Aquaporins (AQPs) are homotetrameric channel proteins allowing the diffusion of water/small solutes across membranes. AQP structures are very similar, with each monomer defining a single pore selective to water/solutes and contributing to form a fifth central pore, whose meaning remains elusive. Nevertheless, AQPs show distinct transport selectivity to water, orthodox AQPs, or glycerol/solutes, aquaglyceroporins. The variety of available AQP 3D-structures is a valuable resource for studying the structure-function relationships within this protein family. A recent comparative analysis allowed specific electrostatic profiles to be associated with the main AQP selectivity to water and glycerol. We exploited this approach to gain some insights into the role of the AQP central pore. Interestingly: the electrostatics of AQP central channels correlates with their main transport function; AQP1 and AQP4 fifth pore has strikingly comparable electrostatics, supporting previous works reporting its involvement in the transport of CO2 across membranes; the central pore of the spinach PIP2;1 shares the same electrostatic profile of the monomeric pore of orthodox AQPs, suggesting that the fifth pore could allegedly represent an alternative/additional path for the transport of water across (at least some) plant AQPs. The hypothesis is being verified experimentally.

We propose a system of first-order ordinary differential equations to describe and understand the physiological mechanisms of the interplay between plasma glucose and insulin and their behaviors in diabetes. The proposed model is based on Hill and step functions which are used to simulate the switch-like behavior that occurs in metabolic regulatory variables when some of the threshold parameters are approached. A simplified piece-wise linear system is also proposed to study the possible equilibria and solutions and used to introduce simple theoretical control mechanisms representing the action of an artificial pancreas and regulating exogenous insulin.

Background Caveolin-1, the main structural protein of caveolae, is involved in cholesterol homoeostasis, transcytosis, endocytosis and signal transduction and thought to play an important role in lipidogenesis. Little is known about the pathophysiological role of caveolin-1 in nonalcoholic fatty liver disease (NAFLD), a condition frequently associated with the metabolic syndrome and characterized by abnormal accumulation of intrahepatic triglycerides with a potentially harmful risk of evolution to liver fibrosis, cirrhosis and hepatocellular carcinoma. Materials and methods Liver steatosis (micro/macrovesicular) was induced in adult rats fed a choline-deficient diet for 14 days and compared with a control normal diet. The expression and subcellular distribution of caveolin-1 was assessed using light and electron microscopy by immunohistochemical and immunocytochemical techniques and by Western blotting. Results Caveolin-1 was mainly associated with the hepatocyte basolateral plasma membrane. Fatty hepatocytes were characterized by a significant increase in the expression of caveolin-1 around and within the lipid droplets as well as in the inner membrane of mitochondria. Conclusions Our data suggest the involvement of caveolin-1 in the case of abnormal lipogenesis and mitochondrial function typical of steatotic hepatocytes in NAFLD. Addressing the role played by caveolin-1 in liver membranes in NAFLD may help future therapeutic choices in a frequent metabolic liver disease.

Hepatocyte mitochondrial ammonia detoxification via ureagenesis is critical for prevention of hyperammonemia and hepatic encephalopathy. Aquaporin-8 (AQP8) channels facilitate the membrane transport of ammonia. Since AQP8 is expressed in hepatocyte inner mitochondrial membranes, we studied whether mitochondrial AQP8 (mtAQP8) plays a role in ureagenesis from ammonia. Primary cultured rat hepatocytes were transfected with siRNAs targeting two different regions of the rat AQP8 molecule or with scrambled control siRNA. After 48 h, the levels of mtAQP8 protein specifically decreased by around 80% (P < 0.05) without affecting cell viability. The mtAQP8-knockdown cells in the presence of ammonium chloride, showed a decrease in ureagenesis of around 30% (P < 0.05). Glucagon strongly stimulated ureagenesis in control hepatocytes (+120%, P < 0.05), whereas in mtAQP8-knockdown cells, it induced no significant stimulation. Contrarily, mtAQP8-silencing induced no significant change in basal and glucagoninduced ureagenesis when glutamine or alanine was used as source of nitrogen. NMR studies using 15N-labeled ammonia, confirmed that glucagon-induced 15N-labeled urea synthesis was markedly reduced in mtAQP8-knockdown hepatocytes (-90%, P < 0.05). In vivo studies in the rat showed that under glucagon-induced ureagenesis, hepatic mtAQP8 protein expression was markedly upregulated (+160% P < 0.05). Moreover, transport studies in liver inner mitochondrial membrane vesicles showed that glucagon increased the diffusional permeability to the ammonia analog [14C]methylamine (+80%, P < 0.05). Conclusion: Hepatocyte mtAQP8 channels facilitate the mitochondrial uptake of ammonia and its metabolism into urea, mainly under glucagon stimulation. This mechanism may be relevant to hepatic ammonia detoxification and in turn, avoid the deleterious effects of hyperammonemia.

