Risk of nervous system cancer among workers exposed to lead

Nutrition and Diet: A Double-Edged Sword in Development and Treatment of Brain Tumors

Brain tumor (BT) is the second most common pediatric cancer, one of the most common cancers among adults, and the major cause of cancer-related morbidity and mortality worldwide. Both genetics and environment can contribute to BT induction. One of the environmental risks is diet which has not been proven as a certain hazard yet. The objective of the current chapter was to review the literature concerning both positive and negative effects of nutrition on BT risk.

Systems Toxicology Approach for Assessing Developmental Neurotoxicity in Larval Zebrafish

Adverse outcomes that result from chemical toxicity are rarely caused by dysregulation of individual proteins; rather, they are often caused by system-level perturbations in networks of molecular events. To fully understand the mechanisms of toxicity, it is necessary to recognize the interactions of molecules, pathways, and biological processes within these networks. The developing brain is a prime example of an extremely complex network, which makes developmental neurotoxicity one of the most challenging areas in toxicology. We have developed a systems toxicology method that uses a computable biological network to represent molecular interactions in the developing brain of zebrafish larvae. The network is curated from scientific literature and describes interactions between biological processes, signaling pathways, and adverse outcomes associated with neurotoxicity. This allows us to identify important signaling hubs, pathway interactions, and emergent adverse outcomes, providing a more complete understanding of neurotoxicity. Here, we describe the construction of a zebrafish developmental neurotoxicity network and its validation by integration with publicly available neurotoxicity-related transcriptomic datasets. Our network analysis identified consistent regulation of tumor suppressors p53 and retinoblastoma 1 (Rb1) as well as the oncogene Krüppel-like factor (Klf8) in response to chemically induced developmental neurotoxicity. The developed network can be used to interpret transcriptomic data in a neurotoxicological context.

Occupational Lead Exposure and Brain Tumors: Systematic Review and Meta-Analysis

(1) Background: Due to inconsistencies in epidemiological findings, there has been uncertainty regarding the association of lead compounds with brain tumors. We performed a meta-analysis of published case-control and cohort studies exploring lead compound exposure and brain tumor risk. (2) Methods: We searched PubMed, Embase^®, and Cochrane to find eligible studies. Eighteen studies were selected for assessment of occupational exposure to lead compound and brain tumor. Pooled estimates of odds ratios (ORs) were obtained using random effects models. We assessed the differences through subgroup analysis according to tumor type, study design, measurements of exposure, and tumor outcome. Statistical tests for publication bias, heterogeneity, and sensitivity analysis were applied. (3) Results: Our systematic review and meta-analysis showed a not significant association with lead exposure and risk of benign and malignant brain tumors (pooled OR = 1.11, 95% Confidence Interval (CI): 0.95–1.29). Including only malignant brain tumors, the risk of brain tumor was significantly increased (pooled OR = 1.13, 95% CI: 1.04–1.24). (4) Conclusions: This meta-analysis provides suggestive evidence for an association between lead compound exposure and brain tumor. In future studies, it will be necessary to identify the effect of lead compounds according to the types of brain tumor.

High incidence of brain and other nervous system cancer identified in two mining counties, 2001-2015

Two counties in Montana, Deer Lodge and Silver Bow (DL/SB), have two Superfund sites, as well as an active copper and molybdenum mine in SB. The population living in proximity to these sites are exposed to additional metals and some have been shown to be neurotoxic, especially for children; thus, this study focused on the incidence of brain and other nervous system cancers. The Montana Central Tumor Registry data was used to identify the cases in DL/SB and the remaining 54 counties of Montana (comparison group). After controlling for sex, cancer stage, and year of diagnosis, we found an incidence rate ratio for DL/SB versus comparison group of 6.28 (95% CI: 2.32-17.02) for children ages birth to 4 years, and 3.95 (95% CI: 1.66-9.38) for adults age 30-34 years. The high incidence rate of the brain cancer in the two age groups requires public health action.

Ecogenetics of lead toxicity and its influence on risk assessment

Lead (Pb) toxicity is a public health problem affecting millions worldwide. Advances in 'omic' technology have paved the way to toxico-genomics which is currently revolutionizing the understanding of interindividual variations in susceptibility to Pb toxicity and its functional consequences to exposure. Our objective was to identify, comprehensively analyze, and curate all the potential genetic and epigenetic biomarkers studied to date in relation to Pb toxicity and its association with diseases. We screened a volume of research articles that focused on Pb toxicity and its association with genetic and epigenetic signatures in the perspective of occupational and environmental Pb exposure. Due to wide variations in population size, ethnicity, age-groups, and source of exposure in different studies, researchers continue to be skeptical on the topic of the influence of genetic variations in Pb toxicity. However, surface knowledge of the underlying genetic factors will aid in elucidating the mechanism of action of Pb. Moreover, in recent years, the application of epigenetics in Pb toxicity has become a promising area in toxicology to understand the influence of epigenetic mechanisms such as DNA methylation, chromatin remodeling, and small RNAs for the regulation of genes in response to Pb exposure during early life. Growing evidences of ecogenetic understanding (both genetic and epigenetic processes) in a dose-dependent manner may help uncover the mechanism of action of Pb and in the identification of susceptible groups. Such studies will further help in refining uncertainty factors and in addressing risk assessment of Pb poisoning.

Cancer incidence among workers with blood lead measurements in two countries

OBJECTIVE

Study carcinogenicity of inorganic lead, classified as 'probably carcinogenic' to humans by the International Agency for Research on Cancer (brain, lung, kidney and stomach).

METHODS

We conducted internal and external analyses for cancer incidence in two cohorts of 29 874 lead-exposed workers with past blood lead data (Finland, n=20 752, Great Britain=9122), with 6790 incident cancers. Exposure was maximum measured blood lead.

RESULTS

The combined cohort had a median maximum blood lead of 29 μg/dL, a mean first blood lead test of 1977, and was 87% male. Significant (p<0.05) positive trends, using the log of maximum blood lead, were found for brain cancer (malignant), Hodgkin's lymphoma, lung cancer and rectal cancer, while a significant negative trend was found for melanoma. Borderline significant positive trends (0.05≤p≤0.10) were found for oesophageal cancer, meningioma and combined malignant/benign brain cancer. Categorical analyses reflected these trends. Significant interactions by country were found for lung, brain and oesophageal cancer, with Finland showing strong positive trends, and Great Britain showing modest or no trends. Larynx cancer in Finland also showed a positive trend (p=0.05). External analyses for high exposure workers (maximum blood lead >40 μg/dL) showed a significant excess for lung cancer in both countries combined, and significant excesses in Finland for brain and lung cancer. The Great Britain data were limited by small numbers for some cancers, and limited variation in exposure.

