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Aging or senescence is the process of an organism’s deterioration with time. Although the process is still not completely understood, it is generally believed that lifespan is decided by a combination of genetic factors and environmental effects.

While the damage that occurs to DNA during life is postulated to play a large role in the processes of aging, less attention has been paid to the damage to other macromolecules, such as proteins, and the role that this might play in aging.

Sara Hueber and Na Li

Recent studies indicate that a decrease in protein turnover rate by the lysosome and ubiquitin-proteasome systems (UPS) not only is involved in the normal aging process, but might also contribute to many diseases such as segmental progeroid syndromes, neurodegenerative diseases, and many other age-related diseases. Thus, the characterization of protein turnover mechanisms may lead to the development of therapies that may be effective in such diseases.

Altered protein turnover may also be involved in the increased life span that has been reported in organisms that are under caloric restriction. Studies have shown that caloric restriction maintains ‘juvenile’ UPS activity in tissues so that decreases the protein damages and wastes in the process of aging. Thus, caloric restriction mimetics might be developed as antidotes for ageing by working on improving protein turnover rate.

Perfluorinated compounds (PFCs) are a family of manmade chemicals that have been used for decades to make products that resist heat, oil, stains, grease and water. Common uses include nonstick cookware, stain-resistant carpets and fabrics, as components of fire-fighting foam, and other industrial applications. PFCs are incredibly resistant to breakdown and persistent in the environment, present in humans, and wide-spread in wildlife.

Although these chemicals have been used since the 1950s in countless familiar products, they’ve been subjected to little government testing. Interest in perfluorinated compounds (PFCs) began in the late 1990’s with the significant advances in analytical chemistry.

Sophia and Na

Perfluorooctanoc acid (PFOA) is one of the most studied PFCs. It is also one of the terminal breakdown products of other PFCs. PFOA has been found in the plasma of occupationally and non-occupationally exposed humans. The PFC family of chemicals is relatively new and is the focus of active scientific research.

Results from laboratory animal studies indicated developmental toxicity, hepatotoxicity, immunotoxicity, carcinogenicity, metabolic and endocrine disrupting potentials of PFOA and related chemicals; but modes of their action are ill-defined. There are not many studies of health effects in people. However, studies of workers exposed to PFOA and other PFCs during manufacturing showed some signs of multi-system toxicity of PFOA. A recent paper about the developmental toxicity of PFOA in mouse will be discussed in the presentation. Human health concerns will also be discussed.

Hematopoiesis is a sequential and tightly regulated process. The cellular events to maintain homeostatic blood formation are critically regulated by hematopoietic stem cells (HSCs). HSCs remain predominantly quiescent in vivo, have the ability to self-renew and can reconstitute all the lineages of mature blood cells. Different blood malignancies are related to functional alterations of HSCs and immature progenitor populations.

The aryl hydrocarbon receptor (AhR) was originally discovered due to its mediation of the toxic, carcinogenic and teratogenic effects of environmental pollutants such as dioxins. Dioxin exposure has been correlated with blood malignancies such as leukemias and lymphomas. Preliminary data indicates that upon dioxin exposure, the numbers of primitive progenitor cells in bone marrow are altered. It was also observed that AhR knock-out mice have constitutively higher numbers of HSCs and higher proliferation in vitro than controls. Currently, no physiological role of this evolutionarily conserved receptor has been established.

This study will characterize the role of the AhR in regulating reconstitution potential, self-renewal activity and quiescency of HSCs. Dysregulation of these critical HSCs properties may predispose more lineage committed populations to acquire malignant behaviors.

Introduced by Fang (Sophia) Fang.

Chronic Obstructive Pulmonary Disease (COPD) is the fourth leading cause of death in the United States. Although there are cases of COPD caused by industrial exposures, the main etiological factor in the development of the disease is cigarette smoke. Current treatments are ineffective and damage due to COPD is irreversible. Therefore, it is essential that COPD research focus on improving our understanding of mechanisms of specific cellular and biochemical pathways altered by smoke within the lung.

