In vitro studies of aquaporins 1 and 3 expression in cultured human proximal tubular cells: upregulation by transferrin but not albumin

Generation and characterization of iPSC-derived renal proximal tubule-like cells with extended stability

The renal proximal tubule is responsible for re-absorption of the majority of the glomerular filtrate and its proper function is necessary for whole-body homeostasis. Aging, certain diseases and chemical-induced toxicity are factors that contribute to proximal tubule injury and chronic kidney disease progression. To better understand these processes, it would be advantageous to generate renal tissues from human induced pluripotent stem cells (iPSC). Here, we report the differentiation and characterization of iPSC lines into proximal tubular-like cells (PTL). The protocol is a step wise exposure of small molecules and growth factors, including the GSK3 inhibitor (CHIR99021), the retinoic acid receptor activator (TTNPB), FGF9 and EGF, to drive iPSC to PTL via cell stages representing characteristics of early stages of renal development. Genome-wide RNA sequencing showed that PTL clustered within a kidney phenotype. PTL expressed proximal tubular-specific markers, including megalin (LRP2), showed a polarized phenotype, and were responsive to parathyroid hormone. PTL could take up albumin and exhibited ABCB1 transport activity. The phenotype was stable for up to 7 days and was maintained after passaging. This protocol will form the basis of an optimized strategy for molecular investigations using iPSC derived PTL.

Aging-Related Functional and Structural Changes in Renal Tissues: Lesson from a Camel Model

Renal aging is a progressive, physiological, and anatomical change that naturally occurs in all animal species. To date, no information is available concerning the aging-related structural and functional changes in camel kidneys. A total of 25 healthy male camels (14 aged 4–6 years and 11 aged 18–22 years) were included in this study. After the camels were slaughtered, samples were collected from all the camels’ kidneys and prepared for histopathological, immunohistochemical, and gene expression evaluations. The most striking observation was the significant decline in the immunohistochemical abundance of podocin and the significant upregulation of smoothening in the aging camels’ kidneys. However, the nonsignificant changes have reported for nephrin, calbindin, autophagy 5 (ATG5), aquaporin 1, and toll-like receptor 9. Furthermore, the mRNA expressions of sirtuin 1, superoxide dismutase 1, superoxide dismutase 2, peroxisome proliferator-activated receptor alpha, B-cell lymphoma 2 (Bcl-2), and erythropoietin were significantly decreased in the aging camels’ kidneys. While the significant upregulation of Bcl-2-associated X protein and the nonsignificant increase in ATG5 expression levels were reported in the aging camels’ kidneys. The present findings provide better understanding of the complex events and initiating factors of aging, allowing for the development of a future therapeutic strategy to preserve adequate renal function throughout life.

Renal proximal tubular epithelial cells: review of isolation, characterization, and culturing techniques

The kidney is a complex organ, comprised primarily of glomerular, tubular, mesangial, and endothelial cells, and podocytes. The fact that renal cells are terminally differentiated at 34 weeks of gestation is the main obstacle in regeneration and treatment of acute kidney injury or chronic kidney disease. Furthermore, the number of chronic kidney disease patients is ever increasing and with it the medical community should aim to improve existing and develop new methods of renal replacement therapy. On the other hand, as polypharmacy is on the rise, thought should be given into developing new ways of testing drug safety. A possible way to tackle these issues is with isolation and culture of renal cells. Several protocols are currently described to isolate the desired cells, of which the most isolated are the proximal tubular epithelial cells. They play a major role in water homeostasis, acid-base control, reabsorption of compounds, and secretion of xenobiotics and endogenous metabolites. When exposed to ischemic, toxic, septic, or obstructive conditions their death results in what we clinically perceive as acute kidney injury. Additionally, due to renal cells' limited regenerative potential, the profibrotic environment inevitably leads to chronic kidney disease. In this review we will focus on human proximal tubular epithelial cells. We will cover human kidney culture models, cell sources, isolation, culture, immortalization, and characterization subdivided into morphological, phenotypical, and functional characterization.

