Immediate early gene expression in PC12 cells exposed to lead: requirement for protein kinase C

MOLECULAR MECHANISMS OF LEAD NEUROTOXICITY

Lead (Pb2+) is a non-essential metal with numerous industrial applications that have led to ts ubiquity in the environment. Thus, not only occupational-exposed individuals' health is compromised, but also that of the general population and in particular children. Notably, although the central nervous system is particularly susceptible to Pb2+, other systems are affected as well. The present study focuses on molecular mechanisms that underlie the effects that arise from the presence of Pb2+ in situ in the brain, and the possible toxic effects that follows. As the brain barriers represent the first target of systemic Pb2+, mechanisms of Pb2+ entry into the brain are discussed, followed by a detailed discussion on neurotoxic mechanisms, with special emphasis on theories of ion mimicry, mitochondrial dysfunction, redox imbalance, and neuroinflammation. Most importantly, the confluence and crosstalk between these events is combined into a cogent mechanism of toxicity, by intertwining recent and old evidences from humans, in vitro cell culture and experimental animals. Finally, pharmacological interventions, including chelators, antioxidants substances, anti-inflammatory drugs, or their combination are reviewed as integrated approaches to ameliorate Pb2+ harmful effects in both developing or adult organisms.

The cationic (calcium and lead) and enzyme conundrum

The environmental toxicant lead (Pb) and the essential element calcium (Ca) play an interactive role in extracellular and intracellular regulatory functions that affect health. Lead's usurping calcium binding sites, as well as its interactions with thiols and phosphates have been suggested to be the basis for adverse effects on many organ systems especially the nervous system. Among regulatory processes controlled by Ca are calmodulin-dependent phosphodiesterase, calmodulin-dependent protein kinases, calmodulin inhibitor sensitive potassium channels, and calmodulin-independent protein kinase C (PKC) activation. This review focused on Pb studies describing the modulation of PKC, which is also regulated by steroids. Steroid hormone regulation may relate to a focal point for the sex differences of Pb and cellular signaling events. Picomolar concentrations of Pb may stimulate partially purified PKC, but higher concentrations inhibit activity. Although knowledge exists regarding Pb and PKC isoforms, especially interaction of Pb with the purified enzyme, there are conflicting reports concerning metal-mediated activation or inhibition of PKC and downstream signaling events. The effect of Pb on PKC in vivo remains elusive. Most reports of Pb and PKC in whole animal and human studies indicated that Pb either inhibits PKC or exerts no significant effect. However, most of the animal studies were performed with males. Recent studies performed with females and males separately revealed that females and males respond to Pb quite differently, and for this reason, it is suggested that future Pb studies of PKC and other biomedical investigations be performed with females and males.

Central nervous system cytokine gene expression: modulation by lead

The environmental heavy metal toxicant, lead (Pb) has been shown to be more harmful to the central nervous system (CNS) of children than to adults, given that Pb exposure affects the neural system during development. Because growth factors and cytokines play very important roles in development of the CNS, we have examined the impact of Pb exposure on the expression of cytokines during CNS development. Cytokine expression was studied in post-natal-day 21 (pnd21) mice by microarray, real-time RT-PCR, Luminex, and ELISA methodologies. BALB/c mouse pups were exposed to Pb through the dam's drinking water (0.1 mM Pb acetate), from gestation-day 8 (gd8) to pnd21. Two cytokines, interleukin-6 (IL-6) and transforming growth factor-β1 (TGF-β1), displayed significantly changed transcript levels in the presence of Pb. IL-6 and TGF-β1 both have signal transduction cascades that can cooperatively turn on the gene for the astrocyte marker glial-fibrillary acidic protein (GFAP). Microarray results indicated that Pb exposure significantly increased expression of GFAP. Pb also modulated IL-6, TGF-β1, and IL-18 protein expression in select brain regions. The deleterious effects of Pb on learning and long-term memory are posited to result from excessive astrocyte growth and/or activation with concomitant interference with neural connections. Differential neural expression of cytokines in brain regions needs to be further investigated to mechanistically associate Pb and neuroinflammation with behavioral and cognitive changes.

Oral lead exposure induces dysbacteriosis in rats

OBJECTIVES

Lead's (Pb(II)) possible role in intestinal pathologies of microbial etiology remains mostly unknown. The aim of this study was to examine the effects of lead on the gut microbial community and its interactions with rat intestinal epithelium.

METHODS

The lead-induced changes in different intestinal microbial groups (lactose-positive lac(+) and -negative lac(-) E.coli strains, lactobacilli and yeasts) were followed separately by the colony-forming unit (CFU) method. Samples were taken from outbred white rats subjected to different exposure schedules. Additionally, the impact of different lead doses on microbial adhesion to cultured intestinal cells (IEC-6) was investigated. Finally, the lead accumulation and distribution were measured by means of atomic absorption spectrometry.

