Phospholipid Secretion Research Articles

In Vivo Monitoring of Glycerolipid Metabolism in Animal Nutrition Biomodel-Fed Smart-Farm Eggs.

Although many studies have examined the biochemical metabolic pathways by which an egg (egg yolk) lowers blood lipid levels, data on the molecular biological mechanisms that regulate and induce the partitioning of hepatic glycerolipids are missing. The aim of this study was to investigate in vivo monitoring in four study groups using an animal nutrition biomodel fitted with a jugular-vein cannula after egg yolk intake: CON (control group, oral administration of 1.0 g of saline), T1 (oral administration of 1.0 g of pork belly fat), T2 (oral administration of 1.0 g of smart-farm egg yolk), and T3 (oral administration of T1 and T2 alternately every week). The eggs induced significant and reciprocal changes in incorporating 14C lipids into the total glycerolipids and releasing 14CO2, thereby regulating esterification and accelerating oxidation in vivo. The eggs increased phospholipid secretion from the liver into the blood and decreased triacylglycerol secretion by regulating the multiple cleavage of fatty acyl-CoA moieties' fluxes. In conclusion, the results of the current study reveal the novel fact that eggs can lower blood lipids by lowering triacylglycerol secretion in the biochemical metabolic pathway of hepatic glycerolipid partitioning while simultaneously increasing phospholipid secretion and 14CO2 emission.

Tetraparesis following thoracic spine surgery in a patient with Klippel–Feil syndrome and ABCB4 mutation: a case report

BackgroundKlippel–Feil syndrome is a rare condition described in 1912 by Maurice Klippel and André Feil. It is defined as a congenital cervical fusion of at least two vertebrae, associated with a classical triad of clinical signs: short neck, low posterior hairline, and limited range of movement. However, Klippel–Feil syndrome manifests with a vast spectrum of phenotypes, ranging from no symptoms to complete triad, with or without other associated malformations. Most commonly, CCF results from sporadic mutations, even though autosomal recessive, autosomal dominant, or even X-linked inheritance can be detected. The ATP-binding cassette subfamily B member 4 is only expressed in the liver and is involved in biliary phospholipid secretion. The clinical spectrum includes various hepatobiliary pathologies, including low phospholipid-associated cholelithiasis, and has never been associated with musculoskeletal anomalies.Case presentationA 55-year-old male Caucasian patient presenting with low phospholipid-associated cholelithiasis syndrome with ATP-binding cassette subfamily B member 4 mutation and liver cirrhosis was referred to our clinic for a liver transplant. A period of 6 months before, the patient underwent a T7–T9 posterior fixation for a T8 osteoporotic fracture. Postoperatively, he was tetraparetic, whereas he was neurologically intact before the operation. At admission to our hospital, he was still tetraparetic and presented with clinical signs of cervical myelopathy. Moreover, he suffered a limitation of cervical range of motion in all directions, short neck, and low posterior hairline. Imaging showed multiple cervical and thoracic vertebral bodies fusion, as well as cervical spine stenosis. Based on the available data, we diagnosed a type 3 Klippel–Feil syndrome according to Samartzis’ classification.ConclusionsThe heterogeneity of KFS and the various potential hereditary links that are known indicate that it is important to highlight all potential cases related to known genetic defects. At present, no association between ATP-binding cassette subfamily B member 4 mutation and congenital cervical fusions has been reported. The other important clinical focus of this case is the appearance of spontaneous tetraparesis after thoracic spine surgery. This mechanism remains unclear, but considering different spinal anatomy it might have been due to difficult intubation and patient’s positioning during his previous operation.

LIRAGLUTIDE ALLEVIATES ACUTE LUNG INJURY AND MORTALITY IN PNEUMONIA-INDUCED SEPSIS THROUGH REGULATING SURFACTANT PROTEIN EXPRESSION AND SECRETION.

