Elevated cPLA2 levels as a mechanism by which the p70 TNF and p75 NGF receptors enhance apoptosis

New developments on the TNFα-mediated signalling pathways

TNFα (tumour necrosis factor α) is an extensively studied pleiotropic cytokine associated with the pathogenesis of a variety of inflammatory diseases. It elicits a wide spectrum of cellular responses which mediates and regulates inflammation, immune response, cell survival, proliferation and apoptosis. TNFα initiates its responses by binding to its receptors. TNFα-induced effector responses are mediated by the actions and interactions among the various intracellular signalling mediators in the cell. TNFα induces both survival and apoptotic signal in a TRADD (TNF receptor-associated DD)-dependent and -independent way. The signals are further transduced via a variety of signalling mediators, including caspases, MAPKs (mitogen-activated protein kinases), phospholipid mediators and miRNA/miR (microRNA), whose roles in specific functional responses is not fully understood. Elucidating the complexity and cross talks among signalling mediators involved in the TNFα-mediated responses will certainly aid in the identification of molecular targets, which can potentially lead to the development of novel therapeutics to treat TNFα-associated disorders and in dampening inflammation.

Increased expression and activation of cytosolic phospholipase A2 in the spinal cord of patients with sporadic amyotrophic lateral sclerosis

Compelling evidence identifies a link between cytotoxic effects of cytosolic phospholipase A2 (cPLA2) activity and neuron death in cell cultures. cPLA2 catalyzes the hydrolysis of membrane phospholipids to produce and release arachidonate, leading to plasma membrane injury, inflammatory response and subsequent cell death. To assess a role for cPLA2 in the pathomechanism of amyotrophic lateral sclerosis (ALS), we performed immunohistochemical, immunoblot, and densitometric analyses of cPLA2 and its active form phosphorylated at S505 (p-cPLA2) on spinal cords obtained at autopsy from ten sporadic ALS patients and ten age-matched controls. On sections, immunoreactivities for cPLA2 and p-cPLA2 were distinct and localized in almost all of the motor neurons, reactive astrocytes, and activated microglia in the ALS cases, while immunoreactivities were only weak or not at all observed in neurons and glia in the control cases. On immunoblots, both the cPLA2/β-actin density ratio and the p-cPLA2/cPLA2 density ratio were significantly increased in the ALS group compared to the control group. There was no significant link between the densitometric data and the clinical phenotypes, age at death or disease duration of the ALS patients. These results provide in vivo evidence for increased expression and activation of cPLA2 in motor neurons, reactive astrocytes, and activated microglia in ALS, suggesting occurrence of arachidonate cascade-induced motor neuron death via cell-autonomous and/or non-cell-autonomous mechanisms.

Ischaemic preconditioning and TNF-alpha-mediated preconditioning is associated with a differential cPLA2 translocation pattern in early ischaemia

Both the cytokine tumour necrosis factor-alpha (TNF-alpha) and the enzyme cytosolic phospholipase A2 (cPLA2) have been implicated in ischaemic injury. Apart from the induction of apoptosis, TNF-alpha also mediates cytoprotection when present in low concentrations. However, the relationship between TNF-alpha and cPLA2 activities during cytoprotection is poorly understood. Therefore, we examined the role of cPLA2 in TNF-alpha-mediated (TNF-PC) and ischaemic preconditioning (IPC) in tolerance to ischaemia (SI) in C2C12 myotubes. Significant decreases in cPLA2 phosphorylation in SI compared with the preconditioned groups were observed. This was also mirrored by the p38 mitogen activated protein kinase (MAPK) phosphorylation pattern. These results correlated with fluorescence- and three-dimensional imaging, demonstrating increased translocation of phospho-cPLA2 to the nuclear region in SI compared to TNF-PC and IPC. These data suggest a p38 driven cPLA2 translocation pattern, with a possible role for cPLA2 in deciding cell fate.

Interactions between neural membrane glycerophospholipid and sphingolipid mediators: A recipe for neural cell survival or suicide

The neural membranes contain phospholipids, sphingolipids, cholesterol, and proteins. Glycerophospholipids and sphingolipids are precursors for lipid mediators involved in signal transduction processes. Degradation of glycerophospholipids by phospholipase A(2) (PLA(2)) generates arachidonic acid (AA) and docosahexaenoic acids (DHA). Arachidonic acid is metabolized to eicosanoids and DHA is metabolized to docosanoids. The catabolism of glycosphingolipids generates ceramide, ceramide 1-phosphate, sphingosine, and sphingosine 1-phosphate. These metabolites modulate PLA(2) activity. Arachidonic acid, a product derived from glycerophospholipid catabolism by PLA(2), modulates sphingomyelinase (SMase), the enzyme that generates ceramide and phosphocholine. Furthermore, sphingosine 1-phosphate modulates cyclooxygenase, an enzyme responsible for eicosanoid production in brain. This suggests that an interplay and cross talk occurs between lipid mediators of glycerophospholipid and glycosphingolipid metabolism in brain tissue. This interplay between metabolites of glycerophospholipid and sphingolipid metabolism may play an important role in initiation and maintenance of oxidative stress associated with neurologic disorders as well as in neural cell proliferation, differentiation, and apoptosis. Recent studies indicate that PLA(2) and SMase inhibitors can be used as neuroprotective and anti-apoptotic agents. Development of novel inhibitors of PLA(2) and SMase may be useful for the treatment of oxidative stress, and apoptosis associated with neurologic disorders such as stroke, Alzheimer disease, Parkinson disease, and head and spinal cord injuries.

