A comparative study on the uptake of alpha-aminoisobutyric acid by normal and immortalized human embryonic kidney cells from proximal tubule

ZFP226 is a novel artificial transcription factor for selective activation of tumor suppressor KIBRA

KIBRA has been suggested as a key regulator of the hippo pathway, regulating organ size, cell contact inhibition as well as tissue regeneration and tumorigenesis. Recently, alterations of KIBRA expression caused by promotor methylation have been reported for several types of cancer. Our current study aimed to design an artificial transcription factor capable of re-activating expression of the tumor suppressor KIBRA and the hippo pathway. We engineered a new gene named 'ZFP226' encoding for a ~23 kDa fusion protein. ZFP226 belongs to the Cys2-His2 zinc finger type and recognizes a nine base-pair DNA sequence 5'-GGC-GGC-GGC-3' in the KIBRA core promoter P1a. ZFP226 showed nuclear localization in human immortalized kidney epithelial cells and activated the KIBRA core promoter (p < 0.001) resulting in significantly increased KIBRA mRNA and protein levels (p < 0.001). Furthermore, ZFP226 led to activation of hippo signaling marked by elevated YAP and LATS phosphorylation. In Annexin V flow cytometry assays ZFP226 overexpression showed strong pro-apoptotic capacity on MCF-7 breast cancer cells (p < 0.01 early-, p < 0.001 late-apoptotic cells). We conclude that the artificial transcription factor ZFP226 can be used for target KIBRA and hippo pathway activation. This novel molecule may represent a molecular tool for the development of future applications in cancer treatment.

Promoter methylation inhibits expression of tumor suppressor KIBRA in human clear cell renal cell carcinoma

BACKGROUND

KIBRA has been suggested as a key regulator of the Hippo signaling pathway, regulating organ size, cell contact inhibition, tissue regeneration as well as tumorigenesis and cystogenesis. We recently reported that human KIBRA expression depends on a complex alternative CpG-rich promoter system. Our current study aimed at the identification of epigenetic mechanisms associated with alterations in KIBRA expression regulation.

RESULTS

We identified two separated methylation-sensitive CpG islands located to independent KIBRA promoter regions. In vitro promoter methylation analysis using human neuroblastoma (SH-SY5Y) and immortalized kidney cells (IHKE) revealed that total promoter methylation by CpG methyltransferase SssI resulted in complete abrogation of transcriptional activity (p < 0.001), while partial methylation by HpaII selectively repressed KIBRA core promoter activity in kidney cells (p < 0.001). Cell culture-based experiments demonstrated that 5-azacitidine may be used to restore KIBRA mRNA and protein levels, while overexpression of transcription factor SP1 also induced KIBRA upregulation (all p < 0.001). Furthermore, SP1 transactivation of KIBRA transcription was largely prevented by methylation of KIBRA regulatory elements (p < 0.001). Analysis of human kidney biopsies revealed that KIBRA promoter methylation was associated with human clear cell renal cell carcinoma (ccRCC; n = 8 vs 16 controls, OR = 1.921, [CI 95% = 1.369-2.695]). The subsequent determination of KIBRA mRNA levels by real-time PCR in a larger patient sample confirmed significantly reduced KIBRA expression in ccRCC (n = 32) compared to non-neoplastic human kidney tissue samples (controls, n = 32, p < 0.001).

CONCLUSION

We conclude that epigenetic downregulation of tumor suppressor KIBRA may involve impaired SP1 binding to functional methylation-sensitive KIBRA promoter elements as observed in human kidney clear cell carcinoma. Our findings provide a pathophysiological basis for future studies on altered KIBRA regulation in clinical disease entities such as renal cancer.

Cryptogenic stroke and small fiber neuropathy of unknown etiology in patients with alpha-galactosidase A -10T genotype

BACKGROUND

Fabry disease (FD) is a multisystemic disorder with typical neurological manifestations such as stroke and small fiber neuropathy (SFN), caused by mutations of the alpha-galactosidase A (GLA) gene. We analyzed 15 patients carrying the GLA haplotype -10C>T [rs2071225], IVS2-81_-77delCAGCC [rs5903184], IVS4-16A>G [rs2071397], and IVS6-22C>T [rs2071228] for potential neurological manifestations.