Downregulation of the S1P Transporter Spinster Homology Protein 2 (Spns2) Exerts an Anti-Fibrotic and Anti-Inflammatory Effect in Human Renal Proximal Tubular Epithelial Cells

Sphingosine kinase (SK) catalyses the formation of sphingosine 1-phosphate (S1P), which acts as a key regulator of inflammatory and fibrotic reactions, mainly via S1P receptor activation. Here, we show that in the human renal proximal tubular epithelial cell line HK2, the profibrotic mediator transforming growth factor β (TGFβ) induces SK-1 mRNA and protein expression, and in parallel, it also upregulates the expression of the fibrotic markers connective tissue growth factor (CTGF) and fibronectin. Stable downregulation of SK-1 by RNAi resulted in the increased expression of CTGF, suggesting a suppressive effect of SK-1-derived intracellular S1P in the fibrotic process, which is lost when SK-1 is downregulated. In a further approach, the S1P transporter Spns2, which is known to export S1P and thereby reduces intracellular S1P levels, was stably downregulated in HK2 cells by RNAi. This treatment decreased TGFβ-induced CTGF and fibronectin expression, and it abolished the strong induction of the monocyte chemotactic protein 1 (MCP-1) by the pro-inflammatory cytokines tumor necrosis factor (TNF)α and interleukin (IL)-1β. Moreover, it enhanced the expression of aquaporin 1, which is an important water channel that is expressed in the proximal tubules, and reverted aquaporin 1 downregulation induced by IL-1β/TNFα. On the other hand, overexpression of a Spns2-GFP construct increased S1P secretion and it resulted in enhanced TGFβ-induced CTGF expression. In summary, our data demonstrate that in human renal proximal tubular epithelial cells, SK-1 downregulation accelerates an inflammatory and fibrotic reaction, whereas Spns2 downregulation has an opposite effect. We conclude that Spns2 represents a promising new target for the treatment of tubulointerstitial inflammation and fibrosis.

Sex-dependent expression of water channel AQP1 along the rat nephron

In the mammalian kidney, nonglycosylated and glycosylated forms of aquaporin protein 1 (AQP1) coexist in the luminal and basolateral plasma membranes of proximal tubule and descending thin limb. Factors that influence AQP1 expression in (patho)physiological conditions are poorly known. Thus far, only angiotensin II and hypertonicity were found to upregulate AQP1 expression in rat proximal tubule in vivo and in vitro (Bouley R, Palomino Z, Tang SS, Nunes P, Kobori H, Lu HA, Shum WW, Sabolic I, Brown D, Ingelfinger JR, Jung FF. Am J Physiol Renal Physiol 297: F1575-F1586, 2009), a phenomenon that may be relevant for higher blood pressure observed in men and male experimental animals. Here we investigated the sex-dependent AQP1 protein and mRNA expression in the rat kidney by immunochemical methods and qRT-PCR in tissue samples from prepubertal and intact gonadectomized animals and sex hormone-treated gonadectomized adult male and female animals. In adult rats, the overall renal AQP1 protein and mRNA expression was ∼80% and ∼40% higher, respectively, in males than in females, downregulated by gonadectomy in both sexes and upregulated strongly by testosterone and moderately by progesterone treatment; estradiol treatment had no effect. In prepubertal rats, the AQP1 protein expression was low compared with adults and slightly higher in females, whereas the AQP1 mRNA expression was low and similar in both sexes. The observed differences in AQP1 protein expression in various experiments mainly reflect changes in the glycosylated form. The male-dominant expression of renal AQP1 in rats, which develops after puberty largely in the glycosylated form of the protein, may contribute to enhanced fluid reabsorption following the androgen- or progesterone-stimulated activities of sodium-reabsorptive mechanisms in proximal tubules.

Tissue-engineered kidney disease models

Renal disease represents a major health problem that often results in end-stage renal failure necessitating dialysis and eventually transplantation. Historically these diseases have been studied with patient observation and screening, animal models, and two-dimensional cell culture. In this review, we focus on recent advances in tissue engineered kidney disease models that have the capacity to compensate for the limitations of traditional modalities. The cells and materials utilized to develop these models are discussed and tissue engineered models of polycystic kidney disease, drug-induced nephrotoxicity, and the glomerulus are examined in detail. The application of these models has the potential to direct future disease treatments and preclinical drug development.

