Epidermal growth factor increases protein and messenger RNA expression levels of Ras GTPase activating protein.

The Ras GTPase activating protein (RasGAP) is a key regulatory enzyme in the Ras signaling pathway, which is crucial for growth factor-induced mitogenesis. In this study, it is shown that epidermal growth factor (EGF) increases RasGAP protein expression by 100-150% in NIH 3T3 cells, which overexpress the human EGF receptor, and mouse keratinocytes but does not increase RasGAP protein expression in A431 cells, where EGF does not have a mitogenic effect. In contrast, EGF does not affect the expression of other signal transduction SH2-containing proteins, such as phospholipase C gamma 1 and the p85 subunit of phosphatidylinositide 3-kinase. The growth factor-induced increase of RasGAP protein parallels an increase in RasGAP activity. EGF stimulates RasGAP protein synthesis and does not affect its degradation rate. The mechanism for RasGAP protein induction by EGF involves an increase in rasGAP mRNA levels. The growth factor-stimulated up-regulation of rasGAP is a delayed response, since the increase in the mRNA levels and protein synthesis rate begin after 3 h and reach maximal values between 9 and 24 h of growth factor treatment. EGF fails to increase rasGAP mRNA levels in the presence of cycloheximide, suggesting that this effect is dependent on de novo protein synthesis. However, cycloheximide alone is able to increase by 6-fold rasGAP mRNA expression. Since EGF does not modify rasGAP mRNA stability, the increase in rasGAP mRNA expression is likely due to an increase in transcription of the rasGAP gene.