Sub-Chronic Microcystin-LR Liver Toxicity in Preexisting Diet-Induced Nonalcoholic Steatohepatitis in Rats

The effect and mechanism of combined exposure of MC-LR and NaNO2 on liver lipid metabolism

Microcystin-LR (MC-LR) and sodium nitrite (NaNO2) co-exist in the environment and are hepatotoxic. The liver has the function of lipid metabolism, but the impacts and mechanisms of MC-LR and NaNO2 on liver lipid metabolism are unclear. Therefore, we established a chronic exposure model of Balb/c mice and used LO2 cells for in vitro verification to investigate the effects and mechanisms of liver lipid metabolism caused by MC-LR and NaNO2. The results showed that after 6 months of exposure to MC-LR and NaNO2, the lipid droplets content was increased, and the activities of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) were raised in the liver (P < 0.05). Moreover, MC-LR and NaNO2 synergistically induced hepatic oxidative stress by decreasing total superoxide dismutase (T-SOD) activity and glutathione (GSH) levels and increasing malondialdehyde (MDA) content levels. In addition, the levels of Nrf2, HO-1, NQO1 and P-AMPK was decreased and Keap1 was increased in the Nrf2/HO-1 pathway. The key factors of lipid metabolism, SREBP-1c, FASN and ACC, were up-regulated in the liver. More importantly, there was a combined effect on lipid deposition of MC-LR and NaNO2 co-exposure. In vitro experiments, MC-LR and NaNO2-induced lipid deposition and changes in lipid metabolism-related changes were mitigated after activation of the Nrf2/HO-1 signaling pathway by the Nrf2 activator tertiary butylhydroquinone (TBHQ). Additionally, TBHQ alleviated the rise of reactive oxygen species (ROS) in LO2 cells induced by MC-LR and NaNO2. Overall, our findings indicated that MC-LR and NaNO2 can cause abnormal liver lipid metabolism, and the combined effects were observed after MC-LR and NaNO2 co-exposure. The Nrf2/HO-1 signal pathway may be a potential target for prevention and control of liver toxicity caused by MC-LR and NaNO2.

DEHP and DBP, common phthalates, induce glucose metabolism disorders in rats via oxidative damage of PI3K/Akt/GLUT4 signaling

Phthalic acid esters (PAEs) are environmental endocrine disruptors thought to interfere with glucose metabolism in humans. Most of the related research has focused on population epidemiological studies, with the underlying mechanisms remaining unresolved. Using an in vivo animal model, we examined the effects of oral administration of two commonly used PAEs [di(2-ethylhexyl) phthalate (DEHP) and dibutyl phthalate (DBP)] on glucose homeostasis and insulin secretion. DEHP (750 mg/kg, 1/40 LD50), DBP (500 mg/kg, 1/40 LD50), and DEHP (750 mg/kg) + DBP (500 mg/kg) exert an influence on glucose metabolism and elicit a reduction in insulin sensitivity in rats. Furthermore, these substances induce detrimental effects on the structure and functionality of pancreatic β-cells. DEHP and/or DBP triggered an increase in plasma malondialdehyde (MDA) and reduction in superoxide dismutase (SOD) activity; a reduction in the phosphorylation of phosphatidyl inositol 3 kinase (PI3K) and phospho-protein kinase B (p-Akt473) proteins; an increase in the relative expression of Bax, Caspase-8, cleaved-Caspase-9, and cleaved-Caspase-3; and a reduction in the relative expression of Bcl-2-related Bax in pancreatic tissue and of gastrocnemius glucose transporter 4 (GLUT4) in the gastrocnemius muscle. Based on these findings, these PAEs can disrupt glucose metabolism, possibly via oxidative damage of the PI3K/Akt/GLUT4 pathway; this damage induces pancreatic β-cell apoptosis, affects pancreatic β-cell function, and affects glucose metabolism and insulin resistance in rats. To the best of our knowledge, this study was the first to show that the combined effect of the two PAEs affects glucose metabolism and insulin resistance in rats that is significantly higher than the effects of each PAE. Thus, safety standards and studies do not consider this effect as a significant oversight when blending PAEs. We assert that this must be addressed and corrected for establishing more impactful and safer standards.

