Lipocortin inhibition of extracellular and intracellular phospholipases A2 is substrate concentration dependent

Oncogenic action of phospholipase A2 in prostate cancer

Mortality from prostate cancer is a result of progression of cancer cells to become androgen-refractory and metastatic. Eicosanoid products of the cyclooxygenase (COX) and lipoxygenase (LOX) pathways are important mediators of the proliferation of prostate cancer cells in culture and regulate tumour vascularisation and metastasis in animal models. Pharmacological agents that block either COX or LOX products effectively reduce the size of prostate cancer xenografts. Recently, phospholipase A(2) (PLA(2)) enzymes, which regulate the provision of arachidonic acid to both COX- and LOX-derived eicosanoids, are found to also regulate the growth of prostate cancer cells and tumours, with one enzyme, secreted PLA(2)-IIA, being increased in prostate cancer tissues. Annexin A1 and A2, known inhibitors of cytosolic phospholipase A(2)-alpha activity, are absent in prostate cancer tissues. We propose that PLA(2) enzyme function is dysregulated by aberrant up regulation of secreted enzymes and downregulation of endogenous inhibitors of cytosolic phospholipase A(2) activity in prostate cancer and that this dysregulation contributes to the pathogenesis of prostate cancer. Thus, in addition to COX and LOX enzymes, PLA(2) enzymes represent important targets for the treatment of prostate cancer.

Phospholipase A2: its role in ADP- and thrombin-induced platelet activation mechanisms

ADP and thrombin are two of the most important agonists of platelet aggregation--a cellular response that is critical for maintaining normal hemostasis. However, aberrant platelet aggregation induced by these agonists plays a central role in the pathogenesis of cardiovascular and cerebrovascular diseases. Agonist-induced primary or secondary activation of phospholipases leads to generation of the second messengers that participate in biochemical reactions essential to a number of platelet responses elicited by ADP and thrombin. Phospholipase A2 (PLA2) has been linked to cardiovascular diseases. However, the mechanism(s) of activation of PLA2 in platelets stimulated by ADP and thrombin has remained less well defined and much less appreciated. The purpose of this review is to examine and compare the molecular mechanisms of activation of PLA2 in platelets stimulated by ADP and thrombin.

Inhibition of secreted phospholipases A2 by annexin V. Competition for anionic phospholipid interfaces allows an assessment of the relative interfacial affinities of secreted phospholipases A2

The ability of annexins, particularly annexin 1 (lipocortin 1), to inhibit phospholipase A2 (PLA2) is well known and a substrate depletion mechanism is now widely accepted as the explanation for most inhibitory studies. In this investigation we have examined the substrate depletion mechanism of annexin V using a variety of phospholipid substrates and secreted PLA2's (sPLA2). The results suggest that the term interfacial competition best describes the inhibitory effect of annexin V although the overall inhibitory process remains one of substrate sequestration by the annexin. We have utilised the competitive nature of the interaction of enzyme and annexin V for a phospholipid interface as a means of quantifying the relative affinity of sPLA2's for anionic phospholipid vesicles. The results highlight the very high affinity of the human non-pancreatic sPLA2 for such vesicles (Kd<<10-(10) M) while the Naja naja venom PLA2 and porcine pancreatic sPLA2 showed lower affinities. Hydrolysis of mixed vesicles containing phosphatidylserine and phosphatidylcholine by the venom and pancreatic enzymes were differentially inhibited by annexin V. This difference must reflect the preference of both annexin V and the pancreatic enzyme for an anionic phospholipid interface. In contrast, the venom enzyme is able to readily hydrolyse phosphatidylcholine domains that would be minimally affected by annexin V. Annexin V was an effective inhibitor of cardiolipin hydrolysis by the pancreatic PLA2, however the inhibition was of a more complex nature than seen with other phospholipids tested. Overall the results highlight the ability of annexin V to inhibit phospholipid hydrolysis by sPLA2's by an interfacial competition (substrate depletion) mechanism. The effectiveness of annexin V as an apparent inhibitor depends on the nature of the enzyme and the phospholipid substrate.

P11, a unique member of the S100 family of calcium-binding proteins, interacts with and inhibits the activity of the 85-kDa cytosolic phospholipase A2

Using a two hybrid system screen of a human cDNA library, we have found that p11, a unique member of the S100 family of calcium-binding proteins, interacts with the carboxyl region of the 85-kDa cytosolic phospholipase A2 (cPLA2). p11 synthesized in a cell-free system interacts with cPLA2 in vitro. The p11-cPLA2 complex is detectable from a human bronchial epithelial cell line (BEAS 2B). Furthermore, p11 inhibits cPLA2 activity in vitro. Selective inhibition of p11 expression in the BEAS 2B cells by antisense RNA results in an increased PLA2 activity as well as an increased release of prelabeled arachidonic acid. This study demonstrates a novel mechanism for the regulation of cPLA2 by an S100 protein.

Dexamethasone inhibits mucous glycoprotein secretion via a phospholipase A2-dependent mechanism in cultured chinchilla middle ear epithelial cells

Inhibition or attenuation of mucous hypersecretion in middle ear epithelium is a key step toward resolution of mucoid otitis media. Mucous hypersecretion induced by platelet-activating factor (PAF) in cultured Chinchilla middle ear epithelial cells is dependent on arachidonic acid metabolites via PAF receptors, suggestive of the role of phospholipase A2 (PLA2) in mucous glycoprotein (MGP) secretion. In this study, dexamethasone added to cultured Chinchilla middle ear epithelial cells inhibited baseline and PAF-induced MGP secretion in a concentration-dependent manner. A definite time lag (16 h) was observed between administration of dexamethasone and MGP inhibition. This inhibition was reversed by the addition of exogenous PLA2 (the rate-limiting enzyme of arachidonic acid metabolism) and actinomycin D (an inhibitor of mRNA synthesis). This suggests that dexamethasone inhibits baseline and PAF-induced MGP secretion via a PLA2-dependent mechanism.

