Aquaporins: Their role in cholestatic liver disease

Hepatocyte aquaporin 8-mediated water transport facilitates bile dilution and prevents gallstone formation in mice

BACKGROUND & AIMS

Although water channel aquaporin-8 (AQP8) has been implicated in hepatic bile formation and liver diseases associated with abnormal bile flow in human and animal studies, direct evidence of its involvement in bile secretion is still lacking. This study aimed to determine the role of AQP8 in bile secretion and gallstone formation.

METHODS

We generated various transgenic knock-in and knockout mouse models and assessed liver AQP8 expression by immunostaining and immunoblotting, hepatic bile secretion by cannulation of the common bile duct, cholesterol gallstone formation by feeding a high-fat lithogenic diet, and identified regulatory small molecules by screening the organic fractions of cholagogic Chinese herbs and performing biochemical characterization.

RESULTS

We identified a novel expression pattern of AQP8 protein in the canalicular membrane of approximately 50% of the liver lobules. AQP8-deficient mice exhibited impaired hepatic bile formation, characterized by the secretion of concentrated bile with a lower flow rate and higher levels of bile lipids than that of wild-type littermates. Aqp8-/- mice showed accelerated gallstone formation, which was rescued by AAV-mediated hepatic expression of AQP8 or AQP1. Moreover, we identified a small molecule, scutellarin, that upregulates hepatocyte AQP8 expression in vitro and in vivo. In Aqp8+/+ mice, scutellarin significantly increased bile flow, decreased bile lipid concentrations, and prevented gallstone formation compared to Aqp8-/- mice. Molecular studies revealed that scutellarin promoted the ubiquitination and degradation of HIF-1α, a negative transcriptional regulator of AQP8, by disrupting its interactions with HSP90.

CONCLUSIONS

AQP8 plays a crucial role in facilitating water transport and bile dilution during hepatic bile formation, thereby mitigating gallstone formation in mice. Small-molecule intervention validated hepatocyte AQP8 as a promising drug target for gallstone therapy.

IMPACT AND IMPLICATIONS

The incidence of gallstone disease is high, and current drug treatments for gallstones are very limited, necessitating the identification of novel drug targets for therapeutic development with universal applicability. To our knowledge, this is the first study to provide direct evidence that the hepatic water channel AQP8 plays a key role in bile dilution and gallstone formation. Modulation of hepatic water transport may provide a universal therapeutic strategy for all types of gallstone diseases.

Expression and Immunolocalization of Aquaporins in the Buffalo Liver and Adipose Tissue

Increasing incidences of fatty liver in humans and animals worldwide is the leading cause of liver related morbidities. Currently, in the face of the growing global increase in fatty liver, and the necessity to explore new factors significantly affecting it, aquaporins (AQPs) have become the focus of interest for many researchers. AQPs are membrane integral proteins involved in the transport of water, glycerol and other small solutes. These are expressed in all tissues and play multiple roles under normal and pathophysiological conditions. Despite ongoing advancements in understanding the involvement of aquaporins in metabolic processes, there remains a notable lack of knowledge concerning cellular and subcellular localization of the AQPs in bovine tissues and organs. Understanding this could provide a new therapeutic target for metabolic syndromes such as fatty liver disease in bovine. In this study, AQPs in bovine liver, adipose tissue and gall bladder are examined using immunohistochemistry. AQP9 immunoreactivity is predominantly detected at the sinusoidal surfaces of hepatocytes. AQP8 is mostly intracellular and localized to the central vein and sinusoid, whereas AQP7 is found around the portal vein. Notably, AQP3 is observed in the bovine gall bladder and adipose tissue but not in the liver. In adipose tissue, AQP7 is also detected in the cytoplasmic membranes of adipocytes. AQPs in liver and adipose tissue were also studied using the western blotting technique. Higher AQP9 and AQP3 expression is observed in the liver and adipose tissue, respectively, indicating they are the dominant aquaporins in these tissues. This suggests they could be potential therapeutic targets for treating fatty liver disease and other metabolic disorders in bovine.

