Cancer Stem Cell Phenotypes in ER+ Breast Cancer Models Are Promoted by PELP1/AIB1 Complexes

Abstract

Proline, glutamic acid, leucine-rich protein 1 (PELP1) is overexpressed in approximately 80% of invasive breast tumors. PELP1 dynamically shuttles between the nucleus and cytoplasm, but is primarily nuclear in normal breast tissue. However, altered localization of PELP1 to the cytoplasm is an oncogenic event that promotes breast cancer initiation and progression. Herein, interacting partners unique to cytoplasmic PELP1 and the mechanisms by which these interactions promote oncogenic PELP1 signaling were sought. AIB1 (amplified in breast cancer 1; also known as SRC-3 or NCOA3) was identified as a novel binding partner of cytoplasmic PELP1 in both estrogen receptor-positive (ER) and ER-negative cell lines. Cytoplasmic PELP1 expression elevated basal phosphorylation levels (i.e., activation) of AIB1 at Thr24, enhanced ALDH tumorsphere formation, and upregulated specific target genes independently of hormone stimulation. Direct manipulation of AIB1 levels using shRNA abrogated cytoplasmic PELP1-induced tumorsphere formation and downregulated cytoplasmic PELP1-specific target genes. SI-2, an AIB1 inhibitor, limited the PELP1/AIB1 interaction and decreased cytoplasmic PELP1-induced tumorsphere formation. Similar results were observed in a murine-derived MMTV-AIB1 tumor cell line. Furthermore, syngeneic tumor studies revealed that PELP1 knockdown resulted in increased survival of tumor-bearing mice as compared with mice injected with control cells. These data demonstrate that cytoplasmic PELP1/AIB1-containing complexes function to promote advanced cancer phenotypes, including outgrowth of stem-like cells, associated with estrogen-independent breast cancer progression. .

PELP1/SRC-3-dependent regulation of metabolic kinases drives therapy resistant ER+ breast cancer

“…Phosphorylation of SRC-3 Ser857 promotes SRC-3 association with transcription factor ATF4 to mediate non-oxidative pentose phosphate pathway and purine synthesis. This study (35) did not evaluate SRC-3 in the context of CSCs, although SRC-3 has been linked to CSC activity (10,11). ATF4 pathway activation was identified by IPA in our studies (Figure 1) and could explain the correlation between PFKFB4 and PELP1/SRC-3driven CSCs.…”

“…STR authentication was performed by ATCC (October 2018). MCF-7 PELP1 and J110 cells were cultured as described (11). MCF-7 (12) and T47D TamR (13) cells were cultured in nM tamoxifen.…”

“…MCF-7 TaxR (14) cells were cultured in 2 μM Taxol. For 3D (tumorsphere) conditions, cells were cultured as described (11).…”

“…We performed RNA-seq on MCF-7 PELP1 models grown as 3D tumorspheres and compared these data to studies conducted in 2D culture (11) to identify candidate genes and pathways differentially regulated in cyto PELP1 expressing cells. Comparison of 3D versus 2D conditions identified 206 upregulated and 114 downregulated genes similarly regulated by >2-fold in all cell lines (LXSN, WT PELP1, cyto PELP1) ( Figure 1C, Supplementary Figure 3).…”

“…xenograft models (10). Our laboratory reported that cytoplasmic complexes composed of PELP1 and SRC-3 mediate breast CSC expansion (11). Targeting SRC-3 using shRNA or pharmacological inhibitors (i.e.…”