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J. Biol. Chem., Vol. 282, Issue 45, 33218-33226, November 9, 2007

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Proteomic and Lipid Characterization of Apolipoprotein B-free Luminal Lipid Droplets from Mouse Liver Microsomes

IMPLICATIONS FOR VERY LOW DENSITY LIPOPROTEIN ASSEMBLY^*

Huajin Wang¹, Dean Gilham², and Richard Lehner^¶3

From the Departments of Cell Biology and ^¶Pediatrics and the Canadian Institutes of Health Research Group on Molecular and Cell Biology of Lipids, University of Alberta, Edmonton, Alberta T6G 2S2, Canada

The assembly of very low density lipoproteins involves the formation of a primordial, poorly lipidated apoB-containing particle in the endoplasmic reticulum, followed by the addition of neutral lipid from luminal lipid droplets (LLD). However, the lipid and protein compositions of LLD have not been determined. We have isolated LLD from mouse liver microsomes and analyzed their lipid and protein compositions. LLD are variably sized particles relatively poor in triacylglycerol (TG) content when compared with the lipid composition of cytosolic lipid droplets (CLD). They are devoid of apoB, adipophilin, and albumin but contain numerous proteins different from those found on CLD, including TG hydrolase (TGH), carboxylesterase 1 (Ces1), microsomal triglyceride transfer protein (MTP), and apoE. Ectopic expression of TGH in McArdle RH7777 hepatoma cells resulted in decreased cellular TG levels, demonstrating a role for TGH in the mobilization of hepatic neutral lipid stores. The isolation and characterization of LLD provide new supporting evidence for the two-step assembly of very low density lipoproteins.

Received for publication, August 16, 2007 , and in revised form, September 11, 2007.

^* This work was supported in part by Canadian Institutes of Health Research Grant MOP-69043. 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.

¹ Supported by the Faculty of Medicine and Dentistry 75th Anniversary Award.

² Supported by the Canadian Institutes of Health Research/Heart and Stroke Foundation of Canada/Industry Strategic Training Program (SCOLAR) during the study. Present address: Dept. of Pathology and Laboratory Medicine, University of Cincinnati, Cincinnati, OH 45237.

³ A Senior Scholar of the Alberta Heritage Foundation for Medical Research. To whom correspondence should be addressed: CIHR Group in Molecular and Cell Biology of Lipids, University of Alberta, 328 Heritage Medical Research Centre, Edmonton, Alberta T6G 2S2, Canada. Tel.: 780-492-2963; Fax: 780-492-3383; E-mail: richard.lehner{at}ualberta.ca.

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