Aquaporin-5 is expressed in adipocytes with implications in adipose differentiation

Effect of Obesity on Aquaporin5 Expression in Human Placental and Uterus Tissues

Background: Obesity and overweight are also becoming more prevalent among women of childbearing age and pregnant women. In maternal obesity, the activation of metabolic, inflammatory, and oxidative stress pathways is proven, which appears to be a key step in the pathological changes observed in placental and uterine function. Several recent studies have evidenced that aquaporins (AQPs) are critical players in adipose tissue biology and are involved in the onset of obesity. Methods: Our studies aimed to investigate the changes in placental volume and vascularization and measure the AQP5 expression and total antioxidant capacity (TAC) in the placenta and uterus tissues in obese and typical-weight mothers. We also aim to measure the AQP5 plasma concentration. Results: We found AQP5 dominance in the uterus and plasma at 34 weeks of normal pregnancy. The placental volume increased and the vascularization decreased in obese mothers compared to the control. The AQP5 expression increased in the uterus of the obese group and did not change in the placenta. The TAC decreased in the plasma of overweight mothers. Conclusions: We hypothesize that increased AQP5 expression prolongs the length of pregnancy and inhibits the onset of contractions. Based on our findings, we can develop diagnostic tests and provide new targets for tocolytic drug development.

Increased Aquaporin-7 Expression Is Associated with Changes in Rat Brown Adipose Tissue Whitening in Obesity: Impact of Cold Exposure and Bariatric Surgery

Glycerol is a key metabolite for lipid accumulation in insulin-sensitive tissues. We examined the role of aquaporin-7 (AQP7), the main glycerol channel in adipocytes, in the improvement of brown adipose tissue (BAT) whitening, a process whereby brown adipocytes differentiate into white-like unilocular cells, after cold exposure or bariatric surgery in male Wistar rats with diet-induced obesity (DIO) (n = 229). DIO promoted BAT whitening, evidenced by increased BAT hypertrophy, steatosis and upregulation of the lipogenic factors Pparg2, Mogat2 and Dgat1. AQP7 was detected in BAT capillary endothelial cells and brown adipocytes, and its expression was upregulated by DIO. Interestingly, AQP7 gene and protein expressions were downregulated after cold exposure (4 °C) for 1 week or one month after sleeve gastrectomy in parallel to the improvement of BAT whitening. Moreover, Aqp7 mRNA expression was positively associated with transcripts of the lipogenic factors Pparg2, Mogat2 and Dgat1 and regulated by lipogenic (ghrelin) and lipolytic (isoproterenol and leptin) signals. Together, the upregulation of AQP7 in DIO might contribute to glycerol influx used for triacylglycerol synthesis in brown adipocytes, and hence, BAT whitening. This process is reversible by cold exposure and bariatric surgery, thereby suggesting the potential of targeting BAT AQP7 as an anti-obesity therapy.

Aqp5-/- mice exhibit reduced maximal body O2 consumption under cold exposure, normal pulmonary gas exchange, and impaired formation of brown adipose tissue

The fundamental body functions that determine maximal O₂ uptake (V̇o_2max) have not been studied in Aqp5^-/- mice (aquaporin 5, AQP5). We measured V̇o_2max to globally assess these functions and then investigated why it was found altered in Aqp5^-/- mice. V̇o_2max was measured by the Helox technique, which elicits maximal metabolic rate by intense cold exposure of the animals. We found V̇o_2max reduced in Aqp5^-/- mice by 20%-30% compared with wild-type (WT) mice. As AQP5 has been implicated to act as a membrane channel for respiratory gases, we studied whether this is caused by the known lack of AQP5 in the alveolar epithelial membranes of Aqp5^-/- mice. Lung function parameters as well as arterial O₂ saturation were normal and identical between Aqp5^-/- and WT mice, indicating that AQP5 does not contribute to pulmonary O₂ exchange. The cause for the decreased V̇o_2max thus might be found in decreased O₂ consumption of an intensely O₂-consuming peripheral organ such as activated brown adipose tissue (BAT). We found indeed that absence of AQP5 greatly reduces the amount of interscapular BAT formed in response to 4 wk of cold exposure, from 63% in WT to 25% in Aqp5^-/- animals. We conclude that lack of AQP5 does not affect pulmonary O₂ exchange, but greatly inhibits transformation of white to brown adipose tissue. As under cold exposure, BAT is a major source of the animals' heat production, reduction of BAT likely causes the decrease in V̇o_2max under this condition.

