3.1. The cholesterol epoxide hydrolase (ChEH).
ChEH is responsible for the hydration of 5,6-ECs to give
cholestane-3β,5α,6β-triol (CT)(Aringer & Eneroth, 1974; Chan & Black,
1974; Nashed, Michaud, Levin & Jerina, 1985; Sevanian & McLeod, 1986)
(Fig 1C). Like other epoxide hydrolases, ChEH was initially considered
as a type II detoxification enzyme to eliminate toxicants (Morisseau &
Hammock, 2005). However, the biological functions epoxide hydrolases
have been extended to the control of the production of bioactive lipids
(Kodani & Hammock, 2015; Morisseau, 2013; Newman, Morisseau & Hammock,
2005) as observed for other families of type two detoxification enzymes
such as glutathione transferases (Board & Menon, 2013). CT was shown to
display certain biological properties suggesting it may have a
physiological role in mammals. CT is an oxysterol that can induce
cytotoxicity in vivo and in vitro in normal and cancerous
cells (Carvalho, Silva, Moreira, Simoes & Sa e Melo, 2010; Carvalho,
Silva, Moreira, Simoes & Sa, 2011; Imai, Werthessen, Subramanyam,
LeQuesne, Soloway & Kanisawa, 1980; Kandutsch, Chen & Heiniger, 1978)
and hypocholesterolemia in animals (Aramaki, Kobayashi, Imai, Kikuchi,
Matsukawa & Kanazawa, 1967). As opposed to other oxysterols such as
25-hydroxycholesterol, CT is not an inhibitor of the HMG-coA reductase,
the rate-controlling enzyme of the cholesterol pathway, (Cavenee,
Gibbons, Chen & Kandutsch, 1979). This hypocholesterolemic property of
CT might be due to its inhibition of post-lanosterol cholesterogenic
enzymes such as DHCR7 (Witiak, Parker, Dempsey & Ritter, 1971) and C4
lanosterol demethylase (Scallen, Dhar & Loughran, 1971), and to a
modulation of the LXRα/SREBP2 axis (Lin et al., 2013). CT stimulates
phospholipid biosynthesis and CTP-phosphocholine Cytidyltransferase in
mammalian cells (Mahfouz, Smith, Zhou & Kummerow, 1996). CT was shown
to inhibit osteoblastic differentiation and the induction of bone marrow
stromal cell apoptosis (Liu, Yuan, Xu, Wang & Zhang, 2005). CT was also
reported to suppress prostate cancer cell proliferation, migration and
invasion (Lin et al., 2013), and to inhibit voltage-gated sodium
channels (Tang et al., 2015; Tang et al., 2018). CT displays chaperone
properties for the Niemann-Pick C1 protein, which is an intracellular
sterol transporter (Ohgane, Karaki, Noguchi-Yachide, Dodo & Hashimoto,
2014)and is also a blood marker for Niemann-Pick C1 Disease (Porter et
al., 2010). This illustrates the fact that CT displays certain
biological properties.
ChEH was characterized in 2010 as being carried out by a
multiproteinaceous hetero-oligomeric complex. This complex includes
enzymes involved in the late stages of cholesterol biosynthesis (de
Medina, Paillasse, Segala, Poirot & Silvente-Poirot, 2010). ChEH is
composed of the 3β-hydroxyterol-Δ8-Δ7-isomerase (D8D7I) and of the
3β-hydroxyterol-Δ7-reductase (DHCR7). D8D7I also known as the emopamyl
binding protein (EBP) is the catalytic subunit, and DHCR7 is a
regulatory subunit of ChEH (Fig 3A). ChEH was characterized as a
pharmacological target of the antitumour drugs tamoxifen and tesmilifene
(de Medina, Paillasse, Segala, Poirot & Silvente-Poirot, 2010; de
Medina et al., 2013; Segala et al., 2013; Sola et al., 2013). The
different structural classes of drugs that inhibit the ChEH activity
were also found to be inhibitors of EBP and/or DHCR7 leading to the
accumulation of Δ8- and/or Δ7-cholesterol intermediates in the
biosynthesis of cholesterol (Kedjouar et al., 2004; Korade et al., 2016;
Segala et al., 2017; Silvente-Poirot & Poirot, 2012) (Fig 3B).
Δ8-cholesterol precursors such as zymostenol (cholest-8-ene-3β-ol) were
shown to induce BC cell arrest in the G1 phase of the cell cycle (Payre
et al., 2008), lysosome biogenesis and autophagy (de Medina et al.,
2009; de Medina, Silvente-Poirot & Poirot, 2009; Segala et al., 2017;
Sola et al., 2013). Inhibition of EBP has been shown to enhance
oligodendrocyte formation and myelination (Allimuthu et al., 2019;
Hubler et al., 2018) suggesting that Δ8 intermediates in cholesterol
biosynthesis play a role in cell differentiation. Truncated
APC-selective inhibitors (TASIN) including EBP and DHCR7 inhibitors that
are under development for colorectal cancer treatment applications
(Cully, 2016; Theodoropoulos et al., 2020; Wang, Zhang, Morlock,
Williams, Shay & De Brabander, 2019; Zhang, Kim, Luitel & Shay, 2018;
Zhang et al., 2016). Interestingly, Δ8- and Δ7-sterols including
zymostenol are unstable sterols that are prone to autoxidation (Kedjouar
et al., 2004; Lamberson, Muchalski, McDuffee, Tallman, Xu & Porter,
2017; Payre et al., 2008; Porter, Xu & Pratt, 2020) to produce B-ring
oxysterols (Lamberson, Muchalski, McDuffee, Tallman, Xu & Porter,
2017). On the other hand, B-ring oxysterols are known as endogenous ChEH
inhibitors (de Medina, Paillasse, Segala, Poirot & Silvente-Poirot,
2010). This and the fact that EBP carries the ChEH activity, suggests
that it will be important to consider these parameters in the cell
differentiation effects of TASIN compounds especially if they are
developed for clinical applications. The recent elucidation of the EBP
structure complexed with tamoxifen (Long, Hassan, Thompson, McDonald,
Wang & Li, 2019) by X-ray crystallography will encourage
structure-function studies to identify amino acid residues responsible
for ChEH activity.
In the clinic, Kaplan-Meier analyses of several BC transcriptome patient
datasets showed that EBP and DHCR7 expression were positively correlated
and were overexpressed in all BC subtypes compared to normal breast
tissue (Voisin et al., 2017). In addition, it was reported that high
levels of expression of EBP and DHCR7 were associated with a lower
survival rate of patients (Voisin et al., 2017). ChEH can therefore
represent an interesting target for the development of anticancer
compounds which deserves further exploration with regard to
redifferentiation therapies (Bizzarri, Giuliani, Cucina & Minini,
2020).