Advances in investigating microcystin-induced liver toxicity and underlying mechanisms

Microcystins (MCs) are a class of biologically active cyclic heptapeptide pollutants produced by the freshwater alga Microcystis aeruginosa. With increased environmental pollution, MCs have become a popular research topic. In recent years, the hepatotoxicity of MCs and associated effects and mechanisms have been studied extensively. Current epidemiological data indicate that long-term human exposure to MCs can lead to severe liver toxicity, acute toxicity, and death. In addition, current toxicological studies on the liver, a vital target organ of MCs, indicate that MC contamination is associated with the development of liver cancer, nonalcoholic fatty liver, and liver fibrosis. MCs produce hepatotoxicity that affects the metabolic homeostasis of the liver, induces apoptosis, and acts as a pro-cancer factor, leading to liver lesions. MCs mainly mediate the activation of signaling pathways, such as the ERK/JNK/p38 MAPK and IL-6-STAT3 pathways, which leads to oxidative damage and even carcinogenesis. Moreover, MCs can act synergistically with other pollutants to produce combined toxicity. However, few systematic reviews have been performed on these new findings. This review systematically summarizes the toxic effects and mechanisms of MCs on the liver and discusses the combined liver toxicity effects of MCs and other pollutants to provide reference for subsequent research. The toxicity of different MC isomers deserves further study. The detection methods and limit standards of MCs in agricultural and aquatic products will represent important research directions in the future. Standard protocols for fish sampling during harmful algal blooms or to evaluate the degree of MC toxicity in nature are lacking. In future, bioinformatics can be applied to offer insights into MC toxicology research and potential drug development for MC poisoning. Further research is essential to understand the molecular mechanisms of liver function damage in combined-exposure toxicology studies to establish treatment for MC-induced liver damage.

Sex, Nutrition, and NAFLD: Relevance of Environmental Pollution

Non-alcoholic fatty liver disease (NAFLD) is the most common form of chronic liver disease and represents an increasing public health issue given the limited treatment options and its association with several other metabolic and inflammatory disorders. The epidemic, still growing prevalence of NAFLD worldwide cannot be merely explained by changes in diet and lifestyle that occurred in the last few decades, nor from their association with genetic and epigenetic risk factors. It is conceivable that environmental pollutants, which act as endocrine and metabolic disruptors, may contribute to the spreading of this pathology due to their ability to enter the food chain and be ingested through contaminated food and water. Given the strict interplay between nutrients and the regulation of hepatic metabolism and reproductive functions in females, pollutant-induced metabolic dysfunctions may be of particular relevance for the female liver, dampening sex differences in NAFLD prevalence. Dietary intake of environmental pollutants can be particularly detrimental during gestation, when endocrine-disrupting chemicals may interfere with the programming of liver metabolism, accounting for the developmental origin of NAFLD in offspring. This review summarizes cause-effect evidence between environmental pollutants and increased incidence of NAFLD and emphasizes the need for further studies in this field.

Subchronic Microcystin-LR Aggravates Colorectal Inflammatory Response and Barrier Disruption via Raf/ERK Signaling Pathway in Obese Mice

Microcystin-LR (MC-LR) is an extremely poisonous cyanotoxin that poses a threat to ecosystems and human health. MC-LR has been reported as an enterotoxin. The objective of this study was to determine the effect and the mechanism of subchronic MC-LR toxicity on preexisting diet-induced colorectal damage. C57BL/6J mice were given either a regular diet or a high-fat diet (HFD) for 8 weeks. After 8 weeks of feeding, animals were supplied with vehicle or 120 μg/L MC-LR via drinking water for another 8 weeks, and their colorectal were stained with H&E to detect microstructural alterations. Compared with the CT group, the HFD and MC-LR + HFD-treatment group induced a significant weight gain in the mice. Histopathological findings showed that the HFD- and MC-LR + HFD-treatment groups caused epithelial barrier disruption and infiltration of inflammatory cells. The HFD- and MC-LR + HFD-treatment groups raised the levels of inflammation mediator factors and decreased the expression of tight junction-related factors compared to the CT group. The expression levels of p-Raf/Raf and p-ERK/ERK in the HFD- and MC-LR + HFD-treatment groups were significantly increased compared with the CT group. Additionally, treated with MC-LR + HFD, the colorectal injury was further aggravated compared with the HFD-treatment group. These findings suggest that by stimulating the Raf/ERK signaling pathway, MC-LR may cause colorectal inflammation and barrier disruption. This study suggests that MC-LR treatment may exacerbate the colorectal toxicity caused by an HFD. These findings offer unique insights into the consequences and harmful mechanisms of MC-LR and provide strategies for preventing and treating intestinal disorders.

