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Clinical Lipidology

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Abnormalities of lipid metabolism, gallstonedisease and gallbladder function

Silvana Zanlungo, Attilio Rigotti, Juan Francisco Miquel & Flavio Nervi

To cite this article: Silvana Zanlungo, Attilio Rigotti, Juan Francisco Miquel & Flavio Nervi (2011)Abnormalities of lipid metabolism, gallstone disease and gallbladder function, Clinical Lipidology,6:3, 315-325

To link to this article: https://doi.org/10.2217/clp.11.22

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315ISSN 1758-429910.2217/CLP.11.22 © 2011 Future Medicine Ltd Clin. Lipidol. (2011) 6(3), 315–325

Review

Abnormalities of lipid metabolism, gallstone disease and gallbladder function

The prevailing views about the gallbladder (GB) are that it is a gastrointestinal organ that concentrates and stores hepatic bile for delivery into the intestinal lumen during the food intake cycles for lipid digestion and absorption, and that it is the organ where gallstones (GS) form, producing GB disease (GBD). However, recent studies on the role of bile salts (BS) as impor-tant signaling molecules that participate in metabolic regulation, have raised the question of whether the GB might also play a direct role in metabolic homeostasis of the whole body.

Cholesterol (CH) GS (defined by >50% CH content by weight) are present in approximately 80% of patients with GBD [1–3]. The remain-ing GS patients harbor black pigment stones, mainly composed of calcium bilirubinate, and brown pigment or mixed stones. Black pigment stones form in patients with hemolytic diseases such as thalassemia, whereas brown pigment GS are frequent in conditions associated with infections of the biliary tree [1–3].

Gallbladder disease secondary to GS is a high burden disease condition that increases mortality in the general population [4]. CH

GS disease is particularly prevalent among Native Americans and Hispanics [5,6] . Cholecystectomy is the best treatment for GBD and is one of the more frequently per-formed surgical procedures performed world-wide. Besides gender, age and family history, a number of metabolic abnormalities included in the metabolic syndrome (abdominal obes-ity, insulin resistance [IR], glucose intoler-ance or diabetes Type 2 and high triglycer-ides, low HDL-C dyslipidemia) are associated with both GBD and arteriosclerotic vascular diseases. The hallmark pathogenic condition that is common to all these disease conditions is IR [7–12], however, how all these metabolic abnormalities favor GS formation is unknown. Consistent with a common epidemiological and pathogenic background, patients with atherosclerotic coronary [13,14] and carotid artery [15,16] diseases have a higher risk of developing GS compared with the general population. However, which metabolic abnor-malities of lipid trafficking at the cellular and molecular levels (particularly hypertriglyceri-demia and low serum HDL-C dyslipidemia

Gallstone disease is highly prevalent with a complex and multifactorial pathogenesis. Gallstones are closely related to the metabolic syndrome – associated disease conditions in which abnormal regulation of lipid metabolism secondary to insulin resistance plays a major pathogenic role. Insulin resistance increases biliary cholesterol secretion and affects gallbladder (GB) motility. Regulation of lipid metabolism and energy homeostasis is complex and the GB has been considered to have a minor regulatory role in both the intestinal absorption of lipids and metabolic homeostasis of the whole body. In fact, ablation of the GB does not affect nutrient absorption or the ability to lead a normal life. GB function regulates the cycling of bile salts through the enterohepatic circulation. Bile salts have important signaling effects that can affect whole body metabolic homeostasis. The GB and intestinal mucosa are rich in the hormone FGF15/19 and the receptor TGR5, which participate in metabolic regulation. Recent evidence supports the hypothesis that cholecystectomy may not be innocuous and that the GB has a significant role in the regulation of hepatic triglyceride metabolism. This article provides information regarding recent advances in the understanding of the interaction between regulation of lipid metabolism, insulin resistance, gallstone disease and GB function.

