Luminal breast cancers account for ~75% of cases and express (ER) and frequently contain a range of progesterone receptor (PR) positive cells. ER and PR are steroid hormone receptors (SR) that rapidly and dynamically shuttle between the cytoplasm and the nucleus, a process that allows for essential crosstalk with cytoplasmic signaling pathways that serve as direct inputs to SR transcriptional functions. During disease progression, aberrant hormone signaling often leads to expression of altered SR gene programs and endocrine resistance. An emerging biomarker of increased risk and aggressive cancer behavior is cytoplasmic PELP1. Our lab was the first to show that PELP1 forms a functional signaling and transcriptional complex with ER and PR to regulate novel estrogen-driven SR target genes associated with endocrine resistance. PELP1 is primarily nuclear in normal breast tissue, but cytoplasmic localization of PELP1 (cyto-PELP1) is observed in 40-58% of invasive tumors. To elucidate the impact of PELP1 mislocalization on SR signaling pathways and transcription programs, we have generated SR+ breast cancer cell models expressing vector, wild-type (nuclear) PELP1, and cyto-PELP1. Our studies show that cells expressing cyto-PELP1 exhibit increased mammosphere formation relative to WT-PELP1 or vector control cells in response to estrogen. Moreover, we have identified AIB1 (ER co-activator) as a robust binding partner for cytoplasmic, but not nuclear PELP1, through mass spectrometry. This interaction occurs in a hormone-dependent manner as shown by co-immunoprecipitation assays. AIB1 activation (i.e. phosphorylation levels) is increased in cyto-PELP1 cells relative to WT-PELP1 or control cells. Knockdown of AIB1 reduces cyto-PELP1 protein levels and leads to decreased gene expression of key pro-inflammatory and immune regulatory cyto-PELP1 target genes. Our results reveal novel cytoplasmic actions of AIB1 and highlight the significance of delineating the impact of dynamic PELP1 shuttling/mislocalization with respect to altered hormone/SR-driven target gene expression in luminal breast cancer progression. We conclude direct signaling inputs to SR complexes are profoundly altered in the context of PELP1 mislocalization and interaction with AIB1. Our findings are relevant to mechanisms of endocrine resistance that may be targeted by inhibitors of the PELP1/SR interaction.

Citation Format: Thu H. Truong, Julie H. Ostrander, Carol A. Lange. PELP1 cytoplasmic mislocalization favors estrogen-induced signaling via AIB1 in ER+ breast cancer models [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3618. doi:10.1158/1538-7445.AM2017-3618