CONCLUSIONS

We found strong positive incidence trends with increasing blood lead level, for several outcomes in internal analysis. Two of these, lung and brain cancer, were sites of a priori interest.

Use of Mendelian andomization for dentifying isk actors for rain umors

Gliomas are a group of primary brain tumors, the most common and aggressive subtype of which is glioblastoma. Glioblastoma has a median survival of just 15 months after diagnosis. Only previous exposure to ionizing radiation and particular inherited genetic syndromes are accepted risk factors for glioma; the vast majority of cases are thought to occur spontaneously. Previous observational studies have described associations between several risk factors and glioma, but studies are often conflicting and whether these associations reflect true casual relationships is unclear because observational studies may be susceptible to confounding, measurement error and reverse causation. Mendelian randomization (MR) is a form of instrumental variable analysis that can be used to provide supporting evidence for causal relationships between exposures (e.g., risk factors) and outcomes (e.g., disease onset). MR utilizes genetic variants, such as single nucleotide polymorphisms (SNPs), that are robustly associated with an exposure to determine whether there is a causal effect of the exposure on the outcome. MR is less susceptible to confounding, reverse causation and measurement errors as it is based on the random inheritance during conception of genetic variants that can be relatively accurately measured. In previous studies, MR has implicated a genetically predicted increase in telomere length with an increased risk of glioma, and found little evidence that obesity related factors, vitamin D or atopy are causal in glioma risk. In this review, we describe MR and its potential use to discover and validate novel risk factors, mechanistic factors, and therapeutic targets in glioma.

A cohort mortality study of lead-exposed workers in the USA, Finland and the UK

OBJECTIVES

To investigate further whether inorganic lead is a carcinogen among adults, or associated with increased blood pressure and kidney damage, via a large mortality study.

METHODS

We conducted internal analyses via Cox regression of mortality in three cohorts of lead-exposed workers with blood lead (BL) data (USA, Finland, UK), including over 88 000 workers and over 14 000 deaths. Our exposure metric was maximum BL. We also conducted external analyses using country-specific background rates.

RESULTS

The combined cohort had a median BL of 26 µg/dL, a mean first-year BL test of 1990 and was 96% male. Fifty per cent had more than one BL test (mean 7). Significant (p<0.05) positive trends, using the log of each worker's maximum BL, were found for lung cancer, chronic obstructive pulmonary disease (COPD), stroke and heart disease, while borderline significant trends (0.05≤p≤0.10) were found for bladder cancer, brain cancer and larynx cancer. Most results were consistent across all three cohorts. In external comparisons, we found significantly elevated SMRs for those with BLs>40 µg/dL; for bladder, lung and larynx cancer; and for COPD. In a small subsample of the US cohort (n=115) who were interviewed, we found no association between smoking and BL.

CONCLUSIONS

We found strong positive mortality trends, with increasing BL level, for several outcomes in internal analysis. Many of these outcomes are associated with smoking, for which we had no data. A borderline trend was found for brain cancer, not associated with smoking.

Lifetime occupational exposure to metals and welding fumes, and risk of glioma: a 7-country population-based case-control study

BACKGROUND

Brain tumor etiology is poorly understood. Based on their ability to pass through the blood-brain barrier, it has been hypothesized that exposure to metals may increase the risk of brain cancer. Results from the few epidemiological studies on this issue are limited and inconsistent.

METHODS

We investigated the relationship between glioma risk and occupational exposure to five metals - lead, cadmium, nickel, chromium and iron- as well as to welding fumes, using data from the seven-country INTEROCC study. A total of 1800 incident glioma cases and 5160 controls aged 30-69 years were included in the analysis. Lifetime occupational exposure to the agents was assessed using the INTEROCC JEM, a modified version of the Finnish job exposure matrix FINJEM.

RESULTS

In general, cases had a slightly higher prevalence of exposure to the various metals and welding fumes than did controls, with the prevalence among ever exposed ranging between 1.7 and 2.2% for cadmium to 10.2 and 13.6% for iron among controls and cases, respectively. However, in multivariable logistic regression analyses, there was no association between ever exposure to any of the agents and risk of glioma with odds ratios (95% confidence intervals) ranging from 0.8 (0.7-1.0) for lead to 1.1 (0.7-1.6) for cadmium. Results were consistent across models considering cumulative exposure or duration, as well as in all sensitivity analyses conducted.

CONCLUSIONS

Findings from this large-scale international study provide no evidence for an association between occupational exposure to any of the metals under scrutiny or welding fumes, and risk of glioma.

Occupational Lead Exposure and Associations with Selected Cancers: The Shanghai Men's and Women's Health Study Cohorts

BACKGROUND

Epidemiologic studies of occupational lead exposure have suggested increased risks of cancers of the stomach, lung, kidney, brain, and meninges; however, the totality of the evidence is inconsistent.

OBJECTIVE

We investigated the relationship between occupational lead exposure and cancer incidence at the five abovementioned sites in two prospective cohorts in Shanghai, China.

METHODS

Annual job/industry-specific estimates of lead fume and lead dust exposure, derived from a statistical model combining expert lead intensity ratings with inspection measurements, were applied to the lifetime work histories of participants from the Shanghai Women's Health Study (SWHS; n = 73,363) and the Shanghai Men's Health Study (SMHS; n = 61,379) to estimate cumulative exposure to lead fume and lead dust. These metrics were then combined into an overall occupational lead exposure variable. Cohort-specific relative hazard rate ratios (RRs) and 95% confidence intervals (CIs) comparing exposed and unexposed participants were estimated using Cox proportional hazards regression and combined by meta-analysis.

RESULTS

The proportions of SWHS and SMHS participants with estimated occupational lead exposure were 8.9% and 6.9%, respectively. Lead exposure was positively associated with meningioma risk in women only (n = 38 unexposed and 9 exposed cases; RR = 2.4; 95% CI: 1.1, 5.0), particularly with above-median cumulative exposure (RR = 3.1; 95% CI: 1.3, 7.4). However, all 12 meningioma cases among men were classified as unexposed to lead. We also observed non-significant associations with lead exposure for cancers of the kidney (n = 157 unexposed and 17 ever exposed cases; RR = 1.4; 95% CI: 0.9, 2.3) and brain (n = 67 unexposed and 10 ever exposed cases; RR = 1.8; 95% CI: 0.7, 4.8) overall.

