Abstract
BackgroundSecreted frizzled-related proteins (SFRPs) are a family of proteins that block the Wnt signaling pathway and loss of SFRP1 expression is found in breast cancer along with a multitude of other human cancers. Activated Wnt signaling leads to inappropriate mammary gland development and mammary tumorigenesis in mice. When SFRP1 is knocked down in immortalized non-malignant mammary epithelial cells, the cells exhibit a malignant phenotype which resembles the characteristics observed in metastatic breast cancer stem-like cells. However, the effects of SFRP1 loss on mammary gland development in vivo are yet to be elucidated. The work described here was initiated to investigate the role of SFRP1 in mammary gland development and whether SFRP1−/− mice exhibit changes in mammary gland morphology and cell signaling pathways shown to be associated with SFRP1 loss in vitro.Results10 week old nulliparous SFRP1−/− mammary glands exhibited branching with clear lobulo-alveolar development, which normally only occurs in hormonally stimulated mid-pregnant wt mammary glands. Explant cultures of SFRP1−/− mammary glands display increased levels of a well known Wnt signaling target gene, Axin2. Histomorphologic evaluation of virgin glands revealed that by 10 weeks of age, the duct profile is markedly altered in SFRP1−/− mice showing a significantly higher density of ducts with distinct alveoli present throughout the mammary gland, and with focal ductal epithelial hyperplasia. These findings persist as the mice age and are evident at 23 weeks of age. Changes in gene expression, including c-Myc, TGFβ-2, Wnt4, RANKL, and Rspo2 early in mammary gland development are consistent with the excessive hyper branching phenotype. Finally, we found that loss of SFRP1 significantly increases the number of mammary epithelial cells capable of mammosphere formation.ConclusionsOur study indicates that SFRP1 gene is critical for maintaining proper mammary gland development, and that reduced levels of SFRP1 results in hyperplastic lesions and its loss may be a critical event in cancer initiation.
Highlights
Secreted frizzled-related proteins (SFRPs) are a family of proteins that block the Wnt signaling pathway and loss of secreted frizzled related protein (SFRP1) expression is found in breast cancer along with a multitude of other human cancers
SFRP1−/− mice exhibit precocious mammary gland development To determine the effect of SFRP1 loss on early mammary gland development, we examined whole mount preparations of mammary glands from female virgin SFRP1+/+ and SFRP1−/−mice and compared our findings with pregnant day 8 (P8) and P15 wt animals (Figure 1)
Differences in the mammary gland morphology could be detected between SFRP1+/+ and SFRP1−/− mice at early stages of puberty
Summary
Secreted frizzled-related proteins (SFRPs) are a family of proteins that block the Wnt signaling pathway and loss of SFRP1 expression is found in breast cancer along with a multitude of other human cancers. The work described here was initiated to investigate the role of SFRP1 in mammary gland development and whether SFRP1−/− mice exhibit changes in mammary gland morphology and cell signaling pathways shown to be associated with SFRP1 loss in vitro. Histomorphologic evaluation of virgin glands revealed that by 10 weeks of age, the duct profile is markedly altered in SFRP1−/− mice showing a significantly higher density of ducts with distinct alveoli present throughout the mammary gland, and with focal ductal epithelial hyperplasia. These findings persist as the mice age and are evident at 23 weeks of age. Pregnancy dependent hormones normally trigger both of these aspects of mammary morphogenesis, but both can be induced prematurely by ectopic expression of Wnt proteins in virgin mice [9,10,11,12]
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