Intrahepatic biliary cholesterol and phospholipid transport in humans: effect of obesity and cholesterol cholelithiasis

Association between weight-adjusted-waist index and gallstones: an analysis of the National Health and Nutrition Examination Survey

BACKGROUND

The weight-adjusted-waist index (WWI) is a novel obesity index, and gallstones are associated with obesity. This study aimed to investigate the possible relationship between WWI and gallstones.

METHODS

The datasets from the National Health and Nutrition Examination Survey (NHANES) 2017-2020 were used in a cross-sectional investigation. Multivariate linear regression models were used to examine the linear connection between WWI and gallstones incidence. Fitted smoothing curves and threshold effect analysis were used to describe the nonlinear relationship.

RESULTS

The study comprised 8004 participants over the age of 20, including 833 reported with gallstones. Participants in the higher WWI tertile tended to have a higher gallstones prevalence. In the final adjusted model, a positive association between WWI and gallstones prevalence was observed (OR = 1.34, 95% CI: 1.20‒1.49). Participants in the highest WWI tertile had a significantly 71% higher risk of gallstones than those in the lowest WWI tertile (OR = 1.71, 95% CI: 1.35‒2.17). A nonlinear correlation was found between the WWI and gallstones prevalence, with an inflection point of 12.7.

CONCLUSIONS

Our study found that higher WWI levels connected with increased prevalence of gallstones. However, more prospective studies are needed to validate our findings.

Stimulation of ABCB4/MDR3 ATPase activity requires an intact phosphatidylcholine lipid

ABCB4/MDR3 is located in the canalicular membrane of hepatocytes and translocates PC-lipids from the cytoplasmic to the extracellular leaflet. ABCB4 is an ATP-dependent transporter that reduces the harsh detergent effect of the bile salts by counteracting self-digestion. To do so, ABCB4 provides PC lipids for extraction into bile. PC lipids account for 40% of the entire pool of lipids in the canalicular membrane with an unknown distribution over both leaflets. Extracted PC lipids end up in so-called mixed micelles. Mixed micelles are composed of phospholipids, bile salts, and cholesterol. Ninety to ninety-five percent of the phospholipids are members of the PC family, but only a subset of mainly 16.0-18:1 PC and 16:0-18:2 PC variants are present. To elucidate whether ABCB4 is the key discriminator in this enrichment of specific PC lipids, we used in vitro studies to identify crucial determinants in substrate selection. We demonstrate that PC-lipid moieties alone are insufficient for stimulating ABCB4 ATPase activity, and that at least two acyl chains and the backbone itself are required for a productive interaction. The nature of the fatty acids, like length or saturation has a quantitative impact on the ATPase activity. Our data demonstrate a two-step enrichment and protective function of ABCB4 to mitigate the harsh detergent effect of the bile salts, because ABCB4 can translocate more than just the PC-lipid variants found in bile.

