Abstract 958: PELP1 regulation of mTOR Axis: Role in breast cancer progression and resistance

Abstract

Abstract Hormonal therapy resistance is a major clinical problem. Emerging evidence suggests that estrogen (ER) participates in extra-nuclear signaling events in the cytoplasm / membrane and such actions may play a role in therapy resistance. Recent studies suggested that mTOR pathway play a critical role in ER-mediated cell proliferation and growth factor signaling crosstalk. Proline, Glutamic-acid and Leucine-rich Protein 1 (PELP1) participates in ER extra-nuclear actions. PELP1 is a prognostic indicator of shorter breast cancer specific survival and PELP1 expression is predominantly in the cytoplasm in a subset of breast tumors. The objective of this application is to test whether cross talk occurs between mTOR-PELP1 signaling axis and whether mTOR targeting drugs can be used to target PELP1 oncogenic functions leading to therapy resistance. We have tested this hypothesis using ER-positive breast cancer cells that over express PELP1 (MCF7-PELP1, ZR75-PELP1) and models cells lacking PELP1 (MCF7-PELP1 shRNA, ZR-75-PELP1 shRNA). Vector transfected ZR75 and MCF7 cells were used as controls. Rapamycin and AZD8055 were used as pharmacological inhibitors to block mTOR pathway. Over expression of PELP1 enhanced the estrogen and Heregulin mediated cell proliferation in breast cancer cells. Rapamycin (10−7M) or AZD8055 (10−8M) treatment significantly reduced PELP1 driven growth in both MCF7 and ZR75 cells that are treated with estrogen. Similarly, Rapamycin and AZD8055 treatment of MCF7 and ZR75 cells significantly reduced PELP1 mediated increase in cell growth of Heregulin stimulated cells. Mechanistic studies using yeast two hybrid screen and reciprocal immunoprecipitations demonstrated that PELP1 directly interacts with mTOR and PP2A. PELP1cyto model cells that uniquely express PELP1 in the cytoplasm, exhibit excessive activation of mTOR signaling pathway upon estrogen and growth factor stimulation. Over expression of PELP1 in the breast cancer model cells increased phosphorylation of mTOR axis components (phos-mTOR, -S6K, -4EBP1,-AKT). Further, Knock down of PELP1 with siRNA significantly reduced the activation of mTOR signaling components upon estrogen and Heregulin stimulation. Immunohistochenistry studies using xenograft tumor tissues with PELP1 overexpression and PELP1cyto driven tumors with PELP1 deregulation showed correlation of PELP1 expression with the activation of mTOR signaling components and that PELP1cyto driven tumors have excessive activation of mTOR signaling components. Combination therapy of Tamoxifen and Rapamycin or AZD8055, sensitized PELP1 deregulated cells to hormonal therapy. Our results suggest that PELP1 driven oncogenic functions involve PELP1 modulation of mTOR signaling and blockade of mTOR signaling render PELP1 driven tumors highly sensitive to therapeutic inhibition. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 958. doi:1538-7445.AM2012-958