Increasing the number of tandem estrogen response elements increases the estrogenic activity of a tamoxifen analogue

Cancer Lett. 1995 May 25;92(1):39-47. doi: 10.1016/0304-3835(95)03755-l.


There have been several reports of women who have tumor relapse while on tamoxifen therapy, followed by tumor regression after tamoxifen withdrawal. In such apparently tamoxifen-stimulated tumors, there is likely a genetic change which increases the estrogenicity of tamoxifen. In this study, we determine if increasing the number of estrogen response elements (EREs) in the promoter region of a reporter gene can alter the agonistic activity of fixed-ring 4-hydroxytamoxifen. We show that increasing the number of EREs in the promotor region increases the transcriptional response of the reporter plasmid to estradiol. We also find that while fixed-ring 4-hydroxytamoxifen is unable to stimulate transcription when one ERE is present, transcriptional activation can occur with multiple EREs. These results demonstrate that ERE amplification could explain the agonistic properties of tamoxifen, and suggests a novel mechanism to explain tamoxifen-stimulated breast cancer growth.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adenocarcinoma / drug therapy*
  • Adenocarcinoma / metabolism
  • Adenocarcinoma / pathology*
  • Base Sequence
  • Breast Neoplasms / drug therapy*
  • Breast Neoplasms / metabolism
  • Breast Neoplasms / pathology*
  • Cell Division / drug effects
  • Estradiol / pharmacology
  • Estrogen Antagonists / pharmacology*
  • Estrogens / genetics
  • Estrogens / metabolism*
  • Female
  • Gene Expression Regulation
  • Humans
  • Luciferases
  • Molecular Sequence Data
  • Promoter Regions, Genetic
  • Receptors, Estrogen / antagonists & inhibitors
  • Receptors, Estrogen / metabolism
  • Tamoxifen / pharmacology*
  • Tamoxifen / therapeutic use
  • Transcription, Genetic
  • Transfection
  • Tumor Cells, Cultured


  • Estrogen Antagonists
  • Estrogens
  • Receptors, Estrogen
  • Tamoxifen
  • Estradiol
  • Luciferases