Characterisation and uptake of paraquat by rat renal proximal tubular cells in primary culture

Reactive oxygen species impair Na+ transport and renal components of the renin-angiotensin-aldosterone system after paraquat poisoning

Paraquat (1,1'-dimethyl-4,4'-bipyridyl dichloride) is an herbicide widely used worldwide and officially banned in Brazil in 2020. Kidney lesions frequently occur, leading to acute kidney injury (AKI) due to exacerbated reactive O2 species (ROS) production. However, the consequences of ROS exposure on ionic transport and the regulator local renin-angiotensin-aldosterone system (RAAS) still need to be elucidated at a molecular level. This study evaluated how ROS acutely influences Na+-transporting ATPases and the renal RAAS. Adult male Wistar rats received paraquat (20 mg/kg; ip). After 24 h, we observed body weight loss and elevation of urinary flow and serum creatinine. In the renal cortex, paraquat increased ROS levels, NADPH oxidase and (Na++K+)ATPase activities, angiotensin II-type 1 receptors, tumor necrosis factor-α (TNF-α), and interleukin-6. In the medulla, paraquat increased ROS levels and NADPH oxidase activity but inhibited (Na++K+)ATPase. Paraquat induced opposite effects on the ouabain-resistant Na+-ATPase in the cortex (decrease) and medulla (increase). These alterations, except for increased serum creatinine and renal levels of TNF-α and interleukin-6, were prevented by 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl (tempol; 1 mmol/L in drinking water), a stable antioxidant. In summary, after paraquat poisoning, ROS production culminated with impaired medullary function, urinary fluid loss, and disruption of Na+-transporting ATPases and angiotensin II signaling.

Glyphosate's Synergistic Toxicity in Combination with Other Factors as a Cause of Chronic Kidney Disease of Unknown Origin

Chronic kidney disease of unknown etiology (CKDu) is a global epidemic. Sri Lanka has experienced a doubling of the disease every 4 or 5 years since it was first identified in the North Central province in the mid-1990s. The disease primarily affects people in agricultural regions who are missing the commonly known risk factors for CKD. Sri Lanka is not alone: health workers have reported prevalence of CKDu in Mexico, Nicaragua, El Salvador, and the state of Andhra Pradesh in India. A global search for the cause of CKDu has not identified a single factor, but rather many factors that may contribute to the etiology of the disease. Some of these factors include heat stroke leading to dehydration, toxic metals such as cadmium and arsenic, fluoride, low selenium, toxigenic cyanobacteria, nutritionally deficient diet and mycotoxins from mold exposure. Furthermore, exposure to agrichemicals, particularly glyphosate and paraquat, are likely compounding factors, and may be the primary factors. Here, we argue that glyphosate in particular is working synergistically with most of the other factors to increase toxic effects. We propose, further, that glyphosate causes insidious harm through its action as an amino acid analogue of glycine, and that this interferes with natural protective mechanisms against other exposures. Glyphosate's synergistic health effects in combination with exposure to other pollutants, in particular paraquat, and physical labor in the ubiquitous high temperatures of lowland tropical regions, could result in renal damage consistent with CKDu in Sri Lanka.

Xenobiotic transporters and kidney injury

Renal proximal tubules are targets for toxicity due in part to the expression of transporters that mediate the secretion and reabsorption of xenobiotics. Alterations in transporter expression and/or function can enhance the accumulation of toxicants and sensitize the kidneys to injury. This can be observed when xenobiotic uptake by carrier proteins is increased or efflux of toxicants and their metabolites is reduced. Nephrotoxic chemicals include environmental contaminants (halogenated hydrocarbon solvents, the herbicide paraquat, the fungal toxin ochratoxin, and heavy metals) as well as pharmaceuticals (certain beta-lactam antibiotics, antiviral drugs, and chemotherapeutic drugs). This review explores the mechanisms by which transporters mediate the entry and exit of toxicants from renal tubule cells and influence the degree of kidney injury. Delineating how transport proteins regulate the renal accumulation of toxicants is critical for understanding the likelihood of nephrotoxicity resulting from competition for excretion or genetic polymorphisms that affect transporter function.

