Increased [3H]phorbol ester binding in rat cerebellar granule cells and inhibition of 45Ca2+ sequestration in rat cerebellum by polychlorinated diphenyl ether congeners and analogs: structure-activity relationships

Integrating data gap filling techniques: A case study predicting TEFs for neurotoxicity TEQs to facilitate the hazard assessment of polychlorinated biphenyls

The application of toxic equivalency factors (TEFs) or toxic units to estimate toxic potencies for mixtures of chemicals which contribute to a biological effect through a common mechanism is one approach for filling data gaps. Toxic Equivalents (TEQ) have been used to express the toxicity of dioxin-like compounds (i.e., dioxins, furans, and dioxin-like polychlorinated biphenyls (PCBs)) in terms of the most toxic form of dioxin: 2,3,7,8-tetrachlorodibenzo-p-dioxin (2,3,7,8-TCDD). This study sought to integrate two data gap filling techniques, quantitative structure-activity relationships (QSARs) and TEFs, to predict neurotoxicity TEQs for PCBs. Simon et al. (2007) previously derived neurotoxic equivalent (NEQ) values for a dataset of 87 PCB congeners, of which 83 congeners had experimental data. These data were taken from a set of four different studies measuring different effects related to neurotoxicity, each of which tested overlapping subsets of the 83 PCB congeners. The goals of the current study were to: (i) evaluate an alternative neurotoxic equivalent factor (NEF) derivations from an expanded dataset, relative to those derived by Simon et al. and (ii) develop QSAR models to provide NEF estimates for the large number of untested PCB congeners. The models used multiple linear regression, support vector regression, k-nearest neighbor and random forest algorithms within a 5-fold cross validation scheme and position-specific chlorine substitution patterns on the biphenyl scaffold as descriptors. Alternative NEF values were derived but the resulting QSAR models had relatively low predictivity (RMSE ∼0.24). This was mostly driven by the large uncertainties in the underlying data and NEF values. The derived NEFs and the QSAR predicted NEFs to fill data gaps should be applied with caution.

Mechanisms of Action Point Towards Combined PBDE/NDL-PCB Risk Assessment

At present, human risk assessment of the structurally similar non-dioxin-like (NDL) PCBs and polybrominated diphenylethers (PBDEs) is done independently for both groups of compounds. There are however obvious similarities between NDL-PCBs and PBDEs with regard to modulation of the intracellular calcium homeostasis (basal calcium levels, voltage-gated calcium channels, calcium uptake, ryanodine receptor) and thyroid hormone (TH) homeostasis (TH levels and transport). which are mechanisms of action related to neurobehavioral effects (spontaneous activity, habituation and learning ability). There also similarities in agonistic interactions with the hepatic nuclear receptors PXR and CAR. Several effects on developmental (reproductive) processes have also been observed, but results were more dispersed and insufficient to compare both groups of compounds. The available mechanistic information is sufficient to warrant a dose addition model for NDL-PCBs and PBDEs, including their hydroxylated metabolites.Although many of the observed effects are similar from a qualitative point of view for both groups, congener or tissue specific differences have also been found. As this is a source of uncertainty in the combined hazard and risk assessment of these compounds, molecular entities involved in the observed mechanisms and adverse outcomes associated with these compounds need to be identified. The systematical generation of (quantitative) structure-activity information for NDL-PCBs and PBDEs on these targets (including potential non-additive effects) will allow a more realistic risk estimation associated with combined exposure to both groups of compounds during early life. Additional validation studies are needed to quantify these uncertainties for risk assessment of NDL-PCBs and PBDEs.

Modulation of Calmodulin and Protein Kinase C Activities by Pencillium Mycotoxins

