Bidirectional transport of cadmium across apical membrane of renal epithelial cell lines via H+-antiporter and inorganic anion exchanger

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.

Multidrug and toxin extrusion proteins mediate cellular transport of cadmium

Cadmium (Cd) is an environmentally prevalent toxicant posing increasing risk to human health worldwide. As compared to the extensive research in Cd tissue accumulation, little was known about the elimination of Cd, particularly its toxic form, Cd ion (Cd²⁺). In this study, we aimed to examine whether Cd²⁺ is a substrate of multidrug and toxin extrusion proteins (MATEs) that are important in renal xenobiotic elimination. HEK-293 cells overexpressing the human MATE1 (HEK-hMATE1), human MATE2-K (HEK-hMATE2-K) and mouse Mate1 (HEK-mMate1) were used to study the cellular transport and toxicity of Cd²⁺. The cells overexpressing MATEs showed a 2-4 fold increase of Cd²⁺ uptake that could be blocked by the MATE inhibitor cimetidine. A saturable transport profile was observed with the Michaelis-Menten constant (K_m) of 130±15.8μM for HEK-hMATE1; 139±21.3μM for HEK-hMATE2-K; and 88.7±13.5μM for HEK-mMate1, respectively. Cd²⁺ could inhibit the uptake of metformin, a substrate of MATE transporters, with the half maximal inhibitory concentration (IC₅₀) of 97.5±6.0μM, 20.2±2.6μM, and 49.9±6.9μM in HEK-hMATE1, HEK-hMATE2-K, and HEK-mMate1 cells, respectively. In addition, hMATE1 could transport preloaded Cd²⁺ out of the HEK-hMATE1 cells, thus resulting in a significant decrease of Cd²⁺-induced cytotoxicity. The present study has provided the first evidence supporting that MATEs transport Cd²⁺ and may function as cellular elimination machinery in Cd intoxication.

Role of the sodium-dependent phosphate co-transporters and of the phosphate complexes of uranyl in the cytotoxicity of uranium in LLC-PK1 cells

Although uranium is a well-characterized nephrotoxic agent, very little is known at the cellular and molecular level about the mechanisms underlying the uptake and toxicity of this element in proximal tubule cells. The aim of this study was thus to characterize the species of uranium that are responsible for its cytotoxicity and define the mechanism which is involved in the uptake of the cytotoxic fraction of uranium using two cell lines derived from kidney proximal (LLC-PK(1)) and distal (MDCK) tubule as in vitro models. Treatment of LLC-PK(1) cells with colchicine, cytochalasin D, concanavalin A and PMA increased the sodium-dependent phosphate co-transport and the cytotoxicity of uranium. On the contrary, replacement of the extra-cellular sodium with N-methyl-D-glucamine highly reduced the transport of phosphate and the cytotoxic effect of uranium. Uranium cytotoxicity was also dependent upon the extra-cellular concentration of phosphate and decreased in a concentration-dependent manner by 0.1-10 mM phosphonoformic acid, a competitive inhibitor of phosphate uptake. Consistent with these observations, over-expression of the rat proximal tubule sodium-dependent phosphate co-transporter NaPi-IIa in stably transfected MDCK cells significantly increased the cytotoxicity of uranium, and computer modeling of uranium speciation showed that uranium cytotoxicity was directly dependent on the presence of the phosphate complexes of uranyl UO(2)(PO(4))(-) and UO(2)(HPO(4))(aq). Taken together, these data suggest that the cytotoxic fraction of uranium is a phosphate complex of uranyl whose uptake is mediated by a sodium-dependent phosphate co-transporter system.

