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Key Event Title
Inhibition of CYP26B1 activity
|Level of Biological Organization|
Key Event Components
Key Event Overview
AOPs Including This Key Event
|Human, rat, mouse||Human, rat, mouse||Moderate||NCBI|
|All life stages||Moderate|
Key Event Description
CYP26B1 and its function
Numerous physiological processes as well as fetal development is dependent on correct levels of retinoic acid (RA). RA homeostasis is tightly regulated and involves, amongst many other factors, breakdown by the Cytochrome P450 enzymes of family 26 (Kedishvili, 2013; White et al, 1997; White et al, 1996). The family consists of the enzymes CYP26A1, CYP26B1, and CYP26C1 (Isoherranen & Zhong, 2019). These RA-metabolizing enzymes convert all-trans-RA to more polar metabolites, including 4-hydroxy-RA and 4-oxo-RA (White et al, 1996; White et al, 2000).
CYP26B1 is particularly important for the clearance of RA in the fetal mouse testis, with knockout models having established its critical role in normal testis development in preventing premature meiotic initiation of male germ cells (Bowles et al, 2018; Bowles et al, 2006; Li et al, 2009; MacLean et al, 2007). CYP26B1 is necessary for steroidogenesis and male reproductive tract formation (Bowles et al, 2018), and may also play a role in maintaining normal spermatogenesis in the adult male mouse (Hogarth et al, 2015). CYP26B1 has been shown to be expressed in the postnatal mouse ovary, where it is proposed to regulate granulosa cell proliferation (Kipp et al, 2011).
In addition to reproductive development, CYP26B1 is important for other aspects of embryonic development. The expression of CYP26B1 is initiated at embryonic day 8 in the mouse hindbrain and is expressed in the limb buds from the beginning of their outgrowth as well as later in many different tissues during organ development including the genital tubercle, craniofacial areas, and spinal cord (Abu-Abed et al, 2002; MacLean et al, 2001; Ross & Zolfaghari, 2011). Consistent with this, Cyp26b1-knockout mice display various developmental defects including severe limb malformations and pups born alive die right after birth due to respiratory distress (Yashiro et al, 2004).
Besides from the important role CYP26B1 plays during fetal development, CYP26B1 is also expressed in multiple tissues in the adult human including adipose tissue, bladder, blood vessels, brain, kidney, adrenals, liver, lung, pancreas, intestines, testis, uterus, and skin (The-Human-Protein-Atlas, 2021; Topletz et al, 2012).
CYP26B1 inhibition as Key Event
The main function of CYP26B1 is to inactivate all-trans RA. The major primary metabolites formed from RA by CYP26B1 are 4-OH-RA and 18-OH-RA (Topletz et al, 2012) and CYP26B1 shows preference for the following substrates: all-trans-RA > 9-cis-RA > 13-cis-RA (Topletz et al, 2012; White et al, 2000). CYP26B1 is regulated by a range of molecular factors but also by RA itself (Isoherranen & Zhong, 2019; White et al, 2000).
How It Is Measured or Detected
There are no OECD validated assays for measuring CYP26B1 inhibition.
CYP26b1 mRNA and protein levels can be measured using various probes, antibodies as well as ELISA kits that are commercially available.
Enzyme activity can be measured in different types of assays including using microsomes measuring conversion of RA to metabolites (Van Wauwe et al, 1988).
Domain of Applicability
CYP26B1 is highly evolutionary conserved with e.g. a human to mouse sequence homology of 93% (Thatcher & Isoherranen, 2009).
This KE is applicable for both sexes, across developmental stages into adulthood, in numerous cells and tissues and across taxa.
Abu-Abed S, MacLean G, Fraulob V, Chambon P, Petkovich M, Dollé P (2002) Differential expression of the retinoic acid-metabolizing enzymes CYP26A1 and CYP26B1 during murine organogenesis. Mech Dev 110: 173-177
Bowles J, Knight D, Smith C, Wilhelm D, Richman J, Mamiya S, Yashiro K, Chawengsaksophak K, Wilson MJ, Rossant J, Hamada H, Koopman P (2006) Retinoid signaling determines germ cell fate in mice. Science 312: 596-600
Foti RS, Diaz P, Douguet D (2016) Comparison of the ligand binding site of CYP2C8 with CYP26A1 and CYP26B1: a structural basis for the identification of new inhibitors of the retinoic acid hydroxylases. J Enzyme Inhib Med Chem 31: 148-161
Hogarth CA, Evans E, Onken J, Kent T, D. M, Petkovich M, Griswold MD (2015) CYP26 Enzymes Are Necessary Within the Postnatal Seminiferous Epithelium for Normal Murine Spermatogenesis. Biol Reprod 93: 19
Kipp JL, Golebiowski A, Rodriguez G, Demczuk M, Kilen SM, Mayo KE (2011) Gene expression profiling reveals Cyp26b1 to be an activin regulated gene involved in ovarian granulosa cell proliferation. Endocrinology 152: 303-312
Li H, MacLean G, Cameron D, Clagett-Dame M, Petkovich M (2009) Cyp26b1 expression in murine Sertoli cells is required to maintain male germ cells in an undifferentiated state during embryogenesis. PLoS One 4: e7501
MacLean G, Abu-Abed S, Dollé P, Tahayato A, Chambon P, Petkovich M (2001) Cloning of a novel retinoic-acid metabolizing cytochrome P450, Cyp26B1, and comparative expression analysis with Cyp26A1 during early murine development. Mech Dev 107: 195-201
Topletz AR, Thatcher JE, Zelter A, Lutz JD, Tay S, Nelson WL, Isoherranen N (2012) Comparison of the function and expression of CYP26A1 and CYP26B1, the two retinoic acid hydroxylases. Biochem Pharmacol 83: 149-163
White JA, Beckett-Jones B, Guo YD, Dilworth FJ, Bonasoro J, Jones G, Petkovich M (1997) cDNA cloning of human retinoic acid-metabolizing enzyme (hP450RAI) identifies a novel family of cytochromes P450. J Biol Chem 272: 18538-18541
White JA, Guo YD, Baetz K, Beckett-Jones B, Bonasoro J, Hsu KE, Dilworth FJ, Jones G, Petkovich M (1996) Identification of the retinoic acid-inducible all-trans-retinoic acid 4-hydroxylase. J Biol Chem 271: 29922-29927
White JA, Ramshaw H, Taimi M, Stangle W, Zhang A, Everingham S, Creighton S, Tam SP, Jones G, Petkovich M (2000) Identification of the human cytochrome P450, P450RAI-2, which is predominantly expressed in the adult cerebellum and is responsible for all-trans-retinoic acid metabolism. Proc Natl Acad Sci U S A 97: 6403-6408
Yashiro K, Zhao X, Uehara M, Yamashita K, Nishijima M, Nishino J, Saijoh Y, Sakai Y, Hamada H (2004) Regulation of retinoic acid distribution is required for proximodistal patterning and outgrowth of the developing mouse limb. Dev Cell 6: 411-422