Glucagon-like peptide 1 (GLP-1) analogs are used to treat type 2 diabetes, and they can regulate insulin secretion, energy homeostasis, inflammation, and immune cell function. This study sought to determine whether the GLP-1 analog liraglutide exerts a beneficial action in an acute lung injury model of pneumonia-induced sepsis. Methods: Wild-type FVB/NJ mice (n = 114) were infected by intratracheal injection with Pseudomonas aeruginosa Xen5 (4 × 10 4 CFU/mouse) or an equal volume (50 μL) of saline (control) with or without a subcutaneous injection of liraglutide (2 mg/kg, 30 min after infection). Mice were killed 24 h after infection. Lung tissues and BALF were analyzed. In separate experiments, the dynamic growth of bacteria and animal mortality was monitored using in vivo imaging system within 48 h after infection. In addition, primary lung alveolar type II cells isolated from mice were used to study the mechanism of liraglutide action. Result: Liraglutide improved survival ( P < 0.05), decreased bacterial loads in vivo , and reduced lung injury scores ( P < 0.01) in septic mice. Liraglutide-treated mice showed decreased levels of inflammatory cells ( P < 0.01) and proinflammatory cytokines (TNF-α and IL-6) ( P < 0.01) in the lung compared with septic controls. Liraglutide significantly increased pulmonary surfactant proteins (SP-A and SP-B) expression/secretion ( P < 0.01) and phospholipid secretion ( P < 0.01) in vivo . Primary alveolar type II cells pretreated with liraglutide improved SP-A and SP-B expression after LPS exposure ( P < 0.01). Conclusion: Liraglutide attenuates mortality and lung inflammation/injury in pneumonia-induced sepsis. The increased surfactant expression/secretion and anti-inflammatory effects of liraglutide represent potential mechanisms by GLP-1 agonists potentiate host defense and maintain alveolar respiratory function in acute lung injury.

Functional Characterization of Lysophospholipids by Proteomic and Lipidomic Analysis of Fibroblast-like Synoviocytes.

Synovial fluid (SF) from human knee joints with osteoarthritis (OA) has elevated levels of lysophosphatidylcholine (LPC) species, but their functional role is not well understood. This in vitro study was designed to test the hypothesis that various LPCs found elevated in OA SF and their metabolites, lysophosphatidic acids (LPAs), modulate the abundance of proteins and phospholipids (PLs) in human fibroblast-like synoviocytes (FLSs), with even minute chemical variations in lysophospholipids determining the extent of regulation. Cultured FLSs (n = 5-7) were treated with one of the LPC species, LPA species, IL-1β, or a vehicle. Tandem mass tag peptide labeling coupled with LC-MS/MS/MS was performed to quantify proteins. The expression of mRNA from regulated proteins was analyzed using RT-PCR. PL synthesis was determined via ESI-MS/MS, and the release of radiolabeled PLs was determined by means of liquid scintillation counting. In total, 3960 proteins were quantified using multiplexed MS, of which 119, 8, and 3 were significantly and reproducibly regulated by IL-1β, LPC 16:0, and LPC 18:0, respectively. LPC 16:0 significantly inhibited the release of PLs and the synthesis of phosphatidylcholine, LPC, and sphingomyelin. Neither LPC metabolite-LPA 16:0 nor LPA 18:0-had any reproducible effect on the levels of each protein. In conclusion, small chemical variations in LPC species can result in the significantly altered expression and secretion of proteins and PLs from FLSs. IL-1β influenced all proteins that were reproducibly regulated by LPC 16:0. LPC species are likely to modulate FLS protein expression only in more advanced OA stages with low IL-1β levels. None of the eight proteins being significantly regulated by LPC 16:0 have been previously reported in OA. However, our in vitro findings show that the CD81 antigen, calumenin, and B4E2C1 are promising candidates for further study, focusing in particular on their potential ability to modulate inflammatory and catabolic mechanisms.

Abstract 447: Cholesterol Excretion In Whole-body And Liver-specific ABCG5 ABCG8-deficient Mice