Aquaporins in Biliary Function: Pathophysiological Implications and Therapeutic Targeting

Aquaporins (AQPs) are transmembrane proteins permeable to water and a series of small solutes. AQPs play a key role in pathways of hepatobiliary secretion at the level of the liver, bile ducts, and gallbladder. AQP8 and -9 are pivotal in facilitating the osmotic water movement of hepatic bile, which is composed of 95% water. In the biliary tract, AQP1 and -4 are involved in the rearrangement of bile composition by mechanisms of reabsorption/secretion of water. In the gallbladder, AQP1 and -8 are also involved in trans-epithelial bidirectional water flow with the ultimate goal of bile concentration. Pathophysiologically, AQPs have been indicated as players in several hepatobiliary disorders, including cholestatic diseases and cholesterol cholelithiasis. Research on AQP function and the modulation of AQP expression is in progress, with the identification of potent and homolog-specific compounds modulating the expression or inhibiting these membrane channels with promising pharmacological developments. This review summarizes the contribution of AQPs in physiological and pathophysiological stages related to hepatobiliary function.

Genipin improves obesity through promoting bile secretion and changing bile acids composition in diet-induced obese rats

OBJECTIVES

Bile acids (BAs), as signaling molecules to regulate metabolism, have received considerable attention. Genipin is an iridoid compound extracted from Fructus Gradeniae, which has been shown to relieve adiposity and metabolic syndrome. Here, we investigated the mechanism of genipin counteracting obesity and its relationship with BAs signals in diet-induced obese (DIO) rats.

METHODS

The DIO rats were received intraperitoneal injections of genipin for 10 days. The body weight, visceral fat, lipid metabolism in the liver, thermogenic genes expressions in brown fat, BAs metabolism and signals, and key enzymes for BAs synthesis were determined.

KEY FINDINGS

Genipin inhibited fat synthesis and promoted lipolysis in the liver, and upregulated thermogenic gene expressions in brown adipose tissue of DIO rats. Genipin increased bile flow rate and upregulated the expressions of aquaporin 8 and the transporters of BAs in liver. Furthermore, genipin changed BAs composition by promoting alternative pathways and inhibiting classical pathways for BAs synthesis and upregulated the expressions of bile acid receptors synchronously.

CONCLUSIONS

These results suggest that genipin ameliorate obesity through BAs-mediated signaling pathways.

Aquaporins: Potential Targets in Inflammatory Diseases

Inflammation involves a long chain of molecular reactions and cellular activity designed to repair tissue damaged by various causes. The inflammatory process and its complex mechanisms have recently become a focus of interest for many researchers. After the onset of inflammation, various adverse conditions that initiate the inflammatory response need to be addressed; however, failure to limit the inflammatory reaction may result in the damage or destruction of host cells. Therefore, inflammatory reactions play a role in many diferent diseases. Aquaporins (AQPs), commonly referred to as water channels, are protein channels responsible for forming pores in the membranes of biological cells. Their main function is to aid in the movement of water between cells. Aquaporins not only regulate transepithelial fluid transport across membranes but also play a role in regulating essential events crucial for the inflammatory response. Aquaporins have been shown in many studies to have important roles in inflammatory diseases. This clearly indicates that AQPs may be potential targets for inflammatory diseases. This review summarizes the research to date on the structure and function of AQPs and provides an update on the relationship between AQPs and various human inflammatory diseases.

Insights into the Function of Aquaporins in Gastrointestinal Fluid Absorption and Secretion in Health and Disease

Aquaporins (AQPs), transmembrane proteins permeable to water, are involved in gastrointestinal secretion. The secretory products of the glands are delivered either to some organ cavities for exocrine glands or to the bloodstream for endocrine glands. The main secretory glands being part of the gastrointestinal system are salivary glands, gastric glands, duodenal Brunner's gland, liver, bile ducts, gallbladder, intestinal goblet cells, exocrine and endocrine pancreas. Due to their expression in gastrointestinal exocrine and endocrine glands, AQPs fulfill important roles in the secretion of various fluids involved in food handling. This review summarizes the contribution of AQPs in physiological and pathophysiological stages related to gastrointestinal secretion.

Aquaporin-8 ameliorates hepatic steatosis through farnesoid X receptor in obese mice

Aquaporin-8(AQP8), is a transmembrane channel protein that abounds in liver, which mainly promotes water transport, modulating bile acid formation. However, its role in hepatic lipid metabolism remains unclear. In this study, we found the expression of AQP8 was reduced in liver specimens of patients with NAFLD, high-fat diet (HFD)-induced mice and genetically obese db/db mice. Knockdown of AQP8 in hepatocytes exacerbated the intracellular lipid accumulation induced by free fatty acid (FFA) mixtures. In contrast, hepatic AQP8 overexpression activated farnesoid X receptor (FXR), inhibiting gene expression associated with lipogenesis, which further reduced intrahepatic triglyceride overload in obese mice. FXR knockout abrogated the ameliorating effect of AQP8 overexpression on NAFLD in mice. These findings indicate that AQP8 overexpression protects against fatty liver through activating the FXR pathway.

