Intracellular trafficking and regulation of canalicular ATP-binding cassette transporters

The bile canaliculus contains at least four ATP-binding cassette (ABC) prot eins 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 l ocalized to the canalicuslus; however their function has not been fully del ineated. The specific amount and function of spgp and mrp2 in the canalicul ar membrane increases in response to taurocholate and cAMP. The mechanism i nvolves increased recruit ment of spgp and mrp2 from Golgi to the canalicul ar membrane by a microtubular and PI, kinase-dependent vesicular traffickin g system. Because the effects of taurocholate and cAMP summate, two distinc t pathways are proposed. Mdr family members traffic either directly to the apical plasma membrane or, in the case of spgp, through a separate intracel lular pool(s); in either case, there is no direct evidence for transcytosis of ABC transporters from Golgi to basolateral plasma membrane and subseque ntly to the canalicular plasma membrane. Direct transfer from Golgi to apic al membrane was demonstrated by in vivo pulse labeling, in vitro membrane l ocalization, and on-line video microscopy in WIFB9 cells that were stably t ransfected with mdr1-GFP. A critical role for 3'-phosphoinositide products of PI3 kinase was demonstrated in the intracellular trafficking of canalicu lar ABC transporters and for optimal transporter activity within the canali cular membrane. These studies suggest that many intracellular components, i ncluding ATP, Ca2+, numerous GTPases, microtubules, cytoplasmic motors, and other unknown factors, are required for physiologic regulation of ABC tran sporter traffic from Golgi to the canalicular membrane. Defects in this com plex system are prostulated to produce an "intrahepatic traffic jam" that r esults in defective ABC transporter function in the canalicular membrane an d, consequently, in cholestasis.