METHODS AND RESULTS

Patients were retrospectively analyzed for stroke, transient ischemic attack (TIA), white matter lesions (WML) and SFN with neuropathic pain. Functional impact of the haplotype was determined by molecular genetic methods including real-time PCR, exon trapping, promoter deletion constructs and electrophoretic mobility shift assays. Symptomatic -10T allele carriers suffered from stroke, TIA, WML, and SFN with neuropathic pain. Patients' mean GLA mRNA expression level was reduced to ~70% (p < 0.0001) and a dose-dependent effect of the -10T allele on GLA mRNA expression was observed in hemi/homozygous compared to heterozygous patients (p < 0.0001). Molecular analyzes revealed that the -10T allele resulted in a reduced promoter activity and an altered transcription factor binding, while a functional relevance of the co-segregated intronic variants was excluded by exon trapping.

CONCLUSIONS

Based on this complementary approach of clinical observation and functional testing, we conclude that the GLA -10T allele could be causal for the observed neurological manifestations. Future studies are needed to clarify whether affected patients benefit from GLA enzyme replacement therapy for end-organ damage prevention.

Soluble Adenylyl Cyclase in Vascular Endothelium

The Ca 2+ - and bicarbonate-activated soluble adenylyl cyclase (sAC) has been identified recently as an important mediator of aldosterone signaling in the kidney. Nuclear sAC has been reported to stimulate cAMP response element–binding protein 1 phosphorylation via protein kinase A, suggesting an alternative cAMP pathway in the nucleus. In this study, we analyzed the sAC as a potential modulator of endothelial stiffness in the vascular endothelium. We determined the contribution of sAC to cAMP response element–mediated transcriptional activation in vascular endothelial cells and kidney collecting duct cells. Inhibition of sAC by the specific inhibitor KH7 significantly reduced cAMP response element–mediated promoter activity and affected cAMP response element–binding protein 1 phosphorylation. Furthermore, KH7 and anti-sAC small interfering RNA significantly decreased mRNA and protein levels of epithelial sodium channel-α and Na + /K + -ATPase-α. Using atomic force microscopy, a nano-technique that measures stiffness and deformability of living cells, we detected significant endothelial cell softening after sAC inhibition. Our results suggest that the sAC is a regulator of gene expression involved in aldosterone signaling and an important regulator of endothelial stiffness. Additional studies are warranted to investigate the protective action of sAC inhibitors in humans for potential clinical use.

Ochratoxin A: Apoptosis and Aberrant Exit from Mitosis due to Perturbation of Microtubule Dynamics?

Ochratoxin A (OTA) is a potent nephrotoxin and causes high incidences of renal tumors in rodents. The molecular events leading to tumor formation by OTA are not well defined. Early pathological changes observed in kidneys of rats treated with OTA in vivo include frequent mitotic and abnormally enlarged cells, detachment of tubule cells, and apoptosis within the S3 segment of the proximal tubule, suggesting that OTA may interfere with molecules involved in the regulation of cell division and apoptosis. In this study, treatment of immortalized human kidney epithelial (IHKE) cells with OTA (0-50 microM) resulted in a time- and dose-dependent increase in apoptosis and activation of c-Jun N-terminal kinase. At the same time, OTA blocked metaphase/anaphase transition and led to the formation of aberrant mitotic figures and giant cells with abnormally enlarged and/or multiple nuclei, sometimes still connected by chromatin bridges. Immunostaining of the mitotic apparatus using an alpha-tubulin antibody revealed defects in spindle formation. In addition, OTA inhibited microtubule assembly in a concentration-dependent manner in a cell-free, in vitro assay. Interestingly, treatment with OTA also resulted in activation of the transcription factor nuclear factor kappa B (NFkappaB), which has recently been shown to promote cell survival during mitotic cell cycle arrest. Based on these observations, we hypothesize that the mechanism by which OTA promotes tumor formation involves interference with microtubuli dynamics and mitotic spindle formation, resulting in apoptosis or-in the presence of survival signals such as stimulation of the NFkappaB pathway-premature exit from mitosis. Aberrant exit from mitosis resulting in blocked or asymmetric cell division may favor the occurrence of cytogenetic abnormalities and may therefore play a critical role in renal tumor formation by OTA.

