At seeming safe concentrations, synergistic effects of PM2.5 and formaldehyde co-exposure induces Alzheimer-like changes in mouse brain

[Advances in the applications of exposomics in the identification of environmental pollutants and their health hazards]

Environmental pollution has become a prominent global problem, and the potential health hazards of pollutants have caused widespread concern. However, revealing the relationship between complex-pollutant exposure and disease development remains an immense challenge. The core of environmental-health research and risk assessment is the identification of contaminants and their effects. Exposomics provides a new approach in the study of the relationship between environmental factors and human health. Both "top-down" and "bottom-up" strategies are employed in exposomics research. The development of new technologies for chemical detection and "multi-omics" has greatly facilitated the implementation of these strategies. Exposomics focuses on the measurement of an individual's lifelong exposure and aims to identify the health effects of such exposure. It involves the dynamic monitoring of external and internal exposure levels at different stages of life through traditional biomonitoring and exposomic methods. It also includes the identification of biomarkers, which indicate specific environmental exposures and the adverse effects of these exposures on health. Compared with traditional environmental-health studies, exposomics can more accurately reflect the diversity of exposure factors such as pollutants, natural factors, and lifestyles in the real environment, as well as the complexity of their in vivo processes and the responses they trigger in an organism. Powerful chemical analytical tools such as high-resolution mass spectrometry (HRMS) are widely used in studies related to the field of exposomics. Liquid chromatography-mass spectrometry (LC-MS) has been applied in the detection and analysis of environmental pollutants. Proteomics and metabolomics, as two important tools for biomarker identification and effects analysis, are widely used to explore the relationship between environmental factors and diseases. Pollutants can lead to pathological changes and even toxic effects by interacting with proteins. In the case of mixed exposure, some contaminants may present joint toxicity. The interaction between contaminants may change their environmental behavior or the amount of each contaminant that enters the human body, which, in turn, affects their health effects.

Overview of PM2.5 and health outcomes: Focusing on components, sources, and pollutant mixture co-exposure

PM_2.5 varies in source and composition over time and space as a complicated mixture. Consequently, the health effects caused by PM_2.5 varies significantly over time and generally exhibit significant regional variations. According to numerous studies, a notable relationship exists between PM_2.5 and the occurrence of many diseases, such as respiratory, cardiovascular, and nervous system diseases, as well as cancer. Therefore, a comprehensive understanding of the effect of PM_2.5 on human health is critical. The toxic effects of various PM_2.5 components, as well as the overall toxicity of PM_2.5 are discussed in this review to provide a foundation for precise PM_2.5 emission control. Furthermore, this review summarizes the synergistic effect of PM_2.5 and other pollutants, which can be used to draft effective policies.

Impacts of combined exposure to formaldehyde and PM2.5 at ambient concentrations on airway inflammation in mice

Asthma is a respiratory disease that can be exacerbated by certain environmental factors. Both formaldehyde (FA) and PM_2.5, the most common indoor and outdoor air pollutants in mainland China, are closely associated with the onset and development of asthma. To date, however, there is very little report available on whether there is an exacerbating effect of combined exposure to FA and PM_2.5 at ambient concentrations. In this study, asthmatic mice were exposed to 1 mg/m³ FA, 1 mg/kg PM_2.5, or a combination of 0.5 mg/m³ FA and 0.5 mg/kg PM_2.5, respectively. Results demonstrated that both levels of oxidative stress and inflammation were significantly increased, accompanied by an obvious decline in lung function. Further, the initial activation of p38 MAPK and NF-κB that intensified the immune imbalance of asthmatic mice were found to be visibly mitigated following the administration of SB203580, a p38 MAPK inhibitor. Noteworthily, it was found that combined exposure to the two at ambient concentrations could significantly worsen asthma than exposure to each of the two alone at twice the ambient concentration. This suggests that combined exposure to formaldehyde and PM_2.5 at ambient concentrations may have a synergistic effect, thus causing more severe damage in asthmatic mice. In general, this work has revealed that the combined exposure to FA and PM_2.5 at ambient concentrations can synergistically aggravate asthma via the p38 MAPK pathway in mice.

