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).