Background: The ABCG5 ABCG8 sterol transporter is the body’s primary defense against the accumulation of dietary sterols. Loss of function mutations in ABCG5 or ABCG8 result in Sitosterolemia, a rare disorder that presents as familial hypercholesterolemia, but is distinguished by recessive genetics and the accumulation of phytosterols in plasma and tissues. Biliary cholesterol is reduced by greater than 80% in ABCG5 ABCG8-deficient mice, but fecal neutral sterols are unaffected, suggesting a G5G8-independent pathway for cholesterol excretion. The objectives of the present study were to determine the ability of whole-body and liver-specific G5G8-deficient mice to maintain fecal neutral sterol output when challenged with cholesterol -enriched diet and determine maximal rates of biliary cholesterol secretion. Methods: Wild-type, whole-body and liver-specific ABCG5 ABCG8-deficient mice were maintained on cereal-based mixed diet, a phytosterol-free purified diet, or a purified diet supplemented with 0.2% cholesterol. Following acclimatization to each diet, fecal neutral sterol output was determined over a three-day period by gas chromatography/mass spectrometry. Maximal biliary cholesterol secretion rates were determined by biliary diversion with bile acid infusion over a range of 0 to 1000 nmol/minute. Biliary lipid secretion was determined by enzymatic colorimetric assays and molecular coupling of cholesterol to bile acid and phospholipid secretion calculated by linear regression. Results &amp; Conclusions: Whole-body G5G8-deficient mice exhibited a 40% reduction in fecal neutral sterol output compared to wild-type mice when maintained on cereal-based or purified, low cholesterol containing diets. Cholesterol feeding resulted in a 10-fold increase in fecal neutral sterols in wild-type mice, an effect not observed in G5G8-deficient mice. These data indicate that ABCG5 ABCG8 is indispensable in opposing the accumulation of dietary cholesterol and that alternative pathways cannot compensate for G5G8 deficiency.

Secreted metabolome of porcine blastocysts encapsulated within in vitro 3D alginate hydrogel culture systems undergoing morphological changes provides insights into specific mechanisms involved in the initiation of porcine conceptus elongation.

The exact mechanisms regulating the initiation of porcine conceptus elongation are not known due to the complexity of the uterine environment. To identify contributing factors for initiation of conceptus elongation in vitro , this study evaluated differential metabolite abundance within media following culture of blastocysts within unmodified alginate (ALG) or Arg-Gly-Asp (RGD)-modified alginate hydrogel culture systems. Blastocysts were harvested from pregnant gilts, encapsulated within ALG or RGD or as non-encapsulated control blastocysts (CONT), and cultured. At the termination of 96h culture, media were separated into blastocyst media groups: non-encapsulated control blastocysts (CONT); ALG and RGD blastocysts with no morphological change (ALG- and RGD-); ALG and RGD blastocysts with morphological changes (ALG+ and RGD+) and evaluated for non-targeted metabolomic profiling by liquid chromatography (LC)-mass spectrometry (MS) techniques and gas chromatography-(GC-MS). Analysis of variance identified 280 (LC-MS) and 1 (GC-MS) compounds that differed (P <0.05), of which 134 (LC-MS) and 1 (GC-MS) were annotated. Metabolites abundance between ALG+ vs ALG-, RGD+ vs RGD-, and RGD+ vs ALG+ were further investigated to identify potential differences in metabolic processes during the initiation of elongation. This study identified changes in phospholipid, glycosphingolipid, lipid signalling, and amino acid metabolic processes as potential RGD-independent mechanisms of elongation and identified changes in lysophosphatidylcholine and sphingolipid secretions during RGD-mediated elongation. These results illustrate changes in phospholipid and sphingolipid metabolic processes and secretions may act as mediators of the RGD-integrin adhesion that promotes porcine conceptus elongation.

Nutritional assessment in patients with progressive familial intrahepatic cholestasis type 3

Rationale: Progressive familial intrahepatic cholestasis (PFIC) is a group of genetic diseases affecting the hepatocellular transport and bile formation. In PFIC3 there is a defect in the ABCB4 gene, which leads to impaired biliary phospholipid secretion, damaged hepatocytes and bile ducts, and heterogeneous clinical presentation. Growth impairment is often described in children with cholestatic liver diseases. The aim of our study was to assess the nutritional status in patients with PFIC3.

