Met signaling regulates growth, repopulating potential and basal cell-fate commitment of mammary luminal progenitors: implications for basal-like breast cancer.

Basal-like breast cancer is an aggressive subtype of mammary carcinoma. Despite expressing basal markers, typical of mammary stem cells, this tumor has been proposed to originate from luminal progenitors, which are downstream of stem cells along the mammary epithelial hierarchy. This suggests that committed luminal progenitors may reacquire basal, stem-like characteristics, but the mechanisms that regulate this transition remain unclear. Using mouse models, we found that luminal progenitors express high levels of the Met receptor for hepatocyte growth factor (HGF), as compared with the other mammary epithelial sub-populations. Constitutive activation of Met led luminal progenitors to attain stem cell properties, including enhanced clonogenic activity in vitro and de novo ability to reconstitute mammary glands in repopulation assays in vivo. Moreover, in response to Met signaling, luminal progenitors gave rise to hyperplastic ductal morphogenesis and preferentially underwent basal lineage commitment at the expense of luminal cell-fate specification. Opposite and symmetric results were produced by systemic pharmacological inhibition of Met. Hence, Met signaling targets luminal progenitors for expansion, impairs their differentiation toward the mature luminal phenotype and enables their commitment toward the basal lineage. These results emphasize a critical role for Met in promoting deregulated proliferation and basal plasticity of normal luminal progenitors in the mammary gland, a complex of events that may be required for sustaining the functional and phenotypic properties of basal-like breast tumors.