Lipid Metabolism and its Significance in Toxicant Exposure
Lipid metabolism, encompassing processes such as synthesis, degradation, and transport of lipids, is intricately linked to toxicant exposure. Upon encountering foreign substances, cells undergo dynamic changes in lipid metabolism as part of their response to the stress induced by toxicants. For instance, exposure to certain chemicals may lead to alterations in lipid synthesis pathways, resulting in the accumulation of specific lipid species or the depletion of essential lipid components. These changes not only reflect the cellular response to toxic insults but also serve as indicators of toxicant exposure levels and potential adverse effects on cellular function.
Understanding the nuances of lipid metabolism in the context of toxicant exposure holds significant implications for toxicological research and risk assessment. By elucidating the intricate pathways involved in lipid metabolism, researchers can gain insights into the mechanisms by which toxicants exert their effects on cellular lipid homeostasis. Moreover, targeting specific components of lipid metabolism may offer novel strategies for mitigating the toxic effects of environmental pollutants or pharmaceutical agents.
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Lipid Changes as Biomarkers of Toxicity
One of the hallmarks of toxicant exposure is the perturbation of lipid profiles within biological systems. Lipidomic analysis, which involves the comprehensive profiling of lipid species in biological samples, has emerged as a powerful tool for identifying lipid changes associated with toxicological insults. These alterations in lipid composition and abundance serve as valuable biomarkers of toxicity, providing early indicators of cellular dysfunction and damage.
By monitoring changes in lipid profiles, researchers can discern patterns indicative of specific toxicological responses, such as oxidative stress, inflammation, or membrane disruption. Moreover, the identification of lipid biomarkers enables the development of targeted diagnostic assays for assessing exposure to environmental contaminants or evaluating the safety and efficacy of pharmaceutical interventions. Thus, lipidomics offers a sensitive and specific approach for biomonitoring and risk assessment in toxicology.
Lipid-Mediated Toxic Pathways: Oxidative Stress and Inflammation
In addition to serving as biomarkers of toxicity, lipids play crucial roles in mediating toxicological pathways implicated in cellular damage and disease progression. Two prominent pathways through which lipids contribute to toxicological responses are oxidative stress and inflammation.
Oxidative stress, characterized by an imbalance between reactive oxygen species (ROS) production and antioxidant defense mechanisms, can lead to oxidative damage to lipids, proteins, and DNA. Lipid peroxidation, a hallmark of oxidative stress, results in the generation of lipid hydroperoxides and reactive aldehydes, which further propagate cellular damage and trigger inflammatory responses.
Furthermore, lipids act as signaling molecules in inflammatory pathways, modulating the expression of pro-inflammatory cytokines and chemokines. Dysregulation of lipid signaling pathways can exacerbate inflammation and contribute to the pathogenesis of various inflammatory diseases.
Lipid Classes
Within the vast landscape of lipids, diverse classes manifest unique structural and functional attributes, imbuing them with distinct roles in cellular physiology and toxicological responses. Foremost among these are phospholipids, integral constituents of cellular membranes pivotal for membrane integrity and cellular signaling. Profiling phospholipid species enables elucidation of membrane dynamics and signaling cascades, crucial for understanding cellular homeostasis and perturbations induced by toxicants.
Sphingolipids, another prominent lipid class, intricately regulate cellular signaling and apoptosis. From ceramides to sphingomyelins, these bioactive molecules wield profound influence over cellular fate and are implicated in various pathological states. Through lipidomic analysis, the intricate interplay between sphingolipid metabolism and toxicological responses is unveiled, presenting avenues for therapeutic intervention and biomarker discovery.
Lipidomics Techniques
Lipidomics, a field at the intersection of biology and analytical chemistry, has become increasingly relevant in toxicological research. It involves the comprehensive study of lipids in biological systems, employing various analytical techniques for detailed lipid profiling.
Central to lipidomics is the utilization of advanced analytical techniques, notably mass spectrometry and chromatography. Mass spectrometry, in particular, stands as a pillar in lipidomic analyses, enabling the precise identification and quantification of lipid species within complex biological matrices. By ionizing lipids and analyzing their mass-to-charge ratios, mass spectrometry facilitates the elucidation of lipid profiles with exceptional sensitivity and specificity. Moreover, tandem mass spectrometry (MS/MS) further enhances the structural elucidation of individual lipid species, unraveling their complex molecular compositions.