CONCLUSIONS

Our findings, though limited by small numbers of cases, suggest that lead is associated with the risk of several cancers in women and men.

CITATION

Liao LM, Friesen MC, Xiang YB, Cai H, Koh DH, Ji BT, Yang G, Li HL, Locke SJ, Rothman N, Zheng W, Gao YT, Shu XO, Purdue MP. 2016. Occupational lead exposure and associations with selected cancers: the Shanghai Men's and Women's Health Study cohorts. Environ Health Perspect 124:97-103; http://dx.doi.org/10.1289/ehp.1408171.

Occupational solvent exposure and risk of meningioma: results from the INTEROCC multicentre case-control study

OBJECTIVE

To examine associations between occupational exposure to selected organic solvents and meningioma.

METHODOLOGY

A multicentre case-control study conducted in seven countries, including 1906 cases and 5565 controls. Occupational exposure to selected classes of organic solvents (aliphatic and alicyclic hydrocarbons, aromatic hydrocarbons, chlorinated hydrocarbons and 'other' organic solvents) or seven specific solvents (benzene, toluene, trichloroethylene, perchloroethylene, 1,1,1-trichloroethylene, methylene chloride and gasoline) was assessed using lifetime occupational histories and a modified version of the FINJEM job-exposure matrix (INTEROCC-JEM). Study participants were classified as 'exposed' when they had worked in an occupation for at least 1 year, with a 5-year lag, in which the estimated prevalence of exposure was 25% or greater in the INTEROCC-JEM. Associations between meningioma and each of the solvent exposures were estimated using conditional logistic regression, adjusting for potential confounders.

RESULTS

A total of 6.5% of study participants were ever exposed to 'any' solvent, with a somewhat greater proportion of controls (7%) ever exposed compared with cases (5%), but only one case was ever exposed to any chlorinated hydrocarbon (1,1,1-trichloroethane). No association was observed between any of the organic solvents and meningioma, in either men or women, and no dose-response relationships were observed in internal analyses using either exposure duration or cumulative exposure.

DISCUSSION

We found no evidence that occupational exposure to these organic solvents is associated with meningioma.

Glutathione S-transferase M1, T1, and P1 polymorphisms and risk of glioma: a meta-analysis

The relationship between genetic polymorphisms of glutathione S-transferase (GST) and the development of glioma has been investigated in several epidemiologic studies. However these studies report inconsistent results. In order to quantitatively summarise the evidence for such a relationship, a meta-analysis is conducted. The PubMed database was searched from inception to January 2012 to identify relevant studies that met pre-stated inclusion criteria. We also reviewed reference lists from retrieved articles. Two researchers evaluated study eligibility and extracted the data independently, and disagreements were resolved by discussion. The principal outcome measure was the odds ratio (OR) with 95 % confidence interval (CI) for the risk of glioma associated with GSTM1, GSTT1, GSTP1 I105V or GSTP1 A114V. This meta-analysis included 11 case-control studies, which included 2,404 glioma cases and 6,379 controls. The combined results based on all studies showed that there was no association between any of the GST variants and the risk of glioma (for GSTM1: pooled OR = 1.03; 95 % CI, 0.92-1.15; for GSTT1: pooled OR = 1.12; 95 % CI, 0.90-1.40; for GSTP1 I105V: pooled OR = 0.92; 95 % CI, 0.64-1.31 and for GSTP1 A114V: pooled OR = 1.14; 95 % CI, 0.97-1.34). Subgroup analyses showed that GSTP1 A114V genotype was associated with an increased risk of other histopathologic glioma except glioblastoma multiforme (GBM) (pooled OR = 1.30; 95 % CI = 1.06-1.60); no relationship was found between other GST variants and histopathologic groups. In conclusion, our meta-analysis suggests no association between GST variants and the risk of glioma. However, the significant risk elevation is present between GSTP1 A114V genotype and other histopathologic glioma except GBM.

The Upper Midwest Health Study: industry and occupation of glioma cases and controls

BACKGROUND

Understanding glioma etiology requires determining which environmental factors are associated with glioma. Upper Midwest Health Study case-control participant work histories collected 1995-1998 were evaluated for occupational associations with glioma. "Exposures of interest" from our study protocol comprise our a priori hypotheses.

MATERIALS AND METHODS

Year-long or longer jobs for 1,973 participants were assigned Standard Occupational Classifications (SOC) and Standard Industrial Classifications (SIC). The analysis file includes 8,078 SIC- and SOC-coded jobs. For each individual, SAS 9.2 programs collated employment with identical SIC-SOC coding. Distributions of longest "total employment duration" (total years worked in jobs with identical industry and occupation codes, including multiple jobs, and non-consecutive jobs) were compared between cases and controls, using an industrial hygiene algorithm to group occupations.

RESULTS

Longest employment duration was calculated for 780 cases and 1,156 controls. More case than control longest total employment duration was in the "engineer, architect" occupational group [16 cases, 10 controls, odds ratio (OR) 2.50, adjusted for age group, sex, age and education, 95% confidence interval (CI) 1.12-5.60]. Employment as a food processing worker [mostly butchers and meat cutters] was of borderline significance (27 cases, 21 controls, adjusted OR: 1.78, CI: 0.99-3.18).

CONCLUSIONS

Among our exposures of interest work as engineers or as butchers and meat cutters was associated with increased glioma risk. Significant associations could be due to chance, because of multiple comparisons, but similar findings have been reported for other glioma studies. Our results suggest some possible associations but by themselves could not provide conclusive evidence.

Occupation and risk of glioma, meningioma and acoustic neuroma: results from a German case-control study (interphone study group, Germany)

BACKGROUND

Several epidemiological studies have investigated the association between occupation and brain tumour risk, but results have been inconclusive. We investigated the association between six occupational categories defined a priori: chemical, metal, agricultural, construction, electrical/electronic and transport, and the risk of glioma, meningioma and acoustic neuroma.

METHODS

In a population-based case-control study involving a total of 844 cases and 1688 controls conducted from 2000 to 2003, detailed information on life-long job histories was collected during personal interviews and used to create job calendars for each participant. Job title, job activity, job number, and the starting and ending dates of the activity were recorded for all activities with duration of at least 1 year. Reported occupational activities were coded according to the International Standard Classification of Occupations 1988 (ISCO 88). For the analyses we focused on six a priori defined occupational sectors, namely chemical, metal, agricultural, construction, electrical/electronic and transport. Multiple conditional logistic regression analysis was used to estimate odds ratios and their 95% confidence intervals.

