Possible molecular targets of halogenated aromatic hydrocarbons in neuronal cells

Polychlorinated Biphenyls (PCBs): Risk Factors for Autism Spectrum Disorder?

Autism spectrum disorder (ASD) includes a group of multifactorial neurodevelopmental disorders defined clinically by core deficits in social reciprocity and communication, restrictive interests and repetitive behaviors. ASD affects one in 54 children in the United States, one in 89 children in Europe, and one in 277 children in Asia, with an estimated worldwide prevalence of 1-2%. While there is increasing consensus that ASD results from complex gene x environment interactions, the identity of specific environmental risk factors and the mechanisms by which environmental and genetic factors interact to determine individual risk remain critical gaps in our understanding of ASD etiology. Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that have been linked to altered neurodevelopment in humans. Preclinical studies demonstrate that PCBs modulate signaling pathways implicated in ASD and phenocopy the effects of ASD risk genes on critical morphometric determinants of neuronal connectivity, such as dendritic arborization. Here, we review human and experimental evidence identifying PCBs as potential risk factors for ASD and discuss the potential for PCBs to influence not only core symptoms of ASD, but also comorbidities commonly associated with ASD, via effects on the central and peripheral nervous systems, and/or peripheral target tissues, using bladder dysfunction as an example. We also discuss critical data gaps in the literature implicating PCBs as ASD risk factors. Unlike genetic factors, which are currently irreversible, environmental factors are modifiable risks. Therefore, data confirming PCBs as risk factors for ASD may suggest rational approaches for the primary prevention of ASD in genetically susceptible individuals.

The developmental neurotoxicity of legacy vs. contemporary polychlorinated biphenyls (PCBs): similarities and differences

Although banned from production for decades, PCBs remain a significant risk to human health. A primary target of concern is the developing brain. Epidemiological studies link PCB exposures in utero or during infancy to increased risk of neuropsychiatric deficits in children. Nonclinical studies of legacy congeners found in PCB mixtures synthesized prior to the ban on PCB production suggest that non-dioxin-like (NDL) congeners are predominantly responsible for the developmental neurotoxicity associated with PCB exposures. Mechanistic studies suggest that NDL PCBs alter neurodevelopment via ryanodine receptor-dependent effects on dendritic arborization. Lightly chlorinated congeners, which were not present in the industrial mixtures synthesized prior to the ban on PCB production, have emerged as contemporary environmental contaminants, but there is a paucity of data regarding their potential developmental neurotoxicity. PCB 11, a prevalent contemporary congener, is found in the serum of children and their mothers, as well as in the serum of pregnant women at increased risk for having a child diagnosed with a neurodevelopmental disorder (NDD). Recent data demonstrates that PCB 11 modulates neuronal morphogenesis via mechanisms that are convergent with and divergent from those implicated in the developmental neurotoxicity of legacy NDL PCBs. This review summarizes these data and discusses their relevance to adverse neurodevelopmental outcomes in humans.

Neurotoxicity of polychlorinated biphenyls and related organohalogens

Halogenated organic compounds are pervasive in natural and built environments. Despite restrictions on the production of many of these compounds in most parts of the world through the Stockholm Convention on Persistent Organic Pollutants (POPs), many "legacy" compounds, including polychlorinated biphenyls (PCBs), are routinely detected in human tissues where they continue to pose significant health risks to highly exposed and susceptible populations. A major concern is developmental neurotoxicity, although impacts on neurodegenerative outcomes have also been noted. Here, we review human studies of prenatal and adult exposures to PCBs and describe the state of knowledge regarding outcomes across domains related to cognition (e.g., IQ, language, memory, learning), attention, behavioral regulation and executive function, and social behavior, including traits related to attention-deficit hyperactivity disorder (ADHD) and autism spectrum disorders (ASD). We also review current understanding of molecular mechanisms underpinning these associations, with a focus on dopaminergic neurotransmission, thyroid hormone disruption, calcium dyshomeostasis, and oxidative stress. Finally, we briefly consider contemporary sources of organohalogens that may pose human health risks via mechanisms of neurotoxicity common to those ascribed to PCBs.

Preventive effects of imperatorin on perfluorohexanesulfonate-induced neuronal apoptosis via inhibition of intracellular calcium-mediated ERK pathway

Early life neuronal exposure to environmental toxicants has been suggested to be an important etiology of neurodegenerative disease development. Perfluorohexanesulfonate (PFHxS), one of the major perfluoroalkyl compounds, is widely distributed environmental contaminants. We have reported that PFHxS induces neuronal apoptosis via ERK-mediated pathway. Imperatorin is a furanocoumarin found in various edible plants and has a wide range of pharmacological effects including neuroprotection. In this study, the effects of imperatorin on PFHxS-induced neuronal apoptosis and the underlying mechanisms are examined using cerebellar granule cells (CGC). CGC were isolated from seven-day old rats and were grown in culture for seven days. Caspase-3 activity and TUNEL staining were used to determine neuronal apoptosis. PFHxS-induced apoptosis of CGC was significantly reduced by imperatorin and PD98059, an ERK pathway inhibitor. PFHxS induced a persistent increase in intracellular calcium, which was significantly blocked by imperatorin, NMDA receptor antagonist, MK801 and the L-type voltage-dependent calcium channel blockers, diltiazem and nifedipine. The activation of caspase-3 by PFHxS was also inhibited by MK801, diltiazem and nifedipine. PFHxS-increased ERK activation was inhibited by imperatorin, MK801, diltiazem and nifedipine. Taken together, imperatorin protects CGC against PFHxS-induced apoptosis via inhibition of NMDA receptor/intracellular calcium-mediated ERK pathway.

