Aquaporin-1: an osmoinducible water channel in cultured mIMCD-3 cells

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.

TonEBP regulates the hyperosmotic expression of aquaporin 1 and 5 in the intervertebral disc

The central region of the intervertebral disc (IVD) is rich in proteoglycans, leading to a hyperosmotic environment, which fluctuates with daily loading. The cells of the nucleus pulposus (NP cells) have adapted to this environment via the function of tonicity enhancer binding protein (TonEBP), and NP cells have been shown to express several water channels known as aquaporins (AQP). We have previously shown that AQP1 and 5 decrease during IVD degeneration. Here, the regulation of AQP1 and 5 by hyperosmotic conditions and the role of TonEBP in this regulation was investigated. AQP1 and 5 gene expression was upregulated by hyperosmotic conditions mimicking the osmolality of the healthy IVD, which was abrogated by TonEBP knockdown. Furthermore, AQP1 and 5 immunopositivity was significantly reduced in TonEBP^Δ/Δ E17.5 mice when compared with wildtype controls, indicating in vivo expression of AQP1 and 5 is controlled at least in part by TonEBP. This hyperosmotic regulation of AQP1 and 5 could help to explain the decreased AQP1 and 5 expression during degeneration, when the osmolality of the NP decreases. Together this data suggests that TonEBP-regulated osmo-adaptation may be disrupted during IVD degeneration when the expression of both AQPs is reduced.

Expression of Aquaporin-1 in the Peritoneal Tissues: Localization and Regulation by Hyperosmolality

Objective

The purpose of this study was to determine the localization of the aquaporin-1 (AQP1) water channel in peritoneal tissues and the effect of hyperosmolality on the peritoneal expression and function of AQP1.

Methods

Immunohistochemical localization of AQP1 was identified in rat peritoneal tissues. Cultured rat peritoneal mesothelial cells (RPMCs) were exposed to hyperosmolality by adding 4% glucose to the culture medium. After 1 hour, 4 hours, 24 hours, and 48 hours, AQP1 was identified by semiquantitative immunoblot and immunocytochemistry. Osmotic water permeability was measured using a light-scattering method.

Results

Immunohistochemistry of rat peritoneal tissues showed the presence of AQP1 in mesothelial cells, venular endothelial cells, and capillary endothelial cells, but not in arteriole and interstitial cells. Semiquantitative immunoblot revealed that exposure to hyperosmolality significantly increased AQP1 expression after 24 hours in whole RPMC lysates (3.3-fold at 24 hours and 3.9-fold at 48 hours). Consistent with the immunoblot, osmotic water permeability of RPMC was augmented 1.7-fold and 2.7-fold after 1 hour and 24 hours, respectively, in a hyperosmotic environment. In RPMC membrane fractions, AQP1 expression was significantly increased after 1 hour of exposure to hyperosmolality (3.9-fold at 1 hour, 7.1-fold at 4 hours, and 8.7-fold at 24 hours). Immunocytochemistry of RPMCs showed that AQP1 was gradually redistributed from the perinuclear area to the peripheral cytoplasm, and then to the plasma membrane after a 1-hour hyperosmotic challenge, suggesting hyperosmolality-induced translocation of AQP1. Upregulation of AQP1 was also observed in the omentum of rats loaded intraperitoneally with hyperosmotic dialysate every day for 10 weeks.

Conclusion

AQP1 is widely distributed in the peritoneal cavity and may provide the major aqueous pathway across the peritoneal barrier. In addition, our findings suggested that hyperosmolality increases AQP1-dependent water permeability in peritoneal tissues by regulating the translocation and synthesis of AQP1 protein.

Role of Aquaporin 1 Signalling in Cancer Development and Progression

Cancer is a major health burden worldwide. Despite the advances in our understanding of its pathogenesis and continued improvement in cancer management and outcomes, there remains a strong clinical demand for more accurate and reliable biomarkers of metastatic progression and novel therapeutic targets to abrogate angiogenesis and tumour progression. Aquaporin 1 (AQP1) is a small hydrophobic integral transmembrane protein with a predominant role in trans-cellular water transport. Recently, over-expression of AQP1 has been associated with many types of cancer as a distinctive clinical prognostic factor. This has prompted researchers to evaluate the link between AQP1 and cancer biological functions. Available literature implicates the role of AQP1 in tumour cell migration, invasion and angiogenesis. This article reviews the current understanding of AQP1-facilitated tumour development and progression with a focus on regulatory mechanisms and downstream signalling pathways.

Structure and function of the thin limbs of the loop of Henle

The thin limbs of the loop of Henle, which comprise the intermediate segment, connect the proximal tubule to the distal tubule and lie entirely within the renal medulla. The descending thin limb consists of at least two or three morphologically and functionally distinct subsegments and participates in transepithelial transport of NaCl, urea, and water. Only one functionally distinct segment is recognized for the ascending thin limb, which carries out transepithelial transport of NaCl and urea in the reabsorptive and/or secretory directions. Membrane transporters involved with passive transcellular Cl, urea, and water fluxes have been characterized for thin limbs; however, these pathways do not account for all transepithelial fluid and solute fluxes that have been measured in vivo. The paracellular pathway has been proposed to play an important role in transepithelial Na and urea fluxes in defined thin-limb subsegments. As the transport pathways become clearer, the overall function of the thin limbs is becoming better understood. Primary and secondary signaling pathways and protein-protein interactions are increasingly recognized as important modulators of thin-limb cell function and cell metabolism. These functions must be investigated under diverse extracellular conditions, particularly for those cells of the deep inner medulla that function in an environment of wide variation in hyperosmolality. Transgenic mouse models of several key water and solute transport proteins have provided significant insights into thin-limb function. An understanding of the overall architecture of the medulla, including juxtapositions of thin limbs with collecting ducts, thick ascending limbs, and vasa recta, is essential for understanding the role of the kidney in maintaining Na and water homeostasis, and for understanding the urine concentrating mechanism.

