Enzymatic isolation and serum-free culture of human renal cells : retaining properties of proximal tubule cells

Role of plasma membrane dicarboxylate transporters in the uptake and toxicity of diglycolic acid, a metabolite of diethylene glycol, in human proximal tubule cells

Diethylene glycol (DEG) mass poisonings have resulted from ingestion of pharmaceuticals mistakenly adulterated with DEG, typically leading to proximal tubular necrosis and acute kidney injury. The metabolite, diglycolic acid (DGA) accumulates greatly in kidney tissue and its direct administration results in toxicity identical to that in DEG-treated rats. DGA is a dicarboxylic acid, similar in structure to metabolites like succinate. These studies have assessed the mechanism for cellular accumulation of DGA, specifically whether DGA is taken into primary cultures of human proximal tubule (HPT) cells via sodium dicarboxylate transporters (NaDC-1 or NaDC-3) like those responsible for succinate uptake. When HPT cells were cultured on membrane inserts, sodium dependent succinate uptake was observed from both apical and basolateral directions. Pretreatment with the NaDC-1 inhibitor N-(p-amylcinnamoyl)anthranilic acid (ACA) markedly reduced apical uptakes of both succinate and DGA. Basolateral uptake of both succinate and DGA were decreased similarly following combined treatment with ACA and the NaDC-3 inhibitor 2,3-dimethylsuccinate. When the cells were pre-treated with siRNA to knockdown NaDC-1 function, apical uptake of succinate and toxicity of apically applied DGA were reduced, while the reduction in basolateral succinate uptake and basolateral DGA toxicity was marginal with NaDC-3 knockdown. DGA reduced apical uptake of succinate, but not basolateral uptake. This study confirmed that primary HPT cells retain sodium dicarboxylate transport functionality and that DGA was taken up by these transporters. This study identified NaDC-1 as a likely and NaDC-3 as a possible molecular target to reduce uptake of this toxic metabolite by the kidney.

Bromate-induced Changes in p21 DNA Methylation and Histone Acetylation in Renal Cells

Bromate (BrO3-) is a water disinfection byproduct (DBP) previously shown to induce nephrotoxicity in vitro and in vivo. We recently showed that inhibitors of DNA methyltransferase 5-aza-2'-deoxycytidine (5-Aza) and histone deacetylase trichostatin A (TSA) increased BrO3- nephrotoxicity whereas altering the expression of the cyclin-dependent kinase inhibitor p21. Human embryonic kidney cells (HEK293) and normal rat kidney (NRK) cells were sub-chronically exposed to BrO3- or epigenetic inhibitors for 18 days, followed by 9 days of withdrawal. DNA methylation was studied using a modification of bisulfite amplicon sequencing called targeted gene bisulfite sequencing. Basal promoter methylation in the human p21 promoter region was substantially lower than that of the rat DNA. Furthermore, 5-Aza decreased DNA methylation in HEK293 cells at the sis-inducible element at 3 distinct CpG sites located at 691, 855, and 895 bp upstream of transcription start site (TSS). 5-Aza also decreased methylation at the rat p21 promoter about 250 bp upstream of the p21 TSS. In contrast, sub-chronic BrO3- exposure failed to alter methylation in human or rat renal cells. BrO3- exposure altered histone acetylation in NRK cells at the p21 TSS, but not in HEK293 cells. Interestingly, changes in DNA methylation induced by 5-Aza persisted after its removal; however, TSA- and BrO3--induced histone hyperacetylation returned to basal levels after 3 days of withdrawal. These data demonstrate novel sites within the p21 gene that are epigenetically regulated and further show that significant differences exist in the epigenetic landscape between rat and human p21, especially with regards to toxicant-induced changes in histone acetylation.

