Rationale:
Removal of bile salts from the liver is the final step of the reverse cholesterol transport pathway. We studied the contribution of Abcb1 (P-glycoprotein), in bile acid efflux. Although a number of endogenous substrates have been postulated for Abcb1 based on
in vitro
evidence, studies using animal models have not supported these claims. Recent studies in mice demonstrated that in the absence of the Bile Salt Efflux Pump (Bsep), Abcb1 is required for removal of bile salts, especially when challenged with a cholic acid containing diet. To date, no study using atherogenic diets has demonstrated the role of Abcb1 in the removal of bile salts in the presence of functional Bsep.
Methods:
We fed male mice lacking both isoforms of Abcb1 (Abcb1a
-/-
/1b
-/-
) and wild-type controls a diet providing either 25% or 45% of the kcal from fat, supplemented with either normal chow or high levels of cholesterol (0.02% w/w or 0.2% w/w respectively) for nine weeks; n=5 per group. On the tenth week, we assessed the efflux of cholesterol, phospholipid and bile acids to the gallbladder. Enzymatic assays were used to measure cholesterol and phospholipid, the pool of bile acids was quantified by HPLC to determine the concentrations of the six most prevalent murine bile acids.
Results:
Abcb1 knockout mice have a >30% reduction in the moles of bile salt normalized to phospholipid relative to wild type mice after administration of diets containing either elevated fat or cholesterol (p<0.05). Neither the efflux of phospholipid, nor the molar composition of the six bile acids was affected by deletion of Abcb1.
Conclusions:
We conclude that Abcb1 is a secondary efflux mechanism required for the removal of bile acids after consumption of diets rich in fat and/or cholesterol. Although Abcb1 knockout mice have reduced total bile acids in the gallbladder, the molar ratio of the specific bile acids is the same as in the wild type mice. These data suggest that Abcb1 effluxes the six bile acids in a non-specific manner, unlike Bsep which preferentially effluxes hydrophobic bile acids. The lack of specificity demonstrated by Abcb1 is desirable for a low- affinity secondary efflux mechanism, which supplements Bsep activity in bile acid output.
Acknowledgments:
Canadian Institutes of Health Research, Michael Smith Foundation for Health Research
Marinobufagenin, a mammalian endogenous cardiotonic steroid, is synthesized from the intermediates in the classical bile acid pathway in Cyp27A1 knockout mice