Calmodulin (CaM), a calcium-binding protein, is found in high concentrations in mammalian brain where it plays a pivotal role in a large number of cellular functions. Protein kinase C (PKC), a multifunctional cytosolic enzyme, in the presence of both Ca2+ and phospholipids, transduce extracellular signals into intracellu-lar events. Both CaM and PKC are partially involved in maintaining Ca2+ homeostasis in the cell. Any fluctuations in the intracel-lular Ca2+ can modulate cellular functions and may contribute to neuronal dysfunction. Hence, the present investigation was initiated to study the effects of some selected penicillium (naturally occurring tremorgenic) mycotoxins like secalonic acid, citreoviridin, and verruculogen on CaM activity, active conformation of CaM and PKC activity. Stimulation of CaM-deflcient bovine brain 3′-5′ phosphodieste rase (PDE) indicated CaM activity. The modification of CaM active conformation was studied by the binding of fluorescent probe N-phenyl-1-napthylamine (NPN) to CaM. Alterations in the fluorescence of dansyl-CaM was used to study the effect of these compounds on complex formation between CaM and PDE. Rat brain cytosolic PKC was studied using 32P-ATP as a measure of altered protein phosphorylation. The concentrations of mycotoxins used were in the range of 10 to 50 μM. All three mycotoxins inhibited CaM-stimulated PDE activity in a concentration-dependent manner. Citreoviridin and secalonic acid inhibited NPN fluorescence and Ca2+-dependent complex formation of dansyl-CaM and PDE. The IC50 values for NPN fluorescence of citreoviridin and secalonic acid were 13 μM and 19 μM respectively. However, verruculogen showed little effect on NPN fluorescence and the Ca2+-dependent complex formation of dansyl-CaM and PDE. These mycotoxins also inhibited PKC activity in a concentration-dependent manner with IC50 values of 19.8, 25.7, and 38.4 μM for secalonic acid, citreoviridin, and verruculogen, respectively. The results of our study suggest that these mycotoxins at very low concentrations are interacting with CaM and PKC. Such an effect could lead to impairment of neurotransmission and result in neurotoxicity.

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.

Quantitative structure-activity relationship (QSAR) studies for predicting activation of the ryanodine receptor type 1 channel complex (RyR1) by polychlorinated biphenyl (PCB) congeners

A quantitative structure-activity relationship (QSAR) was developed to predict the congener specific ryanodine receptor type RyR1 activity of all 209 polychlorinated biphenyl (PCB) congeners. A three-variable QSAR equation was obtained via stepwise forward linear regression on an unsupervised forward selection reduced data set from an initial database. Application of the QSAR towards predicting EC(2x) values for all 209 PCB congeners indicated good agreement in substitution pattern trends between the experimental and estimated data sets. The QSAR model predicts a less than two-fold increase in maximal potency among all congeners outside the experimental database, and it appears that no high-potency PCB congeners with EC(2x) values much less than 0.2 microM exist. Increasing RyR1-neuro toxicity equivalents with increasing homologue number and Aroclor chlorination likely reflect indirect molecular controls on toxicity, since congeners with multiple ortho substituents-the primary structural feature controlling a lack of coplanarity and resulting neurotoxicity-are more likely to be found in higher homologues.

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.

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.

Polychlorinated diphenyl ethers (PCDEs): environmental levels, toxicity and human exposure: a review of the published literature

This paper reviews the state of the science regarding polychlorinated diphenyl ethers (PCDEs), a group of halogenated aromatic compounds, which are structurally related to polychlorinated biphenyls (PCBs) and polychlorinated dibenzofurans (PCDFs). Special attention is paid to the environmental levels, toxicity, and human exposure. PCDEs have been detected in a number of environmental samples, and their widespread occurrence in the environment is mainly the result of their presence as impurities in chlorophenol preparations. In humans, PCDE congeners have been detected in adipose tissue. As for other persistent organic pollutants (POPs), dietary intake is very probably the main route of exposure to PCDEs for the general population. However, data concerning PCDE levels in foodstuffs are very limited. It is concluded that investigations on experimental toxicity, dietary intake, and potential human health effects of PCDEs are clearly necessary.

