Particle Toxicology - Chapter 15 pps

Kleeberger and Reuben Howden National Institute of Environmental Health Sciences, National Institutes of Health CONTENTS 15.1 Introduction ...275 15.2 Susceptibility Factors ...276 15.2.

15 Susceptibility to Particle Effects

Steven R Kleeberger and Reuben Howden

National Institute of Environmental Health Sciences,

National Institutes of Health

CONTENTS

15.1 Introduction 275

15.2 Susceptibility Factors 276

15.2.1 Genetic Background 276

15.2.2 Nongenetic Factors 279

15.3 Conclusions 282

References 282

15.1 INTRODUCTION

Linkage between exposure to particulate matter (PM) !10 mm in aerodynamic diameter (PM10) and a number of acute and chronic health effects throughout the industrialized world has been well established in epidemiological studies (e.g., [1–3]) Acute effects include mortality, hospitalization, increased respiratory symptoms, decreased lung function, increased plasma viscosity, changes in heart rate and heart rate variability (HRV), and pulmonary inflammation [4] Chronic effects associated with particulate exposures include increased mortality rates (e.g., cancer), chronic cardiopulmonary disease, and decreased lung function [4] Understanding the mechanisms through which particulate exposures cause morbidity and mortality continues to be a critical public health concern

It is also clear that susceptibility to particle effects is not the same from one individual to the next, i.e., interindividual differences in response to particulate exposures exist The factors that determine interindividual susceptibility are almost certainly complex, and may include intrinsic (host) and extrinsic (environmental) factors (Figure 15.1) While considerable interest in genetic background as a host factor for susceptibility to pollutants has been generated [5–8], other host factors should also be considered [9] These may include gender, age, pharmacokinetic and phar-macodynamic response parameters (including particle deposition and clearance [10]), pre-existing disease, and nutrition Furthermore, these factors may, and probably do, interact to determine individual responsiveness Numerous examples of gene X gene, gene X environment, and gene X gene X environment interaction in the pathogenesis of lung disease have been described (e.g., [11]) Specific subpopulations may be particularly at risk to the toxic effects of particles These sub-populations include the young and the elderly, patients with pre-existing diseases such as pulmonary hypertension, chronic obstructive pulmonary disease (COPD), asthma, diabetes, and compromised immune systems [4,12,13] Environmental factors that may impact an individual’s response to particle exposure include coexposures and the physical environment (e.g., temperature, altitude)

In this chapter, we identify and briefly discuss factors that have been found to contribute to interindividual susceptibility to particulate exposures We include reports using animal models,

275

human subjects in clinical investigations, and epidemiological studies Our overall objective is to provide the reader with a perspective on the relative importance of those factors that may impact on individual particle susceptibility and to suggest where further investigation is needed to clarify susceptibility factors

15.2 SUSCEPTIBILITY FACTORS

An extensive literature exists that describes interindividual differences in drug metabolism and susceptibility (e.g., [14–17]) Many of these differences have been attributed to polymorphisms in genes that encode metabolism and phase two enzymes (e.g., N-acetyltransferase and G6PD) For example, genetic background has an important role in determining susceptibility to infectious agents and pesticide exposures [18–20] It is well known that many complex diseases cluster in families, and clustering can be explained by genetic background, shared environment, or a com-bination of the two Two broad research strategies have been utilized to identify genes [or quantitative trait loci (QTLs)] that determine disease susceptibility The first is positional cloning or linkage mapping, which exploits within-family associations between marker alleles and putative trait-influencing alleles that arise within families [21] This approach is designed to identify association of a chromosomal interval(s) within the entire genome that may contain genes that are polymorphic and might account for the differential response phenotype under study Linkage mapping is applicable to human populations and animal models, and has had considerable success in Mendelian (single gene) diseases The second approach (candidate gene or association study) chooses loci, which a priori are likely to determine the phenotype of interest Linkage is assessed between the phenotype of interest and markers flanking the candidate genes or the candi-date genes themselves, and evaluates across-family associations The principle underlying the association of genetic polymorphisms not directly involved in disease pathogenesis is that of linkage disequilibrium, which arises from the coinheritance of alleles at loci that are in close physical proximity on an individual chromosome [22] Emergent technologies, including gene and protein expression arrays, have been important developments in the ability to identify candidate susceptibility genes (Figure 15.2)

A number of laboratories have used positional cloning in animal models to determine whether genetic background is an important determinant of pulmonary responses to particulates Ohtsuka et al [23] studied the interstrain variance of lung responses to particle-associated sulfate (acid coated particles, (ACP)) in inbred strains of mice The ACP model was chosen for study

Genetic background Pre-existingdisease

Socioeconomic status Age

Cardiopulmonary responses

Children

Diabetes Atherosclerosis Arrhythmia

Inflammation Coagulation Antioxidant Innate immunity

Education Nutrition Workplace

Phenotypes

Susceptibility

factors

FIGURE 15.1 General susceptibility factors that may influence cardiopulmonary responses to particle exposures

because the particles could be generated reproducibly in the laboratory and all mice could be exposed to the ACP under similar conditions Nine strains of inbred mice were exposed to ACP for 4 h in a nose-only chamber, and inflammation was assessed after 1, 3, 7, or 14 days by bronchoalveolar lavage (BAL) Control exposures were carbon black (CB) or sulfur dioxide alone Innate immune response was assessed by measuring Fc-receptor mediated phagocytosis of BAL alveolar macrophages (i.e., quantitation of ingested sensitized sheep red blood cells) ACP exposure did not cause an appreciable inflammatory response in any of the strains at any time postexposure However, significant interstrain differences were found in Fc-receptor mediated phagocytosis by alveolar macrophages Among the nine strains examined, C3H/HeJ (C3) mice were the most resistant and C57BL/6J (B6) were the most susceptible

The significant interstrain variation in susceptibility indicated a strong genetic component contributed to ACP susceptibility A genome-wide search for linkage of the Fc-receptor mediated phagocytosis phenotype was performed in segregant populations derived from B6 and C3 mice using informative simple sequence length polymorphisms (SSLPs) distributed at approximately 10 centi-Morgan (cM) intervals throughout the genome Interval mapping by simple linear regression identified a susceptibility locus on chromosome 17 between approximately 16–22 cMs [24] An additional QTL was detected on chromosome 11 between D11Mit20 and D11Mit12, and no significant interaction between QTLs was detected The presence of separate susceptibility loci for the phagocytosis phenotype was consistent with the cosegregation analysis for this parameter [24]

Within the chromosome 17 QTL are a number of candidate genes, including the proinflamma-tory cytokine Tnf (tumor necrosis factor-a, TNF-a), multiple histocompatibility loci, and heat shock proteins (HSP) The chromosome 11 QTL also contains a number of candidate genes, including a cluster of inducible cytokines and inducible nitric oxide synthase (iNOS) Interestingly, these QTLs nearly overlapped similar QTLs identified for ozone susceptibility The common linkages suggest that similar genetic mechanisms may contribute to pulmonary responses to ozone-induced inflammation and macrophage phagocytic dysfunction induced by ACP, but further genetic analyses are required to confirm this hypothesis

Particle exposure

Responses

Inflammation Heart rate Innate immunity Heart rate variability Antioxidant defense Arrhythmia Susceptibility factors

Lungs

Cardiovascular

FIGURE 15.2 Schematic of the potential interactions between susceptibility factors (e.g., age, genetic back-ground, pre-existing disease, socioeconomic status) and the route of particle entry into the body and physiological responses to the particles

Tolerance to air pollutant-induced pulmonary inflammatory and hyperpermeability effects has been demonstrated in animal models and human subjects (e.g., [25,26]) Wesselkamper et al [27] evaluated the interstrain variation in the ability of inbred mice to “become tolerant” to the toxic effects of repeated exposure to zinc oxide (ZnO) These investigators found significant interstrain variation in the inflammatory cell and hyperpermeability responses to single and multiple ZnO exposures A genome-scan for susceptibility QTLs for the development of pulmonary tolerance to ZnO in a DBA/2J and Balb/cByJ intercross cohort from mice identified a significant QTL on chromosome 1, and suggestive QTLs on chromosomes 4 and 5 [28] Toll-like receptor 5 (Tlr5) was identified as a candidate susceptibility gene in the chromosome 1 QTL, and functional analysis confirmed a role for Tlr5 in particle tolerance [28] This represents the first attempt to identify the genes responsible for the development of tolerance, and confirmation of candidate genes in additional models and human subjects may have important implications for understanding suscep-tibility and resistance to repeated exposures to pulmonary toxicants