Connexin 43 mediates PFOS-induced apoptosis in astrocytes

Perfluorooctane sulfonate (PFOS) is a man-made environmental pollutant that is toxic to mammals. However, the neurotoxic effects of PFOS remain largely unexplored. In this study, we determined the role of an astrocyte specific gap junction protein, connexin 43 (Cx43), in PFOS-induced apoptosis. The rate of astrocyte apoptosis was higher in cortex astrocytes after PFOS treatment. These astrocytes also showed up-regulated expression of Cx43 and higher levels of cleaved caspase-3. Elevated ROS accumulation and decreased ΔΨm also confirmed the presence of PFOS-induced apoptosis. However, the exposure of astrocytes to PFOS together with carbenoxolone (CBX) significantly reduced both Cx43 and cleaved caspase-3 levels. These results indicate that Cx43 plays a proapoptotic role in PFOS-induced apoptosis in cortex astrocyte cells.

Activation of neuronal nitric oxide synthase (nNOS) signaling pathway in 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced neurotoxicity

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been reported to cause alterations in cognitive and motor behavior during both development and adulthood. In this study, the neuronal nitric oxide synthase (nNOS) signaling pathway was investigated in differentiated pheochromocytoma (PC12) cells to better understand the mechanisms of TCDD-induced neurotoxicity. TCDD exposure induced a time- and dose-dependent increase in nNOS expression. High levels of nitric oxide (NO) production by nNOS activation induced mitochondrial cytochrome c (Cyt-c) release and down-regulation of Bcl-2. Additionally, TCDD increased the expression of active caspase-3 and significantly led to apoptosis in PC12 cells. However, these effects above could be effectively inhibited by the addition of 7-nitroindazole (7-NI), a highly selective nNOS inhibitor. Moreover, in the brain cortex of Sprague-Dawley (SD) rats, nNOS was also found to have certain relationship with TCDD-induced neuronal apoptosis. Together, our findings establish a role for nNOS as an enhancer of TCDD-induced apoptosis in PC12 cells.

PFHxS induces apoptosis of neuronal cells via ERK1/2-mediated pathway

Perfluorohexanesulfonate (PFHxS) is one of the most widely distributed perfluoroalkyl compounds (PFCs) and its possible neurotoxicity has been suggested. However, the effects of PFHxS on neuronal function remain to be elucidated. In this study, the effects of PFHxS on neuronal cell death and the underlying mechanisms were examined. Cerebellar granule cells (CGCs) were isolated from 7-day old rat pups and maintained in culture for additional 7d. The apoptotic effects of PFHxS were determined by caspase-3 activity and TUNEL staining. PFHxS increased the apoptotic death of CGC in concentration-dependent manner. It also increased the activation of ERK1/2, JNK and p38 MAPK with different temporal activation. PD98059, an inhibitor of ERK1/2 pathway, completely blocked PFHxS-induced apoptosis whereas SP600125, a JNK inhibitor, significantly increased the apoptosis, showing their opposite roles in the apoptosis of CGCs. Treatment of antioxidants, Trolox or N-acetylcysteine (NAC), completely blocked ROS generation by PFHxS but neither of these antioxidants prevented PFHxS-induced apoptosis, suggesting that ROS may not play a key role in the process of apoptosis. PD98059 prevented ROS accumulation by PFHxS but the ERK1/2 activation was not affected by Trolox or NAC. These results indicate that ROS is one of downstream targets of ERK1/2, not vice versa. Taken together, PFHxS increased apoptosis of CGC in ERK1/2-dependent manner, where downstream pathway other than ROS may play a major role. This is a first report that PFHxS induces apoptosis of CGC isolated from the developing brain and its possible mode of action is associated with ERK1/2 pathway.

Chlorination of ortho-position on Polychlorinated Biphenyls Increases Protein Kinase C Activity in Neuronal Cells

Polychlorinated biphenyls (PCBs) are persistent and bioaccumulative environmental pollutants. Recently, it is suggested that neurotoxic effects such as motor dysfunction and impairment in memory and learning have been associated with PCB exposure. However, structure relationship of PCB congeners with neurotoxic effects remains unknown. Since PKC signaling pathway is implicated in the modulation of motor behavior as well as learning and memory and the role of PKC are subspecies-specific, we attempted to study the effects of structurally distinct PCBs on the total PKC activity as well as subspecies of PKC in cerebellar granule cell culture model. Cells were exposed to 0, 25 and 50 μM of PCB-126, PCB-169, PCB-114, PCB-157, PCB-52 and PCB-4 for 15 min. Cells were subsequently analyzed by [³H] phorbol ester binding assay or immunoblotted against PKC-α and -ε monoclonal antibodies. While non-dioxin-like-PCB (PCB-52 and PCB-4) induced a translocation of PKC-α and -ε from cytosol to membrane fraction, dioxin-like PCBs (PCB-126, -169, -114, -157) had no effects. [³H] Phorbol ester binding assay also revealed structure-dependent increase similar to translocation of PKC isozymes. While PCB-4 induced translocation of PKC-α and -ε was inhibited by ROS inhibitor, the pattern of translocation was not affected in presence of AhR inhibitor. It is suggested that PCB-4-induced PKC activity may not be mediated via AhR-dependent pathway. Taken together, our findings suggest that chlorination of ortho-position in PCB may be a critical structural moiety associated with neurotoxic effects, which may be preferentially mediated via non-AhR-dependent pathway. Therefore, the present study may contribute to understanding the neurotoxic mechanism of PCBs as well as providing a basis for establishing a better neurotoxic assessment.