Polybrominated diphenyl ether (PBDE) effects in rat neuronal cultures: 14C-PBDE accumulation, biological effects, and structure-activity relationships

Polybrominated diphenyl ethers (PBDEs), widely used as flame-retardants, are now recognized as globally distributed pollutants, and are detected in most environmental and biological samples, including human blood, adipose tissue, and breast milk. Due to their wide use in commercial products and their persistent nature, long-term exposure to PBDEs may pose a human health risk, especially to children. Our previous reports showed that the commercial PBDE mixture, DE-71, affected protein kinase C (PKC) and calcium homeostasis in a similar way to those of a structurally-related polychlorinated biphenyl (PCB) mixture. These intracellular signaling events are associated with neuronal development and learning and memory function. The objectives of the present study were to test whether environmentally relevant PBDE congeners, with different position and number of bromines, affected PKC translocation in cerebellar granule neuronal cultures and compare the potency and efficacy of PBDE congeners with their 14C-accumulation. All the tested PBDE congeners increased 3H-phorbol ester (PDBu) binding, and a significant effect was seen as low as 10 microM. Among the congeners tested, 2,2',4,4'-tetrabromodiphenyl ether (PBDE 47) increased 3H-PDBu binding in a concentration-dependent manner and to a greater extent than other congeners. These effects were seen at concentrations and exposure times where no cytotoxicity was observed. The efficacy of PBDE congeners varied with their structural composition, and the effects seen on 3H-PDBu binding with some PBDE congeners are similar to those of PCB congeners. Cerebellar granule neurons accumulated all three PBDE congeners (PBDEs 47, 99, and 153) following exposure. At the lowest concentration (0.67 microM), about 13-18% of the total dose of 14C-PBDE congeners was accumulated by these neurons. There were distinct differences in the pattern of 14C-PBDE accumulation among the PBDE congeners. The 14C-PBDE accumulation, either represented as percent basis or nanomole basis, was much lower for the 30.69 microM PBDE 99 and 10.69-30.69 microM PBDE 153 than at the lower concentrations, which may be due to low solubility of these congeners. The accumulation pattern with PBDE 47 did not vary with concentration. On a nanomole accumulation basis, PBDEs 47, 99, and 153 accumulation was linear with time. While the nanomole accumulation was linear with concentration for PBDE 47, it is nonlinear for PBDEs 99 and 153. The pattern of PBDE accumulation seems to correlate with the effects on PKC translocation, with regression values of 0.773-0.991. These results indicate that PBDEs affected PKC translocation in neurons in a similar way to those of other organohalogens, some PBDE congeners are equally efficacious as the respective PCB congeners, and PBDE accumulation correlated well with PKC translocation, suggesting a common mode of action for this group of chemicals.

Quantitative structure-activity relationship models for prediction of the toxicity of polybrominated diphenyl ether congeners

Levels of polybrominated diphenyl ethers (PBDEs) are increasing in the environment and may cause long-term health problems in humans. The similarity in the chemical structures of PBDEs and other halogenated aromatic pollutants hints on the possibility that they might share similar toxicological effects. In this work, three-dimensional quantitative structure activity relationships (3-D-QSAR) models, using comparative molecular field analysis (CoMFA) and comparative similarity indices analysis (CoMSIA), were built based on calculated structural indices and a reported experimental toxicology index (aryl hydrocarbon receptor relative binding affinities, RBA) of 18 PBDEs congeners, to determine the factors required for the RBA of these PBDEs. After performing leave-one-out cross-validation, satisfactory results were obtained with cross-validation O2 and R2 values of 0.580 and 0.995 by the CoMFA model and 0.680 and 0.982 by the CoMSIA model, respectively. The results showed clearly that the nonplanar conformations of PBDEs result in the lowest energy level and that the electrostatic index was the main factor reflecting the RBA of PBDEs. The two QSAR models were then used to predict the RBA value of 46 PBDEs for which experimental values are unavailable at present.

Estrogenic endocrine disruptive components interfere with calcium handling and differentiation of human trophoblast cells

During development, calcium (Ca) is actively transported by placental trophoblasts to meet fetal nutritional and the skeletal mineralization needs. Maternal exposure to estrogenic pesticides, such as 1,1-bis(p-chlorophenyl)-2,2,2-trichloroethane (DDT) and methoxychlor (MTC), has been shown to result in reproductive disorders and/or abnormal fetal development. In this study, we have examined the effects of exposure of trophoblastic cells to MTC and DTT, in comparison to 17beta-estradiol (E2) and diethylstilbestrol (DES), to test the hypothesis that cellular Ca handling is a target for these endocrine disruptive components. Treatment with DDT, MTC, DES, or E2 increased cellular Ca uptake, and the expression of trophoblast-specific human Ca binding protein (HCaBP) was down-regulated by both MTC and DDT. Treatment with MTC, DDT, and DES inhibited cell proliferation, induced apoptosis, and suppressed expression of several trophoblast differentiation marker genes. These effects were reversed by overexpression of metallothionein IIa, a gene highly responsive to cadmium and other metals. These results strongly suggest that trophoblast Ca handling functions are endocrinally modulated, and that their alteration by candidate endocrine disruptors, such as MTC and DDT, constitutes a possible pathway of the harmful effects of these components on fetal development.