Inhibition of cytosolic phospholipase A2 by annexin V in differentiated permeabilized HL-60 cells. Evidence of crucial importance of domain I type II Ca2+-binding site in the mechanism of inhibition

Annexin V belongs to a family of proteins that interact with phospholipids in a Ca2+-dependent manner. This protein has been demonstrated to have anti-phospholipase A2 activity. However, this effect has never yet been reported with the 85-kDa cytosolic PLA2 (cPLA2). We studied, in a model of differentiated and streptolysin O-permeabilized HL-60 cells, the effect of annexin V on cPLA2 activity after stimulation by calcium, GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), formyl-Met-Leu-Phe, or phorbol 12-myristate 13-acetate. Both recombinant and human placental purified annexin V inhibit cPLA2 activity whatever the stimulus used. The decrease of arachidonic acid release is of 40 and 50%, respectively, at [Ca2+] of 3 and 10 microM. The mechanism of inhibition was also analyzed. cPLA2 requires calcium and protein kinase C (PKC) or mitogen-activated protein kinase phosphorylation for its activation. As annexin V was shown to be an endogenous inhibitor of PKC, PKC-stimulated cPLA2 activity was analyzed. Using GF109203x, a specific PKC inhibitor, we demonstrated that this pathway is of minor importance in our model. cPLA2 inhibition by annexin V is not linked to PKC inhibition. To test the hypothesis of phospholipid depletion, mutants of annexin V were constructed using mutagenesis directed to Ca2+ site. We demonstrate that the Ca2+ site located in domain I is necessary for the inhibitory effect of annexin V on cPLA2 activity. The site in domain IV is also involved but with less efficiency. In contrast, mutations in site II and III do not modify this effect. Moreover, annexin V mutated on all sites does not inhibit cPLA2. Thus, we propose a predominant role of module (I/IV) in the biological action of annexin V, which, in physiological conditions, may control cPLA2 activity by depletion of the phospholipid substrate.

Annexin I concentration and prostacyclin production in rat lung and alveolar macrophages following irradiation

The purpose of this study was to gather additional evidence in irradiated rat lung on the relationship between annexin I and prostaglandin synthesis. The right hemithorax of the animal was exposed to a single dose of 0 or 30 Gy of X-rays, and the animals were killed 3 months postirradiation. Levels of annexin I and synthesis of prostacyclin (PGI2) were determined in lungs, in cell-free bronchoalveolar lavage (BAL) fluid, and in macrophages lavaged from those lungs. In addition, protein concentration, lactate dehydrogenase (LDH) activity and macrophage count in BAL fluid obtained from irradiated lung were compared with that from sham-irradiated (0 Gy) lung. Levels of annexin I, the putative inhibitor of phospholipase A2, in lung and cell-free BAL fluid were decreased in samples from irradiated animals. By contrast, the level of annexin I in macrophages lavaged from irradiated lung was higher than that in macrophages from sham-irradiated lung. The irradiated lung produced nearly 3.5 times more prostacyclin than did the control lung. However, prostacyclin synthesis by macrophages lavaged from irradiated lung was no different than that of macrophages from sham-irradiated lung. Protein, LDH and macrophage number in BAL fluid from irradiated lungs were significantly higher than in corresponding specimens from sham-irradiated lungs. These data demonstrate that reduced levels of annexin I, as well as increased protein concentration, LDH activity and macrophage numbers in irradiated rat lung are reflected in BAL fluid. Therefore, information obtained from BAL fluid, but not from BAL macrophages, reflects lung status, and may serve as a minimally invasive index of radiation pneumonitis in this model. In irradiated lung, increased PGI2 synthesis coupled with a decreased annexin I level are consistent with the hypothesis of an inhibitory role of annexin I in prostaglandin metabolism. However, this hypothesis is not supported by findings in BAL macrophages, where increased annexin I concentration is not accompanied by a decrease in PGI2 production. In view of the latter findings, and a previous study from our laboratory demonstrating that phospholipase activity in irradiated rat lung is in fact decreased, despite the reduction in annexin I concentration and the hyperproduction of prostanoids, it would seem unlikely that annexin I negatively modulates prostaglandin synthesis via inhibition of phospholipase in this model.

Localization of annexin I (lipocortin I, p35) mRNA in normal and diseased human skin by in situ hybridization

Annexin I is a calcium- and phospholipid-binding protein that is involved in the regulation of cellular differentiation. The aim of the present study was to determine the localization of annexin I mRNA expression in normal and diseased human skin. In situ hybridization with a specific digoxigenin-labelled RNA probe was used throughout. We detected no annexin I mRNA signals in basal and suprabasal cells of normal epidermis, but positive signals were evident in the sudoriferous ducts. Annexin I mRNA expression was detected in the keratinizing squamous cells in keratotic type seborrhoeic keratosis and in keratinocytes at the periphery of the horn pearl in well-differentiated squamous cell carcinoma. Positive signals were also seen at the border between involved and noninvolved skin in psoriasis vulgaris and in dyskeratotic epidermal keratinocytes in keratosis follicularis Darier. By contrast, no annexin I mRNA signals were detected in tumour cells in basal cell carcinoma. The present results suggest that annexin I expression is related to, and may play a role in, keratinization disorders.