Background: Glycerol released from adipocytes flows to the liver where its utilization in supplying hepatocyte gluconeogenesis is rate-limited by the permeation step. An aquaglyceroporin, AQP9, has been often linked to liver uptake of glycerol.However, the truthfulness of this postulation and the potential existence of additional pathways of glycerol import by hepatocytes have never been verified directly. Here, we define the molecular identity and extent of liver glycerol transport andevaluate the correlationbetweenhepatic AQP9 level and glycerol permeability inAQP9+/+ wild type mice in different nutritional states and circulating insulin levels. Materials and methods: In male C57BL/6J wild type or AQP9 knockout mice, were used. Levels of AQP9 mRNA and protein were evaluated by RT-PCR and immunoblotting/immunocytochemistry, respectively. Glycerol permeability (Pgly), Arrhenius activation energy (Ea) and inhibition of glycerol transport were assessed by stopped flow light scattering. Results: Facilitated diffusion of glycerol into hepatocyteswas indicated by the lowArrhenius activationenergy (3.5 kcalper mole) and strong inhibition exerted by phloretin, anAQP9 blocker.While fasting markedly increased hepatic AQP9, a straight parallelism was seen both in quantitative and time-space terms between glycerol permeability and AQP9 protein in AQP9+/+ mice kept in fed or fasted/refed states. The highest glycerol permeability, at 18-h fasting, coincided with the highest percent of phloretin inhibition (63%). Besides being markedly lower than inAQP9+/+ mice the liver Pgly of the AQP9 / mice did not increase during fasting. Conclusions: These results prove experimentally AQP9 plays a major functional significance rolefor AQP9 in maximizing liver glycerol import during states requiring increased glucose production. Refining the understanding of liver AQP9 in metabolic and energy balance may reveal helpfulunravel novel for therapeutic purposesstrategies in several metabolic disorders.

Bile acids (BAs) regulate the absorption of fat-soluble vitamins, cholesterol and lipids but have also a key role as singalling molecules and in the modulation of epithelial cell proliferation, gene expression and metabolism. These homeostatic pathways, when disrupted, are able to promote local inflammation, systemic metabolic disorders and, ultimately, cancer. The effect of hydrophobic BAs, in particular, can be linked with cancer in several digestive (mainly oesophagus, stomach, liver, pancreas, biliary tract, colon) and extra-digestive organs (i.e. prostate, breast) through a complex series of mechanisms including direct oxidative stress with DNA damage, apoptosis, epigenetic factors regulating gene expression, reduced/increased expression of nuclear receptors (mainly farnesoid X receptor, FXR) and altered composition of gut microbiota, also acting as a common interface between environmental factors (including diet, lifestyle, exposure to toxics) and the molecular events promoting cancerogenesis. Primary prevention strategies (i.e. changes in dietary habits and lifestyle, reduced exposure to environmental toxics) mainly able to modulate gut microbiota and the epigenome, and the therapeutic use of hydrophilic BAs to counterbalance the negative effects of the more hydrophobic BAs might be, in the near future, part of useful tools for cancer prevention and management.

