Arbuckle, Nicola Dawn; (1994) The role of ligand in oestrogen receptor dimerisation and DNA binding. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The oestrogen receptor is a member of the nuclear receptor superfamily which stimulates gene expression in response to oestradiol after recognising and binding to palindromic sequences known as oestrogen response elements. The receptor binds to DNA as a preformed dimer and dimerisation is required to stabilise the receptor-DNA complex. Treatment with the class II antioestrogens has been proposed to inhibit DNA binding activity as a consequence of disrupting the dimerisation of the receptor. The ability of the class II antioestrogens to inhibit the DNA binding activity of the mouse oestrogen receptor was found to be dependent on the structure of the aliphatic side-chain which characterises this family of ligands. A direct correlation was observed between the antagonism mediated by the pure antioestrogens in vivo and their ability to inhibit the DNA binding activity of the oestrogen receptor in vitro. The antagonists were found to bind to a similar region of the hormone binding domain to oestradiol and the aliphatic side-chain influences both the ability of the ligands to bind to the receptor and subsequently the efficiency with which each compound can antagonise DNA binding. To study the influence of ligand on the dimerisation of the oestrogen receptor, the hormone binding domain of the mouse protein was expressed as a glutathione S-transferase fusion protein in bacteria and purified to homogeneity. The fusion protein was found to bind agonist and antagonist with a similar affinity to the full length receptor and was subsequently analysed by fast protein liquid chromatography and electron microscopy. The hormone binding domain was observed to dimerise constitutively in vitro and is converted to a monomeric form following treatment with the pure antioestrogen ICI 182,780. This supports the hypothesis that the class II antagonists inhibit the DNA binding of the oestrogen receptor as a consequence of disrupting receptor dimerisation. Using a gel retardation assay, the ability of the antagonists to inhibit DNA binding was found to be sensitive to the inherent stability of oestrogen receptor dimers which appears to differ between different types of cells. As a result, although the pure antioestrogens may prevent the dimerisation of the oestrogen receptor during translation, the ligands may not necessarily disrupt the dimerisation of preformed receptor dimers depending of the strength of the dimerisation interaction.

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