Effects of dexamethasone and transforming growth factor-beta 2 on group II phospholipase A2 mRNA and activity levels in interleukin 1 beta- and forskolin-stimulated mesangial cells

The expression of 14 kDa group II phospholipase A2 [also referred to as secretory PLA2 (sPLA2)] is induced in rat glomerular mesangial cells by exposure to inflammatory cytokines and forskolin, a cAMP elevating agent. Previously we have shown that dexamethasone and transforming growth factor-beta 2 (TGF-beta 2) suppress sPLA2 protein synthesis and enzyme activity induced by cytokines and forskolin. The regulation of sPLA2 by pro-inflammatory cytokines suggests that the enzyme may play a role in glomerular inflammatory reactions. In order to understand the regulation of sPLA, in more detail, we investigated whether dexamethasone and TGF-beta 2 also suppress sPLA, mRNA after its induction by either interleukin-1 beta (IL-1 beta) or forskolin. We found that IL-1 beta-induced sPLA2 mRNA in rat mesangial cells is not down-regulated by pretreatment of the cells with dexamethasone, even at a concentration of 10 microM, which dramatically decreases sPLA2 protein levels and activity. Metabolic labelling experiments indicated that the decreased sPLA2 levels under these conditions can be explained by inhibition of the rate of sPLA2 synthesis from the elevated mRNA levels. In contrast, the forskolin-induced elevation of sPLA, mRNA is inhibited by dexamethasone in a concentration-dependent manner. Likewise, TGF-beta 2 inhibits the elevation of sPLA, mRNAs induced by either IL-1 beta or forskolin. The decrease in sPLA2 mRNA caused by TGF-beta 2 corresponds with the decrease in sPLA2 enzyme levels and activity. These data suggest that cytokine- and forskolin-induced sPLA2, expression is tightly controlled via both transcriptional and post-transcriptional mechanisms. Furthermore, we show that pretreatment of mesangial cells with epidermal growth factor prior to stimulation with IL-1 beta or forskolin had no suppressing effect on sPLA2 levels or enzyme activity, as has been reported previously for osteoblasts.

Phospholipase A2 is secreted by murine keratinocytes after stimulation with IL-1 alpha and TNF-alpha

Phospholipase A2 (PLA2)-catalysed liberation of arachidonic acid is the rate-limiting step in the generation of the lipid mediators prostaglandins and leukotrienes. PLA2 regulation thus represents a pivotal mechanism in the pathogenesis of inflammation. In this study we investigated the effects of TNF alpha and IL-1 alpha on PLA2 activity in cultured murine keratinocytes. Starting 18 h after stimulation, PLA2 activity increased significantly by about 250-320%) in the supernatants and in the cell pellets. This effect was completely inhibited either by preincubation of the cells with dexamethasone 48 h before stimulation or by coincubation with actinomycin D. PLA2 activity detected in the supernatants was blocked by reduction with dithiothreitol, whereas the PLA2 activity in the pellets was dithiothreitol-resistant. We conclude that in murine keratinocytes IL-1 alpha induce de novo synthesis and release of a secretory PLA2 and the induction of a different PLA2 activity in the cytosol. These findings indicate a crucial link between early cytokine effects and the initiation of the lipid mediator cascade in keratinocytes. The observation that PLA2 induction could be completely inhibited by preincubation with dexamethasone allows new insights into the mechanism of steroid effects on epidermal inflammation and renders PLA2 regulation an interesting therapeutic target.

Dexamethasone down-regulates the 85 kDa phospholipase A2 in mouse macrophages and suppresses its activation

We have studied the effects of dexamethasone (dex) (i) on the level of the arachidonate-mobilizing phospholipase A2 (PLA2-85) in macrophages, (ii) on the stimulus-induced activation of this enzyme, and (iii) on the stimulus-induced release of arachidonate. Treatment of macrophages with 10 nM dex led to progressive reduction of PLA2-85 down to approx. 35% of control levels in 20 h in the absence of stimuli. This was accompanied by a partial inhibition of calcium-ionophore-induced arachidonate release. In contrast, the ability of zymosan or phorbol ester to cause both persistent activation of PLA2-85 and arachidonate release was greatly reduced or abolished. However, the protein phosphatase inhibitor okadaic acid, previously shown to cause enhanced phosphorylation and persistent activation of PLA2-85, was still able to exert this effect on the dex-suppressed PLA2-85. This suggests that the effect of okadaic acid was exerted at, or downstream of, the dex-sensitive step(s). Treatment with dex also led to inhibition of the characteristic changes in phosphoprotein labelling induced by phorbol ester or zymosan. However, phorbol-dibutyrate-binding isoforms of protein kinase C were not severely down-regulated. Thus dex was found to down-regulate PLA2-85 and, in addition, to affect one or more component(s) in the signal chain that normally leads to its activation. However, okadaic acid retained the ability to cause activation of PLA2-85.

Levels and localization of group II phospholipase A2 and annexin I in interleukin- and dexamethasone-treated rat mesangial cells: evidence against annexin mediation of the dexamethasone-induced inhibition of group II phospholipases A2

The mechanism by which glucocorticosteroids inhibit the synthesis and secretion of pro-inflammatory arachidonate metabolites is still controversial. Initially it was postulated that glucocorticoids can induce the formation of PLA2 inhibitory proteins termed annexins. We have previously shown that the cytokine-induced 14 kDa PLA2 activity and the synthesis of prostaglandin E2 in rat mesangial cells is dose-dependently blocked by pretreatment of the cells with dexamethasone (Schalkwijk et al. (1991) Biochem. Biophys. Res. Commun. 180, 46-52). Concurrently, the synthesis of 14 kDa group II PLA2 is suppressed. The regulation of PLA2 activity is complex and may well involve superimposable mechanisms. Thus, although the decrease in PLA2 protein levels could in itself explain the dexamethasone-induced decrease in PLA2 activity, a contribution of the glucocorticoid-induced anti-phospholipase A2 protein annexin cannot be ruled out a priori. To investigate this possibility we analyzed the level of annexin I by Western blotting and immunostaining in mesangial cells treated with interleukin-1 beta and/or dexamethasone. Under conditions where 14 kDa group II PLA2 activity and protein levels were dramatically affected by interleukin-1 and dexamethasone, the level of annexin I in the cells remained constant. Dexamethasone also did not induce the secretion of annexin I. In addition, no evidence for dexamethasone-induced translocation of annexin I from the cytosol to membranes, thereby possibly sequestering the substrates for PLA2, was obtained. Immunofluorescence studies localized the cytokine-induced PLA2 to the Golgi area and punctate structures in the cytoplasm. We have also studied the subcellular localization of annexin I in rat mesangial cells using confocal microscopy. These studies located annexin I mainly in the cytoplasma and the nucleus. We conclude from these experiments that the dexamethasone-induced inhibition of 14 kDa group II PLA2 in rat mesangial cells is not mediated by annexin I and is solely due to the suppression of PLA2 gene expression.

