Interleukin-1 generates transmembrane signals from phospholipids through novel pathways in cultured rat mesangial cells

Transient Receptor Potential Ankyrin-1-expressing vagus nerve fibers mediate IL-1β induced hypothermia and reflex anti-inflammatory responses

BACKGROUND

Inflammation, the physiological response to infection and injury, is coordinated by the immune and nervous systems. Interleukin-1β (IL-1β) and other cytokines produced during inflammatory responses activate sensory neurons (nociceptors) to mediate the onset of pain, sickness behavior, and metabolic responses. Although nociceptors expressing Transient Receptor Potential Ankyrin-1 (TRPA1) can initiate inflammation, comparatively little is known about the role of TRPA1 nociceptors in the physiological responses to specific cytokines.

METHODS

To monitor body temperature in conscious and unrestrained mice, telemetry probes were implanted into peritoneal cavity of mice. Using transgenic and tissue specific knockouts and chemogenetic techniques, we recorded temperature responses to the potent pro-inflammatory cytokine IL-1β. Using calcium imaging, whole cell patch clamping and whole nerve recordings, we investigated the role of TRPA1 during IL-1β-mediated neuronal activation. Mouse models of acute endotoxemia and sepsis were used to elucidate how specific activation, with optogenetics and chemogenetics, or ablation of TRPA1 neurons can affect the outcomes of inflammatory insults. All statistical tests were performed with GraphPad Prism 9 software and for all analyses, P ≤ 0.05 was considered statistically significant.

RESULTS

Here, we describe a previously unrecognized mechanism by which IL-1β activates afferent vagus nerve fibers to trigger hypothermia, a response which is abolished by selective silencing of neuronal TRPA1. Afferent vagus nerve TRPA1 signaling also inhibits endotoxin-stimulated cytokine storm and significantly reduces the lethality of bacterial sepsis.

CONCLUSION

Thus, IL-1β activates TRPA1 vagus nerve signaling in the afferent arm of a reflex anti-inflammatory response which inhibits cytokine release, induces hypothermia, and reduces the mortality of infection. This discovery establishes that TRPA1, an ion channel known previously as a pro-inflammatory detector of cold, pain, itch, and a wide variety of noxious molecules, also plays a specific anti-inflammatory role via activating reflex anti-inflammatory activity.

The signal-induced phospholipid degradation cascade and protein kinase C activation

Acting in synergy with diacylglycerol, unsaturated free fatty acids such as arachidonic, oleic, linoleic, linolenic and docosahexaenoic acids dramatically activate some members of the protein kinase C family at the basal level of Ca2+ concentration. It is plausible that phospholipase C and phospholipase A2, and possibly phospholipase D as well, are involved in the activation of protein kinase C. Presumably, this enzyme activation is integrated into the signal-induced membrane phospholipid degradation cascade, prolonging the activation of protein kinase C. The sustained activity of this enzyme appears to be of importance for long-term cellular responses such as development of neuronal plasticity and gene activation.

Phosphatidic acid has a potential to promote hair growth in vitro and in vivo, and activates mitogen-activated protein kinase/extracellular signal-regulated kinase kinase in hair epithelial cells

Phospholipids have recently been discovered to play an important role in cellular regulation. In this study, we focused on phosphatidic acid and lysophosphatidic acid, which are phospholipids known to possess growth-hormonal effects on several types of cells, and examined their growth-promoting effects on murine hair epithelial cells. We discovered that phosphatidic acid possesses intensive growth-promotional effects on hair epithelial cells and epidermal keratinocytes. In contrast, lyso-phosphatidic acid showed lower growth-promoting effects on hair epithelial cells relative to phosphatidic acid and showed minimal or no growth-promoting activity on epidermal keratinocytes. Phosphatidic acid was also shown to have hair-growing activity to induce the anagen phase of the hair cycle in the in vivo murine model. For the purpose of examining the hair-growing mechanisms of phosphatidic acid, we examined its relationship to the mitogen-activated protein kinase cascade linked to cell proliferation and the transforming growth factor beta signal pathway known to be a regulator of catagen induction. We confirmed that phosphatidic acid activates MEK-1/2 and upregulates the expression of MEK-1/2 in cultured murine hair epithelial cells. Addition of transforming growth factor beta1 to hair epithelial cell cultures concentration-dependently decreased cell growth and induced apoptosis; however, addition of phosphatidic acid to the culture neutralized the growth-inhibiting effects of transforming growth factor beta1 and protected the cells from apoptosis. We speculate that the hair-growing activity of phosphatidic acid is at least linked to its growth-promoting effects on hair epithelial cells that follow mitogen-activated protein kinase/extracellular signal-regulated kinase kinase activation and its protective action on transforming-growth-factor-beta1-induced apoptosis that is assumed to trigger catagen induction in the hair cycle.

Phosphatidylcholine and cell death

Phosphatidylcholine (PC) constitutes a major portion of cellular phospholipids and displays unique molecular species in different cell types and tissues. Inhibition of the CDP-choline pathway in most mammalian cells or overexpression of the hepatic phosphatidylethanolamine methylation pathway in hepatocytes leads to perturbation of PC homeostasis, growth arrest or even cell death. Although many agents that perturb PC homeostasis and induce cell death have been identified, the signaling pathways that mediate this cell death have not been well defined. This review summarizes recent progress in understanding the relationship between PC homeostasis and cell death.

Interleukin-1/Toll receptor family members: receptor structure and signal transduction pathways

Interleukin-1 (IL-1) is a central mediator of the inflammatory response. It plays a role in both systemic and local immune responses to invading microbes. There are two receptors (IL-1RI and IL-1RII) that mediate the cellular responses. These receptors belong to a family of receptors based on homologous receptor structure within the intracellular signaling domain. Other family members include the Drosophila protein Toll, the recently discovered mammalian Toll-like receptors (TLR), and the IL-18 receptor. Engagement of these receptors by their diverse ligands results in activation of very similar signal transduction cascades through use of common signaling intermediates. These signal transduction cascades lead to the activation of cellular responses that are known to regulate the innate immune response. Therefore, elucidating the function and redundancy of this receptor family is essential to the understanding of the innate immune response. This review examines each member of this receptor family and emphasizes similarities and potential differences in both receptor structure and signal transduction pathways to further the understanding of this complex receptor family.

Proinflammatory cytokines

STUDY OBJECTIVES

To review the concept of proinflammatory cytokines.

DESIGN

Review of published literature.

SETTING

Academic (university hospital).

RESULTS

Cytokines are regulators of host responses to infection, immune responses, inflammation, and trauma. Some cytokines act to make disease worse (proinflammatory), whereas others serve to reduce inflammation and promote healing (anti-inflammatory). Attention also has focused on blocking cytokines, which are harmful to the host, particularly during overwhelming infection. Interleukin (IL)-1 and tumor necrosis factor (TNF) are proinflammatory cytokines, and when they are administered to humans, they produce fever, inflammation, tissue destruction, and, in some cases, shock and death. Reducing the biological activities of IL-1 and TNF is accomplished by several different, but highly specific, strategies, which involve neutralizing antibodies, soluble receptors, receptor antagonist, and inhibitors of proteases that convert inactive precursors to active, mature molecules. Blocking IL-1 or TNF has been highly successful in patients with rheumatoid arthritis, inflammatory bowel disease, or graft-vs-host disease but distinctly has not been successful in humans with sepsis. Agents such as TNF-neutralizing antibodies, soluble TNF receptors, and IL-1 receptor antagonist have been infused into > 10,000 patients in double-blind, placebo-controlled trials. Although there has been a highly consistent small increase (2 to 3%) in 28-day survival rates with anticytokine therapy, the effect has not been statistically significant.

CONCLUSIONS

Anticytokine therapy should be able to "rescue" the patient whose condition continues to deteriorate in the face of considerable support efforts. Unfortunately, it remains difficult to identify those patients who would benefit from anticytokine therapy for septic shock.