Infantile cholestasis related to ABCB4 gene mutation and cytomegalovirus (CMV) infection: a case report

: Type three progressive familial intrahepatic cholestasis (PFIC3) is a rare disorder which is caused by defect in bile secretion and presents with intrahepatic cholestasis, usually in infancy and childhood. It is autosomal recessive in inheritance. The defect is in adenosine triphosphate binding cassette, sub-family B, member 4 (ABCB4) gene encoding multidrug-resistant protein 3 (MDR3). MDR3 is a p-glycoprotein (pGp) which is a phospholipid translocator and responsible for biliary secretion of phospholipids, predominantly phosphatidylcholine (PC). A case of infant with cholestasis was reported in the article and explored for underlying pathogenesis. A 9-month-old girl, who had recovered from cytomegalovirus (CMV) infection, had elevated bile acids and gamma-glutamyl transpeptidase (GGT) for seven months with unknown reason. She was examined for hereditary and metabolic disease-associated genes. A heterozygous mutation of ABCB4 gene was found. PFIC3 was suspected. It was considered that patient with heterozygous ABCB4 gene mutation had residual gene expression and a relatively low level of MDR3 proteins. Without risk factors, the biliary secretion in the patient would be normal. But CMV infection further down regulated the gene expression, disturbing the bile secretion, leading to cholestasis. After antiviral therapy, oral ursodeoxycholic acid (UDCA) and glutathione were administered and her liver tests improved gradually. Therefore, patients with persistent cholestasis of unknown reasons may undergo genetic sequencing to identify whether there is mutation of genes related to cholestatic liver disease. CMV infection may play a role in persistent cholestasis of the patient with ABCB4 gene variance.

Pharmacological and Metabolic Significance of Bile Acids in Retinal Diseases.

Bile acids (BAs) are amphipathic sterols primarily synthesized from cholesterol in the liver and released in the intestinal lumen upon food intake. BAs play important roles in micellination of dietary lipids, stimulating bile flow, promoting biliary phospholipid secretion, and regulating cholesterol synthesis and elimination. Emerging evidence, however, suggests that, aside from their conventional biological function, BAs are also important signaling molecules and therapeutic tools. In the last decade, the therapeutic applications of BAs in the treatment of ocular diseases have gained great interest. Despite the identification of BA synthesis, metabolism, and recycling in ocular tissues, much remains unknown with regards to their biological significance in the eye. Additionally, as gut microbiota directly affects the quality of circulating BAs, their analysis could derive important information on changes occurring in this microenvironment. This review aims at providing an overview of BA metabolism and biological function with a focus on their potential therapeutic and diagnostic use for retinal diseases.

Depletion of hepatic forkhead box O1 does not affect cholelithiasis in male and female mice

Cholelithiasis is one of the most prevalent gastroenterological diseases and is characterized by the formation of gallstones in the gallbladder. Both clinical and preclinical data indicate that obesity, along with comorbidity insulin resistance, is a predisposing factor for cholelithiasis. Forkhead box O1 (FoxO1) is a key transcription factor that integrates insulin signaling with hepatic metabolism and becomes deregulated in the insulin-resistant liver, contributing to dyslipidemia in obesity. To gain mechanistic insights into how insulin resistance is linked to cholelithiasis, here we determined FoxO1's role in bile acid homeostasis and its contribution to cholelithiasis. We hypothesized that hepatic FoxO1 deregulation links insulin resistance to impaired bile acid metabolism and cholelithiasis. To address this hypothesis, we used the FoxO1LoxP/LoxP-Albumin-Cre system to generate liver-specific FoxO1-knockout mice. FoxO1-knockout mice and age- and sex-matched WT littermates were fed a lithogenic diet, and bile acid metabolism and gallstone formation were assessed in these animals. We showed that FoxO1 affected bile acid homeostasis by regulating hepatic expression of key enzymes in bile acid synthesis and in biliary cholesterol and phospholipid secretion. Furthermore, FoxO1 inhibited hepatic expression of the bile acid receptor farnesoid X receptor and thereby counteracted hepatic farnesoid X receptor signaling. Nonetheless, hepatic FoxO1 depletion neither affected the onset of gallstone disease nor impacted the disease progression, as FoxO1-knockout and control mice of both sexes had similar gallstone weights and incidence rates. These results argue against the notion that FoxO1 is a link between insulin resistance and cholelithiasis.

Bicarbonate: the alternative buffer for peritoneal dialysis.

