3.3.3. DDA is an LXR modulator that induces lethal autophagy in
cancer cells.
DDA was found to induce cell death and differentiation in mouse and
human cancer cells. Further work was done to study the molecular
mechanisms involved in DDA cytotoxicity. It was found that DDA induced
cytotoxicity in cultured cancer cells in a dose- and time-dependent
manner suggesting the implication of a receptor, death was not
apoptotic, and required gene expression and protein neosynthesis (Segala
et al., 2017), as observed with other ChEH inhibitors (de Medina et al.,
2009; Leignadier, Dalenc, Poirot & Silvente-Poirot, 2017; Payre et al.,
2008). However, as opposed to other ChEH inhibitors, DDA cytoxicity was
not inhibited by anti-oxidants because 5,6-EC do not accumulate and are
not second messengers of DDA (de Medina et al., 2009; Leignadier,
Dalenc, Poirot & Silvente-Poirot, 2017; Payre et al., 2008; Segala et
al., 2017). Studies on the DDA molecular mechanism of cytotoxicity
showed that it induced lethal autophagy (Segala et al., 2017) as opposed
to other ChEH inhibitors that induced a protective autophagy
(Leignadier, Dalenc, Poirot & Silvente-Poirot, 2017). ChEH contributed
to autophagy through the accumulation of pro-autophagic Δ8-sterols due
the inhibition of its EBP subunit (de Medina, Silvente-Poirot & Poirot,
2009; Silvente-Poirot, Segala, Poirot & Poirot, 2018). DDA was found to
be a ligand of LXRα and LXRβ receptors as opposed to other ChEH
inhibitors (Segala et al., 2013). Genetic and pharmacological evidences
has been given to confirm that LXRβ was required for lethal autophagy.
Although DDA was a poor modulator of canonical LXR-dependent genes, it
activated via LXRβ the transcription of genes encoding master regulators
of lysosome biogenesis and autophagy such as the transcription factor EB
(TFEB) (Segala et al., 2017). DDA is, to our knowledge, the first
example of an LXR ligand that induces TFEB expression. Such an effect
has not been reported to date by other cytotoxic natural LXR ligands
belonging to the oxysterols family. It was further established that DDA
did not modulate other common nuclear receptors establishing its
selectivity to LXRs (Segala et al., 2017).
It was next established that DDA, at cytotoxic doses, actively inhibit
the growth of human and mouse tumours implanted into immunocompromized
mice with different administration modes, including primary tumours from
patients (Segala et al., 2017). Knock down of LXR receptors in cancer
cells using small interfering RNA (siRNA) or single hairpin RNA (shRNA)
approaches strongly impaired DDA induction of autophagy and its
anticancer activities in vitro and in vivo (Segala et al.,
2017). This showed that LXRβ was required to control the anticancer
activity of DDA. The particular cell death induced by DDA compared to
conventional ChEH inhibitors and LXR modulators is probably due to its
specific LXRβ-dependent regulation of gene expression and induction of
Δ8-sterols accumulation as discussed earlier (Poirot & Silvente-Poirot,
2018). Moreover, this suggests that the control of autophagy might be a
specific physiological function of the LXRβ isoform when activated by
specific ligands such as DDA. This data shows the importance of LXR in
cancer cells as targets for anticancer strategies. Due to the
established importance of LXR in the tumour micro-enrironment (Ma &
Nelson, 2019) and in the control of the immune response (Fessler, 2016),
it will be interesting to determine how autophagy and LXR from immune
cells can contribute to the anti-tumour action of DDA observed in
immunocompetent mice (de Medina et al., 2013). It would be of interest
to determine the impact of combination treatments of DDA with other
chemotherapeutic agents acting through different molecular mechanisms,
and in particular with drugs that have a limited therapeutic outcome or
resistance through the induction of protective autophagy.