Microcystin-LR disrupts insulin signaling by hyperphosphorylating insulin receptor substrate 1 and glycogen synthase

Tacrolimus induces insulin receptor substrate 1 hyperphosphorylation and inhibits mTORc1/S6K1 cascade in HL7702 cells

BACKGROUND

Tacrolimus (FK506) is a key calcineurin inhibitor used to prevent organ transplant rejection and is effective in improving graft survival. However, it is linked to hyperglycemia and insulin resistance, contributing to new-onset diabetes after transplantation and negatively affecting islet function.

AIM

To study the effects of tacrolimus on the insulin signaling pathway of hepatocytes.

METHODS

HL7702 cells were treated with different concentrations of tacrolimus (0.1 mg/L, 1 mg/L, 5 mg/L) for 24 hours. The proteins involved in insulin signaling were detected by Western blotting.

RESULTS

Compared with the control group, phosphorylation of insulin receptor substrate (IRS) 1 at Ser 307 and Ser 323 were increased significantly when the tacrolimus concentration reached 1 and 5 mg/L. Phosphorylation of IRS1 at Ser 1101 was also increased, although not significantly. However, phosphorylation of Ribosomal protein S6 kinase beta-1 at Thr 389 was decreased significantly. The levels of phosphorylated glycogen synthase kinase 3α Ser 21 and Ser 9 were increased. Surprisingly, phosphorylation of glycogen synthase at Ser 641 was increased. There was no significant change in the activity of glycogen phosphorylase.

CONCLUSION

Tacrolimus has no direct effect on hepatic glucose metabolism, but inhibits IRS1-mediated insulin signaling. This may be one of the underlying mechanisms by which tacrolimus induces insulin resistance.

Microcystin-LR in drinking water: An emerging role of mitochondrial-induced epigenetic modifications and possible mitigation strategies

Algal blooms are a serious menace to freshwater bodies all over the world. These blooms typically comprise cyanobacterial outgrowths that produce a heptapeptide toxin, Microcystin-LR (MC-LR). Chronic MC-LR exposure impairs mitochondrial-nuclear crosstalk, ROS generation, activation of DNA damage repair pathways, apoptosis, and calcium homeostasis by interfering with PC/MAPK/RTK/PI3K signaling. The discovery of the toxin's biosynthesis pathways paved the way for the development of molecular techniques for the early detection of microcystin. Phosphatase inhibition-based bioassays, high-performance liquid chromatography, and enzyme-linked immunosorbent tests have recently been employed to identify MC-LR in aquatic ecosystems. Biosensors are an exciting alternative for effective on-site analysis and field-based characterization. Here, we present a synthesis of evidence supporting MC-LR as a mitotoxicant, examine various detection methods, and propose a novel theory for the relevance of MC-LR-induced breakdown of mitochondrial machinery and its myriad biological ramifications in human health and disease.

Dissection of molecular mechanisms of liver injury induced by microcystin-leucine arginine via single-cell RNA-sequencing

The occurrence of poisoning incidents caused by cyanobacterial blooms has aroused wide public concern. Microcystin-leucine arginine (MC-LR) is a well-established toxin produced by cyanobacterial blooms, which is widely distributed in eutrophic waters. MC-LR is not only hazardous to the water environment but also exerts multiple toxic effects including liver toxicity in both humans and animals. However, the underlying mechanisms of MC-LR-induced liver toxicity are unclear. Herein, we used advanced single-cell RNA sequencing technology to characterize MC-LR-induced liver injury in mice. We established the first single-cell atlas of mouse livers in response to MC-LR. Our results showed that the differentially expressed genes and pathways in diverse cell types of liver tissues of mice treated with MC-LR are highly heterogeneous. Deep analysis showed that MC-LR induced an increase in a subpopulation of hepatocytes that highly express Gstm3, which potentially contributed to hepatocyte apoptosis in response to MC-LR. Moreover, MC-LR increased the proportion and multiple subtypes of Kupffer cells with M1 phenotypes and highly expressed proinflammatory genes. Furthermore, the MC-LR increased several subtypes of CD8+ T cells with highly expressed multiple cytokines and chemokines. Overall, apart from directly inducing hepatocytes apoptosis, MC-LR activated proinflammatory Kupffer cell and CD8+ T cells, and their interaction may constitute a hostile microenvironment that contributes to liver injury. Our findings not only present novel insight into underlying molecular mechanisms but also provide a valuable resource and foundation for additional discovery of MC-LR-induced liver toxicity.

