Lecithin:Cholesterol Acyltransferase Deficiency Protects against Cholesterol-induced Hepatic Endoplasmic Reticulum Stress in Mice

Lauren Hager,Lixin Li,Henry Pun,Lu Liu,Mohammad A Hossain,Graham F Maguire,Mark Naples,Chris Baker,Lilia Magomedova,Jonathan Tam,Khosrow Adeli,Carolyn L Cummins,Philip W Connelly,Dominic S Ng

doi:10.1074/jbc.m112.340919

Abstract

We recently reported that lecithin:cholesterol acyltransferase (LCAT) knock-out mice, particularly in the LDL receptor knock-out background, are hypersensitive to insulin and resistant to high fat diet-induced insulin resistance (IR) and obesity. We demonstrated that chow-fed Ldlr-/-xLcat+/+ mice have elevated hepatic endoplasmic reticulum (ER) stress, which promotes IR, compared with wild-type controls, and this effect is normalized in Ldlr-/-xLcat-/- mice. In the present study, we tested the hypothesis that hepatic ER cholesterol metabolism differentially regulates ER stress using these models. We observed that the Ldlr-/-xLcat+/+ mice accumulate excess hepatic total and ER cholesterol primarily attributed to increased reuptake of biliary cholesterol as we observed reduced biliary cholesterol in conjunction with decreased hepatic Abcg5/g8 mRNA, increased Npc1l1 mRNA, and decreased Hmgr mRNA and nuclear SREBP2 protein. Intestinal NPC1L1 protein was induced. Expression of these genes was reversed in the Ldlr-/-xLcat-/- mice, accounting for the normalization of total and ER cholesterol and ER stress. Upon feeding a 2% high cholesterol diet (HCD), Ldlr-/-xLcat-/- mice accumulated a similar amount of total hepatic cholesterol compared with the Ldlr-/-xLcat+/+ mice, but the hepatic ER cholesterol levels remained low in conjunction with being protected from HCD-induced ER stress and IR. Hepatic ER stress correlates strongly with hepatic ER free cholesterol but poorly with hepatic tissue free cholesterol. The unexpectedly low ER cholesterol seen in HCD-fed Ldlr-/-xLcat-/- mice was attributable to a coordinated marked up-regulation of ACAT2 and suppressed SREBP2 processing. Thus, factors influencing the accumulation of ER cholesterol may be important for the development of hepatic insulin resistance.

Highlights

Hepatic endoplasmic reticulum (ER) stress promotes insulin resistance, but the role of cholesterol in this pathway is unknown
LdlrϪ/ϪxLcatϩ/ϩ Mice Are Susceptible to Dietary Cholesterol-induced ER Stress and Impairment of Glucose Tolerance, and LdlrϪ/ϪxLcatϪ/Ϫ Mice Are Protected from These Phenotypes—To investigate the role of cholesterol loading in the induction of hepatic ER stress, we fed the female C57Bl/6 (WT), LdlrϪ/ϪxLcatϩ/ϩ (SKO), and LdlrϪ/ϪxLcatϪ/Ϫ (DKO) mice a 2% high cholesterol diet (HCD) for 10 weeks
We examined the relationship between the hepatic tissue lipid and hepatic ER stress response to the 10-week HCD

Summary

Background

Hepatic ER stress promotes insulin resistance, but the role of cholesterol in this pathway is unknown. Despite the metabolically unfavorable high triglyceride/low HDL phenotypes [2, 7], we recently reported an unexpected observation that LCATdeficient mice are protected from diet-induced obesity and IR in a gender-specific manner with the phenotypes being more pronounced in females [8]. These protective effects were comparatively more pronounced in the low density lipoprotein receptor-null (LdlrϪ/Ϫ) background. LCAT Deficiency Attenuates Sterol-induced Hepatic ER Stress observed that the hepatic expression of unfolded protein response (UPR) markers is up-regulated by ϳ2-fold in the LdlrϪ/ϪxLcatϩ/ϩ mice when compared with the wild type (WT) control under chow-fed conditions. Alteration in the cholesterol esterification pathways in LdlrϪ/ϪxLcatϪ/Ϫ mice has been shown to protect, at least in part, against cholesterol-induced ER cholesterol accumulation and ER stress

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION