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.

Microsomal epoxide hydrolase (mEH) is a detoxifying enzyme for xenobiotic compounds. Enzymatic activity of mEH can be greatly increased by a point mutation, leading to an E404D amino acid exchange in its catalytic triad. Surprisingly, this variant is not found in any vertebrate species, despite the obvious advantage of accelerated detoxification. We hypothesized that this evolutionary avoidance is due to the fact that the mEH plays a dualistic role in detoxification and control of endogenous vascular signaling molecules. To test this, we generated mEH E404D mice and assessed them for detoxification capacity and vascular dynamics. In liver microsomes from these mice, turnover of the xenobiotic compound phenanthrene-9,10-oxide was four times faster compared to WT liver microsomes, confirming accelerated detoxification. mEH E404D animals also showed faster metabolization of a specific class of endogenous eicosanoids, arachidonic acid-derived epoxyeicosatrienoic acids (EETs) to dihydroxyeicosatrienoic acids (DHETs). Significantly higher DHETs/EETs ratios were found in mEH E404D liver, urine, plasma, brain and cerebral endothelial cells compared to WT controls, suggesting a broad impact of the mEH mutant on endogenous EETs metabolism. Because EETs are strong vasodilators in cerebral vasculature, hemodynamics were assessed in mEH E404D and WT cerebral cortex and hippocampus using cerebral blood volume (CBV)-based functional magnetic resonance imaging (fMRI). Basal CBV0 levels were similar between mEH E404D and control mice in both brain areas. But vascular reactivity and vasodilation in response to the vasodilatory drug acetazolamide were reduced in mEH E404D forebrain compared to WT controls by factor 3 and 2.6, respectively. These results demonstrate a critical role for mEH E404D in vasodynamics and suggest that deregulation of endogenous signaling pathways is the undesirable gain of function associated with the E404D variant.Electronic supplementary materialThe online version of this article (doi:10.1007/s00204-016-1666-2) contains supplementary material, which is available to authorized users.

Recent studies suggest a role for the arachidonic acid-derived epoxyeicosatrienoic acids (EETs) in attenuating epileptic seizures. However, their effect on neurotransmission has never been investigated in detail. Here, we studied how 11,12- and 14,15 EET affect excitability and excitatory neurotransmission in mouse hippocampus. 11,12 EET (2 μM), but not 14,15 EET (2 μM), induced the opening of a hyperpolarizing K conductance in CA1 pyramidal cells. This action could be blocked by BaCl, the G protein blocker GDPβ-S and the GIRK1/4 blocker tertiapin Q and the channel was thus identified as a GIRK channel. The 11,12 EET-mediated opening of this channel significantly reduced excitability of CA1 pyramidal cells, which could not be blocked by the functional antagonist EEZE (10 μM). Furthermore, both 11,12 EET and 14,15 EET reduced glutamate release on CA1 pyramidal cells with 14,15 EET being the less potent regioisomer. In CA1 pyramidal cells, 11,12 EET reduced the amplitude of excitatory postsynaptic currents (EPSCs) by 20% and the slope of field excitatory postsynaptic potentials (fEPSPs) by 50%, presumably via a presynaptic mechanism. EEZE increased both EPSC amplitude and fEPSP slope by 40%, also via a presynaptic mechanism, but failed to block 11,12 EET-mediated reduction of EPSCs and fEPSPs. This strongly suggests the existence of distinct targets for 11,12 EET and EEZE in neurons. In summary, 11,12 EET substantially reduced excitation in CA1 pyramidal cells by inhibiting the release of glutamate and opening a GIRK channel. These findings might explain the therapeutic potential of EETs in reducing epileptiform activity.

This review explores the concept of altered glucose metabolic pathways leading to ROS increase and its possible link to NDs, with a special emphasis on Alzheimer's diseases (AD). We also discuss the detailed mechanistic link between hyperglycemia, ROS generation, and neurodegeneration to highlight potential therapeutic avenues for better prevention and treatment.

No abstract