Suppression of nerve growth factor-induced neuronal differentiation of PC12 cells. N-acetylcysteine uncouples the signal transduction from ras to the mitogen-activated protein kinase cascade

Growth inhibitory effect of green tea extract and (-)-epigallocatechin in Ehrlich ascites tumor cells involves a cellular thiol-dependent activation of mitogenic-activated protein kinases

The effect of green tea extract (GTE) in Ehrlich ascites tumor cells (EATC) was studied with respect to changes in the intracellular kinase system involving mitogen-activated protein kinases (MAPKs) and cellular thiol. We have previously shown a reduction in viability of EATC and tyrosine phosphorylation of 42 and 45 kDa proteins by GTE and its polyphenolic component, Epigallocatechin (EGC) (D.O. Kennedy, S. Nishimura, T. Hasuma, Y. Yoshihisa, S. Otani, I. Matsui-Yuasa, Involvement of protein tyrosine phosphorylation in the effect of green tea polyphenols on Ehrlich ascites tumor cells in vitro, Chem. Biol. Interact. 110 (1998) 159-172). Furthermore, GTE and EGC significantly decreased both cellular non-protein and protein sulfhydryl levels in EATC, but replenishing thiol stores with N-acetylcysteine (NAC) caused a recovery in cell viability, and therefore SH groups were identified as a novel target of green tea cytotoxicity (D.O. Kennedy, M. Matsumoto, A. Kojima, I. Matsui-Yuasa, Cellular thiol status and cell death in the effect of green tea polyphenols in Ehrlich ascites tumor cells, Chem. Biol. Interact. 122 (1999) 59-71). In this study, we have observed the stimulation of three forms of MAPK, namely ERK1/2, JNK/SAPK and p38, by EGC, which were dose and time-dependent. These MAPK stimulations were found to be cellular thiol status-dependent events as NAC reversed these stimulations. Furthermore, inhibition of the p38 MAPK pathway using the p38 inhibitor SB203580 caused a marked dose-dependent reduction in the decrease in cell viability caused by EGC treatment. Inhibiting the Erk1/2 MAPK pathway using the MEK inhibitor PD098059 caused a slight change in the decrease in cell viability by EGC. These may suggest that the cytotoxicity associated with EGC was more associated with the other MAPKs than with ERK1/2. This may be the first study of its kind providing a novel evidence of a role for different forms of MAPKs in the antitumor effect of green tea polyphenols, especially EGC, in Ehrlich ascites tumor cells.

Elevated superoxide production by active H-ras enhances human lung WI-38VA-13 cell proliferation, migration and resistance to TNF-alpha

Accumulating evidence has suggested that cellular production of superoxide acts as an intracellular messenger to regulate gene expression and modulate cellular activities. In this report, we set out to investigate the role of active H-ras-mediated superoxide production on tumor cell malignancy in a SV-40 transformed human lung WI-38 VA-13 cell line. Stable transfection and expression of constitutively active mutant V12-H-ras (V12-H-ras) dramatically increased intracellular production of superoxide. The expression of V12-H-ras significantly enhanced cell proliferation, migration and resistance to TNF-alpha treatment compared to that of parental and vector control cells, while expression of wild type H-ras (WT-H-ras) only had modest effects. Upon scavenging by superoxide dismutase and other molecules that decrease the intracellular level of active H-ras mediated superoxide production, cell proliferation, migration and resistance to TNF-alpha were significantly reduced. Furthermore, we demonstrated that the activation of membrane NADPH oxidase activity by expression of active H-ras contributed to the intracellular superoxide production. The causal relationship between membrane superoxide production and increased cell proliferation, migration, and resistance to TNF-alpha by the expression of active H-ras, has provided direct evidence to demonstrate that superoxide acts as an intracellular messenger to cascade ras oncogenic signal relay and to modulate tumor malignant activity.

Inhibitory effect of 2'-O-benzoylcinnamaldehyde on vascular endothelial cell proliferation and migration

PURPOSE

To evaluate the inhibitory effect of the farnesyl transferase inhibitor 2'-O-benzoylcinnamaldehyde (CB 2'-ph) on proliferation and migration of vascular endothelial cells.

