IGF-1R is essential for the maintenance and Akt activation of estrogen receptor-positive breast cancer cells. (A) MCF7 or BT474 breast cancer cells were treated with picropodophyllin (PPP) and harvested to detect Akt phosphorylation with western blot (right panel) or mammosphere formation capability (left panel). Numbers in the left panel indicated relative expression in comparison with dimethylsulfoxide (DMSO)-treated cells. Data of mammosphere formation assay presented as the relative percentage of DMSO-treated cells. *P <0.05; #P <0.01. (B) MCF7 or BT474 cells were transfected with negative control siRNA (Ctrl-siRNA) or insulin-like growth factor 1 receptor (IGF-1R) specific siRNA  for 48 hours and cells were harvested for detection of Akt phosphorylation by western blot (left panel) or mammosphere formation capability. Data presented as relative percentage of Ctrl-siRNA transfected cells. *P <0.05; #P <0.01. All experiments were repeated independently at least twice and results shown were collected from a representative experiment.