Complementing mass spectrometry, chromatography techniques such as liquid chromatography (LC) and gas chromatography (GC) play a pivotal role in lipidomic workflows. Through chromatographic separation, lipids are fractionated based on their physicochemical properties, facilitating the isolation and purification of lipid species prior to mass spectrometric analysis. This synergy between chromatography and mass spectrometry empowers researchers to explore the vast landscape of lipid diversity within biological systems with unprecedented resolution and accuracy.
Lipid profiling assumes paramount significance in toxicological inquiry, serving as a linchpin for deciphering cellular processes and toxicological mechanisms. By delineating alterations in lipid composition and distribution induced by toxicants, lipidomics furnishes invaluable insights into early cellular responses to environmental insults. Moreover, elucidation of lipid-mediated pathways elucidates the molecular underpinnings of toxicity, informing strategies for risk assessment and intervention.
Untargeted lipidomics reveals the toxicity (Marqueo et al., 2021)
Application of Lipidomics in Neurotoxicity Research
Due to the high content of polyunsaturated fatty acids and oxygen demand in the animal nervous system, it is more susceptible to oxidative damage from exposure to toxic substances than other tissues. The deepening degree of cellular oxidative damage further affects cellular metabolism and life processes such as cell death, potentially leading to neurodegenerative diseases. Studies have shown an association between environmental pollutant polybrominated diphenyl ether-47 (PBDE-47) and Parkinson's disease occurrence. Using C57BL/6J mice as a model, significant lipid changes in 105 lipids in the mouse brain were observed after oral administration of BDE-47. The changes in lipids in the midbrain were more significant than those in the frontal cortex. Integration of metabolomics and proteomics data revealed that PBDE-47 exposure-induced metabolic changes in mouse brain tissue could further lead to disruption of neurotransmitter systems, abnormal phosphorylation, mitochondrial dysfunction, and oxidative stress occurrence. Arsenic, as a heavy metal, can induce neurodegenerative diseases, but the related mechanisms remain unclear. Analysis of brain tissues from male mice continuously exposed to arsenic for 16 weeks revealed abnormalities in 118 metabolites and 17 lipid metabolites, indicating neurotoxic effects of arsenic crossing the blood-brain barrier in mice. Additionally, correlation analysis indicated close associations between these differential metabolites and 12 intestinal microbial communities, suggesting a potential link between arsenic exposure-induced metabolic disruptions in mouse brains and disturbances in intestinal microbial metabolism.
Furthermore, utilizing lipidomics techniques, Wang et al. discovered significant reductions in levels of carnitine and acylcarnitines in the serum of newborn monkeys exposed to sevoflurane, along with substantial accumulations of 16:0 and 18:1 fatty acid chains in triglycerides and extensive losses of unsaturated fatty acids in phospholipids and non-esterified fatty acids. Comprehensive lipidomics results indicated that sevoflurane could induce systemic energy deficiency in newborn monkeys, with significant reductions in short-chain acylcarnitines occurring as early indicators of sevoflurane-induced neurotoxicity 2 hours after exposure. In exploring the neurotoxic effects of novel neonicotinoid insecticides, lipidomics has also played a crucial indicative role. Neuro-2a cells exposed to low doses of imidacloprid and clothianidin showed significant disruptions in lipid metabolism, particularly in glycerophospholipid and sphingomyelin metabolism, with 14 lipids identified as potential biomarkers indicating neurotoxic effects of these two novel neonicotinoid insecticides.
Multiple studies have shown that exposure to environmental pollutants, including PM2.5, can lead to the occurrence of neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease, vascular dementia, and multiple sclerosis. One of the reasons is the abnormal accumulation of cerebral vascular lipids after exposure. Neurologists point out that lipids constitute over 50% of the brain's dry weight and play crucial roles in various brain functions such as neuronal growth and neurotransmitter release. Phospholipids such as glycerophospholipids and sphingomyelins are abundant in neural cells. The process of harmful substances entering neural cells may involve a series of reactions with membrane lipids, leading to disturbances in lipid metabolism, such as membrane contraction and alterations in membrane protein binding.