Autoantibodies to annexins: a diagnostic marker for cutaneous disorders?

Annexins/lipocortins are a group of structurally related calcium and lipid binding proteins which have been implicated as mediators of the anti-inflammatory action of corticosteroids. Autoantibodies against annexin-1 have been reported in association with autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis and their presence has been hypothesized as the reason for the steroid resistance phenomenon. In this study we investigated IgG- and IgM-autoantibodies against annexin-1,-2,-3,-4,-5 and -6 in sera of 221 patients with skin disorders and 114 healthy blood donors with newly established ELISAs. Patients were clustered into 5 groups according to their diagnosis: autoimmune diseases, psoriasis, leg ulcer, malignant melanoma, and miscellaneous diseases. Autoantibodies directed against each annexin were detectable in all investigated groups, in the control group as well as in the disease groups, without displaying any significant correlation to any of the disease states. The homogenous distribution of annexin-autoantibodies throughout the control group and all the disease groups studied, do not support the implication of annexin-autoantibodies in pathophysiological states and make them an unlikely candidate for use as a diagnostic marker.

Lipocortin I is not accessible for protein kinase C bound to the cytoplasmic surface of the plasma membrane in streptolysin-O-permeabilized pig granulocytes

We previously observed a 38 kDa protein that was a major protein component of the cytosolic extract of pig granulocytes and the dominant substrate of protein kinase C at supra-physiological Ca2+ concentrations. The purified 38 kDa protein itself required Ca2+ to be phosphorylated by protein kinase C. Now we demonstrate that this protein, which is also present in human granulocytes, is identical to lipocortin I. The identification is based on the chromatographic properties and immunoblot of the purified protein which is also a good substrate for tissue transglutaminase. Phosphorylation of lipocortin I by protein kinase C was investigated in granulocytes permeabilized with streptolysin-O. At physiological intracellular Ca2+ concentrations lipocortin I was not phosphorylated at all. At supra-physiological Ca2+ concentrations (0.5 mM), lipocortin I was also not phosphorylated when protein kinase C was translocated to the membrane by treatment of the cells with phorbol myristate acetate. Its phosphorylation was detectable only in control experiments when protein kinase C was activated in the cytosol by the addition of dioleoylglycerol and phosphatidylserine to the permeabilized cells. The data presented show that, in permeabilized granulocytes, Ca(2+)-lipocortin is not formed at physiological Ca2+ concentrations, and at supra-physiological Ca2+ concentrations the Ca(2+)-lipocortin I is not accessible to protein kinase C bound to the cytoplasmic surface of the plasma membrane.

The effects of methylprednisolone and the ganglioside GM1 on acute spinal cord injury in rats

Recent clinical trials have reported that methylprednisolone sodium succinate (MP) or the monosialic ganglioside GM1 improves neurological recovery in human spinal cord injury. Because GM1 may have additive or synergistic effects when used with MP, the authors compared MP, GM1, and MP+GM1 treatments in a graded rat spinal cord contusion model. Spinal cord injury was caused by dropping a rod weighing 10 gm from a height of 1.25, 2.5, or 5.0 cm onto the rat spinal cord at T-10, which had been exposed via laminectomy. The lesion volumes were quantified from spinal cord Na and K shifts at 24 hours after injury and the results were verified histologically in separate experiments. A single dose of MP (30 mg/kg), given 5 minutes after injury, reduced 24-hour spinal cord lesion volumes by 56% (p = 0.0052), 28% (p = 0.0065), and 13% (p > 0.05) in the three injury-severity groups, respectively, compared to similarly injured control groups treated with vehicle only. Methylprednisolone also prevented injury-induced hyponatremia and increased body weight loss in the spine-injured rats. When used alone, GM1 (10 to 30 mg/kg) had little or no effect on any measured variable compared to vehicle controls; when given concomitantly with MP, GM1 blocked the neuroprotective effects of MP. At a dose of 3 mg/kg, GM1 partially prevented MP-induced reductions in lesion volumes, while 10 to 30 mg/kg of GM1 completely blocked these effects of MP. The effects of MP on injury-induced hyponatremia and body weight loss were also blocked by GM1. Thus, GM1 antagonized both central and peripheral effects of MP in spine-injured rats. Until this interaction is clarified, the authors recommend that MP and GM1 not be used concomitantly to treat acute human spinal cord injury. Because GM1 modulates protein kinase activity, protein kinases inhibit lipocortins, and lipocortins mediate anti-inflammatory effects of glucocorticoids, it is proposed that the neuroprotective effects of MP are partially due to anti-inflammatory effects and that GM1 antagonizes the effects of MP by inhibiting lipocortin. Possible beneficial effects of GM1 reported in central nervous system injury may be related to the effects on neural recovery rather than acute injury processes.

Conformation and interactions of uteroglobin fragments 4-14 and 49-65 in aqueous solution containing surfactant micelles

The conformation of two fragments of rabbit uteroglobin is described. The peptides are PRFAHVIENLL and PQTTRENIMKLTEKIVK, corresponding to helices I and IV in the crystal structure. CD shows that both peptides interact with sodium dodecyl sulfate (SDS) micelles and change their conformation to an alpha-helix. The helical content estimated from the CD band at 222 nm is about 40% in each peptide. Surface tension measurements show that both peptides lower the critical micellar concentration (cmc) of SDS, with a more dramatic effect in the case of helix I. This peptide by itself acts as a surfactant, and is able to interact with SDS even below the observed cmc, forming beta aggregates. Proton magnetic resonance (1H-nmr) suggests that flexible helices are present. The longest helical stretches compatible with 1H-nmr data extend from Phe6 to Leu14 for helix I and from Arg53 to Ile63 for helix IV.

