Canalicular secretion of bile salts is a vital function of the vertebrate liver, yet the molecular identity of the involved ATP-dependent carrier protein has not been elucidated. We cloned the full-length cDNA of the sister of P-glycoprotein (spgp; M r ϳ160,000) of rat liver and demonstrated that it functions as an ATP-dependent bile salt transporter in cRNA injected Xenopus laevis oocytes and in vesicles isolated from transfected Sf9 cells. The latter demonstrated a 5-fold stimulation of ATP-dependent taurocholate transport as compared with controls. This spgp-mediated taurocholate transport was stimulated solely by ATP, was inhibited by vanadate, and exhibited saturability with increasing concentrations of taurocholate (K m Ӎ 5 M). Furthermore, spgp-mediated transport rates of various bile salts followed the same order of magnitude as ATP-dependent transport in canalicular rat liver plasma membrane vesicles, i.e. taurochenodeoxycholate > tauroursodeoxycholate ‫؍‬ taurocholate > glycocholate ‫؍‬ cholate. Tissue distribution assessed by Northern blotting revealed predominant, if not exclusive, expression of spgp in the liver, where it was further localized to the canalicular microvilli and to subcanalicular vesicles of the hepatocytes by in situ immunofluorescence and immunogold labeling studies. These results indicate that the sister of P-glycoprotein is the major canalicular bile salt export pump of mammalian liver.Bile formation is an important function of vertebrate liver (1). It is mediated by hepatocytes that generate bile flow within bile canaliculi by continuous vectorial secretion of bile salts and other solutes across their canalicular (apical) membrane (2). Studies in isolated membrane vesicles of rat and human livers have shown that canalicular bile salt transport is an ATP-dependent process (3-7). However, the molecular identity of the primary active canalicular bile salt transporter or bile salt export pump (BSEP) 1,2 has not yet been elucidated (8, 9). Although the canalicular ecto-ATPase has been proposed as a possible candidate (9, 10), other investigations have provided evidence that BSEP of mammalian liver is an ABC (ATP binding cassette)-type of membrane transporter (11,12). This assumption has recently been further supported by the cloning of an ATP-dependent bile salt transporter from Saccharomyces cerevisiae (13). This yeast bile salt transporter (BAT1) belongs to a subgroup of ABC-type proteins that includes also the canalicular multiorganic anion transporter or multidrug resistance protein MRP2 (human)/mrp2 (rat) (14 -16). Although MRP2/mrp2 mediates canalicular excretion of a broad range of divalent amphipathic anionic conjugates (1, 14, 17), it does not transport primary bile salts such as taurocholate or glycocholate (1, 18). Therefore, we designed degenerate oligonucleotide primers spanning the Walker A and B motifs of ABC proteins and performed reverse transcription-polymerase chain reactin with total rat liver mRNA. One of the amplified fragments revealed an 88% identity with the p...

Bile acids, the water-soluble, amphipathic end products of cholesterol metabolism, are involved in liver, biliary, and intestinal disease. Formed in the liver, bile acids are absorbed actively from the small intestine, with each molecule undergoing multiple enterohepatic circulations before being excreted. After their synthesis from cholesterol, bile acids are conjugated with glycine or taurine, a process that makes them impermeable to cell membranes and permits high concentrations to persist in bile and intestinal content. The relation between the chemical structure and the multiple physiological functions of bile acids is reviewed. Bile acids induce biliary lipid secretion and solubilize cholesterol in bile, promoting its elimination. In the small intestine, bile acids solubilize dietary lipids promoting their absorption. Bile acids are cytotoxic when present in abnormally high concentrations. This may occur intracellularly, as occurs in the hepatocyte in cholestasis, or extracellularly, as occurs in the colon in patients with bile acid malabsorption. Disturbances in bile acid metabolism can be caused by (1) defective biosynthesis from cholesterol or defective conjugation, (2) defective membrane transport in the hepatocyte or ileal enterocyte, (3) defective transport between organs or biliary diversion, and (4) increased bacterial degradation during enterohepatic cycling. Bile acid therapy involves bile acid replacement in deficiency states or bile acid displacement by ursodeoxycholic acid, a noncytotoxic bile acid. In cholestatic liver disease, administration of ursodeoxycholic acid decreases hepatocyte injury by retained bile acids, improving liver tests, and slowing disease progression. Bile acid malabsorption may lead to high concentrations of bile acids in the colon and impaired colonic mucosal function; bile acid sequestrants provide symptomatic benefit for diarrhea. A knowledge of bile acid physiology and the perturbations of bile acid metabolism in liver and digestive disease should be useful for the internist.

