1102 Pharmacologic Blockage of Cholesterol Absorption With Ezetimibe Reveals a Novel Homeostatic Network in Enterocytes

Abstract

Listen

Translate article

BACKGROUND: Ezetimibe is a widely-used drug that lowers plasma cholesterol by blocking NPC1L1-mediated sterol absorption in small intestine. Enterocyte cholesterol homeostasis is controlled by the aggregated rates of sterol synthesis, efflux, and uptake from plasma and gut lumen. Endogenous cholesterol synthesis and low density lipoprotein (LDL) uptake are coordinately regulated by transcription factors termed Sterol Regulatory Element-Binding Proteins (SREBPs). Liver-X-receptors (LXRs), another family of transcription factors, are activated by sterols and control other aspects of sterol metabolism, especially cholesterol efflux. How blockade of cholesterol absorption by ezetimibe (EZE) affects intestinal SREBPs, LXRs, and key effector molecules, such as LDL Receptor (LDLR) and HMG-CoA Reductase (HMGR) is unknown. AIM: To study the effect of EZE on regulatory molecules that control sterol synthesis and LDL uptake in small intestine. METHODS: C57BL/6 mice were fed a chow diet with indicated drugs. Liver and small intestine were harvested. Jejunal enterocytes were isolated, and tissues fractionated for immunoblotting, which were quantified by densitometry. mRNAs were quantified by qPCR and microarray analysis. RESULTS: Feeding EZE increased active nuclear SREBP-2 by 8-fold in intestine, leading to increases in mRNAs for HMGR and LDLR by 1.3-fold +/0.03 (P<0.01) and 2.0-fold +/0.13 (P<0.01), respectively. HMGR and LDLR protein levels increased much more profoundly (24-fold and 6-fold, respectively). Microarray analysis revealed that in jejunum, EZE increased the expression of 20 out of 21 SREBP-2 target genes encoding enzymes required for sterol biosynthesis. The mRNA of a LXR target gene, Inducible Degrader Of the LDLR (IDOL), was reduced by 46% +/6% (P<0.01) in EZE-treated intestine. Coadministration of EZE with an LXR agonist (T0901317) abolished EZE-mediated reduction in IDOL mRNA and prevented its induction of LDLR protein in the intestine. Hepatic LDLR was unaffected in these studies. CONCLUSION: To maintain sterol homeostasis in the face of EZE-induced blockade of cholesterol uptake, enterocytes boost LDL uptake by increasing LDLR levels and boost sterol synthesis by increasing HMGR and other genes needed for sterol synthesis. These changes in gene expression are mediated by an increase in active SREBP-2. For HMGR and LDLR, it is likely that this regulation is mediated by both transcriptional and posttranscriptional mechanisms. The reduced sterol input caused by EZE is associated with a reduction in LXR target genes including IDOL. The concerted increase in LDLR mRNA through SREBP-2 and reduction in IDOL leads to a dramatic increase in LDLR protein levels. These studies reveal a previously unknown homeostatic network in enterocytes engaged by pharmacologic blockage of cholesterol absorption.