Evaluation of biomarkers for in vitro prediction of drug-induced nephrotoxicity: comparison of HK-2, immortalized human proximal tubule epithelial, and primary cultures of human proximal tubular cells

There has been intensive effort to identify in vivo biomarkers that can be used to monitor drug-induced kidney damage and identify injury before significant impairment occurs. Kidney injury molecule-1 (KIM-1), neutrophil gelatinase-associated lipocalin (NGAL), and human macrophage colony stimulating factor (M-CSF) have been validated as urinary and plasma clinical biomarkers predictive of acute and chronic kidney injury and disease. Similar validation of a high throughput in vitro assay predictive of nephrotoxicity could potentially be implemented early in drug discovery lead optimization to reduce attrition at later stages of drug development. To assess these known in vivo biomarkers for their potential for in vitro screening of drug-induced nephrotoxicity, we selected a panel of nephrotoxic agents and examined their effects on the overexpression of nephrotoxicity biomarkers in immortalized (HK-2) and primary (commercially available and freshly in-house produced) human renal proximal tubule epithelial cells. Traditional cytotoxicity was contrasted with expression levels of KIM-1, NGAL, and M-CSF assessed using ELISA and real-time quantitative reverse transcription PCR. Traditional cytotoxicity assays and biomarker assays using HK-2 cells were both unsuitable for prediction of nephrotoxicity. However, increases in protein levels of KIM-1 and NGAL in primary cells were well correlated with dose levels of known nephrotoxic compounds, with limited correlation seen in M-CSF protein and mRNA levels. These results suggest that profiling compounds against primary cells with monitoring of biomarker protein levels may have potential as in vitro predictive assays of drug-induced nephrotoxicity.

Cadherin expression, vectorial active transport, and metallothionein isoform 3 mediated EMT/MET responses in cultured primary and immortalized human proximal tubule cells

BACKGROUND

Cultures of human proximal tubule cells have been widely utilized to study the role of EMT in renal disease. The goal of this study was to define the role of growth media composition on classic EMT responses, define the expression of E- and N-cadherin, and define the functional epitope of MT-3 that mediates MET in HK-2 cells.

METHODS

Immunohistochemistry, microdissection, real-time PCR, western blotting, and ELISA were used to define the expression of E- and N-cadherin mRNA and protein in HK-2 and HPT cell cultures. Site-directed mutagenesis, stable transfection, measurement of transepithelial resistance and dome formation were used to define the unique amino acid sequence of MT-3 associated with MET in HK-2 cells.

RESULTS

It was shown that both E- and N-cadherin mRNA and protein are expressed in the human renal proximal tubule. It was shown, based on the pattern of cadherin expression, connexin expression, vectorial active transport, and transepithelial resistance, that the HK-2 cell line has already undergone many of the early features associated with EMT. It was shown that the unique, six amino acid, C-terminal sequence of MT-3 is required for MT-3 to induce MET in HK-2 cells.

CONCLUSIONS

The results show that the HK-2 cell line can be an effective model to study later stages in the conversion of the renal epithelial cell to a mesenchymal cell. The HK-2 cell line, transfected with MT-3, may be an effective model to study the process of MET. The study implicates the unique C-terminal sequence of MT-3 in the conversion of HK-2 cells to display an enhanced epithelial phenotype.

Aluminum citrate prevents renal injury from calcium oxalate crystal deposition

Calcium oxalate monohydrate crystals are responsible for the kidney injury associated with exposure to ethylene glycol or severe hyperoxaluria. Current treatment strategies target the formation of calcium oxalate but not its interaction with kidney tissue. Because aluminum citrate blocks calcium oxalate binding and toxicity in human kidney cells, it may provide a different therapeutic approach to calcium oxalate-induced injury. Here, we tested the effects of aluminum citrate and sodium citrate in a Wistar rat model of acute high-dose ethylene glycol exposure. Aluminum citrate, but not sodium citrate, attenuated increases in urea nitrogen, creatinine, and the ratio of kidney to body weight in ethylene glycol-treated rats. Compared with ethylene glycol alone, the addition of aluminum citrate significantly increased the urinary excretion of both crystalline calcium and crystalline oxalate and decreased the deposition of crystals in renal tissue. In vitro, aluminum citrate interacted directly with oxalate crystals to inhibit their uptake by proximal tubule cells. These results suggest that treating with aluminum citrate attenuates renal injury in rats with severe ethylene glycol toxicity, apparently by inhibiting calcium oxalate's interaction with, and retention by, the kidney epithelium.