Inhibition of phosphatidylcholine and phosphatidylethanolamine biosynthesis in rat-2 fibroblasts by cell-permeable ceramides

Phospholipids and sphingolipids are important precursors of lipid-derived second messengers such as diacylglycerol and ceramide, which participate in several signal transduction pathways and in that way mediate the effects of various agonists. The cross-talk between glycerophospholipid and sphingolipid metabolism was investigated by examining the effects of cell-permeable ceramides on phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) synthesis in Rat-2 fibroblasts. Addition of short-chain C6-ceramide to the cells resulted in a dose- and time-dependent inhibition of the CDP-pathways for PtdCho and PtdEtn synthesis. Treatment of cells for 4 h with 50 microM C6-ceramide caused an 83% and a 56% decrease in incorporation of radiolabelled choline and ethanolamine into PtdCho and PtdEtn, respectively. Exposure of the cells for longer time-periods (>/= 16 h) to 50 microM C6-ceramide resulted in apoptosis. The structural analogue dihydro-C6-ceramide did not affect PtdCho and PtdEtn synthesis. In pulse-chase experiments, radioactive choline and ethanolamine accumulated in CDP-choline and CDP-ethanolamine under the influence of C6-ceramide, suggesting that synthesis of both PtdCho and PtdEtn were inhibited at the final step in the CDP-pathways. Indeed, cholinephosphotransferase and ethanolaminephosphotransferase activities in membrane fractions from C6-ceramide-treated cells were reduced by 64% and 43%, respectively, when compared with control cells. No changes in diacylglycerol mass levels or synthesis of diacylglycerol from radiolabelled palmitate were observed. It was concluded that C6-ceramide affected glycerophospholipid synthesis predominantly by inhibition of the step in the CDP-pathways catalysed by cholinephosphotransferase and ethanolaminephosphotransferase.

Interleukin-2 causes an increase in saturated/monounsaturated phosphatidic acid derived from 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol

Phosphatidic acid generation through activation of diacylglycerol kinase alpha has been implicated in interleukin-2-dependent T-lymphocyte proliferation. To investigate this lipid signaling in more detail, we characterized the molecular structures of the diradylglycerols and phosphatidic acids in the murine CTLL-2 T-cell line under both basal and stimulated conditions. In resting cells, 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol subtypes represented 44 and 55% of total diradylglycerol, respectively, and both showed a highly saturated profile containing primarily 16:0 and 18:1 fatty acids. 1-O-Alk-1'-enyl-2-acylglycerol represented 1-2% of total diradylglycerol. Interleukin-2 stimulation did not alter the molecular species profiles, however, it did selectively reduce total 1-O-alkyl-2-acylglycerol by over 50% at 15 min while only causing a 10% drop in 1,2-diacylglycerol. When radiolabeled CTLL-2 cells were challenged with interleukin-2, no change in the cellular content of phosphatidylcholine nor phosphatidylethanolamine was observed thereby ruling out phospholipase C activity as the source of diradylglycerol. In addition, interleukin-2 failed to stimulate de novo synthesis of diradylglycerol. Structural analysis revealed approximately equal amounts of 1,2-diacyl phosphatidic acid and 1-O-alkyl-2-acyl phosphatidic acid under resting conditions, both containing only saturated and monounsaturated fatty acids. After acute (2 and 15 min) interleukin-2 stimulation the total phosphatidic acid mass increased, almost entirely through the formation of 1-O-alkyl-2-acyl species. In vitro assays revealed that both 1,2-diacylglycerol and 1-O-alkyl-2-acylglycerol were substrates for 1,2-diacylglycerol kinase alpha, the major isoform in CTLL-2 cells, and that the lipid kinase activity was almost totally inhibited by R59949. In conclusion, this investigation shows that, in CTLL-2 cells, 1,2-diacylglycerol kinase alpha specifically phosphorylates a pre-existing pool of 1-O-alkyl-2-acylglycerol to form the intracellular messenger 1-O-alkyl-2-acyl phosphatidic acid.

Involvement of lipid mediators on cytokine signaling and induction of secretory phospholipase A2 in immortalized astrocytes (DITNC)

Our previous studies demonstrated the ability of proinflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha) and interleukin 1beta (IL-1beta), to stimulate NFkappaB/DNA binding and synthesis of secretory phospholipase A2 (sPLA2) in immortalized astrocytes (DITNC). In this study, we examined possible involvement of lipid mediators in the cytokine action. Using [14C]serine to label sphingomyelin and ceramide in these cells, subsequent exposure of cells to cytokines did not result in alteration of sphingomyelin/ceramide ratio. Furthermore, neither exogenous sphingomyelinase nor cell-permeable ceramides could stimulate NFkappaB/DNA binding. On the other hand, C-2 ceramide (0.3 microM) as well as other lipid mediators, such as lysophosphatidylcholine and arachidonic acid, were able to elicit a small increase in sPLA2 and potentiate the induction of sPLA2 by TNF-alpha. When DITNC cells were prelabeled with [32P]Pi, an increase in labeled phosphatidic acid (PA) was observed on treatment of cells with IL-1beta (200 U/mL). However, despite the ability of phorbol myristate acetate (PMA) to stimulate phospholipase D (PLD) and synthesis of phosphatidylethanol (PEt) in these cells, PLD activity was not affected by IL-1beta. With the [32P]labeled cells, however, PA-phosphohydrolase inhibitors, such as chlorpromazine and propranolol, could elicit large increases in labeled PA, indicating active PA metabolism in these cells. Cytokines also caused an increase in levels of diacylglycerol (DG) in these cells, although the source of this lipid pool is presently not understood. Taken together, these results provide evidence for the participation of PA and DG in cytokine signaling activity. Furthermore, although cytokines did not cause the release of ceramide, lipid mediators, such as lysophospholipids, and AA could modulate cytokine-mediated induction of sPLA2 in astrocytes.

High-resolution separation and quantification of neutral lipid and phospholipid species in mammalian cells and sera by multi-one-dimensional thin-layer chromatography

An improvement of current methods is needed for simple, rapid, and precise quantification of cellular lipids, including rare species of biologically active cellular lipids, such as phosphatidic acid (PA) and diradylglycerol (DG). In addition, further analysis of hydrolyzed acyl chains from these species by methods such as gas chromatography requires complete separations. Methods have been developed for the quantification of neutral lipids and several phospholipids extracted from mammalian cells and sera. Lipid masses were determined for the major classes of the neutral, nonpolar lipids, and of the phospholipids. The lipid classes were separated by a multistep thin-layer chromatography (TLC) procedure in different solvent systems, a method which we have designated as multi-one-dimensional thin-layer chromatography (MOD-TLC). Resolved lipid bands were visualized by the lipophilic dye primulin (direct yellow 59) and scanned by an automated laser-fluorescence detector. The mass of each band was determined by comparing band intensities of unknown samples to dilution curves of authentic standards. With modifications in solvent mixtures and length of separation times, the majority of biological lipids could be resolved and quantified with MOD-TLC methods. Since the detection method is nondestructive, purified lipids could then be recovered by scraping the visualized bands and extracting the lipids from the silica. The structural identities of the recovered lipids were confirmed by fast-atom bombardment and electrospray mass spectrometry. Extracted lipids were also hydrolyzed to release acyl chains and acyl chain species were determined in comparison to authentic standards by gas chromatography. PA and DG levels in ECV.304 cells were found to be 4. 6 and 3.3%, respectively, of PC levels, with a PA/DG ratio of 1.4, which is in accord with published experience using other methods and different cell types. PA in human serum was detected at 0.6% of PC, indicating the sensitivity of the technique. In contrast to two-dimensional thin-layer chromatography, which allows for good resolution of some lipid species, but cannot be used to analyze more than a single experimental point per plate, MOD-TLC allows for direct comparative analysis of multiple samples on a single TLC plate, while still providing good resolution for the quantification of most major classes of lipid species.

Interleukin-1 beta stimulates mitogen-activated protein kinase in U373 astrocytoma cells without the production of lipid second messengers

IL-1 beta is one of the cytokines known to affect astroglial cells in normal brain development, brain injury and neurodegenerative diseases. IL-1 beta causes astrocytes to become more reactive, alter the expression and release of molecules and in some cases to proliferate. We have investigated the mitogenic effect and signal transduction pathway induced by IL-1 beta in U373 cells, a human astrocytoma cell-line. Recombinant human IL-1 beta induced mitogenesis of U373 cells in a dose-dependent fashion as assessed by tritiated thymidine incorporation. The following signal transduction mechanisms, reported to be induced in other systems by IL-1 beta, were investigated in U373 cells: (1) activation of phosphatidylcholine-specific phospholipase C as assayed by incorporation of tritiated choline into cellular phospholipids, (2) production of diacylglycerol, a lipid second messenger, (3) activation of sphingomyelinase, and (4) activation of mitogen-activated protein kinase (MAPK). Of these, IL-1 beta activated only MAPK. In cultured rat astrocytes, IL-1 beta caused activation of MAPK without inducing proliferation.

