TNF-alpha increases ceramide without inducing apoptosis in alveolar type II epithelial cells

Integrated multi-omics analysis reveals the positive leverage of citrus flavonoids on hindgut microbiota and host homeostasis by modulating sphingolipid metabolism in mid-lactation dairy cows consuming a high-starch diet

BACKGROUND

Modern dairy diets have shifted from being forage-based to grain and energy dense. However, feeding high-starch diets can lead to a metabolic disturbance that is linked to dysregulation of the gastrointestinal microbiome and systemic inflammatory response. Plant flavonoids have recently attracted extensive interest due to their anti-inflammatory effects in humans and ruminants. Here, multi-omics analysis was conducted to characterize the biological function and mechanisms of citrus flavonoids in modulating the hindgut microbiome of dairy cows fed a high-starch diet.

RESULTS

Citrus flavonoid extract (CFE) significantly lowered serum concentrations of lipopolysaccharide (LPS) proinflammatory cytokines (TNF-α and IL-6), acute phase proteins (LPS-binding protein and haptoglobin) in dairy cows fed a high-starch diet. Dietary CFE supplementation increased fecal butyrate production and decreased fecal LPS. In addition, dietary CFE influenced the overall hindgut microbiota's structure and composition. Notably, potentially beneficial bacteria, including Bacteroides, Bifidobacterium, Alistipes, and Akkermansia, were enriched in CFE and were found to be positively correlated with fecal metabolites and host metabolites. Fecal and serum untargeted metabolomics indicated that CFE supplementation mainly emphasized the metabolic feature "sphingolipid metabolism." Metabolites associated with the sphingolipid metabolism pathway were positively associated with increased microorganisms in dairy cows fed CFE, particularly Bacteroides. Serum lipidomics analysis showed that the total contents of ceramide and sphingomyelin were decreased by CFE addition. Some differentially abundant sphingolipid species were markedly associated with serum IL-6, TNF-α, LPS, and fecal Bacteroides. Metaproteomics revealed that dietary supplementation with CFE strongly impacted the overall fecal bacterial protein profile and function. In CFE cows, enzymes involved in carbon metabolism, sphingolipid metabolism, and valine, leucine, and isoleucine biosynthesis were upregulated.

CONCLUSIONS

Our research indicates the importance of bacterial sphingolipids in maintaining hindgut symbiosis and homeostasis. Dietary supplementation with CFE can decrease systemic inflammation by maintaining hindgut microbiota homeostasis and regulating sphingolipid metabolism in dairy cows fed a high-starch diet. Video Abstract.

Mitochondria and the NLRP3 Inflammasome in Alcoholic and Nonalcoholic Steatohepatitis

Alcoholic (ASH) and nonalcoholic steatohepatitis (NASH) are advanced stages of fatty liver disease and two of the most prevalent forms of chronic liver disease. ASH and NASH are associated with significant risk of further progression to cirrhosis and hepatocellular carcinoma (HCC), the most common type of liver cancer, and a major cause of cancer-related mortality. Despite extensive research and progress in the last decades to elucidate the mechanisms of the development of ASH and NASH, the pathogenesis of both diseases is still poorly understood. Mitochondrial damage and activation of inflammasome complexes have a role in inducing and sustaining liver damage. Mitochondrial dysfunction produces inflammatory factors that activate the inflammasome complexes. NLRP3 inflammasome (nucleotide-binding oligomerization domain-like receptor protein 3) is a multiprotein complex that activates caspase 1 and the release of pro-inflammatory cytokines, including interleukin-1β (IL-1β) and interleukin-18 (IL-18), and contributes to inflammatory pyroptotic cell death. The present review, which is part of the issue "Mitochondria in Liver Pathobiology", provides an overview of the role of mitochondrial dysfunction and NLRP3 activation in ASH and NASH.

