Contribution of Known Mitogenic Signaling Pathways to Induction of DNA Synthesis in Quiescent Chinese Hamster Fibroblasts

The mitogenic pathways so far identified in mammalian cells fall into three main categories: tyrosine kinase, kinase C, and the cAMP-dependent pathways. In quiescent murine 3T3 fibroblasts, all three signaling pathways synergize with each other to restart DNA synthesis. In order to establish if the same was true in other rodent fibroblast lines we studied the effects of factors, known to modulate the above-mentioned pathways, on DNA synthesis in Chinese hamster embryo fibroblasts (CHEF/18). The factors examined were: (1) EGF and insulin representative of tyrosine kinase-activating growth factors, (2) TPA as specific activator of protein kinase C, (3) cholera toxin, dibutyryl cyclic AMP, and theophylline as compounds increasing cAMP levels. We found that EGF alone is a strong mitogen in CHEF/18 cells, probably because it can modulate by itself all three pathways. Although cAMP acts as a growth enhancer in 3T3 cells, in CHEF/18 where high levels of cAMP were found, increased concentrations of this second messenger produce strong DNA synthesis inhibition and temporal disturbance of ribosomal protein S6 phosphorylation. Possible interpretations of these findings are presented.