HER2 and ERβ2 downregulate estrogen-responsive element-mediated transcription activity of ERα-positive cells in response to estrogen stimulation

Background

Although HER2 expression is more common in ER-negative and PR-negative breast cancers, HER2 overexpression is observed in ER-positive cancers and these cancers seem to have intrinsic resistance to endocrine treatment. There are controversial reports about the effect of HER2 on estrogen dependence. ERβ2 has been reported to act as a dominant-negative regulator of ERα in the breast, and ERβ1 and/or ERβ2 might be co-expressed with ERα in breast cancer. It is therefore necessary to investigate the effect of HER2 and ERβ2 expression on the ERE-mediated transcription activity in the cells co-expressing ERα.

Methods

NIH3T3 cells, T6-17 cells (NIH3T3 cells stably transfected with HER2), and MCF-7 cells were maintained in dextran-coated charcoal-stripped 10% Dulbecco's modified Eagle medium. Transient transfection of constructs (pcDNA3-ERα, pcDNA3-ERβ2, pERE-luc, pcDNA-HER2) into each cell was performed using the Lipofectamine PLUS™ system. The reporter gene assay using estrogen-responsive element (ERE)-luciferase was used to measure the ERα transcriptional activity after treatment of 17-β-estradiol and tamoxifen.

Results

The reporter gene assay using ERE-luciferase showed much less responsiveness to estrogen in HER2-overexpressing T6-17 cells than in NIH3T3 cells, but there was no remarkable difference after treatment of tamoxifen. The responsiveness to estrogen in HER2-transfected MCF-7 cells was a little less, but not remarkable, than that in control MCF-7 cells. However, the responsiveness to estrogen in MCF-7 cells was decreased in a dose-dependent manner of HER2 expression. ERE-mediated transcriptional activity was decreased with the increase of ERβ2 expression; it is much more remarkable in HER2-overexpressing T6-17 cells.

Conclusion

Expression of HER2 and/or ERβ2 reduces estrogen responsiveness in ERα-positive tumor cells. HER2 might be associated with growth dependence of the tumor cells, but ERβ2 seems to be associated with binding affinity to estrogen and DNA.