Inhibition of GPR30 by estriol prevents growth stimulation of triple-negative breast cancer cells by 17β-estradiol.

BACKGROUND: Due to the lack of ERα, triple negative breast cancers (TNBCs) are not susceptible to endocrine therapy using antiestrogens. However, the majority of TNBCs express the membrane bound estrogen receptor GPR30. We have recently shown that knock-down of GPR30 expression prevented growth stimulation of TNBC cell lines by 17β-estradiol. Now we analyzed whether specific inhibition of GPR30 represents a new option for therapy of TNBC.

METHODS: Growth of TNBC cells was assessed using Alamar-blue colorimetric assay. Activation of c-Src and EGF-receptor was assessed using Western blots. Expression of c-fos, cyclin D1 and aromatase was quantified by RT-PCR. Gα-specific signaling of GPR30 was analyzed by electrophoretic mobility shift assay.

RESULTS: HCC1806 cells showed the highest GPR30 expression, in HCC70 cells it was clearly lower, in MDA-MB-231 cells it was lowest. 10-8 M 17β-estradiol significantly increased proliferation of HCC1806 cells to 134 ± 12% of control (p < 0.01). Proliferation of HCC70 cells was slightly increased to 116 ± 8% of control. Estriol significantly reduced cell number of HCC1806 cells to 16 ± 12% (p < 0.01). Cell number of HCC70 cells and of MDA-MB-231 cells was reduced to 68 ± 25% and to 61 ± 10%, respectively.Activity of Src kinase increased to 150 ± 10% (p < 0.05) by 10-8 M 17β-estradiol treatment in HCC1806 and to 220 ± 20% in HCC70 cells (p < 0.01). Estriol treatment completely inhibited 17β-estradiol-induced p-src activation. Transactivation of EGF-receptor increased by estradiol treatment to 350% in HCC1806 and to 280% in HCC70 cells. Estriol completely suppressed EGF-receptor transactivation. c-fos expression increased to 260% and to 190%, respectively. Estriol reduced this induction to 160% (HCC1806) and below control in HCC70 cells. Cyclin D1 was induced to 290% (HCC1806) and 170% (HCC70) and completely inhibited by estriol. 17β-estradiol increased CREB-phosphorylation to 400%. Binding of phospho-CREB to a CRE of cyclin D1 was enhanced to 320%.

CONCLUSION: Specific pharmacological inhibition of GPR30 might become a promising targeted therapy for TNBC in future.