Unwrapping the mechanisms of ceramide and fatty acid-initiated signals leading to immune-inflammatory responses in obesity

Obesity is considered a global epidemic developed in part as a consequence of the overconsumption of high fat diets. One of the main negative outcomes of obesity is the development of low-grade chronic systemic inflammation, induced by dysregulated immune responses, which can lead to multiple obesity-related diseases. Ceramides are a group of bioactive lipids known to be elevated in obesity and obesity-associated conditions, including cardiovascular disease and type II diabetes. Ceramides may be key players in promoting an obesity-induced inflammatory environment due to their ability to activate key pathways such as Toll-like receptor 4 (TLR4) and NLR pyrin domain containing receptor 3 (Nlrp3), while studies have shown that inhibition of ceramide synthesis gives rise to an anti-inflammatory environment. N-3 polyunsaturated fatty acids (n-3 PUFA) have been of interest due to their anti-inflammatory actions and shown to have beneficial effects in obesity-related diseases. This review will highlight the impact of ceramides in promoting an obesity-induced inflammatory microenvironment and discuss how n-3 PUFA could potentially counteract these responses and have a regulatory effect promoting immune homeostasis.

Adipocyte Ceramides-The Nexus of Inflammation and Metabolic Disease

Adipose depots are heterogeneous tissues that store and sense fuel levels. Through the secretion of lipids, cytokines, and protein hormones (adipokines), they communicate with other organ systems, informing them of the organism's nutritional status. The adipose tissues include diverse types of adipocytes (white, beige, and brown) distinguished by the number/size of lipid droplets, mitochondrial density, and thermogenic capacity. Moreover, they include a spectrum of immune cells that modulate metabolic activity and tissue remodeling. The unique characteristics and interplay of these cells control the production of ceramides, a class of nutrient signals derived from fat and protein metabolism that modulate adipocyte function to regulate glucose and lipid metabolism. The excessive accumulation of ceramides contributes to the adipose tissue inflammation and dysfunction that underlies cardiometabolic disease. Herein we review findings on this important class of lipid species and discuss their role at the convergence point that links overnutrition/inflammation to key features of the metabolic syndrome.

Apoptotic sphingolipid signaling by ceramides in lung endothelial cells

The de novo pathway of ceramide synthesis has been implicated in the pathogenesis of excessive lung apoptosis and murine emphysema. Intracellular and paracellular-generated ceramides may trigger apoptosis and propagate the death signals to neighboring cells, respectively. In this study we compared the sphingolipid signaling pathways triggered by the paracellular- versus intracellular-generated ceramides as they induce lung endothelial cell apoptosis, a process important in emphysema development. Intermediate-chain length (C(8:0)) extracellular ceramides, used as a surrogate of paracellular ceramides, triggered caspase-3 activation in primary mouse lung endothelial cells, similar to TNF-alpha-generated endogenous ceramides. Inhibitory siRNA against serine palmitoyl transferase subunit 1 but not acid sphingomyelinase inhibited both C(8:0) ceramide- and TNF-alpha (plus cycloheximide)-induced apoptosis, consistent with the requirement for activation of the de novo pathway of sphingolipid synthesis. Tandem mass spectrometry analysis detected increases in both relative and absolute levels of C(16:0) ceramide in response to C(8:0) and TNF-alpha treatments. These results implicate the de novo pathway of ceramide synthesis in the apoptotic effects of both paracellular ceramides and TNF-alpha-stimulated intracellular ceramides in primary lung endothelial cells. The serine palmitoyl synthase-regulated ceramides synthesis may contribute to the amplification of pulmonary vascular injury induced by excessive ceramides.

Pseudomonas aeruginosa and sPLA2 IB stimulate ABCA1-mediated phospholipid efflux via ERK-activation of PPARalpha-RXR

Bacterial infection triggers an acute inflammatory response that might alter phospholipid metabolism. We have investigated the acute-phase response of murine lung epithelia to Pseudomonas aeruginosa infection. Ps. aeruginosa triggered secretion of the pro-inflammatory lipase, sPLA2 IB (phospholipase A2 IB), from lung epithelium. Ps. aeruginosa and sPLA2 IB each stimulated basolateral PtdCho (phosphatidylcholine) efflux in lung epithelial cells. Pre-treatment of cells with glyburide, an inhibitor of the lipid-export pump, ABCA1 (ATP-binding cassette transporter A1), attenuated Ps. aeruginosa and sPLA2 IB stimulation of PtdCho efflux. Effects of Ps. aeruginosa and sPLA2 IB were completely abolished in human Tangier disease fibroblasts, cells that harbour an ABCA1 genetic defect. Ps. aeruginosa and sPLA2 IB induced the heterodimeric receptors, PPARa (peroxisome-proliferator-activated receptor-a) and RXR (retinoid X receptor), factors known to modulate ABCA1 gene expression. Ps. aeruginosa and sPLA2 IB stimulation of PtdCho efflux was blocked with PD98059, a p44/42 kinase inhibitor. Transfection with MEK1 (mitogen-activated protein kinase/extracellular-signal-regulated kinase kinase 1), a kinase upstream of p44/42, increased PPARa and RXR expression co-ordinately with increased ABCA1 protein. These results suggest that pro-inflammatory effects of Ps. aeruginosa involve release of an sPLA2 of epithelial origin that, in part, via distinct signalling molecules, transactivates the ABCA1 gene, leading to export of phospholipid.