Acute ethanol exposure inhibits renal folate transport, but repeated exposure upregulates folate transport proteins in rats and human cells

Deficiency of folate in heavy-drinking alcoholic populations can occur partly because of an increased urinary folate excretion. Ethanol directly reduces the reabsorption of folate in the renal proximal tubule (PT) by acting on either of 2 folate transport proteins, the reduced folate carrier (RFC) and the folate receptor (FR). This study was designed to determine the effects of ethanol on the transport of folate by PT cells and to examine the effects of ethanol on RFC and the FR protein expression. Normal human PT (HPT) cells were cultured on membrane inserts to study intracellular transport of 5-methyltetrahydrofolate from the apical or basolateral direction in the presence of ethanol [11-109 mmol/L (50-500 mg/dL)]. The long-term effect of ethanol on the renal folate transport protein content was determined by western blot in treated HPT cells and in vivo in rats pair-fed control diets or ethanol-containing liquid diets. A 1-h treatment of HPT cells with ethanol (> or = 65 mmol/L) reduced the apically directed transport of folate by 20-25% without affecting the basolateral transport. A 5-d exposure of HPT cells to ethanol dose-dependently increased the content of both the FR and RFC proteins, with a greater effect on the RFC. Similarly, a 14-d exposure of rats to ethanol increased the in vivo expression of both the RFC and FR. These studies demonstrate that ethanol decreases the reabsorptive transport of folate by renal PT cells, which would increase urinary folate excretion. In contrast, subchronic exposure of PT cells, both in vivo and in vitro, to folate-depleting concentrations of ethanol leads to an upregulation of the 2 folate transport proteins. The increase in folate transporters partly counteracts the inhibitory effects of ethanol on folate transport activity, which explains the lower magnitude of ethanol's effect on transport with subchronic exposure compared with that with acute exposure.

The antimicrobial peptide cathelicidin protects the urinary tract against invasive bacterial infection

The urinary tract functions in close proximity to the outside environment, yet must remain free of microbial colonization to avoid disease. The mechanisms for establishing an antimicrobial barrier in this area are not completely understood. Here, we describe the production and function of the cathelicidin antimicrobial peptides LL-37, its precursor hCAP-18 and its ortholog CRAMP in epithelial cells of human and mouse urinary tract, respectively. Bacterial contact with epithelial cells resulted in rapid production and secretion of the respective peptides, and in humans LL-37/hCAP-18 was released into urine. Epithelium-derived cathelicidin substantially contributed to the protection of the urinary tract against infection, as shown using CRAMP-deficient and neutrophil-depleted mice. In addition, clinical E. coli strains that were more resistant to LL-37 caused more severe urinary tract infections than did susceptible strains. Thus, cathelicidin seems to be a key factor in mucosal immunity of the urinary tract.

Roles of necrosis, Apoptosis, and mitochondrial dysfunction in S-(1,2-dichlorovinyl)-L-cysteine sulfoxide-induced cytotoxicity in primary cultures of human renal proximal tubular cells

S-(1,2-Dichlorovinyl)-L-cysteine (DCVC) is the penultimate nephrotoxic metabolite of the environmental contaminant trichloroethylene. Although metabolism of DCVC by the cysteine conjugate beta-lyase is the most studied bioactivation pathway, DCVC may also be metabolized by the flavin-containing monooxygenase (FMO) to yield DCVC sulfoxide (DCVCS). Renal cellular injury induced by DCVCS was investigated in primary cultures of human proximal tubular (hPT) cells by assessment of time- and concentration-dependent effects on cellular morphology, acute cellular necrosis, apoptosis, mitochondrial function, and cellular glutathione (GSH) status. Confluent hPT cells incubated with as little as 10 microM DCVCS for 24 h exhibited morphological changes, although at least 100 microM DCVCS was required to produce marked changes. Acute cellular necrosis did not occur until 48 h with at least 200 microM DCVCS, indicating that this is a high-dose, late response. The extent of necrosis was similar to that with DCVC. In contrast, apoptosis occurred as early as 1 h with as little as 10 microM DCVCS and the extent of apoptosis was much less than that with DCVC. Mitochondrial function was maintained with DCVCS concentrations up to 100 microM, consistent with hPT cells only being competent to undergo apoptosis at early time points and relatively low concentrations. Marked depletion (>50%) of cellular GSH content was only observed with 500 microM DCVCS. These results, combined with previous studies showing protection from DCVC-induced necrosis and apoptosis by the FMO inhibitor methimazole, suggest that formation of DCVCS plays a significant role in trichloroethylene-induced renal cellular injury in hPT cells.

