A PDZ-Like Motif in the Biliary Transporter ABCB4 Interacts with the Scaffold Protein EBP50 and Regulates ABCB4 Cell Surface Expression

International audienceABCB4/MDR3, a member of the ABC superfamily, is an ATP-dependent phosphatidylcholine translocator expressed at the canalicular membrane of hepatocytes. Defects in the ABCB4 gene are associated with rare biliary diseases. It is essential to understand the mechanisms of its canalicular membrane expression in particular for the development of new therapies. The stability of several ABC transporters is regulated through their binding to PDZ (PSD95/DglA/ZO-1) domain-containing proteins. ABCB4 protein ends by the sequence glutamine-asparagine-leucine (QNL), which shows some similarity to PDZ-binding motifs. The aim of our study was to assess the potential role of the QNL motif on the surface expression of ABCB4 and to determine if PDZ domain-containing proteins are involved. We found that truncation of the QNL motif decreased the stability of ABCB4 in HepG2-transfected cells. The deleted mutant ABCB4-ΔQNL also displayed accelerated endocytosis. EBP50, a PDZ protein highly expressed in the liver, strongly colocalized and coimmunoprecipitated with ABCB4, and this interaction required the QNL motif. Down-regulation of EBP50 by siRNA or by expression of an EBP50 dominant-negative mutant caused a significant decrease in the level of ABCB4 protein expression, and in the amount of ABCB4 localized at the canalicular membrane. Interaction of ABCB4 with EBP50 through its PDZ-like motif plays a critical role in the regulation of ABCB4 expression and stability at the canalicular plasma membrane

Effect of overexpression of the PDZ domains of EBP50.

<p>(A) ABCB4-wt- or ABCB4-ΔQNL-expressing HepG2 cells were transiently transfected with the Flag-tagged EBP50 PDZ domains (PDZ1+PDZ2-Flag) or with the empty vector-Flag (control vector). ABCB4 was detected by immunoblotting from cell lysates. (B) Amounts of ABCB4 were quantified from immunoblots by densitometry. ABCB4 levels were expressed as a percentage of total expression in HepG2 cell transfected with control vector. (C) ABCB4-wt- or ABCB4-ΔQNL-expressing HepG2 cells were transiently transfected with the Flag-tagged EBP50 PDZ domains (PDZ1+PDZ2-Flag). Cells were fixed, permeabilized and stained with the anti-Flag antibody followed by anti-ABCB4 antibody and then incubated with Alexa-Fluor-594-and 488-conjugated secondary antibodies and visualized by confocal microscopy. Representative immunofluorescence images are shown. Asterisks indicate bile canaliculi. Bars, 10 μm. (D) The amount of ABCB4 at the bile canaliculi was quantified in HepG2 transfected cells (PDZ1+PDZ2-Flag(+)) and compared to that observed in control adjacent non-transfected cells (PDZ1+PDZ2-Flag(-)). Means (± SEM) of at least three independent experiments are shown. *<i>P</i><0.01 for ABCB4-wt; n.s., not significant; a.u., arbitrary units.</p

The C-terminus of ABCB4 ends by a PDZ-like motif.

<p>(A) Schematic representation of ABCB4. ABCB4 contains two transmembrane domains (TMD1 and TMD2) and two nucleotide binding domains (NBD1 and NBD2). The two glycosylation sites in the first extracellular loop are indicated. The amino acid sequence of the intracytoplasmic C-terminal domain of the human ABCB4 isoform A (NP_000434.1) and the human ABCB1 (NP_000918.2) are shown. Green letters indicate the PDZ-like motif of ABCB4. (B) A sequence alignment of known PDZ motifs found in ABC family members is shown. ABCC2, ABCC4 and ABCC7 display class I PDZ consensus motifs.</p

Expression and localization of ABCB4-wt and ABCB4-ΔQNL.

<p>(A) ABCB4 was detected by immunoblotting from cell lysates of HepG2 cells stably expressing ABCB4-wt or ABCB4-ΔQNL. (B) HepG2 cells stably expressing ABCB4-wt or ABCB4-ΔQNL were fixed with methanol/acetone, processed for immunofluorescence using the monoclonal P3II-26 antibody and Alexa 488-conjugated anti-mouse IgG and visualized by confocal microscopy. Nuclei were stained with DRAQ 5 (blue). Asterisks indicate bile canaliculi. Cell contours are indicated by dotted lines. Bars, 10 μm. Representative of three experiments.</p

Effect of EBP50 silencing.

<p>(A) HepG2 cells expressing ABCB4-wt or ABCB4-ΔQNL were transfected with EBP50 siRNA or scramble siRNA (Scr). After 60 hours of transfection, cell lysates were subjected to western blot analysis. (B) Amounts of ABCB4 were quantified from immunoblots by densitometry. ABCB4 levels were expressed as a percentage of total expression in HepG2 cell transfected with scramble siRNA. Means (±SEM) of at least three independent experiments are shown. *<i>P</i><0.05 for ABCB4-wt; n.s., not significant.</p

Stability of ABCB4-wt and ABCB4-ΔQNL.

<p>(A) Stability of ABCB4-wt or ABCB4-ΔQNL was analyzed in stably transfected HepG2, after inhibiting protein synthesis with cycloheximide (25 μg/mL). ABCB4 was detected at the indicated time points in cell lysates by immunoblotting, using equal amounts of total proteins per lane. (B) Amounts of ABCB4 were quantified from chase experiments. The amount of ABCB4 at time zero was considered as 100%. Remaining ABCB4 at later time points was expressed as percentage of time zero. Means (± SEM) of three independent experiments are shown. <i>* P</i><b><</b>0.05 at all points.</p

The internalization of ABCB4-ΔQNL is accelerated.

<p>(A) Transiently transfected HEK 293 cells were incubated for 30 minutes at 0°C with anti-myc antibody. After surface labeling, cells were incubated at 37°C for 60 minutes. At the end of the 37°C incubation, non-internalized antibody-antigen complexes were removed by acid washing. Cells were fixed, permeabilized and internalized antibodies were visualized with Alexa-Fluor 488-conjugated secondary antibodies and examined by confocal microscopy. Representative immunofluorescence images are shown. Nuclei were stained with DRAQ 5 (blue). Bars, 10 μm. (B) The amount of internalized ABCB4 was measured in cells transfected with 3xmyc-ABCB4-wt or 3xmyc-ABCB4-ΔQNL using ImageJ 1.41 software. Means (± SEM) of at least three independent experiments are shown. *<i>P</i><0.001. a.u., arbitrary units.</p

ABCB4 colocalizes and coimmunoprecipitates with EBP50.

<p>(A) ABCB4-wt-expressing HepG2 cells were fixed, permeabilized and stained with anti-ABCB4 antibody followed by anti-EBP50 antibody and then incubated with Alexa-Fluor-488-and 594-conjugated secondary antibodies and visualized by confocal microscopy. Nuclei were stained with DRAQ 5 (blue). Asterisks indicate bile canaliculi. Bar, 10 μm. (B) Cell lysates of HepG2 cells stably transfected with ABCB4-wt or ABCB4-ΔQNL or cell lysates of primary human hepatocytes (PhH) were incubated with anti-ABCB4 antibody or mouse imununoglobulin G (IgG) covalently linked to agarose beads. The coimmunoprecipitated complex was immunoblotted with anti-ABCB4 and anti-EBP50 antibodies.</p