Abstract 3572: Unliganded progesterone receptors augment estrogen-induced growth of breast cancer cells via co-regulation of estrogen receptor target genes.

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Abstract The ovarian steroid hormone receptors, Progesterone and Estrogen Receptors (PR, ER), are important drivers of breast cancer proliferation. Recently, ER and IGF1R were shown to function within the same signaling pathways. Notably, PR cross-talk with the IGF signaling pathway has also been reported. Herein, we further probed for signaling cross-talk between ER and PR. Stable expression of PR-B in PR-low, ER+, MCF7 breast cancer cells increased the sensitivity of cells to IGF and estrogen, as measured in MTT and soft agar growth assays in the absence of exogenous progestins. In MCF7 cells, PR co-immunoprecipitated with PELP1, IGF1R, and ERα. PR expressing MCF7 cells exhibited increased IGF-induced signaling relative to vector controls. Genome-wide microarray analyses revealed that in the absence of progestin, stably expressed PR-B contributes to heightened estrogen responsiveness. We identified a PR-dependent, estrogen-induced gene signature; several ER-target genes required PR-B (unliganded) for robust expression, including CathepsinD. Stable PR expression in T47D and MCF7 breast cancer cells lead to induction of CathepsinD, but not pS2, in response to estrogen. Inhibition of IGF1R and PI3K had no effect on estrogen-induced pS2, but blocked CathepsinD upregulation in cells stably expressing PR-B. Notably, estrogen-treated MCF7 cells stably expressing PR-B (but not vector controls) exhibited enhanced ERα recruitment to an ERE located in the CathepsinD distal promoter. In addition to ERα, estrogen treatment induced robust accumulation of PR, IGF1R, and PELP1 on the CathepsinD ERE while complexes containing ERα and PELP1 alone were detected on the pS2 promoter. Importantly, stable knockdown of endogenous PR in unmodified MCF-7-L cells blocked estrogen-mediated CathepsinD induction and decreased estrogen-mediated growth in soft agar. Similarly, in unmodified PR+/ER+ BT474 and MCF7-L cells, estrogen-dependent growth was blocked by the PR antagonist, Onapristone. These data indicate that unliganded PR mediates enhanced growth responses to IGF and estrogen, perhaps by stabilizing a functional signaling complex that contains PR-B, ERα, PELP1, and IGF1R. This complex may serve to facilitate hormone-induced activation of PELP1-associated kinase cascades required for robust expression of transcriptional programs at newly defined “ER/PR” target genes. Taken together, our data provide a strong rationale for targeting PR in addition to ER and IGFR in combined breast cancer therapy. This work was supported by NIH grant R01 CA159712 (to C.A.L). Citation Format: Andrea R. Daniel, Angela L. Gaviglio, Todd P. Knutson, Julie H. Ostrander, Douglas Yee, Carol A. Lange. Unliganded progesterone receptors augment estrogen-induced growth of breast cancer cells via co-regulation of estrogen receptor target genes. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3572. doi:10.1158/1538-7445.AM2013-3572