Journal of Steroid Biochemistry & Molecular Biology 100 (2006) 141–151 Comparison of effects of 4-hydroxy tamoxifen and trilostane on oestrogen-regulated gene expression in MCF-7 cells: Up-regulation of oestrogen receptor beta Stewart Barker ∗ , Sylvanie D.M. Malouitre, Hilary R. Glover, John R. Puddefoot, Gavin P. Vinson School of Biological and Chemical Sciences, Queen Mary, University of London, London E1 4NS, UK Received 19 October 2005; accepted 10 April 2006 Abstract 4-Hydroxy tamoxifen (OHT) and trilostane interact differently with the oestrogen receptor (ER). OHT is a competitive inhibitor whereas trilostane has direct, but non-competitive effects on ER. This study compared the effects of OHT and trilostane, in the presence of 17- oestradiol (E2) on gene expression in MCF-7 breast cancer cells using microarrays each representing nearly 20,000 human genes. Striking differences between the sets of genes affected by these two drugs were observed. Both OHT and trilostane affected transcription of genes involved in cell cycle regulation, cell adhesion and matrix formation, however, only 12.5% of trilostane down-regulated genes and 9.2% of up-regulated genes were similarly regulated by OHT. A selective up-regulation of ER by trilostane, but not OHT, was observed and confirmed by qRT-PCR. Similar up-regulation of this gene by trilostane was observed in the uterus of trilostane-treated (4mg/kg for 7 days) rats, in which ER mRNA (3-fold) and ER protein expression (10-fold) were both increased. These data show that OHT and trilostane regulate the expression of different sets of genes, reflecting their different modes of interaction with ER. Trilostane-specific up-regulation of ER could explain its positive benefit rates in acquired tamoxifen resistance. © 2006 Elsevier Ltd. All rights reserved. Keywords: 4-Hydroxy tamoxifen; Trilostane; MCF-7 cells; Oestrogen receptor; Breast cancer; Microarray 1. Introduction Tamoxifen, and its active metabolite 4-hydroxy tamoxifen (OHT), has been the drug of choice for treatment of hormon- ally responsive breast cancer for many years [1,2]. As a con- sequence of its differential tissue effects – anti-oestrogenic in breast and a partial antagonist in endometrium – it has been regarded as a selective oestrogen receptor modulator (SERM) [3], even though its relative affinities for binding to the two oestrogen receptor (ER) subtypes, ER and ER, are equivalent [4]. Apart from the greater risk of endome- trial cancer in tamoxifen-treated patients there is a much more common problem of acquired resistance to tamoxifen where cancer recurs despite continued treatment [5]. Largely ∗ Corresponding author. Tel.: +44 207 882 6316; fax: +44 208 983 0973. E-mail address: s.barker@qmul.ac.uk (S. Barker). as a result of these limitations, tamoxifen’s status as first line endocrine treatment of choice is now being challenged by a new generation of aromatase inhibitors (AIs) which, rather than selectively competing with oestrogen at the ER, reduce the local supply of oestrogen to the breast tumour [6,7]. AIs are therefore non-selective with respect to the actions of oestrogens through ER and ER and elicit a different profile of side effects, particularly on bone [8]. However, develop- ment of resistance to these AI treatments is also a common occurrence [9]. Tamoxifen competes with 17-oestradiol (E2) for bind- ing in the ligand-binding domain (LBD) of ER. Once bound, tamoxifen causes a specific conformational change in ER that, at least in the breast, prevents the binding of coactiva- tors and reduces gene transcription mediated at oestrogen- response element (ERE)-dependent gene promoters [10]. Other anti-oestrogens, such as raloxifene and faslodex (ICI 0960-0760/$ – see front matter © 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsbmb.2006.04.006