Wesselkamper et al [29] developed another model of particle susceptibility and acute lung injury using nickel sulfate aerosol Continuous exposure of mice to 150 mcg/m3of nickel sulfate causes death in a strain-dependent manner; A/J mice were significantly more susceptible to the aerosol than B6 mice [29] A QTL analysis with backcross mice from A/J and B6 progenitors identified a significant QTL on chromosome 6, and suggestive QTLs on chromosomes 1 and 12 This study suggested that susceptibility to irritant-induced lung injury and subsequent survival was thus dependent on relatively few loci [30] A number of interesting candidate genes were identified

in these QTLs, including transforming growth factor alpha (Tgfa), and proof-of-concept testing in this model has begun This group has also used gene expression arrays to identify candidate genes for particle-induced lung injury [31,32] This approach led to the identification of transforming growth factor beta (Tgfb) and macrophage-stimulating 1 receptor (Mst1r) as candidate suscep-tibility genes, and functional analyses have confirmed an important role for both in the injury that follows exposure to nickel sulfate aerosol

We have also compared the lung injury responses to residual oil fly ash (ROFA) exposure in inbred mouse strains [33] Significant interstrain (genetic) variation was observed in ROFA-induced lung inflammation and hyperpermeability, and C3 mice were most resistant to the ROFA-induced injury responses, while B6 mice were among the most susceptible Interestingly, ROFA-induced lung injury was significantly greater in C3H/HeOuJ mice compared

to C3 C3H/HeOuJ and C3 mice differ only at a loss of function mutation in toll-like receptor 4 (Tlr4) that confers resistance to endotoxin and ozone in the C3 strain ROFA also significantly enhanced transcript and protein levels of lung TLR4 in C3H/HeOuJ, but not in C3 mice Further-more, ROFA activated downstream TLR4 signaling molecules (i.e., MyD88, TRAF6, IRAK-1,

NF-kB, MAPK, AP-1) to a greater extent in C3H/HeOuJ mice than in C3 mice before the development

of pulmonary injury These results support an important contribution of genetic background

to particle-mediated lung injury and suggest that Tlr4 is a candidate susceptibility gene Gilmour

et al [34] also found that pulmonary responses to combustion source PM in hypertensive rats are mediated through TLR4 signaling Interestingly, Hollingsworth et al [35] found that the pulmonary inflammatory responses to ROFA were not different in B6 mice with targeted deletion of Tlr4 compared to wild type mice, and may suggest that the interaction between Tlr4 and genetic back-ground (strain) is an important consideration in pulmonary response to ROFA

It is interesting to note that similar QTLs have been identified for multiple independent models

of susceptibility to pollutant-induced inflammation, injury, and immune dysfunction For example, nearly identical QTLs on chromosomes 17 and 11 have been found to explain a significant portion

of the genetic variance in susceptibility to ACP, ozone-induced inflammation, and ozone-induced lung injury and death Further, these QTLs were also found to have an important role in bleomycin-and radiation-induced lung injury in the mouse [36,37] The common linkage suggests that similar mechanisms control susceptibility to the various environmental agents [38]

Evidence for an important role of genetic background in susceptibility to particle effects in human populations has also emerged For example, Schwartz et al [39] found changes in the high frequency (HF) component of HRV associated with exposure to PM2.5 only in individuals without the glutathione-S-transferase M1 (GSTM1) allele No particle effects on HRV were found in individuals with normal GSTM1 These investigators also found that use of statins reversed the particle effects on HRV in the GSTM1 null subjects This gene X drug X environment interaction on HRV demonstrates the complex nature of susceptibility to particle effects in human populations Adonis et al [40] found an association between a biomarker (1-OH-P) of exposure to PAHs in diesel exhaust and the presence of the CYP1A1*2A genotype, and may be useful in identifying individuals at higher risk among those exposed to diesel exhaust Interestingly, the GSTM1 null genotype was not associated with the exposure biomarker, though an interaction between CYP1A1*2A and GSTM1 may be informative These studies support the notion that oxidative stress may be an important component of particle toxicity, and that individuals with compromised antioxidant defenses may be at enhanced risk to the injurious effects of particles

Factors other than genetic background that are involved in cardiopulmonary susceptibility to particle exposure are also of considerable importance in understanding the etiology of this public health concern (Figure 15.2) It is critical to understand which subsections of the general population are susceptible to particulate exposure in order to reduce the health risks While it is beyond the scope of this chapter to discuss all nongenetic components associated with susceptibility, we will focus on three important subgroups: age; pre-existing disease; socioeconomic status (Table 15.1)

Age Elderly individuals and developing infants are at particular risk from exposure to particu-late air pollution A number of epidemiological and laboratory studies have addressed this concern,