Perfluorooctane sulfonate-induced apoptosis of cerebellar granule cells is mediated by ERK 1/2 pathway

Perfluorooctane sulfonate (PFOS), a ubiquitous environmental pollutant, is considered as a neurotoxicant to mammalian species. However, the underlying mechanism of its neurotoxicity is largely unknown. In the present study, we examined roles of mitogen-activated protein kinases (MAPKs) in PFOS-induced apoptosis of neuronal cells to elucidate the molecular mechanism. Cerebellar granule cells were isolated from 7-d old rats and maintained in culture for additional 7 d. Cells were exposed to PFOS and caspase-3 activity and nuclear morphology were evaluated by enzyme activity assay and Hoechst 33342 staining, respectively, to determine its effects on apoptosis. The treatment with PFOS resulted in caspase-3 activation and nuclear condensation and fragmentation. PFOS exposure selectively increased activation of ERK that remained above control over 6 h. The inhibitor of ERK pathway, PD98059, substantially blocked caspase-3 activation induced by PFOS, whereas inhibitors of JNK and p38 MAPK, SP600125 and SB203580, respectively, had no effect. PKC inhibitors, bisindolylmaleimide I and Gö6976, dampened caspase-3 activity and ERK activation induced by PFOS. Collectively, it is suggested that PKC and ERK play proapoptotic roles in PFOS-induced apoptosis of cerebellar granule cells and PKC act as an upstream regulator of ERK activation.

Effects of polychlorinated biphenyls on the development of neuronal cells in growth period; structure-activity relationship

Polychlorinated biphenyls (PCBs) are accumulated in our body through food chain and cause a variety of adverse health effects including neurotoxicities such as cognitive deficits and motor dysfunction. In particular, neonates are considered as a high risk group for the neurotoxicity of PCBs exposure. The present study attempted to analyze the structure-activity relationship among PCB congeners and the mechanism of PCBs-induced neurotoxicity. We measured total protein kinase C (PKC) activities, PKC isoforms, reactive oxygen species (ROS), and induction of neurogranin (RC-3) and growth associated protein-43 (GAP-43) mRNA in cerebellar granule cells of neonatal rats with phorbol 12, 13-dibutyrate ([(3)H]PDBu) binding assay, western blot, ROS assay, and reverse transcription PCR (RT-PCR) analysis respectively following the different structural PCBs exposure. Only non-coplanar PCBs showed a significant increase of total PKC-α and βII activity as measured with [(3)H]PDBu binding assay. ROS were more increased with non-coplanar PCBs than coplanar PCBs. The mRNA levels of RC-3 and GAP-43 were more induced with non-coplanar PCBs than coplanar PCBs, indicating that these factors may be useful biomarkers for differentiating non-coplanar PCBs from coplanar PCBs. Non-coplanar PCBs may be more potent neurotoxic congeners than coplanar PCBs. This study provides evidences that non-coplanar PCBs, which have been neglected in the risk assessment processes, should be added in the future to improve the quality and accuracy of risk assessment on the neuroendocrinal adverse effects of PCBs exposures.

PKC-δ mediates TCDD-induced apoptosis of chondrocyte in ROS-dependent manner

Exposure to dioxin-like compounds is associated with arthritis in humans. A recent study reported that 2,3,7,8,-tetrachlorodibenzo-p-dioxin (TCDD) induces apoptosis in chondrocytes, which is a critical event in the pathogenesis of cartilage disease. In this study, protein kinase C (PKC) signaling pathway was investigated to determine the mechanism of TCDD-induced rabbit articular chondrocyte apoptosis. TCDD exposure induced glutathione-mediated ROS generation and the translocation of PKC isozymes. Among the PKC isozymes tested, PKC-δ showed the most sensitive translocation. The translocation was then blocked by ROS inhibitors (trolox and N-acetyl cysteine), a PKC-δ inhibitor (rottlerin), a caspase-3 inhibitor (z-DEVD-fmk) or an AhR blocker (α-naphthoflavone). TCDD increased caspase-3 activity, the activating enzyme for PKC-δ, and prior treatment with trolox blocked such an increase. These results suggest that the translocation of PKC-δ was mediated by ROS-dependent caspase-3 activity. Pretreatment with rottlerin or trolox dampened TCDD-induced apoptosis of chondrocyte, as determined by TUNEL staining and ELISA. Taken together, this study suggests that ROS generation is an upstream event for TCDD-induced chondrocyte apoptosis and PKC-δ mediates the apoptotic processes through ROS-dependent caspase-3 activation. This is a first finding demonstrating the role of PKC-δ in chondrocyte apoptosis stimulated by an environmental pollutant. The results may contribute to understanding the mechanism of joint disease associated with the exposure of dioxin-like compounds and identifying a target for the therapeutic interventions.