RESULTS

For the first time it was shown that oral lead exposure causes drastic changes in the gut microbial community. Proportional to the lead dose received, the relative number of lactose-negative E.coli cells increased dramatically (up to 1,000-fold) in comparison to the other microbial groups during 2 wk of exposure. Considering the number of microbes in the intestine, such a shift in intestinal microflora (dysbacteriosis) is very significant. Adhesion studies showed certain stimulating effects of lead on E. coli attachment to rat intestinal epithelium as compared to Lactobacillus attachment.

CONCLUSIONS

The mechanisms providing the apparent competitive success of the lac(-) group are unclear but could be related to changes in surface interactions between microbial and host cells. This study may provide important clues for understanding the pathological effects of metal dietary toxins in human beings.

Activation of protein kinase Calpha signaling prevents cytotoxicity and mutagenicity following lead acetate in CL3 human lung cancer cells

Protein kinase C (PKC) family of serine/threonine protein kinases is sensitive signaling transducers in response to lead acetate (Pb) that could transmit phosphorylation cascade for proliferation and de-differentiation of neural cells. However, little is known as to the impact of PKC on Pb genotoxicity. Here we investigate whether Pb activates the conventional/classical subfamily of PKC (cPKC) signaling to affect cytotoxicity and mutagenicity in CL3 human non-small-cell lung adenocarcinoma cells. Pb specifically promoted membrane localization of the alpha isoform of PKC in CL3 cells. Pb also elicited Raf-1 activation as measured by the induction of phospho-Raf-1S338 and the dissociation from the Raf-1 kinase inhibitor protein. Inhibition of cPKC activity using Gö6976 or depletion of PKCalpha by introducing specific small interfering RNA blocked the induction of phospho-Raf-1S338, phospho-MKK1/2 and phospho-ERK1/2 in cells exposed to Pb. Intriguingly, declining PKCalpha enhanced the Pb cytotoxicity and revealed the Pb mutagenicity at the hprt gene. The results suggest that PKCalpha is obligatory for activation of the Raf-1-MKK1/2-ERK1/2 signaling module and plays a defensive role against cytotoxicity and mutagenicity following Pb exposure. Results obtained in this study also support our previous report showing that ERK1/2 activity is involved in preventing Pb genotoxicity.

Early lead exposure increases the leakage of the blood-cerebrospinal fluid barrier, in vitro

The cell type constructing the blood-brain barrier (BBB) and blood-cerebrospinal fluid barrier (BCB) is entirely different, ie, endothelia in BBB and epithelia in BCB. Nonetheless, both barriers share a common character--the tight junctions (TJ) between adjacent cells. This study investigated the consequence of lead (Pb) exposure on the tightness of BCB. In an in vitro BCB transwell model, using immortalized choroidal epithelial Z310 cells, we found that early exposure to Pb (prior to the formation of tight barrier) at 5 and 10 microM, significantly reduced the tightness of BCB, as evidenced by a 20% reduction in transepithelial electrical resistance (TEER) values (P <0.05), and >20% increase in the paracellular permeability of [(14)C]sucrose (P <0.05). Exposure to Pb after the formation of tight barrier, however, did not cause any detectable barrier dysfunction. RT-PCR and Western blot analyses on typical TJ proteins revealed that Pb exposure decreased both the mRNA and protein levels of claudin-1, with the membrane-bound claudin-1 more profoundly affected than cytosolic claudin-1. Pb exposure, however, had no significant effect on ZO1 and occludin. These data suggest that Pb exposure selectively alters the cellular level of claudin-1, which, in turn, reduces the tightness and augments the permeability of tight blood-CSF barrier. The immature barrier appears to be more vulnerable to Pb toxicity than the mature, well-developed, brain barrier, the fact possibly contributing to Pb-induced neurotoxicity among young children.

Modulation of ERK1/2 and p38(MAPK) by lead in the cerebellum of Brazilian catfish Rhamdia quelen

Lead (Pb2+) is a neurotoxic trace metal, widespread in aquatic environment that can change physiologic, biochemical and behavioral parameters in diverse fish species. Chemical exposure may drive modulation of mitogen-activated protein kinases (MAPKs) that are a family of highly conserved enzymes which comprise ubiquitous groups of signaling proteins playing critical regulatory roles in cell physiology. Extracellular signal-regulated kinases (ERK1/2) and p38(MAPK) control complex programs such as gene expression, embryogenesis, cell differentiation, cell proliferation, cell death and synaptic plasticity. Little information is available about MAPKs in aquatic organisms and their modulation by trace metals. The aim of this work was to determine the modulation of ERK1/2 and p38(MAPK) phosphorylation by Pb2+ in vivo and in vitro, in cerebellar slices of the catfish, Rhamdia quelen. In the in vitro model, slices were incubated for 3 h with lead acetate (1-10 microM). In the in vivo studies, the animals were exposed for 2 days to lead acetate (1 mg L(-1)). ERK1/2 and p38(MAPK) (total and phosphorylated forms) were immunodetected in cerebellar slices by Western blotting. Pb2+ added in vitro at 5 and 10 microM increased significantly the phosphorylation of both MAPKs. The in vivo exposed animals also showed a significant increase of ERK1/2 and p38(MAPK) phosphorylation without changes in the total content of the enzymes. In conclusion, the present work indicates that it is possible to evaluate the ERK1/2 and p38(MAPK) activation in the central nervous system (CNS) of a freshwater fish largely distributed in South America. Moreover, Pb2+, an important environmental pollutant may activate in vitro and in vivo ERK1/2 and p38(MAPK) enzymes. These findings are important considering the functional and ecologic implications associated to Pb2+ exposure of a freshwater fish species, such as R. quelen, and the roles of ERK1/2 and p38(MAPK) in the control of brain development, neuroplasticity and cell death.