Sequential organ failure assessment score can predict mortality in patients with paraquat intoxication

Introduction

Paraquat poisoning is characterized by multi-organ failure and pulmonary fibrosis with respiratory failure, resulting in high mortality and morbidity. The objective of this study was to identify predictors of mortality in cases of paraquat poisoning. Furthermore, we sought to determine the association between these parameters.

Methods

A total of 187 patients were referred for management of intentional paraquat ingestion between January 2000 and December 2010. Demographic, clinical, and laboratory data were recorded. Sequential organ failure assessment (SOFA) and acute kidney injury network (AKIN) scores were collected, and predictors of mortality were analyzed.

Results

Overall hospital mortality for the entire population was 54% (101/187). Using a multivariate logistic regression model, it was found that age, time to hospitalization, blood paraquat level, estimated glomerular filtration rate at admission (eGFR _{first day}), and the SOFA_48-h score, but not the AKIN_48-h score, were significant predictors of mortality. For predicting the in-hospital mortality, SOFA_48-h scores displayed a good area under the receiver operating characteristic curve (AUROC) (0.795±0.033, P<0.001). The cumulative survival rate differed significantly between patients with SOFA_48-h scores <3 and those ≥3 (P<0.001). A modified SOFA (mSOFA) score was further developed by using the blood paraquat level, and this new score also demonstrated a better AUROC (0.848±0.029, P<0.001) than the original SOFA score. Finally, the cumulative survival rate also differed significantly between patients with mSOFA scores <4 and ≥4 (P<0.001).

Conclusion

The analytical data demonstrate that SOFA and mSOFA scores, which are based on the extent of organ function or rate of organ failure, help to predict mortality after intentional paraquat poisoning.

Paraquat poisonings: mechanisms of lung toxicity, clinical features, and treatment

Paraquat dichloride (methyl viologen; PQ) is an effective and widely used herbicide that has a proven safety record when appropriately applied to eliminate weeds. However, over the last decades, there have been numerous fatalities, mainly caused by accidental or voluntary ingestion. PQ poisoning is an extremely frustrating condition to manage clinically, due to the elevated morbidity and mortality observed so far and due to the lack of effective treatments to be used in humans. PQ mainly accumulates in the lung (pulmonary concentrations can be 6 to 10 times higher than those in the plasma), where it is retained even when blood levels start to decrease. The pulmonary effects can be explained by the participation of the polyamine transport system abundantly expressed in the membrane of alveolar cells type I, II, and Clara cells. Further downstream at the toxicodynamic level, the main molecular mechanism of PQ toxicity is based on redox cycling and intracellular oxidative stress generation. With this review we aimed to collect and describe the most pertinent and significant findings published in established scientific publications since the discovery of PQ, focusing on the most recent developments related to PQ lung toxicity and their relevance to the treatment of human poisonings. Considerable space is also dedicated to techniques for prognosis prediction, since these could allow development of rigorous clinical protocols that may produce comparable data for the evaluation of proposed therapies.

Reduction of paraquat-induced renal cytotoxicity by manganese and copper complexes of EGTA and EHPG

Superoxide anion generation plays an important role in the development of paraquat toxicity. Although superoxide dismutase mimetics (SODm) have provided protection against organ injury involving generation of superoxide anions, they often suffer problems, e.g., regarding their bioavailability or potential pro-oxidant activity. The aim here was to investigate and compare the therapeutic potential of two novel SODm, manganese(II) and copper(II) complexes of the calcium chelator ethylenebis(oxyethylenenitrilo)tetraacetic acid (EGTA) and of the contrast agent ethylenebis(hydroxyphenylglycine) (EHPG), against paraquat-induced renal toxicity in vitro. Incubation of renal NRK-52E cells with paraquat (1 mM) for 24 h produced submaximal, yet significant, reduction in cellular viability and cell death and produced significant increases in superoxide anion and hydroxyl radical generation. Manganese and copper complexes of EGTA (10-100 microM) and EHPG (30-100 microM) reduced paraquat-induced renal cell toxicity and reduced superoxide anion and hydroxyl radical generation significantly. Manganese complexes displayed greater efficacy than copper complexes and, at equivalent concentrations, manganese complexed with EHPG provided the greatest protection. Furthermore, these metal complexes did not interfere with the uptake of [methyl-(14)C]paraquat into NRK-52E cells, suggesting that they provided protection against paraquat cytotoxicity via intracellular mechanisms. These complexes did not display cytotoxicity at the concentrations examined. Together, these results suggest that manganese and copper complexes of EGTA and EHPG, and especially the manganese-EHPG complex, could provide benefit against paraquat nephrotoxicity.

