Oncogenic transformation of mammary epithelial cells by transforming growth factor beta independent of mammary stem cell regulation

Abstract

Background: Transforming growth factor beta (TGF beta) is transiently increased in the mammary gland during involution and by radiation. While TGF beta normally has a tumour suppressor role, prolonged exposure to TGF beta can induce an oncogenic epithelial to mesenchymal transition (EMT) program in permissive cells and initiate the generation of cancer stem cells. Our objective is to mimic the transient exposure to TGF beta during involution to determine the persistent effects on premalignant mammary epithelium. Method: CD beta Geo cells, a transplantable mouse mammary epithelial cell line, were treated in vitro for 14 days with TGF beta (5 ng/ml). The cells were passaged for an additional 14 days in media without TGF beta and then assessed for markers of EMT and transformation. Results: The 14-day exposure to TGF beta induced EMT and transdifferentiation in vitro that persists after withdrawal of TGF beta. TGF beta-treated cells are highly tumorigenic in vivo, producing invasive solid de-differentiated tumours (100%; latency 6.7 weeks) compared to control (43%; latency 32.7 weeks). Although the TGF beta-treated cells have initiated a persistent EMT program, the stem cell population was unchanged relative to the controls. The gene expression profiles of TGF beta-treated cells demonstrate de-differentiation with decreases in the expression of genes that define luminal, basal and stem cells. Additionally, the gene expression profiles demonstrate increases in markers of EMT, growth factor signalling, TGF beta 2 and changes in extra cellular matrix. Conclusion: This model demonstrates full oncogenic EMT without an increase in stem cells, serving to separate EMT markers from stem cell markers.