The interaction of specific peptide aptamers with the DNA binding domain and the dimerization domain of the transcription factor Stat3 inhibits transactivation and induces apoptosis in tumor cells.

The transcription factor signal transducer and activator of transcription (Stat) 3 is activated through the interleukin-6 family of cytokines and by binding of growth factors to the epidermal growth factor (EGF) receptor. It plays an essential role in embryonic development and assumes specialized tasks in many differentiated tissues. Constitutively activated Stat3 has been found in tumor cell lines and primary tumors and plays a crucial role in tumor cell survival and proliferation. To inhibit the oncogenic action of Stat3 in tumor cells, we have selected short peptides, so-called peptide aptamers, which specifically interact with defined functional domains of this transcription factor. The peptide aptamers were selected from a peptide library of high complexity by an adaptation of the yeast two-hybrid procedure. Peptide aptamers specifically interacting with the Stat3 dimerization domain caused inhibition of DNA binding activity and suppression of transactivation by Stat3 in EGF-responsive cells. Similarly, a peptide aptamer selected for its ability to recognize the Stat3 DNA binding domain inhibited DNA binding and transactivation by Stat3 following EGF stimulation of cells. Peptide aptamers were expressed in bacteria as fusion proteins with a protein transduction domain and introduced into human myeloma cells. This resulted in dose-dependent growth inhibition, down-regulation of Bcl-x(L) expression, and induction of apoptosis. The inhibition of Stat3 functions through the interaction with peptide aptamers counteracts the transformed phenotype and could become useful in targeted tumor therapy.