Signal transducer and activator of transcription-5 activation and breast cancer prognosis.

Transcription factor signal transducer and activator of transcription-5 (Stat5) promotes breast epithelial cell differentiation. We retrospectively analyzed whether levels of active Stat5 in breast cancer were linked to clinical outcome. Immunohistochemistry was used to detect active, tyrosine-phosphorylated Stat5 in paraffin-embedded breast cancer specimens from three archival tissue microarray materials A, B, and C. Material A included 19 healthy human breast tissues and a progression series of primary lymph node-negative, primary lymph node-positive, and metastatic breast cancer (n = 400). Materials B (n = 785) and C (n = 570) represented two independent arrays of unselected primary breast cancer specimens with clinical follow-up data. Material A demonstrated that Stat5 activation, but not Stat5 protein expression, was gradually lost during cancer progression, with detectable activation in 100% of healthy breast specimens compared with less than 20% of node-positive breast cancers and metastases. Stat5 activation in tumors of material B was associated with favorable prognosis. This observation was confirmed and extended in material C to include both breast cancer-specific survival and disease-free survival. Stat5 activation remained an independent prognostic marker after adjusting for patient age, tumor size, histological grade, estrogen receptor, progesterone receptor, and Her2/neu status by Cox multivariate analysis (hazard ratio, 2.0; P =.029). Stat5 activation was a particularly favorable marker in the lymph node-negative breast cancer subpopulation (hazard ratio, 7.5; P =.003). In our study, active Stat5 distinguishes breast cancer patients with favorable prognosis, and may be a useful marker for selection of more individualized treatment, especially in localized disease. These findings require confirmation in a large prospective study.