Aquaporins in Obesity

Obesity is one of the most important metabolic disorders of this century and is associated with a cluster of the most dangerous cardiovascular disease risk factors, such as insulin resistance and diabetes, dyslipidemia, and hypertension, collectively named Metabolic Syndrome. The role of aquaporins (AQP) in glycerol metabolism facilitating glycerol release from the adipose tissue and distribution to various tissues and organs unveils these membrane channels as important players in lipid balance and energy homeostasis and points to their involvement in a variety of pathophysiological mechanisms including insulin resistance, obesity, and diabetes. This review summarizes the physiologic role of aquaglyceroporins in glycerol metabolism and lipid homeostasis, describing their specific tissue distribution, involvement in glycerol balance, and implication in obesity and fat-related metabolic complications. The development of specify pharmacologic modulators able to regulate aquaglyceroporins expression and function, in particular AQP7 in adipose tissue, might constitute a novel approach for controlling obesity and other metabolic disorders.

Physiological Cooperation between Aquaporin 5 and TRPV4

Aquaporins—among them, AQP5—are responsible for transporting water across biological membranes, which is an important process in all living organisms. The transient receptor potential channel 4 (TRPV4) is a cation channel that is mostly calcium-permeable and can also be activated by osmotic stimuli. It plays a role in a number of different functions in the body, e.g., the development of bones and cartilage, and it is involved in the body’s osmoregulation, the generation of certain types of sensation (pain), and apoptosis. Our earlier studies on the uterus and the literature data aroused our interest in the physiological role of the cooperation of AQP5 and TRPV4. In this review, we focus on the co-expression and cooperation of AQP5 and TRPV4 in the lung, salivary glands, uterus, adipose tissues, and lens. Understanding the cooperation between AQP5 and TRPV4 may contribute to the development of new drug candidates and the therapy of several disorders (e.g., preterm birth, cataract, ischemia/reperfusion-induced edema, exercise- or cold-induced asthma).

Peroxiporins Are Induced upon Oxidative Stress Insult and Are Associated with Oxidative Stress Resistance in Colon Cancer Cell Lines

Oxidative stress can induce genetic instability and change cellular processes, resulting in colorectal cancer. Additionally, adaptation of oxidative defense causes therapy resistance, a major obstacle in successful cancer treatment. Peroxiporins are aquaporin membrane channels that facilitate H₂O₂ membrane permeation, crucial for regulating cell proliferation and antioxidative defense. Here, we investigated four colon cancer cell lines (Caco-2, HT-29, SW620, and HCT 116) for their sensitivity to H₂O₂, cellular antioxidative status, and ROS intracellular accumulation after H₂O₂ treatment. The expression of peroxiporins AQP1, AQP3, and AQP5 and levels of NRF2, the antioxidant transcription factor, and PPARγ, a transcription factor that regulates lipid metabolism, were evaluated before and after oxidative insult. Of the four tested cell lines, HT-29 was the most resistant and showed the highest expression of all tested peroxiporins and the lowest levels of intracellular ROS, without differences in GSH levels, catalase activity, nor NF2 and PPARγ levels. Caco-2 shows high expression of AQP3 and similar resistance as HT-29. These results imply that oxidative stress resistance can be obtained by several mechanisms other than the antioxidant defense system. Regulation of intracellular ROS through modulation of peroxiporin expression may represent an additional strategy to target the therapy resistance of cancer cells.

Aquaporin-3 is involved in NLRP3-inflammasome activation contributing to the setting of inflammatory response

Inflammasomes are large immune multiprotein complexes that tightly regulate the production of the pro-inflammatory cytokines, being dependent on cell regulatory volume mechanisms. Aquaporins (AQPs) are protein channels that facilitate the transport of water and glycerol (aquaglyceroporins) through membranes, essential for cell volume regulation. Although these membrane proteins are highly expressed in monocytes and macrophages, their role in the inflammatory process is still unclear. Here, we investigated the role of aquaglyceroporin AQP3 in NLRP3-inflammasome activation by complementary approaches based either on shRNA silencing or on AQP3 selective inhibition. The latter has been achieved using a reported potent gold-based inhibitor, Auphen. AQP3 inhibition or silencing partially blocked LPS-priming and decreased production of IL-6, proIL-1β, and TNF-α, suggesting the possible involvement of AQP3 in macrophage priming by Toll-like receptor 4 engagement. Moreover, AQP3-dependent cell reswelling increased IL-1β release through caspase-1 activation. NLRP3-inflammasome activation induced by reswelling, nigericin, and ATP was also blocked when AQP3 was inhibited or silenced. Altogether, these data point towards AQPs as potential players in the setting of the inflammatory response.