Keywords: gallbladder function n gallstone disease n insulin resistancen triglyceride metabolism

Silvana Zanlungo1, Attilio Rigotti1, Juan Francisco Miquel1 & Flavio Nervi†1

1Departamento de Gastroenterología, Facultad de Medicina, Pontificia Universidad Católica de Chile, Marcoleta 367, Casilla 114-D, Santiago, Chile †Author for correspondence: Tel.: +56 2686 3820 Fax: +56 2637 7780 [email protected]

Clin. Lipidol. (2011) 6(3)316 future science groupClin. Lipidol. (2011) 6(3)

frequently found in IR) can favor the precipita-tion and accumulation of CH in the GB and the artery wall, respectively is unknown.

The purposes of this article are first, to review some of the metabolic links between abnormalities of lipid metabolism commonly found in IR-associated diseases, GB functions and GBD. Second, to discuss experimental evi-dence supporting a new metabolic role of the GB and speculate on the hypothesis that the GB, besides its well-known role in lipid diges-tion and absorption, could also have important endocrine functions participating in whole body metabolic homeostasis.

GB function & lipid metabolismGallbladder function is integrated in the ‘liver–GB–intestine’ axis, responsible for maintaining the homeostasis of triglycerides (TGs), free fatty acids (FFAs) and CH of the entire organism. The GB plays a central role in the digestion and absorption of lipids present in the diet. It stores and concentrates hepatic bile secreted by the liver during periods of fasting and then sends it to the intestine during food intake, through contrac-tions triggered by duodenal cholecystokinin. The release of the ileal hormone FGF15 (human ortholog FGF19), due to the ileal reabsorption of biliary BS, aids GB filling and at the same time inhibits the synthesis of BS [17,18]. GB bile that is rich in BS and phospholipid mixed micelles, allows intraluminal emulsification and hydrolysis of dietary TGs in the intestine [19]. After intesti-nal uptake, FFAs are re-esterified into new TGs, which are incorporated with CH, phospholipid and apoB, forming chylomicrons, which are then sent to the periphery.

Triglycerides are key metabolites through which energy can be stored in adipose tissue and transported throughout the organism for main-taining energy homeostasis. TGs circulate in plasma incorporated in lipoproteins (preferably chylomicrons and VLDL) and are also found in fat goblets inside of cells as energy stores. There are three fundamental organs that contain TGs: adipose tissue, muscle and the liver. These organs store TGs, hydrolyse them to release FFAs, export them, and finally produce energy for cellular function. There is a continuous flow of FFAs between the three tissues via the blood. Dietary fat enters the plasma from the small intestine in the form of chylomicrons, and they then deliver FFAs from TGs to the tissues that express lipo-protein lipase. Once approximately two thirds of their TGs are unloaded in peripheral tissues,

chylomicron remnants containing the remain-ing TGs and CH esters are quickly taken in by the liver [19]. Abnormal accumulation of hepatic lipids occurs when the amount of FFAs and CH entering the hepatocytes exceeds their exit. FFA flow between the different tissues changes according to the nutritional state of the subject. Similarly, cellular CH content will depend on the uptake from plasma lipoproteins and the level of endogenous synthesis. In the feeding stage, FFAs travel bound to albumin towards the tissues to be stored in adipose tissue. The endogenous secretion of TGs and CH to the plasma is carried out from the liver in the form of VLDL and is regulated by food intake cycles. This regulation is dependent on insulin effects that decrease the hepatic production of VLDL, which is increased in states of IR [20].

The metabolism of FFAs, TGs, CH and BS are highly interrelated processes and are regulated by genes that control synthesis, uptake of lipo-proteins and their production, as well as biliary and fecal excretion of the steroidal ring in the form of neutral sterols and BS. Numerous stud-ies have shown a close functional relationship between the regulation of BS metabolism, as well as TG and CH metabolism [21–26]. For example, hypertriglyceridemia can be observed in patients with 7-a-hydroxylase deficiency [27], and in those with an increased fecal excretion of BS, as hap-pens after removal of the distal ileum, Crohn’s disease and cholestyramine treatment [28–32].