Increased [3H]phorbol ester binding in rat cerebellar granule cells and inhibition of 45Ca(2+) buffering in rat cerebellum by hydroxylated polychlorinated biphenyls

Our previous structure-activity relationship (SAR) studies indicated that the effects of polychlorinated biphenyls (PCBs) on neuronal Ca(2+) homeostasis and protein kinase C (PKC) translocation were associated with the extent of coplanarity. Chlorine substitutions at ortho position on the biphenyl, which increase the non-coplanarity, are characteristic of the most active congeners in vitro. In the present study, we investigated the effects of selected hydroxylated PCBs, which are major PCB metabolites identified in mammals, on the same measures where PCBs had differential effects based on structural configuration. These measures include PKC translocation as determined by [3H]phorbol ester ([3H]PDBu) binding in cerebellar granule cells, and Ca(2+) sequestration as determined by 45Ca(2+) uptake by microsomes isolated from adult rat cerebellum. All the selected hydroxy-PCBs with ortho-chlorine substitutions increased [3H]PDBu binding in a concentration-dependent manner and the order of potency as determined by E(50) (concentration that increases control activity by 50%) is 2',4',6'-trichloro-4-biphenylol (32 +/- 4 microM), 2',5'-dichloro-4-biphenylol (70 +/- 9 microM), 2,2',4',5,5'-pentachloro-4-biphenylol (80 +/- 7 microM) and 2,2',5'-trichloro-4-biphenylol (93 +/- 14 microM). All the selected hydroxy-PCBs inhibited microsomal 45Ca(2+) uptake to a different extent. Among the hydroxy-PCBs selected, 2',4',6'-trichloro-4-biphenylol is the most active in increasing [3H]PDBu binding as well as inhibiting microsomal 45Ca(2+) uptake. 3,5-Dichloro-4-biphenylol and 3,4',5-trichloro-4-biphenylol did not increase [3H]PDBu binding, but inhibited microsomal 45Ca(2+) uptake. This effect was not related to ionization of these two hydroxy-PCBs. Hydroxylated PCBs seemed to be as active as parent PCBs in vitro. These studies indicate that PCB metabolites such as hydroxy-PCBs might contribute significantly to the neurotoxic responses of PCBs.

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.

A perspective on the potential health risks of PBDEs

The polybrominated diphenylethers (PBDEs) are a class of chemicals widely used as flame retardants. Concentrations of PBDEs in some human and marine mammal populations are increasing. The toxicological endpoints of concern for environmental levels of PBDEs are likely to be thyroid hormone disruption, neurodevelopmental deficits and cancer. Unfortunately, the available toxicological evidence for these endpoints is surprisingly limited, given their widespread use, bioaccumulative potential, and structural similarity to thyroid hormones and polychlorinated biphenyls (PCBs). Available evidence suggests that the PBDE congeners likely to bioaccumulate (i.e., those observed in human tissues and other biota) have the propensity to disrupt thyroid hormones, cause neurobehavioral deficits and possibly cause cancer in laboratory animals. It is unclear whether current concentrations of PBDEs in human tissues would be expected to adversely impact human health. Since nearly all individuals are exposed to low levels of PBDEs, the potential health impacts also should include assessment at the population level. This paper summarizes the available toxicological evidence for PBDE-induced thyroid hormone disruption, neurodevelopmental deficits, and, for some congeners, cancer, and provides a perspective on the potential risks of the PBDEs for human health.