METHODS

Bovine lens epithelial cells, bovine corneal endothelial cells, bovine keratocytes, bovine aortic endothelial cells (BAECs) and human umbilical vein endothelial cells (HUVECs) were treated with CB 2'-ph to determine its cell type specificity and antiproliferative effect. For inhibition of vascular endothelial cell growth factor (VEGF)- or basic fibroblast growth factor (bFGF)-induced proliferation of HUVECs, these cells were treated with various concentrations of CB 2'-ph. To assess the proliferation, MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl-tetrazolium bromide) assay was used. The migration assay was also performed to determine the effect of CB 2'-ph on HUVECs. The distance of HUVEC outgrowth was measured from the scraped edge of a monolayer after treatment with CB 2'-ph concentrations of 0, 1.5 and 2.5 microg/ml for 24, 48 and 72 h.

RESULTS

The CB 2'-ph had an inhibitory effect on all tested types of cell proliferation but only HUVEC and BAEC proliferation was specifically inhibited in a dose-dependent manner. In addition, CB 2'-ph inhibited VEGF- or bFGF-induced proliferation and migration of HUVECs in a dose-dependent manner.

CONCLUSIONS

These results indicate that CB 2'-ph, a farnesyl transferase inhibitor is thought to be an effective inhibitor of vascular endothelial cell proliferation and migration.

Does the developmental neurotoxicity of chlorpyrifos involve glial targets? Macromolecule synthesis, adenylyl cyclase signaling, nuclear transcription factors, and formation of reactive oxygen in C6 glioma cells

The widespread use of chlorpyrifos (CPF) has raised major concerns about its potential to cause fetal or neonatal neurobehavioral damage, even at doses that do not evoke acute toxicity. CPF has been shown to inhibit replication of brain cells, to elicit alterations in neurotrophic signaling governing cell differentiation and apoptosis, and to evoke oxidative stress. However, the specific cell types targeted by CPF have not been clarified, an issue of vital importance in establishing the boundaries of the critical period in which the developing brain is vulnerable. In the current study, we evaluated the effects of CPF on C6 glioma cells, a well-established glial model. In undifferentiated C6 cells, CPF inhibited DNA synthesis in a concentration-dependent manner, with greater potency than had been seen previously with neuronal cell lines. Just as found after in vivo CPF treatment or with neuronal cell lines, the effects on cell replication were independent of cholinergic stimulation, as cholinergic antagonists did not block CPF-induced inhibition. CPF interfered with cell signaling mediated through adenylyl cyclase at the level of G-protein function; the effects again were greater in undifferentiated C6 cells but were still detectable in differentiating cells. In contrast, differentiation enhanced the ability of CPF to elicit the formation of reactive oxygen species and to evoke deficits in Sp1, a nuclear transcription factor essential for differentiation. These results indicate that glial-type cells are targeted by CPF through the same multiple mechanisms that have been demonstrated for the effects of CPF on brain development in vivo. Because glial development continues long after the conclusion of neurogenesis, and given that CPF targets events in both glial cell replication and the later stages of differentiation, the vulnerable period for developmental neurotoxicity of CPF is likely to extend well into childhood.

Manganese induces neurite outgrowth in PC12 cells via upregulation of alpha(v) integrins

Previous studies have demonstrated that the divalent cation manganese (Mn) causes PC12 cells to form neurites in the absence of NGF. Since divalent cations modulate the binding affinity and specificity of integrins, and integrin function affects neurite outgrowth, we tested the hypothesis that Mn induces neurite outgrowth through an integrin-dependent signaling pathway. Our studies support this hypothesis. Function-blocking antisera specific for beta(1) integrins block the neurite-promoting activity of Mn by 90-95%. Bioassays and biochemical studies with antisera specific for the alpha(v), alpha(5), or alpha(8) integrin subunit suggest that the alpha(v)beta(1) heterodimer is one of the principal beta(1) integrins mediating the response of PC12 cells to Mn. This is corroborated by studies in which Mn failed to induce neurite outgrowth in a clone of PC12 cells that does not express alpha(v) at levels detectable by immunoprecipitation or immunocytochemistry. SDS-PAGE analysis of biotinylated surface proteins immunoprecipitated from Mn-responsive PC12 cells, as well as confocal laser microscopy of PC12 immunostained for surface alpha(v) indicate that Mn increases the surface expression of alpha(v) integrins. This increase appears to be due in part to synthesis of alpha(v) since specific inhibitors of RNA and protein synthesis block the neurite-promoting activity of Mn. These data indicate that Mn induces neurite outgrowth in PC12 cells by upregulating alpha(v) integrins, suggesting that Mn potentially represents an additional mechanism for regulating the rate and direction of neurite outgrowth during development and regeneration.