A number of studies have shown that GB motility is altered, decreasing GB empty-ing and favoring GS formation in patients with Type 2 diabetes, IR, obesity and hyper-triglyceridemia [33–35]. However, the molecular mechanism(s) responsible for this abnormality is unknown. GB motility is regulated by complex interrelated neuronal, hormonal and paracrine factors [36]. FGF15/19, produced on arrival of BS in the ileum, is also important for GB motility favoring GB filling [19].

In recent years, close relationships have been proved between the enterohepatic circulation of BS, GS, regulation of serum glucose and serum TG concentrations, presence of IR and non-alcoholic fatty liver disease (NAFLD) [21–26]. For instance, BS function as metabolic modula-tors through the activation of the nuclear recep-tor farnesoid X receptor (FXR), which acts as an intracellular sensor of the concentration of BS [26]. The activation of FXR reduces gluco-neogenesis and participates indirectly in the metabolic regulation of FFAs, TGs, CH and

Review | Zanlungo, Rigotti, Miquel & Nervi

www.futuremedicine.com 317future science group

BS. An indirect FXR-mediated effect on meta-bolic homeostasis is also mediated through the induction of FGF15/19 in the ileum and the GB. FGF19 transgenic mice [37] and administration of recombinant FGF19 to mice [38] increase basal metabolic rate, decrease adiposity and serum TG levels, and increase FFA oxidation in brown adipose tissue [39]. Another BS-dependent sign-aling pathway for whole body metabolic regula-tion relates to the interaction between BS and the membrane receptor TGR5 (located in a number of tissues including adipose tissue, muscle and the epithelium of the GB, biliary tree and small bowel), which increases basal metabolic rate [39] and controls glucose homeostasis [40].

Mechanisms of GS formationThe pathogenesis of CH GS formation implies abnormalities of bile secretion, CH solubility and crystal formation, and abnormal function of the GB [2,3]. Normal hepatic bile composition by weight consists of water (90%) and 10% solids: BS (72%), phospholipids (24%) and CH (4%). A number of specific ATP-binding-cassette (ABC) transport proteins expressed at the canalicular membrane are responsible for the secretion of bil-iary lipids into bile: ABCB11 (BS export pump) is the main BS transporter; ABCB4 (multidrug resistant p-glycoprotein MDR3) acts as a ‘flip-pase’ that translocates phospholipids from the inner to the outer leaflet of the canalicular mem-brane as unilamellar vesicles; the heterodimer ABCG5/ABCG8 is responsible for CH transloca-tion into bile. Unilamellar phospholipid vesicles solubilize free CH, and simple BS micelles tend to dissolve them into mixed micelles in the GB, where diluted hepatic bile is concentrated. In CH supersaturated bile, vesicles persist and become enriched in CH translocated by the heterodimer ABCG5/ABCG8. When the CH:phospholipid ratio is greater than 1, CH crystals can nucleate and initiate CH GS formation [2,3].

Cholesterol supersaturation of bile has been considered a sine qua non pathogenic condition for CH GS formation. This is the result of increased hepatic secretion of CH and/or decreased secre-tion of BS and phospholipids into bile [1,3]. CH GS patients usually have increased relative bil-iary CH concentration in GB bile compared with pigment GS patients or normal subject’s bile [3]. However, it is important to note that biliary CH supersaturation is commonly found in nor-mal subjects without GBD [41], reinforcing the concept that posthepatic events, including GB factors, are fundamental for CH GS formation.

Pathogenic GB factors include pronucleating proteins such as mucin and abnormal GB motil-ity with impaired postprandial GB emptying, as found in obese and diabetic patients, and during pregnancy [2,3].

The pathogenesis of pigment GS is related to increased biliary concentration of unconjugated bilirubin as occurs in sickle cell disease [42] and in patients with ileal dysfunction resulting from any medical or surgical cause, such as Crohn’s disease and ileal resection [43]. Brown pigment stones are frequently found in Asian patients with bacterial infection of the biliary tract, which causes high glucuronidase activity in bile [2,3].

