Effects of cholesterol on CCK-1 receptors and caveolin-3 proteins recycling in human gallbladder muscle

An Update on the Lithogenic Mechanisms of Cholecystokinin a Receptor (CCKAR), an Important Gallstone Gene for Lith13

The cholecystokinin A receptor (CCKAR) is expressed predominantly in the gallbladder and small intestine in the digestive system, where it is responsible for CCK's regulation of gallbladder and small intestinal motility. The effect of CCKAR on small intestinal transit is a physiological response for regulating intestinal cholesterol absorption. The Cckar gene has been identified to be an important gallstone gene, Lith13, in inbred mice by a powerful quantitative trait locus analysis. Knockout of the Cckar gene in mice enhances cholesterol cholelithogenesis by impairing gallbladder contraction and emptying, promoting cholesterol crystallization and crystal growth, and increasing intestinal cholesterol absorption. Clinical and epidemiological studies have demonstrated that several variants in the CCKAR gene are associated with increased prevalence of cholesterol cholelithiasis in humans. Dysfunctional gallbladder emptying in response to exogenously administered CCK-8 is often found in patients with cholesterol gallstones, and patients with pigment gallstones display an intermediate degree of gallbladder motility defect. Gallbladder hypomotility is also revealed in some subjects without gallstones under several conditions: pregnancy, total parenteral nutrition, celiac disease, oral contraceptives and conjugated estrogens, obesity, diabetes, the metabolic syndrome, and administration of CCKAR antagonists. The physical-chemical, genetic, and molecular studies of Lith13 show that dysfunctional CCKAR enhances susceptibility to cholesterol gallstones through two primary mechanisms: impaired gallbladder emptying is a key risk factor for the development of gallbladder hypomotility, biliary sludge (the precursor of gallstones), and microlithiasis, as well as delayed small intestinal transit augments cholesterol absorption as a major source for the hepatic hypersecretion of biliary cholesterol and for the accumulation of excess cholesterol in the gallbladder wall that further worsens impaired gallbladder motor function. If these two defects in the gallbladder and small intestine could be prevented by the potent CCKAR agonists, the risk of developing cholesterol gallstones could be dramatically reduced.

Identification of a new genetic variant associated with cholecystitis: A multicenter genome-wide association study

BACKGROUND

The genomic landscape of gallbladder disease remains poorly understood. We sought to examine the association between genetic variants and the development of cholecystitis.

METHODS

The Biobank of a large multi-institutional health care system was used. All patients with cholecystitis were identified using International Statistical Classification of Diseases, 10th Revision, codes and genotyped across six batches. To control for population stratification, data were restricted to that from individuals of European genomic ancestry using a multidimensional scaling approach. The association between single nucleotide polymorphisms and cholecystitis was evaluated with a mixed linear model-based analysis, controlling for age, sex, and obesity. The threshold for significance was set at 5 × 10.

RESULTS

Of 24,635 patients (mean ± SD age, 60.1 ± 16.7 years; 13,022 females [52.9%]), 900 had cholecystitis (mean ± SD age, 65.4 ± 14.3 years; 496 females [55.1%]). After meta-analysis, three single nucleotide polymorphisms on chromosome 5p15 exceeded the threshold for significance (p < 5 × 10). The phenotypic variance of cholecystitis explained by genetics and controlling for sex and obesity was estimated to be 17.9%.

CONCLUSION

Using a multi-institutional genomic Biobank, we report that a region on chromosome 5p15 is associated with the development of cholecystitis that can be used to identify patients at risk.

LEVEL OF EVIDENCE

Prognostic and epidemiological, Level III.

Modern views on development mechanisms and tactics for treatment of patients with gallbladder disease associated with metabolic syndrome

The high prevalence of metabolic syndrome in population has led to the cholelithiasis is important problem of modern health care along with coronary heart disease, arterial hypertension and diabetes. The main role in the formation of gallbladder pathology in patients with metabolic syndrome belongs to insulin resistance, atherogenic dyslipidemia, and non‑alcoholic fatty liver disease. Тhe prescription of ursodeoxycholic acid is pathogenetically justified. This article presents modern views on the mechanisms formation of gallstone disease in the conditions of metabolic disorders and management of this patients based on the results own research.