Expression of nerve growth factor receptors is correlated with progression and prognosis of human pancreatic cancer

BACKGROUND AND AIM

The aim of the present study was to investigate the prognostic value of the two types of nerve growth factor receptors (NGFR), namely high-affinity receptor TrkA and low-affinity receptor p75NGFR, in pancreatic cancer.

METHODS

The mRNA expression of NGFR for TrkA and p75NGFR was examined in 56 human primary pancreatic cancers using real-time quantitative reverse transcription-polymerase chain reaction.

RESULTS

Nerve growth factor (NGF) receptors were found in all tumor specimens. It appears that the growth of pancreatic cancer cells stimulated by NGF depended on the expression levels and the ratio of TrkA to p75NGFR. TrkA and p75NGFR were negatively correlated and both were associated with abdominal or back pain and perineural invasion. Regarding this, patients with high TrkA expression levels exhibited more frequent perineural invasion and a higher degree of pain, whereas the results of p75NGFR were opposite. For Cox univariate analyses in the overall survival study, high expression of p75NGFR was associated with longer overall survival, but TrkA exhibited opposite effects and included an effect on perineural invasion and pain. Histoprognostic grading, tumor size and node involvement were not prognostic factors. In Cox multivariate analyses, TrkA and p75NGFR were both prognostic parameters.

CONCLUSIONS

The present study found that the expression of TrkA in pancreatic cancer is a marker of tumor aggressiveness. Conversely, we also found that elevated p75NGFR expression is associated with a favorable prognosis. We demonstrated that NGF exerts both stimulatory and inhibitory effects on pancreatic cancers, with the overall effect determined by the expression levels and the ratio of TrkA to p75NGFR.

Phospholipase A2 inhibitors in development

To date, three isoforms of phospholipase A2 (PLA2) have been identified. Of these, the two Ca2+-dependent isoforms, secretory (sPLA2) and cytosolic phospholipase A2 (cPLA2), are targets for new anti-inflammatory drugs. The catalytic mechanisms and functions of the third isoform, Ca2+-independent cytosolic phospholipase A2 (iPLA2), are unknown at present. sPLA2 and cPLA2 are both implicated in the release of arachidonic acid and prophlogistic lipid mediators. However, recent findings provide evidence that cPLA2 is the dominant isoform in various kinds of inflammation, such as T-cell-mediated experimental arthritis. A triple function of PLA2-derived lipid mediators has been suggested: causing immediate inflammatory signs, involvement in secondary processes, e.g., superoxide free radical (O2) generation, apoptosis, or tumour necrosis factor-alpha (TNF-alpha)-cytotoxicity, and controlling the expression and activation of pivotal proteins implicated in inflammation and cell development, e.g., cytokines, adhesion proteins, proteinases, NF-kappaB, fos/jun/AP-1, c-Myc, or p21ras. In the past, research predominantly focused on the development of sPLA2 inhibitors; however, present techniques enable discrimination of cPLA2, sPLA2, and iPLA2, and specific inhibitors of each of the three isoforms are likely to appear soon. Over the last decade, between 40 and 50 sPLA2 inhibitors have been described; and the list is growing. However, of these, few have the potential for clinical success, and those that do are predominantly active site-directed inhibitors, e.g., BMS-181162, LY311727, ARL-67974, FPL67047, SB-203347, Ro-23-9358, YM-26734, and IS-741. At present, there are no likely clinical candidates emerging from the ranks of cPLA2 and iPLA2 inhibitors in development. Indications for which PLA2 inhibitors are being pursued include, sepsis, acute pancreatitis, inflammatory skin and bowel diseases, asthma, and rheumatoid arthritis. The three main obstacles to the successful development of PLA2 inhibitors include, insufficient oral bioavailability, low affinity for the enzyme corresponding to low in vivo efficacy and insufficient selectivity.

Integrative role of cPLA2with COX-2 and the effect of non-steriodal anti-inflammatory drugs in a transgenic mouse model of amyotrophic lateral sclerosis

Cyclooxygenase-2 (COX-2) is a key molecule in the inflammatory pathway in amyotrophic lateral sclerosis (ALS). Cytosolic phospholipase A (cPLA2) is an important enzyme providing substrate for cyclooxygenases. We therefore examined cPLA2 expression in human ALS and mutant Cu/Zn superoxide dismutase (SOD1) transgenic mice and its relation to COX-2. Immunohistochemistry and real-time RT-PCR revealed elevated cPLA2 protein and its mRNA levels in the lumbar spinal cord of mutant SOD1 mice. COX-2 immunoreactivity was increased in lumbar spinal cord sections from both familial ALS (FALS) and sporadic ALS (SALS) as compared to controls, and cPLA2 immunoreactivity was increased in a patient with FALS. Oral administration of the non-selective cyclooxygenase (COX) inhibitor, sulindac, extended the survival (by 10%) of G93A SOD1 mice as compared to littermate controls. Sulindac, as well as the selective COX-2 inhibitors, rofecoxib and celecoxib reduced cPLA2 immunoreactivity in the lumbar spinal cord of G93A transgenic mice. Sulindac treatment preserved motor neurons, and reduced microglial activation and astrocytosis, in the spinal cord of G93A SOD1 transgenic mice. These results suggest that cPLA2 plays an important role in supplying arachidonic acid to the COX-2 driven inflammatory pathway in ALS associated with SOD1 mutations.

