Development of cytosolic hypoxia and hypoxia-inducible factor stabilization are facilitated by aquaporin-1 expression

Mechanistic complement of autosomal dominant polycystic kidney disease: the role of aquaporins

Autosomal dominant polycystic kidney disease is a genetic kidney disease caused by mutations in the genes PKD1 or PKD2. Its course is characterized by the formation of progressively enlarged cysts in the renal tubules bilaterally. The basic genetic explanation for autosomal dominant polycystic kidney disease is the double-hit theory, and many of its mechanistic issues can be explained by the cilia doctrine. However, the precise molecular mechanisms underpinning this condition's occurrence are still not completely understood. Experimental evidence suggests that aquaporins, a class of transmembrane channel proteins, including aquaporin-1, aquaporin-2, aquaporin-3, and aquaporin-11, are involved in the mechanism of autosomal dominant polycystic kidney disease. Aquaporins are either a potential new target for the treatment of autosomal dominant polycystic kidney disease, and further study into the physiopathological role of aquaporins in autosomal dominant polycystic kidney disease will assist to clarify the disease's pathophysiology and increase the pool of potential treatment options. We primarily cover pertinent findings on aquaporins in autosomal dominant polycystic kidney disease in this review.

Pathogenesis of diabetic complications: Exploring hypoxic niche formation and HIF-1α activation

Hypoxia is a common feature of diabetic tissues, which highly correlates to the progression of diabetes. The formation of hypoxic context is induced by disrupted oxygen homeostasis that is predominantly driven by vascular remodeling in diabetes. While different types of vascular impairments have been reported, the specific features and underlying mechanisms are yet to be fully understood. Under hypoxic condition, cells upregulate hypoxia-inducible factor-1α (HIF-1α), an oxygen sensor that coordinates oxygen concentration and cell metabolism under hypoxic conditions. However, diabetic context exploits this machinery for pathogenic functions. Although HIF-1α protects cells from diabetic insult in multiple tissues, it also jeopardizes cell function in the retina. To gain a deeper understanding of hypoxia in diabetic complications, we focus on the formation of tissue hypoxia and the outcomes of HIF-1α dysregulation under diabetic context. Hopefully, this review can provide a better understanding on hypoxia biology in diabetes.

Hyperoxic-hypoxic Paradox: Breast Cancer Microenvironment and an Innovative Treatment Strategy

A small therapeutic range of oxygen is required for effective metabolism. As a result, hypoxia (low oxygen concentration) is one of the most potent inducers of gene expression, metabolic alterations, and regenerative processes, such as angiogenesis, stem cell proliferation, migration, and differentiation. The cellular response is controlled by sensing the increased oxygen levels (hyperoxia) or hypoxia via specific chemoreceptor cells. Surprisingly, changes in free oxygen concentration instead of absolute oxygen levels may be regarded as a deficiency of oxygen at the cellular level. Recurrent intermittent hyperoxia may trigger many mediators of cellular pathways typically generated during hypoxia. The dilemma of hyperoxic-hypoxic conditions is known as the hyperoxic-hypoxic paradox. According to the latest data, the hypoxic microenvironment, crucial during cancer formation, has been demonstrated to play a key role in regulating breast cancer growth and metastasis. Hypoxic circumstances cause breast cancer cells to respond in a variety of ways. Transcription factors are identified as hypoxia-inducible factors (HIFs) that have been suggested to be a factor in the pathobiology of breast cancer and a possible therapeutic target, driving the cellular response to hypoxia. Breast cancer has a dismal prognosis due to a high level of resistance to practically all well-known cancer management that has been related to hypoxia-based interactions between tumor cells and the stromal milieu. We attempt to review the enigma by exploring the starring roles of HIFs in breast cancer, the HIF paradox, and the hyperoxic-hypoxic enigma.

Achilles' Heel-The Significance of Maintaining Microenvironmental Homeostasis in the Nucleus Pulposus for Intervertebral Discs

The dysregulation of intracellular and extracellular environments as well as the aberrant expression of ion channels on the cell membrane are intricately linked to a diverse array of degenerative disorders, including intervertebral disc degeneration. This condition is a significant contributor to low back pain, which poses a substantial burden on both personal quality of life and societal economics. Changes in the number and function of ion channels can disrupt the water and ion balance both inside and outside cells, thereby impacting the physiological functions of tissues and organs. Therefore, maintaining ion homeostasis and stable expression of ion channels within the cellular microenvironment may prove beneficial in the treatment of disc degeneration. Aquaporin (AQP), calcium ion channels, and acid-sensitive ion channels (ASIC) play crucial roles in regulating water, calcium ions, and hydrogen ions levels. These channels have significant effects on physiological and pathological processes such as cellular aging, inflammatory response, stromal decomposition, endoplasmic reticulum stress, and accumulation of cell metabolites. Additionally, Piezo 1, transient receptor potential vanilloid type 4 (TRPV4), tension response enhancer binding protein (TonEBP), potassium ions, zinc ions, and tungsten all play a role in the process of intervertebral disc degeneration. This review endeavors to elucidate alterations in the microenvironment of the nucleus pulposus during intervertebral disc degeneration (IVDD), with a view to offer novel insights and approaches for exploring therapeutic interventions against disc degeneration.

The Effect of High-Intensity Interval Exercise on Short-Term Glycaemic Control, Serum Level of Key Mediator in Hypoxia and Pro-Inflammatory Cytokines in Patients with Type 1 Diabetes-An Exploratory Case Study

Type 1 diabetes (T1D) is associated with hyperglycaemia-induced hypoxia and inflammation. This study assessed the effects of a single bout of high-intensity interval exercise (HIIE) on glycaemia (BG) and serum level of pro-inflammatory cytokines, and an essential mediator of adaptive response to hypoxia in T1D patients. The macronutrient intake was also evaluated. Nine patients suffering from T1D for about 12 years and nine healthy individuals (CG) were enrolled and completed one session of HIIE at the intensity of 120% lactate threshold with a duration of 4 × 5 min intermittent with 5 min rests after each bout of exercise. Capillary and venous blood were withdrawn at rest, immediately after and at 24 h post-HIIE for analysis of BG, hypoxia-inducible factor alpha (HIF-1α), tumour necrosis factor alpha (TNF-α) and vascular-endothelial growth factor (VEGF). Pre-exercise BG was significantly higher in the T1D patients compared to the CG (p = 0.043). HIIE led to a significant decline in T1D patients' BG (p = 0.027) and a tendency for a lower BG at 24 h post-HIIE vs. pre-HIIE. HIF-1α was significantly elevated in the T1D patients compared to CG and there was a trend for HIF-1α to decline, and for VEGF and TNF-α to increase in response to HIIE in the T1D group. Both groups consumed more and less than the recommended amounts of protein and fat, respectively. In the T1D group, a tendency for a higher digestible carbohydrate intake and more frequent hyperglycaemic episodes on the day after HIIE were observed. HIIE was effective in reducing T1D patients' glycaemia and improving short-term glycaemic control. HIIE has the potential to improve adaptive response to hypoxia by elevating the serum level of VEGF. Patients' diet and level of physical activity should be screened on a regular basis, and they should be educated on the glycaemic effects of digestible carbohydrates.

Water channel protein AQP1 in cytoplasm is a critical factor in breast cancer local invasion

BACKGROUND

Metastasis of breast cancer grows from the local invasion to the distant colonization. Blocking the local invasion step would be promising for breast cancer treatment. Our present study demonstrated AQP1 was a crucial target in breast cancer local invasion.