Sodium-hydrogen exchangers (NHE) in human cardiovascular diseases: interfering strategies and their therapeutic applications

Sodium-hydrogen exchangers (NHE) are among the main regulators of cell volume and intracellular concentration of hydrogen and sodium ions. By indirectly affecting sodium/calcium exchange across the plasma membrane, NHE can also influence the intracellular concentration of calcium. Excess activation of NHE or inappropriate sodium extrusion due to failure of ATP-dependent Na(+)/K(+) transport system can be deleterious during cardiac or peripheral organ ischemia. Besides being responsible for the regulation of intracellular pH and sodium-calcium inward currents, NHE isoform 1 (NHE-1), which is predominantly expressed in the cardiovascular system, influences the tone of the vessel wall in response to a variety of stimuli, including hypertonic stress. Because of the extensive involvement of NHE-1 in cardiac myocyte contracture and necrosis, stunning, reperfusion arrhythmias, as well as hypertension and myocardial diseases such as diabetic cardiomyopathy, efforts have been made in developing inhibitors of this transporter. We here review the biology and regulation of NHE, focusing on current knowledge of the role of NHE-1 as a potential target in the development of novel compounds that could play a role in cardiovascular homeostasis, both in physiological and pathological conditions.

The Osmopressor Response Is Linked to Upregulation of Aquaporin-1 Tyrosine Phosphorylation on Red Blood Cell Membranes

Studies in patients with an impaired efferent baroreflex led us to discover that ingesting water induces a robust increase in blood pressure and vascular resistance. This response was also present in healthy subjects with intact baroreflexes, described as osmopressor response. This study was to discover the physiology of the osmopressor response by determining functional activation of the aquaporin-1 water channel receptor on red blood cell membranes in young healthy subjects. In a randomized, controlled, crossover fashion, 22 young healthy subjects (age, 19–27 years) ingested either 500 or 50 mL of water. Heart rate, blood pressure, cardiac index, and total peripheral vascular resistance were measured using a Finometer hemodynamic monitor. Blood sampling was performed at 5 minutes before and at 25 and 50 minutes after either the water ingestion or control session. Immunoblotting for aquaporin-1 tyrosine phosphorylation was performed before and after subjects ingested either 500 or 50 mL of water. At 25 minutes after the ingestion of 500 mL of water, total peripheral resistance increased significantly, and plasma osmolality decreased. Functional expression of aquaporin-1 tyrosine phosphorylation on red blood cell membranes increased significantly at 25 and 50 minutes after subjects ingested 500 mL of water compared with that before water ingestion. This study concludes that water ingestion produces upregulation of aquaporin-1 tyrosine phosphorylation on red blood cell, which presents as a novel biological marker that occurs simultaneously with the osmopressor response.

Effect of Poria cocos on hypertonic stress-induced water channel expression and apoptosis in renal collecting duct cells

ETHNOPHARMACOLOGICAL RELEVANCE

A major physiological role of the kidney is to regulate body water and urine concentration. Aquaporin-2 (AQP2), a family of water channels, plays an important role in the urinary concentrating process and regulation of water balance in the kidney. The dried sclerotia of Poria cocos Wolf has been known to have a diuretic effect and used for the treatment of chronic edema and nephrosis.

AIM OF THE STUDY

This study was conducted to evaluate the inhibitory effect of the sclerotia of Poria cocos (WPC) on hypertonic stress-induced AQP2 expression and apoptosis in inner medullary collecting duct cell lines (IMCD-3).

MATERIALS AND METHODS

Hypertonic stress was induced by 175mM NaCl. Inhibitory effect of WPC on hypertonic stress-induced AQP2 expression and apoptosis were determined by western blot, RT-PCR, and immunofluorescence.

RESULTS

Hypertonic stress (175mM NaCl) increased in the levels of AQP2 expression by hypertonicity in IMCD-3 cells. WPC attenuated the hypertonicity-induced increase in protein and mRNA levels of AQP2 in a concentration-dependent manner. Pretreatment with WPC attenuated hypertonicity-induced cell death. Hypertonicity increased serum- and glucocorticoid-inducible protein kinase (Sgk1) phosphorylation, however, WPC attenuated the hypertonicity-induced Sgk1 activation. Tonicity-responsive enhancer binding protein (TonEBP) mRNA was also recovered by WPC under hypertonic stress. Pretreatment with WPC presented the similar effect of PKA inhibitor which decreased hypertonic stress-induced AQP2 expression. Hypertonicity increased cAMP levels and the changes were blocked by WPC. On the other hand, hypertonic stress-induced Bax or caspase-3 expression was decreased by WPC, resulting in anti-apoptotic effect.

CONCLUSIONS

These results provided evidence that the beneficial effect of WPC in water balance against in vitro hypertonic stress of renal collecting ducts. In addition, WPC exhibits anti-apoptotic property response to hypertonic stress. Thus, these data suggests that WPC has benefit for the therapeutic approach to the inhibition of renal disorder.

