Differential phenotypic expression induced in cultured rat astroblasts by acidic fibroblast growth factor, epidermal...

We compared the effects of three growth factors, acidic fibroblast growth factor (aFGF), epidermal growth factor (EGF), and thrombin, on rat astroblast proliferation, morphology, glutamine synthetase-specific activity, and phenotypic expression of proteins. In vitro experiments were made on 20-day-old primary cultures. Astroblast proliferation was stimulated transiently (after 48 h treatment) by the three growth factors, while the cell glutamine synthetase activity began to increase significantly only after 3 days of treatment. Acidic FGF and EGF, but not thrombin, modified the cell morphology. The effects on phenotypic expression were first determined after 5 days of treatment to minimize the mitogenic effect of the factors. Proteins synthesized during the last 18 h of the treatments were separated by two-dimensional polyacrylamide gel electrophoresis. About 600 spots were compared, 54 were modulated by the various treatments, 13 were altered similarly by all three factors, 28 by aFGF and EGF, 7 by only aFGF, 3 by only EGF, and 3 by only thrombin. These results indicate a large similarity of effects between aFGF and EGF (41 proteins) and show that these factors elicit a more extended modulation of the phenotypic expression than thrombin (13 proteins). Each of the three factors has a few specific effects, which suggests that even for aFGF and EGF, which are supposed to elicit their effects through membrane receptor-associated tyrosine kinase activity, some specificity appears in their mechanism of action. A model is proposed to suggest that cell maturation is characterized by the modulation of the synthesis of many proteins which can be grouped into classes. Each class appears to be under the control of one regulatory element. The specificity of the effect of a growth factor should result from the activation of a specific combination of such regulatory elements. Analysis of the proteins after only 18 h of treatment, when neither proliferation nor maturation were significantly affected, showed that 11 proteins were regulated only at that time. These proteins could be related to intermediate steps of the growth factor signal transduction.