An Atherogenic Diet Disturbs Aquaporin 5 Expression in Liver and Adipocyte Tissues of Apolipoprotein E-Deficient Mice: New Insights into an Old Model of Experimental Atherosclerosis

The dysfunction of vascular endothelial cells is profoundly implicated in the pathogenesis of atherosclerosis and cardiovascular disease, the global leading cause of death. Aquaporins (AQPs) are membrane channels that facilitate water and glycerol transport across cellular membranes recently implicated in the homeostasis of the cardiovascular system. Apolipoprotein-E deficient (apoE-/-) mice are a common model to study the progression of atherosclerosis. Nevertheless, the pattern of expression of AQPs in this atheroprone model is poorly characterized. In this study, apoE-/- mice were fed an atherogenic high-fat (HF) or a control diet. Plasma was collected at multiple time points to assess metabolic disturbances. At the endpoint, the aortic atherosclerotic burden was quantified using high field magnetic resonance imaging. Moreover, the transcriptional levels of several AQP isoforms were evaluated in the liver, white adipocyte tissue (WAT), and brown adipocyte tissue (BAT). The results revealed that HF-fed mice, when compared to controls, presented an exacerbated systemic inflammation and atherosclerotic phenotype, with no major differences in systemic methylation status, circulating amino acids, or plasma total glutathione. Moreover, an overexpression of the isoform AQP5 was detected in all studied tissues from HF-fed mice when compared to controls. These results suggest a novel role for AQP5 on diet-induced atherosclerosis that warrants further investigation.

AQP7 mediates post-menopausal lipogenesis in adipocytes through FSH-induced transcriptional crosstalk with AP-1 sites

RESEARCH QUESTION

Fat accumulation is present in most post-menopausal women, but the underlying mechanism remains unclear. Aquaporin 7 (AQP7) is the most important glycerol channel facilitating glycerol efflux in adipocytes. High circulating FSH in post-menopausal women may play an independent role in regulation of the lipogenic effect of AQP7 in adipocytes. This study explored the role of AQP7 in the pathophysiology of post-menopausal lipogenesis mediated by high concentrations of circulating FSH.

DESIGN

Primary adipocytes from post-menopausal and childbearing women were analysed. An in-vivo post-menopausal animal model was established. AQP7 expression, lipid accumulation and glycerol concentration in adipocytes were measured. Luciferase reporter assay and chromatin immunoprecipitation were performed to identify transcriptional crosstalk in AQP7 promoter.

RESULTS

It was found that FSH down-regulated AQP7 expression and glycerol efflux function in mature adipocytes of post-menopausal women and ovariectomized (OVX) mice. In vitro, FSH inhibited lipid accumulation in primary cultured mature adipocytes in a dose-dependent manner and the mechanism was down-regulating AQP7 expression via a FSH receptor pathway. The effect of FSH on AQP7 in adipocytes was through activation of cAMP response element-binding (CREB) protein, which could bind to activator protein-1 (AP-1) sites in the AQP7 promoter, and therefore inhibited the transcriptional activation elicited by c-Jun.

CONCLUSIONS

Down-regulation of AQP7 by FSH mediated post-menopausal lipogenesis, and the role of FSH was based on binding competition for AP-1 sites between CREB and c-Jun.

Aquaporin-7 and aquaporin-12 modulate the inflammatory phenotype of endocrine pancreatic beta-cells

Aquaporins (AQPs) facilitate water and glycerol movement across membranes. AQP7 is the main aquaglyceroporin in pancreatic β-cells and was proposed to play a role in insulin exocytosis. Although AQP7-null mice display adult-onset obesity, impaired insulin secretion and insulin resistance, AQP7 loss-of-function homozygous mutations in humans do not correlate with obesity nor type-2 diabetes. In addition, AQP12 is upregulated in pancreatitis. However, the implication of this isoform in endocrine pancreas inflammation is still unclear. Here, we investigated AQP7 and AQP12 involvement in cellular and inflammatory processes using RIN-m5F beta cells, a model widely used for their high insulin secretion. AQP7 and AQP12 expression were directly associated with cell proliferation, adhesion and migration. While tumor necrosis factor-alpha (TNFα)-induced inflammation impaired AQP7 expression and drastically reduced insulin secretion, lipopolysaccharides (LPS) prompted AQP7 upregulation, and both TNFα and LPS upregulated AQP12. Importantly, cells overexpressing AQP12 are more resistant to inflammation, revealing lower levels of proinflammatory markers. Altogether, these data document AQP7 involvement in insulin secretion and AQP12 implication in inflammation, highlighting their fundamental role in pancreatic β-cell function.