Formaldehyde toxicity in age-related neurological dementia

The primordial small gaseous molecules, such as: NO, CO, H₂S and formaldehyde (FA) are present in the brains. Whether FA as well as the other molecules participates in brain functions is unclear. Recently, its pathophysiological functions have been investigated. Notably, under physiological conditions, learning activity induces a transient generation of hippocampal FA, which promotes memory formation by enhancing N-methyl-D-aspartate (NMDA)-currents. However, ageing leads to FA accumulation in brain for the dysregulation of FA metabolism; and excessive FA directly impairs memory by inhibiting NMDA-receptor. Especially, in Alzheimer's disease (AD), amyloid-beta (Aβ) accelerates FA accumulation by inactivating alcohol dehydrogenase-5; in turn, FA promotes Aβ oligomerization, fibrillation and tau hyperphosphorylation. Hence, there is a vicious circle encompassing Aβ assembly and FA generation. Even worse, FA induces Aβ deposition in the extracellular space (ECS), which blocks the medicines (dissolved in the interstitial fluid) flowing into the damaged neurons in the deep cortex. However, phototherapy destroys Aβ deposits in the ECS and restores ISF flow. Coenzyme Q10, which scavenges FA, was shown to ameliorate Aβ-induced AD pathological phenotypes, thus suggesting a causative relation between FA toxicity and AD. These findings suggest that the combination of these two methods is a promising strategy for treating AD.

Three month inhalation exposure to low-level PM2.5 induced brain toxicity in an Alzheimer's disease mouse model

Although numerous epidemiological studies revealed an association between ambient fine particulate matter (PM2.5) exposure and Alzheimer's disease (AD), the PM2.5-induced neuron toxicity and associated mechanisms were not fully elucidated. The present study assessed brain toxicity in 6-month-old female triple-transgenic AD (3xTg-AD) mice following subchronic exposure to PM2.5 via an inhalation system. The treated mice were whole-bodily and continuously exposed to real-world PM2.5 for 3 months, while the control mice inhaled filtered air. Changes in cognitive and motor functions were evaluated using the Morris Water Maze and rotarod tests. Magnetic resonance imaging analysis was used to record gross brain volume alterations, and tissue staining with hematoxylin and eosin, Nissl, and immunohistochemistry methods were used to monitor pathological changes in microstructures after PM2.5 exposure. The levels of AD-related hallmarks and the oxidative stress biomarker malondialdehyde (MDA) were assessed using Western blot analysis and liquid chromatography-mass spectrometry, respectively. Our results showed that subchronic exposure to environmental levels of PM2.5 induced obvious neuronal loss in the cortex of exposed mice, but without significant impairment of cognitive and motor function. Increased levels of phosphorylated-tau and MDA were also observed in olfactory bulb or hippocampus after PM2.5 exposure, but no amyloid pathology was detected, as reported in previous studies. These results revealed that a relatively lower level of PM2.5 subchronic exposure from the environmental atmosphere still induced certain neurodegenerative changes in the brains of AD mice, especially in the olfactory bulb, entorhinal cortex and hippocampus, which is consistent with the nasal entry and spreading route for PM exposure. Systemic factors may also contribute to the neuronal toxicity. The effects of PM2.5 after a more prolonged exposure period are needed to establish a more comprehensive picture of the PM2.5-mediated development of AD.

Ozone and Particulate Matter Exposure and Alzheimer's Disease: A Review of Human and Animal Studies

Alzheimer's disease (AD), an aging-related neurodegenerative disease, is a major cause of dementia in the elderly. Although the early-onset (familial) AD is attributed to mutations in the genes coding for amyloid-β protein precursor (AβPP) and presenilin1/presenilin 2 (PS1/PS2), the cause for the late-onset AD (LOAD), which accounts for more than 95% of AD cases, remains unclear. Aging is the greatest risk factor for LOAD, whereas the apolipo protein E4 allele (APOEɛ4) is believed to be a major genetic risk factor in acquiring LOAD, with female APOEɛ4 carriers at highest risk. Nonetheless, not all the elderly, even older female APOEɛ4 carriers, develop LOAD, suggesting that other factors, including environmental exposure, must play a role. This review summarizes recent studies that show a potential role of environmental exposure, especially ozone and particulate matter exposure, in the development of AD. Interactions between environmental exposure, genetic risk factor (APOEɛ4), and sex in AD pathophysiology are also discussed briefly. Identification of environmental risk factor(s) and elucidation of the complex interactions between genetic and environmental risk factors plus aging and female sex in the onset of AD will be a key to our understanding of the etiology and pathogenesis of AD and the development of the strategies for its prevention and treatment.