RESULTS

Most of the observed odds ratios were close to 1.0 for ever having worked in the six occupational sectors and risk of glioma, meningioma and acoustic neuroma. Sub-group analyses according to duration of employment resulted in two elevated odds ratios with confidence intervals excluding unity.

CONCLUSIONS

We did not observe an increased risk of glioma or meningioma for occupations in the agricultural, construction, transport, chemical, electrical/electronic and metal sectors. The number of 'significant' odds ratios is consistent with an overall 'null-effect'.

Lead exposure, polymorphisms in genes related to oxidative stress, and risk of adult brain tumors

There is some evidence that oxidative stress plays a role in lead-induced toxicity. Mechanisms for dealing with oxidative stress may be of particular relevance in the brain given the high rate of oxygen metabolism. Using a hospital-based case-control study, we investigated the role of oxidative stress in the potential carcinogenicity of lead through examination of effect modification of the association between occupational lead exposure and brain tumors by single nucleotide polymorphisms in genes with functions related to oxidative stress. The study included 362 patients with glioma (176 of which had glioblastoma multiforme), 134 patients with meningioma, and 494 controls. Lead exposure was estimated by expert review of detailed job history data for each participant. We evaluated effect modification with 142 single nucleotide polymorphisms using likelihood ratio tests that compared nested unconditional logistic regression models that did and did not include a cross-product term for cumulative lead exposure and genotype. When the analyses were restricted to cases with glioblastoma multiforme, RAC2 rs2239774 and two highly correlated GPX1 polymorphisms (rs1050450 and rs18006688) were found to significantly modify the association with lead exposure (P Collapse

Key Words

• glioma
• meningioma
• lead exposure
• oxidative stress
• polymorphism

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MESH Headings

• Adolescent
• Adult
• Aged
• Aged, 80 and over
• Brain Neoplasms/chemically induced
• Brain Neoplasms/genetics
• Female
• Genetic Predisposition to Disease
• Glutathione Peroxidase/genetics
• Humans
• Lead/adverse effects
• Male
• Middle Aged
• Occupational Diseases/genetics
• Occupational Exposure/adverse effects
• Oxidative Stress/genetics
• Polymerase Chain Reaction
• Polymorphism, Single Nucleotide
• Risk Factors
• Young Adult
• rac GTP-Binding Proteins/genetics
• Glutathione Peroxidase GPX1
• RAC2 GTP-Binding Protein

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Grants

• N01CO12400 NCI NIH HHS
• NIH0011946573 Intramural NIH HHS
• Z01 CP010135-12 Intramural NIH HHS
• N01-CO-12400 NCI NIH HHS

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Affiliation(s)

Parveen Bhatti Department of Health ad Human Services, Division of Cancer Epidemiology and Genetics, National Cancer Institute, NIH, Bethesda, Maryland, USA.
Patricia A Stewart
Amy Hutchinson
Nathaniel Rothman
Martha S Linet
Peter D Inskip
Preetha Rajaraman

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Epidemiology of brain tumors

Gliomas account for more than 70% of all brain tumors, and of these, glioblastoma is the most frequent and malignant histologic type (World Health Organization [WHO] grade IV). There is a tendency toward a higher incidence of gliomas in highly developed, industrialized countries. Some reports indicate that Caucasians have a higher incidence than African or Asian populations. With the exception of pilocytic astrocytomas (WHO grade I), the prognosis of glioma patients is still poor. Fewer than 3% of glioblastoma patients are still alive at 5 years after diagnosis, older age being the most significant and consistent prognostic factor of poorer outcome. Gliomas are components of several inherited tumor syndromes, but the prevalence of these syndromes is very low. Many environmental and lifestyle factors including several occupations, environmental carcinogens, and diet have been reported to be associated with an elevated glioma risk, but the only factor unequivocally associated with an increased risk is therapeutic X-irradiation. In particular, children treated with X-irradiation for acute lymphoblastic leukemia show a significantly elevated risk of developing gliomas and primitive neuroectodermal tumors, often within 10 years after therapy. Significant correlation between G:C --> A:T transitions in the TP53 gene and promoter methylation of the O6 -methylguanine-DNA methyltransferase (MGMT) gene in glio-mas have been reported in several studies, suggesting the possible involvement of O6-methylguanine DNA adducts, which may be produced by exogenous or endogenous alkylating agents in the development of gliomas.

Brain tumor epidemiology: consensus from the Brain Tumor Epidemiology Consortium

Epidemiologists in the Brain Tumor Epidemiology Consortium (BTEC) have prioritized areas for further research. Although many risk factors have been examined over the past several decades, there are few consistent findings, possibly because of small sample sizes in individual studies and differences between studies in patients, tumor types, and methods of classification. Individual studies generally have lacked samples of sufficient size to examine interactions. A major priority based on available evidence and technologies includes expanding research in genetics and molecular epidemiology of brain tumors. BTEC has taken an active role in promoting understudied groups, such as pediatric brain tumors; the etiology of rare glioma subtypes, such as oligodendroglioma; and meningioma, which, although it is not uncommon, has only recently been registered systematically in the United States. There also is a pressing need for more researchers, especially junior investigators, to study brain tumor epidemiology. However, relatively poor funding for brain tumor research has made it difficult to encourage careers in this area. In this report, BTEC epidemiologists reviewed the group's consensus on the current state of scientific findings, and they present a consensus on research priorities to identify which important areas the science should move to address.

Meningiomas: causes and risk factors

Meningiomas are among the most common primary intracranial tumors. Although the vast majority of these tumors are considered histologically benign, the incidence of complications can be high. Few studies have investigated the causes and risk factors for meningioma; this review highlights the current state of knowledge. Gaining a better understanding of the origin of this disease is essential so that treatments and outcomes can be improved and prevention strategies can be developed.