Aquaporin-1 translocation and degradation mediates the water transportation mechanism of acetazolamide

Background

Diuretic agents are widely used on the treatment of water retention related diseases, among which acetazolamide (AZA) acts originally as a carbonic anhydrase (CA) inhibitor. Aquaporin-1 (AQP1) being located in renal proximal tubules is required for urine concentration. Previously our lab has reported AZA putatively modulated AQP1. Aim of this study is to testify our hypothesis that regulating AQP1 may mediate diuretic effect of AZA.

Methodology/Principal Findings

For in vivo study, we utilized Sprague Dawley rats, as well as AQP1 knock-out (AQP1^−/−) mice to examine urine volume, and human kidney-2 (HK-2) cell line was used for in vitro mechanism study. In our present study we found that AZA decreased CAs activity initially but the activity gradually recovered. Contrarily, diuretic effect was consistently significant. AQP1 protein expression was significantly decreased on day 7 and 14. By utilizing AQP1^−/− mice, we found diuretic effect of AZA was cancelled on day 14, while urine volume continuously increased in wild-type mice. Surface plasmon resonance (SPR) results indicated AQP1 was physiologically bound by myosin heavy chain (MHC), immunoprecipitation and immunofluorescence results confirmed this protein interaction. In vitro study results proved AZA facilitated AQP1 translocation onto cell membrane by promoting interaction with MHC, dependent on ERK/ myosin light chain kinase (MLCK) pathway activation. MHC inhibitor BDM and ERK inhibitor U0126 both abolished above effect of AZA. Eventually AZA induced AQP1 ubiquitination, while proteasome inhibitor MG132 reversed AZA's down-regulating effect upon AQP1.

Conclusions/Significance

Our results identified AZA exerted diuretic effect through an innovative mechanism by regulating AQP1 and verified its inhibitory mechanism was via promoting MHC-dependent translocation onto cell membrane and then ubiquitin mediated degradation, implicating a novel mechanism and target for diuretic agent discovering.

AQP1 expression analysis in human diseases: implications for proteomic characterization

Aquaporin (AQP)1 belongs to a ubiquitous family of water channel proteins characterized by sequence similarity and the presence of two NPA (Asp-Pro-Ala) motifs existing in almost all organs and tissues. Currently, 13 human AQPs are known and they are divided into two subgroups according to their ability to transport only water molecules, such as AQP1, or also glycerol and other small solutes. The genomic, structural and functional aspects of AQP1 are briefly described. An in-depth discussion is devoted to proteomic approaches that are useful for identifying and characterizing AQP1, mainly through electrophoretic techniques combined with different extraction procedures followed by mass spectrometry analysis. Moreover, the relevance of AQP1 in human diseases is also explained. Its role in human tumors and, in particular, those of the kidney (e.g., clear cell renal carcinoma) is discussed.

Distribution of AQP2 and AQP3 water channels in human tissue microarrays

The objective of this investigation was to use semi-quantitative immunohistochemistry to determine the distribution and expression levels of AQP2 and AQP3 proteins in normal human Tissue MicroArrays. Expression of the vasopressin regulated AQP2 was observed in a limited number of tissues. AQP2 was prominent in the apical and subapical plasma membranes of cortical and medullary renal collecting ducts. Surprisingly, weak AQP2 immunoreactivity was also noted in pancreatic islets, fallopian tubes and peripheral nerves. AQP2 was also localized to selected parts of the central nervous system (ependymal cell layer, subcortical white matter, hippocampus, spinal cord) and selected cells in the gastrointestinal system (antral and oxyntic gastric mucosa, small intestine and colon). These findings corroborate the restricted tissue distribution of AQP2. AQP3 was strongly expressed in many of the human tissues examined particularly in basolateral membranes of the distal nephron (medullary collecting ducts), distal colon, upper airway epithelia, transitional epithelium of the urinary bladder, tracheal, bronchial and nasopharyngeal epithelium, stratified squamous epithelial cells of the esophagus, and anus. AQP3 was moderately expressed in basolateral membranes of prostatic tubuloalveolar epithelium, pancreatic ducts, uterine endometrium, choroid plexus, articular chondrocytes, subchondral osteoblasts and synovium. Low AQP3 levels were also detected in skeletal muscle, cardiac muscle, gastric pits, seminiferous tubules, lymphoid vessels, salivary and endocrine glands, amniotic membranes, placenta and ovary. The abundance of basolateral AQP3 in epithelial tissues and its expression in many non-epithelial cells suggests that this aquaglyceroporin is a major participant in barrier hydration and water and osmolyte homeostasis in the human body.