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.

Applications of In Silico Models to Predict Drug-Induced Liver Injury

Drug-induced liver injury (DILI) is a major cause of the withdrawal of pre-marketed drugs, typically attributed to oxidative stress, mitochondrial damage, disrupted bile acid homeostasis, and innate immune-related inflammation. DILI can be divided into intrinsic and idiosyncratic DILI with cholestatic liver injury as an important manifestation. The diagnosis of DILI remains a challenge today and relies on clinical judgment and knowledge of the insulting agent. Early prediction of hepatotoxicity is an important but still unfulfilled component of drug development. In response, in silico modeling has shown good potential to fill the missing puzzle. Computer algorithms, with machine learning and artificial intelligence as a representative, can be established to initiate a reaction on the given condition to predict DILI. DILIsym is a mechanistic approach that integrates physiologically based pharmacokinetic modeling with the mechanisms of hepatoxicity and has gained increasing popularity for DILI prediction. This article reviews existing in silico approaches utilized to predict DILI risks in clinical medication and provides an overview of the underlying principles and related practical applications.

Hepatic Aquaporin 10 Expression Is Downregulated by Activated NFκB Signaling in Human Obstructive Cholestasis

BACKGROUND AND AIMS

Recent studies reported that the hepatic expression of AQP8 and AQP9 was downregulated in bile duct-ligated (BDL) rats and that overexpression of human AQP1 in the rat liver attenuated cholestasis. However, the hepatic expression of AQP10 and its regulatory mechanism in human cholestasis remain unclear.

METHODS

Serum and liver samples were collected from 34 patients with obstructive cholestasis and from 12 control patients. Eight-week-old male C57BL/6J mice were intravenously injected with an adeno-associated virus 8 (AAV8) encoding human AQP10 driven by a hepatocyte-specific Alb promotor (AAV8-Alb promotor-hAQP10) for functional studies. Constructs of the AQP10 promoter and PLC/PRF/5-ASBT cell lines were used for regulatory mechanism studies.

RESULTS

AQP10 was significantly downregulated in patients with obstructive cholestasis and negatively associated with the serum levels of total bile acid (TBA). The hepatocyte-specific overexpression of hAQP10 significantly attenuated the cholestatic liver injury and intrahepatic bile acids (BA) accumulation in BDL mice. Conjugated BAs, such as TCA and inflammatory factor TNFα, significantly repressed AQP10 expression. Furthermore, NFκB p65/p50 directly bound to the AQP10 promotor and decreased its activity in PLC/RPF/5-ASBT cells and in the livers of patients with obstructive cholestasis. However, these changes were diminished by BAY 11-7082 (a specific inhibitor of NFκB signaling).

CONCLUSION

We are the first to report that AQP10 was significantly decreased in patients with obstructive cholestasis. AQP10 overexpression significantly attenuated cholestatic liver injury in BDL mice. Therefore, overexpression of hAQP10 in the liver may be a valuable strategy for cholestasis intervention.

Hepatobiliary Thyroid Hormone Deficiency Impacts Bile Acid Hydrophilicity and Aquaporins in Cholestatic C57BL/6J Mice

Women are more prone to develop either hypothyroidism or cholesterol gallstones than men. However, a male predominance in cholesterol gallstones under hypothyroidism was reported. Recently, a novel pathogenic link between thyroid hormone (TH) deficiency and cholesterol gallstones has been described in male mice. Here, we investigate if TH deficiency impacts cholesterol gallstone formation in females by the same mechanism. Three-month-old C57BL/6J mice were randomly divided into a control, a TH deficient, a lithogenic, and a lithogenic + TH deficient group and diet-treated for two, four, and six weeks. Gallstone prevalence, liver function tests, bile composition, hepatic gene expression, and gallbladder aquaporin expression and localization were investigated. Cholesterol gallstones were observed in lithogenic + TH deficient but not lithogenic only female mice. Diminished hydrophilicity of primary bile acids due to decreased gene expression of hepatic detoxification phase II enzymes was observed. A sex-specific expression and localization of hepatobiliary aquaporins involved in transcellular water and glycerol permeability was observed under TH deficient and lithogenic conditions. TH deficiency promotes cholesterol gallstone formation in female C57BL/6J mice by the same mechanism as observed in males. However, cholesterol gallstone prevalence was lower in female than male C57BL/6J mice. Interestingly, the sex-specific expression and localization of hepatobiliary aquaporins could protect female C57BL/6J mice to cholestasis and could reduce biliary water transport in male C57BL/6J mice possibly contributing to the sex-dependent cholesterol gallstone prevalence under TH deficiency.