Nerve growth factor-induced up-regulation of cytosolic phospholipase A2alpha level in rat PC12 cells

Nerve growth factor (NGF) regulates various types of gene transcription in neurons. One of the cytosolic phospholipase A(2)s, cPLA(2)alpha, which preferentially cleaves phospholipids at the sn-2 position to arachidonic acid (AA), is involved in neuronal responses including survival. We investigated the effect of NGF on cPLA(2)alpha expression and its signaling pathways in PC12 cells, which differentiate into neuronal-like cells with neurites by NGF treatment. Treatment with NGF increased cPLA(2)alpha mRNA level after 4h and its protein level 24h after NGF addition. The NGF-induced increase in cPLA(2)alpha mRNA was inhibited by actinomycin D. NGF caused phosphorylation of mitogen-activated protein kinases (MAPKs); sustained phosphorylation of extracellular-regulated kinases (ERK1/2) and transient phosphorylation of p38 MAPK. NGF responses (cPLA(2)alpha mRNA and its protein) were inhibited by selective inhibitors for the ERK1/2 pathway, p38 MAPK and c-Jun NH(2)-terminal kinase. Epidermal growth factor, which transiently activates ERK1/2, did not modify cPLA(2)alpha expression. Although phorbol 12-myristate 13-acetate, an activator of protein kinase C (PKC), alone showed no effect, NGF-induced cPLA(2)alpha mRNA expression decreased due to the inhibition of PKC. These findings suggest that NGF-induced cPLA(2)alpha expression is regulated by gene transcription via the ERK1/2, p38 MAPK and PKC pathways in PC12 cells.

A potentially critical role of phospholipases in central nervous system ischemic, traumatic, and neurodegenerative disorders

Phospholipases are a diverse group of enzymes whose activation may be responsible for the development of injury following insult to the brain. Amongst the numerous isoforms of phospholipase proteins expressed in mammals are 19 different phospholipase A2's (PLA2s), classified functionally as either secretory, calcium dependent, or calcium independent, 11 isozymes belonging to three structural groups of PLC, and 3 PLD gene products. Many of these phospholipases have been identified in selected brain regions. Under normal conditions, these enzymes regulate the turnover of free fatty acids (FFAs) in membrane phospholipids affecting membrane stability, fluidity, and transport processes. The measurement of free fatty acids thus provides a convenient method to follow phospholipase activity and their regulation. Phospholipase activity is also responsible for the generation of an extensive list of intracellular messengers including arachidonic acid metabolites. Phospholipases are regulated by many factors including selective phosphorylation, intracellular calcium and pH. However, under abnormal conditions, excessive phospholipase activation, along with a decreased ability to resynthesize membrane phospholipids, can lead to the generation of free radicals, excitotoxicity, mitochondrial dysfunction, and apoptosis/necrosis. This review evaluates the critical contribution of the various phospholipases to brain injury following ischemia and trauma and in neurodegenerative diseases.

Distinct regulation of cytosolic phospholipase A2 phosphorylation, translocation, proteolysis and activation by tumour necrosis factor-receptor subtypes

The hormonally regulated Ca(2+)-dependent enzyme, cytosolic phospholipase A(2) (cPLA(2)) is activated by a range of inflammatory stimuli. Tumour necrosis factor-alpha (TNF) is one of the first known stimuli for cPLA(2) but it is not known whether both TNF receptor subtypes are involved in activating the lipase. In the present study, we show for the first time that both type I 55 kDa TNFR (TNFR1) and type II 75 kDa TNFR (TNFR2) stimulate cPLA(2) enzyme, but with distinct signalling mechanisms. TNFR1 activates mitogen-activated protein kinase (MAPK) and p38MAPK. TNFR1 then phosphorylates and activates cPLA(2) in a MAPK-dependent fashion. Furthermore, TNFR1 causes the translocation and caspase-dependent proteolysis of cPLA(2) as part of its activation profile. TNFR2, on the other hand, does not cause the phosphorylation of cPLA(2) as it does not activate MAPK or p38MAPK, but instead activates cPLA(2) by causing its translocation to plasma membrane and perinuclear subcellular regions. TNFR2 activation causes a delayed, slight increase in [Ca(2+)](i) of <50 nM that may contribute towards the translocation and activation of cPLA(2). Therefore both TNF receptor subtypes play a role in cPLA(2) activation, but by means of separate signal-transduction pathways.

EGR2 induces apoptosis in various cancer cell lines by direct transactivation of BNIP3L and BAK

EGR2 plays a key role in the PTEN-induced apoptotic pathway. Using adenovirus-mediated gene transfer to 39 cancer cell lines, we found that EGR2 could induce apoptosis in a large proportion of these lines by altering the permeability of mitochondrial membranes, releasing cytochrome c and activating caspase-3, -8, and -9. Analysis by cDNA microarray and subsequent functional studies revealed that EGR2 directly transactivates expression of BNIP3L and BAK. Our results helped to clarify the molecular mechanism of the apoptotic pathway induced by PTEN-EGR2, and suggested that EGR2 may be an excellent target molecule for gene therapy to treat a variety of cancers.

