Effects of epidermal growth factor and platelet-derived growth factor on c- fos and c- myc mRNA levels in normal human fibroblasts

Abstract

The mRNA levels of two proto-oncogenes, c- fos and c- myc, were determined in human foreskin fibroblasts exposed to epidermal growth factor (EGF) or platelet-derived growth factor (PDGF) in a serum-free, defined medium (MCDB 104). Untreated, quiescent cells were found to have low or undetectable levels of c- fos and c- myc mRNA. Within 10 min after the addition of EGF or PDGF the c- fos mRNA level increased, reached a peak at 30 min, and then declined to the control level after 60 min. The level of c- myc mRNA increased somewhat later and peaked after 8 h in cultures treated with either of the growth factors. The c- myc mRNA level remained elevated throughout the 24 h of investigation. The concentrations of EGF and PDGF required for a maximal effect on c- fos or c- myc expression were found to be similar to those that give maximal effect on cell proliferation. Both c- fos and c- myc mRNA expression were superinduced by the addition of cycloheximide. The addition of neutralizing PDGF antibodies to cultures that had received PDGF 4 h earlier inhibited the subsequent increase in the c- myc mRNA level, indicating that the effect of PDGF on c- myc expression is not caused by a “hit and run” mechanism. Density-inhibited cells responded to EGF and PDGF by an increase in c- fos and c- myc mRNA levels in the absence of any mitogenic response. The present results conform to the view that the c- fos and c- myc proto-oncogenes may be important (or necessary) but not sufficient for the initiation of DNA synthesis. Moreover, the finding that both EGF and PDGF increase c- fos and c- myc expression supports our previous suggestion that these two growth factors may in part act via a common intracellular pathway in the prereplicative phase of human fibroblasts.