Cadmium induces Wnt signaling to upregulate proliferation and survival genes in sub-confluent kidney proximal tubule cells

Background

The class 1 carcinogen cadmium (Cd²⁺) disrupts the E-cadherin/β-catenin complex of epithelial adherens junctions (AJs) and causes renal cancer. Deregulation of E-cadherin adhesion and changes in Wnt/β-catenin signaling are known to contribute to carcinogenesis.

Results

We investigated Wnt signaling after Cd²⁺-induced E-cadherin disruption in sub-confluent cultured kidney proximal tubule cells (PTC). Cd²⁺ (25 μM, 3-9 h) caused nuclear translocation of β-catenin and triggered a Wnt response measured by TOPflash reporter assays. Cd²⁺ reduced the interaction of β-catenin with AJ components (E-cadherin, α-catenin) and increased binding to the transcription factor TCF4 of the Wnt pathway, which was upregulated and translocated to the nucleus. While Wnt target genes (c-Myc, cyclin D1 and ABCB1) were up-regulated by Cd²⁺, electromobility shift assays showed increased TCF4 binding to cyclin D1 and ABCB1 promoter sequences with Cd²⁺. Overexpression of wild-type and mutant TCF4 confirmed Cd²⁺-induced Wnt signaling. Wnt signaling elicited by Cd²⁺ was not observed in confluent non-proliferating cells, which showed increased E-cadherin expression. Overexpression of E-cadherin reduced Wnt signaling, PTC proliferation and Cd²⁺ toxicity. Cd²⁺ also induced reactive oxygen species dependent expression of the pro-apoptotic ER stress marker and Wnt suppressor CHOP/GADD153 which, however, did not abolish Wnt response and cell viability.

Conclusions

Cd²⁺ induces Wnt signaling in PTC. Hence, Cd²⁺ may facilitate carcinogenesis of PTC by promoting Wnt pathway-mediated proliferation and survival of pre-neoplastic cells.

Cellular distribution and subcellular localization of mCLCA1/2 in murine gastrointestinal epithelia

mCLCA1/2 are members of the CLCA protein family that are widely expressed in secretory epithelia, but their putative physiological role still awaits elucidation. mCLCA1/2 have 95% amino acid identity, but currently no specific antibody is available. We have generated a rabbit polyclonal antibody (pAb849) against aa 424-443 of mCLCA1/2. In HEK293 cells transfected with mCLCA1; pAb849 detected two specific protein bands at approximately 125 kDa and 90 kDa, representing full-length precursor and N-terminal cleavage product, respectively. pAb849 also immunoprecipitated mCLCA1 and labeled the protein by immunostaining. But pAb849 crossreacted with mCLCA3/4/6 despite < or =80% amino acid identity of the antigenic epitope. We therefore investigated the cellular localization of mCLCA1/2 in epithelial tissues, which do not express mCLCA3/4/6 (salivary glands, pancreas, kidney) or express mCLCA3/6 with known localization (mucus cells of stomach and small intestine; villi of small intestine). mCLCA1/2 mRNA and protein expression were found in both parotid and submandibular gland, and immunohistochemistry revealed labeling in parotid acinar cells, in the luminal membrane of parotid duct cells, and in the duct cells of submandibular gland. In exocrine pancreas, mCLCA1/2 expression was restricted to acinar zymogen granule membranes, as assessed by immunoblotting, immunohistochemistry, and preembedding immunoperoxidase and immunogold electron microscopy. Moreover, mCLCA1/2 immunolabeling was present in luminal membranes of gastric parietal cells and small intestinal crypt enterocytes, whereas in the kidney, mCLCA1/2 protein was localized to proximal and distal tubules. The apical membrane localization and overall distribution pattern of mCLCA1/2 favor a transmembrane protein implicated in transepithelial ion transport and protein secretion.