Zinc deficiency increases the susceptibility of human neuroblastoma cells to lead-induced activator protein-1 activation

Lead (Pb2+) is a major environmental pollutant that has severe adverse effects on the nervous system. Similar human populations are at risk of suffering both Pb2+ toxicity and zinc (Zn) deficiency. Thus, in the present study we investigated whether Zn deficiency can increase the susceptibility of human neuroblastoma IMR-32 cells to Pb2+-induced oxidative stress which could trigger the activation of the mitogen-activated protein kinases (MAPKs) c-Jun-N-terminal kinase (JNK) and p38 and subsequently activate transcription factor activator protein-1 (AP-1). After 24 h of incubation, 5-50 microM Pb2+ caused a decrease in cell viability that was markedly higher in the Zn-deficient cells compared to controls. Pb caused a time (2-24 h) and dose (5-50 microM)-dependent increase of cell oxidants, with a significantly higher effect in the Zn-deficient cells. Pb2+ treatment triggered the activation of JNK and p38, measured as the phosphorylation of JNK and p38, only in cells incubated in the Zn-deficient media. The exposure to Pb2+ (2-24 h) led to a higher AP-1 DNA-binding activity and AP-1-dependent gene transactivation, only in the Zn-deficient cells. Results show that Zn deficiency can increase the cytotoxicity of Pb2+ and the susceptibility of neurons to Pb2+-induced oxidative stress, leading to JNK and p38 phosphorylation and, subsequently, AP-1 activation.

Metal-induced oxidative stress and signal transduction

Occupational and environmental exposures to metals are associated with the development of various cancers. Although carcinogenesis caused by metals has been intensively investigated, the mechanisms of action, especially at the molecular level, are still unclear. Accumulating evidence indicates that reactive oxygen species generated by metals may play an important role in the etiology of disease. This review covers recent advances in (1) metal-induced generation of reactive oxygen species; (2) the receptors, kinases, and nuclear transcription factors affected by metals and metal-induced oxidative stress, including growth factor receptors, src kinase, ras signaling, mitogen-activated protein kinases, the phosphoinositide 3-phosphate/Akt pathway, nuclear transcription factor kappaB, activator protein 1, p53, nuclear factor of activated T cells, and hypoxia-inducible factor 1; and (3) global cellular phenomena (signal transduction, cell cycle regulation, and apoptosis) associated with metal-induced ROS production and gene expression.

Lead stimulates ERK1/2 and p38MAPK phosphorylation in the hippocampus of immature rats

Lead (Pb(2+)) is widely recognized as a neurotoxicant whose mechanisms of action are not completely established. We have previously demonstrated that Pb(2+) can activate the p38(MAPK) pathway and increase the phosphorylation of Hsp27 in bovine adrenal chromaffin cells and human SH SY5Y cells over a short incubation period (1 h). In the present work we analyzed the effects of Pb(2+) administered in vivo on the level and the phosphorylation state of ERK1/2 and p38(MAPK) in the hippocampus of immature rats. Rats were treated with lead acetate (2, 8 or 12 mg/kg, i.p.) or saline (control) over the 8th to 12th postnatal days, and hippocampal slices were prepared on the 14th day. The Pb(2+) level in the lead-treated animals increased 2.5-6-fold in the blood (3.0-6.0 microg/dl) and 2.0-3.0-fold in the forebrain (78-103 ng/g wet weight), compared to control (saline). The phosphorylation of both ERK1/2 and p38(MAPK) was significantly increased by prior exposure to Pb(2+) in vivo. In in vitro experiments, hippocampal slices from 14-day-old rats were exposed to Pb(2+) (1-10 microM) for 1 and 3 h. There were no changes in the phosphorylation state of ERK and p38(MAPK) for 1-h incubation, whereas a significant increase of ERK1/2 and p38(MAPK) phosphorylation by Pb(2+) (5 microM) was observed for the 3-h incubation. Cell viability measured using MTT was not modified in any of the conditions tested. These results indicate that the phosphorylation of hippocampal ERK1/2 and p38(MAPK) is stimulated by lead in a period of rapid brain development, an effect that may underlie, at least in part, the neurotoxicty elicited by this metal.

