Changes in promoter utilization in human and mouse c-myc genes upon transformation induction in temperature-sensitive cell lines.

We have previously reported accelerated transcription and rapid accumulation of c-myc mRNAs upon induction of transformation in a temperature-sensitive mouse cell line (Gallant et al., 1989, Oncogene Res., 4:39-46). Here we have used both mouse and human cell lines transformed with a temperature-sensitive mutant of the Simian virus 40 (SV40) virus to investigate whether a shift in promoter utilization within the c-myc gene locus is part of a general mechanism that deregulates c-myc expression during transformation induction. We devised a simple and sensitive method using reverse transcription followed by radioactive polymerase chain reaction (RT-PCR) to measure the relative change in c-myc mRNAs arising from each of the four known promoters. We show that a three to fivefold increase in c-myc transcripts from the P1 and P3 promoters occurs in both human and mouse cell lines within 30 min of the shift to the permissive temperature. The major P2-initiated transcripts are not significantly effected. However, exon 3-containing RNAs increase more gradually up to 24 h postinduction and P1 and P3 transcripts, while remaining elevated, still contribute relatively little to the total c-myc RNA population. These and other results, demonstrating a transient activation of P1 and P3 promoters, suggest an indirect role of the minor transcripts in the deregulated expression of the c-myc gene in transformed cells.