Triglycerides and gallstone formation

Changes in bile acid (BA) metabolism and gallbladder function are critical factors in the pathogenesis of gallstones. Patients with hypertriglyceridemia (HTG) - often overweight and insulin resistant - are at risk for gallstone disease. The question arises whether HTG itself contributes to gallstone formation or whether gallstone disease only associates with this disorder. Triglycerides are formed in response to fluxes of non-esterified fatty acids and glucose. Hypertriglyceridemia results from either overproduction of triglycerides by the liver, impaired lipolysis or a combination of both. Hyperinsulinemia, as observed in the insulin resistant state, stimulates very low-density lipoprotein (VLDL)-triglyceride synthesis. Peroxisome proliferator-activated receptors (PPARs), liver X receptors (LXRs), farnesoid X receptor (FXR) and hepatocyte nuclear factor 4α (HNF4α) are the nuclear receptors involved in the regulation of lipogenesis. Microsomal triglyceride transfer protein (MTP) is involved in the production of VLDL and its activation is also under control of transcription factors as FXR and Forkhead box-O1 (FoxO1). Triglyceride and BA metabolism are linked. There is an inverse relationship between bile acid fluxes and pool size and VLDL production and SHP (small heterodimer partner) and FXR are the link between BAs and TG metabolism. BAs are also ligands for FXR and G-protein-coupled receptors, such as TGR5. FXR activation by BAs suppresses the expression of MTP, transcription factor sterol regulatory element binding protein (SREBP)-1c and other lipogenic genes. LXRs stimulate lipogenesis whereas FXRs inhibit the metabolic process. Synthesis of BAs from cholesterol occurs either via the classical pathway (7α-hydroxylation of cholesterol; CYP7A1) or via the alternate pathway (CYP39A1 or CYP7B1). BAs induce FXR, which inhibits CYP7A1 transcription by activation of SHP and inhibition of HNF4α transactivation. Bile composition (supersaturation with cholesterol), gallbladder dysmotility, inflammation, hypersecretion of mucin gel in the gallbladder and slow large intestinal motility and increased intestinal cholesterol absorption may contribute to the pathogenesis of cholesterol gallstones. In HTG patients supersaturated bile may be related to the presence of obesity rather than to HTG itself. Contraction and relaxation of the gallbladder are regulated by neuronal, hormonal and paracrine factors. Postprandial gallbladder emptying is regulated by cholecystokinin (CCK). Poor postprandial gallbladder contraction may be due to the magnitude of the CCK response and to the amount of CCK receptors in the gallbladder smooth muscle cells. In the fasting state gallbladder motility is associated with the intestinal migrating motor complex (MMC) activity and with elevated plasma motilin levels. Fibroblast growth factor (FGF19), produced on arrival of bile acids in the ileum, is also important for gallbladder motility. Gallbladder motility is impaired in HTG patients compared to BMI matched controls. There is evidence that the gallbladder in HTG is less sensitive to CCK and that this sensitivity improves after reversal of high serum TG levels by use of TG lowering agents. In hypertriglyceridemia TG lowering therapy (fibrates or fish-oil) is essential to prevent cardiovascular disease and pancreatitis. Fibrates, however, also increase the risk for cholelithiasis by increasing biliary cholesterol saturation and by reduction of bile acid synthesis. On the other hand fish-oil decreases biliary cholesterol saturation. Fish-oil may increase bile acid synthesis by activation of 7alpha-hydroxylase and may inhibit VLDL production and secretion through activation of nuclear factors and increased apoB degradation. In HTG patients, gallbladder motility improves during bezafibrate as well as during fish-oil therapy. The question remains whether improvement of gallbladder motility and increased lithogenicity of bile by bezafibrate therapy counteract each other or still result in gallstone formation in HTG patients.

Effect of membrane cholesterol on BSEP/Bsep activity: species specificity studies for substrates and inhibitors

The efflux transporter responsible for the canalicular elimination of bile salts from the hepatocytes is the bile salt export pump (BSEP, ABCB11). Absence or inhibition of this transporter leads to bile salt retention in the hepatocyte and in turn can lead to cholestatic liver disease. We expressed the BSEP/Bsep protein from three species (human, rat, and mouse) in a baculovirus-infected Sf9 system. Vesicles prepared from these cells were used to evaluate bile salt transport of four conjugated bile salts. Because the Sf9 system contains less membrane cholesterol than the liver canalicular membrane, the effect of added cholesterol on the kinetics of BSEP/Bsep-mediated bile salt transport was also investigated. Cholesterol treatment increased the V(max) values in all the species, with the most pronounced effect observed in the rat transporter. In contrast, K(m) values, with the exception of glycochenodeoxycholate, remained largely unchanged. The species-specific bile salt transport inhibition potential of three compounds known to cause clinical cholestasis was investigated in vesicles containing BSEP/Bsep. Troglitazone and glibenclamide inhibited the BSEP/Bsep-mediated transport of different bile salts with similar affinities, whereas the potential of cyclosporine A to inhibit bile salt transport showed species- and bile salt-specific variations. In conclusion, the cholesterol-loaded Sf9 vesicles overexpressing BSEP/Bsep seem to be a useful system for the identification of potential cholestatic compounds and can also be used for the investigation of species specificity. We observed greater differences in IC(50) values for inhibitors than in K(m) values for substrates between species.