MC-LR Aggravates Liver Lipid Metabolism Disorders in Obese Mice Fed a High-Fat Diet via PI3K/AKT/mTOR/SREBP1 Signaling Pathway

Obesity, a metabolic disease caused by excessive fat accumulation in the body, has attracted worldwide attention. Microcystin-LR (MC-LR) is a hepatotoxic cyanotoxin which has been reportedly to cause lipid metabolism disorder. In this study, C57BL/6J mice were fed a high-fat diet (HFD) for eight weeks to build obese an animal model, and subsequently, the obese mice were fed MC-LR for another eight weeks, and we aimed to determine how MC-LR exposure affects the liver lipid metabolism in high-fat-diet-induced obese mice. The results show that MC-LR increased the obese mice serum aspartate aminotransferase (AST) and alanine aminotransferase (ALT), indicating damaged liver function. The lipid parameters include serum triglyceride (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-c), and liver TG, which were all increased, whilst the high-density lipoprotein cholesterol (HDL-c) was decreased. Furthermore, after MC-LR treatment, histopathological observation revealed that the number of red lipid droplets increased, and that steatosis was more severe in the obese mice. In addition, the lipid synthesis-related genes were increased and the fatty acid β-oxidation-related genes were decreased in the obese mice after MC-LR exposure. Meanwhile, the protein expression levels of phosphorylation phosphatidylinositol 3-kinase (p-PI3K), phosphorylation protein kinase B (p-AKT), phosphorylation mammalian target of rapamycin (p-mTOR), and sterol regulatory element binding protein 1c (SREBP1-c) were increased; similarly, the p-PI3K/PI3K, p-AKT/AKT, p-mTOR/mTOR, and SREBP1/β-actin were significantly up-regulated in obese mice after being exposed to MC-LR, and the activated PI3K/AKT/mTOR/SREBP1 signaling pathway. In addition, MC-LR exposure reduced the activity of superoxide dismutase (SOD) and increased the level of malondialdehyde (MDA) in the obese mice's serum. In summary, the MC-LR could aggravate the HFD-induced obese mice liver lipid metabolism disorder by activating the PI3K/AKT/mTOR/SREBP1 signaling pathway to hepatocytes, increasing the SREBP1-c-regulated key enzymes for lipid synthesis, and blocking fatty acid β-oxidation.

Utility of Human Relevant Preclinical Animal Models in Navigating NAFLD to MAFLD Paradigm

Fatty liver disease is an emerging contributor to disease burden worldwide. The past decades of work established the heterogeneous nature of non-alcoholic fatty liver disease (NAFLD) etiology and systemic contributions to the pathogenesis of the disease. This called for the proposal of a redefinition in 2020 to that of metabolic dysfunction-associated fatty liver disease (MAFLD) to better reflect the current understanding of the disease. To date, several clinical cohort studies comparing NAFLD and MAFLD hint at the relevancy of the new nomenclature in enriching for patients with more severe hepatic injury and extrahepatic comorbidities. However, the underlying systemic pathogenesis is still not fully understood. Preclinical animal models have been imperative in elucidating key biological mechanisms in various contexts, including intrahepatic disease progression, interorgan crosstalk and systemic dysregulation. Furthermore, they are integral in developing novel therapeutics against MAFLD. However, substantial contextual variabilities exist across different models due to the lack of standardization in several aspects. As such, it is crucial to understand the strengths and weaknesses of existing models to better align them to the human condition. In this review, we consolidate the implications arising from the change in nomenclature and summarize MAFLD pathogenesis. Subsequently, we provide an updated evaluation of existing MAFLD preclinical models in alignment with the new definitions and perspectives to improve their translational relevance.