The Pathological Mechanisms of Estrogen-Induced Cholestasis: Current Perspectives

Estrogens are steroid hormones with a wide range of biological activities. The excess of estrogens can lead to decreased bile flow, toxic bile acid (BA) accumulation, subsequently causing intrahepatic cholestasis. Estrogen-induced cholestasis (EIC) may have increased incidence during pregnancy, and within women taking oral contraception and postmenopausal hormone replacement therapy, and result in liver injury, preterm birth, meconium-stained amniotic fluid, and intrauterine fetal death in pregnant women. The main pathogenic mechanisms of EIC may include deregulation of BA synthetic or metabolic enzymes, and BA transporters. In addition, impaired cell membrane fluidity, inflammatory responses and change of hepatocyte tight junctions are also involved in the pathogenesis of EIC. In this article, we review the role of estrogens in intrahepatic cholestasis, and outlined the mechanisms of EIC, providing a greater understanding of this disease.

Aquaporins implicated in the cell proliferation and the signaling pathways of cell stemness

Aquaporins (AQPs) are water channel proteins facilitating passive transport of water and other small molecules across biomembranes. Regulation of osmotic homeostasis via AQPs is accompanied by dynamic participation of various cellular signaling pathways. Recently emerging evidence reveals that functional roles of AQPs are further extended from the osmotic regulation via water permeation into the cell proliferation and differentiation. In particular, anomalous expression of AQPs has been demonstrated in various types of cancer cells and cancer stem-like cells and it has been proposed as markers for proliferation and progression of cancer cells. Thus, a more comprehensive view on AQPs could bring a great interest in the cell stemness accompanied by the expression of AQPs. AQPs are broadly expressed across tissues and cells in a cell type- and lineage-specific manner during development via spatiotemporal transcriptional regulation. Moreover, AQPs are expressed in various adult stem cells and cells associated with a stem cell niche as well as cancer stem-like cells. However, the expression and regulatory mechanisms of AQP expression in stem cells have not been well understood. This review highlighted the AQPs expression in stem cell niches/stem cells and the involvement of AQPs in the cell proliferation and signaling pathways associated with cell stemness.

Hepatic bile formation: bile acid transport and water flow into the canalicular conduit

Advances in molecular biology identifying the many carrier-mediated organic anion transporters and advances in microscopy that have provided a more detailed anatomy of the canalicular conduit make updating the concept of osmotically determined canalicular flow possible. For the most part water flow is not transmembrane but via specific pore proteins in both the hepatocyte and the tight junction. These pores independently regulate the rate at which water flows in response to an osmotic gradient and therefore are determinants of canalicular bile acid concentration. Review of the literature indicates that the initial effect on hepatic bile flow of cholestatic agents such as Thorazine and estradiol 17β-glucuronide are on water flow and not bile salt export pump-mediated bile acid transport and thus provides new approaches to the pathogenesis of drug-induced liver injury. Attaining a micellar concentration of bile acids in the canaliculus is essential to the formation of cholesterol-lecithin vesicles, which mostly occur in the periportal region of the canalicular conduit. The other regions, midcentral and pericentral, may transport lesser amounts of bile acid but augment water flow. Broadening the concept of how hepatic bile flow is initiated, provides new insights into the pathogenesis of canalicular cholestasis.