Synthesis of polybrominated diphenyl ethers and their capacity to induce CYP1A by the Ah receptor mediated pathway

Polybrominated diphenyl ethers (PBDEs) have become widely distributed as environmental contaminants due to their use as flame retardants. Their structural similarity to other halogenated aromatic pollutants has led to speculation that they might share toxicological properties such as hepatic enzyme induction. In this work we synthesized a number of PBDE congeners, studied their affinity for rat hepatic Ah receptor through competitive binding assays, and determined their ability to induce hepatic cytochrome P-450 enzymes by means of EROD (ethoxyresorufin-O-deethylase) assays in human, rat, chick, and rainbow trout cells. Both pure PBDE congeners and commercial PBDE mixtures had Ah receptor binding affinities 10(-2)-10(-5) times that of 2,3,7,8-tetrachlorodibenzo-p-dioxin. In contrast with polychlorinated biphenyls, Ah receptor binding affinities of PBDEs could not be related to the planarity of the molecule, possibly because the large size of the bromine atoms expands the Ah receptor's binding site. EROD activities of the PBDE congeners followed a similar rank order in all cells. Some congeners, notably PBDE 85, did not follow the usual trend in which strength of Ah receptor binding affinity paralleled P-450 induction potency. Use of the gel retardation assay with a synthetic oligonucleotide indicated that in these cases the liganded Ah receptor failed to bind to the DNA recognition sequence.

Possible molecular targets of halogenated aromatic hydrocarbons in neuronal cells

Halogenated aromatic hydrocarbon including polychlorinated biphenyls (PCBs) are persistent and bioaccumulative environmental toxicants. Although health effects associated with exposure to these chemicals, including motor dysfunction and impairment in memory and learning, have been identified, their molecular site of action is unknown. Previous study from this laboratory demonstrated that, while ortho PCBs perturbed intracellular signaling mechanisms including Ca2+ homeostasis, receptor-mediated inositol phosphate production and translocation of PKC, non-ortho PCBs did not. Since PKC signaling pathway is implicated in the modulation of motor behavior, as well as learning and memory, and the roles of PKC are isoform-specific, we have now studied the effects of two structurally distinct PCBs on isoforms of PKC in cerebellar granule cell culture model. Cells were exposed to 2,2'-dichlorobiphenyl (ortho PCB; 2,2'-DCB) or 4,4'-dichlorobiphenyl (non-ortho PCB; 4,4'-DCB) for 15 min, respectively, and subsequently fractionated and immunoblotted against the selected PKC monoclonal antibodies (alpha, gamma, delta, epsilon, lambda, iota). While 2,2'-DCB induced a translocation of PKC-alpha [cytosol (% control): 54 +/- 12 at 25 microM and 66 +/- 10 at 50 microM; membrane (% control): 186 +/- 37 at 25 microM and 200 +/- 48 at 50 microM] and -epsilon [cytosol (% control): 92 +/- 12 at 25 microM and 97 +/- 15 at 50 microM; membrane (% control): 143 +/- 23 at 25 microM and 192 +/- 24 at 50 microM] from cytosol to membrane fraction in a concentration-dependent manner, 4,4'-DCB had no effects. 2,2'-DCB induced translocation of PKC-alpha was blocked by pretreatment with sphingosine, suggesting a possible role of sphingolipid pathway. Although reports on implication of PKC-gamma with learning and memory are relatively extensive, the expression of this particular isoform in the primary cerebellar granule cells was below the detectable level. PKC-delta, -lambda and -iota were present in these cells, but were not altered by PCB exposure. These results suggest that the effects of 2,2'-DCB on PKC is isoform-dependent and PKC-alpha as well as PKC-epsilon may be target molecules for ortho-PCBs in neuronal cells.

Repeated exposure of adult rats to Aroclor 1254 causes brain region-specific changes in intracellular Ca2+ buffering and protein kinase C activity in the absence of changes in tyrosine hydroxylase