Application of LC-NMR and HR-NMR to the characterization of biphenyl impurities in the synthetic route development for vestipitant, a novel NK1 antagonist

Vestipitant (1) is a novel NK1 antagonist currently under investigation for the treatment of CNS disorders and emesis. The first synthetic step comprised a Grignard synthesis. An impurity was identified and initially expected to be a symmetric biphenyl. This paper reports the work to synthesise the supposed structure and the spectroscopic analyses (LC-NMR and HR-NMR) to correctly identify the real structure and understand the chemical pathway of the impurity.

Endocrine disrupting polyhalogenated organic pollutants interfere with thyroid hormone signalling in the developing brain

Persistent polyhalogenated organic pollutants are present worldwide and accumulate along the food chain. They interfere with human and animal health and are particularly harmful for pre- and perinatal neurodevelopment. The mechanisms behind the observed effects vary depending on the specific compound investigated. Co-planar polychlorinated biphenyls (PCBs) can act via the arylhydrocarbon receptor while many ortho-substituted PCBs disrupt intracellular Ca(2+) homeostasis. A common mechanism for a wide variety of PCBs is interference with thyroid hormone (TH) signalling in developing brain, by changing intracellular TH availability or by interacting directly at the level of the TH receptors. Studies on gene expression in cortex and cerebellum revealed both hypothyroid- and hyperthyroid-like effects. However, since THdependent gene expression plays a crucial role in the coordination of neuronal proliferation, migration, synaptogenesis, myelination, etc., both reduced/delayed and increased/premature expression may result in permanent structural changes in neuronal communication networks, leading to lifelong deficits in cognitive performance, motor functions, and psychobehavior. In a similar way, PCBs are able to interfere with estrogen- and androgen-dependent brain development and in some studies neurobehavioral outcome was shown to be gender-specific. Other persistent organohalogens like polychlorinated dibenzo-p-dioxins (PCDDs) and polybrominated diphenyl ethers (PBDEs) also act as endocrine disrupters in the developing brain. Several of the mechanisms involved are similar to those of PCBs, but each group also works via own specific pathways. The fact that persistent organohalogens can amplify the neurotoxic effects of other environmental pollutants, such as heavy metals, further increases their risk for human and animal neurodevelopment.

Effect of aroclor 1248 and two pure PCB congeners on phospholipase D activity in rat renal tubular cell cultures

This paper elucidates the effect of different polychlorinated biphenyls (PCBs) on the phospholipase D (PLD) activity in soluble and particulate fractions of rat renal proximal tubular culture cells. Treatment with Aroclor 1248 (a commercial PCB mixture) caused a marked increase in the activity of PLD in intact renal tubular cells. The PLD activity was increased by Aroclor 1248 in the particulate fraction while the enzyme activity was unaffected in the soluble fraction. This work also shows that PCB 153 (2,2',4,4',5,5'-hexachlorobiphenyl, a di-ortho-substituted nonplanar congener) can increase the activity of PLD only in the particulate fraction. The exposure of cell cultures to PCB 77 (3,3',4,4'-tetrachlorobiphenyl, a non-ortho-substituted planar congener) does not alter PLD activity. These results suggest that PCB effects are structure dependent. Therefore, in order to clarify the molecular mechanism of activation of PLD by PCBs, the contents of immunoreactive PLD were examined by immunoblot analysis. Renal tubular cells expressed a PLD protein of 120 kDa corresponding with the PLD1 mammalian isoform in both the particulate and the soluble fraction. Aroclor 1248, PCB 153, and PCB 77 do not induce changes in the levels of PLD protein. These data indicate that PCBs, particularly nonplanar congeners, increase PLD activity. Moreover, these changes could not be demonstrated in the enzyme content in rat renal tubular cell cultures.

TCDD alters PKC signaling pathways in developing neuronal cells in culture

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to induce neurodevelopmental deficits such as poor cognitive development and motor dysfunction. However, the mechanism of TCDD-mediated neurotoxicity remains unclear. Since PKC signaling is one of the most pivotal events involved in neuronal function and development, we analyzed the effects of TCDD on the PKC signaling pathway in cerebellar granule cells derived from PND-7 rat brain. Immunoblot analysis revealed the presence of PKC-alpha, betaII, delta, epsilon, lambda and iota in both cytosol and membrane fractions of cerebellar granule cells, but PKC-gamma was below the detectable level. TCDD induced a significant translocation of PKC-alpha, -betaII and -epsilon from cytosol to membrane fraction (p<0.05) and a marginal translocation of PKC-delta at high dose only (p<0.1). It also increased RACK-1, an adaptor protein for PKC, in a dose-dependent manner. Exposure to TCDD induced a dose-dependent increase of both [3H] PDBu binding and the intracellular calcium level. The results suggest that the selective PKC isozymes and RACK-1 are involved in TCDD-mediated signaling pathway and these proteins may be possible molecular targets in neuronal cells for TCDD exposure. Our study provides basic data to understand mechanism of TCDD-induced neurotoxicity with respect to PKC signaling pathway and a scientific basis for improving the health risk assessment of neurotoxicants by identifying intracellular target molecules in neuronal cells.