Molecular and ionic mimicry and the transport of toxic metals

Despite many scientific advances, human exposure to, and intoxication by, toxic metal species continues to occur. Surprisingly, little is understood about the mechanisms by which certain metals and metal-containing species gain entry into target cells. Since there do not appear to be transporters designed specifically for the entry of most toxic metal species into mammalian cells, it has been postulated that some of these metals gain entry into target cells, through the mechanisms of ionic and/or molecular mimicry, at the site of transporters of essential elements and/or molecules. The primary purpose of this review is to discuss the transport of selective toxic metals in target organs and provide evidence supporting a role of ionic and/or molecular mimicry. In the context of this review, molecular mimicry refers to the ability of a metal ion to bond to an endogenous organic molecule to form an organic metal species that acts as a functional or structural mimic of essential molecules at the sites of transporters of those molecules. Ionic mimicry refers to the ability of a cationic form of a toxic metal to mimic an essential element or cationic species of an element at the site of a transporter of that element. Molecular and ionic mimics can also be sub-classified as structural or functional mimics. This review will present the established and putative roles of molecular and ionic mimicry in the transport of mercury, cadmium, lead, arsenic, selenium, and selected oxyanions in target organs and tissues.

Effects of P-glycoprotein inhibitors on transepithelial transport of cadmium in cultured renal epithelial cells, LLC-PK1 and LLC-GA5-COL 150

The purpose of this study using LLC-PK(1) cells and P-glycoprotein (P-gp) overexpressed LLC-PK(1) cells (LLC-GA5-COL 150 cells) was to investigate the secretory transport of cadmium (Cd) via endogenous and overexpressed P-gp, respectively. Cell monolayers cultured on permeable membranes were incubated at 37 degrees C for 60 min with 1 microM CdCl(2) from either the apical or the basolateral side. The basolateral-to-apical transport of Cd was 1.7 times higher than the apical-to-basolateral transport of Cd in LLC-GA5-COL 150 cells, while the transport from apical and basolateral sides was almost the same in LLC-PK(1) cells. Treatment with a P-gp monoclonal antibody, UIC2, significantly decreased the basolateral-to-apical transport of Cd in LLC-PK(1) and LLC-GA5-COL 150 cells, and significantly increased the apical-to-basolateral transport of Cd in both cells. The effects of UIC2 were more marked in LLC-GA5-COL 150 cells than in LLC-PK(1) cells. Furthermore, typical P-gp inhibitors such as cyclosporin A, and doxorubicin decreased the basolateral-to-apical transport of Cd slightly in LLC-PK(1) cells and significantly in LLC-GA5-COL 150 cells. These results suggest that Cd is extruded from the apical membrane of LLC-PK(1) and LLC-GA5-COL 150 cells, probably depending on the level of P-gp expression.

Cadmium induces both pyruvate kinase and Na+/H+exchanger activity through protein kinase C mediated signal transduction, in isolated digestive gland cells ofMytilus galloprovincialis(L.)

The present study investigates the transduction pathway mediated by cadmium in isolated digestive gland cells of mussel Mytilus galloprovincialis. The effects of cadmium treatment on a key glycolytic enzyme, pyruvate kinase (PK), and on Na+/H+ exchanger activity were examined. Cadmium (50 μmol l–1) caused a significant elevation of intracellular pH (pHi) and a rise (176%) of Na influx relative to control values. The amiloride analogue, EIPA (20 nmol l–1), a Na+/H+ exchanger blocker,together with cadmium, significantly reduced the effect of treatment by cadmium alone on both Na+ influx and pHi. In addition, PK activity was significantly increased after treatment with cadmium. PK activity was inhibited after treatment of cells with amiloride or EIPA together with cadmium. Moreover, phorbol-ester (PMA), a potent activator of protein kinase C(PKC), caused a significant rise in both pHi and PK activity, while staurosporine or calphostin C reversed both events. Adrenaline, isoprenaline and phenylephrine alone or together with cadmium also significantly increased the pHi and PK activity of isolated digestive gland cells. The latter effectors in combination with cadmium showed a synergistic effect on pHi and PK. These responses seem to be blocked by propranolol, metoprolol and prazosin. Our findings suggest a hormone-like effect of cadmium on digestive gland cells. The signal transduction pathway induced by cadmium involves the stimulation of PK, PKC and Na+/H+ exchanger in isolated digestive gland cells of Mytilus galloprovincialis.

