Proteomic profiling of secretome and adherent plasma membranes from distinct mammary epithelial cell subpopulations

The stem cell niche comprises stem cells (SCs), stromal cells, soluble factors, extracellular matrix constituents and vascular networks. The ability to identify signals that regulate SC self‐renewal and differentiation is confounded by the difficulty in isolating pure SC niche components in sufficient quantities to enable their biochemical characterisation. Here, we report the extracellular (secretome) and adherent plasma membrane proteomes of three distinct epithelial cell subpopulations isolated and immortalized from the mouse mammary gland – basal and mammary stem cell (basal/MaSC), luminal progenitor (LP) and mature luminal (ML) cell lines. GeLC‐MS/MS‐based proteomic profiling revealed a distinct switch in components modulating Wnt and ephrin signalling, and integrin‐mediated interactions amongst the three cell subpopulations. For example, expression of ephrin B2, ephrin receptors A1, and A2, as well as integrins α2β1 and α6β4 were shown to be enriched in basal/MaSCs, relative to LP and ML cells. Conspicuously, Wnt10a was uniquely detected in basal/MaSCs, and may modulate the canonical Wnt signalling pathway to maintain basal/MaSC activity. By contrast, non‐canonical Wnt signalling might be elevated in ML cells, as evidenced by the high expression levels of Wnt5a, Wnt5b, and the transmembrane tyrosine kinase Ror2.