Bile acid signaling in renal water regulation

Emerging evidence has shown that bile acids play important roles in renal physiology and diseases by activating two major receptors, the nuclear farnesoid X receptor (FXR) and the membrane G protein-coupled bile acid receptor-1 (Gpbar1; also known as TGR5). Both FXR and TGR5 have been identified in human and rodent kidneys, where they are deeply involved in renal water handling. In mice, FXR- or TGR5-related gene deficiency has been associated with reduced aquaporin-2 expression accompanied with impaired urinary concentration ability. In this mini-review, we briefly discuss the current understanding of FXR/TGR5 signaling in the kidneys, with a special focus on the regulation of aquaporin-2 expression by bile acids in the collecting ducts and its potential significance in disease conditions.

Aquaporin water channels: New perspectives on the potential role in inflammation

Aquaporins (AQPs) are a family of membrane water channel proteins that osmotically modulate water fluid homeostasis in several tissues; some of them also transport small solutes such as glycerol. At the cellular level, the AQPs regulate not only cell migration and transepithelial fluid transport across membranes, but also common events that are crucial for the inflammatory response. Emerging data reveal a new function of AQPs in the inflammatory process, as demonstrated by their dysregulation in a wide range of inflammatory diseases including edematous states, cancer, obesity, wound healing and several autoimmune diseases. This chapter summarizes the discoveries made so far about the structure and functions of the AQPs and provides updated information on the underlying mechanisms of AQPs in several human inflammatory diseases. The discovery of new functions for AQPs opens new vistas offering promise for the discovery of mechanisms and therapeutic opportunities in inflammatory disorders.

Animal models of cholestasis: An update on inflammatory cholangiopathies

Cholestasis is a frequent clinical condition initiating or complicating chronic liver diseases, particularly cholangiopathies, where the biliary epithelium is the primary target of the pathogenetic sequence. Until a few decades ago, understanding of cholestasis relied mostly on the experimental model of bile duct ligation in rodents. However, a simple model of biliary obstruction cannot reproduce the complex mechanisms and networks leading to cholestasis in cholangiopathies. These networks are underpinned by an intricate dysregulation of pro-inflammatory and pro-fibrotic signals involving besides cholangiocytes, multiple cell elements of both innate and adaptive immunity. Therefore, in the last years, a wide range of animal models of biliary injury have been developed, mostly in mice, following three main approaches, chemical induction, immunization and genetic manipulation. In this review, we will give an update of the animal models of the two main cholangiopathies, primary sclerosing cholangitis and primary biliary cholangitis, which have provided us with the most relevant insights into the pathogenesis of these still controversial diseases.

A structural preview of aquaporin 8 via homology modeling of seven vertebrate isoforms

Background

Aquaporins (AQPs) facilitate the passage of small neutral polar molecules across membranes of the cell. In animals there are four distinct AQP subfamilies, whereof AQP8 homologues constitute one of the smallest subfamilies with just one member in man. AQP8 conducts water, ammonia, urea, glycerol and H₂O₂ through various membranes of animal cells. This passive channel has been connected to a number of phenomena, such as volume change of mitochondria, ammonia neurotoxicity, and mitochondrial dysfunction related to oxidative stress. Currently, there is no experimentally determined structure of an AQP8, hence the structural understanding of this subfamily is limited. The recently solved structure of the plant AQP, AtTIP2;1, which has structural and functional features in common with AQP8s, has opened up for construction of homology models that are likely to be more accurate than previous models.

Results

Here we present homology models of seven vertebrate AQP8s. Modeling based on the AtTIP2;1 structure alone resulted in reasonable models except for the pore being blocked by a phenylalanine that is not present in AtTIP2;1. To achieve an open pore, these models were supplemented with models based on the bacterial water specific AQP, EcAqpZ, creating a chimeric monomeric model for each AQP8 isoform. The selectivity filter (also named the aromatic/arginine region), which defines the permeant substrate profile, comprises five amino acid residues in AtTIP2;1, including a histidine coming from loop C. Compared to AtTIP2;1, the selectivity filters of modelled AQP8s only deviates in that they are slightly more narrow and more hydrophobic due to a phenylalanine replacing the histidine from loop C. Interestingly, the models do not exclude the existence of a side pore beneath loop C similar to that described in the structure of AtTIP2;1.

Conclusions

Our models concur that AQP8s are likely to have an AtTIP2;1-like selectivity filter. The detailed description of the expected configuration of residues in the selectivity filters of AQP8s provides an excellent starting point for planning of as well as rationalizing the outcome of mutational studies. Our strategy to compile hybrid models based on several templates may prove useful also for other AQPs for which structural information is limited.

