The signal transducer and activator of transcription Stat5 is transiently activated by
growth factor and
cytokine signals in normal cells, but its persistent activation has been observed in a wide range of human
tumors. Aberrant Stat5 activity was initially observed in
leukemias, but subsequently also found in
carcinomas. We investigated the importance of Stat5 in human tumor cell lines.
shRNA mediated downregulation of Stat5 revealed the dependence of prostate and
breast cancer cells on the expression of this
transcription factor. We extended these inhibition studies and derived a
peptide aptamer (PA)
ligand, which directly interacts with the
DNA-binding domain of Stat5 in a yeast-two-hybrid screen. The Stat5 specific PA sequence is embedded in a
thioredoxin (hTRX) scaffold
protein. The resulting
recombinant protein S5-DBD-PA was expressed in bacteria, purified and introduced into
tumor cells by
protein transduction. Alternatively, S5-DBD-PA was expressed in the
tumor cells after
infection with a S5-DBD-PA encoding gene transfer vector. Both strategies impaired the
DNA-binding ability of Stat5, suppressed Stat5 dependent transactivation and caused its intracellular degradation. Our experiments describe a
peptide based inhibitor of
Stat5 protein activity which can serve as a lead for the development of a clinically useful compound for
cancer treatment.