An intriguing finding is that patients with pigment stones as well as those with CH GS (both types of GS have very different pathogenic mechanisms) present higher than normal serum TG levels [44], a serum abnormality commonly found in IR states. This interesting epidemiologi-cal finding suggests that abnormal GB function as a consequence of GS disease could affect serum and hepatic TG metabolism.

IR, abnormal lipid metabolism & GBDInsulin resistance is characterized by hyper-insulinemia with progressive tendency to hyper-glycemia, hypertriglyceridemia and low HDL-C, and is considered the central pathogenic mecha-nism of the metabolic syndrome [7,8] and its clini-cal constituents, as shown in Figure 1. Several studies have indicated that IR predisposes to subsequent CH GBD [9–12], a link that supports the hypothesis that insulin plays a significant role in the regulation of CH and TG metabolism and the enterohepatic circulation of lipids.

Gallbladder diease is strongly associated with the metabolic syndrome [9–12], suggesting that IR could play a key pathogenic role in GS formation. Because abnormal regulation of GB function has an important pathogenic role in GBD [1–3] it is possible that IR also affects GB function. In fact, IR causes GB dysmotility in humans [34]. The two more common abnormalities of biliary lipid metabolism that induce CH supersaturated bile in CH GBD are increased biliary CH secretion and decreased BS pool and biliary secretion rate [1–3]. To understand the pathogenic mechanism lead-ing to CH GS formation in patients with IR, it is important to know the role of insulin on biliary CH, BS and phospholipids secretion rates and GB function.

A number of experimental data support the concept that insulin plays a major role in the regulation of CH metabolism. For instance,

Abnormalities of lipid metabolism, gallstone disease & gallbladder function | Review

Clin. Lipidol. (2011) 6(3)318 future science group

IR increases CH synthesis, decreases intestinal CH absorption [45], and increases VLDL pro-duction [20]. Insulin also inhibits the expression of CYP7A1, the enzyme that controls the rate-liming step of BS synthesis [46]. The metabolic abnormality linking IR, CH-supersaturated GB bile and CH GS formation is the increase of body CH synthesis and hypersecretion of biliary CH [47]. Recent experimental studies have shown that mice with hepatic IR due to disruption of the insulin receptor (LIRKO) had increased biliary CH output and saturation [48]. These animals developed a higher incidence of GBD when exposed to a lithogenic diet compared with controls. LIRKO mice with specific hepatic IR

presented hypersecretion of CH into bile as a consequence of the hepatic overexpression of CH transporters ABCG5 and ABCG8 in the canaliculi [48]. These animals have increased GB volume presumably as a consequence of increased bile flow, conditions that could also favor GS formation [49]. Interestingly, LIRKO mice also developed atherosclerosis when exposed to this lithogenic diet [50], a finding consistent with the epidemiological evidence linking CH GS with arteriosclerotic cardiovascular disease and the metabolic syndrome. Contrary to human GS patients with IR states that usually develop with higher than normal serum TG levels, LIRKO mice presented lower than normal serum TG

Figure 1. Metabolic abnormalities of insulin resistance state and its relationship with gallbladder disease, nonalcoholic fatty liver disease and cardiovascular disease. The major IR organs are adipose tissue, muscle and liver. Patients with IR have hyperinsulinemia, a tendency to hyperglycemia and hypertriglyceridemia, and increased serum FFA levels secondary to increased lipolysis from adipose tissue. They also have increased body CH synthesis and biliary CH supersaturation, and decreased gallbladder motility, conditions that favor gallstone formation and gallbladder disease. Insulin decreases the expression levels of the canalicular CH transporters ABCG5/8, which are overexpressed in IR, favoring biliary CH hypersecretion and gallstone formation. nonalcoholic fatty liver disease usually results from increased hepatic TG synthesis in IR. The increased synthesis of hepatic VLDL in IR favors the production of atherogenic lipoproteins (LDL and VLDL remnants) and the development of arteriosclerotic cardiovascular disease. CH: Cholesterol; FFA: Free fatty acid; IR: Insulin resistance; TG: Triglyceride.