Recent research has shown that the genetic ablation of nuclear factor erythroid 2 related factor 2, Nrf2, caused earlier onset and more severe cigarette smoke induced emphysema in mice. Nrf2 is an oxidant sensitive transcription factor responsible for the regulation of antioxidants and phase II metabolic genes. Under basal conditions Nrf2 is sequestered in the cytoplasm by the inhibitory protein Keap1, but can be induced to translocate to the nucleus upon exposure to endogenous and environmental oxidants. Inducers modify key residues on Keap1 and activate kinases that phosphorylate Nrf2. Interestingly, it has also been recently shown that cigarette smoke, which contains many oxidants capable of inducing Nrf2, fails to upregulate antioxidant and phase II genes with chronic exposures.

Preliminary data from our lab shows that Nrf2 is sequestered in the cytoplasm in human lung samples collected from patients with COPD and smokers without the disease. In addition, Nrf2 sequestration has been shown in primary human small airway epithelial cells (SAEC) with prolonged exposures to cigarette smoke extract, while Nrf2 was activated at shorter time points.

We propose that cigarette smoke alters Nrf2 signaling by both enzymatic (kinase-dependent) and/or non-enzymatic (thiol-dependent) mechanisms, reducing Nrf2 functioning, leading to decreased phase II metabolic and antioxidant defense mechanisms. We also propose that genetic ablation of Nrf2 leads to excessive lung inflammation, which is the hallmark of COPD whereas functional reconstitution or upregulation of Nrf2 leads to reversal of cigarette smoke-induced lung inflammation in mouse.  Therefore, understanding the molecular mechanism of cigarette smoke mediated regulation of Nrf2 will provide a better understanding of the susceptibility to and progression of COPD.

Chronic obstructive pulmonary disease (COPD) is characterized by a progressive and irreversible obstruction of airflow and an exaggerated inflammatory response in the lungs to inhaled gaseous pollutants and cigarette smoke.

Glucocorticoids, used as therapeutic agents to dampen the inflammatory response in COPD, bind to the glucocorticoid receptor where they translocate to the nucleus, recruit a critical histone deacetylase (HDAC2) as part of a co-repressor complex to NF-κB-driven pro-inflammatory gene promoters and suppress gene expression. Cigarette smoke-induced oxidative stress, the major etiological factor in the pathogenesis of COPD, has also been implicated in increased bone loss in smokers and patients with Graves’ disease. Cigarette smoke is known to induce an impaired response to glucocorticoids both in-vivo in COPD patients, severe asthmatic patients who smoke and in-vitro in macrophage cells.

This impaired response to glucocorticoids has been attributed to a selective decrease in HDAC2 which correlates with increased severity of COPD.

Several post translational modifications to HDAC2 such as nitrosylation and aldehyde-adduct formation have been implicated in the loss of deacetylase activity as possible mechanisms for impaired inflammatory response to glucocorticoids. However the mechanism by which HDAC2 is lost in response to cigarette smoke-mediated oxidative stress is unknown.

We propose that cigarette smoke modifies HDAC2 leading to an increased degradation of HDAC2 via the ubiquitin-proteasomal pathway and that hyperphosphorylation might be a trigger for this event.

In preliminary experiments utilizing the monocyte/macrophage cell line (MM6) as an in-vitro model to study the molecular mechanisms by which cigarette smoke triggers the loss of HDAC2 in the lungs, we observe a loss of HDAC2 protein and a dose response increase in HDAC2 hyperphosphorylation in response to cigarette smoke that also correlates with increased release of pro-inflammatory cytokines. These cells also show an impaired response to Dexamethasone when exposed to cigarette smoke. Preliminary in-vivo studies also show a loss of HDAC2 in lung homogenates of rats exposed to cigarette smoke.