Vascular endothelial growth factor mediates vasogenic edema in acute lead encephalopathy

Brain injury from inorganic Pb(2+) is considered the most important environmental childhood health hazard worldwide. The microvasculature of the developing brain is uniquely susceptible to high level Pb(2+) toxicity (ie, Pb(2+) encephalopathy) characterized by cerebellar hemorrhage, increased blood-brain barrier permeability, and vasogenic edema. However, the specific molecular mediators of Pb(2+) encephalopathy have been elusive. We found that Pb(2+) induces vascular endothelial growth factor/vascular permeability factor (VEGF) in cultured astrocytes (J Biol Chem, 2000;275:27874-27882). The study presented here asks if VEGF dysregulation contributes mechanistically to Pb(2+) encephalopathy. Neonatal rats exposed to 4% Pb-carbonate develop the histopathological features of Pb(2+) encephalopathy seen in children. Cerebellar VEGF expression increased approximately twofold (p < 0.01) concurrent with the development of cerebellar microvascular hemorrhage, enhanced vascular permeability to serum albumin, and vasogenic cerebellar edema (p < 0.01). No change in VEGF expression occurred in cerebral cortex that does not develop these histopathological complications of acute Pb(2+) intoxication. Pb(2+) exposure increased phosphorylation of cerebellar Flk-1 VEGF receptors and the Flk-1 inhibitor CEP-3967 completely blocked cerebellar edema formation without affecting microhemorrhage formation or blood-brain barrier permeability. This establishes that Pb(2+)-induced vasogenic edema formation develops via a Flk-1-dependent mechanism and suggests that the vascular permeability caused by Pb(2+) is Flk-1 independent.

Lead-induced cell signaling cascades in GT1-7 cells

The effects of lead on the signal transduction pathways that may be involved in the release of gonadotropin-releasing hormone (GnRH) from neurons in the hypothalamus have not been well defined. Using the GT1-7 cell line, an in vitro model for GnRH-secreting neurons, we examined signal transduction pathways directly affected by lead. We found that lead-induced phosphorylation of extracellular signal-regulated kinase 1 and 2 (ERK1 and ERK2), as well as p90RSK and cAMP response element-binding protein (CREB), but did not induce IkappaB degradation. MEK1/2 inhibitor (PD98059) suppressed lead-induced ERK and p90RSK activation. Neither PKC inhibitors (Go6983, Go6976) nor CaMKII inhibitor (KN-62) had a pronounced effect on lead-induced ERK1 and ERK2 phosphorylation. However, MEK1/2 inhibitor, CaMKII inhibitor, and PKC inhibitor significantly suppressed lead-induced CREB phosphorylation. These results indicate that lead-activated PKC, CaMKII and MEK/ERK/p90RSK pathways simultaneously, all of which contributed to CREB phosphorylation. Our results also indicate that lead-induced p90RSK and CREB activation does not alter expression of early response genes like c-fos. We conclude that lead activates PKC, CaMKII or MEK-ERK-p90RSK pathways in GT1-7 cells, leading to CREB phosphorylation and modulation of gene expression.

Oligodendroglia in developmental neurotoxicity

The developing nervous system has been long recognized as a primary target for a variety of toxicants. To date, most efforts to understand the impact of neurotoxic agents on the brain have focused primarily on neurons and to a lesser degree astroglia as cellular targets. The role of oligodendroglia, the myelin-forming cells in the central nervous system (CNS), in developmental neurotoxicity has been emphasized only in recent years. Oligodendrocytes originate from migratory, mitotic progenitors that mature progressively into postmitotic myelinating cells. During differentiation, oligodendroglial lineage cells pass through a series of distinct phenotypic stages that are characterized by different proliferative capacities and migratory abilities, as well as dramatic changes in morphology with sequential expression of unique developmental markers. In recent years, it has become appreciated that oligodendrocyte lineage cells have important functions other than those related to myelin formation and maintenance, including participation in neuronal survival and development, as well as neurotransmission and synaptic function. Substantial knowledge has accumulated on the control of oligodendroglial survival, migration, proliferation, and differentiation, as well as the cellular and molecular events involved in oligodendroglial development and myelin formation. Recently, studies have been initiated to address the role of oligodendrocyte lineage cells in neurotoxic processes. This article examines recent progress in oligodendroglial biology, focuses attention on the characteristic features of the oligodendrocyte developmental lineage as a model system for neurotoxicological studies, and explores the role of oligodendrocyte lineage cells in developmental neurotoxicity. The potential role of oligodendroglia in environmental lead neurotoxicity is presented to exemplify this thesis.

