Soluble epoxide hydrolase in human tissues: Distribution, subcellular localization and novel endogeneous substrates
Soluble epoxide hydrolase (sEH) is a phase I xenobiotic metabolizing enzyme. Endogenous substrates include arachidonic acid epoxides (EETs) which are generated by cytochrome P450 epoxygenases. sEH has been suggested to be involved in various biological processes such as cell signaling, regulation of blood pressure and inflammation. Most of these effects have been attributed to regulation of the relatively well studied EET substrates. Recently, sEH was found to possess an N-terminal phosphatase domain in addition to the C-terminal epoxide hydrolase domain; however, data regarding potential endogenous substrates for the N-terminal domain is limited. sEH is known to exist predominantly in the liver and kidney. However, previous studies of its subcellular localization in these tissues have been controversial. In this study, we tested the hypothesis that sEH has a broad distribution in human tissues and that its subcellular localization may vary from one tissue to another. Indeed, our results show that sEH and CYP450 epoxygenases are widely expressed in various human tissues such as liver, kidney, intestine, adrenal gland, pituitary gland, prostate, vascular wall, lymphoid follicles and pancreas. In addition, previous evidence suggestive of a potential role of these CYPs and their EET products in carcinogenesis led us to evaluate the expression of these enzymes in neoplastic human tissue samples. The subcellular localization results interestingly showed cell/tissue-specific compartmentalization of sEH. In human liver and kidney sEH was found to be both cytosolic and peroxisomal, whereas it was found to be exclusively cytosolic in other sEH-expressing tissues. The peroxisomal subcellular localization of sEH in human liver led us to evaluate the peroxisomal isoprenoid phosphate intermediates as potential substrates of the N-terminal phosphatase domain. Several isoprenoid phosphates have been found to be substrates for the N-terminal domain such as farnesyl monophosphate that had a Km of 24 μM and Vmax of 684 nmol/min/mg. Subsequently, we were able to identify several isoprenoid derived compounds as novel inhibitors of the N-terminal domain with IC50 values from 0.84 μM to 55 μM. Our results are suggestive of a potential role for sEH in the regulation of isoprenoid dependent pathways such as cholesterol biosynthesis and G-protein isoprenylation.