Polychlorinated biphenyls (PCBs) are ubiquitous environmental contaminants, some of which may be neurotoxic. In vitro studies from this laboratory indicated that noncoplanar PCBs perturbed intracellular signal transduction mechanisms including Ca2+ homeostasis, receptor-mediated inositol phosphate production, and translocation of protein kinase C (PKC). In the present study, we examined the effects of PCBs in vivo by dosing adult male Long-Evans rats orally with Aroclor 1254 (0, 10, or 30 mg/kg/day; 5 days/week for 4 weeks) in corn oil. At 24 h after the last dose, rats were tested for motor activity in a photocell device for 30 min. Immediately, the rats were euthanized, blood was collected for thyroid hormone analysis, and brains were removed, dissected into regions (cerebellum, frontal cortex, and striatum), and subcellular fractions were obtained for neurochemical analysis. Following Aroclor 1254 treatment, body weight gain in the high-dose group was significantly lower than the control and low-dose groups. Horizontal motor activity was significantly lower in rats dosed with 30 mg/kg Aroclor 1254. Ca2+ buffering by microsomes was significantly lower in all three brain regions from the 30 mg/kg group. In the same dose group, mitochondrial Ca2+ buffering was affected in cerebellum but not in cortex or striatum. Similarly, total cerebellar PKC activity was decreased significantly while membrane-bound PKC activity was significantly elevated at 10 and 30 mg/kg. PKC activity was not altered either in cortex or the striatum. Neurotransmitter levels in striatum or cortex were slightly altered in PCB-exposed rats compared to controls. Furthermore, repeated oral administration of Aroclor 1254 to rats did not significantly alter forebrain tyrosine hydroxylase immunoreactivity or enzymatic activity. Circulating T4 (total and free) concentrations were severely depressed at both doses in Aroclor 1254-exposed rats compared to control rats, suggesting a severe hypothyroid state. These results indicate that (1) in vivo exposure to a PCB mixture can produce changes in second messenger systems that are similar to those observed after in vitro exposure of neuronal cell cultures; (2) second messenger systems seem to be more sensitive than alterations in neurotransmitter levels or tyrosine hydroxylase involved in dopamine synthesis during repeated exposure to PCBs; and (3) the observed motor activity changes were independent of changes in striatal dopamine levels.

Ortho-substituted polychlorinated biphenyls activate respiratory burst measured as luminol-amplified chemoluminescence in human granulocytes

The effect of polychlorinated biphenyls (PCBs) on the activation of respiratory burst measured as luminol-amplified chemoluminescence in human granulocytes is elucidated here. Chemoluminescence was stimulated in a concentration-dependent manner (ED50 approximately 10 microM) by ortho-substituted PCB congeners, while meta- and para-substituted congeners had no significant effect. Two ortho-substituted PCB congeners were chosen for the mechanistic studies, namely 2,2',4,4'-TeCB and 2,2'-DCB, since they have been used in previous studies by others. In the absence of extracellular calcium, the respiratory burst in response to 2,2'-DCB and 2,2',4,4'-TeCB was reduced by 63% and 82%, respectively. Bisindolylmaleimide, which inhibits protein kinase C, reduced activated chemoluminescence by 2,2'-DCB, 2,2',4,4'-TeCB, N-formyl-methionyl-leucyl-phenylalanine, and phorbol 12-myristate 13-acetate. Neomycin, which inhibits phospholipase C, had a slight, but significant, effect on the 2,2',4,4'-TeCB-activated chemoluminescence but had a more pronounced effect on the 2,2'-DCB-activated chemoluminescence. 2,2'-DCB and 2,2',4,4'-TeCB significantly increased phospholipase D (PLD) activity measured as the amount of 14C-phosphatidylbutanol formed. Ethanol (1%), a phospholipase D modulator, reduced the response to 2,2'-DCB and 2,2',4,4'-TeCB by 72% and 75%, respectively. Furthermore, wortmannin (25 nM), a phosphatidylinositol 3-kinase, and genistein, a more unspecific tyrosine kinase inhibitor, reduced chemoluminescence in response to PCB. In conclusion, our results indicate that PCB-activated chemoluminescence is dependent on the Ca(2+)-dependent phospholipase D or phospholipase C, phosphatidylinositol 3-kinase, and protein kinase C activation prior to activation of the NADPH oxidase. Defects in neutrophhil functions upon exposure to PCB may render a greater susceptibility in the host to invading microorganisms or evoke inappropriate inflammatory responses leading to tissue injury.