Ceramide alters endothelial cell permeability by a nonapoptotic mechanism

Ceramide is a lipid second messenger that was recently identified as mediator of pulmonary edema in vivo. Here, we investigated the effect of ceramide on the permeability of confluent endothelial cell monolayers. In monolayers of bovine pulmonary artery and human microvascular pulmonary endothelial cells, incubation with C6-ceramide for 3 h elevated permeability in a concentration-dependent manner, whereas dihydroceramide was without effect. After 3 h of incubation with ceramide, we found no signs of necrosis (release of lactate dehydrogenase, loss of thiazylyl blue reduction) or apoptosis (ssDNA, caspase-8 activity). The increased endothelial permeability in response to ceramide was attenuated by the Ser/Thr protein kinase inhibitors K252a, K252b and H-7, as well as by the phosphatidylinositol-specific phospholipase C inhibitor L108. Since in some systems sphingosine-1-phosphate (S1P) acts antagonistic to ceramide, the effect of S1P was studied. S1P transiently increased endothelial cell resistance, whether it was given together with ceramide or 90 min thereafter. These data provide a novel example of the antagonism between S1P and ceramide. Our findings further suggest that ceramide alters vascular permeability by activation of pathways dependent on unidentified phospholipase C and Ser/Thr kinase isoenzymes.

Transcriptional repression of the CTP:phosphocholine cytidylyltransferase gene by sphingosine

We examined the effects of the bioactive lipid, sphingosine, on the expression of the rate-limiting enzyme involved in surfactant phosphatidylcholine synthesis, CCTalpha (CTP:phosphocholine cytidylyltransferase alpha). Sphingosine decreased phosphatidylcholine synthesis by inhibiting CCT activity in primary alveolar type II epithelia. Sphingosine decreased CCTalpha protein and mRNA levels by approx. 50% compared with control. The bioactive lipid did not alter CCTalpha mRNA stability, but significantly inhibited its transcriptional rate. In murine lung epithelia, sphingosine selectively reduced CCTalpha promoter-reporter activity when transfected with a 2 kb CCTalpha promoter/luciferase gene construct. Sphingosine also decreased transgene expression in murine type II epithelia isolated from CCTalpha promoter-reporter transgenic mice harbouring this 2 kb proximal 5'-flanking sequence. Deletional analysis revealed that sphingosine responsiveness was mapped to a negative regulatory element contained within 814 bp upstream of the coding region. The results indicate that bioactive sphingolipid metabolites suppress surfactant lipid synthesis by inhibiting gene transcription of a key surfactant biosynthetic enzyme.

Pulmonary phosphatidic acid phosphatase and lipid phosphate phosphohydrolase

The lung contains two distinct forms of phosphatidic acid phosphatase (PAP). PAP1 is a cytosolic enzyme that is activated through fatty acid-induced translocation to the endoplasmic reticulum, where it converts phosphatidic acid (PA) to diacylglycerol (DAG) for the biosynthesis of phospholipids and neutral lipids. PAP1 is Mg(2+) dependent and sulfhydryl reagent sensitive. PAP2 is a six-transmembrane-domain integral protein localized to the plasma membrane. Because PAP2 degrades sphingosine-1-phosphate (S1P) and ceramide-1-phosphate in addition to PA and lyso-PA, it has been renamed lipid phosphate phosphohydrolase (LPP). LPP is Mg(2+) independent and sulfhydryl reagent insensitive. This review describes LPP isoforms found in the lung and their location in signaling platforms (rafts/caveolae). Pulmonary LPPs likely function in the phospholipase D pathway, thereby controlling surfactant secretion. Through lowering the levels of lyso-PA and S1P, which serve as agonists for endothelial differentiation gene receptors, LPPs regulate cell division, differentiation, apoptosis, and mobility. LPP activity could also influence transdifferentiation of alveolar type II to type I cells. It is considered likely that these lipid phosphohydrolases have critical roles in lung morphogenesis and in acute lung injury and repair.