Lipolytic glycerol, released from adipocytes, flows through bloodstream to liver where it is transported into hepatocytes to supply gluconeogenesis. AQP9, an aquaporin membrane channel, has been often linked to the uptake of glycerol in hepatocytes; however, no direct experimental evidence has been provided, yet. Here, we address the question by employing a stopped flow light scattering approach using vesicles of hepatocyte basolateral plasma membrane (BLPM) prepared from mouse livers in various metabolic states.The movement of glycerol across the hepatocyte BLPM vesicles occurred by facilitated diffusion as indicated by the low Arrhenius activation energy (3.5 kcal/mol) characterizing the transport and the strong (>60%) inhibition seen after incubation with phloretin or HgCl2, two known AQP9 blockers. Fasting markedly increased both AQP9 expression and glycerol membrane permeability (Pgly) of BLPM in a time-dependent manner. In line with these results, the plasma glycerol levels of the examined animals changed accordingly to the extents of Pgly and AQP9 expression values. This was also in line with additional experiments where the Pgly of the liver vesicles of AQP9 knockout mice resulted markedly lower than the Pgly of wild type mice counterpart (5.43±0.2 vs. 10.03±0.6 x 10­3 mm/s, respectively; P<0.01). Overall, these results prove major functional relevance for AQP9 in hepatocyte glycerol uptake indicating AQP9 as a new player in metabolic and energy homeostasis.

Assessing the selectivity, regulation and physiological relevance of aquaporin membrane channels (AQPs) requires structural and functional studies of wild type and modified proteins. In particular, when characterizing their transport properties, reconstitution in isolation from native cellular or membrane processes is of pivotal importance. Here, we describe rapid and efficient incorporation of OsPIP1;1, a rice AQP, in liposomes at analytical scale. PIP1;1 was produced as a histidine-tagged form, 10His-OsPIP1;1, in an Escherichia coli-based expression system. The recombinant protein was purified by affinity chromatography and incorporated into liposomes by a micro-batchwise technology using egg-yolk phospholipids and the non-polar Amberlite resin. PIP1;1 proteoliposomes and control empty liposomes had good size homogeneity as seen by quasi-elastic light scattering and electron microscopy analyses. By stoppedflow light scattering, indicating correct protein folding of the incorporated protein, the osmotic water permeability exhibited by the PIP1;1 proteoliposomes was markedly higher than empty liposomes. Functional reconstitution of OsPIP1;1 was further confirmed by the low Arrhenius activation energy (3.37 kcal/mol) and sensitivity to HgCl2, a known AQP blocker, of the PIP1;1-mediated osmotic water conductance. These results provide a valuable contribution in fully elucidating the regulation and waterconducting property of PIP1;1, an AQP that needs to hetero-multimerize with AQPs of the PIP2 subgroup to reach the native plasma membrane and play its role. The micro-batchwise methodology is suitable for the functional reconstitution of whichever AQPs and other membrane transport proteins. © 2014 Elsevier Masson SAS. All rights reserved.

The voltage-dependent anion channel (VDAC) is a membrane channel mediating the flux of metabolites and water across the mitochondrial outer membrane. Upon lethal stress, VDAC can mediate mitochondrial membrane permeabilization and cell death. Here, we demonstrate that in mouse models of non-alcoholic fatty liver disease (NAFLD), VDAC exhibits reduced threonine phosphorylation linked to a lack of interaction with the anti-apoptotic protein Bcl- XL and the serine/threonine kinase GSK3 in mitochondrial membrane. VDAC dephosphorylation is observed in in vitro models of lipotoxicity, as well as mice with NAFLD and patients with morbid fatty liver disease. This post-translational modification increases the influx of water and calcium into mitochondria and sensitizes the organelle to matrix swelling, depolarization and cytochrome c release. Moreover, in the NAFDL models, NADH oxidase activity and channel conductance of VDAC are sensitized to calcium regulation in steatosis models. In conclusion, VDAC acts as an early sensor of lipid toxicity and its GSK3-mediated phosphorylation status controls outer mitochondrial membrane permeabilization in hepatosteatosis.

