MEKK1 activation of human estrogen receptor alpha and stimulation of the agonistic activity of 4-hydroxytamoxifen in endometrial and ovarian cancer cells

Mol Endocrinol. 2000 Nov;14(11):1882-96. doi: 10.1210/mend.14.11.0554.


Estrogens are mitogens that stimulate the growth of both normal and transformed epithelial cells of the female reproductive system. The effect of estrogens is mediated through the estrogen receptors, which are ligand-regulated transcription factors. Tamoxifen, a selective estrogen receptor modulator, functions as an estrogen receptor antagonist in breast but an agonist in uterus. In the current study, we show that coexpression of a constitutively active MEKK1, but not RAF or MEKK2, significantly increases the transcriptional activity of the receptor in endometrial and ovarian cancer cells. The expression of wild-type MEKK1 and an active Rac1, which functions upstream of MEKK1, also increased the activity of the receptor while coexpression of dominant negative MEKK1 blocked the Rac1 induction, indicating that endogenous MEKK1 is capable of activating the receptor. Additional experiments demonstrated that the MEKK1-induced activation was mediated through both Jun N-terminal kinases and p38/Hog1 and was independent of the known phosphorylation sites on the receptor. p38, but not Jun N-terminal kinases, efficiently phosphorylated the receptor in immunocomplex kinase assays, suggesting a differential involvement of the two kinases in the receptor activation. More importantly, the expression of the constitutively active MEKK1 increased the agonistic activity of 4-hydroxytamoxifen to a level comparable to that of 17beta-estradiol and fully blocked its antagonistic activity. These findings suggest that the uterine-specific agonistic activity of the tamoxifen compound may be determined by the status of kinases acting downstream of MEKK1.

Publication types

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

MeSH terms

  • Endometrial Neoplasms / drug therapy*
  • Endometrial Neoplasms / metabolism
  • Enzyme Activation
  • Enzyme Inhibitors / pharmacology
  • Estrogen Antagonists / pharmacology*
  • Estrogen Receptor alpha
  • Female
  • Flavonoids / pharmacology
  • Humans
  • Imidazoles / pharmacology
  • JNK Mitogen-Activated Protein Kinases
  • MAP Kinase Kinase Kinase 1*
  • MAP Kinase Kinase Kinase 2
  • MAP Kinase Kinase Kinases / drug effects
  • MAP Kinase Kinase Kinases / genetics
  • MAP Kinase Kinase Kinases / metabolism
  • Mitogen-Activated Protein Kinases / antagonists & inhibitors
  • Mitogen-Activated Protein Kinases / metabolism
  • Ovarian Neoplasms / drug therapy*
  • Ovarian Neoplasms / metabolism
  • Phosphorylation
  • Protein-Serine-Threonine Kinases / genetics
  • Protein-Serine-Threonine Kinases / metabolism*
  • Proto-Oncogene Proteins c-raf / drug effects
  • Proto-Oncogene Proteins c-raf / genetics
  • Proto-Oncogene Proteins c-raf / metabolism
  • Pyridines / pharmacology
  • Receptors, Estrogen / drug effects
  • Receptors, Estrogen / genetics
  • Receptors, Estrogen / metabolism*
  • Tamoxifen / analogs & derivatives*
  • Tamoxifen / pharmacology
  • Transcription, Genetic
  • Tumor Cells, Cultured
  • p38 Mitogen-Activated Protein Kinases


  • Enzyme Inhibitors
  • Estrogen Antagonists
  • Estrogen Receptor alpha
  • Flavonoids
  • Imidazoles
  • Pyridines
  • Receptors, Estrogen
  • Tamoxifen
  • afimoxifene
  • Protein-Serine-Threonine Kinases
  • Proto-Oncogene Proteins c-raf
  • JNK Mitogen-Activated Protein Kinases
  • Mitogen-Activated Protein Kinases
  • p38 Mitogen-Activated Protein Kinases
  • MAP Kinase Kinase Kinase 1
  • MAP Kinase Kinase Kinase 2
  • MAP Kinase Kinase Kinases
  • MAP3K1 protein, human
  • MAP3K2 protein, human
  • SB 203580
  • 2-(2-amino-3-methoxyphenyl)-4H-1-benzopyran-4-one