2.2. Mechanisms of hypoxia-mediated regulation of CYP
2.2.1. Nuclear receptors
2.2.1.1. Pregnane X receptor (PXR) and constitutive androstane receptor
(CAR)
PXR and CAR are members of the orphan receptor subfamily. The targets
and regulatory functions of PXR and CAR are similar in many ways.
Activation of PXR and CAR also induces CYP expression (Bautista-Olivier
et al., 2022; Smutny et al., 2021). Hypoxia affected PXR expression Yuan
et al. found that the expression of PXR mRNA in the rat liver
substantially decreased by 63%, 96%, 86%, and 85% after 12, 24, 36,
and 48 h of exposure to acute hypoxia, respectively. PXR protein
expression increased substantially by 93% and 99% after 36 and 48 h of
exposure to hypoxia, respectively (Yuan et al., 2019). Such changes in
PXR or CAR under hypoxic conditions may be the potential mechanisms
mediating CYP changes during hypoxia. Legendre et al. found that the
expression of CAR and PXR is downregulated, whereas the activities of
CYP3A4, CYP1A2, CYP2E1, and CYP2C9 are reduced during hypoxia (Legendre
et al., 2009). Our study also confirmed that hypoxia inhibited the
expression of CYP isoforms through the PXR and CAR regulatory pathways.
Hypoxia is effective in inhibiting the expression of CYP isoforms, PXR,
and CAR proteins and mRNAs in both rat and HepG2 cells. We conducted
additional research in vitro using substances that either inhibit or
activate PXR and CAR. Our findings indicate that hypoxia and inhibitors
suppress the expression of CYP450 isoforms, while activators increase
the expression of CYP isoforms, PXR, and CAR. Furthermore, hypoxia
counteracts the activating effect of activators, suggesting that the
decrease in expression of CYP1A2, CYP2C9, CYP2E1, and CYP3A4 under
hypoxia is primarily due to the inhibition of the regulatory effects of
PXR and CAR (Duan et al., 2020).
2.2.1.2. Aryl hydrocarbon receptor (AHR)
AHR promotes the transcription of its target genes, including CYP1A1 and
CYP1A2, by binding to the aryl hydrocarbon receptor nuclear translocator
(ARNT) to form the AHR/ARNT complex (Dolly et al., 2023), which also
occurs under hypoxic conditions. Zhang et al. measured CYP1A1 mRNA in human lung microvascular endothelial cells exposed to hypoxia
and 25 nM AHR siRNA, and found that hypoxia substantially reducedCYP1A1 and AHR mRNA, and AHR siRNA also
substantially reduced CYP1A1 mRNA. Hypoxia, when paired with AHR
siRNA, resulted in a greater reduction of CYP1A1 mRNA compared to
hypoxia alone with AHR siRNA. This suggests that hypoxia and AHR
inhibition may have a shared route for lowering CYP1A1 expression (Zhang
et al., 2007). This regulatory role was further confirmed in a later
study using the AHR activator-like dioxin PCB (PCB126), where Vorrink et
al. found that exposure to hypoxia (1% O2)
substantially inhibited CYP1A1 mRNA and protein expression in human
HepG2 and HaCaT cells. CYP1A1 promoter-luciferase reporter activity
after PCB126 stimulation was also substantially reduced by hypoxic
stimulation, and hypoxic preconditioning reduced PCB126-induced AHR
binding to the promoters of CYP1 target genes (Coelho et al., 2020;
Jacob et al., 2015; Vorrink et al., 2014).
2.2.1.3. Peroxisome proliferator-activated receptor (PPAR)
PPARα is a member of the peroxisome proliferator-activated receptor
family, which is involved in the regulation of a variety of biological
processes. PPARα activates members of CYP1A1, CYP2E1, CYP2J2, CYP4A, and
CYP7A1 (Cizkova et al., 2012), and further studies on the role of PPARα
ligands will provide insight into the link between inhibition and CYP
expression. Treatment with PPARα ligands, such as Wy-14643 or fibrate
downregulated CYP2C9 and CYP2C44 expression and reduced the biosynthesis
of the CYP epoxide metabolite, EET, in humans and mice, respectively.PPAR is a key gene in hypoxia adaptation, is highly susceptible
to hypoxia regulation, and promotes adaptation to hypoxia (Kudo et al.,
2023; S. Wang et al., 2022). This change in PPAR under hypoxic
conditions also regulates CYP expression. Bonazzi et al. found that in
rabbit corneal epithelial cells, hypoxia increased PPAR-inducedCYP4B1 mRNA (Bonazzi et al., 2000). van Malenstein et al. found
that in human HepG2 hepatocytes chronically hypoxic for 72 h at 2%
O2, a PPARα/RXRα expression signal was affected and the
hepatic CYP450-specific detoxification pathway was downregulated (van
Malenstein et al., 2012).
2.2.1.4. Hepatocyte nuclear factor (HNF)
HNF1α and HNF4α are direct trans-activators of CYP genes involved in
regulating the constitutive expression of most CYP genes in the human
liver. Knockdown of HNF4α and HNF1α alters mRNA levels of CYP2B6 ,CYP2C8 , CYP2C9 , CYP2C19 , CYP2E1 ,CYP1A2 and CYP3A4 (Chen et al., 2018; Chiang et al.,
2014). Hypoxia inhibits HNF expression, and Mazure et al. initially
found that hypoxia downregulates HNF4 protein and gene expression in
HepG2 cells (Mazure et al., 2001). Sato et al. found that hypoxia
decreased HNF4α protein expression and stability in β-cells (Sato et
al., 2017). Changes in the expression of HNF1α and HNF4α in hypoxia may
be a potential regulatory mechanism for the changes in CYP expression
under hypoxia. Yang showed that HNF1α and HNF4α synergistically
downregulate CYP1A2 through mutual inhibition under hypoxia; the
regulatory mechanism of CYP under hypoxia may be different from that
under normoxia, where HNF1α and HNF4α inhibit each other and
synergistically downregulate CYP1A2; and there is a protein-protein
interaction between HNF1α and HNF4α. HNF1A binds to the HNF1A binding
site (-2892 bp to -2886 bp) in the CYP1A2 promoter, thereby activating
the CYP1A2 promoter and the effect of HNF4A on CYP1A2 is not in the
promoter region (Yang et al., 2022).