We intend to probe further the relationship between post translational modifications to HDAC2 and the subsequent loss of corepressor interactions critical to HDAC2 deacetylase activity, whether cigarette smoke increases the expression of proteins responsible for HDAC2 degradation via the ubiquitin-proteasomal pathway, potential phosphorylation sites on HDAC2 in response to cigarette smoke and using an in-vivo mouse model to study the inflammatory response to glucocorticoids after smoking cessation. In conclusion, this study intends to stimulate the investigation of novel therapies to complement glucocorticoids presently used as a frontline medication in the fight against COPD.

“The dose makes the poison” is a central dogma in toxicology and so the typical dose-response relationship of a certain chemical has traditionally been depicted as an S-shaped curve. In recent years, however, many scientists have joined a revolt against this longstanding belief by propagating a concept of hormesis.

Hormesis has traditionally been defined as a stimulatory / beneficial effect of a substance / factor at low doses while the substance / factor is toxic at high doses to an organism. In recent years, much debate has been going on with regard to the definition, evidences, mechanism, generality, and implications in public health and clinical practices.

By citing data from several laboratory and epidemiologic studies (such as in radiation, exercise, caloric restriction, moderate alcohol drinking, etc.), this talk will give a brief overview on several issues related to hormesis, including the evolving concept, data interpretation issues, implications to risk assessment, and some evolutionary/ecological perspectives. It is concluded that the definition of hormesis needs further updates and that the application of hormesis in risk assessment and public health should be treated case by case.

Bone remodeling is the concerted interplay of two cellular activities: osteoclastic bone resorption and osteoblastic bone formation. A decrease in bone density results when there is relatively more resorption than formation. This leads to an increased risk of fractures and osteoporosis, a skeletal disorder characterized by compromised bone strength and fractures.

Epidemiological studies reveal a strong association between cigarette smoke inhalation and osteoporosis^1-3. Other research demonstrates that smoke inhalation is highly associated with reduced alveolar bone density in the jaw⁴.

There are few in vivo preclinical models to study the effects of cigarette smoking on bone loss, so the mechanisms of smoke-induced bone loss remain unknown. At the University of Rochester, The Center for Musculoskeletal Research along with the Pulmonary Toxicology Program have developed a mouse model that demonstrates that cigarette smoke-exposed mice have reduced cortical and trabecular bone content and increased bone ductility. This model will be used to determine which cells/tissues are the main targets of cigarette smoke, and which components of cigarette smoke facilitate these effects.

The mechanism of lead-induced bone loss has been more extensively studied and is an important co-morbid factor with cigarette smoke^{5, 6}.   In the United States, the lower socioeconomic groups are more likely to smoke (and to be lead exposed), and are thus at a greater risk of exhibiting bone loss. This new model will help to determine the mechanism of smoke-induced bone loss, and assess risk of osteoporosis by cigarette-smoke and co-morbid factors.

Pulmonary fibrosis (lung scarring) can be caused by a variety of factors including exposure to environmental and occupational toxicants (silica and coal dust), drug toxicity, auto-immune disease and radiation therapy. Pulmonary fibrosis leads to severe impairment of lung structure and function and causes significant disability and even death. Currently there are few, if any, effective therapies for these diseases.

Fibroblasts respond to the pro-fibrogenic cytokine transforming growth factor beta (TGF-β. TGF-β induces the differentiation of fibroblasts to myofibroblasts which initiates the deposition of excess extracellular matrix components (ECM) including collagen and fibronectin. Peroxisome Proliferator Activated Receptor-gamma (PPARγ) is a nuclear transcription factor that is known to play a role in adipogenesis and insulin sensitization.

Recent data also implicates activation of the PPARγ pathway in down-regulation of inflammation in a variety of tissues. We hypothesize that PPAR-γ ligands have a role as anti-fibrotic agents through inhibition of key TGFβ-stimulated fibrogenic activities.

