Susumu Kohno

While loss of function (LOF) of retinoblastoma 1 (RB1) tumor suppressor is known to drive initiation of small‐cell lung cancer and retinoblastoma, RB1 mutation is rarely observed in breast cancers at their initiation. In this study, we investigated the impact on untransformed mammary epithelial cells given by RB1 LOF. Depletion of RB1 in anon‐tumorigenic MCF10A cells induced reversible growth arrest (quiescence) featured by downregulation of multiple cyclins and MYC, upregulation of p27 KIP1 , and lack of expression of markers which indicate cellular senescence or epithelial–mesenchymal transition (EMT). We observed a similar phenomenon in human mammary epithelial cells (HMEC) as well. Additionally, we found that RB1 depletion attenuated the activity of RAS and the downstream MAPK pathway in an RBL2/p130‐dependent manner. The expression of farnesyltransferase β, which is essential for RAS maturation, was found to be downregulated following RB1 depletion also in an RBL2/p130‐dependent manner. These findings unveiled an unexpected mechanism whereby normal mammary epithelial cells resist to tumor initiation upon RB1 LOF.