Application of Lipidomics in Hepatotoxicity Research
The liver, primarily a metabolic organ in humans and animals, is also a target organ for various toxic substances. During the process of liver function impairment, there is a decrease in lipid transport capacity, leading to lipid metabolism disorders, lipid accumulation, and even the development of liver diseases such as fatty liver.
In a study by Yang et al., newborn mice continuously exposed to diisononyl phthalate (DINP) at a dose of 4.8 mg/kg body weight per day for 21 days showed significantly increased levels of glycerides, triglycerides, and cholesterol esters in the liver, closely associated with hyperlipidemia. These changes may induce hepatic steatosis in mice, providing scientific evidence for understanding the impact of DINP on chronic liver diseases. Dimethylformamide (DMF) has been classified as a Group 2A carcinogen by the International Agency for Research on Cancer and can cause liver damage, but the mechanism of its hepatotoxicity remains unclear. Combined studies using proteomics, lipidomics, and metabolomics techniques revealed significant differences in various lipid contents in human liver cells after exposure to 10-160 mmol/L DMF for 24 hours compared to the control group. Pathway analysis further identified disruption of polysaccharide synthesis, abnormal bile acid metabolism, mitochondrial dysfunction, and glutathione deficiency as the major pathways affected by DMF toxicity. Similarly, using human liver cells as a model, lipidomics techniques were employed to explore the toxic effects of polyethylene microplastics and polychlorinated biphenyls (PCB101 and PCB126) alone and in combination. Both PCBs primarily caused significant changes in glycerophospholipids and glycosphingolipids, essential components of cell membranes crucial for stabilizing membrane composition and increasing membrane fluidity. On the other hand, exposure to polyethylene microplastics mainly interfered with triglyceride metabolism, leading to elevated triglyceride levels. Combined exposure showed enhanced toxicity, with significant changes observed in various sphingomyelins in addition to similar differential lipids such as triglycerides and phosphatidylcholines, revealing the synergistic toxic effects of polyethylene microplastics and polychlorinated biphenyls contamination.
Perfluorooctanesulfonic acid (PFOS), a novel organic pollutant, has gained widespread attention for its toxic effects on organisms. Research on male BALB/c mice as a model revealed that the liver is the most significantly affected target organ by PFOS accumulation, followed by the lungs, kidneys, spleen, heart, and brain. Lipid analysis of mouse liver after PFOS exposure identified glycerophospholipids and sphingomyelins as the main disturbed lipid classes. Additionally, upregulation of neuroamide and hemolytic phosphatidylcholine suggested PFOS-induced apoptosis of liver cells, while decreased total triglyceride content in the liver might lead to energy deficiency in mice and subsequent morphological liver damage. Bisphenol A (BPA) and bisphenol F (BPF) are two commonly present environmental pollutants. Marqueño et al. utilized non-targeted lipidomics to investigate the effects of these substances on zebrafish liver cell metabolism. Both BPA and BPF significantly increased the content of dihydroceramides and ether-linked triacylglycerols in cells, indicating inhibitory effects on cell growth. Moreover, BPA increased the content of saturated and low unsaturated fatty acid triglycerides, while BPF reduced the content of triglycerides containing polyunsaturated fatty acids. Overall, both BPA and BPF increased the content of saturated and monounsaturated fatty acids in zebrafish liver cells, with significant upregulation of two genes closely related to fatty acid synthesis confirmed by qPCR. This study fully elucidated the impact of BPA and BPF on lipid homeostasis and provided a basis for further investigation into the toxic mechanisms of these substances.
The process of lipid metabolism in the liver mainly includes fatty acid uptake or de novo synthesis, fatty acid β-oxidation, and triglyceride synthesis and secretion. Excessive entry of exogenous substances into hepatocytes can further affect lipid metabolism by influencing the activity of enzymes such as acetyl-CoA and fatty acid synthase, leading to phenomena such as increased fatty acid levels, lipid accumulation, ultimately resulting in liver necrosis.