Tumour necrosis factor-alpha- vs. growth factor deprivation-promoted cell death: different receptor requirements for mediating nerve growth factor-promoted rescue

Physiological and pathological aging of the central nervous system (CNS) is characterized by functional neuronal impairments which may lead to perturbed cell homeostasis and eventually to neuronal death. Many toxic events may underlie age-related neurodegeneration. These include the effects of beta amyloid, Tau and mutated presenilin proteins, free radicals and oxidative stress, pro-inflammatory cytokines and lack of growth factor support, which can be individually or collectively involved. Taken individually, these toxicants can induce very diverse cell responses, thus requiring individually targeted corrective interventions upstream of common cell death (apoptotic) pathways. Recent preliminary evidence suggests that the pro-inflammatory cytokine tumour necrosis factor alpha (TNFalpha) and growth factor withdrawal can both activate a common apoptotic pathway in nerve growth factor (NGF)-responsive PC12 cells involving caspase 3, albeit through very distinct upstream pathways: the former through active signalling and the latter through passive or lack of survival signalling. Here, we show that NGF can rescue PC12 cells from both growth factor withdrawal- and TNFalpha-promoted cell death. However, NGF rescue from growth factor withdrawal requires NGF signalling through the high-affinity tyrosine kinase receptor (TrkA), while NGF rescue from TNFalpha-promoted cell death requires NGF signalling through the low-affinity p75NTR receptor. These results strengthen the idea that prevention of age- or pathology-associated neurodegeneration may require varied molecular approaches reflecting the diversity of the toxicants involved, possibly acting simultaneously.

TNF receptor subtype signalling: differences and cellular consequences

Tumour necrosis factor-alpha (TNF alpha) is a multifunctional cytokine belonging to a family of ligands with an associated family of receptor proteins. The pleiotropic actions of TNF range from proliferative responses such as cell growth and differentiation, to inflammatory effects and the mediation of immune responses, to destructive cellular outcomes such as apoptotic and necrotic cell death mechanisms. Activated TNF receptors mediate the association of distinct adaptor proteins that regulate a variety of signalling processes including kinase or phosphatase activation, lipase stimulation, and protease induction. Moreover, the cytokine regulates the activities of transcription factors, heterotrimeric or monomeric G-proteins and calcium ion homeostasis in order to orchestrate its cellular functions. This review addresses the structural basis of TNF signalling, the pathways employed with their cellular consequences, and focuses on the specific role played by each of the two TNF receptor isotypes, TNFR1 and TNFR2.

TNF-alpha receptors simultaneously activate Ca2+ mobilisation and stress kinases in cultured sensory neurones

The cytokine tumour necrosis factor-alpha (TNF) has been implicated in autoimmune diseases and may play an indirect role in activation of pain pathways. In this study we have investigated the possibility that TNF directly activates cultured neonatal rat dorsal root ganglion (DRG) neurones and provides a signalling pathway from cells in the immune system such as macrophages to sensory neurones. Expression of TNF receptor subtypes (TNFR1 and TNFR2) on sensory neurones was identified using immunohistochemistry, fluorescence-activated cell sorting analysis and RT-PCR. Biochemical and immunocytochemical analysis showed that TNF activated p38 mitogen-activated protein kinase (p38MAPK) and c-Jun N-terminal kinase (JNK) but not p42/p44 MAPK. TNF treatment evoked transient Ca2+-dependent inward currents in 70% of DRG neurones. These TNF-evoked currents were significantly attenuated by ryanodine or thapsigargin or by inclusion of BAPTA in the patch pipette solution. Responses were also evoked in subpopulations of cultured DRG neurones by human mutant TNFs that cross-reacted with rat receptors and selectively activated TNFR1 or TNFR2 subtypes. TNF-evoked transient increases in [Ca2+]i were also detected in 34% of fura-2-loaded DRG neurones. The link between TNF receptor activation and Ca2+ release from stores remains to be elucidated. However, responses to TNF were mimicked by sphingolipids, including sphingosine-1-phosphate, which evoked a transient rises in [Ca2+]i in a pertussis toxin-insensitive manner in fura-2-loaded DRG neurones. We conclude that distinct receptors TNFR1 and TNFR2 are expressed on cultured DRG neurones and that they are functionally linked to intracellular Ca2+ mobilisation, a response that may involve sphingolipid signalling.

Type II tumour necrosis factor-alpha receptor (TNFR2) activates c-Jun N-terminal kinase (JNK) but not mitogen-activated protein kinase (MAPK) or p38 MAPK pathways

