ABSTRACT
The bile canaliculus contains at least four ATP-binding cassette (ABC) proteins responsible
for ATP-dependent transport of bile acids (spgp), nonbile acid organic anions (mrp2),
organic cations (mdr1), and phosphatidylcholine (mdr2). Other ABC transporters (including
mrp3) have also been partially localized to the canaliculus; however, their function
has not been fully delineated. The specific amount and function of spgp and mrp2 in
the canalicular membrane increases in response to taurocholate and cAMP. The mechanism
involves increased recruitment of spgp and mrp2 from Golgi to the canalicular membrane
by a microtubular and PI3 kinase-dependent vesicular trafficking system. Because the effects of taurocholate
and cAMP summate, two distinct pathways are proposed. Mdr family members traffic either
directly to the apical plasma membrane or, in the case of spgp, through a separate
intracellular pool(s); in either case, there is no direct evidence for transcytosis
of ABC transporters from Golgi to basolateral plasma membrane and subsequently to
the canalicular plasma membrane. Direct transfer from Golgi to apical membrane was
demonstrated by in vivo pulse labeling, in vitro membrane localization, and on-line
video microscopy in WIFB9 cells that were stably transfected with mdr1-GFP. A critical
role for 3′-phosphoinositide products of PI3 kinase was demonstrated in the intracellular trafficking of canalicular ABC transporters
and for optimal transporter activity within the canalicular membrane. These studies
suggest that many intracellular components, including ATP, Ca3
+, numerous GTPases, microtubules, cytoplasmic motors, and other unknown factors, are
required for physiologic regulation of ABC transporter traffic from Golgi to the canalicular
membrane. Defects in this complex system are postulated to produce an ``intrahepatic
traffic jam'' that results in defective ABC transporter function in the canalicular
membrane and, consequently, in cholestasis.
KEYWORD
hepatocyte - bile canaliculus - ATP-binding cassette transporter - trafficking