Electronic supplementary material

The online version of this article (10.1186/s12900-018-0081-8) contains supplementary material, which is available to authorized users.

Function of aquaporins in sepsis: a systematic review

BACKGROUND

Sepsis is a common cause of death in intensive care units worldwide. Due to the high complexity of this immunological syndrome development of novel therapeutic strategies is urgent. Promising drug targets or biomarkers may depict aquaporins (AQPs) as they regulate crucial key mechanisms of sepsis.

MAIN BODY

Here we report on base of the current literature that several AQPs are involved in different physiological processes of sepsis. In immune system mainly AQPs 3, 5 and 9 seem to be important, as they regulate the migration of different immune cells. Several studies showed that AQP3 is essential for T cell function and macrophage migration and that AQP5 and AQP9 regulate neutrophil cell migration and impact sepsis survival. Additionally, to the function in immune system AQPs 1 and 5 play a role in sepsis induced lung injury and their downregulation after inflammatory stimuli impair lung injury. By contrast, AQP4 expression is up-regulated during brain inflammation and aggravates brain edema in sepsis. In kidney AQP2 expression is downregulated during sepsis and can cause renal failure. Some studies also suggest a role of AQP1 in cardiac function.

CONCLUSION

In conclusion, AQPs are involved in many physiological dysfunctions in sepsis and their expressions are differently regulated. Additional research on the regulatory mechanisms of aquaporins may identify potential therapeutic targets.

Aquaporins and Gland Secretion

Aquaporins (AQPs ) are expressed in most exocrine and endocrine secretory glands. Consequently, summarizing the expression and functions of AQPs in secretory glands represents a daunting task considering the important number of glands present in the body, as well as the number of mammalian AQPs - thirteen. The roles played by AQPs in secretory processes have been investigated in many secretory glands. However, despite considerable research, additional studies are clearly needed to pursue our understanding of the role played by AQPs in secretory processes. This book chapter will focus on summarizing the current knowledge on AQPs expression and function in the gastrointestinal tract , including salivary glands, gastric glands, Duodenal Brunner's gland, liver and gallbladder, intestinal goblets cells, exocrine and endocrine pancreas, as well as few other secretory glands including airway submucosal glands, lacrimal glands, mammary glands and eccrine sweat glands.

Aquaporins in Health and Disease: An Overview Focusing on the Gut of Different Species

Aquaporins (AQPs) play a pivotal role in gut homeostasis since their distribution and function is modulated both in physiological and in pathophysiological conditions. The transport of water and solutes through gut epithelia is essential for osmoregulation and digestive and absorptive functions. This passage is regulated by different AQP isoforms and characterized by their peculiar distribution in the gastrointestinal tract. To date, AQP localization has been identified in the gut and associated organs of several mammalian species by different techniques (immunohistochemical, western blotting, and RT-PCR). The present review describes the modulation of AQP expression, distribution, and function in gut pathophysiology. At the same time, the comparative description of AQP in animal species sheds light on the full range of AQP functions and the screening of their activity as transport modulators, diagnostic biomarkers, and drug targets. Moreover, the phenotype of knockout mice for several AQPs and their compensatory role and the use of specific AQP inhibitors have been also reviewed. The reported data could be useful to design future research in both basic and clinical fields.

A comprehensive analysis of aquaporin and secretory related gene expression in neonate and adult cholangiocytes

Canalicular bile is secreted by hepatocytes and then passes through the intrahepatic bile ducts, comprised of cholangiocytes, to reach the extrahepatic biliary system. In addition to providing a conduit for bile to drain from the liver, cholangiocytes play an active role in modifying bile composition. Bile formation is the result of a series of highly coordinated intricate membrane-transport interactions. Proper systematic regulation of solute and water transport is critical for both digestion and the health of the liver, yet our knowledge of cholangiocyte water and ion transporters and their relative expression patterns remains incomplete. In this report, we provide a comprehensive expression profile of the aquaporin (AQP) family and three receptors/channels known to regulate ion transport in the murine cholangiocyte. In murine intrahepatic cholangiocytes, we found mRNA expression for all twelve of the members of the AQP family of proteins and found temporal changes in the expression profile occurring with age. Using AQP4, an established marker within cholangiocyte physiology, we found that AQP2, AQP5 and AQP6 expression levels to be significantly different between the neonatal and adult time points. Furthermore, there were distinct temporal expression patterns, with that of AQP12 unique in that its expression level decreased with age, whilst the majority of AQPs followed an increasing expression level trend with age. Of the three receptors/channels regulating ion transport in the murine cholangiocyte, only the cystic fibrosis transmembrane conductance regulator was found to follow a consistent trend of decreasing expression coincident with age. We have further validated AQP3 and AQP8 protein localization in both hepatocytes and cholangiocytes. This study emphasizes the need to further appreciate and consider the differences in cholangiocyte biology when treating neonatal and adult hepatobiliary diseases.