Insulin

Liver

IR

IR

IR

Fat

Lipolysis

Muscle

Insulin

FFAs

Bile CH

G5/G8

Gallbladderdisease

Fatty liverdisease

TG and CHsynthesis

Cardiovasculardisease

TG-VLDL

Atherogeniclipoproteins



levels. These observations indicate that the mechanism of hypertriglyceridemia commonly found in human GBD and IR-associated disease conditions is more complex compared with the LIRKO mouse model of hepatic IR.

Hepatic TG content and VLDL production are increased in normal weight GS patients with normal liver histology, compared with individu-als without GS, suggesting that GS formation, or altered GB function could affect hepatic TG metabolism [51]. NAFLD usually presents higher than normal serum TG levels and GBD [9,35,52] and it would appear that IR is the principal path-ogenic factor [53,54]. NAFLD is the main cause of cryptogenic cirrhosis with a major impact on healthcare systems [55,56]. Cirrhosis is also associated with GBD [57–59].

The cause of the increase of hepatic and serum TGs in GBD has not been completely deduced. However, it is known that IR is a fundamen-tal factor that can alter each of the processes that regulate the concentration of TGs in blood and liver as well as CH secretion into bile. IR increases lipolysis from adipocytes, liberating FFAs, which are quickly taken up by the liver. In the IR state, the activity of the peripheral lipopro-tein lipase is diminished, producing an increase of chylo microns and VLDL remnants, which are also taken up by the liver, thus increasing TGs and CH mass in the organ and increasing the availability of hepatic free CH for secretion into bile.

Genetics of GBD & abnormalities of lipid metabolismThe genetics of GS disease is complex and GS disease is polygenic, as are obesity, diabetes and arteriosclerosis. It is well known that an impor-tant component of GBD depends on genetic fac-tors [60–64] giving an estimate that a number of genes could contribute approximately 25% of the development of symptomatic GBD in popula-tions with a relatively low prevalence of GBD [64]. However, in populations with a high prevalence of GBD the genetic contribution could be as high as 60% [65]. Genetic factors play an important role not only in CH GS pathogenesis, but also in pigment GS formation [66].

Candidate genes for GBD, the so-called Lith genes, have long been known in mice. Quantitative trait locus mapping was employed to identify more than 40 candidate genes for susceptibility to GS [67]. Most of these genes are involved in lipid metabolism and traffic, as well as in GB function. However, in contrast

to the plentiful information on mouse Lith genes, information in humans is more scarce. However, more than ten candidate genes have been described with an attractive mechanistic hypothesis [68,69]. Through studies of genome-wide association and linkage in affected sibling pairs the ABCG8 D19H variant was identi-fied as a common genetic factor contributing approximately 10% of the total risk for the development of GS [70]. Confirming these results, a recent study in Swedish twins has shown that heterozygous or homozygous twins carrying the ABCG8 D19H genotype have a significantly increased risk of GS disease [70]. Genetic variants in the ABCG8 and ABCG5 genes have also been found to be associated with CH metabolism and individual responsiveness of plasma CH to dietary and pharmaceutical interventions for hypercholesterolemia [71].