Inhibition of Na(+)-dependent transporters in cystine-loaded human renal cells: electrophysiological studies on the Fanconi syndrome of cystinosis

Cystinosis is the most common cause of the renal Fanconi syndrome in children, leading to severe electrolyte disturbances and growth failure. A defective lysosomal transporter, cystinosin, results in intralysosomal accumulation of cystine. Loading cells with cystine dimethyl ester (CDME) is the only available model for this disease. This model was used to present electrophysiologic studies on immortalized human kidney epithelial (IHKE-1) cells that had been derived from the proximal tubule with the slow whole-cell patch clamp technique. Basal membrane voltages (V(m)) of IHKE-1 cells were -30.7 +/- 0.4 mV (n = 151). CDME concentration-dependently altered V(m) with an initial depolarization (2.7 +/- 0.2 mV;n = 76; 1 mM CDME) followed by a more pronounced hyperpolarization (-9.9 +/- 1.0 mV;n = 49). Three Na(+)-dependent transporters were examined. Alanine (1 mM) depolarized IHKE-1 cells by 17.6 +/- 0.7 mV (n = 59), and phosphate (1.8 mM) depolarized by 9.7 +/- 1.1 mV (n = 18). Acidification of IHKE-1 cells with propionate (20 mM) resulted in a depolarization of V(m) by 7.1 +/- 0.3 mV (n = 21) followed by a repolarization by 2.9 +/- 0.3 mV/min (n = 17), reflecting Na(+)/H(+)-exchanger activity. Acute addition of 1 mM CDME did not alter the alanine- and propionate-induced changes in V(m), but it reduced the phosphate-induced depolarization by 37 +/- 9% (n = 10). Incubation with 1 mM CDME reduced the activity of all three transporters. Depolarizations by alanine and phosphate and the repolarization after propionate were inhibited by 57 +/- 4% (n =30), 45 +/- 9% (n = 9), and 78 +/- 15% (n = 8), respectively. In conclusion, this study demonstrates that CDME acutely alters V(m) of IHKE-1 cells and that at least three Na(+)-dependent transporters are inhibited, the Na(+)-phosphate cotransporter most sensitively. This might suggest that phosphate depletion and dissipation of the Na(+)-gradient are involved in the development of the Fanconi syndrome of cystinosis.

A novel cGMP-regulated K+ channel in immortalized human kidney epitheliall cells (IHKE-1)

1. K+ channels from the apical membrane of immortalized human kidney epithelial (IHKE-1) cells were investigated in the cell-attached membrane configuration as well as in excised membranes using the patch clamp technique. 2. In cell-attached membrane patches the open probability (Po) of the K+ channel was 0.42 +/- 0.06 (mean +/- s.e.m. , n = 22) and its conductance was 94 +/- 5 pS with 145 mM K+ in the pipette (n = 25). In excised membrane patches the Po of the channel was 0.55 +/- 0.03 (n = 86) and its conductance was 65 +/- 2 pS (n = 68) with 145 mM K+ on one side of the membrane and 3.6 mM K+ on the other. The I-V curve of the K+ channel was not rectifying. 3. The channel was inhibited by several blockers of K+ channels such as 1 mM Ba2+ (cell-attached membrane: 78 +/- 8 %, n = 9; excised: 80 +/- 4 %, n = 26), 10 mM TEA+ (excised inside-out: 48 +/- 5 %, n = 34; excised outside-out: 100 +/- 0 %, n = 26), 0.1 mM verapamil (excised: 73 +/- 9 %, n = 12), and 10 nM charybdotoxin (excised outside-out: 67 +/- 9 %, n = 9). 4. The K+ channel was activated by depolarization and rising cytosolic Ca2+. Half-maximal activity occurred at a cytosolic Ca2+ concentration of 200 nM. In the cell-attached membrane configuration the K+ channel was inhibited in a concentration-dependent manner by atrial natriuretic peptide (ANP). Powas blocked equally well by 10 nM ANP (52 +/- 7 %, n = 10), brain natriuretic peptide (BNP; 37 +/- 11 %, n = 6) and C-type natriuretic peptide (CNP; 44 +/- 13 %, n = 8). 8-Bromoguanosine 3',5' cyclic monophosphate (8-Br-cGMP, 0.1 mM) also inhibited Poof this K+ channel, by 70 +/- 10 % (n = 5). 5. In excised membrane patches cGMP inhibited Po of this K+ channel in a concentration-dependent manner. The first significant effects were measured at a concentration of 1 microM (22 +/- 7 %, n = 6), and greatest effects were obtained at 0.1 mM (34 +/- 5 %, n = 15). cAMP (0.1 mM, n = 5) as well as GTP (0.1 mM, n = 5) had no significant effects on Po of this K+ channel. ATP (0.1 mM) had a weak inhibitory effect (17 +/- 5 %, n = 14). Addition of Mg-ATP to cGMP did not increase the inhibitory effect (30 +/- 4 %, n = 14). KT5823 (1 microM), a specific inhibitor of cGMP-dependent protein kinases, did not significantly alter the cGMP-induced reduction in Po of the K+ channel in three excised membrane patches. 6. The results present the first electrophysiological characterization of a mammalian K+ channel that is directly regulated by cGMP.

