Current Drug Targets - Immune, Endocrine & Metabolic Disorders - Volume 5, Issue 2, 2005

New Insights Into Bile Formation: from Secretion to Therapeutic Implications

Biliary lipid secretion is driven by bile salts, the primary metabolites of cholesterol. Transport of bile salts as well as phospholipids and cholesterol is mediated by ATP Binding Cassette (ABC) transporters. Expression of these transporters is regulated in a coordinate fashion by a set of nuclear hormone receptors explaining the old observation of coupling between bile salt secretion and biliary lipid secretion. Although it is now clear which proteins are involved, the molecular mechanism of biliary lipid secretion is still unresolved. In addition, scarce information is available about the systems responsible for intracellular transport of cholesterol, phospholipid and bile salt. These issues form the subject of the review.

Formation of bile and generation of bile flow are driven by the active secretion of bile salts (BS), lipids and electrolytes into the canalicular and bile duct lumens followed by the osmotic movement of water. Although the transporting proteins involved in solute secretion have been cloned and their coordinated interplay defined both in health and disease, boosted by the discovery of the aquaporin water channels, only recently has considerable attention been addressed to the mechanism by which water, the major component of bile (>95%), moves across the hepatobiliary epithelia. This review summarizes the novel acquisitions in liver membrane water transport and functional participation of aquaporin water channels in multiple aspects of hepatobiliary fluid balance. Emerging evidences suggesting involvement of aquaporins in the metabolic homeostasis of the hepatobiliary tract are also discussed.

The function of the gallbladder is not only to store bile, but also to concentrate it during the interdigestive phase by means of salt-dependent water reabsorption. On the contrary, secretions of water and salt take place during the digestive phase. Dysregulation of ion absorption or secretion are common in many gallbladder diseases, such as colelithiasis. Transepithelial absorptions are determined by the Na +/K+ pump on the basolateral membrane, and by several apical membrane Na+-coupled transporters. Among these, some isoforms of Na+/H+ and Cl-/HCO3 - exchangers have been studied. The presence of a Na+-Cl- simport has been molecularly and functionally characterized in some animal species. The ion transepithelial secretion is mainly dependent on an apical chloride transport attributable to a CFTR-like cAMPactivated channel with high permeability to HCO3 -. The apical membrane electrical potential is one of the factors influencing anion secretion and is maintained by the activity of cAMP-dependent K+ channels. The regulation of the activity of these channels is complex, because of their sensitivity to voltage, and to intracellular calcium and pH. The coordinated interplay underlying the regulation of transporters and channels needs to be clarified yet, as well as the interactions between transporters, channels and aquaporins.

At least one third of the bile flow is driven osmotically by the amount of hepatic glutathione excreted into canalicular spaces. Beyond the importance of this secretory mechanism for bile formation, the excretion of glutathione is an important way to discharge toxic anionic compounds deriving from liver metabolism of exogenous and endogenous substances. Thus, biliary secretion of glutathione and its conjugates really works as a major detoxification system for the hepatocytes. Derangement of hepatic and/or biliary glutathione status can occur in several experimental animal models of liver injury and in human diseases. In the present review, we will focus on mechanisms of bile glutathione efflux and changes associated with cholestatic conditions. Novel findings on the role of water channels and of the multidrug resistant proteins in bile salt-independent bile formation, will also be discussed. New routes of intervention to modify bile flow for therapeutic purposes are considered.

Family and twin studies as well as animal studies indicate that gallstone disease is, in part, genetically determined. Recently new single gene defects have been identified in specific patients with cholesterol and pigment gallstones. Examples include low phospholipid-associated cholelithiasis due to mutations of the gene encoding the hepatocanalicular phosphatidylcholine transporter, and pigment stones in association with mutations of the ileal bile salt transporter gene. Here we summarize the evidence for common genetic determinants of human gallstone disease in general and provide an inventory of human lithogenic genes. The precise understanding of such genes and their molecular mechanisms will establish new targets for rational drug design for this exceptionally prevalent and economically significant digestive disease.

