Apolipoprotein A-I in bile inhibits cholesterol crystallization and modifies transcellular lipid transfer through cultured human gall-bladder epithelial cells.
BACKGROUND
Apolipoprotein A-I (Apo A-I), conventionally purified by several steps including organic solvent-delipidation from plasma, inhibits cholesterol crystallization in bile. To observe a significant effect in vitro, however, supraphysiological concentrations above 100 microg/mL are required. For this reason, this protein has not been considered to play a physiological role in vivo. In the present study, we examined the cholesterol crystal growth-inhibiting effect of biliary Apo A-I at its physiological concentration, the modification of transcellular transfer of biliary lipids through cultured human gall-bladder epithelial cells (GBEC) by Apo A-I at its physiological concentration and the binding and secretion of Apo A-I by GBEC.


RESULTS
We purified biliary Apo A-I to near homogeneity using immobilized artificial membrane chromatography. At 5 microg/mL, biliary Apo A-I reduced cholesterol crystal mass by 50%, whereas plasma-derived, solvent-delipidated Apo A-I had no effect. Using an antibody-capture enzyme-linked immunosorbent assay, we found reduced Apo A-I concentrations in bile samples from gallstone patients when compared with bile samples from gallstone-free controls (medians, 2.35 and 9.4 microg/mL, respectively). In a GBEC line, Apo A-I (5 microg/mL) enhanced transfer of phospholipid and cholesterol from the mucosal to the serosal side of cell monolayers by approximately 50%. These cells appear to bind Apo A-I reversibly in a dose- and time-dependent manner, compatible with receptor-type binding. Cultured human gall-bladder epithelial cells also showed basal secretion of Apo A-I, which was greatly increased by exposure to model bile solutions.


CONCLUSIONS
Apolipoprotein A-I in bile, thus, has both a direct effect on cholesterol crystal formation and enhances lipid removal from gall-bladder bile by GBEC. This effect may be specific and receptor mediated. These observations support two separate roles for human biliary Apo A-I and suggest that this protein may be important in preventing the formation of cholesterol crystals (the initial step in gallstone formation) in supersaturated bile.
Secknus
R
R
Gastrointestinal Research Unit, Cleveland Clinic Foundation, Ohio, USA. rsec@server3.medizin.uni-leipzig.de
Darby
G H
GH
Chernosky
A
A
Juvonen
T
T
Moore
E W
EW
Holzbach
R T
RT
D016632:Apolipoprotein A-I / Q000302:isolation & purification / Q000378:metabolism / Q000494:pharmacology / Q000502:physiology*; D001646:Bile / Q000737:chemistry; D001667:Binding, Competitive; D001692:Biological Transport; D002478:Cells, Cultured; D002784:Cholesterol / Q000378:metabolism*; D003460:Crystallization; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism; D005704:Gallbladder / Q000187:drug effects / Q000378:metabolism*; D006801:Humans; D050356:Lipid Metabolism; D013997:Time Factors
D016632:Apolipoprotein A-I; D002784:Cholesterol