Synthetic double-stranded DNA with high affinity for a target transcription factor can be introduced into target cells as decoy cis- elements to bind the factor and alter gene transcription. The CRE (cyclic AMP response element)-transcription factor complex is a pleiotropic activator that participates in the induction of a wide variety of cellular and viral genes. Because the CRE cis-element TGACGTCA is palindromic, a synthetic single stranded oligonucleotide composed of the CRE sequence, which will self-hybridize to form either a duplex or hairpin, when introduced into a cell, can act as a decoy for the transcription factor. We have investigated the CRE-palindromic and -hairpin forming oligonucleotides as transcription factor decoys and the biological effects thereof. Herein we report that the CRE- palindromic oligonucleotide penetrated into cells, competed with CRE enhancers for binding transcription factors, and specifically interfered with CRE- and Ap-1-directed transcription in vivo. The 24 mer CRE palindrome oligonucleotide produced potent growth inhibition in a variety of cancer cells including breast, prostate, lung, ovarian, colon, and epidermoid carcinomas, and multidrug-resistant cancer cell lines of MCF7-TH (MDR-breast cancer) and HCT-15 (MDR-colon carcinoma) (IC50, 100-200 nM). In contrast, the growth of normal human mammary epithelial cell and lung epithelial cell lines was not affected by the CRE oligonucleotide. Treatment of nude mice bearing HCT-15 MDR colon carcinoma with 24 mer CRE oligonucleotide (0.1 mg/0.1 ml saline/mouse ip, 5x/week for 4 weeks) resulted in >85% inhibition of tumor growth. The CRE-oligonucleotide-induced growth inhibition accompanied changes in cell morphology and the appearance of apoptotic nuclei. By comparison, two base-mismatched control oligonucleotide or a palindromic oligonucleotide containing no CRE sequence had no effect on either CRE-directed transcription or cell growth. The mechanism by which the blockade of CRE-gene transcription brings about the selective inhibition of tumor cell growth but not normal cell growth is under investigation. Our data show that the CRE-transcription factor decoy can modulate in vivo gene transcription and restrain tumor growth in vivo. Thus, this technology offers great promise as a tool for defining cellular regulatory processes and treating diseased conditions. - Cancer, Transcription, cyclic AMP response element, growth arrest, oligonucleotide, transcription factor-decoy, AP-1,