Fatty acid bile acid conjugates (FABACs) are novel synthetic lipid molecules, which were designed for the treatment of cholesterol gallstones. The rationale was to combine a cholesterol solubilizing moiety (a saturated fatty acid) with a bile acid (cholic acid) as a vehicle to enable secretion into bile and entry into the enterohepatic circulation. An amide bond was used to provide stability against intestinal degradation. Initial in vitro studies showed that FABACs are indeed cholesterol solubilizers, able to prevent biliary cholesterol crystallization. Arachidyl-amido-cholanoic acid (Aramchol) was found to be the most potent FABAC in these studies. Animal studies revealed that Aramchol was absorbed after oral administration and could prevent cholesterol crystallization as well as dissolve preformed crystals in rodents fed a lithogenic diet. In gallstone susceptible mice, Aramchol prevented gallstone formation and dissolved gallstones. FABACs were found to be metabolically active substances, also able to decrease blood cholesterol, atherosclerotic plaques and fat accumulation in the liver in several animal species. The underlying mechanisms of action are under active investigation, and several effects, e.g. on cholesterol and bile salt metabolizing enzymes as well as cholesterol efflux from cells have been discovered. These findings are, however, only the beginning of our understanding of the metabolic actions as well as the potential of use of FABACs as therapeutic agents.

The first step in cholesterol gallstone disease is precipitation of cholesterol crystals in bile. In gallbladder bile, cholesterol is normally solubilized together with bile salts and phospholipids to form mixed micellar structures. When cholesterol in bile is in excess, vesicles (i.e. phospholipid-cholesterol globular structures: liquid crystals) form which become supersaturated in cholesterol. Early aggregation and precipitation of cholesterol molecules into sub microscopic nuclei occurs from these super saturated vesicles. This crucial step is followed by precipitation and agglomeration of cholesterol crystals which then become visible at light microscopy. Here we describe the mechanism of cholesterol crystallization and its modulation in vivo and in vitro. Recent advances on the role of ursodeoxycholate as an agent preventing the precipitation of cholesterol crystals in bile will be highligthed.

Gallstone disease in the Western world has an estimated prevalence of 10-15% and more than 75% are cholesterol-enriched gallstones. Defective gallbladder motility has been identified as an important pathogenic factor for cholesterol gallstone disease. Various agents may enhance or impair postprandial gallbladder motility, and their effects on interdigestive gallbladder and intestinal motility should also be taken into account. Patients in high-risk situations for gallstone disease, and those chronically treated with drugs inhibiting gallbladder motility (e.g. somatostatin analogues) may benefit from improving gallbladder motility with prokinetic agents. Whether such a strategy can really prevent gallstone formation is still unknown, long-term studies are lacking so far. The efficacy of bile acid therapy with UDCA for gallstone dissolution or for prevention in high risk patients is limited and hampered by high recurrence rates. The efficacy of UDCA in prevention of colics or gallstone related complications in symptomatic patients with gallbladder stones with contraindications for operation or on the waiting list should be explored further, since several retrospective studies showed favourable outcomes with this strategy.

Acute biliary pancreatitis, caused by macroscopic cholesterol gallstones or microlithiasis, is often a severe disease with considerable morbidity and mortality. Formation of cholesterol gallstones and microlithiasis is caused by cholesterol crystallization from cholesterol supersaturated gallbladder bile. Particularly patients with fast and extensive crystallization, due to highly concentrated bile, low biliary phospholipid contents and gallbladder mucin hypersecretion seem at risk for pancreatitis. Patients who suffered from acute biliary pancreatitis should undergo cholecystectomy as secondary prevention strategy. For patients at high surgical risk, endoscopic sphincterotomy may be an appropriate alternative. Pharmacological manipulation of biliary lipids by the hydrophilic bile salt ursodeoxycholic acid is reserved for patients with recurrent pancreatitis despite previous cholecystectomy or sphincterotomy, or with contraindications to surgical and endoscopic treatment. Maintenance therapy with ursodeoxycholic acid is however a very effective secondary prevention strategy. Potentially, secondary prevention of acute biliary pancreatitis could also be achieved through decreasing biliary mucin contents by UDCA, NSAIDs or N-acetylcystein, or through achieving bile dilution (currently not feasible).

Gene therapy constitutes a great promise for the treatment of inherited diseases as well as cancer. Although the principle is extremely elegant, reality proves that several important problems remain to be solved before gene therapy becomes a standard application for these conditions. Meanwhile, and because of these problems alternatives are being considered as well. For the treatment of hepatic inherited disorders, hepatocyte transplantation has proven to be an attractive alternative, although this form of therapy also remains experimental at this moment. Problems and possibilities are discussed with the inherited disease, Progressive Familial Intrahepatic Cholestasis, as an example.

Gallstone disease is exceptionally common, occurring especially in Western populations, with cholesterol gallstones predominating. Currently, it is believed that obesity is the most consistent and important risk factor for the development of cholesterol gallstones. Obesity has been shown to be associated with the supersaturation of bile with cholesterol because of increased hepatic secretion of the sterol. In accord with current information from experimental studies, leptin appears to be involved in biliary cholesterol secretion and cholesterol gallstone formation in humans. This review summarizes the current information on the role of obesity in biliary lipid secretion as well as the effect of leptin and its potential consequences for gallstone formation and therapy in the obese.