Alveolar sphingolipids generated in response to TNF-alpha modifies surfactant biophysical activity

Sphingolipids represent a diverse group of bioactive lipid species that are generated intracellularly in response to tumor necrosis factor-alpha (TNF-alpha) and are implicated as potential mediators of acute lung injury. The purpose of these studies was to determine whether there was an extracellular, TNF-alpha-regulated pool of sphingolipids in the alveolus that modulates the surface tension lowering capacity of pulmonary surfactant. Intratracheal instillation of TNF-alpha in adult rats led to a twofold increase in the amount of surfactant-associated ceramide and tended to decrease levels of sphingomyelin without significantly altering sphingosine or sphinganine content. TNF-alpha induction of alveolar ceramide was associated with nearly an 80% increase in acid sphingomyelinase activity recovered in cell-free alveolar lavage. Ceramide administered in a dose-dependent manner potently antagonized the surface tension lowering effects of natural surfactant in vitro. Intratracheal TNF-alpha and ceramide treatment of rats significantly increased lung permeability, as was evidenced by extravasation of Evans blue dye into alveolar lavage and lung tissue. Thus these studies are the first to demonstrate the existence of a cytokine-regulated alveolar pool of sphingomyelin hydrolysis products that impairs the biophysical properties of the alveolar surfactant film. The results also suggest the presence of a secretory alveolar sphingomylinase that is TNF-alpha responsive and mediates effects of the cytokine on alveolar sphingolipid metabolism.

TNF-alpha inhibits SP-A gene expression in lung epithelial cells via p38 MAPK

Surfactant protein A (SP-A), the major lung surfactant-associated protein, mediates local defense against pathogens and modulates inflammation in the alveolus. Tumor necrosis factor (TNF)-alpha, a proinflammatory cytokine, inhibits SP-A gene expression in lung epithelial cells. Inhibitors of the phosphatidylinositol 3-kinase pathway, i.e., wortmannin, LY-294002, and rapamycin, did not block the inhibitory effects of TNF-alpha on SP-A mRNA levels. An inhibitor of the p44/42 mitogen-activated protein kinase (MAPK) pathway, PD-98059, was also ineffective. PD-169316 and SB-203580, inhibitors of p38 MAPK, blocked the TNF-alpha-mediated inhibition of SP-A mRNA levels. TNF-alpha increased the phosphorylation of p38 MAPK within 15 min. Anisomycin, an activator of p38 MAPK, increased p38 MAPK phosphorylation and decreased SP-A mRNA levels in a dose-dependent manner. Finally, TNF-alpha increased the phosphorylation of ATF-2, a transcription factor that is a p38 MAPK substrate. We conclude that TNF-alpha downregulates SP-A gene expression in lung epithelial cells via the p38 MAPK signal transduction pathway.

Pulmonary-specific expression of tumor necrosis factor-alpha alters surfactant lipid metabolism

Tumor necrosis factor (TNF)-alpha is a major cytokine implicated in inducing acute and chronic lung injury, conditions associated with surfactant phosphatidylcholine (PtdCho) deficiency. Acutely, TNF-alpha decreases PtdCho synthesis but stimulates surfactant secretion. To investigate chronic effects of TNF-alpha, we investigated PtdCho metabolism in a murine transgenic model exhibiting lung-specific TNF-alpha overexpression. Compared with controls, TNF-alpha transgenic mice exhibited a discordant pattern of PtdCho metabolism, with a decrease in PtdCho and disaturated PtdCho (DSPtdCho) content in the lung, but increased levels in alveolar lavage. Transgenics had lower activities and increased immunoreactive levels of cytidylyltransferase (CCT), a key PtdCho biosynthetic enzyme. Ceramide, a CCT inhibitor, was elevated, and linoleic acid, a CCT activator, was decreased in transgenics. Radiolabeling studies revealed that alveolar reuptake of DSPtdCho was significantly decreased in transgenic mice. These observations suggest that chronic expression of TNF-alpha results in a complex pattern of PtdCho metabolism where elevated lavage PtdCho may originate from alveolar inflammatory cells, decreased surfactant reuptake, or altered surfactant secretion. Reduced parenchymal PtdCho synthesis appears to be attributed to CCT enzyme that is physiologically inactivated by ceramide or by diminished availability of activating lipids.