We are currently investigating whether PPARγ ligands are capable of inhibiting TGFβ-mediated myofibroblast differentiation and extracellular matrix production in human fibroblasts. Our previous data indicate that PPAR&gamm; ligands inhibit TGF-β driven myofibroblast differentiation, in vitro (AJP Lung 2005. 288:L1146-53). These data also indicate that the mechanism responsible for inhibiting myofibroblast differentiation is at least partially independent of PPARγ. Triterpenoids are a novel class of PPARγ ligands that show higher affinity for the receptor than other classes of PPARγ ligands such as the thiazolidinediones. Triterpenoids also show greater PPARγ-independent activities, and a greater ability to inhibit the fibroblast to myofibroblast transition at much lower concentrations than other PPARγ ligands.

Therefore, we are investigating the efficacy and mechanism of action of these and other PPAR-γ ligands at inhibiting myofibroblast differentiation in vitro and in vivo. This research has the potential to lead to novel therapeutic interventions for individuals with fibrosis of the lung as well as other tissues. Several PPARγ ligands are currently in use for the treatment of diabetes, and this may facilitate the translation of our studies to patients with lung scarring

In 1996, JP-8 (jet propellant-8) became the primary jet fuel used by the United States Air Force, Army, and the North Atlantic Treaty Organization (NATO). Since then, 60 billion gallons of JP-8 fuel have been used and it is estimated that 4.5 billion gallons are used by the US and NATO military organizations each year. Its main use is to power aircraft, but it is also used as a fuel source for heaters, stoves, tanks, and other military vehicles and serves as a coolant in engines and other aircraft components.

JP-8 is a kerosene-based fuel and has a strong smell. Since its emergence as the primary fuel source in the US military, workers have complained about the smell and even tasting JP-8 hours after work. On further evaluation, workers showed signs of fatigue, headaches, blocked nasal passages, ear infections, nausea, and skin irritation. Epidemiological studies on Air Force bases show elevated levels of JP-8 constituents in exhaled breath from workers who perform routine maintenance on aircraft. In fact, JP-8 is recognized as the single largest chemical exposure for military personnel according to the Department of Defense.

Many studies have shown that JP-8 dermal exposure leads to immune suppression. More specifically, dermal exposure leads to the inhibition of delayed-type hypersensitivity reactions in C3H/HeNCr (MTV-) mice. Exposure of rat lung alveolar type II epithelial cells to JP-8 induces biochemical and morphological markers of apoptotic cell death, ROS generation, and decreases intracellular GSH.

The results from these studies indicate that JP-8 exposure is a viable health concern. This seminar will focus on the epidemiological studies conducted on Air Force Bases across the country and experimental studies investigating how dermal exposure of JP-8 leads to immune suppression.

Cardiovascular diseases are the leading cause of morbidity and mortality in the world. The exposure to heavy metals, such as cadmium (Cd), has been suspected to be a key contributor for the development of cardiovascular diseases. Cadmium is considered one of the most toxic substances in the environment due to its wide range of organ toxicity and long elimination half life of 10-30 years¹.

Classic cadmium poisoning (known as itai-itai disease in Japan) has been characterized by multiple fractures, osteomalacia, bone pain, and osteoporosis that occur along with renal dysfunction². Although there has been much research in this area, there is still no established evidence to support linkage of Cd exposure with cardiovascular diseases in humans.

This study will investigate some of the possible effects and mechanisms of Cd on the cardiovascular system in vivo and in vitro and the possible antagonizing effect from Zinc. In a three-month animal study, Cd and Zinc(Zn) were administrated to rats via drinking water. The blood pressure of Cd-exposed rats (50mg/L) was significantly higher compared to that of control rats. At the same time, a significant change of Renin-Angiotensin-Aldersterone-System (RAAS) levels and alteration of heart tissue morphology, determined by electron microscopy, were also found in Cd-exposed groups. These findings may be related to the effects of Cd on sodium retention, cardiac output or RAAS. Zn inhibited the effects of Cd on blood pressure, possibly due to its competition with Cd in absorption and transport. The exposure of cultured cardiomycytes to Cd demonstrated increased beating rate and the release of Angiotensin. Plasma Membrane Calcium ATPase of cardiomyocytes was inhibited and its expression was down-regulated by Cd. Zn partly antagonized the effects of Cd.