For a long time bicarbonate, the physiological buffer of the body, was suggested to be the best buffer for peritoneal dialysis. However, since the production of bicarbonate-containing solutions is associated with technical problems, lactate was favored. To avoid the well-known disadvantages of lactate solution concerning biocompatibility and possible metabolic side effects, different attempts have been made to use bicarbonate as a buffer in peritoneal dialysis. One of the major approaches was the total replacement of lactate by bicarbonate combined with storage of the fluid in a specially designed double-chamber bag. Further solutions of the above-mentioned problem were the on-line preparation of bicarbonate fluids for intermittent peritoneal dialysis, the addition of bicarbonate just before use, the combination of bicarbonate with organic acids, or its combination with the dipeptide glycylglycine as a stabilizing agent. By now, the beneficial effect of the neutral bicarbonate fluid, for example, on cell viability and cell functions, has been demonstrated in many different in vitro and animal studies. However, only few reports on clinical experience have been published. These investigations demonstrated independently that bicarbonate fluids diminish inflow pain, are well tolerated by the patients, and may correct metabolic acidosis of uremic patients. A controlled randomized multicenter trial using 34 mmol/L bicarbonate for at least three months confirmed that bicarbonate is as efficacious as lactate in equimolar concentrations. Concomitant investigations on energy metabolism and redox state of red blood cells and phospholipid secretion of mesothelial cells additionally demonstrated the improvement of cell function with bicarbonate solutions. For some patients with severe metabolic acidosis the bicarbonate concentration used in the multicenter trial seemed to be too low. Thus, a fluid containing a higher bicarbonate concentration was tested in a pilot study resulting in the expected significant increase of arterial bicarbonate levels. In summary, bicarbonate-containing peritoneal dialysis solutions are a promising alternative to lactate, especially if bicarbonate concentrations are adjusted individually to the patient's need.

Dysregulated FXR-FGF19 signaling and choline metabolism are associated with gut dysbiosis and hyperplasia in a novel pig model of pediatric NASH.

To investigate the role of bile acids (BAs) in the pathogenesis of diet-induced nonalcoholic steatohepatitis (NASH), we fed a "Western-style diet" [high fructose, high fat (HFF)] enriched with fructose, cholesterol, and saturated fat for 10 wk to juvenile Iberian pigs. We also supplemented probiotics with in vitro BA deconjugating activity to evaluate their potential therapeutic effect in NASH. Liver lipid and function, cytokines, and hormones were analyzed using commercially available kits. Metabolites, BAs, and fatty acids were measured by liquid chromatography-mass spectrometry. Histology and gene and protein expression analyses were performed using standard protocols. HFF-fed pigs developed NASH, cholestasis, and impaired enterohepatic Farnesoid-X receptor (FXR)-fibroblast growth factor 19 (FGF19) signaling in the absence of obesity and insulin resistance. Choline depletion in HFF livers was associated with decreased lipoprotein and cholesterol in serum and an increase of choline-containing phospholipids in colon contents and trimethylamine-N-oxide in the liver. Additionally, gut dysbiosis and hyperplasia increased with the severity of NASH, and were correlated with increased colonic levels of choline metabolites and secondary BAs. Supplementation of probiotics in the HFF diet enhanced NASH, inhibited hepatic autophagy, increased excretion of taurine and choline, and decreased gut microbial diversity. In conclusion, dysregulation of BA homeostasis was associated with injury and choline depletion in the liver, as well as increased biliary secretion, gut metabolism and excretion of choline-based phospholipids. Choline depletion limited lipoprotein synthesis, resulting in hepatic steatosis, whereas secondary BAs and choline-containing phospholipids in colon may have promoted dysbiosis, hyperplasia, and trimethylamine synthesis, causing further damage to the liver.NEW & NOTEWORTHY Impaired Farnesoid-X receptor (FXR)-fibroblast growth factor 19 (FGF19) signaling and cholestasis has been described in nonalcoholic fatty liver disease (NAFLD) patients. However, therapeutic interventions with FXR agonists have produced contradictory results. In a swine model of pediatric nonalcoholic steatohepatitis (NASH), we show that the uncoupling of intestinal FXR-FGF19 signaling and a decrease in FGF19 levels are associated with a choline-deficient phenotype of NASH and increased choline excretion in the gut, with the subsequent dysbiosis, colonic hyperplasia, and accumulation of trimethylamine-N-oxide in the liver.