Astaxanthin Alleviates Hepatic Lipid Metabolic Dysregulation Induced by Microcystin-LR

Microcystin-LR (MC-LR), frequently generated by cyanobacteria, has been demonstrated to raise the likelihood of liver disease. Few previous studies have explored the potential antagonist against MC-LR. Astaxanthin (ASX) has been shown to possess various beneficial effects in regulating lipid metabolism in the liver. However, whether ASX could alleviate MC-LR-induced hepatic lipid metabolic dysregulation is as yet unclear. In this work, the important roles and mechanisms of ASX in countering MC-LR-induced liver damage and lipid metabolic dysregulation were explored for the first time. The findings revealed that ASX not only prevented weight loss but also enhanced liver health after MC-LR exposure. Moreover, ASX effectively decreased triglyceride, total cholesterol, aspartate transaminase, and alanine aminotransferase contents in mice that were elevated by MC-LR. Histological observation showed that ASX significantly alleviated lipid accumulation and inflammation induced by MC-LR. Mechanically, ASX could significantly diminish the expression of genes responsible for lipid generation (Srebp-1c, Fasn, Cd36, Scd1, Dgat1, and Pparg), which probably reduced lipid accumulation induced by MC-LR. Analogously, MC-LR increased intracellular lipid deposition in THLE-3 cells, while ASX decreased these symptoms by down-regulating the expression of key genes in the lipid synthesis pathway. Our results implied that ASX played a crucial part in lipid synthesis and effectively alleviated MC-LR-induced lipid metabolism dysregulation. ASX might be developed as a novel protectant against hepatic impairment and lipid metabolic dysregulation associated with MC-LR. This study offers new insights for further management of MC-LR-related metabolic diseases.

PI3K/AKT/mTOR pathway mediated-cell cycle dysregulation contribute to malignant proliferation of mouse spermatogonia induced by microcystin-leucine arginine

Environmental cyanotoxin exposure may be a trigger of testicular cancer. Activation of PI3K/AKT/mTOR signaling pathway is the critical molecular event in testicular carcinogenesis. As a widespread cyanotoxin, microcystin-leucine arginine (MC-LR) is known to induce cell malignant transformation and tumorigenesis. However, the effects of MC-LR on the regulatory mechanism of PI3K/AKT/mTOR pathway in seminoma, the most common testicular tumor, are unknown. In this study, mouse spermatogonia cell line (GC-1) and nude mice were used to investigate the effects and mechanisms of MC-LR on the malignant transformation of spermatogonia by nude mouse tumorigenesis assay, cell migration invasion assay, western blot, and cell cycle assay, and so forth. The results showed that, after continuous exposure to environmentally relevant concentrations of MC-LR (20 nM) for 35 generations, the proliferation, migration, and invasion abilities of GC-1 cells were increased by 120%, 340%, and 370%, respectively. In nude mice, MC-LR-treated GC-1 cells formed tumors with significantly greater volume (0.998 ± 0.768 cm³ ) and weight (0.637 ± 0.406 g) than the control group (0.067 ± 0.039 cm³ ; 0.094 ± 0.087 g) (P < .05). Furthermore, PI3K inhibitor Wortmannin inhibited the PI3K/AKT/mTOR pathway and its downstream proteins (c-MYC, CDK4, CCND1, and MMP14) activated by MC-LR. Blocking PI3K alleviated MC-LR-induced cell cycle disorder and malignant proliferation, migration and invasive of GC-1 cells. Altogether, our findings suggest that MC-LR can induce malignant transformation of mouse spermatogonia, and the PI3K/AKT/mTOR pathway-mediated cell cycle dysregulation may be an important target for malignant proliferation. This study provides clues to further reveal the etiology and pathogenesis of seminoma.

