We speculate that in vivo serum GH of pituitary

We speculate that in vivo, serum GH of pituitary and placental origin may be responsible for the expansion of mammary stem/progenitor gonadotropin releasing hormone agonist during developmental windows when the mammary gland grows isometrically (intrauterine life, childhood). In adulthood, when GH secretion from the pituitary gland decreases, this hormone is secreted locally in the mammary gland, in response to progesterone stimulation. This process may contribute to additional expansion of the stem/progenitor cell populations during the lutheal phase of the menstrual cycle and during pregnancy, when the mammary gland does not grow isometrically. In a process reminiscent of “niche formation,” ER− stem/progenitor cells generate sensory ER+ cells (Honeth et al., 2014). A paracrine signaling loop is initiated when estrogen peaks at the end of follicular phase and upregulates PR. Subsequently, progesterone peaks in the lutheal phase and activates PR molecular targets. We propose that GH is secreted as long as progesterone levels are high. Mitogenic signals are conveyed by GH to neighboring GHR+ stem/progenitor cells (Figures 7A and 7B). If these cycling progenitor cells divide further and generate differentiated cells, the ratio between stem/progenitor and differentiated cells will be maintained relatively constant. Unlike other tissues, however, proliferation of breast cells follows the cyclic fluctuation of ovarian hormones (Russo and Russo, 2006). The cumulative effect of prolonged exposure to steroid hormones and/or GH during many menstrual cycles would lead eventually to changed ratios between progenitor and differentiated cells, in favor of the former. An expanded undifferentiated cell population with a higher rate of proliferation would be at higher risk for transformation through oncogenic hits (Figure 7C). Animals treated with GH develop hyperproliferative lesions in the mammary gland and have increased incidence of spontaneous mammary tumors (Törnell et al., 1992; van Garderen and Schalken, 2002; Waters and Barclay, 2007). Conversely, animals deficient in GH signaling, such as spontaneous dwarf rats or Ghr KO mice, are resistant to carcinogen-induced cancers, including mammary cancer (Swanson and Unterman, 2002). Blocking GH signaling in mice xenografted with the ER-positive breast cancer cell line MCF7 considerably reduced tumor size and prolonged latency of tumor formation (Divisova et al., 2006). In our study, we utilized an xenograft model based on orthotopic implantation of a triple-negative breast cancer propagated in vivo only. The tumorigenic cell population of this tumor is represented by cells with ALDH activity (Ginestier et al., 2007). In this model, a significant reduction in tumor growth was observed in animals treated with GHA, compared to controls.
Experimental Procedures
Acknowledgments
Introduction Hematopoiesis is a tightly regulated process orchestrated by cell-autonomous and non-cell-autonomous signals emanating from a variety of cell types within specialized bone marrow (BM) microenvironments (Wang and Wagers, 2011; Frenette et al., 2013). Coordinated signals instruct hematopoietic stem cells (HSCs) to maintain their undifferentiated status or to commit and differentiate into mature hematopoietic cells (Kiel and Morrison, 2008; Wilson and Trumpp, 2006). A number of recent reports suggest that signaling by hypoxia-inducible transcription factors (HIFs) regulate HSC maintenance in a cell-autonomous manner. HIF factors are heterodimeric transcription factors composed of α and β subunits: the β subunit (ARNT) is constitutively expressed, whereas the α subunit is degraded through an oxygen-dependent mechanism and is stabilized at low oxygen concentrations (Schofield and Ratcliffe, 2004). Three α subunits have been identified: HIF-1α, HIF-2α, and HIF-3α, with HIF-1α and HIF-2α being the most extensively characterized (Keith et al., 2012). Despite sharing a high degree of sequence identity, HIF-1α and HIF-2α are not redundant, because they are expressed at least partly in a tissue-specific manner and regulate a number of unique target genes (Ratcliffe, 2007; Keith et al., 2012). In hypoxic conditions, HIF transcription factors trigger a variety of adaptive responses that include induction of anaerobic metabolism, cell migration, and neo-angiogenesis (Semenza, 2003). More recently, HIF factors are being increasingly implicated in regulating stem cells homeostasis (Mohyeldin et al., 2010; Suda et al., 2011), particularly in the hematopoietic system where HIF-1α is expressed in HSC (Takubo et al., 2010) and promotes HSC maintenance by enforcing a glycolytic metabolic state (Takubo et al., 2013).