Non-Alcoholic Fatty Liver Disease (NAFLD), a pathology caused by excessive accumulation of triglycerides (TG) within hepatocytes, is recognized as the leading cause of chronic liver disease in adults and children worldwide. NAFLD is often associated with obesity and diabetes and mostly closely linked to insulin resistance. Investigation into NAFLD pathogenesis has increased exponentially in the last years. Main pathways include increased visceral adipose tissue and insulin resistance, altered hepatic fatty acid export, oxidation, and desaturation within the liver, and the initiation and subsequent effects of lipotoxicity. Altered uptake of glycerol by hepatocytes is also a major intersecting component, however, the underlying mechanism has begun to be understood only recently after Aquaporin-9 (AQP9), an aquaglyceroporin regulated by insulin and leptin, was found to mediate liver glycerol permeability. AQP9 is dysregulated in the liver of morbidly obese patients with NAFLD associated with insulin resistance and diabetes and in animal models of NAFLD. The reduction in AQP9 expression and consequent decrease of glycerol influx into steatotic hepatocytes is hypothesized to be a compensatory mechanism to avoid further infiltration of TGs in liver parenchyma. Besides being a new important player in metabolic homeostasis AQP9 may prove a novel target to treat therapeutically NAFLD, a common feature of metabolic syndrome.

Aquaporins (AQPs) are channel proteins largely present in mammals where they facilitate the permeation of water and a variety of substrates across cellular membranes. AQPs exert pleiotropic roles in both health and disease. This chapter addresses the most recent acquisitions in terms of expression and regulation, as well as physiological and pathophysiological rolesof AQPs in the hepatobiliary tract, salivary glands and pancreas. A number of AQPs are found in liver, bile ducts and gallbladder where they are playing roles in bile formation, secretion and reabsorption. Liver AQPs are also implicated in energy homeostasis by acting in hepatic gluconeogenesis and fat metabolism, and in important processes such as ammonia detoxification and mitochondrial release of hydrogen peroxide. Hepatobiliary AQPs are involved in clinical disorders including cholestasis, gallstone formation, insulin resistance, fatty liver disease, hepatic cirrhosis and hepatocarcinoma. Salivary and exocrine pancreas AQPs exert main roles in fluid secretion and contribute to the pathogenesis of xerostomia and pancreatic insufficiencies. Endocrine pancreas AQPs are suggested to be key regulators of intra-islet glycerol content as well as insulin production and secretion, and to contribute to the pathogenesis of diabetes. This body of knowledge represents the mainstay of present and future research in a rapidly expanding field.

Mucins are high molecular weight epithelial proteins, strongly glycosylated, and are the main component of the mucus. Since mucus secretion can be altered in diseases, colon mucins can be regarded as a biomarker of chronic inflammatory bowel diseases or preneoplastic changes. Conventional histochemistry and lectin histochemistry combined with chemical treatment and enzymatic digestion were carried out to analyze the colon mucins in mice fed a high-fat diet for 25 weeks, a period sufficient to induce simple liver steatosis, to check whether the carbohydrate features of mucus can be altered by an inadequate diet. An increase in the sialo/sulfomucins ratio with respect to control mice, assessed by computerized image analysis, was observed in the colon, although differences in sialic acid acetylation between control and mice fed a high-fat diet were not found. High-fat diet was also associated with altered lectin-binding pattern of the mucus, with a probable shortening of oligosaccharide chains of glycoproteins. This pattern was leading to over-expression of Galβ1,3GalNA c terminal dimers (TF antigen) and GalNA c terminal residues (Tn antigen). This altered composition of mucins can be related to a defect in the process of glycosylation, or to incomplete maturation of goblet cells, and may be an early indication of preneoplastic and neoplastic changes.