METHODS

Mass spectrometry combined with bioinformatics analysis was used to identify AQP1 associated proteins ANXA2 and Rab1b. Co-immunoprecipitation, immunofluorescence assays and cell functional experiments were carried out to define the relationship among AQP1, ANXA2 and Rab1b and their re-localization in breast cancer cells. The Cox proportional hazards regression model was performed toward the identification of relevant prognostic factors. Survival curves were plotted by the Kaplan-Meier method and compared by the log-rank test.

RESULTS

Here, we show that the cytoplasmic water channel protein AQP1, a crucial target in breast cancer local invasion, recruited ANXA2 from the cellular membrane to the Golgi apparatus, promoted Golgi apparatus extension, and induced breast cancer cell migration and invasion. In addition, cytoplasmic AQP1 recruited cytosolic free Rab1b to the Golgi apparatus to form a ternary complex containing AQP1, ANXA2, and Rab1b, which induced cellular secretion of the pro-metastatic proteins ICAM1 and CTSS. Cellular secretion of ICAM1 and CTSS led to the migration and invasion of breast cancer cells. Both in vivo assay and clinical analysis data confirmed above results.

CONCLUSIONS

Our findings suggested a novel mechanism for AQP1-induced breast cancer local invasion. Therefore, targeting AQP1 offers promises in breast cancer treatment.

AQP1 in the Gastrointestinal Tract of Mice: Expression Pattern and Impact of AQP1 Knockout on Colonic Function

Aquaporin 1 (AQP1) is one of thirteen known mammalian aquaporins. Its main function is the transport of water across cell membranes. Lately, a role of AQP has been attributed to other physiological and pathological functions including cell migration and peripheral pain perception. AQP1 has been found in several parts of the enteric nervous system, e.g., in the rat ileum and in the ovine duodenum. Its function in the intestine appears to be multifaceted and is still not completely understood. The aim of the study was to analyze the distribution and localization of AQP1 in the entire intestinal tract of mice. AQP1 expression was correlated with the hypoxic expression profile of the various intestinal segments, intestinal wall thickness and edema, as well as other aspects of colon function including the ability of mice to concentrate stools and their microbiome composition. AQP1 was found in a specific pattern in the serosa, the mucosa, and the enteric nervous system throughout the gastrointestinal tract. The highest amount of AQP1 in the gastrointestinal tract was found in the small intestine. AQP1 expression correlated with the expression profiles of hypoxia-dependent proteins such as HIF-1α and PGK1. Loss of AQP1 through knockout of AQP1 in these mice led to a reduced amount of bacteroidetes and firmicutes but an increased amount of the rest of the phyla, especially deferribacteres, proteobacteria, and verrucomicrobia. Although AQP-KO mice retained gastrointestinal function, distinct changes regarding the anatomy of the intestinal wall including intestinal wall thickness and edema were observed. Loss of AQP1 might interfere with the ability of the mice to concentrate their stool and it is associated with a significantly different composition of the of the bacterial stool microbiome.

Non-Transport Functions of Aquaporins

Although it has been more than 20 years since the first aquaporin was discovered, the specific functions of many aquaporins are still under investigation, because various mice lacking aquaporins have no significant phenotypes. And in many studies, the function of aquaporin is not directly related to its transport function. Therefore, this chapter will focus on some unexpected functions of aquaporins, such the decreased tumor angiogenesis in AQP1 knockout mice, and AQP1 promotes cell migration, possibly by accelerating the water transport in lamellipodia of migrating cells. AQP transports glycerol, and water regulates glycerol content in epidermis and fat, thereby regulating skin hydration/biosynthesis and fat metabolism. AQPs may also be involved in neural signal transduction, cell volume regulation, and organelle physiology. AQP1, AQP3, and AQP5 are also involved in cell proliferation. In addition, AQPs have also been reported to play roles in inflammation in various tissues and organs. The functions of these AQPs may not depend on the permeability of small molecules such as water and glycerol, suggesting AQPs may play more roles in different biological processes in the body.

Signaling Mechanisms and Pharmacological Modulators Governing Diverse Aquaporin Functions in Human Health and Disease

The aquaporins (AQPs) are a family of small integral membrane proteins that facilitate the bidirectional transport of water across biological membranes in response to osmotic pressure gradients as well as enable the transmembrane diffusion of small neutral solutes (such as urea, glycerol, and hydrogen peroxide) and ions. AQPs are expressed throughout the human body. Here, we review their key roles in fluid homeostasis, glandular secretions, signal transduction and sensation, barrier function, immunity and inflammation, cell migration, and angiogenesis. Evidence from a wide variety of studies now supports a view of the functions of AQPs being much more complex than simply mediating the passive flow of water across biological membranes. The discovery and development of small-molecule AQP inhibitors for research use and therapeutic development will lead to new insights into the basic biology of and novel treatments for the wide range of AQP-associated disorders.

Aquaporin-1 in breast cancer

The canonical function of aquaporin (AQP) water channels is to facilitate passive transport of water across cellular membranes making them essential in the regulation of body water homeostasis. Moreover, AQPs, including AQP1, have been found to be overexpressed in multiple cancer types, including breast cancer, where AQP1 overexpression is associated with poor prognosis. AQPs have been shown to affect cellular processes associated with cancer progression and spread including cell migration, angiogenesis, and proliferation. Moreover, AQPs can regulate levels of adhesion proteins at cell-cell junctions, a regulatory role, which is still largely unexplored in cancer. Understanding the molecular mechanisms of how AQP1 contributes to breast cancer progression and metastatic processes is essential to establish AQP1 as a biomarker and to develop targeted anticancer treatments for breast cancer patients. This mini-review focuses on the role of AQP1 in breast cancer.

Dose Response of Bumetanide on Aquaporins and Angiogenesis Biomarkers in Human Retinal Endothelial Cells Exposed to Intermittent Hypoxia

Aquaporins (AQPs) are important for regulating cellular water, solute transport, and balance. Recently, AQPs have also been recognized as playing a key role in cell migration and angiogenesis. In the retina, hypoxia induces vascular endothelial growth factor (VEGF), a potent angiogenic and vascular permeability factor, resulting in retinal edema, which is facilitated by AQPs. Bumetanide is a diuretic agent and AQP 1–4 blocker. We tested the hypothesis that bumetanide suppression of AQPs ameliorates intermittent hypoxia (IH)-induced angiogenesis and oxidative stress in human microvascular retinal endothelial cells (HMRECs). HMRECs were treated with a low-dose (0.05 µg/mL) or high-dose (0.2 µg/mL) of bumetanide and were exposed to normoxia (Nx), hyperoxia (50% O₂), or IH (50% O₂ with brief hypoxia 5% O₂) for 24, 48, and 72 h. Angiogenesis and oxidative stress biomarkers were determined in the culture media, and the cells were assessed for tube formation capacity and AQP-1 and -4 expression. Both doses of bumetanide significantly decreased oxidative stress and angiogenesis biomarkers. This response was reflected by reductions in tube formation capacity and AQP expression. These findings confirm the role of AQPs in retinal angiogenesis. Therapeutic targeting of AQPs with bumetanide may be advantageous for IH-induced aberrant retinal development.

Effects of Hyperoxia on Mitochondrial Homeostasis: Are Mitochondria the Hub for Bronchopulmonary Dysplasia?

Mitochondria are involved in energy metabolism and redox reactions in the cell. Emerging data indicate that mitochondria play an essential role in physiological and pathological processes of neonatal lung development. Mitochondrial damage due to exposure to high concentrations of oxygen is an indeed important factor for simplification of lung structure and development of bronchopulmonary dysplasia (BPD), as reported in humans and rodent models. Here, we comprehensively review research that have determined the effects of oxygen environment on alveolar development and morphology, summarize changes in mitochondria under high oxygen concentrations, and discuss several mitochondrial mechanisms that may affect cell plasticity and their effects on BPD. Thus, the pathophysiological effects of mitochondria may provide insights into targeted mitochondrial and BPD therapy.

