Kidney is a target organ for heavy metals. They accumulate in several segments of the nephron and cause profound alterations in morphology and function. Acute intoxication frequently causes acute renal failure. The effects of chronic exposure have not been fully disclosed. In recent years increasing awareness of the consequences of their presence in the kidney has evolved. In this review we focus on the alterations induced by heavy metals on the intercellular junctions of the kidney. We describe that in addition to the proximal tubule, which has been recognized as the main site of accumulation and injury, other segments of the nephron, such as glomeruli, vessels, and distal nephron, show also deleterious effects. We also emphasize the participation of oxidative stress as a relevant component of the renal damage induced by heavy metals and the beneficial effect that some antioxidant drugs, such as vitamin A (all-trans-retinoic acid) and vitamin E (
Kidney plays the most important role in elimination of xenobiotics, including drugs and toxic environmental agents. Due to its secretory mechanisms, tubular proximal cells are often exposed to higher concentrations of toxic substances than those occurring in plasma or extracellular fluids. In addition to these circumstances, kidney depicts several mechanisms to absorb and excrete xenobiotics. They use molecular protein components located at the cell membrane, that are involved in the transport through the epithelial cells (transcellular route) and those located between cells (paracellular route). In this review we focus on the alterations induced by heavy metals on the proteins that participate in the paracellular transport (junctional adhesion molecules, claudins, occludin, and zonula occludens). Awareness of the damage to these proteins and the relevant consequences, it has on renal function and morphology, has been recently developed.
Heavy metals are accumulated in the kidney and liver, for this reason it is more accurate to estimate degree and duration of exposition to them, by measuring their concentrations in renal and hepatic tissues than in blood. Other options to measure them are in nails and hair. Kidney is the target of heavy metals and the proximal tubule has been recognized for a long time, as the main site of accumulation and damage; however, in this review we will describe that other segments of the nephron are also damaged after exposition to them, and that this damage leads to severe alterations in renal function, that may be permanent or reversible.
The toxic effect of heavy metals on human body has been recognized. In the past century due to the development of industrialized world, large amounts of these elements were produced and most of them were not biodegradable, remaining in the environment for long periods of time. The problem of disposal of toxic products is more extended in emerging economies. In Latin America despite of strict legal regulations, high levels of these elements are found in soil and sediments, leading to the risk of chronic exposure in the general population. In addition, in those countries with volcanic areas, such as Mexico, heavy metals are present in fossil waters that are extracted for human and animal use [
Some heavy metals are necessary for vital functions of the human body, such as iron (Fe), cobalt (Co), copper (Cu), manganese (Mn), molybdenum (Mb), and zinc (Zn). Although it is unknown whether metals such as lead (Pb), cadmium (Cd), and arsenic (As) have a physiological role, they adversely affect several organs, including the liver, lungs, and kidney, being the latest particularly sensitive to their toxic effects even at low levels, due to its ability to reabsorb and accumulate these divalent metals [
Reabsorption of filtered solutes and water is one of the vital functions performed by the kidney. It occurs in several segments of the nephron, the functional unit of the kidney. Reabsorption occurs through two pathways, transcellular and paracellular. The transcellular route is dependent on specialized molecules, such as transporters, ionic channels, and water channels. The paracellular reabsorption is mediated by the proteins that form the TJ: junctional adhesion molecules (JAMs), occludin, claudins, tricellulin, and scaffolding proteins zonula occludens (ZO-1, ZO2, and ZO-3), for reviews see [
From 60 to 70% of the absorption of the glomerular filtered load of ions, organic compounds, and water occurs in proximal tubules, through both, transcellular and paracellular routes [
Distribution of TJ proteins along the nephron has been shown to be similar in several mammal species, such as mouse, rat, and rabbit, and has been related to the permeability characteristics of the segments that constitute the nephron [
Renal distribution of claudins, occludin, and zonula occludens (ZO-1 and ZO-2) along the nephron. Proteins of tight junctions (TJs) display combined expression patterns along the nephron. G: glomerulus; PT: proximal tubule; DTL: descending thin limb of Henle; ATL: ascending thin limb of Henle; TAL: thick loop of Henle; DT: distal tubule; CD: collecting duct.
