Breast cancers have a poorer prognosis if estrogen receptor expression was lost during recurrence. It is unclear whether this conversion is cell autonomous or whether it can be promoted by the microenvironment during cancer dormancy. Bone marrow is the most common site for luminal breast cancer recurrence. We explored whether the HS5 bone marrow stromal cell line that is known to arrest co-cultured breast cancer cells could suppress estrogen receptor alpha (ER) expression during arrest, and whether it could prime the emergence of estrogen-independent breast cancer clones. We demonstrated that paracrine signaling from HS5 cells downregulated ER at the mRNA level and also decreased ER protein stability in T47D and MCF7 breast cancer cells. Conditioned medium (CM) from HS5 arrested the cells in G0/G1 in part through interleukin-1 (IL1), inhibiting mammosphere formation and 2D-growth despite activation of proliferative pathways (Erk and AKT) by the CM. Reversal of stromal-induced growth arrest by blockade of signaling by IL1 in HS5-CM enabled the outgrowth of ER-negative breast cancer cells that were fulvestrant-resistant and estrogen-independent. Outgrowth depended on AKT signaling. Our results describe pro- and anti-proliferative paracrine signals arising from a bone marrow stromal cell line. Blockade of the growth suppressive signaling unmasked stromal facilitation of ER conversion and anti-estrogen resistance in breast cancer cells.

Citation Format: Richard Steinman, Jingjing Huang, Paul Woods, Daniel Normolle, Julie Goff, Christine Stehle. Stromalcell suppression of estrogen receptor in growth-arrested breast cancer cellsand promotion of outgrowth of estrogen-independent clones. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 4100.