1. ¹Department of Microbiology and Molecular Genetics, Stony Brook University, Stony Brook, NY, USA
2. ²Department of Physiology and Biophysics, Stony Brook University, Stony Brook, NY, USA
3. ³Department of Genetics, Biology and Biochemistry, University of Turin, Turin, Italy

Correspondence: Dr NC Reich, Department of Microbiology and Molecular Genetics, Stony Brook University, Nicolls Road, Lifesciences Rm 168, Stony Brook, New York 11794-8691, USA. E-mail: nreich@notes.cc.sunysb.edu

Received 1 December 2005; Revised 8 February 2006; Accepted 9 February 2006; Published online 27 March 2006.

Abstract

Breast tumor kinase (Brk) is a non-receptor tyrosine kinase distantly related to the Src family kinase. It is expressed in more than 60% of breast tumors, but the biological role of this kinase remains to be determined. Only a limited number of substates have been identified for Brk, and the link of Brk to tumorigenesis remains largely unknown. In this study, we provide evidence that the signal transducer and activator of transcription 3, STAT3, is a physiological target of Brk. Activation of STAT3 previously has been linked to oncogenesis, and results in this study demonstrate that STAT3 is tyrosine phosphorylated and transcriptionally activated in cells expressing endogenous Brk. Signal transducer and activator of transcription 3 is specifically targeted since other STAT members are not responsive to Brk expression. Signal transducer and activator of transcription 3 activation requires the catalytic activity of Brk, and expression of both STAT3 and Brk stimulate cellular proliferation. In addition, we have identified a negative regulator of Brk, the suppressor of cytokine signaling, SOCS3. The SOCS3 protein is known to block signaling mediated by cytokine receptors, and here we find that SOCS3 is able to repress the activity of the Brk non-receptor tyrosine kinase.