Dietary fish oil potentiates bile acid-induced cholesterol secretion into bile in rats1

Recently we demonstrated that dietary fish oil (FO) causes changes in intrahepatic cholesterol transport and hypersecretion of cholesterol into bile in rats (J Clin. Znuest. 88: 943-951, 1991). We have now investigated in more detail the relationship between cholesterol and bile acid secretion in rats with chronic bile diversion fed purified diets supplemented (9% wt/wt) with either FO or corn oil (CO) for 2 weeks. Effects of FO on biliary cholesterol secretion (+ 400% as compared to co tion of this process are not fully understood. Biliary output of cholesterol and phosphofipids appears to be regulated, at least in part, by bile acid secretion. stimulation of bile acid output, within physiological limits, generally evokes an increase in lipid secretion (I). The between bile acid and lipid output appears to be accomDlished at the level of the bile canaliculus (2. 3). A rela, I I after l4 days) were much more Pronounced than PreviouslY Ob;ionship between the hydrophobicity of the different bile served in rats with intact enterohepatic circulation (+50%). Biliary bile acid (+30%) and phospholipid (+120%) secretion were increased to a much lesser extent than that of cholesterol acid species and their capacity to induce biliary lipid secretion has been reported (4-6). However, this resulting in the formation of bile supersaturated with cholesterol. The biliary cholesterol/bile acid molar ratio was 0.069 and 0.032 in FOand CO-fed rats, respectively, at noon of day 14. This ratio increased to 0.108 in FO-fed rats at midnight, when bile acid output was maximal, but remained unchanged in CO-fed rats during the day-night cycle. Intravenous administration of taurochenodeoxycholic acid (15 pmol/kg) resulted in a 2-fold increase in bile acid output and a simultaneous 1.6-fold stimulation of cholesterol secretion in both groups, implying that administration of the bile acid induced the secretion of 2-3 times as much cholesterol in FOthan in CO-fed rats. Likewise, administration of bilirubin ditaurate (30 pmollkg), an inhibitor of bile acid-induced biliary lipid secreship is highly species-specific, indicating that the physicochemical characteristics of bile acids are not the only determinants of bile acid-induced lipid secretion. Recent data from our laboratory suggest that species-dependent differences in the magnitude of the bile acid-independent fraction of bile flow also play a role (7). In addition, it is well established that cholesterol output can be dissociated from bile acid output by pharmacological and dietary means. Various hydrophilic organic anions inhibit biliary cholesterol and phospholipid secretion in experimental animals without affecting bile acid secretion (2, 3, 8-10) tion, reduced cholesterol output in both groups by about 50% while bile acid output remained unchanged. It is concluded that, in rats, dietary fish oil increases the disposition of cholesterol into bile by potentiating bile acid-dependent cholesterol secretion, presumably by facilitating the recruitment of bile-destined cholesterol.-Smit, M. J., H. J. Verkade, R. by interacting with canalicular bile acids (2, 3) . Feeding of a cholesterol-enriched diet reduces cholesterol output relative to that of bile acids (and phospholipids) in the rat (I1). On the Other h a n d y treatment with progesterone (l2)j acute cessation of lovastatin treatment (13), and diets con. , Having% RJVonk G. LScherPhof, G. In 't Veld, and *a Kuipers. Dietary fish oil potentiates bile acid-induced cholesterol secretion into bile in rats. J Lipid Res. 1994. 35: ?ni-?in taining fish oil (14, 15) or plant sterols such as diosgenin (16) lead to a marked hypersecretion of cholesterol in this species. These latter observations indicate that, in addi.,".,&". tion to the "driving force" of bile acids, factors exist that Supplementary key words acid docosahexaenoic acid chenodeoxycholic acid bilirubin ~ ditaurate dietary fats corn oil eicosapentaenoic Abbreviations: FO, fish oil; CO, corn oil; EPA, eicosapentaenoic acid; DHA, docosahexaenoic acid; WDC, taurochenodeoxycholic acid; B m , bilirubin ditaurate; VLDL, very low density lipoprotein; LDL, low den'This work was presented at the Annual Meeting of the American Gastroenterological Association, May 15-21 1993, Boston, MA, and has been published in abstract form (Garfromferolo~. 1993. 104: A993). The hepatobiliary pathway is the main route for sity lipoprotein; ACAT, acyl-CoA:cholesterol acyltransferase. removal of cholesterol from the body, either after conversion to The mechanism of biliary cholesterol secretion and the regulaacids or in the form Of free 2To whom correspondence should be addressed. Journal of Lipid Research Volume 35, 1994 301 at P E N N S T A T E U N IV E R S IT Y , on F ebuary 3, 2013 w w w .j.org D ow nladed fom coregulate the amount of cholesterol secreted into the bile. We have recently compared the effects on hepatic cholesterol metabolism of a diet supplemented (9% wt/wt) with fish oil (FO), enriched in the highly unsaturated n-3 fatty acids eicosapentaenoic acid (EPA, C20:5) and docosahexaenoic acid (DHA, C22:6), with those of corn oil (CO, n-6 unsaturated fatty acids) in rats (14). We found that FO feeding reduced plasma cholesterol and triglyceride levels by 38% and 69%, respectively, presumably due to the well-established inhibitory effects of FO on hepatic VLDL production (17). The plasma lipid-lowering effects of FO were associated with an increase in bile acid pool size by 28%; a 4-fold increase in the cholic acid/chenodeoxycholic acid ratio in bile; a preferential use of endocytosed cholesterol for bile acid synthesis; and an increase in biliary cholesterol secretion by 51%. To gain further insight in the mechanism(s) involved in FO-induced hypersecretion of cholesterol, we have now studied the relationship between bile acid and cholesterol secretion in detail in FOand CO-fed rats with chronic bile diversion (18). We have recently published a detailed account of the effects of prolonged interruption of the enterohepatic circulation on hepatic cholesterol and bile acid metabolism and biliary lipid secretion in rats (19). This experimental set-up allowed us to study cholesterol secretion in a longitudinal fashion under the conditions of low, stable bile acid output (derived from de novo synthesis only) in the absence of bile acid flux through the liver. Using this model, we investigated the effects of the experimental diets on plasma lipids and biliary lipid composition, on the day-night variation in biliary cholesterol output and on the capacity of taurochenodeoxycholic acid ( X D C ) and bilirubin ditaurate (BDT) to stimulate and to inhibit, respectively, biliary cholesterol output.