Fig. 1. IGF-IR expression levels and signaling in MCF-7-derived cell lines. (a) Expression of the IGF-IR was detected in MCF-7, MCF-7/IGF-IR/WT and MCF-7/IGF-IR/DK cells by western blotting of 20 µg of total protein with an antibody against the C-terminus of the IGF-IR ß subunit. The arrows show the position of the ß subunit in the mature receptor (95 kDa) and its precursors: prepro-(190 kDa) and pro-receptor (185 kDa). (b) Top panel: phosphotyrosine blot of MCF-7/IGF-IR/WT and MCF-7/IGF-IR/DK cells stimulated with 50 ng/ml IGF-I. Cell lysates were collected from serum-starved cells before (0 min) and after stimulation with IGF-I for 5, 15 and 60 minutes. Arrows show the position of the IGF-IR ß subunit (95 kDa) and IRS-1 (175 kDa). Protein molecular weight markers are indicated in kDa on the left of the blot. Lower panel: quantification of sample loading by western blot for the IGF-IR.