AQP3 Facilitates Proliferation and Adipogenic Differentiation of Porcine Intramuscular Adipocytes

The meat quality of animal products is closely related to the intramuscular fat content. Aquaglyceroporin (AQP) defines a class of water/glycerol channels that primarily facilitate the passive transport of glycerol and water across biological membranes. In this study, the AQP3 protein of the AQP family was mainly studied in the adipogenic function of intramuscular adipocytes in pigs. Here, we found that AQP3 was increased at both mRNA and protein levels upon adipogenic stimuli in porcine intramuscular adipocytes in vitro. Western blot results showed knockdown of AQP3 by siRNA significantly suppressed the expression of adipogenic genes (PPARγ, aP2, etc.), repressed Akt phosphorylation, as well as reducing lipid accumulation. Furthermore, deletion of AQP3 by siRNA significantly downregulated expression of cell cycle genes (cyclin D, E), and decreased the number of EdU-positive cells as well as cell viability. Collectively, our data indicate that AQP3 is of great importance in both adipogenic differentiation and proliferation in intramuscular adipocytes, providing a potential target for modulating fat infiltration in skeletal muscles.

Aquaglyceroporins Are Differentially Expressed in Beige and White Adipocytes

Browning of white adipocytes has been proposed as a powerful strategy to overcome metabolic complications, since brown adipocytes are more catabolic, expending energy as a heat form. However, the biological pathways involved in the browning process are still unclear. Aquaglyceroporins are a sub-class of aquaporin water channels that also permeate glycerol and are involved in body energy homeostasis. In the adipose tissue, aquaporin-7 (AQP7) is the most representative isoform, being crucial for white adipocyte fully differentiation and glycerol metabolism. The altered expression of AQP7 is involved in the onset of obesity and metabolic disorders. Herein, we investigated if aquaglyceroporins are implicated in beige adipocyte differentiation, similar to white cells. Thus, we optimized a protocol of murine 3T3-L1 preadipocytes browning that displayed increased beige and decreased white adipose tissue features at both gene and protein levels and evaluated aquaporin expression patterns along the differentiation process together with cellular lipid content. Our results revealed that AQP7 and aquaporin-9 (AQP9) expression was downregulated throughout beige adipocyte differentiation compared to white differentiation, which may be related to the beige physiological role of heat production from oxidative metabolism, contrasting with the anabolic/catabolic lipid metabolism requiring glycerol gateways occurring in white adipose cells.

Pituitary Hormones (FSH, LH, PRL, and GH) Differentially Regulate AQP5 Expression in Porcine Ovarian Follicular Cells

This study aimed to examine the effect of follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin (PRL), and growth hormone (GH) on Aquaporin 5 (AQP5) expression in granulosa (Gc) and theca cells (Tc) from medium (MF) and large (LF) ovarian follicles of pigs. The results showed that GH significantly decreased the expression of AQP5 in Gc from MF in relation to the control. In the Gc of large follicles, PRL stimulated the expression of AQP5. However, the increased expression of AQP5 in the Tc of LF was indicated by GH and PRL in relation to the control. A significantly higher expression of the AQP5 protein in the Gc from MF and LF was indicated by FSH and PRL. In co-cultures, an increased expression of AQP5 was observed in the Gc from LF incubated with LH, PRL, and GH. A significantly increased expression of AQP5 was also observed in co-cultures of Tc from all type of follicles incubated with LH, whereas PRL stimulated the expression of AQP5 in Tc from MF. Moreover, AQP5 protein expression increased in the co-culture isolated from MF and LF after treatment with FSH, LH, PRL, and GH. AQP5 immunoreactivity was observed in the cytoplasm, mainly in the perinuclear region and endosomes, as well as in the cell membranes of Gc and Tc from the LF and MF.

Lipid Profile and Aquaporin Expression under Oxidative Stress in Breast Cancer Cells of Different Malignancies

Breast cancer is the major cause of tumor-associated mortality in women worldwide, with prognosis depending on the early discovery of the disease and on the type of breast cancer diagnosed. Among many factors, lipids could contribute to breast cancer malignancy by participating in cellular processes. Also, aquaporins are membrane channels found aberrantly expressed in cancer tissues that were correlated with tumor aggressiveness, progression, and metastasis. However, the differences in lipid profile and aquaporin expression between cell types of different malignant potential have never been investigated. Here, we selected three breast cancer cell lines representing the three major breast cancer types (hormone positive, HER2^NEU positive, and triple negative) and analyzed their lipid profile and steady state lipid hydroperoxide levels to correlate with cell sensitivity to H₂O₂. Additionally, the expression profiles of AQP1, AQP3, and AQP5 and the Nrf2 transcription factor were evaluated, before and after oxidative challenge. We found that the lipid profile was dependent on the cell type, with the HER2-positive cells having the lowest level PUFA, whereas the triple negative showed the highest. However, in triple-negative cancer cells, a lower level of the Nrf2 may be responsible for a higher sensitivity to H₂O₂ challenge. Interestingly, HER2-positive cells showed the highest increase in intracellular ROS after oxidative challenge, concomitant with a significantly higher level of AQP1, AQP3, and AQP5 expression compared to the other cell types, with AQP3 always being the most expressed isoform. The AQP3 gene expression was stimulated by H₂O₂ treatment in hormone-positive and HER2^NEU cells, together with Nrf2 expression, but was downregulated in triple-negative cells that showed instead upregulation of AQP1 and AQP5. The lipid profile and AQP gene expression after oxidative challenge of these particularly aggressive cell types may represent metabolic reprogramming of cancer cells and reflect a role in adaptation to stress and therapy resistance.

