Fig. 3

The magnitude of STAT activation varies with physiologic conditions. IL-2-dependent NKL cells were cultured in IL-2 containing growth medium (lanes 1 and 3) or were starved of IL-2 for 16 hr (lanes 3 and 4). Cells were left untreated (lanes 1 and 3) or were stimulated with supplemental IL-2 (lanes 2 and 4). Tyrosine-phosphorylated STAT1 and STAT5 were determined by performing a Western blot with an antibody that recognizes the activated, tyrosine-phosphorylated form of both proteins. IL-2 treatment of starved cells, which mimics the signaling response in transformed cells, leads to a much greater level of STAT1 and STAT5 activation than that seen in cells growing under physiologic conditions.