The pleitropic actions of tumour necrosis factor-alpha (TNF) are transmitted by the type I 55 kDa TNF receptor (TNFR1) and type II 75 kDa TNF receptor (TNFR2), but the signalling mechanisms elicited by these two receptors are not fully understood. In the present study, we report for the first time subtype-specific differential kinase activation in cell models that respond to TNF by undergoing apoptotic cell death. KYM-1 human rhabdomyosarcoma cells and HeLa human cervical epithelial cells, engineered to overexpress TNFR2, displayed c-Jun N-terminal kinase (JNK) activation by wild-type TNF, a TNFR1-specific TNF mutant and a TNFR2-specific mutant TNF in combination with an agonistic TNFR2-specific monoclonal antiserum. A combination of the TNFR2-specific mutant and agonistic antiserum elicited maximal endogenous or exogenous TNFR2 responsiveness. Moreover, alternative expression of a TNFR2 deletion mutant lacking its cytoplasmic domain rendered the cells unable to activate JNK activity through this receptor subtype. The profile of JNK activation by TNFR1 was more transient than that of TNFR2, with TNFR2-induced JNK activity also being more sensitive to the caspase inhibitor, benzyloxycarbonyl-Val-Ala-DL-Asp-fluoromethylketone. Conversely, only activation of the TNFR1 could stimulate mitogen-activated protein kinase (MAPK) or p38 MAPK activities in a time-dependent manner. The role of TNFR2 activation in enhanced apoptotic cell death was confirmed with agonistic monoclonal antisera in cells expressing high levels of TNFR2. Activation of TNFR2 alone elicited cell death, but full TNF-induced death required stimulation of both receptor types. These findings indicate that efficient activation of TNFR2 by soluble TNFs is achievable with co-stimulation by antisera, and that both receptors differentially modulate extracellular signal-regulated kinases contributing to the cytokine's cytotoxic response.

Modulated kinase activities in cells undergoing tumour necrosis factor-induced apoptotic cell death

Tumour necrosis factor-alpha (TNF) has a variety of cellular effects including apoptotic and necrotic cytotoxicity. TNF activates a range of kinases, but their role in cytotoxic mechanisms is unclear. HeLa cells expressing elevated type II 75 kDa TNF receptor (TNFR2) protein, analysed by flow cytometry and Western analysis, showed altered c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (p38MAPK; but not MAPK) protein content and activation. There was greater JNK activation, but reduced p38MAPK activation in dying cells compared to those still to enter TNF-induced apoptosis. Moreover, cells displaying more rapid apoptosis possess higher levels of type I 55 kDa TNFR1 receptor isoform, but less TNFR2. These findings reveal differential kinase activation in TNF-induced apoptotic death.

Stimulation of stress-activated but not mitogen-activated protein kinases by tumour necrosis factor receptor subtypes in airway smooth muscle

The multifunctional cytokine tumour necrosis factor-alpha (TNF) displays many physiological effects in a variety of tissues, especially proliferative and cytotoxic actions in immunological cells. Recently, we uncovered an important new mechanism by which TNF can sensitise airway smooth muscle (ASM) to a fixed intracellular Ca2+ concentration which in vivo would produce a marked hypercontractility of the airways. Here, we report that both 50-60 kDa type I TNFR (TNFR1) and 70-80 kDa type II TNFR (TNFR2) receptor subtypes were expressed in ASM cells and selectively activated the stress kinases, c-Jun N-terminal kinase and p38 mitogen-activated protein kinase (p38 MAPK). However, TNF caused no activation of p42/p44 MAPK or cytosolic phospholipase A(2) activity. In contrast, TNF stimulation of the TNFR1, but not the TNFR2, elicited nuclear factor-kappaB transcription factor function, a species known to be important in mediation of certain inflammatory cellular responses. This is the first report of TNF receptor subtypes in ASM cells causing selective kinase activation, which may prove important in therapeutic strategies for treating immune airway disorders such as chronic obstructive pulmonary disease and asthma.

Cytokine/neurotrophin interaction in the aged central nervous system

Age-associated neurodegenerative diseases such as Alzheimer's disease are characterised by neuronal impairment that leads to cognitive deficits. As certain affected neurons depend on trophic factors such as neurotrophins (NTs), impairment in NT function has been suggested to be a component of neuronal damage associated with such disorders. Age-related neurodegenerative diseases are also characterised by high levels of proinflammatory cytokines such as tumour necrosis factor alpha (TNFalpha) in the CNS. Because TNFalpha receptors and certain NT receptors share a high degree of homology and are capable of activating similar signalling pathways, one possibility is that altered cytokine levels may affect NT function in the aged or diseased CNS. Here we wish briefly to review the evidence suggesting a role for cytokine and NT in the onset of age-associated neurodegenerative diseases. We propose that cytokine/NT interactions may alter neuronal homeostasis, thus possibly contributing to some of the neuronal degeneration occurring during such age-associated CNS diseases.

Unmodified calcium concentrations in tumour necrosis factor receptor subtype-mediated apoptotic cell death

Tumour necrosis factor-alpha (TNF) receptors mediate a variety of effects dependent on cell type. A role for Ca2+ in TNF-induced death remains uncertain. Here we investigated restricting intracellular/extracellular Ca2+ in HeLa epithelial carcinoma cells expressing low and high levels of p75TNFR receptor subtype and KYM-1 rhabdomyosarcoma cells, models of rapid TNF-induced apoptosis. Ca2+ -chelators EGTA and BAPTA-AM as well as microsomal Ca2+ -ATPase inhibitor thapsigargin, did not alter TNF-induced death. TNF was also unable to alter resting [Ca2+]i levels which remained < 200 nM even during times when these cells were undergoing apoptotic cell death. These findings indicate no role for modulated Ca2+ concentrations in TNF-induced apoptotic cell death.