Ginsenoside Rg3 attenuates cell migration via inhibition of aquaporin 1 expression in PC-3M prostate cancer cells

Ginsenoside Rg3 (Rg3), one of the bioactive extracts found in ginseng root, was reported to have anti-cancer activity in various cancer models. The anti-proliferation effect of Rg3 on prostate cancer cells has been well reported. To test whether Rg3 has an anti-metastatic effect on prostate cancer, we treated a highly metastatic PC-3M prostate cancer cell line with Rg3. We found that Rg3 (10μM) led to remarkable inhibition of PC-3M cell migration. Simultaneously, exposure to Rg3 suppressed expression of the aquaporin 1 (AQP1) water channel protein, which has previously been reported to be involved in cell migration. Overexpression of AQP1 attenuated Rg3-induced inhibition of cell migration, and introduction of a shRNA targeting AQP1 abrogated the inhibitory effect of Rg3, although the basal level of cell migration was decreased by RNA interference. In mechanism study, estrogen receptor- and glucocorticoid receptor-dependent pathways are proved uninvolved in the AQP1 regulation by Rg3. However, Rg3 treatment triggered the activation of p38 MAPK; and SB202190, a specific inhibitor of p38 MAPK, antagonized the Rg3-induced regulation of AQP1 and cell migration, suggesting a crucial role for p38 in the regulation process. Deletion analysis of the promoter region of AQP1 was also conducted using dual-luciferase assay, which indicated that the -1000 bp to -200 bp promoter region was involved in the AQP1 regulation by Rg3. In all, we conclude that Rg3 effectively suppresses migration of PC-3M cells by down-regulating AQP1 expression through p38 MAPK pathway and some transcription factors acting on the AQP1 promoter.

Osmoadaptation of Mammalian cells - an orchestrated network of protective genes

In mammals, the cells of the renal medulla are physiologically exposed to interstitial osmolalities several-fold higher that found in any other tissue. Nevertheless, these cells not only have the ability to survive in this harsh environment, but also to function normally, which is critical for maintenance of systemic electrolyte and fluid homeostasis. Over the last two decades, a substantial body of evidence has accumulated, indicating that sequential and well orchestrated genomic responses are required to provide tolerance to osmotic stress. This includes the enhanced expression and action of immediate-early genes, growth arrest and DNA damage inducible genes (GADDs), genes involved in cell cycle control and apoptosis, heat shock proteins, and ultimately that of genes involved in the intracellular accumulation of nonperturbing organic osmolytes. The present review summarizes the sequence of genomic responses conferring resistance against osmotic stress. In addition, the regulatory mechanisms mediating the coordinated genomic response to osmotic stress will be highlighted.

The expression of aquaporin-1 in the medulla of the kidney is dependent on the transcription factor associated with hypertonicity, TonEBP

Expression of aquaporin-1 (AQP1) and -2 (AQP2) channels in the kidney are critical for the maintenance of water homeostasis and the operation of the urinary concentrating mechanism. Hypertonic stress induced in inner medullary (IMCD3) cells by addition of NaCl to the medium substantially up-regulated the mRNA and protein expression of AQP1, suggesting that its activation occurs at a transcriptional and a translational levels. In contrast, no up-regulation of AQP1 was observed when these cells were exposed to the same tonicity by addition of urea. To explore the transcriptional activation of aqp1 under hypertonic stress, we examined the role of the transcription factor associated with hypertonicity, TonEBP. Treatment of IMCD3 cells with the TonEBP inhibitor rottlerin or silencing its expression with specific shRNA technology led to a substantial reduction in AQP1 expression under hypertonic conditions. Moreover, we defined a conserved TonEBP binding site located 811 bp upstream of the aqp1 exon that is essential for its expression. Single site-directed mutation of this TonE site led to a 54 ± 5% (p < 0.01) decrease in AQP1 luciferase-driven activity under hypertonic stress. TonEBP mutant mice display marked decrement in the expression of AQP1 in the inner medulla. In conclusion, these data demonstrate that TonEBP is necessary for the regulation of AQP1 expression in the inner medulla of the kidney under hypertonic conditions.

Angiotensin II and hypertonicity modulate proximal tubular aquaporin 1 expression

Aquaporin 1 (AQP1) is the major water channel in the renal proximal tubule (PT) and thin descending limb of Henle, but its regulation remains elusive. Here, we investigated the effect of ANG II, a key mediator of body water homeostasis, on AQP1 expression in immortalized rat proximal tubule cells (IRPTC) and rat kidney. Real-time PCR on IRPTC exposed to ANG II for 12 h revealed a biphasic effect AQP1 mRNA increased dose dependently in response to 10(-12) to 10(-8) M ANG II but decreased by 50% with 10(-7) M ANG II. The twofold increase of AQP1 mRNA in the presence of 10(-8) M ANG II was abolished by the AT(1) receptor blocker losartan. Hypertonicity due to either NaCl or mannitol also upregulated AQP1 mRNA by three- and twofold, respectively. Immunocytochemistry and Western blotting revealed a two- to threefold increase in AQP1 protein expression in IRPTC exposed concomitantly to ANG II (10(-8)M) and hypertonic medium (either NaCl or mannitol), indicating that these stimuli were not additive. Three-dimensional reconstruction of confocal images suggested that AQP1 expression was increased by ANG II in both the apical and basolateral poles of IRPTC. In vivo studies showed that short-term ANG II infusion had a diuretic effect, while this effect was attenuated after several days of ANG II infusion. After 10 days, we observed a twofold increase in AQP1 expression in the PT and thin descending limb of Henle of ANG II-infused rats that was abolished when rats were treated with the selective AT(1)-receptor antagonist olmesartan. Thus ANG II increases AQP1 expression in vitro and in vivo via direct interaction with the AT(1) receptor, providing an important regulatory mechanism to link PT water reabsorption to body fluid homeostasis via the renin-angiotensin system.

