The Wnt Receptor, Lrp5, Is Expressed by Mouse Mammary Stem Cells and Is Required to Maintain the Basal Lineage

Abstract

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.

From milk to malignancy: the role of mammary stem cells in development, pregnancy and breast cancer

“…Notably, when the MaSC marker CD29 is deleted from the basal compartment of the mammary gland, the mammary epithelial cells can no longer reconstitute a new mammary gland, but they can form alveoli late in pregnancy [18], suggesting a distinct MaSC population. Similar results were also obtained when the Wnt receptor LPR5 was deleted [23]. A recent study showed that using a GFP reporter driven by the s-SHIP promoter, GFP + replicating "active" MaSCs can be identified in cap cells in puberty and basal alveolar bud cells in pregnancy, but not in adult virgin animals, or in mammary tissues during lactation or involution stages [16].…”

“…For instance, in the Wnt pathway the receptor LRP5 can enrich for MaSCs on its own (although to a lesser degree than CD24 and CD29/ CD49f) and is functionally required for maintaining stem cell populations [23]. Overexpression of Wnt1 using the mammary-specific MMTV promoter resulted in a 6.4-fold increase in the number of MaSCs [7].…”

“…p63 is expressed in primarily two different isoforms, ∆N-p63 and TA-p63. The ∆N-p63 isoform is expressed in MaSCs of the human [27] and mouse [36,37], where its expression can be induced by Wnt signaling [23] and promotes the expression of self-renewal genes. The TAp63 isoform is expressed in luminal progenitor populations and promotes the expression of hedgehog pathway components necessary for progenitor cell function [37].…”

LRP5/6 directly bind to Frizzled and prevent Frizzled-regulated tumour metastasis

“…LRP5/6 are presently thought to be required for mammary outgrowths and often overexpressed in breast tumours [4][5][6][7] . Higher expressions of LRP5/6 were also observed in metastatic 4T1 cells than nonmetastatic 168FARN cells (Supplementary Fig.…”

“…The canonical Wnt1 gene was originally identified as a preferential integration site for the mouse mammary tumour virus in virally induced breast tumours 3 . Thereafter, LRP5/6 have been identified to be key for Wnt1-dependent tumour development in the Wnt1 transgenic mouse model [4][5][6] and LRP6 is often overexpressed in human breast tumours 7 . Thus, the components of the canonical pathway including LRP5/6 are commonly thought to be involved in cancer development as oncogenes.…”