Regulation of c-myc expression by granulocyte-macrophage colony-
stimulating factor in human leukemia cells
EL Schwartz, H Chamberlin and AB Brechbuhl
Department of Oncology, Albert Einstein College of Medicine, Bronx, NY
10467.
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, but did not
affect the stimulation of c-myc transcription by GM-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.
Volume 77,
Issue 12,
pp. 2716-2723,
06/15/1991
Copyright © 1991 by The American Society of Hematology