Cadmium-induced DNA damage triggers G(2)/M arrest via chk1/2 and cdc2 in p53-deficient kidney proximal tubule cells

Carcinogenesis is a multistep process that is frequently associated with p53 inactivation. The class 1 carcinogen cadmium (Cd(2+)) causes renal cancer and is known to inactivate p53. G(2)/mitosis (M) arrest contributes to stabilization of p53-deficient mutated cells, but its role and regulation in Cd(2+)-exposed p53-deficient renal cells are unknown. In p53-inactivated kidney proximal tubule (PT) cells, comet assay experiments showed that Cd(2+) (50-100 microM) induced DNA damage within 1-6 h. This was associated with peak formation of reactive oxygen species (ROS) at 1-3 h, measured with dihydrorhodamine 123, and G(2)/M cell cycle arrest at 6 h, which were abolished by the antioxidant alpha-tocopherol (100 microM). Cd(2+)-induced G(2)/M arrest was enhanced approximately twofold on release from cell synchronization (double thymidine block or nocodazole) and resulted in approximately twofold increase of apoptosis, indicating that G(2)/M arrest mirrors DNA damage and toxicity. The Chk1/2 kinase inhibitor UCN-01 (0.3 microM), which relieves G(2)/M transition block, abolished Cd(2+)-induced G(2) arrest and increased apoptosis. This was accompanied by prevention of Cd(2+)-induced cyclin-dependent kinase cdc2 phosphorylation at tyrosine 15, as shown by immunofluorescence microscopy and immunoblotting. The data indicate that in p53-inactivated PT cells Cd(2+)-induced ROS formation and DNA damage trigger signaling of checkpoint activating kinases ataxia telangiectasia-mutated kinase (ATM) and ataxia telangiectasia and Rad3-related kinase (ATR) to cause G(2)/M arrest. This may promote survival of premalignant PT cells and Cd(2+) carcinogenesis.

Cadmium and cellular signaling cascades: to be or not to be?

The cellular effects of the toxic metal cadmium (Cd) are manifold. A large proportion of the cellular reactions affected by ionic Cd(2+) are mediated by cellular signaling cascades. The aim of this review is to provide a principal understanding of the known physiological signaling cascades, which are recruited by Cd(2+), and to highlight the fact that Cd(2+), similarly to other toxic metals, disrupts physiological signal transduction. In principle, second messengers are generated at the time of receptor activation, are short-lived, and act specifically in space and time through non-covalent binding on effectors to transiently alter their activity. Signaling dysregulation induced by Cd(2+) is reflected by a permanent disruption of transducing modules, resulting in low and/or elevated and constant levels of second messengers, which overwhelm the control mechanisms of signaling. This disturbs physiological cellular functions, gene transcription and regulation and may result in cell death and/or stress-induced adaptation and survival as well as carcinogenesis. The impact of Cd(2+) on Ca(2+)-, cAMP-, NO-, ROS-, MAP-kinase-, PKB/Akt-, nuclear factor-kappa B-, and developmental signaling is critically discussed. The hierarchical as well as cooperative and integrative character of signaling cascades activated by Cd(2+) is illustrated in the kidney proximal tubule, a major target of Cd(2+) toxicity. This review also aspires to pinpoint new avenues of research that may contribute to a more differentiated view of the complex mechanisms underlying Cd(2+) toxicity in target tissues and eventually lead to rationales and strategies for prevention and therapy of Cd(2+) toxicity.

Organic cation transporters OCT1, 2, and 3 mediate high-affinity transport of the mutagenic vital dye ethidium in the kidney proximal tubule

The positively charged fluorescent dyes ethidium (Et+) and propidium (Pr2+) are widely used as DNA and necrosis markers. Et+is cytotoxic and mutagenic. The polyspecific organic cation transporters OCT1 (SLC22A1), OCT2 (SLC22A2), and OCT3 (SLC22A3) mediate electrogenic facilitated diffusion of small (≤500 Da) organic cations with broad specificities. In humans, OCT2 mediates basolateral uptake by kidney proximal tubules (PT), whereas in rodents OCT1/2 are involved. In mouse kidney, perfused Et+accumulated predominantly in the S2/S3 segments of the PT, but not Pr2+. In cells stably overexpressing human OCTs (hOCTs), Et+uptake was observed with Kmvalues of 0.8 ± 0.2 μM (hOCT1), 1.7 ± 0.5 μM (hOCT2), and 2.0 ± 0.5 μM (hOCT3), whereas Pr2+was not transported. Accumulation of Et+was inhibited by OCT substrates quinine, 3-methyl-4-phenylpyridinium (MPP+), cimetidine, and tetraethylammonium (TEA+). For hOCT1 and hOCT2, the IC50values for MPP+, TEA+, and cimetidine were higher than for inhibition of previously tested transported substrates. For hOCT2, the inhibition of Et+uptake by MPP+and cimetidine was shown to be competitive. Et+also inhibited transport of 0.1 μM [3H]MPP+by all hOCT isoforms with IC50values between 0.4 and 1.3 μM, and the inhibition of hOCT1-mediated uptake of MPP+by Et+was competitive. In Oct1/2−/−mice, Et+uptake in the PT was almost abolished. The data demonstrate that Et+is taken up avidly by the PT, which is mediated by OCT1 and/or OCT2. Considering the high affinity of OCTs for Et+and their strong expression in various organs, strict safety guidelines for Et+handling should be reinforced.

