CHAPTER OUTLINE
Enterohepatic Circulation and Bile Acid Modification
Regulation of Bile Acid Homeostasis
High-Yield Terms
Classic (neutral) pathway: major pathway for bile acid synthesis, initiated from cholesterol via the action of CYP7A1, both cholic and chenodeoxycholic acid synthesized via the classic pathway
Acidic pathway: alternative, minor pathway for bile acid synthesis initiated via the action of sterol 27-hydroxylase (CYP27A1), only chenodeoxycholic acid synthesized via the acidic pathway
CYP7A1: 7α-hydroxylase, rate-limiting enzyme in the classic pathway of bile acid synthesis
Primary bile acids: the end products of hepatic bile acid synthesis and includes cholic acid and chenodeoxycholic acid
Secondary bile acids: products of the actions of intestinal bacteria on the primary bile acids generating deoxycholate (from cholate) and lithocholate (from chenodeoxycholate)
Enterohepatic circulation: the circulation of bile acids, or other substances, from the liver to the gallbladder, followed by secretion into the small intestine, absorption by intestinal enterocytes, and transport back to the liver
Farnesoid X receptors (FXR): nuclear receptors for bile acids and bile acid metabolites; regulate the expression of genes involved in bile acid synthesis
Guggulsterone: the term for any resin collected by tapping the trunk of a tree is called guggul (or guggal) and the lipid component of this extract is called guggulsterone (also called guggul lipid)
Although the acidic pathway is not a major route for human bile acid synthesis, it is an important one as demonstrated by the phenotype presenting in a newborn harboring a mutation in the CYP7B1 gene. This infant presented with severe cholestasis (blockage in bile flow from liver) with cirrhosis and liver dysfunction.
Bile Acid Synthesis Pathways
The end products of cholesterol utilization are the bile acids. Indeed, synthesis of bile acids is one of the predominant mechanisms for the excretion of excess cholesterol. However, the excretion of cholesterol in the form of bile acids is insufficient to compensate for an excess dietary intake of cholesterol.
Although several of the enzymes involved in bile acid synthesis are active in many cell types, the liver is the only organ where their complete biosynthesis can occur. Although bile acid synthesis constitutes the route of catabolism of cholesterol, these compounds are also important in the solubilization of dietary cholesterol, lipids, and essential nutrients, thus promoting their delivery to the liver. Synthesis of a full complement of bile acids requires 17 individual enzymes and occurs in multiple intracellular compartments that include the cytosol, endoplasmic reticulum (ER), mitochondria, and peroxisomes. The genes encoding several of the enzymes of bile acid synthesis are under tight regulatory control to ensure that the necessary level of bile acid production is coordinated to changing metabolic conditions. Given the fact that many bile acid metabolites are cytotoxic, it is understandable why their synthesis needs to be tightly controlled.
The major pathway for the synthesis of the bile acids is initiated via hydroxylation of cholesterol at the 7 position via the action of cholesterol 7α-hydroxylase (CYP7A1) which is an ER-localized enzyme. CYP7A1 is a member of the cytochrome P450 family of metabolic enzymes. This pathway is depicted in highly abbreviated fashion in Figure 27-1. The pathway initiated by CYP7A1 is referred to as the “classic” or “neutral” pathway of bile acid synthesis.
FIGURE 27-1: Synthesis of the 2 primary bile acids, cholic acid (CA) and chenodeoxycholic acid (CDCA). The reaction catalyzed by the 7α-hydroxylase (CYP7A1) is the rate-limiting step in bile acid synthesis. Expression of CYP7A1 occurs only in the liver. Conversion of 7α-hydroxycholesterol to the bile acids requires several steps not shown in detail in this image. Only the relevant cofactors needed for the synthesis steps are shown. Sterol 12α-hydroxylase (CYB8B1) controls the synthesis of cholic acid and as such is under tight transcriptional control (see text). Reproduced with permission of themedicalbiochemistrypage, LLC.
