Constitutive activation of Stat3 by the Src and JAK tyrosine kinases participates in growth regulation of human breast carcinoma cells

Constitutive activation of signal transducer and activator of transcription (STAT) proteins has been detected in a wide variety of human primary tumor specimens and tumor cell lines including blood malignancies, head and neck cancer, and breast cancer. We have previously demonstrated a high frequenc y of Stat3 DNA-binding activity that is constitutively-induced by an unknow n mechanism in human breast cancer cell lines possessing elevated EGF recep tor (EGF-R) and c-Src kinase activities. Using tyrosine kinase selective in hibitors, we show here that Src and JAK family tyrosine kinases cooperate t o mediate constitutive Stat3 activation in the absence of EGF stimulation i n model human breast cancer cell lines. Inhibition of Scr or JAKs results i n dose-dependent suppression of Stat3 DNA-binding activity, which is accomp anied by growth inhibition and induction of programmed cell death. In addit ion, transfection of a dominant-negative form Stat3 leads to growth inhibit ion involving apoptosis of breast cancer cells. These results indicate that the biological effects of the Src and JAK tyrosine kinase inhibitors are a t least partially mediated by blocking Stat3 signaling. While EGF-R kinase activity is not required for constitutive Stat3 activation in breast cancer cells, EGF stimulation further increases STAT DNA-binding activity, consis tent with an important role for EGF-R in STAT signaling and malignant progr ession. Analysis of primary breast tumor specimens from patients with advan ced disease revealed that the majority exhibit elevated STAT DNA-binding ac tivity compared to adjacent non-tumor tissues. Our findings, taken together , suggest that tyrosine kinases transduce signals through Stat3 protein tha t contribute to the growth and survival of human breast cancer cells in cul ture and potentially in vivo.