Regulation of c-myc expression by granulocyte-macrophage colony-stimulating factor in human leukemia cells

Granulocyte-macrophage colony-stimulating factor (GM-CSF) stimulates both the proliferation and functional properties of normal and leukemic myeloid cells via cell surface receptors. The postreceptor mechanisms for these two actions, and the extent to which they represent overlapping biochemical pathways, have not been fully clarified. We have examined the actions of GM-CSF on the expression of c-myc, an early response oncogene associated with the proliferative stimulus of growth factors. GM-CSF reduced the population doubling time of HL-60 leukemia cells from 32 hours to 25 hours, and, at concentrations that were correlated with mitogenicity, induced a rapid twofold increase in the level of c-myc mRNA. Nuclear runoff studies indicated that GM-CSF approximately doubled the transcription rate of c-myc by reversing the transcription attenuation that occurs at the exon 1-intron 1 junction. GM-CSF had no effect on the half-life of c-myc messenger RNA. The biochemical basis for the modulation of c-myc expression by GM-CSF was explored. GM-CSF treatment caused intracellular alkalinization of the cells as measured using the fluorescent probe 2′,7′-bis-(2-carboxyethyl)-5(and-6) carboxyfluorescein (BCECF). The sodium channel blocker amiloride prevented the GM-CSF-induced change in pH_i, but did not affect the stimulation of c-myc transcription by GNI-CSF. Agents that increase cellular cyclic adenosine monophosphate (cAMP) levels (prostaglandin E₂ and cholera toxin) blocked the actions of GM-CSF on c-myc; however, these agents also reduced the basal level of c-myc expression. GM-CSF caused a rapid (5 minutes) and transient decline in cellular cyclic guanosine monophosphate (cGMP) levels, and a slower (30 minutes) and transient decrease in cellular cAMP levels. These observations are consistent with the hypothesis that the declines in cAMP and cGMP are associated with a stimulation of HL-60 proliferation, while previously reported manipulations that elevate cyclic nucleotides are related to an inhibition of HL-60 proliferation and the potentiation of differentiation.