Tumor necrosis factor cytotoxicity is associated with phospholipase D activation

The triangle of death of neurons: Oxidative damage, mitochondrial dysfunction, and loss of choline-containing biomolecules in brains of mice treated with doxorubicin. Advanced insights into mechanisms of chemotherapy induced cognitive impairment ("chemobrain") involving TNF-α

Cancer treatments are developing fast and the number of cancer survivors could arise to 20 million in United State by 2025. However, a large fraction of cancer survivors demonstrate cognitive dysfunction and associated decreased quality of life both shortly, and often long-term, after chemotherapy treatment. The etiologies of chemotherapy induced cognitive impairment (CICI) are complicated, made more so by the fact that many anti-cancer drugs cannot cross the blood-brain barrier (BBB). Multiple related factors and confounders lead to difficulties in determining the underlying mechanisms. Chemotherapy induced, oxidative stress-mediated tumor necrosis factor-alpha (TNF-α) elevation was considered as one of the main candidate mechanisms underlying CICI. Doxorubicin (Dox) is a prototypical reactive oxygen species (ROS)-generating chemotherapeutic agent used to treat solid tumors and lymphomas as part of multi-drug chemotherapeutic regimens. We previously reported that peripheral Dox-administration leads to plasma protein damage and elevation of TNF-α in plasma and brain of mice. In the present study, we used TNF-α null (TNFKO) mice to investigate the role of TNF-α in Dox-induced, oxidative stress-mediated alterations in brain. We report that Dox-induced oxidative stress in brain is ameliorated and brain mitochondrial function assessed by the Seahorse-determined oxygen consumption rate (OCR) is preserved in brains of TNFKO mice. Further, we show that Dox-decreased the level of hippocampal choline-containing compounds and brain phospholipases activity are partially protected in TNFKO group in MRS study. Our results provide strong evidence that Dox-targeted mitochondrial damage and levels of brain choline-containing metabolites, as well as phospholipases changes decreased in the CNS are associated with oxidative stress mediated by TNF-α. These results are consistent with the notion that oxidative stress and elevated TNF-α in brain underlie the damage to mitochondria and other pathological changes that lead to CICI. The results are discussed with reference to our identifying a potential therapeutic target to protect against cognitive problems after chemotherapy.

Lack of alpha-synuclein modulates microglial phenotype in vitro

Alpha (α)-synuclein neuronal effects are continually being defined although its role in regulating glial phenotypes remains unclear. An ability to regulate microglial activation was investigated using primary cultures from wild type and α-synuclein deficient mice (Snca-/-). Snca-/- microglia demonstrated increased secretion of the cytokine tumor necrosis factor-alpha (TNF-α), impaired phagocytic ability, elevated prostaglandin levels, and increased protein levels of key enzymes in lipid-mediated signaling events, cytosolic phospholipase (cPLA(2)), cyclooxygenase-2 (Cox-2) and phospholipase D2 (PLD2) when compared to wild type cells. Increased cytokine secretion and cPLA(2) and Cox-2 levels in Snca-/- microglia were partially attenuated by inhibiting PLD-dependent signaling with n-butanol treatment.

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.

Phospholipase D1 plays a key role in TNF-alpha signaling

The primary characteristic features of any inflammatory or infectious lesions are immune cell infiltration, cellular proliferation, and the generation of proinflammatory mediators. TNF-alpha is a potent proinflammatory and immuno-regulatory cytokine. Decades of research have been focused on the physiological/pathophysiological events triggered by TNF-alpha. However, the signaling network initiated by TNF-alpha in human leukocytes is still poorly understood. In this study, we report that TNF-alpha activates phospholipase D1 (PLD1), in a dose-dependent manner, and PLD1 is required for the activation of sphingosine kinase and cytosolic calcium signals. PLD1 is also required for NFkappaB and ERK1/2 activation in human monocytic cells. Using antisense oligonucleotides to reduce specifically the expression of PLD isozymes showed PLD1, but not PLD2, to be coupled to TNF-alpha signaling and that PLD1 is required to mediate receptor activation of sphingosine kinase and calcium transients. In addition, the coupling of TNF-alpha to activation of the phosphorylation of ERK1/2 and the activation of NFkappaB were inhibited by pretreating cells with antisense to PLD1, but not to PLD2; thus, demonstrating a specific requirement for PLD1. Furthermore, use of antisense oligonucleotides to reduce expression of PLD1 or PLD2 demonstrated that PLD1 is required for TNF-alpha-induced production of several important cytokines, such as IL-1beta, IL-5, IL-6, and IL-13, in human monocytes. These studies demonstrate the critical role of PLD1 in the intracellular signaling cascades initiated by TNF-alpha and its functional role for coordinating the signals to inflammatory responses.