Vasoactive intestinal peptide and helodermin inhibit phospholipase A2 activity in vitro

We recently reported that the widely distributed neuropeptide vasoactive intestinal peptide (VIP) reduces inflammatory lung injury due to a variety of agents and inhibits the associated generation of cyclo-oxygenase and lipoxygenase products. We therefore investigated whether VIP may inhibit phospholipase A2 activity, thus reducing the release of arachidonic acid, the common precursor of all eicosanoids. VIP dose-dependently inhibited PLA2 of porcine pancreas and of Naja naja venom, as assessed by the release of free [3H]oleic acid from labeled Escherichia coli phospholipids. The potency of VIP was similar to that of mepacrine, with 50% inhibition at 400-500 microM. The closely related peptide helodermin produced 50% inhibition at 200 microM, but secretin and peptide histidine isoleucineamide produced little or no inhibition. The results suggest that VIP and helodermin selectively inhibit PLA2 in vitro. If this activity is exerted in vivo, it may contribute to the anti-inflammatory activity of these two peptides.

Effect of the choline acetyltransferase inhibitor (2-benzoylethyl)-trimethylammonium iodide (BETA) on human placental prostaglandin release and phospholipase A2 activity

We investigated the effect of an inhibitor of acetylcholine (ACh) synthesis, (2-benzoylethyl)trimethylammonium iodide (BETA), on prostaglandin (PG)E2 and PGF2 alpha release from incubated placental explants in the presence or absence of ACh or arachidonic acid (AA). BETA alone (100 microM) significantly reduced both PGE2 and PGF2 alpha release. However, this inhibitory effect of BETA was not reversed in the presence of ACh (10 microM to 1 mM). The addition of AA (10 to 100 microM) increased both PGE2 and PGF2 alpha release, and simultaneously overcame the inhibition of PGE2 but not PGF2 alpha release by BETA. These results are compatible with the hypothesis that BETA may be inhibiting the phospholipase A2 (PLA2) step rather than prostaglandin H synthase (PGHS) step in the enzymatic pathway of PG generation. This hypothesis was supported by evidence showing a lack of effect of BETA (10 and 100 microM) on ovine placental microsome PGHS activity. Moreover, human placental homogenate PLA2 activity was reversibly inhibited by BETA (100 microM). In the presence of BETA (100 microM), addition of exogenous ACh (100 microM) had no significant effect on placental PLA2 activity. These results indicate that the inhibitory effect of BETA on placental PG release was unlikely to be via an action on ACh synthesis, but rather via a reversible effect on PLA2 activity.

Inhibition of human skin phospholipase A2 by "lipocortins" is an indirect effect of substrate/lipocortin interaction

Proteins of the annexin/lipocortin family have been claimed to mediate the anti-inflammatory action of glucocorticosteroids by the inhibition of phospholipases A2. This hypothesis has been challenged by the finding that annexins do not directly interact with the enzyme in a classical enzyme/inhibitor behavior, but more likely block the access of the phospholipase A2 to its substrate by binding to phospholipids. Because former studies with skin phospholipase A2 suggested a specific regulation by annexin-1, we investigated the substrate dependence of this effect. For this purpose phospholipase A2 activities in human epidermis and dermis homogenates were measured in the presence of various amounts of annexins-1, -2, or -5. The respective annexin was preincubated in separate series either with the substrate or with the enzyme. We found a partial inhibition of both epidermal and dermal phospholipase A2 activities with all annexins tested (annexin-5 >> annexin-2 > annexin-1). The inhibitory effect was absolutely dependent on the annexin/phospholipid ratio and occurred only at very high annexin concentrations relative to the amount of substrate. Our data demonstrate that the inhibition of human skin phospholipase A2 by annexins depends on the substrate concentrations, as has been shown for phospholipases A2 of other origins as well. All observations can be explained by the current "substrate depletion model" characterizing the indirect effects of annexins on phospholipase A2 activities. It is therefore rather unlikely that annexins are directly involved in the regulation of phospholipase A2 activity of human skin under physiologic conditions.

Immunohistochemical localization of lipocortins in normal and psoriatic human skin

The distribution of lipocortin I, a steroid-induced inhibitory protein of phospholipase A2, was examined in normal and psoriatic human skin. Using immunoblotting analysis with specific antibody against human lipocortin I purified from human placenta, lipocortin I was detected as a 37 kDa protein in cultured epidermal cells, whole skin and epidermis. In the dermis and stratum corneum, lipocortin I was only weakly detectable by Western blotting. In contrast to normal skin, much less lipocortin I was detected by Western blotting analysis in psoriatic skin. Using immunoperoxidase immunohistochemical analysis, lipocortin I was demonstrated in the cytoplasm of keratinocytes in the upper and middle layers of the epidermis and in some infiltrating cells in the dermis in normal skin. In involved psoriatic skin, by contrast, lipocortin I was almost undetectable in the epidermis, although it was demonstrated in some infiltrating cells in the dermis. No immunostaining of lipocortin I was observed in the stratum corneum of normal or psoriatic skin. These results, together with the finding that phospholipase A2 activity is higher in psoriatic epidermis than in normal epidermis, suggest that lipocortin I plays an important role in the regulation of differentiation and proliferation of epidermal keratinocytes.