Proteomic approach to identify candidate effector molecules during the in vitro immune exclusion of infective Teladorsagia circumcincta in the abomasum of sheep

In the present study we have employed an in vitro organ challenge model to study the post-challenge responses in parasite naïve and immune gastric tissue of sheep, in an attempt to identify the host derived factors involved in immune exclusion of Teladorsagia circumcincta larvae. Proteins present in the epithelial cells and mucus from ovine abomasa following parasite challenge in previously naïve and immune animals were analysed through Matrix Assisted Laser Desorption/Ionization-Time of Flight (MALDI-Tof)-MS and shotgun proteomics. MALDI-ToF analysis of epithelial cell lysates revealed that a number of proteins identified were differentially expressed in naïve and immune cells. These included intelectin and lysozymes, which were present at higher levels in epithelial cell lysates derived from immune samples. A large number of proteins were identified in the mucosal wash from immune tissue which were not present in the mucosal wash of the naïve tissue. Some of these proteins were present in washes of immune tissue prior to the parasite challenge including immunoglobulin A, galectin 14 and 15 and sheep mast cell protease 1. However, other proteins, such as calcium activated chloride channel and intelectin were only detected in the washings from the challenged tissue. The latter may be related to an enhanced mucus release, which may result in entrapment of infective larvae and thus reduced establishment in tissue that has been previously challenged with the parasite. In conclusion, several proteins have been identified which may be involved, either directly or indirectly, in the exclusion and immune elimination of incoming infective larvae. In the present study, the usefulness of the in vitro model has been confirmed, and the global proteomic approach has identified proteins that had not previously been associated with parasite exclusion from abomasal mucosa, such as the calcium activated chloride channel.

Evidence for KCNQ1 K+ channel expression in rat zymogen granule membranes and involvement in cholecystokinin-induced pancreatic acinar secretion

Secretion of enzymes and fluid induced by Ca(2+) in pancreatic acini is not completely understood and may involve activation of ion conductive pathways in zymogen granule (ZG) membranes. We hypothesized that a chromanol 293B-sensitive K(+) conductance carried by a KCNQ1 protein is expressed in ZG membranes (ZGM). In suspensions of rat pancreatic ZG, ion flux was determined by ionophore-induced osmotic lysis of ZG suspended in isotonic salts. The KCNQ1 blocker 293B selectively blocked K(+) permeability (IC(50) of approximately 10 microM). After incorporation of ZGM into planar bilayer membranes, cation channels were detected in 645/150 mM potassium gluconate cis/trans solutions. Channels had linear current-voltage relationships, a reversal potential (E(rev)) of -20.9 +/- 0.9 mV, and a single-channel K(+) conductance (g(K)) of 265.8 +/- 44.0 pS (n = 39). Replacement of cis 500 mM K(+) by 500 mM Na(+) shifted E(rev) to -2.4 +/- 3.6 mV (n = 3), indicating K(+) selectivity. Single-channel analysis identified several K(+) channel groups with distinct channel behaviors. K(+) channels with a g(K) of 651.8 +/- 88.0 pS, E(rev) of -22.9 +/- 2.2 mV, and open probability (P(open)) of 0.43 +/- 0.06 at 0 mV (n = 6) and channels with a g(K) of 155.0 +/- 11.4 pS, E(rev) of -18.3 +/- 1.8 mV, and P(open) of 0.80 +/- 0.03 at 0 mV (n = 3) were inhibited by 100 microM 293B or by the more selective inhibitor HMR-1556 but not by the maxi-Ca(2+)-activated K(+) channel (BK channel) inhibitor charybdotoxin (5 nM). KCNQ1 protein was demonstrated by immunoperoxidase labeling of pancreatic tissue, immunogold labeling of ZG, and immunoblotting of ZGM. 293B also inhibited cholecystokinin-induced amylase secretion of permeabilized acini (IC(50) of approximately 10 microM). Thus KCNQ1 may account for ZG K(+) conductance and contribute to pancreatic hormone-stimulated enzyme and fluid secretion.