cGMP-dependent and -independent inhibition of a K+ conductance by natriuretic peptides: molecular and functional studies in human proximal tubule cells

In immortalized human kidney epithelial (IHKE-1) cells derived from proximal tubules, two natriuretic peptide receptors (NPR) were identified. In addition to NPR-A, which is bound by atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and urodilatin (URO), a novel form of NPR-B that might be bound by C-type natriuretic peptide (CNP) was identified using PCR. This novel splice variant of NPR-B (NPR-Bi) was also found in human kidney. Whereas ANP, BNP, and URO increased intracellular cGMP levels in IHKE-1 cells in a concentration-dependent manner, CNP had no effect on cGMP levels. To determine the physiologic responses to these agonists in IHKE-1 cells, the membrane voltage (Vm) was monitored using the slow whole-cell patch-clamp technique. ANP (10 nM), BNP (10 nM), and URO (16 nM) depolarized these cells by 3 to 4 mV (n = 47, 7, and 16, respectively), an effect that could be mimicked by 0.1 mM 8-Br-cGMP (n = 15). The effects of ANP and 8-Br-cGMP were not additive (n = 4). CNP (10 nM) also depolarized these cells, by 3+/-1 mV (n = 28), despite the absence of an increase in cellular cGMP levels, indicating a cGMP-independent mechanism. In the presence of CNP, 8-Br-cGMP further depolarized Vm significantly, by 1.6+/-0.3 mV (n = 5). The depolarizations by ANP were completely abolished in the presence of Ba2+ (1 mM, n = 4) and thus can be related to inhibition of a K+ conductance in the luminal membrane of IHKE-1 cells. The depolarizations attributable to CNP were completely blocked when genistein (10 microM, n = 6), an inhibitor of tyrosine kinases, was present. These findings indicate that natriuretic peptides regulate electrogenic transport processes via cGMP-dependent and -independent pathways that influence the Vm of IHKE-1 cells.

Cloning and characterization of two human polyspecific organic cation transporters

Previously we cloned a polyspecific transporter from rat (rOCT1) that is expressed in renal proximal tubules and hepatocytes and mediates electrogenic uptake of organic cations with different molecular structures. Recently a homologous transporter from rat kidney (rOCT2) was cloned but not characterized in detail. We report cloning and characterization of two homologous transporters from man (hOCT1 and hOCT2) displaying approximately 80% amino acid identity to rOCT1 and rOCT2, respectively. Northern blots showed that hOCT1 is mainly transcribed in liver, while hOCT2 is found in kidney. Using in situ hybridization and immunohistochemistry, expression of hOCT2 was mainly detected in the distal tubule where the transporter is localized at the luminal membrane. After expression in Xenopus laevis oocytes, hOCT1 and hOCT2 mediate tracer influx of N-1-methylnicotinamide (NMN), tetraethylammonium (TEA), and 1-methyl-4-phenylpyridinium (MPP). For cation transport by hOCT2 apparent K(m) and K(i) values were determined in tracer flux measurements. In addition, electrical measurements were performed with voltage-clamped oocytes. Similar to rOCT1, cation transport by hOCT2 was pH independent, electrogenic, and polyspecific; however, the cation specificity was different. In voltage-clamped hOCT2-expressing oocytes, inward currents were induced by superfusion with MPP, TEA, choline, quinine, d-tubocurarine, pancuronium, and cyanine863. Cation transport in distal tubules is indicated for the first time. Here hOCT2 mediates the first step in cation reabsorption. hOCT1 may participate in hepatic excretion of organic cations.