Modulation of aquaporin gene expression by n-3 long-chain PUFA lipid structures in white and brown adipose tissue from hamsters

EPA (20 : 5n-3) and DHA (22 : 6n-3) fatty acids have weight-reducing properties with physiological activity depending on their molecular structure – that is, as TAG or ethyl esters (EE). Aquaporins (AQP) are membrane protein channels recognised as important players in fat metabolism, but their differential expression in white adipose tissue (WAT) and brown adipose tissue (BAT), as well as their modulation by dietary n-3 long-chain PUFA (LCPUFA) such as EPA and DHA, has never been investigated. In this study, the transcriptional profiles of AQP3, AQP5, AQP7 and selected lipid markers of WAT (subcutaneous and visceral) and BAT (interscapular) from hamsters fed diets containing n-3 LCPUFA in different lipid structures such as fish oil (FO, rich in EPA and DHA in the TAG form) and FO-EE (rich in EPA and DHA in the EE form) were used and compared with linseed oil (LSO) as the reference group. A clear effect of fat depot was observed for AQP3 and leptin (LEP), with the lowest values of mRNA found in BAT relative to WAT. The opposite occurred for PPARα. AQP7 was affected by diet, with FO-fed hamsters having higher mRNA levels compared with LSO-fed hamsters. The relative gene expression of AQP5, adiponectin (ADIPO), GLUT4 and PPARγ was influenced by both fat tissue and diet. Taken together, our results revealed a differential expression profile of AQP and some markers of lipid metabolism in both WAT and BAT in response to feeding n-3 LCPUFA in two different structural formats: TAG v. EE.

Pancreatic Aquaporin-7: A Novel Target for Anti-diabetic Drugs?

Aquaporins comprise a family of 13 members of water channels (AQP0-12) that facilitate a rapid transport of water across cell membranes. In some cases, these pores are also permeated by small solutes, particularly glycerol, urea or nitric oxide, among other solutes. Several aquaporins have been identified in the pancreas, an exocrine and endocrine organ that plays an essential role in the onset of insulin resistance and type 2 diabetes. The exocrine pancreas, which accounts for 90% of the total pancreas, secretes daily large volumes of a near-isotonic fluid containing digestive enzymes into the duodenum. AQP1, AQP5, and AQP8 contribute to fluid secretion especially from ductal cells, whereas AQP12 allows the proper maturation and exocytosis of secretory granules in acinar cells of the exocrine pancreas. The endocrine pancreas (10% of the total pancreatic cells) is composed by the islets of Langerhans, which are distributed in α, β, δ, ε, and pancreatic polypeptide (PP) cells that secrete glucagon, insulin, somatostatin, ghrelin and PP, respectively. AQP7, an aquaglyceroporin permeated by water and glycerol, is expressed in pancreatic β-cells and murine studies have confirmed its participation in insulin secretion, triacylglycerol synthesis and proliferation of these endocrine cells. In this regard, transgenic AQP7-knockout mice develop adult-onset obesity, hyperinsulinemia, increased intracellular triacylglycerol content and reduced β-cell mass in Langerhans islets. Moreover, we have recently reported that AQP7 upregulation in β-cells after bariatric surgery, an effective weight loss surgical procedure, contributes, in part, to the improvement of pancreatic steatosis and insulin secretion through the increase of intracytoplasmic glycerol in obese rats. Human studies remain scarce and controversial, with some rare cases of loss-of function mutations of the AQP7 gene being associated with the onset of type 2 diabetes. The present Review is focused on the role of aquaporins in the physiology and pathophysiology of the pancreas, highlighting the role of pancreatic AQP7 as a novel player in the control of β-cell function and a potential anti-diabetic-drug.

Gastric Plication Improves Glycemia Partly by Restoring the Altered Expression of Aquaglyceroporins in Adipose Tissue and the Liver in Obese Rats

BACKGROUND

Gastric plication is a minimally invasive bariatric surgical procedure, where the greater curvature is plicated inside the gastric lumen. Our aims were to analyze the effectiveness of gastric plication on the resolution of obesity, impaired glucose tolerance, and fatty liver in an experimental model of diet-induced obesity (DIO) and to evaluate changes in glycerol metabolism, a key substrate for adiposity and gluconeogenesis, in adipose tissue and the liver.