Microcystin-LR aerosol induces inflammatory responses in healthy human primary airway epithelium

Harmful algal blooms plague bodies of freshwater globally. These blooms are often composed of outgrowths of cyanobacteria capable of producing the heptapeptide Microcystin-LR (MC-LR) which is a well-known hepatotoxin. Recently, MC-LR has been detected in aerosols generated from lake water. However, the risk for human health effects due to MC-LR inhalation exposure have not been extensively investigated. In this study, we exposed a fully differentiated 3D human airway epithelium derived from 14 healthy donors to MC-LR-containing aerosol once a day for 3 days. Concentrations of MC-LR ranged from 100 pM to 1 µM. Although there were little to no detrimental alterations in measures of the airway epithelial function (i.e. cell survival, tissue integrity, mucociliary clearance, or cilia beating frequency), a distinct shift in the transcriptional activity was found. Genes related to inflammation were found to be upregulated such as C-C motif chemokine 5 (CCL5; log2FC = 0.57, p = 0.03) and C-C chemokine receptor type 7 (CCR7; log2FC = 0.84, p = 0.03). Functionally, conditioned media from MC-LR exposed airway epithelium was also found to have significant chemo-attractive properties for primary human neutrophils. Additionally, increases were found in the concentration of secreted chemokine proteins in the conditioned media such as CCL1 (log2FC = 5.07, p = 0.0001) and CCL5 (log2FC = 1.02, p = 0.046). These results suggest that MC-LR exposure to the human airway epithelium is capable of inducing an inflammatory response that may potentiate acute or chronic disease.

Antioxidant Therapy Significantly Attenuates Hepatotoxicity following Low Dose Exposure to Microcystin-LR in a Murine Model of Diet-Induced Non-Alcoholic Fatty Liver Disease

We have previously shown in a murine model of Non-alcoholic Fatty Liver Disease (NAFLD) that chronic, low-dose exposure to the Harmful Algal Bloom cyanotoxin microcystin-LR (MC-LR), resulted in significant hepatotoxicity including micro-vesicular lipid accumulation, impaired toxin metabolism as well as dysregulation of the key signaling pathways involved in inflammation, immune response and oxidative stress. On this background we hypothesized that augmentation of hepatic drug metabolism pathways with targeted antioxidant therapies would improve MC-LR metabolism and reduce hepatic injury in NAFLD mice exposed to MC-LR. We chose N-acetylcysteine (NAC, 40 mM), a known antioxidant that augments the glutathione detoxification pathway and a novel peptide (pNaKtide, 25 mg/kg) which is targeted to interrupting a specific Src-kinase mediated pro-oxidant amplification mechanism. Histological analysis showed significant increase in hepatic inflammation in NAFLD mice exposed to MC-LR which was attenuated on treatment with both NAC and pNaKtide (both p ≤ 0.05). Oxidative stress, as measured by 8-OHDG levels in urine and protein carbonylation in liver sections, was also significantly downregulated upon treatment with both antioxidants after MC-LR exposure. Genetic analysis of key drug transporters including Abcb1a, Phase I enzyme-Cyp3a11 and Phase II metabolic enzymes-Pkm (Pyruvate kinase, muscle), Pklr (Pyruvate kinase, liver, and red blood cell) and Gad1 (Glutamic acid decarboxylase) was significantly altered by MC-LR exposure as compared to the non-exposed control group (all p ≤ 0.05). These changes were significantly attenuated with both pNaKtide and NAC treatment. These results suggest that MC-LR metabolism and detoxification is significantly impaired in the setting of NAFLD, and that these pathways can potentially be reversed with targeted antioxidant treatment.

Safety assessment of graphene oxide and microcystin-LR complex: a toxicological scenario beyond physical mixture

Background

Nanomaterials have been widely used in electrochemistry, sensors, medicine among others applications, causing its inevitable environmental exposure. A raising question is the “carrier” effect due to unique surface properties of nanomaterials, which may collectively impact the bioavailability, toxicokinetic, distribution and biological effects of classic toxicants. Noteworthy, this aspect of information remains largely unexplored.

Methods

Here, we deliberately selected two entities to mimic this scenario. One is graphene oxide (GO), which is made in ton quantity with huge surface-area that provides hydrophilicity and π–π interaction to certain chemicals of unique structures. The other is Microcystin-LR (MCLR), a representative double-bond rich liver-toxic endotoxin widely distributed in aquatic-system. Firstly, the adsorption of GO and MCLR after meeting under environmental conditions was explored, and then we focused on the toxicological effect and related mechanism of GO-MCLR complex on human skin cutin forming cells (HaCaT cells) and normal liver cells (L02 cells).