Interleukin-1

Interleukin-1 (IL-1) is the prototypic pro-inflammatory cytokine. There are two forms of IL-1, IL-1alpha and IL-1beta and in most studies, their biological activities are indistinguishable. IL-1 affects nearly every cell type, often in concert with another pro-inflammatory cytokine, tumor necrosis factor (TNF). Although IL-1 can upregulate host defenses and function as an immunoadjuvant, IL-1 is a highly inflammatory cytokine. The margin between clinical benefit and unacceptable toxicity in humans is exceedingly narrow. In contrast, agents that reduce the production and/or activity of IL-1 are likely to have an impact on clinical medicine. The synthesis, processing, secretion and activity of IL-1, particularly IL-1beta, are tightly regulated events. A unique aspect of cytokine biology is the naturally occurring IL-1 receptor antagonist (IL-1Ra). IL-1Ra is structurally similar to IL-1beta but lacking agonist activity is used in clinical trials to reduce disease severity. In addition, regulation of IL-1 activity extends to low numbers of surface receptors, circulating soluble receptors and a cell surface "decoy" receptor to down-regulate responses to IL-1beta. This review updates the current knowledge on IL-1.

Interleukin-1-induced ether-linked diglycerides inhibit calcium-insensitive protein kinase C isotypes. Implications for growth senescence

It is hypothesized that inflammatory cytokines and vasoactive peptides stimulate distinct species of diglycerides that differentially regulate protein kinase C isotypes. In published data, we demonstrated that interleukin-1, in contrast to endothelin, selectively generates ether-linked diglyceride species (alkyl, acyl- and alkenyl, acylglycerols) in rat mesangial cells, a smooth muscle-like pericyte in the glomerulus. We now demonstrate both in intact cell and in cell-free preparations that these interleukin-1 receptor-generated ether-linked diglycerides inhibit immunoprecipitated protein kinase C delta and epsilon but not zeta activity. Neither interleukin-1 nor endothelin affect de novo protein expression of these protein kinase C isotypes. As down-regulation of calcium-insensitive protein kinase C isotypes has been linked to antimitogenic activity, we investigated growth arrest as a functional correlate for IL-1-generated ether-linked diglycerides. Cell-permeable ether-linked diglycerides mimic the effects of interleukin-1 to induce a growth-arrested state in both G-protein-linked receptor- and tyrosine kinase receptor-stimulated mesangial cells. This signaling mechanism implicates cytokine receptor-induced ether-linked diglycerides as second messengers that inhibit the bioactivity of calcium-insensitive protein kinase C isotypes resulting in growth arrest.

Staurosporine, but not Ro 31-8220, induces interleukin 2 production and synergizes with interleukin 1alpha in EL4 thymoma cells

Protein kinase C (PKC) has been implicated in interleukin 1 (IL1) signal transduction in a number of cellular systems, either as a key event in IL1 action or as a negative regulator. Here we have examined the effects of two PKC inhibitors, staurosporine and the more selective agent Ro 31-8220, on IL1 responses in the murine thymoma line EL4.NOB-1. A 1 h pulse of staurosporine was found to strongly potentiate the induction of IL2 by IL1alpha in these cells. In contrast, neither a pulse nor prolonged incubation with Ro 31-8220 affected the response to IL1alpha. Both agents blocked the response to PMA, however. A 1 h pulse of staurosporine was also found to induce IL2 production on its own, activate the transcription factor nuclear factor kappaB (NFkappaB) and increase the expression of a NFkappaB-linked reporter gene. It synergized with IL1alpha in all of these responses. Ro 31-8220 was again without effect, although both staurosporine and Ro 31-8220 blocked the activation of NFkappaB by PMA. Finally, staurosporine caused the translocation of PKC-alpha and -epsilon, and to a lesser extent PKC-beta, but not PKC-θ or -zeta, from the cytosol to the membrane, although a similar effect was observed with Ro 31-8220. The results suggest that PKC is not involved in IL1alpha signalling in EL4 cells. Furthermore, the potentiating effect of staurosporine on IL1alpha action does not involve PKC inhibition, and is likely to be at the level of NFkappaB activation.

Group II phospholipase A2 as an autocrine growth factor mediating interleukin-1 action on mesangial cells

The proliferation of mesangial cells plays a central role in the progression of glomerulonephritis. We studied the role of group II phospholipase A2 in interleukin-1-stimulated proliferation of mesangial cells. Cultured rat mesangial cells secreted 5.3 units group II phospholipase A2/24 h per 10(5) cells in response to stimulation of 200 U/ml of interleukin-1. Northern hybridization analysis showed that mRNA for group II phospholipase A2 was induced by exogenously added group II phospholipase A2 (15 U/ml) as well as interleukin-1. The pretreatment of quiescent mesangial cells with interleukin-1 augmented [3H]thymidine incorporation caused by platelet derived growth factor. Exogenous group II phospholipase A2 (5-36 U/ml) purified homogeneously from rat spleen also increased [3H]thymidine incorporation by platelet derived growth factor-stimulated mesangial cells in a dose dependent manner (36 U/ml phospholipase A2; 1.9-fold). The stimulatory effect of interleukin-1 on DNA synthesis of mesangial cells was specifically blunted by immunoglobulin raised against group II phospholipase A2. Group II phospholipase A2 (16 U/ml) amplified a platelet derived growth factor-stimulated increase in the mesangial cell number by 1.5-fold. Among the products of the phospholipase A2-catalyzed reaction, lysophospholipids including lysophosphatidylcholine, lysophosphatidylethanolamine and lysophosphatidic acid, but not fatty acids, mimicked the stimulatory effect of interleukin-1 and phospholipase A2. These results suggest that group II phospholipase A2 acts as a signaling molecule that mediates interleukin-1-induced growth of rat mesangial cells through yielding lysophospholipids.

Activation of phosphatidylcholine-selective phospholipase C by I1-imidazoline receptors in PC12 cells and rostral ventrolateral medulla

The I1-imidazoline receptor is expressed in the rostral ventrolateral medulla (RVLM) where it mediates vasodepression, and in PC12 pheochromocytoma cells where it elicits generation of diacylglycerol independent of phosphatidylinositol turnover or activation of phospholipase D. We hypothesized that the I1-imidazoline receptor couples to a phosphatidylcholine-selective phospholipase C (PC-PLC). The I1-agonist moxonidine elicited diacyglyceride accumulation and release of [3H]phosphocholine from PC12 cells prelabeled with [3H]choline. The PC-PLC inhibitor D609 abolished both responses. Microinjection of D609 into the RVLM of hypertensive rats blocked the vasodepressor response to intravenous moxonidine. These data implicate PC-PLC in cellular and organismic responses to I1-receptor stimulation.

Interleukin-1-induced growth inhibition of human melanoma cells. Interleukin-1-induced antizyme expression is responsible for ornithine decarboxylase activity down-regulation

Interleukin (IL)-1 is a multi-functional cytokine and regulates cell growth either positively or negatively. Previous studies have shown that IL-1-induced ornithine decarboxylase (ODC) activity down-regulation is involved in the anti-proliferative effect of IL-1 on human A375 melanoma cells. In this study, we examined the IL-1alpha-induced molecular events resulting in ODC activity down-regulation in C2-1, a A375 cell line stably transfected with human type I IL-1 receptor. Recombinant human (rh) IL-1alpha inhibited the growth and down-regulated the ODC activity of C2-1 cells in a dose-dependent manner. Kinetics studies showed that both the DNA synthesis and ODC activity of C2-1 cells progressively decreased from 12 h after IL-1 addition. Northern hybridization showed that IL-1 had no influence on ODC mRNA level. However, rhIL-1 induced both a decrease of ODC protein and an ODC-inhibiting activity in IL-1-treated C2-1 cells. IL-1 specifically up-modulated the mRNA level of antizyme, a protein essential for ODC regulation, but had little effect on its stability. IL-1-induced antizyme up-modulation preceded IL-1-induced down-regulation of ODC protein, ODC activity, and DNA synthesis in C2-1 cells. Run-on transcription analysis confirmed that the increased antizyme mRNA expression was due to elevated antizyme gene transcription. Furthermore, the action of IL-1 to inhibit the ODC activity and growth of C2-1 cells was blocked by expressing the antisense RNA of human antizyme in C2-1 cells. These results suggest that IL-1-induced antizyme expression is responsible for IL-1-induced ODC activity down-regulation in human melanoma cells.