Effect of hyperosmotic stimulation on aquaporins gene expression in chick kidney

Birds can produce hyperosmotic urine, but their renal morphology differs from that of mammals. Recent studies in mammals, suggested that various aquaporins (AQPs) are present in the kidney and play crucial roles in urine production. To elucidate the role of AQPs in the avian kidney, we first examined for the presence of AQP1, 2, 3, 4, 7 and 9 mRNAs in the chick (Gallus gallus) kidney by RT-PCR analysis. Next, we quantified variations of AQPs mRNAs levels in chick kidney after hyperosmotic stimulation (water-deprivation or salt-loading) by real-time RT-PCR analysis. Our study showed that in addition to AQP1, 2, 3, 4 and 7, chick kidney also expressed AQP9 and that hyperosmotic stimulation induced changes in AQPs expression. In particular, water-deprivation increased AQP2 and AQP3 mRNAs levels, whereas salt-loading induced a significant increase in AQP1, AQP2 and AQP9 mRNAs levels. AQP4 and AQP7 mRNA levels were not affected by any hyperosmotic stimulation. Taken together, these results indicated that the presence of AQPs in chick kidney is similar to that in mammals, that the chick kidney has an additional AQP9 and that AQP1, 2, 3 and 9 may play a crucial but different role in water permeability in this organ.

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.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Calcineurin-NFATc signaling pathway regulates AQP2 expression in response to calcium signals and osmotic stress

The aquaporin (AQP)2 channel mediates the reabsorption of water in renal collecting ducts in response to arginine vasopressin (AVP) and hypertonicity. Here we show that AQP2 expression is induced not only by the tonicity-responsive enhancer binding protein (TonEBP)/nuclear factor of activated T cells (NFAT)5-mediated hypertonic stress response but also by the calcium-dependent calcineurin-NFATc pathway. The induction of AQP2 expression by the calcineurin-NFATc pathway can occur in the absence of TonEBP/NFAT5. Mutational and chromatin immunoprecipitation analyses revealed the existence of functional NFAT binding sites within the proximal AQP2 promoter responsible for regulation of AQP2 by NFATc proteins and TonEBP/NFAT5. Contrary to the notion that TonEBP/NFAT5 is the only Rel/NFAT family member regulated by tonicity, we found that hypertonicity promotes the nuclear translocation of NFATc proteins for the subsequent induction of AQP2 expression. Calcineurin activity was also found to be involved in the induction of TonEBP/NFAT5 expression by hypertonicity, thus further defining the signaling mechanisms that underlie the TonEBP/NFAT5 osmotic stress response pathway. The coordinate regulation of AQP2 expression by both osmotic stress and calcium signaling appears to provide a means to integrate diverse extracellular signals into optimal cellular responses.

Hypertonic induction of aquaporin-5: novel role of hypoxia-inducible factor-1alpha

Aquaporin-5 (AQP5) is a water channel protein expressed on the apical surface of alveolar epithelial type I cells in distal rat lung, suggesting a role for AQP5 in regulating alveolar surface liquid tonicity and/or cell volume. We investigated the molecular mechanisms underlying hypertonic induction of AQP5 in primary rat alveolar epithelial cells (AEC). Steady-state levels of AQP5 mRNA and protein were increased by exposure to sorbitol (200 mM in culture fluid) for 24 h. The increase in AQP5 was not accompanied by changes in mRNA half-life. Transduction of mouse lung epithelial (MLE-15) cells and primary rat AEC with lentivirus vectors encoding AQP5-luciferase demonstrated transcriptional activation of the reporter by exposure to hypertonic sorbitol solution. Hybridization of proteins from sorbitol-treated cells to a transcription factor DNA array demonstrated induction of hypoxia-inducible factor-1alpha (HIF-1alpha) by hypertonicity, which was confirmed by quantitative RT-PCR. Cotransfections of AQP5-luciferase with HIF-1alpha and HIF-1beta expression plasmids in MLE-15 cells led to dose-dependent transcriptional enhancement, which was partially abrogated by mutagenesis of putative HIF-1alpha binding sites in the proximal AQP5 promoter. Importantly, hypertonic induction of AQP5 was significantly inhibited by preventing HIF-1alpha induction with small interfering RNA. Hypertonicity induced activation of a transiently transfected vascular endothelial growth factor (VEGF) hypoxia response element-driven luciferase construct and increased expression of endogenous VEGF. These results demonstrate that hypertonic induction of both AQP5 and VEGF is transcriptionally regulated and mediated, at least in part, by HIF-1alpha, suggesting a novel role for HIF-1alpha in modulating cellular adaptive responses to osmotic stress.

Chronic hyperglycemia modulates osteoblast gene expression through osmotic and non-osmotic pathways

Insulin dependent diabetes mellitus (IDDM; type I) is a chronic disease stemming from little or no insulin production and elevated blood glucose levels. IDDM is associated with osteoporosis and increased fracture rates. The mechanisms underlying IDDM associated bone loss are not known. Previously we demonstrated that osteoblasts exhibit a response to acute (1 and 24 h) hyperglycemia and hyperosmolality. Here we examined the influence of chronic hyperglycemia (30 mM) and its associated hyperosmolality on osteoblast phenotype. Our findings demonstrate that osteoblasts respond to chronic hyperglycemia through modulated gene expression. Specifically, chronic hyperglycemia increases alkaline phosphatase activity and expression and decreases osteocalcin, MMP-13, VEGF and GAPDH expression. Of these genes, only MMP-13 mRNA levels exhibit a similar suppression in response to hyperosmotic conditions (mannitol treatment). Acute hyperglycemia for a 48-h period was also capable of inducing alkaline phosphatase and suppressing osteocalcin, MMP-13, VEGF, and GAPDH expression in differentiated osteoblasts. This suggests that acute responses in differentiated cells are maintained chronically. In addition, hyperglycemic and hyperosmotic conditions increased PPARgamma2 expression, although this increase reached significance only in 21 days chronic glucose treated cultures. Given that osteocalcin is suppressed and PPARgamma2 expression is increased in type I diabetic mouse model bones, these findings suggest that diabetes-associated hyperglycemia may modulate osteoblast gene expression, function and bone formation and thereby contribute to type I diabetic bone loss.