Adaptive regulation of taurine and beta-alanine uptake in a human kidney cell line from the proximal tubule

1. The underlying mechanisms involved in the adaptive regulation of beta-amino acid uptake in the human proximal tubule were examined by use of an immortalized human embryonic kidney epithelial cell line (IHKE). 2. The results indicated that the adaptive response to maintain whole-body taurine homeostasis occurs predominantly via changes in the activity of the high-affinity taurine transport system by alterations in the uptake capacity and with an unaffected half-saturation constant. An adaptive response was not observed for the structurally related beta-alanine. 3. Only colchicine, which interferes with microtubule organization, was capable of blocking the response to alterations of taurine in cell medium, whereas inhibition of protein and nucleic acid synthesis by cycloheximide and actinomycin D, respectively, did not change the adaptive pattern. 4. Phorbol 12-myristate 13-acetate (PMA), mimicking the effects of diacylglycerol, induced inhibition of both beta-alanine and taurine uptake. By contrast, the Ca2(+)-ionophore A23187, mimicking the effects of IP3, only stimulated the uptake of taurine but not the influx of beta-alanine. However, the effect of PMA down-regulation and A23187 up-regulation was rapid and short-lived in contrast to the adaptive response, suggesting that the inositol phospholipid pathway involving diacetylglycerol and IP3 is less likely to be linked directly to the adaptive regulation, but rather plays a role in short-term regulation.

Uptake of neutral alpha- and beta-amino acids by human proximal tubular cells

The transport characteristics of amino acids in primary cell cultures from the proximal tubule of human adults (AHKE cells) were examined, using alpha-aminoisobutyric acid (AIB) and beta-alanine as representatives of alpha- and beta-amino acids, respectively. The Na(+)-gradient dependent influx of AIB occurred by a single, saturable transport system, whereas the Na(+)-gradient dependent uptake data for beta-alanine could be described in terms of two-independent transport components as well as one-transport one-leak model with identical kinetic constants for the high-affinity system. Competition experiments revealed that all the neutral amino acids tested reduced the uptake of AIB, whereas there was no effect of taurine, L-aspartic acid, and L-arginine. By contrast, the influx of beta-alanine was only drastically reduced by beta-amino acids, whereas the inhibition by neutral alpha-amino acids was relatively low. Nor did L-arginine and L-aspartic acid affect the uptake of beta-alanine into AHKE cells. Comparison with the results obtained for normal (NHKE) and immortalized (IHKE) embryonic cells suggested an unaltered expression of the types of transport carriers for neutral alpha- and beta-amino acids in the embryonic and AHKE cells. However, the uptake capacity of the above-mentioned transport proteins was relatively smaller in the embryonic kidney compared with the adult human kidney, which may explain, at least partly, the phenomenon of physiologic amino aciduria in neonates.

Taurine and beta-alanine transport in an established human kidney cell line derived from the proximal tubule

The transport mechanisms of taurine and beta-alanine by an immortalized human embryonic kidney epithelial cell line (IHKE) were examined. The uptake of these beta-amino acids was characterized by two Na(+)-dependent transport components, whereas an inwardly directed H(+)-gradient only stimulated amino acid influx to a small extent and in the absence of sodium. Competition experiments revealed that taurine and beta-alanine drastically reduced the uptake of one another by the high-affinity Na(+)-dependent transport system. However, some alpha-amino acids could also compete with the beta-amino acids, but with a low affinity. Examinations of the effect of different anions on the Na(+)-dependent uptake of taurine at a low amino acid concentration (240 nM) revealed a specific requirement for Cl-, whereas Cl- had no measurable effect at a higher concentration (1.0 mM) of taurine. In addition, activation of taurine transport as a function of Na+ and Cl- concentration indicated a probable coupling ratio of 3 Na+/1 Cl-/1 taurine for the high-affinity carrier. Finally, cellular regulation of taurine transport was indicated by the finding that pretreatment with taurine containing media decreased the activity of the taurine transporter(s).