Nerve growth factor-induced neuronal differentiation requires generation of Rac1-regulated reactive oxygen species

Nerve growth factor (NGF) stimulation of pheochromocytoma PC12 cells transiently increased the intracellular concentration of reactive oxygen species (ROS). This increase was blocked by the chemical antioxidant N-acetylcysteine and a flavoprotein inhibitor, diphenylene iodonium. NGF responses of PC12 cells, including neurite outgrowth, tyrosine phosphorylation, and AP-1 activation, was inhibited when ROS production was prevented by N-acetylcysteine and diphenylene iodonium. The expression of dominant negative Rac1N17 blocked induction of both ROS generation and morphological differentiation by NGF. The ROS produced appears to be H(2)O(2), because the introduction of catalase into the cells abolished NGF-induced neurite outgrowth, ROS production, and tyrosine phosphorylation. These results suggest that the ROS, perhaps H(2)O(2), acts as an intracellular signal mediator for NGF-induced neuronal differentiation and that NGF-stimulated ROS production is regulated by Rac1 and a flavoprotein-binding protein similar to the phagocytic NADPH oxidase.

N-acetylcysteine suppresses TNF-induced NF-kappaB activation through inhibition of IkappaB kinases

Here, we used a reductant, N-acetyl-L-cysteine (NAC), to investigate the redox-sensitive step(s) in the signalling pathway from the tumor necrosis factor (TNF) receptor to nuclear factor kappaB (NF-kappaB). We found that NAC suppressed NF-kappaB activation triggered by TNF or by overexpression of either the TNF receptor-associated death domain protein, TNF receptor-associated factor 2, NF-kappaB-inducing kinase (NIK), or IkappaB kinases (IKKalpha and IKKbeta). NAC also suppressed the TNF-induced activation of IKKalpha and IKKbeta, phosphorylation and degradation of IkappaB, and nuclear translocation of NF-kappaB. Furthermore, NAC suppressed the activation of IKKalpha and IKKbeta triggered by the overexpression of NIK. These results indicate that IKKalpha and IKKbeta are subject to redox regulation in the cells, and that NAC inhibits NF-kappaB activation through the suppression of these kinases.

Epidermal growth factor receptor is modulated by redox through multiple mechanisms. Effects of reductants and H2O2

The cellular redox state has been shown to play an essential role in cellular signaling systems. Here we investigate the effects of reductants and H2O2 on the signaling of epidermal growth factor (EGF) in cells. H2O2 induced the phosphorylation of the EGF receptor and the formation of a receptor complex comprising Shc, Grb2, Sos, and the EGF receptor. Dimerization or oligomerization of the EGF receptor was not induced by H2O2. Protein tyrosine phosphatase (PTP) assay showed that H2O2 suppressed dephosphorylation of the EGF receptor in cell lysates, suggesting that inactivation of PTP was involved in H2O2-induced activation of the EGF receptor. In contrast, the reductants N-acetyl-L-cysteine [Cys(Ac)] and dithiothreitol markedly suppressed EGF-induced dimerization and activation of the EGF receptor in cells. In accordance with suppression of the EGF receptor, Cys(Ac) suppressed EGF-induced activation of Ras, phosphatidylinositol 3-kinase and mitogen-activated protein kinase. Dithiothreitol completely inhibited EGF binding and kinase activation of the EGF receptor both in vitro and in vivo. In contrast, Cys(Ac) suppressed high-affinity EGF-binding sites on the cells, but had no effect on low-affinity binding sites. Furthermore, Cys(Ac) did not suppress EGF-induced kinase activation or dimerization of the EGF receptor in vitro, indicating that it suppressed the EGF receptor through a redox-sensitive cellular process or processes. Thus, the EGF receptor is regulated by redox through multiple steps including dephosphorylation by PTP, ligand binding, and a Cys(Ac)-sensitive cellular process or processes.

