Mithramycin blocks transcriptional initiation of the c-myc P1 and P2 promoters.

The c-myc protooncogene plays an important role in the regulation of cellular proliferation. Mithramycin, a DNA binding antibiotic which binds G-C-rich DNA, inhibits c-myc expression in both differentiating and nondifferentiating cells. The G-C-rich nature of the c-myc promoter suggests that mithramycin may act by directly inhibiting promoter function. The mithramycin binding sites in the c-myc promoter regions were determined by DNAse I footprinting. Particularly prominent mithramycin binding is noted in the regions just 5' of the P1 and P2 promoter TATA boxes. Gel retardation experiments performed in the presence of mithramycin demonstrate that drug binding can prevent the formation of discrete complexes between HeLa cell nuclear proteins and c-myc promoter DNA fragments. Mithramycin also directly blocks the binding of the transcription factor Sp1 to the P1 promoter region. In vitro run-off transcription demonstrates that mithramycin can completely inhibit the in vitro function of both the P1 and P2 promoters. These data suggest that mithramycin inhibits transcription of the c-myc protooncogene by blocking the binding of important regulatory factors, thus preventing formation of the c-myc transcription initiation complex.