Molecular handling of cadmium in transporting epithelia

Cadmium (Cd) is an industrial and environmental pollutant that affects adversely a number of organs in humans and other mammals, including the kidneys, liver, lungs, pancreas, testis, and placenta. The liver and kidneys, which are the primary organs involved in the elimination of systemic Cd, are especially sensitive to the toxic effects of Cd. Because Cd ions possess a high affinity for sulfhydryl groups and thiolate anions, the cellular and molecular mechanisms involved in the handling and toxicity of Cd in target organs can be defined largely by the molecular interactions that occur between Cd ions and various sulfhydryl-containing molecules that are present in both the intracellular and extracellular compartments. A great deal of scientific data have been collected over the years to better define the toxic effects of Cd in the primary target organs. Notwithstanding all of the new developments made and information gathered, it is surprising that very little is known about the cellular and molecular mechanisms involved in the uptake, retention, and elimination of Cd in target epithelial cells. Therefore, the primary purpose of this review is to summarize and put into perspective some of the more salient current findings, assertions, and hypotheses pertaining to the transport and handling of Cd in the epithelial cells of target organs. Particular attention has been placed on the molecular mechanisms involved in the absorption, retention, and secretion of Cd in small intestinal enterocytes, hepatocytes, and tubular epithelial cells lining both proximal and distal portions of the nephron. The purpose of this review is not only to provide a summary of published findings but also to provide speculations and testable hypotheses based on contemporary findings made in other areas of research, with the hope that they may promote and serve as the impetus for future investigations designed to define more precisely the cellular mechanisms involved in the transport and handling of Cd within the body.

Arabidopsis and the Genetic Potential for the Phytoremediation of Toxic Elemental and Organic Pollutants

In a process called phytoremediation, plants can be used to extract, detoxify, and/or sequester toxic pollutants from soil, water, and air. Phytoremediation may become an essential tool in cleaning the environment and reducing human and animal exposure to potential carcinogens and other toxins. Arabidopsis has provided useful information about the genetic, physiological, and biochemical mechanisms behind phytoremediation, and it is an excellent model genetic organism to test foreign gene expression. This review focuses on Arabidopsis studies concerning: 1) the remediation of elemental pollutants; 2) the remediation of organic pollutants; and 3) the phytoremediation genome. Elemental pollutants include heavy metals and metalloids (e.g., mercury, lead, cadmium, arsenic) that are immutable. The general goal of phytoremediation is to extract, detoxify, and hyperaccumulate elemental pollutants in above-ground plant tissues for later harvest. A few dozen Arabidopsis genes and proteins that play direct roles in the remediation of elemental pollutants are discussed. Organic pollutants include toxic chemicals such as benzene, benzo(a)pyrene, polychlorinated biphenyls, trichloroethylene, trinitrotoluene, and dichlorodiphenyltrichloroethane. Phytoremediation of organic pollutants is focused on their complete mineralization to harmless products, however, less is known about the potential of plants to act on complex organic chemicals. A preliminary survey of the Arabidopsis genome suggests that as many as 700 genes encode proteins that have the capacity to act directly on environmental pollutants or could be modified to do so. The potential of the phytoremediation proteome to be used to reduce human exposure to toxic pollutants appears to be enormous and untapped.

