Outgrowth of single oncogene-expressing cells from suppressive epithelial environments

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Abstract
The earliest stages of tumorigenesis are mimicked in a three-dimensional model of mammary epithelial cells, showing that oncogenes that can promote cell translocation can also drive clonal outgrowth. How a single mutant cell in otherwise normal tissue escapes its tightly regulated environment to become cancerous is still not clear. In a new three-dimensional cell culture model of mammary epithelial tissue, Cheuk Leung and Joan Brugge have been able to mimic the first stages of tumorigenesis. They introduce oncogenes into single cells and show that those that promote translocation of targeted cells into the lumen by perturbing local matrix adhesion, such as ERBB2, can also drive clonal outgrowth if the cells are capable of anchorage-independent survival. Blocking translocation inhibits outgrowth, demonstrating that translocation out of an epithelial layer allows cells that have acquired sporadic oncogenic mutations to escape the growth-suppressive microenvironment conferred by a normal epithelium and potentially initiate tumorigenesis. Tumorigenesis is a clonal evolution process that is initiated from single cells within otherwise histologically normal tissue1. It is unclear how single, sporadic mutant cells that have sustained oncogenic alterations evolve within a tightly regulated tissue environment. Here we investigated the effects of inducing oncogene expression in single cells in organotypic mammary acini as a model to elucidate the processes by which oncogenic alterations initiate clonal progression from organized epithelial environments. Sporadic cells induced to overexpress oncogenes that specifically perturb cell-cycle checkpoints (for example, E7 from human papilloma virus 16, and cyclin D1), deregulate Myc transcription or activate AKT signalling remained quiescent within growth-arrested acini. By contrast, single cells that overexpress ERBB2 initiated a cellular cascade involving cell translocation from the epithelial layer, as well as luminal outgrowth that is characteristic of neoplastic progression in early-stage epithelial tumours. In addition, ERBB2-mediated cell translocation to the lumen was found to depend on extracellular-regulated kinase and matrix metalloproteinase activities, and genetic alterations that perturb local cell–matrix adhesion drove cell translocation. We also provide evidence that luminal cell translocation may drive clonal selection by promoting either the death or the expansion of quiescent oncogene-expressing cells, depending on whether the pre-existing alterations allow anchorage-independent survival and growth. Our data show that the initial outgrowth of single oncogene-expressing cells from organized epithelial structures is a highly regulated process, and we propose that a cell translocation mechanism allows sporadic mutant cells to evade suppressive micro-environments and elicits clonal selection for survival and proliferative expansion outside the native niches of these cells.