TABLE 15.1

Representative Investigations of the Effects of Age, Pre-Existing Disease, and

Socioeconomic Status on Responsivity to Particle Effects

Subgroup Study type Conclusion Reference Age Epidemiology Elderly individuals with high airway

hyperresponsiveness and IgE were susceptible to air pollution including PM 10

Boezen et al [47]

Mouse The regulation of heart rate was altered when senescent

AKR/J mice were exposed to carbon black

Tankersley [53] Epidemiology The risk of respiratory mortality in postneonatal infants

increased in relation to PM 10 increases

Ha et al [45] Pre-existing

disease Epidemiology Individuals with congestive heart failure, arrhythmia oratherosclerosis are at risk Brook et al [55]

Epidemiology COPD and asthma increase susceptibility via elevated

oxidative stress Li et al [61] Human subjects Higher fine particle deposition was found in subjects with

obstructive lung disease Kim and Kang[64] Socioeconomic Epidemiology Higher particle associated mortality rates were found in

low socioeconomic regions Jerret et al [67] Epidemiology Low education levels and employment in manufacturing

present additional particle related health risks Levy et al [68] Field study Antioxidant supplementation attenuated lung function

responses to particle exposure

Romieu et al [70]

but the health effects associated with infant exposure to particulate matter has not been studied in sufficient detail Pre- and perinatal infants can be affected by particulate air pollution, including increases in fetal mortality [41] and low birth weight [42,43] These effects of particulate air pollution are of great concern since, for example, the relationship between low birth weight and infant mortality is well known Evidence for infant health risk associated with particle exposure was also provided by Woodruff et al [44] In this study, increased levels of PM10 were linked to respiratory illness and sudden infant death syndrome in 1–11 month old children Another epide-miological studies reported that postneonatal infants are highly susceptible to increases in respiratory mortality in relation to particulate exposure [45] In this study, daily mortality records from 1995 to 1999 in Seoul were used to establish subsections in the Korean population that were susceptible to air pollution Subsections of the population were divided into 3 age groups:

1 month–1 year; 2–64 years; and individuals 65 years and over The authors reported that post-neonates had the highest relative risk (1.142) of total or respiratory mortality upon exposure to

PM10when compared to elderly individuals (1.023) and the intermediate age group (1.008) This suggests that infants are at greater risk than the elderly, and that particle related illness at a young age could lead to important developmental consequences that may affect these individuals in later life

The aged as a second subgroup of the general population are more susceptible to particles than younger adults The elderly share many of the same responses to particle exposure as children and they include those already discussed above in addition to airway hyperresponsiveness (AHA) and allergic reaction mediated by immunoglobulin E (IgE) Furthermore, women may be at greater risk because they have higher AHA than men [46] Concomitant AHA and high levels of IgE have been reported to result in higher susceptibility to PM10exposure than either AHA or high levels of IgE alone [47] However, the mechanisms involved in these responses to particle air pollution are currently not clear

Recently, a number of epidemiological and observational experimental studies have investi-gated a possible association between particulate air pollution exposure and changes in the regulation of the heart as a risk phenotype An association between changes in HRV and increased cardiovascular (CV) risk has been established for some time [48] For example, myocardial infarction patients with low HRV are more likely to suffer complications than patients with normal HRV Therefore, reduced HRV during or following exposure to particles could provide insight into the potential mechanisms involved in susceptibility Epidemiological studies have shown changes in HRV that were associated with changes in ambient particulate matter Such changes in HRV are believed to indicate a greater risk of life threatening arrhythmia or the occurrence of fatal CV events in those with pre-existing CV or cardiopulmonary disease Reduced HRV in the elderly has been reported during or following periods of higher ambient

PM [49,50] It has been suggested that health status may be an important factor in determining the degree of PM induced HRV changes [51] It is important to note that methods designed for HRV measurement require controlled laboratory conditions for ECG recording Since epidemiologists are forced to use ambulatory ECG data for HRV, results from these studies can be difficult to interpret Controlled studies using mice have shown reductions in heart rate (HR) and HRV following exposure to ultrafine particles [52] These responses occurred within an hour of the bolus exposure but were transient and values returned to baseline rapidly Moreover, in these young and healthy mice, baseline HR and HRV did not predict the response to PM exposure In aged mice, carbon black (CB) induced changes in the autonomic nervous system that differed according to the degree of aging [53] In older, but healthy mice, CB exposure resulted in changes

in the sympathetic nervous system Conversely, in terminally senescent mice changes in the parasympathetic nervous system were observed following CB exposure and these changes were related to HR regulation