Glycerophospholipids in brain: their metabolism, incorporation into membranes, functions, and involvement in neurological disorders

Neural membranes contain several classes of glycerophospholipids which turnover at different rates with respect to their structure and localization in different cells and membranes. The glycerophospholipid composition of neural membranes greatly alters their functional efficacy. The length of glycerophospholipid acyl chain and the degree of saturation are important determinants of many membrane characteristics including the formation of lateral domains that are rich in polyunsaturated fatty acids. Receptor-mediated degradation of glycerophospholipids by phospholipases A(l), A(2), C, and D results in generation of second messengers such as arachidonic acid, eicosanoids, platelet activating factor and diacylglycerol. Thus, neural membrane phospholipids are a reservoir for second messengers. They are also involved in apoptosis, modulation of activities of transporters, and membrane-bound enzymes. Marked alterations in neural membrane glycerophospholipid composition have been reported to occur in neurological disorders. These alterations result in changes in membrane fluidity and permeability. These processes along with the accumulation of lipid peroxides and compromised energy metabolism may be responsible for the neurodegeneration observed in neurological disorders.

Suppression of acute experimental inflammation by antisense oligonucleotides targeting secretory phospholipase A2 (sPLA2) in vitro and in vivo experiments

In HepG2 cells phosphorothioate modified antisense oligonucleotides against a sequence in the Ca2+ binding domain (AS-Ca2+) of type II sPLA2 mRNA restrained IL-6-induced synthesis of sPLA2 protein, sPLA2 mRNA (northern blot), and abolished IL-6 stimulated PGE2 release. An antisense oligonucleotide corresponding to a sequence in the catalytic domain (AS-Cat) of sPLA2 was less effective. The antisense oligonucleotides did not affect albumin synthesis in HepG2 cells, additionally demonstrating their specificity. The corresponding AS-Ca2+ against a homologous part of the rat sPLA2 mRNA depressed rat carrageenin oedema for 60-70%. Identical suppression was achieved by specific low molecular weight inhibitors of sPLA2. Since cyclo- and 5-lipoxygenase inhibitors exerted similar reductions of carrageenin oedema type II sPLA2 dependent eicosanoid formation seems to be a key cascade in this type of inflammation.

Selective down-regulation of the G(q)alpha/G11alpha G-protein family in tumour necrosis factor-alpha induced cell death

Investigations into the regulation of heterotrimeric GTP-binding protein alpha-subunits in models of tumour necrosis factor-alpha (TNF)-induced cell death, revealed the selective down-regulation of the G(q)alpha/G11alpha family of G-proteins. The human HeLa and murine L929 cells treated with recombinant human TNF for up to 24 h displayed down-regulated G(q)alpha/G11alpha family protein levels, but not G(s)alpha, G(i)alpha and G(o)alpha protein levels as determined by Western analyses. This effect of TNF was observed in a concentration--and time-dependent manner, consistent with the profiles of TNF-induced cell death observed. Moreover, the functioning of G(q)alpha/G11alpha family proteins were found to be impaired in TNF-treated cells, as measured by agonist-induced [Ca2+]i release. In contrast, G(s)alpha activity was unaltered by TNF-treatment, determined by measurement of agonist-induced intracellular cyclic AMP generation. These findings in TNF-induced cytotoxic models, indicate a novel 'cross-talk' mechanism by which TNF alters Ca2+-signalling mechanisms, which may contribute towards the apoptotic and necrotic cell death.

Cytosolic phospholipase A2 is induced in reactive glia following different forms of neurodegeneration

Many recent studies have emphasized the deleterious role of inflammation in CNS injury. Increases in free fatty acids, eicosanoids, and products of lipid peroxidation are known to occur in various conditions of acute and chronic CNS injury, including cerebral ischemia, traumatic brain injury, and Alzheimer's disease. Although an inflammatory response can be induced by many different means, phospholipases, such as cytosolic phospholipase A(2) (cPLA(2)), may play an important role in the production of inflammatory mediators and in the production of other potential second messengers. cPLA(2) hydrolyzes membrane phospholipids and its activity liberates free fatty acids leading directly to the production of eicosanoids. We investigated the cellular localization of cytosolic phospholipase A(2) in the CNS following: (1) focal and global cerebral ischemia, (2) facial nerve axotomy, (3) human cases of Alzheimer's disease, (4) transgenic mice overexpressing mutant superoxide dismutase, a mouse model of amyotrophic lateral sclerosis, and (5) transgenic mice overexpressing mutant amyloid precursor protein, which exhibits age-related amyloid deposition characteristic of Alzheimer's disease. We show that in every condition evaluated, cytosolic phospholipase A(2) is present in reactive glial cells within the precise region of neuron loss. In conditions where neurons did not degenerate or are protected from death, cytosolic phospholipase A(2) is not observed. Both astrocytes and microglial cells are immunoreactive for cytosolic phospholipase A(2) following injury, with astrocytes being the most consistent cell type expressing cytosolic phospholipase A(2). The presence of cytosolic phospholipase A(2) does not merely overlap with reactive astroglia, as reactive astrocytes were observed that did not exhibit cytosolic phospholipase A(2) immunoreactivity. In most conditions evaluated, inflammatory processes have been postulated to play a pivotal role and may even participate in neuronal cell death. These results suggest that cytosolic phospholipase A(2) may prove an attractive therapeutic target for neurodegeneration.

