanism of inhibition suggested by the binding assay is that sPLA2 binds with affinity to the SPH interface, after either phase segregation at the assay temperature or on the pure SPH liposomes added to the incubation medium. Cholesterol is shown to suppress the binding affinity of the enzyme for the SPH interface. A model for inhibition is suggested in which binding of the sphingosine moiety is competitive for sPLA2 (inhibition) or for cholesterol (release of the enzyme).

Generation of lyso-phospholipids from surfactant in acute lung injury is mediated by type-II phospholipase A2 and inhibited by a direct surfactant protein A-phospholipase A2 protein interaction

Lyso-phospholipids exert a major injurious effect on lung cell membranes during Acute Respiratory Distress Syndrome (ARDS), but the mechanisms leading to their in vivo generation are still unknown. Intratracheal administration of LPS to guinea pigs induced the secretion of type II secretory phospholipase A2 (sPLA2-II) accompanied by a marked increase in fatty acid and lyso-phosphatidylcholine (lyso-PC) levels in the bronchoalveolar lavage fluid (BALF). Administration of LY311727, a specific sPLA2-II inhibitor, reduced by 60% the mass of free fatty acid and lyso-PC content in BALF. Gas chromatography/mass spectrometry analysis revealed that palmitic acid and palmitoyl-2-lyso-PC were the predominant lipid derivatives released in BALF. A similar pattern was observed after the intratracheal administration of recombinant guinea pig (r-GP) sPLA2-II and was accompanied by a 50-60% loss of surfactant phospholipid content, suggesting that surfactant is a major lung target of sPLA2-II. In confirmation, r-GP sPLA2-II was able to hydrolyze surfactant phospholipids in vitro. This hydrolysis was inhibited by surfactant protein A (SP-A) through a direct and selective protein-protein interaction between SP-A and sPLA2-II. Hence, our study reports an in vivo direct causal relationship between sPLA2-II and early surfactant degradation and a new process of regulation for sPLA2-II activity. Anti-sPLA2-II strategy may represent a novel therapeutic approach in lung injury, such as ARDS.

Isolation and properties of a novel phospholipase A from rat brain that hydrolyses fatty acids at sn-1 and sn-2 positions

A Ca(2+)-independent phospholipase A that releases various fatty acids from sn-1 and sn-2 positions was partially purified from rat brain soluble fraction. The enzyme showed an approximate molecular mass of 300 kDa on gel filtration column chromatography. Its enzymatic properties are distinct from those of well characterized phospholipase A2 enzymes; by using a series of synthetic phosphatidylcholines, the enzyme cleaved oleic, linoleic, and arachidonic acids like phospholipase A2, and released palmitic and stearic acids like phospholipase A1. Phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, and phosphatidic acid were hydrolysed with almost equal efficiencies by this enzyme. These results indicate that the enzyme isolated is a novel Ca(2+)-independent intracellular phospholipase A that might be responsible for production of various fatty acids from membrane phospholipids.

Ca2+ -independent cytosolic phospholipase A in HL-60 cells differentiating to granulocytes

The release of various fatty acids (FAs) from permeabilized HL-60 cells, predominantly oleic acid (OA) rather than arachidonic acid, was greatly enhanced by GTP-gamma-S and vanadate [Tsujishita, Y., Asaoka, Y. and Nishizuka, Y., Proc. Natl. Acad. Sci. USA 91 (1994) 6274-62781. The present study shows that phospholipase A (A2/A1) activity which cleaves the acyl group from both sn-2 and sn-1 positions of phosphatidylethanolamine (PtdEtn) is increased in HL-60 cells during differentiation to granulocyte-like cells. This enzyme does not require Ca2+ and releases various FAs, preferentially OA from PtdEtn and, to lesser extent, from lysoPtdEtn. Other phospholipids including phosphatidylcholine and phosphatidic acid serve as very poor substrates. Although further studies are necessary to show the direct link of this enzyme activation to receptor stimulation, the results described here imply that this enzyme is responsible for the release of various FAs, particularly OA, from permeabilized HL-60 cells.

Modulation of human type II secretory phospholipase A2 by sphingomyelin and annexin VI

