title: Tamoxifen activation of the estrogen receptor/AP-1 pathway: potential origin for the cell-specific estrogen-like effects of antiestrogens.
abstract: We find that tamoxifen is a potent activator of estrogen receptor (ER)- mediated induction of promoters regulated by AP-1 sites including the human collagenase gene promoter and constructs in which an AP-1 site is fused to the herpes thymidine kinase promoter. This contrasts with the inability of tamoxifen to activate otherwise identical promoters bearing classical estrogen response elements. Tamoxifen agonism at AP-1 sites is cell type specific, occurring in cell lines of uterine, but not of breast, origin. It thus parallels tamoxifen agonism in vivo. AP-1 proteins such as Jun or Jun/Fos are needed for tamoxifen stimulation, and tamoxifen increases the transcriptional efficiency of these proteins even when they are provided at optimal amounts. The DNA binding domain (DBD) of ER is required for tamoxifen activation at AP-1 sites. In contrast, estrogen activation is partially independent of this domain. This suggests the existence of two pathways of ER action at AP-1: an alpha (DBD-dependent) pathway activated by tamoxifen, and a beta (DBD-independent) pathway activated by estrogen. Fusing VP16 transcriptional activation functions to ER potentiates the beta, but not the alpha, pathway. We discuss models for the two pathways and the possibility that the AP-1 pathway is a major route by which ER affects target tissue growth and differentiation in vivo.
authors.affiliation: Metabolic Research Unit, University of California, San Francisco 94143, USA.
authors.forename: P
authors.initials: P
authors.lastname: Webb
authors.forename: G N
authors.initials: GN
authors.lastname: Lopez
authors.forename: R M
authors.initials: RM
authors.lastname: Uht
authors.forename: P J
authors.initials: PJ
authors.lastname: Kushner
mesh_terms: D016475:3T3 Cells / Q000187:drug effects; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D001940:Breast / Q000187:drug effects*; D001943:Breast Neoplasms / Q000473:pathology; D016466:CHO Cells / Q000187:drug effects; D017364:Collagenases / Q000096:biosynthesis / Q000235:genetics; D016384:Consensus Sequence; D006224:Cricetinae; D003412:Cricetulus; D004717:Endometrium / Q000187:drug effects*; D004965:Estrogen Antagonists / Q000494:pharmacology*; D005260:Female; D005786:Gene Expression Regulation / Q000187:drug effects; D006367:HeLa Cells / Q000187:drug effects; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D009928:Organ Specificity; D011401:Promoter Regions, Genetic / Q000187:drug effects; D011960:Receptors, Estrogen / Q000037:antagonists & inhibitors / Q000737:chemistry / Q000187:drug effects*; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis; D015398:Signal Transduction / Q000187:drug effects*; D013629:Tamoxifen / Q000494:pharmacology*; D018808:Transcription Factor AP-1 / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects; D015533:Transcriptional Activation / Q000187:drug effects; D014407:Tumor Cells, Cultured
chemical_list: D004965:Estrogen Antagonists; D011960:Receptors, Estrogen; D011993:Recombinant Fusion Proteins; D018808:Transcription Factor AP-1; D013629:Tamoxifen; D017364:Collagenases