A new identity of microcystins: Environmental endocrine disruptors? An evidence-based review

Microcystins (MCs) are widely distributed cyanobacterial toxins in eutrophic waters. At present, the endocrine-disrupting effects of MCs have been extensively studied, but whether MCs can be classified as environmental endocrine disruptors (EDCs) is still unclear. This review is aimed to evaluate the rationality for MCs as to be classified as EDCs based on the available evidence. It has been identified that MCs meet eight of ten key characteristics of chemicals that can be classified as EDCs. MCs interfere with the six processes, including synthesis, release, circulation, metabolism, binding and action of natural hormones in the body. Also, they are fit two other characteristics of EDC: altering the fate of producing/responding cells and epigenetic modification. Further evidence indicates that the endocrine-disrupting effect of MCs may be an important cause of adverse health outcomes such as metabolic disorders, reproductive disorders and effects on the growth and development of offspring. Generally, MCs have endocrine-disrupting properties, suggesting that it is reasonable for them to be considered EDCs. This is of great importance in understanding and evaluating the harm done by MCs on humans.

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.

Update on the adverse effects of microcystins on the liver

Microcystins (MCs) are the most common cyanobacteria toxins in eutrophic water, which have strong hepatotoxicity. In the past decade, epidemiological and toxicological studies on liver damage caused by MCs have proliferated, and new mechanisms of hepatotoxicity induced by MCs have also been discovered and confirmed. However, there has not been a comprehensive and systematic review of these new findings. Therefore, this paper summarizes the latest advances in studies on the hepatotoxicity of MCs to reveal the effects and mechanisms of hepatotoxicity induced by MCs. Current epidemiological studies have confirmed that symptoms or signs of liver damage appear after human exposure to MCs, and a long time of exposure can even lead to liver cancer. Toxicological studies have shown that MCs can affect the expression of oncogenes by activating cell proliferation pathways such as MAPK and Akt, thereby promoting the occurrence and development of cancer. The latest evidence shows that epigenetic modifications may play an important role in MCs-induced liver cancer. MCs can cause damage to the liver by inducing hepatocyte death, mainly manifested as apoptosis and necrosis. The imbalance of liver metabolic homeostasis may be involved in hepatotoxicity induced by MCs. In addition, the combined toxicity of MCs and other toxins are also discussed in this article. This detailed information will be a valuable reference for further exploring of MCs-induced hepatotoxicity.

The effects of short-term treatment of microcystin-LR on the insulin pathway in both the HL7702 cell line and livers of mice

Our previous work indicated exposure of Human liver cell 7702 (HL7702) cells to Microcystin-leucine-arginine (MC-LR) for 24 hours can disrupt insulin (INS) signaling by the hyperphosphorylation of specific proteins. For further exploring the time-dependent effect posed by MC-LR on this pathway, in the current study, HL7702 cells together with mice were exposed to the MC-LR with different concentrations under short-term treatment, and then, protein phosphatase 2A (PP2A) activity and expression of proteins related to INS signaling, as well as the characteristics of their action in the liver, were investigated. The results indicated, in HL7702 cells with 0.5, 1, and 6 hours of treatment by MC-LR, PP2A activity showed an obvious decrease in a time and concentration-dependent manner. While the total protein level of Akt, glycogen synthase kinase 3 (GSK-3), and glycogen synthase remained unchanged, GSK-3 and Akt phosphorylation increased significantly. In livers of mice with 1 hour of intraperitoneal injection with MC-LR, a similar change in these proteins was observed. In addition, the levels of total IRS1 and p-IRS1 at serine sites showed decreasing and increasing trends,respectively, and the hematoxylin and eosin staining showed that liver tissues of mice in the maximum-dose group exhibited obvious hepatocyte degeneration and hemorrhage. Our results further proved that short-term treatment with MC-LR can inhibit PP2A activity and disrupt INS signaling proteins' phosphorylation level, thereby interfering with the INS pathway. Our findings provide a helpful understanding of the toxic effects posed by MC-LR on the glucose metabolism of liver via interference with the INS signaling pathway.

Regulation of insulin resistance and glucose metabolism by interaction of PIM kinases and insulin receptor substrates

Insulin resistance is caused by various environmental and genetic factors leading to a number of serious health issues. Due to its multifactorial origin, molecular characterization may provide better tools for its effective treatment. On molecular level, dysregulation of signaling pathway by insulin receptor substrates (IRSs) is one of the most common reasons of this disease. IRSs are regulated by >50 serine/threonine kinases, which may have positive or negative effects on insulin sensitivity. Among these serine/threonine kinases, PIM kinases have garnered much attention as they not only affect insulin sensitivity by phosphorylating IRSs directly and/or indirectly but also alter the activities of their downstream molecules like PI3K, AKT, and mTOR. In this review, interactions of PIM kinases with IRSs and their downstream proteins and their action mechanism in the regulation of insulin resistance are elaborated. Furthermore, this review offers fundamental understandings of the role of PIM kinases in this signaling pathway.