Conjectural results have been reported on the capacity of inflammatory secreted phospholipase A2 (sPLA2) to hydrolyse mammalian membrane phospholipids. Development of an assay based on the release of non-esterified fatty acids by the enzyme acting on the organized phospholipid mixture constituting the membrane matrix has led to the identification of two prominent effectors, sphingomyelin (SPH) and annexin. Recombinant human type II sPLA2 hydrolyses red-cell membrane phospholipids with a marked preference for the inner leaflet. This preference is apparently related to the high content of SPH in the outer leaflet, which inhibits sPLA2. This inhibition by SPH is specific for sPLA2. Cholesterol counteracts the inhibition of sPLA2 by SPH, suggesting that the SPH-to-cholesterol ratio accounts in vivo for the variable susceptibility of cell membranes to sPLA2. Different effects were observed of the presence of the non-hydrolysable D-alpha-dipalmitoyl phosphatidylcholine (D-DPPC), which renders the membranes rigid but does not inhibit sPLA2. Annexin VI was shown, along with other annexins, to inhibit sPLA2 activity by sequestering the phospholipid substrate. The present study has provided the first evidence that annexin VI, in concentrations that inhibit hydrolysis of purified phospholipid substrates, stimulated the hydrolysis of membrane phospholipids by sPLA2. The activation requires the presence of membrane proteins. The effect is specific for type II sPLA2 and is not reproducible with type I PLA2. The activation by annexin VI of sPLA2 acting on red cell membranes results in the preferential release of polyunsaturated fatty acids. It suggests that type II sPLA2, in conjunction with annexin VI, might be involved in the final step of endocytosis and/or exocytosis providing the free polyunsaturated fatty acids acting synergistically to cause membrane fusion.

Cloning, chromosomal mapping, and expression of a novel human secretory phospholipase A2

Secretory phospholipases A2 (sPLA2s) represent a rapidly expanding family of structurally related enzymes found in mammals as well as in insect and snake venoms. In this report, a cDNA coding for a novel sPLA2 has been isolated from human fetal lung, and its gene has been mapped to chromosome 16p13.1-p12. The mature sPLA2 protein has a molecular mass of 13.6 kDa, is acidic (pI 5.3), and made up of 123 amino acids. Key structural features of the sPLA2 include: (i) a long prepropeptide ending with an arginine doublet, (ii) 16 cysteines located at positions that are characteristic of both group I and group II sPLA2s, (iii) a C-terminal extension typical of group II sPLA2s, (iv) and the absence of elapid and pancreatic loops that are characteristic of group I sPLA2s. Based on these structural properties, this sPLA2 appears as a first member of a new group of sPLA2s, called group X. A 1.5-kilobase transcript coding for the human group X (hGX) sPLA2 was found in spleen, thymus, and peripheral blood leukocytes, while a less abundant 0.8-kilobase transcript was detected in the pancreas, lung, and colon. When the hGX sPLA2 cDNA was expressed in COS cells, sPLA2 activity preferentially accumulated in the culture medium, indicating that hGX sPLA2 is an actively secreted enzyme. It is maximally active at physiological pH and with 10 mM Ca2+. hGX sPLA2 prefers phosphatidylethanolamine and phosphatidylcholine liposomes to those of phosphatidylserine.

Cytosolic phospholipase A2 is coupled to muscarinic receptors in the human astrocytoma cell line 1321N1: characterization of the transducing mechanism

The cholinergic agonist carbachol induced the release of arachidonic acid in the 1321N1 astrocytoma cell line, and this was blocked by atropine, suggesting the involvement of muscarinic receptors. To assess the mechanisms of signalling involved in the response to carbachol, a set of compounds characterized by eliciting responses through different mechanisms was tested. A combination of 4beta-phorbol 12beta-myristate 13alpha-acetate and thapsigargin, an inhibitor of endomembrane Ca2+-ATPase that induces a prolonged elevation of cytosolic Ca2+ concentration, induced an optimal response, suggesting at first glance that both protein kinase C (PKC) and Ca2+ mobilization were involved in the response. This was consistent with the observation that carbachol elicited Ca2+ mobilization and PKC-dependent phosphorylation of cytosolic phospholipase A2 (cPLA2; phosphatide sn-2-acylhydrolase, EC 3.1.1.4) as measured by a decrease in electrophoretic mobility. Nevertheless, the release of arachidonate induced by carbachol was unaltered in media containing decreased concentrations of Ca2+ or in the presence of neomycin, a potent inhibitor of phospholipase C which blocks phosphoinositide turnover and Ca2+ mobilization. Guanosine 5'-[gamma-thio]triphosphate added to the cell-free homogenate induced both [3H]arachidonate release and cPLA2 translocation to the cell membrane fraction in the absence of Ca2+, thus suggesting the existence of an alternative mechanism of cPLA2 translocation dependent on G-proteins and independent of Ca2+ mobilization. From the combination of experiments utilizing biochemical and immunological tools the involvement of cPLA2 was ascertained. In summary, these data indicate the existence in the astrocytoma cell line 1321N1 of a pathway involving the cPLA2 which couples the release of arachidonate to the occupancy of receptors for a neurotransmitter, requires PKC activity and G-proteins and might operate in the absence of Ca2+ mobilization.