Bile acids and bile alcohols in the form of their conjugates are amphipathic end products of cholesterol metabolism with multiple physiological functions. The great variety of bile acids and bile alcohols that are present in vertebrates are tabulated. Bile salts have an enterohepatic circulation resulting from efficient vectorial transport of bile salts through the hepatocyte and the ileal enterocyte; such transport leads to the accumulation of a pool of bile salts that cycles between the liver and intestine. Bile salt anions promote lipid absorption, enhance tryptic cleavage of dietary proteins, and have antimicrobial effects. Bile salts are signaling molecules, activating nuclear receptors in the hepatocyte and ileal enterocyte, as well as an increasing number of G-protein coupled receptors. Bile acids are used therapeutically to correct deficiency states, to decrease the cholesterol saturation of bile, or to decrease the cytotoxicity of retained bile acids in cholestatic liver disease.

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The TGR5 receptor (or GP-BAR1, or M-BAR) was characterized ten years ago as the first identified G-coupled protein receptor specific for bile acids. TGR5 gene expression is widely distributed, including endocrine glands, adipocytes, muscles, immune organs, spinal cord, and the enteric nervous system. The effect of TGR5 activation depends on the tissue where it is expressed and the signalling cascade that it induces. Animal studies suggest that TGR5 activation influences energy production and thereby may be involved in obesity and diabetes. TGR5 activation also influences intestinal motility. This review provides an overview of TGR5-bile acid interactions in health as well as the possible involvement of TGR5 in human disease.

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conceptions are rare, and provocative judgments stand the test of time. Let us also hope that this effort will be both useful and entertaining. Each of the topics discussed merits at least a full length article, so there will be, of necessity in many instances, consideration of only what we perceive to be the highlights. THE EBB AND FLOW OF MEDICAL INTEREST IN BILE ACIDSAfter the elucidation of the true chemical structure of bile acids in 1932 (see below), there was little interest in bile acids in the Western world. One exception to this statement was the laboratory of Siegfried Thannhauser, who wrote the fi rst textbook of metabolic biochemistry in Germany. He studied cholesterol and bile acid balance in the biliary fi stula dog ( 1 ). During this time, bile acids were sold as liver tonics and laxatives, but there were no placebo-controlled studies showing effi cacy. Indeed, bile acids were considered by the medical profession to have no useful therapeutic properties. The tri-oxo derivative of cholic acid (called "dehydrocholic acid") was known to induce bile fl ow in animals ( 2 ), and was occasionally used to stimulate bile fl ow in patients; but again there were no controlled studies showing effi cacy in hepatobiliary disease.

TGR5, a metabotropic receptor that is G-protein-coupled to the induction of adenylate cyclase, has been recognized as the molecular link connecting bile acids to the control of energy and glucose homeostasis. With the aim of disclosing novel selective modulators of this receptor and at the same time clarifying the molecular basis of TGR5 activation, we report herein the biological screening of a collection of natural occurring bile acids, bile acid derivatives, and some steroid hormones, which has resulted in the discovery of new potent and selective TGR5 ligands. Biological results of the tested collection of compounds were used to extend the structure-activity relationships of TGR5 agonists and to develop a binary classification model of TGR5 activity. This model in particular could unveil some hidden properties shared by the molecular shape of bile acids and steroid hormones that are relevant to TGR5 activation and may hence be used to address the design of novel selective and potent TGR5 agonists.