Microcystin-LR exposure disrupts the insulin signaling pathway in C2C12 mice muscle cell line

Microcystin-LR (MC-LR) is a widely produced monocyclic heptapeptides in eutrophication waterbodies. MC-LR can induce various toxic effects in different cells. Our previous studies have found that MC-LR exposure can disrupt insulin signaling pathway in human liver cells (HL 7702). Skeletal muscle is one of the major organs for glucose disposal and responsive to insulin. However, the effects of MC-LR on insulin signaling pathway in muscle cells have not been fully explored. By using C2C12 mice muscle cells, this study aims to investigate the toxic effects of MC-LR in muscle cells with a focus on its effects on insulin signaling pathways. It was found that MC-LR entered into cells and inhibited protein phosphatase 2A (PP2A) significantly. Furthermore, MC-LR increased phosphorylation of Ser302, Ser307, Ser612 of insulin receptor substrate 1, AKT-Ser473, GSK3α-Ser21, and S6K1-Thr389 by inhibiting the activity of PP2A. The results in this study demonstrate that exposure of MCLR can disrupt the insulin pathway in muscle cells.

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

Microcystin-LR (MCLR) is a hepatotoxic cyanotoxin reported to cause a phenotype similar to nonalcoholic steatohepatitis (NASH). NASH is a common progressive liver disease that advances in severity due to exogenous stressors such as poor diet and toxicant exposure. Our objective was to determine how sub-chronic MCLR toxicity affects preexisting diet-induced NASH. Sprague-Dawley rats were fed one of three diets for 10 weeks: control, methionine and choline deficient (MCD), or high fat/high cholesterol (HFHC). After six weeks of diet, animals received vehicle, 10 µg/kg, or 30 µg/kg MCLR via intraperitoneal injection every other day for the final 4 weeks. Incidence and severity scoring of histopathology endpoints suggested that MCLR toxicity drove NASH to a less fatty and more fibrotic state. In general, expression of genes involved in de novo lipogenesis and fatty acid esterification were altered in favor of decreased steatosis. The higher MCLR dose increased expression of genes involved in fibrosis and inflammation in the control and HFHC groups. These data suggest MCLR toxicity in the context of preexisting NASH may drive the liver to a more severe phenotype that resembles burnt-out NASH.

Nonalcoholic fatty liver disease alters microcystin-LR toxicokinetics and acute toxicity

Microcystin-LR (MCLR) is a cyanotoxin produced by blue-green algae that causes liver and kidney toxicities. MCLR toxicity is dependent on cellular uptake through the organic anion transporting polypeptide (OATP) transporters. Nonalcoholic fatty liver disease (NAFLD) progresses through multiple stages, alters expression of hepatic OATPs, and is associated with chronic kidney disease. The purpose of this study was to determine whether NAFLD increases systemic exposure to MCLR and influences acute liver and kidney toxicities. Rats were fed a control diet or two dietary models of NAFLD; methionine and choline deficient (MCD) or high fat/high cholesterol (HFHC). Two studies were performed in these groups: 1) a single dose intravenous toxicokinetic study (20 μg/kg), and 2) a single dose intraperitoneal toxicity study (60 μg/kg). Compared to control rats, plasma MCLR area under the concentration-time curve (AUC) in MCD rats doubled, whereas biliary clearance (Clbil) was unchanged; in contrast, plasma AUC in HFHC rats was unchanged, whereas Clbil approximately doubled. Less MCLR bound to PP2A was observed in the liver of MCD rats. This shift in exposure decreased the severity of liver pathology only in the MCD rats after a single toxic dose of MCLR (60 μg/kg). In contrast, the single toxic dose of MCLR increased hepatic inflammation, plasma cholesterol, proteinuria, and urinary KIM1 in HFHC rats more than MCLR exposed control rats. In conclusion, rodent models of NAFLD alter MCLR toxicokinetics and acute toxicity and may have implications for liver and kidney pathologies in NAFLD patients.