Blocking Sodium-Taurocholate Cotransporting Polypeptide Stimulates Biliary Cholesterol and Phospholipid Secretion in Mice.

Active secretion of bile salts into the canalicular lumen drives bile formation and promotes biliary cholesterol and phospholipid output. Disrupting hepatic bile salt uptake, by inhibition of sodium‐taurocholate cotransporting polypetide (NTCP; Slc10a1) with Myrcludex B, is expected to limit bile salt flux through the liver and thereby to decrease biliary lipid excretion. Here, we show that Myrcludex B–mediated NTCP inhibition actually causes an increase in biliary cholesterol and phospholipid excretion whereas biliary bile salt output and bile salt composition remains unchanged. Increased lysosomal discharge into bile was excluded as a potential contributor to increased biliary lipid secretion. Induction of cholesterol secretion was not a consequence of increased ATP‐binding cassette subfamily G member 5/8 activity given that NTCP inhibition still promoted cholesterol excretion in Abcg8 −/− mice. Stimulatory effects of NTCP inhibition were maintained in Sr‐b1 −/− mice, eliminating the possibility that the increase in biliary lipids was derived from enhanced uptake of high‐density lipoprotein–derived lipids. NTCP inhibition shifts bile salt uptake, which is generally more periportally restricted, toward pericentral hepatocytes, as was visualized using a fluorescently labeled conjugated bile salt. As a consequence, exposure of the canalicular membrane to bile salts was increased, allowing for more cholesterol and phospholipid molecules to be excreted per bile salt. Conclusion: NTCP inhibition increases biliary lipid secretion, which is independent of alterations in bile salt output, biliary bile salt hydrophobicity, or increased activity of dedicated cholesterol and phospholipid transporters. Instead, NTCP inhibition shifts hepatic bile salt uptake from mainly periportal hepatocytes toward pericentral hepatocytes, thereby increasing exposure of the canalicular membrane to bile salts linking to increased biliary cholesterol secretion. This process provides an additional level of control to biliary cholesterol and phospholipid secretion.

New treatments/targets for primary biliary cholangitis.

SummaryPrimary biliary cholangitis (PBC) is an autoimmune, cholestatic, chronic liver disease that ultimately progresses towards cirrhosis and liver failure if untreated. While ursodeoxycholic acid has been established as standard of care for PBC in the last few decades, significant advances in second-line treatment options have recently been made and new therapeutic developments are currently under evaluation. The purpose of this article is to provide the clinician with an overview of the current treatment options and future opportunities for patients with PBC.

Enhancing effect of taurohyodeoxycholate on ABCB4-mediated phospholipid efflux

Hydrophilic bile salts, ursodeoxycholate and hyodeoxycholate, have choleretic effects. ABCB4, a member of the ABC transporter family, is essential for the secretion of phospholipids from hepatocytes into bile. In this study, we assessed the effects of taurine- or glycine-conjugated cholate, ursodeoxycholate and hyodeoxycholate on the ABCB4-mediated phosphatidylcholine (PC) efflux using Abcb4 knockout mice and HEK293 cells stably expressing ABCB4. To evaluate the effects of bile salts on bile formation in Abcb4+/+ or Abcb4−/− mice, the bile was collected during intravenous infusion of saline or bile salts. The biliary PC secretion in Abcb4+/+ mice was significantly increased by the infusions of all tested bile salts, especially taurohyodeoxycholate. On the other hand, Abcb4−/− mice exhibited extremely low secretion of PC into bile, which was not altered by bile salt infusions. We also showed that the PC efflux from ABCB4-expressing HEK293 cells was stimulated by taurohyodeoxycholate much more strongly than the other tested bile salts. However, taurohyodeoxycholate did not restore the activities of ABCB4 mutants. Furthermore, light scattering measurements demonstrated a remarkable ability of taurohyodeoxycholate to form mixed micelles with PC. Therefore, the enhancing effect of taurohyodeoxycholate on the ABCB4-mediated PC efflux may be due to the strong mixed micelle formation ability.

