An electrophysiological freeze fracture assessment of cadmium nephrotoxicity in vitro

ZIP8 expression in human proximal tubule cells, human urothelial cells transformed by Cd+2 and As+3 and in specimens of normal human urothelium and urothelial cancer

Background

ZIP8 functions endogenously as a Zn⁺²/HCO₃^- symporter that can also bring cadmium (Cd⁺²) into the cell. It has also been proposed that ZIP8 participates in Cd-induced testicular necrosis and renal disease. In this study real-time PCR, western analysis, immunostaining and fluorescent localization were used to define the expression of ZIP8 in human kidney, cultured human proximal tubule (HPT) cells, normal and malignant human urothelium and Cd⁺² and arsenite (As⁺³) transformed urothelial cells.

Results

It was shown that in the renal system both the non-glycosylated and glycosylated form of ZIP8 was expressed in the proximal tubule cells with localization of ZIP8 to the cytoplasm and cell membrane; findings in line with previous studies on ZIP8. The studies in the bladder were the first to show that ZIP8 was expressed in normal urothelium and that ZIP8 could be localized to the paranuclear region. Studies in the UROtsa cell line confirmed a paranuclear localization of ZIP8, however addition of growth medium to the cells increased the expression of the protein in the UROtsa cells. In archival human samples of the normal urothelium, the expression of ZIP8 was variable in intensity whereas in urothelial cancers ZIP8 was expressed in 13 of 14 samples, with one high grade invasive urothelial cancer showing no expression. The expression of ZIP8 was similar in the Cd⁺² and As⁺³ transformed UROtsa cell lines and their tumor transplants.

Conclusion

This is the first study which shows that ZIP8 is expressed in the normal urothelium and in bladder cancer. In addition the normal UROtsa cell line and its transformed counterparts show similar expression of ZIP8 compared to the normal urothelium and the urothelial cancers suggesting that the UROtsa cell line could serve as a model system to study the expression of ZIP8 in bladder disease.

Cadmium and transport of ions and substances across cell membranes and epithelia

Toxic metals such as cadmium (Cd(2+)) pose serious risks to human health. However, even though the importance of Cd(2+) as environmental health hazards is now widely appreciated, the specific mechanisms by which it produces its adverse effects have yet to be fully elucidated. Cd(2+) is known to enter cells, it binds and interacts with a multitude of molecules, it may indirectly induce oxidative stress and interfere with gene expression and repair of DNA. It also interacts with transport across cell membranes and epithelia and may therefore disturb the cell's homeostasis and function. Interaction with epithelial transport, especially in the kidney and the liver, may have serious consequences in general health. A lot of research still needs to be done to understand the exact way in which Cd(2+) interferes with these transport phenomena. It is not always clear whether Cd(2+) has primary or secondary effects on cell membrane transport. In the present review we try to summarize the work that has been done up to now and to critically discuss the relevance of the experimental work in vitro with respect to the in vivo situation.

The unique N-terminal sequence of metallothionein-3 is required to regulate the choice between apoptotic or necrotic cell death of human proximal tubule cells exposed to Cd+2

This laboratory has shown that MT-3 expression determines the choice between apoptotic or necrotic cell death in Cd(+2)-exposed human proximal tubule cells. Human proximal tubule cells that express MT-3 undergo necrosis when exposed to Cd(+2), while cells that have no basal expression of MT-3 undergo apoptotic cell death. It was also shown that cells which express MT-3 were more sensitive to Cd(+2)-induced cell death than those having no basal expression. In the present study, site directed mutagenesis was used to determine if the unique N-terminal sequence of MT-3 was required for these activities regarding toxicity and cell death. The results demonstrated that HK-2 cells stably transfected with MT-3 that had been modified by converting the 2 prolines at amino acid positions 7 and 9 to threonines was no longer active in promoting necrotic cell death at lower levels of Cd(+2) exposure. This was shown in comparison to cells containing the wild type MT-3 sequence and blank vector controls as regards the % of DAPI-stained fragmented nuclei, DNA laddering, LDH release, caspase-9, and caspase-3 activation. This study demonstrates that the unique N-terminal sequence of MT-3 is required to elicit an effect on the mechanism of Cd(+2)-induced death of the proximal tubule cell. This is the identical sequence that has been shown to be responsible for the growth inhibitory activity of MT-3 in the neural system.

ClC-2 chloride secretion mediates prostaglandin-induced recovery of barrier function in ischemia-injured porcine ileum

BACKGROUND & AIMS

Ischemia results in the breakdown of the intestinal barrier, predisposing patients to sepsis and multiple organ failure. Prostaglandins play a critical role in mediating recovery of barrier function in ischemia-injured intestine through a mechanism involving stimulation of Cl - secretion. In the present study, we investigated the contributory role of individual Cl - channels in the recovery of barrier function in ischemia-injured porcine ileum.

METHODS

Ischemia-injured porcine ileal mucosa was mounted in Ussing chambers. Short-circuit current (Isc) and transepithelial resistance (TER) were measured in response to prostaglandin E 2 (PGE 2 ) and pharmacologic inhibitors of epithelial Cl - channels. Immunoassays were used to assess the expression and localization of ion channels.

