Beyond detoxification: a role for mouse mEH in the hepatic metabolism of endogenous lipids

Abstract

Microsomal and soluble epoxide hydrolase (mEH and sEH) fulfill apparently distinct roles: Whereas mEH detoxifies xenobiotics, sEH hydrolyzes fatty acid (FA) signaling molecules and is thus implicated in a variety of physiological functions. These epoxy FAs comprise epoxyeicosatrienoic acids (EETs) and epoxy-octadecenoic acids (EpOMEs), which are formed by CYP epoxygenases from arachidonic acid (AA) and linoleic acid, respectively, and then are hydrolyzed to their respective diols, the so-called DHETs and DiHOMEs. Although EETs and EpOMEs are also substrates for mEH, its role in lipid signaling is considered minor due to lower abundance and activity relative to sEH. Surprisingly, we found that in plasma from mEH KO mice, hydrolysis rates for 8,9-EET and 9,10-EpOME were reduced by 50% compared to WT plasma. This strongly suggests that mEH contributes substantially to the turnover of these FA epoxides—despite kinetic parameters being in favor of sEH. Given the crucial role of liver in controlling plasma diol levels, we next studied the capacity of sEH and mEH KO liver microsomes to synthesize DHETs with varying concentrations of AA (1–30 μM) and NADPH. mEH-generated DHET levels were similar to the ones generated by sEH, when AA concentrations were low (1 μM) or epoxygenase activity was curbed by modulating NADPH. With increasing AA concentrations sEH became more dominant and with 30 μM AA produced twice the level of DHETs compared to mEH. Immunohistochemistry of C57BL/6 liver slices further revealed that mEH expression was more widespread than sEH expression. mEH immunoreactivity was detected in hepatocytes, Kupffer cells, endothelial cells, and bile duct epithelial cells, while sEH immunoreactivity was confined to hepatocytes and bile duct epithelial cells. Finally, transcriptome analysis of WT, mEH KO, and sEH KO liver was carried out to discern transcriptional changes associated with the loss of EH genes along the CYP-epoxygenase–EH axis. We found several prominent dysregulations occurring in a parallel manner in both KO livers: (a) gene expression of Ephx1 (encoding for mEH protein) was increased 1.35-fold in sEH KO, while expression of Ephx2 (encoding for sEH protein) was increased 1.4-fold in mEH KO liver; (b) Cyp2c genes, encoding for the predominant epoxygenases in mouse liver, were mostly dysregulated in the same manner in both sEH and mEH KO mice, showing that loss of either EH has a similar impact. Taken together, mEH appears to play a leading role in the hydrolysis of 8,9-EET and 9,10-EpOME and also contributes to the hydrolysis of other FA epoxides. It probably profits from its high affinity for FA epoxides under non-saturating conditions and its close physical proximity to CYP epoxygenases, and compensates its lower abundance by a more widespread expression, being the only EH present in several sEH-lacking cell types.Electronic supplementary materialThe online version of this article (doi:10.1007/s00204-017-2060-4) contains supplementary material, which is available to authorized users.

Epoxide hydrolase 1 (EPHX1) hydrolyzes epoxyeicosanoids and impairs cardiac recovery after ischemia

“…Compared to EPHX2, EPHX1 is 10s-to 1000s-fold slower in in vitro EET hydrolysis assays, and is also less abundant in vivo (1,7,8). In some tissues, such as the brain, which has low EPHX2 expression, EPHX1 is believed to be responsible for hydrolysis of only a small fraction of its preferred substrate, 11,12-EET (1, 8,9). Three other enzymes, EPHX3, EPHX4, and PEG1/MEST, were identified based on homology to the catalytic sites of EPHX1 and EPHX2 (7).…”

The Multifaceted Role of Epoxide Hydrolases in Human Health and Disease

“…However, the reality seems to be more complex. More recently, EPHX1 was shown to play a leading role in the hydrolysis of different fatty acid epoxides [26,27]. EPHX1 and EPHX2 indeed have partly redundant functions, with a significant overlap in substrate selectivity, and an identical catalytic triad to convert epoxides into diols.…”

“…EPHX1 and EPHX2 indeed have partly redundant functions, with a significant overlap in substrate selectivity, and an identical catalytic triad to convert epoxides into diols. These overlapping functions are illustrated in a murine model of cardiac ischemia [26,27]. In this model, hypoxia induces cytochrome P450 epoxygenase-mediated production of AA-derived EETs, which have cardioprotective properties.…”

“…Endogenous substrates include epoxides of fatty acids, which are important regulatory lipid mediators. These epoxy fatty acids comprise epoxyeicosatrienoic acids (EETs) and epoxy-octadecenoic acids (EpOMEs), which are formed by CYP epoxygenases from arachidonic acid (AA) and linoleic acid (LA), respectively [26][27][28]. EPHX1 catalyzes their hydrolysis to their respective diols, the so-called dihydroxyeicosatrienoic acids (DHETs) and dihydroxyoctadecanoic acids (DiHOMEs).…”

Metabolic/inflammatory/vascular comorbidity in psychiatric disorders; soluble epoxide hydrolase (sEH) as a possible new target

“…In a variety of bioassays, and particularly in inflammation driven models, epoxides of EPA and DHA have proven to have anti-inflammatory activities (Kodani et al, 2015). Given the crucial role of sEH, and to a lesser extent the microsomal epoxide hydorolase (Marowsky et al, 2017), in the metabolism of the anti-inflammatory omega-3 epoxides, it is likely that omega-3 PUFA (particularly EPA), in combination with a sEH inhibitor, would be a novel therapeutic approach for depression (Hashimoto, 2016).…”