The involvement of lipid activators of protein kinase C in the induction of ZIF268 in PC12 cells exposed to lead

Lead (Pb) induces the expression of immediate early genes (IEG) in PC12 cells by a mechanism that involves protein kinase C (PKC). To define the mechanisms, the involvement of two commonly observed lipid activators of PKC, diacylglycerols, and phosphatidylinositols, were examined. A dose-dependent increase in the expression of the IEG zif268 was observed in PC12 cells exposed to Pb. The PKC inhibitor Ro-31-8220 blocked the induction. An increase in levels of diacylglycerols was observed in PC12 cells exposed to Pb, but the increase was inhibited by Ro-31-8220. The phosphatidylinositol 3-kinase inhibitor Wortmannin, but not the inhibitor LY 294002, blocked the induction zif268 in Pb-exposed cells. Small increases in phosphatidylinositol 3-kinase activity were observed after exposure to Pb. In summary, diacylglycerols are elevated in PC12 cells exposed to Pb by a mechanism that requires PKC. It is possible that diacylglycerols contribute to the induction of zif268 by Pb by sustaining PKC activation.

Molecular targets of lead in brain neurotoxicity

The detrimental effects of lead poisoning have been well known since ancient times, but some of the most severe consequences of exposure to this metal have only been described recently. Lead [Pb(II)] affects the higher functions of the central nervous system and undermines brain growth, preventing the correct development of cognitive and behavioral functions. As an established neurotoxin, Pb(II) crosses the blood-brain barrier rapidly and concentrates in the brain. The mechanisms of lead neurotoxicity are complex and still not fully understood, but recent findings recognized that both Ca(II) dependent proteins and neurotransmitters receptors represent significant targets for Pb(II). In particular, acute and chronic exposure to lead would predominantly affect two specific protein complexes: protein kinase C and the N-methyl-D-aspartate subtype of glutamate receptor. These protein complexes are deeply involved in learning and cognitive functions and are also thought to interact significantly with each other to mediate these functions. This review outlines the most recent hypotheses and evidences that link lead poisoning to impairment of these protein functions, as well as the in vitro experimental approaches that are most likely to provide information on basic mechanicistic processes.

Suppression of Egr-1 expression in human oral squamous carcinoma cells by okadaic acid

We examined the expression of early growth response-1 (Egr-1) gene in human oral squamous carcinoma cell lines SCCKN and SCC-25 cells and human osteoblastic cell lines Saos-2 and MG63 cells treated with okadaic acid, a potent inhibitor of protein phosphatases type 1 and type 2A. Western blot analysis revealed that Egr-1 was strongly expressed in SCCKN cells and that okadaic acid decreased the expression of Egr-1 protein in these cells. However, Egr-1 was expressed at lower levels in SCC-25, Saos-2, and MG63 cells and transiently increased with the okadaic acid treatment. Suppression of Egr-1 protein expression in okadaic acid-treated SCCKN cells stemmed from the suppression of the Egr-1 mRNA level, as determined by the RT-RCR method. Formaldehyde-fixed and alcohol-permeabilized cultured SCCKN cells were reacted with the anti-Egr-1 antibody using immunohistochemical methods. Intense fluorescence was observed in the nuclei of the control SCCKN cells interacted with anti-Egr-1 antibody. However, only a weak reaction was observed in the nuclei in SCCKN cells treated with okadaic acid. A gel mobility shift assay showed that treatment of SCCKN cells with okadaic acid suppressed Egr-1 binding to the DIG-labeled Egr-1 consensus oligonucleotide probe. The present results indicate that the alteration of phosphorylation states in SCCKN cells regulates Egr-1 binding to its consensus sequence and its expression at the transcriptional level.

In vitro vs in vivo Pb effects on brain protein kinase C activity

Alteration of normal protein kinase C (PKC) function by environmental Pb exposure during neurodevelopment is hypothesized to be an important mechanism of toxicity underlying neurologic impairment. Previous studies have reported widely varying effects of Pb on PKC, possibly in part because of differences in in vitro and in vivo models used in those studies. Therefore, we tested the hypothesis that, with comparable tissue Pb levels, the effects of in vitro Pb exposure on brain PKC are the same as the effects caused by in vivo Pb exposure of intact animals. For chronic in vivo Pb exposure, female Long-Evans rats were exposed to Pb or vehicle from postnatal days 1 to 34-36 (n=10/treatment). For in vitro Pb exposure, homogenate of the frontal cortex region was exposed directly to Pb in an amount comparable to that accumulated in brain during chronic in vivo Pb exposure. Brain Pb levels were measured using ultraclean techniques and inductively coupled plasma mass spectrometry. PKC activity was subsequently determined in cytosolic and membrane subcellular fractions in the frontal cortex, hippocampus, and remaining brain regions. Results indicate that brain Pb levels following in vivo Pb exposure were increased approximately 20-fold above those of nonexposed animals (vehicle group [Pb] approximately 130ng Pb/g dry wt.). However, in vivo Pb exposure did not measurably alter brain PKC activity in the regions tested. In contrast, in vitro Pb exposure significantly increased PKC activity by approximately 20% in the frontal cortex homogenate membrane subcellular fraction. These results indicate that Pb added in vitro caused more dramatic effects than those produced by a comparable amount of Pb in the tissue from in vivo exposure. While the mechanisms underlying these outcomes are not clear, they suggest that in vitro models might not accurately reflect effects of chronic low-level in vivo Pb exposure.

