the secretion of bile salts and xenobiotics across the canalicular membrane of hepatocytes. These include members of the family of multidrug resistance (MDR) P‐glycoproteins such as MDR1 (ABCB1), MDR3 (ABCB4), and the bile salt export pump (BSEP or ABCB11). Within the family of MDR proteins, BSEP and MDR3 are two highly conserved members that are involved in the secretion of cholephilic compounds from the liver cell into the bile canaliculus.
BSEP constitutes the predominant bile salt efflux system of hepatocytes and mediates the cellular excretion of numerous conjugated bile salts such as taurine‐ or glycine‐conjugated cholate, chenodeoxycholate, and deoxycholate. MDR3 works as an ATP‐dependent phospholipid flippase, translocating phosphatidylcholine from the inner to the outer leaflet of the canalicular membrane. Canalicular phospholipids are then solubilized by canalicular bile salts to form mixed micelles, thereby protecting cholangiocytes from the detergent properties of bile salts.
In addition to these processes, MRP2, the only canalicular member of the MDR‐associated protein family, mediates the canalicular transport of glucuronidated and sulfated bile salts. MRP2 is the main driving force for bile salt‐independent bile flow through canalicular excretion of reduced glutathione. Furthermore, MRP2 transports a wide spectrum of organic anions, including bilirubin diglucuronide, glutathione conjugates, leukotriene C4, and divalent bile salt conjugates, as well as drug substrates such as chemotherapeutic agents and antibiotics.
MDR1 contributes to the canalicular excretion of drugs and other xenobiotics into bile, although its exact contribution has yet to be established. Its broad substrate specificity and its physiologic expression in various tissues with excretory and protective functions make MDR1 one of the major determinants of drug disposition and toxicity. Substrates are neutral and positively charged organic compounds and include various chemotherapeutic and immunosuppressant agents, antiarrhythmic drugs, HIV protease inhibitors, and antifungals.
Transcriptional regulation of BSEP and MDR3 is mediated by FXR and their activation leads to increased bile salt efflux and the formation of mixed micelles in the biliary tree during cholestatic episodes, thereby preventing the toxic effects of bile salts on hepatocytes and cholangiocytes. In addition, FXR has been shown to induce MRP2 expression, which might constitute another compensatory mechanism during cholestasis. In contrast, MDR1 is upregulated via the pregnane X receptor (PXR), which in addition to endogenous ligands is activated by different xenobiotics. This pathway is part of a general cellular mechanism of detoxification, because MDR1 is the key transporter protein involved in the cellular efflux of numerous drugs and xenobiotics.
Drug Metabolism
The liver has a major role in drug metabolism. The main hepatocyte enzymes involved in metabolism belong to the cytochrome P450 group, a large family of related enzymes housed in the smooth endoplasmic reticulum of the hepatocyte. Metabolism is often divided into two phases of biochemical reaction. Phase 1 involves reduction, hydrolysis or oxidation of the drug, the latter being the most common process. After phase 1 reactions, the resulting drug metabolite is still often chemically active. Phase 2 metabolism involves conjugation with glutathione, methyl or acetyl groups, which usually occurs in the cytoplasm of the hepatocyte and makes the metabolite more hydrosoluble. This facilitates excretion as well as decreasing the pharmacologic activity. Some drugs may undergo just phase 1 or just phase 2 metabolism, but more often the drug will undergo phase 1 and then phase 2 sequentially.
Many factors can affect liver metabolism of drugs. The numbers of hepatocytes and enzyme activity can decline, with a reduction in the metabolic potential of the liver, following aging, acute and chronic liver disease, and conditions that affect hepatic blood flow. Metabolism can also be altered due to genetic deficiency of a particular enzyme and secondary to the use of other drugs as well as dietary and environmental factors. Capillarization of sinusoids during chronic liver disease increases the bioavailability of drugs at high hepatic extraction, possibly increasing the side effects. Drug‐induced liver injury is a major clinical problem, is often favored by exposure to a combination of drugs and, at times, may be mediated by immunologic mechanisms.
Bile Formation, Secretion and the Enterohepatic Circulation
Bile is a complex secretion that originates from hepatocytes and is modified distally by absorptive and secretory transport systems in the bile duct epithelium. Bile formation by the hepatocytes involves secretion of osmotically active inorganic and organic anions into the canalicular lumen, followed by passive water movement. Bile then enters the gallbladder where it is concentrated or is delivered directly to the bowel.
Bile comprises about 95% water in which are dissolved a number of endogenous constituents, including bile salts, bilirubin, phospholipid, cholesterol, amino acids, steroids, enzymes, porphyrins, vitamins, and heavy metals, as well as exogenous drugs, xenobiotics and environmental toxins [4]. Lipophilic constituents are in solution in mixed micelle composed of BAs, phospholipids, and cholesterol.
Bile is essential for several important functions:
the excretion of potentially harmful exogenous lipophilic substances, as well as the excretion of endogenous substrates such as bilirubin and bile salts;
the digestion and absorption of lipid in the gut by bile salts;
cholesterol homeostasis, by facilitating intestinal cholesterol absorption and, on the other hand, promoting cholesterol elimination;
the excretion of immunoglobulin A (IgA) and inflammatory cytokines, thus protecting the organism from enteric infections;
signaling properties of the BAs in the liver and the intestine, which are mediated by nuclear BA receptors such as FXR, PXR and vitamin D receptor (VDR), as well as by membrane a5b1 integrin, epidermal growth factor receptor, and sphingosine‐1‐phosphate receptor 2.
Our understanding of cholestatic liver diseases has been profoundly advanced by the discovery of nuclear receptors for BA signaling and their role in hepatobiliary excretory function and the adaptive changes counteracting the liver injury caused by retained, potentially toxic, and proinflammatory BAs. Ligand‐activated nuclear receptors such as FXR control a broad range of metabolic processes, including hepatic BA transport and metabolism, lipid and glucose metabolism, drug disposition, as well as liver regeneration, inflammation, fibrosis, cell differentiation, and tumor formation. Moreover, FXR has anti‐inflammatory and immunomodulatory actions and controls intestinal integrity and permeability, as well as gut microbiota. Conversely, the gut microbiota metabolizes BAs, with formation of secondary BAs that, in turn, modulate BA signaling. Based on these broad physiologic effects in the liver and intestine, drugs targeting FXR and TGR5 therefore open important perspectives for pharmacotherapy of cholestatic and metabolic liver disorders, including the complications of liver cirrhosis such as portal hypertension and hepatocellular carcinoma (HCC).
In addition, BAs stimulate glucagon‐like peptide (GLP)‐1 production via TGR5 activation. GLP‐1 is known to promote insulin secretion and thus regulate glucose homeostasis. Because GLP‐1 mimetics and receptor agonists are currently under clinical development and have shown promise in improving glucose homeostasis in diabetes, BA‐based TGR5 agonists may be a potential therapeutic to stimulate GLP‐1 secretion in diabetic patients.
Bile Acid Synthesis and Metabolism
BAs are synthesized from cholesterol. In humans, the “primary” BAs are cholic acid (CA) and chenodeoxycholic acid (CDCA). Before secretion into the bile, both CA and CDCA are conjugated to the amino group of taurine or glycine. Conjugation enhances the hydrophilicity of the BA, the major function of this being to decrease the passive diffusion of BAs across the cell membranes during their transit through the biliary tree and intestine. Therefore, conjugated BAs are absorbed