Brain prostaglandin formation is increased by alpha-synuclein gene-ablation during global ischemia

We have previously demonstrated that alpha-synuclein (Snca) gene ablation reduces brain arachidonic acid (20:4n-6) turnover rate in phospholipids through modulation of endoplasmic reticulum-localized acyl-CoA synthetase activity. Although 20:4n-6 is a precursor for prostaglandin (PG), Snca effect on PG levels is unknown. In the present study, we examined the effect of Snca ablation on brain PG level at basal conditions and following 30s of global ischemia. Brain PG were extracted with methanol, purified on C(18) cartridges, and analyzed by LC-MS/MS. We demonstrate, for the first time, that Snca gene ablation did not affect brain PG mass under normal physiological conditions. However, total PG mass and masses of individual PG were elevated approximately 2-fold upon global ischemia in the absence of Snca. These data are consistent with our previously observed reduction in 20:4n-6 recycling through endoplasmic reticulum-localized acyl-CoA synthetase in the absence of Snca, which may result in the increased 20:4n-6 availability for PG production in the absence of Snca during global ischemia and suggest a role for Snca in brain inflammatory response.

Alpha-synuclein expression modulates microglial activation phenotype

Recent Parkinson's disease research has focused on understanding the function of the cytosolic protein, alpha-synuclein, and its contribution to disease mechanisms. Within neurons, alpha-synuclein is hypothesized to have a role in regulating synaptic plasticity, vesicle release, and trafficking. In contrast, glial-expressed alpha-synuclein remains poorly described. Here, we examine the consequence of a loss of alpha-synuclein expression on microglial activation. Using a postnatal brain-derived culture system, we defined the phenotype of microglia from wild-type and knock-out alpha-synuclein mice (Scna-/-). Scna-/- microglia displayed a basally increased reactive phenotype compared with the wild-type cells and an exacerbated reactive phenotype after stimulation. They also exhibited dramatic morphologic differences compared with wild-type, presenting as large, ramified cells filled with vacuole-like structures. This corresponded with increased protein levels of activation markers, CD68 and beta1 integrin, in the Scna-/- cells. More importantly, Scna-/- microglia, after stimulation, secreted elevated levels of proinflammatory cytokines, TNFalpha (tumor necrosis factor alpha) and IL-6 (interleukin-6), compared with wild type. However, despite the reactive phenotype, Scna-/- cells had impaired phagocytic ability. We demonstrate for the first time that alpha-synuclein plays a critical role in modulating microglial activation state. We suggest that altered microglial alpha-synuclein expression will affect their phenotype as has already been demonstrated in neurons. This has direct ramifications for the contribution of microglia to the pathophysiology of disease, particularly in familial cases linked to altered alpha-synuclein expression.

Phosphatidylcholine-derived phosphatidic acid and diacylglycerol are involved in the signaling pathways activated by docetaxel

Taxanes are known to activate several cellular signals including mitogen-activated protein kinase (MAPK) and nuclear factor kappa B (NF-kappa B), tyrosine phosphorylation of Shc, and serine phosphorylation of Bcl-2. However, the mediators of these signaling pathways are unknown. Using U937 leukemic cells, we evaluated the effect of docetaxel on phosphatidylcholine (PC) and its metabolites, phosphatidic acid (PA) and diacylglycerol (DAG), and their impact on MAPK and NF-kappa B activation, as well as on Raf-1 and Bcl-2 phosphorylation. Metabolic labeling studies showed that docetaxel (10 nM) induced two waves of PA production (130-140%), which were detected at 1 and 10 min. Docetaxel also stimulated DAG production (130%), which followed the first PA wave. The initial PA burst was due to phospholipase D (PLD)-mediated PC hydrolysis. Subsequent DAG production was inhibited by the phosphatidate phosphohydrolase (PAP) inhibitor, propranolol. R59949, a DAG kinase inhibitor, increased DAG accumulation and blocked the second PA wave. These results suggest that docetaxel triggers a metabolic cascade consisting in PLD-mediated PC hydrolysis, PA release, PAP-dependent DAG production, and DAG kinase stimulation, leading to DAG conversion back to PA. Neither R59949 nor propranolol influenced docetaxel-induced Raf-1/ERK activation. However, R59949 abrogated both NF-kappa B activation and Bcl-2 phosphorylation, suggesting that DAG and/or DAG-derived PA contribute in regulating these events.