Results

Abiotically, our study demonstrated that GO could effectively adsorb MCLR through hydrogen bonding and π–π interaction, the oxidation degree of GO-MCLR decreased significantly and surface defect level raised. Compared to GO or MCLR, GO-MCLR was found to induce more remarkable apoptosis and ferroptosis in both HaCaT and L02 cells. The underlying mechanism was that GO-MCLR induced stronger intracellular reactive oxygen species (ROS) and mtROS generation, followed by Fe²⁺ accumulation, mitochondrial dysfunction and cytoskeletal damage.

Conclusions

These results suggest that the GO-MCLR complex formed by GO adsorption of MCLR may exhibit more toxic effects than the single material, which demonstrates the necessity for assessing nano-toxicant complexity. Our discovery may serve as a new toxicological paradigm in which nanomaterial mediated surface adsorption effects could impact the degree of cytotoxicity and toxicological mechanisms of classic toxins.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12989-022-00466-x.

As We Drink and Breathe: Adverse Health Effects of Microcystins and Other Harmful Algal Bloom Toxins in the Liver, Gut, Lungs and Beyond

Freshwater harmful algal blooms (HABs) are increasing in number and severity worldwide. These HABs are chiefly composed of one or more species of cyanobacteria, also known as blue-green algae, such as Microcystis and Anabaena. Numerous HAB cyanobacterial species produce toxins (e.g., microcystin and anatoxin—collectively referred to as HAB toxins) that disrupt ecosystems, impact water and air quality, and deter recreation because they are harmful to both human and animal health. Exposure to these toxins can occur through ingestion, inhalation, or skin contact. Acute health effects of HAB toxins have been well documented and include symptoms such as nausea, vomiting, abdominal pain and diarrhea, headache, fever, and skin rashes. While these adverse effects typically increase with amount, duration, and frequency of exposure, susceptibility to HAB toxins may also be increased by the presence of comorbidities. The emerging science on potential long-term or chronic effects of HAB toxins with a particular emphasis on microcystins, especially in vulnerable populations such as those with pre-existing liver or gastrointestinal disease, is summarized herein. This review suggests additional research is needed to define at-risk populations who may be helped by preventative measures. Furthermore, studies are required to develop a mechanistic understanding of chronic, low-dose exposure to HAB toxins so that appropriate preventative, diagnostic, and therapeutic strategies can be created in a targeted fashion.

Spirulina platensis protects against microcystin-LR-induced toxicity in rats

Microcystis aeruginosa produces an abundant cyanotoxin (microcystins (MCs) in freshwater supplies. MCs have adverse health hazards to animals and humans. Microcystin-leucine-arginine (microcystin-LR or MC-LR) is the most studied among these MCs due to their high toxicity. So, this study was designed to evaluate the possible therapeutic role of the natural algal food supplement, Spirulina platensis (SP), against MC-LR-induced toxic effects in male Wistar rats. Forty rats were randomly divided into five groups. Control and SP groups orally administered distilled water and SP (1000 mg/kg/daily), respectively, for 21 days. MC-LR group was intraperitoneally injected with MC-LR (10 μg/kg/day) for 14 days. MC-LR-SP₅₀₀ and MC-LR-SP₁₀₀₀ groups were orally treated with SP (500 and 1000 mg/kg, respectively) for 7 days and concomitantly with MC-LR for 14 days. MC-LR induced oxidative hepatorenal damage, cardiotoxicity, and neurotoxicity greatly, which was represented by reduction of reduced glutathione content and the activities of glutathione peroxidase, catalase, and superoxide dismutase and elevation of concentrations of nitric oxide and malondialdehyde in renal, hepatic, brain, and heart tissues. In addition, it increased serum levels of urea, creatinine, tumor necrosis factor-alfa, interleukin-1beta and interleukin-6 and serum activities of alkaline phosphatase, aspartate aminotransferase, alanine aminotransferase, lactate dehydrogenase, creatine kinase, and creatine kinase-MB. However, S. platensis restored normal levels of measured serum parameters, ameliorated MC-LR-induced oxidative damage, and normalized tissue antioxidant biomarkers. In conclusion, SP alleviated MC-induced organ toxicities by mitigating oxidative and nitrosative stress and lipid peroxidation.