A 78-kDa glucose-regulated protein is involved in the decrease of interleukin-6 secretion by lead treatment from astrocytes

Interleukin (IL)-6 is a cytokine produced mainly by microglia and astrocytes and plays a pleiotropic role in the central nervous system. In this study, we cloned rat IL-6 cDNA into an enhanced green fluorescent protein (EGFP) or a red fluorescent protein (DsRed2) vector and rat 78-kDa glucose-regulated protein (GRP78) cDNA into an EGFP vector to construct IL-6-EGFP, IL-6-DsRed2, and GRP78-EGFP chimeras for the investigation of the mechanism of IL-6 secretion from astrocytes. The data showed that constructed IL-6-EGFP and IL-6-DsRed2 chimeras retained the secretory property, and the secretion of IL-6-EGFP from astrocytes could be attenuated by GRP78 depletion with double-stranded RNA interference. Coexpression of IL-6-DsRed2 and dysfunctional GRP78-EGFP abolished IL-6-DsRed2 secretion, and two chimeric proteins colocalized inside living astrocytes. Coimmunoprecipitation analysis indicated that IL-6 and GRP78 resided in the same complex. The data further revealed that IL-6-EGFP secretion from astrocytes was blocked by the heavy metal lead (Pb) in a concentration-dependent manner. Analysis of the Pb interaction with protein on a Pb-affinity column demonstrated that Pb bound to GRP78 but failed to bind to IL-6. Therefore, these data suggest that IL-6-EGFP or IL-6-DsRed2 chimeras can be used as imaging probes to study IL-6 secretion from living cells, that GRP78 is involved in IL-6 secretion from astrocytes, and that Pb can block IL-6 secretion from astrocytes via targeting GRP78.

Lead, genetic susceptibility, and risk of adult brain tumors

BACKGROUND

Although few etiologic factors for brain tumors have been identified, limited data suggest that lead may increase the risk of brain tumors, particularly meningioma. The ALAD G177C polymorphism affects the toxicokinetics of lead and may confer genetic susceptibility to adverse effects of lead exposure.

METHODS

We examined occupational exposure to lead and risk of brain tumors in a multisite, hospital-based, case-control study of 489 patients with glioma, 197 with meningioma, and 799 non-cancer controls frequency matched on hospital, age, sex, race/ethnicity, and residential proximity to hospital. ALAD genotype was assessed by a Taqman assay for 355 glioma patients, 151 meningioma patients, and 505 controls. Exposure to lead was estimated using a rigorous questionnaire-based exposure assessment strategy incorporating lead measurement and other occupational data abstracted from published articles and reports.

RESULTS

Increased risk of meningioma with occupational lead exposure (estimated by odds ratios and 95% confidence intervals) was most apparent in individuals with the ALAD2 variant allele, for whom risk increased from 1.1 (0.3-4.5) to 5.6 (0.7-45.5) and 12.8 (1.4-120.8) for estimated cumulative lead exposures of 1 to 49 microg/m3-y, 50 to 99 microg/m3-y, and >or=100 microg/m3-y, respectively, compared with unexposed individuals (two-sided P trend = 0.06). This relationship became stronger after excluding occupational lead exposures characterized by a low confidence level or occurring in the 10 years before meningioma diagnosis. Occupational lead exposure was not associated with glioma risk.

CONCLUSIONS

Although our results indicate that lead may be implicated in meningioma risk in genetically susceptible individuals, these results need to be interpreted with caution given the small numbers of exposed cases with a variant genotype.

Brain cancer mortality and potential occupational exposure to lead: findings from the National Longitudinal Mortality Study, 1979-1989

We evaluated the association between potential occupational lead exposure and the risk of brain cancer mortality in the National Longitudinal Mortality Study (NLMS), which is a prospective census-based cohort study of mortality among the noninstitutionalized United States population (1979-1989). The present study was limited to individuals for whom occupation and industry were available (n = 317,968). Estimates of probability and intensity of lead exposure were assigned using a job-exposure matrix (JEM). Risk estimates for the impact of lead on brain cancer mortality were computed using standardized mortality ratio (SMR) and proportional hazards and Poisson regression techniques, adjusting for the effects of age, gender and several other covariates. Brain cancer mortality rates were greater among individuals in jobs potentially involving lead exposure as compared to those unexposed (age- and gender-adjusted hazard ratio (HR) = 1.5; 95% confidence interval (CI) = 0.9-2.3) with indications of an exposure-response trend (probability: low HR = 0.7 (95% CI = 0.2-2.2), medium HR = 1.4 (95% CI = 0.8-2.5), high HR = 2.2 (95% CI = 1.2-4.0); intensity: low HR = 1.2 (95% CI = 0.7-2.1), medium/high HR = 1.9 (95% CI = 1.0-3.4)). Brain cancer risk was greatest among individuals with the highest levels of probability and intensity (HR = 2.3; 95% CI = 1.3-4.2). These findings provide further support for an association between occupational lead exposure and brain cancer mortality, but need to be interpreted cautiously due to the consideration of brain cancer as one disease entity and the absence of biological measures of lead exposure.

The involvement of copper transporter in lead-induced oxidative stress in astroglia

Lead (Pb), depositing primarily in astroglia in the brain, is a well-known neurotoxicant and a risk factor for neurologic disorders. Pb has been reported to induce oxidative stress by probably the disturbance of copper (Cu) homeostasis in astroglia. Thus, we hypothesized that Pb-induced oxidative stress is initiated by interfering with Cu transporter in astroglia. In this study, we observed Pb-induced oxidative stress as indicated by reactive oxygen species (ROS) augmentation and GRP78 and GRP94 protein induction, and it was parallel to Cu accumulation intracellularly by Pb. To further address Cu transporter as a potential Pb target, a heavy metal-binding (HMB) domain of Cu-transporting ATPase (Atp7a) was overexpressed and purified. Evidence showed that one molecule of HMB chelated 11 Pb ions or seven Cu ions and that Pb competed with Cu for binding to HMB. These findings suggest that Pb-induced oxidative stress results from the impairment of Cu metabolism by Pb targeting of Atp7a.