An update on the pathogenesis of cholesterol gallstone disease

PURPOSE OF REVIEW

Gallstone disease is a major epidemiologic and economic burden worldwide, and the most frequent form is cholesterol gallstone disease.

RECENT FINDINGS

Major pathogenetic factors for cholesterol gallstones include a genetic background, hepatic hypersecretion of cholesterol, and supersaturated bile which give life to precipitating cholesterol crystals that accumulate and grow in a sluggish gallbladder. Additional factors include mucin and inflammatory changes in the gallbladder, slow intestinal motility, increased intestinal absorption of cholesterol, and altered gut microbiota. Mechanisms of disease are linked with insulin resistance, obesity, the metabolic syndrome, and type 2 diabetes. The role of nuclear receptors, signaling pathways, gut microbiota, and epigenome are being actively investigated.

SUMMARY

Ongoing research on cholesterol gallstone disease is intensively investigating several pathogenic mechanisms, associated metabolic disorders, new therapeutic approaches, and novel strategies for primary prevention, including lifestyles.

Recent advances in understanding and managing cholesterol gallstones

The high prevalence of cholesterol gallstones, the availability of new information about pathogenesis, and the relevant health costs due to the management of cholelithiasis in both children and adults contribute to a growing interest in this disease. From an epidemiologic point of view, the risk of gallstones has been associated with higher risk of incident ischemic heart disease, total mortality, and disease-specific mortality (including cancer) independently from the presence of traditional risk factors such as body weight, lifestyle, diabetes, and dyslipidemia. This evidence points to the existence of complex pathogenic pathways linking the occurrence of gallstones to altered systemic homeostasis involving multiple organs and dynamics. In fact, the formation of gallstones is secondary to local factors strictly dependent on the gallbladder (that is, impaired smooth muscle function, wall inflammation, and intraluminal mucin accumulation) and bile (that is, supersaturation in cholesterol and precipitation of solid crystals) but also to "extra-gallbladder" features such as gene polymorphism, epigenetic factors, expression and activity of nuclear receptors, hormonal factors (in particular, insulin resistance), multi-level alterations in cholesterol metabolism, altered intestinal motility, and variations in gut microbiota. Of note, the majority of these factors are potentially manageable. Thus, cholelithiasis appears as the expression of systemic unbalances that, besides the classic therapeutic approaches to patients with clinical evidence of symptomatic disease or complications (surgery and, in a small subgroup of subjects, oral litholysis with bile acids), could be managed with tools oriented to primary prevention (changes in diet and lifestyle and pharmacologic prevention in subgroups at high risk), and there could be relevant implications in reducing both prevalence and health costs.

Functions of the Gallbladder

The gallbladder stores and concentrates bile between meals. Gallbladder motor function is regulated by bile acids via the membrane bile acid receptor, TGR5, and by neurohormonal signals linked to digestion, for example, cholecystokinin and FGF15/19 intestinal hormones, which trigger gallbladder emptying and refilling, respectively. The cycle of gallbladder filling and emptying controls the flow of bile into the intestine and thereby the enterohepatic circulation of bile acids. The gallbladder also largely contributes to the regulation of bile composition by unique absorptive and secretory capacities. The gallbladder epithelium secretes bicarbonate and mucins, which both provide cytoprotection against bile acids. The reversal of fluid transport from absorption to secretion occurs together with bicarbonate secretion after feeding, predominantly in response to an adenosine 3',5'-cyclic monophosphate (cAMP)-dependent pathway triggered by neurohormonal factors, such as vasoactive intestinal peptide. Mucin secretion in the gallbladder is stimulated predominantly by calcium-dependent pathways that are activated by ATP present in bile, and bile acids. The gallbladder epithelium has the capacity to absorb cholesterol and provides a cholecystohepatic shunt pathway for bile acids. Changes in gallbladder motor function not only can contribute to gallstone disease, but also subserve protective functions in multiple pathological settings through the sequestration of bile acids and changes in the bile acid composition. Cholecystectomy increases the enterohepatic recirculation rates of bile acids leading to metabolic effects and an increased risk of nonalcoholic fatty liver disease, cirrhosis, and small-intestine carcinoid, independently of cholelithiasis. Among subjects with gallstones, cholecystectomy remains a priority in those at risk of gallbladder cancer, while others could benefit from gallbladder-preserving strategies. © 2016 American Physiological Society. Compr Physiol 6:1549-1577, 2016.

