Figure 4

Cleavage of STAT3α in human breast cancer cell lines. Whole cell extracts from one normal human breast cell line (HB4A) and 7 breast cancer cell lines with varying levels of activated, tyrosine phosphorylated, STAT3α were immunoblotted with either the anti-STAT3 amino terminal antibody (upper panel, 48 kDa cleaved fragment) or the anti-STAT3 PY705 antibody (lower panel, 43 kDa cleaved fragment). Over exposure of the upper blot revealed an additional cleavage product of approximately 25 kDa that was most apparent in the GI-101 cell line which has the highest level of tyrosine phosphorylated STAT3α in the breast cancer cell lines examined. Extracts from C57Bl/6 mouse mammary glands were loaded as controls: 5d Gest = day 5 pregnancy; 10d Lac = 10 days of lactation; 96 Hr In = 96 hours following forced weaning at day 10 lactation. Arrows indicate cleaved fragments.