Air Pollution Neurotoxicity in the Adult Brain: Emerging Concepts from Experimental Findings

Epidemiological studies are associating elevated exposure to air pollution with increased risk of Alzheimer's disease and other neurodegenerative disorders. In effect, air pollution accelerates many aging conditions that promote cognitive declines of aging. The underlying mechanisms and scale of effects remain largely unknown due to its chemical and physical complexity. Moreover, individual responses to air pollution are shaped by an intricate interface of pollutant mixture with the biological features of the exposed individual such as age, sex, genetic background, underlying diseases, and nutrition, but also other environmental factors including exposure to cigarette smoke. Resolving this complex manifold requires more detailed environmental and lifestyle data on diverse populations, and a systematic experimental approach. Our review aims to summarize the modest existing literature on experimental studies on air pollution neurotoxicity for adult rodents and identify key gaps and emerging challenges as we go forward. It is timely for experimental biologists to critically understand prior findings and develop innovative approaches to this urgent global problem. We hope to increase recognition of the importance of air pollution on brain aging by our colleagues in the neurosciences and in biomedical gerontology, and to support the immediate translation of the findings into public health guidelines for the regulation of remedial environmental factors that accelerate aging processes.

A mechanistic view on the neurotoxic effects of air pollution on central nervous system: risk for autism and neurodegenerative diseases

Many reports have shown a strong association between exposure to neurotoxic air pollutants like heavy metal and particulate matter (PM) as an active participant and neurological disorders. While the effects of these toxic pollutants on cardiopulmonary morbidity have principally been studied, growing evidence has shown that exposure to polluted air is associated with memory impairment, communication deficits, and anxiety/depression among all ages. So, these toxic pollutants in the environment increase the risk of neurodegenerative disease, ischemia, and autism spectrum disorders (ASD). The precise mechanisms in which air pollutants lead to communicative inability, social inability, and declined cognition have remained unknown. Various animal model studies show that amyloid precursor protein (APP), processing, oxidant/antioxidant balance, and inflammation pathways change following the exposure to constituents of polluted air. In the present review study, we collect the probable molecular mechanisms of deleterious CNS effects in response to various air pollutants.

Combined exposure to formaldehyde and PM2.5: Hematopoietic toxicity and molecular mechanism in mice

PM2.5 and formaldehyde (FA) are major outdoor and indoor air pollutants in China, respectively, and both are known to be harmful to human health and to be carcinogenic. Of all the known chronic health effects, leukaemia is one of the most serious health risks associated with these two pollutants. To explore the influence and underlying mechanisms of exposure to formaldehyde and PM2.5 on hematopoietic toxicity, we systematically studied the toxicity induced in hematopoietic organs: bone marrow (BM); spleen; and myeloid progenitor cells (MPCs). Male Balb/c mice were exposed to: PM2.5 (20, 160 μg/kg·d) at a dose of 40 μL per mouse or formaldehyde (0.5, 3.0 mg/m3) for 8 h per day for 2 weeks or co-exposed to formaldehyde and PM2.5 (20 μg/kg·d PM2.5 + 0.5 mg/m3 FA, 20 μg/kg·d PM2.5 + 3 mg/m3 FA, 160 μg/kg·d PM2.5 + 0.5 mg/m3 FA, 160 μg/kg·d PM2.5 + 3 mg/m3 FA) for 2 weeks. Similar toxic effects were found in the formaldehyde-only and PM2.5-only groups, including significant decrease of blood cells and MPCs, along with decreased expression of hematopoietic growth factors. In addition, individual exposure of formaldehyde or PM2.5 increased oxidative stress, DNA damage and immune system disorder by destroying the balance of Th1/Th2, and Treg/Th17. DNA repair was markedly inhibited by deregulating the mammalian target of rapamycin (mTOR) pathway. Combined exposure to PM2.5 and formaldehyde led to more severe effects. Administration of Vitamin E (VE) was shown to attenuate these effects. In conclusion, our findings suggested that PM2.5 and formaldehyde may induce hematopoietic toxicity by reducing the expression of hematopoietic growth factors, increasing oxidative stress and DNA damage, activating the 'immune imbalance' pathway and suppressing the DNA-repair related mTOR pathway. The hematopoietic toxicity induced by combined exposure of PM2.5 and formaldehyde might provide further insights into the increased incidence of hematological diseases, including human myeloid leukaemia.