Adaptive redistribution of NBCe1-A and NBCe1-B in rat kidney proximal tubule and striated ducts of salivary glands during acid-base disturbances

The cellular distribution of the NH2-terminal electrogenic Na+-HCO3(-) cotransporter (NBCe1) variants NBCe1-A and NBCe1-B has been investigated in rat kidney and submandibular gland (SMG) under physiological conditions and after systemic acid-base perturbations. Moreover, the in vivo data were complemented in vitro by using an immortalized cell line derived from the S1 segment of the proximal tubule (PT) of normotensive Wistar-Kyoto rats (WKPT-0293 Cl.2). NBCe1-A was basolaterally localized in PT cells, whereas NBCe1-B exhibited intracellular and basolateral distribution. SMG showed transcript and protein expression for NBCe1-A and NBCe1-B. NBCe1-B was basolaterally localized in duct cells; NBCe1-A was found intracellularly in salivary striated ducts and apically in main duct cells. Acute metabolic acidosis significantly increased cells that showed basolateral NBCe1-A in the PT, indicating increased HCO3(-) reabsorption, and significantly decreased cells that exhibited basolateral NBCe1-B in the salivary ducts, suggesting decreased HCO3(-) secretion. Chronic acidosis had no effect on NBCe1 distribution in PT but significantly increased the percentage of cells with basolateral NBCe1-A in salivary striated duct cells, suggesting increased HCO3(-) reabsorption. In contrast, chronic alkalosis caused adaptive redistribution of NBCe1-A and NBCe1-B in renal PT, favoring decreased HCO3(-) reabsorption. In vitro, WKPT-0293 Cl.2 cells expressed key acid-base transporters. Extracellular alkalosis downregulated NBCe1-A protein. WKPT-0293 Cl.2 cells are therefore a useful model to study renal acid-base regulation in vitro. The results propose redistribution of the transporters as a potential posttranslational regulation modus during acid-base disturbances. Moreover, the data demonstrate that renal PT and salivary duct epithelia respond to acid-base disturbances by an opposite redistribution pattern for NBCe1-A and NBCe1-B, reflecting specialized functions as the HCO3(-)-reabsorbing and HCO3(-)-secreting epithelium, respectively.

Cadmium-induced ceramide formation triggers calpain-dependent apoptosis in cultured kidney proximal tubule cells

A major target of cadmium (Cd2+) toxicity is the kidney proximal tubule (PT) cell. Cd2+-induced apoptosis of PT cells is mediated by sequential activation of calpains at 3–6 h and caspases-9 and -3 after 24-h exposure. Calpains also partly contribute to caspase activation, which emphasizes the importance of calpains for PT apoptosis by Cd2+. Upstream processes underlying Cd2+-induced calpain activation remain unclear. We describe for the first time that 10–50 μM Cd2+ causes a significant increase in ceramide formation by ∼22% (3 h) and ∼72% (24 h), as measured by diacylglycerol kinase assay. Inhibition of ceramide synthase with fumonisin B1 (3 μM) prevents ceramide formation at 3 h and abolishes calpain activation at 6 h, which is associated with significant attenuation of apoptosis at 3–6 h with Hoechst 33342 nuclear staining and/or 3(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2 H-tetrazolium bromide (MTT) death assays. This indicates that Cd2+ enhances de novo ceramide synthesis and that calpains are a downstream target of ceramides in apoptosis execution. Moreover, addition of C6-ceramide to PT cells increases cytosolic Ca2+ and activates calpains. Apoptosis mediated by C6-ceramide at 24 h is significantly reduced by caspase-3 inhibition, which supports cross talk between calpain- and caspase-dependent apoptotic pathways. We conclude that Cd2+-induced apoptosis of PT cells entails endogenous ceramide elevation and subsequent Ca2+-dependent calpain activation, which propagates kidney damage by Cd2+.