Growth factor-mediated signal transduction and redox balance in isolated digestive gland cells from Mytilus galloprovincialis Lam

In mammalian cells, a growing body of evidence indicates a relationship between cellular redox balance and tyrosine kinase-mediated cell signalling. The phosphorylative cascade activated by extracellular signals is inhibited by reducing conditions and stimulated by oxidative stress, in particular at the level of mitogen activated protein kinase (MAPK) activation. The mussel Mytilus typically shows variations in antioxidant defence systems and decreases in glutathione content in response to both natural and contaminant environmental stressors. In isolated mussel digestive gland cells, both epidermal growth factor (EGF) and insulin-like growth factor-I (IGF-I) have been recently demonstrated to activate tyrosine kinase receptors leading to multiple responses; among these, stimulation of the key glycolytic enzymes phosphofructokinase (PFK) and pyruvate kinase (PK). The present study investigates the possible relationship between the tyrosine kinase-mediated metabolic effects of growth factors and cellular redox balance in mussel cells. The results demonstrate that the effects of growth factors on glycolytic enzymes were abolished by cell pretreatment with the antioxidant N-acetyl-cysteine (NAC). On the other hand, in cells where the glutathione content and synthesis were lowered either in vitro (by cell pretreatment with buthionine sulfoximine (BSO)), or in vivo (by mussel exposure to Cu(2+)) the metabolic effects of growth factors were unaffected. Moreover, the results show that, in both control and glutathione-depleted cells, growth factors can also regulate the level of glutathione apparently by modulating, via phosphorylative mechanisms involving MAPK activation, the activity of gamma-glutamylcysteine synthetase (GCS), the rate limiting enzyme in GSH biosynthesis. Overall, this study extends the hypothesis that cell signalling is intimately related to redox balance in marine invertebrate cells.

Developmental neurotoxicity of chlorpyrifos in vivo and in vitro: effects on nuclear transcription factors involved in cell replication and differentiation

Chlorpyrifos is a widely used organophosphate insecticide that is a suspected developmental neurotoxin. Although chlorpyrifos exerts some effects through cholinesterase inhibition, recent studies suggest additional, direct actions on developing cells. We assessed the effects of chlorpyrifos on nuclear transcription factors involved in cell replication and differentiation using in vitro and in vivo models. HeLa nuclear protein extracts were incubated with the labeled consensus oligonucleotides for AP-1 and Sp1 transcription factors in the presence and absence of chlorpyrifos. In concentrations previously shown to affect cell development, chlorpyrifos reduced AP-1, but not Sp1 DNA-binding activity. Next, chlorpyrifos was incubated with PC12 cells either during cell replication or after initiation of differentiation with NGF. Chlorpyrifos evoked stage-specific interference with the expression of the transcription factors: Sp1 was reduced in replicating and differentiating cells, whereas AP-1 was affected only during differentiation. Finally, neonatal rats were given apparently subtoxic doses of chlorpyrifos either on postnatal days 1-4 or 11-14 and the effects were evaluated in the forebrain (an early-developing, cholinergic target region) and cerebellum (late-developing region, poor in cholinergic innervation). Again, chlorpyrifos evoked stage-specific changes in transcription factor expression and binding activity, with greater effects on Sp1 during active neurogenesis, and effects on AP-1 during differentiation. The changes were present in both forebrain and cerebellum and were gender-specific. These results indicate that chlorpyrifos interferes with brain development, in part by multiple alterations in the activity of transcription factors involved in the basic machinery of cell replication and differentiation. Noncholinergic actions of chlorpyrifos that are unique to brain development reinforce the need to examine endpoints other than cholinesterase inhibition.

Oxidative stress and gene regulation

Reactive oxygen species are produced by all aerobic cells and are widely believed to play a pivotal role in aging as well as a number of degenerative diseases. The consequences of the generation of oxidants in cells does not appear to be limited to promotion of deleterious effects. Alterations in oxidative metabolism have long been known to occur during differentiation and development. Experimental perturbations in cellular redox state have been shown to exert a strong impact on these processes. The discovery of specific genes and pathways affected by oxidants led to the hypothesis that reactive oxygen species serve as subcellular messengers in gene regulatory and signal transduction pathways. Additionally, antioxidants can activate numerous genes and pathways. The burgeoning growth in the number of pathways shown to be dependent on oxidation or antioxidation has accelerated during the last decade. In the discussion presented here, we provide a tabular summary of many of the redox effects on gene expression and signaling pathways that are currently known to exist.

