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J. Biol. Chem., Vol. 280, Issue 8, 6766-6779, February 25, 2005

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

Daniel Wüstner

From the Theoretical Biophysics Group, Max-Delbrück Center for Molecular Medicine, Robert-Rössle-Strasse 10, D-13125 Berlin, Germany

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.

Received for publication, November 23, 2004

^* This work was supported by a postdoctoral fellowship of the Max-Delbrück Center. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org) contains supplementary data.

To whom correspondence should be addressed: Theoretical Biophysics Group, Max-Delbrück Center for Molecular Medicine, Robert-Rössle-Str. 10, D-13125 Berlin, Germany. Tel.: 49-30-9406-3713; Fax: 49-30-9406-2548; E-mail: da.wuestner{at}mdc-berlin.de.

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