Differential expression of aquaporins in the kidneys of streptozotocin-induced diabetic mice

BACKGROUND AND AIM

Aquaporins (AQPs) are members of the water channel family and are important in renal physiology as it affects urinary concentration. The downregulation of aquaporins is often observed in polyuria associated with acquired nephrogenic diabetes insipidus. In this study, we examined the expression of AQP1, AQP2, AQP3 and AQP4 in streptozotocin (STZ)-induced diabetic mice.

RESULTS

By semiquantitative reverse transcription-polymerase chain reaction, we detected no change in the gene expression of AQP1 or AQP4 in whole kidney among STZ-induced diabetic mice (STZ mice) and sham (control group that received citrate buffer injection only). In contrast, we found less AQP2 or AQP3 mRNA expression in the whole kidney from STZ mice. Immunoblotting studies confirmed no difference in AQP1 or AQP4 protein expression of whole kidney between STZ mice and sham. However, there was less AQP2 or AQP3 protein expression in the whole kidney from STZ mice as compared to sham. By immunochemical staining, the reduction of AQP2 protein was localized to the principle cells of the collecting ducts. The expression of cortical AQP3 (especially the outer cortex, the S1 and S2 segments of the proximal tubules) was downregulated in STZ mice whereas the expression of AQP3 protein in medullary collecting ducts was similar to that of sham.

CONCLUSION

Our results reveal that the water transport in urinary concentration involves the downregulation of AQP2 and AQP3 expression in STZ mice.

Bronchiolar expression of aquaporin-3 (AQP3) in rat lung and its dynamics in pulmonary oedema

Aquaporins (AQPs) are water channel proteins that permit osmotically driven water movement. To determine their dynamics in pulmonary oedema, we examined the expression of mRNA and protein for AQP1, AQP3, AQP4, and AQP5 in the lungs of normal and thiourea-treated rats. In the thiourea group, lung water content increased significantly (vs. controls) with the peak at around 4 h. Semi-quantitative RT-PCR showed that AQP3 mRNA in the thiourea group rose significantly, peaking at around 4-8 h. The expression of AQP1, AQP4, AQP5, ENaC and CFTR mRNA each decreased significantly some time after the peak in lung water content. Immunoblot analysis showed that glycosylated AQP3 protein was increased 4-10 h after treatment. Expression of the other AQP proteins was not significantly altered, except for that of AQP4. Immunohistochemical examination revealed that AQP1 was expressed in endothelia, AQP3 in the basal cells of the large airways and in cuboidal cells in the bronchioles, AQP4 in the basolateral membrane of airway cells and AQP5 in type-I pneumocytes. Our results suggest that AQP3 is expressed not only in large airways, but also in bronchioles, and is related to water movement in pulmonary oedema.

Aquaporin Expression in Normal Human Kidney and in Renal Disease

ABSTRACT. Aquaporins (AQPs), membrane-inserted water channel proteins, play a highly important role in the reabsorption of water from the renal tubular fluid. Experimentally, both in rats and mice, failure to insert functional AQP molecules into renal tubular membranes leads to nephrogenic diabetes insipidus. In humans, most forms of renal disease lead to a reduction in the water handling capacity of the kidney. AQP distribution in various forms of human renal disease has not been documented. Immunohistochemical studies of biopsy samples from a wide range of renal diseases revealed a substantial and striking upregulation of AQP-1 in the glomeruli of most diseased kidneys. AQP-1 expression remained prominent in proximal tubules in all lesions. In contrast, there was judged qualitatively to be a reduction in the amounts of AQP-2 and AQP-3 expression, especially in lesions with substantial interstitial fibrosis and nephron loss, as compared with a healthy region of normal kidneys. The results were quantitatively confirmed by real-time reverse transcriptase–PCR. This is the first documentation of altered AQP expression in human renal disease. The significance of the increased AQP-1 expression requires further studies. E-mail: rob.walker@stonebow.otago.ac.nz

Albumin stimulates interleukin-8 expression in proximal tubular epithelial cells in vitro and in vivo

