BackgroundEctopic Wnt signaling induces increased stem/progenitor cell activity in the mouse mammary gland, followed by tumor development. The Wnt signaling receptors, Lrp5/6, are uniquely required for canonical Wnt activity. Previous data has shown that the absence of Lrp5 confers resistance to Wnt1-induced tumor development.Methodology/Principal FindingsHere, we show that all basal mammary cells express Lrp5, and co-express Lrp6 in a similar fashion. Though Wnt dependent transcription of key target genes is relatively unchanged in mammary epithelial cell cultures, the absence of Lrp5 specifically depletes adult regenerative stem cell activity (to less than 1%). Stem cell activity can be enriched by >200 fold (over 80% of activity), based on high Lrp5 expression alone. Though Lrp5 null glands have apparent normal function, the basal lineage is relatively reduced (from 42% basal/total epithelial cells to 22%) and Lrp5−/− mammary epithelial cells show enhanced expression of senescence-associated markers in vitro, as measured by expression of p16Ink4a and TA-p63.Conclusions/SignificanceThis is the first single biomarker that has been demonstrated to be functionally involved in stem cell maintenance. Together, these results demonstrate that Wnt signaling through Lrp5 is an important component of normal mammary stem cell function.

Homeostatic temperature regulation is fundamental to mammalian physiology and is controlled by acute and chronic responses of local, endocrine and nervous regulators. Here, we report that loss of the heparan sulfate proteoglycan, syndecan-1, causes a profoundly depleted intradermal fat layer, which provides crucial thermogenic insulation for mammals. Mice without syndecan-1 enter torpor upon fasting and show multiple indicators of cold stress, including activation of the stress checkpoint p38α in brown adipose tissue, liver and lung. The metabolic phenotype in mutant mice, including reduced liver glycogen, is rescued by housing at thermoneutrality, suggesting that reduced insulation in cool temperatures underlies the observed phenotypes. We find that syndecan-1, which functions as a facultative lipoprotein uptake receptor, is required for adipocyte differentiation in vitro. Intradermal fat shows highly dynamic differentiation, continuously expanding and involuting in response to hair cycle and ambient temperature. This physiology probably confers a unique role for Sdc1 in this adipocyte sub-type. The PPARγ agonist rosiglitazone rescues Sdc1−/− intradermal adipose tissue, placing PPARγ downstream of Sdc1 in triggering adipocyte differentiation. Our study indicates that disruption of intradermal adipose tissue development results in cold stress and complex metabolic pathology.

Background:The RNA binding protein, coding region determinant-binding protein (CRD-BP), is expressed by tumor cells and protects key mRNAs. Results: This study identifies a novel variant of CRD-BP and finds that CRD-BP is required for breast tumor cell clonogenicity. Conclusion: CRD-BP has tumorigenic activity and is ubiquitously expressed in breast epithelium. Significance: Under-reporting of CRD-BP isoforms suggests that published studies may be incomplete.

Background: Brisken et al (2000) showed that Wnt4 null mammary glands were deficient in early lobuloalveolar mammary outgrowth during pregnancy, and implicated Wnt4 as an effector for the progesterone-induced mammary growth program. Though ectopic Wnt1 signaling is known to be mitogenic and oncogenic, no endogenously expressed Wnt ligands have ever been directly implicated in mammary growth and morphogenesis. Therefore, we generated conditional transgenic mice to test whether Wnt4 can stimulate mammary epithelial cell growth.

Xenograft model studies have shown that tumor-associated, or genetically modified, activated stromal cells can promote tumor cell growth. Here, we examined mammary tumors arising in response to two different transgene-mediated Wnt signaling effectors: Wnt1 (a ligand with cell-nonautonomous effects) and #NB-catenin (a constitutively active form of the intracellular effector). Although the route of tumor development has been shown to be similar for these two models, histologic analysis shows that Wnt1-induced tumors are associated with tracts of activated stroma, whereas most #NB-catenin-induced tumors are solid adenocarcinomas. Furthermore, quantification of the ''reactive stroma index'' indicates that abundant activated stroma correlates with accelerated tumor progression. Wnt1-expressing mammary epithelial cells induce Wnt-specific target gene expression in local stromal cells (Wnt1-induced secreted protein 1/CCN4) but also induce long-range effects. Thus, mice with rapid tumor progression have 2-fold more circulating endothelial progenitor cells in peripheral blood than control or #NB-catenin transgenic mice. Using tagged bone marrow (BM) transplants, we show that BM-derived cells are massively recruited to infiltrate the stroma of Wnt1-induced tumors where they differentiate into multiple cell types. Thus, localized ectopic expression of the proto-oncogene Wnt1 in mammary glands induces systemic responses, and we propose that this response modifies the tumorigenic outcome. [Cancer Res 2008;68(24):10145-53]