Screening of TP53 mutations by DHPLC and sequencing in brain tumours from patients with an occupational exposure to pesticides or organic solvents

The aetiology of brain tumours remains unclear. Occupational exposures to pesticides and organic solvents are suspected risk factors. The case-control study CEREPHY (221 cases, 442 controls) carried in the Departement de la Gironde in France revealed a significantly increased risk of brain tumours for subjects most exposed to pesticides. In some cancers, TP53 mutations could reflect exposure to specific carcinogens. These mutations are present in approximately 30% of astrocytic brain tumours. In a pilot study, we explored the hypothesis that pesticide or solvent exposure could raise the frequency of TP53 mutations in brain tumour cells. We investigated TP53 mutations in exons 2-11 by denaturing high performance liquid chromatography (DHPLC) and sequencing, and p53 accumulation by immunohistochemistry in brain tumour of the 30 patients from CEREPHY study with a history of occupational exposure to pesticides (n = 21) and/or organic solvents (n = 14) for whom tumoral tissue was available. Included cases concerned 27% of CEREPHY cases exposed to pesticides and, based on the cumulative index of occupational exposure, they were more exposed to pesticides. There were 12 gliomas, 6 meningiomas, 7 neurinomas, 2 central nervous system lymphomas and 3 tumours of other histological types. We detected TP53 mutations in three tumours, which is similar to the expected number (3.3) calculated from 46 published studies referenced in the IARC TP53 mutations database, taking into account histological types. Considering TP53 mutations previously detected in the laboratory by DHPLC and the frequency of TP53 polymorphisms detected in this sample (similar to published data), the TP53 mutations rate is probably not underestimated. These preliminary results, even if it was on a limited number of tumours, are not in favour of the role of pesticide or organic solvent exposure in the occurrence of TP53 mutations in brain tumours.

Epidemiology and etiology of gliomas

Gliomas of astrocytic, oligodendroglial and ependymal origin account for more than 70% of all brain tumors. The most frequent (65%) and most malignant histological type is the glioblastoma. Since the introduction of computerized tomography and magnetic resonance imaging, the incidence rates of brain tumors have been rather stable, with a tendency of higher rates in highly developed, industrialized countries. Some reports indicate that Caucasians have higher incidence than black or Asian populations, but to some extent, this may reflect socio-economic differences and under-ascertainment in some regions, rather than a significant difference in genetic susceptibility. With the exception of pilocytic astrocytomas, the prognosis of glioma patients is still poor. Less than 3% of glioblastoma patients are still alive at 5 years after diagnosis, higher age being the most significant predictor of poor outcome. Brain tumors are a component of several inherited tumor syndromes, but the prevalence of these syndromes is very low. Several occupations, environmental carcinogens, and diet (N-nitroso compounds) have been reported to be associated with an elevated glioma risk, but the only environmental factor unequivocally associated with an increased risk of brain tumors, including gliomas, is therapeutic X-irradiation. In particular, children treated with X-irradiation for acute lymphoblastic leukemia show a significantly elevated risk of developing gliomas and primitive neuroectodermal tumor (PNET), often within 10 years after therapy. TP53 mutations are frequent in low-grade gliomas and secondary glioblastomas derived therefrom. Approximately 60% of mutations are located in the hot spot codons 248 and 273, and the majority of these are G:C-->A:T transitions at CpG sites. TP53 mutations are significantly more frequent in low-grade astrocytomas with promoter methylation of the O(6)-methylguanine-DNA methyltransferase repair gene, suggesting that, in addition to deamination of 5-methylcytosine, exogenous or endogenous alkylation in the O(6) position of guanine may contribute to the formation of these mutations.

Inguinal hernias, endometriosis, and other adverse outcomes in rhesus monkeys following lead exposure

The Harlow Center for Biological Psychology (HCBP) has a cohort of rhesus monkeys that were exposed to low concentrations of lead acetate in utero or as infants. The lead-exposed animals have been followed for 19 years and have developed four cases of inguinal hernia (males), three cases of endometriosis (females), and one case of immunoblastic lymphoma (male). Retrospective analysis of the data from the original lead-exposed cohort indicates that there is a significant association between lead exposure and the development of inguinal hernia (P=.04). Endometriosis was not significantly associated with lead exposure (P=.36). A case control study also was done to determine the significance of neonatal lead exposure as a risk factor for the development of inguinal hernia and endometriosis. The risk of developing inguinal hernia was significantly increased in lead-exposed animals (OR=20.0, P=.009). The association between endometriosis and lead exposure was also strong (OR=10.13, P<.001). No unmatched variables were associated with inguinal hernia, including body weight, history of diarrhea, constipation, or intussusception. No unmatched variables were highly associated with endometriosis, including body weight, age at first parity, and history of stillbirths. However, parity and the number of stillbirths were associated with lead exposure (P=.011 and P=.041, respectively). There was an association between endometriosis and a history of hysterotomy (OR=2.09) but it was not statistically significant (P=.38). No other cases of lymphoma in unexposed animals were identified using HCBP animal health records. These data indicate that early lead-exposed rhesus monkeys may develop illnesses later in life, especially inguinal hernia and endometriosis, more frequently than unexposed monkeys. Studies of human populations with early lead exposure are warranted to determine their incidence of inguinal hernia, endometriosis, and hematologic neoplasia.

In vitro genotoxicity of lead acetate: induction of single and double DNA strand breaks and DNA-protein cross-links

Lead is present in the natural and occupational environment and is reported to interact with DNA, but the mechanism of this interaction is not fully understood. Using the alkaline comet assay we showed that lead acetate at 1-100 microM induced DNA damage in isolated human lymphocytes measured the change in the comet tail length. At 1 and 10 microM we observed an increase in the tail length, whereas at 100 microM a decrease was seen. The former effect could follow from the induction of DNA strand breaks and/or alkali-labile sites (ALS), the latter from the formation of DNA-DNA and/or DNA-protein cross-links. No difference was observed between tail length for the alkaline and pH 12.1 versions of the assay, which indicates that strand breaks and not ALS are responsible for the tail length increase induced by lead. The neutral version of the test revealed that lead acetate induced DNA double-strand breaks at all concentrations tested. The presence of spin traps, 5,5-dimethyl-pyrroline N-oxide (DMPO) and N-tert-butyl-alpha-phenylnitrone (PBN) did not influence the level of DNA damage induced by lead. Post-treatment of the lead-damaged DNA (at 100 microM treatment concentration) by endonuclease III (Endo III) and formamidopyrimidine-DNA glycosylase (Fpg), enzymes recognizing oxidized DNA bases, as well as 3-methyladenine-DNA glycosylase II, an enzyme recognizing alkylated bases, gave rise to a significant increase in the extent of DNA damage. Proteinase K caused an increase in comet tail length, suggesting that lead acetate might cross-link DNA with nuclear proteins. Vitamin A, E, C, calcium chloride and zinc chloride acted synergistically on DNA damage evoked by lead. The results obtained suggest that lead acetate may induce single-strand breaks (SSB) and double-strand breaks (DSB) in DNA as well as DNA-protein cross-links. The participation of free radicals in DNA-damaging potential of lead is not important and it concerns other reactive species than could be trapped by DMPO or PBN.

