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,Roger A Davis

doi:10.1194/jlr.m300020-jlr200

Abstract

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.

Highlights

This study shows, for the first time, that blocking hepatic apolipoprotein B (apoB) secretion in vivo in mice results in a rapid homeostatic degradation pathway that prevents the accumulation of apoB in the endoplasmic reticulum (ER) without inducing heat shock or unfolded protein responses
Blocking Microsomal triglyceride transfer protein (MTP) function by Cre-mediated gene disruption or chemical inhibition reduced plasma lipid levels markedly without causing massive fatty liver. These findings suggest that MTP could be an effective therapeutic target for hyperlipidemia
Since MTP is localized to the proximal portion of the secretory pathway [42], we have focused these studies on examining how blocking MTP affects the ER content of apoB and of lumenal proteins that participate in the unfolded protein response

Summary

Introduction

There have been no published studies that have examined how the liver in vivo responds to loss of MTP in regard to the accumulation of apoB and possible induction of unfolded protein and/or heat shock responses. Our studies revealed that the inactivation of MTP (by gene disruption or chemical inactivation) led to a block in the secretion of apoB and prevented its accumulation in the ER, as well as the induction of unfolded protein and/or heat shock responses.

Results

Conclusion