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.

中文翻译：

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除排毒之外：小鼠mEH在内源性脂质肝脏代谢中的作用。

微粒体和可溶性环氧化物水解酶（mEH和sEH）扮演着明显不同的角色：尽管mEH解毒异种生物，但sEH水解脂肪酸（FA）信号分子，因此涉及多种生理功能。这些环氧FA包含环氧二十碳三烯酸（EET）和环氧十八烯酸（EpOMEs），它们分别由CYP环氧酶由花生四烯酸（AA）和亚油酸形成，然后水解为它们各自的二醇，即DHET和DiHOMEs。尽管EET和EpOMEs也是mEH的底物，但由于其丰度和活性均低于sEH，因此其在脂质信号传导中的作用被认为是次要的。出乎意料的是，我们发现在mEH KO小鼠的血浆中，与WT血浆相比，8,9-EET和9,10-EpOME的水解速率降低了50％。这有力地表明，尽管动力学参数有利于sEH，但mEH实质上有助于这些FA环氧化物的周转。考虑到肝脏在控制血浆二醇水平中的关键作用，我们接下来研究了sEH和mEH KO肝微粒体合成具有不同浓度的AA（1-30μM）和NADPH的DHET的能力。当AA浓度低（1μM）或通过调节NADPH抑制环氧酶活性时，mEH产生的DHET水平与sEH产生的水平相似。随着AA浓度的增加，sEH变得更加占优势，并且与mEH相比，30μM的AA产生的DHET水平高出一倍。C57BL / 6肝切片的免疫组织化学进一步揭示了mEH表达比sEH表达更广泛。在肝细胞，枯否细胞，内皮细胞中检测到mEH免疫反应性，胆管上皮细胞和胆管上皮细胞，而sEH免疫反应仅限于肝细胞和胆管上皮细胞。最后，对WT，mEH KO和sEH KO肝脏进行转录组分析，以识别与沿CYP-环氧酶-EH轴的EH基因丢失相关的转录变化。我们发现在两个KO肝脏中并行发生几种突出的异常调节：（a）sEH KO中Ephx1（编码mEH蛋白）的基因表达增加了1.35倍，而Ephx2（编码sEH蛋白）的表达增加了1.4。 mEH KO肝中的3倍；（b）在sEH和mEH KO小鼠中，编码小鼠肝脏中主要环氧酶的Cyp2c基因大多以相同的方式失调，表明任一EH的丧失都具有相似的影响。综上所述，mEH似乎在8的水解中起着主导作用，9-EET和9,10-EpOME，也有助于其他FA环氧化物的水解。它可能是由于在非饱和条件下对FA环氧化物的高亲和力以及与CYP环氧酶的物理接近性而获益，并通过更广泛的表达来补偿其较低的丰度，这是在几种缺少sEH的细胞类型中唯一的EH。