Lead-induced endoplasmic reticulum (ER) stress responses in the nervous system

Lead (Pb) poisoning continues to be a significant health risk because of its pervasiveness in the environment, its known neurotoxic effects in children, and potential endogenous exposure from Pb deposited in bone. New information about mechanisms by which Pb enters cells and its organelle targets within cells are briefly reviewed. Toxic effects of Pb on the endoplasmic reticulum (ER) are considered in detail, based on recent evidence that Pb induces the expression of the gene for 78-kD glucose-regulated protein (GRP78) and other ER stress genes. GRP78 is a molecular chaperone that binds transiently to proteins traversing through the ER and facilitates their folding, assembly, and transport. Models are presented for the induction of ER stress by Pb in astrocytes, the major cell type of the central nervous system, in which Pb accumulates. A key feature of the models is disruption of GRP78 function by direct Pb binding. Possible pathways by which Pb-bound GRP78 stimulates the unfolded protein response (UPR) in the ER are discussed, specifically transduction by IRE1/ATF6 and/or IRE1/JNK. The effect of Pb binding to GRP78 in the ER is expected to be a key component for understanding mechanisms of Pb-induced ER stress gene expression.

Occupation, exposure to chemicals and risk of gliomas and meningiomas in Sweden

BACKGROUND

Occupational exposures may be related to the development of brain cancer. The objective was to estimate occupational-specific risk of gliomas and meningiomas among Swedish men and women gainfully employed in 1970 over the period 1971-1989, and the influence of occupational exposure to chemical substances.

METHODS

A dataset linking cancer diagnoses from the Swedish national cancer register to occupational and demographical data obtained in the 1970 census was used to fit log-linear Poisson models, in order to obtain relative risks adjusted by age, period, geographical area and town size. Exposure to 13 chemicals was assessed using a Swedish job-exposure matrix.

RESULTS

The main findings of this study among men were the increased risk of glioma with occupational exposure to arsenic, mercury, and petroleum products and of meningioma with lead. Women in occupational sectors with a higher socio-economic status showed an increased incidence of both, gliomas and meningiomas.

CONCLUSIONS

Occupational exposure to some chemicals appeared to be related with an increased risk of glioma and meningioma in men. Exposures involved in glioma and meningioma development seemed to be different.

Inorganic lead activates the mitogen-activated protein kinase kinase-mitogen-activated protein kinase-p90(RSK) signaling pathway in human astrocytoma cells via a protein kinase C-dependent mechanism

We have previously reported that lead acetate activates protein kinase Calpha (PKCalpha) and induces DNA synthesis in human 1321N1 astrocytoma cells. In this study, we investigated the ability of lead to activate the mitogen-activated protein kinase (MAPK) cascade. We found that exposure to lead acetate (1-50 microM) resulted in concentration- and time-dependent activation of MAPK (extracellular signal responsive kinase 1/2), as shown by increased phosphorylation and increased kinase activity. This effect was significantly reduced by the PKC-specific inhibitor bisindolylmaleimide (GF109203X), by down-regulation of PKC with 12-O-tetradecanoyl-phorbol 13-acetate, by a pseudosubstrate to PKCalpha, and by selective down-regulation of PKCalpha by prior lead exposure. Lead was also shown to activate MAPK kinase (MEK1/2), and this effect was mediated by PKC. Two MEK inhibitors, 2-(2'-amino-3'-methoxyphenol)-oxanaphthalen-4-one (PD98059) and 1,4-diamino-2,3-dicyano-1,4-bis[2-aminophenylthio]butadiene (UO126), blocked lead-induced MAPK activation and inhibited lead-induced DNA synthesis, as measured by incorporation of [methyl-3H]thymidine into cell DNA. The 90 kDa ribosomal S6 protein kinase, p90(RSK), a substrate of MAPK, was also found to be activated by lead in a PKC- and MAPK-dependent manner. Stimulation of DNA synthesis by lead in astrocytoma cells may be of interest in light of the observed association between exposure to lead and an increased risk of astrocytomas.

Inorganic lead stimulates DNA synthesis in human astrocytoma cells: role of protein kinase Calpha

As lead has been shown to activate protein kinase C (PKC), and gliomas are reported to be highly dependent on PKC for their proliferation, this study was undertaken to investigate whether lead may act as a mitogen in human astrocytoma cells, and to determine the role of PKC in this effect. Lead acetate (from 100 nM to 100 microM) induced a concentration- and time-dependent increase in DNA synthesis, as measured by incorporation of [methyl-3H]thymidine into cell DNA, without causing any cytotoxicity. Flow cytometric analysis showed that lead was able to stimulate the cell cycle transition from the G0/G1 phase to the S/G2 phase, resulting in increased percentage of cells in the latter phase. Western blot analyses showed that lead induced translocation of PKCalpha, but not of PKCepsilon or PKCzeta, from the cytosolic to the particulate fraction, with a concomitant increase in PKC enzyme activity. Prolonged exposure to lead caused down-regulation of PKCalpha, but not of PKCepsilon. The effect of lead on DNA synthesis was mediated through PKC as evidenced by the finding that two PKC inhibitors, GF 109203X and staurosporine, as well as down-regulation of PKC through prolonged treatment with 12-O-tetradecanoylphorbol 13-acetate, blocked lead-induced DNA synthesis. Further experiments using a pseudosubstrate peptide targeting classical PKCs and selective down-regulation of specific PKC isoforms indicated that the effect of lead on DNA synthesis was mediated by PKCalpha. Altogether, these results suggest that lead stimulates DNA synthesis in human astrocytoma cells by a mechanism that involves activation of PKCalpha.

Intracellular signal transduction pathways as targets for neurotoxicants

The multiple cascades of signal transduction pathways that lead from receptors on the cell membrane to the nucleus, thus translating extracellular signals into changes in gene expression, may represent important targets for neurotoxic compounds. Among the biochemical steps and pathways that have been investigated are the metabolism of cyclic nucleotides, the formation of nitric oxide, the metabolism of membrane phospholipids, the activation of a multitude of protein kinases and the induction of transcription factors. This brief review will focus on the interactions of three known neurotoxicants, lead, ethanol and polychlorinated biphenyls, with signal transduction pathways, particularly the family of protein kinase C isozymes, and discusses how such effects may be involved in their neurotoxicity.