METHODS

Male Wistar DIO rats (n = 58) were subjected to surgical (sham operation and gastric plication) or dietary interventions [fed a normal diet (ND) or high-fat diet (HFD) or pair-fed to the amount of food eaten by gastric-plicated animals]. The expression of aquaglyceroporins (AQPs) in epididymal (EWAT) and subcutaneous (SCWAT) fat and the liver was analyzed by real-time PCR and Western blot.

RESULTS

Gastric plication did not result in a significant weight loss in DIO rats, showing a modest reduction in whole-body adiposity and hepatic steatosis. However, gastric-plicated animals exhibited an improvement in basal glycemia and glucose clearance, without changes in hepatic gluconeogenic genes. DIO was associated with an increase in glycerol, higher AQP3 and AQP7 in EWAT and SCWAT, and a decrease in hepatic AQP9. Gastric plication downregulated AQP3 in both fat depots without changes in adipose AQP7 and hepatic AQP9.

CONCLUSION

Gastric plication results in a modest reduction in adiposity and hepatosteatosis but restores glycemia by downregulating AQP3, which entails lower efflux of glycerol from fat, lower plasma glycerol availability, and a reduced use of glycerol as a substrate for hepatic gluconeogenesis.

Endothelial Aquaporins and Hypomethylation: Potential Implications for Atherosclerosis and Cardiovascular Disease

Aquaporins (AQPs) are transmembrane channels that facilitate water and glycerol permeation through cell membranes. Recently, the water channel AQP1 was suggested to contribute to endothelial homeostasis and cardiovascular health. Less is known about endothelial aquaglyceroporins expression and its implication in cardiovascular disease (CVD). We have previously used cultured human endothelial cells under a hypomethylating environment to study endothelial dysfunction and activation, a phenotype implicated in the establishment of atherosclerosis and CVD. Here, we used the same cell model to investigate aquaporin’s expression and function in healthy or pro-atherogenic phenotype. We first confirmed key features of endothelium dysfunction and activation in our cell model, including an augmented endothelial transmigration under hypomethylation. Subsequently, we found AQP1 and AQP3 to be the most predominant AQPs accounting for water and glycerol fluxes, respectively, in the healthy endothelium. Moreover, endothelial hypomethylation led to decreased levels of AQP1 and impaired water permeability without affecting AQP3 and glycerol permeability. Furthermore, TNF-α treatment-induced AQP1 downregulation suggesting that the inflammatory NF-κB signaling pathway mediates AQP1 transcriptional repression in a pro-atherogenic endothelium, a possibility that warrants further investigation. In conclusion, our results add further support to AQP1 as a candidate player in the setting of endothelial dysfunction and CVD.

Genome-Wide Mapping of In Vivo ERα-Binding Sites in Male Mouse Efferent Ductules

As an important nuclear hormone receptor, estrogen receptor α (ERα), which is encoded by the Esr1 gene, regulates the expression of hundreds of genes in a stimulus-specific, temporal, and tissue-specific fashion, mainly by binding to specific DNA sequences called estrogen response elements (EREs). As an important estrogen target tissue in males, the function of the efferent ductules relies on the presence of the ERα protein, but the underlying regulatory mechanisms are poorly illustrated. In this study, genome-wide ERα-binding sites in mouse efferent ductules were mapped by chromatin immunoprecipitation sequencing. In total, 12,105 peaks were identified, and a majority of them were located far from the annotated gene transcription start site. Motif analysis revealed that ∼80% of the ERα-binding peaks harbored at least one ERE, whereas androgen response element-like sequences were the most overrepresented motif in the peaks without any EREs. A number of candidate transcription factor motifs adjacent to the EREs were significantly enriched, including AP2 and GRE, implying the involvement of these putative adjacent factors in the global regulation of ERα target genes. Unexpectedly, more than 50% of the ERα-binding peaks in mouse efferent ductules overlapped with those binding peaks previously identified in mouse uterus, suggesting the conserved mechanism of ERα action in these two tissues. Cobinding of ERα target genes by androgen receptor was further confirmed for Slc9a3 gene, which was responsible for fluid resorption in the efferent ductules. Taken together, our study provides a useful reference set for future work aimed at exploring the mechanism of ERα action in physiological conditions.

Role of aquaporin-7 in ghrelin- and GLP-1-induced improvement of pancreatic β-cell function after sleeve gastrectomy in obese rats

BACKGROUND/OBJECTIVES

Glycerol is a key metabolite for lipid accumulation in insulin-sensitive tissues as well as for pancreatic insulin secretion. We examined the role of aquaporin-7 (AQP7), the main glycerol channel in β-cells, and AQP12, an aquaporin related to pancreatic damage, in the improvement of pancreatic function and steatosis after sleeve gastrectomy in diet-induced obese rats.

SUBJECTS/METHODS

Male Wistar obese rats (n=125) were subjected to surgical (sham operation and sleeve gastrectomy) or dietary (pair-fed to the amount of food eaten by sleeve-gastrectomized animals) interventions. The tissue distribution and expression of AQPs in the rat pancreas were analyzed by real-time PCR, western blotting and immunohistochemistry. The effect of ghrelin isoforms and glucagon-like peptide 1 (GLP-1) on insulin secretion, triacylglycerol (TG) accumulation and AQP expression was determined in vitro in RIN-m5F β-cells.

RESULTS

Sleeve gastrectomy reduced pancreatic β-cell apoptosis, steatosis and insulin secretion. Lower ghrelin and higher GLP-1 concentrations were also found after bariatric surgery. Acylated and desacyl ghrelin increased TG content, whereas GLP-1 increased insulin release in RIN-m5F β-cells. Sleeve gastrectomy was associated with an upregulation of AQP7 together with a normalization of the increased AQP12 levels in the rat pancreas. Interestingly, ghrelin and GLP-1 repressed AQP7 and AQP12 expression in RIN-m5F β-cells. AQP7 protein was negatively correlated with intracellular lipid accumulation in acylated ghrelin-treated cells and with insulin release in GLP-1-stimulated β-cells.

CONCLUSIONS

AQP7 upregulation in β-cells after sleeve gastrectomy contributes, in part, to the improvement of pancreatic steatosis and insulin secretion by increasing intracellular glycerol used for insulin release triggered by GLP-1 rather than for ghrelin-induced TG biosynthesis.

Aquaporins in Obesity

Obesity is one of the most important metabolic disorders of this century and is associated with a cluster of the most dangerous cardiovascular disease risk factors, such as insulin resistance and diabetes , dyslipidemia and hypertension , collectively named Metabolic Syndrome. The role of aquaporins in glycerol metabolism facilitating glycerol release from the adipose tissue and distribution to various tissues and organs, unveils these membrane channels as important players in lipid balance and energy homeostasis and points to their involvement in a variety of pathophysiological mechanisms including insulin resistance, obesity and diabetes.This review summarizes the physiologic role of aquaglyceroporins in glycerol metabolism and lipid homeostasis, describing their specific tissue distribution, their involvement in glycerol balance and their implication in obesity and fat-related metabolic complications. The development of specify pharmacologic modulators able to regulate aquaglyceroporins expression and function , in particular AQP7 in adipose tissue, might constitute a novel approach for controlling obesity and other metabolic disorders.

Involvement of JNK/NFκB Signaling Pathways in the Lipopolysaccharide-Induced Modulation of Aquaglyceroporin Expression in 3T3-L1 Cells Differentiated into Adipocytes

Aquaglyceroporins, belonging to the family of aquaporins (AQPs), are integral plasma membrane proteins permeable to water and glycerol that have emerged as key players in obesity. The aim of this study was to investigate the expression profile of AQPs in undifferentiated and differentiated 3T3-L1 cells and to investigate the changes in expression of aquaglyceroporins in 3T3-L1 cells differentiated into adipocytes and subjected to lipopolysaccharide (LPS) mimicking inflammation occurring during obesity. Furthermore, the study aimed at identifying the signaling cascade involved in the regulation of aquaglyceroporins expression upon LPS stimulation. 3T3-L1 cells were grown as undifferentiated cells (UDC; preadipocytes) or cells differentiated into adipocytes (DC, adipocytes). DC were incubated in the presence or absence of LPS with or without inhibitors of various protein kinases. AQPs mRNA expression levels were measured by real-time quantitative polymerase chain reaction (RT-qPCR). AQP1, AQP2, AQP3, AQP9 and AQP11 mRNA were expressed in both UDC and DC, whereas AQP4, AQP7 and AQP8 mRNA were expressed only in DC. In DC, LPS up-regulated AQP3 mRNA levels (p < 0.05) compared to control; these effects were inhibited by CLI095, SP600125 and BAY11-7082 (p < 0.05). LPS decreased both AQP7 and AQP11 mRNA levels (p < 0.01) in DC as compared to control; this decrease was inhibited by CLI095 and BAY11-7082 (p < 0.05) and additionally by SP00125 for AQP7 (p < 0.05). SB203580 had no effect on LPS-induced AQP3, AQP7 and AQP11 mRNA levels modulations. In conclusion, our results clearly show that many AQPs are expressed in murine 3T3-L1 adipocytes. Moreover, in DCs, LPS led to decreased AQP7 and AQP11 mRNA levels but to increased AQP3 mRNA levels, resulting from the Toll-like receptor 4 (TLR4)-induced activation of JNK and/or NFκB pathway.