Intracellular supply of phospholipids for biliary secretion: evidence for a nonvesicular transport component

Phospholipids (PL) for biliary secretion could be supplied from the endoplasmic reticulum (ER) to the plasma membrane by cytosolic transfer proteins or transport vesicles. Therefore, we studied whether biliary secretions of PL and apolipoprotein A-I (apo A-I), as markers for the ER-to-Golgi vesicular transport pathway, are tightly coupled in isolated perfused rat livers with enhanced secretion (+60%) of PL after withdrawal of the cholesterol synthesis inhibitor pravastatin (0.1% of chow, fed for 7 days). Blocking agents dissociated the secretion of apo A-I and PL. Brefeldin A as well as cycloheximide inhibited biliary secretion of apo A-I (-52%; -68%), however, not of PL. Both bilirubin ditaurate and taurodehydrocholic acid reduced biliary secretion of PL (-27%; -79%), but not of apo A-I. The data support the concept that PL destined for biliary secretion bypass the vesicular transport pathway of apo A-I through the Golgi compartment, most likely via cytosolic transfer proteins.

Demonstration and partial characterisation of phospholipid methyltransferase activity in bile canalicular membrane from hamster liver

BACKGROUND/AIMS

Methylation of phosphatidylethanolamine to phosphatidylcholine predominantly takes place in mitochondrial-associated membrane and the endoplasmic reticulum of the liver. The transport of the phospholipids from endoplasmic reticulum to the bile canalicular membrane is via vesicular and protein transporters. In the bile canalicular membrane a flippase enzyme helps to transport phosphatidylcholine specifically to the biliary leaflet. The phosphatidylcholine then enters the bile where it accounts for about 95% of the phospholipids. We postulated that the increased proportion of phosphatidylcholine in the bile canalicular membrane and the bile compared to the transport vesicles may be due to a methyltransferase activity in the bile canalicular membrane which, using s-adenosyl methionine as the substrate, converts phosphatidylethanolamine on the cytoplasmic leaflet to phosphatidylcholine, which is transported to the biliary leaflet. The aim of our study was to demonstrate and partially characterise methyltransferase activity in the bile canalicular membrane.

METHODS

Organelles were obtained from hamster liver by homogenisation and separation by sucrose gradient ultracentrifugation. These, along with phosphatidylethanolamine, were incubated with radiolabelled s-adenosyl methionine. Phospholipids were separated by thin-layer chromatography and radioactivity was counted by scintigraphy.

RESULTS

We demonstrated methyltransferase activity (nmol of SAMe converted/mg of protein/h at 37 degrees C) in the bile canalicular membrane of 0.442 (SEM 0.077, n=8), which is more than twice that found in the microsomes at 0.195 (SEM 0.013, n=8). The Km and pH optimum for the methyltransferase in the bile canalicular membrane and the microsomes were similar (Km 25 and 28 microM, respectively, pH 9.9 for both). The Vmax was different at 0.358 and 0.168 nmol of SAMe converted/mg of protein/h for the bile canalicular membrane and the microsomes, respectively.

CONCLUSION

The presence of the methyltransferase activity in the bile canalicular membrane may be amenable to therapeutic manipulation.

Hepatocellular transport and secretion of biliary lipids

Bile is the route for elimination of cholesterol from the body. Recent studies have begun to elucidate hepatocellular, molecular and physical-chemical mechanisms whereby bile salts stimulate biliary secretion of cholesterol together with phospholipids, which are enriched (up to 95%) in phosphatidylcholines. Active translocation of bile salts and phosphatidylcholines across the hepatocyte's canalicular plasma membrane provides the driving force for biliary lipid secretion. This facilitates physical-chemical interactions between detergent-like bile salt molecules and the ectoplasmic leaflet of the canalicular membrane, which result in biliary secretion of cholesterol and phosphatidylcholines as vesicles. Within the hepatocyte, separate molecular pathways function to resupply bile salts, phosphatidylcholines and cholesterol to the canalicular membrane for ongoing biliary lipid secretion.