Interleukin 1-beta-induced protein kinase C-zeta activation is mimicked by exogenous phospholipase D

Interleukin 1-beta (IL1-beta) is a pleiotropic cytokine that stimulates a number of signal transduction pathways in cells, leading to different cellular responses. In this study we investigated the signal transduction pathways activated by IL1-beta in two different human cell lines: RD/TE671, a rhabdomyosarcoma, and EJ, a bladder-derived carcinoma. We showed that this cytokine induced the activation of protein kinase C-zeta (PKC-zeta) and the accumulation of a putative physiological PKC-zeta activator, phosphatidic acid [Limatola, Schaap, Moolenaar and van Blitterswijk (1994) Biochem. J. 304, 1001-1008]. Exogenously supplied phospholipase D, which generated cellular phosphatidic acid, was able to mimic the cytokine effect, supporting the hypothesis that this lipid second messenger might contribute to cytokine-induced PKC-zeta activation. In addition, we show that IL1-beta stimulation of BOSC23 cells, transiently overexpressing PKC-zeta, induced an increase in PKC-zeta autophosphorylation. These results give the first direct evidence that IL1-beta can activate this atypical PKC isoform and suggest that this enzyme might be involved in mediating some of the biological effects induced by IL1-beta.

Protein kinase C in IL-2 signal transduction

Interleukin-2 (IL-2), secreted principally by activated helper T-cells, plays a pivotal role in the generation and regulation of the immune response. The various biologic functions of IL-2 have been the focus of intensive study over the years and have been well worked out. By contrast, an understanding of the intracellular signals coupled to the IL-2 receptor and responsible for mediating IL-2 effects in T-cells is far less developed, and the role that protein kinase C (PKC) may play in the various cellular responses to IL-2 receptor activation is unclear. In this article we will discuss IL-2, its receptors, and IL-2 signal transduction in relation to the physiological roles PKC activation may play in IL-2-mediated activation of T-cells and other hematopoietic cells.

Interleukin-1 signal transduction

Interleukin-1 (IL-1) is primarily an inflammatory cytokine, although it is capable of mediating a wide variety of effects on many different cell types. Nearly every known signal transduction pathway has been reported to be activated in response to IL-1. However, the significance of many of these signaling events is unclear, due to the use of different and sometimes unique cell lines in studying IL-1-initiated signal transduction. Complicating matters further is the lack of association in many studies between identified IL-1-induced signals and subsequent biological responses. In this article, we review what is known about IL-1 receptor signaling and, whenever possible, correlate signaling events to biological responses.

Regulation of phospholipase D by protein kinase C

In nearly all mammalian cells and tissues examined, protein kinase C (PKC) has been shown to serve as a major regulator of a phosphatidylcholine-specific phospholipase D (PLD) activity. At least 12 distinct isoforms of PKC have been described so far; of these enzymes only the alpha- and beta-isoforms were found to regulate PLD activity. While the mechanism of this regulation has remained unknown, available evidence suggests that both phosphorylating and non-phosphorylating mechanisms may be involved. A phosphatidylcholine-specific PLD activity was recently purified from pig lung, but its possible regulation by PKC has not been reported yet. Several cell types and tissues appear to express additional forms of PLD which can hydrolyze either phosphatidylethanolamine or phosphatidylinositol. It has also been reported that at least one form of PLD can be activated by oncogenes, but not by PKC activators. Similar to activated PKC, some of the primary and secondary products of PLD-mediated phospholipid hydrolysis, including phosphatidic acid, 1,2-diacylglycerol, choline phosphate and ethanolamine, also exhibit mitogenic/co-mitogenic effects in cultured cells. Furthermore, both the PLD and PKC systems have been implicated in the regulation of vesicle transport and exocytosis. Recently the PLD enzyme has been cloned and the tools of molecular biology to study its biological roles will soon be available. Using specific inhibitors of growth regulating signals and vesicle transport, so far no convincing evidence has been reported to support the role of PLD in the mediation of any of the above cellular effects of activated PKC.

A comparison of the effects of signal transduction inhibitors on oxidative burst and degranulation in IL-I beta stimulated bovine neutrophils

There is little information available on IL-1 mediated signal transduction in neutrophils from species other than humans. In this study, signal transduction pathway inhibitors were used to compare signaling pathways for the oxidative burst and degranulation in bovine neutrophils stimulated with rBoIL-1 beta. Protein kinase C inhibitors (staurosporine and chelerythine), DL-propranolol, pertussis toxin (PT), genistein and verapamil significantly inhibited rBoIL-1 beta (10 ng/ml) stimulated luminol-dependent chemiluminescence (LDCL) in a dose-dependent manner, while indomethacin and zileuton had no effect. Propranolol significantly decreased both primary and secondary granule release in response to rBoIL-1 beta. Staurosporine enhanced secondary but not primary granule release, and PT increased primary and secondary granule release. In addition, propranolol inhibited the shape change induced by rBoIL-1 beta and zymosan-activated serum, whereas PT markedly decreased the response induced by zymosan-activated serum, but not rBoIL-1 beta. These findings suggest that rBoIL-1 beta stimulation of the oxidative burst, degranulation, and shape change of bovine neutrophils are mediated through distinct signal transduction pathways.

Interleukin-1 signal transduction

Interleukin 1 (IL1) is a potent pro-inflammatory cytokine which has been implicated in the pathogenesis of chronic inflammatory disease. Despite much effect, the signal transduction pathway activated by IL1 has remained obscure. Recently, much attention has focussed on IL1 receptors and early events triggered by IL1 in cells, including activation of transcription factors and serine/threonine protein kinases. Two main types of IL1 receptors have been described, IL1RI and IL1RII. They appear to belong to a family of proteins which include most notably a Drosophila protein, Toll. Following receptor binding IL1 has been shown to increase protein phosphorylation in cells, and much effort has been made to identify the protein kinases responsible. Novel enzymes have been discovered, including a family of MAP kinase--like enzymes which are also activated by a range of stresses such as hypertonic stress and heat shock. Attention has also been focussed in the activation of the transcription factor NF kappa B, which is rapidly activated by IL1. This review will describe our current understanding of how IL1 activated cells and will particularly describe more recent work on IL1 receptors and early post-receptors events.

Differential regulation of sphingomyelinase and ceramidase activities by growth factors and cytokines. Implications for cellular proliferation and differentiation

Sphingosine is a product of sphingolipid metabolism that has been linked to a protein kinase C-independent mitogenic response. In previously published data, utilizing an in vitro model system for platelet-derived growth factor (PDGF)-induced vascular smooth muscle proliferation, we have demonstrated that sphingosine is increased at the expense of a concomitant decrease in ceramide formation, implicating an altered ceramidase activity. To explore mechanisms of growth factor-stimulated sphingosine formation, we have developed and investigated a cell-free model system assessing ceramidase activity. We now report that an alkaline, membrane-associated, ceramidase activity in the rat glomerular mesangial cell, a smooth muscle-like pericyte, is up-regulated by growth factors, apparently via a tyrosine kinase phosphorylation mechanism. PDGF also stimulated sphingomyelinase activity which generates sufficient substrate to drive the subsequent ceramidase reaction. Inflammatory cytokines, including interleukin-1, and tumor necrosis factor-alpha, stimulated sphingomyelinase but not ceramidase activity, a result consistent with the cellular accumulation of the ceramide, apoptidic, differentiating second messenger. Mitogenic vasoconstrictor peptides such as endothelin-1 stimulated neither sphingomyelinase nor ceramidase activities. An inhibitor of ceramidase activity, N-oleoylethanolamine, reduced PDGF- but not endothelin-1-stimulated proliferation. Thus, we conclude that, in mesangial cells, growth factors but not vasoconstrictor peptides or cytokines induce mitogenesis, in part, through ceramidase-mediated sphingosine formation.

