1. Introduction.
Metabolic reprogramming has emerged as a hallmark in cancer, impacting
gene expression, cell de-differentiation and tumour microenvironment
(Pavlova & Thompson, 2016). It is well established that cancer cells
must rewire the cellular metabolism to satisfy the demands of growth and
proliferation, most notably by providing energy, reducing equivalents
and building blocks while several metabolites exert a signalling
functions promoting tumour growth and progression (Danhier et al.,
2017). The exploration of cancer metabolism for clinical benefits is
required to identify metabolic pathways that are limiting for tumour
progression (Vander Heiden & DeBerardinis, 2017). Cholesterol is a
major lipid that is crucial as a building block for membrane formation
and protein structuration (Grouleff, Irudayam, Skeby & Schiott, 2015).
It is also the precursor of steroid hormones, bile acids and oxysterols
(Schroepfer, 2000). Cholesterol biosynthesis is a multi-step process
(Nes, 2011) that is subject to homeostasis and finely regulated in cells
(Luo, Yang & Song, 2020). In cancer cells several deregulations have
recently emerged, opening up new therapeutic strategies (Huang, Song &
Xu, 2020; Kuzu, Noory & Robertson, 2016).
Recent epidemiological studies have shown that breast cancer (BC) still
represent the world leading female cancer in terms of incidence and
mortality (Bray, Ferlay, Soerjomataram, Siegel, Torre & Jemal, 2018;
Global Burden of Disease Cancer et al., 2019). Thus, there is an urgent
need to find and validate new therapeutic targets in order to improve
patient survival and tumour recurrence. BC is a heterogeneous pathology
and several molecular BC subtypes have been described driving
therapeutic treatments. We can distinguish the following three major
subtypes: 1) Estrogen receptor positive breast cancers (ER(+)BC) are the
most frequent BC and are treated with selective ER modulators (SERM) and
aromatase inhibitors (AI). SERM block the mitogenic effects of
17β-oestradiol (E2) at the ER level, and AI inhibit E2 neosynthesis in
BC (Jordan & Brodie, 2007). 2) ER(-)negative and HER2(+) BC are treated
using anti-HER2 therapeutic antibodies that block the activation of
HER2-dependent mitogenic pathways with or without conventional
chemotherapy (Goldhirsch et al., 2011); 3) triple negative BC (TNBC)
that do not express steroid hormone receptors and HER2 are treated by
conventional chemotherapy with non-selective cytotoxic drugs (Goldhirsch
et al., 2011).
Epidemiological population studies have identified links between
cholesterol and cancer. Meta-analysis of clinical trials have shown an
inverse relationship between circulating cholesterol levels and BC
(Touvier et al., 2015), while hypercholesterolemia has been proposed as
a risk factor for BC recurrence (Nelson, 2018), implying that
cholesterol metabolism deregulations occured in BC and that targeting
cholesterol metabolism deregulations may be of interest for BC treatment
and chemoprevention (Garcia-Estevez & Moreno-Bueno, 2019).
At the molecular level, recent studies have shpwn that certain
oxysterols display either tumour promoter but also tumour suppressor
properties (Fig 1A). 27-hydroxycholesterol (27-HC) has been shown to
stimulate ER(+)BC proliferation and invasiveness through the modulation
of ER and LXRβ respectively (Nelson et al., 2013) . It has also been
shown that the pro-metastatic action of 27-HC in mice required myeloid
immune cell functions such as polymorphonuclear-neutrophils and γδ-T
cells at distal metastatic sites (Baek et al., 2017). In mice, it has
been shown that the CXCR2 receptor was involved in this effect (Baek et
al., 2017; Raccosta et al., 2013; Raccosta, Fontana, Traversari &
Russo, 2013) . It has been shown that other side-chain oxysterols
displayed similar properties, possibly after sulfation by the
sulfotransferase SULT2B1b (Moresco et al., 2018; Raccosta et al., 2013).
These observations led to the proposal that combination therapies
associating the inhibition of 27-HC biosynthesis at the 27-hydroxylase
level (CYP27A1) and the use of ERα and LXR antagonist could increase the
efficacy of treatments against ER(+)-BC (Nelson, 2018). In human,
clinical studies from the EPIC-Heidelberg cohort showed that high level
of circulating 27-HC were associated with a decreased BC risk in
postmenauposal women suggesting that 27-HC could prevent BC in these
cases (Le Cornet et al., 2020; Lu, Le Cornet, Sookthai, Johnson, Kaaks
& Fortner, 2019). In contrast, strategies aiming to target 27HC
biosynthesis as well as its effectors, as proposed by Nelson et al,
should be limited to ER(+)-BC patients after an endocrine therapy to
protect them against BC recurrence .
Certain B-ring oxysterols such as 7-hydroperoxycholesterol and
5,6-epoxycholesterol (5,6-EC) (Fig.1A) have retained the attention of
researchers during the last century as they are major autoxidation and
photo-oxidation products of cholesterol (Smith, 1981; Smith & Johnson,
1989), and are suspected to be alkylating substances and thus possibly
mutagenic and carcinogenic. These oxysterols were shown to induce
mutagenicity in some yeast strains (Ansari, Walker, Smart & Smith,
1982; Smith, Smart & Ansari, 1979) and chinese hamster V79 cells
(Chang, Jone, Trosko, Peterson & Sevanian, 1988; Peterson, Peterson,
Spears, Trosko & Sevanian, 1988; Sevanian & Peterson, 1984; Sevanian
& Peterson, 1986) in vitro. However in vivo tests failed
to show any carcinogenic potencies for 5,6-EC (el-Bayoumy et al., 1996).
Meanwhile, recent studies have revealed that 5,6-EC are involved in a
metabolic branch clearly involved in carcinogenesis and identified new
5,6-EC metabolites with opposite properties regarding BC oncogenesis. 1)
5,6α-EC can give metabolites with antiproliferative and cancer cell
redifferentiation properties: 5,6α-EC can be sulphated by the
sulfotransferase SULT2B1b in BC cells to produce
5,6α-epoxy-cholesterol-3β-sulfate (5,6-ECS) (Fig 1A) and 5,6-ECS was
shown to induce BC cell death and BC cell redifferentiation activitiesin vitro . In normal breast tissue 5,6α-EC was shown to be
conjugated to histamine to give Dendrogenin A (Fig 1A,B), a steroidal
alkaloid that displays tumour suppressive properties (de Medina et al.,
2013; Poirot & Silvente-Poirot, 2018; Segala et al., 2017;
Silvente-Poirot & Poirot, 2014) (Fig 2A). 2) 5,6α-EC can be transformed
into a tumour promoter: 5,6-ECs were shown to give a secondary
metabolite named oncosterone (6-oxo-cholestan-3,6- diol,
cholestan-3,6-diol-6-one, OCDO) (Fig 1A,C) with tumour promoter
properties in ER(+)BC and TN BC (Poirot, Soules, Mallinger, Dalenc &
Silvente-Poirot, 2018; Silvente-Poirot, Dalenc & Poirot, 2018; Voisin
et al., 2017) (Fig 2B).