Pre-existing disease Pre-existing disease increases susceptibility to environmental pollution such as particles While many disease states are likely to be involved, there is some evidence

available to suggest that CV and respiratory illnesses and diabetes are important In the previous section responses to PM exposure were discussed, and these biologic systems may become over-whelmed if underlying disease exists Moreover, early development of disease can be accelerated

by frequent exposure to particles For example, Sun et al [54] demonstrated that long-term exposure to PM2.5altered vascular tone, induced vascular inflammation, and potentiated athero-sclerosis in ApoE-/- mice

PM has been associated with increased hospital admissions and CV mortality due to CV disease Those with congestive heart failure, arrhythmia and atherosclerosis appear to be most at risk [55] Park et al [56] reported that a family history of ischaemic heart disease (IHD), hyperten-sion, and diabetes are all associated with susceptibility to PM Increases in blood coagulation factors are also known to occur with PM exposure, and could have fatal consequences in individuals with IHD, hypertension, and atherosclerosis One study used a marker of potential myocardial ischaemia and found an increased risk of S-T segment depression during an exercise test that associated with ultra fine particles and PM2.5[57] Furthermore, additional risk has been associated with a high concentration of plasma fibrinogen [58], giving the impression that susceptibility to particles may be partly dependant upon the number of these risk factors that are present together Cardiovascular injury or risk, in association with particle exposure, has also been linked to diabetes Zanobetti and Schwartz [59] reported that the percentage increase in PM10-related CV hospital admission in diabetics was twice as high as those of nondiabetics The authors suggested that this observation might be associated with upregulation of inflammatory activity in diabetics, which may confer heightened susceptibility to PM Patients with diabetes also present several risk factors associated independently with CV disease and exposure to PM If compensatory mechanisms have a baseline reduction in capacity because of pre-existing disease, subsequent exposure to PM may overwhelm these systems causing severe illness or sudden death [60] Pre-existing COPD and asthma are known to increase susceptibility to particle exposure perhaps due to a significant, additional oxidative stress [61] In COPD patients, all cause associations with particle exposure were 10 times greater when compared to all subjects in a recent study [62] Potential mechanisms for this high level of susceptibility in these patients include decreased antioxidant defenses [63] and higher fine particle deposition in the lungs of patients with obstructive airways disease [64] Induction or exacerbation of asthmatic symptoms

by exposure to particles has also been well established In a recent study, fractional exhaled nitric oxide (FENO) was used as a noninvasive method of estimating airway inflammation [65] FENOwas measured over a 12-day period while local particle levels were monitored FENO levels were associated with changes in PM10and PM2.5in the elderly asthmatic patients These data were in agreement with an earlier study by this group that showed similar results in asthmatic children [66] Socioeconomic status Subsections of a citywide population in an intraurban area with low socioeconomic status have been associated with higher mortality rates in relation to ambient air pollution [67] Low education levels and employment in manufacturing present additional particle-related health risks [68] Proposed explanations for this effect of socioeconomic status and education attainment include higher workplace particle exposures for those working in manufacturing material deprivation and poor material conditions [69] Also, lower socioeconomic status has been associated with less exposure measurement error since there individuals are less mobile [69] Since these groups must tolerate a disproportionate level of susceptibility to air pollution related health risks, they would account for a large percentage of emissions control benefits [68] Low socioeconomic status may also affect the ability of people to achieve adequate nutrition Poor nutrition status has been suggested as another particle susceptibility factor [69] Inadequate nutrition could compromise antioxidant defenses thus increasing particle suscep-tibility—a recent study showed reduced particulate effects on lung function after dietary supplementation with antioxidants [70] The importance of this factor becomes apparent when considering the fact that poor nutritional status is possible across all socioeconomic classes

15.3 CONCLUSIONS

It is clear from the above discussion that genetics background is a critical component to inter-individual susceptibility to particle exposure Further investigation is of great importance if our understanding of the genetic contribution to particle associated health risks is to be improved For example, several QTLs are known to be involved, but few candidate genes have been rigorously tested Little is known about how age, pre-existing disease, or socioeconomic status modifies the genetic components to particle susceptibility Susceptibility to morbidity and mortality associated with particle exposure appears to be highly complex and it likely includes interactions between many of these components, both genetic and nongenetic It is also clear that the mechanisms involved in many of these factors are poorly understood and much work is needed to reduce this important public health concern

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