Aquaglyceroporin-3's Expression and Cellular Localization Is Differentially Modulated by Hypoxia in Prostate Cancer Cell Lines

Aquaporins are required by cells to enable fast adaptation to volume and osmotic changes, as well as microenvironmental metabolic stimuli. Aquaglyceroporins play a crucial role in supplying cancer cells with glycerol for metabolic needs. Here, we show that AQP3 is differentially expressed in cells of a prostate cancer panel. AQP3 is located at the cell membrane and cytoplasm of LNCaP cell while being exclusively expressed in the cytoplasm of Du145 and PC3 cells. LNCaP cells show enhanced hypoxia growth; Du145 and PC3 cells display stress factors, indicating a crucial role for AQP3 at the plasma membrane in adaptation to hypoxia. Hypoxia, both acute and chronic affected AQP3′s cellular localization. These outcomes were validated using a machine learning classification approach of the three cell lines and of the six normoxic or hypoxic conditions. Classifiers trained on morphological features derived from cytoskeletal and nuclear labeling alongside corresponding texture features could uniquely identify each individual cell line and the corresponding hypoxia exposure. Cytoskeletal features were 70–90% accurate, while nuclear features allowed for 55–70% accuracy. Cellular texture features (73.9% accuracy) were a stronger predictor of the hypoxic load than the AQP3 distribution (60.3%).

AQP1 Is Up-Regulated by Hypoxia and Leads to Increased Cell Water Permeability, Motility, and Migration in Neuroblastoma

The water channel aquaporin 1 (AQP1) has been implicated in tumor progression and metastasis. It is hypothesized that AQP1 expression can facilitate the transmembrane water transport leading to changes in cell structure that promote migration. Its impact in neuroblastoma has not been addressed so far. The objectives of this study have been to determine whether AQP1 expression in neuroblastoma is dependent on hypoxia, to demonstrate whether AQP1 is functionally relevant for migration, and to further define AQP1-dependent properties of the migrating cells. This was determined by investigating the reaction of neuroblastoma cell lines, particularly SH-SY5Y, Kelly, SH-EP Tet-21/N and SK-N-BE(2)-M17 to hypoxia, quantitating the AQP1-related water permeability by stopped-flow spectroscopy, and studying the migration-related properties of the cells in a modified transwell assay. We find that AQP1 expression in neuroblastoma cells is up-regulated by hypoxic conditions, and that increased AQP1 expression enabled the cells to form a phenotype which is associated with migratory properties and increased cell agility. This suggests that the hypoxic tumor microenvironment is the trigger for some tumor cells to transition to a migratory phenotype. We demonstrate that migrating tumor cell express elevated AQP1 levels and a hypoxic biochemical phenotype. Our experiments strongly suggest that elevated AQP1 might be a key driver in transitioning stable tumor cells to migrating tumor cells in a hypoxic microenvironment.

Effect of Dexmedetomidine on Postoperative Lung Injury during One-Lung Ventilation in Thoracoscopic Surgery

Objective

To investigate the effect of dexmedetomidine on postoperative lung injury in patients undergoing thoracoscopic surgery.

Methods

From March 2019 to October 2019, 40 patients were randomly divided into two groups: dexmedetomidine group (group D) and control group (group C). Except recording the general condition of the patients in both groups preoperatively and intraoperatively, the oxygenation index (OI) and alveolar-arterial oxygen partial pressure difference (A-aDO₂) were monitored at admission (T0), immediately after one-lung ventilation (T1), 0.5 h after one-lung ventilation (T2), and 15 minutes after inhaling air before leaving the room (T3). The content of IL-8 in arterial blood was measured by enzyme-linked immunosorbent assay (ELISA) at T0 and T2, and the expression of AQP1 protein in isolated lung tissue was measured by immunohistochemistry and Western blot. The incidence of postoperative pulmonary complications (atelectasis, pneumonia, and acute respiratory distress syndrome) was used as the index of lung injury.

Results

There was no significant difference in the general condition before and during operation between the two groups. There was no significant difference in arterial blood IL-8 content between the two groups at the T0 time point, but the arterial blood IL-8 content at the T2 time point was significantly higher than that at the T0 time point, especially in group C. The results of immunohistochemistry and Western blot showed that the expression level of AQP1 protein in the isolated lung tissue of group D was significantly higher than that of group C (P < 0 05). At T3, the OI of group D was significantly higher than that of group C, and the A-aDO₂ of group D was significantly lower than that of group C (P < 0.05). There was no significant difference in the incidence of postoperative PPCs between the two groups.

Conclusion

Dexmedetomidine can reduce the level of plasma IL-8 and upregulate the expression of AQP1 in the lung tissue of patients undergoing thoracoscopic surgery under one-lung ventilation, but it has no significant effect on the incidence of postoperative PPCs. Dexmedetomidine can be safely used in thoracoscopic surgery and has a certain protective effect on lung injury.

The Hyperoxic-Hypoxic Paradox

Effective metabolism is highly dependent on a narrow therapeutic range of oxygen. Accordingly, low levels of oxygen, or hypoxia, are one of the most powerful inducers of gene expression, metabolic changes, and regenerative processes, including angiogenesis and stimulation of stem cell proliferation, migration, and differentiation. The sensing of decreased oxygen levels (hypoxia) or increased oxygen levels (hyperoxia), occurs through specialized chemoreceptor cells and metabolic changes at the cellular level, which regulate the response. Interestingly, fluctuations in the free oxygen concentration rather than the absolute level of oxygen can be interpreted at the cellular level as a lack of oxygen. Thus, repeated intermittent hyperoxia can induce many of the mediators and cellular mechanisms that are usually induced during hypoxia. This is called the hyperoxic-hypoxic paradox (HHP). This article reviews oxygen physiology, the main cellular processes triggered by hypoxia, and the cascade of events triggered by the HHP.

The Hyperoxic-Hypoxic Paradox

Effective metabolism is highly dependent on a narrow therapeutic range of oxygen. Accordingly, low levels of oxygen, or hypoxia, are one of the most powerful inducers of gene expression, metabolic changes, and regenerative processes, including angiogenesis and stimulation of stem cell proliferation, migration, and differentiation. The sensing of decreased oxygen levels (hypoxia) or increased oxygen levels (hyperoxia), occurs through specialized chemoreceptor cells and metabolic changes at the cellular level, which regulate the response. Interestingly, fluctuations in the free oxygen concentration rather than the absolute level of oxygen can be interpreted at the cellular level as a lack of oxygen. Thus, repeated intermittent hyperoxia can induce many of the mediators and cellular mechanisms that are usually induced during hypoxia. This is called the hyperoxic-hypoxic paradox (HHP). This article reviews oxygen physiology, the main cellular processes triggered by hypoxia, and the cascade of events triggered by the HHP.

Ultraviolet B Downregulated Aquaporin 1 Expression via the MEK/ERK pathway in the Dermal Fibroblasts

Background

Aquaporin 1 (AQP1) is a transmembrane channel protein that allows rapid transposition of water and gases, in recent discoveries of AQP1 function involve cell proliferation, differentiation, wound healing, inflammation and infection in different cell types, suggesting that AQP1 plays key roles in diverse biologic process. Until now, less is known about the function of AQP1 on ultraviolet radiation induced photoaged skin.

Objective

In this study we set out to examine whether AQP1 expression may be influenced by repeated irradiation of ultraviolet B (UVB) in cultured dermal fibroblasts.

Methods

To elucidate the function of AQP1 in skin photoaging, human dermal fibroblasts (HS68) were irradiated by a series of 4 sub-cytotoxic doses of UVB which are known as UV-induced cell premature senescence model. Reverse transcription polymerase chain reaction and Western blotting were conducted to detect AQP1 expression from different groups. Then, cells were transfected with AQP1-targeting small interfering RNA. The activities of signaling proteins upon UVB irradiation were investigated to determine which pathways are involved in AQP1 expression.

