J. Biol. Chem., Vol. 261, Issue 18, 8158-8162, 06, 1986
c-fos and c-myc gene activation, ionic signals, and DNA synthesis in thymocytes
JP Moore, JA Todd, TR Hesketh and JC Metcalfe
Among the earliest responses to mitogens that have been detected in normal
quiescent cells are ionic changes: we have described rapid increases in the
cytosolic free Ca2+ concentration ([Ca]i) and in the intracellular pH (pHi)
in mitogen-stimulated thymocytes and fibroblasts (Hesketh, T. R., Moore, J.
P., Morris, J. D. H., Taylor, M. V., Rogers, J., Smith, G. A., and
Metcalfe, J. C. (1985) Nature 313, 482-484). Here we investigate the
relationship between these ionic signals and the subsequent expression of
the c-fos and c-myc proto-oncogenes in murine thymocytes. We show that the
plant lectin concanavalin A (ConA), the phorbol ester 12-O-tetradecanoyl
phorbol 13-acetate (TPA) and the Ca2+- ionophore A23187 each causes a rapid
increase in both c-fos and c-myc mRNAs. The activation of both genes is
completely dependent on the extracellular Ca2+ concentration ([Ca]o) for
A23187 and independent of [Ca]o for TPA. Activation of c-myc, but not
c-fos, by ConA is partially dependent on [Ca]o. The pHi increases generated
by ConA or TPA are not necessary for expression of mRNA from either gene in
response to these mitogens. Exogenous 8-bromo-cyclic AMP (but not
8-bromo-cyclic GMP) inhibits the c-myc responses to ConA and TPA. The data
also show that neither early c-fos nor c-myc expression is sufficient to
commit the cells to DNA synthesis.