Chronic ethanol ingestion potentiates TNF-alpha-mediated oxidative stress and apoptosis in rat type II cells

In septic patients, chronic alcohol abuse increases the incidence of the acute respiratory distress syndrome (ARDS). Because alveolar type II cell viability is critical for epithelial repair, our objective was to determine if chronic ethanol ingestion increased the sensitivity of type II cells to the inflammatory mediators upregulated during sepsis. In rats chronically fed ethanol, type II cell mitochondrial GSH was depleted, and tumor necrosis factor-alpha (TNF-alpha)-induced generation of mitochondrial reactive oxygen species (ROS) and apoptosis were potentiated. When added to the ethanol diet, the GSH precursor (-)-2-oxo-4-thiazolidinecarboxylic acid (Procysteine; Pro) but not N-acetylcysteine (NAC) normalized type II cell mitochondrial GSH. Likewise, Pro but not NAC normalized TNF-alpha-induced mitochondrial ROS and apoptosis. This suggested that chronic ethanol ingestion potentiated TNF-alpha-induced apoptosis in type II cells via mitochondrial GSH depletion. This may be particularly relevant in ARDS when type II cell viability is critical to repair of the damaged alveolar epithelium and may have important ramifications for the treatment of ARDS patients with a history of alcohol abuse.

Sphingomyelin metabolites inhibit sphingomyelin synthase and CTP:phosphocholine cytidylyltransferase

Tissue injury in inflammation involves the release of several cytokines that activate sphingomyelinases and generate ceramide. In the lung, the impaired metabolism of surfactant phosphatidylcholine (PC) accompanies this acute and chronic injury. These effects are long-lived and extend beyond the time frame over which tumor necrosis factor (TNF)-alpha and interleukin-1beta are elevated. In this paper, we demonstrate that in H441 lung cells these two processes, cytokine-induced metabolism of sphingomyelin and the inhibition of PC metabolism, are directly interrelated. First, metabolites of sphingomyelin hydrolysis themselves inhibit key enzymes necessary for restoring homeostasis between sphingomyelin and its metabolites. Ceramide stimulates sphingomyelinases as effectively as TNF-alpha, thereby amplifying the sphingomyelinase activation, and TNF-alpha, ceramide, and sphingosine all inhibit PC:ceramide phosphocholine transferase (sphingomyelin synthase), the enzyme that restores homeostasis between sphingomyelin and ceramide pools. Second, ceramide inhibits PC synthesis, probably because of its effects on CTP:phosphocholine cytidylyltransferase, the rate-limiting enzymatic step in de novo PC synthesis. The data presented here suggest that TNF-alpha may be an inhibitor of phospholipid metabolism in inflammatory tissue injury. These actions may be amplified because of the ability of metabolites of sphingomyelin to inhibit the pathways that should restore the normal ceramide-sphingomyelin homeostasis.

CTP:phosphocholine cytidylyltransferase inhibition by ceramide via PKC-alpha, p38 MAPK, cPLA2, and 5-lipoxygenase

In a companion paper (Vivekananda J, Smith D, and King RJ. Am J Physiol Lung Cell Mol Physiol 281: L98-L107, 2001), we demonstrated that tumor necrosis factor (TNF)-alpha inhibited the activity of CTP:phosphocholine cytidylyltransferase (CT), the rate-limiting enzyme in the de novo synthesis of phosphatidylcholine (PC), and that its actions were likely exerted through a metabolite of sphingomyelin. In this paper, we explore the signaling pathway employed by TNF-alpha using C2 ceramide as a cell-penetrating sphingolipid representative of the metabolites induced by TNF-alpha. We found that in H441 cells, as reported in other cell types, cytosolic phospholipase A2 (cPLA2) is activated by TNF-alpha. We also observed that the inhibiting action of C2 ceramide on CT requires protein kinase C-alpha, p38 mitogen-activated protein kinase, and cPLA2. The actions of C2 ceramide on CT activity can be duplicated by adding 2 microM lysoPC to these cells. Furthermore, we found that the effects of C2 ceramide are dependent on 5-lipoxygenase but that cyclooxygenase II is unimportant. We hypothesize that CT activity is inhibited by the lysoPC generated as a consequence of the activation of cPLA2 by protein kinase C-alpha and p38 mitogen-activated protein kinase. The other product of the activation of cPLA2, arachidonic acid, is a substrate for the synthesis of leukotrienes, which raise intracellular Ca2+ levels and complete the activation of cPLA2.