In conclusion, long term exposure of Cd induces hypertension in rats. The alteration of RAAS, calcium transport system and the expression of some related genes may contribute to the mechanisms of the toxic effects of Cd on cardiovascular system. Zn partly antagonizes the effects of Cd, which may be related to its competition with Cd for absorption and transport.

References

1. Toxicological Profile for Cadmium. US. Department of Health & Human Services. ATSDR, 1999

2. Patrick L. Toxic metals and antioxidants: Part II. The role of antioxidants in arsenic and cadmium toxicity. Altern Med Rev. 2003 May;8(2):106-128.

By definition, neurodegeneration refers to a progressive neuronal death, which usually affects a specific population of nerve cells. The vulnerability of these subpopulations to a set of complex and multifaceted insults will determine the clinical manifestations of a particular neurodegenerative disease. One of the main challenges to researchers is to determine which of the observed phenomena represent causes and which are merely effects of neurodegenerative diseases.

While the pathologies of diseases such as Alzheimer’s (AD), Parkinson’s (PD), Amyotrophic lateral sclerosis (ALS), and transmissible spongiform encephalopathies (prion disease) are characterized by distinct genetic and environmental factors, a closer examination of the data reveals some reoccurring themes, including protein aggregation and oxidative stress-induced injury. Current research has noted distinct changes in metal ion homeostasis associated with neurodegenerative diseases. Tissue levels of metals such as Cu, Mn, and Fe are altered in AD, PD, and prion diseases. These metal ions are also associated with the protein aggregates that characterize these diseases.

Metal ions are unusually reactive species: they can participate in reduction or oxidation reactions, acid-base chemistry, or ligand coordination reactions. These chemical properties allow the essential metals to catalyze a variety of biochemical reactions and physiological processes. However, when metal homeostasis is altered in the above disease states, these same properties allow them to contribute to the generation of reactive or toxic intermediates that may aid in the progression of disease. This seminar will examine the evidence for the involvement of metals in neurodegenerative diseases, and explore the possible mechanisms by which metals may induce or facilitate the progression of these diseases.

References

Carri MT, et al. (2003) Neurodegeneration in amyotrophic lateral sclerosis: the role of oxidative stress and altered homeostasis of metals. Brain Res. Bull. 61:365-374.

Gaeta A and Hider RC. (2005) The crucial role of metal ions in neurodegeneration: the basis for a promising therapeutic strategy. British Journal of Pharmacology 146:1041-1059.

Thackray AM, et al. (2002) Metal imbalance and compromised antioxidant function are early changes in prion disease. Biochem. J. 362:253-258.

The two tumor necrosis factor-α (TNF-α) receptors TNFR1 and TNFR2, exist in both membrane-bound and soluble forms, both capable of binding TNF-α- a pleiotropic cytokine implicated in the pathogenesis of numerous chronic inflammatory diseases such as pulmonary fibrosis. Solubilization of the receptors occurs following activation of metalloproteases such as TNF-α converting enzyme (TACE). Recently, in vivo studies demonstrate soluble TNF receptors are detectable in the serum of normal patients while patients suffering from TNF-α-mediated diseases such as acute respiratory distress syndrome (ARDS) or rheumatoid arthritis (RA) have elevated levels. It is therefore evident that TNF receptor shedding can occur under both physiologic and pathological conditions in vivo. However, the regulation and function of the shed receptors are still poorly understood. Our laboratory has begun to investigate stimuli leading to receptor shedding and the mechanisms underlying both basal and stimulant-induced release.