Renal tubulointerstitial injury is characterized by inflammatory cell infiltrate; however, the stimuli for leukocyte recruitment are not fully understood. IL-8 is a potent chemokine produced by proximal tubular epithelial cells (PTECs). Whether nephrotic proteins stimulate tubular IL-8 expression remains unknown. Acute exposure of human PTECs to albumin induced IL-8 gene and protein expression time- and dose-dependently. Apical albumin predominantly stimulated basolateral IL-8 secretion. Electrophoretic mobility shift assay demonstrated nuclear translocation of NF-kappaB, and the p65/p50 subunits were activated. NF-kappaB activation and IL-8 secretion were attenuated by the NF-kappaB inhibitors pyrrolidine dithiocarbamate and cell-permeable peptide. Albumin upregulated intracellular reactive oxygen species (ROS) generation, while exogenous H2O2 stimulated NF-kappaB translocation and IL-8 secretion. Albumin-induced ROS generation, NF-kappaB activation, and IL-8 secretion were endocytosis- and PKC-dependent as these downstream events were abrogated by the PI3K inhibitors LY294002 and wortmannin, and the PKC inhibitors GF109203X and staurosporin, respectively. In vivo, IL-8 mRNA expression was localized by in situ hybridization to the proximal tubules in nephrotic kidney tissues. The intensity of IL-8 immunostaining was higher in nephrotic than non-nephrotic subjects. In conclusion, albumin is a strong stimulus for tubular IL-8 expression, which occurs via NF-kappaB-dependent pathways through PKC activation and ROS generation.

Enhancement of aquaporin-3 by vasoactive intestinal polypeptide in a human colonic epithelial cell line

BACKGROUND

Vasoactive intestinal polypeptide (VIP) plays an important role in water transport in the intestine. Several specialized channels termed aquaporins (AQP) facilitate water transport in the gastrointestinal tract. Aquaporin-3 localizes to epithelial cells in the human small intestine and colon. However, the regulatory mechanisms underlying the functions of AQP3 remain unclear. To characterize the regulation of AQP3 expression by VIP, we studied messenger (m)RNA expression, protein expression and DNA binding activity in a human colonic epithelial cell line, HT-29.

METHOD

Human colonic epithelial cells, HT-29, were incubated with VIP (10-12-10-7 M). The cells were treated with protein kinase-A (PK-A) inhibitors (H-89, H-9) or chloride channel-blockers (diphenylamine-2-carboxylate (DPC), 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPD)). The expression of AQP3 mRNA and protein was determined by Northern blot and Western blot, respectively. The DNA-binding activities of cyclic adenosine monophosphate (cAMP) response elements/activating transcription factor (CRE/ATF)) in the nuclear extract were determined by electrophoretic mobility shift assay.

RESULTS

Aquaporin-3 mRNA was up-regulated at a concentration of 10-10 M VIP. The expression of AQP3 protein was enhanced at 3 h after addition of VIP. The PK-A inhibitors (H-89, H-9) inhibited the expression of AQP3 mRNA enhanced by VIP and cAMP. The gel shift assay of CRE/ATF in HT-29 cells revealed a single band.

CONCLUSION

These results indicate that VIP upregulated the expression of AQP3 mRNA and protein, and that a cAMP-dependent pathway mediated this effect in a human colonic epithelial cell line, HT-29.

Down-regulation of aquaporin 3 in bronchiectatic airways in vivo

Bronchiectasis is characterized pathologically by permanent abnormal bronchial dilation, and clinically by chronic sputum production. Aquaporin 3 (AQP3), a recently described water channel that is also found in large airway cell membrane, could play a role in the pathogenesis and particularly that of bronchorrhea in bronchiectasis. However, little is known of its in vivo distribution and physiological role in human airways. We have, therefore, performed this quantitative immunohistochemistry study on endobronchial biopsies to evaluate the expression and clinical relevance of AQP3 in patients with idiopathic bronchiectasis (n = 25, 15 F, 64.3 +/- 11.5 years) and control subjects (n = 14, 5 F, 57.5 +/- 12.0 years). Quantitative image analysis was performed to evaluate the expression of AQP3 in the bronchial epithelial cells. Our results show that AQP3 was predominantly expressed in the basal cells of the epithelial layer in both groups. Expression of AQP3 was significantly reduced in the basal, but not columnar, epithelial cells in bronchiectasis compared with control airways (p = 0.02, 0.35). Only bronchiectatic patients with regular sputum production, but not their counterparts, had significant downregulation of epithelial AQP3 expression compared with control airways (p = 0.004, 0.24). Our findings suggest that AQP3 could have an important role in the pathogenesis of increased mucus production in bronchiectasis.