MCLR-elicited hepatic fibrosis and carcinogenic gene expression changes persist in rats with diet-induced nonalcoholic steatohepatitis through a 4-week recovery period

Nonalcoholic steatohepatitis (NASH) causes liver extracellular matrix (ECM) remodeling and is a risk factor for fibrosis and hepatocellular carcinoma (HCC). Microcystin-LR (MCLR) is a hepatotoxin produced by fresh-water cyanobacteria that causes a NASH-like phenotype, liver fibrosis, and is also a risk factor for HCC. The focus of the current study was to investigate and compare hepatic recovery after cessation of MCLR exposure in healthy versus NASH animals. Male Sprague-Dawley rats were fed either a control or a high fat/high cholesterol (HFHC) diet for eight weeks. Animals received either vehicle or 30 μg/kg MCLR (i.p: 2 weeks, alternate days). Animals were euthanized at one of three time points: at the completion of the MCLR exposure period and after 2 and 4 weeks of recovery. Histological staining suggested that after four weeks of recovery the MCLR-exposed HFHC group had less steatosis and more fibrosis compared to the vehicle-exposed HFHC group and MCLR-exposed control group. RNA-Seq analysis revealed dysregulation of ECM genes after MCLR exposure in both control and HFHC groups that persisted only in the HFHC groups during recovery. After 4 weeks of recovery, MCLR hepatotoxicity in pre-existing NASH persistently dysregulated genes related to cellular differentiation and HCC. These data demonstrate impaired hepatic recovery and persistent carcinogenic changes after MCLR toxicity in pre-existing NASH.

Effects of environmental contaminants in water resources on nonalcoholic fatty liver disease

The global prevalence of nonalcoholic fatty liver disease (NAFLD) has been increasing rapidly in recent years, which is now estimated to be over 25%. NAFLD is one of the most common chronic liver diseases in the world. At present, with the rapid development of economy and industrialization, many chemicals are released into the environment. These chemical contaminants in the environment might cause harm to human health and result in lipid metabolism disorder during long-term exposure. Moreover, the incentive of many NAFLD cases is unknown, and the environmental risk factors of NAFLD need to be urgently identified. Hence, we focus on the impacts of several popular environmental contaminants in water environment on the development and progression of NAFLD. These contaminants mainly include microcystins (MCs), disinfection by-products (DBPs), heavy metals (HMs), dioxins and polychlorinated biphenyls (PCBs). Through analyzing a great many epidemiological and toxicological studies, we have found positive associations between NAFLD and chronic exposure to these contaminants at the environmental levels. This review may enhance the understanding of liver damage caused by environmental pollutants, which are considered as tangible environmental risk factors for NAFLD.

Microcystin Toxicokinetics, Molecular Toxicology, and Pathophysiology in Preclinical Rodent Models and Humans

Microcystins are ubiquitous toxins produced by photoautotrophic cyanobacteria. Human exposures to microcystins occur through the consumption of contaminated drinking water, fish and shellfish, vegetables, and algal dietary supplements and through recreational activities. Microcystin-leucine-arginine (MCLR) is the prototypical microcystin because it is reported to be the most common and toxic variant and is the only microcystin with an established tolerable daily intake of 0.04 µg/kg. Microcystin toxicokinetics is characterized by low intestinal absorption, rapid and specific distribution to the liver, moderate metabolism to glutathione and cysteinyl conjugates, and low urinary and fecal excretion. Molecular toxicology involves covalent binding to and inhibition of protein phosphatases, oxidative stress, cell death (autophagy, apoptosis, necrosis), and cytoskeleton disruption. These molecular and cellular effects are interconnected and are commonly observed together. The main target organs for microcystin toxicity are the intestine, liver, and kidney. Preclinical data indicate microcystins may also have nervous, pulmonary, cardiac, and reproductive system toxicities. Recent evidence suggests that exposure to other hepatotoxic insults could potentiate microcystin toxicity and increase the risk for chronic diseases. This review summarizes the current knowledge for microcystin toxicokinetics, molecular toxicology, and pathophysiology in preclinical rodent models and humans. More research is needed to better understand human toxicokinetics and how multifactorial exposures contribute to disease pathogenesis and progression.