Prevention of gallbladder hypomotility via FATP2 inhibition protects from lithogenic diet-induced cholelithiasis

Gallstone disease is a widespread disorder costing billions for annual treatment in the United States. The primary mechanisms underlying gallstone formation are biliary cholesterol supersaturation and gallbladder hypomotility. The relative contribution of these two processes has been difficult to dissect, as experimental lithogenic diets cause both bile supersaturation and alterations in gallbladder motility. Importantly, there is no mechanistic explanation for obesity as a major risk factor for cholelithiasis. We discovered that lithogenic diets induce ectopic triacylglycerol (TAG) accumulation, a major feature of obesity and a known muscle contraction impairing condition. We hypothesized that prevention of TAG accumulation in gallbladder walls may prevent gallbladder contractile dysfunction without impacting biliary cholesterol saturation. We utilized adeno-associated virus-mediated knock down of the long-chain fatty acid transporter 2 (FATP2; Slc27A2), which is highly expressed by gallbladder epithelial cells, to downregulate lithogenic diet-associated TAG accumulation. FATP2-knockdown significantly reduced gallbladder TAG, but did not affect key bile composition parameters. Importantly, measurements with force displacement transducers showed that contractile strength in FATP2-knockdown gallbladders was significantly greater than in control gallbladders following lithogenic diet administration, and the magnitude of this effect was sufficient to prevent the formation of gallstones. FATP2-driven fatty acid uptake and the subsequent TAG accumulation in gallbladder tissue plays a pivotal role in cholelithiasis, and prevention of this process can protect from gallstone formation, even in the context of supersaturated bile cholesterol levels, thus pointing to new treatment approaches and targets.

Endogenous elevation of plasma cholecystokinin does not prevent gallstones

BACKGROUND

Regular gall bladder contraction reduces bile stasis and prevents gallstone formation. Intraduodenal administration of exogenous pancreatic secretory trypsin inhibitor-I (PSTI-I, also known as monitor peptide) causes cholecystokinin (CCK) secretion.

DESIGN

We proposed that stimulation of CCK release by PSTI would produce gall bladder contraction and prevent gallstones in mice fed a lithogenic diet. Therefore, we tested the effect of overexpression of rat PSTI-I in pancreatic acinar cells on plasma CCK levels and gall bladder function in a transgenic mouse line (TgN[Psti1]; known hereafter as PSTI-I tg).

RESULTS

Importantly, PSTI tg mice had elevated fasting and fed plasma CCK levels compared to wild-type (WT) mice. Only mice fed the lithogenic diet developed gallstones. Both fasting and stimulated plasma CCK levels were substantially reduced in both WT and PSTI-I tg mice on the lithogenic diet. Moreover, despite higher CCK levels PSTI-I tg animals developed more gallstones than WT animals.

CONCLUSIONS

Together with the previously observed decrease in CCK-stimulated gall bladder emptying in mice fed a lithogenic diet, our findings suggest that a lithogenic diet causes gallstone formation by impaired CCK secretion in addition to reduced gall bladder sensitivity to CCK.