PCBs reduce long-term potentiation in the CA1 region of rat hippocampus

Prenatal exposure to polychlorinated biphenyls (PCBs) has been associated with a lower IQ in childhood. We have examined the effects of acute exposure to PCB mixtures and two single congeners on synaptic transmission between Schaffer collaterals and CA1 neurons of the rat hippocampus as well as posttetanic potentiation (PTP), paired pulse facilitation (PPF), and long-term potentiation (LTP). PTP and PPF represent transient increases in transmitter release immediately after stimulation, while LTP is a measure of long-term changes in synaptic plasticity that has been related to learning and memory. LTP, but neither PTP nor PPF, was reduced by Aroclor 1016 in a dose-dependent fashion at concentrations that had little effect on general synaptic transmission. The more highly chlorinated Aroclor 1254 at low concentrations specifically blocked LTP, but at higher concentrations also reduced synaptic transmission. The mono-ortho PCB congener 2,4,4'-trichlorobiphenyl and the coplanar congener 3,3',4,4'-tetrachlorobiphenyl also blocked LTP without effect on PTP or PPF. We conclude that PCBs selectively impair the process of LTP in CA1 neurons of the hippocampus.

Polychlorinated diphenylethers: origin, analysis, distribution, and toxicity in the marine environment

PCDEs are present as impurities in chlorophenol preparations, which are often used as wood preservatives. Other sources of PCDEs such as municipal waste incinerators seem to be of minor importance. It is estimated that, worldwide 250-2500 metric tons of PCDEs have been produced, which is a lesser amount compared to the production figures of polychlorinated biphenyls (PCBs), polybrominated diphenylethers (PBDEs), or polychlorinated naphthalenes (PCNs). The required analytical methods are rather complex and are comparable to methods used for polychlorinated dibenzo-p-dioxins (PCDDs) and furans (PCDFs). Gas chromatography with low resolution or high resolution mass spectrometry or with electron-capture detection is recommended for the final determination of PCDEs. There is only limited information on the presence of PCDEs in the marine environment. High concentrations of PCDEs were found in Baltic white-tailed sea eagles (up to 13 mg/kg lipid weight per congener). Total PCDE concentrations of 50-300 micrograms/kg were reported in cod liver oil samples. PCDEs are immunotoxic and show a clear potency as inducers of hepatic microsomal AHH and EROD activity. Although PCDE concentrations in marine organisms are expected to be lower than those of PCBs, PBDEs, and PCNs, because of their toxic properties, persistence, and hydrophobic and bioaccumulative character and because of the limited information on these concentrations in marine organisms, further research on these concentrations in marine organisms is recommended.

Maternal and developmental toxicity of polychlorinated diphenyl ethers (PCDEs) in Swiss-Webster mice and Sprague-Dawley rats

Polychlorinated diphenyl ethers (PCDEs) are industrial byproducts found in many ecosystems at low levels. PCDEs are not markedly toxic to adult rodents, but their developmental toxicity has not previously been examined. We evaluated the maternal and perinatal toxicity of nine PCDE congeners to outbred mice when compounds were administered from gestation day (GD) 6 through GD 15. 2,2',4,4',5,6'-hexaCDE and 2,3',4',6-tetraCDE decreased the number of pups born per female mated and the number of pups surviving per litter born. 2,2',4,4',5,5'-hexaCDE and 2,2',4,5,6'-pentaCDE decreased the number of litters born per female mated, without decreasing postnatal survival. The other PCDEs did not decrease survival either pre- or postnatally. None of the PCDEs caused absence of Harderian glands in surviving offspring at the doses administered. Neither induction of cytochromes P450 nor tissue residues of individual congeners correlated well with developmental toxicity. Three PCDEs were also evaluated in outbred (Sprague-Dawley) rats: 2,2',4,5,6'-pentaCDE and 2,3',4',6-tetraCDE, because of their toxicity to mice; 2,2',4,4',5,5'-hexaCDE, because it should exhibit PCB-like toxicity. Each congener was administered at three dose levels from GD 6 through GD 15. 2,2',4,5,6'-pentaCDE decreased the number of litters born at 100 mg/kg/day, and the survival of pups in litters carried to term, at both 50 and 100 mg/kg per day. Postnatal weight gain was also reduced. In contrast to its action in mice, 2,3',4',6-tetraCDE decreased neither the numbers of litters born nor postnatal survival of rat offspring, but did suppress postnatal weight gain at least through PD 5. As in mice, induction of cytochromes P450 was not well correlated with the developmental toxicity of individual congeners.