Objective: Glycerol constitutes an important metabolite for the control of lipid accumulation and glucose homeostasis. The impact of obesity and obesity-associated type 2 diabetes as well as the potential regulatory role of insulin and leptin on aquaglyceroporins (AQP) 3, 7, and 9 were analyzed. Research DesignandMethods: The tissue distributionandexpression ofAQPin biopsies of omental and sc adipose tissue as well as liver were analyzed in lean and obese Caucasian volunteers (n63). The effect of insulin (1, 10, and 100 nmol/liter) and leptin (0.1, 1, and 10 nmol/liter) on the expression of the glycerol channels was determined in vitro in human omental adipocytes and HepG2 hepatocytes. The translocation of AQP in response to insulin and isoproterenol was analyzed by immunocytochemistry. Results: In addition to the well-known expression of AQP7 in adipose tissue, AQP3 and AQP9 were also expressed in both omental and sc adipose tissue. Obese type 2 diabetes patientsshowedhigher expression of AQP in visceral adipose tissue and lower expression of AQP7 in sc adipose tissue and hepatic AQP9. The staining of AQP9 in the plasma membrane of adipocytes was reinforced by insulin, whereas isoproterenol induced the translocation ofAQP3andAQP7from the lipid droplets to the plasma membrane. Insulin up-regulated all AQP, whereas leptin up-regulated AQP3 and down-regulated AQP7 and AQP9 in adipocytes and hepatocytes. These effects were abrogated by both the phosphatidylinositol 3-kinase inhibitor wortmannin and the mammalian target of rapamycin inhibitor rapamycin. Conclusions: Our findings show, for the first time, that insulin and leptin regulate theAQPthrough the phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin pathway in human visceral adipocytes and hepatocytes. AQP3 and AQP7 may facilitate glycerol efflux from adipose tissue while reducing the glycerol influx into hepatocytes via AQP9 to prevent the excessive lipid accumulation and the subsequent aggravation of hyperglycemia in human obesity.

Aims: Non-Alcoholic Fatty Liver Disease (NAFLD) is the leading cause of chronic liver disease in adults and children. It is characterized by triglycerides (TG) over accumulation in hepatocytes and ranges from simple fatty liver (steatosis) to nonalcoholic steatohepatitis (NASH) and to cirrhosis. Here, we study the expression and function of AQP9, an aquaporin regulated by insulin and leptin mediating the entry of glycerol into hepatocytes, in the liver of obese women with NAFLD undergoing bariatric surgery. Methods: Obese women were classified into three groups: normoglycemia (NG; n=15), impaired glucose tolerance (IGT; n=13) and type 2 diabetes mellitus (T2DM; n=9). Blood assays were from plasma samples after an overnight fast. Liver biopsies were used to assess the levels of 1) AQP9 mRNA (qPCR) and protein (immunoblotting and immunohistochemistry), and 2) glycerol permeability (Pgly) by stopped flow light scattering. Results:The livers of obese IGT and T2DM patients showed considerably lower Pgly and AQP9 expression compared with NG women. Consistent with these data, the plasma levels of glycerol were significantly higher in the IGT and T2DM patients (T2DM >> IGT) than NG women. A similar scenario was observed in animal models of NAFLD. Conclusions:The AQP9 downregulation and consequent reduction in hepatic glycerol import may be a compensatory mechanism by which the liver contrasts further TG accumulation within its parenchyma as well as reduces hepatic gluconeogenesis.

Aims. Non-Alcoholic Fatty Liver Disease (NAFLD) is the leading cause of chronic liver disease in adults and children. It is characterized by triglycerides (TG) over accumulation in hepatocytes and ranges from simple fatty liver (steatosis) to nonalcoholic steatohepatitis (NASH) and to cirrhosis. Here, we study the expression and function of AQP9, an aquaporin regulated by insulin and leptin mediating the entry of glycerol into hepatocytes, in the liver of obese women with NAFLD undergoing bariatric surgery. Methods. Obese women were classified into three groups: normoglycemia (NG; n=15), impaired glucose tolerance (IGT; n=13) and type 2 diabetes mellitus (T2DM; n=9). Blood assays were from plasma samples after an overnight fast. Liver biopsies were used to assess the levels of 1) AQP9 mRNA (qPCR) and protein (immunoblotting and immunohistochemistry), and 2) glycerol permeability (Pgly) by stopped flow light scattering. Results. The livers of obese IGT and T2DM patients showed considerably lower Pgly and AQP9 expression compared with NG women. Consistent with these data, the plasma levels of glycerol were significantly higher in the IGT and T2DM patients (T2DM >> IGT) than NG women. A similar scenario was observed in animal models of NAFLD. Conclusions. The AQP9 downregulation and consequent reduction in hepatic glycerol import may be a compensatory mechanism by which the liver contrasts further TG accumulation within its parenchyma as well as reduces hepatic gluconeogenesis.

