Lomustine In this study we transplanted human iHeps into

In this study, we transplanted human iHeps into the livers of Gunn rats. Proliferation of the transplanted Lomustine was Lomustine induced by preconditioning a single liver lobe by hepatic X-irradiation (HIR). HIR enhances the engraftment of transplanted cells by transiently disrupting the sinusoidal endothelial barrier. Additionally, reduction of the mitotic capacity of the irradiated host hepatocytes provides a competitive proliferative advantage to the engrafted cells (Guha et al., 2002; Yamanouchi et al., 2009). Here, as with patients, to increase the safety of HIR, we treated only one liver lobe, representing 30% of the liver mass, to achieve regional hepatic repopulation by the transplanted cells.

Results and Discussion

Experimental Procedures

Author Contributions

Acknowledgments

Introduction
The terminal ductal lobular units in the adult human mammary gland and their lobuloalveoli counterparts in the mouse are key hormone-sensitive structures (Cardiff and Wellings, 1999). They are also foci of milk-secreting cells following pregnancy and represent major sites of breast cancer development in both species. Increased progesterone levels that occur both during the reproductive cycle and pregnancy trigger a dynamic growth response in these structures, resulting in a documented marked expansion in the number of stem and progenitor cells in the mammary glands of mice (Asselin-Labat et al., 2010; Joshi et al., 2010). These primitive cells lack estrogen and progesterone receptors (ER–PR–) and therefore must respond to these hormones through indirect mechanisms via receipt of critical signals from other types of cells within the mammary stem cell niche that are ER+PR+ (Joshi et al., 2012). WNT signaling is thought to contribute to the regulation of stem cell self-renewal and differentiation responses in many tissues (Nusse et al., 2008), including the mouse mammary gland (van Amerongen et al., 2012; Zeng and Nusse, 2010). However, the specific mechanisms that control the ability of mammary stem and progenitor cells to respond to WNT ligands have remained largely undefined.
In this study, we show that the Receptor Activator of Nuclear factor Kappa B (RANK) ligand and WNT paracrine signals are conserved in adult mouse and human mammary tissue and fluctuate similarly during the cyclic progenitor expansion seen in both species. By exploiting a combination of genetic and pharmacological approaches, we also reveal an interaction between the RANK and WNT pathways that provides the molecular circuitry essential for the WNT response and the expansion of ER–PR– mammary progenitors in the adult mouse mammary gland.

Results

Discussion
In this study, we provide evidence that human breast luminal progenitors undergo a similar type of progesterone-mediated expansion of their numbers during the luteal phase of the menstrual cycle as previously documented in the mouse (Asselin-Labat et al., 2010; Joshi et al., 2010). This cyclical rise in luminal progenitor numbers would be anticipated to pose repeated opportunities for mutagenic events to accumulate, and thus offer an explanation for an increasing risk of breast cancer with more menstrual cycles (Kelsey et al., 1993). Luminal progenitors have been implicated as the cell of origin in BRCA1-related breast cancers (Lim et al., 2009; Molyneux et al., 2010), and in more recent work we have shown that these cells possess short dysfunctional telomeres and higher levels of reactive oxygen species, consistent with a higher propensity for transformation (Kannan et al., 2013, 2014). Previous studies investigating the effects of progesterone on human mammary cells have relied on progestin treatment of cultured human breast cells or organoids (Graham et al., 2009; Tanos et al., 2013). In a recent study, ex vivo progestin treatment of human mammary epithelial organoids or microstructures did not shift the distribution of epithelial subpopulations, although increased RANKL was observed (Tanos et al., 2013). The significant differences that we observed may reflect the fact that the cells we analyzed were obtained directly from tissues removed from women in progesterone-high and progesterone-low menstrual phases, thus pointing to the potential importance of avoiding in vitro treatments that may not faithfully recapitulate conditions operative in vivo. Our data also provide a snapshot of RANKL and WNT activity during the luteal phase in the human breast, thus indicating their involvement in mediating the physiological changes in circulating progesterone levels on the human luminal progenitor compartment.