Outgrowth of Single Oncogene-expressing Cells from Suppressive Epithelial Environments

Tumorigenesis is a clonal evolution process initiated from single cells within otherwise histologically normal tissue(1). How single, sporadic mutant cells that have sustained oncogenic alterations evolve within tightly regulated tissue environment remains elusive. Here, we investigated the effects of inducing oncogene expression in single cells within organotypic mammary acini as a model to elucidate the processes by which oncogenic alterations initiate clonal progression from organized epithelial environments. We found that sporadic cells induced to overexpress oncogenes that specifically perturb cell-cycle checkpoints (HPV16-E7 and cyclinD1), deregulate c-Myc transcription, or activate AKT signaling, remain quiescent within growth-arrested acini. In contrast, single ErbB2-overexpressing cells initiate a cellular cascade involving cell translocation from the epithelial layer and luminal outgrowth characteristic of neoplastic progression in early-stage epithelial tumors. ErbB2-mediated luminal cell translocation is dependent on ERK and matrix metalloproteinase (MMP) activities, and genetic alterations that perturb local cell-matrix adhesion can drive cell translocation. We further provide evidence that luminal cell translocation may drive clonal selection by promoting either death or expansion of quiescent oncogene-expressing cells depending on whether preexisting 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 microenvironments and provokes clonal selection for survival and proliferative expansion outside their native niches.