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Keywords:

  • antiestrogens;
  • estrogen receptor;
  • bone morphogenetic protein 6;
  • osteoblasts

Abstract

The effects of 17β-estradiol (E2) and ICI 182,780 (ICI) on activity of a BMP-6 promoter were compared in osteoblast-like and breast cancer cells transiently transfected with ERα. E2 but not ICI stimulated BMP-6 reporter activity in breast cancer cells, whereas the opposite was observed in osteoblast-like cells, associated with lack of AF-2 dependence of the response, and absent intranuclear localization of ERα, suggesting the involvement of a distinct ERα-dependent response mechanism in osteoblasts.

Introduction: Previous studies suggest that the tissue-selective effect of antiestrogens on bone reflects the ability of these compounds to target certain osteoblast regulatory genes. To explore this hypothesis, we examined whether antiestrogens preferentially stimulate the bone morphogenetic protein 6 (BMP-6) promoter in bone cells, and if so, whether this activity is associated with a distinct estrogen receptor (ER)α-dependent response mechanism to that in other cell types.

Materials and Methods: We compared the effects of 17β-estradiol (E2) and ICI 182,780 (ICI) on activity of a 4.3-kb BMP-6 reporter construct in osteoblast-like cells (human MG63 and SaOS-2 cells and rat ROS 17/2.8 cells), human MCF-7 and T47-D breast cancer cell lines, and HepG2 hepatoma cells, after transient transfection with ERα, ERβ, and mutant ER constructs.

Results: E2, but not ICI, stimulated BMP-6 reporter activity by approximately 100% in MCF-7, T47-D cells, and HepG2 cells when transfected with ERα. In contrast, in ERα-transfected osteoblast-like cells, an increase in reporter activity of approximately 75% was observed after treatment with ICI but not E2. The response of MG63 cells to ICI and MCF-7 cells to E2 both required ERα as opposed to ERβ and the ERα activation function (AF)-1 activation domain. However, whereas the AF-2 domain was also required for E2 to stimulate reporter activity in MCF-7 cells, the response to ICI in MG63 cells was AF-2 independent. In further studies where we compared the intracellular distribution of ERα associated with these responses, E2-dependent stimulation of the BMP-6 reporter in MCF-7 cells was associated with intranuclear localization of ERα, whereas extranuclear localization was seen in rat osteosarcoma cells (ROS) cells treated with ICI.

Conclusions: Antiestrogens selectively stimulate BMP-6 reporter activity in osteoblast-like cells through a distinct ERα-dependent mechanism characterized by independence of the AF-2 domain and extranuclear localization of ERα.