Claudin 5 in glomerulus and in medullary vessels from rat kidney. Claudin 5 is expressed at the endothelial tight junctions (TJs) of glomerular capillaries (a) and in medullary vessels (b), in a renal section from a normal rat. Bar = 50
Expression of claudin 2 in Bowman’s capsule and proximal tubule in kidney from rat. In renal sections from a normal rat, claudin 2 (green label) costained with claudin 5 (red label), is expressed in Bowman’s capsule (arrow heads, (a)). Claudin 2 (green label) costained with dipeptidyl peptidase (red label) shows a typical “chicken fence” pattern in proximal tubules (arrows, (b)). Bar = 50
In general terms, distribution of claudins along the nephron follows the pattern of TER present in this structure. It increases from the proximal tubule to collecting duct and in opposite direction, paracellular permeability decreases from proximal to collecting duct [
The absorption of heavy metals, such as Cd and Pb, is carried out in the small intestine by a divalent metal transporter characterized as DMT-1. This transporter is expressed in the duodenum, red blood cells, liver, and in the proximal convoluted tubular cells of the kidney. This protein transports Fe and displays a high affinity for other divalent metals such as Cd, Nickel (Ni), Pb, Co, Mn, Zn, and Cu [
Once absorbed, heavy metals are accumulated in liver where they bind to metallothioneins (MTs). These proteins are widely expressed through the body and have a particular feature; they contain a large quantity of cysteine, which confers both a high affinity and a storage capability to heavy metals, such as Zn, Cd, Mercury (Hg), Cu, Pb, Ni, Co, and Fe. The main role of MT is to transfer heavy metals to metalloproteins, transcription factors, and enzymes [
Cadmium is one the most important toxic elements to which general population is exposed. The most common exposition circumstance occurs by contact with tobacco smoke, contaminated water, and foodstuffs such as vegetables, grains, and molluscs. Cadmium progressively accumulates in the body compartments, notably in liver and kidney, given rise to its long half-life, which is more than 20 years in human [
Once absorbed and dissociated from MT by the action of gastric environment, Cd binds to albumin and then it is transported to liver, where it binds to glutathione (GSH) and MT-1. This complex has a low molecular weight (<7 kDa), and then it easily filtrated by the glomerulus being almost totally uptaken in the S1 segment of proximal tubule, in a process mediated by megalin and cubilin [
Cadmium induces cellular damage through several mechanisms, one of the best characterized is that it accumulates in mitochondria, where it blocks the respiratory chain at complex III resulting in an increased production of free radicals that enhances caspase activity leading to apoptosis [
Comparative claudin 5 distribution in tubules and vessels in kidney from a control rat and a dam rat intoxicated with Cd during pregnancy. In renal sections from a normal dam rat claudin 5 (green label) costained with VE-cadherin (red label) is expressed in glomerular capillaries (arrow heads (a)) and in surrounding vessels (arrow, (a)). Expression of claudin 5 at the glomerular capillaries (arrow heads, (b)) and in surrounding vessels (arrow, (b)) is severely altered in a dam rat exposed to Cd. Bar = 50
The best characterized clinical findings of Cd renal toxicity are low molecular weight proteinuria, aminoaciduria, bicarbonaturia, glycosuria, and phosphaturia [
Nephrotoxic effects are severe and include the development of a Fanconi-like syndrome, with glucosuria, aminoaciduria, and phosphaturia. Hazen-Martin et al. [
This metal is accumulated in the kidney, as other heavy metals. The toxic effects of this accumulation have been recognized mostly in acute intoxication episodes. Much less is known for chronic exposure. In a model of acute renal failure induced by potassium dichromate, Perez et al. [
Exposure to hexavalent chromium (Cr6+) causes mutagenic, carcinogenic, and toxic effects, some of which have been associated with its oxidative capacity. In the kidney, TJs are especially sensitive to oxidative stress. In this sense, Basuroy et al. [
Chromium induces variations in serum creatinine, creatinine clearance (Ccr), and fractional excretion of sodium (FeNa), and the latest can be mediated by the mislocation of claudin 2 and occludin induced by Cr [
Comparative occludin distribution in tubules and vessels in kidney from a control rat and a rat treated with potassium dichromate. In control rats, occludin in vessel shows a clearly defined endothelial punctuated label (arrow heads, (a)). In tubules, label for occludin shows a typical “chicken fence” distribution (arrows, (a)). Rats treated with potassium dichromate (15 mg/kg, single dose) show severe alteration of occludin distribution in tubules (arrows, (b)), and in a vessel (arrow head, (b)). Bar = 50
