Over-expression of cleaved
cyclin E in
breast tumors is closely associated with
tumor progression and resistance to
antiestrogens. 17β-Estradiol (E2) has been recently shown to induce
cyclin E processing in
breast cancer cells.
Tamoxifen has been used in patients with
estrogen-sensitive
breast cancer, yet resistance to
antiestrogens and recurrence will appear in some of the patients after its continued use. We therefore addressed possible effects of
tamoxifen on the generation of cleaved
cyclin E and its signal mechanism(s) in
estrogen-responsive MCF-7
breast cancer cells that express both
G protein-coupled
protein (GPR) 30 and
estrogen receptor α (ERα).
4-Hydroxytamoxifen (OHT,
tamoxifen's active form) failed to prevent E2-induced proteolysis of
cyclin E and migration, but rather triggered
cyclin E cleavage coincident with augmented migration. OHT-induced
cyclin E truncation also occurred in SK-BR-3 cells that express GPR30 and lack ERα, but not in MDA-MB-231 cells that express neither GPR30 nor ERα. G1, a specific GPR 30 agonist, caused dramatic proteolysis of
cyclin E and enhanced migration. Furthermore, OHT-stimulated cleavage of
cyclin E and migration were tremendously attenuated by G15, a GPR30 antagonist, or
siRNA against GPR30. In addition, inhibitors for EGFR or ERK1/2 remarkably suppressed OHT-induced truncation of
cyclin E, suggesting involvement of EGFR signaling. Collectively, our data indicate that OHT contributes to the production of proteolyzed
cyclin E via GPR30 with augmented migration in MCF-7 cells.