Autocrine regulation of cell proliferation by estrogen receptor-alpha in estrogen receptor-alpha-positive breast cancer cell lines

Estrogen receptor-alpha (ERalpha) is essential for mammary gland development and is a major oncogene in breast cancer. Since ERalpha is not colocalized with the cell proliferation marker Ki-67 in the normal mammary glands and the majority of primary breast tumors, it is generally believed that paracrine regulation is involved in ERalpha mediated cell proliferation. In the paracrine model, ERalpha-positive cells don't proliferate but will release some paracrine growth factors to stimulate the neighboring cells to proliferate. In a subpopulation of cancer cells in some primary breast tumors, however, ERalpha does colocalize with the cell proliferation marker Ki-67, suggesting an autocrine regulation by ERalpha in some primary breast tumors.

Our data indicate that ERalpha can mediate estrogen-induced cell proliferation in an autocrine mode in ERalpha-positive breast cancer cell lines. All of the three ERalpha-positive cell lines used in our study showed colocalization of ERalpha and Ki-67, indicating that these cell lines might be originated from primary tumor cells with autocrine regulation.

Colocalization of ERalpha with Ki-67 in ERalpha-positive breast cancer cell lines (MCF-7, T47D, and ZR75-1) was evaluated by immunofluorescent staining. Cell cycle phase dependent expression of ERalpha was determined by co-immunofluorescent staining of ERalpha and the major cyclins (D, E, A, B), and by flow cytometry analysis of ERalphahigh cells. To further confirm the autocrine action of ERalpha, MCF-7 cells were growth arrested by ICI182780 treatment, followed by treatment with EGFR inhibitor, before estrogen stimulation and analyses for colocalization of Ki-67 and ERalpha and cell cycle progression.

Colocalization of ERalpha with Ki-67 was present in all three ERalpha-positive breast cancer cell lines. Unlike that in the normal mammary glands and the majority of primary breast tumors, ERalpha is highly expressed throughout the cell cycle in MCF-7 cells. Without E2 stimulation, MCF-7 cells released from ICI182780 treatment remain at G1 phase. E2 stimulation of ICI182780 treated cells, however, promotes the expression and colocalization of ERalpha and Ki-67 as well as the cell cycle progressing through the S and G2/M phases. Inhibition of EGFR signaling does not inhibit the autocrine action of ERalpha.