Anisomycin uses multiple mechanisms to stimulate mitogen-activated protein kinases and gene expression and to inhibit neuronal differentiation in PC12 phaeochromocytoma cells

Treatment of PC12 cells with nerve growth factor (NGF) stimulates extracellular signal-regulated kinases (ERKs), as well as stress-activated c-Jun N-terminal kinases (JNKs) and p38 kinase, and induces neuronal differentiation. While the pivotal role of ERKs in NGF-induced morphological differentiation is well established, the contribution of JNK- and p38-pathways is less clear. The role of the JNK- and p38-pathway in PC12 cells was analysed by using anisomycin, a protein synthesis inhibitor that activates JNKs and p38. Non-toxic concentrations of anisomycin were found to stimulate these enzyme activities as well as the expression of the early response genes c-jun, c-fos and zif268, and to inhibit NGF-induced neurite formation. These effects of anisomycin appear to be mediated by the generation of reactive oxygen species (ROS), which in turn act through both TrkA/Ras-dependent and -independent signalling pathways. In addition, cross-talk between the p38- and ERK-pathways appears to play a role in the action of anisomycin.

Serotonin stimulates mitogen-activated protein kinase activity through the formation of superoxide anion

Our previous studies have shown that, through an active transport process, serotonin (5-HT) rapidly elevates O(-)(2). formation, stimulates protein phosphorylation, and enhances proliferation of bovine pulmonary artery smooth muscle cells (SMCs). We presently show that 1 microM 5-HT also rapidly elevates phosphorylation and activation of the mitogen-activated protein (MAP) kinases extracellular signal-regulated kinase (ERK) 1 and ERK2 of SMCs, and the enhanced phosphorylation is blocked by the antioxidants Tiron, N-acetyl-L-cysteine (NAC), and Ginkgo biloba extract. Inhibition of MAP kinase with PD-98059 failed to block enhanced O(-)(2). formation by 5-HT. Chinese hamster lung fibroblasts (CCL-39 cells), which demonstrate both 5-HT transporter and receptor activity, showed a similar response to 5-HT (i.e., enhanced mitogenesis, O(-)(2). formation, and ERK1 and ERK2 phosphorylation and activation). Unlike SMCs, they also responded to 5-HT receptor agonists. We conclude that downstream signaling of MAP kinase is a generalized cellular response to 5-HT that occurs secondary to O(-)(2). formation and may be initiated by either the 5-HT transporter or receptor depending on the cell type.

MAP kinase pathways

MAP kinases help to mediate diverse processes ranging from transcription of protooncogenes to programmed cell death. More than a dozen mammalian MAP kinase family members have been discovered and include, among others, the well studied ERKs and several stress-sensitive enzymes. MAP kinases lie within protein kinase cascades. Each cascade consists of no fewer than three enzymes that are activated in series. Cascades convey information to effectors, coordinates incoming information from other signaling pathways, amplify signals, and allow for a variety of response patterns. Subcellular localization of enzymes in the cascades is an important aspect of their mechanisms of action and contributes to cell-type and ligand-specific responses. Recent findings on these properties of MAP kinase cascades are the major focus of this review.

Redox regulation of cellular signalling

Extracellular stimuli elicit a variety of responses, such as cell proliferation and differentiation, through the cellular signalling system. Binding of growth factors to the respective receptor leads to the activation of receptor tyrosine kinases, which in turn stimulate downstream signalling systems such as mitogen-activated protein (MAP) kinases, phospholipase Cgamma (PLCgamma) and phosphatidylinositol 3-kinase. These biochemical reactions finally reach the nucleus, resulting in gene expression mediated by the activation of several transcription factors. Recent studies have revealed that cellular signalling pathways are regulated by the intracellular redox state. Generation of reactive oxygen species (ROS), such as H2O2, leads to the activation of protein tyrosine kinases followed by the stimulation of downstream signalling systems including MAP kinase and PLCgamma. The activation of PLCgamma by oxidative radical stress elevates the cellular Ca2+ levels by flux from the intracellular Ca2+ pool and from the extracellular space. Such reactions in the upstream signalling cascade, in concert, result in the activation of several transcription factors. On the other hand, reductants generally suppress the upstream signalling cascade resulting in the suppression of transcription factors. However, it is well known that cysteine residues in a reduced state are essential for the activity of many transcription factors. In fact, in vitro, oxidation of NFkappaB results in its activation, whereas reductants promote its activity. Thus, cellular signalling pathways are generally subjected to dual redox regulation in which redox has opposite effects on upstream signalling systems and downstream transcription factors. Not only are the cellular signalling pathways subjected to redox regulation, but also the signalling systems regulate the cellular redox state. When cells are activated by extracellular stimuli, the cells produce ROS, which in turn stimulate other cellular signalling pathways, indicating that ROS act as second messengers. It is thus evident that there is cross talk between the cellular signalling system and the cellular redox state. Cell death and life also are subjected to such dual redox regulation and cross talk. Death signals induce apoptosis through the activation of caspases in the cells. Oxidative radical stress induces the activation of caspases, whereas the oxidation of caspases results in their inactivation. Furthermore, some cell-death signals induce the production of ROS in the cells, and the ROS produced in turn stimulate the cell-death machinery. All this evidence shows that the cell's fate is determined by cross talk between the cellular signalling pathways and the cellular redox state through a complicated regulation mechanism.