Polychlorinated biphenyls disrupt the actin cytoskeleton in hippocampal neurons

It is well known that developmental exposure to polychlorinated biphenyls (PCBs) could cause learning and memory deficits, but the underlying mechanisms are not clear. Actin cytoskeleton is directly involved in synaptic plasticity which is considered critical to learning and memory formation by LIM kinase 1 (LIMK-1)/cofilin pathway. To determine whether PCBs could alter actin cytoskeleton, we exposed the cultured hippocampal neurons to PCBs mixture Aroclor 1254 (A 1254). By biochemical measurement, fluorimetric assay and fluorescence microscopy, we found that A 1254 elicited a loss of filamentous actin, which preceded cytotoxicity. Western blots showed that a concentration-dependent decrease in the phosphorylation of cofilin and a decrease in LIMK-1 were induced by A 1254. We concluded that PCBs induced actin depolymerization in hippocampal neurons, probably by inhibiting the LIMK-1/cofilin signaling pathway. The above findings offer new perspectives for the understanding of PCBs-induced learning and memory deficits.

Neurochemical targets and behavioral effects of organohalogen compounds: an update

Organohalogen compounds (OHCs) have been used and still are used extensively as pesticides, flame retardants, hydraulic fluids, and in other industrial applications. These compounds are stable, most often lipophilic, and may therefore easily biomagnify. Today these compounds are found distributed both in human tissue, including breast milk, and in wildlife animals. In the late 1960s and early 1970s, high levels of the polychlorinated biphenyls (PCBs) and the pesticide dichlorodiphenyl trichloroethane (DDT) were detected in the environment. In the 1970s it was discovered that PCBs and some chlorinated pesticides, such as lindane, have neurotoxic potentials after both acute and chronic exposure. Although the use of PCBs, DDT, and other halogenated pesticides has been reduced, and environmental levels of these compounds are slowly diminishing, other halogenated compounds with potential of toxic effects are being found in the environment. These include the brominated flame retardants, chlorinated paraffins (PCAs), and perfluorinated compounds, whose levels are increasing. It is now established that several OHCs have neurobehavioral effects, indicating adverse effects on the central nervous system (CNS). For instance, several reports have shown that OHCs alter neurotransmitter functions in CNS and Ca2+ homeostatic processes, induce protein kinase C (PKC) and phospholipase A2 (PLA2) mobilization, and induce oxidative stress. In this review we summarize the findings of the neurobehavioral and neurochemical effects of some of the major OHCs with our main focus on the PCBs. Further, we try to elucidate, on the basis of available literature, the possible implications of these findings on human health.

Translocation of PKC-betaII is mediated via RACK-1 in the neuronal cells following dioxin exposure

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to induce neurotoxic effects. However, the mechanism of TCDD-mediated signaling pathways and its possible molecular targets in neurons remains unknown. In this study, we analyzed effects of TCDD on neurofilament subunits, receptor for activated C kinase-1 (RACK-1), and PKC-betaII activity in developing neuronal cells. TCDD induced a significant increase of RACK-1, an adaptor protein for protein kinase C (PKC), in cerebellar granule cells in both dose- and time-dependent manner, indicating that RACK-1 is a sensitive molecular target in neuronal cells for TCDD exposure. TCDD induced a dose-dependent translocation of PKC-betaII from cytosol to membrane fractions. However, when RACK-1 induction was blocked by antisense oligonucleotide or alpha-naphthoflavone, Ah receptor (AhR) inhibitor, the translocation of PKC-betaII was inhibited. Our data suggests that TCDD activates PKC-betaII via RACK-1 in an AhR-dependent manner. This is the first report identifying RACK-1 as a target molecule involved in TCDD-mediated signaling pathways. TCDD exposure also increased the level of neurofilament-H mRNA. These results suggest that identification of target molecules may contribute to improve our understanding of TCDD-mediated signaling pathway and the risk assessment of TCDD-induced neurotoxicities.

Neurotoxicity of persistent organic pollutants: possible mode(s) of action and further considerations

Persistent organic pollutants (POPs) are long-lived toxic organic compounds and are of major concern for human and ecosystem health. Although the use of most POPs is banned in most countries, some organochlorine pesticides are still being used in several parts of the world. Although environmental levels of some POPs such as polychlorinated biphenyls (PCBs) have declined, newly emerging POPs such as polybrominated diphenyl ethers (PBDEs) have been increasing considerably. Exposure to POPs has been associated with a wide spectrum of effects including reproductive, developmental, immunologic, carcinogenic, and neurotoxic effects. It is of particular concern that neurotoxic effects of some POPs have been observed in humans at low environmental concentrations. This review focuses on PCBs as a representative chemical class of POPs and discusses the possible mode(s) of action for the neurotoxic effects with emphasis on comparing dose-response and structure-activity relationships (SAR) with other structurally related chemicals. There is sufficient epidemiological and experimental evidence showing that PCB exposure is associated with motor and cognitive deficits in humans and animal models. Although several potential mode(s) of actions were postulated for PCB-induced neurotoxic effects, changes in neurotransmitter systems, altered intracellular signalling processes, and thyroid hormone imbalance are predominant ones. These three potential mechanisms are discussed in detail in vitro and in vivo. In addition, SAR was conducted on other structurally similar chemicals to see if they have a common mode(s) of action. Relative potency factors for several of these POPs were calculated based on their effects on intracellular signalling processes. This is a comprehensive review comparing molecular effects at the cellular level to the neurotoxic effects seen in the whole animal for environmentally relevant POPs.

