Bile Acid Transport in Sister of P-Glycoprotein (ABCB11) Knockout Mice

Abstract

In vertebrates, bile flow is essential for movement of water and solutes across liver canalicular membranes. In recent years, the molecular motor of canalicular bile acid secretion has been identified as a member of the ATP binding cassette transporter (ABC) superfamily, known as sister of P-glycoprotein (Spgp) or bile salt export pump (Bsep, ABCB11). In humans, mutations in the BSEP gene are associated with a very low level of bile acid secretion and severe cholestasis. However, as reported previously, because the spgp(-)(/)(-) knockout mice do not express severe cholestasis and have substantial bile acid secretion, we investigated the "alternative transport system" that allows these mice to be physiologically relatively normal. We examined the expression levels of several ABC transporters in spgp(-)(/)(-) mice and found that the level of multidrug resistance Mdr1 (P-glycoprotein) was strikingly increased while those of Mdr2, Mrp2, and Mrp3 were increased to only a moderate extent. We hypothesize that an elevated level of Mdr1 in the spgp(-)(/)(-) knockout mice functions as an alternative pathway to transport bile acids and protects hepatocytes from bile acid-induced cholestasis. In support of this hypothesis, we showed that plasma membrane vesicles isolated from a drug resistant cell line expressing high levels of P-glycoprotein were capable of transporting bile acids, albeit with a 5-fold lower affinity compared to Spgp. This finding is the first direct evidence that P-glycoprotein (Mdr1) is capable of transporting bile acids.

Evaluation of the Contribution of the ATP Binding Cassette Transporter, P-glycoprotein, to in Vivo Cholesterol Homeostasis

“…Our finding that Pgp knockout mice do not maintain gallbladder bile composition (Figure 6) compliments the recent report by Ling et al who used triple knockout mice to demonstrate that Pgp acts as a compensatory bile salt efflux transporter in the absence of the Bile Salt Export Pump (BSEP) 36,44 . Our gallbladder bile data suggest that Pgp may also be required for bile salt homeostasis under physiologically relevant dietary conditions, even in the presence of BSEP.…”

The bile salt export pump

“…This observation may indicate that mouse Mdr1a could act as a salvage transporter for bile salts in conditions of high intracellular bile salt concentration. Transport studies using canalicular plasma membrane vesicles from Bsep knockout animals revealed a residual ATP-dependent transport activity for taurocholate [52]. Plasma membrane vesicles isolated from a drug-resistant Chinese hamster ovary cell line expressing high levels (about 15% of total plasma membrane proteins) of class I P-glycoprotein also exhibit ATP-dependent bile salt transport, albeit with lower affinity than Bsep [52].…”

“…Transport studies using canalicular plasma membrane vesicles from Bsep knockout animals revealed a residual ATP-dependent transport activity for taurocholate [52]. Plasma membrane vesicles isolated from a drug-resistant Chinese hamster ovary cell line expressing high levels (about 15% of total plasma membrane proteins) of class I P-glycoprotein also exhibit ATP-dependent bile salt transport, albeit with lower affinity than Bsep [52]. Hence, Mdr1 may act as a salvage system for bile salts in case of nonfunctional or impaired Bsep.…”

Recent insights into the function and regulation of the bile salt export pump (ABCB11)

“…Furthermore, mice with an ablated Bsep gene do not develop severe cholestasis on a normal diet (77), but need to be put on a high cholate diet (78). As a consequence of absent Bsep, novel, more hydrophilic bile salts have been detected in the bile of knockout animals (77) as well as an upregulation of p-glyocoprotein, which could provide a salvage pathway for bile salts (79). Nevertheless, Bsep knockout mice can be used to develop novel therapeutic approaches, such transplantation of cells to correct consequences of non-functional BSEP (80).…”