Stimulation of aquaporin-5 and transepithelial water permeability in human airway epithelium by hyperosmotic stress

Osmotic water permeability (P(f )) was measured in spheroid-shaped human nasal airway epithelial explants pre-exposed to increasing levels of hyperosmotic stress. The fluid-filled spheroids, derived from nasal polyps, were lined by a single cell layer with the ciliated apical cell membrane facing the outside. The P(f ) was determined from diameter changes of the spheroids in response to changes in bathing medium osmolarity forth and back between 300 and 225 mOsm x l(-1). Continuous diameter measurements also allowed determination of spontaneous fluid absorption. Hyperosmotic pretreatment (increase from 300 up to 600 mOsm x l(-1)) caused a time- and osmolarity-dependent increase (up to approximately 1.5 times) in epithelial P(f ) which was of similar magnitude in cystic fibrosis (CF) and non-CF spheroids. The effect saturated at approximately 450 mOsm x l(-1) and at approximately 24 h. Expression of aquaporin-5 (AQP5), studied by immunofluorescence and confocal microscopy, showed an increase in parallel with the increase in P(f ) following hyperosmotic stress. The AQP5 was localized both in cytoplasmic vesicles and in apical cell membranes. Spontaneous fluid absorption rates were equal in CF and non-CF spheroids and were not significantly influenced by hyperosmotic stress. The results suggest that hyperosmotic stress is an important activator of AQP-5 in human airway epithelium, leading to significantly increased transepithelial water permeability.

Increased aquaporin-1 expression in choroid plexus epithelium after systemic hyponatremia

The expression of aquaporin-1 (AQP1) protein, a water channel, in the choroid plexus epithelium was examined in the rat to determine the role of AQP1 in the pathophysiology of systemic hyponatremia. Systemic hyponatremia was produced by coadministration of 30 ml hypotonic dextrose solution (140 mM) intraperitoneally and 3 microg 1-deamino-8-d-arginine vasopressin (dDAVP) subcutaneously. This hydration protocol produced severe systemic hyponatremia: 2h after drug administration, normonatremic serum osmolarity and Na(+) concentration (296+/-5mOsm/L and 140+/-5 mM) were reduced to 252+/-5mOsm/L and 117+/-2 mM, respectively. AQP1 expression in the third ventricle choroid plexus epithelium was determined by using immunohistochemistry and quantitative immunoblot analysis. After 2h of systemic hyponatremia, AQP1 expression was significantly increased by 28.2+/-2.7% (n=6, P<0.05), whereas it was attenuated to 15.7+/-2.2% (n=6, P<0.05) after 6h of systemic hyponatremia. In addition, TUNEL staining revealed apoptotic granule cells in the dentate gyrus (DG) adjacent to the third ventricle with a peak measured 2h after the start of the systemic hyponatremia protocol, while virtually no apoptotic granule cells left 6h after the start of the hyponatremic treatment. These findings suggest that AQP1-mediated excessive water influx into the brain through the blood-cerebrospinal fluid barrier during systemic hyponatremia may elevate the intracranial pressure and result in the apoptotic brain cell death. Thus, AQP1 in the choroid plexus may play an important role in the pathogenesis of systemic hyponatremia.

Effect of water deprivation on aquaporin 4 (AQP4) mRNA expression in chickens (Gallus domesticus)

Aquaporin (AQP) 4 is a member of the AQP gene family of water-selective transport proteins. We studied the effect of water deprivation on AQP4 gene expression in chickens. The nucleotide sequence of a chicken aquaporin 4 (AQP4) cDNA that encodes a protein of 335 amino acids showed high homology to mammalian AQP4. Using Northern blotting analysis, AQP4 mRNA in chickens was observed as a band of approximately 5.5 kb in several tissues in addition to the hypothalamus, proventriculus, kidney, and breast muscle. Quantitative analysis by real-time RT-PCR analysis showed that the mRNA expression of AQP4 in the hypothalamus significantly increased after dehydration. On the other hand, the mRNA expression of AQP4 in the kidney significantly decreased after dehydration. This suggests that AQP4 may play a pivotal role in osmoregulation in the chicken brain.

Clinical update on renal aquaporins

Following the discovery of the aquaporin-1 water channel over a decade ago, molecular techniques have been developed to examine the role of renal aquaporin water channels under numerous physiological and pathological conditions. The present article reviews current knowledge regarding the function and dysfunction of renal aquaporins in disorders of water metabolism.

