Molecular and Cellular Pathobiology Basal but not Luminal Mammary Epithelial Cells Require PI3K/mTOR Signaling for Ras-Driven Overgrowth

Themammary ducts of humans andmice are comprised of twomainmammary epithelial cell (MEC) subtypes: a surrounding layer of basal MECs and an inner layer of luminal MECs. Breast cancer subtypes show divergent clinical behavior that may reflect properties inherent in their MEC compartment of origin. How the response to a cancer-initiating genetic event is shaped byMEC subtype remains largely unexplored. Using themousemammary gland, we designed organotypic three-dimensional culture models that permit challenge of discrete MEC compartments with the same oncogenic insult. Mammary organoids were prepared from mice engineered for compartment-restricted coexpression of oncogenic H-RAS together with a nuclear fluorescent reporter. Monitoring of H-RAS-expressing MECs during extended live cell imaging permitted visualization of Rasdriven phenotypes via video microscopy. Challenging either basal or luminal MECs with H-RAS drove MEC proliferation and survival, culminating in aberrant organoid overgrowth. In each compartment, Ras activation triggered modes of collective MEC migration and invasion that contrasted with physiologic modes used during growth factor–initiated branching morphogenesis. Although basal and luminal Ras activation produced similar overgrowth phenotypes, inhibitor studies revealed divergent use of Ras effector pathways. Blocking either the phosphoinositide 3-kinase or the mammalian target of rapamycin pathway completely suppressed Ras-driven invasion and overgrowth of basal MECs, but only modestly attenuated Ras-driven phenotypes in luminal MECs. We show thatMEC subtype defines signaling pathway dependencies downstreamofRas. Thus, cells-of-originmay critically determine the drug sensitivity profiles ofmammary neoplasia. Cancer Res; 72(22); 5856–66. 2012 AACR.