Expression of aquaporin-3 in human peritoneal mesothelial cells and its up-regulation by glucose in vitro

BACKGROUND

Aquaporin-3 (AQP3) is a member of the water channel family that is selective for the passage of not only water, but also glycerol and urea. Our recent study demonstrated the presence of aquaporin-1 in human peritoneal mesothelial cells (HPMC). Although transcripts encoding for AQP3 has been detected by reverse transcription-polymerase chain reaction (RT-PCR) in murine peritoneal mesothelium, to date there is no documentation of protein expression on peritoneal mesothelial cells.

METHOD

Our present study was designed to explore the gene and protein expression of AQP3 in HPMC and its regulation under different concentrations of glucose.

RESULTS

AQP3 protein was detected in the human peritoneal tissue by immunohistological staining using specific, affinity-purified polyclonal anti-AQP3 antibodies. AQ3 transcripts and protein expression in cultured HPMC were investigated by RT-PCR and immunoblotting analysis respectively. Cell permeability to glycerol (flux) was measured using [(14)C]glycerol incorporation. AQP3 transcript and protein were weakly expressed in HPMC constitutively. The gene expression of AQP3 and its protein biosynthesis in HPMC were inducible following exposure to glucose in a dose- and time-dependent manner (P < 0.0001). Glucose at a concentration of 200 mmol induced glycerol flux by 4.82-fold above the control value (P < 0.0001) and its effect was significantly inhibited by mercuric chloride (P < 0.01).

CONCLUSION

Our novel observation demonstrated the AQP3 expression and biosynthesis in HPMC and in vitro studies revealed that glycerol permeability in HPMC was up-regulated by glucose. Further study is warranted to elucidate the role of AQP3 in HPMC for maintaining the ultrafiltration of the peritoneal membrane.

Transferrin up-regulates chemokine synthesis by human proximal tubular epithelial cells: implication on mechanism of tubuloglomerular communication in glomerulopathic proteinura

BACKGROUND

The pathogenesis of glomerulosclerosis and tubulointerstitial fibrosis in proteinuric renal disease is obscure. We recently showed that transferrin, a key proteinuric component, mediates proximal tubular epithelial cell (PTEC) C3 synthesis. To further examine whether proteinuric tubular injury may induce glomerular inflammation and to characterize the role of transferrin in activating PTEC, glomerular mesangial cells (MC) were exposed to transferrin-activated PTEC culture supernatant and their proliferative and profibrotic responses analyzed.

METHODS

Human PTEC and MC were obtained by primary culture. Confluent, transferrin-stimulated PTEC were grown in serum-free medium to produce a "conditioned" medium that was incubated with quiescent MC. The proliferative response of MC was then assessed by thymidine uptake, and the expression of fibrogenic factors measured by reverse transcription-polymerase chain reaction (RT-PCR) and enzyme-linked immunosorbent assay (ELISA). The chemokine profile in PTEC after transferrin treatment was examined by RT-PCR and ELISA.

RESULTS

"Conditioned" supernatant from PTEC, which contained the highest amounts of platelet-derived growth factor (PDGF), stimulated MC proliferation compared with serum-free (P = 0.03) or transferrin-containing (P = 0.009) control media. This proliferative response was partially abrogated by treating MC with anti-PDGF. MC expression of PDGF, but not transforming growth factor-beta or intercellular cell adhesion molecule-1, was up-regulated by conditioned PTEC medium. Transferrin up-regulated monocyte chemoattractant peptide-1, interleukin-8, and macrophage migration inhibitory factor expression in a time- and dose-dependent fashion, but had no effect on RANTES expression by PTEC.

CONCLUSIONS

These results provide experimental evidence suggesting that there is a tubuloglomerular "cross-talk" mechanism in the proteinuric state. PTEC-secreted PDGF, which further induces mesangial PDGF, could partially account for the mesangial proliferation frequently observed in proteinuric renal disease. Transferrin is one of the culprit nephrotic proteins leading to tubular overexpression of various proinflammatory chemokines, which may explain the interstitial changes observed in proteinuric states.