RESULTS

Application of PGE 2 to ischemia-injured ileal mucosa stimulated increases in Isc, an indicator of Cl - secretion, that was followed by marked increases in TER, an indicator of barrier function recovery. In vitro studies revealed that although PGE 2 induced Cl - secretion via at least 3 distinct secretory pathways, recovery of barrier function was initiated by Cl - secretion via ClC-2 Cl - channels co-expressed with occludin and localized to tight junctions within restituting epithelium. Intravenous administration of furosemide to pigs subjected to 1 hour of ileal ischemia impaired recovery of barrier function, as evidenced by decreased TER and increased mucosal-to-serosal 3 H-mannitol flux after a 2-hour reperfusion/recovery period, confirming an important role for Cl - secretory pathways in vivo.

CONCLUSIONS

ClC-2-mediated intestinal Cl - secretion restores TER in ischemia-injured intestine. These data may provide the basis for targeted pharmacologic therapy for diseases associated with impaired barrier function.

Effects of cadmium chloride on the paracellular barrier function of intestinal epithelial cell lines

In the present study we characterized the functional and structural disruption of the paracellular barrier of intestinal epithelium in vitro in relation to cytotoxicity after apical Cd2+ exposure. For that purpose filter-grown Caco-2 and IEC-18 cells were apically exposed to 5 to 100 microM CdCl2 for 4 or 14 h. It was found that the effects of Cd2+ on the epithelial barrier were concentration- and time-dependent. The first detected effects of Cd2+ in Caco-2 cells after 4 h exposure were a decrease in transepithelial electrical resistance, increased permeabilities of mannitol and PEG-4000, and changes in intercellular localization of ZO-1, occludin, and e-cadherin. The effects were far more pronounced after prolonged exposure. The disruption of the paracellular barrier by 5 to 30 microM Cd2+ was detected without a significant loss of viability of the Caco-2 cells. In the IEC-18 cells, Cd2+ concentrations affecting the barrier (50 and 100 microM) also affected cell viability. In both cell lines the effects on the cell layers continued to develop after removal of extracellular Cd2+. This correlated with the cellular retention of Cd2+, which was high for the 12 h following 4 h accumulation. This study showed that the decreased epithelial barrier function of intestinal epithelial cells is accompanied by tight junction disruption. It is concluded that Cd2+ causes increased paracellular permeability by disruption of junctional function and structure. The initial junctional effects of Cd2+ suggest that Cd2+ increases its own bioavailability by causing disruption of the intestinal paracellular barrier.

Heterogeneity in the amount of ionic cadmium necessary to elicit cell death in independent cultures of human proximal tubule cells

Eleven separate isolates of human renal proximal tubule cells (HPT) were analyzed for toxic response to ionic cadmium exposure over a 16-day period in vitro. This study demonstrates a heterogeneous response to Cd2+ exposure in isolates from different individuals with some individuals nearly 3-times more sensitive to ionic cadmium exposure than others. There was no apparent correlation to the race, sex or age of the individuals in the response to cadmium. In addition, readily identifiable culture artifacts, i.e., culture age, passage number, had no influence on the response to Cd2+ exposure and the different isolates had homogeneous baseline HPT properties and morphology. This difference in response to Cd2+ may reflect a heterogeneous response within the human population to cadmium exposure.

Aminoglycoside antibiotics alter the electrogenic transport properties of cultured human proximal tubule cells

Monolayers of human proximal tubule (HPT) cells, when grown on permeable supports and mounted in Ussing chambers, spontaneously display a transepithelial potential difference (PD) and short-circuit current (Isc). These electrical parameters were used in the present study to determine if aminoglycoside exposure altered electrogenic sodium transport by HPT cells. The results of this determination demonstrated that exposure to gentamicin, at levels below that producing cell necrosis, caused a marked reduction in Isc and that this reduction followed the known in vivo nephrotoxicities of the aminoglycosides streptomycin, gentamicin, and neomycin. It was concluded through a similar analysis on a total of 14 isolates of HPT cells that the aminoglycosides repeatably reduced the electrogenic sodium transport of HPT cells. It was further determined that this alteration in electrogenic transport by gentamicin was mediated through exposure of the drug to the basolateral cell surface and that apical exposure had little effect. Evidence was obtained against the involvement of Na+, K(+)-ATPase, adenosine 3',5'-cyclic monophosphate, and sodium-coupled substrate transport in this alteration in electrogenic transport by the aminoglycosides. The basolaterally located Na+: CO3(-2):HCO3(-1) symporter is a possible site for aminoglycoside-induced nephrotoxicity.

Renal proximal tubular cells in suspension or in primary culture as in vitro models to study nephrotoxicity

The kidney forms a frequent target for xenobiotic toxicity. The complex biochemical mechanisms underlying nephrotoxicity are best studied in vitro provided that reliable and relevant in vitro models are available. Since most nephrotoxicants affect primarily the cells of the proximal tubules (PTC), much effort has been directed towards the development of in vitro models of PTC. This review focuses on the preparation of PTC and the use of these cells. Discussed are important criteria such as the viability (survival time) of the cells and the parameters to assess toxicity. Recent studies have shown that isolated PTC in suspension are especially suitable for studies on the biochemical mechanisms of 'acute' nephrotoxicity, whereas PTC in primary culture may be used to investigate mechanisms of nephrotoxic damage at very low concentrations, upon prolonged exposure. PTC cultured on porous filter membranes provide new possibilities to study toxicity in relation to cell and transport polarity. Primary cell cultures of human PTC have been set up. Although a further characterization of these systems is needed, recent data indicate their usefulness.