Here we demonstrate TNFR1 and TNFR2 shedding is differentially regulated in a murine pulmonary alveolar type II-like cell line (MLE-15s). MLE-15 cells spontaneously shed both TNF receptors into the culture medium, however release of TNFR2 is approximately two times greater than TNFR1 even though cell associated content of the receptors is similar. Addition of TNF-α significantly increases the shedding of TNFR2, whereas cellular and shed TNFR1 content are decreased. Interestingly, although TNFR2 shedding increases, cell content is not altered suggesting shedding does not appear to be a mechanism by which cell receptor content, and therefore sensitivity to TNF-α, is decreased. Interleukin-1β (IL-1β), another pro-inflammatory cytokine produced under similar conditions as TNF-α, also enhances TNF receptor cell associated content and shedding, specifically TNFR2. Additional investigation into the transcriptional regulation of the receptors showed TNFR2 mRNA levels were increased in the presence of either cytokine whereas TNFR1 transcript levels remained unchanged. Together these findings suggest a strict and unique regulation of TNF receptor production and shedding in lung epithelial cells.

It is well known that both TNF-α and IL-1β induce the formation of reactive oxygen species (ROS). Our preliminary research indicates ROS may be responsible for the activation of TNF receptor sheddases. Future studies will more closely examine the involvement of ROS and other potential intracellular signaling molecules.

Modulation of TNF receptor shedding and production on lung epithelial cells suggest regulable cellular responses to TNF-α. Understanding such regulations could give further insight into TNF-α-mediated pathological conditions, including inflammatory pulmonary diseases and their treatment.

Reduced glutathione (GSH; l-g-glutamyl-l-cysteinylglycine) is the most abundant cellular thiol and plays a central role in a multitude of biochemical processes. Disturbances in glutathione homeostasis are implicated in the etiology and progression of a number of diseases. Glutathione is synthesized intracellularly from its precursor amino acids by the ATP requiring enzymes, g-glutamylcysteine synthetase and GSH synthetase. After its synthesis, some of glutathione is delivered to other intracellular compartments or consumed in various reactions, but most of it is exported into the extracellular space for degradation. Thus, intracellular biosynthesis and export from the cell are the two major mechanisms that regulate intracellular glutathione level.

Whereas GSH biosynthesis has been studied extensively, relatively little is known about the proteins involved in GSH transport. Recent studies implicate proteins belonging to the MRP/CFTR and the OATP transporter families in GSH efflux. However, some data indicate that additional GSH transport systems are likely to exist, but have not yet been identified at the molecular level.

The overall goal of this ongoing project is to identify additional human GSH transporters and to functionally characterize any identified genes. The experimental approach involves functional complementation of a yeast strain that is deficient in glutathione uptake (hgt1D stain). This presentation will provide an update on the current state of the project.

Before

Mechanical stimuli serve as essential regulators of function in a variety of organ systems. In the respiratory system, the lungs are subjected to numerous mechanical forces. Mechanical strain is both necessary for normal lung growth and development and sometimes a sequela to respiratory failure in the form of mechanical ventilation. Mechanical ventilation is an essential tool in treating patients with a variety of respiratory diseases, however, it brings with it iatrogenic consequences that can cause additional damage to the lungs.

The mechanisms of strain-induced cellular responses resulting in damage are poorly understood. One of the most widely observed inflammatory responses to mechanical strain and ventilation is the induction of chemokines. A number of studies have found that mechanical strain from high tidal volume ventilation induces the production of αCXC chemokines interleukin-8 (IL-8) and its mouse homolog macrophage inflammatory protein-2 (MIP-2) in both in vitro and in vivo models. IL-8 and MIP-2 serve as potent neutrophil chemoattractants.

Our laboratory has shown that mechanical strain can induce an oxidant response in pulmonary epithelial cells in vitro, and murine ventilation with a physiologic tidal volume produces an inflammatory response in vivo. We hypothesize that mechanical strain-induced ROS production plays a role in strain-induced inflammatory and antioxidant responses in the lung.