Lead nephrotoxicity has been recognized for more than a century; however, cell mechanisms involved have not been fully disclosed. This is more evident in the case of effect on intercellular junctions. Navarro-Moreno et al. [
Lead is one of the most frequent divalent metals that induces nephrotoxicity. It has been demonstrated that even at low doses, Pb increases cardiovascular morbility [
Although, the maximum nontoxic dose of Pb blood levels has not been established, there is increasing evidence that even previously considered nontoxic Pb levels increase morbility and mortality rates in general population [
Once absorbed by intestine, lung and to a lesser extent through the skin, Pb binds to erythrocyte proteins at above 90%, and then it is distributed to soft tissues and bone. The latest is the main reservoir for Pb in the body. Its bloodstream levels increase during augmented bone turnover, notably during adolescence and pregnancy [
Lead binds to low molecular weight proteins in a proportion lower than 1%. For this reason it is freely filtered at the glomerulus and it is reabsorbed in the proximal tubular cells by a mechanism mediated by endocytosis. Once into the cell, Pb induces mitochondrial damage, uncoupling of respiratory chain, intracellular depletion of GSH, oxidative stress, and apoptosis [
It has been demonstrated that Pb enhances proinflammatory processes through activation of nuclear factor kappa B (NF
The clinical findings of Pb toxicity can be divided in two categories depending on duration of exposure; acute toxicity manifested as aminoaciduria, glycosuria, hyperphosphatemia, haemolytic anemia, gout, and encephalopathy [
Effects of Hg on proximal tubule were recognized long time ago leading to its use as diuretic. Recognition of its toxic effects precluded this therapeutic application. To our knowledge to date there is no available information on the effects of Hg exposure on intercellular renal TJs. As in the case of Cr toxicity, in Hg-intoxicated rats vitamin E protected against the renal damage and development of interstitial renal fibrosis [
Mercuric chloride (HgCl2) causes acute oxidant renal failure that affects mainly proximal tubules. Proteinuria induced by chronic exposure to Hg and the relationship between urinary Hg and renal damage were explored in rats. The results showed that the primary site of damage was the proximal renal tubule and that the glomerulus was eventually involved, due to the capability of Hg to be freely filtered at the glomerulus. The tubular ultrastructural analysis revealed that the lysosome was the most sensitive organelle to Hg, and there was a close relationship between the excretion of urinary Hg and the Hg detoxication mechanisms of the kidney. There is evidence that Hg was accumulated also in endothelia and mesangia of the glomeruli [
The mechanisms involved in renal damage (membranous nephropathy) induced by Hg are in relationship with the deposit of immune complexes directed both against laminin
Experimental models of HgCl2-induced renal glomerular injury showed alterations in urine osmolality, volume, and protein levels were seen within 24 h in response to 1 mg/kg of HgCl2 [
Toxicity of this metal on renal attachment mechanisms has been only scarcely reported. Acute renal failure induced by bismuth (Bi) has been reported [
Nickel compounds are associated with several human diseases mainly in lung and kidney cancers [
Subsequently, Horak and Sunderman [
Accordingly, administration of Ni (6 mg per kg, i.p., three days) significantly enhanced the urinary excretion of alkaline phosphatase (ALP), lactate dehydrogenase (LDH), glutamate oxaloacetate transaminase (GOT), amino acids, and proteins. In addition, it increased the activity of serum ALP, GOT, and glutamate pyruvate transaminase (GPT) [
Renal toxicity induced by heavy metals has been recognized for over a century; however, the intracellular mechanisms of this nephrotoxicity remain unclear. Recent studies performed
Other phenomena such as the loss of the function of transporters, ATPases and ion channels, deranged metabolism, cytoskeleton, and cell polarity destabilization with loss of cell membrane integrity are present. Increased synthesis of MT proteins, upregulation of heat shock proteins (Hsp), increase in cytoplasmic concentration of Ca2+, impaired endocytosis, enhancement of ion conductances, and structural and functional damage in mitochondria have been described in the nephrotoxicity induced by heavy metals such as Cd, Hg, and Cr.