Nitric oxide and N-acetylcysteine inhibit the activation of mitogen-activated protein kinases by angiotensin II in rat cardiac fibroblasts

Angiotensin II acts on the cardiac fibroblast to produce a mitogenic response. Nitric oxide and N-acetylcysteine have been used to determine if oxidative stress influenced the effects of angiotensin II on the cardiac fibroblast. Angiotensin II activated the mitogen-activated protein kinases designated extracellular signal-regulated kinases within 5 min by interacting with the AT1 receptor. This activation was completely independent of protein kinase C and was inhibited when farnesylation was blocked, implicating Ras involvement. Pretreatment of cardiac fibroblasts with either N-acetylcysteine for 8 h or nitric oxide for 10 min suppressed this activation by angiotensin II in a dose-dependent manner. However, when both agents were added, inhibition was essentially complete. This combined effect of N-acetylcysteine and nitric oxide to block ERKs activation also was found if the activity was stimulated by either another growth factor (platelet-derived growth factor) or by the addition of phorbol ester, suggesting the effect was not limited to the receptor site alone. The results are consistent with the hypothesis that hormonal activation of mitogenic steps such as ERKs is influenced by increased oxidative stress, which is reduced by the combined effects of N-acetylcysteine and nitric oxide.

Prevention of PC12 cell death by N-acetylcysteine requires activation of the Ras pathway

We have shown that N-acetylcysteine (NAC) promotes survival of sympathetic neurons and pheochromocytoma (PC12) cells in the absence of trophic factors. This action of NAC was not related to its antioxidant properties or ability to increase intracellular glutathione levels but was instead dependent on ongoing transcription and seemed attributable to the action of NAC as a reducing agent. Here, we investigate the mechanism by which NAC promotes neuronal survival. We show that NAC activates the Ras-extracellular signal-regulated kinase (ERK) pathway in PC12 cells. Ras activation by NAC seems necessary for survival in that it is unable to sustain serum-deprived PC12 MM17-26 cells constitutively expressing a dominant-negative form of Ras. Promotion of PC12 cell survival by NAC is totally blocked by PD98059, an inhibitor of the ERK-activating MAP kinase/ERK kinase, suggesting a required role for ERK activation in the NAC mechanism. In contrast, LY294002 and wortmannin, inhibitors of phosphatidylinositol 3-kinase (PI3K) that partially block NGF-promoted PC12 cell survival, have no effect on prevention of death by NAC. We hypothesized previously that the ability of NAC to promote survival correlates with its antiproliferative properties. However, although NAC does not protect PC12 MM17-26 cells from loss of trophic support, it does inhibit their capacity to synthesize DNA. Thus, the antiproliferative effect of NAC does not require Ras activation, and inhibition of DNA synthesis is insufficient to mediate NAC-promoted survival. These findings highlight the role of Ras-ERK activation in the mechanism by which NAC prevents neuronal death after loss of trophic support.

