Relationship between biliary lipid and protoporphyrin secretion; potential role of mdr2 P-glycoprotein in hepatobiliary organic anion transport

Erythropoietic protoporphyria, caused by ferrochelatase deficiency, leads to protoporphyrin accumulation in the liver. Therapeutic attempts to increase the secretion of ths hydrophobic organic anion into bile are hampered by a lack of understanding of the secretory mechanism(s) involved. We have investigated biliary secretion of protoporphyrin in rats and mice, primarily targeted on the role of biliary lipids in this process. Methods: Gel permeation chromatography was applied to investigate the association of porphyrins with lipid fractions in bile. Secretion of endogenous porphyrins was studied in (GY mutant) rats and mdr2 P-glycoprotein deficient mice, under conditions of widely varying biliary lipid secretion rates. Results: Gel permeation chromatography revealed that, in native human and rat bile, protoporphyrin associated with cholesterol/phospholipid vesicles upon elution with bile salt-free buffer. In contrast, the more hydrophilic corporpyrin isomers I and III were found only in bile salt/organic anion hybrid particles and smaller aggregates. Interruption of the enterohepatic circulation in normal Wistar rats resulted in a parallel decrease of endogenous protoporphyrin-, lipid-, and bile salt secretion, but did not alter the secretion of coproporphyrin I and III. Uncoupling of lipid- from bile salt secretion by sulfated taurolithocholate resulted in imparied secretion into bile of protoporpyrin only. Conversely, secretion of coproporphyrin I and III, but not that of protoporphyrin, was impaired in mutant Gronigen Yellow rats with defective ATP-dependent hepatobiliary organic anions transport. In mice homozygous for a disruption of the mdr2 P-glycoprotein gene, resulting in complete absence of phospholipids in bile and strongly reduced cholesterol output, secretion of protoporphyrin was reduced by 90%, whereas that of coproporpyrin I and III was affected to a much lesser extent. Conclusions: Our data demonstrate a close association between protoporphyrin and lipid secretion into bile, indicating that these processes are, at least functionally coupled. This finding implicates a role of mdr2 P-glycoprotein activity in hepatobiliary removal of the hydrophobic organic anion protoporphyrin. Hence, it may be speculated that protoporphyrin secretion can be influenced by drugs, diet or other means that affect biliary lipid secretion