In this review we focus on the deleterious effects induced by heavy metals on the renal TJ structure and function and the participation of oxidative stress and renal cells response to these toxicants. It is known that TJ structure is compromised under oxidative stress conditions [
Multiple pathways participate in Cd-induced nephrotoxicity [
Cadmium can selectively damage the TJ in LLCPK1 cultures [
Cr6+ compounds are oxidizing agents that directly induce tissue damage. The kidney is the main target for Cr accumulation, which might result in acute tubular necrosis in humans after oral or dermal absorption [
The effects of Hg are highly dependent on the different chemical forms of this metal. Dental amalgam is a major source of Hg ingestion [
Renal toxicity by Pb was recognized in 19th century; however, the cellular mechanisms involved in the renal damage induced by chronic exposure to this metal remain largely unknown. Navarro-Moreno et al. [
Oxidative stress is a state in which the cell undergoes altered intracellular redox homeostasis; that is, the balance between oxidants and antioxidants is lost. This imbalance is caused by excessive production of ROS and/or deficiency in antioxidant mechanisms. ROS and reactive nitrogen species (RNS) often act together to create a state of oxidative stress. The three molecular targets more susceptible to be damaged by ROS are DNA, lipids, and proteins [
Oxidative stress has been identified as one of the factors that trigger certain pathologies such as asthma, cardiopulmonary disorders, hypertension, Parkinson, and Alzheimer and plays an important role in the development of kidney diseases [
Metal-mediated formation of free radicals causes various modifications to DNA bases, augmented lipid peroxidation, and altered Ca2+ and sulfhydryl homeostasis. Lipid peroxides, formed by the attack of radicals on polyunsaturated fatty acid residues of phospholipids, can further react with redox metals, finally producing mutagenic and carcinogenic malondialdehyde, 4-hydroxynonenal, and other exocyclic DNA adducts (etheno and/or propano adducts). Whilst Fe, Cu, Cr, Vanadium (V), and Co undergo redox-cycling reactions, for a second group of metals, Hg, Cd and Ni, the primary route for their toxicity is depletion of glutathione and binding to sulfhydryl groups of proteins. Arsenic (As) is thought to bind directly to critical thiols; however, other mechanisms, involving formation of hydrogen peroxide under physiological conditions, have been proposed. The unifying factor in determining toxicity and carcinogenicity for all these metals is the generation of ROS and RNS. Common mechanisms involving the Fenton reaction, generation of the superoxide radical (
Lipids in the kidney suffer damage due to oxidative stress generated by exposure to certain metals such as Cr (Table
Biomarkers of oxidative stress and antioxidant capacity in heavy metals-induced nephrotoxicity.