Results

AQP1 expression was increased by 100 mJ/cm² of UVB irradiation, but decreased by 200 mJ/cm². Depletion of the AQP1 increased the apoptotic sensitivity of cells to UVB, as judged by upregulation of the p53, p21, poly (adenosine diphosphate [ADP]-ribose) polymerase and Bax together with the increased Bax/Bcl2 ratio. UVB induced downregulation of AQP1 was significantly attenuated by pretreatment with the MEK/ERK inhibitor (PD98059).

Conclusion

We concluded that AQP1 expression was down-regulated by repeated exposure of UVB via MEK/ERK activation pathways. The AQP1 reduction by UVB lead to changes of physiological functions in dermal fibroblasts, which might be associated with the occurrence and development of UVB induced photoaging.

Aquaporin 1 alleviates acute kidney injury via PI3K-mediated macrophage M2 polarization

BACKGROUND

Lipopolysaccharide (LPS)-induced acute kidney injury (AKI) is associated with an abnormal immune response. Accumulating evidence has demonstrated that aquaporin 1 (AQP1) prevents kidney tissue injury in LPS-induced AKI by mediating immune response. However, the underlying mechanisms remain obscure. Macrophages as immune cells with multiple phenotypes are important mediators in tissue homeostasis and host defense. We propose that macrophage polarization is implicated in AQP1-mediated immune response.

METHODS

Herein we established sepsis-induced AKI model rats through intraperitoneal injection of LPS into Wistar rats to reveal immune mechanism of damage. We also used LPS-induced mouse RAW264.7 cells to elucidate the molecular mechanism of macropage polarization.

RESULTS

Histopathology showed that renal tubular epithelial cells in the model group were swollen, inflammatory exudation was obvious and the inflammatory factors, interleukin-6 (IL-6) and tumor necrosis factor α (TNF-α) were increased. Western blotting showed PI3K was upregulated in the model group. Serum creatinine and urea nitrogen increased after LPS injection. Renal AQP1 mRNA is downregulated and serum AQP1 protein increased first and then decreased in LPS-induced AKI rats. M2 macrophage markers (Arg-1, CD206) were increased in repair stage. In addition, treatment of murine macrophages (RAW264.7) with AQP1 siRNA resulted in decreased PI3K activation and M2 polarization, but increased IL-6 and TNF-α. Moreover, inhibiting PI3K with wortmannin imitated the results of AQP1 silencing.

CONCLUSIONS

Macrophage M2 polarization is likely the cellular mechanism underlying the anti-AKI property of AQP1, and PI3K activation is involved in the AQP1-induced M2 phenotype switch.

Ion Channel Dysregulation in Head and Neck Cancers: Perspectives for Clinical Application

Head and neck cancers are a highly complex and heterogeneous group of malignancies that involve very diverse anatomical structures and distinct aetiological factors, treatments and clinical outcomes. Among them, head and neck squamous cell carcinomas (HNSCC) are predominant and the sixth most common cancer worldwide with still low survival rates. Omic technologies have unravelled the intricacies of tumour biology, harbouring a large diversity of genetic and molecular changes to drive the carcinogenesis process. Nonetheless, this remarkable heterogeneity of molecular alterations opens up an immense opportunity to discover novel biomarkers and develop molecular-targeted therapies. Increasing evidence demonstrates that dysregulation of ion channel expression and/or function is frequently and commonly observed in a variety of cancers from different origin. As a consequence, the concept of ion channels as potential membrane therapeutic targets and/or biomarkers for cancer diagnosis and prognosis has attracted growing attention. This chapter intends to comprehensively and critically review the current state-of-art ion channel dysregulation specifically focusing on head and neck cancers and to formulate the major challenges and research needs to translate this knowledge into clinical application. Based on current reported data, various voltage-gated potassium (K_v) channels (i.e. K_v3.4, K_v10.1 and K_v11.1) have been found frequently aberrantly expressed in HNSCC as well as precancerous lesions and are highlighted as clinically and biologically relevant features in both early stages of tumourigenesis and late stages of disease progression. More importantly, they also emerge as promising candidates as cancer risk markers, tumour markers and potential anti-proliferative and anti-metastatic targets for therapeutic interventions; however, the oncogenic properties seem to be independent of their ion-conducting function.

The Evolution of Cholesterol-Rich Membrane in Oxygen Adaption: The Respiratory System as a Model

The increase in atmospheric oxygen levels imposed significant environmental pressure on primitive organisms concerning intracellular oxygen concentration management. Evidence suggests the rise of cholesterol, a key molecule for cellular membrane organization, as a cellular strategy to restrain free oxygen diffusion under the new environmental conditions. During evolution and the increase in organismal complexity, cholesterol played a pivotal role in the establishment of novel and more complex functions associated with lipid membranes. Of these, caveolae, cholesterol-rich membrane domains, are signaling hubs that regulate important in situ functions. Evolution resulted in complex respiratory systems and molecular response mechanisms that ensure responses to critical events such as hypoxia facilitated oxygen diffusion and transport in complex organisms. Caveolae have been structurally and functionally associated with respiratory systems and oxygen diffusion control through their relationship with molecular response systems like hypoxia-inducible factors (HIF), and particularly as a membrane-localized oxygen sensor, controlling oxygen diffusion balanced with cellular physiological requirements. This review will focus on membrane adaptations that contribute to regulating oxygen in living systems.

Aqp-1 Gene Knockout Attenuates Hypoxic Pulmonary Hypertension of Mice

Objective—

Hypoxic pulmonary hypertension (HPH) is characterized by proliferative vascular remodeling. Abnormal pulmonary artery smooth muscle cells proliferation and endothelial dysfunction are the primary cellular bases of vascular remodeling. AQP1 (aquaporin-1) is regulated by oxygen level and has been observed to play a role in the proliferation and migration of pulmonary artery smooth muscle cells. The role of AQP1 in HPH pathogenesis has not been directly determined to date. To determine the possible roles of AQP1 in the pathogenesis of HPH and explore its possible mechanisms.

Approach and Results—

Aqp1 knockout mice were used, and HPH model was established in this study. Primary pulmonary artery smooth muscle cells, primary mouse lung endothelial cells, and lung tissue sections from HPH model were used. Immunohistochemistry, immunofluorescence and Western blot, cell cycle, apoptosis, and migration analysis were performed in this study. AQP1 expression was upregulated by chronic hypoxia exposure, both in pulmonary artery endothelia and medial smooth muscle layer of mice. Aqp1 deficiency attenuated the elevation of right ventricular systolic pressures and mitigated pulmonary vascular structure remodeling. AQP1 deletion reduced abnormal cell proliferation in pulmonary artery and accompanied with accumulation of HIF (hypoxia-inducible factor). In vitro, Aqp1 deletion reduced hypoxia-induced proliferation, apoptosis resistance, and migration ability of primary cultured pulmonary artery smooth muscle cells and repressed HIF-1α protein stability. Furthermore, Aqp1 deficiency protected lung endothelial cells from apoptosis in response to hypoxic injury.

Conclusions—

Our data showed that Aqp1 deficiency could attenuate hypoxia-induced vascular remodeling in the development of HPH. AQP1 may be a potential target for pulmonary hypertension treatment.

