3.3.1. DDA is enzymatically produced in normal tissues
Researchers have hypothesized that 5,6-EC could be metabolized into inducers of cell-differentiation such as dendrogenin A for several reasons: 1) ChEH contains the microsomal anti-oestrogen binding site (AEBS) which controls BC cell death and differentiation, showing that 5,6-EC and 5,6-EC metabolites mediated BC cell differentiation(de Medina et al., 2009; Payre et al., 2008; Segala et al., 2013; Silvente-Poirot & Poirot, 2012); 2) ChEH subunits EBP and DHCR7 are required for human development programs (Herman, 2003; Porter & Herman, 2011) opening up the possibilities that ChEH activity and 5,6-EC metabolites could contribute to development programs as inducers of cell differentiation; 3) at the chemical level, 5,6α-EC was found to be reactive towards nucleophilic groups such as mercaptant and amines only in the presence of a catalyst and give a single conjugated stereoisomer amongst multiple possibilities(Silvente-Poirot, de Medina, Record & Poirot, 2016). This supported the possible existence of a new metabolic branch based on 5,6α-EC stereospecific conjugation since homochirality is a common occurence in life (Blackmond, 2019; Fujii & Saito, 2004); 4) the AEBS binds 5,6-EC and nucleophilic substances such as histamine or polyamines opening up a possibility of conjugation reaction at the ChEH level (Leignadier, Dalenc, Poirot & Silvente-Poirot, 2017). The chemical synthesis of 5,6α-EC conjugation products with certain biogenic amines including histamine and polyamine has been performed(de Medina, Paillasse, Payre, Silvente-Poirot & Poirot, 2009). As expected, 5,6α-EC conjugates displayed strong cell differentiation properties in vitro (de Medina, Paillasse, Payre, Silvente-Poirot & Poirot, 2009). Amongst these conjugates, the histaminic adduct called dendrogenin A (DDA), was found to induce cell differentiation properties in pluripotent undifferentiated cells (de Medina, Paillasse, Payre, Silvente-Poirot & Poirot, 2009) and normal progenitor cells (Khalifa, de Medina, Erlandsson, El-Seedi, Silvente-Poirot & Poirot, 2014). DDA was reported to induce cell differentiation and death in mouse and human cells of various tissue origins (Bauriaud-Mallet et al., 2019; de Medina, Paillasse, Payre, Silvente-Poirot & Poirot, 2009; de Medina et al., 2013). More specifically DDA was found to induce BC cell re-differentiation to produce breast epithelial-like cells (Bauriaud-Mallet et al., 2019; de Medina, Paillasse, Payre, Silvente-Poirot & Poirot, 2009; de Medina et al., 2013). The production of chemical tools such as radio- and deuterium-labelled DDA, and the development of analytical methods has made possible the detection and quantification of DDA in mammalian tissues which has led to the demonstration that DDA exists in mammals (de Medina et al., 2013; Soules et al., 2019). DDA can be formed in mammalian tissue from 5,6α-EC and histamine and this reaction was shown to require a proteinaceous enzyme that remains to be identified (de Medina et al., 2013). Cultured normal breast epithelial cells produced DDA, while DDA was not detectable in BC cell lines (de Medina et al., 2013). This suggests that a deregulation in DDA metabolism could occur during oncogenesis and it may result from an alteration of the DDA metabolism in epithelial cells. This was confirmed in patients by comparing DDA levels in BC tumours and normal adjacent tissues (de Medina et al., 2013). Interestingly, thanks to the improvement of analytical methods for DDA quantification, dendrogenin B which is a 5,6α-EC conjugate of spermidine with neurodifferentiation and neurone regeneration properties (Dalenc, Poirot & Silvente-Poirot, 2015; de Medina, Paillasse, Payre, Silvente-Poirot & Poirot, 2009; Fransson, de Medina, Paillasse, Silvente-Poirot, Poirot & Ulfendahl, 2015; Khalifa, de Medina, Erlandsson, El-Seedi, Silvente-Poirot & Poirot, 2014) was found to exist as a mammalian metabolite (Soules et al., 2019). This showed that a new metabolic branch existed in the cholesterol pathway and this branch is centered around 5,6α-EC conjugation and produces inducers of cell differentiation.