Bile formation and secretion

Bile is a unique and vital aqueous secretion of the liver that is formed by the hepatocyte and modified down stream by absorptive and secretory properties of the bile duct epithelium. Approximately 5% of bile consists of organic and inorganic solutes of considerable complexity. The bile-secretory unit consists of a canalicular network which is formed by the apical membrane of adjacent hepatocytes and sealed by tight junctions. The bile canaliculi (∼1 μm in diameter) conduct the flow of bile countercurrent to the direction of portal blood flow and connect with the canal of Hering and bile ducts which progressively increase in diameter and complexity prior to the entry of bile into the gallbladder, common bile duct, and intestine. Canalicular bile secretion is determined by both bile salt-dependent and independent transport systems which are localized at the apical membrane of the hepatocyte and largely consist of a series of adenosine triphosphate-binding cassette transport proteins that function as export pumps for bile salts and other organic solutes. These transporters create osmotic gradients within the bile canalicular lumen that provide the driving force for movement of fluid into the lumen via aquaporins. Species vary with respect to the relative amounts of bile salt-dependent and independent canalicular flow and cholangiocyte secretion which is highly regulated by hormones, second messengers, and signal transduction pathways. Most determinants of bile secretion are now characterized at the molecular level in animal models and in man. Genetic mutations serve to illuminate many of their functions.

Physiological and molecular biochemical mechanisms of bile formation

This review considers the physiological and molecular biochemical mechanisms of bile formation. The composition of bile and structure of a bile canaliculus, biosynthesis and conjugation of bile acids, bile phospholipids, formation of bile micellar structures, and enterohepatic circulation of bile acids are described. In general, the review focuses on the molecular physiology of the transporting systems of the hepatocyte sinusoidal and apical membranes. Knowledge of physiological and biochemical basis of bile formation has implications for understanding the mechanisms of development of pathological processes, associated with diseases of the liver and biliary tract.

Evidence of drug-drug interactions through uptake and efflux transport systems in rat hepatocytes: implications for cellular concentrations of competing drugs

For drugs with hepatobiliary transport across hepatocytes, the interplay between uptake and efflux transporters determines hepatic concentrations of drugs, but the evolution over time of these concentrations is difficult to measure in humans other than with magnetic resonance imaging contrast agents in the liver. Gadobenate dimeglumine (BOPTA) is a contrast agent used in liver magnetic resonance imaging that enters into human hepatocytes through organic anion transporting polypeptides (OATP) and exits unchanged into bile through the multiple resistance-associated protein 2 (MRP2). Rifampicin (RIF) is transported by the same membrane proteins and may compete with BOPTA for hepatic uptake. Simultaneous drug-drug interactions through uptake and efflux transport systems in hepatocytes according to the cellular concentrations of competing drugs were never investigated. In perfused rat liver preparations, we demonstrate how the drug-drug interactions through transporters determine cellular concentrations of the competing drugs BOPTA and RIF, and we show that the cellular concentrations by modulating transport through membranes regulate the rat Oatp-Mrp2 interplay. Moreover, drug interactions through transporters change greatly over time.

The molecular pathogenesis of cholestasis in sepsis

Sepsis-induced cholestasis is a complication of infection. Infections cause systemic and intrahepatic increase in proinflammatory cytokines which result in impaired bile flow ie. cholestasis. Several other mediators of impairment in bile flow have been identified under conditions of sepsis such as increased nitric oxide production and decreased aquaporin channels. The development of cholestasis may also further worsen inflammation. The molecular basis of normal bile flow and mechanisms of impairment in sepsis are discussed.