Interleukin-1 and endothelin stimulate distinct species of diglycerides that differentially regulate protein kinase C in mesangial cells

Diglycerides are phospholipid-derived second messengers that serve as cofactors for protein kinase C activation. We have previously shown that, in rat glomerular mesangial cells, the cytokine, interleukin-1 alpha, and the vasoactive peptide, endothelin, generate diglycerides from unique phospholipid precursors. However, neither the molecular species of these diglycerides nor their biological actions were determined. It is now hypothesized that interleukin-1- and endothelin-treated mesangial cells form distinct molecular species of diglycerides which may serve different roles as intracellular signaling molecules. Diglyceride molecular species were resolved and quantified by TLC and high performance liquid chromatography as diglyceride-[14C]acetate derivatives. Endothelin stimulates predominantly ester-linked species (diacylglycerols) in contrast to interleukin-1 which stimulates only ether-linked species (alkyl, acyl- and alkenyl,acylglycerols). In support of these data, interleukin-1-treated mesangial cells hydrolyze ethanolamine plasmalogens, vinyl ether-linked phospholipids. It has been reported that ether-linked, in contrast to ester-linked, diglyceride species do not activate protein kinase C activity. Thus, we next assessed membrane protein kinase C activity in endothelin- or interleukin-1-treated mesangial cells. Even though interleukin-1 has no effect upon basal protein kinase C activity, this cytokine, through the formation of ether-linked diglyceride second messengers, inhibits endothelin, platelet-activating factor, or arginine vasopressin-stimulated protein kinase C activity. We further demonstrate that ester-linked diacylglycerols but not alkyl,acyl- or alkenyl,acylglycerols substitute for phorbol esters in a cell-free protein kinase C assay. In addition, alkenyl,acylglycerols inhibit diacylglycerol-stimulated immunoprecipitated protein kinase C alpha activity in vitro and total protein kinase C activity in permeabilized mesangial cells ex vivo. Taken together, these data suggest that interleukin-1-induced formation of ether-linked diglycerides may physiologically serve to down-regulate receptor-mediated protein kinase C activity and that individual molecular species of diglycerides may serve different roles as intracellular signaling molecules.

Regulation of manganese superoxide dismutase in glomerular epithelial cells: mechanisms for interleukin 1 induction

Reactive oxygen species have been implicated as mediators of tissue injury in glomerular inflammation. The expression of the antioxidant enzyme, manganese superoxide dismutase (MnSOD), was examined in primary cultures of rat glomerular epithelial cells (GEC) in response to inflammatory mediators. The results demonstrate that GEC respond to interleukin-1 (IL-1) and bacterial lipopolysaccharride (LPS) with an increase in MnSOD steady-state mRNA levels. The IL-1 alpha-mediated induction of MnSOD mRNA levels was both time- and dose-dependent. Maximal levels approximately 40-fold above controls, were observed at 12 hours with 2 ng/ml of IL-1 alpha. MnSOD protein levels were also markedly elevated by IL-1 alpha. The induction of MnSOD mRNA by IL-1 alpha required de novo transcription as well as some degree of protein synthesis. To elucidate the potential intracellular signal that mediates IL-1 alpha-dependent MnSOD expression, three classical signaling pathways were examined. We found no evidence that MnSOD induction by IL-1 alpha is mediated by either the cyclooxygenase or lipoxygenase pathway or via activation of protein kinase C. Based on the presence of IL-1 alpha in several forms of glomerular inflammation, the observed increase in MnSOD expression by this immunoregulatory cytokine must have an important role in the antioxidant defense of glomerular epithelial cells.

Tyrosine kinase activation is necessary for inducible nitric oxide synthase expression by interleukin-1 beta

The inflammatory cytokine interleukin-1 (IL-1) induces the inducible form of nitric oxide synthase (iNOS) with an increase in nitric oxide in rat mesangial cells. However, the cellular mechanisms that underlie the induction of iNOS by IL-1 beta in mesangial cells has not been clarified. Because we have shown that tyrosine kinase inhibitors attenuate IL-1 beta-induced cyclooxygenase expression and prostaglandin production, we investigated the effect of tyrosine kinase inhibitors on IL-1 beta-induced nitrite production and iNOS mRNA expression in rat mesangial cells. The tyrosine kinase inhibitors genistein and herbimycin A attenuated IL-1 beta-induced nitrite production in a dose-dependent manner. In addition, both of these inhibitors blocked IL-1 beta-induced iNOS mRNA expression. These data suggest that tyrosine kinase(s) plays a central role in IL-1 beta signaling to induce iNOS in rat mesangial cells.

Interleukin-1 beta-induced stimulation of insulin release in mouse pancreatic islets is related to diacylglycerol production and protein kinase C activation

The aim of the present study was to investigate the mechanisms responsible for the acute, stimulatory effects of interleukin-1 beta (rIL-1 beta; 1 ng/ml) on insulin release from mouse pancreatic islets. For this purpose, mouse islets were exposed for 60-120 min to rIL-1 beta and their function and metabolism characterized during this period. The cytokine did not increase insulin release in the presence of 1.7 mM glucose, but both in the presence of 5.6 or 16.7 mM glucose, or 10 mM leucine + 2 mM glutamine, it induced a 60-100% increase in insulin release. Moreover, rIL-1 beta also enhanced the effects of 1 mu/ml glipizide on insulin release, but failed to increase insulin release induced by 30 mM KCl or by glucose plus phorbol ester (TPA; 100 nM). These early stimulatory effects of rIL-1 beta on insulin release were neither accompanied by major increases in glucose or amino acid metabolism, nor by modifications in islet cAMP content, and they were prevented by mannoheptulose, diazoxide or verapamil. rIL-1 beta potentiation of glucose-induced insulin release was not accompanied by modifications in [Ca2+]i, but the cytokine increased diacylglycerol production and induced protein kinase C (PKC) activation. Down-regulation of PKC completely prevented the stimulatory effects of rIL-1 beta on glucose-induced insulin release. In conclusion, rIL-1 beta induces an early stimulation of insulin release in mouse beta-cells by a mechanism independent of glucose metabolism, cAMP generation or modifications in [Ca2+]i. This effect is probably related to diacylglycerol formation and stimulation of PKC.

IL-1 beta regulates rat mesangial cyclooxygenase II gene expression by tyrosine phosphorylation

The pro-inflammatory cytokine, interleukin-1 beta, induces the mRNA for prostaglandin endoperoxide synthase II gene in renal mesangial cells. This inductive effect is selective for prostaglandin endoperoxide synthase II and not prostaglandin endoperoxide synthase I. In the present experiments IL-1 beta increased COX II mRNA, and this was inhibited by genistein and herbimycin A, both inhibitors of protein tyrosine kinases. The dose dependent effect of genistein on inhibition of mRNA for COX II correlated with the inhibition of the release of PGE2 into the media. Induction of COX II by interleukin-1 beta was mimicked by incubating the cells in the presence of a protein tyrosine phosphatase inhibitor, vanadate. These experiments also illustrate selective induction of COX II mRNA without induction of COX I mRNA. Western analysis utilizing antiphosphotyrosine antibodies demonstrated in whole lysates of mesangial cells treated with interleukin-1 beta that the transient phosphorylation of several proteins occurred. Interleukin-1 beta induced the transient phosphorylation of a protein of about 39/40 kD. Similarly, vanadate also produced a rapid and transient phosphorylation of a protein of about 39/40 kD in addition to other proteins. Immunoprecipitation of mesangial cell lysates with agarose conjugated antiphosphotyrosine antibody and Western analysis of precipitated proteins with anti-ERK2 antibody demonstrate that the 39/40 kD protein phosphorylated on tyrosine is ERK2 and suggests participation of one of the MAP kinase family of extracellular receptor kinases in IL-1 beta stimulated induction of the COX II gene.