Modulation of Hypertonicity-Induced Aquaporin-1 by Sodium Chloride, Urea, Betaine, and Heat Shock in Murine Renal Medullary Cells

Aquaporin-1 (AQP1) expression is induced by hypertonicity in renal medullary cells. The purpose of the present study was to elucidate the role of sodium chloride (NaCl), urea, betaine, and heat shock on hypertonicity-induced AQP1 expression in cultured murine renal medullary-K2 (mIMCD-K2) cells. AQP1 expression was maximally induced under mild hypertonic medium supplemented with 100 mM NaCl (N100), whereas severe hypertonic medium supplemented with 150 mM NaCl (N150) caused little AQP1 induction. The reduction of AQP1 expression in N150 was associated with reduced cell viability. When cells were exposed continuously to N100, hypertonicity-induced AQP1 expression was elevated, whereas the return to isotonic medium reduced AQP1 expression in a time-dependent manner. The half-life of AQP1 protein in isotonic conditions was approximately 4 h, whereas hypertonicity markedly increased its half-life. These results indicate that hypertonicity plays an important role in AQP1 induction, stability, and degradation. On the contrary, urea inhibited hypertonicity-induced AQP1 expression in a dose-dependent manner. The addition of organic osmolyte betaine in N150 enhanced hypertonicity-induced AQP1 expression, whereas it decreased AQP1 expression in N100. This suggests that the excessive accumulation of betaine may counteract hypertonic stress and thus attenuate hypertonicity-induced AQP1 expression. Heat shock treatment promoted hypertonicity-induced AQP1 and heat shock protein 70 (HSP70) expression in both N100 and N150, suggesting an effect on the stability of hypertonicity-induced AQP1 expression. Taken together, NaCl, urea, betaine, and heat shock that regulate hypertonicity-induced AQP1 expression are potentially important factors in urinary concentration and contribute to the steady-state level of AQP1 expression.

Regulation of brain aquaporins

Emerging evidence suggests that brain aquaporins (AQP) play important roles for the dynamic regulation of brain water homeostasis and for the regulation of cerebrospinal fluid production. This review deals with the short- and long-term regulation of AQP4 and AQP9, both expressed in astrocytes, and of AQP1, expressed in the choroid plexus. AQP1 and 4 have in other cell types been shown to be regulated by phosphorylation. Phosphorylation affects the gating of AQP4 and the trafficking and insertion into membrane of AQP1. Mercury inhibits the water permeability of AQP1 and AQP9, but not AQP4. The permeability of AQP4 is increased by lead. AQP4 is also regulated by protein-protein interaction. The assembly between AQP4 and syntrophin is required for the proper localization of AQP4 in the astrocyte plasma membrane that faces capillaries. There is evidence from studies on peripheral tissues that steroid hormones regulate the expression of AQP1, AQP4 and AQP9. There is also evidence that the expression of AQP1 can be regulated by ubiquitination, and that osmolality can regulate the expression of AQP1, AQP4 and AQP9. Further insight into the mechanisms by which brain AQPs are regulated will be of utmost clinical importance, since perturbed water flow via brain AQPs has been implicated in many neurological diseases and since, in brain edema, water flow via AQP4 may have a harmful effect.

Hyperosmolar mannitol simulates expression of aquaporins 4 and 9 through a p38 mitogen-activated protein kinase-dependent pathway in rat astrocytes

The membrane pore proteins, aquaporins (AQPs), facilitate the osmotically driven passage of water and, in some instances, small solutes. Under hyperosmotic conditions, the expression of some AQPs changes, and some studies have shown that the expression of AQP1 and AQP5 is regulated by MAPKs. However, the mechanisms regulating the expression of AQP4 and AQP9 induced by hyperosmotic stress are poorly understood. In this study, we observed that hyperosmotic stress induced by mannitol increased the expression of AQP4 and AQP9 in cultured rat astrocytes, and intraperitoneal infusion of mannitol increased AQP4 and AQP9 in the rat brain cortex. In addition, a p38 MAPK inhibitor, but not ERK and JNK inhibitors, suppressed their expression in cultured astrocytes. AQPs play important roles in maintaining brain homeostasis. The expression of AQP4 and AQP9 in astrocytes changes after brain ischemia or traumatic injury, and some studies have shown that p38 MAPK in astrocytes is activated under similar conditions. Since mannitol is commonly used to reduce brain edema, understanding the regulation of AQPs and p38 MAPK in astrocytes under hyperosmotic conditions induced with mannitol may lead to a control of water movements and a new treatment for brain edema.

Hypertonicity-induced aquaporin-1 (AQP1) expression is mediated by the activation of MAPK pathways and hypertonicity-responsive element in the AQP1 gene

Aquaporin-1 (AQP1) is a water channel that is induced by hypertonicity. The present study was undertaken to clarify the osmoregulation mechanism of AQP1 in renal medullary cells. In cultured mouse medullary (mIMCD-3) cells, AQP1 expression was significantly induced by hypertonic treatment with impermeable solutes, whereas urea had no effect on AQP1 expression. This result indicates the requirement of a hypertonic gradient. Hypertonicity activated ERK, p38 kinase, and JNK in mIMCD-3 cells. Furthermore, all three MAPKs were phosphorylated by the upstream activation of MEK1/2, MKK3/6, and MKK4, respectively. The treatments with MEK inhibitor U0126, p38 kinase inhibitor SB203580, and JNK inhibitor SP600125 significantly attenuated hypertonicity-induced AQP1 expression in mIMCD-3 cells. In addition, hypertonicity-induced AQP1 expression was significantly reduced by both the dominant-negative mutants of JNK1- and JNK2-expressing mIMCD-3 cells. NaCl-inducible activity of AQP1 promoter, which contains a hypertonicity response element, was attenuated in the presence of U0126, SB203580, and SP600125 in a dose-dependent manner and was also significantly reduced by the dominant-negative mutants of JNK1 and JNK2. These data demonstrate that the activation of ERK, p38 kinase, and JNK pathways and the hypertonicity response element in the AQP1 promoter are involved in hypertonicity-induced AQP1 expression in mIMCD-3 cells.