One form of liver steatosis, namely Non-Alcoholic Fatty Liver Disease (NAFLD), is a worrisome health problem worldwide characterized by intrahepatic triacylglycerol (TG) overaccumulation. NAFLD is a common feature of metabolic syndrome being often associated with obesity, dyslipidemia and diabetes and mostly closely linked to insulin resistance. The mechanism of NAFLD pathogenesis is object of intense investigation especially regarding complex systems ultimately resulting in excessive TG deposition in hepatocytes. However, scarce is the attention about the relevance of hepatic import of glycerol, the other primary source (as glycerol-3-phosphate) of increased TG in hepatocytes. Obese leptin-deficient (ob/ ob) mice, an animal model of NAFLD, were used to evaluate the functional involvement of Aquaporin-9 (AQP9), the major pathway of liver glycerol entry, in hepatosteatosis. By RT-PCR and qPCR, the level of Aqp9 mRNA in the liver of starved obese mice was comparable with the corresponding control lean littermates. By immunoblotting, the AQP9 protein at the hepatocyte sinusoidal plasma membrane of obese mice was markedly lower (33%) than lean mice, a finding fully confirmed by immunohistochemistry. By stopped-flow light scattering, the liver glycerol permeability of ob/ob mice was significantly lower (53%) than lean mice, a finding consistent with both the observed down-regulation of AQP9 protein and increased level of plasma glycerol characterizing obese mice. In summary, our results suggest implication of AQP9 in liver steatosis. The reduction of hepatocyte AQP9 and, consequently, glycerol permeability might be a defensive mechanism to counteract further fat infiltration in liver parenchyma.

Understanding the selectivity of aquaporin (AQP) membrane channels will require structural and functional studies of wild type and modified proteins; however, expression systems have not previously yielded AQPs in the necessary milligrams quantities. Cell free (CF) systems have emerged in recent years as fast, efficient and versatile technologies for the production of membrane proteins. Here, we establish a convenient method to make large amounts of efficient human AQP3, a physiologically relevant aquaglyceroporin permeating glycerol, water and urea.AQP3 was produced in an E. coli extract based CF system using the D-CF mode (CF membrane protein expression in presence of detergent). Milligrams amounts of a recombinant histidine-tagged AQP3 protein (hAQP3-6His) per ml of CF reaction were synthesized in presence of the nonionic detergent Brij 98 and purified by Ni-NTA chromatography. Purified hAQP3-6His was incorporated into liposomes and analyzed functionally by stopped flow light scattering. The glycerol permeability (Pgly) of the proteoliposomes resulted four times higher than the Pgly of the empty (control) liposomes (7.08±0.7 vs 1.74±0.3 x 10-3 mm/s, respectively). Consistently, the Pgly of the proteoliposomes was markedly reduced (-50%) after incubation with 0.5 mM phloretin, a known AQP3 blocker. AQP3 is the first aquaglyceroporin produced in preparative scale by a CF system, which could serve as basis for further structural/functional studies and biomimetic technologies.

Obesity has increased dramatically during the past decade and is an established risk factor for the development of chronic kidney disease. In this study we focused our attention on the potential dysregulation of renal aquaporins (AQPs) in the kidney of a mouse model of obesity and type 2 diabetes mellitus. Mice were fed a high fat (HFD) or normal (ND) diet and analyzed at 20 weeks and at 34 weeks. At 20 weeks, western blotting of total lysates from HFD mice revealed higher immunoreactivity for AQP1 whereas no significant change in AQP2, AQP3 and AQP4 abundance was observed. However, AQP1, AQP2, AQP3 and AQP4 immunoreactivity decreased in a crude kidney membrane preparation suggesting a reduced cell surface expression of all these AQPs. Stopped flow light scattering studies showed a reduced osmotic water permeability (Pf) of the whole renal plasma membranes from 254.8±30.5 mm/s to 169.8±16.2 (P&lt;0.0001) in ND versus HFD mice, respectively, consistent with the observed decrease in AQPs cell surface expression. At 34 weeks, western blotting of total lysates or of a crude membrane preparation from HFD mice revealed significant downregulation of AQP1 and AQP2 whereas AQP3 and AQP4 immunoreactivity was unchanged in both preparations compared to ND mice. Altogether, these data reveal altered AQPs expression and osmotic water permeability of HFD mice kidney. These observations suggest a patho-physiological relevance for aquaporins in renal complications associated with metabolic syndrome.

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.