Differential effects of PCBs on the induction of apoptosis machinery and PKCα translocation in rat renal tubular cell cultures

We have demonstrated previously [Pérez-Reyes, P.L., Sánchez-Alonso, J.A., López-Aparicio, P., Recio, M.N., Pérez-Albarsanz, M.A., 2001. Different molecular capacity in the induction of apoptosis by polychlorinated biphenyl congeners in rat renal tubular cell cultures. Biosci. Rep. 6, 765-778] that the polychlorinated biphenyls (PCBs) cause loss of cell viability and accelerate apoptosis in cell kidney cultures. Further investigations are necessary to elucidate the mechanism of apoptosis induction. In this way, we have analyzed in the present work the effects of PCBs on protein kinase C (PKC, a protein family intimately involved in the regulation of cell survival) and the expression of two proapoptotic (caspase-3 and Bax) and one antiapoptotic (Bcl-2) proteins. Aroclor 1248 (a commercial PCB mixture with 48% chlorine by weight), PCB 153 (2,2',4,4',5,5'-hexachlorobiphenyl, a di-ortho-substituted nonplanar congener) and PCB 77 (3,3',4,4'-tetrachlorobiphenyl, a non-ortho-substituted planar congener), significantly increased PKCalpha activity compared to control cells in the cytosolic and particulate cell fractions, and increased the PKCalpha protein content in the particulate fraction. The nonplanar PCB 153 showed stronger effects than the coplanar congener PCB 77. In addition, Aroclor 1248 decreased both, procaspase-3 levels and the Bcl-2/Bax protein ratio. These findings indicate that PCBs, particularly nonplanar congeners, can induce apoptosis in primary renal tubular cells through the PKCalpha, caspase-3 and Bcl-2/Bax pathway.

Involvement of Na+-Ca2+ exchanger in intracellular Ca2+ increase and neuronal injury induced by polychlorinated biphenyls in human neuroblastoma SH-SY5Y cells

In SH-SY5Y, a human neuroblastoma cell line, Aroclor 1254 (A1254), induced a dose-dependent (10-50 microg/ml) intracellular calcium concentration ([Ca2+]i) increase. Two rather specific sodium-calcium (Na+-Ca2+) exchanger (NCX) inhibitors, bepridil (10 microM) and KB-R7943 [2-[2-[4-(4-nitrobenzyloxy) phenyl]ethyl]isothiourea methanesulfonate] (10 microM), reduced A1254-induced [Ca2+]i increase. A 24-h exposure to 30 microg/ml A1254 caused remarkable SH-SY5Y neuroblastoma cell damage. It is noteworthy that both bepridil and KB-R7943 counteracted A1254-induced neuronal injury. These results indicate that NCX contributes to [Ca2+]i increase and neuronal injury induced by A1254. RT-PCR experiments revealed in SH-SY5Y neuroblastoma cells the expression of NCX1 and NCX3 isoforms. To investigate which isoform was involved in [Ca2+]i increase and neuronal damage induced by A1254, we used specific antisense oligodeoxynucleotides (ODNs) to reduce NCX1 or NCX3 protein expression. The results showed that only NCX1 ODN reduced [Ca2+]i increase and neuronal injury induced by A1254. In conclusion, these results indicate that NCX1 may participate to [Ca2+]i increase and neurotoxicity evoked by A1254 in SH-SY5Y neuroblastoma cells.

Hydroxylated metabolites of polychlorinated biphenyls inhibit thyroid-hormone-dependent extension of cerebellar Purkinje cell dendrites

Thyroid hormones (THs) are important for brain development, and polychlorinated biphenyl (PCB) accumulation in humans is a serious problem because PCBs may affect TH functions. To determine the effects of hydroxylated metabolites of PCBs (OH-PCBs) on brain development, we performed mouse cerebellar culture assays. 4-OH-2',3,3',4',5'-pentachlorobiphenyl and 4-OH-2',3,3',4',5,5'-hexachlorobiphenyl significantly inhibited the TH-dependent extension of Purkinje cell dendrites even at 5 x 10(-11) M and 5 x 10(-12) M, respectively. OH-PCBs may disturb TH-dependent brain development.