Urea stress is more akin to EGF exposure than to hypertonic stress in renal medullary cells

Although urea is considered to be a cell stressor even in renal medullary cells perpetually exposed to this solute in vivo by virtue of the renal concentrating mechanism, aspects of urea signaling resemble that of a peptide mitogen. Urea was compared with epidermal growth factor and hypertonic NaCl or hypertonic mannitol using a large-scale expression array-based approach. The expression profile in response to urea stress more closely resembled that of EGF treatment than hypertonic stress, as determined by hierarchical cluster analysis; the effect of urea+NaCl was equidistant from that of either solute applied individually. Among the most highly urea- and hypertonicity-responsive transcripts were genes that had previously been shown to be responsive to these solutes, validating this approach. Increased expression of the activating transcription factor 3 by urea was newly detected via expression array and confirmed via immunoblot analysis. Earlier, we noted an abrogation of tonicity-dependent gene regulation by urea, primarily in a transient transfection-based model (Tian W and Cohen DM. Am J Physiol Renal Physiol 280: F904-F912, 2001). Here we applied K-means cluster analysis to demonstrate that the genes most profoundly up- or downregulated by hypertonic stress were partially restored toward basal levels in the presence of urea pretreatment. These global expression data are consistent with our earlier biochemical studies suggesting that urea affords cytoprotection in this context. In the aggregate, these data strongly support the hypothesis that the urea effect in renal medullary cells resembles that of a peptide mitogen in terms of the adaptive program of gene expression and in terms of cytoprotection from hypertonicity.

Identification and characterization of a novel hypertonicity-responsive element in the human aquaporin-1 gene

The AQP1 gene is transcriptionally upregulated in response to hypertonicity. However, the molecular mechanism in hypertonicity-induced transcription in the AQP1 gene has been poorly understood. Here we report a novel hypertonicity response element (HRE) in the AQP1 gene. A critical cis-acting element to hypertonicity in the AQP1 promoter is located at -54 to -46 and the change from GCTCCCCCC to GCTTTCCCC completely abolished the hypertonic induction and osmotic response, thus implicating the importance of the first CC sequence in the region. Two prominent DNA-protein complexes were observed electrophoretic mobility shift assay (EMSA), and the band intensities in nuclear extracts from osmotically stressed cells were much higher than that of isotonic nuclear extracts. The EMSA supershift assay with anti-Sp1 antibody showed that two retarded bands did not bind to anti-Sp1 antibody, suggesting that the DNA binding proteins bound to the HRE are not Sp1 family proteins. These data suggest that the transcription of the AQP1 by hypertonicity in renal cells is upregulated by the interaction with putative DNA binding proteins to a novel HRE located at -54 to -46 in the AQP1 gene.

Osmotic stress up-regulates aquaporin-3 gene expression in cultured human keratinocytes

Of ten members of the aquaporin family (AQP), the mRNA expression and regulation of AQP1, AQP3, AQP4 and AQP9 in cultured human keratinocytes were examined by an RNase protection assay. AQP3 mRNA was expressed in growing and differentiating cells, while AQP9 mRNA was only detected in differentiating cells. The epidermis in skin-equivalent cultures expressed both AQP3 and AQP9 mRNA. However, neither AQP1 nor AQP4 mRNA was detectable in either monolayer or skin-equivalent cultures. Incubation of keratinocytes in sorbitol-added hypertonic medium increased AQP3 mRNA expression. This was confirmed using other solutes such as NaCl, mannitol, glucose and sucrose. The effect of sorbitol was reversible, dose-dependent and maximal at 24 h after addition. However, AQP1, AQP4 and AQP9 mRNA expression were unchanged under any of the hypertonic conditions examined. These findings indicated that osmotic stress up-regulates AQP3 gene expression in cultured keratinocytes.

Hypertonicity-induced expression of aquaporin 3 in MDCK cells

Aquaporins (AQPs) are water channel proteins that participate in water transport. In the principal cells of the kidney collecting duct, water reabsorption is mediated by the combined action of AQP2 in the apical membrane and both AQP3 and AQP4 in the basolateral membrane, and the expression of AQP2 and AQP3 is regulated by antidiuretic hormone and water restriction. The effect of hypertonicity on AQP3 expression in Madin-Darby canine kidney (MDCK) epithelial cells was investigated by exposing the cells to hypertonic medium containing raffinose or NaCl. Northern blot and immunoblot analyses revealed that the amounts of AQP3 mRNA and AQP3 protein, respectively, were markedly increased by exposure of cells to hypertonicity. These effects were maximal at 12 and 24 h, respectively. Immunofluorescence and immunoelectron microscopy also demonstrated that the abundance of AQP3 protein was increased in cells incubated in hypertonic medium and that the protein was localized at the basolateral plasma membrane. These results indicate that the expression of AQP3 is upregulated by hypertonicity.

Altered ubiquitination and stability of aquaporin-1 in hypertonic stress

Aquaporin-1 (AQP1) water channel protein expression is increased by hypertonic stress. The contribution of changes in protein stability to hypertonic induction of AQP1 have not been described. Incubation of BALB/c fibroblasts spontaneously expressing AQP1 with proteasome inhibitors increased AQP1 expression, suggesting basal proteasome-dependent degradation of the protein. Degradation by the proteasome is thought to be triggered by polyubiquitination of a target protein. To determine whether AQP1 is ubiquitinated, immunoprecipitation with anti-AQP1 antibodies was performed, and the resultant samples were probed by protein immunoblot for the presence of ubiquitin. Immunoblots demonstrated ubiquitination of AQP1 under control conditions that increased after treatment with proteasome inhibitors (MG132, lactacystin). Exposure of cells to hypertonic medium for as little as 4 h decreased ubiquitination of AQP1, an effect that persisted through 24 h in hypertonic medium. Using metabolic labeling with [(35)S]methionine, the half-life of AQP1 protein under isotonic conditions was found to be <4 h. AQP1 protein half-life was markedly increased by exposure of cells to hypertonic medium. These observations provide evidence that aquaporins are a target for ubiquitination and proteasome-dependent degradation. Additionally, these studies demonstrate that reduced protein ubiquitination and increased protein stability lead to increased levels of AQP1 expression during hypertonic stress.