Application of Lipidomics in Immunotoxicity Research
Upon entry into the body, harmful substances may exert toxic effects by damaging immune cells, thereby reducing the body's immune capacity. With the continuous development of lipidomics technology, its application in studies related to immunotoxicity has begun to emerge.
As an important structural analog of BPA, bisphenol S (BPS) has been used as a substitute for BPA in industrial production. Using macrophages as a model, lipidomics technology has been employed to explore the immunotoxicity of BPS exposure. It was found that BPS exposure not only activated macrophages to produce various immune-related inflammatory factors but also significantly interfered with the metabolism of intracellular sphingomyelins, neuroamides, glycerophospholipids, and glycerides. This result also indicates that lipids play an important role in the cellular response to the toxic effects of harmful substances, thereby generating immune regulation processes. Many toxic substances can exert toxic effects on multiple target organs. For example, studies have shown that PFOS has hepatorenal toxicity and reproductive toxicity. Recent research has found that PFOS can also exert toxic effects on the immune system of organisms. In this study, human lymphocytes were used as a model, and transcriptomics and lipidomics technologies were combined to reveal the immunotoxic effects of PFOS. The results showed that the three most abundant lipid classes in human lymphocytes were fatty acyls, glycerolipids, and glycerophospholipids. Compared with the control group, exposure to PFOS resulted in the identification of 37 differential lipids, including diglycerides, thromboxanes, glucosylceramides, and lactosylceramides, which play important roles in the immune response process. Inflammation is an important stage in which organisms exert immune function. To explore the acute inflammatory stimulation caused by indole alkaloids, targeted lipidomics analysis was conducted on adult male rats injected with toad venom capsule aqueous solution. The results showed that 121 inflammation-related lipids were detected in the rat hind paw samples. These inflammatory lipids can be produced through the cyclooxygenase, lipoxygenase, and cytochrome P450 pathways and are metabolites of linoleic acid, α-linolenic acid, docosahexaenoic acid, and eicosapentaenoic acid.
The immune response is a defensive response of the body to exogenous substances or mutations in cells, aging and dying cells, or other harmful components. The activation of abnormal immune responses is closely related to the occurrence of various diseases, such as dilated cardiomyopathy, viral myocarditis, and other cardiovascular diseases. Using lipidomics technology to explore the immunotoxicity caused by exposure to exogenous substances, important lipid biomarkers can be screened, which plays a crucial indicative role in the early diagnosis of related diseases.
Application of Lipidomics in Endocrine Disrupting Toxicity Studies
Endocrine disruptors are chemicals from external sources that interfere with the body's hormone synthesis, secretion, transportation, and metabolism, disrupting normal physiological functions. Research suggests that exposure to these disruptors can lead to abnormal behaviors in animals and even trigger cancer, particularly prostate and breast cancer, which are closely linked to disruptions in the endocrine system. Recently, there has been growing interest in understanding how lipid metabolism disturbances contribute to the development of these cancers.
For instance, in studies using DU145 prostate cancer cells, researchers found that prolonged exposure to substances like polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT) not only caused significant cell migration but also disrupted lipid metabolism to varying degrees. They identified specific lipids that were altered in response to exposure, indicating potential targets for treating prostate cancer. Similarly, investigations into the relationship between the endocrine disruptor PBDE-47 and breast cancer revealed significant metabolic disturbances in mice exposed to PBDE-47, suggesting its potential role in promoting breast tumor growth.
Endocrine disruptors, which include substances like estradiol and phthalates found in industrial waste, exert their toxic effects through long-term, low-dose exposure. They primarily disrupt lipid metabolism by affecting transcription factors, enzyme activities, protein levels, expression of endocannabinoids and cannabinoid receptors, and epigenetic modifications. Further research using lipidomics technology is needed to fully understand how these disruptors interact with lipid metabolism pathways.
Reference
- Marqueo, Anna , et al. "Untargeted lipidomics reveals the toxicity of bisphenol A bis(3-chloro-2- hydroxypropyl) ether and bisphenols A and F in zebrafish liver cells." Ecotoxicology and Environmental Safety 219(2021):112311-.