Chronic exposure to ammonia induces isoform-selective alterations in the intracellular distribution and NMDA receptor-mediated translocation of protein kinase C in cerebellar neurons in culture

Hyperammonemia is responsible for most neurological alterations in patients with hepatic encephalopathy by mechanisms that remain unclear. Hyperammonemia alters phosphorylation of neuronal protein kinase C (PKC) substrates and impairs NMDA receptor-associated signal transduction. The aim of this work was to analyse the effects of hyperammonemia on the amount and intracellular distribution of PKC isoforms and on translocation of each isoform induced by NMDA receptor activation in cerebellar neurons. Chronic hyperammonemia alters differentially the intracellular distribution of PKC isoforms. The amount of all isoforms (except PKC zeta) was reduced (17-50%) in the particulate fraction. The contents of alpha, beta1, and epsilon isoforms decreased similarly in cytosol (65-78%) and membranes (66-83%), whereas gamma, delta, and theta; isoforms increased in cytosol but decreased in membranes, and zeta isoform increased in membranes and decreased in cytosol. Chronic hyperammonemia also affects differentially NMDA-induced translocation of PKC isoforms. NMDA-induced translocation of PKC alpha and beta is prevented by ammonia, whereas PKC gamma, delta, epsilon, or theta; translocation is not affected. Inhibition of phospholipase C did not affect PKC alpha translocation but reduced significantly PKC gamma translocation, indicating that NMDA-induced translocation of PKC alpha is mediated by Ca2+, whereas PKC gamma translocation is mediated by diacylglycerol. Chronic hyperammonemia reduces Ca+2-mediated but not diacylglycerol-mediated translocation of PKC isoforms induced by NMDA.

Polychlorinated biphenyl mixtures (Aroclors) induce apoptosis via Bcl-2, Bax and caspase-3 proteins in neuronal cell cultures

Polychlorinated biphenyls (PCBs) are a group of persistent and widely dispersed environmental pollutants, some of which may be neurotoxic. In the present study, we have investigated the effect of PCB commercial mixtures (Aroclors) on neuronal cell cultures by assessing cell viability and apoptotic cell death. We have combined morphological and biochemical techniques to establish the relevance of apoptosis in neuronal cell death induced by Aroclors. Treatment with both Aroclor 1248 and Aroclor 1260 caused the loss of cell viability and accelerated apoptosis both in a concentration- and time-dependent manner. However, the extent of apoptosis resulted greater for Aroclor 1248 than for Aroclor 1260. This is correlated with the loss of cell viability since Aroclor 1248 is more cytotoxic. The apoptosis induced by Aroclors involves the increase of caspase-3 activity. To correlate the caspase-3 activity with respect to changes in protein processing, caspase-3 precursor protein (procaspase-3) was evaluated by Western blot analysis. Also, Bcl-2 and Bax protein were assessed in order to elucidate the cell death machinery induced in cortical neuronal cell cultures by Aroclor 1248. The results indicate that the increase in Aroclor-induced apoptosis correlates with a reduction in the expression of antiapoptotic Bcl-2 and an increase in the expression of proapoptotic Bax. These results suggest that, with our experimental conditions, Aroclors induce apoptosis in primary cultures of cortical neurons via proteins of the Bcl-2 and caspase families.

Alterations in brain protein kinase C isoforms following developmental exposure to a polychlorinated biphenyl mixture

PCBs have been shown to alter several neurochemical end-points and are implicated in the etiology of some neurological diseases. Recent in vivo studies from our laboratory indicated that developmental exposure to a commercial PCB mixture, Aroclor 1254, caused perturbations in calcium homeostasis and changes in protein kinase C (PKC) activities in rat brain. However, it is not known which molecular substances are targets for PCB-induced developmental neurotoxicity. Since the PKC signaling pathway has been implicated in the modulation of motor behavior as well as learning and memory, and the roles of PKC are subspecies specific, the present study attempted to analyze the effects on selected PKC isozymes in the cerebellum and the hippocampus following developmental exposure (gestational day 6 through postnatal day 21) to a PCB mixture, Aroclor 1254. The results indicated that the developmental exposure to PCBs caused significant hypothyroxinemia and age-dependent alterations in the translocation of PKC isozymes; the effects were greatly significant at postnatal day (PND) 14. Immunoblot analysis of PKC-alpha (alpha) from both cerebellum and hippocampus revealed that developmental exposure to Aroclor 1254 caused a significant decrease in cytosolic fraction and an increase in particulate fraction. There was no significant difference between these two brain regions on the level of fractional changes. However, the ratio between the fractions (particulate/cytosol) from cerebellum only was increased in a dose-dependent manner. Analysis of PKC-gamma (gamma) in cerebellum on PND14 showed a decrease in cytosolic fraction in both dose groups and an increase in particulate fraction at high dose (6 mg/kg) only. The ratio between the two fractions was increased in a dose-dependent manner. In the hippocampus, there was a significant decrease in PKC-gamma in cytosolic fraction of the high-dose group and a significant increase in particulate fraction of the low-dose group. But, the ratio between the fractions showed a significant increase (2.6-fold increase in high dose on PND14). Analysis of PKC-epsilon (epsilon) in cerebellum showed a significant decrease in cytosolic fraction at PND14, while particulate PKand an increase in ratio between fractions at 6 mg/kg on PND14. The results from this study indicate that the patterns of subcellular distributions of PKC isoforms following a developmental PCB exposure were PKC isozyme- and developmental stage-specific. Considering the significant role of PKC signaling in motor behavior, learning and memory, it is suggested that altered subcellular distribution of PKC isoforms at critical periods of brain development may be a possible mechanism of PCB-induced neurotoxic effects and that PKC-alpha, gamma, and epsilon may be among the target molecules implicated with PCB-induced neurological impairments during developmental exposure. It is believed that this is the first report successfully identifying PKC isoforms responding to PCBs during developmental exposure.