Protein kinase A-dependent phosphorylation of aquaporin-1

The molecular mechanisms for regulating water balance in many tissues are unknown. Like the kidney, the eye contains multiple water channel proteins (aquaporins) that transport water through membranes, including two (AQP1 and AQP4) in the ciliary body, the site of aqueous humor production. Previous results from our laboratory demonstrated that water channel activity of AQP1 was significantly increased by protein kinase A (PKA) activators such as cyclic-AMP (cAMP) and forskolin. The purpose of this study is to determine whether PKA-dependent protein phosphorylation is involved in the regulation of water channel activity of AQP1. Results presented here suggest that catalytic subunit of protein kinase A significantly increased the amount of phosphorylated AQP1 protein. In addition, these results indicated that cAMP-responsive redistribution of AQP1 may be regulated by phosphorylation of AQP1. Moreover, they provide new insights on the molecular mechanisms for regulating water balance in several tissues involving rapid water transport such as ciliary epithelium. In addition, they suggest important potential roles for AQP1 in several clinical disorders involving rapid water transport such as glaucoma.

Hyperosmotic NaCl and urea synergistically regulate the expression of the UT-A2 urea transporter in vitro and in vivo

The UT-A2 urea transporter is involved in the recycling of urea through the kidney, a process required to maintain high osmotic gradients. Dehydration increases UT-A2 expression in vivo. The tissue distribution of UT-A2 suggested that hyperosmolarity, and not vasopressin, might mediate this effect. We have analyzed the regulation of UT-A2 expression by ambiant osmolarity both in vitro (mIMCD3 cell line) and in vivo (rat kidney medulla). The UT-A2 mRNA was found to be synergistically up-regulated by a combination of NaCl and urea. Curiously, the UT-A2 protein was undetectable in this hypertonic culture condition, or after transfection of the UT-A2 cDNA, whereas it could be detected in HEK-293 transfected cells. Treating rats with furosemide, a diuretic which decreases the kidney interstitium osmolarity without affecting vasopressin levels, led to decreased levels of the UT-A2 protein. Our results show that the UT-A2 urea transporter is regulated by hyperosmolarity both in vitro and in vivo.

Hypertonic induction of aquaporin-5 expression through an ERK-dependent pathway

Aquaporin-5 (AQP5) is a water channel protein expressed in lung, salivary gland, and lacrimal gland epithelia. Each of these sites may experience fluctuations in surface liquid osmolarity; however, osmotic regulation of AQP5 expression has not been reported. This study demonstrates that AQP5 is induced by hypertonic stress and that induction requires activation of extracellular signal-regulated kinase (ERK). Incubation of mouse lung epithelial cells (MLE-15) in hypertonic medium produced a dose-dependent increase in AQP5 expression; AQP5 protein peaked by 24 h and returned to baseline levels within hours of returning cells to isotonic medium. AQP5 induction was observed only with relatively impermeable solutes, suggesting an osmotic pressure gradient is required for induction. ERK was selectively activated in MLE-15 cells by hypertonic stress, and inhibition of ERK activation with two distinct mitogen-activated extracellular regulated kinase kinase (MEK) inhibitors, U0126 and PD98059, blocked AQP5 induction. AQP5 induction was also observed in the lung, salivary, and lacrimal glands of hyperosmolar rats, suggesting potential physiologic relevance for osmotic regulation of AQP5 expression. This report provides the first example of hypertonic induction of an extrarenal aquaporin, as well as the first association between mitogen-activated protein kinase signaling and aquaporin expression.

Aquaporin-1 expression in proximal tubule epithelial cells of human kidney is regulated by hyperosmolarity and contrast agents

Primary cells of renal proximal tubule epithelium (S1 segment) of human kidney (HRPTE cells) up-regulate aquaporin-1 (AQP-1) expression in response to hyperosmolarity. NaCl and D(+)-raffinose increased (2-2.5 fold) AQP-1 expression when medium osmolarity was 400 and 500 mOsm/kg.H2O. Urea did not have this effect. Unlike our previous findings with mIMCD-3 cells, vasopressin (10(-8)M) did not affect AQP-1 expression in HRPTE cells in isosmolar or NaCl-enriched hyperosmolar conditions. Furthermore, HRPTE cells increased (3-4 fold) AQP-1 expression when exposed to hyperosmolar Reno-60 and Hypaque-76 (diatrizoates, ionic) contrast agents at 400 and 500 mOsm/kg.H2O. Isosmolar (290 mOsm/kg H2O) Visipaque (iodixanol, non-ionic) at 10% (v/v) concentrations also increased AQP-1 expression, and 25% v/v of Visipaque rendered morphological alterations of HRPTE cells and a 3-fold increase in AQP-1 expression after 24h exposure. Finally, semi-quantitative RT-PCR of HRPTE cells subjected to various isosmolar or hyperosmolar conditions demonstrated up-regulation of AQP-1 mRNA and protein levels. Our results suggest AQP-1 up-regulation in HRPTE cells exposed to environmental stresses such as hyperosmolarity and high doses of isosmolar contrast agents.