Lead and cancer in humans: where are we now?

BACKGROUND

Lead is only weakly mutagenic, but in vitro it inhibits DNA repair and acts synergistically with other mutagens. Lead acetate administered orally, cutaneously, or intraperitoneally causes kidney cancer, brain cancer (gliomas), and lung cancer in rodents, and acts synergistically with other carcinogens. Most cytogenetic studies of exposed workers have shown increases in chromosome aberrations or sister chromatid exchange, including some studies with positive-exposure response trends. There are eight studies of cancer mortality or incidence among highly exposed workers; most are cohort studies of lead smelter or battery workers exposed decades ago.

METHODS

We reviewed the epidemologic studies with regard to cancer.

RESULTS

These studies provide some evidence of increased risk of lung cancer (RR = 1.30, 1.15-1.46, 675 observed deaths) and stomach cancer (combined RR = 1.34, 1.14-1.57, 181 observed). However, the lung cancer findings are not consistent across studies, and confounding by arsenic may affect the study with the highest lung cancer RR. Exclusion of that study yields a combined lung cancer RR of 1.14 (1.04-1.73). There is little evidence of increased risk of kidney cancer (combined RR = 1.01, 0. 72-1.42, 40 observed) or brain cancer (combined RR = 1.06, 0.81-1.40, 69 observed). However, two studies show a two-fold increase in kidney cancer, and one study shows a significant excess of gliomas. IARC classified lead as a "possible human carcinogen" based on sufficient animal data and insufficient human data in 1987. Six of the eight studies cited above have been published since 1987.

CONCLUSIONS

Overall, there is only weak evidence associating lead with cancer; the most likely candidates are lung cancer, stomach cancer, and gliomas.

Increased AP-1 DNA-binding activity and nuclear REF-1 accumulation in lead-exposed primary cultures of astrocytes

Pb was shown to perturb neuronal and glial function either directly by interacting with protein thiol groups or indirectly by mimicking Ca(2+) and increasing oxidative stress. In view of the potential action of Pb on cellular redox homeostasis we studied the regulation of activator protein-1 (AP-1) DNA binding. A 1h incubation of astrocyte primary cultures with 10 microM Pb caused a 2.5 fold increase in AP-1 DNA binding. An assessment of how Pb elicited this increase revealed the involvement of 1. transcriptional and 2. posttranslational processes. The first one was documented by an increase of c-jun mRNA content after 15 to 30 min of 10 microM Pb exposure. The second one was suggested by an enhanced nuclear accumulation of redox factor-1 after 30 to 60 min of 10 microM Pb exposure. The Pb-elicited increase of the reduction/oxidation-sensitive AP-1 signal transduction may regulate target genes operative in cell survival or cell death.

Nervous system cancer mortality in an industrialized area of Brazil 1980-1993

OBJECTIVES

The industrialization process and nervous system cancer (NSC) mortality in a urban region of Brazil.

METHOD

From registries of the State System of Data Analysis Foundation (SEADE), 103 males deaths by NSC (ICD-9) in Baixada Santista (BS), from 1980 to 1993 were selected. Mortality ratios were calculated comparing the standardized mortality rate for ages over 10 years old (G1) and for the age group from 35 to 64 years old, in the industrialized and non-industrialized areas in three periods: 1980-1993, 1980-86, 1987-93.

RESULTS

A statiscally significant high mortality was observed in the industrialized area, for ages over 10 in all periods and only from 1980 to 1993 for ages from 34 to 64. The highest mortality ratio occurred from 1980-86 for ages over 10 - 4.12 (CI 1.79-9.42).

CONCLUSION

High mortality was probably related to the environmental and occupational exposure to many organic and inorganic chemical substances, considered carcinogenics, such as aliphatic and aromatic hydrocarbons, organochlorinated, formaldehyde, nitrogenated compounds and heavy metals, found in the port and industrial complex. We discuss the importance of case-control studies in characterizing the association of these and other risk factors in the determination of NSC.

Risk factors for meningioma in adults: a case-control study in northeast China

A hospital case-control study of meningioma was conducted in Heilongjiang Province in northeast China between September 1989 and December 1996. It included 183 cases of newly diagnosed primary meningioma and 366 individually matched hospital controls with non-neoplastic and non-neurological disease selected from six major hospitals. Cases and controls were matched by sex, age and area of residence and interviewed in the hospital wards to obtain information on medical history, occupation and lifestyle. No association with liquor or beer consumption was apparent. Cigarette smoking was positively associated with meningioma risk in women but not in men. In women, compared with non-smokers, the adjusted OR for pack-years of smoking above the median (124) was 6.2 (CI 2.04-18.87). Both of these observations contrast with the results of a study of glioma in the same population, using similar methods. The risk of meningioma was positively associated with reported occupational exposure to lead, tin, cadmium and ionising radiation in both genders.

Brain cancer and occupational exposure to lead

A recent report in the literature suggested a link between occupational exposure to lead and brain cancer. To explore the hypothesis, we applied a job-exposure matrix for lead to the occupation and industry codes given on the death certificate of 27,060 brain cancer cases and 108,240 controls who died of non-malignant diseases in 24 US states in 1984-1992. Brain cancer risk increased by probability of exposure to lead among Caucasian men and women with high-level exposure, with a significant twofold excess among Caucasian men with high probability and high level of exposure to lead (odds ratio = 2.1; 95% confidence interval, 1.1-4.0). Risks were also elevated in the low- and medium-probability cells for African-American men with high-level exposure to lead. Trend by intensity level was statistically significant among African-American men (all probabilities combined). Although exposure assessment was based solely on the occupation and industry reported on the death certificate, these results add to other epidemiologic and experimental findings in lending some support to the hypothesis of an association between occupational exposure to lead and brain cancer risk. Analytic studies are warranted to further test this hypothesis.

Signal transduction in environmental neurotoxicity

Signal transduction is the process by which specific information is transferred from the cell surface to the cytosol and ultimately to the nucleus, leading to changes in gene expression. Since these chains of biochemical and molecular steps control the normal function of each cell, disruption of these processes would have a significant impact on cell physiology. Some of the major signal transduction pathways are briefly reviewed. The interactions of four chemicals (lead, ethanol, polychlorinated biphenyls, and trimethyltin) with different cell signaling systems, particularly the phospholipid hydrolysis/protein kinase C pathway, are discussed. The possible causal relationship of such cellular and molecular interactions with known signs and symptoms of neurotoxicity are highlighted.