Occupational exposure to formaldehyde and risk of lung cancer: A systematic review and meta-analysis

BACKGROUND

Formaldehyde exposure is associated with nasopharyngeal cancer and leukemia. Previously-described links between formaldehyde exposure and lung cancer have been weak and inconsistent. We performed a systematic review and meta-analysis to evaluate quantitatively the association between formaldehyde exposure and lung cancer.

METHODS

We searched for articles on occupational formaldehyde exposure and lung cancer in PubMed, EMBASE, Web of Science, and CINAHL databases. In total, 32 articles were selected and 31 studies were included in a meta-analysis. Subgroup analyses and quality assessments were also performed.

RESULTS

The risk of lung cancer among workers exposed to formaldehyde was not significantly increased, with an overall pooled risk estimate of 1.04 (95% confidence interval [CI], 0.97-1.12). The pooled risk estimate of lung cancer was increased when higher exposure studies were considered (1.19; 95% CI, 0.96-1.46). More statistically robust results were obtained when high quality (1.13; 95% CI, 1.08-1.19) and recent (1.13; 95% CI, 1.07-1.19) studies were used in deriving pooled risk estimates.

CONCLUSIONS

No significant increase in the risk of lung cancer was evident in the overall pooled risk estimate; even in higher formaldehyde exposure groups. Our findings do not provide strong evidence in favor of formaldehyde as a risk factor for lung cancer. However, since risk estimates were significantly increased for high-quality and recent studies, the possibility that exposure to formaldehyde can increase the risk of lung cancer might still be considered.

Vitamin E reduces the extent of mouse brain damage induced by combined exposure to formaldehyde and PM2.5

Exposure to specific air pollutants has been demonstrated to induce damage in the brain. However, these studies ignore the effects of a combination of contaminants, and there is a high likelihood that people will be exposed to a mixture of contaminants in daily life. Our previous study showed that co-exposure to formaldehyde (FA) and PM2.5 induced damage in the mouse brain at the safe exposure level for FA or PM2.5 exposure alone, and that oxidative stress and inflammation may be involved in the toxicity mechanisms. A universal strategy to protect people exposed to FA and PM2.5 is urgently needed. To explore whether an exogenous substance could counteract the negative effects of exposure to these pollutants, we administered vitamin E (Vit E) to the experimental animals. The results showed that administration of Vit E in tandem with the FA and PM2.5 co-exposure, reduced the extent of damage to the mouse brain. Down-regulation of oxidative stress and inflammation were proposed to explain the protective effects of Vit E. This research provides a universal strategy to effectively protect people who are exposed to FA and PM2.5 simultaneously.

Formaldehyde, Epigenetics, and Alzheimer's Disease

Alzheimer's disease (AD) is the most common form of dementia. The accumulation of β-amyloid plaques and intracellular neurofibrillary tangles of hyperphosphorylated tau protein are two hallmarks of AD. The β-amyloid and tau proteins have been at the center of AD research and drug development for decades. However, most of the clinical trials targeting β-amyloid have failed. Whereas the safety and efficacy of most tau-targeting drugs have not yet been completely assessed, the first tau aggregation inhibitor, LMTX, failed in a late-stage trial, leading to further recognition of the complexities of AD and reconsideration of the amyloid hypothesis and perhaps the tau hypothesis as well. Multilevel complex interactions between genetic, epigenetic, and environmental factors contribute to the occurrence and progression of AD. Formaldehyde (FA) is a widespread environmental organic pollutant. It is also an endogenous metabolite in the human body. Recent studies suggest that elevation of FA in the body by endogenous and/or exogenous exposure may play important roles in AD development. We have demonstrated that FA reduces lysine acetylation of cytosolic histones, thereby compromising chromatin assembly and resulting in the loss of histone content in chromatin, a conserved feature of aging from yeast to humans. Aging is an important factor for AD progression. Therefore, FA-induced inhibition of chromatin assembly and the loss of histones may contribute to AD initiation and/or development. This review will briefly summarize current knowledge on mechanistic insights into AD, focusing on epigenetic alterations and the involvement of FA in AD development. The exploration of chemical exposures as contributing factors to AD may provide new insights into AD mechanisms and could identify potential novel therapeutic targets.