Knockdown of endosomal/lysosomal divalent metal transporter 1 by RNA interference prevents cadmium-metallothionein-1 cytotoxicity in renal proximal tubule cells

Chronic exposure to Cd2+ causes renal proximal tubular (PT) damage. Cd2+ reaches the PT mainly as cadmium-metallothionein 1 (CdMT-1) complexes that are filtered at the glomerulus and then internalized in part via endocytosis mediated by megalin and cubulin. Subsequently, Cd2+ is thought to be released in the cytosol to activate cell death pathways. The proton-coupled divalent metal transporter DMT1 also transports Cd2+ and is expressed exclusively in endosomes/lysosomes in rat PT cells. Using vector-based RNA interference with short-hairpin small-interfering RNAs (shRNAs) to downregulate DMT1 in the rat renal PT cell line WKPT-0293 Cl.2, we tested the hypothesis that endosomal/lysosomal DMT1 is involved in CdMT-1 nephrotoxicity. One out of 5 shRNAs tested (sh3) significantly reduced expression of DMT1 protein detected by immunoblotting and DMT1 mRNA as determined by RT-PCR by 45.1 +/- 9.6 and 36.9 +/- 14.4% (n = 5-6), respectively. Similarly, sh3 reduced perinuclear DMT1 immunostaining in transfected cells. Consistent with the assumed role of DMT1 in CdMT-1 cytotoxicity, sh3, but not the empty vector or sh5, significantly attenuated cell death induced by a 24-h exposure to 14.3 microM CdMT-1 by 35.6 +/- 4.2% (n = 13). In contrast, neither fluorescently labeled metallothionein-1 (MT-1) uptake nor free Cd2+ toxicity was altered by the effective DMT1 shRNA (sh3), indicating that cellular uptake of metal-MT-1 complexes and Cd2+-induced cell death signaling are not affected by DMT1 knockdown. Thus we conclude that endosomal/lysosomal DMT1 plays a role in renal PT CdMT-1 toxicity.

Highly Efficient Neural Differentiation of Human Somatic Stem Cells, Isolated by Minimally Invasive Periodontal Surgery

Neural stem cells (NSCs) are potential sources for cell therapy of neurodegenerative diseases and for drug screening. Despite their potential benefits, ethical and practical considerations limit the application of NSCs derived from human embryonic stem cells (ES) or adult brain tissue. Thus, alternative sources are required to satisfy the criteria of ready accessibility, rapid expansion in chemically defined media and reliable induction to a neuronal fate. We isolated somatic stem cells from the human periodontium that were collected during minimally invasive periodontal access flap surgery as part of guided tissue regeneration therapy. These cells could be propagated as neurospheres in serum-free medium, which underscores their cranial neural crest cell origin. Culture in the presence of epidermal growth factor (EGF) and fibroblast growth factor-2 (FGF-2) under serum-free conditions resulted in large numbers of nestin-positive/Sox-2-positive NSCs. These periodontium-derived (pd) NSCs are highly proliferative and migrate in response to chemokines that have been described as inducing NSC migration. We used immunocytochemical techniques and RT-PCR analysis to assess neural differentiation after treatment of the expanded cells with a novel induction medium. Adherence to substrate, growth factor deprivation, and retinoic acid treatment led to the acquisition of neuronal morphology and stable expression of markers of neuronal differentiation by more than 90% of the cells. Thus, our novel method might provide nearly limitless numbers of neuronal precursors from a readily accessible autologous adult human source, which could be used as a platform for further experimental studies and has potential therapeutic implications.

