Mathematical Analysis of Hepatic High Density Lipoprotein Transport Based on Quantitative Imaging Data

Daniel Wüstner

doi:10.1074/jbc.m413238200

Abstract

Hepatocytes internalize high density lipoprotein (HDL) at the basolateral membrane. Most HDL is recycled while some is shuttled to the canalicular membrane by transcytosis. Here, transport of HDL was analyzed by mathematical modeling based on measurements in polarized hepatic HepG2 cells. Recycling of HDL from basolateral sorting endosomes was modeled by applying the rapid equilibrium approach. Analytical expressions were derived, which describe in one model the transport of HDL to the subapical compartment/apical recycling compartment, the biliary canaliculus (BC), and to late endosomes and lysosomes (LE/LYS). Apical endocytosis of HDL predicted by the model was confirmed for rhodamine-dextran and fluorescent asialoorosomucoid, markers for LE/LYS in living HepG2 cells. Budding of endocytic vesicles from the BC was directly observed by time lapse imaging of a fluorescent lipid probe. Based on fitted kinetic parameters and their covariance matrix a Monte Carlo simulation of HDL transport in hepatocytes was performed. The model was used to quantitatively assess release of HDL-associated free cholesterol by scavenger receptor BI. It is shown that only 6% of HDL-associated sterol reaches the BC as a constituent of the HDL particles, whereas the remaining sterol is rapidly released from HDL and shuttled to the BC by non-vesicular transport.

Highlights

Hepatic transport of high density lipoprotein (HDL)1 and processing of its associated lipids including sterols is a key step
Some fluorescent HDL was transported to the SAC/ARC whereas, with a lag-time, Alexa-488-HDL was found in the biliary canaliculus (BC)
To model intrahepatic transport of HDL based on this kinetic data two assumptions have been made: (i) internalized fluorescent HDL was targeted from the basolateral membrane to sorting endosomes (SE) from where rapid recycling occurred, (ii) some HDL escaping the recycling pathway was transcytosed to the BC and/or targeted to the SAC/ARC

Summary

Introduction

Hepatic transport of high density lipoprotein (HDL)1 and processing of its associated lipids including sterols is a key step. Analytical solutions for a 4-compartment model were derived and fitted simultaneously to the experimental data for transport of fluorescent HDL to the BC, to the SAC/ARC as well as to LE/LYS.

Results

Conclusion