Aquaporins and Their Regulation after Spinal Cord Injury

After injury to the spinal cord, edema contributes to the underlying detrimental pathophysiological outcomes that lead to worsening of function. Several related membrane proteins called aquaporins (AQPs) regulate water movement in fluid transporting tissues including the spinal cord. Within the cord, AQP1, 4 and 9 contribute to spinal cord injury (SCI)-induced edema. AQP1, 4 and 9 are expressed in a variety of cells including astrocytes, neurons, ependymal cells, and endothelial cells. This review discusses some of the recent findings of the involvement of AQP in SCI and highlights the need for further study of these proteins to develop effective therapies to counteract the negative effects of SCI-induced edema.

Early redox activities modulate Xenopus tail regeneration

Redox state sustained by reactive oxygen species (ROS) is crucial for regeneration; however, the interplay between oxygen (O2), ROS and hypoxia-inducible factors (HIF) remains elusive. Here we observe, using an optic-based probe (optrode), an elevated and steady O2 influx immediately upon amputation. The spatiotemporal O2 influx profile correlates with the regeneration of Xenopus laevis tadpole tails. Inhibition of ROS production but not ROS scavenging decreases O2 influx. Inhibition of HIF-1α impairs regeneration and stabilization of HIF-1α induces regeneration in the refractory period. In the regeneration bud, hypoxia correlates with O2 influx, ROS production, and HIF-1α stabilization that modulate regeneration. Further analyses reveal that heat shock protein 90 is a putative downstream target of HIF-1α while electric current reversal is a de facto downstream target of HIF-1α. Collectively, the results show a mechanism for regeneration via the orchestration of O2 influx, ROS production, and HIF-1α stabilization.

Combined effects of aquaporin-4 and hypoxia produce age-related hydrocephalus

Aquaporin-4, present in ependymal cells, in glia limiting and abundantly in pericapillary astrocyte foot processes, and aquaporin-1, expressed in choroid plexus epithelial cells, play an important role in cerebrospinal fluid production and may be involved in the pathophysiology of age-dependent hydrocephalus. The finding that brain aquaporins expression is regulated by low oxygen tension led us to investigate how hypoxia and elevated levels of cerebral aquaporins may result in an increase in cerebrospinal fluid production that could be associated with a hydrocephalic condition. Here we have explored, in young and aged mice exposed to hypoxia, whether aquaporin-4 and aquaporin-1 participate in the development of age-related hydrocephalus. Choroid plexus, striatum, cortex and ependymal tissue were analyzed separately both for mRNA and protein levels of aquaporins. Furthermore, parameters such as total ventricular volume, intraventricular pressure, cerebrospinal fluid outflow rate, ventricular compliance and cognitive function were studied in wild type, aquaporin-1 and aquaporin-4 knock-out animals subjected to hypoxia or normoxia. Our data demonstrate that hypoxia is involved in the development of age-related hydrocephalus by a process that depends on aquaporin-4 channels as a main route for cerebrospinal fluid movement. Significant increases in aquaporin-4 expression that occur over the course of animal aging, together with a reduced cerebrospinal fluid outflow rate and ventricular compliance, contribute to produce more severe hydrocephalus related to hypoxic events in aged mice, with a notable impairment in cognitive function. These results indicate that physiological events and/or pathological conditions presenting with cerebral hypoxia/ischemia contribute to the development of chronic adult hydrocephalus.

Prognostic implication of aquaporin 1 overexpression in resected lung adenocarcinoma

OBJECTIVES

Aquaporins (AQPs) are a group of transmembrane water-selective channel proteins thought to play a role in the regulation of water permeability for plasma membranes. Indeed, high AQP levels have been suggested to promote the progression, invasion and metastasis of tumours. Specifically, AQP1 and AQP5 overexpression in lung adenocarcinoma (AC) have been suggested to be involved in molecular mechanisms in lung cancer. The aim of this retrospective cohort single-centre study was to assess both the levels of expression and therein the prognostic significance, regarding outcome of AQP1 and AQP5 in resected AC patients.

METHODS

Patients with histological diagnoses of lung AC submitted to pulmonary resection were included in this cohort study. Tissue microarrays containing cores from 185 ACs were prepared. AQP1 and AQP5 expressions were assessed by immunohistochemistry. Results were scored as either low (Score 0-2) or high (Score 3-9). Clinical data, pathological tumour-node-metastasis staging and follow-up were recorded. Multivariate Cox survival analysis and Fisher's t-test were performed.

RESULTS

AQP1 overexpression was detected in 85 (46%) patients, while AQP5 overexpression was observed in 45 (24%) patients. AQP1 did not result being significantly correlated with clinical and pathological parameters, while AQP5 resulted more expressed in AC with mucinous and papillary predominant patterns. Patients with AQP1 overexpression had shorter disease-free survival (P = 0.001) compared with patients without AQP1 overexpression. Multivariate analysis confirmed that AQP1 overexpression was significantly associated with shorter disease-free survival (P = 0.001).

CONCLUSIONS

Our results evidenced that AQP1 overexpression resulted in a shorter disease-free survival in lung AC patients. Being so, AQP1 overexpression might be an important prognostic marker in lung AC.

The Expression of AQP1 IS Modified in Lung of Patients With Idiopathic Pulmonary Fibrosis: Addressing a Possible New Target

Activation of the epithelial-mesenchymal transition process (EMT) by which alveolar cells in human lung tissue undergo differentiation giving rise to a mesenchymal phenotype (fibroblast/miofibroblasts) has been well recognized as a key element in the origin of idiopathic pulmonary fibrosis (IPF). Here we analyzed expression of AQP1 in lung biopsies of patients diagnosed with IPF, and compared it to biopsies derived from patients with diverse lung pneumonies, such as hypersensitivity pneumonitis, sarcoidosis or normal lungs. Immunostaining for AQP1 showed a clear increment of AQP1 localized in the alveolar epithelium in biopsies from IPF patients alone. Moreover, to examine the possible participation of AQP1 in the pathophysiology of IPF, we evaluated its role in the pro-fibrotic transformation induced by transforming growth factor (TGF-β) in vitro. Human alveolar epithelial cells (A549), and fibroblasts derived from an IPF patient (LL29), or fibroblasts from healthy normal lung tissue (MRC-5), were treated with TGF-β, and levels of expression of AQP1, as well as those of E-cadherin, vimentin, α-SMA and collagen were analyzed by RT-qPCR, western blot and immunohistochemistry. An increase of AQP1 mRNA and protein after TGF-β treatment (4–72h) was observed either in A549 or IPF fibroblast-LL29 but not in MRC-5 fibroblasts. A gradual reduction of E-cadherin, and increased expression of vimentin, with no changes in α-SMA levels were observed in A549. Whereas in LL29 and MRC-5, TGF-β1 elicited a large production of collagen and α-SMA that was significantly greater in IPF fibroblast-LL29. Changes observed are consistent with activation of EMT by TGF-β, but whether modifications in AQP1 expression are responsible or independent events occurring at the same time is still unknown. Our results suggest that AQP1 plays a role in the pro-fibrotic TGF-β action and contributes to the etiology and pathophysiology of IPF. Understanding AQP1's role will help us comprehend the fate of this disease.

pH-mediated upregulation of AQP1 gene expression through the Spi-B transcription factor

Background

Bicarbonate-based peritoneal dialysis (PD) fluids enhance the migratory capacity and damage-repair ability of human peritoneal mesothelial cells by upregulating AQP1. However, little is known about the underlying molecular mechanisms.

Results

Here we used HEK-293T cells to investigate the effect of pH on AQP1 gene transcription levels. We found that AQP1 mRNA levels increases with pH. Transfection of HEK-293T cells with luciferase reporter vectors containing different regions of the AQP1 promoter identified an upstream region in the AQP1 gene between − 2200 and – 2300 bp as an enhancer required for pH-mediated regulation of AQP1 expression. Site-directed mutagenesis of this specific promoter region revealed a critical region between − 2257 and − 2251 bp, and gene knock-down experiments and ChIP assays suggested that the Spi-B transcription factor SPIB is involved in pH-mediated regulation of AQP1 expression.