Tumour necrosis factor-alpha activates a calcium sensitization pathway in guinea-pig bronchial smooth muscle

1. The effects of tumour necrosis factor-alpha (TNF) on guinea-pig bronchial smooth muscle contractility were investigated. 2. The Ca2+-activated contractile response of permeabilized bronchial smooth muscle strips was significantly increased after incubation with 1 microgram ml-1 TNF for 45 min. This TNF-induced effect was not due to a further increase in intracellular Ca2+. 3. The TNF-induced Ca2+ sensitization was, at least partly, the result of an increase in myosin light chain20 phosphorylation. 4. The intracellular signalling pathway involved in this effect of TNF was further investigated. Sphingomyelinase, a potential mediator of TNF, had no effect on Ca2+ sensitivity of permeabilized bronchial smooth muscle. Also, p42/p44 mitogen-activated protein kinase (p42/p44mapk), activated by TNF in some cell types, did not show an increased activation in bronchial smooth muscle after TNF treatment. 5. In conclusion, TNF may activate a novel signalling pathway in guinea-pig bronchial smooth muscle leading to an increase in myosin light chain20 phosphorylation and a subsequent increase in Ca2+ sensitivity of the myofilaments. This pathway does not appear to involve sphingomyelinase-liberated ceramides or activation of p42/p44mapk. Given the importance of TNF in asthma, this TNF-induced Ca2+ sensitization of the myofilaments may represent a mechanism responsible for airway hyper-responsiveness.

Perturbations in the control of cellular arachidonic acid levels block cell growth and induce apoptosis in HL-60 cells

Our previous studies demonstrated that inhibitors of arachidonate-phospholipid remodeling [i.e. the enzyme CoA-independent transacylase (CoA-IT)] decrease cell proliferation and induce apoptosis in neoplastic cells. The goal of the current study was to elucidate the molecular events associated with arachidonate-phospholipid remodeling that influence cell proliferation and survival. Initial experiments revealed the essential nature of cellular arachidonate to the signaling process by demonstrating that HL-60 cells depleted of arachidonate were more resistant to apoptosis induced by CoA-IT inhibition. In cells treated with CoA-IT inhibitors a marked increase in free arachidonic acid and AA-containing triglycerides were measured. TG enrichment was likely due to acylation of arachidonic acid into diglycerides and triglycerides via de novo glycerolipid biosynthesis. To determine the potential of free fatty acids to affect cell proliferation, HL-60 cells were incubated with varying concentrations of free fatty acids; exogenously provided 20-carbon polyunsaturated fatty acids caused a dose-dependent inhibition of cell proliferation, whereas oleic acid was without effect. Blocking 5-lipoxygenase or cyclooxygenases had no effect on the inhibition of cell proliferation induced by arachidonic acid or CoA-IT inhibitors. An increase in cell-associated ceramides (mainly in the 16:0-ceramide fraction) was measured in cells exposed to free arachidonic acid or to CoA-IT inhibitors. This study, in conjunction with other recent studies, suggests that perturbations in the control of cellular arachidonic acid levels affect cell proliferation and survival.

Upregulation of cytosolic phospholipase A2 correlates with apoptosis in mouse superior cervical and dorsal root ganglia neurons

The involvement of cytosolic phospholipase A2 (cPLA2) in apoptosis of adult mouse superior cervical and dorsal root ganglia neurons has been investigated by the use of immunohistochemistry for cPLA2 and DNA nick-end labeling for apoptotic cells, respectively. cPLA2 immunoreactivity was strongly upregulated in neurons of both preparations during in vitro culturing. By double labeling it was unequivocally demonstrated that cPLA2 was present and upregulated only in neurons undergoing apoptosis. A similar picture emerged when cPLA2 immunoreactivity was compared with staining with Fluoro-Jade, a novel fluorochrome marker for neuronal degeneration. The preferential presence of cPLA2 in apoptotic and degenerating cells suggests that the enzyme is important for some mechanism involved in or intimately coupled to neuronal cell death.

Tumor necrosis factor-alpha mediates both apoptotic cell death and cell proliferation in a human hematopoietic cell line dependent on mitotic activity and receptor subtype expression

The TF-1 human erythroleukemic cell line exhibits opposing physiological responses toward tumor necrosis factor-alpha (TNF) treatment, dependent upon the mitotic state of the cells. Mitotically active cells in log growth respond to TNF by rapidly undergoing apoptosis whereas TNF exposure stimulates cellular proliferation in mitotically quiescent cells. The concentration-dependent TNF-induced apoptosis was monitored by cellular metabolic activity and confirmed by both DNA epifluorescence and DNA fragmentation. Moreover, these responses could be detected by measuring extracellular acidification activity, enabling rapid prediction (within approximately 1.5 h of TNF treatment) of the fate of the cell in response to TNF. Growth factor resupplementation of quiescent cells, resulting in reactivation of cell cycling, altered TNF action from a proliferative stimulus to an apoptotic signal. Expression levels of the type II TNF receptor subtype (p75TNFR) were found to correlate with sensitivity to TNF-induced apoptosis. Pretreatment of log growth TF-1 cells with a neutralizing anti-p75TNFR monoclonal antibody inhibited TNF-induced apoptosis by greater than 80%. Studies utilizing TNF receptor subtype-specific TNF mutants and neutralizing antisera implicated p75TNFR in TNF-dependent apoptotic signaling. These data show a bifunctional physiological role for TNF in TF-1 cells that is dependent on mitotic activity and controlled by the p75TNFR.