Several processes are involved in control of plasma cholesterol levels, e.g., intestinal cholesterol absorption, endogenous cholesterol synthesis and transport and bile acid synthesis. Adaptation of either of these processes allows the body to adapt to changes in dietary cholesterol intake. Disturbances herein may lead to hypercholesterolemia and increase the risk for atherosclerosis. Several approaches are available for plasma-cholesterol lowering therapies, particularly aimed at reduction of low-density lipoprotein (LDL) cholesterol. Currently used therapies aim to decrease (hepatic) cholesterol synthesis, to inhibit cholesterol absorption or to stimulate bile acid synthesis. The latter can be achieved by reducing bile acid absorption to alleviate the negative feedback control exerted by bile acids circulating in the body. Approaches to directly stimulate bile acid synthesis may become available. Novel drugs should be tested on the efficiency to influence their actual targets. Several techniques are available to measure cholesterol absorption, cholesterol synthesis and bile acid synthesis and absorption in vivo in human subjects. The most reliable techniques are based on the use of stable isotopes and mass spectrometry. This paper provides a condensed background on physiological parameters that determine cholesterol homeostasis, and potential new mechanisms of drug action and focuses, especially, on new techniques to monitor the effects of drugs in humans.

Cardiovascular disease is a major cause of mortality and morbidity in individuals with obesity, type 2 diabetes and the metabolic syndrome. The mechanisms for this are partially understood, but include increased atherosclerosis, hypercoagulability and increased hypertension. Epidemiological data suggests however, that a component of the excess cardiovascular mortality occurs independently of underlying coronary artery disease. Indeed, diabetes is an independent risk factor for the development of heart failure and the mechanisms responsible remain to be clarified. Insulin resistance in skeletal muscle, adipose tissue and the liver are widely recognized features of obesity and type 2 diabetes, and contribute to the pathogenesis of impaired glucose homeostasis. Insulin resistance has also been described in the vasculature, and may contribute to endothelial dysfunction and atherosclerosis. The heart is an insulin responsive organ and less is known about whether or not the heart becomes insulin resistant in diabetes and what the pathogenic consequences of this might be. This review will discuss the currently available evidence from human and animal studies, that the heart may become insulin resistant in obesity and type 2 diabetes. The potential consequences of this on cardiac structure, function and metabolism will be discussed as well as recent data from transgenic mice with perturbed cardiac insulin sensitivity that have shed interesting new insight into potential mechanisms linking cardiac insulin resistance with myocardial dysfunction in diabetes.

The pathogenesis of type 1 diabetes is multifactorial, involving genetic susceptibility, autoimmune mechanisms, and environmental factors. This article will focus on two main strategies for altering the underlying disease process in type 1 diabetes. The first strategy is to identify individuals at risk for the development of diabetes and to halt the immune process before it leads to overt clinical disease. Promising in vitro and animal studies with nicotinamide, parenteral insulin, and oral insulin led to large clinical prevention studies, such as the European Nicotinamide Diabetes Intervention Trial and the Diabetes Prevention Trial (DPT-1). These studies failed to show that nicotinamide and insulin prevented the disease in at risk relatives of patients with type 1 diabetes and left many questions unanswered. The second strategy focuses on intervention shortly after diagnosis in order to arrest the destruction of b cells and to preserve residual b-cell function as long as possible. Cyclosporin was an effective immunosuppressive but was rejected as a potential treatment for type 1 diabetes because of its renal toxicity. Recently, more attention has been focused on an anti-CD3 antibody, on DiaPep277, and on glutamic acid decarboxylase (GAD). Animal studies and small short-term human trials with these compounds have suggested that they may be effective interventions in patients recently diagnosed with type 1 diabetes.

The innate immune system is the oldest mammalian defence against invading micro-organisms and provides the first line of defence against them, however until recently a detailed understanding of its complexity has been lacking. This review describes recent advances that have been made in understanding the components of the innate immune system, including the pathogen sensing mechanisms, receptor and intracellular signalling pathways, linkage to the acquired immune system, and effectors of the innate immune response. These discoveries have created an opportunity for the development of novel drugs through the identification of targets for rational drug design. The opportunity for the development of novel anti-inflammatory and antimicrobial drugs through modulation of pro-inflammatory or antimicrobial signals within the innate immune system, are discussed. A more detailed understanding of the effectors of the innate immune system is providing an opportunity for the design of effector mimetics as novel antimicrobial drugs. The innate immune system is providing the basis for much-needed alternative approaches to controlling infection and inflammation in human medicine.