Metal | Damaged target | Biomarkers of oxidative stress | Biomarkers of antioxidant capacity | Sources |
---|---|---|---|---|
Cr6+ |
Lipid peroxidation |
MDA |
SOD |
[ |
| ||||
Pb |
Lipid peroxidation | TBARS | GSH |
[ |
| ||||
Pb and Hg |
MTs | GSH | ||
Mitochondria | Hsp25 | GST |
[ | |
Apoptosis | Hsp72 | |||
Grp78 | ||||
| ||||
Hg |
MTs | MEL | ||
Mitochondria | Hsp72 |
[ | ||
Grp75 | ||||
iNOS | ||||
| ||||
Cd |
Lipid peroxidation |
TBARS |
SOD |
[ |
CAT: catalase; DNPH: 2,4-dinitrophenylhydrazine; GPx: glutathione peroxidase; GR: glutathione reductase; Grp: glucose-regulated proteins; GSH: reduced glutathione; GSSG: oxidized glutathione; GST: glutathione-S-transferase; HO-1: Heme oxygenase-1; Hsp: heat shock proteins; iNOS: inducible nitric oxide synthase; MDA: malondialdehyde; MEL: melatonin; MT: metallothionein; NO2−/NO3−: nitrate/nitrite; 3-NT: 3-nitrotyrosine; SOD: superoxide dismutase; TBARS: thiobarbituric acid reactive substances.
Renal tissue has high requirements for metabolic energy and relies heavily on aerobic metabolism for the production of ATP through oxidative phosphorylation. The reduction of molecular O2 along the electron transport chain within mitochondria is vital for renal cellular function, yet with a potentially devastating long-term effect [
Chromium also causes damage to the kidney through oxidative stress. Potassium dichromate administered intraperitoneally to Sprague-Dawley rats for 5 days at doses of 2.5, 5.0, 7.5, and 10 mg/kg body weight per day caused a significant increase of ROS production in both liver and kidney, with increments in superoxide dismutase and catalase activities and DNA damage [
Potassium dichromate (K2Cr2O7) is a chemical compound that is widely used in metallurgy, chrome plating, chemical industry, textile manufacture, wood preservation, photography, photoengraving, stainless steel industry, and cooling systems. Its nephrotoxicity is associated with oxidative and nitrosative stress [
Cd is a toxic trace metal that is absorbed through lung, gastrointestinal tract, and skin. Following absorption, Cd is taken up by the hepatocytes and also circulates in blood, bound to MT. The Cd-MT complex, due to its small molecular size, is easily filtered through the glomerular membrane and taken up by renal tubular cells. In the cell this complex induces oxidative stress by interaction of Cd with mitochondrial structures and causes renal damage [
Hg and its compounds are environmental and industrial agents that induce severe nephrotoxicity, both in man and animals. HgCl2 administration is a classic model for the study of the pathogenesis of inorganic Hg toxicity, both
Exposure to Pb is ubiquitous, with the highest levels found in some environmental exposures that have been associated with a number of serious systemic adverse effects involving the nervous system, blood-forming organs, lungs, and kidneys. Pb is also one of the most prevalent and nephrotoxic metals to human. It induces oxidative stress conditions in specific organ targets not only through generation of ROS but also by decrement of cellular antioxidant mechanisms [
A proposed mechanism through which heavy metals disrupt the structure and function of the TJ and cause renal dysfunction is shown in Figure
Mechanisms of oxidant stress and nephrotoxicity induced by heavy metals. Metal exposure produces an increase in reactive oxygen species (ROS) and reactive nitrogen species (RNS) activating signaling pathways, such as nuclear factor kappa B (NF
Renal toxicity induced by heavy metals is a circumstance that may occur at any age and disrespect of gender. Acute exposition may cause renal failure, but chronic exposure results in a great variety of alterations, depending on the dose and duration of exposure. Occupational and environmental sources are of relevance. Damage induced by heavy metals on the proteins that constitute TJs is under current investigation, as well as the severe physiological consequences. It should be emphasized that most heavy metals depict oxidative stress in the kidney and this is one of the mechanisms involved in disruption of TJs. Therefore, to disclose the role of antioxidants in the prevention of oxidative stress might prove helpful in the prevention of the disassembly of the TJ structure.
This review was partially supported by Consejo Nacional de Ciencia y Tecnologia (CONACyT), Mexico (Grant 0179870). Eduardo Molina-Jijón, Yazmin Debray-García, and Pablo Bautista-García have postgraduate fellowships from CONACyT.