Characterization of graf, the GTPase-activating protein for rho associated with focal adhesion kinase. Phosphorylation and possible regulation by mitogen-activated protein kinase

Graf is a GTPase-activating protein for Rho that interacts with focal adhesion kinase and co-localizes with the actin cytoskeleton (Hildebrand, J. D., Taylor, J. M. and Parsons, J. T. (1996) Mol. Cell. Biol. 16, 3169-3178). We examined the expression and regulation of Graf as a prelude to understanding the role of Graf in mediating signal transduction in vivo. We demonstrated that Graf is a ubiquitously expressed 95-kDa protein with high levels observed in heart and brain and cells derived from these tissues. Stimulation of PC12 cells with epidermal growth factor or nerve growth factor induced a phosphatase-reversible mobility shift upon gel electrophoresis, indicative of phosphorylation. In vitro, purified mitogen-activated protein (MAP) kinase catalyzed the phosphorylation of Graf on serine 510, suggesting that Graf phosphorylation may be mediated through MAP kinase signaling. In addition, the mutation of serine 510 to alanine inhibited the epidermal growth factor-induced mobility shift of mutant Graf protein in vivo, consistent with serine 510 being the site of in vivo phosphorylation. Based on these data we suggest that phosphorylation of Graf by MAP kinase or related kinases may be a mechanism by which growth factor signaling modulates Rho-mediated cytoskeletal changes in PC12 and perhaps other cells.

Oxygen radicals and signaling

Recent evidence suggests that reactive oxygen species, such as superoxide anions and hydrogen peroxide, function as intracellular second messengers. This review will discuss the progress in understanding the intracellular pathways leading from ligand stimulation to the generation of oxidants, as well as some of the increasing number of cellular processes that appear to be subject to redox regulation.

Membrane targeting of p21-activated kinase 1 (PAK1) induces neurite outgrowth from PC12 cells

Rho-family GTPases regulate cytoskeletal dynamics in various cell types. p21-activated kinase 1 (PAK1) is one of the downstream effectors of Rac and Cdc42 which has been implicated as a mediator of polarized cytoskeletal changes in fibroblasts. We show here that the extension of neurites induced by nerve growth factor (NGF) in the neuronal cell line PC12 is inhibited by dominant-negative Rac2 and Cdc42, indicating that these GTPases are required components of the NGF signaling pathway. While cytoplasmically expressed PAK1 constructs do not cause efficient neurite outgrowth from PC12 cells, targeting of these constructs to the plasma membrane via a C-terminal isoprenylation sequence induced PC12 cells to extend neurites similar to those stimulated by NGF. This effect was independent of PAK1 ser/thr kinase activity but was dependent on structural domains within both the N- and C-terminal portions of the molecule. Using these regions of PAK1 as dominant-negative inhibitors, we were able to effectively inhibit normal neurite outgrowth stimulated by NGF. Taken together with the requirement for Rac and Cdc42 in neurite outgrowth, these data suggest that PAK(s) may be acting downstream of these GTPases in a signaling system which drives polarized outgrowth of the actin cytoskeleton in the developing neurite.

Reactive oxygen species and antioxidants in signal transduction and gene expression

Reactive oxygen species (ROS) are produced by cellular metabolic reactions, and have been implicated in the pathogenesis of several diseases, including atherosclerosis, cancer, and Alzheimer's disease. Interestingly, clinical and epidemiologic studies have, in some cases, indicated that antioxidant nutrients may be effective in disease prevention. However, the efficacy of specific antioxidants in disease prevention is often both controversial and inconclusive. In an effort to elucidate the role of ROS and antioxidants in disease development and prevention, the chemistries of ROS and antioxidants have been examined extensively. Recently, molecular and cellular approaches have demonstrated that ROS and antioxidants can directly affect the cellular signaling apparatus and, consequently, the control of gene expression. This new research provides the link between ROS and antioxidant chemistries and the mechanisms of disease processes and prevention. This review illustrates how ROS function as potential intracellular and extracellular signaling molecules and how antioxidants can affect this process.

Redox regulation of lipopolysaccharide (LPS)-induced interleukin-8 (IL-8) gene expression mediated by NF kappa B and AP-1 in human astrocytoma U373 cells

LPS-induced expression of the IL-8 gene was markedly enhanced by H2O2 or by deprivation of the cellular antioxidant glutathione by L-buthionine-(S,R)-sulfoximine (BSO) in human astrocytoma U373 cells. In contrast, it was markedly suppressed by the reductant N-acetyl-L-cysteine (NAC) and other antioxidants. Transient expression analysis using the chloramphenicol acetyltransferase assay revealed that activation of the IL-8 promoter by LPS was stimulated by BSO and was suppressed by NAC; likewise LPS-induced activation of both NF kappa B and AP-1 was enhanced by BSO and inhibited by NAC. These results suggest that LPS-induced IL-8 gene expression is regulated by cellular redox via modulation of these transcription factors.