Evaluation of the in vivo role of p-glycoprotein in the maintenance of cholesterol homeostasis

Removal of cholesterol from the body is the final step in reverse cholesterol transport, the process by which excess cholesterol is moved from peripheral tissues to the liver for excretion. Several provocative studies led us to probe the contributions of P-glycoprotein (Pgp), an ATP-dependent transporter with broad range of hydrophobic substrates, to the removal of cholesterol and its bile acid metabolites from the body. We used knockout mice lacking both isoforms of Pgp (Abcb1a⁺/1b⁺) fed test diets containing elevated fat and/or cholesterol to determine if Pgp deletion affects murine cholesterol homeostasis. The knockout mice had the same cholesterol phenotype as wild-type mice, measured by cholesterol absorption, circulating lipoproteins, and hepatic storage of cholesterol. We examined the activation of two key regulatory pathways that maintain cholesterol homeostasis, Sterol Regulatory Element Binding Protein-2 (SREBP-2) and Liver X Receptor (LXR), to determine if the knockout mice maintain cholesterol balance through compensatory mechanisms. The knockout mice had decreased expression of SREBP-2-target genes in the jejunum and increased expression of LXR-target genes in the liver. These perturbations to cholesterol regulatory networks are indirect evidence that the knockout mice lacked a cholesterol efflux mechanism. We evaluated efflux of bile acids from the liver to the gallbladder in mice after nine weeks on the test diets. Knockout mice fed elevated fat or cholesterol had a 30-50% reduction in bile acid efflux capacity compared to wild type mice, suggesting that Pgp contributes to murine bile acid efflux. Subsequent analysis revealed that Pgp did not display specificity for a particular class of bile acids. In summary, we provide evidence that deletion of Pgp did not alter the cholesterol phenotype of mice; however, compensatory shifts in cholesterol regulatory networks maintained homeostasis in the knockout. Additionally, we show that Pgp knockout mice had a reduced capacity to efflux bile acids to the gallbladder. The nature of the changes to the cholesterol regulatory networks and the perturbations to bile acid efflux suggest that Pgp is a non-specific efflux transporter that recognizes both cholesterol and bile acids as substrates.