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

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 Ͼ 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, glucoseregulated 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. VLDLs produced by the liver are the major source of LDLs in the plasma, which are causally related to the development of atherosclerotic cardiovascular disease (1). The importance of the hepatic secretion of apolipoprotein B (apoB) in cardiovascular disease was recognized in early studies of abetalipoproteinemic patients, who lack the ability to secrete apoB-containing lipoproteins (2) and are markedly resistant to cardiovascular disease (3). Subsequent studies showed that specific mutations in the microsomal triglyceride transfer protein (MTP) gene are responsible for abetalipoproteinemia (4,5). MTP exists in a functional complex with protein disulfide isomerase (PDI) (6, 7). The loss of MTP function blocks the secretion of apoB-containing lipoproteins from both the liver and the intestine (5). Apart from neuropathy, which can be prevented by vitamin E supplements (8) and moderate steatosis in enterocytes and hepatocytes, abetalipoproteinemia is not usually associated with liver failure or cirrhosis (3). The absence of severe symptoms has suggested that inactivation of MTP might be a useful strategy for combating hyperlipidemia and cardiovascular disease.Several chemical inhibitors of the lipid transfer activity of MTP have been developed (9-12). Many of these inhibitors lower plasma lipid levels in animal models (11-13). Especially encouraging results were obtained in Watanabe-heritable hyperlipidemic rabbits, a model of homozygous familial hypercholesterolemia (13). Administration of an MTP inhibitor to Watan...

Microsomal Triglyceride Transfer Protein Inhibition Induces Endoplasmic Reticulum Stress and Increases Gene Transcription via Ire1α/cJun to Enhance Plasma ALT/AST

“…1D). Increased hepatic triglyceride and free cholesterol is consistent with previous studies (17,20). Increases in hepatic free cholesterol after MTP inhibition occur as accumulation of cholesterol ester inhibits their synthesis and enhances cellular free cholesterol accumulation (20).…”

“…Western diet fed (WDF) C57Bl6J mice were utilized because MTP inhibition and gene deletion in chow fed mice did not cause ER stress or increase plasma ALT/AST levels (17,18). Second, MTP inhibitor therapy is under evaluation for treatment of hyperlipidemia (4,19).…”

Coordinate Transcriptional Repression of Liver Fatty Acid-binding Protein and Microsomal Triglyceride Transfer Protein Blocks Hepatic Very Low Density Lipoprotein Secretion without Hepatosteatosis

“…We now show that treating L35 cells with PPAR␣-RXR␣ agonists markedly enhanced the secretion of de novo synthesized 35 S-labeled apoB (Fig. 4C).…”

“…Vivo-Chemical inhibition of MTP, a strategy developed to ameliorate hyperlipidemia (47)(48)(49)(50), is of limited use because of the development of hepatic steatosis (35,51). Although L35 cells do not express MTP or secrete apoBcontaining lipoproteins, they do not accumulate triglycerides even when challenged with fatty acids (e.g.…”

“…fasting), fatty acids supplied by adipose tissue can provide sufficient substrate for the glycerolipid synthesis and VLDL assembly/secretion (34). Variations in hepatic expression levels of both L-FABP (9) and MTP (35) control the flux of fatty acids into glycerolipid biosynthesis and VLDL assembly/secretion.…”