3.2.1. Arachidonic acid
AA is the most well-characterized polyunsaturated fatty acid substrate and can be used to generate various eicosanoids via the cyclooxygenase (COX), lipoxygenase (LOX), and CYP pathways. The COX and LOX pathways are well studied; however, an increasing number of studies have recently found that eicosanoids derived from the CYP pathway also have important biological roles, particularly in regulating the development and growth of high-altitude diseases. CYP monooxygenases consist of cyclooxygenases (CYP2 family) and ω-hydroxylases (CYP4 family). Epoxidation of AA by epoxidase produces four epoxyeicosatrienoic acid (EET) regional isomers: 5,6-EET, 8,9-EET, 11,12-EET, and 14,15-EET. Subsequently, it is metabolized by soluble epoxide hydrolase (sEH) to the corresponding dihydroxyeicosatrienoic acid (DHET), which has lower potency. ω-hydroxylase oxidizes AA to 19- or 20-hydroxyeicosatetraenoic acid (HETE).
EET is an endothelium-derived hyperpolarizing factor that regulates vascular tone, influences cell proliferation and angiogenesis, and modulates cellular responses to hypoxia. Vascular endothelial growth factor (VEGF) is integral to the entire process of angiogenesis. Suzuki et al. found that the overexpression of CYP2C8 in human hepatocellular carcinoma cells Hep3B and human umbilical artery endothelial cells (HUAEC) induced the production of VEGF and erythropoietin under hypoxic conditions, whereas the hypoxia response element luciferase promoter activity of VEGF was inhibited after the use of the CYP2C8 inhibitor, sulfaphenazole. Exogenous 11,12-EET and 14,15-DHET induced reporter gene activity in HUAEC and Hep3B cells, along with an increase in the level of HIF-1α, suggesting the importance of EETs and DHETs in the response to hypoxia (Suzuki et al., 2008). This effect was further confirmed by Zhao et al. who found that 11,12-EET promoted HUVEC cell angiogenesis, an effect that was attenuated by the EET antagonist, 14,15-EEZE, and 11,12-EET up-regulated the expression of VEGF-A, HIF-1α, and bFGF under both normoxic and hypoxic conditions (Zhang et al., 2021; Zhao et al., 2018). EET expression was modulated by CYP changes during hypoxia, as shown by Michaelis et al. in cultured bovine retinal endothelial cells under normoxic and hypoxic (1% O2) conditions. They found that hypoxic treatment for 6–48 h enhanced CYP2C protein expression and EET formation (Michaelis et al., 2005; Michaelis et al., 2008). In vivo, a 3-week hypoxia exposure of sEH knockout (sEH-KO) and wild-type mice revealed hypoxia-induced downregulation of sEH and upregulation of CYP2C9, accompanied by elevation of CYP-derived superoxide, which resulted in enhanced lung EET in hypoxic mice, with substantially higher levels in sEH-KO mice, suggesting that hypoxia drives the increase in EET (Kandhi et al., 2019). HETE is also affected by hypoxia. Zhu et al. found that 15-HETE was activated in the pulmonary arteries (PA) of neonatal rabbits maintained in a hypoxic environment for 9 days and that the formation of 15-HETE was attenuated by lipoxygenase inhibitors. 15-HETE results in concentration-dependent contraction of the PA ring in animals exposed to hypoxia, whereas lipoxygenase inhibitors reduce phenylephrine-induced contraction of the PA ring (Zhu et al., 2003). In addition, similar results were obtained in an in vitro culture model of corneal organoids developed by Vafeas et al., in which hypoxia considerably stimulated the synthesis of 12(R)-HETE. Further studies revealed that CYP4B1 is involved in the synthesis of these eicosanoids in the corneal epithelium in response to hypoxia (Mastyugin et al., 1999; Vafeas et al., 1998).