Fetal Estetrol (E4) Inhibits Growth Hormone (GH) Signal Transduction in Breast Cancer Cell Lines

Objective: Inhibition of GH signaling has been associated with increased longevity and cancer resistance. As the fetal estrogen estetrol was also linked to longevity and cancer resistance, we hypothesized that these effects of estetrol are mediated by inhibition of GH activity. Design: The CNMm-6TR mammary cell line, stably transfected with doxycycline (DOX)-inducible GHR (CNMm-cGHR cells), HEK293 cell line transfected with rGHR and the human T47D cell line were used to assess effects of estetrol on GH signal transduction. Proliferation was measured using the colorimetric 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide assay (MTT). Spi-luciferase reporter activity, responsive to phosphorylated Stat5, was used to assess GH signaling. GHR mRNA expression was normalized to a reference gene. Results: Treatment of CNMm-cGHR cells with GH resulted in decreased proliferation rates. No effect was seen by the co-inhibition with estetrol. Using the spi-luc reporter activity, it was shown that GH activates the GHR/Jak2/ Stat5 pathway. Estetrol partially inhibited this activation. In the HEK-rGHR cells, estetrol was able to inhibit the GH induced activation of the spi-luc reporter, but only in the presence of an ERα expression plasmid. Effects of estetrol on the expression of GHR mRNA was assessed in T47D cells, a cell-line expressing ER, PR and GHR under natural promoters. After incubation with estetrol, a mean 4-fold inhibition of GHR mRNA expression was observed. Conclusion: Using different cell systems, it was shown that estetrol is able to inhibit GH signaling, a process dependent on the presence of the Estrogen Receptor (ER). Estetrol may therefore be used to inhibit GH-induced effects on breast cancer proliferation