Musashi1 Modulates Mammary Progenitor Cell Expansion through Proliferin-Mediated Activation of the Wnt and Notch Pathways

Abstract

The RNA-binding protein Musashi1 (Msi1) is a positive regulator of Notch-mediated transcription in Drosophila melanogaster and neural progenitor cells and has been identified as a putative human breast stem cell marker. Here we describe a novel functional role for Msi1: its ability to drive progenitor cell expansion along the luminal and myoepithelial lineages. Expression of Msi1 in mammary epithelial cells increases the abundance of CD24 hi Sca-1 ؉ , CD24 hi CD29 ؉ , CK19, CK6, and double-positive CK14/CK18 progenitor cells. Proliferation is associated with increased proliferin-1 (PLF1) and reduced Dickkopf-3 (DKK3) secretion into the conditioned medium from Msi-expressing cells, which is associated with increased colony formation and extracellular signal-regulated kinase (ERK) phosphorylation. Treatment with the MEK inhibitor U0126 inhibits ERK activation and decreases Notch and ␤-catenin/T-cell factor (TCF) reporter activity resulting from Msi1 expression. Reduction of DKK3 in control cells with a short hairpin RNA (shRNA) increases Notch and ␤-catenin/TCF activation, whereas reduction of PLF1 with a shRNA in Msi1-expressing cells inhibits these pathways. These results identify Msi1 as a key determinant of the mammary lineage through its ability to coordinate cell cycle entry and activate the Notch and Wnt pathways by a novel autocrine process involving PLF1 and DKK3.

The Musashi1 RNA-Binding Protein: A Critical Regulator in Glioblastoma

“…Consistent with its role in self-renewal, Msi1 expression is positively correlated with a labelretaining and side population human breast epithelial cells enriched in ERα, p21 Cip1 , CK19 and double-positive CK14/CK18 progenitor cells (Smalley and Clarke, 2005;Clarke et al, 2005). When Msi1 is overexpressed in murine mammary epithelial cells, CD24 hi /Sca-1 + , CD24 hi /CD29 + , CK14 + /CK18 + and CK6 + and CK19 + expression is enhanced in mammary stem and progenitor cells Wang et al, 2008). Msi1 acts in a unique autocrine pathway which consists of increased secretion of the growth factor Proliferin, loss of the Wnt inhibitor DKK3, activation of Wnt and Notch signaling Wang et al, 2008).…”

“…Although Msi1 and Msi2 have differing patterns of expression and roles within the cell (Aubert et al, 2003;Chan et al, 2006;Siddall et al, 2006;Sugiyama-Nakagiri et al, 2006;Sgubin et al, 2007;Kharas et al, 2010), both Msi1 and Msi2 are required for brain stem cell self-renewal (Sakakibara et al, 2002). In mammalian cells, Msi1 denotes multipotent stem cells in the brain (Sakakibara et al, 1996;Keyoung et al, 2001;McGuckin et al, 2004;Uchida et al, 2004), intestine (Kayahara et al, 2003;Nishimura et al, 2003;Sakatani et al, 2005) , breast (Clarke, 2005;Wang et al, 2008), and hair follicles (Sugiyama-Nakagiri et al, 2006).…”

“…When Msi1 is overexpressed in murine mammary epithelial cells, CD24 hi /Sca-1 + , CD24 hi /CD29 + , CK14 + /CK18 + and CK6 + and CK19 + expression is enhanced in mammary stem and progenitor cells Wang et al, 2008). Msi1 acts in a unique autocrine pathway which consists of increased secretion of the growth factor Proliferin, loss of the Wnt inhibitor DKK3, activation of Wnt and Notch signaling Wang et al, 2008). This results in a gene expression profile indicative of the cell cycle, growth factor signaling, invasion, adhesion, survival and embryonic stem cells.…”

Notch Signaling in Hair Follicle Development

“…Furthermore, as well as other stem/progenitor cells, putative human breast stem cells were discovered in the mammary gland using specific antibodies for Msi1 and p21 WAF1 (23). Wang et al found that much more committed progenitor cells in the mammary gland also express Msi1 protein, although luminal and myoepithelial progenitor cells do not express it (47). Likewise, SugiyamaNakagiri et al reported that Msi1 and Msi2 are expressed in the epidermis and hair follicles of mice from E14.5 until adulthood (24).…”

Musashi Proteins in Neural Stem/Progenitor Cells

“…Although Wang et al (2008) proposed that the Msi1 protein is involved in both Notch and Wnt signaling pathways as a novel autocrine process (Nagata et al, 2006;Glazer et al, 2008), details of the mechanism remain unclear, and direct regulators of Msi1 have not been identified.…”