MDR2 P-glycoprotein-mediated lipid secretion and its relevance to biliary drug transport

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Rapid nonvesicular transport of sterol between the plasma membrane domains of polarized hepatic cells

We studied the transport of the fluorescent cholesterol analog dehydroergosterol (DHE) in polarized HepG2 human hepatoma cells. DHE delivered via methyl-beta-cyclodextrin was delivered to both the apical and basolateral membranes and became concentrated in the apical membrane within 1 min. Intracellular DHE was targeted mainly to vesicles of the subapical compartment or apical recycling compartment (SAC/ARC), where it colocalized with fluorescent transferrin and fluorescent analogs of phosphatidylcholine and sphingomyelin. In contrast, transport of DHE from the plasma membrane to the trans-Golgi network was found to be very low. Vesicles containing DHE traversed the cells in both directions, but vesicular export of DHE from the SAC/ARC to the plasma membrane domains was low. Disruption of the microtubule cytoskeleton disturbed vesicular transport of DHE but not its enrichment in the apical (canalicular) membrane. Transport of DHE to the canalicular membrane after photobleaching was very rapid (t(12) = 1.6 min) and was largely ATP-independent in contrast to enrichment of DHE in the SAC/ARC. Release of DHE from the canalicular membrane was also ATP-independent but slower than the enrichment of sterol in the biliary canaliculus (t(12) = 5.4 min). Canalicular DHE could completely redistribute to the basolateral plasma membrane but could not transfer from one cell to the other cell of an HepG2 couplet. We conclude that sterol shuttles rapidly among the plasma membrane domains and other membrane organelles and that this nonvesicular pathway includes fast transbilayer migration.

Effect of P-glycoprotein modulators on the pharmacokinetics of camptothecin using microdialysis

1. By performing microdialysis, this study investigated the pharmacokinetics of unbound camptothecin in rat blood, brain and bile in the presence of P-glycoprotein mediated transport modulators (cyclosporin A, berberine, quercetin, naringin and naringenin). Pharmacokinetic parameters of camptothecin were assessed using a non-compartmental model. 2. Camptothecin rapidly crosses the blood-brain barrier (BBB) within 20 min after camptothecin administration. The disposition of camptothecin in rat bile appeared to have a slow elimination phase and a peak concentration after 20 min of camptothecin administration. The area under the concentration versus time curve (AUC) for camptothecin in bile significantly surpassed that in blood, suggesting active transport of hepatobiliary excretion. 3. In the presence of cyclosporin A camptothecin AUC, in the brain, was significantly elevated but no significant change in the presence of berberine, quercetin, naringin and naringenin. 4. With treatment by smaller doses of quercetin (0.1 mg x kg(-1)), naringin (10 mg x kg(-1)) and naringenin (10 mg x kg(-1)), they significantly diminished the camptothecin AUC in bile, but was not altered by the treatment of berberine (20 mg x kg(-1)), a higher dose of quercetin (10 mg x kg(-1)), and cyclosporin A treated (20 mg x kg(-1)) and pretreated groups. 5. The distribution ratio (AUC(bile)/AUC(blood)) of camptothecin in bile was decreased in the cyclosporin A, quercetin, naringin and naringenin treated groups. However, the distribution ratio in the brain was increased in the cyclosporin A groups, but was decreased in the groups treated with quercetin, naringin and naringenin. These results revealed that P-glycoprotein might modulate hepatobiliary excretion and BBB penetration of camptothecin.