Modeling Diet-Induced NAFLD and NASH in Rats: A Comprehensive Review

Nonalcoholic fatty liver disease (NAFLD) is the most common chronic liver disease, characterized by hepatic steatosis without any alcohol abuse. As the prevalence of NAFLD is rapidly increasing worldwide, important research activity is being dedicated to deciphering the underlying molecular mechanisms in order to define new therapeutic targets. To investigate these pathways and validate preclinical study, reliable, simple and reproducible tools are needed. For that purpose, animal models, more precisely, diet-induced NAFLD and nonalcoholic steatohepatitis (NASH) models, were developed to mimic the human disease. In this review, we focus on rat models, especially in the current investigation of the establishment of the dietary model of NAFLD and NASH in this species, compiling the different dietary compositions and their impact on histological outcomes and metabolic injuries, as well as external factors influencing the course of liver pathogenesis.

Sub-chronic microcystin-LR renal toxicity in rats fed a high fat/high cholesterol diet

Microcystin-LR (MCLR) is a liver and kidney toxin produced by cyanobacteria. Recently, it was demonstrated that MCLR exposure drives the progression of high fat/high cholesterol (HFHC) induced nonalcoholic fatty liver disease (NAFLD) to a more severe state. NAFLD is also a risk factor for chronic kidney disease (CKD), and the current study investigated MCLR renal toxicity in the context of an HFHC diet. Sprague Dawley rats were fed either a control diet or an HFHC diet for 10 weeks. After 6 weeks of diet, animals were administered either vehicle, 10 μg/kg, or 30 μg/kg MCLR via intraperitoneal injection every other day for 4 weeks. HFHC diet alone increased the renal glomerular change histopathology score, and 30 μg/kg MCLR exposure increased this score in both the control group and the HFHC group. In contrast, 30 μg/kg MCLR caused greater proteinuria and cast formation and decreased protein phosphatase 1 and 2A protein expression in the HFHC group. Urinary excretion of KIM-1 increased, but albumin and tamm-horsfall protein did not change after MCLR exposure. The general concordance between KIM-1, polyuria, proteinuria, and renal casts after MCLR exposure suggests that proximal tubule cell damage contributed to these connected pathologies. The control group adapted to repeated MCLR exposure by increasing the urinary elimination of MCLR and its metabolites, whereas this adaptation was blunted in the HFHC group. These data suggest an HFHC diet may increase the severity of certain MCLR-elicited renal toxicities.

Metabolomics safety assessments of microcystin exposure via drinking water in rats

Drinking water exposure to microcystin-leucine-arginine (MC-LR), the most widely occurring cyanotoxins, poses a highly potential risk for human health. However, the health risk of MC-LR exposure at current guideline value in drinking water has not yet entirely evaluated. In the current study, we used ¹H NMR-based metabolomics combined with targeted metabolic profiling by GC/LC-MS to explore the toxic effects of MC-LR exposure at environmentally relevant concentrations via drinking water in rats. The results revealed that multiple biological consequences of MC-LR exposure on host metabolism in rats. Both relatively low and high doses of MC-LR used here induced hepatic lipogenesis and inflammation. While only relatively high dose MC-LR (10 μg/L) in drinking water caused more metabolic disorders including inhibition of gluconeogenesis and promotion of β-oxidation of fatty acid. Although the dose of 1.0 μg/L MC-LR is extremely low for rats, alterations of metabolic profiles were unexpectedly found in rat liver and serum, alarming potential health risk of MC-LR at the WHO guideline level.

Cylindrospermopsin-Microcystin-LR Combinations May Induce Genotoxic and Histopathological Damage in Rats

Cylindrospermopsin (CYN) and microcystins (MC) are cyanotoxins that can occur simultaneously in contaminated water and food. CYN/MC-LR mixtures previously investigated in vitro showed an induction of micronucleus (MN) formation only in the presence of the metabolic fraction S9. When this is the case, the European Food Safety Authority recommends a follow up to in vivo testing. Thus, rats were orally exposed to 7.5 + 75, 23.7 + 237, and 75 + 750 μg CYN/MC-LR/kg body weight (b.w.). The MN test in bone marrow was performed, and the standard and modified comet assays were carried out to measure DNA strand breaks or oxidative DNA damage in stomach, liver, and blood cells. The results revealed an increase in MN formation in bone marrow, at all the assayed doses. However, no DNA strand breaks nor oxidative DNA damage were induced, as shown in the comet assays. The histopathological study indicated alterations only in the highest dose group. Liver was the target organ showing fatty degeneration and necrotic hepatocytes in centrilobular areas, as well as a light mononuclear inflammatory periportal infiltrate. Additionally, the stomach had flaking epithelium and mild necrosis of epithelial cells. Therefore, the combined exposure to cyanotoxins may induce genotoxic and histopathological damage in vivo.

