Placental supernatants' enhancement of the metastatic potential of breast cancer cells: is estrogen receptor (ERα) essential for this phenomenon?

Purpose Pregnancy-associated breast cancer (PABC) is usually diagnosed at an advanced stage in comparison to non-pregnant women. The placenta secretes hormones and cytokines, which affect breast cancer progression. Previously, we demonstrated that human placental secretome facilitates the survival and migration of ER alpha+ breast cancer cells (BCCL), but pregnant women have a relatively high frequency of ER alpha-negative tumors. In the current study, we analyzed the effect of placental secretome on ER alpha-negative BCCL. Methods BCCL [MCF-7(estrogen/progesterone receptor positive (ER alpha+/PR+), ER alpha reduced MCF-7 (siRNA, MCF-7 ER alpha-), HS-578 and BT-549 cells (both ER-/PR-)] were exposed to supernatants (collected from first trimester human placental explants and from control BCCL) or to E2 + P4 (estrogen + progesterone) in placental supernatant concentrations and then tested for cell proliferation (number, cell cycle, PCNA), cell-death, cell migration, STAT3 pathway activation and functionality. Results Silencing ER alpha in the MCF-7 cells negated the placental supernatant and E2 + P4 enhancement of cell migration (> 130%, p < 0.05), number (> 120%) and survival (similar to 130%). However, it had no such effect on MCF-7-ER- migration, which was still elevated in the presence of placental secretome. ER-/PR- BCCL were unaffected by the hormones, but placental secretome significantly elevated their migration (115%), number (140-170%), STAT3 phosphorylation (similar to 180%) and BT-549 STAT3 level. These effects were negated by the STAT3 inhibitor. Conclusions Placental supernatant facilitates BCCL malignant characteristics by activating ER alpha in estrogen responsive cells and STAT3 in ER alpha- BCCL. This indicates a possible mechanism that may underlie PABC's advanced state and suggests STAT3 pathway as a therapeutic target for PABC.