Conclusions

We identified an upstream region in the AQP1 gene and the transcription factor SPIB that are critically involved in pH-mediated regulation of AQP1 expression. These findings provide the basis for further studies on the pH- and buffer-dependent effects of PD fluids on peritoneal membrane integrity and function.

Electronic supplementary material

The online version of this article (10.1186/s12867-018-0104-9) contains supplementary material, which is available to authorized users.

The Effect of Aquaporin 1-Inhibition on Vasculogenic Mimicry in Malignant Mesothelioma

Malignant mesothelioma (MM) is an aggressive malignancy of the serosal membranes, with poor overall survival and quality of life. Limited targeted treatment strategies exist due to restricted knowledge of pathogenic pathways. Vasculogenic mimicry (VM) is a newly described phenomenon associated with increased aggressiveness in other malignancies, and has been characterized in MM. Normal mesothelium expresses aquaporin 1 (AQP1) and retained expression has been associated with improved survival in MM. AQP1 is expressed by normal vascular endothelium and is involved in mediating MM cell motility and proliferation. We investigated the role of AQP1 in VM, and its interaction with the pro-angiogenic factor vascular endothelial growth factor A (VEGFA), which is variably expressed in MM. Matrigel VM assays were performed using NCI-H226 and NCI-H28 MM cell lines and primary cells in hypoxia and normoxia. The synthetic blocker AqB050 and siRNA were used to inhibit AQP1, and bevacizumab was used to inhibit VEGF. Inhibition of AQP1 resulted in increased VEGFA secretion by MM cells and reduced VM in MM cell lines in hypoxia but not normoxia. No change in VM was seen in MM primary cells. Combined inhibition of AQP1 and VEGF had no effect on VM in normoxia. In a heterotopic xenograft mouse model, AqB050 treatment did not alter vessel formation. AQP1 may interact with VEGFA and play a role in VM, especially under hypoxic conditions, but the heterogeneity of MM cells may result in different dominant pathways between patients.

Water permeability is a measure of severity in acute appendicitis

Acute appendicitis is the most common indication for pediatric abdominal emergency surgery. Determination of the severity of appendicitis on clinical grounds is challenging. Complicated appendicitis presenting with perforation, abscess or diffuse peritonitis is not uncommon. The question remains why and when acute appendicitis progresses to perforation. The aim of this study was to assess the impact of water permeability on the severity of appendicitis. We show that AQP1 expression and water permeability in appendicitis correlate with the stage of inflammation and systemic infection parameters, leading eventually to perforation of the appendix. AQP1 is also expressed within the ganglia of the enteric nervous system and ganglia count increases with inflammation. Severity of appendicitis can be correlated with water permeability measured by AQP1 protein expression and increase of ganglia count in a progressive manner. This introduces the question if regulation of water permeability can present novel curative or ameliorating therapeutic options.

Increased Lung Ischemia-Reperfusion Injury in Aquaporin 1-Null Mice Is Mediated via Decreased Hypoxia-Inducible Factor 2α Stability

Aquaporin (AQP) 1, a water channel protein expressed widely in vascular endothelia, has been shown to regulate cell migration, angiogenesis, and organ regeneration. Even though its role in the pathogenesis of lung ischemia-reperfusion (IR) injury has been defined, the functional role of AQP1 during long-term IR resolution remains to be clarified. Here, we found that AQP1 expression was increased at late time points (7-14 d) after IR and colocalized with endothelial cell (EC) marker CD31. Compared with IR in wild-type mice, IR in Aqp1(-/-) mice had significantly enhanced leukocyte infiltration, collagen deposition, and microvascular permeability, as well as inhibited angiogenic factor expression. AQP1 knockdown repressed hypoxia-inducible factor (HIF)-2α protein stability. HIF-2α overexpression rescued the angiogenic factor expression in pulmonary microvascular ECs with AQP1 knockdown exposed to hypoxia-reoxygenation. Furthermore, AQP1 knockdown suppressed cellular viability and capillary tube formation, and enhanced permeability in pulmonary microvascular ECs, which were partly rescued by HIF-2α overexpression. Thus, this study demonstrates that AQP1 deficiency delays long-term IR resolution, partly through repressing angiogenesis mediated by destabilizing HIF-2α. These results suggest that AQP1 participates in long-term IR resolution, at least in part by promoting angiogenesis.

Aquaporin 8 involvement in human cervical cancer SiHa migration via the EGFR-Erk1/2 pathway

Overexpression of aquaporins (AQPs) has been reported in several human cancers. Epidermal growth factor receptor (EGFR)-extracellular signal-regulated kinases 1/2 (Erk1/2) are associated with tumorigenesis and cancer progression and may upregulate AQP expression. In this study, we demonstrated that EGF (epidermal growth factor) induces SiHa cells migration and AQP8 expression. Wound healing results showed that cell migration was increased by 2.79-1.50-fold at 24 h and 48 h after EGF treatment. AQP8 expression was significantly increased (3.33-fold) at 48 h after EGF treatment in SiHa cells. An EGFR kinase inhibitor, PD153035, blocked EGF-induced AQP8 expression and cell migration and AQP8 expression was decreased from 1.59-fold (EGF-treated) to 0.43-fold (PD153035-treated) in SiHa. Furthermore, the MEK (MAPK (mitogen-activated protein kinase)/Erk (extracellular signal regulated kinase)/Erk inhibitor U0126 also inhibited EGF-induced AQP8 expression and cell migration. AQP8 expression was decreased from 1.21-fold (EGF-treated) to 0.43-fold (U0126-treated). Immunofluorescence microscopy further confirmed the results. Collectively, our findings show that EGF induces AQP8 expression and cell migration in human cervical cancer SiHa cells via the EGFR/Erk1/2 signal transduction pathway.

Significance of oxygen transport through aquaporins

Aquaporins are membrane integral proteins responsible for the transmembrane transport of water and other small neutral molecules. Despite their well-acknowledged importance in water transport, their significance in gas transport processes remains unclear. Growing evidence points to the involvement of plant aquaporins in CO2 delivery for photosynthesis. The role of these channel proteins in the transport of O2 and other gases may also be more important than previously envisioned. In this study, we examined O2 permeability of various human, plant, and fungal aquaporins by co-expressing heterologous aquaporin and myoglobin in yeast. Two of the most promising O2-transporters (Homo sapiens AQP1 and Nicotiana tabacum PIP1;3) were confirmed to facilitate O2 transport in the spectrophotometric assay using yeast protoplasts. The over-expression of NtPIP1;3 in yeasts significantly increased their O2 uptake rates in suspension culture. In N. tabacum roots subjected to hypoxic hydroponic conditions, the transcript levels of the O2-transporting aquaporin NtPIP1;3 significantly increased after the seven-day hypoxia treatment, which was accompanied by the increase of ATP levels in the apical root segments. Our results suggest that the functional significance of aquaporin-mediated O2 transport and the possibility of controlling the rate of transmembrane O2 transport should be further explored.

Protective effects of puerarin on acute lung and cerebrum injury induced by hypobaric hypoxia via the regulation of aquaporin (AQP) via NF-κB signaling pathway

OBJECTIVE

Hypobaric hypoxia, frequently encountered at high altitude, may lead to lung and cerebrum injury. Our study aimed to investigate whether puerarin could exert ameliorative effects on rats exposed to hypobaric hypoxia via regulation of aquaporin (AQP) and NF-κB signaling pathway in lung and cerebrum.