The in vitro effects of Pb acetate on NO production by C6 glial cells

The CNS neurotoxic effects of lead (Pb) are well documented; however, the molecular toxicity targets have not been clearly delineated. Astroglial cells, which are the most abundant cells in the brain and provide critical support to the neurons, are known to accumulate Pb. Although NO generated by inducible NO synthase (iNOS) in glial cells has been associated with many neurotoxic events, it can also serve to protect by modulating blood flow, increase antimicrobial and tumoricidal activities, and promote immune responses following injury or insult. The present investigations were designed to test the hypothesis that Pb exposure may perturb cytokine signal transduction pathways leading to NO production by astroglial cells. Pretreatment with Pb acetate (500 nM-10 microM) attenuated the generation of NO in a concentration-dependent manner up to 90%, and suppressed iNOS protein expression, as well as interfered with the homeostatic functions of calcium in the cytokine-induced NO signal transduction pathway. In addition, pretreatment with staurosporine, a serine-threonine kinase inhibitor, or KT5720, a specific protein kinase A inhibitor (PKA), inhibited cytokine-induced NO production in a concentration-dependent manner with IC(50) values of 26.3 and 346.7 nM, respectively. Therefore, Pb may impede events within the PKA signal transduction pathway; although, based on results from a gel shift assay, Pb does not directly affect PKA enzyme activity. Taken together, these results suggest the possibility that the suppressive effect of Pb acetate on cytokine-induced NO production in glial cells may be implicated in the neurophysiologic changes noted following occupational or environmental exposure to Pb.

Exposure to lead elevates induction of zif268 and Arc mRNA in rats after electroconvulsive shock: the involvement of protein kinase C

Exposure to lead is well known to impair cognitive function in young children. Because of the importance of gene regulation for neurodevelopment, we examined the effect of lead on the induction of the mRNA of the immediate early genes zif268 and Arc. The time course for the induction of zif268 mRNA and Arc mRNA by electroconvulsant shock (ECS) was altered in the area of the dentate gyrus of the hippocampus in rats exposed to lead from postnatal days (PND) 1 to 28. Other areas of the hippocampus were not affected by lead. The effects on the induction of zif268 mRNA were observed at blood lead levels as low as 12 microg/dl. No change in the induction of zif268 mRNA was observed in the hippocampus of rats exposed to lead from PND 28 to PND 56. Because of the possible involvement of protein kinase C (PKC) in the effect of lead, activation of different isoforms of PKC was investigated. An increase in the amount of PKC epsilon and PKC gamma was observed at 60 min after ECS in the membrane fraction from hippocampus, indicating activation of these isoforms. The amount of PKC epsilon in membranes was higher in rats exposed to lead than in rats not exposed to lead after ECS. Taken together, the data suggest that lead may disturb regulation of specific immediate early genes by activating PKC epsilon.

Protein kinase C activation is required for the lead-induced inhibition of proliferation and differentiation of cultured oligodendroglial progenitor cells

Lead (Pb) is a common neurotoxicant of major public health concern. Previous studies revealed that cultured oligodendrocyte progenitor cells (OPCs) are highly vulnerable to Pb toxicity. The present study examines the effect of Pb on the survival, proliferation and differentiation of OPCs in vitro. Dose-response studies showed that> or = l5-10 microM Pb is cytotoxic to OPCs within 24 h. However, 1 microM of Pb was found to inhibit the proliferation and differentiation of OPCs without affecting cell viability. Pb markedly decreased the proliferative capability of OPCs and inhibited cell-intrinsic lineage progression of OPCs at a late progenitor stage. The Pb-induced decrease of proliferation and differentiation was abolished by inhibition of protein kinase C (PKC) with bisindolylmaleimide I, while the effect of the PKC-activating agent phorbol-12,13-didecanoate was potentiated by Pb. Furthermore, Pb exposure of OPCs caused the translocation of PKC from the cytoplasm to membrane without an increase in total cellular PKC enzymic activity. These results indicate that Pb inhibits the proliferation and differentiation of oligodendrocyte lineage cells in vitro through a mechanism requiring PKC activation.