Other attractive candidates as genetic fac-tors linking lipid metabolism, IR and GBD are the nuclear receptors that control the expres-sion of transporters and key enzymes of lipid metabolism. The nuclear receptor FXR acts as an intracellular BS sensor [72,73] and induces the expression of Abcb11 and Abcb4 and represses BS synthesis by SHP-mediated CYP7A1 inhibition. Results obtained by Moschetta et al. confirmed the relevance of FXR in CH GD development in mouse models. FXR-deficient mice fed with a lithogenic diet showed decreased biliary phos-pholipid and BS concentrations and increased CH GS formation. In contrast, treatment of wild-type mice with an agonist of FXR decreased CH GS susceptibility [74]. In addition, FXR regulates lipogenesis, plasma VLDL-TG and plasma TG export and turnover [25,26]. Moreover, FXR is also involved in the regulation of hepatic gluconeo-genesis, glycogen synthesis and insulin sensitiv-ity [26]. Interestingly, a common polymorphism in the FXR gene was associated with decreased hepatic lipid gene expression of some FXR-target genes, suggesting that genetic FXR modulation could affect the susceptibility to conditions such as GBD, arteriosclerosis and diabetes.

The liver X receptor (LXR), which is an oxysterol intracellular sensor, regulates master genes related to sterols, lipids and BS home-ostasis. LXR are also known to induce the expression of CH and phospholipid eff lux transporters such as ABCG5 and ABCG8, as well as ABCA1, a basolateral ABC transporter that effluxes both CH and phospholipids [75]. Although increased expression in LXR as well as ABCG5 and ABCG8 has been found in



nonobese Chinese GS patients, no correlations were found between SREBP1c and LXR [76]. Interestingly, activation of LXR augmented mice susceptibility to a lithogenic diet, increas-ing the CH saturation index in bile and CH GS formation [77]. Further studies are required to establish whether nuclear receptors are rel-evant as genetic link factors between GBD, lipid abnormalities and IR in humans. Some well-documented monogenic mutations that occur very rarely in the general population clearly predispose to CH GBD [78]. These monogenic diseases mainly include monogenic deficiency of Abcb4, responsible for the secretion of phos-pholipids into the canaliculi, and deficiency of Cyp7A1 [27], the rate-limiting step of BS synthe-sis that determines familiar early-onset GBD associated with hyperlipidemia.

GB function & hepatic TG metabolismIt is generally accepted that cholecystectomy is the best treatment for GBD without any negative impact on metabolic regulation or on normal life. In fact, the digestion and absorp-tion of liposoluble vitamins and dietary fats is normal after a cholecystectomy, due to the fact that the BS pool size remains normal and the bile is stored in the proximal portion of the small intestine during fasting periods [79]. Cholecystectomy determines an increase in the number of times the pool of BS travels the enterohepatic circulation, exposing the mucosa of the distal ileum and hepatocytes and biliary tree to an increase in the mass flux of BS per day. Due to hormonal connections between the distal ileum, the liver and the GB it is important to explore the possibility of an inter-relationship between the regulation of hepatic metabolism of lipids, energy balance and predisposition to the development of the metabolic syndrome, associated diseases and the presence or absence of the GB in the enterohepatic circuit.

Recent studies from our group have found increased serum and hepatic TG levels, increased hepatic VLDL production, and increased activity of hepatic microsomal TG transfer protein after ablation of the GB in mice [80]. In these experiments, BS pool size, composition and synthesis remained unchanged after cholecystectomy. This observation indi-cated that the mechanisms responsible for the metabolic changes of TG metabolism after ablation of the GB were different from those observed in conditions that determine altered BS metabolism, as occurs in patients with

CYP7A1 deficiency [27], ileal resection and Crohn’s disease [28–30], and in subjects receiving cholestyramine treatment [31,32].

