Transcription of the Human Microsomal Epoxide Hydrolase Gene (EPHX1) Is Regulated by PARP-1 and Histone H1.2. Association with Sodium-Dependent Bile Acid Transport.

Hui Peng,Shuping Zhong,Daniel Levy,Qin-Shi Zhu,Makoto Makishima

doi:10.1371/journal.pone.0125318

Hui Peng, Shuping Zhong + Show 3 more

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https://doi.org/10.1371/journal.pone.0125318

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Journal: PloS one	Publication Date: May 20, 2015
Citations: 17	License type: CC BY 4.0

Affiliation: University of Southern California

Abstract

Microsomal epoxide hydrolase (mEH) is a bifunctional protein that plays a central role in the metabolism of numerous xenobiotics as well as mediating the sodium-dependent transport of bile acids into hepatocytes. These compounds are involved in cholesterol homeostasis, lipid digestion, excretion of xenobiotics and the regulation of several nuclear receptors and signaling transduction pathways. Previous studies have demonstrated the critical role of GATA-4, a C/EBPα-NF/Y complex and an HNF-4α/CAR/RXR/PSF complex in the transcriptional regulation of the mEH gene (EPHX1). Studies also identified heterozygous mutations in human EPHX1 that resulted in a 95% decrease in mEH expression levels which was associated with a decrease in bile acid transport and severe hypercholanemia. In the present investigation we demonstrate that EPHX1 transcription is significantly inhibited by two heterozygous mutations observed in the Old Order Amish population that present numerous hypercholanemic subjects in the absence of liver damage suggesting a defect in bile acid transport into the hepatocyte. The identity of the regulatory proteins binding to these sites, established using biotinylated oligonucleotides in conjunction with mass spectrometry was shown to be poly(ADP-ribose)polymerase-1 (PARP-1) bound to the EPHX1 proximal promoter and a linker histone complex, H1.2/Aly, bound to a regulatory intron 1 site. These sites exhibited 71% homology and may represent potential nucleosome positioning domains. The high frequency of the H1.2 site polymorphism in the Amish population results in a potential genetic predisposition to hypercholanemia and in conjunction with our previous studies, further supports the critical role of mEH in mediating bile acid transport into hepatocytes.

Highlights

Microsomal epoxide hydrolase is a 48-kDa bifunctional protein that is expressed on the hepatocyte endoplasmic reticulum membrane in two distinct topological orientations [1] where the type I form plays a central role in the metabolism of numerous xenobiotics [2]
The expression of Microsomal epoxide hydrolase (mEH) and its subsequent functional role in xenobiotic metabolism and sodium-dependent bile acid transport is determined by several transcription factors binding to the proximal promoter
Previous studies have demonstrated that the transcriptional regulation of EPHX1 is mediated, in part, by GATA-4 [16], and complexes composed of C/EBPα-NF/Y [17] and HNF-4α/CAR/RXR/PSF [18]

Summary

Introduction

Microsomal epoxide hydrolase (mEH) is a 48-kDa bifunctional protein that is expressed on the hepatocyte endoplasmic reticulum membrane in two distinct topological orientations [1] where the type I form plays a central role in the metabolism of numerous xenobiotics [2]. Defects in bile salt transporters are involved in the etiology of numerous hepatobiliary disorders [15] Previous studies from this laboratory have demonstrated that GATA-4 [16], a C/EBPα-NF/Y complex [17] and an HNF-4α/CAR/RXR/PSF complex [18] play critical roles in regulating the transcription of the mEH gene (EPHX1). Studies have identified mutations in human EPHX1 that resulted in a 95% decrease in mEH expression that was associated with a significant decrease in bile acid uptake across the sinusoidal plasma membrane resulting in a 100-fold increase in serum bile salt levels (hypercholanemia) in the absence of liver damage [19]. The Ntcp mRNA and protein expression levels in this subject were normal with no mutations in the amino acid sequence [20]

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