Cadmium induces nuclear translocation of beta-catenin and increases expression of c-myc and Abcb1a in kidney proximal tubule cells

Cadmium (Cd2+) induces renal proximal tubular (PT) damage, including disruption of the E-cadherin/beta-catenin complex of adherens junctions (AJs) and apoptosis. Yet, chronic Cd2+ exposure causes malignant transformation of renal cells. Previously, we have demonstrated that Cd(2+)-mediated up-regulation of the multidrug transporter Abcb1 causes apoptosis resistance in PT cells. We hypothesized that Cd2+ activates adaptive signaling mechanisms mediated by beta-catenin to evade apoptosis and increase proliferation. Here we show that 50 microM Cd2+, which induces cell death via apoptosis and necrosis, also causes a decrease of the trans-epithelial resistance of confluent WKPT-0293 Cl.2 cells, a rat renal PT cell model, within 45 min of Cd2+ exposure, as measured by electric cell-substrate impedance sensing. Immunofluorescence microscopy demonstrates Cd(2+)-induced decrease of E-cadherin at AJs and redistribution of beta-catenin from the E-cadherin/beta-catenin complex of AJs to cytosol and nuclei after 3 h. Immunoblotting confirms Cd(2+)-induced decrease of E-cadherin expression and translocation of beta-catenin to cytosol and nuclei of PT cells. RT-PCR shows Cd(2+)-induced increase of expression of c-myc and of the isoform Abcb1a at 3 h. The data prove for the first time that Cd2+ induces nuclear translocation of beta-catenin in PT cells. We speculate that Cd2+ activates beta-catenin/T-cell factor signaling to trans-activate proliferation and apoptosis resistance genes and promote carcinogenesis of PT cells.

Caspase-dependent and -independent pathways for cadmium-induced apoptosis in cultured kidney proximal tubule cells

The nephrotoxic metal cadmium at micromolar concentrations induces apoptosis of rat kidney proximal tubule (PT) cells within 3–6 h of exposure. The underlying cell death pathways remain poorly defined. Using Hoechst 33342/ethidium bromide nuclear staining and 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) cell death assays, 10–50 μM cadmium induced apoptosis of immortalized rat kidney cells derived from the S1-segment of PT at 6 and 24 h, but necrosis at 24 h only. Cadmium (10–50 μM) also caused mitochondrial cytochrome c (cyt. c)- and apoptosis-inducing factor release at 24 h, but not at 6 h, as measured by immunofluorescence imaging and immunoblotting. Caspases-9 and -3 were activated only by 10 μM cadmium for 24 h, and accordingly apoptosis was significantly reduced by the respective inhibitors (z-LEHD-fmk, z-DEVD-fmk; 10 μg/ml) at 24 h, but not at 6 h, without affecting necrosis. At 6 h, 10 μM cadmium increased the activity of the calcium-activated protease calpain, but not at 24 h, and calpain inhibitors (ALLN, PD 150606; 10–30 μM) blocked apoptosis by 10 μM cadmium at 3–6 h. However, PD-150606 also attenuated caspase-3 activity and apoptosis at 24 h, suggesting calpain-dependent caspase activation. Thus cadmium-induced apoptosis of PT cells involves a complex and sensitive interplay of signaling cascades involving mitochondrial proapoptotic factors, calpains and caspases, whose activation is also determined by cadmium concentration and the duration of cadmium exposure.

Divalent metal transporter 1 in the kidney proximal tubule is expressed in late endosomes/lysosomal membranes: implications for renal handling of protein-metal complexes

The H+-coupled polyligand transport protein divalent metal transporter 1 (DMT1) plays a key role in mammalian iron homeostasis. It has a widespread pattern of expression including tissues associated with iron acquisition and storage. Interestingly, it is also highly expressed in the kidney, yet its function in this tissue is unknown. The aim of this study was to determine the cellular location of DMT1 in proximal tubule cells as a first step to determining the role of this protein in the kidney. To do this we performed RT-PCR and immunostaining experiments using rat kidney and the S1 proximal tubule-derived WKPT-0293 Cl.2 cell line. RT-PCR revealed that mRNAs encoding all four DMT1 splice variants were present in RNA extracted from rat kidney cortex or WKPT-0293 Cl.2 cells. Immunostaining of rat kidney cortex or WKPT-0293 Cl.2 cells showed that DMT1 protein was expressed intracellularly and was not present in the plasma membrane. Expression of DMT1 partially colocalized with the late endosomal/lysosomal proteins LAMP1 and cathepsin-L. Using immunogold labeling, DMT1 was shown to be expressed in the membranes of late endosomes/lysosomes. Uptake of Alexa Fluor 546-transferrin was only observed following application to the apical membrane of WKPT-0293 Cl.2 cells. Within these cells, Alexa Fluor 546-transferrin colocalized with DMT1. In conclusion, renal proximal tubular cells express DMT1 in the membranes of organelles, including late endosomes/lysosomes, associated with processing of apically sequestered transferrin. These findings have implications for renal iron handling and possibly for the handling of nephrotoxic metals that are also DMT1 ligands, including Cd2+.

Megalin-Dependent Internalization of Cadmium-Metallothionein and Cytotoxicity in Cultured Renal Proximal Tubule Cells

Chronic cadmium (Cd2+) exposure results in renal proximal tubular cell damage. Delivery of Cd2+ to the kidney occurs mainly as complexes with metallothionein-1 (molecular mass approximately 7 kDa), freely filtered at the glomerulus. For Cd2+ to gain access to the proximal tubule cells, these complexes are thought to be internalized via receptors for small protein ligands, such as megalin and cubilin, followed by release of Cd2+ from metallothionein-1 in endosomal/lysosomal compartments. To investigate the role of megalin in renal cadmium-metallothionein-1 reabsorption, megalin expression and dependence of cadmium-metallothionein-1 internalization and cytotoxicity on megalin were studied in a renal proximal tubular cell model (WKPT-0293 Cl.2 cells). Expression of megalin was detected by reverse transcriptase-polymerase chain reaction and visualized by immunofluorescence both at the cell surface (live staining) and intracellularly (permeabilized cells). Internalization of Alexa Fluor 488-coupled metallothionein-1 was concentration-dependent, saturating at approximately 15 microM. At 14.3 microM, metallothionein-1 uptake could be significantly attenuated by 30.9 +/- 6.6% (n = 4) by 1 muM of the receptor-associated protein (RAP) used as a competitive inhibitor of cadmium-metallothionein-1 binding to megalin and cubilin. Consistently, cytotoxicity of a 24-h treatment with 7.14 muM cadmium-metallothionein-1 was significantly reduced by 41.0 +/- 7.6%, 61.6 +/- 3.4%, and 26.2 +/- 1.8% (n = 4-5 each) by the presence of 1 microM RAP, 400 microg/ml anti-megalin antibody, or 5 microM of the cubilin-specific ligand, apo-transferrin, respectively. Cubilin expression in proximal tubule cells was also confirmed at the mRNA and protein level. The data indicate that renal proximal tubular cadmium-metallothionein-1 uptake and cell death are mediated at least in part by megalin.

A role for mitochondrial aquaporins in cellular life-and-death decisions?

Mitochondria dominate the process of life-and-death decisions of the cell. Continuous generation of ATP is essential for cell sustenance, but, on the other hand, mitochondria play a central role in the orchestra of events that lead to apoptotic cell death. Changes of mitochondrial volume contribute to the modulation of physiological mitochondrial function, and several ion permeability pathways located in the inner mitochondrial membrane have been implicated in the mediation of physiological swelling-contraction reactions, such as the K+ cycle. However, the channels and transporters involved in these processes have not yet been identified. Osmotic swelling is also one of the fundamental characteristics exhibited by mitochondria in pathological situations, which activates downstream cascades, culminating in apoptosis. The permeability transition pore has long been postulated to be the primary mediator for water movement in mitochondrial swelling during cell death, but its molecular identity remains obscure. Inevitably, accumulating evidence shows that mitochondrial swelling induced by apoptotic stimuli can also occur independently of permeability transition pore activation. Recently, a novel mechanism for osmotic swelling of mitochondria has been described. Aquaporin-8 and -9 channels have been identified in the inner mitochondrial membrane of various tissues, including the kidney, liver, and brain, where they may mediate water transport associated with physiological volume changes, contribute to the transport of metabolic substrates, and/or participate in osmotic swelling induced by apoptotic stimuli. Hence, the recent discovery that aquaporins are expressed in mitochondria opens up new areas of investigation in health and disease.