Intratracheal instillation of lipopolysaccharide in mice induces apoptosis in bronchial epithelial cells: no role for tumor necrosis factor-alpha and infiltrating neutrophils

This study investigated apoptosis in lungs after local exposure to lipopolysaccharide (LPS). Mice were instilled intratracheally with 5 microg LPS, which corresponds to the amount acquired by smoking approximately 25 cigarettes, and killed at different time points after exposure. Our data demonstrate that local LPS exposure resulted in apoptosis in lungs from 2 h and peaked at 24 h, as detected by ligation-mediated polymerase chain reaction. Morphologic examination and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate nick-end label staining demonstrated apoptosis in bronchial epithelial cells early after intratracheal (IT) LPS challenge, whereas infiltrating neutrophils displayed positive staining at 24 and 72 h after exposure. Apoptosis in lungs clearly preceded pulmonary neutrophil infiltration, confirming that neutrophils did not contribute to pulmonary apoptosis at early time points. Further, using three experimental approaches--namely, anti-tumor necrosis factor (TNF)-alpha treatment, IT TNF-alpha instillation, and TNF/lymphotoxin-alpha knockout mice--we demonstrate that TNF-alpha, which was elevated in lungs at both messenger RNA and protein levels after IT LPS challenge, was no primary mediator in LPS-induced apoptosis at early time points. Thus, local LPS exposure in mice resulted in early apoptosis of bronchial epithelial cells independent of infiltrating neutrophils and TNF-alpha, which suggests that apoptosis of bronchial epithelium may be involved in airway injury during exposure to LPS.

Neutrophils induce apoptosis of lung epithelial cells via release of soluble Fas ligand

Neutrophils release soluble Fas ligand (sFasL), which can induce apoptosis in certain Fas-bearing cell types (Liles WC, Kiener PA, Ledbetter JA, Aruffo A, and Klebanoff SJ. J Exp Med 184: 429-440, 1996). We hypothesized that neutrophils could induce alveolar epithelial apoptosis via release of sFasL. A549 pulmonary adenocarcinoma cells expressed surface Fas and underwent cell death (10 +/- 7% viability) and DNA fragmentation (354 +/- 98% of control cells) when incubated with agonistic CD95/Fas monoclonal antibody (P < 0.05). Coincubation with human neutrophils induced significant A549 cell death at 48 (51 +/- 9% viability; P < 0.05) and 72 h (25 +/- 10%; P < 0.05) and increased DNA fragmentation (178 +/- 42% of control cells; P < 0.05), with morphological characteristics of apoptosis. The addition of antioxidants did not inhibit apoptosis. sFasL concentrations were maximally increased in coculture medium at 24 h (4.9 +/- 0.7 ng/ml; P < 0.05). Neutrophil-induced A549 cell apoptosis was blocked by inhibitory anti-Fas (42 +/- 6% of control cells; P < 0.05) and anti-FasL monoclonal antibodies (29 +/- 3%; P < 0.05). Human neutrophils and Fas similarly affected murine primary alveolar epithelial cell bilayers, and caspase activation occurred in response to Fas exposure. We conclude that neutrophils undergoing spontaneous apoptosis induce A549 cell death and DNA fragmentation, independent of the oxidative burst, that is mediated by sFasL.

Ceramide-enhanced urokinase-type plasminogen activator (uPA) release is mediated by protein kinase C in cultured microglia

As described previously, a relatively high dose of neurotrophins increased the release of urokinase-type plasminogen activator (uPA) from cultured microglia. This biological response is suggested to be caused by ceramide, which is a metabolite of nerve growth factor low-affinity receptor (NGFRp75)-associated sphingomyelin turnover. Therefore, in the present study, we examined the effect of ceramide on the release of uPA from cultured microglia. Treatment of the cells with permeable C8-ceramide (D-erythro-Sphingosine, N-octanoyl-) enhanced uPA release in a dose-dependent manner. This effect of C8-ceramide was mimicked by treatment with bacterial sphingomyelinase. A pharmacological study using a specific PKC activator, phorbol-12-myristate-13-acetate, and a protein kinase C (PKC) inhibitor, bisindolylmaleimide, showed that PKC activation is required in order to release uPA from ceramide-stimulated microglia as well as from nonstimulated microglia. Further study using a specific conventional PKC (cPKC) activator, 1-oleoyl-2-acetyl-sn-glycerol (OAG), and a specific cPKC inhibitor, Gö 6976, suggested that PKC-delta and/or -epsilon is involved in uPA release. As opposed to the apoptotic pathway, however, no activation of c-Jun N-terminal kinase and nuclear factor kappa B was observed in C8-ceramide-stimulated microglia. The findings suggest that uPA release from microglia is regulated by a mechanism in which PKC-delta and/or -epsilon are activated and further signals are transduced subsequently.