Interleukin-11 induces phosphatidic acid formation and activates MAP kinase in mouse 3T3-L1 cells

Interleukin-11 (IL-11) stimulated [3H]phosphatidic acid (PA) formation in [3H]arachidonic acid (AA) prelabelled quiescent mouse 3T3-L1 cells. When IL-11 stimulated 3T3-L1 cells were incubated with NaF, a phosphatidic acid phosphohydrolase (PAP) inhibitor, increased PA formation was observed. In the presence of ethanol, phosphatidylethanol accumulated at the expense of PA. These results indicated that the formation of PA upon IL-11 stimulation was a result of phospholipase D (PLD) activation. Endogenous accumulation of PA by NaF treatment or exogenously added PA enhanced tyrosine phosphorylation of two proteins of 44 KDa (p44) and 47 KDa (p47) whereas tyrosine phosphorylation of other proteins was not affected. Among various PA species, dipalmitoyl PA was found to be most effective in enhancing tyrosine phosphorylation of these proteins. p44 and p47 cross reacted with anti-MAP kinase monoclonal antibody (MoAb) in both immunoprecipitation and western blot analysis. Lysates from IL-11-induced or PA-induced cells stimulated phosphorylation of a synthetic peptide substrate for MAP kinase, indicating the activation of MAP kinase in the induced cells. These studies suggest that one of the cellular signalling mechanisms of IL-11 in 3T3-L1 cells involves the activation of phospholipase D to produce the second messenger PA. The increased level of PA enhances tyrosine phosphorylation of p44 and p47 which belong to the members of MAP kinase family and thus transduces some of the mitogenic signals of IL-11 in this cell line.

Differential release of tumor necrosis factor and IL-6 from adrenal zona glomerulosa cells in vitro

In previous studies, rat adrenal zona glomerulosa (ZG) cells were demonstrated to release interleukin-6 (IL-6). In the current study, cultures of ZG cells and bioassays for tumor necrosis factor (TNF) and IL-6 were used to determine if ZG cells release TNF and to define more fully the factors that regulate ZG IL-6 release. ZG cells released IL-6 and TNF, and this release was stimulated by lipopolysaccharide, interleukin-1 alpha, interleukin-1 beta, a protein kinase C activator, and a calcium ionophore without affecting intracellular adenosine 3', 5'-cyclic monophosphate (cAMP) content. In contrast, adrenocorticotropic hormone (ACTH) increased the intracellular cAMP content, increased basal and secretagogue-stimulated IL-6 release but decreased basal and secretagogue-stimulated TNF release. The effects of ACTH on IL-6 and TNF release may be mediated by increases in intracellular cAMP because ACTH and dibutyryl cAMP modified IL-6 and TNF release in an identical manner. Therefore, IL-6 and TNF release from ZG cells can be differentially regulated. Because IL-6 and TNF modify adrenal steroid release, the adrenal production of these cytokines may have a role in the stress response.

Phorbol ester selectively stimulates the phospholipase D-mediated hydrolysis of phosphatidylethanolamine in multidrug-resistant MCF-7 human breast carcinoma cells

The phospholipase D (PLD)-mediated synthesis of phosphatidylethanol (PtdEtOH) and the hydrolysis of phosphatidylethanolamine (PtdEtn) and phosphatidylcholine (PtdCho) were examined in drug-sensitive and multidrug-resistant lines of MCF-7 human breast carcinoma cells. In drug-sensitive (MCF-7/WT) cells, the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) failed to enhance either the synthesis of PtdEtOH or the hydrolysis of either phospholipid. In the drug-resistant (MCF-7/MDR) cells, 100 nM PMA greatly enhanced both the synthesis of PtdEtOH (approximately 21-fold) and the hydrolysis of PtdEtn (approximately 29-fold), but had no effect on the hydrolysis of PtdCho. The PLD activators sphingosine and H2O2 were found to elicit only a slight (1.28-1.4-fold) stimulatory effect on PtdCho hydrolysis in both the MCF-7/WT and MCF-7/MDR cell types, and had only a small effect on PtdEtn hydrolysis in the MCF-7/WT cells as well. However, these agents significantly (approximately 2.6-3.5-fold) stimulated PtdEtn hydrolysis in the MCF-7/MDR cells. These data indicate that MCF-7/MDR cells contain a PtdEtn-specific PLD activity which can be selectively stimulated by PMA, sphingosine and H2O2.

Captopril inhibits the agonist-induced increase of cytosolic free Ca2+ in glomerular mesangial cells

To evaluate the underlying mechanism of the putative renal protective effects of angiotensin converting enzyme (ACE) inhibitors, the modulatory action of captopril on the angiotensin II (Ang II) and platelet-derived growth factor (PDGF)-induced increase of cytosolic free calcium concentration ([Ca2+]i) was investigated in cultured glomerular mesangial cells (MC) from spontaneously hypertensive rats from the Münster strain (SHR) and normotensive Wistar-Kyoto rats (WKY). Resting [Ca2+]i was not affected by captopril in MC from either SHR or WKY. Captopril inhibited the Ang II-induced [Ca2+]i increase in MC from both SHR and WKY in a dose-dependent and time-dependent fashion. The preincubation of MC with 1 mumol/liter captopril for 40 minutes significantly reduced the Ang II-induced [Ca2+]i increase in SHR from 167 +/- 30 nmol/liter (N = 17) to 74 +/- 20 nmol/liter (N = 8, P < 0.05) and in WKY from 102 +/- 42 nmol/liter (N = 14) to 43 +/- 12 nmol/liter (N = 7, P < 0.05). After removal of external calcium there was no significant effect of captopril on the Ang II-induced [Ca2+]i increase. With the Mn2+ quenching technique, it was confirmed that captopril affects Ca2+ influx. Phospholipase C activity as estimated by diacylglycerol formation was not changed by captopril. The preincubation of MC with 1 mumol/liter captopril for 40 minutes significantly reduced the PDGF-induced [Ca2+]i increase in SHR from 166 +/-54 nmol/liter (N = 9) to 31 +/- 19 nmol/liter (N = 6, P < 0.01) and in WKY from 127 +/- 31 nmol/liter (N = 11) to 61 +/- 32 nmol/liter (N = 5, P < 0.05). Similarly captopril reduced the [Ca2+]i increase induced by endothelin and vasopressin. The results indicate that the actions of Ang II and PDGF on MC are modulated by captopril, probably resulting in the impairment of the calcium dependent contractile response of mesangial cells.

Phosphatidylcholine breakdown and signal transduction

PC hydrolysis by PLA2, PLC or PLD is a widespread response elicited by most growth factors, cytokines, neurotransmitters, hormones and other extracellular signals. The mechanisms can involve G-proteins, PKC, Ca2+ and tyrosine kinase activities. Although an agonist-responsive cytosolic PLA2 has been purified, cloned and sequenced, the agonist-responsive form(s) of PC-PLC has not been identified and no form of PC-PLD has been purified or cloned. Regulation of PLA2 by Ca2+ and MAPK is well established and involves membrane translocation and phosphorylation, respectively. PKC regulation of the enzyme in intact cells is probably mediated by MAPK. The question of G-protein control of PLA2 remains controversial since the nature of the G-protein is unknown and it is not established that its interaction with the enzyme is direct or not. Growth factor regulation of PLA2 involves tyrosine kinase activity, but not necessarily PKC. It may be mediated by MAPK. The physiological significance of PLA2 activation is undoubtedly related to the release of AA for eicosanoid production, but the LPC formed may have actions also. There is much evidence that PKC regulates PC-PLC and PC-PLD and this is probably a major mechanism by which agonists that promote PI hydrolysis secondarily activate PC hydrolysis. Since no agonist-responsive forms of either phospholipase have been isolated, it is not clear that PKC exerts its effects directly on the enzymes. Although it is assumed that a phosphorylation mechanism is involved, this may not be the case, and regulation may be by protein-protein interactions. G-protein control of PC-PLD is well-established, although, again, it has not been demonstrated that this is direct, and the nature of the G-protein(s) involved is unknown. In some cell types, there is evidence of the participation of a soluble protein, which may be a low Mr GTP-binding protein. What role this plays in the activation of PC-PLD is obscure. Agonist activation of PC hydrolysis in cells is usually Ca(2+)-dependent, but the step at which Ca2+ is involved is unclear, since PC-PLD and PC-PLC per se are not influenced by physiological concentrations of the ion. Most growth factors promote PC hydrolysis and this is mainly due to activation of PKC as a result of PI breakdown. However, in some cases, PC breakdown occurs in the absence of PI hydrolysis, implying another mechanism that does not involve PI-derived DAG.(ABSTRACT TRUNCATED AT 400 WORDS)

