An essential role of CBL and CBL-B ubiquitin ligases in mammary stem cell maintenance

The ubiquitin ligases CBL and CBL-B are negative regulators of tyrosine kinase signaling with established roles in the immune system. However, their physiological roles in epithelial tissues are unknown. Here, we used MMTV-Cre-mediated gene deletion on a null background, as well as a tamoxifen-inducible mammary stem cell (MaSC)-specific and double knockout ( DKO) using Lgr5-EGFP-IRES-CreERT2, to demonstrate a mammary epithelial cell-autonomous requirement of CBL and CBL-B in the maintenance of MaSCs. Using a newly engineered tamoxifen-inducible and deletion model with a dual fluorescent reporter ( ), we show that DKO in mammary organoids leads to hyperactivation of AKT-mTOR signaling with depletion of MaSCs. Chemical inhibition of AKT or mTOR rescued MaSCs from DKO-induced depletion. Our studies reveal a novel, cell-autonomous requirement of CBL and CBL-B in epithelial stem cell maintenance during organ development and remodeling through modulation of mTOR signaling.