There is an alternative pathway that involves hydroxylation of cholesterol at the 27 position by the mitochondrial enzyme sterol 27-hydroxylase (CYP27A1). This alternative pathway is referred to as the “acidic” pathway of bile acid synthesis. In humans this alternative bile acid synthesis pathway accounts for no more than 6% of total bile acid production. The bile acid intermediates generated via the action of CYP27A1 are subsequently hydroxylated on the 7 position by oxysterol 7α-hydroxylase (CYP7B1). Numerous disorders have been identified that result from mutations in the genes of bile synthesis (Clinical Box 27-1 and Table 27-1)
CLINICAL BOX 27-1: INBORN ERRORS IN BILE ACID SYNTHESIS
Several inborn errors in metabolism are due to defects in genes of bile acid synthesis and are associated with liver failure in early childhood to progressive neuropathies in adults. Metabolic disorders associated with bile acid synthesis and metabolism are broadly classified as primary or secondary disorders. Primary disorders involve inherited deficiencies in enzymes responsible for catalyzing key reactions in the synthesis of cholic and chenodeoxycholic acids. Bile acid disorders classified as secondary refer to metabolic defects that impact primary bile acid synthesis but that are not due to defects in the enzymes responsible for synthesis. Secondary disorders of bile acid metabolism include peroxisomal disorders such as Zellweger syndrome and related peroxisomal biogenesis disorders and Smith-Lemli-Opitz syndrome which results from a deficiency of 7-dehydrocholesterol reductase (DHCR7) (Table 27-1).
High-Yield Concept
The hydroxyl group on cholesterol at the 3 position is in the β-orientation and must be epimerized to the α-orientation during the synthesis of the bile acids. This epimerization is initiated by conversion of the 3β-hydroxyl to a 3-oxo group catalyzed by 3β-hydroxy-Δ5-C27-steroid oxidoreductase (HSD3B7). Evidence that this reaction is critical for bile acid synthesis and function is demonstrated in children harboring mutations in the HSD3B7 gene. These children develop progressive liver disease that is characterized by cholestatic jaundice.
Following the action of HSD3B7 the bile acid intermediates can proceed via two pathways whose end products are chenodeoxycholic acid (CDCA) and cholic acid (CA). The distribution of these 2 bile acids is determined by the activity of sterol 12α-hydroxylase (CYP8B1). The intermediates of the HSD3B7 reaction that are acted on by CYP8B1 become CA and those that escape the action of the enzyme will become CDCA. Therefore, the activity of the CYP8B1 gene will determine the ratio of CA to CDCA. The CYP8B1 gene is subject to regulation by bile acids themselves via their ability to regulate the action of the nuclear receptor FXR.
The most abundant bile acids in human bile are chenodeoxycholic acid (45%) and cholic acid (31%). These are referred to as the primary bile acids. Before the primary bile acids are secreted into the canalicular lumen they are conjugated via an amide bond at the terminal carboxyl group with either of the amino acids glycine or taurine. These conjugation reactions yield glycoconjugates and tauroconjugates, respectively (Figure 27-2). This conjugation process increases the amphipathic nature of the bile acids making them more easily secretable as well as less cytotoxic. The conjugated bile acids are the major solutes in human bile.
FIGURE 27-2: Structures of the conjugated forms of cholic acid.
Enterohepatic Circulation and Bile Acid Modification
Following secretion by the liver, the bile acids enter the bile canaliculi which join with the bile ductules which then form the bile ducts (Figure 27-3). Bile acids are carried from the liver through these ducts to the gallbladder, where they are stored for future use. In the gallbladder bile acids are concentrated up to 1000 fold. Excess bile accumulation in the bile canaliculi and the gallbladder is a primary cause of gallstones (Clinical Box 27-2). Following stimulation by food intake the gallbladder releases the bile into the duodenum, via the action of the gut hormone cholecystokinin, CCK (Chapter 44), where they aid in the emulsification of dietary lipids.
FIGURE 27-3: Structure of a liver portal lobule. Liver lobules represent small subdivisions of the liver defined at the histological level as opposed to the four major anatomic lobes. Kuppffer cells are specialized liver-resident macrophages. Reproduced with permission of themedicalbiochemistrypage, LLC.
Within the intestines the primary bile acids are acted upon by bacteria and undergo a deconjugation process that removes the glycine and taurine residues. The deconjugated bile acids are either excreted (only a small percentage) or reabsorbed by the gut and returned to the liver. Anaerobic bacteria present in the colon modify the primary bile acids converting them to the secondary bile acids, identified as deoxycholate (from cholate) and lithocholate (from chenodeoxycholate). Both primary and secondary bile acids are reabsorbed by the intestines and delivered back to the liver via the portal circulation. Indeed, as much as 95% of total bile acid synthesized by the liver is absorbed by the distal ileum and returned to the liver. This process of secretion from the liver to the gallbladder and then to the intestines and finally reabsorption is termed the enterohepatic circulation.
Regulation of Bile Acid Homeostasis
Bile acids, in particular chenodeoxycholic acid (CDCA) and cholic acid (CA), can regulate the expression of genes involved in their synthesis, thereby creating a feedback loop. This regulatory pathway involves a class of nuclear receptors called the farnesoid X receptors, FXR. The FXR genes are expressed at highest levels in the intestine and liver. Bile acids and bile acid metabolites bind to and activate the transcriptional activity of FXR.
