Linkage of progestin and epidermal growth factor signaling: Phosphorylation of progesterone receptors mediates transcriptional hypersensitivity and increased ligand-independent breast cancer cell growth

Department of Medicine, Division of Hematology, Oncology, and Transplantation and Pharmacology, University of Minnesota Cancer Center, Minneapolis, MN 55455, United States.
Steroids (Impact Factor: 2.72). 03/2007; 72(2):188-201. DOI: 10.1016/j.steroids.2006.11.009
Source: PubMed

ABSTRACT Progesterone receptor (PR) action is linked to epidermal growth factor (EGF) initiated signaling pathways at multiple levels; mitogen-activated protein kinases (MAPKs) are key mediators of this important cross-talk. Herein, we probed the effects of EGF on PR function and regulation of breast cancer cell growth. EGF stimulated rapid and transient phosphorylation of PR-B Ser294 relative to persistent phosphorylation of this site induced by the synthetic progestin, R5020. EGF induced nuclear translocation and DNA binding of unliganded wild-type, but not mutant PRs containing an Ala at position 294 (S294A). However, EGF alone induced little to no PR-B transcriptional activity; S294A PR-B was transcriptionally impaired. In contrast, pretreatment of cells with EGF (30min) significantly increased the potency and efficacy of wild-type, but not S294A PR transcriptional activity in response to progestin, and enhanced ligand-dependent downregulation of wild-type but not S294A PR. Replacement of Ser294 with aspartic acid (S294D) to mimic phosphorylation at this site decreased receptor stability and, as predicted, heightened progestin-induced transcription relative to wild-type PR-B. RT-PCR demonstrated the Ser294 phosphorylation-dependence of selected PR target genes (TGFalpha and HB-EGF). Surprisingly, PR-B expressing cells growing in soft agar were highly responsive to EGF or progestin, and this was further stimulated by the combination of both hormones. Cells expressing S294A PR exhibited reduced soft agar growth, and were also sensitive to R5020 alone, but failed to respond to EGF. These results suggest that PR Ser294 is an important "sensor" for growth factor inputs that affects PR function and breast cancer cell growth in the absence of progestin or in the presence of low or "sub-threshold" progestin concentrations. PR function likely contributes to breast cancer progression when EGFR family members or their ligands are overexpressed, a condition that predicts low abundance, but highly active and nuclear PR.

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Available from: Julie Ostrander, Jul 30, 2015
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    • "nregulation of the PR protein and this event is required for the transcriptional activity of PR . These results , cou - pled with the fact that breast cancers frequently have elevated MAPK , have led to the proposal that cross talk between growth fac - tor receptors and PR contributes to deregulated proliferation and progression of breast cancer ( Daniel et al . , 2007 , 2009 ) . Loss of detectable PR protein , or low PR , in breast cancers with elevated MAPK or growth factor receptors maybe due to a hyper - activated persistently downregulated PR that contributes to tumor progres - sion during transition to more aggressive PR negative tumors . Again , an important piece of information that is lacking"
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    ABSTRACT: This paper reviews work on progesterone and the progesterone receptor (PR) in the mouse mammary gland that has been used extensively as an experimental model. Studies have led to the concept that progesterone controls proliferation and morphogenesis of the luminal epithelium in a tightly orchestrated manner at distinct stages of development by paracrine signaling pathways, including receptor activator of nuclear factor κB ligand (RANKL) as a major paracrine factor. Progesterone also drives expansion of stem cells by paracrine signals to generate progenitors required for alveologenesis. During mid-to-late pregnancy, progesterone has another role to suppress secretory activation until parturition mediated in part by crosstalk between PR and prolactin/Stat5 signaling to inhibit induction of milk protein gene expression, and by inhibiting tight junction closure. In models of hormone-dependent mouse mammary tumors, the progesterone/PR signaling axis enhances pre-neoplastic progression by a switch from a paracrine to an autocrine mode of proliferation and dysregulation of the RANKL signaling pathway. Limited experiments with normal human breast show that progesterone/PR signaling also stimulates epithelial cell proliferation by a paracrine mechanism; however, the signaling pathways and whether RANKL is a major mediator remains unknown. Work with human breast cancer cell lines, patient tumor samples and clinical studies indicates that progesterone is a risk factor for breast cancer and that alteration in progesterone/PR signaling pathways contributes to early stage human breast cancer progression. However, loss of PR expression in primary tumors is associated with a less differentiated more invasive phenotype and worse prognosis, suggesting that PR may limit later stages of tumor progression.
    Molecular and Cellular Endocrinology 12/2011; 357(1-2):4-17. DOI:10.1016/j.mce.2011.10.030 · 4.24 Impact Factor
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    • "MO to PR - B Lys388 ( Daniel et al . , 2007a ) . Underphosphorylated PRs are heavily sumoylated and transcriptionally repressed . Conversely , generation of a Ser294 phospho - mimic receptor by replacement of Ser294 with aspartic acid ( S294D ) resulted in hyperactive progestin - induced transcription with increased PR turnover relative to wt PR ( Daniel et al . , 2007b ) ; S294D PRs are pre - dicted to be undersumoylated ( Daniel et al . , 2007a ) ."
    Hormones, Brain and Behavior, 3 edited by DW Pfaff, AP Arnold, AM Etgen, SE Fahrbach, R Rubin, 01/2009: chapter Molecular genomics of progestin actions: pages 1439-1465; Academic Press.
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    • "Interestingly, ligand-activated S294A PR is a weak transcription factor when stably expressed in breast cancer cells and fails to respond to agents that activate MAPK [39]. Conversely, generation of a Ser294 phospho-mimic receptor by replacement of Ser294 with aspartic acid (S294D) resulted in hyperactive progestin-induced transcription with increased PR turnover relative to wt PR [53]. Thus, reversible phosphorylation of PR Ser294 couples increased transcriptional activity to rapid down-regulation of PR protein by the ubiquitinproteosome pathway. "
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    ABSTRACT: Recent discoveries suggest that several protein kinases are rapidly activated in response to ligand binding to cytoplasmic steroid hormone receptors (SRs), including progesterone receptors (PRs). Thus, PRs act as ligand-activated transcription factor "sensors" for growth factor-initiated signaling pathways in hormonally regulated tissues, such as the breast. Induction of rapid signaling upon progestin binding to PR-B provides a means to ensure that receptors and co-regulators are appropriately phosphorylated as part of optimal transcription complexes. Alternatively, PR-B activated kinase cascades provide additional avenues for progestin-regulated gene expression independent of PR nuclear action. Herein, an overview of progesterone/PR and signaling cross-talk in breast cancer models is provided. Kinases are emerging as key mediators of PR action. Cross-talk between SR and membrane-initiated signaling events suggests a mechanism for coordinate regulation of gene subsets by mitogenic stimuli in hormonally responsive normal tissues, and is suspected to contribute to cancer biology.
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