MATERIALS AND METHODS

40 Sprague Dawley rats were divided into four groups (normal control group, hypobaric hypoxia group, puerarin group and dexamethasone group). Wet/dry ratio, blood gas, pathological changes of lung and cerebrum and spatial memory were observed in each group. Inflammatory cytokines in bronchoalveolar lavage fluid (BALF) were determined with ELISA and expression of AQP1, AQP4, NF-κB signaling pathway in lung and cerebrum with western blot RESULTS: Puerarin showed significant preventative effects on tissue injury and behavioral changes, as evidenced by histopathological findings and Morris water maze. In addition, levels of inflammatory cytokines in BALF decreased in the two preventative groups compared with those of hypobaric hypoxia group. AQP in lung and cerebrum increased under the condition of hypobaric hypoxia while was down regulated in both two preventative groups. NF-κB and IκB was also inhibited by puerarin.

CONCLUSION

Our study suggested that lung and cerebrum injury, increased inflammatory cytokines in BALF and increased AQP1, AQP4 and NF-κB signaling pathway occurred under the condition of hypobaric hypoxia. Moreover, puerarin could prevent lung and cerebrum injury of rats exposed to hypobaric hypoxia via down-regulation of inflammatory cytokines, AQP1 and AQP4 expression and NF-κB signaling pathway.

Maximal Oxygen Consumption Is Reduced in Aquaporin-1 Knockout Mice

We have measured maximal oxygen consumption (V˙_O2,max) of mice lacking one or two of the established mouse red-cell CO₂ channels AQP1, AQP9, and Rhag. We intended to study whether these proteins, by acting as channels for O₂, determine O₂ exchange in the lung and in the periphery. We found that V˙_O2,max as determined by the Helox technique is reduced by ~16%, when AQP1 is knocked out, but not when AQP9 or Rhag are lacking. This figure holds for animals respiring normoxic as well as hypoxic gas mixtures. To see whether the reduction of V˙_O2,max is due to impaired O₂ uptake in the lung, we measured carotid arterial O₂ saturation (S_O2) by pulse oximetry. Neither under normoxic (inspiratory O₂ 21%) nor under hypoxic conditions (11% O₂) is there a difference in S_O2 between AQP1_null and WT mice, suggesting that AQP1 is not critical for O₂ uptake in the lung. The fact that the % reduction of V˙_O2,max is identical in normoxia and hypoxia indicates moreover that the limitation of V˙_O2,max is not due to an O₂ diffusion problem, neither in the lung nor in the periphery. Instead, it appears likely that AQP1_null animals exhibit a reduced V˙_O2,max due to the reduced wall thickness and muscle mass of the left ventricles of their hearts, as reported previously. We conclude that very likely the properties of the hearts of AQP1 knockout mice cause a reduced maximal cardiac output and thus cause a reduced V˙_O2,max, which constitutes a new phenotype of these mice.

Hyperglycemia Induces Cellular Hypoxia through Production of Mitochondrial ROS Followed by Suppression of Aquaporin-1

We previously proposed that hyperglycemia-induced mitochondrial reactive oxygen species (mtROS) generation is a key event in the development of diabetic complications. Interestingly, some common aspects exist between hyperglycemia and hypoxia-induced phenomena. Thus, hyperglycemia may induce cellular hypoxia, and this phenomenon may also be involved in the pathogenesis of diabetic complications. In endothelial cells (ECs), cellular hypoxia increased after incubation with high glucose (HG). A similar phenomenon was observed in glomeruli of diabetic mice. HG-induced cellular hypoxia was suppressed by mitochondria blockades or manganese superoxide dismutase (MnSOD) overexpression, which is a specific SOD for mtROS. Overexpression of MnSOD also increased the expression of aquaporin-1 (AQP1), a water and oxygen channel. AQP1 overexpression in ECs suppressed hyperglycemia-induced cellular hypoxia, endothelin-1 and fibronectin overproduction, and apoptosis. Therefore, hyperglycemia-induced cellular hypoxia and mtROS generation may promote hyperglycemic damage in a coordinated manner.

Increased neuronal and astroglial aquaporin-1 immunoreactivity in rat striatum by chemical preconditioning with 3-nitropropionic acid

Aquaporin-1 (AQP1) is a water channel expressed in the choroid plexus and participates in forming cerebrospinal fluid. Interestingly, reactive astrocytes also express AQP1 in the central nervous system under some pathological conditions. On the other hand, 3-nitropropionic acid (3NP) is a mitochondrial toxin that causes selective degeneration of striatum; however, its chemical preconditioning is neuroprotective against cerebral ischemia. We previously reported that mild 3NP application is accompanied with numerous reactive astrocytes in rat striatum devoid of typical necrotic lesions. Therefore, we studied whether AQP1 in the rat striatum could be upregulated with reactive astrocytosis using the 3NP model. Immunohistochemical or immunofluorescence analysis showed that reactive astrocytosis in the striatum, which upregulates glial fibrillary acidic protein and glutamine synthetase, was induced by mild doses of 3NP administration. Intriguingly, after 3NP treatment, AQP1 was intensely expressed not only by the subpopulation of astroglia but also by neurons. The AQP1 immunoreactivity became more intensified at the early-subtoxic stage (ES: 24-48h), but not as much in the delayed-subtoxic stage (DS: 96-120h). In contrast, AQP4 expression in the striatum was downregulated after 3NP treatment, in particular during the ES stage. AQP1 upregulation/AQP4 downregulation induced under subtoxic 3NP treatment may play a pivotal role in water homeostasis and cell viability in the striatum.

Oxygen-dependent regulation of aquaporin-3 expression

The purpose of this study was to investigate whether aquaporin-3 (AQP3) expression is altered in hypoxia and whether hypoxia-inducible transcription factor (HIF)-1 regulates the hypoxic expression. AQP3 mRNA expression was studied in L929 fibrosarcoma cells and in several tissues derived from mice that were subjected to hypoxia. Computational analysis of the AQP3 promoter revealed conserved HIF binding sites within close proximity to the translational start site, and chromatin immunoprecipitation assays confirmed binding of HIF-1α to the endogenous hypoxia response elements. Furthermore, hypoxia resulted in increased expression of AQP3 mRNA in L929 fibrosarcoma cells. Consistently, shRNA-mediated knockdown of HIF-1α greatly reduced the hypoxic induction of AQP3. In addition, mRNA analysis of organs from mice exposed to inspiratory hypoxia demonstrated pronounced hypoxia-inducible expression of AQP3 in the kidney. Overall, our findings suggest that AQP3 expression can be regulated at the transcriptional level and that AQP3 represents a novel HIF-1 target gene.

Aquaporin 1 and 5 expression decreases during human intervertebral disc degeneration: Novel HIF-1-mediated regulation of aquaporins in NP cells

Objectives of this study were to investigate whether AQP1 and AQP5 expression is altered during intervertebral disc degeneration and if hypoxia and HIF-1 regulate their expression in NP cells. AQP expression was measured in human tissues from different degenerative grades; regulation by hypoxia and HIF-1 was studied using promoter analysis and gain- and loss-of-function experiments. We show that both AQPs are expressed in the disc and that mRNA and protein levels decline with human disease severity. Bioinformatic analyses of AQP promoters showed multiple evolutionarily conserved HREs. Surprisingly, hypoxia failed to induce promoter activity or expression of either AQP. While genomic chromatin immunoprecipitation showed limited binding of HIF-1α to conserved HREs, their mutation did not suppress promoter activities. Stable HIF-1α suppression significantly decreased mRNA and protein levels of both AQPs, but HIF-1α failed to induce AQP levels following accumulation. Together, our results demonstrate that AQP1 and AQP5 expression is sensitive to human disc degeneration and that HIF-1α uniquely maintains basal expression of both AQPs in NP cells, independent of oxemic tension and HIF-1 binding to promoter HREs. Diminished HIF-1 activity during degeneration may suppress AQP levels in NP cells, compromising their ability to respond to extracellular osmolarity changes.

