Effects of IL-3 on promoter usage, attenuation and antisense transcription of the c-myc oncogene in the IL-3-dependent Ba/F3 early pre-B cell line.

The c-myc gene uses two major promoters, P1 and P2, for production of mRNA. In most proliferating normal cells, transcripts initiated from P2 predominate over the ones from P1. Furthermore, transcription of normal and translocated c-myc genes is bidirectional and overlapping. In this study, we have measured the effects of interleukin 3 (IL-3) deprivation and restimulation on c-myc promoter usage in the IL-3-dependent pre-B cell line Ba/F3. The rapid drop in c-myc mRNA expression observed in Ba/F3 cells upon IL-3 deprivation is reversible upon restimulation with interleukin. The use of P1 and P2 promoters, as judged by P1/P2 ratio, shifted from 0.2 in cells in exponential growth or in quiescence to 1.3 following IL-3 stimulation. This change was not due to selective instability of one of the two myc transcripts. In vitro nuclear run-on experiments indicated that IL-3 addition resulted in a large release of transcriptional attenuation, as well as a significant increase in transcriptional initiation. These results are consistent with the hypothesis that P1/P2 promoter usage is involved in the control of transcriptional elongation. Furthermore, deprivation of IL-3 resulted in a dramatic increase in antisense transcription, whereas little change was observed in the rate of initiation of elongation of c-myc mRNA precursors. This correlation suggests a negative role for antisense transcription in expression of c-myc mRNA levels in IL-3-deprived Ba/F3 cells.