Activation of estrogen receptor-mediated gene transcription by IGF-I in human breast cancer cells.
Estrogen and IGF-I are potent mitogens for most breast cancer cell lines, and although their signaling pathways contrast, there is considerable interaction between them. Recent evidence indicating that IGF-I can alter estrogen receptor (ER) action led us to investigate whether an inhibitor of IGF-I action. IGF-binding protein-1 (IGFBP-1), could affect transcriptional activation of ER. First, we confirmed that tamoxifen (TAM) could inhibit IGF-I-mediated proliferation of MCF-7 cells. Although TAM can increase IGFBP-3 expression in MCF-7 cells, and this binding protein has been shown to be able to inhibit IGF action, TAM had no effect on IGF-I-stimulated tyrosine phosphorylation of IGF-I receptor or the downstream signaling molecule, insulin receptor substrate-1. Therefore, to confirm that IGF-I was affecting transcriptional activation of ER, we utilized a gene reporter assay using a single consensus estrogen response element (ERE-tk-luc) upstream of luciferase. As expected, estradiol (E2; 1nM) increased transcriptional activation three- to fivefold from the ERE in three ER-positive breast cancer cell lines (MCF-7, ZR-75 and T47D). A 2.5-to 4-fold increase was also seen with IGF-I (5 nM). TAM (1 microM) effectively blocked activation by E2 and IGF-I, indicating disruption of ER-mediated transcription. As expected, human recombinant IGFBP-1 (80 nM) completely inhibited IGF-I-mediated activation of ER, however, IGFBP-1 also caused a significant decrease in E2-mediated activation. We also noticed that IGF-I increased the activity of all plasmids that we cotransfected including TATA-luc, SV40-luc and pGL Basic. This effect was post-transcriptional, as it was not affected by actinomycin D (2 micrograms/ml), while we were able to completely inhibit E2-mediated transcriptional activation of ERE-tk-luc. Unlike the complete inhibition of ER-mediated transcriptional activation by actinomycin D, IGF-I-mediated transactivation was reduced by only 50%, indicating that the activation by IGF-I represented both transcriptional and post-transcriptional effects. This study confirmed that IGF-I can cause transcriptional activation of endogenous ER in human breast cancel cells, and inhibition of ER action by IGFBP-1 suggests that IGF-1 signaling may be necessary for maximal ER activation.
Lee
A V
AV
Department of Medicine, University of Texas Health Science Center at San Antonio 78284-7884, USA.
Weng
C N
CN
Jackson
J G
JG
Yee
D
D
D001943:Breast Neoplasms / Q000235:genetics* / Q000473:pathology; D002455:Cell Division / Q000187:drug effects; D003609:Dactinomycin / Q000494:pharmacology; D004958:Estradiol / Q000494:pharmacology; D004965:Estrogen Antagonists / Q000494:pharmacology; D005260:Female; D006801:Humans; D018970:Insulin-Like Growth Factor Binding Protein 1 / Q000494:pharmacology; D007334:Insulin-Like Growth Factor I / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D010957:Plasmids; D011500:Protein Synthesis Inhibitors / Q000494:pharmacology; D011960:Receptors, Estrogen / Q000235:genetics*; D011994:Recombinant Proteins / Q000494:pharmacology; D015398:Signal Transduction / Q000187:drug effects; D013629:Tamoxifen / Q000494:pharmacology; D015533:Transcriptional Activation / Q000187:drug effects*; D014407:Tumor Cells, Cultured
D004965:Estrogen Antagonists; D018970:Insulin-Like Growth Factor Binding Protein 1; D011500:Protein Synthesis Inhibitors; D011960:Receptors, Estrogen; D011994:Recombinant Proteins; D013629:Tamoxifen; D003609:Dactinomycin; D004958:Estradiol; D007334:Insulin-Like Growth Factor I