Activation of tubular epithelial cells in diabetic nephropathy

Previous studies have shown that renal function in type 2 diabetes correlates better with tubular changes than with glomerular pathology. Since advanced glycation end products (AGEs; AGE-albumin) and in particular carboxymethyllysine (CML) are known to play a central role in diabetic nephropathy, we studied the activation of nuclear factor kappaB (NF-kappaB) in tubular epithelial cells in vivo and in vitro by AGE-albumin and CML. Urine samples from healthy control subjects (n = 50) and type 2 diabetic patients (n = 100) were collected and tested for excretion of CML and the presence of proximal tubular epithelial cells (pTECs). CML excretion was significantly higher in diabetic patients than in healthy control subjects (P < 0.0001) and correlated with the degree of albuminuria (r = 0.7, P < 0.0001), while there was no correlation between CML excretion and HbA(1c) (r = 0.03, P = 0.76). Urine sediments from 20 of 100 patients contained pTECs, evidenced by cytokeratin 18 positivity, while healthy control subjects (n = 50) showed none (P < 0.0001). Activated NF-kappaB could be detected in the nuclear region of excreted pTECs in 8 of 20 patients with pTECs in the urine sediment (40%). Five of eight NF-kappaBp65 antigen-positive cells stained positive for interleukin-6 (IL-6) antigen (62%), while only one of the NF-kappaB-negative cells showed IL-6 positivity. pTECs in the urine sediment correlated positively with albuminuria (r = 0.57, P < 0.0001) and CML excretion (r = 0.55, P < 0.0001). Immunohistochemistry in diabetic rat kidneys and a human diabetic kidney confirmed strong expression of NF-kappaB in tubular cells. To further prove an AGE/CML-induced NF-kappaB activation in pTECs, NF-kappaB activation was studied in cultured human pTECs by electrophoretic mobility shift assays (EMSAs) and Western blot. Stimulation of NF-kappaB binding activity was dose dependent and was one-half maximal at 250 nmol/l AGE-albumin or CML and time dependent at a maximum of activation after 4 days. Functional relevance of the observed NF-kappaB activation was demonstrated in pTECs transfected with a NF-kappaB-driven luciferase reporter plasmid and was associated with an increased release of IL-6 into the supernatant. The AGE- and CML-dependent activation of NF-kappaBp65 and NF-kappaB-dependent IL-6 expression could be inhibited using the soluble form of the receptor for AGEs (RAGE) (soluble RAGE [sRAGE]), RAGE-specific antibody, or the antioxidant thioctic acid. In addition transcriptional activity and IL-6 release from transfected cells could be inhibited by overexpression of the NF-kappaB-specific inhibitor kappaBalpha. The findings that excreted pTECs demonstrate activated NF-kappaB and IL-6 antigen and that AGE-albumin and CML lead to a perpetuated activation of NF-kappaB in vitro infer that a perpetuated increase in proinflammtory gene products, such as IL-6, plays a role in damaging the renal tubule.

Expression of MT-3 mRNA in human kidney, proximal tubule cell cultures, and renal cell carcinoma

The human metallothionein 3 (MT-3) gene has recently been identified and characterized as a brain-specific MT having growth inhibitory activity for neuronal cells. One objective of the present study was to determine if MT-3 is brain-specific or also present in the renal system, a site for chronic toxicity due to heavy metal exposure. Using RT-PCR methodology, MT-3 mRNA was shown to be expressed in the human renal system at levels below mRNA for the beta-actin gene. MT-3 mRNA was shown to be expressed in all samples obtained from both the developing and adult renal systems, from 20 weeks of fetal age to 72 years. Cultures of human proximal tubule (HPT) cells were used to determine if MT-3 mRNA expression is influenced by metal exposure. Exposure of HPT cells to either Zn2+ or Cd2+ resulted in an early (within 24 h), but unsustained increase in MT-3 mRNA. The demonstration of MT-3 mRNA expression in the kidney indicates that MT-3 may play an important early role in the response of the cell to metal exposure. MT-3 mRNA expression was also examined in tissues and cells from three cases of renal cell carcinoma. MT-3 was found to be expressed in all three cases at levels similar to those found for normal kidney, providing evidence that MT-3 mRNA expression is not altered in this cancer.