Structural analogues of roscovitine rescue the intracellular traffic and the function of ER-retained ABCB4 variants in cell models

Adenosine triphosphate binding cassette transporter, subfamily B member 4 (ABCB4) is the transporter of phosphatidylcholine at the canalicular membrane of hepatocytes. ABCB4 deficiency, due to genetic variations, is responsible for progressive familial intrahepatic cholestasis type 3 (PFIC3) and other rare biliary diseases. Roscovitine is a molecule in clinical trial that was shown to correct the F508del variant of cystic fibrosis transmembrane conductance regulator (CFTR), another ABC transporter. In the present study, we hypothesized that roscovitine could act as a corrector of ABCB4 traffic-defective variants. Using HEK and HepG2 cells, we showed that roscovitine corrected the traffic and localisation at the plasma membrane of ABCB4-I541F, a prototypical intracellularly retained variant. However, roscovitine caused cytotoxicity, which urged us to synthesize non-toxic structural analogues. Roscovitine analogues were able to correct the intracellular traffic of ABCB4-I541F in HepG2 cells. Importantly, the phospholipid secretion activity of this variant was substantially rescued by three analogues (MRT2-235, MRT2-237 and MRT2-243) in HEK cells. We showed that these analogues also triggered the rescue of intracellular traffic and function of two other intracellularly retained ABCB4 variants, i.e. I490T and L556R. Our results indicate that structural analogues of roscovitine can rescue genetic variations altering the intracellular traffic of ABCB4 and should be considered as therapeutic means for severe biliary diseases caused by this class of variations.

Molecular Mechanisms for Protection of Hepatocytes against Bile Salt Cytotoxicity.

Biliary lipids consist mainly of bile salts, phospholipids and cholesterol, which form mixed micelles and vesicles. Bile salts play various physiological roles but have damaging effects on cell membranes due to their detergent properties. The cytotoxicity of bile salts on hepatocytes leads to liver injuries and is largely determined by the bile salt species, the concentrations of bile salts, phospholipids and cholesterol, and the lipid composition of cell membranes. In bile, monomers and simple micelles of bile salts coexist with mixed micelles and vesicles in dynamic equilibrium, and contribute to the cytotoxicity on hepatocytes. The ATP-binding cassette (ABC) transporter family members, ABCB11, ABCB4 and ABCG5/ABCG8, mediate the biliary secretion of bile salts, phospholipids and cholesterol, respectively. Mutations in ABCB4 result in severe cholestatic diseases, and the biliary phospholipids are necessary for the attenuation of bile salt cytotoxicity. On the other hand, cholesterol reverses the cytoprotective effects of phospholipids against bile salts. In addition, phosphatidylethanolamine N-methyltransferase increases the cell resistance to bile salts by changing the phospholipid composition and structures of the apical membranes. In this review, we focus on the molecular mechanisms for the protection of hepatocytes against bile salt cytotoxicity. Further understanding of these mechanisms will help to develop new therapeutic strategies for cholestatic liver diseases.

Lack of phospholipids in bile enhances cholesterol cholelithogenesis in the ATP‐binding cassette transporter B4 (Abcb4) knockout mice