Interleukin-1 stimulates phosphatidic acid-mediated phospholipase D activity in human mesangial cells

Previous studies suggest that signal transduction mediated by interleukin-1 (IL-1), acting through an IL-1 receptor type found on T-cells and mesangial cells, may use phosphatidylethanolamine (PE) as a signaling molecule. Evidence presented here indicates that stimulation of human mesangial cells by IL-1 results in activation of a phospholipase D (PLD) that hydrolyzes PE to phosphatidic acid (PA). PLD acts on a subfraction of PE enriched in 1-o-alkyl and 1-o-alkenyl, sn-2-unsaturated species, generating a unique PA subspecies 30-120 s after stimulation. This PA species is subsequently converted to diradylglycerols by phosphatidate phosphohydrolase. The PE-directed PLD activity is abolished by antibodies against the IL-1 type I receptor and against IL-1. This specific PLD activity is also stimulated by low concentrations of 1,2-sn-dilinoleoyl PA, but not by high concentrations of 1-palmitoyl or 1-oleoyl lyso-PA. Blockade of PLD activation by IL-1 antibodies or antibody against the IL-1 receptor is bypassed by stimulation of human mesangial cells with 1,2-sn-dilinoleoyl PA. A novel system of signal cytokine mediation through PA self-amplification is indicated.

Down-regulation of cytokine action

The study of cytokines that regulate all areas of cellular communication has expanded over the past few years. The control and modulation of the complex network of cytokine action remains an area of intense interest. Agents that will modulate cytokine signal transduction at the cellular level will assist in the understanding of the molecular basis of cytokine cellular activation and in the design of drugs for the management of clinical disease. Recent work has demonstrated the existence of complex mechanisms of negative regulation of cytokine action. New methods utilizing isolated protein products that participate in immunomodulation may prove useful for clinical regulation of the host response to cytokine up-regulation. Currently, most interest in soluble cytokine receptors, natural cytokine inhibitors, genetically engineered cytokine antagonists and single or combinations of anti-inflammatory cytokines has focused on the possibility that they may become standard pharmacological agents for the treatment of inflammatory complications of clinical disease. Specifically, TNF and IL-1 inhibitors and the cytokines IL-10 and TGF-beta, alone or in combination may be effective for the inhibition of severe clinical inflammation. Soluble receptors for other cytokines such as IL-6 may prove to be carrier proteins that enhance cytokine action and will require cautious investigation. Because most cytokines are pleiomorphic in their activities, down-regulation through the utilization of direct inhibitors or anti-inflammatory cytokines may cause immunosuppression, making the host susceptible to opportunistic infection. Selective and short-term inhibition of inflammatory cytokine action may be necessary to prevent unwanted clinical side-effects.

Interleukin 1 alpha causes rapid activation of cytosolic phospholipase A2 by phosphorylation in rat mesangial cells

We have shown previously that interleukin 1 (IL-1) stimulates eicosanoid production in glomerular mesangial cells (MC) by de novo synthesis of a 14-kD, group II phospholipase A2 (PLA2). IL-1-stimulated prostaglandin E2 synthesis precedes expression of this enzyme, suggesting that another PLA2 isoform must be more rapidly activated. In the presence but not absence of calcium inophore, [3H]arachidonate release is increased significantly as early as 5 min after addition of IL-1, and IL-1 concurrently stimulates a Ca(2+)-dependent phospholipase activity, which was characterized as the cytosolic form of PLA2 (cPLA2). IL-1 does not alter either cPLA2 mRNA expression or mass in serum-stimulated MC, suggesting that cPLA2 activity is increased by a posttranslational modification. IL-1 treatment for 30 min doubles 32P incorporation into immunoprecipitable cPLA2 protein, concordant with the increase in enzyme activity. Immunoblot analysis of extracts derived from IL-1-treated (30 min) cells demonstrates a decreased mobility of cPLA2, and treatment of MC lysates with acid phosphatase significantly reduces cytokine-activated cPLA2 activity, further indicating that IL-1 stimulates phosphorylation of the enzyme. IL-1 treatment (24 h) of serum-deprived MC doubled cPLA2 mRNA, protein, and activity. In summary, IL-1 increases cPLA2 activity in a biphasic, time-dependent manner both by posttranslational modification and de novo synthesis. We consider cPLA2 activation a key step in IL-1-stimulated synthesis of pro-inflammatory, lipid mediators, and an integral event in the phenotypic responses induced in target cells by this cytokine.

Interleukin-1 increases protein kinase A activity by a cAMP-independent mechanism in AtT-20 cells

A recent study from this laboratory has shown that the inflammatory mediator, interleukin-1 alpha (IL-1 alpha), stimulates protein kinase A (PKA) activity and adrenocorticotropic hormone (ACTH) secretion from AtT-20 cells without any detectable increase in intracellular cAMP accumulation. The present studies were conducted to determine if cAMP is involved in IL-1 alpha activation of PKA and if PKA is responsible for IL-1 alpha-induced ACTH release from AtT-20 cells. The data are consistent with a novel mechanism of PKA activation that does not involve cAMP. Inhibition of adenylate cyclase with 2'5'-dideoxyadenosine (2'5'-DDA) did not affect IL-1 alpha-induced increases in PKA activity and ACTH secretion. In contrast, CRF-stimulated PKA activity and ACTH secretion were inhibited by 2'5'-DDA. Additional evidence was obtained using the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). IBMX did not alter IL-1 alpha-induced PKA activity or ACTH secretion, yet IBMX potentiated CRF-induced cAMP accumulation. Inhibition of PKA with the PKA inhibitor, H-8, blocked activation of PKA and ACTH secretion by both IL-1 alpha and CRF in AtT-20 cells. These observations demonstrate that 1) the mechanism of IL-1 alpha activation of PKA is independent of adenylate cyclase or cAMP and 2) PKA is used by IL-1 alpha to induce ACTH secretion from AtT-20 cells.

Cell-specific regulation of type II phospholipase A2 expression in rat mesangial cells

IL-1 stimulates mesangial cells to synthesize specific proteins, including a non-pancreatic (Type II) phospholipase A2 (PLA2). We have studied the regulation of PLA2 by proinflammatory mediators, implicated in the pathogenesis of glomerulonephritis, and have assessed whether the activation of second messenger systems modulates or mimics PLA2 gene expression by cytokines. IL-1 alpha and beta, TNF alpha, and LPS, but not serum, IL-2, or PDGF, potently induce PLA2 mRNA, and enzyme expression. IL-1-stimulated mesangial cells express a 1.0 kB PLA2 mRNA transcript that is induced in a dose- and time-dependent manner. IL-1-stimulated increases in steady-state PLA2 mRNA abundance result from a moderate increase in PLA2 transcription rate that is amplified by the prolonged persistence of the transcript. Forskolin and dibutyryl cAMP potentiate IL-1-induced PLA2 mRNA and enzyme expression, but have no effect in the absence of cytokine. 12-tetradecanoyl phorbol 13-acetate, sn-1, 2-dioctanoyl glycerol or 1-oleoyl-2-acetyl-sn-glycerol fail to induce PLA2 expression or to alter the effect of IL-1 when coincubated with the cytokine. In contrast, serum deprivation for 24 h specifically enhances IL-1-stimulated PLA2. Genistein potentiates PLA2 mRNA expression in cells exposed to both IL-1 and serum. The inhibitory effect of serum on IL-1-induced PLA2 mRNA abundance is reproduced by PDGF but not dexamethasone. These data demonstrate that the signaling pathways directly engaged by IL-1 to induce PLA2 expression in mesangial cells interact with several second messenger systems in a cell-specific manner. We speculate that IL-1 induces specialized changes in mesangial cell structure and function through direct activation of a transcription factor(s), that result in induction of a specific gene set.