Identification of calcium-dependent and -independent signaling pathways involved in polychlorinated biphenyl-induced cyclic AMP-responsive element-binding protein phosphorylation in developing cortical neurons

Cyclic AMP (cAMP)-responsive element-binding protein (CREB) is a transcription factor important in developing nervous system cells and is activated by a variety of signaling molecules. Aroclor 1254 (A1254), a polychlorinated biphenyl mixture, perturbs Ca(2+) homeostasis and increases CREB phosphorylation in rat neonatal cortical cell cultures in a time- and concentration-dependent manner. The present experiments determined that the cell type responding to A1254 with Ca(2+) increases and phosphorylated CREB (phospho-CREB) was predominantly of neuronal morphology and microtubule-associated protein (MAP2)-positive phenotype. Similarly, glutamate (100 microM) increased phospho-CREB immunoreactivity selectively in MAP2-immunopositive cells. Using Western blotting and immunocytochemical techniques, we identified key signal transduction pathways operative in phosphorylating CREB in cortical cell cultures and examined their participation in 3 ppm A1254-induced CREB activation. Cortical cultures treated with glutamate, forskolin or the phorbol ester phorbol 12-myristate 13-acetate exhibited robust increases in phospho-CREB. Tetrodotoxin (1 microM) completely inhibited CREB phosphorylation by A1254, suggesting that synaptic activity is involved in A1254-induced CREB activation. Buffering [Ca(2+)](i) with bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester in the absence of extracellular Ca(2+) partially inhibited A1254-induced CREB phosphorylation. Inhibition of mitogen-activated protein kinase (10 microM U0126) or protein kinase C (PKC; bisindoylmaleimide, 5 microM) activation did not inhibit A1254-induced CREB phosphorylation. By contrast, inhibition of protein kinase A (PKA) with 100 microM PKA inhibitor peptide, PKI, blocked A1254-induced CREB phosphorylation. Thus, we examined whether A1254 activates PKA by increasing cAMP; 10 microM forskolin, but not A1254, elevated intracellular cAMP levels. These results indicate that in neocortical cells in culture, CREB phosphorylation occurs via Ca(2+)-, PKA-, and PKC-dependent pathways. Furthermore, A1254-induced CREB phosphorylation occurs predominantly in neurons, is dependent on synaptic activity and mediated by Ca(2+)- and PKA-dependent pathways.

Gene expression patterns predict exposure to PCBs in developing Xenopus laevis tadpoles

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants that pose global ecological and human health problems. Although it is well established that PCBs are associated with a variety of adverse health effects in wildlife and in humans, it is often difficult to determine direct cause-and-effect relationships between exposure and specific health outcomes. In this study, gene expression signatures were used to relate exposure to PCBs with altered physiological responses and/or specific health effects. Real-time PCR was used to measure gene expression levels for 10 genes in Xenopus laevis tadpoles (18 days postfertilization, PF) after acute exposure (2 days) to the PCB mixture Aroclor 1254. Specific gene expression signatures correlated with exposure and were predictive of adverse health effects. Exposure to low levels of Aroclor 1254 (5-50 ppb) significantly increased expression of six genes, independent of any health effects; exposure to midlevel concentrations (300-400 ppb) significantly decreased expression levels of two genes, NGF and beta-actin, prior to the onset of observable health effects; exposure to higher doses (500-700 ppb) significantly decreased NGF and beta-actin expression concomitant with the appearance of gross morphological abnormalities, behavioral deficits, and a statistically significant decrease in survival. This study expands upon our previous work that demonstrated an age-dependent susceptibility to Aroclor 1254 in Xenopus laevis tadpoles and that defined specific gene expression signatures as useful bioindicators of exposure and as predictors of overt or impending health effects.

The polychlorinated biphenyl mixture aroclor 1254 induces death of rat cerebellar granule cells: the involvement of the N-methyl-D-aspartate receptor and reactive oxygen species

Polychlorinated biphenyls (PCBs) are widespread persistent environmental contaminants that display a complex spectrum of toxicological properties, including neurotoxicity. The present study investigates the effects of the PCB mixtures Aroclor 1242 (A1242) and Aroclor 1254 (A1254), and the PCB congeners 126 (3,3',4,4',5,-PeCB) and 153 (2,2',4,4',5,5'-HxCB) on formation of reactive oxygen species (ROS) and cell death in cultured rat cerebellar granule cells. The increase of ROS and induction of cell death were assayed using the fluorescent probe 2,7-dichlorofluorescin diacetate (DCFH-DA) and the trypan blue exclusion assay, respectively. A1242 and A1254 and PCB 153 induced a concentration-dependent increase in cell death and ROS formation. A1254 was selected for mechanistic studies. When the cerebellar granule cells were exposed to 15 microM A1254 for 12 h, 95% of the cells died. Both PCB-mediated cell death and the increase of the ROS formation were inhibited by MK-801, demonstrating the importance of the N-methyl-D-aspartate receptor. Inhibitors of nitric oxide synthase and phospholipase A2 led to a significant reduction of the DCF fluorescence and cell death. The mitochondrial permeability transition pore blocker cyclosporin A and the antioxidant vitamin E also increased survival and reduced ROS formation. The results show a connection between cell death and free radical formation.