Transport of paraquat by human organic cation transporters and multidrug and toxic compound extrusion family

Paraquat (N,N-dimethyl-4-4'-bipiridinium; PQ), a widely used herbicide, when ingested accidentally or intentionally can cause major organ toxicities in lung, liver, and kidney. Because PQ is primarily eliminated in the kidney, renal elimination, including tubular transport, plays a critical role in controlling systemic exposure to the herbicide. The goal of this study was to determine the molecular identities of the transporters involved in the renal elimination of PQ. Using stably transfected human embryonic kidney (HEK)-293 cells, we examined the role of human organic cation transporters (hOCTs, SLC22A1-3) and human multidrug and toxic compound extrusion (hMATE)1 in the cellular accumulation and cytotoxicity of PQ. We found that overexpression of hOCT2 but not hOCT1 and hOCT3 in HEK-293 cells significantly enhanced the accumulation and cytotoxicity of PQ (-fold increase for uptake was 12 +/- 0.5, p < 0.01; -fold increase of cytotoxicity was 18 +/- 1.5, p < 0.001). The kinetics of PQ transport was altered in cells expressing a genetic polymorphism of hOCT2 (A270S) in comparison with those expressing the reference hOCT2. In addition, the cellular accumulation and cytotoxicity of PQ were also enhanced in cells expressing hMATE1 (-fold increase for uptake was 18 +/- 3.7, p < 0.0001; -fold increase of cytotoxicity was 5.7 +/- 0.5, p < 0.0001). These results suggest that hOCT2 and hMATE1 mediate PQ transport. These transporters may play an important role in the accumulation and renal excretion of PQ, and they may serve as molecular targets for the prevention and treatment of PQ-induced nephrotoxicity.

High blood pressure is one of the symptoms of paraquat-induced toxicity in rats

This study investigated whether paraquat (Pq)-induced lipidic peroxidation (LP) is accompanied by changes in blood pressure and heart rate (HR) in rats. Groups of adult male Wistar rats were studied 2 and 12 h after Pq (35 mg/kg, i.p.) administration. The LP was evaluated by monitoring thiobarbituric acid reactive substances (TBARS) in the kidneys, liver and lungs, and validated by including a group treated with an antioxidant, superoxide dismutase (CuZnSOD 50,000 IU/kg), in the study. The TBARS levels were significantly higher (p<0.05) in the kidneys of the rats studied 2 h after Pq than in their respective controls. Similarly, systolic and diastolic blood pressure (DBP) were higher (p<0.05), while HR was lower (p<0.05) than basal levels 2 and 12 h after Pq administration. In contrast, the group treated simultaneously with Pq and CuZnSOD exhibited lower levels of TBARS (p<0.05) in all studied organs compared to the control group, while the mean arterial pressure and HR did not differ from those seen in the control group. These findings indicate that acute Pq poisoning symptoms include high blood pressure.

Molecular and cellular physiology of renal organic cation and anion transport

Organic cations and anions (OCs and OAs, respectively) constitute an extraordinarily diverse array of compounds of physiological, pharmacological, and toxicological importance. Renal secretion of these compounds, which occurs principally along the proximal portion of the nephron, plays a critical role in regulating their plasma concentrations and in clearing the body of potentially toxic xenobiotics agents. The transepithelial transport involves separate entry and exit steps at the basolateral and luminal aspects of renal tubular cells. It is increasingly apparent that basolateral and luminal OC and OA transport reflects the concerted activity of a suite of separate transport processes arranged in parallel in each pole of proximal tubule cells. The cloning of multiple members of several distinct transport families, the subsequent characterization of their activity, and their subcellular localization within distinct regions of the kidney now allows the development of models describing the molecular basis of the renal secretion of OCs and OAs. This review examines recent work on this issue, with particular emphasis on attempts to integrate information concerning the activity of cloned transporters in heterologous expression systems to that observed in studies of physiologically intact renal systems.