Effects of postnatally administered inorganic lead on the tyrosine hydroxylase immunoreactive norepinephrinergic neurons of the locus ceruleus of the rat

The neurotoxic effects of inorganic lead are known to include peripheral neuropathy in adults and encephalopathy in children. The purpose of this study was to determine the effect of inorganic lead (PbCl2) administration on norepinephrinergic neurons of the locus ceruleus in neonatal rats by immunocytochemical and electron microscopic analyses. Lead chloride solutions, 0.05%, 0.1% and 0.2% in concentrations, were prepared in distilled water and administered orally via drinking water. After 4, 8, or 12 weeks of continuous administration, the rats were sacrificed and brains were immunostained with the tyrosine hydroxylase antibody. The number of immunoreactive cell bodies in the locus ceruleus was estimated. Densitometric analysis of immunoreactive profiles visualized by electron microscopy was performed using an image analyzer. The numbers of immunoreactive neurons in the locus ceruleus were increased statistically by lead administration. The intensity of the immunoreaction, both under the light and electron microscopes was also increased. Degenerative changes, including intra-axonal vacuole formation and widening of the extracellular spaces, were found by electron microscopy in and around the tyrosine hydroxylase immunoreactive axons. Increased tyrosine hydroxylase immunoreactivity may correlate with the hyper-reactivity of lead intoxicated children. Degenerative changes may account for the reported deficits in intellectual attainment and achievement in lead intoxicated children.

Lead-stimulated p38MAPK-dependent Hsp27 phosphorylation

Lead (Pb2+) is a cytotoxic metal ion whose mechanism of action is not established. However, Pb2+ is known to interact with a wide variety of molecules involved in signal transduction. In this study the effect of Pb2+ on protein phosphorylation in bovine adrenal chromaffin cells and human SH SY5Y cells was examined. Cells were incubated with 32P(i) for 1 h in the presence of Pb2+ (1-10 microM) and the proteins were separated by two-dimensional PAGE. An increase in the phosphorylation of a number of proteins was observed in response to Pb2+, including three spots, MW 25 kDa, and pI's in the range 4.0-4.5. These proteins were immunoidentified as three isoforms of the heat-shock protein 27 kDa (Hsp27), and the identity of the most basic spot was confirmed by amino acid sequencing. Phosphorylation of p38MAPK was increased by Pb2+ and the effect of Pb2+ on Hsp27 phosphorylation was blocked by the p38MAPK inhibitor SB203580 (1 microM). The results were similar for bovine chromaffin cells and human SH SY5Y cells. This is the first report showing that Pb2+ can modulate the phosphorylation state of Hsp27 via activation of the p38MAPK pathway.

Multiple pathways of Pb(2+) permeation in rat cerebellar granule neurones

The pathways of lead (Pb(2+)) uptake were studied in fura-2-loaded cerebellar granule cells from 8-day-old rats. In a nominal Ca-free external bath, Pb(2+) (5-50 microM) determined an increase of the fluorescence emission ratio (R = E(340)/E(380)) even in the absence of any specific stimulus. This rise was dose-dependent, was not significantly affected by mM Mg(2+) or Ca(2+), but it was readily reversed by the membrane-permeant heavy metal chelator tetrakis(2-pyridylmethyl) ethylene-diamine (TPEN, 100 microM), indicating that it was due to Pb(2+) influx. The rate of rise, dR/dt, was increased up to a factor of 5 by depolarizing high-KCl solution, indicating a sizeable permeation through voltage-dependent channels. This effect was neither antagonized by nimodipine, nor enhanced by BayK8644, but it was slackened by omega-agatoxin IVA (200 nM), suggesting an involvement of non-L-type calcium channels. Pb(2+) influx was also stimulated by glutamic acid or NMDA in the presence of 10-30 microM glycine, but only in Mg-free solution, suggesting that glutamate channels of the NMDA type are an additional pathway of Pb(2+) uptake. Pb(2+) caused a time-, dose- and stimulus-dependent saturation of the dye, whose intracellular concentration is approximately 10 microM, indicating that intracellular Pb(2+) can readily reach a concentration in the micromolar range. These results indicate that the particular vulnerability of neurones to Pb(2+) poisoning is linked to the presence of specific transport systems, which mediate the rapid uptake of Pb(2+) into the neurone.

Microarray analysis of differential gene expression in lead-exposed astrocytes

The toxic metal lead is a widespread environmental health hazard that can adversely affect human health. In an effort to better understand the cellular and molecular consequences of lead exposure, we have employed cDNA microarrays to analyze the effects of acute lead exposure on large-scale gene expression patterns in immortalized rat astrocytes. Our studies identified many genes previously reported to be differentially regulated by lead exposure. Additionally, we have identified novel putative targets of lead-mediated toxicity, including members of the family of calcium/phospholipid binding annexins, the angiogenesis-inducing thrombospondins, collagens, and tRNA synthetases. We demonstrate the ability to distinguish lead-exposed samples from control or sodium samples solely on the basis of large-scale gene expression patterns using two complementary clustering methods. We have confirmed the altered expression of candidate genes and their encoded proteins by RT-PCR and Western blotting, respectively. Finally, we show that the calcium-dependent phospholipid binding protein annexin A5, initially identified as a differentially regulated gene by our microarray analysis, is directly bound and activated by nanomolar concentrations of lead. We conclude that microarray technology is an effective tool for the identification of lead-induced patterns of gene expression and molecular targets of lead.

