Non‐competitive steroid inhibition of oestrogen receptor functions

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Abstract
Currently available antioestrogens, such as tamoxifen, are competitive inhibitors that bind to the ligand binding sites of oestrogen receptors, ERα and ERβ. The search for alternative anti-hormone therapies is prompted by the need for drugs that are effective when tumours become tamoxifen resistant. The existence of different receptor isoforms also raise the possibility of improving selectivity. Earlier use of the 3β-hydroxysteroid dehydrogenase inhibitor, trilostane (4α,5- epoxy-17β-hydroxy-3-oxo-5α-androstane-2α-carbonitrile), suggested that it had beneficial actions in breast cancer that were only partially attributable to inhibition of steroidogenesis. The present studies on the interactions of trilostane with oestrogen receptors show that it (i) inhibits oestrogen-stimulated proliferation in MCF-7 breast cancer cells, (ii) enhances the affinity of oestradiol binding to ER in rat uteri and specifically increases oestradiol binding to an ERβ-like isoform, (iii) inhibits ERα and ERβ binding to the classical vitellogenin gene oestrogen response element (ERE) and (iv) inhibits oestrogen-stimulated gene transcription in ERE-linked reporter systems in MCF-7 cells. The results demonstrate a novel, presumably allosteric, mode of antioestrogen action. The beneficial actions of trilostane in breast cancer may be attributed to the combination of this antioestrogen effect with its well documented suppression of steroidogenesis.
Funding Information
  • Bioenvision, Inc.