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Journal of Lipid Research, Vol. 44, 978-985, May 2003
Copyright © 2003 by Lipid Research, Inc.

Blocking microsomal triglyceride transfer protein interferes with apoB secretion without causing retention or stress in the ER

Wei Liao^*, To Y. Hui^*, Stephen G. Young and Roger A. Davis^1,*

^* Mammalian Cell and Molecular Biology Laboratory, San Diego State University, San Diego, CA 92182-4614
Gladstone Institute of Cardiovascular Disease, Cardiovascular Research Institute, and Department of Medicine, University of California, San Francisco, CA 94110

¹ To whom correspondence should be addressed. e-mail: rdavis{at}sunstroke.sdsu.edu

Microsomal triglyceride transfer protein (MTP) is an intraluminal protein in the endoplasmic reticulum (ER) that is essential for the assembly of apolipoprotein B (apoB)-containing lipoproteins. In this study, we examine how the livers of mice respond to two distinct methods of blocking MTP function: Cre-mediated disruption of the gene for MTP and chemical inhibition of MTP activity. Blocking MTP significantly reduced plasma levels of triglycerides, cholesterol, and apoB-containing lipoproteins in both wild-type C57BL/6 and LDL receptor-deficient mice. While treating LDL receptor-deficient mice with an MTP inhibitor for 7 days lowered plasma lipids to control levels, liver triglyceride levels were increased by only 4-fold. Plasma levels of apoB-100 and apoB-48 fell by >90% and 65%, respectively, but neither apoB isoform accumulated in hepatic microsomes. Surprisingly, loss of MTP expression was associated with a nearly complete absence of apoB-100 in hepatic microsomes. Levels of microsomal luminal chaperone proteins [e.g., protein disulfide isomerase, glucose-regulated protein 78 (GRP78), and GRP94] and cytosolic heat shock proteins (HSPs) (e.g., HSP60, HSC, HSP70, and HSP90) were unaffected by MTP inhibition. These findings show that the liver responds rapidly to inhibition of MTP by degrading apoB and preventing its accumulation in the ER.

The rapid degradation of secretion-incompetent apoB in the ER may block the induction of proteins associated with unfolded protein and heat shock responses.

Abbreviations: ALLN, acetylated leucine, leucine, norleucal; ER, endoplasmic reticulum; GRP, glucose-regulated protein; HSP, heat shock protein; MTP, microsomal triglyceride transfer protein; PDI, protein disulfide isomerase

Supplementary key words apolipoprotein B • endoplasmic reticulum • hyperlipidemia • liver • inflammation • unfolded protein response

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