Aquaporins: Their role in gastrointestinal malignancies

Aquaporins (AQPs) are small (~30 kDa monomers) integral membrane water transport proteins that allow water to flow through cell membranes in reaction to osmotic gradients in cells. In mammals, the family of AQPs has thirteen (AQP0-12) unique members that mediate critical biological functions. Since AQPs can impact cell proliferation, migration and angiogenesis, their role in various human cancers is well established. Recently, AQPs have been explored as potential diagnostic and therapeutic targets in gastrointestinal (GI) cancers. GI cancers encompass multiple sites including the colon, esophagus, stomach and pancreas. Research in the last three decades has revealed biological aspects and signaling pathways critical for the development of GI cancers. Since the majority of these cancers are very aggressive and rapidly metastasizes, identifying effective targets is crucial for treatment. Preclinical studies have utilized inhibitors of specific AQPs and knock down of AQP expression using siRNA. Although several studies have explored the role of AQPs in colorectal, esophageal, gastric, hepatocellular and pancreatic cancers, there is no comprehensive review compiling the available information on GI cancers as has been published for other malignancies such as ovarian cancer. Due to the similarities and association of various sites of GI cancers, it is helpful to consider these results collectively in order to better understand the role of specific AQPs in critical GI cancers. This review summarizes the current knowledge of the role of AQPs in GI malignancies with particular focus on diagnosis and therapeutic applications.

Role of aquaporins in cell proliferation: What else beyond water permeability?

In addition to the extensive data demonstrating the importance of mammalian AQPs for the movement of water and some small solutes across the cell membrane, there is now a growing body of evidence indicating the involvement of these proteins in numerous cellular processes seemingly unrelated, at least some of them in a direct way, to their canonical function of water permeation. Here, we have presented a broad range of evidence demonstrating that these proteins have a role in cell proliferation by various different mechanisms, namely, by allowing fast cell volume regulation during cell division; by affecting progression of cell cycle and helping maintain the balance between proliferation and apoptosis, and by crosstalk with other cell membrane proteins or transcription factors that, in turn, modulate progression of the cell cycle or regulate biosynthesis pathways of cell structural components. In the end, however, after discussing all these data that strongly support a role for AQPs in the cell proliferation process, it remains impossible to conclude that all these other functions attributed to AQPs occur completely independently of their water permeability, and there is a need for new experiments designed specifically to address this interesting issue.

Overexpression of AQP3 Modifies the Cell Cycle and the Proliferation Rate of Mammalian Cells in Culture

Abnormal AQP3 overexpression in tumor cells of different origins has been reported and a role for this enhanced AQP3 expression in cell proliferation and tumor processess has been indicated. To further understand the role AQP3 plays in cell proliferation we explore the effect that stable over expression of AQP3 produces over the proliferation rate and cell cycle of mammalian cells. The cell cycle was analyzed by flow cytometry with propidium iodide (PI) and the cell proliferation rate measured through cell counting and BrdU staining. Cells with overexpression of AQP3 (AQP3-o) showed higher proliferation rate and larger percentage of cells in phases S and G2/M, than wild type cells (wt). Evaluation of the cell response against arresting the cell cycle with Nocodazole showed that AQP3-o exhibited a less modified cell cycle pattern and lower Annexin V specific staining than wt, consistently with a higher resistance to apoptosis of AQP3-overexpressing cells. The cell volume and complexity were also larger in AQP3-o compared to wt cells. After transcriptomic analysis, RT-qPCR was performed to highlight key molecules implicated in cell proliferation which expression may be altered by overexpression of AQP3 and the comparative analysis between both type of cells showed significant changes in the expression of Zeb2, Jun, JunB, NF-kβ, Cxcl9, Cxcl10, TNF, and TNF receptors. We conclude that the role of AQP3 in cell proliferation seems to be connected to increments in the cell cycle turnover and changes in the expression levels of relevant genes for this process. Larger expression of AQP3 may confer to the cell a more tumor like phenotype and contributes to explain the presence of this protein in many different tumors.

Beyond water homeostasis: Diverse functional roles of mammalian aquaporins

BACKGROUND

Aquaporin (AQP) water channels are best known as passive transporters of water that are vital for water homeostasis.

SCOPE OF REVIEW

AQP knockout studies in whole animals and cultured cells, along with naturally occurring human mutations suggest that the transport of neutral solutes through AQPs has important physiological roles. Emerging biophysical evidence suggests that AQPs may also facilitate gas (CO2) and cation transport. AQPs may be involved in cell signalling for volume regulation and controlling the subcellular localization of other proteins by forming macromolecular complexes. This review examines the evidence for these diverse functions of AQPs as well their physiological relevance.

MAJOR CONCLUSIONS

As well as being crucial for water homeostasis, AQPs are involved in physiologically important transport of molecules other than water, regulation of surface expression of other membrane proteins, cell adhesion, and signalling in cell volume regulation.

GENERAL SIGNIFICANCE

Elucidating the full range of functional roles of AQPs beyond the passive conduction of water will improve our understanding of mammalian physiology in health and disease. The functional variety of AQPs makes them an exciting drug target and could provide routes to a range of novel therapies.

Aquaporin-1 Deficiency Protects Against Myocardial Infarction by Reducing Both Edema and Apoptosis in Mice

Many studies have determined that AQP1 plays an important role in edema formation and resolution in various tissues via water transport across the cell membrane. The aim of this research was to determine both if and how AQP1 is associated with cardiac ischemic injury, particularly the development of edema following myocardial infarction (MI). AQP1^+/+ and AQP1^−/− mice were used to create the MI model. Under physiological conditions, AQP1^−/− mice develop normally; however, in the setting of MI, they exhibit cardioprotective properties, as shown by reduced cardiac infarct size determined via NBT staining, improved cardiac function determined via left ventricular catheter measurements, decreased AQP1-dependent myocardial edema determined via water content assays, and decreased apoptosis determined via TUNEL analysis. Cardiac ischemia caused by hypoxia secondary to AQP1 deficiency stabilized the expression of HIF-1α in endothelial cells and subsequently decreased microvascular permeability, resulting in the development of edema. The AQP1-dependent myocardial edema and apoptosis contributed to the development of MI. AQP1 deficiency protected cardiac function from ischemic injury following MI. Furthermore, AQP1 deficiency reduced microvascular permeability via the stabilization of HIF-1α levels in endothelial cells and decreased cellular apoptosis following MI.

Pharmacology of acute mountain sickness: old drugs and newer thinking

Pharmacotherapy in acute mountain sickness (AMS) for the past half century has largely rested on the use of carbonic anhydrase (CA) inhibitors, such as acetazolamide, and corticosteroids, such as dexamethasone. The benefits of CA inhibitors are thought to arise from their known ventilatory stimulation and resultant greater arterial oxygenation from inhibition of renal CA and generation of a mild metabolic acidosis. The benefits of corticosteroids include their broad-based anti-inflammatory and anti-edemagenic effects. What has emerged from more recent work is the strong likelihood that drugs in both classes act on other pathways and signaling beyond their classical actions to prevent and treat AMS. For the CA inhibitors, these include reduction in aquaporin-mediated transmembrane water transport, anti-oxidant actions, vasodilation, and anti-inflammatory effects. In the case of corticosteroids, these include protection against increases in vascular endothelial and blood-brain barrier permeability, suppression of inflammatory cytokines and reactive oxygen species production, and sympatholysis. The loci of action of both classes of drug include the brain, but may also involve the lung as revealed by benefits that arise with selective administration to the lungs by inhalation. Greater understanding of their pluripotent actions and sites of action in AMS may help guide development of better drugs with more selective action and fewer side effects.