[Constitutive activity of MAP kinase cascades in REF cells transformed by E1A and cHa-ras oncogenes]

Proteins of Ras family play an important role in regulation of cell growth and proliferation, and their mutations can lead to growth factor-independent proliferation due to constitutive activity of various signal transduction cascades. In the present work, we studied the activity of ERK, JNK and p38 MAP-kinase cascades in rat embryo fibroblast cells transformed with oncogenes E1A and cHa-ras. These transformed cells are characterized by a high and non-regulated activity of transcription factor AP-1 involved in the regulation of cell proliferation. Since phosphorylation of AP-1 depends on the activity of relevant MAP-kinase cascades (ERK, JNK and p38), we analysed the expression of non-phosphorylated forms of the kinases and their phosphorylated state in E1A + cHa-ras cells using antibodies specific to non-phosphorylated and phosphorylated proteins. It has been established that transformed cells contain higher amounts of non-phosphorylated ERK, JNK and p38 kinases, thus implying a reduced degradation of these and other proteins in the transformants. The content of phosphorylated (active) forms studied in Western blot-analysis with phosphoantibodies was shown to be also higher in exponentially growing E1A + cHa-ras cells. But serum stimulation of the starved cells gave insignificant rise to an increase of ERK, JNK and p38 phosphorylation. Nevertheless, an in vitro kinase assay performed with the kinases, either immunoprecipitated by antibody or bound to GST-fusion substrates, enabled us to show a certain level of stimulation of c-Jun-associated (JNK) and MEF2A-associated (p38) kinase activity in serum stimulated E1A + cHa-ras cells. Thus, the obtained results show that transformation of fibroblasts with E1A and ras oncogenes may contribute to constitutive activation of ERK, JNK and p38 kinase cascades responsible for a high and non-regulated activity of MAP-kinase-dependent transcription factors, in particular AP-1.