Apoptosis of lung epithelial cells in response to TNF-alpha requires angiotensin II generation de novo

Recent work from this laboratory demonstrated that apoptosis of pulmonary alveolar epithelial cells (AEC) in response to Fas requires angiotensin II (ANGII) generation de novo and binding to its receptor (Wang et al., 1999b, Am J Physiol Lung Cell Mol Physiol 277:L1245-L1250). These findings led us to hypothesize that a similar mechanism might be involved in the induction of AEC apoptosis by TNF-alpha. Apoptosis was detected by assessment of nuclear and chromatin morphology, increased activity of caspase 3, binding of annexin V, and by net cell loss inhibitable by the caspase inhibitor ZVAD-fmk. Purified human TNF-alpha induced dose-dependent apoptosis in primary type II pneumocytes isolated from rats or in the AEC-derived human lung carcinoma cell line A549. Apoptosis in response to TNF-alpha was inhibited in a dose-dependent manner by the nonselective ANGII receptor antagonist saralasin or by the nonthiol ACE inhibitor lisinopril; the inhibition of TNF-induced apoptosis was maximal at 50 microgram/ml saralasin (101% inhibition) and at 0.5 microgram/ml lisinopril (86% inhibition). In both cell culture models, purified TNF-alpha caused a significant increase in the mRNA for angiotensinogen (ANGEN), which was not expressed in unactivated cells. Transfection of primary cultures of rat AEC with antisense oligonucleotides against ANGEN mRNA inhibited the subsequent induction of TNF-stimulated apoptosis by 72% (P < 0.01). Exposure to TNF-alpha increased the concentration of ANGII in the serum-free extracellular medium by fivefold in A549 cell cultures and by 40-fold in primary AEC preparations; further, exposure to TNF-alpha for 40 h caused a net cell loss of 70%, which was completely abrogated by either the caspase inhibitor ZVAD-fmk, lisinopril, or saralasin. Apoptosis in response to TNF-alpha was also completely inhibited by neutralizing antibodies specific for ANGII (P < 0.01), but isotype-matched nonimmune immunoglobulins had no significant effect. These data indicate that the induction of AEC apoptosis by TNF-alpha requires a functional renin/angiotensin system (RAS) in the target cell. They also suggest that therapeutic control of AEC apoptosis in response to TNF-alpha is feasible through pharmacologic manipulation of the local RAS.

Effects of intratracheal instillation of TNF-alpha on surfactant metabolism

Tumor necrosis factor-alpha (TNF-alpha) has been shown to play an integral role in the pathogenesis of the acute respiratory distress syndrome. This disorder is characterized by a deficiency of alveolar surfactant, a surface-active material that is composed of key hydrophobic proteins and the major lipid disaturated phosphatidylcholine (DSPC). We investigated how TNF-alpha might alter DSPC content in rat lungs by instilling the cytokine (2.5 microg) intratracheally for 10 min and then assaying parameters of DSPC synthesis and degradation in alveolar type II epithelial cells, which produce surfactant. Cells isolated from rats given TNF-alpha had 26% lower levels of phosphatidylcholine compared with control. TNF-alpha treatment also decreased the ability of these cells to incorporate [(3)H]choline into DSPC by 45% compared with control isolates. There were no significant differences in the levels of choline substrate or choline transport between the groups. However, TNF-alpha produced a 64% decrease in the activity of cytidylyltransferase, the rate-regulatory enzyme required for DSPC synthesis. TNF-alpha administration in vivo also tended to stimulate phospholipase A(2) activity, but it did not alter other parameters for DSPC degradation such as activities for phosphatidylcholine-specific phospholipase C or phospholipase D. These observations indicate that TNF-alpha decreases the levels of surfactant lipid by decreasing the activity of a key enzyme involved in surfactant lipid synthesis. The results do not exclude stimulatory effects of the cytokine on phosphatidylcholine breakdown.