PLD pathway involved in carbachol-induced Cl- secretion: possible role of TNF-alpha

In a previous study, it was found that exposure to tumor necrosis factor-alpha (TNF-alpha) potentiated the electrophysiological response to carbachol in a time-dependent and cycloheximide-sensitive manner. It was deduced that the potentiation could be due to protein kinase C activity because of increased 1,2-diacylglycerol. It was also observed that propranolol could decrease the electrophysiological response to carbachol (Oprins JC, Meijer HP, and Groot JA. Am J Physiol Cell Physiol 278: C463-C472, 2000). The aim of the present study was to investigate whether the phospholipase D (PLD) pathway plays a role in the carbachol response and the potentiating effect of TNF-alpha. The transphosphatidylation reaction in the presence of the primary alcohol 1-butanol [leading to stable phosphatidylbutanol (Pbut) formation] was used to measure activity of PLD. The phosphatidic acid (PA) levels were also measured. Muscarinic stimulation resulted in an increased formation of Pbut and PA. TNF-alpha decreased levels of PA.

Tumor necrosis factor receptor and Fas signaling mechanisms

Four members of the tumor necrosis factor (TNF) ligand family, TNF-alpha, LT-alpha, LT-beta, and LIGHT, interact with four receptors of the TNF/nerve growth factor family, the p55 TNF receptor (CD120a), the p75 TNF receptor (CD120b), the lymphotoxin beta receptor (LT beta R), and herpes virus entry mediator (HVEM) to control a wide range of innate and adaptive immune response functions. Of these, the most thoroughly studied are cell death induction and regulation of the inflammatory process. Fas/Apo1 (CD95), a receptor of the TNF receptor family activated by a distinct ligand, induces death in cells through mechanisms shared with CD120a. The last four years have seen a proliferation in knowledge of the proteins participating in the signaling by the TNF system and CD95. The downstream signaling molecules identified so far--caspases, phospholipases, the three known mitogen activated protein (MAP) kinase pathways, and the NF-kappa B activation cascade--mediate the effects of other inducers as well. However, the molecules that initiate these signaling events, including the death domain- and TNF receptor associated factor (TRAF) domain-containing adapter proteins and the signaling enzymes associated with them, are largely unique to the TNF/nerve growth factor receptor family.

Amyloid beta-induced neurotoxicity is associated with phospholipase D activation in cultured rat hippocampal cells

The role of phospholipase D (PLD) in amyloid beta (Abeta)-induced neurotoxicity was studied by comparing the effects of Abeta (1-40) on PLD activity and release of lactate dehydrogenase (LDH) from cultured rat hippocampal cells. PLD activity was determined in [3H]myristic acid-labeled cells by measuring the formation of [3H]phosphatidylethanol in the presence of ethanol (0.5%), and LDH activity in the cell media was measured via colorimetric assay. Abeta (50 microM), aged for 3 days to allow for peptide aggregation, acutely (1 h) stimulated PLD activity. Unaged Abeta (50 microM) had no acute (1 h) effect on PLD activity, but significantly stimulated PLD activity by 87% when incubated with cells for 1-3 days. Abeta (50 microM)-induced PLD activity was closely correlated with Abeta (50 microM)-induced LDH release over a time course of 1-3 days. These data suggest that PLD activation may be involved in Abeta-induced neurotoxicity.

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)

Molecular mechanisms of tumor necrosis factor-induced cytotoxicity. What we do understand and what we do not

Although TNF plays an important role in several physiological and pathological conditions, the hallmark of this important cytokine has been its selective cytotoxic activity on tumor cells. Since its cloning in 1984, understanding of how TNF selectively kills tumor cells has been the subject of research in many laboratories. Here we review TNF-induced post-receptor signaling mechanisms which seem to be involved in the pathway to cytotoxicity.