MDA-MB-231 and MCF7 cells were purchased from ATCC (Mannassas, VA, USA). MCF7 cells were grown in Dulbecco’s modified Eagle’s medium (Invitrogen, Carlsbad, CA, USA), MDA-MB-231 cells were cultured in ATCC-formulated Leibovitz’s L-15 medium (ATCC), supplemented with 10% fetal bovine serum (PAA Laboratories, Pasching, Austria) and 10 U/ml penicillin/streptomycin (Invitrogen). Cells were cultured in Earle’s Balanced Salt Solution (EBSS; Sigma, St Louis, MO, USA) to induce nutrient starvation.