Intracellular transport of bile acids

Intracellular transport of bile acids mainly takes place through the cytoskeleton and intracellular structures (Golgi apparatus, endoplasmic reticulum). Here, too, cholestatic factors can prove to be damaging. Microfilaments are contractile elements not only is the intracellular transport of the bile acids disturbed, but the peristaltic activity of the canaliculi (so-called paralytic cholestasis within the lolsules) is also reduced if the functional capacity of those microfilaments becomes diminished. 

Agellon, L.B., Torchia, E.C. 2000. Intracellular transport of bile acids. Biochim. Biophys. Acta 1486 ... 

Bile acids, which have been taken up by the liver, are transported across the hepatocyte and secreted into the bile canaliculus. Newly synthesized bile acids, in a small amount just sufftcient to balance the fraction lost by fecal excretion, join recycled bile acids for biliary secretion. Intracellular bile acid transport may be mediated by carrier proteins (B24, S42). The detailed mechanism of biliary secretion of bile acids and other organic anions into the bile canaliculus is not yet clear (B24). Possible mechanisms include vectorial vesicular transport, fticilitated diflusion, or an energy-requiring carrier-mediated transport process (B24). 

The relative content of the dietary fat components varies with different sources but generally the physico-chemical properties are rather similar. For absorption to take place the physico-chemical properties of the fat have to be changed. This takes place as a consequence of the lipolytic activity in the intestinal tract and the addition of bile to chyme. Through lipolytic enzymes the dietary lipids are converted to more polar products. Bile contributes bile salt-phospholipid-cholesterol aggregates to the intestinal content (cf. Chapter 13). The concerted action of these agents is the formation of lipid products in a physical state which allows them to be transported into the enterocyte membrane and onwards for further metabolism in the cell. Bile salts are involved in the proper function of some of these enzymatic reactions and in the formation of product phases on which a normal uptake process is based. Little is known at present of the importance of bile salts for the intracellular reactions following uptake of fat into the enterocyte. Different aspects of intestinal lipid absorption have been reviewed in recent years by Patton [7], Thomson and Dietschy [8], Carey [9], Carey et al. [10], Wells and Direnzo [11], and Grundy [12]. The role of bile acids in fat absorption has been discussed by Holt [13]. 

Another nuclear receptor, called FXR, is activated by the binding of bile acids. Expressed in hepatocytes and intestinal epithelial cells, FXR plays a key role in regulating the en-terohepatlc circulation of bile acids. Bile acid-activated FXR stimulates the expression of Intracellular bile acid-binding protein (I-BABP) and of transport proteins (e.g., ABCBll, NTCP) that mediate cellular export and Import of bile acids (see Figure 18-11). In contrast, active FXR represses the expression of cholesterol 7a-hydroxylase, thereby decreasing the synthesis of bile acids from cholesterol in the liver—another example of end-product Inhibition of a metabolic pathway. Both FXR and LXR function as heterodimers with the nuclear receptor RXR. 

Bile acid-binding resin therapy Oral administration of a bile acid-binding resin, or sequestrant (D), increases the loss of bile acids from the body by preventing their absorption by intestinal epithelial cells through the IBAT transport protein and reduces bile acids delivered to the blood (0) and then to the liver (0) by the transporter NTCR Step The lower levels of cytoplasmic bile acids reduce the amount of bile acid bound to the nuclear hormone receptor EXP (0) and its suppression (0) of the expression of cholesterol 7a-hydroxylase. The consequent increased levels of expression and activity of cholesterol 7a-hydroxylase (B) reduce the levels of intracellular cholesterol (0). As with the statin treatment, the reduced cellular cholesterol levels (EHB) increase LDLR activity, lower plasma LDL levels, and protect against atherosclerosis. [Part (a) adapted from M. S. Brown and J. L. Goldstein, 1986, Sdence 232 34.]... 