BackgroundBiliary phospholipids play a key role in transporting cholesterol (Ch) by forming unilamellar and multilamellar vesicles that display a significantly higher ability to solubilize Ch in bile compared to simple and mixed micelles. The ATP‐binding cassette transporter B4 (ABCB4) is responsible for hepatic phospholipid secretion and its deletion in the liver causes a lack of phospholipids in bile of mice. Our aim is to explore whether targeted deletion of the Abcb4 gene in the liver leads to more rapid Ch crystallization and gallstone formation in Abcb4 knockout (KO) mice fed even a low Ch diet compared to wild‐type (WT) mice.MethodsThe critical role of biliary phospholipids in Ch crystallization and gallstone formation was investigated by physical‐chemical methods in male Abcb4 KO vs WT mice fed a chow diet (on day 0) and a special diet containing 0.1% Ch, 15% butterfat and 0.25% cholic acid for 4 wk.ResultsOn the chow diet (day 0), biliary phospholipids are 18.1±0.3 mM in pooled gallbladder bile of WT mice. However, no phospholipids are detected in Abcb4 KO mice fed either the chow or the special diet. After 1 wk of feeding the special diet, phase diagram analysis finds that lipid composition of pooled gallbladder bile of Abcb4 KO mice is located in crystallization region A. Moreover, many needle‐like Ch crystals, typical Ch monohydrate crystals, sandy stones and small gallstones (0.1–0.2 mm in diameter), but not liquid crystals, are detected in bile by phase contrast and polarizing light microscopy. This shows that lack of phospholipids in bile produces anhydrous crystalline metastable intermediates that rapidly evolve to Ch monohydrate crystals from supersaturated bile. In contrast, lipid composition of pooled gallbladder bile of WT mice is still within the micellar zone, indicating that bile is unsaturated with Ch. After 4 wk of feeding the special diet, lipid composition of pooled gallbladder bile of Abcb4 KO mice still stays in crystallization region A, with many larger gallstones (0.3–0.6 mm in diameter) being formed in the gallbladder. Notably, lipid composition of pooled gallbladder bile of WT mice enters crystallization region C, as analyzed by phase diagram. Thus, many liquid crystals (i.e., unilamellar and multilamellar vesicles) and classical solid plate‐like Ch monohydrate crystals, but not gallstones, are found in the gallbladder of WT mice. These results indicate that solid Ch crystals are formed through the liquid crystalline pathway in WT mice.ConclusionsLack of biliary phospholipids caused by the Abcb4 deletion in the mouse liver is a critical risk factor for Ch gallstone disease by significantly reducing Ch solubility in bile through disrupting the liquid crystalline pathway, whereas it leads to more rapid Ch crystallization via the anhydrous crystalline pathway. Our findings provide novel insight into the pathophysiological mechanisms of why Ch gallstones are rapidly formed in patients with the ABCB4 mutations, thereby leading to low phospholipid‐associated cholelithiasis, a rare biliary disease caused by a single‐gene mutation.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Biliary cholesterol secretion: more than a simple ABC.

Biliary cholesterol secretion is a process important for 2 major disease complexes, atherosclerotic cardiovascular disease and cholesterol gallstone disease. With respect to cardiovascular disease, biliary cholesterol secretion is regarded as the final step for the elimination of cholesterol originating from cholesterol-laden macrophage foam cells in the vessel wall in a pathway named reverse cholesterol transport. On the other hand, cholesterol hypersecretion into the bile is considered the main pathophysiological determinant of cholesterol gallstone formation. This review summarizes current knowledge on the origins of cholesterol secreted into the bile as well as the relevant processes and transporters involved. Next to the established ATP-binding cassette (ABC) transporters mediating the biliary secretion of bile acids (ABCB11), phospholipids (ABCB4) and cholesterol (ABCG5/G8), special attention is given to emerging proteins that modulate or mediate biliary cholesterol secretion. In this regard, the potential impact of the phosphatidylserine flippase ATPase class I type 8B member 1, the Niemann Pick C1-like protein 1 that mediates cholesterol absorption and the high density lipoprotein cholesterol uptake receptor, scavenger receptor class B type I, is discussed.

Phospholipid secretions of organ cultured ciliary body

Homeostasis of intraocular pressure (IOP) is important for the maintenance of anterior eye anatomic integrity, minimizing pressure-associated damage to the optic nerve, and maintaining a pressure gradient for blood flow to the eye. IOP is regulated by equilibrium between aqueous humor (AH) production and its outflow. The ciliary body (CB) is thought to actively secrete AH. However, whether AH composition and in particular, its phospholipids are entirely due to CB secretion remains uncertain. Comparison of phospholipids released by cultured CB, phospholipids present within CB tissue, within AH, and within blood and serum are consistent with release of most phospholipids into the AH by the CB. Treatment of CB in culture with timolol, a non-specific beta-adrenergic antagonist, alters the release of phospholipids by CB into the media. However, dorzalamide, a carbonic anhydrase inhibitor that reduces production of AH, does not affect phospholipid release thereby suggesting timolol, which also decreases IOP through decreased AH outflow, affects other physiological homeostatic mechanisms regulating aqueous outflow. These outflow changes also affect the composition of secreted phospholipids. We present evidence that release of lipids by the CB has a prolonged survival effect on cultured primary TM cells and TM tissue.