The pravastatin-induced decrease of biliary cholesterol secretion is not directly related to an inhibition of cholesterol synthesis in humans

3-Hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase inhibitors have been reported to suppress biliary cholesterol secretion and saturation. It remains unproven whether this is mediated by inhibition of cholesterol synthesis. Therefore, the effect of a long-term administration of pravastatin on cholesterogenesis and on biliary lipid secretion was investigated in seven healthy volunteers. Placebo or 40 mg of pravastatin were taken daily at bedtime for 5 weeks using a double-blind crossover design. During the last week, 12 hours after the last drug intake 0.04 mmol [1-13C]acetate/kg. h and 0.5 g polyethylene glycol 4,000/h were infused intraduodenally for 15 hours. Plasma and duodenal bile samples were collected hourly. Thereafter, the decay of [13C]labeled plasma cholesterol was measured during the following 3 days. The fractional and absolute syntheses of plasma and biliary cholesterol were determined by gas chromatography mass spectrometry using mass isotopomer distribution analysis. At the end of the pravastatin period plasma total and low-density lipoprotein (LDL) cholesterol had decreased by 20% and 24%, respectively. Similarly, pravastatin suppressed biliary secretion rates of cholesterol, total bile acids and phospholipids (P <.05) by 46%, 36%, and 51%. As a consequence, cholesterol saturation index remained unchanged. However, fractional syntheses of cholesterol were comparable (P >.05) on placebo compared with pravastatin with 3.1% versus 4.0% in plasma and 4.3% versus 5.2% in bile after 15 hours, respectively. The mean absolute synthesis rates amounted to 0.3 mg/kg/h on placebo versus 0.4 on pravastatin (P >. 05). In conclusion, the pravastatin-induced reduction of biliary cholesterol secretion is not directly related to an inhibition of cholesterol synthesis.

Physical and metabolic factors in gallstone pathogenesis

Gallstones form when the tenuous balance of solubility of biliary lipids tips in favor of precipitation of cholesterol, unconjugated bilirubin, or bacterial degradation products of biliary lipids. For cholesterol gallstones, metabolic alterations in hepatic cholesterol secretion combine with changes in gallbladder motility and intestinal bacterial degradation of bile salts to destabilize cholesterol carriers in bile and produce cholesterol crystals. For black pigment gallstones, changes in heme metabolism or bilirubin absorption lead to increased bilirubin concentrations and precipitation of calcium bilirubinate. In contrast, mechanical obstruction of the biliary tract is the major factor leading to bacterial degradation and precipitation of biliary lipids in brown pigment stones. Further understanding of the physical and metabolic factors of cholesterol and black pigment formation is likely to provide interventions to interrupt the earliest stages of gallstone formation.

Evidence for an ATP-independent long-chain phosphatidylcholine translocator in hepatocyte membranes

Transport of phosphatidylcholine (PC) molecules across canalicular plasma membranes of the liver is essential for their secretion into bile. To test for evidence of protein-mediated translocation of natural long-chain PCs, we investigated whether hepatocyte membrane subfractions reconstituted into proteoliposomes promoted transmembrane translocation of radiolabeled PCs. Translocation of PC molecules in proteoliposomes was measured by an assay that employed multilamellar acceptor vesicles and the specific PC transfer protein purified from liver. As inferred from the percentage of radiolabel removed from proteoliposomes, facilitated PC translocation occurred in microsomes and canalicular and basolateral plasma membranes from rat liver but not in erythrocyte ghosts, microsomes, homogenates of COS and H35 cells, or Xenopus laevis oocytes. Heat denaturation in the presence of 2-mercaptoethanol and Pronase digestion of solubilized membrane proteins inhibited translocation. In contrast to the mdr2 gene product (Mdr2), which promotes ATP-dependent, verapamil-inhibitable PC translocation, ATP did not enhance and verapamil failed to block PC translocation. These data support the possibility that an ATP-independent PC translocator, possibly distinct from Mdr2, may be present in hepatocyte canalicular plasma membranes.