Current work in our laboratory focuses on using the in vivo ventilation model to identify strain-induced ROS and inflammatory signaling molecules in the lung, and the in vitro strain system to determine signaling pathways utilized by these molecules. Understanding these mechanisms and pathways is important in the design of clinical interventions aimed at reducing injury associated with mechanical ventilation.

After

Aflatoxins are a structurally related group of toxic secondary metabolites produced by the fungi Aspergillus flavus and A. paraciticus. These Aspergilli flourish around the globe and can feed on an astonishingly wide variety of grains and nuts subsequently contaminating them with toxin.

While human exposure occasionally reaches levels high enough to illicit acute toxicity resulting in hundreds of deaths each year, of greater concern is the staggering degree of lower level exposure that occurs. Low level aflatoxin exposure has been has been epidemiologically linked to hepatocellular carcinoma in humans and experimentally linked in a preponderance of animal models, and the CDC estimates that over 4.5 billion people are chronically exposed.

Aflatoxin is metabolized in the liver by cytochrome P450 enzymes. Research shows that different metabolites may be responsible for acute and chronic effects. Acute effects are due to generalized covalent bonding of a hemiacetal derivative creating widespread nonspecific interference. Investigation into the effects of the epoxide derivative on DNA damage, p53 modulation, and ROS production will elucidate the mechanistic progression of carcinogenesis and may elucidate potential therapies and preventative strategies to be employed in combating aflatoxin exposure.

Nanotechnology has become a common term in today’s lexicon. These materials are finding wide use in common items such as suncreens, cosmetics, textiles, and construction materials. They are also being investigated for use by the military to create safer and/or better fuels as well as advances in soldier technology. (NOS).

With the increase in the use of these compounds have also come advances in engineering technologies to produce these items with more complex structures with tighter precision in size and composition. This increase in number and variety of products with novel properties presents an opportunity for unique toxicological investigations.

One of the main questions is do the unique physical and chemical properties of nanomaterials lead to unexpected toxic effects in their environment and if they do, through what mechanism? To help answer this question we have undertaken in vitro investigations to help evaluate the toxic potential of a number of these materials. Preliminary nanoparticle research and previous research with ultrafine particles shows that the mechanism of damage appears to be reactive species mediated. Reactive Oxygen Species (ROS) have been linked to a number of different toxicities such as mitochondrial damage, genetic lesions, and/or lipid peroxidation which can compromise membrane integrity.

Research has shown that nanoparticles themselves are reactive and are able to produce reactive oxygen species in the absence of biological material. The ability to produce ROS is enhanced when cells are present. The reactive potential of nanoparticles is based on a number of different factors including chemical composition, surface chemistry, and crystallinity. Many of these factors are considered as research continues in the developing field of nanotoxicology.

Nitric oxide (NO) is a key mediator for several processes such as vasodilation and neurotransmission; however, it also plays a role in antimicrobial processes and possibly immune cell homeostasis. It is synthesized from a family of enzymes called the nitric oxide synthases (NOS).

There are three isoforms of NOS, two constitutive forms found in endothelium and neuronal cells (eNOS and nNOS) and an inducible form (iNOS) found in a variety of cell types. iNOS is found in several different cell types including macrophages.

Inflammation and autoimmune processes are known to induce iNOS. Production of nitric oxide involves macrophage activation by proinflammatory and inflammatory mediators and proper protein dimerization that occurs via a heme group and other cofactors.

Lead (Pb), a toxic heavy metal that adversely affects several organ systems, is believed to be interfering with iNOS function by suppressing nitric oxide production. The proximal target of Pb-mediated toxicity is a subpopulation of macrophages that are GR1+ and cd11b+. This population of macrophages produces NO and appears to be playing a role in T cell homeostasis by regulating T cell proliferation.

Our laboratory has previously reported that Pb exposure in our mixed lymphocyte reaction caused an increase in T cell proliferation. This phenomenon has been referred to as allo-enhancement. However, this increase in proliferation actually appears to be a release of suppression.