2. The 5,6-epoxycholesterols (5,6-ECs)
5,6-ECs exist as two different diastereoisomers: the 5,6α-EC and the 5,6β-EC (Fig 1A). They stem from the mono-oxygenation of cholesterol on its Δ5,6-double bond. In biological systems they can be produced as a mixture via a free radical lipid peroxidation process (Iuliano, 2011; Poirot & Silvente-Poirot, 2013; Porter, Xu & Pratt, 2020; Yin, Xu & Porter, 2011). The stereo-selective synthesis of 5,6β-EC can be achieve using porphyrins, that mimic cytochrome p450 monooxygenases(Poirot & Silvente-Poirot, 2013), and a stereoselective biosynthesis of 5,6α-EC by a yet unidentified cytochrome p450 have been described in bovine adrenals (Watabe & Sawahata, 1979). 5,6-EC are known as major autoxidation and photo-oxidation products of cholesterol (Smith, 1981). They are present in biological fluids and solid tissues from mammals (Poirot & Silvente-Poirot, 2013; Schroepfer, 2000).
The epoxide ring, is well known by chemists for its reactivity towards nucleophilic groups and is widely used for this reason in organic chemistry (Gorzynski Smith, 1984; Parker & Isaacs, 1959). 5,6α -EC was suspected to be involved in skin photocarcinogenesis as it accumulated in skin after UV exposure (Black & Douglas, 1973; Black & Lo, 1971; Lo & Black, 1973; Lo & Black, 1972) and activated a ChEH activity to accelerate the hydrolysis and elimination of 5,6-EC (Chan & Black, 1974). Meanwhile, 5,6-EC were found to be chemically stable towards nucleophiles ruling out a direct carcinogenic activity (Paillasse, Saffon, Gornitzka, Silvente-Poirot, Poirot & de Medina, 2012).
In the presence of a catalyst 5,6α-EC was the only of the two 5,6-ECs that reacts with nucleophiles to give a single product of addition with a 5α-6β stereochemistry through a trans-diaxial ring opening (Paillasse, Saffon, Gornitzka, Silvente-Poirot, Poirot & de Medina, 2012; Poirot & Silvente-Poirot, 2013) (Fig 1B). 5,6-EC were reported to be modulators of nuclear receptors such as Liver-X-Receptors α (LXRα) and β (LXRβ) (Berrodin, Shen, Quinet, Yudt, Freedman & Nagpal, 2010; Song, Hiipakka & Liao, 2001) showing that these oxysterols are biologically active oxysterols. The metabolism of 5,6-EC has been previously reviewed and showed that 5,6α-EC and 5,6β-EC were differentially metabolized in mammals (Poirot & Silvente-Poirot, 2013). Interestingly, 5,6β-EC was reported to accumulate in breast nipple fluids from patients with BC and preneoplastic breast lesions supporting that the 5,6-EC metabolism was linked to breast carcinogenesis (Gruenke, Wrensch, Petrakis, Miike, Ernster & Craig, 1987; Petrakis, Gruenke & Craig, 1981; Wrensch et al., 1989).