Targeting of proteins to granule subsets is determined by timing and not by sorting: The specific granule protein NGAL is localized to azurophil granules when expressed in HL-60 cells

The mechanism of protein targeting to individual granules in cells that contain different subsets of storage granules is poorly understood. The neutrophil contains two highly distinct major types of granules, the peroxidase positive (azurophil) granules and the peroxidase negative (specific and gelatinase) granules. We hypothesized that targeting of proteins to individual granule subsets may be determined by the stage of maturation of the cell, at which the granule proteins are synthesized, rather than by individual sorting information present in the proteins. This was tested by transfecting the cDNA of the specific granule protein, NGAL, which is normally synthesized in metamyelocytes, into the promyelocytic cell line HL-60, which is developmentally arrested at the stage of formation of azurophil granules, and thus does not contain specific and gelatinase granules. Controlled by a cytomegalovirus promoter, NGAL was constitutively expressed in transfected HL-60 cells. This resulted in the targeting of NGAL to azurophil granules as demonstrated by colocalization of NGAL with myeloperoxidase, visualized by immunoelectron microscopy. This shows that targeting of proteins into distinct granule subsets may be determined solely by the time of their biosynthesis and does not depend on individual sorting information present in the proteins.

Tyrosine kinase activity modulates catalysis and translocation of cellular 5-lipoxygenase

Tyrosine kinase activity, a determinant of Src homology domain interactions, has a prominent effect on cellular localization and catalysis by 5-lipoxygenase. Six separate inhibitors of tyrosine kinase each inhibited 5(S)-hydroxyeicosatetraenoic acid formation by HL-60 cells stimulated with calcium ionophore, in the presence or absence of exogenous arachidonic acid substrate, indicating that they modulated cellular 5-lipoxygenase activity. The tyrosine kinase inhibitors also blocked the translocation of 5-lipoxygenase from cytosol to membranes during cellular activation, consistent with their effects on its catalytic activity. These results fit a model which postulates that Src homology domain interactions are a molecular determinant of the processes which coordinate the subcellular localization and functions of 5-lipoxygenase. In addition, we demonstrate that activated leukocytes contain two molecularly distinct forms of 5-lipoxygenase: a phosphorylated form and a nonphosphorylated form. In activated HL-60 cells the pool of phosphorylated 5-lipoxygenase accumulates in the nuclear fraction, not with the membrane or cytosolic fractions. The amount of phosphorylated 5-lipoxygenase is a small fraction of the total. Overall, equilibrium reactions involving the nuclear localizing sequence, the proline-rich SH3 binding motif, and the phosphorylation state of 5-lipoxygenase may each influence its partnership with other cellular proteins and any novel functions derived from such partnerships.

Short-term and long-term-effects of phorbol 12-myristate 13-acetate and different inhibitors on the ability of bone marrow cells to form colonies in vitro

The process of signal transduction responsible for the phorbol 12-myristate 13-acetate mediated increase in the colony-forming potential of murine (CBA) bone marrow cells was studied using known modulators of the mitogenic signal. Pretreatment of cells for 60 minutes with staurosporine (1 mumol/l), an inhibitor of protein kinase C, completely prevented colony formation in the control group of cells and significantly reduced the number of colonies formed in the phorbol ester-treated group. Brief exposure (60 min) of cells to the phospholipase A2 inhibitors, mepacrine (500 mumol/l) and heparin (1 g/l), reduced the number of colonies formed in the control group and completely abolished the increase in the number of colonies formed after treatment of the cells with phorbol ester. When inhibitors of protein kinase C or phospholipase A2 were present during the entire period of the colony forming assay (7 days), no colonies could be scored in either the control or phorbol ester-treated groups of bone marrow cells. Long-term treatment or temporary exposure (60 min) of cells to indomethacin (50 mumol/l), an inhibitor of cyclooxygenase, or nordihydroguaiaretic acid (50 mumol/l), an inhibitor of lipoxygenase, had no effect on colony formation in both groups. Pretreatment of cells for 45 min with calcium ionophore A23187 (10 mumol/l) failed to increase the number of colonies, compared with the control group. Moreover, simultaneous treatment of cells for 45 min with phorbol ester (500 nmol/l) and A23187 (10 mumol) did not produce any further increase in the number of colonies, compared with the phorbol ester-treated group, suggesting that elevation of intracellular calcium is unimportant in the phorbol ester-mediated response. Dibutyryl cyclic adenosine monophosphate (50 mumol/l) in the presence or absence of phorbol ester, failed to stimulate colony formation, indicating that cyclic AMP-dependent protein kinases are not involved in the signalling process. Temporary exposure (75 min) of bone marrow cells to okadaic acid (1 mumol/l), a potent inhibitor of serine/threonine phosphatases, or to tyrphostine AG-115 (20 mumol/l), a tyrosine kinase inhibitor, did not effect colony growth in the control or phorbol ester-treated group. The results indicate that phospholipase A2 activation is involved in the phorbol ester-mediated increase in colony formation, since, of the different agents applied, only staurosporine, an inhibitor of protein kinase C, and mepacrine and heparin, putative inhibitors of phospholipase A2, were capable of abolishing phorbol ester-mediated effects.