Impaired cholecystokinin-induced gallbladder emptying incriminated in spontaneous "black" pigment gallstone formation in germfree Swiss Webster mice

"Black" pigment gallstones form in sterile gallbladder bile in the presence of excess bilirubin conjugates ("hyperbilirubinbilia") from ineffective erythropoiesis, hemolysis, or induced enterohepatic cycling (EHC) of unconjugated bilirubin. Impaired gallbladder motility is a less well-studied risk factor. We evaluated the spontaneous occurrence of gallstones in adult germfree (GF) and conventionally housed specific pathogen-free (SPF) Swiss Webster (SW) mice. GF SW mice were more likely to have gallstones than SPF SW mice, with 75% and 23% prevalence, respectively. In GF SW mice, gallstones were observed predominately in heavier, older females. Gallbladders of GF SW mice were markedly enlarged, contained sterile black gallstones composed of calcium bilirubinate and <1% cholesterol, and had low-grade inflammation, edema, and epithelial hyperplasia. Hemograms were normal, but serum cholesterol was elevated in GF compared with SPF SW mice, and serum glucose levels were positively related to increasing age. Aged GF and SPF SW mice had deficits in gallbladder smooth muscle activity. In response to cholecystokinin (CCK), gallbladders of fasted GF SW mice showed impaired emptying (females: 29%; males: 1% emptying), whereas SPF SW females and males emptied 89% and 53% of volume, respectively. Bilirubin secretion rates of GF SW mice were not greater than SPF SW mice, repudiating an induced EHC. Gallstones likely developed in GF SW mice because of gallbladder hypomotility, enabled by features of GF physiology, including decreased intestinal CCK concentration and delayed intestinal transit, as well as an apparent genetic predisposition of the SW stock. GF SW mice may provide a valuable model to study gallbladder stasis as a cause of black pigment gallstones.

Cholesterol gallstone disease: focusing on the role of gallbladder

Gallstone disease (GSD) is one of the most common biliary tract diseases worldwide in which both genetic and environmental factors have roles in its pathogenesis. Biliary cholesterol supersaturation from metabolic defects in the liver is traditionally seen as the main pathogenic factor. Recently, there have been renewed investigative interests in the downstream events that occur in gallbladder lithogenesis. This article focuses on the role of the gallbladder in the pathogenesis of cholesterol GSD (CGD). Various conditions affecting the crystallization process are discussed, such as gallbladder motility, concentrating function, lipid transport, and an imbalance between pro-nucleating and nucleation inhibiting proteins.

Association of caveolin-3 and cholecystokinin A receptor with cholesterol gallstone disease in mice

AIM: To investigate the role of caveolin-3 (CAV3) and cholecystokinin A receptor (CCKAR) in cholesterol gallstone disease (CGD).

METHODS: To establish a mouse model of CGD, male C57BL/6 mice were fed with a lithogenic diet containing 1.0% cholic acid, 1.25% cholesterol and 15% fat; a similar control group was given a normal diet. The fresh liver weights and liver-to-body weight ratio were compared between the two groups after one month. Serum lipid profile and bile composition were determined with an autoanalyzer. The Cav3 and Cckar mRNA and CAV3 and CCKAR protein levels in the liver and gallbladder were determined via real-time polymerase chain reaction and Western blot, respectively.

RESULTS: Establishment of the mouse CGD model was verified by the presence of cholesterol gallstones in mice fed the lithogenic diet. Compared with mice maintained on a normal diet, those fed the lithogenic diet had significantly higher mean liver-to-body weight ratio (0.067 ± 0.007 vs 0.039 ± 0.007, P < 0.01), serum total cholesterol (4.22 ± 0.46 mmol/L vs 2.21 ± 0.11 mmol/L, P < 0.001), bile total cholesterol (1.33 ± 0.33 mmol/L vs 0.21 ± 0.11 mmol/L, P < 0.001), and bile phospholipid concentrations (3.55 ± 1.40 mmol/L vs 1.55 ± 0.63 mmol/L, P = 0.04), but lower total bile acid concentrations (726.48 ± 51.83 μmol/L vs 839.83 ± 23.74 μmol/L, P = 0.007). The lithogenic diet was also associated with significantly lower CAV3 in the liver and lower CAV3 and CCKAR in the gallbladder compared with the control mice (all P < 0.05).