Water channel proteins in bile formation and flow in health and disease: when immiscible becomes miscible

An essential function of the liver is the formation and secretion of bile, a complex aqueous solution of organic and inorganic compounds essential as route for the elimination of body cholesterol as unesterified cholesterol or as bile acids. In bile, a considerable amount of otherwise insoluble cholesterol is solubilized by carriers including two other classes of lipids, namely phospholipid and bile acids. Formation of bile and generation of bile flow are driven by the active secretion of bile acids, lipids and electrolytes into the canalicular and bile duct lumens followed by the parallel movement of water. Thus, water has to cross rapidly into and out of the cell interior driven by osmotic forces. Bile as a fluid, results from complicated interplay of hepatocyte and cholangiocyte uptake and secretion, concentration, by involving a number of transporters of lipids, anions, cations, and water. The discovery of the aquaporin water channels, has clarified the mechanisms by which water, the major component of bile (more than 95%), moves across the hepatobiliary epithelia. This review is focusing on novel acquisitions in liver membrane lipidic and water transport and functional participation of aquaporin water channels in multiple aspects of hepatobiliary fluid balance. Involvement of aquaporins in a series of clinically relevant hepatobiliary disorders are also discussed.

Estrogen prevents increased hepatic aquaporin-9 expression and glycerol uptake during starvation

In starvation, glycerol is released from adipose tissue and serves as an important precursor for hepatic gluconeogenesis. By unknown sex-specific mechanisms, women suppress the endogenous glucose production better than men and respond to metabolic stress with higher plasma glycerol levels. Hepatic glycerol uptake is facilitated by aquaporin-9 (AQP9), a broad-selectivity neutral solute channel, and represents an insulin-regulated step in supplying gluconeogenesis with glycerol. In the present study, hepatic AQP9 abundance was increased 2.6-fold in starved male rats as assessed by immunoblotting and immunohistochemistry. By contrast, starvation had no significant effect on hepatic AQP9 expression in female rats. Coordinately, plasma glycerol levels remained unchanged with starvation in male rats, whereas it was increased in female rats. The different responses to starvation were paralleled by higher glycerol permeability in basolateral hepatocyte membranes from starved male rats compared with starved females. Ovariectomy led to a starvation-response pattern identical to that observed in male rats with increased hepatic AQP9 expression and unchanged plasma glycerol levels. In cultured hepatocytes, 17β-estradiol and the selective estrogen receptor α-agonist, propyl pyrazole triol, caused a decrease in AQP9 expression. Our results support that a sex-specific regulation of the hepatic glycerol channel AQP9 during starvation contributes to the higher plasma glycerol levels observed in women during fasting and possibly results in a lower cytosolic availability of glycerol. Furthermore, the sexual dimorphism in the hepatic handling of glycerol during starvation might be explained by 17β-estradiol preventing the starvation-induced increase in hepatic AQP9 abundance.

How organic anions accumulate in hepatocytes lacking Mrp2: evidence in rat liver

In the liver, the accumulation of hepatobiliary contrast agents is a crucial issue to understand the images of liver scintigraphy or magnetic resonance (MR) imaging. Thus, depending on the regulation of uptake and exit membrane systems in normal and injured hepatocytes, these contrast agents will accumulate differently within cells. Gadobenate dimeglumine (Gd-BOPTA) is a hepatobiliary MR contrast agent that distributes to the extracellular space and enters into rat hepatocytes through the sinusoidal transporters, organic anion-transporting polypeptides. Gd-BOPTA is not metabolized during its transport to the canalicular membrane where it is excreted into bile through multiple resistance protein-2 (Mrp2). It is not well known how Gd-BOPTA accumulates in normal livers and in livers lacking Mrp2. We perfused livers from normal rats and from rats lacking Mrp2 with (153)Gd-BOPTA at increasing concentrations and assessed the hepatic accumulation of this agent using a gamma probe placed above the livers. By use of a pharmacokinetic model that best described the amounts of Gd-BOPTA in perfusate, bile, and hepatic tissue over time, we showed how increasing concentrations and the absence of Mrp2 modify the hepatic accumulation of the contrast agent. It is noteworthy that despite the absence of Gd-BOPTA bile excretion and a similar efflux back to sinusoids in livers lacking Mrp2, the maximal hepatic accumulation of contrast agent was similar to normal rats. We also showed how hepatic accumulation relies on the concomitant entry into and exit from hepatocytes. Such information improves our understanding of liver imaging associated with the perfusion of hepatobiliary contrast agents, which was recently introduced in clinical practice.