C3 and C4 gene expression and interferon-gamma-mediated regulation in human glomerular mesangial cells

The glomerular mesangial cell (GMC) plays a key role in the maintenance of glomerular structure and function and in the mediation of glomerular injury. To explore the potential of this cell to produce complement and react to local inflammatory signals, we studied the synthesis and regulation of the third and fourth components of complement in cultured human GMC. Using metabolic labelling and immunoprecipitation, we found that C3 and C4 polypeptide chains were synthesized and secreted by GMC. Interferon-gamma (IFN-gamma) led to an increase in C4 protein synthesis, but not C3 synthesis. There was a corresponding increase in C4 mRNA in IFN-gamma-activated cells, but no increase in C3 mRNA, as determined by semi-quantitative polymerase chain reaction (PCR) estimation. These results demonstrate that human GMC can synthesize C3 and C4 proteins, and that regulation of expression of the C4 gene is mediated by IFN-gamma. We hypothesize that GMC production of complement could influence the clearance of immune aggregates by the kidney and the mediation of glomerular injury.

Platelet-activating factor stimulates phosphatidic acid formation in cultured rat mesangial cells: roles of phospholipase D, diglyceride kinase, and de novo phospholipid synthesis

Platelet-activating factor (PAF) stimulates phospholipase C (PLC)-induced hydrolysis of phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2). Yet, PAF-stimulated diglycerides (DG) are still elevated at time points where inositol polyphosphates have returned to basal levels. Thus, other signal transduction pathways that hydrolyze phosphatidylcholine (PtdCho) or phosphatidylethanolamine (PtdEth) and form DG and phosphatidic acids (PA) through either PLC or phospholipase D (PLD) may also mediate PAF-stimulated cellular responses. Initially the effects of PAF upon 32P-PA generation in mesangial cells (MC) were assessed. PA formation may be indicative of several metabolic pathways including PLD and DG kinase activities as well as de novo phospholipid synthesis. PAF (10(-7) M) increased 32P-PA formation as early as 5 seconds and this elevation persisted up to 15 minutes. When MC were pretreated with the DG kinase inhibitor-R59022, PAF-induced 32P-PA formation was diminished at early but not late time points, demonstrating that the initial component of PA formation may be due, in part, to PLC activation and subsequent phosphorylation of DG. The reciprocal reaction, PA phosphohydrolase, which dephosphorylates PA to from DG was not stimulated by PAF, suggesting that the sustained elevation of DG induced by PAF is primarily a reflection of PLC. 3H-glycerol pulse-labeling experiments suggest that PAF also stimulates de novo phospholipid synthesis which also contributes to PA formation. Conclusive proof for PLD in the generation of PA was obtained by assessing the formation of 3H-phosphatidyl-ethanol (PEt) from 3H-alkyl-lyso-glycero phosphocholine (GPC) and exogenous ethanol. PAF stimulated alkyl-PEt generation in the presence but not the absence of 0.5% ethanol. Also, PAF induced a concomitant elevation of alkyl-PA at 15 minutes and this elevation of alkyl-PA was reduced when the cells were exposed to exogenous ethanol, reflecting the formation of PEt. Corroborating evidence suggests that PAF stimulates 3H-choline and 3H-ethanolamine release, suggesting that PtdCho and PtdEth are substrates for PLD. Thus, these data demonstrate that MC respond to PAF with elevated PLD and DG kinase activities as well as with an increased rate of de novo lipid synthesis which increases PA, a potential intracellular signal.

Interleukin-1-mediated phospholipid breakdown and arachidonic acid release in human synovial cells

OBJECTIVE

Interleukin-1 (IL-1), an important mediator contributing to joint destruction in rheumatoid arthritis, is known to stimulate the release of arachidonic acid (AA) and prostaglandin E2 (PGE2) from adherent synoviocytes. To study the intracellular pathways involved in these functions, we stimulated cultures of human synovial cells with recombinant IL-1 beta.

METHODS

AA liberation was measured after labeling synovial cells with 3H-AA, and PGE2 levels were determined by high performance liquid chromatography or radioimmunoassay. Identification of 3H-AA-labeled phospholipids was performed by thin layer chromatography. Cell-associated phospholipase A2 (PLA2) enzymatic activity was determined by an assay with cell-free systems and exogenous substrates.

RESULTS

Stimulation of synovial cells with recombinant IL-1 beta induced a decrease in phosphatidylcholine (PC), phosphatidylinositol (PI), and phosphatidylethanolamine (PE), and a marked increase in cell-associated PLA2 activity as compared with controls. In the presence of either quinacrine, an inhibitor of PLA2 pathway activation, or neomycin, which binds to PI mono- and biphosphate thus blocking their degradation by phospholipases, AA and PGE2 secretion were reduced in a dose-dependent manner. Kinetic studies revealed that quinacrine had little blocking activity on the IL-1-mediated AA release after 1 hour of stimulation but completely abolished it after 5 or 8 hours. In contrast, neomycin exerted a partial but significant inhibitory effect from the first hour of stimulation onward. Addition of quinacrine was also demonstrated to abolish the IL-1-induced hydrolysis of PC and PE but not PI, indicating that PC and PE are the preferred substrates for PLA2 enzymatic activity in human synovial cells.

CONCLUSION

Our findings strongly suggest that AA and PGE2 production by IL-1-triggered synoviocytes are largely dependent upon PLA2-mediated hydrolysis of PC and PE and to a lesser extent upon the earlier degradation of PI.

Platelet-activating factor stimulates multiple signaling pathways in cultured rat mesangial cells

We have previously reported that platelet-activating factor (PAF) elevates cytosolic free calcium concentration ([Ca2+]i) in fura-2-loaded glomerular mesangial cells. To confirm that this increase in [Ca2+]i is a result of receptor-mediated activation of phospholipase C, we investigated hydrolysis of phosphatidylinositol-4,5-bisphosphate (PtdIns-4,5-P2) in PAF-treated mesangial cells. PAF (10(-7) M) stimulated a rapid and transient formation of inositol trisphosphate. In concomitant experiments, PAF stimulated a biphasic accumulation of 3H-arachidonate-labeled 1,2-diacylglycerol (DAG). The secondary elevation in DAG was coincident with a rise in 3H-phosphorylcholine (PC) and 3H-phosphorylethanolamine (PE) suggesting that PAF stimulates delayed phospholipase activities which hydrolyze alternate phospholipids besides the polyphosphoinositides. This PAF-stimulated elevation in 3H-water soluble phosphorylbases was seen at 5 min but not at 15 sec suggesting that the initial rise in DAG as well as the initial elevation in [Ca2+]i are due primarily to PtdIns-4,5-P2 hydrolysis. PAF also stimulated PGE2 as well as 3H-arachidonic acid and 3H-lyso phosphatidylcholine (PtdCho) formation. We suggest that arachidonate released specifically from PtdCho via phospholipase A2 is a source of this PAF-elevated PGE2. It has been postulated that anti-inflammatory prostaglandins may antagonize the contractile and proinflammatory effects of PAF via activation of adenylate cyclase. Surprisingly, exogenous PAF reduced basal and receptor-mediated cAMP concentration indicating that PAF-stimulated transmembrane signaling pathways may oppose receptor-mediated activation of adenylyl cyclase. We have taken advantage of the different sensitivities of phospholipases A2 and C(s) to PMA, EGTA, and pertussis toxin to dissociate phospholipase A2 and C activities. Acute PMA-treatment enhanced PAF-stimulated PGE2 formation, reduced PAF-induced elevations in [Ca2+]i and had no effect upon PAF-stimulated 3H-PE. We have also demonstrated that phospholipase A2, but not PtdIns-specific phospholipase C, was sensitive to external calcium concentration. The role of a GTP-binding protein to couple PAF-receptors to the PtdIns-specific phospholipase C was confirmed as GTP gamma S synergistically elevated PAF-stimulated inositol phosphate formation. We also demonstrated that pertussis toxin ADP-ribosylates a single protein of an apparent 42 kD mass and that PAF pretreatment reduced subsequent ADP-ribosylation in a time-dependent manner. However, pertussis toxin had no effect upon phospholipase C-generated water soluble phosphorylbases or inositol phosphates. In contrast, PAF-stimulated phospholipase A2 and PAF-inhibited adenylyl cyclase activities were sensitive to pertussis toxin.(ABSTRACT TRUNCATED AT 400 WORDS)