CONCLUSION: CAV3 and CCKAR may be involved in cholesterol gallstone disease.

Roles of cholesterol and bile salts in the pathogenesis of gallbladder hypomotility and inflammation: cholecystitis is not caused by cystic duct obstruction

A large number of human and animal studies have challenged the hypothesis that cystic duct obstruction by gallstones causes cholecystitis. These studies suggest that lithogenic bile that can deliver high cholesterol concentrations to the gallbladder wall causes hypomotility and creates a permissive environment that allows normal concentrations of hydrophobic bile salts to inflame the mucosa and impair muscle function inhibiting gallbladder emptying. High concentrations of cholesterol increase its diffusion rates through the gallbladder wall where they are incorporated into the sarcolemmae of muscle cells by caveolin proteins. High caveolar cholesterol levels inhibit tyrosine-induced phosphorylation of caveolin proteins required to transfer receptor-G protein complexes into recycling endosomes. The sequestration of these receptor-G protein complexes in the caveolae results in fewer receptors recycling to the sarcolemmae to be available for agonist binding. Lower internalization and recycling of CCK-1 and other receptors involved in muscle contraction explain gallbladder hypomotility. PGE2 receptors involved in cytoprotection are similarly affected. Cells with a defective cytoprotection failed to inactivate free radicals induced by normal concentrations of hydrophobic bile salts resulting in chronic inflammation that may lead to acute inflammation. Ursodeoxycholic acid salts (URSO) block these bile salts effects thereby preventing the generation of free radicals in muscle cells in vitro and development of cholecystitis in the ligated common bile duct in guinea pigs in vivo. Treatment with URSO improves muscle contraction and reduces the oxidative stress in patients with symptomatic cholesterol gallstones by lowering cholesterol concentrations and blocking the effects of hydrophobic bile salts on gallbladder tissues.

Physiology and Pathophysiology of the Biliary Tract: The Gallbladder and Sphincter of Oddi—A Review

The biliary tract collects, stores, concentrates, and delivers bile secreted by the liver. Its motility is controlled by neurohormonal mechanisms with the vagus and splanchnic nerves and the hormone cholecystokinin playing key roles. These neurohormonal mechanisms integrate the motility of the gallbladder and sphincter of Oddi (SO) with the gastrointestinal tract in the fasting and digestive phases. During fasting most of the hepatic bile is diverted toward the gallbladder by the resistance of the SO. The gallbladder allows the gradual entry of bile relaxing by passive and active mechanisms. During the digestive phase the gallbladder contracts, and the SO relaxes allowing bile to be released into the duodenum for the digestion and absorption of fats. Pathological processes manifested by recurrent episodes of upper abdominal pain affect both the gallbladder and SO. The gallbladder motility and cytoprotective functions are impaired by lithogenic hepatic bile with excess cholesterol allowing the hydrophobic bile salts to induce chronic cholecystitis. Laparoscopic cholecystectomy is the standard treatment. Three types of SO dyskinesia also cause biliary pain. Their pathophysiology is not completely known. The pain of types I and II usually respond to sphincterotomy, but the pain due to type III usually does not.

Disruption of gallbladder smooth muscle function is an early feature in the development of cholesterol gallstone disease

BACKGROUND; Decreased gallbladder smooth muscle (GBSM) contractility is a hallmark of cholesterol gallstone disease, but the interrelationship between lithogenicity, biliary stasis, and inflammation are poorly understood. We studied a mouse model of gallstone disease to evaluate the development of GBSM dysfunction relative to changes in bile composition and the onset of sterile cholecystitis.

METHODS

BALB/cJ mice were fed a lithogenic diet for up to 8 weeks, and tension generated by gallbladder muscle strips was measured. Smooth muscle Ca(2+) transients were imaged in intact gallbladder.