Annexin 5 as a potential regulator of annexin 1 phosphorylation by protein kinase C. In vitro inhibition compared with quantitative data on annexin distribution in human endothelial cells

In vitro phosphorylation of annexin 1 by purified rat brain protein kinase C (PKC) has been studied in the presence of annexin 5, which is not a substrate for PKC. Annexin 5 promoted a dose-dependent inhibition of annexin 1 phosphorylation, which could be overcome by increasing the concentration of phosphatidylserine (PtdSer). In addition, a close relationship was found between the amount of PtdSer uncovered by annexin 5 and the residual phosphorylation of annexin 1. These data fit with the 'surface depletion model' explaining the antiphospholipase activity of annexins. In order to check the possibility that the in vitro effect of annexin 5 could be of some physiological relevance, annexins 1, 2, and 5, as well as the light chain of calpactin 1 (p11), have been quantified in human endothelial cells by measuring the radioactivity bound to the proteins after Western blotting with specific antibodies and 125I-labelled secondary antibody. Our data indicate that annexins 1 and 5, PKC and PtdSer are present in human endothelial cells in relative amounts very similar to those used in vitro under conditions permitting the detection of the inhibitory effect of annexin 5. Since annexin 1 remained refractory to PKC-dependent phosphorylation in intact cells, we suggest that annexin 5 might exert its inhibitory effect towards PKC in vivo, provided that its binding to phospholipids can occur at physiological (micromolar) concentrations of Ca2+. This was previously shown to occur in vitro using phosphatidylethanolamine/phosphatidic acid vesicles [Blackwood and Ernst (1990) Biochem. J. 266, 195-200]. Using identical assay conditions, which also allowed expression of PKC activity, annexin 5 again inhibited annexin 1 phosphorylation without interfering with PKC autophosphorylation. These data suggest that annexins 1 and 5 might interact with each other on the lipid surface, resulting in a specific inhibition of annexin 1 phosphorylation by PKC. Whether a similar mechanism also occurs in vivo remains to be determined.

Presence of lipocortins I and IV, but not II and VI, in human platelets

The present investigation revealed the presence of lipocortins I and IV, but not lipocortins II and VI, in human platelets. Lipocortin I was found in the Triton-soluble fraction of both resting and thrombin-activated platelets and was not covalently bound to skeletal components. Without detergents, when resting platelets were lysed and fractionated in the absence of Ca2+, lipocortin I was found only in the cytosolic fraction, whereas, in the presence of Ca2+, lipocortin I was associated only with the crude particulate and not with the membrane nor the cytosolic fractions.

Effects of catecholamines on eicosanoid synthesis with special reference to prostanoid/leukotriene ratio

Catecholamines (adrenaline, dopamine, and noradrenaline) stimulate prostanoid synthesis by acting as "cosubstrates." On the other hand, many inhibitors of leukotriene synthesis, such as nordihydroguaiaretic acid and caffeic acid, have a catecholic structure. Catecholamines have opposite effects on prostanoid and leukotriene synthesis in human polymorphonuclear leukocytes and whole blood. Basic phenols (catechol, hydroquinone, and phenol) also increase the prostanoid/leukotriene ratio in polymorphonuclear leukocytes. These actions correlate to their antioxidant capacities and oxidation potentials, and they are not mediated via adrenergic receptors. There is only limited knowledge about the effects of natural catecholamines on the prostanoid/leukotriene ratio in vitro and in vivo. Indirect data suggest that catecholamines could increase prostanoid production in physiological or pathological situations, such as heavy physical exercise, myocardial infarction, and surgical stress. This interaction may also be of clinical importance in asthma, gastric ulcer, and psoriasis, where decreased prostanoid/leukotriene ratios have been reported.

Annexins in cardiac tissue: cellular localization and effect on phospholipase activity

Stimulation of cardiac phospholipid metabolism has diverse biological effects, ranging from subtle changes in cellular function to severe cellular damage. Accordingly, knowledge of the factors governing the activity of cardiac phospholipases is of great biological importance. A possible role of annexins, intracellular proteins that bind to membranes in a calcium dependent manner, as modulators of phospholipase activity has been proposed. In this study we investigated the cell type specific distribution of Annexin V and VIII in the heart. Recombinant Annexin V was used to examine the effect of this type of Annexin on cardiac phospholipase activity. Western blot analysis shows that annexin V is abundantly present in the heart. Using isolated myocytes and cultured cardiac endothelial and fibroblast-like cells, it is demonstrated that the localization of Annexin V is confined to non-myocytes. No positive bands matching the Mw of recombinant Annexin VIII are found in any of the cell types examined. In vitro studies demonstrate that recombinant Annexin V potently inhibits the activity of cardiac membrane-bound phospholipases, acting on their natural surrounding substrate, in a calcium dependent manner. Interestingly, annexin V also inhibits triacylglycerol hydrolysis. In conclusion, the expression of annexins is cell-type specific and suggests a cell-type specific function of these proteins in the heart. The absence of Annexin V in cardiac myocytes dismisses involvement of this annexin in cardiomyocyte phospholipid metabolism. The presence of Annexin V in cardiac endothelial and fibroblasts suggests a regulating role in the phospholipid homeostasis of non-myocyte cell types in the heart.

Inhaled budesonide regimen enhances serotonin- and arachidonic acid-induced platelet aggregation

The influence of inhaled budesonide regimen (400 micrograms x 2 for 7 days), on agonist-induced platelet aggregation and secretion, was investigated in 18 volunteers. Platelet activation induced by serotonin and arachidonic acid was significantly enhanced after budesonide, as demonstrated by an increase in aggregation velocity (Vmax) and amplitude (Amax), and in arachidonic acid-induced ATP-secretion. We found no change in platelet aggregation induced by ADP, epinephrine, and A23187. With the exception of epinephrine-induced platelet aggregation, which was inhibited by 10(-5)-10(-4) M budesonide, in vitro studies revealed no influence of 10 min budesonide preincubation (10(-9)-10(-4) M) on agonist-induced platelet activation, suggesting that the ex vivo enhancement of platelet function was mediated by secondary corticosteroid mechanisms. A tentative explanation of the increased arachidonic acid-induced platelet activation, may be a budesonide-induced stimulation of cyclooxygenase. The enhanced serotonin-induced platelet aggregation may be a reflection of exogenous corticosteroid stimulation of the 5-HT2-receptor.

