Overexpression of
receptor tyrosine kinases including the
epidermal growth factor receptor (
EGF-R) as well as nonreceptor
tyrosine kinases, such as Src, have been implicated in the formation of human
lung cancers. In addition,
cytokines like
interleukin-6 (IL-6) have been demonstrated to modulate
lung cancer cell growth and elevated levels of
IL-6 have been shown to be an adverse prognostic factor for patients with
lung cancer. Despite a large body of evidence pointing to their potential importance, few direct studies into the role of signal transducers and activators of transcription (STAT) pathways in human
lung cancer have been undertaken. Here we demonstrate that multiple
nonsmall cell lung cancer cell lines demonstrate constitutive Stat3
DNA-binding activity. Stat3
DNA-binding activity is specifically upregulated by the addition of
epidermal growth factor (
EGF),
IL-6, and hepatocyte-derived
growth factor (HGF). Furthermore, the stimulation of Stat3
DNA-binding activity by
EGF requires the activity of
EGF-R
tyrosine kinase as well as
Src-kinase, while the upregulation of Stat3 activity by
IL-6 or HGF requires only
Src-kinase activity. Treatment of A549
lung cancer cells with
PD180970 or
SU6656, both pharmacological inhibitors of
Src-kinase, resulted in reduced Src and Stat3 activity, cell cycle arrest in G2, and reduced viability of cells accompanied by induction of apoptosis. Treatment of Stat3-positive A549 and H358 cells with antisense Stat3
oligonucleotides results in complete loss of Stat3
DNA-binding activity and apoptosis, while Stat3-positive H1299 cells remained healthy. Finally, an adenoviral vector expressing a dominant-negative Stat3
isoform results in loss of Stat3
DNA-binding activity, apoptosis, and reduced cellular viability. These results demonstrate a role of Stat3 in transducing survival signals downstream of
tyrosine kinases such as Src,
EGF-R, and c-Met, as well as
cytokines such as
IL-6, in human nonsmall cell
lung cancers.