KEY RESULTS

Lipid composition of bile was altered lithogenically as early as 1 week, with increased hydrophobicity and cholesterol saturation indexes; however, inflammation was not detectable until the fourth week. Agonist-induced contractility was reduced from weeks 2 through 8. GBSM normally exhibits rhythmic synchronized Ca(2+) flashes, and their frequency is increased by carbachol (3 μm). After 1 week, lithogenic diet-fed mice exhibited disrupted Ca(2+) flash activity, manifesting as clustered flashes, asynchronous flashes, or prolonged quiescent periods. These changes could lead to a depletion of intracellular Ca(2+) stores, which are required for agonist-induced contraction, and diminished basal tone of the organ. Responsiveness of Ca(2+) transients to carbachol was reduced in mice on the lithogenic diet, particularly after 4-8 weeks, concomitant with appearance of mucosal inflammatory changes.

CONCLUSIONS & INFERENCES

These observations demonstrate that GBSM dysfunction is an early event in the progression of cholesterol gallstone disease and that it precedes mucosal inflammation.

Augmented cholesterol absorption and sarcolemmal sterol enrichment slow small intestinal transit in mice, contributing to cholesterol cholelithogenesis

Cholesterol gallstones are associated with slow intestinal transit in humans as well as in animal models, but the molecular mechanism is unknown. We investigated in C57L/J mice whether the components of a lithogenic diet (LD; 1.0% cholesterol, 0.5% cholic acid and 17% triglycerides), as well as distal intestinal infection with Helicobacter hepaticus, influence small intestinal transit time. By quantifying the distribution of 3H-sitostanol along the length of the small intestine following intraduodenal instillation,we observed that, in both sexes, the geometric centre (dimensionless) was retarded significantly (P <0.05) by LD but not slowed further by helicobacter infection (males, 9.4±0.5 (uninfected), 9.6±0.5 (infected) on LD compared with 12.5±0.4 and 11.4±0.5 on chow). The effect of the LD was reproduced only by the binary combination of cholesterol and cholic acid. We inferred that the LD-induced cholesterol enrichment of the sarcolemmae of intestinal smooth muscle cells produced hypomotility from signal-transduction decoupling of cholecystokinin (CCK), a physiological agonist for small intestinal propulsion in mice. Treatment with ezetimibe in an amount sufficient to block intestinal cholesterol absorption caused small intestinal transit time to return to normal. In most cholesterol gallstone-prone humans, lithogenic bile carries large quantities of hepatic cholesterol into the upper small intestine continuously, thereby reproducing this dietary effect in mice. Intestinal hypomotility promotes cholelithogenesis by augmenting formation of deoxycholate, a pro-lithogenic secondary bile salt, and increasing the fraction of intestinal cholesterol absorbed.

Pathogenesis of cholesterol and pigment gallstones: an update

Phase separation of cholesterol crystals from supersaturated bile is still considered the key event in cholesterol gallstone formation. In this review, we will first provide a basal framework of the interactions between the sterol, bile salts and phospholipids in aqueous solutions and then summarize new developments. The hepatocytic apical membrane harbours specific transport proteins for these lipids. Polymorphisms in the gene encoding the cholesterol transporter ABCG5-G8 have been found to increase overall gallstone risk, whereas functional mutations in the gene encoding the phospholipid floppase ABCB4 lead to the rare clinical syndrome of low phospholipid associated cholelithiasis. Expression of bile salt and phospholipid transport proteins is regulated bij the bile salt nuclear receptor Farnesoid X receptor (FXR), while the Liver X Receptor (LXR) α regulates ABCG5-G8. Although data from murine experiments suggest a critical role of FXR in gallstone formation, its role in human lithogenesis remains controversial. Variants of the gene encoding UGT1A1 (uridine 5'-diphosphate (UDP)-glucuronosyltransferase 1A1) responsible for bilirubin conjugation were recently associated with risk of gallstones as well as stone bilirubin content, suggesting common factors in cholesterol and pigment gallstone pathogenesis.