Long-term environmental exposure to microcystins increases the risk of nonalcoholic fatty liver disease in humans: A combined fisher-based investigation and murine model study

Microcystins (MCs) produced by cyanobacteria pose serious threats to human health. However, the contribution of long-term exposure to MCs to the development of nonalcoholic fatty liver disease (NAFLD) remains poorly documented. In this study, we estimated the environmental uptake of MCs by a small population of fishers who have lived for many years on Meiliang Bay of Lake Taihu, where cyanobacterial blooms occur frequently. Serum biochemical indices of liver function and their relationships with MC contamination in these people were also investigated. Moreover, to mimic the long-term effects of MC on the livers of fishers, an animal model was established in which mice were exposed to MC-LR at an environmentally relevant level, a reference level (the no-observed adverse effect level, NOAEL), and three times the NOAEL through drinking water for 12 months. We estimated the total daily intake of MCs by fishers through contaminated lake water and food to be 5.95 μg MC-LReq, far exceeding the tolerable daily intake (2.40 μg MC-LReq) proposed by the World Health Organization (WHO). More than 80% of participants had at least one abnormal serum marker. The indices of aspartate aminotransferase (AST)/alanine aminotransferase (ALT), triglyceride (TG), globulin (GLB), and lactate dehydrogenase (LDH) had close positive associations with MC contamination, indicating that both liver damage and lipid metabolism dysfunction were induced by chronic MC exposure. Furthermore, the animal experimental results showed that long-term exposure to MC-LR at the environmentally relevant level led to hepatic steatosis with molecular alterations in circadian rhythm regulation, lipid metabolic processes, and the cell cycle pathway. Exposure to MC-LR at or above the NOAEL worsened the pathological phenotype towards nonalcoholic steatohepatitis disease (NASH) or fibrosis. These results suggest that prolonged exposure to the reference level (NOAEL) of MC-LR could cause severe liver injury to mammals. People with long-term environmental exposure to MCs might be at high risk for developing NAFLD.

MicroRNA-181a regulates endoplasmic reticulum stress in offspring of mice following prenatal microcystin-LR exposure

Microcystin-LR (MCLR) was commonly regarded as a potent hepatotoxin and has been reported to cause neurotoxicity. This study was aimed to investigate how maternal MCLR exposure during pregnancy alters behavioral responses in offspring mice and the possible molecular mechanism involved in this procedure. Three doses of MCLR solutions (0, 3 or 15 μg/kg body weight) were administered subcutaneously to pregnant C57bl/6 from gestation day (GD) 6-19. Our results showed that MCLR prenatal exposure led to the impairment of learning and memory function in offspring on postnatal days (PND) 35, accompanied by endoplasmic reticulum (ER) stress and neuronal apoptosis in hippocampal CA1 regions of mice. Sixteen miRNAs in hippocampus of pups on PND 35 were significantly affected by MCLR exposure with the markedly decreased transcription of miR-181a-5p. We then found that miR-181a-5p was down-regulated, accompanied by activation of ER stress after prenatal exposure to MCLR using qPCR analysis. Furthermore, glucose-regulated protein, 78kDa/binding immunoglobulin protein (Grp78/BIP), a major ER chaperone and signaling regulator, was identified as a target of miR-181a-5p. Our study showed that miR-181a could lead to a decrease in the mRNA expression and protein levels of Grp78 by directly binding to its 3'-untranslated region (3'-UTR) in primary hippocampal neurons. Our findings indicate that the up-regulation of Grp78 mediated by inhibition of miR-181a-5p is a possible mechanism resulting in ER stress and cognitive impairment in pups following prenatal MCLR exposure.