Biliary cholesterol excretion: a novel mechanism that regulates dietary cholesterol absorption

Abstract

The regulation of dietary cholesterol absorption was examined in C57BL/6 and transgenic mice with liver overexpression of the scavenger receptor BI (SR-BI Tg). In C57BL/6 animals, feeding 0.02 to 1% (wt/wt) dietary cholesterol resulted in a dose-dependent decrease in the percentage of dietary cholesterol absorbed. A plot of total daily mass of dietary cholesterol absorbed versus the percentage by weight of cholesterol in the diet yielded a curve suggesting a saturable process with a Km of 0.4% (wt/wt) and a Vmax of 0.65 mg cholesterol/g body weight per day. Dietary cholesterol suppressed hepatic 3-hydroxy-3-methylglutaryl CoA reductase activity, stimulated cholesterol 7alpha-hydroxylase activity, and enhanced fecal excretion of bile acids, but none of these changes correlated with the percentage of dietary cholesterol absorption. Dietary cholesterol also caused an increase in biliary cholesterol concentration, and in this case the concentration of biliary cholesterol was strongly and inversely correlated with the percentage dietary cholesterol absorption (r = -0.63, P < 0.0001). Biliary cholesterol concentration was also directly correlated with daily cholesterol intake, dietary cholesterol mass absorption, and liver cholesterol ester content. Transgene-induced overexpression of SR-BI resulted in a stimulation of excretion of cholesterol into the bile and suppressed percentage dietary cholesterol absorption. Furthermore, biliary cholesterol levels in SR-BI Tg mice were strongly and inversely correlated with the percentage of dietary cholesterol absorbed (r = -0.99, P < 0.0008). In summary, these results suggest that the excretion of cholesterol into the bile plays an important role in regulating the percentage absorption of dietary cholesterol.

Figure 1

Daily fecal excretion of [³H]β-sitostanol and [¹⁴C]cholesterol. C57BL/6 males were gavaged with [³H]β-sitostanol and [¹⁴C]cholesterol and placed in metabolic cages, and feces was collected daily for 4 consecutive days. The percentage of daily [³H] and [¹⁴C] excretion out of the total of each label recovered in the feces over 4 days is plotted (mean ± SD, n = 5).

Figure 2

Effect of dietary cholesterol on cholesterol absorption. (A) Five different groups of C57BL/6 males were fed for 3 weeks with dietary 0.02–1% wt/wt cholesterol. Mice received a gastric bolus of 100 μl of corn oil containing 1.67 μCi [¹⁴C]cholesterol and 0.67 μCi [³H]β-sitostanol, they were placed in metabolic cages, and feces were collected for 24 h. The ratio of [¹⁴C]/[³H] labels in the fecal lipid extracts was determined and percentage absorption was calculated (mean ± SD, n = 5 animals in each group). ∗, P < 0.03 and ∗∗, P < 0.0001 vs. 0.02%. (B) The mass of absorbed cholesterol was calculated as the daily cholesterol intake divided by the body weight and multiplied by percentage cholesterol absorption (mean of 5–10 animals per group). Nonlinear regression shows saturable cholesterol absorption.

Figure 3

Effect of an acute dietary cholesterol switch on percentage cholesterol absorption. C57BL/6 mice were preconditioned for 3 weeks with 0.5% cholesterol and cholesterol absorption was measured (solid bar). The same animals were fed for additional 3 weeks with the same diet, gavaged with radiolabeled mixture, and switched immediately afterwards into 0.02% cholesterol. During the next 24 h, while consuming a 0.02% cholesterol diet, feces were collected and cholesterol absorption was measured (open bar). (Mean ± SD, n = 5.)

Figure 4

Effect of dietary cholesterol on hepatic HMGR and 7α-hydroxylase activities and bile composition. C57BL/6 mice were fed with increasing amounts of cholesterol for 3 weeks. Gallbladder bile was aspirated and analyzed. The activities of HMGR (■) and cholesterol 7α-hydroxylase (○) were determined in isolated hepatic microsomes. Biliary cholesterol (•) and bile acids (▴) were measured as described in Methods (mean ± SD, n = 5).

Figure 5

Relation of percentage of absorption to biliary cholesterol. C57BL/6 mice were fed for 3 weeks with 0.02–1% cholesterol. Cholesterol absorption was measured and gallbladder bile was aspirated and measured for cholesterol content.