EGFR dependent expression of STAT3 (but not STAT1) in breast cancer.

STAT proteins constitute a family of transcription factors whose activation by cytokine and non-cytokine receptors leads to tyrosine phosphorylation, dimerization and translocation from the cytoplasm to the nucleus. In the nucleus they activate the transcription of specific genes by binding to consensus DNA elements. STATs 1 and 3 can be activated by both cytokine and non-cytokine receptors, and bind as homodimers or heterodimers to viral simian sarcoma virus (sis)-inducible elements such as that found in the c-fos promoter. Activation of c-Src and EGF receptor tyrosine kinases is associated with progression of breast cancer. Both these events lead to activation of STAT proteins, Src kinases activate STAT3 dependent transcription in mammary epithelial cells and EGF receptor activation can lead to activation of STATs 1 and 3. STAT3 activation has been demonstrated to have a role in oncogenesis and increasingly, activated STAT proteins are found to be activated in human cancer. In this study we describe detailed immunohistochemical analysis of nuclear and cytoplasmic STATs 1 and 3 expression in primary breast carcinomas and correlate this with EGFR, HER2, p53, ER, PR, p21/waf1, Bcl-XL and Ki-67 expression. We also compared expression between normal and tumor tissue. We report here a highly significant correlation between nuclear STAT3 expression and breast cancers compared to normal tissue. We also report a very strong correlation between nuclear STAT3 and EGFR expression in breast cancers. These data clearly demonstrate a strong association between STAT3 activation and breast tumorigenesis and strengthen the assertion that STAT3 activation may play an important role in the tumorigenic conversion of breast tissue mediated by tyrosine kinase signaling pathways.