Regulation of ileal bile acid transport: Desirability of measuring transport function as well as transporter activity

Arrese et al.1 reported a careful study aimed at testing the hypothesis that bile acid load influences carrier mediated ileal bile acid transport, which is mediately largely by the apical sodiumdependent bile salt transporter (ABST). To do this, in rats, they decreased the load to the ileal enterocyte either by bile duct ligation or by feeding a bile acid binding resin. They showed that neither regimen changed taurocholate transport (using isolated brush border membrane vesicles), the level of messenger RNA (mRNA) for ABST (by Northern blotting), and the protein expression of ABST and the ileal lipid binding protein (by Western blotting). They concluded that ‘‘ileal [bile acid] transport does. not seem to be regulated by the bile acid load. ’’ This work is not in agreement with the studies of Stravitz et al.,2 who found in rats that cholic acid feeding for 10 days increased mRNA, the ABST protein, and its mRNA content threefold; sodium dependent transport by brush border membrane vesicles was increased twofold. Using organ culture of ileum explants, Kanda et al.3 showed that luminal bile caused a dose-dependent increase in mRNA for the ileal bile acid binding protein. These workers also found that transcript for this protein was greatly increased when bile was added to the culture medium. Neither the findings of Arrese et al. nor those of Stravitz et al. are in agreement with the interpretation of experiments reported from my laboratory in 1993.4 Lillienau et al. observed that in the guinea pig, feeding of taurocholate or cholylsarcosine (an analogue of glycocholate that is resistant to bacterial deconjugation or dehydroxylation) downregulated ileal bile acid transport by a factor of two and that feeding of cholestyramine up-regulated ileal bile acid transport by a factor of three. In the rat, in possible agreement with Arrese et al., changes induced by bile acid or cholestyramine feeding were very modest. In the guinea pig, up-regulation and down-regulation required 3 to 4 days, a value similar to that of the life expectancy of the ileal enterocyte. We concluded that in the rodent, ileal bile acid transport is regulated by negative feedback in the same way that calcium and iron absorption are also regulated by negative feedback. In our experiments, we measured total ileal bile acid transport by ileal perfusion in the biliary fistula animal. By using a high perfusion rate, we kept the intraluminal concentration constant and, thus, could define a true value for ileal clearance of taurocholate.5 We did not measure the expression of ABST or its mRNA, because at that time, this transporter had not yet been cloned.6 Jan Lillienau returned to Sweden and confirmed this result. Moreover, by slicing the distal guinea pig intestine and defining taurocholate uptake for individual slices, he could show that up-regulation of bile acid transport was obtained by proximal recruitment of ileal enterocytes.7 Therefore, I suggest that measurement of transporter activity in isolated brush border membranes or measurement of transporter activity (by mRNA or expression of the actual protein) may not give valid results, because these assays measure activity per unit of protein and such indices may not change in systems that are zonally regulated. In my judgement, it is desirable, and perhaps even essential, to measure total intestinal transport by a perfusion procedure. In our studies, the magnitude of the change in ileal bile acid clearance induced by varying the bile acid load was much smaller in the rat than in the guinea pig, and there remains the possibility that there are major species differences (or even strain differences in rats) in the mechanisms of regulation. In mice, Torchia et al.8 showed that cholestyramiiae feeding causes a threefold increase in ABST mRNA, and taurocholate caused a 50% decrease, results that are in agreement with our perfusion results; no details are given in the article as to whether total ileum or terminal ileum was used for mRNA isolation. In humans, there are numerous studies showing that bile acid feeding results in relatively small changes in the size of the bile acid pool or the rate of bile acid secretion (see for example, Northfield et al.,9 Einarsson and Grundy,10 and LaRusso et al.11). If there were substrate induction of ABST, the bile acid pool and bile acid secretion would continue to increase progressively until no further induction occurred. This does not appear to happen. Arrese et al. also cite the report of Chazouilleres et al.,12 noting that the absorption of a ‘‘Se tagged analogue of taurocholate is unchanged in cholestatic disease,’’ which is in support of their interpretation that the ileal bile salt load does not influence ileal bile acid transport. However, the limited studies that have been reported on bile acid secretion in cholestatic disease suggest that it is similar to that in health until cholestatic disease is very advanced.13,14 Thus, our results in the guinea pig that were obtained by ileal perfusion are in agreement with human studies showing that bile acid feeding does not increase the enterohepatic flux of bile acids. If up-regulation of ileal transport results from recruitment of more proximal enterocytes, then measurement of transporter expression in a terminal ileal segment will give misleading results. Indeed, if ileal bile acid transport is regulated by zonal expression, it can be argued that the results of Arrese et al., which show no change in ABST per unit protein, are completely in agreement with the findings of Lillienau et al.4 Perfusion of the human ileum can be done, and measurements of ileal bile acid clearance have been made in healthy humans.15 Therefore, it should be possible to define the effects of increasing and decreasing the bile acid load on ileal bile acid clearance in humans. Until this is done, it seems reasonable to continue to believe that the enterohepatic flux of bile acids is homeostatically controlled at the level of the ileal enterocyte. Control of the enterohepatic flux of bile acids at the level of the intestine protects the hepatocyte from receiving a load of bile acids that might exceed the transport of the canalicular bile salt export pump and lead to accumulation in the hepatocyte, which, in turn, could lead to necrosis and/or apoptosis.