Efficient killing of SW480 colon carcinoma cells by a signal transducer and activator of transcription (STAT) 3 hairpin decoy oligodeoxynucleotide--interference with interferon-gamma-STAT1-mediated killing.

The signal transducers and activators of transcription (STATs) convey signals from the membrane to the nucleus in response to cytokines or growth factors. STAT3 is activated in response to cytokines involved mostly in cell proliferation; STAT1 is activated by cytokines, including interferon-gamma, involved in defence against pathogens and the inhibition of cell proliferation. STAT3, which is frequently activated in tumour cells, is a valuable target with respect to achieving inhibition of tumour cell proliferation. Indeed, its inhibition results in cell death. We previously observed that inhibition of the transcription factor nuclear factor-kappaB, a key regulator of cell proliferation, with decoy oligodeoxynucleotides results in cell death. We used a similar approach for STAT3. A hairpin STAT3 oligodeoxynucleotide was added to a colon carcinoma cell line in which it induced cell death as efficiently as the STAT3 inhibitor stattic. The hairpin STAT3 oligodeoxynucleotide co-localized with STAT3 within the cytoplasm, prevented STAT3 localization to the nucleus, blocked a cyclin D1 reporter promoter and associated with STAT3 in pull-down assays. However, the same cells were efficiently killed by interferon-gamma. This effect was counteracted by the STAT3 oligodeoxynucleotide, which was found to efficiently inhibit STAT1. Thus, although it can inhibit STAT3, the hairpin STAT3 oligodeoxynucleotide appears also to inhibit STAT1-mediated interferon-gamma cell killing, highlighting the need to optimize STAT3-targeting oligodeoxynucleotides.