MTOR and breast cancer: (b) Dexamethasone up-regulates the expression of p27 in both estrogen receptor-positive and negative human breast cancer cells in vitro by down-regulating phosphorylation of 4E-BP1 and this down-regulation is mediated primarily by upstream 5'-AMP-activated kinase/tuberous sclerosis complex proteins/mammalian target of rapamycin (AMPK/TSC/mTOR) protein kinase signaling pathway (pathway #2 in Figures 5a, 5b and 7).