ACAT transfers amino-acyl groups from one molecule to another. ACAT is an important enzyme in bile acid synthesis, and catalyses the intracellular esterification of cholesterol and formation of cholesteryl esters. ACAT-mediated esterification of cholesterol limits its solubility in the cell membrane and thus promotes accumulation of cholesterol ester in the fat droplets within the cytoplasm this process is important in preventing the toxic accumulation of free cholesterol that would otherwise damage ceU-membrane structure and function. Most of the cholesterol absorbed during intestinal transport undergoes ACAT-mediated esterification before incorporation into chylomicrons. In the liver, ACAT-mediated esterification of cholesterol is involved in the production and release of apo-B-containing lipoproteins. 

Changes in intracellular calcium homeostasis produced by active metabolites of xenobiotics may cause disruption of the dynamic cytoskeleton. There are a few toxins that cause disruption of the cytoskeleton through mechanisms independent of biotransformation. Microcystin is one of these toxins. Microcystin is produced by the cyanobacterium Microcystis aeruginosa. Similar toxins are produced by other species of cyanobacteria. The hepatocyte is the specific target of microcystin, which enters the cell through a bile-acid transporter. Microcystin covalently binds to serine/threonine protein phosphatase, leading to the hyperphosphorylation of cytoskeletal proteins and deformation of the cytoskeleton (Treinen-Moslen, 2001). 

Bile salts can also displace BSP from plasma albumin and hepatic intracellular protein (A14), which may contribute to the reduced uptake of the dye. BSP can reduce the transport maximal rate for bile acids... 

Control of these mechanisms occurs at several levels. The uptake of VLDL remnants and LDL by the apo B/E receptor has been shown to be Unked to the excretion of biliary UC and bile acids [122], whereas the formation of intracellular CE by the ACAT reaction seems to be inversely proportionate to the excretion of the same bile components [123]. The uptake of chylomicron remnants does not seem to be related to bile formation and excretion. Instead, when large amounts of remnant CE enter hepatocytes, as is seen in some species in diet-induced hypercholesterolemia, the CE is hydrolyzed within lysosomes, then reformed by the ACAT reaction and recirculated in the plasma as VLDL CE. Less is known about the control of LCAT secretion by the Ever. In rats and in humans the activity of LCAT in the plasma seems to be hnked not only to the transport of cholesterol, but also to the transport of essential fatty acids [124]. 

In a number of in vivo studies by Hollander and co-workers the concurrent feeding of polyunsaturated fatty acids has been shown to depress absorption of all species of fat-soluble vitamin [109]. How this comes about is not clear it does not depend on displacement of the vitamins from bile salt micelles but presumably involves an interaction at the cell membrane or at some later step in absorption such as the binding to intracellular transport proteins. 

In the intestine the ileal bile acid transporter (IBAT) imports bile acids from the lumen into intestinal epithelial cells (step 2]). IBAT is a Na -linked symporter (see Figure 7-21) that uses the energy released by the movement of Na down its concentration gradient to power the uptake of about 95 percent of the bile acids. Those bile acids Imported on the apical side of intestinal epithelial cells move intracellularly with the aid of intestinal bile acid-binding protein (I-BABP) to the basolateral side. There, they are exported into the blood by poorly characterized transport proteins (step 3]) and eventually returned to liver cells by another Na -linked... 

When applied to intact cells, different factors should be considered. The microcystins, cantharidin, and endothal are not permeable across the plasmalem-ma but may be taken up by hepatocytes via the bile acid transport system (Eriksson et al., 1990). A derivative of endothal, endothal thioanhydride, is permeable across the cell membrane (M. Hirano and F. Er-dddi, unpublished observations). Thus permeability across lipid bilayers is one consideration. In addition, the potency of inhibitors used with intact cells always appears less than the in vitro assays. The potency with intact cells is often 10- to 100-fold less sensitive. For example, with 3T3 fibroblasts, external concentrations above 10 nM were required to elicit shape changes (Chartier et al., 1991). This difference in dose dependence could be due to inefficient uptake by the cells or preferential localization of the inhibitor with lipids. Cohen et al. (1989) suggested that the higher concentrations required with intact cells reflect the intracellular concentration of the targeted phosphatase. 

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