The growth of both normal and transformed epithelial cells of the female reproductive system is stimulated by
estrogens, mainly through the activation of
estrogen receptor alpha (
ERalpha), which is a
ligand-regulated
transcription factor. The selective ER modulator
tamoxifen (TAM) has been widely used as an ER antagonist in
breast tumor; however, long-term treatment is associated with an increased risk of
endometrial cancer. To provide new insights into the potential mechanisms involved in the agonistic activity exerted by TAM in the uterus, we evaluated the potential of
4-hydroxytamoxifen (OHT), the active metabolite of TAM, to transactivate wild-type
ERalpha and its splice variant expressed in Ishikawa and HEC1A endometrial
tumor cells, respectively. OHT was able to antagonize only the activation of
ERalpha by 17beta-estradiol (E2) in Ishikawa cells, whereas it up-regulated c-fos expression in a rapid manner similar to E2 and independently of
ERalpha in both cell lines. This stimulation occurred through the
G protein-coupled receptor named GPR30 and required Src-related and
epidermal growth factor receptor tyrosine kinase activities, along with the activation of both ERK1/2 and
phosphatidylinositol 3-kinase/AKT pathways. Most importantly, OHT, like E2, stimulated the proliferation of Ishikawa as well as HEC1A cells. Transfecting a GPR30 antisense expression vector in both
endometrial cancer cell lines, OHT was no longer able to induce growth effects, whereas the proliferative response to E2 was completely abrogated only in HEC1A cells. Furthermore, in the presence of the inhibitors of MAPK and
phosphatidylinositol 3-kinase pathways,
PD 98059 and
wortmannin, respectively, E2 and OHT did not elicit growth stimulation. Our data demonstrate a new mode of action of E2 and OHT in
endometrial cancer cells, contributing to a better understanding of the molecular mechanisms involved in their uterine agonistic activity.