Both TNF receptors are required for direct TNF-mediated cytotoxicity in microvascular endothelial cells

The conditions under which tumor necrosis factor-alpha (TNF) induces apoptosis in primary microvascular endothelial cells (MVEC) were investigated. In the absence of sensitizing agents, TNF induced apoptosis after 3 days of incubation in confluent MVEC. In contrast, upon addition of the transcriptional inhibitor actinomycin D (Act. D), confluence was no longer required and apoptosis occurred already after 16 h. To assess the role of either TNF receptor (TNFR) type in apoptosis, MVEC isolated from mice genetically deficient in TNFR1 (Tnfr1o mice) or TNFR2 (Tnfr2o mice) were incubated with TNF in the presence or absence of Act. D. Under sensitized conditions, Tnfr2o MVEC were lysed like controls, whereas Tnfr1o MVEC were completely resistant, indicating an exclusive role for TNFR1. In contrast, in the absence of Act. D, confluent monolayers of wild-type cells were lysed by TNF, but both Tnfr1o and Tnfr2o MVEC were resistant to TNF-mediated toxicity, indicating a requirement for both TNFR types. Overexpression of the anti-apoptotic protein bcl-xL in MVEC led to a protection against the direct, but not the sensitized cytotoxicity of TNF. In conclusion, in pathophysiologically relevant conditions, both TNFR appear to be required for TNF-induced apoptosis in MVEC.

Apoptosis and necrosis in toxicology: a continuum or distinct modes of cell death?

Mounting evidence indicates that apoptosis rather than necrosis predominates in many cytolethal toxic injuries. Associated cell death models of apoptosis and necrosis are either: (1) totally separate death modes, (2) a continuum whereby they are extremes of biochemically overlapping death pathways, or (3) essentially distinct processes with only limited molecular and cell biology overlap. We conclude that the current balance of evidence favours the third of these options. The established axiom that, even when considering the same toxicant, injury amplitude (dose) is a primary determinant of whether cells die via active cell death (apoptosis) or failure of homeostasis (necrosis) remains valid. Tissue selectivity of toxicants can stem from the apoptotic or necrotic thresholds at which different cells die, as well as targeting factors such as toxicokinetics, receptor recognition, bioactivation, and cell-specific lesions.

Circulating tumor necrosis factor-alpha correlates with electrodiagnostic abnormalities in Guillain-Barré syndrome

Autoimmune damage to peripheral nerves, mediated by activated T lymphocytes and macrophages, underlies the pathogenesis of inflammatory demyelination in Guillain-Barré syndrome. Both T lymphocytes and macrophages secrete tumor necrosis factor-alpha, a cytokine that exerts toxic effects on myelin, Schwann cells, and endothelial cells. The reportedly high serum levels of this cytokine in patients with Guillain-Barré syndrome may reflect the degree of immune activation rather than a direct pathogenic effect. We compared serum levels of tumor necrosis factor-alpha, interleukin-1 beta, and soluble interleukin-2 receptor with well-established electrodiagnostic criteria for primary demyelination in 23 patients with Guillain-Barré syndrome, to assess the relationship between these cytokines and peripheral myelin damage. High serum levels of tumor necrosis factor-alpha were associated with prolonged distal motor latencies and slowed motor conduction velocities, prolonged or absent F-wave responses, and reduced amplitude of distal compound muscle action potentials. No significant correlation was observed between electrodiagnostic criteria for primary demyelination and serum levels of interleukin-1 beta or soluble interleukin-2 receptor. These findings suggest a putative role of tumor necrosis factor-alpha in the pathogenesis of peripheral nerve demyelination in Guillain-Barré syndrome.

Cytosolic phospholipase A2 (PLA2), but not secretory PLA2, potentiates hydrogen peroxide cytotoxicity in kidney epithelial cells

Phospholipase A2 (PLA2) and reactive oxygen species have been implicated both individually and synergistically in various forms of cellular injury. The form(s) of PLA2 important for cell injury and the implications of enhanced activity of the enzyme, however, have not been discerned. Previous studies reveal an increase in PLA2 activity associated with cell injury, but this association does not establish a causal relationship between the increase in activity and the injury. LLC-PK1 cell lines were created that express either the cytosolic PLA2 or a group II PLA2. The susceptibility of these cells to hydrogen peroxide toxicity was determined in order to evaluate the relative importance of these two forms of PLA2 in oxidant injury. Expression of cytosolic PLA2 in the LLC-cPLA2 cell line was associated with a 50-fold increase in PLA2 activity in the cytosolic fraction, an increase in agonist-stimulated arachidonate release, and immunodetection of the cytosolic PLA2 protein that was undetectable in control cells. Exposure to hydrogen peroxide or menadione, but not mercuric chloride, resulted in significantly greater lactate dehydrogenase release in LLC-cPLA2 cells when compared with control cells. Exogenous arachidonic acid (150 microM) did not enhance hydrogen peroxide-induced injury. The intracellular calcium chelator, 1,2-bis-(o-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid/tetra(acetoxymethyl) ester, protected the cells against injury, but the calcium ionophore, A23187, did not increase injury. Glycine conferred no protective effect against hydrogen peroxide toxicity. By contrast to these results with cytosolic PLA2-expressing cells, secretory PLA2 expression to very high levels did not increase susceptibility to hydrogen peroxide. Thus, cytosolic PLA2 may an be an important mediator of oxidant damage to renal epithelial cells.