c-myc gene is transcribed at high rate in G0-arrested fibroblasts and is post-transcriptionally regulated in response to growth factors

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Abstract
There is increasing evidence that at least some of the cellular homologues to retroviral oncogenes (c-onc or proto-oncogenes) are directly linked to the control of cell growth (for a review see ref. 1). Among these, c-myc, the cellular homologue to the avian myelocytomatosis virus (MC29) oncogene, has been shown to express high levels of mRNA during early G0/G1 phase after mitogenic stimulation of T lymphocytes2 by concanavalin A or of fibroblasts by platelet-derived growth factor (PDGF)2 or serum3. An attractive model proposed for this regulation is that the c-myc gene is strongly repressed in cells arrested in the G0 phase of the cell cycle by a growth factor-sensitive represser4. We have investigated an alternative model of post-transcriptional regulation. This latter model leads to two testable predictions. First, that c-myc mRNA should be unusually unstable, which we have confirmed5. And second, that there would be a high level of constitutive expression, a situation opposite to that implied by the represser model. Here we report that c-myc gene is indeed transcribed at a high rate in G0-arrested chinese hamster lung fibroblasts, although the level of mature c-myc mRNA is barely detectable. The early and dramatic increase in c-myc mRNA levels when these resting cells are stimulated by growth factors is not accompanied by any appreciable change in the transcription rate of c-myc gene. Taken together these findings support a model of post-transcriptional regulation of c-myc expression at the level of mRNA degradation.