Suppression of insulin-like growth factor type I receptor by a triple-helix strategy inhibits IGF-I transcription and tumorigenic potential of rat C6 glioblastoma cells

Homopurine (AG) and homopyrimidine (CT) oligodeoxyribonucleotides predicted to form triple-helical (triplex) structures have been shown to specifically suppress gene expression when supplied to cultured cells. Here we present evidence that homopurine RNA (effector) sequences designed to form a triplex with a homopurine⋅homopyrimidine sequence 3′ to the termination codon of the insulin-like growth factor type I receptor (IGF-IR) structural gene can efficiently suppress IGF-IR gene transcription. Transfection vectors were constructed to drive transcription of either AG or CT variant triplex-forming strands. To increase the probability of obtaining stable transfectants with adequate expression of effector sequences, these were designed to be transcribed together with cDNA sequences conferring neomycin resistance as a fusion transcript. Rat C6 glioblastoma cells transfected with the AG variant showed dramatic reduction of IGF-IR transcripts compared with untransfected cells. The AG transfectants also exhibited marked down-regulation of the IGF-I, and an enhanced accumulation of serine protease inhibitor nexin-I mRNA. Similar changes in gene expression were observed following transfection of C6 cells with constructs transcribing antisense RNA to IGF-IR transcripts, but were not observed in C6 cells transfected with either the CT triplex variant or with vector lacking triplex-forming sequences. Moreover, C6 cells transfected with AG triplex variant displayed a dramatic inhibition of tumor growth when injected into nude mice. The results suggest that a triple-helix strategy can be used to inhibit transcription elongation of the IGF-IR gene, and emphasize the efficacy of triplex-mediated gene inhibition in an animal model.