Prolonged Starvation Causes Up-Regulation of AQP1 in Adipose Tissue Capillaries of AQP7 Knock-Out Mice

Aquaporins (AQPs) are membrane proteins involved in the regulation of cellular transport and the balance of water and glycerol and cell volume in the white adipose tissue (WAT). In our previous study, we found the co-expression of the AQP1 water channel and AQP7 in the mouse WAT. In our present study, we aimed to find out whether prolonged starvation influences the AQP1 expression of AQP7 knock-out mice (AQP7 KO) in the WAT. To resolve this hypothesis, immunoperoxidase, immunoblot and immunogold microscopy were used. AQP1 expression was found with the use of immunohistochemistry and was confirmed by immunogold microscopy in the vessels of mouse WAT of all studied groups. Semi-quantitative immunoblot and quantitative immunogold microscopy showed a significant increase (by 2.5- to 3-fold) in the abundance of AQP1 protein expression in WAT in the 72 h starved AQP7 KO mice as compared to AQP7+/+ (p < 0.05) and AQP7−/− (p < 0.01) controls, respectively. In conclusion, the AQP1 water channel located in the vessels of WAT is up-regulated in response to prolonged starvation in the WAT of AQP7 KO mice. The present data suggest that an interaction of different AQP isoforms is required for maintaining proper water homeostasis within the mice WAT.

Aquaporin-5: from structure to function and dysfunction in cancer

Aquaporins, a highly conserved group of membrane proteins, are involved in the bidirectional transfer of water and small solutes across cell membranes taking part in many biological functions all over the human body. In view of the wide range of cancer malignancies in which aquaporin-5 (AQP5) has been detected, an increasing interest in its implication in carcinogenesis has emerged. Recent publications suggest that this isoform may enhance cancer cell proliferation, migration and survival in a variety of malignancies, with strong evidences pointing to AQP5 as a promising drug target and as a novel biomarker for cancer aggressiveness with high translational potential for therapeutics and diagnostics. This review addresses the structural and functional features of AQP5, detailing its tissue distribution and functions in human body, its expression pattern in a variety of tumors, and highlighting the underlying mechanisms involved in carcinogenesis. Finally, the actual progress of AQP5 research, implications in cancer biology and potential for cancer detection and prognosis are discussed.

Revisiting the adipocyte: a model for integration of cytokine signaling in the regulation of energy metabolism

Adipose tissue constitutes an extremely active endocrine organ with a network of signaling pathways enabling the organism to adapt to a wide range of different metabolic challenges, such as starvation, stress, infection, and short periods of gross energy excess. The functional pleiotropism of adipose tissue relies on its ability to synthesize and release a huge variety of hormones, cytokines, complement and growth factors, extracellular matrix proteins, and vasoactive factors, collectively termed adipokines. Obesity is associated with adipose tissue dysfunction leading to the onset of several pathologies including type 2 diabetes, dyslipidemia, nonalcoholic fatty liver, or hypertension, among others. The mechanisms underlying the development of obesity and its associated comorbidities include the hypertrophy and/or hyperplasia of adipocytes, adipose tissue inflammation, impaired extracellular matrix remodeling, and fibrosis together with an altered secretion of adipokines. Recently, the potential role of brown and beige adipose tissue in the protection against obesity has been also recognized. In contrast to white adipocytes, which store energy in the form of fat, brown and beige fat cells display energy-dissipating capacity through the promotion of triacylglycerol clearance, glucose disposal, and generation of heat for thermogenesis. Identification of the morphological and molecular changes in white, beige, and brown adipose tissue during weight gain is of utmost relevance for the identification of pharmacological targets for the treatment of obesity and its associated metabolic diseases.

Sexual Dimorphism of Adipose and Hepatic Aquaglyceroporins in Health and Metabolic Disorders

Gender differences in the relative risk of developing metabolic complications, such as insulin resistance or non-alcoholic fatty liver disease (NAFLD), have been reported. The deregulation of glycerol metabolism partly contributes to the onset of these metabolic diseases, since glycerol constitutes a key substrate for the synthesis of triacylglycerols (TAGs) as well as for hepatic gluconeogenesis. The present mini-review covers the sex--related differences in glycerol metabolism and aquaglyceroporins (AQPs) and its impact in the control of adipose and hepatic fat accumulation as well as in whole-body glucose homeostasis. Plasma glycerol concentrations are increased in women compared to men probably due to the higher lipolytic rate and larger AQP7 amounts in visceral fat as well as the well-known sexual dimorphism in fat mass with women showing higher adiposity. AQP9 represents the primary route for glycerol uptake in hepatocytes, where glycerol is converted by the glycerol-kinase enzyme into glycerol-3-phosphate, a key substrate for de novo synthesis of glucose and TAG. In spite of showing similar hepatic AQP9 protein, women exhibit lower hepatocyte glycerol permeability than men, which might contribute to their lower prevalence of insulin resistance and NAFLD.