Transport of cadmium across the apical membrane of epithelial cell lines

Cadmium (Cd) uptake and secretion across the apical membrane of epithelial cells was studied using LLC-PK1 cells cultured on Petri dishes and permeable membranes, respectively. Cd accumulation in cells from the apical medium was decreased by low temperature and metabolic inhibitors. A saturable tendency was observed between initial Cd accumulation and increased concentrations of Cd in the apical medium at 37 degrees C, but not at 4 degrees C. Co-incubation with ZnCl2 or CuCl2 competitively decreased Cd accumulation at 37 degrees C. A decrease in the pH of the apical medium markedly decreased Cd accumulation. Pretreatment of cells with an inorganic anion-exchange inhibitor significantly decreased Cd uptake at pH 7.4 in the presence of bicarbonate, but only marginally in its absence. A decrease in the pH of the apical medium increased the secretory (basolateral-to-apical) transport of Cd, with a concomitant decrease in the cellular accumulation of Cd. Co-incubation with Cd and tetraethylammonium, a typical substrate of the organic cation transporter, decreased Cd transport, with a concomitant increase in cellular Cd accumulation. The uptake and secretion of Cd across the apical membrane appear to be partly mediated via an inorganic anion exchanger and a H+ antiport of the organic cation transport system, respectively. Therefore, a decrease in pH of the apical medium markedly decreases Cd accumulation, possibly as a result of not only the decrease in Cd uptake via an inorganic anion exchanger, but also the increase in Cd secretion via the Cd2+/H+ antiport. Further evidence of the antiport was obtained from experiments using brush border membrane vesicles isolated from rat kidney and small intestine. In addition, passive diffusion of Cd appears to be decreased by low temperature and a decrease in pH.

Secretory transport of cadmium through intestinal brush border membrane via H(+)-antiport

The effect of pH on the secretory transport of Cd through the intestinal brush border membrane was investigated using isolated rat intestinal brush border membrane vesicles (BBMV) and the Caco-2 intestinal epithelial cell line. BBMV equilibrated at pH 5.5 or 7.5 (pH(in)) were mixed with an experimental buffer at pH 5.5 or 7.5 (pH(out)) containing CdCl(2). The initial accumulation of Cd in BBMV incubated for 1 or 3 min at pH(in) 5.5 and pH(out) 7.5 (outwardly directed H(+)-gradient) was significantly higher than that at pH(in)=pH(out)=7.5, but the equilibrated Cd accumulation incubated for 30 min was marginally lower. Carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP), a protonophore, diminished the increasing effect of the H(+)-gradient on the initial Cd accumulation. Caco-2 cell monolayers cultured on permeable membranes were incubated with CdCl(2) from the basolateral medium, and the transport of Cd from the basolateral to apical medium and the accumulation of Cd in the monolayers were measured. Cd transport was increased by lowering the pH of the apical medium, and was accompanied by a decrease in the Cd accumulation. Coincubation with CdCl(2) and tetraethylammonium, a typical substrate for H(+)-antiport of the renal organic cation transporter, from the basolateral medium slightly but significantly decreased the basolateral-to-apical transport of Cd, with a concomitant increase in the Cd accumulation. These findings suggest the secretory transport of Cd through the intestinal brush border membrane not only via passive diffusion but also via H(+)-antiport of the putative organic cation transporter.

Carrier-mediated uptake of cisplatin by the OK renal epithelial cell line

The purpose of this study was to investigate whether transport of cis-diamminedichloroplatinum II (cisplatin) across renal epithelial cell line OK cells is mediated by the organic cation transport system. OK cell monolayers cultured on permeable membranes were incubated with 100 microM cisplatin on the apical or basolateral side, and the cellular accumulation and the transport of cisplatin across the monolayer were measured. The accumulation from the basolateral medium and the basolateral-to-apical transport of cisplatin were higher than the accumulation from the apical medium and the apical-to-basolateral transport, respectively. The cell monolayers were incubated with different concentrations of cisplatin (0.02 approximately 3 mM) in the basolateral medium. The relationship between the cisplatin concentrations in the medium and in the cells revealed that cisplatin accumulation tended to be saturable. The basolateral-to-apical transport of cisplatin was increased when the pH of the apical medium was decreased, with a concomitant decrease in the accumulation of cisplatin. Coincubation of cisplatin with tetraethylammonium (TEA), a typical substrate for the organic cation transporter, significantly decreased the accumulation and transport of cisplatin from the basolateral medium. These results suggest that the uptake and basolateral-to-apical transport of cisplatin are mediated by not only simple diffusion but also by the organic cation transport system.