Transforming growth factors type-beta and dexamethasone attenuate group II phospholipase A2 gene expression by interleukin-1 and forskolin in rat mesangial cells

Treatment of rat mesangial cells with interleukin-1 beta (IL-1 beta) and forskolin induced, in a synergistic fashion, the expression of group II phospholipase A2 (PLA2) mRNA, with subsequent increased synthesis and secretion of PLA2. In contrast, interleukin-6 did not increase PLA2 mRNA levels of PLA2 activity. Transforming growth factor (TGF) beta 1, TGF beta 2 and TGF beta 3 equipotently attenuated the IL-1 beta- and forskolin-induced elevation of PLA2 mRNA, as well as PLA2 synthesis and secretion. The glucocorticoid dexamethasone only partially suppressed the IL-1 beta- and forskolin-induced elevation of PLA2 mRNA, but totally inhibited PLA2 synthesis and secretion.

Conformation of uteroglobin fragments

The conformation of three fragments of uteroglobin in aqueous solution and in the presence of SDS micelles is described. Two of these fragments correspond to helix II and helix III of uteroglobin, the crystal structure of which is made of four helices. The third peptide comprises helices II and III, with the connecting beta-turn. While helix II does not interact strongly with the micelles, helix III adopts a rather clear alpha-helix in this system. The elongation of helix III with the addition of helix II at the N-terminus somewhat stabilizes the ordered structure. It is possible that the beta-turn found in the crystal is also present in solution.

Annexin II inhibits calcium-dependent phospholipase A1 and lysophospholipase but not triacyl glycerol lipase activities of rat liver hepatic lipase

A member of the annexin family (the heterotetrameric annexin II2p11(2) complex purified from porcine intestinal epithelium) was tested for its ability to affect different calcium-dependent intrinsic lipolytic activities of rat liver hepatic lipase (HL). Whereas annexin II in the presence of calcium failed to interfere with HL triacyl glycerol lipase (EC 3.1.1.3) activity, it inhibited HL phospholipase A1 (EC 3.1.1.32) and lysophospholipase (EC 3.1.1.5) activities. Inhibition could be overcome by increasing the substrate concentration. Under phospholipase A1 assay conditions, annexin II did not bind to the purified HL enzyme. These results therefore suggest that only inhibitor/substrate interactions lead to inhibition of HL phospholipase A1 and lysophospholipase activities, an obviously general mechanism of phospholipase inhibition by annexins. Possible implications of HL inhibition in vivo by annexins are discussed.

The induction of cellular group II phospholipase A2 by cytokines and its prevention by dexamethasone

Treatment of rat glomerular mesangial cells with interleukin-1 beta, tumor necrosis factor or forskolin resulted in the secretion of phospholipase A2 activity into the culture medium. Essentially all of this secreted phospholipase A2 activity was recognized by monoclonal antibodies elicited against rat liver mitochondrial 14 kDa group II phospholipase A2. Immunoblot analysis and gel filtration confirmed the presence of only 14 kDa phospholipase A2 in the culture supernatant. This enzyme could hardly be detected in unstimulated mesangial cells and after a lag period of 6 to 8 hours becomes detectable in both cells and culture medium. The results indicate that the increased phospholipase A2 activity upon treatment of the cells with cytokines is not due to activation of an existing cellular pool of enzyme but is caused by induced synthesis of group II phospholipase A2. Pretreatment of the cells with dexamethasone, a known inhibitor of prostaglandin synthesis, dose-dependently inhibits cytokine-induced phospholipase A2 activity. Western immunoblot analysis of cells and culture medium demonstrates that this is not due to inhibition of existing phospholipase A2 but because dexamethasone prevents the cytokine-induced synthesis of phospholipase A2 protein.

Mode of action of the lanthionine-containing peptide antibiotics duramycin, duramycin B and C, and cinnamycin as indirect inhibitors of phospholipase A2

Effects of the lanthionine-containing peptide antibiotics duramycin, duramycin B, duramycin C and cinnamycin on the activity of phospholipase A2 from six different sources were studied, and their mode of action was investigated. The four antibiotics inhibited potently all tested phospholipases A2, with IC50 values of around 1 microM, using phosphatidylethanolamine or [1-14C]oleate-labelled Escherichia coli, whose phospholipids are rich in phosphatidylethanolamine, as substrates. No inhibition was observed when the substrate was phosphatidylcholine. Binding of the antibiotics to the lipid fraction of E. coli could be demonstrated by co-sedimentation with whole, but not with lipid-depleted E. coli. In addition, preincubation of duramycin B with vesicles of phosphatidylethanolamine, but not those of phosphatidylcholine, prevented the inhibition of phospholipase A2 activity. The interaction of duramycin B and C, but not that of the biologically inactive compounds actagardine and the duramycin B trisulphoxide, with phosphatidylethanolamine was demonstrated using circular dichroism studies. On the other hand, no interaction of duramycin B with phosphatidylcholine could be demonstrated. A strict correlation between the physico-chemical interaction of the studied lantibiotics, demonstrated by circular dichroism spectroscopy, and their inhibition of phospholipase A2 was observed. These results suggest that lanthionine-containing peptide antibiotics inhibit phospholipase A2 indirectly by specifically sequestering the substrate phosphatidylethanolamine. This mode of action is analogous to the one described for the protein lipocortin.

Cytokine- and forskolin-induced synthesis of group II phospholipase A2 and prostaglandin E2 in rat mesangial cells is prevented by dexamethasone

We have previously described that treatment of rat glomerular mesangial cells with interleukin-1 beta, tumor necrosis factor or forskolin stimulates the synthesis and secretion of prostaglandin E2 and group II phospholipase A2. We now report that pretreatment of the mesangial cells with dexamethasone dose-dependently suppresses the cytokines- and forskolin-induced synthesis of prostaglandin E2 as well as the induced synthesis and secretion of group II phospholipase A2. These observations implicate that the inhibition of the cellular or secreted phospholipase A2 activity by dexamethasone in rat mesangial cells is not due to induced synthesis of phospholipase A2 inhibitory proteins but caused by direct inhibition of phospholipase A2 protein expression.