Use of cultured cells of kidney origin to assess specific cytotoxic effects of nephrotoxins

During drug discovery, assessment of renal safety for a compound is important for further development of a candidate drug. In this study, we describe an in vitro cell-based assay capable of discerning nephrotoxicity. Three cell types, two of kidney origin and one of liver origin, were used to examine the effects of nephrotoxins. The cell types were the porcine normal kidney tubular epithelial cell line (LLC-PK1), the primary human renal proximal tubular epithelial cells (hRPTEC) and the human liver cell line (HepG2). Cytotoxicity was measured using a luciferin/luciferase assay that measures cellular ATP levels. Four known nephrotoxins, 4-aminophenol, cisplatin, cyclosporin A and paraquat, were tested in this cell-based assay to evaluate cytotoxicity on drug exposure. Kidney-derived LLC-PK1 cells and hRPTECs were found to be sensitive to selected nephrotoxins while liver-derived HepG2 cells were insensitive. Human RPTEC cells obtained from three individual donors demonstrated highly reproducible effects on drug exposure. With respect to drug discovery efforts, integration of the cell models described here are valuable for evaluation of nephrotoxic potentials during lead selection and optimization processes.

The characterisation and uptake of paraquat in cultured baboon kidney proximal tubule cells (bPTC)

A primary culture of baboon proximal tubule cells (bPTC) was prepared and characterised using LLC-PK1 cells of proximal tubule origin and MDCK cells of distal tubule origin, as positive and negative references, respectively. The proximal tubular origin of the bPTC was determined by morphological studies, immunoperoxidase staining and the expression of proximal tubule markers alkaline phosphatase and gammaglutamyltransferase. The hypothesis that paraquat (PQ) is transported by the bPTC was investigated. The cytotoxic threshold for PQ in these cells was determined and compared to the LLC-PK1 and MDCK cells. Furthermore, this study investigated the transport of the monovalent cation tetraethyl ammonium (TEA) and the polyvalent cation cimetidine in the bPTC and demonstrated their effect on the cellular uptake of PQ. The cytotoxic threshold of PQ in the bPTC, determined by cellular viability studies using the method of Trypan blue exclusion, is 0.05 mM at 2 h incubation. The LC50 after 24 h is 76, 61 and 455 microM for the bPTC, LLC-PK1 and MDCK cells, respectively. This indicates that proximal tubule cells are more susceptible to PQ toxicity compared to distal tubule cells, which is consistent with clinical PQ toxicity where renal damage is found predominantly in the proximal renal tubules. The cations PQ and cimetidine were actively transported by the bPTC. The uptake of PQ (0.05 mM) commenced after 15 min whereas cimetidine (0.5 mM) uptake was evident after 2 min. Furthermore, cimetidine was shown to compete with PQ for uptake in the bPTC. Coincubating PQ (0.05 mM) and cimetidine (0.5 mM) for 60 min resulted in an approximate 50% decrease in PQ uptake. The cation TEA was not transported by the bPTC suggesting either a genetic mutation or complete absence of the transporter for TEA in the cells. The results suggest that PQ may be transported by the same cation transporter as cimetidine and not TEA, indicating PQ uptake in the bPTC to be via a polyvalent organic cation transporter.

Transport of paraquat by isolated renal proximal tubular segments from rabbits

Paraquat is a non-selective herbicide, which induces lung, liver and kidney damage in mammalian species. Because paraquat is mainly eliminated by the kidneys, the induced kidney damage may suppress excretion and enhance toxicity of paraquat in other organs. Since proximal tubules appear to be the target segment of the nephron, this study focuses on transport of paraquat by isolated proximal tubular segments from rabbits. Proximal tubules were isolated using a combined magnetic iron perfusion and collagenase method. Incubations were carried out at 25 degrees under 100% oxygen or nitrogen for varying times at different concentrations of paraquat. Proximal tubules accumulated paraquat by a slow process, which was non-saturable in the concentration range (0.1-5 microM) examined. Tubular excretion of cations involves transport across both basolateral and luminal membranes of the cell. The basolateral uptake of paraquat was inhibited by low temperature, low medium pH and quinine. In contrast to quinine, tetraethylammonium enhanced paraquat accumulation probably by trans-stimulating the basolateral uptake. Incubation under nitrogen enhanced paraquat accumulation possibly by reducing the transport out of the cell at the luminal membrane. Thus, this study shows that proximal tubules accumulate paraquat by an active process related to the cation transport mechanism.