Cell Growth & Differentiation, Vol 2, Issue 9 465-473, Copyright © 1991 by American Association of Cancer Research
Cell cycle analysis of Krox-20, c-fos, and JE expression in proliferating NIH3T3 fibroblasts
J Cortner and PJ Farnham
McArdle Laboratory for Cancer Research, University of Wisconsin, Madison 53706.
We have shown that early growth response genes which were identified on the
basis of their expression during the G0 to G1 transition can also be
induced in proliferating fibroblasts. The expression of Krox-20 and c-fos
mRNAs increased dramatically upon stimulation of cell populations of
increasing density and correlated with the percentage of cells in the G0-G1
stages of the cell cycle. However, fractionation of serum-stimulated
cultures into cell cycle stage-specific subpopulations using
fluorescence-activated cell sorting revealed that the levels of Krox-20 and
c-fos mRNAs were equal in all stages of the cell cycle. This result was
corroborated by serum and cycloheximide stimulation of stage-specific
fractions separated by centrifugal elutriation. Expression of the
immediate-early gene JE was also induced in all stages of the cell cycle in
the elutriated fractions. Thus, although the amount of induced Krox-20,
c-fos, and JE mRNA correlated with the number of quiescent cells in a
proliferating population, expression of all three genes occurred to the
same extent in all stages of the cell cycle at a given culture density.
Possible explanations for the density-associated increase in induced
expression are discussed. We have demonstrated that there are both
transcriptional and posttranscriptional components to the stimulated
expression of the Krox-20 and c-fos genes in proliferating fibroblasts.
Since the increased expression of these genes has the same steady-state and
transcription rate kinetics as serum response factor-mediated induction, we
are currently investigating the role of serum response factor in
serum-induced expression in proliferating cells.