Analytical procedures for a cryptic messenger RNA that mediates translational control of prostaglandin synthase by glucocorticoids

Expression of the enzyme prostaglandin H synthase in cultured vascular smooth muscle cells required epidermal growth factor (EGF) and type beta transforming growth factor (TGF-beta) and was inhibited by cycloheximide but not actinomycin D. Preincubation with the glucocorticoid dexamethasone (0.5 microM) blocked the EGF-induced expression of prostaglandin H (PGH) synthase. Following dexamethasone addition, levels of hybridizable mRNA for PG synthase were reduced by over 90% within 1 h. After dexamethasone was removed, PG synthase mRNA recovered within 3 h by a process that was not inhibited by actinomycin D. These observations, together with other findings, suggested that the mRNA was being converted into some nonextractable and nontranslated form, probably by binding of a glucocorticoid-induced protein to the conserved 3' untranslated region. In order to investigate further the nature of this phenomenon, seven different literature procedures were evaluated for extracting and determining the PG synthase mRNA. Five of the seven procedures failed to detect hybridizable PG synthase mRNA in glucocorticoid-treated cells. Two procedures, however, recovered mRNA in both glucocorticoid-treated and control cells. A comparison of the protocols indicated that only those methods that incorporate a cationic detergent (sodium N-lauroylsarcosine), instead of anionic detergents in the lysis or homogenization buffers, successfully extract the glucocorticoid-suppressed PG synthase mRNA. Based upon these results two procedures are described, one that optimizes the extraction and determination of the glucocorticoid-suppressed (cryptic) form of the mRNA, and another which optimizes the analysis of normal mRNA without extracting the cryptic form. The results indicate that translational control of PG synthase by glucocorticoids is regulated by converting the mRNA into a cryptic form that is more firmly tissue bound than normal mRNA.

Ca(2+)-dependent phospholipid and arachidonic acid binding by the placental annexins VI and IV

Using an assay system in which phospholipids were immobilised on phenyl-Sepharose, we examined the affinities of the placental annexins VI and IV for binding to specific phosphatidylserine, phosphatidylethanolamine and phosphatidylinositol at Ca2+ concentrations of 0.6, 0.4 and 3.5 microM, respectively, compared to values of 4.5, 4.5 and 20 microM Ca2+, respectively for purified annexin IV. These values did not change significantly in the presence of other proteins from the family. Neither annexin VI or IV bound to phosphatidylinositol bisphosphate and phosphatidylcholine, even at millimolar concentrations of Ca2+. However, both proteins bound to arachidonic acid, oleic acid and palmitic acid in a Ca(2+)-dependent manner, using the same assay system. The level of binding for both proteins was significantly increased when mixtures of phosphatidylcholine and arachidonic acid were examined. A dose-dependent inhibition of phospholipase A2 by both annexins VI and IV, at millimolar concentrations of Ca2+ was observed when phosphatidylcholine liposomes were used as a substrate. These results raise questions about the interpretation of experiments in which the release of arachidonic acid is used as a measure of lipase activity, and of the validity of the substrate-depletion model for the inhibition of phospholipases by the annexins.

Lung calcium-dependent phospholipid-binding proteins: structure and function

Distinct peptide maps of two rabbit lung Ca2(+)-dependent phospholipid-binding proteins (PLBPs), 36,000 and 33,000, were generated by cyanogen bromide (CNBr) cleavage, trypsin or Staphylococcus aureus V8 proteinase digestion. The amino acid sequence of a CNBr-cleaved peptide of the 36,000 PLBP was aligned to the amino terminus of human lipocortin I with more than 77% identity, but had no identity with the known amino terminal sequence of other known annexins. Partial amino acid sequence of a 33,000 PLBP peptide demonstrated a close (56%) relationship to endonexin II, human placental anticoagulant protein, and porcine intestine protein II, but shared only 32% identity with lipocortin I, 30% with lipocortin II. Antiserum generated against purified 36,000 PLBP reacted strongly with the 33,000 PLBP, but did not react with any other rabbit lung cytosolic proteins. Both PLBPs inhibited the phospholipase A2 reaction when dioleoyl phosphatidylcholine and phosphatidylglycerol vesicles or monolayers were used as substrates. In the vesicle assay, the phospholipase A2 reaction was inhibited at lower substrate phospholipid concentrations but not at nearly saturating substrate concentrations. In the monolayer assay, the phospholipid-binding proteins did not inhibit phospholipase A2 at a low phospholipid surface concentration of 3.8.10(-3) molecules/A2, but they did at higher surface concentrations between 1.1 x 10(-2) and 3.8 x 10(-2) molecules/A2. The inhibition of phospholipase A2 by rabbit lung phospholipid-binding proteins is most likely due to the prevention of penetration by phospholipase A2 into the interface, a requirement for the enzyme to act on the substrate.

Selective regulation of cellular cyclooxygenase by dexamethasone and endotoxin in mice

We have studied the effect of glucocorticoids administered in vivo on the activity and synthesis of the cyclooxygenase enzyme (COX) in mice treated with or without concurrent intravenous administration of LPS. Mouse peritoneal macrophages from LPS-treated animals showed a two to three fold increase in COX activity determined by the production of PGE2 and PGI2 after stimulation of the cells with exogenous arachidonate. Dexamethasone injected simultaneously with LPS, 12 h before killing of the animal and removal of the macrophages, completely blocked the LPS-induced increase COX activity in peritoneal macrophages. The regulation observed in COX activity by LPS and dexamethasone are due primarily to changes in COX mass as determined by immunoprecipitation of [35S]methionine endogenously labeled enzyme. In contrast, the COX present in the nonadherent cells and in renal medullary microsomes obtained from the same animals, showed no significant changes between treatments. These results indicate that LPS in vivo stimulates COX synthesis in the peritoneal macrophages but not in the kidney. The effect of dexamethasone to inhibit COX synthesis is selective to the LPS-induced enzyme.