Bile salts, which circulate faster through the enterohepatic circulation after cholecystectomy, are signaling molecules for a number of recep-tors that control important metabolic pathways, including FXR and TGR5 [21,22,24,25]. For exam-ple, feeding BS prevents hepatic TG accumula-tion and VLDL production in mouse models of hypertriglyceridemia through reduction of Srebp1c and its target lipogenic genes [81]. The FXR–SHP–SREBP1c pathway was normally expressed in cholecystectomized mice, there-fore if the changes observed in TG metabo-lism in cholecystectomized mice were also BS-dependent, other unknown target(s) and signaling factors for BS molecules should be responsible for the accumulation of TGs in the liver and the increase in hepatic VLDL produc-tion [80]. It is plausible that metabolic effects mediated by the hormone FGF15/19 [21,22,82] or by the membrane-bound TGR5 recep-tor [21,22,39,83,84], which are both expressed in the GB mucosa [84,85] could be affected after ablation of the GB. We postulate that changes in FGF15/19 secretion or altered function of the membrane-bound TGR5 due to increased enterohepatic cycling of BS after cholecystec-tomy, may be critical mechanisms underly-ing the metabolic phenotype observed after surgical removal of the GB. Cholecystectomy could decrease the serum levels of FGF15/19, favoring the increase in serum and hepatic TG concentrations, as found in cholecystectomized mice or overstimulation of TGR5 secondary to increased enterohepatic cycling of BS (Figure 2). Supporting this hypothesis, for example, are the observations that FGF19 transgenic mice have reduced serum TG concentration and adi-posity [37] and administration of recombinant FGF19 to mice induces important metabolic changes in the whole animal, including a reduc-tion in serum TG levels, body weight, adiposity and insulin sensitivity, and an increase in energy expenditure [38].

It is remarkable that findings in cholecystect-omized mice are consistent with a number of stud-ies in humans. The majority of epidemiological studies aimed at the identification of associated risk factors of GBD show that higher than nor-mal serum TG concentration is associated with GBD [1–3], a finding that has been interpreted as a marker of the underlying metabolic abnor-mality that favors the production of lithogenic



bile during CH GS formation. However, Thijs et al. demonstrated that higher than normal serum TG concentration was an associated risk factor related to both pigmented and CH GS, diseases that have different pathogeneses [44]. This finding is not consistent with the prevail-ing view that higher than normal serum TG levels represents a metabolic marker of GS for-mation, or a risk factor of CH GS formation. In contrast, it suggests that the presence of GS and GB inflammation could alter a GB-derived

regulatory factor(s) of whole body metabolism. Experiments in mice showing increased hepatic VLDL production, serum and hepatic TG con-centration in cholecystectomized mice [80], are consistent with the study by Juvonen et al. show-ing increased serum VLDL-apoB and IDL-apoB, 3 years after cholecystectomy in GS patients [86]. We have also demonstrated that Chilean patients with normal weight and symptomatic GBD dis-ease have increased serum apoB, and TG-VLDL levels and increased hepatic TG concentration

Figure 2. effect of experimental cholecystectomy on triglyceride metabolism. Ablation of the gallbladder (GB) increases serum and hepatic triglyceride levels and VLDL production in mice [80]. Bile salt pool size, synthesis and composition remain unchanged, but the pool circulates faster in cholecystectomized mice. This experimental observation allows speculation that ablation of the GB could have important consequences for whole body metabolic homeostasis. The signaling factors responsible for this effect are still unknown. Since the GB is rich in the hormone FGF15/19 and FGR5, which are targeted by bile salts, it is possible that ablation of the GB decreases serum FGF15/19 and or alters the activity of FGR5 in the intestine and biliary tree, because of the increased cycling of the bile acid pool in cholecystectomized mice. This potential effect mediated by the absence of the GB could affect the energy balance of the whole body. It is possible that a decrease in serum FGF15/19 or increased bile acid interaction with TGR5 during the fast period of the diurnal cycle might determine a lower basal metabolic rate, favoring increased flux of FFAs to the liver, TG accumulation and increase VLDL production, as shown in cholecystectomized mice [80]. Experiments with transgenic FGF19 mice [37] and administration of recombinant FGF19 to mice [38], support the contention that ablation of the GB may not be innocuous and could affect whole body metabolic regulation, favoring metabolic syndrome-associated disease conditions. The dashed arrows represent a theoretical possibility. CH: Cholesterol; FFA: Free fatty acid; PL: Phospholipid; RM: VLDL remnant; TG: Triglyceride.

FGF15/19 TGR5?