Lead exposure activates nuclear factor kappa B, activator protein-1, c-Jun N-terminal kinase and caspases in the rat brain

How lead manifests its neurotoxicity is not well understood. The hypothesis that lead may activate nuclear transcription factors NF-kappaB, activator protein-1 (AP-1), c-Jun N-terminal kinase (JNK), mitogen-activated protein kinase kinase (MAPKK) and caspases in the rat brain leading to the manifestation of its neurotoxic effects, was tested in 21-day-old male Long-Evans rats exposed to 50 ppm Pb in drinking water for 90 days. After the 90-day exposure, blood lead levels of the rats in control group were 4+/-0.2 microg/dl, while those of the Pb-exposed group were 18+/-0.3 microg/dl (n=50). Similarly, at the end of the exposure period, the Pb-exposed group showed significantly higher accumulation of Pb in brain regions such as, frontal cortex (FC), brain stem (BS), striatum (ST), and hippocampus (HIP) (338.6+/-7.7, 391.6+/-3.8, 288.3+/-6.7, and 382.3+/-3.3 ng/g wet tissue, respectively, in FC, BS, ST, and HIP) than the control group (126.6+/-2.7, 127.6+/-1.8, 201.3+/-9.4, and 180.3+/-4.4 ng/g wet tissue, respectively, in FC, BS, ST, and HIP). There was a 3-4-fold increase in NF-kappaB and AP-1 level in all the four regions of the brain of lead-treated animals. All four regions showed 4-10-fold activation of JNK and a 5-6-fold activation of MAPKK. As indicated by poly(ADP ribose) polymerase cleavage, lead exposure induced the activation of caspases in all four regions. Overall our results indicate that lead exposure induces the activation of NF-kappaB, AP-1, JNK, MAPKK, and caspases in the brain, which may contribute to its neurotoxic effects.

The biological chemistry of lead

Recent biophysical studies on the interactions between lead and recombinant proteins and peptides that naturally bind zinc or calcium have provided unparalleled insights into the biological chemistry and molecular toxicology of lead. These studies lay the foundation for the rational design of improved methods for detecting and treating lead poisoning.

Lead exposure alters Egr-1 DNA-binding in the neonatal rat brain

Zinc finger proteins (ZFP) contain a structural motif (Cys-2 His-2) found in a large family of eukaryotic transcriptional regulatory proteins, such as Sp1. Previous studies have shown that Sp1 DNA-binding was disrupted by exposure to lead (Pb), due to action on its zinc finger domain. In this paper, we discuss the results of studies with another ZFP, Egr-1, an early growth response gene, which is functionally involved in cell proliferation and differentiation. Egr-1 DNA-binding was studied by gel shift mobility assays in several brain regions of developing rat pups. We observed a distinct developmental profile of Egr-1 DNA-binding with a gradual increase from the early to late postnatal days in all the brain regions examined. Lactational exposure to Pb resulted in a modulation of Egr-1 DNA binding manifested by premature peaks in DNA-binding reminiscent of the in vivo changes previously reported for Sp1. These data are consistent with earlier findings that exposure to Pb both in vivo and in vitro causes a modulation in the DNA-binding of ZFP such as Sp1, Egr-1 and TFIIIA. The commonality by which Pb exposure alters the DNA-binding patterns of ZFP suggests that divalent Pb may be interacting directly with the Zn moiety of these proteins.

Induction of vascular endothelial growth factor in human astrocytes by lead. Involvement of a protein kinase C/activator protein-1 complex-dependent and hypoxia-inducible factor 1-independent signaling pathway

The mechanism(s) underlying lead neurotoxicity are not fully elucidated. cDNA expression microarray analysis identified lead-sensitive genes in immortalized human fetal astrocytes (SV-FHA). Of the represented genes expressed, vascular endothelial growth factor (VEGF) was one of the most sensitive. Lead induced VEGF mRNA 3-fold and VEGF protein approximately 2-fold with maximum mRNA induction following incubation with 10 micrometer lead acetate for 24 h. Phorbol 12-myristate 13-acetate (PMA), a potent protein kinase C (PKC) activator, increased VEGF mRNA 2-fold and PKC inhibition by GF-109203 completely blocked VEGF induction by lead. Expression of dominant-negative PKC-epsilon, but not PKC-alpha, completely inhibited VEGF mRNA induction by lead. Lead activated the transcription factor AP-1 and increased AP-1-dependent luciferase expression >2-fold. Transfection of cells with a c-jun dominant-negative effectively inhibited both AP-1 activation and VEGF mRNA induction by lead. Hypoxia-inducible factor 1 (HIF-1) activity in SV-FHAs was moderately increased by lead (86%) and PMA (96%). Pretreatment with GF-109203 completely inhibited these effects of lead and PMA. However, lead did not alter HIF-1-dependent luciferase expression and a HIF-1alpha dominant-negative had no effects on the induction of VEGF mRNA by lead. These findings indicate that lead induces VEGF expression in SV-FHAs via a PKC/AP-1-dependent and HIF-1-independent signaling pathway.