Basal metabolic rate

Adipose tissue

FFAs

VLDL remnants + FFAs

TGs + apoB + PL + CH VLDL

VLDL

RM

VLDL

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and microsomal TG transfer protein activity [51]. These observations in humans are consistent with the results observed in cholecystectomized mice and support the hypothesis that a still unknown GB factor has signaling effects on sys-temic metabolism and on regulation of hepatic TG content and VLDL production.

Conclusion & future perspectiveIt is apparent that the pathogenesis of CH GS is very complex and frequently associated with the metabolic syndrome. Abnormalities of lipid metabolism in GBD are related to both primary abnormalities in the regulation of biliary lipid secretion and GB function, and abnormalities of whole body lipid metabolism commonly found in IR states, particularly obesity, Type 2 diabetes and NAFLD. The series of studies in humans and mice showing increased serum VLDL-apoB and serum TG concentrations after ablation of the GB, suggest that cholecystectomy and GBD disease may impact negatively on the metabolic homeostasis of the whole body. These observa-tions indicate that GB function has an impor-tant role in regulating systemic TG metabolism and energy balance. Thus, cholecystectomy, one of the most frequently performed surgical proce-dures worldwide, may indeed favor the develop-ment of the metabolic syndrome and associated

highly prevalent chronic disease conditions. These considerations raise the important ques-tion as to whether cholecystectomy might have a serious negative impact on public health [80,87]. Future prospective epidemiological and inter-vention studies should help to address the actual cause–effect relationship between abnormalities of serum lipids, hepatic TG metabolism, chole-cystectomy and GB function that emerge from epidemiological surveys in humans and experi-mental data in mice. Another important issue for further study is defining whether GB-derived FGF15/19 and/or altered TGR5 receptor activity in the enterohepatic organs and adipose tissue induce systemic effects on energy balance and metabolic regulation.

Financial & competing interests disclosureThis work was supported by Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT, Grants #1100020 to Flavio Nervi, #1110310 to Silvana Zanlungo, #1080325 to Juan Francisco Miquel and #1110712 to Attilio Rigotti). The authors have no other relevant affiliations or financial involvement with any organization or entity with a finan-cial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.

executive summary

Gallbladder function & lipid metabolism�n Gallbladder (GB) filling and contraction depends on fed–fasting cycling and regulates the dynamics of the enterohepatic circulation of

bile salts (BS). �n After feeding, the GB contracts and delivers concentrated bile into the intestine for digestion and absorption of lipids.�n Cholecystectomy does not affect lipid absorption.

Mechanisms of gallstone formation�n The primary pathogenic event is cholesterol (CH) supersaturation of bile. This occurs by increased hepatic secretion of CH and/or

decreased secretion of BS into bile. �n Pigment gallstones are related to an increased biliary concentration of unconjugated bilirubin.�n Pronucleating biliary proteins (mucins) facilitate CH crystal formation.�n Abnormal GB motility (frequent in obesity and diabetes) facilitates gallstone growth.

Insulin resistance, abnormal lipid metabolism & GB disease�n GB disease is more frequent in insulin resistance states.�n Insulin resistance increases biliary CH secretion and decreases GB motility.

Genetics of GB disease & abnormalities of lipid metabolism�n Lith genes have been identified in mice. GB disease is associated with polygenic factors.�n Genes could contribute 25% to the development of symptomatic GB disease in humans.�n Altered function of nuclear receptors (Farnesoid X receptor, liver X receptor) could favor the production of both dyslipidemia and biliary

CH supersaturation.

GB function & hepatic triglyceride metabolism�n In mice, ablation of GB increases VLDL production serum and hepatic triglyceride content.

Conclusion & future perspective�n Studies in humans could confirm the relevance of a GB factor(s) in the regulation of hepatic triglyceride metabolism.�n Cholecystectomy might not be innocuous and could favor metabolic syndrome-associated abnormalities.�n A GB-dependent factor could have a major role in the regulation of whole body metabolic homeostasis.



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